The gases formed by gasification are more or less contaminated depending on the process and the feedstock. Depending on the gas utilization, some kind of gas cleaning must thus be applied to prevent eventual problems in downstream process equipment such as plugging, erosion, corrosion, and catalyst poisoning, but also to prevent pollution of the environment. Table 6.3 shows the main requirements for a general syngas.

The product gas from biomass gasification will contain fine carbon-containing ash particles, which are difficult to remove only by cyclones. Barrier filtration methods using e. g. sintered metal or ceramic filters are therefore employed. The differential pressure over the filter tends to increase as deposits build up over time. Problems with filter clogging by soot resulting from the thermal cracking of tars both in the gas phase and on the filter surface. This problem can partly be handled by cooling the gas to <500°C and lowering of the gas velocities through the filter.

However, temperatures should not be allowed to fall below 400°C due to the potential problem of tar condensation and subsequent clogging.

Hot gas cleanup is to be preferred if the sensible heat of the gas has to be retained and thus are low temperature scrubbing systems for tar removal often avoided.

Tar concentrations in the gas are mainly a function of the gasification temperature, the higher the temperature the lower the tar concentration. Tar levels and tar characteristics are not only a function of temperature though, but also of the feedstock, gasifier configuration and processing conditions. The tars formed in pyrolysis are thermally cracked in most environments to refractory tars, soot and gases. However, the problems associated with tar during gasification of biomass differ from those when gasifying coal or peat and thus are the methods for tar handling developed for coal gasification not directly transferable to biomass gasification applications.