Mild combustion of methane-derived fuel mixtures: Natural gas and biogas

Mild combustion (also called diluted or flameless combustion) is an innovative combustion process able to strongly reduce pollutant emissions with respect to traditional combustion processes; it is characterized by high preheating of the combustion air and massive recycle of burned gases. While it is well known that mild combustion leads to lowering NOx emissions when pure fuels (such as methane) are used, no much quantitative information are available concerning the effectiveness of mild combustion when using fuel mixtures. In this work a laboratory-scale burner for mild combustion has been used to investigate mild combustion of two fuel mixtures mainly constituted by methane: natural gas and biogas. While the first allowed to investigate the effect of the presence of small quantities of a more reactive compound, namely, ethane, the latter gave information on the possibility of using low-BTU fuels in mild combustors. Operating conditions for stable mild combustion have been identified and operating parameters maps have been deduced for both the fuels. Moreover, the behavior of mild combustion with respect to pollutants other than NOx (namely, soot and PAHs) has been investigated. It has been found that not only mild combustion can be effectively used for a wide range of low-BTU fuels, but also that it is able to reduce soot and PAHs emissions.