Air-blown CFB gasification of forest residues, demolition wood and crushed wheat straw pellets

Several Circulating Fluidised-Bed (CFB) gasifiers have been recently built in Finland to replace the use of fossil fuels in boilers and lime kilns. When clean wood feedstocks are used, the hot raw gas is usually led directly from the recycle cyclone to the boiler or kiln. In the case of contaminated wastederived fuels or high-alkali biomasses, filtration of the gas is usually required to protect the boiler or kiln from gas contaminants. In CFB gasification, the bed material, gasification temperature, the location of the fuel feeding point and the operation conditions of the filter unit are important design parameters, which must be optimised for the target fuel and end-application. High-alkali biomasses, like straw, are usually rather reactive but have to be gasified at relatively low temperature, and often special bed materials are needed to avoid ash sintering and deposition problems. The results from extensive test campaigns carried out at the CFB gasification Process Development Unit (PDU) of VTT are presented in this paper. The gasification tests were carried out using three different feedstocks: clean forest wood residues, demolition wood waste and crushed wheat straw pellets. The gasification tests with forest residues were focused on studying the effects of bed material, air distribution and fuel feeding point on the formation of tars and nitrogen compounds. The difference of using sand and limestone as the bed material was very significant as the CaO was effectively catalyzing tar decomposition. The fuel feeding point also had a strong effect on the gasification results especially on the tar yields. The tests with straw and demolition wood were on the other hand focused on finding stable gasifier operation conditions and on the other hand on the removal of gas contaminants by filtration.