CATALYTIC DECOMPOSITION OF BIOMASS TARS WITH NICKEL CATALYSTS

Abstract

Tar is a viscous black liquid derived from pyrolysis of organic matter and a complex mixture of hydrocarbons. The presence of tar in the product gas may cause blockage and corrosion of equipments and be responsible for fouling or reducing overall efficiency of processes. By far, tar removal is the most problematic during biomass gasification. Hence, a successful implementation of gasification technology for gas engines, gas turbines or fuel cells based power projects depend much on the effective and efficient removal or conversion of tar from the product gas. Beside that, the catalytic steam reforming tar is one of the most promising methods to suppress the problems. The biomass product gas is usually low calorific value; therefore, an enhancement of the product gas quality is other important target. We propose a research topic that to utilise nickel–loaded brown coal char as a new catalyst for decomposing tar from biomass gasification in a fluidized bed gasifier. The method is promising to get benefits from the use of low rank coal char as a catalyst support material, a high catalyst activity of nickel and an enhancement of product gas quality. In this study, yallourn brown coal has been selected to prepare the catalyst support. The coal that is a low rank with high moisture content, a low heat value and high oxygen content is hard to use for generating energy. However, it has a lot of outstanding features such as less ash and sulfur content, and with abundant of oxygen–containing functional groups such as carboxyl and phenol groups which are available for ion–exchange with metals. In this study, the nickel–loaded brown coal char (Ni/BCC) was prepared by an ion–exchange method, dried at 380 K in nitrogen for 24 h, and then pyrolysed at 923 K in nitrogen for 90 min. The works have been carried out is the use of the Ni/BCC catalyst to decompose tar in pyrolysis and steam gasification processes. The experiments were performed on a two–stage fixed–bed reactor and a laboratory–scale fluidized–bed gasifier under mild conditions (temperatures, steam, space velocities, operation time). Inside of the gasifier was constructed by two beds, the primary one is a fluidized bed with sand, and the second one is a catalyst bed. The Ni/BCC catalyst has shown high catalytic activity and stable activity and given the high quality of the product gas. In the presence of steam, approximately 90% of biomass tar was decomposed and useful gas components (CH4, CO, and H2) yields were higher than those of a conventional Ni/Al2O3 catalyst. The Ni/BCC catalyst was characterized and exhibited a good dispersion of nickel particles, ultra–fine Ni less than 15 nm and a large surface area (Approximately 350 m2/g). Moreover, at the end of its life span, the catalyst can be easily disposed by gasifying or burning the coal char, during which the energy value of the char support can be recovered. Further, the agglomerated nickel residues can be also used as functional materials for powder metallurgy and battery development. The general results suggest that the Ni/BCC catalyst offers a potential to be used as a high activity catalyst for steam reforming of tar in biomass gasification.