The reactor types suitable for biomass gasification can be simply categorized into fixed beds and fluidized beds according to the relative motion between solid and gas and the bulk density of the solid-phase in the reactor. The principal types of gasifiers are: downdraft or co-current fixed beds, updraft or counter-current fixed beds, cross-draft fixed bed, bubbling fluidized beds and fast or circulating fluidized beds. Additional types include the entrained bed and the fluidized bed twin-reactor concept (Bridgwater, 1995). In general one gasifier technology is not appropriate for the full range of applications, i. e. there is a suitable range of applications for each gasifier technology. In the case of the fluidized bed technology and the entrained flow, the range is >10 MW and >100 MW, respectively.

6.4.1.1 Fixed bed

In a fixed bed gasifier, the fuel is supported on a grate where the fuel moves down in the reactor as a plug. This type of gasifier is generally suitable for small-scale operation in the range of 10kW-10MW In Table 6.2 the three main types of fixed bed technologies are compared.

6.4.1.1.1 Updraft gasifiers

The fixed bed updraft gasifier has been the principal gasifier of use for coal for about 150 years. A schematic of an updraft gasifier is shown in Figure 6.6a. In this gasification concept, the biomass is fed into the top of the reactor, and the air or oxygen into the lower zone. The fuel (and the resulting char) flows slowly down and passes sequentially through drying, pyrolysis, gasifica­tion and combustion zones by gravity. In parallel is the pyrolysis vapors carried upward by the up-flowing hot product gas. Ashes are removed by an ash discharge system at the bottom of the gasifier.

In the lower gasification zone, the solid char from pyrolysis and tar cracking is partially oxidized by the incoming air or oxygen. Steam may also be added to provide a higher level of hydrogen in the gas. The sensible heat of the product gas is recovered by direct heat exchange with the entering fuel feed, which thus is dried, preheated and pyrolyzed before entering the gasifier. The product gas temperatures are relatively low, 300-600°C, since the heat is used for preheating, drying and pyrolysis of the incoming fuel.

The tars in the pyrolysis vapor will either condense on the cool descending fuel particles or be carried out of the reactor with the product gas, thus contributing to its high tar content. The extent of this tar bypassing may be up to 20% of the pyrolysis products. The condensed tars are recycled back to the reaction zones, where they are further cracked to gas and char. The product gas from an updraft gasifier contains a significant proportion of tars and hydrocarbons, and these contribute to the relatively high heating value of the gas. The product gas requires substantial clean-up if further processing of the product gas is to be performed.

The dust content of the gas is, however, relatively low because of the filtering effect of the drying and pyrolysis zones and the moderate gas velocities.

Updraft gasifiers can handle fuels with moisture contents up to 60 wt% and particle sizes between 5 and 100 mm, thus covering a wide span of fuels and fuel sizes. In principle is there no upper limitation with respect to size, this in contrast to the downdraft gasifier. The principal advantages of updraft gasifiers are their simple construction and high thermal efficiency.