Improving char and tar conversion in fluidized bed gasification
Conference contribution, 2010
Low char conversion and high tar concentration in the gas are the two main drawbacks of fluidized bed gasifiers (FBG), processing biomass and waste. In this type of units the temperature is maintained relatively low to avoid agglomeration and this reduces the carbon conversion and generates a gas with relatively high concentration of heavy and stable tars. As a result, the cold gasification efficiency (chemical energy from the fuel into gas) is reduced, and utilization of the gas for other applications than direct combustion of the hot gas is difficult. In the present work, char and tar conversion processes in FBG are reviewed to provide an understanding on the most influencing factors in these processes. Solids and gas mixing in FB are qualitatively analyzed to identify the interaction between flow motion and chemical conversion in FBG. Fuel conversion processes are tracked by adopting a sequential scheme of primary (intraparticle) and secondary conversion (in the reactor) to simplify the analysis. Devolatilization of single fuel particles, which generates primary char and tar, and their subsequent conversion by secondary reactions in the bed are reviewed. Chemical reaction of char particles and comminution processes, such as fragmentation, attrition, and entrainment are also dealt with, since they determine the degree of solid carbon converted in an FBG. Staged reforming/gasification is explored, as it seems to be a feasible way to yield high carbon conversion and to produce a gas almost free of heavy tars. Some process developments made during the last decades to conduct gasification for simultaneous optimization of tar and char conversion are discussed. A recent concept to achieve tar and char conversion by tree-stage gasification in an FB is presented.