An investigation of the chemical kinetics of biogas combustion

Abstract

Chemical kinetic aspects of the combustion of biogas (containing CH4,CO2 and possibly H2) have been investigated. Five reaction mechanisms were considered and tested with respect to the ignition delay times of H2–CO2–O2 and CH4–CO2–O2 mixtures measured in shock tubes. While the GRI (Gas Research Institute) mechanism 3.0 could not reproduce the first set of measurements, it brought up the best match for the second one directly relevant to biogas. Consequently it was employed for predicting the amounts of CO and NO produced under the same conditions. It was found that for stoichiometric and lean mixtures an increase in the initial CO2 concentration lowers the production of NO and raises that of CO at higher temperatures. For rich mixtures, the production of NO is far smaller and does not follow this simple pattern. Kinetically, the reaction H+CO2→OH+CO plays a greater role in the presence of CO2 and must hence be accurately known. For some conditions, the environmentally problematic N2O is produced. The effects of hydrogen addition on biogas have also been predicted using GRI 3.0. The addition of 2% of H2 always raises the concentration of produced CO when compared to the mixture without hydrogen. The formation of NO is increased for higher H2 amounts at p=1bar but decreased at p=10bar. Overall, the present study supports the viability of the GRI-mechanism for the simulation of biogas combustion.