Fluidised bed co-gasification of coal and biomass under oxy-fuel conditions

Fluidised bed gasifiers are the preferred option to utilise low value coal, biomass and waste. However, fluidised bed gasifiers are traditionally air rather than oxygen-blown to avoid high temperatures in the gasifier leading to ash melting and loss of fluidisation. Therefore the flue gas of a possible FB-IGCC plant would be diluted by nitrogen, making expensive N ₂-CO₂ separation technology necessary for subsequent capture and storage of the CO₂ (CCS). To overcome this disadvantage, an oxy-fuel process is proposed, where the bed is fluidised with recycled flue gas (mainly CO₂) and oxygen. A laboratory scale fluidised bed gasifier capable to operate up to 1000°C and 20 bar was set up to study the implications of oxy-fuel firing on flue gas composition and overall operability of the gasifier. High carbon conversions were achieved at 950°C with CO ₂ as gasification agent. At that temperature, the addition of O ₂ generated some agglomeration in the bed, leading to lower conversions. At lower temperatures, O₂ addition increased carbon conversion, but not the heating value of the gases. The addition of steam enabled the gasifier to operate at lower temperatures without decreasing conversion and to increase the hydrogen content of the fuel gas. Pressure on the other hand produced a decrease in carbon conversion, mainly above 10 bars, due to a larger formation of secondary char with the subsequent loss in reactivity. These results show that oxy-fuel firing of a fluidised bed gasifier could be a promising route to avoid N₂ dilution of the fuel gas and enable integration of fluidised bed gasification with CCS technology.