Use of Bed Materials in Dual Fluidized Bed (DFB) Systems

Sammanfattning: Dual Fluidized Bed (DFB) gasification is one alternative for the production of biofuels. In a DFB gasification process, the bed material plays a crucial role. Primary role of the bed material is to provide the heat needed for the gasification reaction. If the bed material, used in the system, is catalytically active it can improve the quality of the produced gas. In addition, inorganics originate from the biomass are retained in the system in the form of ash, which interacts with the bed material, thus representing catalytic potential. As a consequence of being circulated between oxidative and reductive environments, the bed material releases some inorganics into the gas phase, thereby influencing the final composition of the produced gas. The goal of the present work is to increase our understanding as to how changes in the bed material related to reactions with ash components can be utilized to improve the performance of the system. In this way, it should be possible to optimize the gasification step, thereby decreasing the cost of biofuel production. Moreover, by choosing naturally occurring bed materials not only the cost of the process is decreased, but also negative impact on the environment which comes with the disposal of the material. This work deals with the application of catalytic bed materials in dual fluidized bed systems, both as a primary measure in the gasification process itself and as a secondary measure for the reforming of the raw gas. Applied directly in the gasification step, the bed material interacts with the inorganics derived from the fuel. As a result of this interaction, the bed material changes its physical and chemical properties, which has impacts for the gasification process. In the present study, four naturally occurring materials were evaluated. In the Chalmers 2–4-MWth gasifier, the effects on the gasification process of interactions between the ash and the bed material were studied for quartz-sand and olivine. The effects on the process were evaluated with respect to: 1) the composition of the produced gas; 2) tar content; and 3) the physicochemical properties of the bed material. The studies yield information about the transport of inorganics between the reactors and their influences on the gas composition. Whereas the release of alkali during gasification implies a potential for enhancement of the gas quality, the presence of these same species represents a risk for agglomeration during combustion. In the Chalmers 12–MWth boiler, ilmenite was tested as the bed material and as an alkali getter material. The adsorption of potassium to the ilmenite is shown to be non-reversible, and ilmenite shows great promise as a material for decreasing bed agglomeration in fluidized bed boilers. As a secondary measure, downstream of the gasifier, manganese ore was evaluated for its tar-reforming capability in a dual fluidized bed reactor system, in a process known as chemical looping reforming. This material has the ability to reduce tar levels by as much as 72%, while having high activity towards hydrogen production.