From the article:
During the already long existence of gasifiers, many quite different designs have been proposed and actually used. In essence the design is a simple one; a broad tubular reactor, a grate on which the fuel lays, one or more entrances for air, and an exit for the gas. Feed is coming in at top, the remaining ash (commonly) goes out at the bottom. The construction may be simple, the actual chemical and technical process is rather complicated, and still not completely understood. To have gasification occurring is easy, to execute it optimally, with high. efficiency and high gas quality is still difficult. The more so because basic principles are not completely clear, and never quantitatively described. Hence the many designs and different approaches. The controlling of the actual gasification process is still more an art than science. It is nevertheless tried to obtain some general rules which can guide the designing process. There are several typical grand designs for gasifiers, upstream, downstream, a combination of both "double fired", cross flow and fluidized! entrained bed gasifiers. The crucial factor is the flow of air, and produced gas. The solid material flows, normally, downwards.
Dependent on the direction of this gas flow the relative position of the various zones: pyrolysis, oxidation, reduction is different, and therefore the sequence the gasfallows through these zones. (see fig 1).
When using tar-free fuels, as cokes and charcoal, all types are feasible. The pyrolysis zone is of no consequence. For tar-producing fuels as [anthracite (removed because not suitable for this type of gasification)] brown coal, wood, peat and other biomass, that place of the pyrolysis zone is all important. The tar containing gas produced in that zone, should thereafter pass completely through a much hotter zone, viz. the oxidation zone, to minimize tar content. The downstream type of gasifier (together with its variation: the double fired of course) is the only possible in that case. For wood it is indeed the only encountered one, as well for stationary energy production as for traction purposes.