The effect of variable fuel composition on a swirl-stabilised producer gas combustor

Producer gas is a form of synthesis gas formed from the gasification of biomass. Its composition varies largely depending on the gasifier type, up-draft, down-draft and entrained flow to name a few, and oxidiser, typically air, steam or pure oxygen. The producer gas in question is derived from an air fed down-draft gasifier containing Grade-A wood waste, and as such has comparatively low calorific value and hydrogen content of around 6MJ/Nm³ and 11% respectively. The viability of producing thermal energy on an industrial scale was proved via the swirl-stabilised combustion of forementioned producer gas in a 1.5MW rated swirl burner, directly attached to a proprietary gasification plant. A largely stable combustion was achieved; however, a few instances of flame extinction did occur which were attributed to varying fluctuations in gasifier throat temperature. A relationship between throat temperature and fuel composition is believed to be the cause of these extinctions. Although these fluctuations were shown to have little effect on the producer gases stoichiometric air-to-fuel ratio and calorific value, there was particular variance in hydrogen content. Simulation using CHEMKIN showed that this variable hydrogen content had a significant effect on flame speed, with an increase from 5% to 15% hydrogen increasing laminar flame speed from 8.01cm/s to 17.70cm/s. Industrial-scale data was also compared to that taken with a 100kW laboratory-scale combustor in an attempt to establish whether burner optimisation could be effectively achieved in a controlled laboratory environment. A relationship was found between exhaust temperature and introduction of axial air, which extended across both burners. Flame speed and flame temperature simulations also showed that air is an acceptable diluent of natural gas when substituting for producer gas.