The positive effect of calcite as bed material for dual fluidized bed steam gasification

Abstract

A promising key technology for the sustainable (renewable) production of electricity, heat, synthetic chemicals and fuels is steam gasification of biomass in a dual fluidized bed (DFB) gasification system. Therefore, gasification of softwood at temperatures between 750 – 850 oC was carried out at an advanced 100 kWth pilot plant at TU Wien to investigate the effects of calcite admixtures to different bed materials on the performance of the DFB steam gasification process regarding product gas composition and the concentration of condensable hydrocarbons (tars) in it. In this respect, olivine, silica sand and feldspar were used as pure bed materials and also mixed with calcite in mass ratios of 100/0, 90/10, 50/50 and 0/100. The gasifier system consists of gasification and combustion reactor, which are thermodynamically connected via a circulating bed material. The bed material has two functions (i) as a heat carrier from the combustion to the gasification reactor and (ii) as a catalyst for gasification reactions to improve the product gas quality. The product gas consists mainly of H2, CO, CO2, CH4 and C2H4. For the use as synthesis gas in subsequent processes tar, char and inorganic fines in the product gas have to be removed by suitable cleaning and filtering equipement. Pure steam was used as fluidization and gasification agent at a range of steam to fuel ratios between 0.7 – 1 kg_(H_2 O)/kg_(fuel,daf). During the gasification operation, extensive gas analyses were performed at the outlet of the gasification reactor (sample point A). Experimental results were validated via IPSEpro software by calculation of mass- and energy balances. A good agreement between simulation and experimental results was achieved. It was observed that the admixture of calcite to olivine, silica sand and feldspar as bed material shifted the product gas compositions towards higher hydrogen and carbon dioxide and lower carbon monoxide contents. Additionally, the tar content (GC-MS tar and gravimetric tar) in the product gas was decreased and the tar composition changed, resulting in lower calculated tar dew points. The pure calcite as bed material led to the product gas with the highest H2 and the lowest tar content as well as the lowest calculated tar dew point compared to all mass ratios of other bed materials. These results can be explained by enhanced water-gas shift reactions and improved steam reforming reactions of hydrocarbons. Pure calcite compared to pure olivine, pure silica sand and pure feldspar as bed material caused an increase of the H2 content of up to 25 %. The tar content was decreased up to 93 %. Additionally, the lowest content of higher hydrocarbons, CH4 and C2H6, in the product gas was found by applying pure calcite as bed material.