Catalytic removal of biomass tars : olivine as prospective in-bed catalyst for fluidized-bed biomass gasifiers

With respect to global issues of sustainable energy and reduction of CO2, biomass is getting increased attention as a potential source of renewable energy. Among all the biomass conversion processes, gasification is one of the most promising. One of the major issues in biomass gasification is how to deal with the tar formed during the process. The average tar concentration in the exit gas from a fluidized bed gasifier is about 10 g m-3 (STP). The minimum allowable limit for tar, however, is highly dependent on the end user application. The preferable tar and dust loads in gases for engines must be lower than 10 mg m-3 (STP). Considerable efforts have been directed on tar removal from the fuel gas. Tar removal technologies can broadly be divided into two approaches; hot gas cleaning after the gasifier (secondary methods), and treatments inside the gasifier (primary methods). Primary methods are gaining much attention as these may eliminate the need for downstream cleanup. The objective of the present research is to find an appropriate catalyst that can be used as an active in-bed material for biomass gasification processes. Olivine is considered as prospective in-bed additive and catalytic activity of olivine is investigated in this thesis. The present research mainly includes fundamental aspects of catalytic tar decomposition, the kinetics and tar decomposition behavior over olivine. The performance of olivine under different gasification medium is also evaluated. Several characterization techniques have been utilized to understand the properties of olivine. Olivine used in this study, is a naturally occurring silicate mineral from Norway, containing oxides of magnesium, iron and silicon. Preliminary experiments are performed at ECN to compare the catalytic activity of olivine with dolomite. Total tar conversion of only 46% is observed at 900oC over olivine, which is lower than that of dolomite. Naphthalene is identified as one of the major tar compounds that contributes to more than 60% of total tar hence it is considered as a model biomass tar compound during this research. A fixed-bed reactor has been used to investigate catalytic decomposition of naphthalene over olivine. It is observed that pre-treatment of olivine with air at 900oC, could improve its activity towards tar removal significantly. Segregation of iron at the surface of olivine is observed due to pre-treatment. This results in a higher activity of olivine. Also the oxidation state of iron changes from 2 to 3. Crystalline hematite is formed, which increases with the increase in the pretreatment time. A naphthalene conversion of more than 81% is obtained over 10 h pretreated olivine, which is observed to be the optimum among the selected range of pre-treatment times. The effect of the presence of various reactive gases is investigated as well. Naphthalene conversion in gas mixture containing H2 and CO is found to be lower than that of only in presence of steam and CO2. The presence of H2 has a negative effect in terms of tar conversion. Steam and dry reforming reactions of naphthalene are mainly responsible for formation of valuable gaseous products such as H2 and CO. Cracking reactions lead to formation of other lower tars. Polymerization reactions form higher tars which further condense leading to soot formation. Olivine shows an attrition resistance, which is higher than that of sand and dolomite. The properties of olivine mentioned above make it a prospective candidate as an in-bed tar removal catalyst for fluidized-bed biomass gasifiers.