The downdraft gasifier was originally developed for gasifying high volatile fuels, for example wood and biomass. The reactor is schematically shown in Figure 6.6b. The fuel enters the gasifier at the top of the reactor and its main feature is the concurrent flow of gases through a slowly downwards descending packed bed of solids.

The bed is supported by a constriction in the middle section known as the throat, where most of the gasification reactions occur. It is in this section that the air or steam/oxygen is added. The reaction products are intimately mixed in the turbulent high-temperature region around the throat. Most of the tar cracking takes place in this high-temperature region. Some tar cracking also occurs below the throat on the residual charcoal bed, where the gasification process is completed. In order to minimize radial temperature gradients, it is of importance that the oxidizing agent is distributed homogeneously throughout the whole cross-section of the throat. In the bottom, there is a system to recover the ash and other non-volatile components.

Fixed bed downdraft gasification is relatively simple, reliable and proven and it results in a high conversion of the pyrolysis intermediates and hence giving a relatively clean gas. It is suitable for dry fuels (up to 25 wt% moisture), blocks or lumps with a low ash content (<6 wt%) and containing a low proportion of fine and coarse particles, preferably between 10 and 300 mm (in the longest dimension). Owing to the low content of tars in the gas, this configuration is generally favored for small-scale electricity generation with an internal combustion (IC) engine. The physical limitations of the throat diameter and particle size relation result in that there is a practical upper limit to the capacity of this configuration of about 500kg/h or 500 kWe. The temperature of the gas is approximately 700°C.