Abstract
This article investigates technical possibilities and performances of flexible integrated gasification polygeneration (IG-PG) facilities equipped with CO2 capture for the near future. These facilities can produce electricity during peak hours, while switching to the production of chemicals during off-peak hours. Several simulations were performed to investigate the influence of substituting
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feedstock and production on IG-PG facility output, load and efficiency. These simulations were done using a detailed AspenPlus simulation model of a Shell entrained flow gasifier combined with conversion facilities. In this model carbon-rich feedstocks (oil residues, coal and biomass) were converted to a variety of products (H2, electricity, FT-liquids, methanol and urea) using state-of-the-art technology. The size of the gasifier was limited to the equivalent of 2000 MWth Il #6 coal input. Overall efficiency of the simulated non-flexible configurations to convert pure coal or pure wood pellets to electricity (40%HHV vs 38%HHV), FT-liquids (60%HHV vs 55%HHV), methanol (53%HHV vs 49%HHV) or urea (51%HHV vs 47%HHV) are in good agreement with the literature. Using torrefied wood pellets instead of pure wood pellets reduces the penalty drop in efficiency compared to coal. Moreover, torrefied wood pellets have superior energetic density, handling and feeding compared to wood pellets. In this analysis, the H2:CO ratio of the sweet syngas was fixed to match FT-liquids criterion. As a result, overall CO2 capture rates are low, around 56–65%, depending on the feedstock used. Still, especially with FT-liquids and methanol production, CO2 emissions at the facility are significantly reduced; less than 20% of the carbon feedstock entering the facility is emitted with the flue gas. Applying biomass and CO2 capture shows great opportunities to produce CO2-neutral electricity or chemicals. When the biomass fraction exceeds 40% on an energy basis, production is CO2-neutral, independent of what is produced. Biomass can be co-fed up till 50% on an energy basis. Higher fractions cause significant fouling on cooling equipment. A small part-load penalty is observed during the substitution of coal by biomass. When changing from pure coal to pure wood pellets, the power case suffers a 2.5% efficiency drop, while all three chemical cases have an efficiency drop of less than 1%. At the same time total output is reduced to 67–69%, mainly because of the lower energy density of biomass. By over-dimensioning the gasifier and gas cleanup and optimisation section this drop can be eliminated. The syngas can be tailored to the desired composition regardless of the used feedstock. Therefore, the chemical conversion sections only have to cope with a reduction in syngas flow and not with a change in syngas composition. Altering production between chemicals and electricity is possible, although the load of the conversion sections should remain between 40% and 100% to prevent operational problems. This gives a high degree of flexibility. Complete substitution between chemical and power production while using the same feedstock is possible for the methanol and urea cases. The FT-liquids case is restricted to 60–100% load of the chemical conversion section to prevent that the gas turbine load is reduced below 40%. The economic aspects of flexible IG-PG facilities are addressed in part B.
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