Modelling of Physical Method for Tar Elimination from Producer Gas at Low Temperature

Abstract

Combining a methanation step with biomass gasification requires an intensive tar removal process that reduces its content down to 1 mg/Nm3. In this paper, low temperature processes are suggested for treating tar and reducing its content to the acceptable limit. However, the gas must be pre-treated prior to the low temperature process in order to reduce its temperature, its moisture content and the heavy tar fraction. Two water scrubbers are placed prior to the low temperature tar removal process. Benzene and toluene remains in the producer gas after the scrubbers. Those two components can be valorised if separated from the producer gas at low temperature. The tar removal, at low temperature, induces the water vapour frosting remaining in the producer gas. The simultaneous frosting and condensation takes place over a cold plate. Modelling simultaneously the multi-component frost growth and condensation requires complex mathematical approach. A parametric study is completed to assess the impact of the wall temperature and the producer gas flow rate on the tar removal process function of time and plate length. Results showed that a wall temperature of -20 (C is required to lead to the benzene frost formation for an initial molar fraction of 0.024. After one hour, the thicknesses of benzene and ice frost layers reach 1.5 and 2.5 mm, respectively. Therefore, the heat transfer between the gas and the cold surface is reduced.