Chemical equilibrium based combustion model to evaluate the effects of H2 addition to biogases with different CO2 contents

Abstract

This research addresses the influence of hydrogen addition on adiabatic flame temperature, and energy-based amounts of biogas combustion products. The evaluations were conducted with the developed solving scheme based on the chemical equilibrium approach for fuel blends of methane (CH4) –carbon dioxide (CO2), and hydrogen (H2) structures. Thus, the biogases, composed of CH4 and CO2, including different levels of CO2 (10, 25 and 40% by volume) were evaluated by blending H2 in the range of 0–40% with a 5% increment by volume. Besides, the analyses were carried out at different equivalence ratios (ϕ) varying from 0.6 to 1.4 to determine the H2 effect in lean, stoichiometric and rich mixture conditions. The results show that the effect of H2 on the adiabatic flame temperatures and the energy-based amounts of CO2, CO, and NO, change heavily depending on the CO2 concentration of biogas and the equivalence ratios. By adding a 40% H2 to biogas involving 40% CO2, the adiabatic flame temperatures enhanced by 55.8 K, 67.9 K, and 114.7 K at φ = 0.6, 1.0, and 1.4, respectively. The H2 addition significantly contributes to decreasing the energy-based amount of CO2. The maximum improvements in the CO2 amounts were obtained as 25.36%, 26.86%, and 32.38% at φ = 0.8, 1.0, and 1.4, respectively, by 40% H2 addition. The CO amounts are tendency to decrease with H2 addition at φ = 1.0 to 1.4 corresponding to stoichiometric and rich mixtures. However, the H2 addition slightly increased the CO amount at the lean mixtures in which the amounts of CO are already quietly low level. Regarding the energy-based amounts of NO, the H2 addition gave rise to increasing the NO amounts. However, the effect of H2 on the NO amount decreased at rich mixtures independent of CO2 content in biogas. The maximum increase in the NO amount by 40% H2 addition is about 0.3 g/MJ at the φ = 0.6 while 0.006 g/MJ at φ = 1.4.