Operational strategies to control the gas composition in dual fluidized bed biomass gasifiers
Abstract: Steam gasification of biomass can increase the share of renewable energy and material resources in the energy sector, transportation and different industries. Prior its application, the raw gas produced in biomass gasifiers needs to be cleaned from impurities. In gasifiers operating at mild temperature, such as fluidized bed steam gasifiers, tar is an impurity of major concern due to the operational problems that it can cause. Tar species can condensate at temperatures as high as 300°C, causing the clogging of pipes and coolers, deactivating downstream catalysts, and forcing unplanned shut-downs. Thus, it is necessary to control the tar and gas compositions in gasifiers to ensure the technical reliability of the technology. This work investigates measures to control biomass conversion in dual fluidized bed (DFB) steam gasifiers and, thereby, contribute to the rational operation and design of these types of units. A parametric experimental investigation of the influences of operating conditions on gas and tar compositions is presented. The examined parameters are: fluidization velocity; steam-to-fuel ratio (S/F); circulation rate of the bed material; temperature; and active bed materials. The bed materials tested include silica sand, olivine, bauxite, and feldspar, as well as the oxygen-carrying materials ilmenite and manganese. The work was carried in the Chalmers 2–4-MWth DFB gasifier using woody biomass as the fuel. The gasification technology applied in this work is similar to that of the existing gasifiers at the Güssing, Senden, Oberwart, and GoBiGas plants. Within the operating window investigated, optimization of the bed material activity was the main tool for controlling tar conversion, which can be improved using additives. The levels of effectiveness of the in-bed catalysts were linked to the destruction of tar precursors. It is proposed that both homogeneous and heterogeneous catalysis of tar reactions occur in systems where alkali is expected in the gas phase. With active bed materials, temperature changes in the range of 700°–830°C were found to affect primarily the composition of the tar, and to a lesser extent, the tar yield. Finally, it is shown that a simple gasifier design with on-bed feeding ensures that at least 50% of the volatiles come in contact with the catalytic bed material when the bed is well-fluidized. Extensive experimental results and their implications for the design and operation of a DFB gasifier are discussed throughout this thesis.
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