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Problems With Determining Oxygen Deficiencies in Ratios Used for Assessing
Spontaneous Combustion Activity
Darren Brady
Manager OHECC
Simtars
Department of Mines and Energy
Spontaneous Combustion Ratios
Several ratios commonly used to indicate spontaneous combustion, compare products of oxidation with the amount of oxygen consumed.
Spontaneous Combustion Ratios
These ratios are used to measure the intensity of any oxidation of the coal that may be occurring. As the coal gets hotter the oxidation reaction becomes more efficient and more of the oxygen is converted to products of oxidation, such as carbon monoxide and carbon dioxide.
Spontaneous Combustion Ratios
Ratios such as Graham’s, Young’s and Jones-Trickett’s all divide products of combustion by the amount of oxygen consumed to give a quantifiable measure of how much oxygen was used to generate the amount of combustion products measured.
DC2.CAD
Oxygen Deficiency
Oxygen deficiency is the term given to the amount of oxygen used (consumed/removed) from the inlet air stream by any activity as it undergoes reactions and interactions with the coal.
What Can Go Wrong?
• More than one source of oxygen depletion
What Can Go Wrong?
• Equation used for calculating the oxygen consumed by any oxidation/absorption-adsorption
What Can Go Wrong?
• The measurement technique
What Can Go Wrong?
• Instrument inaccuracies
What Can Go Wrong?
• Unreliable for samples where oxygen deficiencies are less than 0.3%
More Than One Source of Oxygen Depletion
• If there is more than one source of oxygen depletion then these ratios will be under estimated as it appears that more oxygen was used to produce the products than was really the case
Equations
Graham’s ratio is often expressed as
Where:
= Graham’s ratio
= final carbon monoxide concentration (%)
= final nitrogen concentration (%)
= final oxygen concentration (%)
ff ONCO
GRf
22265.0100
GR
fCOfN 2
fO2
Equation 1
Equations
• Enables calculation without actually knowing what the initial gas concentrations were.
• The denominator in Equation 1 is the oxygen deficiency.
Initial Oxygen Calculation
fi NO 22 265.0
iO2
fN 2
If initial gas entering an area has a fresh air ratio of 20.95% O2 to 79.02% N2 (20.95/79.02 = 0.265), Equation 2 can be used to calculate the initial O2 concentration by using the amount of N2 determined to be present in the sample
Where:= initial oxygen concentration (%) = final nitrogen concentration (%)
Equation 2
Initial Oxygen Calculation
• Based on the assumption that nitrogen, being an inert gas, will not be consumed or created.
• Only valid for samples where the initial gas has the same O2 to N2 ratio as fresh air and where N2 and Ar results are combined (79.02%).
• Eliminates most problems with dilution because the measured N2 will also been diluted.
Oxygen Deficiency
ff ONOD 22265.0
Where:= oxygen deficiency (%)
= final nitrogen concentration (%) = final oxygen concentration (%)
OD
fN 2
fO2
Equation 3
The measured oxygen concentration in the sample is then subtracted from the calculated initial oxygen to give the oxygen deficiency
Equations
• Problems when the oxygen deficiency is large. • Analysis is done on a percentage volume basis, if O2
is being consumed/removed and nothing replaces it, the nitrogen concentration increases.
• The elevated nitrogen concentration results in over estimation of initial oxygen concentration and therefore oxygen deficiency.
Equations
O2
(%)
N2(+ Ar)
(%)
Initial O2
(%) Eq 2
OD( %)Eq 3
OD (%)O2 *(%)
2.3 81.8 21.7 19.4 18.65
9.2 80.4 21.3 12.1 11.75
15.7 83.1 22.0 6.3 5.25
8.1 89.1 23.6 15.5 12.85
*calculated assuming initial oxygen 20.95%.
Equations
GR
fi OO
COCOGR
if
22
100
Where:= Graham’s ratio= final carbon monoxide concentration (%)= initial carbon monoxide concentration (%)= initial oxygen concentration (%)= final oxygen concentration (%)
fCO
iO2
fO2
iCO
Equation 4
If initial gas results are available Graham’s ratio is often calculated using;
Equations
• Used when a tube bundle sampling point located in an intake
• Problems with calibration or drift of the oxygen analyser are negated as they are common to both measurements.
• Any dilution with seam gas between locations is seen as oxygen deficiency and over estimates oxygen deficiency.
Equations
Graham’s ratio calculations using Equation 4
CH4
(%)
O2i
(%)
O2f
(%)
COf
(%)
GR
0 20.95 20.8 0.0005 0.333
3% 20.95 20.8x0.97 = 20.18
0.0005x0.97=0.00049
0.063
6% 20.95 20.8x0.94= 19.55
0.0005x0.94=0.00047
0.034
Equations
fi
i
fOO
N
NOD 22
2
2
OD = oxygen deficiency (%)
fN 2 = final nitrogen concentration (%)
iN 2 = initial nitrogen concentration (%)
iO2 = initial oxygen concentration (%)
fO2 = final oxygen concentration (%)
Where:
Equations
Where:
= Graham’s ratio
= final carbon monoxide concentration (%)
= initial carbon monoxide concentration (%)
= final nitrogen concentration (%)
= initial nitrogen concentration (%)
= initial oxygen concentration (%)
= final oxygen concentration (%)
GR
fi
fi
i
f
ONNO
NNCOCO
GRif
22
22
2
2100
fCO
iCOfN 2
iN 2
iO2
fO2
Equations
Graham’s ratio calculations using Equation 6
CH4
(%)
N2f
(%)
O2f
(%)
COf
(%)
GR
0 78.8 20.8 0.0005 0.55
3 78.8x0.97 =76.44
20.8x0.97 =20.18
0.0005x0.97 =0.00049
0.55
6 78.8x0.94 =74.07
20.8x0.94 =19.55
0.0005x0.94 =0.00047
0.55
Equations
• The use of the fresh air N2 concentration of 79.02% includes 0.9% Ar in the amount and is used for techniques that are unable to differentiate the two gases.
• If the two are reported separately, the fresh air ratio is 20.95% oxygen to 78.1% nitrogen (20.95/78.1=0.268).
Equations
• GC analysis determines Ar and N2 separately• Equations 1, 2 and 3 must be modified for GC results
Equations
• Equation 1 becomes:
ff ON
COGR
f
22268.0
100
Equation 7
Equations
• Equation 2 becomes:
fi NO 22 268.0 Equation 8
Equations
• Equation 3 becomes:
ff ONOD 22268.0 Equation 9
Equations
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
24/03/2006 0:00 13/05/2006 0:00 2/07/2006 0:00 21/08/2006 0:00 10/10/2006 0:00 29/11/2006 0:00 18/01/2007 0:00 9/03/2007 0:00 28/04/2007 0:00 17/06/2007 0:00
Oxygen Deficiency calculated using Nitrogen as 79.02%
Oxygen Deficiency calculated with Nitrogen as 78.1%
Measurement Technique
• Tube bundle and real time systems don’t measure N2
• It’s calculated by subtracting the sum of the measured gases from 100.
• GC actually measures N2
• Influences which equation must be used
Real Time vs Tube Bundle Oxygen Measurements
20.3
20.4
20.5
20.6
20.7
20.8
20.9
21
21.1
21.2
21.3
16/ 07/ 2006 21:36 17/ 07/ 2006 0:00 17/ 07/ 2006 2:24 17/ 07/ 2006 4:48 17/ 07/ 2006 7:12 17/ 07/ 2006 9:36 17/ 07/ 2006 12:00
Time
Oxy
gen
Co
nce
ntr
atio
n(%
)
Realtime Tube
Real Time vs Tube Bundle Oxygen Deficiencies
-0.6-0.5
-0.4-0.3
-0.2-0.1
00.1
0.20.3
0.40.5
0.60.7
16/ 07/ 2006 0:00 16/ 07/ 2006 4:48 16/ 07/ 2006 9:36 16/ 07/ 2006 14:24 16/ 07/ 2006 19:12 17/ 07/ 2006 0:00 17/ 07/ 2006 4:48 17/ 07/ 2006 9:36 17/ 07/ 2006 14:24
Oxy
gen
Def
icie
ncy
(%
)
Real Time Tube Bundle
Real Time vs Tube Bundle Graham’s Ratio
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
7/07/2006 0:00 7/07/2006 12:00 8/07/2006 0:00 8/07/2006 12:00 9/07/2006 0:00 9/07/2006 12:00 10/07/2006 0:00 10/07/2006 12:00 11/07/2006 0:00
Time
Gra
ha
m's
Ra
tio
Real Time Tube
Fresh Air Oxygen as Measured by Tube Bundle
20.45
20.5
20.55
20.6
20.65
20.7
20.75
20.8
20.85
20.9
3/03/2007 0:00 4/03/2007 0:00 5/03/2007 0:00 6/03/2007 0:00 7/03/2007 0:00 8/03/2007 0:00 9/03/2007 0:00
% O
xyg
en
Tube Bundle 1 Tube Bundle 2
Tube Bundle
• Measurement of oxygen using paramagnetic analysers is flow rate dependent so flows from all tubes must be balanced.
Tube Bundle
• Two locations with the same oxygen concentration could read differently because more resistance in one of the tubes results in a slower flow and subsequently a lower reading than a location with the same concentration but flowing through the instrument at a faster rate.
Instrument Inaccuracies
• Slight inaccuracies in all measurements no matter how well the analysis is done and how good the instrument performing the analysis is.
• These slight variations can cause problems in samples with no significant oxygen deficiency whenever we get a slightly higher O2 (or slightly lower N2 measurement by GC analysis), and apply the known fresh air O2 to N2 ratio to determine the oxygen deficiency.
Instrument Inaccuracies
O2
(%)
N2
(%)
Oxygen Deficiency
(%)
20.61 76.63 -0.07
20.55 76.73 0.01
Instrument Inaccuracies
• It can appear that oxygen has actually been created (very unlikely underground).
• Really indicates that the ratio has stayed the same. • Difference comes totally from the acceptable
inaccuracies (tolerance) of the measurement technique.
Instrument Inaccuracies
• Calibration gas suppliers certify each component as the likely concentration within limits eg the O2 concentration in a recently supplied certified calibration gas is 19.6±0.5%.
• The true concentration may be as low as 19.1% or as high as 20.1%.
• When used to calibrate an instrument to 19.6% any difference will result in all oxygen measurements being high or low, but analytically acceptable.
• A change in calibration gas can lead to a step change in values measured by the sensor/instrument calibrated with that gas.
Instrument Inaccuracies
Oxygen analysers, at best, are accurate to 1% of full
scale. Thus a measured value of 20.7% for O2 would
be +/- 0.2%
A sample measured as: 10 ppm CO, 0.1% CO2,
20.7% O2 and 79.2% N2 (by difference) could thus
vary between 20.5% and 20.9% O2 and conversely
N2 would be between 79.4 and 79.0%
Thus Graham’s Ratio would range between:
GR =100 x 0.001 / (0.265 x 79.4 - 20.5)
= 0.1 / (21.04 - 20.5)
= 0.1 / 0.54 = 0.18
and
GR = 0.001 x 100 / (0.265 x 79.0 -20.9)
= 0.1 / (20.94 - 20.9)
= 0.1 / 0.04 = 2.86
Instrument Inaccuracies
Oxygen Deficiencies Less Than 0.3%
• When oxygen deficiencies are less than 0.3% the variation between readings can significantly affect the calculated ratios.
Oxygen Deficiencies Less Than 0.3%
Measured O2
(%) O2 Def
(%) CO (%)
Graham’s Ratio
20.94 0.01 0.0003 3.00 20.92 0.03 0.0003 1.00 20.90 0.05 0.0003 0.60 20.88 0.07 0.0003 0.43 20.86 0.09 0.0003 0.33 20.84 0.11 0.0003 0.27 20.82 0.13 0.0003 0.23 20.80 0.15 0.0003 0.20 20.78 0.17 0.0003 0.18 20.76 0.19 0.0003 0.16 20.74 0.21 0.0003 0.14 20.72 0.23 0.0003 0.13 20.70 0.25 0.0003 0.12 20.68 0.27 0.0003 0.11 20.66 0.29 0.0003 0.10 20.64 0.31 0.0003 0.10 20.60 0.35 0.0003 0.09 20.58 0.37 0.0003 0.08
Conclusions
• Despite the problems, ratios incorporating oxygen deficiencies can still be useful but anyone doing interpretation must be aware of all of these implications.
Conclusions
• Care must be taken when calculating oxygen deficiencies to ensure that the calculation is correct and representative for the sample and analysis technique.
Conclusions
• Interpretation of data is best done looking at trends rather than one off samples. Even if the ratio is being underestimated, any increase in intensity should result in an increase in the trend although the rate of change may not match the increase in intensity.