Oxy-Fuel Gasification in Fluidised Beds

Rising energy demand, up coming shortage of natural gas and oil and increasing awareness of the effect of global warming are the driving forces behind the development of new, highly efficient processes with integrated carbon capture and storage, using fuels with predicted long term availability. Gasification and especially fluidised bed gasification is seen as a promising near zero emissions route to utilise low value coal, waste and biomass for energy production via the integrated gasification combined cycle process (IGCC). The integration of a carbon capture and storage (CCS) technology into the design of IGCC plants changes the design framework for the gasification process as a pure stream of CO2 at the downstream end of the plant is now available. Therefore a novel oxy-fuel fluidised bed gasification process is proposed in which the gasifier is operated as part of an IGCC plant with pure O2 and recycled CO2. Steam is used to enhance the carbon conversion if necessary. A laboratory scale fluidised bed gasifier capable to operate up to 1000°C and 20 bar was used to study the implications of oxy-fuel firing on flue gas composition and operability of the gasifier. Maintaining a constant feed rate (1 - 6 g/min) a stable fuel gas composition was obtained for up to 30 min. Using a lignite as feedstock, carbon conversions higher than 95% and a fuel gas with a medium heating value were obtained under oxy-fuel conditions. With the addition of steam, the operating temperature could be reduced from 950°C to 850°C while maintaining the gasification performance. The decrease in carbon conversion with increasing pressure was investigated using experimental data from a wire mesh rector and optical char surface analysis. Strategies to optimize the gasification performance were developed for atmospheric and pressurised operations. Besides the lignite, two bituminous coals and different mixtures of biomass and lignite were tested for their use in an oxy-fuel fluidised bed gasifier. Analysis of the tar, H2S and NH3 content of the fuel gas provided information about the influence of operation conditions on the emissions of pollutants. The results show that oxy-fuel firing of a fluidised bed gasifier could be a promising route to avoid N2 dilution of the fuel gas and reduce the costs of integrating CCS technology. In order to assist further research, the results obtained in this study are used to derive design recommendations for large scale testing facilities