Tar removal from biomass gasification processes

With respect to global issues of sustainable energy, biomass is getting increased\nattention as a renewable energy source. Biomass gasification is the only process that\nproduces a syn-gas mixture which has numerous opportunities for applications such as\nfor gas turbines, engines, fuel cells, hydrocarbon synthesis, hydrogen production etc. One\nof the major issues in biomass gasification is to deal with the formation of tar. Tar is a\nterm used to describe a complex mixture of condensable hydrocarbons and is undesirable\ndue to its condensation causing blockage in the process equipments. The successful\nimplementation of biomass gasification depends much on the effective and efficient tar\nremoval from the producer gas. This paper addresses several issues on tar treatment with\nspecial emphasis on use of olivine as tar removal catalyst and use of non-thermal plasma\nfor tar removal. In order to facilitate the study tar conversion, naphthalene is chosen as a\nmodel biomass tar compound which is one of the most stable and therefore the most\ndifficult tar to decompose. Olivine, a naturally occurring mineral, is a very attractive inbed\ncatalyst for fluidized bed biomass gasifiers as it shows high attrition resistance. A\nsimple method of pre-treatment of olivine can improve the catalytic activity of olivine\nsignificantly. A naphthalene conversion of more than 81% is observed over olivine\ncatalyst which is pre-treated with air for 10 hours at a temperature of 900°C. Chemical\nconversion of tar requires high temperatures to initiate the reactions. Apart from\ntemperature, another approach to generate this reactive environment is by means of gas\ndischarges. Atmospheric pressure gas discharges or non-thermal plasma as is commonly\nreferred to, generates high energy electrons which dissociate molecules and thereby\ncreates the necessary reactive environment. The major advantage of using non-thermal\nplasma is to do chemical conv~rsion of tars at low temperatures. The experimental results\nhave indicated complete conversion of tar by pulsed corona processing at much lower\ntemperature. The energy density requirement is observed to be 400-600JIL for\nnaphthalene removal from a fuel gas mixture at a temperature of 200°C. This could be\nsignificantly reduced to 200-250JIL by increasing the temperature to 400°C. The\ninvestigations also indicate that the gaseous environment has strong influence on tar\ncracking reactions. For both the methods presented here, it is observed that presence of\nH, and CO in the gas mixture, do not improve the tar removal efficiency of the processes\nfor high temperature processes.