Influence of water vapour and tar compound on laminar flame speed of gasified biomass gas

Biomass can be converted to a gaseous fuel through gasification in order to be used in higher efficiency conversion equipment. Combustion of a gaseous fuel generally allows for higher combustion temperatures than that of a solid fuel leading to the higher efficiency. However, the gasified biomass gas (GBG) contains condensable compounds, such as water vapour and tars, which both will affect the subsequent combustion process with respect to emission levels and flame stability. Cleaning of the GBG prior to combustion is very costly and therefore further research is needed on direct combustion of GBG containing these condensable compounds, in order to develop stable combustion techniques for GBG. The laminar flame speed is a main parameter that relates to other important flame properties such as stability, extinction limit and flashback. Each of GBG components have different chemical and transport properties, which then influences the laminar flame speed of GBG. In this study, the individual effect of water vapour (H2O) and tar compound addition in simulated GBG on laminar flame speed is investigated at atmospheric pressure. The tar compound used is benzene (C6H6) and simulated GBG used is CO/H2/CH4/CO2/N2 mixture. Experiments were carried out with conical burner stabilized flame and a Schlieren photography system. The volume fraction of additives in the fuel mixture was varied: for H2O from 0% to 5% and for C6H6 from 0% to 10%. The unburned fuel air mixture was preheated and the temperature was maintained at Ti = 398 K to avoid condensation of the liquid. It was found that measured laminar flame speed of GBG–air mixture decreases with addition of H2O in the fuel mixture. While, non-monotonic behaviour is shown with addition of C6H6. Initially, as the volume fraction of C6H6 incremented, the laminar flame speed decreases, reaching a minimum value, and then increase.

► Water and benzene vapours were added separately in the gasified biomass gas (GBG).
► The laminar flame speed, SL of GBG decreases with water vapour addition.
► The SL of GBG shows a non-monotonic behaviour with benzene vapour addition.