Now showing 1 - 2 of 2
  • Publication
    Bubbling fluidised bed gasification of wheat straw-gasifier performance using mullite as bed material
    The adoption of wheat straw as a fuel for gasification processes has been hindered due to a lack of experience and its propensity to cause bed agglomeration in fluidised bed gasifiers. In this study wheat straw was gasified in a small scale, air blown bubbling fluidised bed using mullite as bed material. The gasifier was successfully operated and isothermal bed conditions maintained at temperatures up to 750 ◦C. Below this temperature, the gasifier was operated at equivalence ratios from 0.1 to 0.26. The maximum lower heating value of the producer gas was approximately 3.6 MJm−3 at standard temperature and pressure (STP) conditions and was obtained at an equivalence ratio of 0.165. In general, a producer gas with a lower heating value of approximately 3 MJm−3 at STP could be obtained across the entire range of equivalence ratios operated. The lower heating value tended to fluctuate, however, and it was considered more appropriate for use in heat applications than as a fuel for internal combustion engines. The concentration of combustibles in the producer gas was lower than that obtained from the gasification of wheat straw in a dual distributor type gasifier and a circulating fluidised bed. These differences were associated with reactor design and, in the case of the circulating fluidised bed, with higher temperatures. Equilibrium modelling at adiabatic conditions, which provides the maximum performance of the system, showed that the gasifier was operating at suboptimal equivalence ratios to achieve greatest efficiencies. The maximum calculated theoretical cold gas efficiency of 73% was obtained at an equivalence ratio of 0.35.
      1040Scopus© Citations 8
  • Publication
    Analysis of bed agglomeration during gasification of wheat straw in a bubbling fluidised bed gasifier using mullite as bed material
    The quantity and composition of the ash content of straw poses technical challenges to its thermal conversion and have been widely reported to cause severe ash sintering and bed agglomeration during fluidised bed gasification. Literature indicates that a combination of reactor design and bed material measures is required to avoid defluidisation at temperatures above 800 °C. Using scanning electron microscopy and energy dispersive X-ray spectroscopy this study investigated the initial agglomeration of a mullite bed during the gasification of wheat straw in a small scale, air blown bubbling fluidised bed. The results show that the temperatures along the height of the bed converge prior to any marked drop in pressure or heating of the lower freeboard. This convergence was seen to occur at temperatures close to 750 °C in repeated gasification experiments. Energy dispersive X-ray spectroscopy indicates coating-induced agglomeration caused by the reaction of alkali metals with silica. Scanning electron microscopy under high magnification revealed a layered structure to the agglomerates, where ash particles are subsumed into a fused material. This suggests the formation of agglomerates by the three step agglomeration process postulated by other authors. Analysis of indices used to predict agglomeration on the basis of a fuel's ash content and composition indicates that the Alkali Index is the most accurate, successfully predicting agglomeration for 7 of the 9 fuels where agglomeration was observed.
      1024Scopus© Citations 30