Cyclic variations and control of a batch-loaded biomass gasifier-combustor

Uncontrolled batch-loaded biomass gasification systems exhibit large variations in gasification rate during the fuel cycle. A portable batch-loaded biomass gasification-combustion system has been improved by a control system regulating the supply of air to the gasifier ('primary air'). The resulting time-varying gasification characteristic has been investigated by continuous measurements of air supply flow rates and the major flue gas species concentrations. It was found that a simple control system may use combustor exit temperature to monitor overall Excess Air Value (EXAV). With the control system maintaining a reasonably steady EXAV throughout the fuel cycle (requiring considerable variation in primary air), gasification rate falls while higher heating value (HHV) rises, causing thermal input to reach a minimum in mid-cycle. HHV was evaluated using a correlation accurate for a range of biomass materials. The correlation was found to be well approximated by considering carbon flux only.