Tar removal from biomass gasification processes

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