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  • 1. Davidsson, K.O.
    et al.
    Åmand, L.-E.
    Steenari, B.-M.
    Elled, A.-L.
    University of Borås, School of Engineering.
    Eskilsson, D.
    Leckner, B.
    Countermeasures against alkali-related problems during combustion of biomass in a circulating fluidized bed boiler2008In: Chemical Engineering Science, ISSN 0009-2509, E-ISSN 1873-4405, Vol. 63, no 21, p. 5314-5329Article in journal (Refereed)
    Abstract [en]

    The purpose of this work was to study different ways to mitigate alkali-related problems during combustion of biomass in circulating fluidized beds. Wood chips and wood pellets were fired together with straw pellets, while the tendency to agglomerate and form deposits was monitored. In addition to a reference case, a number of countermeasures were applied in related tests. Those were addition of elemental sulphur, ammonium sulphate and kaolin to a bed of silica sand, as well as use of olivine sand and blast-furnace slag as alternative bed materials. The agglomeration temperature, composition and structure of bed-ash samples were examined. The flue-gas composition, including gaseous alkali chlorides, was measured in the hot flue gases and in the stack. Particles in the flue gas were collected and analysed for size distribution and composition. Deposits were collected on a probe in hot flue gases and their amount and composition were analysed. Addition of kaolin was found to be the best method to counteract the agglomeration problem. The deposition problem is effectively counteracted with addition of ammonium sulphate, while kaolin is too expensive to be used commercially against deposits, and sulphur is less effective than ammonium sulphate.

  • 2.
    Johansson, Andreas
    et al.
    University of Borås, School of Engineering.
    Johnsson, Filip
    Leckner, Bo
    Solids back-mixing in CFB boiler2007In: Chemical Engineering Science, ISSN 0009-2509, E-ISSN 1873-4405, Vol. 62, no 1-2, p. 561-573Article in journal (Refereed)
    Abstract [en]

    The furnaces of circulating fluidized bed (CFB) boilers have substantially lower height-to-width ratios than most of the laboratory CFB risers from which experimental investigations on riser fluid dynamics are published. Therefore, it is uncertain whether literature expressions for prediction of the distribution of solids and the thickness of the wall layers, derived from investigations in laboratory units, are applicable to boilers. In this work, data from laboratory units are compared with boiler data, and the accuracy of literature expressions for prediction of solids distribution, solids flow and wall layer thickness is reviewed. It is found that the height-to-width ratio does have a significant impact on the flow properties. Correlations based on experiments in laboratory units show poor agreement when applied to boilers. Modifications of literature expressions for prediction of solids flow and solids distribution are proposed, and an improved expression is formulated for prediction of the thickness of the wall layer.

  • 3.
    Johansson, Andreas
    et al.
    University of Borås, School of Engineering.
    Johnsson, Filip
    Niklasson, Fredrik
    Åmand, Lars-Erik
    Dynamics of furnace process in a CFB boiler2007In: Chemical Engineering Science, ISSN 0009-2509, E-ISSN 1873-4405, Vol. 62, no 1-2, p. 550-560Article in journal (Refereed)
    Abstract [en]

    With the aim of understanding the dynamics of combustion, this work examines simultaneous fluctuations in fluid dynamic parameters and gas composition measured in a CFB furnace operated with coal as a fuel. The fluid dynamic parameters investigated are pressure and air flow entering the furnace. Gas composition was recorded by a zirconia-cell probe and a gas suction probe connected to a mass spectrometer having a high time resolution (10 Hz). The principal fluctuations detected are around I Hz and below 0.3 Hz. The fluctuations below 0.3 Hz mostly originate from variations in the fuel-feed rate. These variations create periods of reducing conditions caused by a momentarily high fuel input accompanied by a pressure rise in the furnace and a reduction of the air feed, which occurs concurrent with the release of an enhanced quantity of volatiles. Modelled pressure fluctuations based on the relation between volatile release and pressure in the furnace give similar pressure fluctuations as the measured pressure fluctuations, with respect to amplitude and characteristic time scale of the fluctuations. There is also a correlation between reducing conditions and the concentration of hydrocarbons. (c) 2006 Elsevier Ltd. All rights reserved.

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