Experimental study on the combustion of torrefied biomass in drop tube furnace and cyclonic TGA

Co-firing of coal and biomass for power generation is an interesting topic in recent years. However as compared to coal, untreated biomass has a low energy density, high moisture content, a poor grind-ability and is inhomogeneous. A good option to solve these issues is torrefaction of the biomass. An experimental study on the torrefaction technology has been performed by researchers at the University of Twente. In this novel approach the combustion reactivity of solid torrefied biomass under air and oxyfuel conditions is studied in detail.

The objective of this thesis is to investigate the effect of the torrefaction degree and the feed gas composition on the combustion reactivity of solid torrefied biomass products.

To approach this goal, a series of experiments in a cyclonic thermogravimetric analyzer (TGA) and a drop tube furnace (DTF) were performed. A single particle combustion model was developed to predict the thermal and mass loss history of the particle in the DTF. Beech wood has been selected as the feed stock and Russian bituminous coal as reference material.

It has been found out that torrefaction has the ability to improve the biomass combustion reactivity towards the property of coal. This improvement is especially pronounced during the pyrolysis stage. However, improving the biomass char reactivity, by torrefaction is weak. It is noticeable that severe torrefaction has the best ability on improving the biomass char reactivity, more studies are necessary. Oxyfuel combustion did not show real improvement compared to air combustion because the char CO2 gasification reaction was not triggered under these experimental conditions.