Adequate tar removal is a recurrent challenge for biomass gasification. Materials such as char and activated char are promising catalysts for tar reforming because of their activity, inexpensiveness and constant production during gasification. Although the behaviour of char and activated char as catalyst has been previously studied, an evaluation of the thermodynamic efficiencies of the tar reforming process using char as a catalyst still lacking. This work analyses the performance of a two-stage system, where gasification is followed by tar reforming using char catalysts. For the study, a model based in a combination of equilibrium thermodynamics and chemical kinetics was developed. The first stage, where gasification occurs, was simulated with a thermodynamic equilibrium model. Gasification equilibrium models available in literature only predict the fractions of H2, CO, CO2 and CH4; the model developed for this work also predicts the formation of three model tars with different characteristics (benzene, toluene and naphthalene), providing information on the stability of formed tars. The second stage, simulated using kinetics from literature, consists on reforming the tars with catalysts made of residual char. The effects of the reactor temperature, equivalence ratio, and residence time were assessed via the gas quality, based on the gas lower heating value and tar concentration, and process efficiency, based on the energy and exergy efficiencies. Results showed that using char or activated char catalysts increases the heating value of the gas while reducing its tar concentration. Moreover, the process benefits thermodynamically (i.e. less exergy is destroyed) from low gasification temperatures and high reforming temperatures. Simulations indicate that a tarless gas with a lower heating value of more than 8 MJ/Nm3 can be produced from gasification at 1023K with an equivalence ratio of 0.15 and subsequent reforming at 1123K with a residence time in the catalyst bed of 1 second.
- thermodynamic equilibrium model
- tar reforming
- char catalyst
- exergy analysis