Wood combustion and ash separation in a cyclone for gas turbine applications

A gas turbine with direct combustion of pulverized wood is an economically and environmentally attractive process for cogeneration in a small scale (5--50 MW{sub th}). It is expected that limiting of deposition and corrosion caused by the high alkali levels of the fuel is a key issue in the assessment of this technology. Alkali levels can be reduced by utilization of a two stage combustor, where the first stage is a cyclone separator. Experiments have been performed at atmospheric pressure in such a combustor with a fuel load of 30-100 kW. Pulverized, direct wood (mainly pine), which is commercially available in Sweden was used as fuel. Results from the experiments show that significant ash separation is possible in a combined combustion/separation cyclone stage. About 60% of the input alkali is separated from the gas flow. The alkali levels are still orders of magnitude higher than what is usually required by turbine manufacturers for conventional fuels. However the results show that the ash melting point is higher than for conventional fuels. The wood ashes do not melt in temperatures up to 1,040 C. This indicates possibilities to avoid hard deposits and corrosion in the turbine despite the high alkali levels.