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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

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Spontaneous Combustion Ratios Several ratios commonly used to indicate spontaneous combustion, compare products of oxidation with the amount of oxygen consumed.

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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.

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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.

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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.

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What Can Go Wrong? More than one source of oxygen depletion

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What Can Go Wrong? Equation used for calculating the oxygen consumed by any oxidation/absorption-adsorption

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What Can Go Wrong? The measurement technique

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What Can Go Wrong? Instrument inaccuracies

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What Can Go Wrong? Unreliable for samples where oxygen deficiencies are less than 0.3%

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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

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Equations Graham’s ratio is often expressed as Where: = Graham’s ratio = final carbon monoxide concentration (%) = final nitrogen concentration (%) = final oxygen concentration (%) Equation 1

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Equations Enables calculation without actually knowing what the initial gas concentrations were. The denominator in Equation 1 is the oxygen deficiency.

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Initial Oxygen Calculation If initial gas entering an area has a fresh air ratio of 20.95% O 2 to 79.02% N 2 (20.95/79.02 = 0.265), Equation 2 can be used to calculate the initial O 2 concentration by using the amount of N 2 determined to be present in the sample Where: = initial oxygen concentration (%) = final nitrogen concentration (%) Equation 2

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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 O 2 to N 2 ratio as fresh air and where N 2 and Ar results are combined (79.02%). Eliminates most problems with dilution because the measured N 2 will also been diluted.

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Oxygen Deficiency Where: = oxygen deficiency (%) = final nitrogen concentration (%) = final oxygen concentration (%) Equation 3 The measured oxygen concentration in the sample is then subtracted from the calculated initial oxygen to give the oxygen deficiency

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Equations Problems when the oxygen deficiency is large. Analysis is done on a percentage volume basis, if O 2 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.

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Equations O 2 (%) N 2 (+ Ar) (%) Initial O 2 (%) Eq 2 OD( %) Eq 3 OD (%) O 2 *(%) *calculated assuming initial oxygen 20.95%.

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Equations Where: = Graham’s ratio = final carbon monoxide concentration (%) = initial carbon monoxide concentration (%) = initial oxygen concentration (%) = final oxygen concentration (%) Equation 4 If initial gas results are available Graham’s ratio is often calculated using;

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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.

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Equations Graham’s ratio calculations using Equation 4 CH 4 (%) O 2i (%) O 2f (%) CO f (%) GR % x0.97 = x0.97 = % x0.94 = x0.94 =

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Equations Where:

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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 (%)

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Equations Graham’s ratio calculations using Equation 6 CH 4 (%) N 2f (%) O 2f (%) CO f (%) GR x0.97 = x0.97 = x0.97 = x0.94 = x0.94 = x0.94 =

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Equations The use of the fresh air N 2 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).

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Equations GC analysis determines Ar and N 2 separately Equations 1, 2 and 3 must be modified for GC results

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Equations Equation 1 becomes: Equation 7

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Equations Equation 2 becomes: Equation 8

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Equations Equation 3 becomes: Equation 9

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Equations

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Measurement Technique Tube bundle and real time systems don’t measure N 2 It’s calculated by subtracting the sum of the measured gases from 100. GC actually measures N 2 Influences which equation must be used

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Real Time vs Tube Bundle Oxygen Measurements

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Real Time vs Tube Bundle Oxygen Deficiencies

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Real Time vs Tube Bundle Graham’s Ratio

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Fresh Air Oxygen as Measured by Tube Bundle

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Tube Bundle Measurement of oxygen using paramagnetic analysers is flow rate dependent so flows from all tubes must be balanced.

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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.

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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 O 2 (or slightly lower N 2 measurement by GC analysis), and apply the known fresh air O 2 to N 2 ratio to determine the oxygen deficiency.

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Instrument Inaccuracies O 2 (%) N 2 (%) Oxygen Deficiency (%)

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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.

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Instrument Inaccuracies Calibration gas suppliers certify each component as the likely concentration within limits eg the O 2 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.

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Instrument Inaccuracies Oxygen analysers, at best, are accurate to 1% of full scale. Thus a measured value of 20.7% for O 2 would be +/- 0.2% A sample measured as: 10 ppm CO, 0.1% CO 2, 20.7% O 2 and 79.2% N 2 (by difference) could thus vary between 20.5% and 20.9% O 2 and conversely N 2 would be between 79.4 and 79.0%

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Thus Graham’s Ratio would range between: GR =100 x / (0.265 x ) = 0.1 / ( ) = 0.1 / 0.54 = 0.18 and GR = x 100 / (0.265 x ) = 0.1 / ( ) = 0.1 / 0.04 = 2.86 Instrument Inaccuracies

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Oxygen Deficiencies Less Than 0.3% When oxygen deficiencies are less than 0.3% the variation between readings can significantly affect the calculated ratios.

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Oxygen Deficiencies Less Than 0.3%

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Conclusions Despite the problems, ratios incorporating oxygen deficiencies can still be useful but anyone doing interpretation must be aware of all of these implications.

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Conclusions Care must be taken when calculating oxygen deficiencies to ensure that the calculation is correct and representative for the sample and analysis technique.

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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.

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