Accuracy of Heat-Release Rate Measured in Microscale Combustion Calorimetry

The ASTM standard method for measuring heats of combustion of plastics in microscale combustion calorimetry (MCC) by the oxygen-consumption principle uses only the volumetric flow rate and oxygen volume fraction exiting a premixed combustor in the calculation. The carbon dioxide (CO2) generated by complete combustion replaces some or all of the oxygen consumed from the dry gas stream depending on the atomic composition of the fuel, so it can change the volumetric flow rate and affect the flow-meter response, which is typically calibrated for pure nitrogen. Consequently, the presence of CO2 in the combustion stream causes a systematic error (bias) in the heat-of-combustion measurement that increases monotonically with the initial oxygen concentration. Accounting for volume changes using the combustion stoichiometry is sensitive to the atomic composition of the fuel and is still subject to the CO2 bias in flow-meter response, which can be up to 3 %. Accounting for volume changes using both the initial and final flow rates in the calculation and correcting the flow-meter response for CO2 using an average combustion stoichiometry is more accurate and less sensitive to material composition. Heats of combustion computed by the new method are in quantitative agreement with theoretical values.