Updraft gasifiers are the oldest and simplest type of gasifier. In these reactors, the gasifying gas travels upward while the solid fuels move downward as shown in Fig. 10.1a. The major advantage of this type of gasifier is its simple structure and design, low capital cost, and high char burn-out, which leads to low gas exit temper­atures and high equipment efficiency, as well as the possibility to process feedstock of various shapes [66].

On the other hand, poor heat and mass transfer can increase the risk of “chan­nelling” in the equipment, which may lead to an oxygen breakthrough and rapid gas-phase combustion reactions (and, possibly, an explosion). Fuels that are prone to agglomeration during gasification are not suitable for these types of reactors due to poor heat and mass transfer. However, high-ash, high-moisture or low-volatile feedstocks are suitable fuels for updraft gasifiers. Also, there are problems asso­ciated with high tar production that require gas cleaning operations. This is of minor importance, though, if the gas is used for direct heat applications, where the tar simply can be burnt. Nonetheless, this is not recommended for engine applications.