FUEL CONVERSION IN A DUAL FLUIDIZED BED GASIFIER
Doctoral thesis, 2014
The present work is motivated by increasing demands and political goals to establish the commercial production of biofuels. Dual fluidized bed (DFB) gasification is a promising route for the production of biofuels through synthesis. The efficiency of biofuel production is limited by the conversion of biomass into syngas. The goal of the present work is to contribute to the understanding and description of the DFB gasification process, so as to facilitate the efficient conversion of biomass to a syngas. Towards this goal, an evaluation procedure is proposed that enables a comprehensive evaluation of the fuel conversion and efficiency of DFB gasifiers. This procedure is used to evaluate the Chalmers 2–4-MWth gasifier, and it is shown that important parameters, such as the yield of organic compounds (OC), char conversion, oxygen transport, and syngas yield, can be quantified online. Further, the dynamics of the loop seals were investigate to quantify the steam entering the gasifier and to ensure that there is no gas leakage through the loop seals into the gasifier.
To increase the fuel conversion in the Chalmers gasifier, it is investigated how individual changes in the level of fluidization, bed material, and layout of the gasifier affect the fuel conversion rate and heat demands of the process. The results obtained show that increasing the level of steam used for fluidization has a positive effect on the conversion of OC. However, increasing the level of steam also increases the heat demand of the system, and only when sufficient heat is available does this measure have a positive effect on the chemical efficiency of the process. Another way to reduce the yield of OC is to use a catalytic material. In the present work, catalytic metal-oxide bed materials are tested and compared with silica sand, used as reference. Metal-oxide materials can transport oxygen from the combustion side to the gasification side of the DFB system. If too much oxygen is transported, the efficiency of the gasifier suffers, since part of the gas is combusted. The investigation show that ilmenite transports too much oxygen to be used in a DFB gasifier without additional measures, while bauxite and olivine show good potentials with lower oxygen-carrying capacities than ilmenite and higher OC conversion rates than silica sand. To increase the conversion of char, a change in the layout of the gasifier was investigated that involved the addition of a baffle, which was placed across the surface of the bubbling bed in the gasifier. After introduction of the baffle, the degree of char conversion was effectively increased by 8%–15%, which can be explained by an increase in average residence time and of by forcing the char into areas with low levels of volatiles.
Using the evaluation procedure proposed in this work, different measures that affect the performance of the gasifier can be assessed, and the results can be exploited to reveal the optimal design and operational parameters for DFB gasifiers to ensure efficient production of biofuels based on biomass.