Solid Fuel Conversion in Dual Fluidized Bed Gasification - Modelling and Experiments

Doktorsavhandling, 2018

Dual fluidized bed gasification (DFBG) is the initial step towards the transformation of ligno-cellulosic materials into a raw gas, which can be further upgraded into transportation fuels, such as substitute natural gas, Fischer-Tropsch diesel, dimethyl ether, and methanol. DFBG units can be operated in two distinctly different ways, depending on whether the main product is a gas (to be refined into a transportation fuel) or heat and power (with gas as a by-product). For efficient operation in either mode, the degree of char conversion in the gasification chamber needs to be controlled and optimised. For this purpose, extensive knowledge is required regarding how the degree of char gasification is affected by different parameters.

The aim of this thesis is to identify and fill key knowledge gaps regarding how different parameters influence solid fuel conversion in the gasification chamber of a DFBG unit, using a combination of laboratory-scale experiments and semi-empirical modelling. In addition, the possibility of ensuring adequate fuel conversion for either of the target modes described above is investigated.

The results of the experiments presented in this thesis confirm that the laboratory-scale conditions applied in the experimental determination of reactivity data aimed at modelling fluidized bed gasification should, as much as possible, mimic the conditions of the end-scale reactor to be modelled. In particular, the effects of fuel axial mixing and of catalytic bed materials on char gasification were found to be significant.

A validated semi-empirical 1D model of the gasification chamber of a DFBG unit has been formulated that: 1) accounts for the effect of fuel axial mixing on the char gasification rate; and 2) introduces a computationally efficient method for describing fuel conversion in fluidized beds. The modelling results show that the dominance of fuel convection over fuel dispersion increases with scale. Satisfactory fuel conversion is easily achieved when heat and power are the main products, with gas as a by-product. However, when the main goal is to improve the efficiency of gas production, a combination of baffles, properly chosen operational conditions, and/or the use of an active bed material is likely necessary to achieve sufficient fuel conversion.