Downdraft gasifiers

Fixed bed gasifiers are generally used for small-scale operation with gas engines, having an electrical output of about 80-500 kWe. They divide into updraft and downdraft gasifiers. 

TABLE 9.10 Effects of Moisture Content of Poplar Wood Chips on Product Yield, Gas Composition, and Thermal Efficiency in a Fixed-Bed, Air-Blown, Downdraft Gasifier"... 

Atmospheric Downdraft Gasifiers (ADG) are attractive for small scale applications (<1.5 MWth) as there is a very big market not only in developed but developing economies too (19). However, the problem of efficient tar removal is still a major problem to be addressed and there is a need for more automated operation especially for small scale industrial applications. Nevertheless, recent progress in catalytic conversion of tar (see below) gives credible options and ADG can therefore be considered of average technical strength. 

Milligan, J.B., Evans, G.D. Bridgwater, A. V. (1993) Results from a transparent open-core downdraft gasifier. Advances in Thermochemical Biomass Conversion, Proceedings of the International Conference on Advances in Thermochemical Biomass Conversion, 11-15 may 1992, Interlaken, Switzerland. 

Di Blasi, C. (2000) Dynamic behaviour of stratified downdraft gasifiers. Chemical Engineering Science 55, pp 2931-2944. 

Coirqpared to conventional downdraft gasifiers the economics of the process are improved, provided that ambient air is used as gasifying medium... 

A small-scale stratified downdraft gasifier coupled to a gas engine for combined heat and power production... 

A small scale stratified downdraft gasifier is erected at our laboratory. The thermal input is 30 kW, supplied by S kg/h of wood pellets. 

Reed, T.B. Das, A, (1988). Handbook of biomass downdraft gasifier engine systems. The Biomass Energy Foundation Press, 1810 Smith Rd, Golden, CO. 80401. 

Reed, T.B. Levie, B. (1984). A simplified model of the stratified downdraft gasifier. International Bio-Energy Directory and Handbook, 1984, pp,379-390. 

In conventional downdraft gasifiers, air passes down through the fuel mass, then in the flaming pyrolysis zone bums the volatiles and tars while making charcoal and pyrolysis gas. The charcoal then further reduces the CO2 and H2O combustion products back to CO and H2 fuel. 

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