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Appendix 3 of the EPA's guidance note AG2 provides detail on the correction of emissions data to reference conditions and should be consulted for further information, and can also refer to Annex 1 of AG3 for some further examples. This is a vital aspect of correctly reporting emissions data and any emissions monitoring being carried out must measure all necessary parameters (e.g. oxygen, moisture, temperature) to allow the required corrections to be applied.
Conversion from ppm to mg/m3 :
The molar volume employed is typically the molar volume of a gas at 273.15 K (temperature) and 101.325 kPa (pressure), i.e. ‘Normal’ temperature and pressure. In this case the molar volume is 22.41 litres. By using this figure the calculated mg/m3 value is effectively corrected to 273.15 K and 101.325 kPA, and no further correction is therefore required for temperature or pressure. Concentrations which are presented at normal temperature and pressure are often notated as mg/Nm3.
Example : Propane (C3H8) concentration of 150 ppm (e.g. output from VOC analyser calibrated with propane). Molecular weight of propane = (12 x 3) + (8 x 1) = 44 g/mol.
Therefore, mg/Nm3 concentration = 150 ppm x (44/22.41) = 294.51 mg/Nm3 propane.
However, in this case VOC emission limit values are normally required as mgC/Nm3 (carbon equivalent), therefore to convert from ppm to mgC/Nm3, a slightly different formula is employed:
Using the example above of an output from a VOC analyser of 150 ppm propane equivalent, the VOC (as C) concentration can be calculated as follows:
mgC/Nm3 = 150 x ((3 X 12.011)/22.41) = 240 mgC/Nm3.
Temperature and Pressure Correction:
To convert a concentration measured at stack conditions to a concentration reference to 273.15 K (equivalent 0 oC), multiply by the following factor Ft:
Where T is the measured temperature in the stack in Kelvin.
To convert a concentration measured at stack conditions to a concentration reference to 101.325 kPa, multiply by the following factor Fp:
Where P is the measured pressure in the stack in kPa.
N.B. To correct a volumetric flow value (m3/hour) to 273.15K or 101.325kPa, the reciprocal of the above formulas should be applied.
Moisture and Oxygen Correction:
Emissions of stack gases are usually expressed on a dry gas basis so that variations in the moisture content of the stack gas do not affect the assessment of the emissions.
Similarly oxygen corrections may be applied to reference periodic emissions monitoring data to a constant oxygen reference value:
Note that if the licence conditions also require moisture correction as well as oxygen correction, then the measured oxygen value should itself be corrected to a ‘dry’ value where the oxygen monitoring technique employed to measure oxygen measures on a ‘wet’ basis.
N.B. The above two equations relate to correction of concentration values, for correction volumetric flow data the reciprocal equations should be employed.
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