Abstract
In this work, a two-stage gasification process enabling low tar content of gas was studied by experimental investigation and mathematical modelling. A computer model capable of predicting mass and energy balances of both steps, producer gas composition, and kinetics of thermal decomposition was developed. As input data, the characteristics of raw material estimated in the laboratory were used. Kinetic and heat transfer data were derived from literature. A mixture of different types of ligno-cellulosic waste biomass was assumed to be gasified in an industrial scale pyrolysis/gasification system with a pyrolysis reactor, a char gasification reactor and a catalytic volatile gasification reactor. Gasification of a mixture of wheat straw, corn stalks, corn leaves, barley straw and wooden chips was modelled under various conditions. Temperature in the pyrolysis reactor was 550 oC and that in the catalytic volatile gasification reactor at optimal air to biomass mass ratio was 850 oC. As the gasification agent, air, oxygen enriched air and pure oxygen were used. The amount and composition of producer gas were determined. All types of waste biomass were studied also by thermogravimetric (TGA) analysis, differential scanning calorimetry (DSC), elemental analysis and bomb calorimetry. Elemental composition of pyrolysis char and solid fraction and also the composition of gases from the pyrolysis stage were measured experimentally. Optimal oxidising medium to biomass mass ratio was estimated to be 1.5 for air, 0.45 for oxygen rich air (70 % oxygen), and 0.35 for technically pure oxygen (99 %). Lower heating value (LHV) of gas from the gasification stage calculated based on its composition was 6.6 MJ/Nm3, 10.3 MJ/Nm3 and 11.0 MJ/Nm3 for air, oxygen rich air and pure oxygen.
