Investigation of Solid Fuel Conversion in a Fluidised Bed Gasifier – Modelling and Experiments

Licentiate thesis, 2015

A substantial proportion of Sweden’s greenhouse gas emissions originates from the transportation sector, and the Swedish government has set the goal that the entire Swedish vehicle fleet will be independent from fossil fuels by 2030. One of the strategies investigated to achieve this goal is biomass gasification, which is a technology that can be used to transform lignocellulosic materials into a raw gas. This gas can be further upgraded into a transportation fuel, such as substitute natural gas (SNG), Fischer-Tropsch diesel, dimethyl ether, or methanol. Three major techniques can be used for biomass gasification: entrained-flow gasification; single fluidised bed gasification; and dual fluidised bed gasification (DFBG). This thesis focuses on DFBG with SNG as the end-product. For this process, there is an optimal overall efficiency of SNG production for a certain degree of char conversion in the gasification chamber. The aim of the work of this thesis is to elucidate how the degree of fuel conversion in the gasifier of a DFBG unit is influenced by different parameters. This knowledge is valuable for the design, upscaling, and optimisation of such units. For this purpose, semi-empirical modelling is combined with experimental work. The model is used to identify the key parameters that affect char gasification in a DFBG unit and to provide the corresponding sensitivity analyses. Furthermore, a general approach for optimising the definition of the conversion classes used in modelling the fuel population balance is proposed and evaluated. Experiments conducted at the laboratory scale examine how the conversion conditions of a fuel particle (fuel vertical mixing, fuel concentration, fuel size, pyrolysis atmosphere, and cooling of the char after pyrolysis) affect the char gasification rate. Experiments are also used to determine the particular kinetic and structural parameters of the biomass fuel used in the Chalmers DFBG unit. The 1D model, combined with the developed discretisation method for the fuel conversion classes and the experimentally determined kinetic and structural parameters, gave results that were in good agreement with the experimental data for the char conversion degree in the gasification chamber of the Chalmers DFBG unit. Furthermore, the experiments showed that the position of the fuel during pyrolysis and char gasification had a significant effect on the char gasification rate, for conditions relevant for DFBG. Particle size was also identified as an important parameter. Thus, when carrying out laboratory-scale tests to generate fuel reactivity data to be used for modelling large-scale units, it is important to replicate the conditions experienced by the fuel particles in the large-scale unit and to use similar fuel sizes.