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Energy & Fuels 2006, 20, 890-895
Articles Elemental Iron as a Tar Breakdown Catalyst in Conjunction with Atmospheric Fluidized Bed Gasification of Biomass: A Thermodynamic Study Thomas Nordgreen,* Truls Liliedahl, and Krister Sjo¨stro¨m Department of Chemical Engineering and Technology, Chemical Technology, KTH, SE-100 44 Stockholm, Sweden ReceiVed July 19, 2005. ReVised Manuscript ReceiVed February 24, 2006
Metallic iron as a catalyst for tar cracking in biomass gasification has been investigated. Based on previous studies showing that iron must be in its elemental form to catalyze the tar breakdown reactions, thermodynamic calculations suggest the existence of an operating window where iron is neither oxidized nor contaminated by carbon deposits. A straightforward biomass gasification model has been derived and used in conjunction with thermodynamics for making plots that illustrate the mentioned operating window, which is achievable under real conditions. Experiments made under these specific calculated conditions confirm that elemental iron effectively acts as a tar breakdown catalyst, resulting in an improved gas yield and a decrease in tar concentration. The desired operating window is governed mainly by adjusting the oxygen input (i.e., the equivalence ratio) and the temperature.
Introduction Biomass-fueled combined heat and power generation plants based on the gasification technique are a promising candidate for power production in the current effort to replace fossil fuels by renewable counterparts.1 In this process, a biomass gasifier is coupled prior to a gas turbine or an engine for electricity production. The principles for biomass gasification can be described in sequential steps. The first step in the reaction scheme is drying the biomass particle to evaporate the moisture, followed by pyrolysis, which results in primary products such as gas, vaporized tars, and a solid char residue. The relative yields of the various pyrolysis products are primarily dependent on the heating rate and the final temperature. The third step is a partial oxidation of the pyrolysis products to give secondary products, such as permanent gases (CO, CO2,H2), volatile hydrocarbons, and contaminants such as small char particles, small amounts of tar, and ash. Tar is a complex mixture of condensable hydrocarbons containing predominantly polyaromatics with up to five individual (dehydrogenated) rings. Moreover, heteroatoms such as oxygen are also frequently found in tar samples and contribute to the overall tar content. At the meeting on a tar measurement protocol between the European Union (EU), the International Energy Agency (IEA), and the United States Department of Energy (US-DOE), held in Brussels in 1998, several experts agreed to define tar as all * Author to whom correspondence should be addressed. E-mail: thomasn@ ket.kth.se. (1) Bridgewater, A. V. The technical and economic feasibility of biomass gasification for power generation. Fuel 1995, 74, 631-653.
organic contaminants with a molecular weight greater than that of benzene. Consequently, benzene is not considered to be a tar constituent in this paper, i.e., it is not included in the total chromatographically determined tar content. However, in some cases, it may illustrate and quantify the decomposition of tars; for example, when the bonds between the methyl groups and the benzene ring in toluene and xylene are broken, the concentration of benzene usually increases. Benzene also resists decomposition at moderate temperature, because of ring stabilization. Thus, it can be used as a model compound in catalyst examinations. However, at present, it is inevitable to obtain byproducts such as tars during the gasification process and this is one of the main obstacles involved in gasification processes; if the tars reach their dew point, condensation occurs, which normally happens at temperatures of
