Tar evolution profiles obtained from gasification of biomass and coal

The tar content of the product gases from gasification of biomass is one of the major factors affecting the subsequent process stages. In this work, evolution profiles of the main tar constituents, i.e. benzene, toluene, indene, naphthalene and phenol were obtained during about 1 h gasification runs of biomass and coal in a pressurised fluidised-bed at 700 and 900°C, 0.4 MPa. Sampling and analysis was achieved, using the solid-phase adsorption (SPA) method, previously developed in our laboratory. Our main objectives were: (1) to illustrate the usefulness of the SPA method; (2) to shed new light on the main factors governing tar evolution. It was found that temperature and the type of feedstock used mainly affected tar yields. For both biomass and coal the concentration of tar products decreased with increasing run time at a rate that was fastest initially. This behaviour, which was much more pronounced for coal, provides evidence that char catalytically affects tar evolution. Accordingly, char accumulates in the bed to a various extent depending on fuel and gradually approaching steady state. Biomass char, contrary to coal char, is readily oxidised during gasification, and thus only small steady-state amounts are available to catalyse tar cracking reactions.