A downdraft gasifier is a co-current reactor where air enters the gasifier at a certain height below the top. The product gas flows downward (giving the name downdraft) and leaves through a bed of hot ash (Figures 6.4 and 6.5). Since it passes through the high-temperature zone of hot ash, the tar in the product gas finds favorable conditions for cracking (see Chapter 4). For this reason, a downdraft gasifier, of all types, has the lowest tar production rate.

Air from a set of nozzles, set around the gasifier’s periphery, flows down­ward and meets with pyrolyzed char particles, developing a combustion zone (zone III shown schematically in Figure 6.5 and described in the discussion of throatless downdraft gasifiers that follows) of about 1200 to 1400 °C. Then the

gas descends further through the bed of hot char particles (zone IV), gasifying them. The ash produced leaves with the gas, dropping off at the bottom of the reactor.

Downdraft gasifiers work well with internal-combustion engines. The engine suction draws air through the bed of fuel, and gas is produced at the end. Low tar content (0.015-3 g/nm3) in the product gas is another motivation for their use with internal-combustion engines. A downdraft gasifier requires a shorter time (20-30 minutes) to ignite and bring the plant up to working tem­perature compared to the time required by an updraft gasifier.

There are two principal types of downdraft gasifier. The throatless (or open core) type is illustrated in Figure 6.5. Reactions in different zones and at dif­ferent temperatures are plotted on the right. The throated (or constricted) type is shown in Figure 6.4.