Composition and stoichiometry effects of biogas as fuel in spark ignition engine

Abstract

The most important advantage of biogas is that one can retrofit easily the conventional combustion engines to be used for the new fuel but remain the problem of pollutant emission. In this paper, the effect of the biogas composition on the emissions of CO and NOx in an SI engine is investigated numerically. Three compositions were studied (CH4:40 %, CO2:60 %), (CH4:50 %, CO2:50 %), (CH4:60 %, CO2:40 %), at equivalence ratio (ϕ) from lean to rich. The non-adiabatic partially premixed combustion model with probability density function (PDF) method and Zimont model of the turbulent flame speed are used. It was found that increasing CO2 in the biogas lowers NOx emissions significantly, whereas, CO was found to be very low, and appears only at high CH4 concentration. A biogas with (CH4:60 %, CO2:40 %), burning at about (ϕ = 0.9), i.e. slightly lean mixture, and advanced spark timing (between 10 to 20°.) seems to be a very a realistic combination in terms of biogas composition, stoichiometry and spark timing. It is recommended that, to accomplish some upgrading, in case of biogases with low CH4 percentage, to get it up to 60 %.