Characteristics of an Air-Blown Fixed-Bed Downdraft Biomass Gasifier

An air-blown fixed-bed, stratified downdraft biomass gasifier was built to investigate the key operating parameters that affect the operational characteristics of a fixed bed. The main purposes of this investigation include studying the effects of operation parameters, such as air flow rate, air preheating temperature, air/fuel ratio, and fuel moisture content, on the fuel conversion rate, specific gasification rate, producer gas heating value, and H/C ratio, as well as cold gas efficiency and hot gas efficiency. The gasifier was designed to operate with air flow in the range between 6 and 18 Nm(3) h(-1). Wooden cubes (15 x 15 x 15 mm) of Red Lauan and White Lauan woods were used as fuel for the gasification experiments. The burning rate of wooden cubes was found to increase with an increasing air flow rate. The producer gas heating value showed a trend of increase with an increasing air flow rate up to 15 Nm(3) h(-1), and a further increase in air flow resulted in a decrease in the gas heating value, because of the increase in conversion of CO and H-2 with oxygen as the air flow rate was increased. The optimum mean higher heating value (HHV) of 5.68 MJ Nm(-3) was obtained in this study when feeding the air unheated and using wooden cubes containing 18% moisture. Preheating the air up to 573 K can effectively shorten the time required to attain the steady-state condition but appears to have marginal effects on the producer gas heating value. The fuel moisture content can significantly lower the fuel conversion rate and the producer gas heating value. The one-dimensional steady-state model developed is able to predict the axial profiles of temperature and gas composition in a downdraft biomass gasifier with reasonable accuracy.