Formation and catalytic decomposition of tars from fluidized-bed gasification

This paper describes studies concerning the formation of tars in a pressurized fluidized-bed gasifier and their catalytic decomposition in a separate reactor. In the gasification tests, the effects of fuel and freeboard temperature on tar yields and composition were measured. Finnish peat, saw dust and German brown coal were used as fuels. The tar yields decreased with increasing freeboard temperature. The total tar concentration in wood gasification at 900°C was in the range of 10-18 g/m³n. In peat gasification the concentration at the same conditions was only 3-6 g/m³n and with brown coal less than 1.5 g/m³n. The main components of tar for all the fuels were benzene and naphthalene.

Catalytic decomposition of tar was studied with a bench-scale tube reactor. Sample gas from the pressurized fluidized-bed gasifier was used as feed. Carbonate rocks (dolomites, ankerite), commercial nickel catalyst and inert reference material were used as bed materials. The treatment temperature range studied was 750-900°C. The most effective material was nickel catalyst, although the dolomites performed nearly as well. At 900°C, a considerable increase in the thermal value of dry gas was achieved, due to an increase in H₂ and CO contents. A long-term test (160 h) was carried out with the nickel catalyst, during which the activity of the catalyst remained almost constant.