Real-time measurement of nitrogen monoxide in … Health and Safety Real-time measurement of nitrogen monoxide in tunnels and its oxidation rate in diluted diesel exhaust Prepared

Executive Health and Safety

Real-time measurement of nitrogen monoxide in tunnels and its oxidation rate in diluted diesel exhaust

Prepared by the Health and Safety Laboratory for the Health and Safety Executive 2009

RR757 Research Report


Real-time measurement of nitrogen monoxide in tunnels and its oxidation rate in diluted diesel exhaust

Paul Dowker & Peter Walsh Health and Safety Laboratory Harpur Hill Buxton Derbyshire SK17 9JN

Exposure to oxides of nitrogen (NOx which denotes the mixture of nitrogen monoxide, NO, and nitrogen dioxide, NO2) commonly arises in the tunnelling industry from diesel engine exhaust emissions and from the use of explosives. The British Tunnelling Society (BTS) guidance levels for NO are 5 ppm for an 8 hour time weighted average (TWA) and 15 ppm for a 15 minute short term exposure limit (STEL). Real-time monitors are used by the construction industry as they provide a means of checking that controls are effective. Previous laboratory work at HSL evaluated various commercial detectors potentially suitable as portable monitors and studied the conversion rate of NO to NO2 in air using pure gases (ie NO and air). This current project investigated:

1 the field use of NO and NO monitors in a small sewer tunnel under construction to assist the determination 2

and application of effective controls in order to maintain a safe working environment. Furthermore, a large road tunnel under construction was visited to study the fixed NO monitoring system installed there.

2 the conversion rate of NO to NO2 using air-diluted diesel exhaust.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

HSE Books

© Crown copyright 2009

First published 2009

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording or otherwise) without the prior written permission of the copyright owner.

Applications for reproduction should be made in writing to:Licensing Division, Her Majesty’s Stationery Office,St Clements House, 2-16 Colegate, Norwich NR3 1BQor by e-mail to [email protected]

ACKNOWLEDGEMENTS

HSL are very grateful to Mr M Thomas and Mr P Shannon of Wessex Water; Mr R Stretton and Mr M Slater of Specialist Engineering Services Ltd; Mr P Hoyland, Mr A Watt and Mr R Bridge of Balfour Beatty; and Mr K Dawson and Mr M Rowland of Trolex Ltd. for their assistance and co-operation.

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CONTENTS

1 INTRODUCTION......................................................................................... 1

2 BRISTOL TUNNEL MONITORING............................................................. 22.1 The sewer tunnel ..................................................................................... 22.2 Monitoring of oxides of nitrogen in diesel fume and after explosions....... 22.3 Background monitoring of oxides of nitrogen in the tunnel ...................... 6

3 HINDHEAD ROAD TUNNEL VISIT .......................................................... 303.1 Introduction............................................................................................ 303.2 The road tunnel .................................................................................... 303.3 Gas monitoring ...................................................................................... 32

4 RATE OF REACTION OF NOX IN DIESEL EXHAUST FUME ................. 384.1 Laboratory investigation......................................................................... 384.2 Effects of other factors on NO conversion ............................................. 514.3 Recommendations................................................................................. 52

5 APPENDICES........................................................................................... 535.1 Appendix 1: Dust concentrations ........................................................... 535.2 Appendix 2: Determination of correction factors .................................... 60

6 REFERENCES.......................................................................................... 62

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EXECUTIVE SUMMARY

Background

Exposure to oxides of nitrogen (NOx which denotes the mixture of nitrogen monoxide, NO, and nitrogen dioxide, NO2) commonly arises in the tunnelling industry from diesel engine exhaust emissions and from the use of explosives. The British Tunnelling Society (BTS) guidance levels for NO are 5 ppm for an 8 hour time weighted average (TWA) and 15 ppm for a 15 minute short term exposure limit (STEL). Real-time monitors are used by the construction industry as they provide a means of checking that controls are effective. Previous laboratory work at HSL evaluated various commercial detectors potentially suitable as portable monitors and studied the conversion rate of NO to NO2 in air using pure gases (ie NO and air). This current project investigated:

1. the field use of NO and NO2 monitors in a small sewer tunnel under construction to assist the determination and application of effective controls in order to maintain a safe working environment. Furthermore, a large road tunnel under construction was visited to study the fixed NO monitoring system installed there.

2. the conversion rate of NO to NO2 using air-diluted diesel exhaust.

Objectives

The objectives were to:

1. evaluate the suitability of previously identified NO (and NO2) personal monitors for the accurate measurement of worker exposure in typical locations in various tunnels under construction.

2. investigate the factors influencing the oxidation of NO to NO2 in air-diluted diesel exhaust under controlled, laboratory conditions.

3. generate knowledge for the development of monitoring and control guidance for HSE and the construction industry relating to NOx, particularly NO.

Main Findings

Measurement of NO in tunnels

The monitor used (RAE Systems MultiRAE) appears robust enough to cope with the hostile environment of a tunnel under construction, both as a portable personal monitor and a fixed background monitor. The resolution of the datalogger in the MultiRAE (0.1 ppm) is effective for monitoring sub-ppm levels of NOx under such conditions.

The TWA exposure to NO of a tunnel engineer not constantly working in the immediate vicinity of diesel-powered equipment was just above 1 ppm. The TWA exposure level to NO of a tunnel engineer working constantly in the immediate vicinity of diesel-powered equipment, inferred from the data taken from a fixed monitor mounted on the equipment in which the engineer was working and not from actual personal exposure measurements, would be expected to be around 5 ppm.

Measurements of NO and dust concentrations were made continuously during a period of a few weeks using real-time monitors logging at 1 s intervals to allow estimations of the background

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concentrations of these species following blasting and from diesel exhaust emissions during normal working.

Background concentrations, ie concentrations between explosions due principally to diesel exhaust, adjusted for all of the peaks caused by blasting, ranged between 2 ppm and 8 ppm NO along the tunnel.

The average peak maximum due to the explosions reduces as distance from the blast face increases: from approximately 120 ppm (∼ 100 m from face) to 30 ppm (∼ 650 m from face).

The average time for which the NO concentration remains above 1 ppm following an explosion is approximately between 60 min and 90 min. The average time for which the NO concentration remains above 5 ppm following an explosion is approximately between 30 min and 50 min.

NO conversion rate in diluted diesel exhaust

The methods examined and compared in this investigation to determine the changes in oxides of nitrogen (NOx) over a 24 hour duration from initial concentrations of 5 ppm, 10 ppm and 50 ppm NO are:

• NO in diesel fume diluted with air. • pure NO in nitrogen mixed with nitrogen and oxygen mixtures. • mathematical modelling based on pure gas mixtures.

Each method showed similar results, allowing for the uncertainties involved in the measurements. The rate constants determined using the diesel exhaust fume for reactions at each concentration are similar to those determined from the modelled results over the first 3 hours of the tests. However, those determined from the tests using pure NO in O2 are not comparable except for the lowest concentration of 5 ppm NO over a less than 1 hour.

A literature evaluation indicates that: • exposure to UV light reduces the overall rate of conversion of NO to NO2 because of the

reformation of NO from NO2 resulting in higher levels of NO than would occur in the absence of UV.

• changes in temperature, pressure and water vapour (up to 90% humidity) have negligible effect on the overall rate of conversion of NO to NO2.

Recommendations

1. Instruments with a resolution of 0.1 ppm are preferable for effective monitoring of the background NO concentrations, particularly around 1 ppm, as found in the tunnels under construction. They should have adequate immunity from interferences (eg NO2, temperature) and long-term stability at this level.

2. Developments in NO sensor technology now make more precise 0.1 ppm resolution a possibility; this should be translated by instrument manufacturers into the next generation of portable and fixed NO monitors for tunnels.

3. NO in tunnels should be measured directly rather than being inferred from measured NO2 concentrations as it converts into NO2 only slowly, in the absence of ozone.

4. Adequate estimations of the conversion rate of NO to NO2 in air contaminated with diesel exhaust, in the absence of ozone, can be obtained through the modelled rate equation based on pure gases.

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5. The calculations of NO conversion rates in air-diluted diesel fume (and in pure gases) require the use of correction factors due in part to the use of electrochemical NO sensors which consumed a small quantity of gas. The use of a ‘non-destructive’ analyser such as Fourier Transform InfraRed (FTIR) for NO would reduce some of the uncertainty in conversion rate estimation, although correction for adsorption on the reaction chamber walls would still be necessary.

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1 INTRODUCTION

Exposure to oxides of nitrogen (NOx), which comprise nitrogen monoxide (also known as nitric oxide - NO) and nitrogen dioxide (NO2), commonly arises in the tunnelling industry from diesel engine exhaust emissions and from the use of explosives. The British Tunnelling Society (BTS) guidance levels for NO are currently 5 ppm for an 8 hour time weighted average (TWA) and 15 ppm for a 15 minute short term exposure limit (STEL) (BTS, 2008). Real-time monitors, both portable and fixed, can provide a means of checking that controls are effective in order to maintain a safe working environment. Real-time portable NOx monitors and fixed instruments are commercially available but have not been tested for operating accurately and on a routine basis at sub-ppm levels in the hostile environment of a tunnel under construction.

The measurement and control strategy for NO and NO2 will depend to an extent on the relative amounts of each. However, this could be complicated by the environmental fate of NO and NO2. NO oxidises slowly to NO2 in the presence of oxygen (and absence of ozone). The kinetics of oxidation of NO to NO2, while understood from laboratory experiments on pure gases, are not as clear when considered in the field where there are additional factors to consider, eg the emission source (diesel exhaust and explosives, principally) which is a mixture of various gases and particulate matter; environmental factors (temperature, humidity, UV light, other pollutants such as ozone); and dispersion (natural and forced ventilation).

Guidance is therefore required to develop an appropriate monitoring strategy, particularly for personal exposure, based on (a) the type of gas monitors suitable for the application; and (b) the environmental fate of NO and NO2 when emitted from the exhaust (and explosives), and its subsequent dispersion and reaction in the workplace atmosphere.

A previous report (Dowker et al, 2007) focused on a laboratory evaluation of NO monitors in the field, and measurements of NO oxidation using pure gases (NO and air) in the laboratory. This report focuses on:

• Field evaluation of portable and fixed NO monitors in tunnels under construction, ie a small sewer tunnel and a large road tunnel.

• NO oxidation rate in air-diluted diesel exhaust fume in the laboratory.

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2 BRISTOL TUNNEL MONITORING

2.1 THE SEWER TUNNEL

2.1.1 Background

The information in section 2.1 is taken from the Wessex Water website (Wessex Water, 2008).

The tunnel in which the investigations were undertaken is being excavated by Wessex Water to alleviate sewer flooding in parts of Bristol city centre. The completed tunnel will be 850 m in length with an approximate cross section of 3 m by 3.5 m, and a drop of 1.25 mm/m. The tunnel entrance is around 10 m below ground level and, due to the relief of the area, the deepest part of the tunnel is 60 m below ground level.

2.1.2 Excavation/Construction Methods

Explosives are used as part of the construction process in a controlled manner to minimise the effects felt at ground level. Each blast takes up to 10 seconds to complete and involves the controlled use of explosives in a civil engineering application to break hard rock, which is conducive to the excavation of approximately 20 m of tunnel per week. The blasting operations take place 24 hours a day, seven days a week and were expected to continue until November 2008. The debris from the blast is removed from the tunnel into skips using a diesel powered excavator known as a ‘scoop tram’ .

The type of explosive used in the excavation of this tunnel is Peruit 28.

2.1.3 Safety Procedures

A siren is sounded before each detonation and measures are in place ensure the area around the site has been evacuated before an explosion occurs. Two minutes prior to blasting a red light is lit at either end of the working compound and an air horn sounds for three seconds. One minute prior to blasting an air horn emits two three second blasts with three second intervals. Immediately prior to blasting an air horn emits three second blasts and on completion of blasting a single five second blast is emitted.

The blast vibrations are monitored by a vibrograph placed in various buildings above the tunnel.

2.2 MONITORING OF OXIDES OF NITROGEN IN DIESEL FUME AND AFTER EXPLOSIONS

2.2.1 Introduction

This phase of the investigation was to evaluate the suitability of the MultiRAE multigas monitor as an effective ‘personal’ NOx monitor when operating in the hostile environment of a tunnel under construction. The MultiRAE was previously identified as having potential for this application (Dowker et al, 2007; Lamont et al, 2008).

2.2.2 Instrumentation

Two MultiRAE multigas monitors were selected for use in this part of the investigation due to their robust build and capability of sub-ppm resolution in the datalog facility (the display, however, only shows ppm resolution). These monitors were setup identically with NO, NO2 and O2 sensors installed and were calibrated to the relevant gases prior to the start of the

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investigation and the calibration checked on completion of the investigation. They were operated with external inline filters fitted to protect them as far as practicably possible from dust and damp ingress. The instrument serial numbers and colour codings, allocated for easier identification, are listed in Table 2.1

Table 2.1: Instrument serial numbers and allocated colour codings Instrument Serial No. Colour Coding MultiRAE 095-106763 blue MultiRAE 095-106772 brown

2.2.3 Experimental

Two different locations were selected in which to position MultiRAE monitors in order to observe different effects on the operation of the monitor.

The blue MultiRAE was mounted on a shot firer working at the face of the tunnel, his specific duties were concerned with mixing and installing the explosive charges in the blast face, and his general duties involved shoring the tunnel and hanging the ventilation pipe (‘bagging’). This MultiRAE would experience the general movement and exposure of the tunnel engineer over a normal working shift.

The brown MultiRAE was mounted on the ‘scoop tram’ used to clear the rubble created by the blast at the closed end (face) of the tunnel. This MultiRAE would experience the vibrations of the engine, sharp movements of the vehicle, would be in constant close proximity to the fume source and would be expected to be exposed to higher and more consistent levels of NOx than the blue MultiRAE. Figure 2.1 shows photographs of the ‘scoop tram’ clearing the tunnel and the position of the brown MultiRAE.

MultiRAE

MultiRAE

Figure 2.1: Photographs of the ‘scoop tram’ clearing the tunnel and the position of the brown MultiRAE

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The rubble is moved away from the face after the previous blast into specially excavated side bays along the length of the tunnel. This allows preparation for the next blast to take place while the rubble is removed the rest of the way out of the tunnel into the awaiting skips. The excavator therefore travels various distances into the tunnel to collect the rubble, returning to deposit the rubble into the skip at the tunnel entrance, each round trip taking approximately 15 minutes. The excavator is also used to transport shoring materials to the face as they are required where it would be in close proximity to the tunnel engineers working at the face.

The MultiRAEs were monitoring the exposure levels in the two positions constantly from approximately 9:40 to 16:10 (6.5 hrs).

2.2.4 Results

The real time concentrations of NO, NO2 and O2 detected by the two MultiRAEs are shown in Figures 2.2 and 2.3. The average concentrations of NO and NO2 detected by the MultiRAE mounted on the ‘scoop tram’ (brown) are shown for the whole shift, but the average concentrations of NO and NO2 detected by the MultiRAE on the operative (blue) do not include the time from approximately 10:15 to 11:15 when the engineer was off site.

Figure 2.2: Concentrations of NO, NO2 & O2 detected by MultiRAE mounted on scoop tram (brown MultiRAE) during typical working shift in tunnel

Inspection of the NO trace from the MultiRAE mounted on the scoop tram in Figure 2.2 shows the main minima to be approximately 15 minutes apart, which correspond to the time expected for each round trip as stated in the previous section. Most of the maxima range between 6 ppm and 12 ppm with an average concentration over the 6.5 hr shift of 5.0 ppm. The minimum values of NO concentration rarely reduced below 1 ppm. The vast majority of the NO2 maxima are less than 1 ppm with an average concentration over the 6.5 hr shift of 0.2 ppm.

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NO Ave 5.0 ppm

NO2 Ave0.2 ppm

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Figure 2.3: Concentrations of NO, NO2 & O2 detected by MultiRAE mounted on tunnel engineer (Blue MultiRAE) during typical working shift in tunnel

Inspection of the NO trace from the MultiRAE mounted on the tunnel engineer in Figure 2.3 shows the maxima to be at irregular intervals with no obvious correlation to the movements of the scoop tram, which can be expected as for most of the time it was operating a considerable distance away, covering the full 330 m length of the tunnel. The times at which the higher NO maxima occur may coincide with the times when the excavator was clearing the rubble from positions closer to the blast face or was at the blast face delivering equipment. The average concentration of NO over the 6.5 hr shift (not considering the time for which the operative was off site) is 1.1 ppm. The vast majority of the NO2 maxima are less than 0.6 ppm with an average concentration over the 6.5 hr shift of 0.1 ppm.

2.2.5 Conclusions

The MultiRAE appears robust enough to cope with the hostile environment of a tunnel under construction, from the vibrations and jerky movements of the Scooper tram and the associated constant close proximity of the fume source to the general movement of a tunnel engineer over a normal working shift. However, this is only over a short time period; the instrument would be required to function over several months between calibration and maintenance periods

The resolution of the datalogger in the MultiRAE seems also to be effective to monitor sub ppm levels of NOx under such conditions.

The TWA exposure level to NO of a tunnel engineer not constantly working in the immediate vicinity of the excavator is just above 1 ppm. The TWA exposure level to NO of a tunnel engineer working constantly in the immediate vicinity of the excavator, inferred from the data taken from a monitor mounted on the vehicle in which the engineer was working and not from actual personal exposure measurements, would be expected to be around 5 ppm. The British Tunnelling Society (BTS) guidance levels for NO are 5 ppm for an 8 hour time weighted average (TWA) and 15 ppm for a 15 minute short term exposure limit (STEL) (BTS, 2008).

For NO2 the TWA exposure levels in both cases are well below 0.5 ppm. There are no BTS guidance levels for NO2 exposure.

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2.3 BACKGROUND MONITORING OF OXIDES OF NITROGEN IN THE TUNNEL

2.3.1 Introduction

This phase of the investigation was designed to monitor the NOx concentrations along the length of the tunnel produced by the controlled explosions and the diesel powered machinery involved in the excavation of the tunnel. As the temperature and humidity of the air in the tunnel and the dust caused by the explosion may affect the operation of the instruments monitoring the NOx levels, these were also monitored at the same positions in the tunnel. Two visits were undertaken.


MultiRAE multigas monitors were selected to monitor the NOx concentrations due to their robust build and capability of sub-ppm resolution. They were operated with external inline filters fitted to protect them as far as practicably possible from dust and damp ingress. Tiny Tag temperature and humidity monitors and Thermo Electron Personal DataRAM (PDR) respirable dust monitors (Thermo, 2008) were used to monitor other conditions at the same positions in the tunnel which it was thought may affect the operation of the MultiRAEs. These monitors were setup identically and were calibrated to the relevant gases or conditions prior to the start of the investigation and the calibration checked on completion of the investigation. It should be noted that these instruments are only intended to be portable monitors and are not designed for the more extreme conditions they were subjected to in this study.

The MultiRAEs were calibrated at 5 ppm NO, 5.7 ppm NO2 and 20.9% O2 in order to ensure greater accuracy at the lower exposure levels which are of prime interest here. The accuracy of higher values detected in these tests will therefore be subject to greater uncertainty and as such will be analysed qualitatively rather than quantitatively.

The PDR dust monitors respond essentially (but not exactly) to the respirable fraction of dust. They were calibrated using 5 mg/m3 respirable silica dust (Arizona road dust) in a dust tunnel at HSL. The detected reading is therefore the ‘silica’ equivalent of this dust in the tunnel and not the actual respirable dust concentration of the tunnel dust.

The instrument serial numbers and colour codings allocated for easier identification are listed in Table 2.2.

Table 2.2: Instrument serial numbers and allocated colour codings Instrument Serial No. Colour Coding MultiRAE 500871 black MultiRAE 095-106710 green MultiRAE 095-106778 pink MultiRAE 095-106720 purple

PDR 02548 blue PDR 02314 orange PDR 02535 red

Tiny Tag 1 139386 blue Tiny Tag 2 139422 green Tiny Tag 3 177528 red

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The internal memory limitations of the MultiRAEs and the PDRs meant that data recording periods of 4 minutes and 2 minutes respectively would have to be utilised in order to record the data for the full duration of each investigation of 20 and 10 days. Also, to allow more temporal resolution and therefore more accurate analysis of the results, each of the instruments was connected to an external Antilog datalogger which had the capability to record the data from that instrument at 1 second data sampling periods for the full duration of each investigation. The internal memory limitations of the Tiny Tags also meant that data recording periods 2 minutes had to be utilised in order to record the temperature and humidity data for the full investigation periods, but the changes in these values were not expected to be fast enough to require better resolution.

The MultiRAEs, PDRs and the Antilog dataloggers were mains-powered to enable them to operate for the duration of the investigation. The Tiny Tags operated for the duration of the investigation powered by their own internal batteries.

Six metal mesh cages were produced in which to house the instruments to afford protection from the hostile environment in which they were to be operating. Each cage had its own mains lead with dual socket into which the supply to the instruments were connected as required. The extension lead was connected to the 110 V mains supply of the tunnel. The cages were covered by a solid metal top which overhung the sides with a longer overhang above the mains socket position. Figure 2.4 shows photographs of a cage containing a Tiny Tag, a MultiRAE and an Antilog, the MultiRAE and Antilog being connected to the 110 V powered dual socket via their dedicated power leads. The PDRs and the associate Antilogs were housed in similar cages. The cages were located at such a height and position that they would not restrict the work practices of, and could not easily be damaged by, vehicles or engineers working in the tunnel.

The data from the internal memories of the MultiRAEs and PDRs show the average concentrations of NOx or dust over the sampling period of 4 minutes and 2 minutes respectively (1 second data values averaged over that period). Any times stated will therefore have a minimum temporal uncertainty equal to those sampling periods, and will not show the actual peak values. The data from the dataloggers, however, will be accurate to 1 second and will show the actual peak values.

It was possible to load the data stored in the internal memories of the MultiRAEs, PDRs and Tiny Tags straight into Microsoft Excel for analysis, but the amount of data stored in the Antilog dataloggers at 1 second intervals (approximately 1,728,000 data points for 20 day investigation and 864,000 data points for 10 day investigation) was far too much for Microsoft Excel to cope with. It was therefore necessary to separate this data into datasets smaller than the 32,000 data point limit with which Excel is able to create a chart, to allow analysis of graphical representations of the MultiRAE or PDR data from each of the dataloggers. This introduced difficulties when determining values over the full duration of the investigations from this more accurate data, and in these cases the slightly less accurate but more easily manipulated data from the internal memory of the instruments was used.

The concentrations of NOx and respirable dust shown in the figures are derived from the data stored in the internal memories of the MultiRAEs and the PDRs and have the temporal and quantitative limitations previously stated, however, these single figures give a visual representation of the concentrations of NOx and dust produced by the explosions and the machinery over the full duration of the investigations. The figures do allow comparison of the magnitude and position of these peaks and the background concentrations between them at each position in the tunnel. It is doubtful whether this can be represented with any greater accuracy in one figure by the 1 second data from the dataloggers due to the limitations of the available graphical software and the pixels resolution on the monitor and printer.

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The average background concentrations at each position along the tunnel are taken over the full duration of the investigations and are therefore are determined from the averaged data samples logged in the memories of the MultiRAEs and the PDRs. The averaging of the data should result in a similar level of accuracy as would be achieved if averages were taken from the less easily manipulated 1 second data stored on the Antilog dataloggers.

The 1 second data from the datalogger shown in the results allow a more accurate analysis of the data observed over smaller time scales and is use to determine the detection delays and peak widths where possible.

Figure 2.4: Cage containing a Tiny Tag, a MultiRAE and an Antilog

2.3.3 Visit 1: 05/06/08 – 24/06/08 (20 days)

2.3.3.1 Experimental

The approximate tunnel length throughout this part of the investigation ranged from 330 m to 380 m.

Two protective cages were located at each test position in the tunnel, one cage housed a Tiny Tag temperature and humidity monitor, and a MultiRAE multigas monitor connected to an Antilog datalogger, while the other housed a PDR dust monitor connected to an Antilog datalogger. Table 2.3 shows the instruments located at each position along the tunnel.

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Table 2.3: Instrument location along the tunnel Test Position

Number Distance from face (at start of investigation)

Distance from entrance

Instruments in Cage 1

Instruments in Cage 2

1 100 m 230 m green MultiRAE Antilog 1 blue Tiny Tag 1

blue PDR Antilog 4

2 170 m 160 m purple MultiRAE Antilog 2 green Tiny Tag 2

orange PDR Antilog 5

3 295 m 35 m black MultiRAE Antilog 3 red Tiny Tag 3

red PDR Antilog 6

These positions were not altered throughout the duration of the investigation, however, the blast face moved approximately 50 m over the course of the 20 day investigation. Figure 2.5 shows photographs of the tunnel and the method of mounting the cages.

Figure 2.5: The tunnel and the instrument cages

The instruments were left in place for 20 days after which time they were recovered and returned to HSL Buxton for the analysis.

Times at which the explosions were carried out over the 20 day period were provided by Wessex Water and are listed in Table 2.4 along with the weight of explosive used.

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Table 2.4: Explosion times and weight of explosive used Date Time Weight (kg) Thur 05/06/08 AM / /

PM 14:29 102 Fri 06/06/08 AM 06:42 79

PM / / Sat 07/06/08 AM / /

PM / / Sun 08/06/08 AM / /

PM 13:29 70.6 Mon 09/06/08 PM 12:20 72.2

PM 22:06 66.2 Tue 10/06/08 AM 11:04 65.8

PM 22:37 65.0 Wed 11/06/08 AM 11:50 62.8

PM 23:06 63.8 Thur 12/06/08 AM 10:55 63.4

PM 22:19 63.6 Fri 13/06/08 AM 10:56 63.4

PM / / Sat 14/06/08 AM / /

PM / / Sun 15/06/08 AM / /

PM 13:00 66.0 Mon 16/06/08 AM / /

PM 18:14 83.2 Tue 17/06/08 AM / /

PM 14:01 76.6 Wed 18/06/08 AM 07:15 72.4

PM 22:54 76.8 Thur 19/06/08 AM / /

PM 15:11 86.4 Fri 20/06/08 AM 09:10 84.0

PM / / Sat 21/06/08 AM / /

PM / / Sun 22/06/08 AM / /

PM / / Mon 23/06/08 AM 08:22 88.6

PM / / Tue 24/06/08 AM 03:34 63.8

PM / /

2.3.3.2 Results

The day after the investigation started a message from the tunnel sub-contractors, Specialist Engineering Services Ltd (SES), was received stating that the MultiRAE alarms had started sounding immediately after the first explosion had occurred at around 14:30 the previous day, and were still sounding 12 hours later. The audible volume of these alarms was intolerable to the tunnel engineers and may have masked other audible warnings. As the alarms signalled a pump malfunction, which at that time was expected to stop the instrument working effectively, instructions were given to SES to turn off the MultiRAEs.

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On returning to the tunnel 20 days later it was discovered that two of the MultiRAEs had been turned off but the green MultiRAE positioned 100m from the face was still alarming.

Inspection of the data stored on each of the MultiRAEs and the respective dataloggers showed that, while the black and the purple MultiRAEs had stopped logging when they had been turned off and hence contained no useful data, the green MultiRAE positioned 100m from the face had been logging to its internal memory and the Antilog datalogger for the duration of the investigation.

The PDRs and the Tiny Tags had been unaffected by the explosions and were also found to have been logging for the duration of the investigation.

The red PDR positioned 295 m from the face was found to have logged to the Antilog datalogger intermittently, which, due to the way the datalogger stores data, prevented any meaningful analysis of this data. Further investigation showed the problem to have been an intermittent fault in the connecting lead between the two instruments, rather than the effects of the explosions.

In an attempt to evaluate the data recorded by the green MultiRAE positioned 100 m from the face, and to allow other MultiRAEs to be used in similar conditions if required, calibration checks were performed on the green, pink and purple MultiRAEs with the pumps de-activated and with the alarms sounding (muted by blocking the audio outlet). The detected values when exposed to various concentrations of NO and NO2 were found to be lower with the pump deactivated. The values were repeatable and had linear relationships to the actual concentrations, allowing correction factors to be determined for each MultiRAE when operating without the pump. It should be stated that RAE do not mention the use of MultiRAEs with de-activated pumps in any of their literature.

Table 2.5 shows the determined correction factors which need to be applied to any NO or NO2 data obtained from the MultiRAEs with the pumps de-activated.

Table 2.5: Correction factors for NO or NO2 data obtained from MultiRAEs with the pumps deactivated

MultiRAE NO correction factor NO2 correction factor Green 1.6 2.4 Pink 1.4 2.8 Purple 1.6 2.1

All of the results from data obtained from the MultiRAEs with the pump de-activated will be displayed as corrected values after application of the relevant correction factors in Table 2.5

11

NOx Concentrations

Figures 2.6 and 2.7 show the corrected NO and NO2 concentrations logged at 4 minute datalog intervals in the internal memory of the green MultiRAE located 100 m from the blast face over the 20 day period.

Figure 2.6:

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Figure 2.7: Corrected NO2 concentrations detected over 20 days 100m from face (green MultiRAE)

These data were analysed using the parameters defined in Table 2.6

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Table 2.6: Definition of parameters by which the data obtained in the tunnel were analysed Parameter Definition

Detection delay Time between blast occurrence and time at which the associated NO and NO2 concentrations rose above 1 ppm at predetermined distances along the tunnel

Peak width > 1 ppm * Time for which NO and NO2 concentration is above 1 ppm after each blast at predetermined distances along the tunnel

Peak width > 5 ppm * Time for which NO and NO2 concentration is above 5 ppm after each blast at predetermined distances along the tunnel

Peak Value The maximum concentration of NO and NO2 after each blast at predetermined distances along the tunnel

Background Concentration The concentration of NO and NO2 due to effects other than the immediate effects of the blasts at predetermined distances along the tunnel

*The NOx and dust concentrations before the explosions were generally less than 1 ppm or 1 mg/m3 but often did not reduce to 1 ppm or 1 mg/m3 after the explosions due to other sources (diesel powered equipment etc.) beginning to operate in the tunnel as the concentration reached ‘acceptable’ levels. The peak width times were therefore calculated using values extrapolated from the data in these situations.

Tables 2.7 and 2.8 show the detection times and corrected concentrations of NO and NO2 recorded in Antilog 1 from the green MultiRAE located 100m from the blast face over the 20 day period.

Table 2.7: NO detection times, peak widths and concentrations taken from 1 s data recorded 100m from face (Antilog 1 from green MultiRAE)

Date Time Explosive weight

(kg)

Detection delay (m:ss)

Peak width (> 1ppm) (h:mm:ss)

Peak width (> 5ppm) (h:mm:ss)

Peak Value (ppm)

Thur 05/06/08 14:29 102 6:25 0:59:55 0:31:17 568 Fri 06/06/08 06:42 79 7:12 1:25:48 0:50:08 326 Sun 08/06/08 13:29 70.6 5:41 0:55:19 0:34:05 259 Mon 09/06/08 12:20 72.2 6:27 1:03:33 0:39:33 158

22:06 66.2 5:52 1:33:08 0:48:17 113 Tue 10/06/08 11:04 65.8 5:56 1:05:04 0:34:48 65

22:37 65.0 6:07 2:01:53 0:47:48 140 Wed 11/06/08 11:50 62.8 6:35 1:03:25 0:48:16 110

23:06 63.8 6:43 1:47:17 0:48:21 126 Thur 12/06/08 10:55 63.4 6:52 1:28:08 0:43:01 72

22:19 63.6 7:07 1:48:53 0:44:27 87 Fri 13/06/08 10:56 63.4 7:27 1:56:33 0:48:03 125 Sun 15/06/08 13:00 66.0 8:03 1:36:57 0:39:50 64 Mon 16/06/08 18:14 83.2 7:55 2:08:05 0:52:46 66 Tue 17/06/08 14:01 76.6 7:17 1:51:43 0:42:15 89 Wed 18/06/08 07:15 72.4 6:51 1:53:09 0:47:06 146

22:54 76.8 7:19 1:28:41 0:43:19 169 Thur 19/06/08 15:11 86.4 7:38 1:56:22 0:46:59 140 Fri 20/06/08 09:10 84.0 7:57 1:57:03 0:52:57 109 Mon 23/06/08 08:22 88.6 8:25 1:29:35 0:44:23 99 Tue 24/06/08 03:34 63.8 7:37 1:18:23 0:36:54 145 Average Values 73.1 7:01 1:33:45 0:44:02 151.2

σ = 10.8 σ = 0:46 σ = 0:22:43 σ = 0:06:04 σ = 114.5

13

Table 2.8: NO2 detection times, peak widths and concentrations taken from 1 s data recorded 100m from face (Antilog 1 from green MultiRAE)

Date Time Explosive Detection Peak width Peak width Peak Value weight (kg) delay (m:ss) (> 1ppm) (> 5ppm) (ppm)

(h:mm:ss) (h:mm:ss) Thur 05/06/08 14:29 102 6:25 0:41:35 0:18:04 275 + Fri 06/06/08 06:42 79 7:18 0:40:42 0:28:22 275 + Sun 08/06/08 13:29 70.6 5:46 0:45:14 0:26:49 275 + Mon 09/06/08 12:20 72.2 6:26 0:03:34 0:26:49 219

22:06 66.2 6:07 0:32:53 0:18:43 74 Tue 10/06/08 11:04 65.8 6:13 0:41:47 0:19:33 46

22:37 65.0 6:15 1:07:45 0:14:15 85 Wed 11/06/08 11:50 62.8 6:41 0:26:19 0:15:09 60

23:06 63.8 7:00 0:32:00 0:15:39 88 Thur 12/06/08 10:55 63.4 6:53 0:29:12 0:18:06 42

22:19 63.6 7:19 0:48:41 0:19:52 50 Fri 13/06/08 10:56 63.4 7:57 0:41:03 0:15:10 105 Sun 15/06/08 13:00 66.0 8:24 0:51:36 0:16:47 36 Mon 16/06/08 18:14 83.2 8:10 0:37:50 0:18:40 32 Tue 17/06/08 14:01 76.6 7:32 0:36:28 0:15:20 53 Wed 18/06/08 07:15 72.4 6:51 0:53:09 0:12:30 101

22:54 76.8 7:39 0:28:21 0:12:25 129 Thur 19/06/08 15:11 86.4 7:54 0:41:06 0:16:26 115 Fri 20/06/08 09:10 84.0 8:06 0:49:54 0:17:14 82 Mon 23/06/08 08:22 88.6 8:30 0:44:30 0:17:56 60 Tue 24/06/08 03:34 63.8 7:48 0:33:12 0:10:54 115 Average Values 73.1 7:12 0:39:22 0:17:51 110.3

σ = 10.8 σ = 0:49 σ =0:12:44 σ = 0:04:38 σ = 80.4

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Figure 2.8: Typical NOx peaks detected in tunnel, 100m from blast face

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Figure 2.8 shows typical NO and NO2 exposure curves after an explosion in the tunnel. The rise time can be seen to be short reaching a concentration above 5 ppm approximately 15 s after initial detection.

With two exceptions (13:29 on 08/06/08 and 12:20 on 09/06/08) the peak NO concentrations created in the explosions were an average of 1.5 times (σ = 0.3) greater than the peak NO2 concentrations.

After an explosion the average duration for which the concentration of NO 100 m from the face was above 1 ppm was 1 hr 34 min (σ = 22.5 min), and above 5 ppm was 44 min (σ = 6 min), while the average duration for which the concentration of NO2 was above 1 ppm was 39.5 min (σ = 12.5 min), and above 5 ppm was 18 min (σ = 4.5 min).

Figure 2.9 shows that the intensities of the NOx peaks 100 m from the face caused by the explosions showed a slight tendency to increase with the weight of the explosive used, however, the width of the peaks showed no such tendencies. The spread of results may be due to inconsistent packing density of the explosive.

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Figure 2.9: Relationship between explosive quantity and concentration of NOx detected over 20 days in tunnel by green MultiRAE positioned 100m from face

The average time delay between the explosion and the initial detection (> 1 ppm) of the NOx by the MultiRAE positioned 100 m away was 7 min (σ = 0.5 min). For the NOx to travel 100m in 7 mins (σ = 0.5 mins) estimating a plug flow it will travel at a velocity of 0.24 m/s (σ = 0.03 m/s), which constitutes flow rate of 2.5 m3/s (σ = 0.3 m3/s) in the tunnel with a cross sectional area of 10.5 m2. These values are much less than the values stated by the tunnel engineers of an extraction air velocity in the tunnel of 0.9 m/s and an air flow rate of 20 m3/s. As no time stamp was initiated on the MultiRAE data (and therefore the Antilog data) at the end of the investigation prior to switching off (due to the expectation that the pump alarm would prevent the instrument from operating) the accuracy of this delay time is uncertain. The tolerances in the initial measurements, inconsistencies in the tunnel dimensions along the length and the irregularly timed increase in tunnel length by 30 m over the 20 day duration of the investigation also add considerable uncertainty to the air velocity and flow rate calculated from the

15

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experimental data. The actual delay time, however, is very unlikely to be anywhere near the time required to travel 100m at 0.9 m/s of 1.9 mins. the tunnel by a different mechanism, or the initial concentration of NO reaching the sensor due to the explosion is less than the background level (which the short rise time evident in Figure 2.8 suggests is unlikely , or the flow rate/velocity of the air in the tunnel is less than stated. Further investigations into these discrepancies would be necessary before conclusions could be made regarding the transport mechanism.

Figure 2.10 shows the expanded scale at lower concentrations of NO monitoring at 100 m from the blast face over the 20 day period. This allows the corrected background concentration of

to be observed.

Figure 2.10: Corrected background concentrations of NO detected over 20 days in tunnel by green MultiRAE positioned 100m from face

concentrations shown in Table 2.9 are calculated from the The peak concentrations caused by the blasts are reduced by allowing for

average peak width times for NO concentrations above 1 ppm (1.5 hrs) and 5 ppm (1 hr) taken from Tables 2.7 and 2.8 from the initial detection time after each blast.

Corrected background concentrations of NO detected over 20 days in tunnel by green MultiRAE positioned 100m from face

ppm) NO2(ppm)

Ave Peak Width Time (hr)

Background NO (ppm)

Background NO2 (ppm)

1 (> 5 ppm) 3.3 σ = 3.7

1.9 σ = 0.4

1.5 (> 1 ppm) 3.1 σ = 3.6

1.8 σ = 0.4

Dust Concentrations

Details of this part of the investigation can be found in Appendix 1.

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Temperature and Humidity

The Tiny Tag monitors were selected for use in this investigation mainly because of their robust construction and ability to operate in the hostile environment of the tunnel. The accuracy of these instruments is uncertain, however, especially in the higher humidity evident in the tunnel, and the results should be analysed as qualitative rather than quantitative data. More accurate instrumentation is available at HSL but it is doubtful that it would have operated effectively in the hostile tunnel environment. The following conclusions were ascertained from the data recorded by the Tiny Tags:

• any sudden change in temperature or relative humidity (RH) due to the explosions is unlikely to be recorded due to the 2 minute sampling period of the Tiny Tags

• there is no obvious correlation between the temperature or relative humidity and distance from the face

• the temperature and relative humidity profiles were found to be very consistent over the duration of the test at all 3 positions along the tunnel

All of these effects prevented any useful observation of the effects of changes in temperature and humidity on the NOx concentrations in the tunnel.

2.3.4 Visit 2: 06/10/08 – 15/10/08 (10 days)

2.3.4.1 Experimental The approximate tunnel length throughout this part if the investigation ranged from 692 m to 717 m.

This part of the investigation was concerned only with the determination of NOx concentrations along the tunnel occurring as a background due to normal operating conditions and as higher concentrations due to the use of explosives at the face.

A protective cage was again located at each test position in the tunnel, each housing a Tiny Tag datalogger, and a mains powered MultiRAE multigas monitor connected to an Antilog datalogger. These positions were not altered throughout the duration of the investigation, however, the blast face moved approximately 25 m over the course of the 10 day investigation.

The datalogging periods of the instruments were the same as in the previous section. Table 2.14 shows the instruments located at each position along the tunnel. The pump in each MultiRAE was purposely de-activated and the alarms muted by blocking the audio outlet.

Table 2.14: Instrument location along the tunnel Test Position

Number Distance from face (at start of investigation)

Distance from entrance

Instruments in Cage

1 107 m 585 m Purple MultiRAE Antilog 3 red Tiny Tag 3

2 447 m 245 m Pink MultiRAE Antilog 2 green Tiny Tag 2

3 657 m 35 m Green MultiRAE Antilog 1 blue Tiny Tag 1

17

The instruments were left in place for 10 days after which time they were recovered and returned to HSL Buxton for the analysis.

Times at which the explosions were carried out over the 10 day period were again provided by Wessex Water and are listed in Table 2.15 along with the weight of explosive used.

Table 2.15: Explosion times and weight of explosive used Date Time Wt (kg)

Mon 06/10/08 AM / / PM 16:12 92.8

Tue 07/10/08 AM / / PM 20:10 56

Wed 08/10/08 AM 11:18 66.2 PM / /

Thu 09/10/08 AM 8:36 40 PM / /

Fri 10/10/08 AM 9:05 43.4 PM / /

Sat 11/10/08 AM / / PM 21:35 88.4

Sun 12/10/08 AM / / PM 15:10 77

Mon 13/10/08 AM / / PM 22:00 42

Tue 14/10/08 AM / / PM 14:45 28.6

Wed 15/10/08 AM 6:45 40.6 PM / /

2.3.4.2 Results

On arrival at the tunnel after 10 days of monitoring all of the instruments seemed to be working satisfactorily with the exception of Antilog 2 which was not receiving data from the Pink MultiRAE.

Inspection of the data stored on each of the instruments showed that the data had been logged by all 3 Tiny Tags, all 3 MultiRAEs and 2 of the Antilogs (1 and 3) successfully, but as suspected no data had been logged by Antilog 2. Further investigation showed the problem to have been an intermittent fault in the connecting lead between the pink MultiRAE and Antilog 2.

A time stamp was recorded on each of the operational Antilog dataloggers using an external Oregon Scientific digital travel radio frequency (RF) controlled clock model RH823 (Oregon Scientific, 2008) as soon as the instruments were removed from the tunnel and before they were switched off.

Correction factors determined in the previous section were applied to the results obtained by the MultiRAEs with the pumps de-activated.

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NOx Concentrations Detected Figures 2.11 to 2.16 show the corrected NO and NO2 concentrations logged in the internal memory of the green, pink and purple MultiRAEs located 107 m, 447 m and 657 m respectively from the blast face over the 10 day period.

Figure 2.11: Corrected NO concentrations detected over 10 days 107 m from face (purple MultiRAE)

Figure 2.12: Corrected NO2 concentrations detected over 10 days 107 m from face (purple MultiRAE)

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Figure 2.14: Corrected NO2 concentrations detected over 10 days 447 m from face (pink MultiRAE)

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MultiRAE)

The 1 second data from the 2 operational dataloggers are shown in Tables 2.16 (NO) and 2.19 (NO2) (Antilog 3 from purple MultiRAE 107 m from face) and Tables 2.18 (NO) and 2.21 (NO2) (Antilog 1 from green MultiRAE 657 m from face) to allow a more precise analysis of the data observed. The temporally and quantitatively less precise data from the pink MultiRAE memory (245 m from face) is shown in Tables 2.17 (NO) and 2.20 (NO2).

21

These data were analysed using the parameters defined in Table 2.6

Table 2.16: NO detection times and concentrations taken from 1 s data recorded 107 m from face (Antilog 3 from purple MultiRAE,)

Date Time Explosive wt (kg)

Detection delay

(mm:ss) (± 30 s)

Peak width (> 1ppm)

(h:mm:ss) (± 30 s)

Peak width (> 5ppm)

(h:mm:ss) (± 30 s)

Peak Value (ppm)

(± 30 s)

Mon 06/10/08 16:12 92.8 03:24 0:43:02 0:17:27 199.5 Tue 07/10/08 20:10 56.0 02:26 0:58:34 0:18:18 215.7 Wed 08/10/08 11:18 66.2 04:12 0:52:48 0:20:27 162.4 Thu 09/10/08 08:36 40.0 04:09 1:19:51 1:05:27 58.4 Fri 10/10/08 09:05 43.4 -00:46 1:02:20 0:22:23 81.0 Sat 11/10/08 21:35 88.4 05:33 0:47:27 0:24:24 70.6 Sun 12/10/08 15:10 77.0 03:54 1:22:06 0:29:01 44.2 Mon 13/10/08 22:00 42.0 01:55 1:10:05 0:16:07 141.4 Tue 14/10/08 14:45 28.6 -00:35 0:44:35 0:15:25 116.6 Wed 15/10/08 06:45 40.6 -25:38 1:15:38 0:19:33 129.3 Average Values 57.5 03:39 1:01:39 0:24:51 121.9

σ = 22.4 σ = 01:13 σ = 0:14:42 σ = 0:14:50 σ = 59.0

Table 2.17: NO detection times and concentrations taken from 4 min data recorded 245 m from face (internal memory of pink MultiRAE)


Detection delay

(mm:ss) (± 4min)

Peak width (> 1ppm)

(h:mm:ss) (± 4min)

Peak width (> 5ppm)

(h:mm:ss) (± 4min)

Peak Value (ppm)

(± 4min)

Mon 06/10/08 16:12 92.8 09:00 1:09:00 0:34:00 49.0 Tue 07/10/08 20:10 56.0 03:00 1:15:00 0:44:00 48.3 Wed 08/10/08 11:18 66.2 04:00 1:20:00 1:00:00 51.0 Thu 09/10/08 08:36 40.0 10:00 1:24:00 0:35:00 22.0 Fri 10/10/08 09:05 43.4 04:00 2:01:00 1:02:00 28.8 Sat 11/10/08 21:35 88.4 13:00 1:16:00 0:43:00 29.5 Sun 12/10/08 15:10 77.0 10:00 1:28:00 1:00:00 24.9 Mon 13/10/08 22:00 42.0 06:00 2:04:00 0:52:00 40.0 Tue 14/10/08 14:45 28.6 05:00 4:10:00 1:17:00 45.2 Wed 15/10/08 06:45 40.6 -20:00 1:25:00 0:36:00 41.6 Average Values 57.5 07:07 1:29:07 0:50:18 38.0

σ = 22.4 σ = 03:29 σ = 0:19:48 σ = 0:14:17 σ = 10.8

Table 2.18: NO detection times and concentrations taken from 1 s data recorded 657 m from face (Antilog 1 from green MultiRAE)


Detection delay

(mm:ss) (± 30 s)

Peak width (> 1ppm)

(h:mm:ss) (± 30 s)

Peak width (> 5ppm)

(h:mm:ss) (± 30 s)

Peak Value (ppm)

(± 30 s)


σ = 22.4 σ = 02:56 σ = 24:46 σ = 0:03:31 σ = 5.8

22

Table 2.19: NO2 detection times and concentrations taken from 1 s data recorded 107 m from face (Antilog 3 from purple MultiRAE,)


Detection delay

(mm:ss) (± 30 s)

Peak width (> 1ppm)

(h:mm:ss) (± 30 s)

Peak width (> 5ppm)

(h:mm:ss) (± 30 s)

Peak Value (ppm)

(± 30 s)

Mon 06/10/08 16:12 92.8 04:14 0:13:33 0:08:12 69.7 Tue 07/10/08 20:10 56.0 02:33 0:33:41 0:12:16 202.7 Wed 08/10/08 11:18 66.2 04:16 0:42:44 0:15:13 140.9 Thu 09/10/08 08:36 40.0 04:35 0:23:26 0:14:31 39.9 Fri 10/10/08 09:05 43.4 -00:40 0:28:56 0:16:45 69.3 Sat 11/10/08 21:35 88.4 05:58 0:26:28 0:18:46 54.2 Sun 12/10/08 15:10 77.0 04:51 0:38:11 0:22:32 39.7 Mon 13/10/08 22:00 42.0 02:13 0:16:44 0:08:50 102.9 Tue 14/10/08 14:45 28.6 -00:20 0:14:54 0:10:42 74.6 Wed 15/10/08 06:45 40.6 -25:31 0:21:56 0:12:38 88.4 Average Values 57.5 04:06 0:26:03 0:14:03 88.2

σ = 22.4 σ = 01:19 σ = 0:09:54 σ = 0:04:29 σ = 50.4

Table 2.20: NO2 detection times and concentrations taken from 4 min data recorded 245 m from face (internal memory of pink MultiRAE)


Detection delay

(mm:ss) (± 4 min)

Peak width (> 1ppm)

(h:mm:ss) (± 4 min)

Peak width (> 5ppm)

(h:mm:ss) (± 4 min)

Peak Value (ppm)

(± 4 min)


σ = 22.4 σ = 03:03 σ = 0:03:20 σ = 0:03:22 σ = 9.9

Table 2.21: NO2 detection times and concentrations taken from 1 s data recorded 657 m from face (Antilog 1 from green MultiRAE)


Detection delay

(mm:ss) (± 30 s)

Peak width (> 1ppm)

(h:mm:ss) (± 30 s)

Peak width (> 5ppm)

(h:mm:ss) (± 30 s)

Peak Value (ppm)

(± 30 s)

Mon 06/10/08 16:12 92.8 20:02 0:32:08 0:17:47 22.3 Tue 07/10/08 20:10 56.0 18:12 0:34:28 0:18:58 24 Wed 08/10/08 11:18 66.2 20:21 0:35:52 0:19:22 21.8 Thu 09/10/08 08:36 40.0 20:17 0:29:00 0:12:50 10.2 Fri 10/10/08 09:05 43.4 16:21 0:32:39 0:15:22 11.5 Sat 11/10/08 21:35 88.4 23:28 0:32:58 0:17:03 12.2 Sun 12/10/08 15:10 77.0 23:42 0:34:49 0:12:39 7.4 Mon 13/10/08 22:00 42.0 18:50 0:27:36 0:13:55 15.6 Tue 14/10/08 14:45 28.6 16:20 0:28:20 0:15:08 14.9 Wed 15/10/08 06:45 40.6 -04:37 0:28:35 0:15:30 17 Average Values 57.5 19:44 0:31:38 0:15:51 15.7

σ = 22.4 σ = 02:40 σ = 0:03:02 σ = 0:02:23 σ = 5.6

23

The rise times were again found to be of a short duration after initial detection.

The peak width determined from the data recorded 245 m from the face (internal memory of pink MultiRAE) from the explosion at 14:45 on Tuesday 14/10/08 is much longer than any of the other data recorded by any of the MultiRAEs at any time in the duration of these tests and can therefore be considered spurious and discounted from the average peak width calculated for concentrations above 1 ppm from the data recorded 245 m (pink MultiRAE) from the face.

The averages of the detection delay times do not include the slightly negative values observed on Friday 10th and Tuesday 14th October in the data logged 107 m from the face (Antilog 3 from purple MultiRAE) or the much higher negative values observed on Wednesday 15th October in the data logged at all positions.

The time stamp initiated on each of the instruments at the end of the investigation showed the time on both Antilogs were within 30 s of each other and of that on the Oregon Scientific digital travel radio frequency (RF) controlled clock model RH823. This suggests that the times stated for the explosions on Friday 10th and Tuesday 14th October are suspect and for the explosion on Wednesday 15th October is in error.

All of the peak NO concentrations in the 3 positions investigated created in the explosions were an average of 1.5 times (σ = 0.4) greater than the peak NO2 concentrations.

The intensity of the detected concentrations of NOx decreases as the distance from the blast face increases, but due to the lack of temporal resolution in the internal memory of the pink MultiRAE, any reduction factor cannot be determined as a function of distance with any reasonable accuracy.

The approximate average durations for which the concentration of NOx at each position in the tunnel is above 1 ppm and 5 ppm after an explosion are summarised in Table 2.22.

Table 2.22: Average durations for which the concentration of NOx at each position in the tunnel is above 1 ppm and 5 ppm after an explosion.

Approx distance from face (m)

Duration (min)

NO > 1ppm NO > 5ppm NO2 > 1ppm NO2 > 5ppm 107 62 (σ = 14.5) 25 (σ = 14.5) 26 (σ = 10.0) 14 (σ = 4.5) 245 89 (σ = 19.5) 50 (σ = 14.0) 28.5 (σ = 3.5) 18 (σ = 3.5) 657 62 (σ = 24.5) 28 (σ = 3.5) 31.5 (σ = 3.0) 16 (σ = 2.5)

The average duration for which the concentration of NOx was above 1 ppm and 5 ppm was approximately comparable within associated standard deviations, along the length of the tunnel, suggesting that, unlike the dust in the previous tests, the NOx ‘cloud’ has not increased in length as it has moved along the tunnel from the face, which would be expected from the evident reduction in concentration.

Figure 2.17 shows that the intensities of the NOx peaks 107 m from the face caused by the explosions again show a very slight tendency to increase with the weight of the explosive used, however, the width of the peaks again show no correlation at all.

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Figure 2.17: Relationship between explosive quantity and concentration of NOx detected over 10 days 107m from face (purple MultiRAE)

The approximate average time delays between the explosion and the initial detection (> 1 ppm) of the NO by the purple and green MultiRAEs and recorded on Antilogs 3 and 1 respectively are summarised in Table 2.23. The approximate air velocities and flow rates (assuming plug flow, which will not be the case) along the tunnel with an approximate cross sectional area of 10.5 m2 calculated from the data from each of the instruments are also shown.

The limited accuracy of the data stored in the memory of the pink MultiRAE results in uncertainties greater than the actual values and therefore has no value when determining air velocities and flow rates within the tunnel.

Table 2.23: Average time delays between the explosion and the initial detection of the NO and calculated air velocities and flow rates

Approx distance NO delay Air velocity (m/s) Air flow rate (m3/s) from face (m) (min)

107 3.5 0.6 5.7 σ = 1 σ = 0.2 σ = 2.2

657 17.5 0.6 6.8 σ = 3 σ = 0.1 σ = 1.2

These values are again much less than the values stated by the tunnel engineers of an extraction air velocity in the tunnel of 0.9 m/s and an air flow rate of 20 m3/s in a ducting of 900mm diameter.

As in the previous section, tolerances in the initial measurements, inconsistencies in the tunnel dimensions along the length and the irregularly timed increase in tunnel length by 25 m over the 10 day duration of the investigation also add considerable uncertainty to the air velocity and flow rate calculated from the experimental data. Again the data suggests that the NOx is being carried down the tunnel by a different mechanism, or the initial concentration of NO reaching the sensor due to the explosion is less than the background level (which again is unlikely due to the short rise time after initial detection), or the flow rate/velocity of the air in the tunnel is less

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than stated. Again, further investigations into these discrepancies would be necessary before conclusions could be made regarding the transport mechanism. Figures 2.18 to 2.20 show the expanded scale at lower concentrations of NOx monitoring at each position in tunnel over the 10 day period, allowing the corrected background concentration of NOx to be observed.

Figure 2.18: Corrected background NO and NO2 concentrations detected over 10 days 107m from face (purple MultiRAE)

Figure 2.19: Corrected background NO and NO2 concentrations detected over 10 days 447m from

face (pink MultiRAE)

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Figure 2.20: Corrected background NO and NO2 concentrations detected over 10 days 657m from face (green MultiRAE)

The average background NOx concentrations shown in Table 2.24 are calculated from the 4 min MultiRAE data. The peak concentrations caused by the blasts are excluded by allowing for average peak width times for NOx concentrations above 1 ppm (∼1.5 hrs) and 5 ppm (∼ 1 hr) taken from Tables 2.16 to 2.19 from the initial detection time after each blast.

Table 2.23: Background concentrations of NO and NO2 over 10 day period detected at each position along tunnel.

Ave Peak Width Time

(hr)

107 m from face

NO (ppm)

107 m from face

NO2 (ppm)

447 m from face

NO (ppm)

447 m from face

NO2 (ppm)

657 m from face

NO (ppm)

657 m from face

NO2 (ppm) 1 2.0

σ = 2.1 0.1

σ = 0.1 8.2

σ = 4.3 0.1

σ = 0.1 6.0

σ = 3.8 0.4

σ = 0..3 1.5 2.0

σ = 2.1 0.1

σ = 0.1 8.3

σ = 4.3 0.1

σ = 0.1 6.0

σ = 3.7 0.4

σ = 0..3

The average background concentration of NO approximately 107 m from the face is above 1 ppm but less than 5 ppm, but is above 5 ppm at the other locations monitored along the tunnel.

The average background concentration of NO2 is less than 0.5 ppm at all locations monitored along the tunnel.

Temperature and Humidity

As in Section 2.3.3 the uncertainty in the accuracy of the Tiny Tag temperature and humidity monitors means that the results should be analysed as qualitative rather than quantitative data.

The conclusions ascertained from the data recorded by the Tiny Tags were the same as in Section 2.3.3:

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• any sudden change in temperature or relative humidity (RH) due to the explosions is unlikely to be recorded due to the 2 minute sampling period of the Tiny Tags

• there is no obvious correlation between the temperature or relative humidity and distance from the face

• the temperature and relative humidity profiles were found to be very consistent over the duration of the test at all 3 positions along the tunnel

Again, all of these effects prevented any useful observation of the effects of changes in temperature and humidity on the NOx concentrations in the tunnel.

2.3.5 Comparisons between results from the first and second investigations

The test locations, which are comparable in both visits to the tunnel, are a position close to the blast face (approximately 100 m from the blast face), a position close to the entrance (approximately 35 m from the entrance) and a position approximately central to both.

Due to the problems experienced with the equipment in the first visit during the explosions, effective comparisons of NOx concentrations are limited to the test positions located 100 m from the blast face. The NOx concentrations and event times from test point positioned 100 m from blast face in first and second visits are shown in Table 2.26.

Table 2.26: NOx concentrations and event times from test point positioned ∼ 100 m from blast face in first and second visits

Condition Visit 1 Visit 2 MultiRAE Green Purple

Antilog No. 1 No. 3 Dist from Face 100 m 107 m Ave Explosive Wt 73.1 kg (σ = 10.8 kg) 57.5 kg (σ = 22.4 kg) Ave Detection Delay 7 min 01 s (σ = 46 s) 3 min 39 s (σ = 73 s) Ave Peak Concentration NO: 151.2 ppm (σ = 114.5 ppm)

NO2: 110.3 ppm (σ = 80.4 ppm) NO: 121.9 ppm (σ = 59.0 ppm) NO2: 88.2 ppm (σ = 50.4 ppm)

Ave Peak Width (>1 ppm)

NO: 93 min 45 s (σ = 22 min 43 s) NO2: 39 min 22 s (σ = 12 min 44 s)


Ave Peak Width (>5 ppm)



Ave Background Concentration (1 hr peak allowance)

NO: 3.3 ppm (σ = 3.7 ppm) NO2: 1.9 ppm (σ = 0.4 ppm)


Ave Background Concentration (1.5 hrs peak allowance)



The peak concentrations of NO created in the explosions were an average of 1.5 times greater than peak concentrations of NO2 in both tests.

The average peak concentrations of both NO and NO2 created in the explosions in the first tests were 1.2 times greater than the average peak concentrations of NO and NO2 created in the explosions in the second tests.

The average weight of explosive used throughout the first visit (73 kg) is approximately 1.3 times greater than that used throughout the second visit (57.5 kg).

All of the concentrations of NOx and the event times are considerably less over the duration of the second visit than the first including:

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• The average background concentration of NO throughout the second visit (NO = 2 ppm) is approximately two thirds of that throughout the first visit (NO = 3 ppm)

• The average background concentration of NO2 throughout the second visit (NO2 = 0.1 ppm) is approximately one twentieth of that throughout the first visit (NO2 = 1.9 ppm)

• The average detection delay time throughout the second visit (3.5 min) is approximately half of that throughout the first visit (7 min)

• The average peak widths (> 1 ppm) of NO and NO2 throughout the second visit (NO = 61.5 min, NO2 = 26 min) are approximately two thirds of those throughout the first visit (NO = 94.5 min, NO2 = 39.5 min )

• The average peak widths (> 5 ppm) of NO and NO2 throughout the second visit (NO = 25 min, NO2 = 14 min) are approximately half and three quarters respectively of those throughout the first visit (NO = 44 min, NO2 = 18 min )

The average background concentrations of NO are comparable within the uncertainties in the system and are both above 1 ppm but below 5 ppm over the durations of each of the investigations. These NO concentrations are well below the average levels detected at the two the further positions along the tunnel over the duration of the second visit of 8 ppm and 6 ppm.

The average background concentrations of NO2, however, are very different, being almost 2 ppm over the duration of the first visit and 0.1 ppm over the duration of the second visit. The NO2 concentrations detected at the two the further positions along the tunnel over the duration of the second visit confirm a lower average NO2 background concentration of less than 0.5 ppm along the length of the tunnel.

Comparisons between the results from the 2 visits suggest that the amount of explosive used may cause in an increase in concentrations of NOx detected and the event durations. The only other detected change in conditions is the increase in relative humidity throughout the second visit. Perhaps this may have some effect on these parameters.

2.3.6 Overall recommendations

Instruments with a resolution of 0.1 ppm are preferable for effective monitoring of the background NO concentrations, particularly around 1 ppm, as found in the tunnels under construction. They should have adequate immunity from interferences (eg NO2, temperature) and long-term stability at this level.

29

3 HINDHEAD ROAD TUNNEL VISIT

3.1 INTRODUCTION

Dr. Donald Lamont (HSE), Dr. Peter Walsh (HSL) and Dr. Paul Dowker (HSL) were invited by Mr. Paul Hoyland (Project Director) of Balfour Beatty Civil Engineering to visit a tunnel under excavation at Hindhead to observe the operational methods and gas monitoring, particularly NO, which were in place. This visit was undertaken on 19/11/08. Tony Watt (Health and Safety Manager) and Roger Bridge (Tunnel Construction Manager) were also present.

3.2 THE ROAD TUNNEL

3.2.1 Background

The following tunnel information in this section is taken from the Highways Agency website (Highways, 2008)

The A3 Hindhead project is one of the Government's programme of major road building schemes, the principle contractor being Balfour Beatty. The new road will be 6.5 km (4 miles) long and includes 1.8 km (1.1 miles) twin bored tunnels under the Devil's Punch Bowl Site of Special Scientific Interest. Figure 3.1 shows maps, a photograph and a projected image of tunnel route and position.

Figure 3.1: Maps, photograph and projected image of tunnel route and position 30

The maximum depth of tunnel below ground will be about 65m (measured to the top of the tunnel). The tunnel will have two separate bores, including a 7.3 m wide 2 lane carriageway with 1.2m wide verges on each side. The bores will be approximately parallel and and linked by pedestrian cross-passages at approximately every 100 m throughout the tunnel as shown in the artsists impression in Figure 3.2.

Figure 3.2: Artists impression of tunnel system

Work started on the 7th of January 2007, the main tunnelling works started in February 2008, and the tunnel is planned to be open for traffic in mid 2011.

The scheme budget has increased from the initial estimation due to various factors including changes to workplace exposure limits (WELs) for nitrogen oxides and respirable crystalline silica for workers involved on tunnelling works. The changes to WELs were introduced by the Health and Safety Executive and the new limits were significantly lower than those at the time the contract was awarded 1. Nitrogen oxides are emitted by diesel machinery used in the tunnel construction and respirable crystalline silica is found in the dust caused by excavating the sandstone through which the tunnel passes. To comply with the new limits the contractor needs to use different machinery and provide more ventilation. This means more expensive machinery and slower rates of progress and increased costs.

The constitution of the ground is such that the tunnel is excavated by mechanical methods without need for the use of explosives, as shown in Figure 3.3.

Figure 3.3: Excavation of tunnel using mechanical methods

1 The new guidance limits were Chemical Hazard Awareness Notices (CHANs) with 8 hour limits for both NO and NO2 of 1 ppm [7]. These were withdrawn in 2006. There are currently no workplace exposure limits (WELs) for NO and NO2.

31

3.3 GAS MONITORING

3.3.1 General

Levels of NO and carbon monoxide (CO) are monitored and logged at regular intervals by Trolex Sentro 8 monitors (Trolex, 2008) which are calibrated at 6 monthly intervals using dedicated gas cylinders. The monitors are located in various positions along the tunnel approximately 200 m apart. The approximate positions around the time of the visit (taken from a schematic provided by Balfour Beatty on the day of the visit) are shown in the schematic in Figure 3.4. The excavations were progressing from north to south and had reached a position south of the sensor position 7 where at least the top heading of the tunnel had been excavated and lined with concrete.

N7 N6

N1

N4 N2N3N5

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S3S4 S2

S8

S5

∼200 m

S7 S6

N8

Nor

th P

orta

l

Sout

h Po

rtal

Northbound Traffic

Southbound Traffic

∼1777 m

Figure 3.4: Locations of the NO and CO sensors (S1 to S8 in southbound tunnel and N1 to N8 in northbound tunnel) in the tunnel around the time of the visit

Readings are taken every minute and transmitted to a Scada display panel in the control room where they are averaged over 24 hour periods. Alarms operate on instantaneous concentrations rather than time averaged concentrations and will sound whenever the NO or CO concentration rises above the designated safety limit of 2 ppm and 15 ppm respectively in the location of the instrument.

Information received from Trolex confirmed that the Trolex Sentro 8 instruments are capable of a resolution limit of 0.5 ppm for both NO and CO sensors but in this case they are only required to log at a resolution of 1 ppm. This means that actual analogue concentrations up to 0.4 ppm will be detected as 0 ppm by the ADC in the instrument and recorded as such, while concentrations between 0.5 ppm and 1.4 ppm will be detected as 1 ppm and recorded as such. The accuracy of the detected concentrations is therefore ± 0.5 ppm.

3.3.2 NO Levels

The detected levels of NO averaged over 24 hour periods at the various sensor locations in each tunnel between 31/10/08 and 18/11/08 are shown in Figures 3.5 and 3.6 and Tables 3.1 and 3.2. An alarm level of 2 ppm NO has been set in the tunnel, this is an instantaneous alarm and not a time weighted average.

32

33

Figure 3.5: Detected levels of NO averaged over 24 hour periods at the various sensor locations in the northbound tunnel between 31/10/08 and 18/11/08

Figure 3.6: Detected levels of NO averaged over 24 hour periods at the various sensor locations in the southbound tunnel between 31/10/08 and 18/11/08

In the northbound tunnel the levels of NO rise above the designated safety limit of 2 ppm (shown in bold in Tables 3.1 and 3.2) on 2 occasions, on 1/11/08 at position N5 and on 6/11/08 at position N3. In the southbound tunnel the levels of NO rise above the designated safety limit of 2 ppm on occasions, at positions S3, S4, and S5.

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NO Limit (ppm)S1 N0 (ppm)S2 N0 (ppm)S3 N0 (ppm)S4 N0 (ppm)S5 N0 (ppm)S6 N0 (ppm)S7 N0 (ppm)S8 N0 (ppm)

Table 3.1: Detected levels of NO averaged over 24 hour periods at the various sensor locations in the northbound tunnel between 31/10/08 and 18/11/08 Date & Concentration of NO (ppm) in northbound tunnel

Sensor 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Position Oct Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov N1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 N2 0.98 0.83 0.00 0.98 0.00 0.00 1.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.49 0.44 0.00 N3 0.51 1.16 0.00 0.51 0.12 0.87 2.05 0.00 0.94 0.01 0.20 0.14 0.08 0.20 0.70 0.03 0.11 0.69 1.29 N4 1.12 1.95 0.00 1.12 1.20 0.72 0.76 0.75 0.75 0.79 0.62 0.68 0.71 0.65 0.00 0.38 0.58 0.00 0.63 N5 1.55 2.40 0.00 1.55 1.63 1.64 0.00 0.40 0.61 0.33 0.44 0.29 0.31 0.47 0.00 0.32 0.44 0.00 0.00 N6 0.69 1.56 0.00 0.69 0.80 2.00 2.00 2.00 2.00 2.00 2.00 2.00 1.07 1.13 1.00 1.03 0.04 1.00 1.00 N7 0.63 1.00 0.00 0.63 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.55 0.63 0.77 0.00 0.72 0.16 0.00 0.00 N8 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Table 3.2: Detected levels of NO averaged over 24 hour periods at the various sensor locations in the southbound tunnel between 31/10/08 and 18/11/08 Date & Concentration of NO (ppm) in southbound tunnel

Sensor 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Position Oct Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov S1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.15 0.01 0.00 0.00 S2 1.07 0.11 1.07 0.00 1.90 0.62 0.00 0.00 0.00 0.00 0.51 0.00 0.00 0.11 0.40 0.03 0.00 1.12 S3 2.11 2.05 2.11 0.00 2.59 1.04 0.46 2.10 1.85 2.13 2.26 1.93 1.04 1.74 0.56 0.77 0.00 1.90 S4 1.68 2.67 1.68 0.00 2.54 0.77 0.93 1.52 1.33 1.82 2.02 2.28 1.24 1.22 0.53 1.11 0.00 1.78 S5 0.69 1.75 0.69 0.00 0.80 1.32 0.68 1.30 1.40 1.44 1.65 0.28 1.03 0.88 0.27 0.78 0.00 2.17 S6 1.00 1.00 1.00 0.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.00 0.82 S7 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.83 S8 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

34

35

CO Levels The detected levels of CO averaged over 24 hour periods at the various sensor locations in each tunnel between 31/10/08 and 18/11/08 are shown in Figures 3.7 and 3.8 and Tables 3.3 and 3.4. An alarm level of 15 ppm CO has been set in the tunnel 2.

Figure 3.7: Detected levels of CO averaged over 24 hour periods at the various sensor locations in the northbound tunnel between 31/10/08 and 18/11/08

Figure 3.8: Detected levels of CO averaged over 24 hour periods at the various sensor locations in the southbound tunnel between 31/10/08 and 18/11/08

2 The Workplace Exposure Limits (WELs) for CO are 30 ppm (8 hr) and 200 ppm (15 min)

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CO Limit (ppm)S1 C0 (ppm)S2 C0 (ppm)S3 C0 (ppm)S4 C0 (ppm)S5 C0 (ppm)S6 C0 (ppm)S7 C0 (ppm)S8 C0 (ppm)

Table 3.3: Detected levels of CO averaged over 24 hour periods at the various sensor locations in the northbound tunnel between 31/10/08 and 18/11/08 Date & Concentration of CO (ppm) in northbound tunnel

Sensor 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Position Oct Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov N1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 N2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.44 0.00 N3 0.92 1.25 0.00 0.92 1.79 2.85 0.15 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 N4 0.05 0.62 0.00 0.05 1.37 1.45 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 N5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 N6 0.00 0.07 0.00 0.00 0.69 3.00 3.00 2.80 2.50 3.00 3.00 2.00 2.00 0.98 0.00 0.06 1.98 0.00 0.00 N7 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 N8 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Table 3.4: Detected levels of CO averaged over 24 hour periods at the various sensor locations in the southbound tunnel between 31/10/08 and 18/11/08 Date & Concentration of CO (ppm) in southbound tunnel

Sensor 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Position Oct Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov S1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 S2 0.35 0.00 0.35 0.00 0.80 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 S3 0.71 0.22 0.71 0.00 1.18 0.22 0.23 0.50 0.33 0.81 0.79 0.53 0.32 0.68 0.00 0.66 0.00 0.00 S4 0.00 0.55 0.00 0.00 0.46 0.00 0.00 0.00 0.00 0.00 0.19 0.93 0.22 0.44 0.00 0.01 0.00 0.00 S5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.38 0.00 0.00 0.00 0.01 0.00 0.00 S6 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.11 0.00 S7 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.16 0.00 S8 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

36

3.3.3 Developments in NO monitoring

Discussions took place with Trolex (Ken Dawson and Mark Rowland) who supply the Sentro monitoring system installed at Hindhead Tunnel. The Sentro system uses electrochemical NO sensors capable of a resolution limit of 0.5 ppm (see Section 3.3.1). There are, however, new electrochemical sensors available which can readily monitor at sub-ppm levels of NO. Initial results from tests carried out at Trolex were promising, although further work is required on stability, cross-sensitivity etc. before they could be incorporated into an instrument.

3.3.4 Recommendation

Developments in NO sensor technology now make more precise 0.1 ppm resolution a possibility; this should be translated by instrument manufacturers into the next generation of portable and fixed NO monitors for tunnels.

37

4 RATE OF REACTION OF NOX IN DIESEL EXHAUST FUME

4.1 LABORATORY INVESTIGATION

4.1.1 Introduction

This phase of the investigation was designed to observe the changes in NOx concentrations in diesel exhaust fume under laboratory conditions.


Two MultiRAE multigas monitors, determined as the most suitable of all of the types of NOx monitors tested in a previous study (Dowker et al, 2007), were selected for use in this part of the investigation. These monitors were setup identically with NO, NO2 and O2 sensors installed and were calibrated to the relevant gases prior to the start of the investigation and the calibration checked on completion of the investigation. They were operated with external inline filters fitted to protect them as far as practicably possible from dust and damp ingress.

Calibrated Tinytag combined temperature and humidity monitors were used to record the temperature and humidity inside the airtight glass vessel (Dowker et al, 2007). The instrument serial numbers and colour codings allocated for easier identification are listed in Table 4.1.

Table 4.1: Instrument serial numbers and allocated colour codings Instrument Serial No. Colour Coding MultiRAE 500871 black MultiRAE 502967 red

Tiny Tag 3 177528 red

A remote monitoring system allowed observation of the detected concentrations of NOx and O2 from a position outside the cabin during the tests (see section 4.1.4).

4.1.3 Diesel Fume Source

A diesel powered generator, model DG 3600E made by United Power Equipment Co. Ltd. (United Power, 2008) shown in Figure 4.1 was purchased from Powerland (Powerland, 2008) for use in these investigations. The diesel fuel used in the generator was Esso extra low sulphur diesel, UN 1202.

38

Figure 4.1: Diesel powered generator (model DG 3600E, United Power Equipment Co. Ltd)

The generator was used at its normal running speed of 3600 rpm under no load conditions in all of the tests.

4.1.4 Environmental Test Cabin Facility

The tests were performed in the environmental test cabin facility located in the environmental test suite at HSL, Buxton. This cabin has the facility to extract ambient exhaust fume from the cabin at a predetermined flow rate and therefore maintain a safe working environment throughout the duration of these tests.

Initial checks on the effectiveness of the extraction system in the environmental test cabin, for a worst case scenario of all of the diesel exhaust fume entering the cabin direct from the generator, showed the NOx concentration to be below detectable limits (0.1 ppm) at all positions between 0.5 and 2 m above ground level and to be a worst case concentration of 6 ppm NO and 3 ppm NO2 at ground level, 1 m in front of the exhaust outlet.

Additional safety measures included placing NO and NO2 detectors in the cabin at all times, placing signs at the entrance to the cabin prohibiting any personnel not involved in the tests from entering while a test was being performed, ensuring that personal NOx monitors were worn by personnel entering the cabin to adjust the experimental settings during the tests, and remotely monitoring the concentrations detected by the MultiRAEs to reduce the time spent inside the cabin during tests.

4.1.5 Experimental

The equipment, set up shown as a schematic in Figure 4.2 and a photograph in Figure 4.3, allowed the diesel fume to be mixed with air to give similar NO content in the same airtight glass vessel as in the experiments undertaken in previous investigations with mixtures of pure NO and O2 (Dowker et al, 2007). Locating a MultiRAE in the glass cylinder allowed comparisons between the NOx changes in both setups with time. The water bath was included in the system to keep the pipes to a temperature at which the plastic taps and joints would not be impaired.

39

Air PumpAir-tight

AirExhaust Fume & Air

Exhaust Fume & Air

Extraction

Tap 1

Tap 2

Tap 3Tap 4Tap 5

Water Bath

MultiRAE

Diesel Generator

Glass Vessel

Exhaust Fume

Exhaust Fume

Exhaust Fume

Figure 4.2: Schematic diagram of set up for mixing diesel fume and air

Figure 4.3: Photograph of set up for mixing diesel fume and air

4.1.5.1 NOx and O2 Concentrations in Pure Diesel Fume from Generator

It was necessary to determine the content of NOx and O2 in the undiluted fume exhausted from the generator to allow mixing of the diesel fume with air in order to achieve the required concentrations of NO. This was achieved by feeding the exhaust fume from the generator through the air tight glass vessel, in which the black MultiRAE was situated, via taps 2, 4 and 5 and closing taps 1 and 3 shown in Figure 4.2. For safety reasons the concentration of NOx detected by the MultiRAE was also monitored remotely on a PC outside the booth until reasonably constant, the results from which are shown in Figure 4.4. (The dip in the profile at ∼ 25 minutes was where the pipe came free from the cylinder and the fume was not entering the cylinder).

40

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NO∼200ppm

O2∼16%

100

120

140

160

180

200

220

240

30 50 55 60 Time (m

ppmNO2(ppmOXY

Figure 4.4: Concentration of NO, NO2 and O2 detected in diesel exhaust fume from generator

The approximate ‘steady-state’ concentrations of NOx and O2 detected in the diesel exhaust fume are summarised in table 4.1.

Table 4.1: Concentrations of NOx and O2 detected in the diesel exhaust fume Gas Detected Concentration NO 200ppm NO2 115 ppm NOx 315 ppm O2 16%

4.1.5.2 Conversion Rate Measurements

Procedure

The diesel fume and air was mixed by adjustment of the respective flow rates through taps 1, 2 and 3 in Figure 4.2 to give NO concentrations of 5 ppm, 10 ppm and 50 ppm of NO in air comprising O2 concentrations as close as possible to 21%. The mixing quantities required to achieve these concentrations are shown in Figure 4.5.

41

200ppm NO 115ppm NO2

2

Air 2

Reduce to 2.5%

Reduce to 97.5%

5ppm NO 3ppm NO2

2

Air 2

5ppm NO 3ppm NO2

2


2

Air 2

Reduce to 5%

Reduce to 95%

10ppm NO 6ppm NO2

2

Air 2

10ppm NO 6ppm NO2

2


2

Air 2

Reduce to 25%

Reduce to 75%

50ppm NO 29ppm NO2

2

Air 2

50ppm NO 29ppm NO2

2

Diesel Fume

16% O

20.9% O

Diesel Fume

0.4% O

20.4% O

Diesel Fume & Air

20.8% O

Diesel Fume

16% O

20.9% O

Diesel Fume

0.8% O

19.9% O

Diesel Fume & Air

20.7% O

Diesel Fume

16% O

20.9% O

Diesel Fume

4 % O

15.7% O

Diesel Fume & Air

19.7% O

Diesel Fume & Air Mix to Achieve 5 ppm NO



Figure 4.5: Diesel Fume & Air Mix to Achieve 5 ppm 10 ppm and 50 ppm NO in 21% O2

The red MultiRAE was placed inside the airtight glass vessel along with the red Tiny Tag temperature & humidity monitor. A small fan was included to ensure good mixing of the fume and the air entering the cylinder.

Each concentration of the diesel fume/air mix in turn was made to flow through the airtight glass vessel and out of the booth via the extraction mechanism. When the required NO concentrations were achieved and were seen to be constant the airtight glass vessel was sealed by closing taps 4 and 5 in Figure 4.2 and the set up left for 24 hours. This procedure was performed at least twice at each NO concentration to show the repeatability of the procedure.

The monitoring equipment and the glass vessel used here has been previously shown (Dowker et al, 2007) to have inherent effects on the changes in NOx concentrations due to the combined effects of adsorption onto the inner surface of the airtight glass vessel and the conversion of the NOx by the sensors. Experimental investigations into these effects are reported in Appendix 2, and correction factors for the resulting changes in NOx concentrations have been determined and included in the results from the diesel fume/air mixture tests shown in Figures 4.6 to 4.8.

4.1.6 Results

Figures 4.6 to 4.8 show the change in NOx concentrations in the diesel fume & air mixes of the NO and O2 concentrations previously stated over 24 hours, before and after application of relevant correction factors.

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tA d ust ed NO p pm 5 ppm NO T Adjusted NO 5 ppm NO T2 Adjusted NO2(ppm 5 ppm NO T1 Adjusted NO2(ppm 5 ppm NO T2 Detect ed NO ppm 5 ppm NO T1 Det ect ed NO 5 ppm NO T2 Detected NO2(ppm 5 ppm NO T1 Det ect ed NO2(ppm 5 ppm NO T2

Detected NO

Detected NO

Figure 4.6: Change in NOx concentrations in diesel fume (5 ppm NO) & air (20.8% O2) mix before and after application of correction factors (Duplicate measurements T1 and T2)

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11

12

0 1 2 3 4 5 6 7 8 9 ( )

Det

ecte

d C

once

ntra

tions

(ppm

) .

( j (( ) j )

( ) ( )) )

Adjusted NO2

Adjusted NO

2

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Time hrs

Adjusted NO ppm) 10 ppm NO T1 Ad usted NO ppm) 10 ppm NO T2 Adjusted NO2 ppm 10 ppm NO T1 Ad usted NO2(ppm 10 ppm NO T2 Detected NO ppm 10 ppm NO T1 Detected NO ppm 10 ppm NO T2 Detected NO2(ppm 10 ppm NO T1 Detected NO2(ppm 10 ppm NO T2

Detected NO

Detected NO


43

0

5

10

15

20

25

30

35

40

45

50

55

0 1 2 3 4 5 6 7 8 9 Ti )

Det

ecte

d C

once

ntra

tions

(ppm

) .

j ( )) )

( ) (ppm)( ) )

Adjusted NO2

Adjusted NO

2

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 me (hrs

Ad usted NO(ppm) 50 ppm NO T1 Adjusted NO ppm 50 ppm NO T2 Adjusted NO2(ppm 50 ppm NO T1 Adjusted NO2(ppm 50 ppm NO T2 Detected NO ppm 50 ppm NO T1 Detected NO 50 ppm NO T2 Detected NO2 ppm 50 ppm NO T1 Detected NO2(ppm 50 ppm NO T2

Detected NO Detected NO


As in previous sections the uncertainty in the accuracy of the Tiny Tag temperature and humidity monitors means that the results should be analysed as qualitative rather than quantitative data.

The Temperature and RH concentrations in the glass vessel were reasonably constant throughout the investigations preventing any comparison of the effects of these conditions on the operational characteristics of the MultiRAEs

4.1.6.1 Conclusions

It can be seen that NO, when emitted in diesel fume into the atmosphere, persists at concentrations above 1 ppm for several hours, the time increasing as the initial NO concentration increased from 5 to 50 ppm. The initial rate of decrease in the NO concentration is dependent on the initial NO concentration, as was found for the pure gas experiments (Dowker et al, 2006).

The NO concentration does not reduce monotonically with time and levels off after several hours. This may be due to competing reactions which generate NO (see Section 4.2).

4.1.7 Analysis of rate of conversion of NO

4.1.7.1 Rate equations

In ambient air, the principal reactions of NO which reduce its concentration are with oxygen and ozone, with the concomitant formation of nitrogen dioxide and, subsequently, nitrous and nitric acid by reaction of reactants and products with water vapour (Boström, 1993). Thermal oxidation of NO with oxygen is much slower than with ozone which is much more reactive. In a tunnel, where ozone levels are typically very low compared to NO levels, in contrast to the outdoor environment, see for example, Leighton (1961), then the thermal oxidation reaction dominates. The basic reaction is:

44

2NO + O2 → 2NO2 (4.1)

This proceeds as a third-order reaction where the rate of NO reaction (equal to the rate of NO2 production) is given by:

Rate = 2k[NO]2[O2] (4.2)

where k is the rate constant (approximately 7.103 litre2.mole-2.s-1) and [NO] and [O2] are the NO and oxygen concentrations respectively (Tsukahara et al, 1999). The reaction is therefore proportional to the square of the NO concentration if the oxygen concentration is assumed to be constant. When ppm units are used for NO, NO2 and oxygen concentration (strictly not the concentration but the volume ratio, and in normal air the oxygen ‘concentration’ is 2.1x105

ppm), then in equation (4.2) above, the rate constant is converted to:

k1 = k (P/RT)2.10-12 (4.3)

where P is the total atmospheric pressure in bar, R is the ideal gas constant (0.0821 litre.bar.mole-1.K-1) and T is the temperature (K). Consequently, at 298 K (25 ºC)

Rate (ppm.s-1) = 4.92x10-6.[NO]2 (4.4)

The NO reaction rate (or NO2 production rate) as a function of time can be obtained by integrating equation (4.2), which gives:

[NO]t = 1/{2k[O2]0t + 1/[NO]0} (4.5)

where [NO]t is the concentration of NO (ppm) at time t (s) and the initial concentration (ppm) is[NO]0.

This can be rearranged to express the time t:

t = {(f/(1-f)}/(4.92x10-6[NO]0) (4.6)

at which a fraction f of NO, whose initial concentration (ppm) is [NO]0, has become NO2, where the definition of f is:

f = [NO2]/ [NO]0 (4.7)

and the concentration of NO remaining after a time t is given by

[NO]t = (1-f) [NO]0 (4.8)

The time required for the conversion of a certain fraction of NO to NO2 (eg the half-life, where f = 0.5) is inversely proportional to the initial NO concentration and the rate constant.

Figure 4.10 shows the oxidation rate of NO for various initial concentrations using equation 4.5.

45

0.1

1

10

0 Ti

NO

con

cent

ratio

n (p

pm)

100

10 20 30 40 50 60 70 80 90 100 me (hr)

100 ppm NO

10 ppm NO

1 ppm NO

0.3 ppm NO

Figure 4.10: Rate of reaction of NO with oxygen (20.9%) for various initial NO concentrations

4.1.7.2 Comparison of theory and experiment

Figures 4.11 to 4.13 compare the changes in each of the adjusted NO concentrations in the diesel fume and air mixes (ie the data from this study), the NO in pure gases N2 and 20.9% O2 mixtures in the cylinder (ie the data from the previous study – Dowker et al, 2007) and the relevant modelled values (from eqn. 4.5 above) over the 24 hour period.

The slight rise shown in the adjusted NO concentrations towards the end of the tests are probably due to a minor mismatch in the reduction rates between the NO concentrations in the various mixes and the NO in N2 determined in the tests reported in Appendix 2, causing the resulting correction factor to be slightly inaccurate as the rate of reduction in NO concentration reduces.

The adjusted results show the NO2 concentration to increase as the NO concentration decreases as predicted by theory (Tsukahara et al, 1999). The results show good repeatability in each set of tests allowing for the slight differences in initial NO concentrations.

Previous investigations into the change in similar concentrations of NO in N2 mixed with 20.9% O2 in an air-tight cylinder (Dowker et al, 2006) also allow comparisons with these results. As the curves of the detected NO concentrations of the untreated data in both the diesel fume experiments and the pure gas experiments are similar, the same relevant correction factors are applied to the detected NO concentrations in both cases.

The NO2 curves of the untreated data of the diesel fume experiments and the pure gas experiments are quite different, however. In the diesel fume experiments, the initial NO2 concentration is non-zero and is more than half the initial NO concentration. The NO2 concentration then reduces very quickly. In the pure gas experiments the initial NO2 concentration is zero and rises for 2 to 3 hours before reducing at a much slower rate (Dowker et al, 2006). This prevents the previously determined correction factors being applied to the pure gas untreated data. As there is no other way that the inherent effects of the equipment on NO2

46

concentrations in these experiments can be determined, an effective comparison between this and the treated data from the diesel fume experiments cannot be made.

0

1

2

3

4

5

0 1 2 3 4 5 6 7 8 9 Ti )

) .

j )

j )

j )

ll ( )

0.5

1.5

2.5

3.5

4.5

5.5

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 me (hrs

Con

cent

ratio

ns (p

pm

Ad usted NO(ppm) 5 ppm NO (Fume T1

Ad usted NO(ppm) 5 ppm NO (Fume T2

Ad usted NO(ppm) 5 ppm NO (Gas

Mode ed NO ppm 5 ppm NO

Figure 4.11: Comparison of change in NO concentrations in 5 ppm NO in diesel fume & air (20.8% O2) mix, 5 ppm NO in N2 & 20.9% O2 in cylinder mix and modelled value over 24 hours

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0 1 2 3 4 5 6 7 8 9 )

Con

cent

ratio

ns (p

pm)

.

(ppm) ( )

(ppm) ( ) T2

j

ll

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Time (hrs

Adjusted NO 10 ppm Fume NO T1

Adjusted NO 10 ppm NO Fume

Ad usted NO(ppm) 10 ppm NO (Gas) T2

Mode ed NO (ppm) 10 ppm NO


47

0

5

10

15

20

25

30

35

40

45

50

55

0 1 2 3 4 5 6 7 8 9 11 13 14 15 17 18 19 20 21 22 23 24 25 )

ions

(ppm

) .

(ppm) ( )

(ppm) ( )

(ppm) ( )

ll

10 12 16 Time (hrs

Con

cent

rat

Adjusted NO 50 ppm NO Fume T1

Adjusted NO 50 ppm NO Fume T2

Adjusted NO 50 ppm NO Gas

Mode ed NO (ppm) 50 ppm NO


The change in NO concentration in the diesel fume experiments and in the pure gas experiments follow the same trend for each initial NO concentration and reach reasonably comparable minimum concentrations over the 24 hour period allowing for the slight differences in initial maximum values. The rate of change in the modelled data is similar to the diesel experiments but is faster than the pure gas experiments, reaching noticeably lower values over the 24 hour period, but still follows the same general trend allowing for the slight differences in initial maximum values. Allowing for the limitations of the experimental set ups, external influences, and the uncertainties involved in the measurements, the results obtained in all three methods utilised here are comparable. They all show the trend for (a) the initial NO conversion rate to increase with initial NO concentration; and (b) the rate to reduce after several hours where the concentration does not change significantly with time and is at appreciable fraction of the initial concentration.

Suitable manipulation of equation 4.5 into a linear format of the form:

1/[NO]t = {2k[O2]0}t + 1/[NO]0} (4.9)

allows the determination of the rate constant, k, from the gradients of the line for each condition investigated. Plotting a graph of the inverse of the detected NO concentration against time for each condition should create a straight line graph the gradient of which allows the determination of the rate constant and the intercept on the vertical axis gives the inverse of the initial NO concentration.

48

Figures 4.14 to 4.16 show the inverse in each of the adjusted NO concentrations in the diesel fume & air mixes, the NO in N2 & 20.9% O2 mixes in cylinder and the relevant modelled values over the 24 hour period.

0 0 1 2 3 4 5 6 7 8 9

( )

Con

cent

ratio

ns (p

pm)

.

j ) )

j ) )

l ( )

0.1

0.2

0.3

0.4

0.5

0.6

0.7

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Time hrs

Inverse of Ad usted NO(ppm 5 ppm NO (Fume

Inverse of Ad usted NO(ppm 5 ppm NO (Gas

Inverse of Model ed NO ppm 5 ppm NO

1/Conc = 0.010t+0.20

1/Conc = 0.018t+0.20

1/Conc = 0.015t+0.21

Figure 4.14: Comparison of inverse values of change in NO concentrations in 5 ppm NO in diesel fume & air (20.8% O2) mix, 5 ppm NO in N2 & 20.9% O2 in cylinder mix and modelled value over

24 hours

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6 7 8 9 Ti )

Con

cent

ratio

ns (p

pm)

.

j )

j )

ll )

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 me (hrs

Inverse Ad usted NO(ppm) 10 ppm NO (Fume

Inverse of Ad usted NO(ppm) 10 ppm NO (Gas

Inverse of Mode ed NO (ppm 10 ppm NO

1/Conc = 0.018t+0.10

1/Conc = 0.019t+0.10

1/Conc = 0.004t+0.08


24 hours

49

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6 7 8 9 )

Con

cent

ratio

ns (p

pm)

. j )

j ( )

l ( )

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time (hrs

Inverse Ad usted NO(ppm) 50 ppm NO (Fume

Inverse of Ad usted NO ppm) 50 ppm NO (Gas

Inverse of Model ed NO ppm 50 ppm NO

1/Conc = 0.018t+0.02

1/Conc = 0.017t+0.02

1/Conc = 0.006t+0.02


24 hours

With the exception of the modelled values the graphs only remain as straight lines for around 3 hours after the start of the tests, and then reach a plateau. The gradients and the associated rate constants are only compared therefore over the first 3 hours of each test and are summarised in Table 4.2

Table 4.2: Gradients and associated rate constants determined from Figures 4.14 to 4.16 NOI = 5 ppm NOI = 10 ppm NOI = 50 ppm

Condition Gradient (2kOI)

(ppm-1 hr-1)

Rate constant k

(ppm-2 s-1)

Gradient (2kOI) (ppm-1

hr-1)

Rate constant k (ppm-2 s-1)

Gradient (2kOI) (ppm-1

hr-1)

Rate constant k (ppm-2 s-1)

Diesel fume 0.015 1.0 × 10-11 0.019 1.3 × 10-11 0.017 1.1 × 10-11

Pure NO 0.010 0.7 × 10-11 0.004 0.3 × 10-11 0.006 0.4 × 10-11

Modelled 0.018 1.2 × 10-11 0.018 1.2 × 10-11 0.018 1.2 × 10-11

The rate constants determined from the tests using diesel fume are similar to the ‘ideal’ modelled rate constant of 1.2 ppm-2s-1 over the first 3 hours of the tests, but those determined from the tests using pure NO in O2 are not similar over that initial time scale or for any part of the test duration for 10 and 50 ppm NO. There is a suggestion, however, that for the lowest NO concentration of 5 ppm, the pure gas rate and the modelled rate are similar over the first hour.

4.1.8 Discussion and Conclusions

All three methods utilised in this investigation (NO in diesel fume mixed with air, pure NO in N2 mixed with 20.9% O2 and modelling) to determine the changes in NOx over a 24 hour duration from initial concentrations of 5 ppm, 10 ppm and 50 ppm NO, show comparable results allowing for the limitations of the experimental set ups, external influences, and the uncertainties involved in the measurements. The initial rate constants determined using the diesel exhaust fume for reactions at each concentration are similar to those determined from the modelled results over the first 3 hours of the tests, but those determined from the tests using pure NO in O2 are only similar for the lowest NO concentration of 5 ppm.

50

4.2

Previous studies on NO conversion from diesel exhaust (Mogan et al, 1979; Smailys and Strazdauskiene, 2005) also found relatively good agreement between experimental and calculated rates of conversion of NO. In Mogan et al (1979), initial NO concentrations were between 25 and 125 ppm generated from a diesel engine test bed sealed in a 50 m3 test cabin. Initial rates were compared with theory over very short timescales: after 4 min and up to 24 min. The initial rates of NO conversion for both diesel exhaust and pure gases (100 ppm NO) were similar. Again, the NO concentration was found to level off after an initial fast decrease.

Smailys and Strazdauskiene (2005) confirmed that the reaction of NO with oxygen is slow compared to the atmosphere where the ozone/NO ratio is much higher. The NO and NO2 analyses were performed using samples and wet chemistry taken from raw diesel exhaust, ie the combined NO and NO2 (ie NOx) concentrations are of the order of 1000 ppm, much higher than used in our study. The rate of conversion as expressed by the ratio NO2/NOx was found to follow the theoretical curve based on the third order reaction (2nd order in NO and 1st order in oxygen) over a period of at least 3 hours.

Guidance by the Highways Agency (Highways Agency, 1999) on ventilation for tunnels for road users has been produced which assumes that the initial emission is solely NO and that it subsequently converts to NO2 with a half-life of 1 week. Using equation 4.6 above, this would be equivalent to an initial NO concentration of approximately 0.4 ppm. This concentration is roughly comparable to those recorded in the Hindhead Tunnel as shown in Figs 3.5 and 3.6 in Section 3.3.2.

EFFECTS OF OTHER FACTORS ON NO CONVERSION

The rate of conversion of NO is dependent on various factors in addition to the concentrations of NO and oxygen as expressed in equation 4.2 (Lamont et al, 2008). These include:

• temperature, which affects the rate constant k. • presence of other gaseous species, eg water vapour, oxidants other than oxygen, reaction

products other than NO2. • presence of aerosols and other surfaces. • sunlight, a source of UV, which dissociates (ie photolyses) NO2 to form NO.

The rate constant k appears to be slightly negatively dependent on temperature (Tsukahara et al, 1999; Lindqvist et al, 1982). Around ambient temperature, the rate decreases by less than 1%/ºC rise. Also, most studies (Tsukahara et al, 1999) agree that the rate constant is independent of variations in the total pressure, the absolute NO concentration or the NO:O2 ratio (up to a thousandfold variation).

For water vapour, the majority of laboratory studies (Tsukahara et al, 1999) found that the rate constant is not affected by humidity (up to 90 %RH). Since water vapour reacts with NO2 but not with NO, it is expected that the presence of water in NO/O2 reaction systems will shift the equilibrium (of reaction equation 4.1) towards products such as oxyacids of nitrogen (eg nitrous and nitric acid) but will not change the rate at which NO is oxidised.

The influence of other factors is relatively unknown. A study (Lindqvist et al, 1982) of NO conversion in the atmosphere around Gothenburg indicated that the presence of aerosols, at concentrations likely to be encountered in practice, does not substantially influence the rate of oxidation of NO. This is possibly in agreement with our study and others (Mogan et al, 1979; Smailys and Strazdauskiene, 2005) where the NO conversion rate in diesel exhaust, which contains an appreciable quantity of particulate matter, was similar to the modelled rate

51

4.3

developed for pure gases (ie aerosol-free). Street surface materials (salt, snow, road dust) had a more significant influence, typically increasing the rate by 50% at -2 ºC (Lindqvist et al, 1982).

Lindqvist et al (1982) also found that sunlight appeared not to affect the rate. However, laboratory studies using pure gases (Bufalini and Stephens, 1965) irradiated by UV, showed that the simple second order rate expression does not adequately describe the system. The inclusion of a rate equation for the photolysis of NO2 (ie its conversion into NO by reaction with UV radiation) explained the slowing down of the NO thermal oxidation rate in the later stages of the reaction when sufficient NO2 has been formed, due to the regeneration of NO from NO2.

RECOMMENDATIONS

1. NO in tunnels should be measured directly rather than being inferred from measured NO2 concentrations as it converts into NO2 only slowly, in the absence of ozone, and appears to reach a steady-state concentration after several hours.

2. Adequate estimations of the conversion rate of NO to NO2 in air contaminated with diesel exhaust, in the absence of ozone, can be obtained through the modelled rate equation based on pure gases for up to approximately 8 hours for NO concentrations up to 10 ppm but for 4 hours at 50 ppm NO. After this time the NO concentration remains relatively constant.

3. The calculations of NO conversion rates in air-diluted diesel fume (and in pure gases) require the use of correction factors due in part to the use of electrochemical NO sensors which consumed a small quantity of gas. The use of a ‘non-destructive’ analyser such as Fourier Transform InfraRed (FTIR) for NO would reduce some of the uncertainty in conversion rate estimation, although correction for adsorption on the reaction chamber walls would still be necessary.

52

53

5 APPENDICES

5.1 APPENDIX 1: DUST CONCENTRATIONS Figures 5.1 to 5.3 show the respirable dust concentrations logged in the internal memory of the blue, orange and red PDRs located 100 m, 170 m and 295 m respectively from the blast face over the 20 day duration of the investigation.

Figure 5.1: Peak concentrations of dust caused by explosions detected 100 m from face over 20 day period (by blue PDR)

Figure 5.2: Peak concentrations of dust caused by explosions detected 170 m from face over 20 day period (by orange PDR)

0102030405060708090

100110120130140150

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3

Det

ecte

d D

ust C

once

ntra

tion

(mg/

m^3

) . Orange PDR (mg/m^3)

Wed

18t

h Ju

n

Wed

11t

h Ju

n

Thur

5th

Jun

Fri 6

th J

un

Mon

9th

Jun

Sat

7th

Jun

Tue

10th

Jun

Sun

8th

Jun

Fri 1

3th

Jun

Thu

12th

Jun

Tue

17th

Jun

Mon

16t

h Ju

n

Sun

15t

h Ju

n

Sat

14t

h Ju

n

Sun

22n

d Ju

n

Sat

21s

t Jun

Fri 2

0th

Jun

Thu

19th

Jun

0102030405060708090

100110120130140150

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

Det

ecte

d D

ust C

once

ntra

tion

(mg/

m^3

) . Blue PDR (mg/m³)

Wed

11t

h Ju

n

Thur

5th

Jun

Fri 6

th J

un

Mon

9th

Jun

Sat 7

th J

un

Tue

10th

Jun

Sun

8th

Jun

Fri 1

3th

Jun

Thu

12th

Jun

Wed

18t

h Ju

n

Tue

17th

Jun

Mon

16t

h Ju

n

Sun

15th

Jun

Sat 1

4th

Jun

Sun

22nd

Jun

Sat 2

1st J

un

Fri 2

0th

Jun

Thu

19th

Jun

Mon

23r

d Ju

n

Tue

24th

Jun

54

Figure 5.3: Peak concentrations of dust caused by explosions detected 295 m from face over 20 day period (by red PDR)

The 1 second data from the dataloggers shown in Tables 5.2 and 5.3 allows a more accurate analysis of the data from the blue and orange PDRs, while due to the problems with the connection between the red PDR and its associated Antilog the data shown in Table 5.4 is taken from the 2 min data from the memory of the red PDR. These data were analysed using the parameters defined in Table 5.1.

Table 5.1: Definition of parameters by which the data obtained in the tunnel were analysed Parameter Definition

Detection delay Time between blast occurrence and time at which the associated respirable dust concentrations rose above 1 mg/m3 at predetermined distances along the tunnel

Peak width > 1 mg/m3* Time for which respirable dust concentration is above 1 mg/m3 after each blast at predetermined distances along the tunnel

Peak Value The maximum concentration of respirable dust after each blast at predetermined distances along the tunnel

Background Concentration The concentration of respirable dust due to effects other than the immediate effects of the blasts at predetermined distances along the tunnel

0102030405060708090

100110120130140150

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

Det

ecte

d D

ust C

once

ntra

tion

(mg/

m^3

)

.Red PDR (mg/m³)

Wed

18t

h Ju

n

Wed

11t

h Ju

n

Thur

5th

Jun

Fri 6

th J

un

Mon

9th

Jun

Sat

7th

Jun

Tue

10th

Jun

Sun

8th

Jun

Fri 1

3th

Jun

Thu

12th

Jun

Tue

17th

Jun

Mon

16t

h Ju

n

Sun

15t

h Ju

n

Sat

14t

h Ju

n

Sun

22n

d Ju

n

Sat

21s

t Jun

Fri 2

0th

Jun

Thu

19th

Jun

Mon

23r

d Ju

n

Tue

24th

Jun

Table 5.2: Detection times, peak widths and concentrations of respirable dust taken from 1 s data recorded 100 m from face (in Antilog 4 from blue PDR)


Peak Value (mg/m3)


Peak width (mm:ss)

Thu 05/06/08 14:29 102 112.9 6:56 13:41 Fri 06/06/08 06:42 79 148.5 8:15 26:56 Sun 08/06/08 13:29 70.6 86.7 6:34 25:55 Mon 09/06/08 12:20 72.2 47.1 9:00 26:53

22:06 66.2 84.9 7:00 26:36 Tue 10/06/08 11:04 65.8 34.3 7:35 24:20

22:37 65.0 35.1 6:38 17:37 Wed 11/06/08 11:50 62.8 47.3 6:57 20:03

23:06 63.8 64.4 7:11 20:24 Thur 12/06/08 10:55 63.4 31.5 7:46 26:40

22:19 63.6 29.8 7:38 35:31 Fri 13/06/08 10:56 63.4 67.4 7:59 17:23 Sun 15/06/08 13:00 66.0 22.6 8:48 24:16 Mon 16/06/08 18:14 83.2 8.9 9:31 35:42 Tue 17/06/08 14:01 76.6 50.6 8:37 19:04 Wed 18/06/08 07:15 72.4 72.6 7:07 15:38

22:54 76.8 85.3 6:57 16:55 Thurs19/06/08 15:11 86.4 47.2 8:21 17:32 Fri 20/06/08 09:10 84.0 31.6 9:59 19:53

Mon 23/06/08 08:22 88.6 104.4 7:49 26:30 Tue 24/06/08 03:34 no data no data no data

Average Values 73.6 60.7 7:50 22:52 σ = 10.8 σ = 34.9 σ = 0:58 σ = 06:06

Table 5.3: Detection times, peak widths and concentrations of respirable dust taken from 1 s data recorded 170 m from face (in Antilog 5 from orange PDR)


Peak Values (mg/m3)


Peak width (mm:ss)

Thu 05/06/08 14:29 102 110.0 8:18 16:22 Fri 06/06/08 06:42 79 99.3 9:58 32:39 Sun 08/06/08 13:29 70.6 94.4 7:40 36:52 Mon 09/06/08 12:20 72.2 66.4 9:27 29:00

22:06 66.2 81.1 8:11 28:00 Tue 10/06/08 11:04 65.8 40.6 8:48 27:05

22:37 65.0 41.4 8:02 19:32 Wed 11/06/08 11:50 62.8 42.3 8:36 20:59

23:06 63.8 65.6 8:51 22:22 Thur 12/06/08 10:55 63.4 33.2 9:09 29:05


22:54 76.8 76.6 8:27 18:48 Thurs19/06/08 15:11 86.4 66.8 9:08 21:06 Fri 20/06/08 09:10 84.0 52.8 9:49 24:46

Mon 23/06/08 08:22 no data no data no data Tue 24/06/08 03:34 no data no data no data


55

Table 5.4: Detection times, peak widths and concentrations of respirable dust taken from 2 min data recorded 295 m from face (in internal memory of red PDR)


Peak Value (mg/m3)

Detection delay (mm:ss)

Peak width (mm:ss)

Thu 05/06/08 14:29 102 56.6 11:06 22:00 Fri 06/06/08 06:42 79 69.9 14:06 44:00 Sun 08/06/08 13:29 70.6 42.0 11:06 56:00 Mon 09/06/08 12:20 72.2 32.8 14:06 32:00

22:06 66.2 40.3 12:06 30:00 Tue 10/06/08 11:04 65.8 20.9 13:06 28:00

22:37 65.0 19.1 12:06 22:00 Wed 11/06/08 11:50 62.8 21.5 12:06 30:00

23:06 63.8 29.3 13:06 28:00 Thur 12/06/08 10:55 63.4 17.0 13:06 30:00


22:54 76.8 36.9 12:06 23:00 Thurs19/06/08 15:11 86.4 36.5 13:06 24:00 Fri 20/06/08 09:10 84.0 27.6 14:36 26:30

Mon 23/06/08 08:22 88.6 45.2 14:06 30:00 Tue 24/06/08 03:34 no data no data no data


The peak concentrations of respirable dust reaching the blue PDR (positioned 100 m from the face) and the orange PDR (positioned 170 m from the face) show no distance dependence, the average values over the entire number of explosions of 60.7 mg/m3 and 58.5 mg/m3 respectively being comparable within the tolerances of the experimental set up. The lack of temporal resolution prevents useful comparison with the intensity of the concentration data stored in the internal memory of the red PDR.

After an explosion the average duration for which the concentration of respirable dust 100 m from the face was above 1 mg/m3 was 23 min (σ = 6 min), while the average duration for which the concentration of respirable dust 170 m from the face was above 1 mg/m3 was 27.5 min (σ = 8.5 min). This suggests that the dust ‘packet’ has increased in length between the 2 sensor positions of 100 m and 170 m from the face, which would be expected to cause a reduction in concentration along that length, which the small reduction evident from the data does not support. However, unlike the NOx which must originate from the blast face, dust may be picked up by the ventilating air from all along the tunnel, which could explain the comparable concentrations detected at both positions.

The average time for which the concentration of respirable dust 100 m from the face was above 1 mg/m3 (23 min) is around a quarter of that for which the concentration of NO was above 1 ppm (94 min), around a half of that for which the concentration of NO was above 5 ppm (44 min) and the NO2 was above 1 ppm (40 min), and one and a quarter times longer than that for which the concentration of NO2 was above 5 ppm (18 min). The concentrations of dust, therefore, show neither temporal nor quantitive relationships.

The average time delay between the explosion and the initial detection (> 1 mg/m3) of the respirable dust by the PDR positioned 100 m away was approximately 8 min (σ = 1 min), and

56

57

by the PDR positioned 170 m away was approximately 9 min (σ = 1 min). For the respirable dust to travel 100 m in 8 min and 170 m in 9 min estimating a plug flow it will travel with a velocity of 0.21 m/s (σ = 0.03 m/s) and 0.3 m/s (σ = 0.03 m/s) respectively which constitutes approximate flow rates of 2.2 m3/s (σ = 0.03 m/s) and 3.3 m3/s (σ = 0.03 m/s) respectively in the tunnel with a cross sectional area of 10.5 m2. The flow rate should be constant along the tunnel and the velocity should be constant for a tunnel of consistent volume. The lack of a time stamp at the end of the investigation prior to switching the instrument off, the tolerances in the initial measurements, inconsistencies in the tunnel dimensions along the length and the irregularly timed increase in tunnel length by 30 m over the 20 day duration of the investigation may account for the discrepancies in the velocities and flow rates at each of the 2 sensor positions. These delay times and associated air velocities and flow rates compare well with the delay times (7 min) and associated air velocities (0.24 m/s) and flow rates (2.5 m3/s) observed from the NOx data taken from the green MultiRAE allowing for the uncertainties in the set up and are again much less than the values stated by the tunnel engineers of an extraction air velocity in the tunnel of 0.9 m/s and an air flow rate of 20 m3/s. It can also be seen from the dust concentrations detected in this part of the investigations that neither the intensities nor widths of the peaks caused by the explosions are dependent on the weight of the explosive used. The background concentrations of respirable dust at each position in the tunnel over the 20 day period are shown in Figures 5.4 to 5.6.

Figure 5.4: Background concentration of dust detected 100 m from face over 20 day period (by blue PDR)

00.20.40.60.8

11.21.41.61.8

22.22.42.62.8

3

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

13:

00:5

1 0

1:00

:51

Det

ecte

d D

ust C

once

ntra

tion

(mg/

m^3

) . Blue PDR (mg/m³)

Wed

11t

h Ju

n

Thur

5th

Jun

Fri 6

th J

un

Mon

9th

Jun

Sat

7th

Jun

Tue

10th

Jun

Sun

8th

Jun

Fri 1

3th

Jun

Thu

12th

Jun

Wed

18t

h Ju

n

Tue

17th

Jun

Mon

16t

h Ju

n

Sun

15t

h Ju

n

Sat

14t

h Ju

n

Sun

22n

d Ju

n

Sat

21s

t Jun

Fri 2

0th

Jun

Thu

19th

Jun

Mon

23r

d Ju

n

Tue

24th

Jun

58

Figure 5.5: Background concentration of dust detected 170 m from face over 20 day period (by orange PDR)

Figure 5.6: Background concentration of dust detected 295 m from face over 20 day period (by red PDR)

The average background dust concentrations are shown in Table 5.5. These are calculated from the PDR data and appear to be reasonably constant at all three positions in the tunnel. The peak concentrations caused by the blasts are excluded by allowing for average peak width times for dust concentrations above 1 mg/m3 (30 mins) taken from Tables 5.2 to 5.4 from the initial detection time after each blast.

00.20.40.60.8

11.21.41.61.8

22.22.42.62.8

3

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3 0

1:00

:23

13:

00:2

3

Det

ecte

d D

ust C

once

ntra

tion

(mg/

m^3

) .

Orange PDR (mg/m^3)

Wed

18t

h Ju

n

Wed

11t

h Ju

n

Thur

5th

Jun

Fri 6

th J

un

Mon

9th

Jun

Sat

7th

Jun

Tue

10th

Jun

Sun

8th

Jun

Fri 1

3th

Jun

Thu

12th

Jun

Tue

17th

Jun

Mon

16t

h Ju

n

Sun

15t

h Ju

n

Sat

14t

h Ju

n

Sun

22n

d Ju

n

Sat

21s

t Jun

Fri 2

0th

Jun

Thu

19th

Jun

00.20.40.60.8

11.21.41.61.8

22.22.42.62.8

3

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

13:

00:0

6 0

1:00

:06

Det

ecte

d D

ust C

once

ntra

tion

(mg/

m^3

)

.

Red PDR (mg/m³)

Wed

11t

h Ju

n

Thur

5th

Jun

Fr

i 6th

Jun

Mon

9th

Jun

Sat

7th

Jun

Tue

10th

Jun

Sun

8th

Jun

Fri 1

3th

Jun

Thu

12th

Jun

Wed

18t

h Ju

n

Tue

17th

Jun

Mon

16t

h Ju

n

Sun

15t

h Ju

n

Sat

14t

h Ju

n

Sun

22n

d Ju

n

Sat

21s

t Jun

Fri 2

0th

Jun

Thu

19th

Jun

Mon

23r

d Ju

n

Tue

24th

Jun

Table 5.5 Background concentrations of dust over 20 day detected at each position in tunnel by PDRs Dist from Face (m) PDR Concentration

(mg/m3) 100 Blue 0.2

σ = 0.3 170 Orange 0.3

σ = 0.5 295 Red 0.3

σ = 0.5

59

5.2 APPENDIX 2: DETERMINATION OF CORRECTION FACTORS

Determination of correction factors to allow for inherent effects on the changes in NOx concentrations due to the combined effects of adsorption onto the inner surface of the airtight glass vessel and the small fraction of conversion by the sensors, ie the reactant gas consumed by the electrode to generate the measurement current.

These investigations were performed in a fume cupboard in the HSL, Buxton.

A MultiRAE was placed inside the airtight glass vessel and a small fan was included to ensure good mixing of the NO and N2. 5 ppm, 10 ppm and 50 ppm concentrations of NO in N2, and NO2 in air were in turn made to flow through the airtight glass vessel until the ambient air was flushed out and the required NO or NO2 concentrations were achieved and were seen to be constant. The airtight glass vessel was then sealed by closing the inlet and outlet ports and the changes in NOx concentrations with time monitored over 24 hours.

Figures 5.7 and 5.8 show the changes on NO and NO2 respectively in the airtight glass vessel over a 24 hour period after the inlet and outlet ports had been closed.

0

5

10

15

20

25

30

35

40

45

50

55

0 1 2 3 4 5 6 7 8 9 11 12 13 15 16 17 20 21 24 )

Det

ecte

d C

once

ntra

tions

NO

(ppm

) .

))

((((

51.6

50.4

10.5

38.2

6.7 6.4

10.6

2.5

5.1

4.7 2.6

40.2

10 14 18 19 22 23 Time (hrs

5 ppm NO Test 1 (ppm 5 ppm NO Test 2 (ppm10 ppm NO Test 1 ppm) 10 ppm NO Test 2 ppm) 50 ppm NO Test 1 ppm) 50 ppm NO Test 2 ppm)

Figure 5.7: Changes in 5 ppm, 10 ppm and 50 ppm concentrations of NO in N2 over 24 hours in the airtight cylinder

The changes in the NO concentration from each starting concentration shown in Figure 5.7 can be approximated to a linear function, allowing the correction factor in each case to be determined by calculated the average rate of change. The resulting correction factors are therefore calculated as:

For initial concentration of 5 ppm NO, correction factor is 0.1 ppm/hr



60

These correction factors must be multiplied by the times of the data samples and added to the concentration detected at that sample time.

0

5

10

15

20

25

30

35

40

45

50

55

0 1 2 3 4 5 6 7 8 9 10 12 14 15 17 19 20 22 24 )

Det

ecte

d C

once

ntra

tions

NO

2 (p

pm)

.

) )

) )))

11 13 16 18 21 23 Time (hrs

5 ppm NO2 Test 1 (ppm5 ppm NO2 Test 2 (ppm10 ppm NO2 Test 1 (ppm10 ppm NO2 Test 2 (ppm 50 ppm NO2 Test 1 (ppm 50 ppm NO2 Test 2 (ppm

Figure 5.8: Changes in 5 ppm, 10 ppm and 50 ppm concentrations of NO2 in air over 24 hours in the airtight cylinder

The changes in the NO2 concentration from each starting concentration shown in Figure 5.8 cannot be approximated to any simple function. It was therefore necessary to determine the fractional change in detected concentration from the initial concentration at each data point in the data set for each of the initial concentrations of NO2 in air. This same fractional change must then be applied to the corresponding initial concentrations of the NO2 in diesel fume mixes, at each data point and added to the detected concentration of NO2 at that point.

61

6 REFERENCES

Bostrom C-E (1993). Nitrogen oxides in ambient air – properties, sources and concentrations. Scand. J. Environ. Health, 19, suppl. 2, 9-13.

BTS (2008). www.britishtunnelling.org.uk (accessed 2008).

Bufalini J.J. and Stephens E.R. (1965). The thermal oxidation of nitric oxide in the presence ofultraviolet light. Int. J. Air Wat. Poll. 9, 123-128.

Dowker K.P., Fletcher B. and Ledin S., (2007). Real Time Monitoring and Environmental Fate of Oxides of Nitrogen in the Construction Industry, HSL Report No.RR546.

Highways (2008). www.highways.gov.uk/roads/projects/3832.aspx. (accessed 2008).

Highways Agency (1999). Design Manual for roads and bridges. Vol. 2. Section 2. Part 9, BD78/79 Design of road tunnels. Chapter 5 Ventilation. (1999) http://www.archive2.official-documents.co.uk/document/deps/ha/dmrb/vol2/sect2/bd7899b.pdf.

Lamont D., Walsh P., Dowker P. and Hoyland P., (2008). UK response to changes in exposure limits for nitrogen monoxide. Proc. World Tunnel Congress-2008, 22-24 Sep, Agra, India.

Leighton P.A., (1961). Photochemistry of Air Pollution, Academic Press, New York.

Lindqvist O., Ljungström E. and Svensson R., (1982). Low temperature thermal oxidation of nitric oxide in polluted air. Atmospheric Environment 16, 1957-1972.

Mogan J.P., Stewart D.B. and Dainty E D., (1979). Oxidation of the nitric oxide fraction of diluted diesel exhaust. Canadian J. Chem. Engineering. 57, 378-380.

Oregon Scientific (2008). www.oregonscientific.com (accessed 2008).

Powerland (2008). www.powerland.co.uk (accessed 2008).

Smailys V. and Strazdauskiene R., (2005). NO conversion in wet samples of raw diesel exhaustgas. Environmental Research Engineering and Management (Lithuania). No. 4 (34), 97-100.

Thermo (2008). www.thermo.com (accessed 2008).

Trolex (2008). www.trolex.com (accessed 2008).

Tsukahara H., Ishida T. and Mayumi M., (1999). Gas-Phase Oxidation of Nitric Oxide: Chemical Kinetics and Rate Constant. NITRIC OXIDE: Biology and Chemistry 3,191-198.

United Power (2008). www.unitedpower.en.alibaba.com (accessed 2008).

Wessex Water (2008). www.wessexwater.co.uk/about/threecol.aspx?id=148 (accessed 2008).

Published by the Health and Safety Executive 12/09


Real-time measurement of nitrogen monoxide in tunnels and its oxidation rate in diluted diesel exhaust Exposure to oxides of nitrogen (NOx which denotes the mixture of nitrogen monoxide, NO, and nitrogen dioxide, NO2) commonly arises in the tunnelling industry from diesel engine exhaust emissions and from the use of explosives. The British Tunnelling Society (BTS) guidance levels for NO are 5 ppm for an 8 hour time weighted average (TWA) and 15 ppm for a 15 minute short term exposure limit (STEL). Real-time monitors are used by the construction industry as they provide a means of checking that controls are effective. Previous laboratory work at HSL evaluated various commercial detectors potentially suitable as portable monitors and studied the conversion rate of NO to NO2 in air using pure gases (ie NO and air). This current project investigated:

1 the field use of NO and NO monitors in a 2

small sewer tunnel under construction to assist the determination and application of effective controls in order to maintain a safe working environment. Furthermore, a large road tunnel under construction was visited to study the fixed NO monitoring system installed there.

2 the conversion rate of NO to NO2 using air-diluted diesel exhaust.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

RR757

www.hse.gov.uk

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