Biomass gasification: Influence of torrefaction on syngas production and tar formation

The paper contains results of comparative gasification of standard wood biomass pellets, torrefied pellets and sawdust in a robust industrial fixed-bed gasifier. Parameters such as process stability, operating difficulties, gas parameters and tar content in syngas were analysed. The operating conditions were optimised to maximise production of liquid hydrocarbons, which can be both a problematic by-product and a valuable component. In order to collect the data concerning quantity and composition of the tars, the experimental set-up was equipped with a syngas cooler. The test runs conducted with sawdust and ordinary pellets did not cause any operational problems. The most complicated part of the experiment was maintaining process stability during gasification of torrefied pellets. The stabilisation effect of grinding of torrefied pellets and blending these pellets with wet sawdust were tested. It was concluded that effective and stable gasification of torrefied pellets in the tested type of fixed-bed gasifier is possible, but this type of fuel is much more suitable for co-gasification. The cleaned syngas from standard pellets had a relatively stable composition and calorific values in the range of 4.8-5.6 MJ/Nm3. Cold gas efficiencies of the process were in the range of 0.72-0.77 MJ/Nm3. Using torrefied pellets as a feedstock led to a higher calorific value of syngas, but the cold gas efficiency remained similar (0.75). For sawdust both the calorific value of syngas (LHV = 3.0 MJ/Nm3) and cold gas efficiency (0.57) were significantly lower than for pellets. The collected condensates contained a water fraction with dissolved organic compounds and thick viscous organic substances tar. It was observed that tar production from torrefied pellets is slower, characterised by lower yield, and technically more difficult in comparison to untreated biomass. The effectiveness of liquid hydrocarbon collection (tar to fuel ratio) varied between 0.0138 [kg tar/kg fuel] for torrefied pellets and 0.0213 [kg/kg] for sawdust. The main component of water fractions were organic acids. The content of organic acids in these fractions was as follows: 79.5% from South African pellets, 67% from Polish pellets, 64% from Polish sawdust and 59% from torrefied pellets respectively. The main organic species in tar from torrefied biomass remained acids, whereas in other cases tars were composed of alkylophenols, linear and cyclic aliphatic oxygenates and polyfunctional aromatic oxygenates.