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|The counter-current fixed bed ("up draft")
gasifier consists of a fixed bed of carbonaceous
fuel (e.g. coal or biomass) through which the
"gasification agent" (steam, oxygen and/or air)
flows in counter-current configuration. The ash is
either removed dry or as a
. The slagging gasifiers have a lower ratio
of steam to carbon, achieving
temperatures higher than the ash fusion temperature.
The nature of the gasifier means that the fuel must
have high mechanical strength and must ideally be
non-caking so that it will form a permeable bed,
although recent developments have reduced these
restrictions to some extent. The throughput for this
type of gasifier is relatively low. Thermal
efficiency is high as the gas exit temperatures are
relatively low. However, this means that tar and
methane production is significant at typical
operation temperatures, so product gas must be
extensively cleaned before use. The tar can be
recycled to the reactor.
The updraft gasifier consists of a top fed fuel bed through which the "gasification agent" (steam, oxygen and/or air) flows in from the bottom and exits through the top as gas. Updraft gasifiers are thermally efficient because the ascending gases pyrolyze and dry the incoming biomass, transferring heat so that the exiting gases leave very cool.
The updraft gasifier has been the standard
of coal gasification for 150 years and it's also
popular in biomass cook stoves.