Modelling of Gasification of Biomass in Dual Fluidized Beds

The thesis investigated tar formation in fluidised bed gasification systems by starting with an introduction to gasification technology and the composition of biomass. Biomass consists of cellulose, hemicellulose, and lignin, which are referred to as pseudo-compounds. All the pseudo-compounds form tar, but lignin is the primary source for phenolic tar.

The tar formation and evolution in gasification is similar to that of pyrolysis. The phenolic tar is degraded to phenol, which undergoes unimolecular removal of CO to form cyclopentadienyl. Cyclopentadienyl is the basis for repolymerisation to form polyaromatic hydrocarbons. A pre-torrefaction experiment has been conducted.

It showed a considerable tar release during torrefaction. In combination with dual fluidised bed systems, the tar formation in the gasifier will be reduced by separation of the torrefaction gas. To minimise the loss of energy of the separated tar filled gas, the gas may be used as fuel for the combustor or the co-production of valuable chemicals.

With the understanding of the tar release from biomass and tar evolution, the reactor system is simulated to understand the effect of process parameters on the tar yield. This is done by the use of three mathematical models. One for the devolatilisation of biomass particles under gasification conditions, one for the evolution of tar under gasification conditions, and one for the fluid dynamic and coking rate.

The models are used to argue for a novel idea for tar removal by coke formation. Instead of converting the tar to useful product gasses, tar is catalytically reformed to coke and transported to the combustor and used as a fuel for heating the bed material. The gas phase model shows that tar composition is primarily affected by the severity of the temperature.

To a small degree some of the formation of larger tar species like naphthalene may be reduced by introducing radical forming gases that react with cyclopentadienyl to form benzene or light C2 and C4 gases. It is, however, unlikely that this will be economically feasible. Simulation of recirculation of the gasification product gas shows an increase of naphthalene if the catalyst for reforming is not utilised.