A novel dual-bed for steam gasification of biomass

A unique indirect steam gasification system for biomass has been developed and tested. The unit features a bubbling bed gasifier equipped with a “sore thumb” and central standpipe, which circulate a mixture of bed material and char through a U-bend to a riser in which char combustion occurs. Re-circulated silica sand from the riser is returned to the bubbling bed through a trickle valve at sufficiently high temperatures to drive biomass gasification. Factors which govern re-circulation of solids are explored and pressure profiles demonstrated. Gasification tests using wood pellets fed at 10–15 kg/h showed that syngas with heating value of about 12 MJ/m³ and nitrogen contents < 5% can be produced. Trends of gas composition versus bed temperature are established. The char combustor required a continuous flow of auxiliary fuel (natural gas) to compensate for high heat losses in the system. Circulation rates were such that riser top temperatures roughly 100 °C above those of the bubbling bed were adequate to provide the gasification heat, without auxiliary fuel to the gasification stage.