Small Scale Propane Blending Plant Services.pdf · 2019. 5. 9. · Small Scale Propane Blending Plant 1 WRAP’s vision is a world in which resources are used sustainably. Our mission

Final Report

Small Scale Propane Blending

Plant

This report provides extensive details about

development of a Small Scale Propane Blending plant

for Biomethane CV enhancement based on volume

production Automotive and Industrial Engine

components.

Project code: OIN001-020

Research date: Jan 2014 to Sept 2015 Date: March 2016

Small Scale Propane Blending Plant 1

WRAP’s vision is a world in which resources are used sustainably. Our mission is to accelerate the move to a sustainable resource-efficient economy through re-inventing how we design, produce and sell products; re-thinking how we use and consume products; and re-defining what is possible through recycling and re-use.

Find out more at www.wrap.org.uk

This report was commissioned and financed as part of WRAP’s Driving Innovation in AD

programme. The report remains entirely the responsibility of the author and WRAP

accepts no liability for the contents of the report howsoever used. Publication of the

report does not imply that WRAP endorses the views, data, opinions or other content

contained herein and parties should not seek to rely on it without satisfying themselves of

its accuracy.

Written by Terry Williamson

Front cover photography: Automotive part in a non-automotive application

While we have tried to make sure this report is accurate, we cannot accept responsibility or be held legally responsible for any loss or damage arising out of or in connection with this

information being inaccurate, incomplete or misleading. This material is copyrighted. You can copy it free of charge as long as the material is accurate and not used in a misleading

context. You must identify the source of the material and acknowledge our copyright. You must not use material to endorse or suggest we have endorsed a commercial product or

service. For more details please see our terms and conditions on our website at www.wrap.org.uk

http://www.wrap.org.uk/


1. Executive Summary

Within the renewable energy industry there is a mature development of systems where

biogas is produced in an anaerobic digester then turned into biomethane by removal of

contaminates and excess CO2. The biomethane may then be injected into the UK gas grid

for use at remote locations as a renewable fuel. Before acceptance for injection into the gas

grid the gas properties are checked and CV (energy content) measured, there is usually a

need to increase the CV to match the properties of the gas already in the grid. This CV

enhancement is achieved by blending in small quantities of propane. Gas quality

measurement and propane blending systems currently sell for £450 to £500k.

Equipment at this value is only viable for large scale plant where a reasonable return on

investment may be achieved. The UK, having an extensive established gas grid network,

presents an opportunity for small scale distributed gas injection locations where grid systems

are localised and operate at low pressures. If equipment may be developed that presents a

reasonable return for small scale systems then renewable opportunities may be considerably

extended.

This report describes the construction and test of a small scale, low cost biomethane blending

and gas CV measurement plant that may be used as a module in an overall biomethane

production system.

Currently propane blending systems utilise high cost process industry components with gas

quality measurement undertaken by Gas Chromatograph systems. In this study we are

utilising components from the automotive industry both for systems operation and gas

measurement, this equipment is extremely robust, presents excellent value for money and

accepted where stringent emissions control and measurement is needed every day in our

road transport.


Contents

1. Executive Summary ........................................... 2

2. Introduction ...................................................... 5

3. Design and Technology ...................................... 7

Design Objectives ...................................................................................... 7 2.1.

Design Requirements ................................................................................. 7 2.2.

Design Detail ............................................................................................ 9 2.3.

i. Biomethane CV enhancement - Propane source ..................................... 9

ii. Propane transfer to the blending unit ................................................... 9

Design to build ........................................................................................ 13 2.4.

Factory testing ........................................................................................ 17 2.5.

Test Facilities .......................................................................................... 17 2.6.

Propane source ....................................................................................... 18 2.7.

Propane vaporisation and pressure control ................................................ 19 2.8.

Propane consumption use ........................................................................ 19 2.9.

Gas detection and measurement .............................................................. 20 2.10.

Propane heating ...................................................................................... 20 2.11.

Regulated air supply ................................................................................ 21 2.12.

Biomethane supply and propane mixing .................................................... 22 2.13.

Gas sensing ............................................................................................ 23 2.14.

Data gathering and control output ............................................................ 23 2.15.

4. Key lessons learned from the pilot plant

demonstration ................................................................. 25

5. Summary ......................................................... 30

Appendix 1 Document Revision/Approval Record .......... 32

Appendix 2 Proposed P&I, Refer to drawing .................. 33

Appendix 3 Control Philosophy ...................................... 35

Appendix 4 Correspondence with Dynament ................. 37

Appendix 5 Test log at Biogas Products ......................... 39


Contents of tables and figures

Table 1 Air compressor .......................................................................................................................... 21

Table 2 Flow meter ................................................................................................................................ 22

Table 3 Methane and Carbon Dioxide sensors ..................................................................................... 23

Figure 1 Propane vapour blending system ............................................................................................. 8

Figure 2 P&ID Wrap Propane blending ................................................................................................. 14

Figure 3 Calculation of propane required to enhance CV of biomethane ........................................... 15

Figure 4 Vapour pressure graph............................................................................................................ 18

Figure 5 Impco Model K fork lift Truck converter ................................................................................. 19

Figure 6 Industrial weight scale, accuracy 0.1g .................................................................................... 20

Figure 7 Electric heater and circulating pump ...................................................................................... 21

Figure 8 HMI display .............................................................................................................................. 24

Figure 9 Allowed Calorific Value for Biomethane to be injected into the grid .................................... 27


2. Introduction

When processing biogas intended for injection into the gas grid it is first cleaned of

contaminates then upgraded to biomethane (97% methane) by removal of the CO2 content

and other impurities such as hydrogen sulphide, nitrogen etc. However at this stage the

energy content (gross CV) of the biomethane is still not sufficient for injection into the gas

grid and requires further enhancement by addition of small amounts of propane so requires a

propane blending module.

Currently technology exists for blending the required quantity of propane for CV

enhancement using complex and therefore expensive equipment. This equipment will be

typical to that used for gas flow and quality measurement in large gas injection plant

undertaking flows many times larger than even the largest existing biomethane plant so is

disproportionate in scale.

To provide a sustainable and affordable technology for the future allowing very small scale

and distributed gas production systems to be developed significant cost reduction in plant

must be achieved.

This project is a module in the cost reduction train and is intended to demonstrate that

suitable plant may be developed using product from other industries, namely automotive and

industrial engines. Such components have to survive in applications significantly more

demanding than most gas plant and being manufactured in high volumes allow excellent

product quality at very low cost.

Manufacture of plant at low cost needs to take a reasonable view on safety issues. Typically

gas handling plant design follows a method whereby any component that comes into contact

with gas that has a source of ignition (electrical component), must follow a practice that

insists on the use of ATEX compliant products. In common practice throughout the world we

all drive cars and operate machinery operating on gaseous fuels (propane and natural gas)

and liquid fuels (petrol and diesel) without resorting to ATEX regulations. The use of

automotive product manufactured with the intention of using the above mentioned fuels

needs to resolve a design that meets safety requirements without resorting to expensive

equipment. A solution is to provide protection by monitoring for gas leakage with adequate

ventilation in the same was as gas engine Combined Heat and Power (CHP) units do as a

compromise for safe operation.


Demonstration of this new approach will assess whether similar accuracy and control is

achievable when compared to full cost equipment. Should it not, but accuracy is reasonable,

then it will indicate whether there is scope for relaxation of industry requirements for very

small scale plant i.e. < 1MW (circa 100m3/hr) gas energy injection and make this affordable

for development.


3. Design and Technology

A review of the process to develop, build, test and evaluate a small scale propane blending

plant to enhance the CV of biomethane with sufficient accuracy to allow injection into the gas

grid.

Design Objectives 2.1.

The biomethane industry today is maturing to the extent where economies of scale and cut

off points for affordable development is becoming more evident. This is in the region of 250

Sm3/hr minimum injection to the grid or 2.7MW of gas energy, this threshold will raise with

the lowering of RHI tariffs and particularly with the tiring of rates that will have the effect of

narrowing the maximum and minimum plant size where affordable development is attractive.

Small Scale is therefore represented at flows below this, is an area attracting attention and

has the potential for significant growth if the right product becomes available. From other

studies involving the dairy and beef industries show that there are significant numbers of

locations that could be developed if return on investment is realistic, this represents the 5 to

50 Sm3/hr biomethane production rate so represents true small scale and where this

feasibility study is targeted.

Existing technology for propane blending and grid injection has stabilised around the

£500,000 mark for plant processing 1000 Sm3/hr. Within this plant the propane blending

system costs some £50,000 but relies on gas chromatographs and associated equipment

included in the main system so the overall cost including this is likely to be closer to

£200,000. The challenge is to make this affordable for a 50m3/hr plant so needs to be in the

£5,000 to £10,000 range.

Design Requirements 2.2.

A propane blending unit requires the following components:-

a) propane source, typically a bulk tank provided under contract from one of the

national suppliers

b) a means of delivering propane to the blending unit under all seasonal conditions. For

full scale plant this involves pumping the propane liquid using electricity, small scale

plant would seek to remove this need to save cost and energy


c) a device to ensure that propane gas is converted from liquid (if necessary), ensures

the gas is pressure and flow controlled before delivery to the blending unit

d) a blending device, usually a tube with baffles in it, no moving parts

e) a means to measure the CV (gas energy) content of the unenriched biomethane and

a second device to again measure the CV after enrichment

f) a control system that trims the quantity of propane flowing to the blending device by

looking at the before and after enrichment signals

Figure 1 Propane vapour blending system


Design Detail 2.3.

i. Biomethane CV enhancement - Propane source

Biomethane CV enhancement generally requires about 4% by volume or 15% by energy from

the propane. A 50 Sm3/hr flow would need 2 Sm3/hr of propane or 3.9kg/hr. A 2 Tonne

domestic bulk tank last some 20 days.

ii. Propane transfer to the blending unit

At normal UK temperatures the bulk tank will contain a vapour pressure of 4 to 15 bar

(propane freezes at -40degC so any temperature above that gas will form and possess a

vapour pressure). This vapour pressure will transfer propane as a gas or liquid at reasonable

flows and distance without mechanical assistance. (This requirement is site specific so may

on some occasions be required, for the demonstration unit we have not included it and will

use a liquid take-off gas cylinder typically found on industrial fork lift trucks. This is easily

transportable and contains sufficient for this purpose).

Conversion of propane liquid to gas is achieved by passing the liquid through a heat

exchanger and warming with hot water.

The warming of the liquid causes expansion at the ratio of 300:1 significantly increasing

pressure, this pressure rise requires controlling and delivering to the blending unit at a

precise level irrespective of the flow rate demanded.

Various products were looked at from the automotive and industrial market and offered by a

number of suppliers in the USA, Italy, Canada, Japan, Argentina and Brazil.

The limiting factors are flow rate capacity and pressure limit. For this study where we are

looking to blend 4% of propane in 5m3/hr of biomethane at a nominal 20 to 100mBar

pressure so requires about 200 L/hr of propane gas (0.7L/hr of propane liquid), most

available units would suffice.

From previous experience of this industry the Impco Model K was chosen being one of the

smallest available, is widely available and has a capacity of 7000 L/min of gas so well in

excess of our requirement.

The secondary diaphragm and casing will handle up to 1000mBar of pressure so again able

to handle the 20 to 100mBar required. This regulator is equipped with a vaporisation baffles


that accepts hot water to turn the liquid propane into a vapour, has two stages of pressure

reduction and is intrinsically safe in that no electrical components are used and in its

quiescent state will not flow gas as the outlet pressure is with respect to atmosphere. It

requires a positive pressure on the second stage diaphragm from the control unit to flow gas.

Delivery of hot water to the vaporiser needs to be controlled at a level with sufficient energy

to compensate for the latent heat of vaporisation of the propane (425 kJ/kg) and warm the

gas to the required temperature to ensure condensation will not occur (Specific heat

1.6kJ/kg), the resultant energy demand is 100W.

The easiest way to carry this out is with a simple hot water circuit with a target gas

temperature and a means of varying the heat introduced into the system.

For this project we selected a low wattage (150W) domestic electric heating element found in

almost any bathroom towel rail. The remainder of the heating circuit contains an expansion

tank and circulating pump, again sourced from domestic products. The temperature of the

hot water is controlled to a level by measuring the propane gas temperature at the exit of the

regulator/vaporiser. Adjusting the electrical supply (and therefore heat) to the heating

system allows gas temperature to maintained at a constant level irrespective of the rate of

propane flow

Propane flow control can be achieved using a number of different technologies used in the

automotive and manufacturing industry.

Most petrol vehicles are now equipped with drive by wire throttle bodies that allow cruise

control, self-parking, traction control etc. These are electronically operated devices where

flow is adjusted using demand signals. The CHP (Combined Heat and Power) industry uses

them for generator load and speed control however all are relatively expensive, over large for

our purpose and complex to integrate outside their industry.

A simple solution is employed in vehicle turbocharger boost control systems whereby a small

low cost 3 port valve diverts air pressure form the turbocharger to allow control of the level

of boost (and therefore torque developed in the engine). These are inexpensive, widely

available and only need a control signal and a supply of clean air.

For this project propane gas pressure is controlled by applying this air supply to the

atmospheric or control port of the regulator/vaporiser by pushing on the second stage

diaphragm this overcomes the closing effort of the diaphragm closing spring and allows


propane to flow. The rate of flow is governed by the pressure difference between the

propane pressure and the pressure in the biomethane line within the mixer.

This pressure difference is obtained by delivering the right pressure of air through the 3 port

valve, it in turn controlled by the time it is switched on by the control system.

Air is supplied by a small DIY type air compressor

The blending device is a tube assembly fitted with baffles so that the biomethane and

propane are thoroughly mixed. This is an easy and uncomplicated device to produce. The

mixing system is no different to that seen in an engine carburettor and the use of such a

device was considered as it is able to provide a purely mechanical system for proportional

flow control, the additional control described above would simply be to trim the ratio and

improve stability. However flows being small in this case would require investigation of

lawnmower sized devices, the additional effort at this stage not considered worthwhile.

Gas flow measurement to a reasonable accuracy is expensive, it is possible to use automotive

devices to measure pressure difference with temperature across a fixed orifice and with

calibration produce a reliable flow meter.

Low cost domestic gas meters are readily available with a flow signal output and represents

excellent value for money so a standard unit was selected. These meters are positive

displacement devices so if we know the gas composition, pressure and temperature we can

calculate with reasonable accuracy biomethane flow and energy content. Note, these meters

are extensively used to measure domestic gas demand and therefore billing so there should

be no reason why the same device cannot be used to deliver gas into the gas grid.

Biomethane is typically 95% methane with the balance containing CO2, Nitrogen and

Oxygen. Our main interest is Methane and CO2, if we measure just these two and deduct

those measurements from 100% we can not only calculate the energy content of the gas,

know the % of CO2 and by deduction know the sum of those not measured, the N2 and O2.

We use the basis of CO2 for CV measurement after blending as the % of this will alter due to

the inclusion of propane. Gas pressure and temperature is measured by standard industrial

devices in this instance.

Knowing gas volume flow through the domestic meter, pressure, temperature and

composition we can calculate with reasonable accuracy the corrected gas flow and energy

content.


The reasonable accuracy is suggested as we know the methane content, say 95%, CO2

content, say 2% so the balance of 3 % must comprise N2 and O2. The energy content is

derived from methane only.

Gas composition meters are widely used in garage MOT test stations, boiler service and in the

biogas and landfill industries. MOT, boiler and biogas units all tend to use sensors based on

infra-red technology, these sensors are UK manufactured and readily available. It suggests

that incorporating these detectors into the overall system control will save on control devices

and save cost.

An electronic conversion board is available for these devices converting gas analysis signals

to a level suitable for entering into a standard industrial display device or programmable logic

control.

We considered the use of an automotive ECU, proprietary devices are available with

programming software in the motorsport industry and have huge capacity, well in excess of

our needs. However programming knowledge is somewhat specialised so a standard

industrial PLC unit was adopted, this takes all the available signals mentioned above, provides

a display of parameters and achieves overall control of the blended gas CV.

The basis of control is as follows, evolved from an overview of automotive and industrial

systems: - Biomethane pressure, temperature, methane and CO2 content is measured.

Biomethane flow is measured in the gas flow meter, this is achieved by timing the rate of

emptying and filling of the positive displacement chamber in the gas meter the resultant

calculation provides the effective rate of flow. By using pressure and temperature correction

(to standard conditions 1013mBar and 15 deg C) we can calculate the effective rate of flow.

Knowing the methane composition of this gas we can then calculate the Gross and Net

Calorific Value (CV) of the gas.

After propane is introduced into the gas stream we again measure the CO2 level, this will

have reduced in % terms indicating that an additional gas has been introduced into the gas

stream. The proportion of the original gasses will not have changed so we know how much

propane has been added. We know the CV of the biomethane before blending, we know the

CV of the propane added so we can therefore calculate the CV of a corrected M3 of blended

gas.

To achieve the correct CV of the blended gas we need to introduce the correct amount of

propane. To do this we must enter a target CV value, say 39 MJ/Sm3. At start-up we


measure the CV of the biomethane with the propane turned off, it will be typically 36MJ/Sm3,

the difference is 3MJ, the amount of propane to introduce.

With the propane turned on we monitor the change in CO2 and adjust the propane rate of

flow by slowly altering air pressure on the 3 port valve, this increases the propane flow to a

point where the calculated blended CV = target CV. There is likely to be a period of settling

till the system stabilisers.

Design to build 2.4.

A provisional system P&ID (Process and Instrument Diagram) was sketched out and

proceeded through a series of iterations till the design shown in the following diagram 1 was

agreed.

The design was based on a selection of main components illustrated in the materials list –

Appendix 2. In addition a control philosophy was drawn up, also Appendix 2 indicating how

the system is proposed to operate. Finally an operation calculator was produced in Excel and

is shown in List 1.

Figure 1 indicates all active components in the system, it shows the biomethane route from

the upgrade plant to the blended outlet to following systems. The propane route from

storage tanks through the vaporisation and control to the blending unit. The gas sensing

components and all of the pressure and temperatures sensors used in flow control.


Figure 2 P&ID Wrap Propane blending


Figure 3 Calculation of propane required to enhance CV of biomethane



Factory testing 2.5.

Simulated testing was carried out at the Biogas Products Workshop in Brierley Hill,

Staffordshire on Tuesday 24th March 2015. This testing sought to indicate whether the

equipment functioned as intended and be suitable for transporting to the demonstration

phase as a selected location away from the workshop. Test results are located in Appendix 5.

Test Facilities 2.6.

Speciality gasses provided. Nitrogen (N2) 99% pure, Carbon Dioxide (CO2) and Methane

(CH4) supplied by Air Liquide.

Gas flowmeters for CO2 and CH4, pressure and flow control equipment to permit controlled

flow of gasses into the unit under test.

Standard workshop equipment, pressure and temperatures indicators, power supply.

Setting up procedure

All references to components within this text can be found in diagram 1


The propane blending unit comprises of a number of systems, they all need setting up or

validating functionally, there are:

a propane source

propane vaporisation

pressure control system

propane consumption use

propane heating

regulated air supply

biomethane supply and propane mixing

gas sensing

data gathering and control output

Propane source 2.7.

Propane source in the test unit is a standard vapour take-off cylinder normally used for water

heater, barbeques etc. The pressure for operating is typically that of domestic supply around

20mBar, the system as designed will cope with up to 500mBar. A propane cylinder at UK

ambient temperatures will deliver a vapour pressure upwards of 4 Bar so is adequate for this

purpose.

Propane Vapour Pressure Curve. (Air Liquide). Operation will be in the 270 to 300 degK

range, 4 to 10 bar pressure.

Figure 4 Vapour pressure graph


Propane vaporisation and pressure control 2.8.

Vaporisation and pressure control is undertaken within a single unit, a standard Impco Model

K fork lift Truck unit. It takes liquid or vapour propane from the cylinder at up to 16 Bar and

reduces this pressure to -4mBar so under quiescent conditions no propane will flow and is

intrinsically safe. (This is a design feature for a vehicle in that should the ignition be left on

no propane will flow till the engine is turning and air intake depression to the cylinders

overcomes this outlet depression and draws gas into the cylinders). There are no setup

procedures required for this other than to observe correct assembly.

Figure 5 Impco Model K fork lift Truck converter

Propane consumption use 2.9.

There are two indicators for propane use. One is a direct reading vertical flowmeter FE100

that provides an indication of flow located in the outlet of the vaporiser/regulator. The

second is a need for recording actual propane usage for cost and billing. Conventionally this

would be handled by a flow meter with pressure and temperature correction and based on a

known energy content of the propane supplied. However during component review for this

study no small scale flow meter was found at an acceptable price so a weighing scale has

been employed. With this a start/finish weight may be recorded, the density and energy

content of the propane is known so an energy flow during the period may be calculated. No

setup procedure is required other than understanding scale accuracy and calibration.


Figure 6 Industrial weight scale, accuracy 0.1g

Gas detection and measurement 2.10.

The gas sensors fitted for CO2 and CH4 have an operating detection range of 0 to 100%. To

set the sensors to 0% detection the system was first flushed through with N2, this process

eliminates the presence of any other gas, particularly CO2 and CH4. After a period of

stabilisation the sensors were zeroed by manually setting this point so that CO2 and CH4

sensors now read out 0%. Following this the system was flushed through with 100% CO2

and the sensor span set to 100% after a period of stabilisation. Similarly 100% CH4 was

then flushed through and the span for that sensor set to 100%. Following this calibration

procedure the sensors should detect and indicate the relevant gas in any content between 0

and 100%. For instance if the trial gas introduced contained 60% CH4 and 40% CO2 the

sensors would display that result.

Propane heating 2.11.

It is desirable when mixing two gasses together that mixing performance is improved if the

two gasses are similar temperature. Within this proviso is that the propane temperature

must not re-condense after vaporisation, with existing plant blending at 7 Bar or so re-

condensing is an issue to protect against, so propane gas temperature is maintained above

the condensing temperature till the moment of mixing where the partial pressure laws apply.

In this instance pressure is very low and well above the point of re-condensing.

Heating is carried out by a small domestic 150W electrical element, heat delivery by a micro

water pump used in solar systems and control through the HMI (Human Machine Interface)

unit from thermocouples inserted in the biomethane and propane delivery lines.

The target propane temperature point is derived from the biomethane gas sensor TE101,

power delivered to the heater HE001 and temperature of the propane observed at TE100.


Power at HE001 is modulated by the HMI unit to keep TE100 = TE101. A simple PID control

is employed to achieve this, the process is not critical but stability will assist the plant to

achieve it target performance. The remainder of the heating system employs conventional

domestic components. Setting up is limited to observing that temperatures are close to

desired values.

Figure 7 Electric heater and circulating pump

Regulated air supply 2.12.

Air demand for propane pressure control is relatively small and for convenience sake a small

DIY compressor is used. Pressure from this is stored in a cylinder at up to 8 bar so the

compressor only runs to replenish store, how much air is used is a matter for observation.

Air demand during testing is at 90 mBar, the standard compressor equipped regulator has

insufficient control for long term use but sufficient for testing. Other than connecting up the

compressor to a mains 230V supply and monitoring the delivered pressure to the control

system no additional setup is required.

Table 1 Air compressor

Power consumption [kW] 1.5

Voltage [V] 230

Current [A] 7.5

Discharge Pressure [bar] 8

Restart Pressure [bar] 4.8


Air displacement [cfm] 9.6

Biomethane supply and propane mixing 2.13.

In normal use the module would receive biomethane after a clean-up and purification process

where most CO2 and impurities are removed, typically Hydrogen sulphide (H2S) and dried to

remove water. In the test example we replicate a gas using methane (CH4) and Carbon

Dioxide (CO2) introduced to synthesise biomethane by using pressure and flow control. Our

target mix was 80L/min of methane and 3L/min of CO2, this provides a mol% blend of

96.4% Methane and 3.6% CO2. This is injected upstream of the mixer MX001, pressure and

temperature are monitored at PE103 and TE101. Direction control of the biomethane in

pass-through mode where the plant is bypassed or blending mode may be selected using

valves V006 and V007 and solenoid valves in automatic mode SV002 and SV003. No setting

up is required other than ensuring pressure and temperature sensors are functioning and in

calibration.

Biomethane flow is measured prior to blending in a conventional domestic flow meter seen in

every home connected to the gas grid.

This is a positive displacement meter and operates by filling a void, once filled the void “flips”

to present a second void of the same capacity, every time the void “flips” a pulse is available,

the volume of the void is known and the period between “flips” can be timed, the sum of

which may be displayed at a rate of flow. The meter has a totalizer providing the flow

volume throughput. Gas pressure and temperature is measured and by using a formulae the

rate of flow may be standardised to the UK model of Standard Meters Cubed Sm3 (15 degC

and 1013mBar). The meter is BS and CE marked so requires no additional calibration, the

pressure and temperature sensors require standard calibration. All calculation is carried out

in the HMI unit.

Table 2 Flow meter

System Volume [dm3] 20

Max. flow rata [scmh] 60

Total pressure loss [Pa] 2000

Max. operating pressure [kPa] 100


Operating ambient temp. [degC] -10- +40

Gas sensing 2.14.

This is carried out by sensors QE100 to 103, two detect Methane and two CO2 at the entry

and exit of the blending unit.

Table 3 Methane and Carbon Dioxide sensors

Operating Voltage [V] 3-5

Current [A]

80

Operating temp. [degC] -20- +50

Calibration and correct function of these are critical to the control accuracy of the quantity of

propane to introduce and the resultant energy content of the resultant gas. Over blending

results in additional cost, under blending risks rejection by the gas grid under present

regulations.

Setting up involves subjecting the sensors to a series of pure gasses to set zero point (no gas

present of the type for detection) and span (100% of gas for detection present). The

sensors need to be flooded with the gas selected at a minimum flow rate monitored in

floating ball vertical scaled indicators, flow adjusted in a needle valve to the desired value.

Much of the testing undertaken involved setting these up. Full test procedure and results are

in Appendix 4.

Data gathering and control output 2.15.

All inputs from sensors are converted to engineering units as appropriate for display on the

HMI unit.


Functions that require adjustment are also available on the screen for manual intervention

and inputting set points during calibration.

Figure 8 HMI display


4. Key lessons learned from the pilot plant

demonstration

The propane blending plant system had to undergo certain modifications in order to achieve

a consistent performance. Pilot studies are a critical step in selecting and testing a system

that can deliver consistent quality of the enriched Biomethane. The view is that one more

iteration taking lessons learned will move the design forward significantly from where we are

today.

1) The initial design intended that propane delivered to the regulator/vaporiser was in

liquid form. However, during the pilot demonstration the test rig used gas form (the

propane supply cylinder issues by the system assembler was incorrectly assumed by

the writer to have been equipped with a dip-tube for liquid extraction), this meant

that flow is in the order of approx. 300% less that it could be. The system capability

was severely restricted in its ability to deliver and control the correct propane ratio.

2) Gas sensors. Those involved in the design and manufacture of gas monitoring

equipment would have learned this during their development phase but coming into

the new design it was not evident how important certain design features are.

The trapped volume of gas in the detection equipment is vital for speed of response.

Time taken for a changed gas sample to enter the detection head when added to the

settle time of the sensor changes the dynamics of the overall detection system.

A significant reduction in volume is required to produce a more practical device, this

includes filtration, transfer pipes, detection head enclosure and rate of gas flow

introduced into the detection system. In this example detected gas was vented to

atmosphere, it is also necessary to explore how to return detected gas to the main

feed so no venting occurs. (This seems to be a feature on gas sampling equipment in

general) Removing the need to vent will help in emissions reduction in general.

The CH4 sensor with a greater accuracy ideally could be installed to help with the

initial assessment of Biomethane CV entering the system. The Infra-Red sensor head

comes from the same British manufacturer used in many approved gas measuring

systems indicating that the method of use in the prototype requires better design.

The experience with gas detection measurement indicates that in order to move

forward the system should include an existing gas detection system to remove design

issues allowing more focus on the propane handling and blending system,


nevertheless in theory reducing design and verification time it is likely to increase

overall system cost.

During the final test the diaphragm failure in the regulator/vaporiser can be explained

as a result of over-pressurisation, an improved protection circuit with the use of a

flow restrictor (to prevent air pressure inrush) and a more appropriate pressure relief

valve will help to prevent failure.

The software included a number of fixed values as this reduced the time to build the

package, this turned out to be a constraint to development, and changes were time

consuming. For development ideally all values should be applied through a look up

table or similar allowing greater “what if” scenarios to be exercised. It may become

evident that some values ultimately may become fixed allowing for a more

streamlined software solution but this is part of the process to bring to manufacture.

3) CV calculation- Comparison with GC system. Gas detection sensors used in this

demonstration are manufactured by Dynament Ltd and their technical specification

indicate the following:

Methane. Resolution 0.1% in the 10 to 100% volume (we are typically 95 to 100%)

and an accuracy of +/-2% indicating that at 96% CH4 level the display could range

between 97.92% to 94.08% ( represented as 36.91 to 35.47 MJ/Sm3, assuming

100% CH4 is 37.7Mj/Sm3 gross CV)

Carbon Dioxide. The 0 to 5% CO2 sensor indicates an accuracy of +/-2% with a

zero and span repeatability of 500ppm (.05%). Suggesting that a 3% CO2

composition the accuracy is 3.06 to 2.94%. Using this to predict Biomethane CV will

provide results shown below.

A better understanding of the actual performance in an application is required to

assess the suitability of such a product in this situation. The likely key is good

system design coupled with using the correct level of gasses for calibration. The

nearer the calibration point is to the gas level being measured the more accurate the

system.

By comparison an Encal 3000 Gas Chromatograph complying with ISO6976 has a

repeatability of 0.01%, indicating that for an actual CH4 level of 96% will display

between 96.01% and 95.99% (represented as 36.2 and 36.19MJ/Sm3). These


systems are calibrated twice, using a proprietary gas mix and an OFGEM supplied

mix. A similar system would be employed whatever the measurement device.

The current gas act regulations allow for a tolerance of gas energy flowing over a 24

hour period. This is termed as FWAC (Flow Weighted Average Calculation). The

current tolerance is +/- 1 MJ.

With a nominal CV of 39MJ/Sm3 flowing in the grid we are permitted to vary the CV

between 38 and 40Mj/Sm3. (or expressed as an accuracy tolerance of +/- 2.56%)

Figure 9 Allowed Calorific Value for Biomethane to be injected into the grid

It will be noted that the % deviation permitted in this case is 2.56% and therefore

within the accuracy of the proposed system at +/-2%. At the unenriched input side

we are measuring methane and CO2. Assume methane is 96%, CO2 is 3% and the

balance of nitrogen and oxygen in the gas is 1%, the latter constituents are not

measured but assumed.


To control the level of propane enrichment we are measuring the change of CO2

level, the assumption being that the CO2 level will reduce. If we add 4% propane to

the gas, the unenriched % levels of 96% methane, 3% CO2 and 1% balance will

result in the following normalised figures> Methane, 92.31%, CO2 2.88%, other

0.96%, propane 3.85%.

We now expect to see the CO2 measurement drop to 2.88%. Allowing for

measurement accuracy of 2% this would be displayed as between 2.94% and

2.82%.

When compared with the original CO2 level before blending of 3.06% and 2.94% and

comparing the four maximum and minimum possibilities we have the change in CO2

volume as 0%, 4%, 4.2% and 8.5%. Indicating that the propane addition varies

between 0 and 8.2%.


With a CO2 change of 8.5% and taking the CO2 value of 2.82%, normalising the

methane content we arrive at a blended Biomethane CV of 41.25MJ. Assuming we

have a target figure of 39MJ the system will think propane injection is too high and

reduce injection to what it believes it should be, i.e. 3.85% that shows a CO2 value

of 2.88%. This results in a CV of 38.76MJ/Sm3 so within the +/- 1MJ requirement.

Taking the opposite extreme of 0% change in the CO2 level the system will think no

propane is being injected so will increase flow till what it believed the target of 39Mj

is reached, this actually calculates at 39.78MJ, again within the +/- 1MJ set by the

FWAC.


5. Summary

Use of IR instrumentation indicates that propane blending can be measured to fall within the

FWAC requirements and that risk of exceeding limits is low so worth development particularly

when considering the low cost compared to existing systems. More work will be required on

control iteration and demonstration of actual accuracy of these instruments to arrive at a

level of confidence needed by the grid operators.

There are two properties within this proposed system that have advantages, one is speed of

operation, current GC systems only measure gas composition every three to four minutes and

the control system is slow to react (up to twenty minutes for stabilisation). This proposed

system monitors continuously so results are available almost immediately and stability take

milliseconds (the requirements in the automotive industry to achieve good emissions).

A method of reducing FWAC risk is to adopt the principal of grid gas blending. This

technique is currently permitted where the flow into the grid is negligible compared to the

actual grid flow and where the blending of unenriched Biomethane does not permit the

blended gas to fall outside the FWAC levels. Use of a low cost propane system will allow a

lower ratio of Biomethane/natural gas to be used as a level of propane enhancement reduces

FWAC risk.

This indicates that the combination of low cost instrumentation and blending is a viable

solution.

With such a scheme we could reduce the cost of gas quality measurement from a basis of

£60,000 to under £1000 and a complete Grid Entry System from £500,000 to perhaps less

than £50,000 providing a real incentive to small scheme development.

An alternative low risk option for this system is where Biomethane used as a vehicle fuel.

There are issues to consider when using Biomethane as a vehicle fuel where the use of

propane blending is beneficial.

Vehicles designed for operation on natural gas take their fuel from the gas grid system (that

may contain Biomethane), the UK gas has a CV generally in the region of 39 to 41 MJ/Sm3

and contains methane, and various levels of higher hydrocarbons (propane, butane, ethane

etc.)

Biomethane without propane blending is generally in the 36MJ/Sm3 region so contains less

energy per m3 than natural gas, this results in two possible issues, a) the vehicle will not

travel as far between fuel fills and b) there may be issues with the engine operation (and


emissions) due to the lower energy content and different composition of the fuel. This may

be particularly evident should the user alter fuel purchased between natural and bio gasses

sources. The addition of propane by blending will reduce differences in the fuels making

source transparent. A low cost solution to gas CV enhancement is unlikely to require the

same level of control that used for grid injection so this technology has potential when the

direct use of Biomethane as a vehicle fuel become attractive.

From the experience of this study the writer believes that the proposed system has a place in

driving down costs and with reasonable economy of scale derived from good design would

support the expansion of small scale Biomethane systems. The gas industry would need to

be comfortable with the risks from reduced accuracy measurement but the overall effect

within the gas grid can be minimised and kept proportional to the flows from small scale

production.


Appendix 1 Document Revision/Approval Record

Record of Issue

Rev Date Description Update

By

Checked

By

O 4/8/2014 First Issue MLN

1 15/04/2016 Second Issue


Appendix 2 Proposed P&I, Refer to drawing

Tag Title

Part

FH001 Fuel Hose, LPG Cylinder to Bulkhead Fitting

BH001 Bulkhead Fitting T-Piece Impco - 63 0000 0444

PRV001 Pressure Relief Valve – 16Bar Impco – 54 3865 000

V001 Manual Isolation Valve

PI100 Pressure Gauge – 0 to 25bar RS 176214

PRV002 Pressure Relief Valve – 16Bar Impco – 54 3865 000

FILT001 LPG Filter

RV001 Vaporiser Pressure Regulator Impco – Cobra Series

PRV003 Pressure Relief Valve – Expansion Tank Screwfix - 78086

PI004 Pressure Gauge – Expansion Tank

V003 Non Return Valve



MX001 Gas Mixer







V010 Manual Control Valve


VO15 Manual Air Bleed Valve

FILT002 Gas Filter

FILT003 Gas Filter

PRV005 Pressure Regulation Valve

PRV004 Hydrostatic Pressure Relief Valve

SV004 Turbo Boost Control Unit Pierberg – 7.22240.13.0



CP001 Control Panel

PS101 Pressure Switch – 8bar Setpoint RS 5184462

SV001 Gas Solenoid Valve Hamilton

HE001 Heater Element – 150W Screwfix 73098

P001 Water Pump Lowara – DC Solar

EV001 Water Expansion Tank Screwfix 52162

PE103 Pressure Transducer – 0 to 50mbar RS 4106231

TE101 Temperature Sensor

FE100 Flow Meter - Gas

QE100 CH4 – Gas Sensor Dynament

QE102 CH4 – Gas Sensor Dynament

QE101 CO2 – Gas Sensor Dynament

QE103 CO2 – Gas Sensor Dynament

FE101 Flow Meter - Biomethane

SV002 Gas Solenoid Valve

SV003 Gas Solenoid Valve

C001 Air Compressor

SV005 Air Solenoid Valve

PS102 Pressure Switch – Min 2bar, Max 7 bar




Appendix 3 Control Philosophy

The system is semi-auto in design and requires manual attendance to operate.

Start Up – (Biomethane available and flowing)

Manually power up the Control Panel

Manually open valve V001

PS101 If Liquid Propane pressure above 8bar – LP available

If Liquid Propane pressure below 8bar - Alarm

Manually switch on Air Compressor

PS102 If Air pressure above 2bar – Air available

If Air pressure below 2bar - Alarm

Manually select Water Heating System ON (On/off selector switch)

P001 Water pump starts

HE001 Heater powered up

Water is circulated and temperature measured by TE102. Circulate hot water for 5 minutes at

50 degC.

SV003 Manually open (Selector Switch)

SV002 Manually close (Selector Switch)

PE103 If Biomethane pressure above 15mbar – Proceed

If Biomethane pressure below 15mbar – Alarm

Biomethane quality now being measured at QE100, 101,102,103

QE100 (CH4), QE101 (CO2) – Biomethane at inlet

QE102 (CH4), QE103 (CO2) – Biomethane/Propane mix at outlet

QE102 has a manually adjustable setpoint (typically 39Mj/sm3)


Biomethane available

Liquid Propane available

Air available

Hot water available (5mins at 50 degC)

SV001 Manually open (Selector switch) to allow flow of LP to RV001

(regulator/vaporiser)

If QE102 is below set point:-

SV005 Opens to allow air pressure onto RV001 (regulator/vaporiser)

PRV004 Modulates and increases the air pressure until the CV set point is measured

at QE102 (manually adjust control valve V016)

The set point is to be maintained at CV +/- 0.5MJ

HE001 The water heater is modulated so that the water temperature measured at

TE102 is equal to the biomethane temperature measured at TE001.

Maintain water temperature to within +/- 2 degC of Biomethane temperature. If outside this

parameter for more than 5minutes then alarm. (Do not shut down)

If after a period of 5 minutes the CV set point required at QE102 is not achieved then initiate

an alarm.

All Alarms initiated are to be manually assessed and dealt with. If a shutdown is required

then this will be done manually.

FE100 Measures PG flow into the gas Mixer

FE101 Measures Biomethane flow into the gas Mixer

Information to be trended and recorded

QE100 CH4 content in Biomethane at inlet to gas Mixer

QE102 CH4 content in Biomethane/LPG at outlet of gas Mixer

FE100 Measures LPG flow into the gas Mixer

FE101 Measures Biomethane flow into the gas Mixer


Appendix 4 Correspondence with Dynament

From: support [mailto:[email protected]]

Sent: 20 October 2013 11:59

To: Terry.Williamson

Subject: Re: Gas sensors

Dear Terry,

Thank you for your email.

I could offer you the following sensors for your application:

1. For Methane measurement it would be best to use the High Resolution Methane sensor.

This sensor provides linearized and temperature compensated output which can be either

read with the use of the analogue voltage output or digital interface.

Please use the following part code: MSH-P/HR/5/V/P 0-100%VOL CH4 = 0.4-2.0V. I assume

that you would prefer the certified sensor due to high volume Methane present. Some

problems my occur after adding Propane gas to the mixture, however, the sensor has

capability to read over range up to 200% for the full scale. If you add up to 4% Propane you

may expect the Methane readings to increase by about 15% on the Methane scale.

2. For the CO2 it seems most convenient to use the following sensor: MSH-P/CO2/5/V/P 0-

2%VOL CO2 = 0.4-2.0V. It is possible to increase the measuring range if you may expect

higher concentrations of the CO2 in the mixture.

These sensors are available at the price of £120 each. You may benefit from the

Configuration Unit or two, one from each sensor. This will allow you to connect the sensors to

the PC. You will be able to see the readings and configure and calibrate the sensors if

needed. The part code is P/CONFIG/POS and it is available at the price of £99 each. We also

offer suitable sockets for the sensors as well as round PCB's with the sockets loaded. You

could also benefit from the sensor mounting systems.

Please see attached price list and the data sheet with accessories for more information.

I hope that helps.

Best regards,


Tomasz Malota.

On 11/10/2013 14:41, Terry. Williamson wrote:

Thomas

I am looking to build a control system to blend a small % of propane into biomethane to

enhance the CV to a set value +/-.

The prototype unit does not need to exhibit absolute accuracy but we need reasonable

consistency, sensors can be calibrated against a span gas to assess repeatability.

Ideally we need to measure before and after CV but perhaps we can arrive at a useable

system detecting HC's and CO2, we will also have N2 and O2 present but I note you don't

have suitable sensors.

We are building this with a PLC control so can accept analogue signals to represent values.

Typical gas properties:

CH4 97 to 98%

CO2 approx 2%

N2 + O2 balance

Dewpoint -10 deg C at 7 Bar

System pressure 50mBar

Temperature 5 to 15 C, we could stabilise if this helps, what range is tolerable?

Typical propane blending volume will be 4 to 6%

If you can help out or have ideas?

Price, availability and what we need...

Many thanks

Terry

Terry Williamson


Appendix 5 Test log at Biogas Products











Documents

Small Scale Propane Blending Plant Services.pdf · 2019. 5. 9. · Small Scale Propane Blending Plant 1 WRAP’s vision is a world in which resources are used sustainably. Our mission