Choice of bed material: a critical parameter in the optimization of dual fluidized bed systems

Doctoral thesis, 2016

Dual Fluidized Bed (DFB) gasification is a promising alternative method for the production of biofuels. In the DFB gasification process, the bed material, which is a crucial component of the process, has two possible roles: 1) to provide the heat needed for the gasification reaction; and 2) if it has catalytic properties, to improve the quality of the produced gas. Ash-forming elements that are introduced together with the biomass into the system can interact with the bed material, and therefore have catalytic potential. As they alternate between the different environments, and depending on the nature of the bonds formed between the bed material and ash elements, some of the inorganic compounds may be released in the gas phase, thereby influencing the final composition of the produced gas. The present work advances current understanding of how biomass ash-induced changes alter the impact of the bed material on the performance of a DFB gasifier. Thus, understanding the bed material-biomass ash interactions can be used to improve the performance of the system. By improving the gasification step, which is the main step in the biomass conversion, the costs associated with the downstream processes, and consequently, the overall cost of the biomass to biofuel pathway can be reduced. In addition, utilizing naturally occurring materials as catalytic bed materials is advantageous given their relatively low price and low environmental impact which in terms of disposal of the material post-process. The work for this thesis was conducted at the Chalmers facility. The possibilities for process improvement were investigated by the use of different bed materials in the gasifier, as well as in a secondary upgrading step. When applied to the gasification step, the interactions between inorganics and bed material alter the chemical properties of the bed, thereby influencing the gasification process. Within the investigations presented in this thesis, six naturally occurring materials were tested in the beds. Tests with silica sand, olivine sand, and bauxite were performed in the Chalmers 2–4-MWth gasifier. For each tested material, the process was evaluated with respect to: 1) the composition of the produced gas; 2) the tar content; and 3) the physicochemical properties of the bed material. The key finding is that the transport of alkalis from the boiler to the gasifier, where they are released, is the major factor influencing the gas composition. An improved understanding the importance of potassium transport was crucial in salvaging the commissioning and start-up of the GoBiGas gasification system, which had suffered from the problem of insufficient gas quality. Even though a positive effect of alkali on the gasification is known, high levels of alkali entail a risk for agglomeration. In this regard, a natural ore of iron, ilmenite, was used in the Chalmers 12–MWth boiler as a material for fluidized bed combustion. It was found that ilmenite has the capability to store potassium in a non-releasable form, thereby decreasing the risk of bed agglomeration in fluidized bed boilers. To enable further upgrading of the gas produced by biomass gasification, two naturally occurring materials were tested in a Chemical Looping Reforming system: a manganese ore; and feldspar. Both materials were able to reform tars and persisted throughout the process with negligible decrease in activity.