The conversion of solid fuels via gasification is a viable method to produce valuable fuels and chemicals or electricity while also offering the option of carbon capture. Fluidized bed gasifiers are most suitable for abundantly available low-rank coal. The design of these gasifiers requires well-developed kinetic models of gasification. Numerous studies deal with single aspects of char gasification, like influence of gas compositions or pre-treatment. Nevertheless, no unified theory for the gasification mechanisms exists that is able to explain the reaction rate over the full range of possible temperatures, gas compositions, carbon conversion, etc. This study aims to demonstrate a rigorous methodology to provide a complete char gasification model for all conditions in a fluidized bed gasifier for one specific fuel. The non-isothermal thermogravimetric method was applied to steam and CO2 gasification from 500 °C to 1100 °C. The inhibiting effect of product gases H2 and CO was taken into account. All measurements were evaluated for their accuracy with the Allan variance. Two reaction models (i.e., Arrhenius and Langmuir–Hinshelwood) and four conversion models (i.e., volumetric model, grain model, random pore model and Johnson model) were fitted to the measurement results and assessed depending on their coefficient of determination. The results for the chosen char show that the Langmuir–Hinshelwood reaction model together with the Johnson conversion model is most suitable to describe the char conversion for both steam and CO2 gasification of the tested lignite. The coefficient of determination is 98% and 95%, respectively.

The integration of intermittent renewable energy sources into the electricity market requires flexible and efficient technologies that compensate for the fluctuating electricity demand. A circulating fluidized bed (CFB) boiler is a suitable solution due to its fuel flexibility, but the thermal inertia of the fluidized bed can have negative effects on the load following capabilities. This study investigates the influence of the operating parameters of the fire side on the speed of load changes on the waterside. Co-combustion of lignite, straw, and refuse derived fuel (RDF) was carried out. In a 1 MWth pilot CFB combustor fifteen load changes were performed with a varying step input of the primary air, the secondary air, and the fuel mass flow. The step input of the primary air had a large influence on the load ramps, as it strongly affects the solids concentration in the upper furnace. The step size of the fuel mass flow had a positive effect on the load change rate. Based on the results, concepts were developed to accelerate load ramping by controlling the hydrodynamic conditions and the temperature on the fireside.

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