Inorganic compounds, such as Cl and S-containing material, HCN, COS, and am­monia, can be removed by means of physical and chemical washing techniques. Removal of ammonia from biomass is necessary in many situations since they are converted to NOX when gas is burned. Ammonia can be removed by catalytic destruc­tion using catalysts similar to those used for tar cracking and also by wet scrubbing where low temperature product gases are acceptable. Catalysts, such as dolomite, nickel-based steam reforming catalysts, and iron-based catalysts, have been used for ammonia removal with > 99 % efficiency. Catalytic removal is economically an attractive option since it has the potential to remove tar and ammonia from product gas while keeping its heat [60].

In systems where product gas is first cooled, ammonia can be removed by wet scrubbing. The ammonia recovered from the scrubber is reinjected into the gasifier to reduce ammonia production through equilibrium.

H2S can be removed by different sorptions, such as metal oxides, Cu- and Ca-based sorbents. For example, the Selexol process uses dimethyl ethers of polyethylene glycol and the Rectisol process employs methanol as a solvent to remove H2S and COS and simultaneously remove CO2 from syngas [61].