Decomposition kinetics of model tar compounds over chars with different internal structure to model hot tar removal in biomass gasification

Abstract

Catalytic decomposition of two model tars (toluene and naphthalene) over three different char materials was investigated. The influence of temperature, steam concentration and internal structure of the char, on the rate of decomposition of the two tars and its evolution with time was investigated. Three chars were used: commercial coconut and coal chars, and a char from dried sewage sludge prepared in our laboratory. Tests were conducted in a laboratory fixed bed reactor varying the temperature and inlet volume fraction of steam in the gas between 750–950°C and 0–25%, respectively. The residence time of the gas in the char bed was 0.3s for all tests. The composition of the two tars and light gases was measured by taking several samples during the 75min long tests. The three chars studied gave similar results for the conversion rate and its evolution with time, despite having very different internal structures. At initial time, the two tars were completely converted within the char bed for the three char materials provided that the temperature was higher than 850°C. However, significant catalytic deactivation of char with time was observed for certain operating conditions. It was found that char deactivation was due to soot deposition on active sites of the original char. The increase in temperature and steam concentration enhanced soot gasification, making it faster than carbon deposition, so that the rate of char deactivation was reduced. The initial internal structure of the chars was found to play a minor role in the deactivation process. Kinetics for the naphthalene and toluene decomposition over the three chars was determined, accounting for char deactivation with time.