Effect of Magnesite as Bed Material in a 100 kWth Steam−Oxygen Blown Circulating Fluidized-Bed Biomass Gasifier on Gas Composition and Tar Formation

Because of the fibrous nature of woody and herbaceous biomass and the widely distributed appearance of the feedstock with respect to its size, the circulating fluidized bed (CFB) is a very suitable type of reactor for thermal conversion of this potentially renewable and sustainable energy source. Of the thermal conversion processes, combustion and gasification are the most well-known. The advantage of gasification above combustion is that it produces a secondary gaseous energy carrier (i.e., product gas or syngas) from solid biomass. This secondary energy carrier can subsequently be upgraded to, for example, hydrogen-rich fuel gas for fuel cells, converted to liquid transportation fuels [such as Fisher−Tropsch (FT) diesel or dimethyl ether (DME)], or used for synthesis of other chemicals. During operation, the CFB internally recirculates a certain amount of inert material, also referred to as “bed material”. This bed material accumulates a part of the energy released during (partial) combustion of biomass and distributes it along the reactor, ensuring a nearly constant temperature throughout the bed. A variety of bulk solids can be used as bed material, with quartz sand being the mostly well-known. The drawback of quartz sand and other bed materials containing a significant amount of silica is that they are prone to agglomerate formation when herbaceous biomass with high alkali and chlorine content in the ash is being used as fuel. These agglomerates can cause a blockage inside the reactor and, therefore, unscheduled maintenance interruptions in the operation. To overcome this problem, alternative bed materials have been proposed, with one of them being magnesite. Because of its low silica content (