Numerical investigation of the asymmetrical vortex combustor running on biogas

Abstract

Biogas is an attractive renewable fuel to combat greenhouse gas (GHG) emissions. However, the low and variable quality of un-upgraded biogas present some challenges in terms of holding a stable flame and resulting emissions. Meanwhile, asymmetrical vortex combustor (AVC) has been shown to possess excellent flame stability even at very lean conditions. This work aims to study the suitability and potential of AVC to burn biogas by investigating its combustion characteristics and explaining its ultra-stable flame behavior. A numerical study was done where the flame stability was studied by looking at the flow field. Synthetic biogas of various compositions that reflect palm oil mill effluent (POME) biogas was used in this research. The combustion characteristics was studied for equivalence ratio ranging from very lean to very rich that explores the limit of the AVC. The results showed that this combustor is very capable of stably burning biogas of different compositions with correspondingly low emissions. This allows ready use of un-upgraded biogas which would have been cost prohibitive previously. The flame stability envelope is extended beyond that of pure methane with lean limit reaching down to 0.3 and the rich limit going beyond 2 compared to the published range of between 0.46 to 1.6. Flame stability was found to be due to two effects; the strong tangential vortex containing the flame region, and also the trapped vortex effect of the backward facing step which provides a secondary stable flame. Thus, it can be concluded that raw biogas can be utilized as fuel without costly upgrading by burning it in an AVC.