Basically, biomass gasifiers can be categorized into several reactor design groups: a descending bed of biomass, often referred to as a moving or fixed bed, with countercurrent gas flow (updraft); a descending bed of biomass with cocurrent gas flow (downdraft); a descending bed of biomass with crossflow of gas; a fluidized bed of biomass with rising gas; an entrained-flow circulating bed of biomass; and tumbling beds. Many reactor designs have been evaluated under a broad range of operating conditions. The designs include fixed-bed, moving-bed, suspended-bed, and fluid-bed reactors; entrained-feed solids reac-

tors; stationary vertical-shaft reactors; inclined rotating kilns; horizontal-shaft kilns; high-temperature electrically heated reactors with gas-blanketed walls; single and multihearth reactors; ablative, ultrafast, and flash pyrolysis reactors; and several other designs. There are clearly numerous reactor designs and configurations for biomass gasification, probably more than in the case of coal gasification systems because of the relative ease of thermal biomass conversion.

Fixed-bed, updraft gasifiers are simple to construct and can consist of carbon steel shells equipped with a grate at the bottom fed by a process air manifold, a lockhopper at the top to feed material, and a manifold to remove gas at the top (с/. Miller, 1987). These units are simple to construct and operate and are relatively inexpensive. The gas exiting the gasifier tends to be cool because it has percolated up through the bed and therefore usually contains a fair fraction of lower molecular weight hydrocarbons. Much of the sensible heat has been lost, the feeds are limited to wood chips, and the size is usually not more than 50 million GJ/h. Fixed-bed, downdraft gasifiers consist of two concentric shells. The inner shell holds the material on the grate; the outer shell is used to transport the gas. The gas is drawn out from under the grate through the outer shell to the outside of the system. The gas exits at the combustion zone, and because it is hot, it contains few longer chain hydrocarbons and particulates. The system, however, is more expensive to construct than fixed-bed, updraft gasifiers, and is also limited to sizes up to about 10 to 20 GJ/h and chip feeds. Fluid-bed systems afford more efficient gasification because hot spots are eliminated, diverse feedstocks can be charged, the exit gas has a high sensible heat content, and the gasifiers are capable of scale-up to relatively large sizes. However, the units are more expensive to construct and product gas quality must be carefully monitored because of its higher particulate content.