CO2 gasification of biomass: The effect of lime concentration in a fluidised bed

Fluidised bed (FB) technology can be advantageously used for the gasification of solid fuels. Calcined carbonate materials, such as limestone or dolomite, can be used directly in the fluidised bed of the gasification reactor to reform tars in situ and to enhance carbon conversion and cold gas efficiency of the gasification process. However, they exhibit poorer mechanical stability, they tend to be attrited and carried over from the reactor, and their catalytic activity decreases over time. Therefore, a portion of the material has to be replenished continuously or periodically. To decrease the amount of carbonate material that has to be replenished, a lower amount of lime (calcined limestone or dolomite) can be used in the FB, diluted by a mechanically robust material, such as silica sand or olivine. According to the literature, even concentrations in the order of 10–35% wt. of carbonate material in the FB of silica sand have a substantial effect on the decomposition of tars during steam or air gasification. However, the effect of the concentration of lime in the FB has not yet been described for CO2 gasification. In this paper, we focus on the effect of the ratio of calcined dolomitic limestone and silica sand in the FB (0%, 25%, 50% and 100% vol. of dolomitic limestone) for CO2+O2 gasification of biomass and compare it with H2O+O2 gasification at the temperature of 850 °C. The experiments were performed in a semi-autothermal spouting FB reactor, gasifying 1.4 kg h−1 of woody biomass. The effects of the concentration of dolomitic lime in the fluidised bed differed for H2O+O2 and CO2+O2 gasification. When gasifying with H2O+O2, optimal results were found with 50% vol. (35% wt.) lime in the FB, when the yield of tar was similar to the use of pure lime in the FB. When gasifying with CO2+O2, a substantial decrease in tar yield was observed when using 50% vol. (35% wt.) lime in the FB (compared to the use of pure silica sand); nevertheless, the use of undiluted lime in the FB remains the best option to attain minimal tar yield when gasifying with a CO2+O2 gasifying agent. In this case, the tar yield was decreased 8.7-fold, and the tar dew point was decreased by 124 °C (to 71 °C) compared to the non-catalysed case with silica sand being the fluidised bed of the gasifier.