| OCR Text |
Show 2. 1 .2 INTRODUCTION The volatile fraction of biomass fuels contributes a significant amount of the total energy released during combustion. For the optimal design of fluidized-bed and travelling-grate combustors, and gasifiers, it is essential to understand the mechanism of devolatilization and to be able to predict the transient release of volatiles in different atmospheric conditions. Major modelling efforts of devolatilization have been centred on coal /14/ and especially on small pre-dried coal particles. When the particles are large, as usually with biomass fuels, the particles are heated non-isothermally, and the transient conduction and convective heat transfer inside the particle greatly affect the pyrolysis. The pyrolysis of biomass involves a comlex series of parallell and successive reactions, and the local generation of volatiles is usually described by a simple Arrhenius type rate equation /1,2, 3,4,5/. In this paper it is assumed that the particles are large enough so that the diffusion of heat becomes the limiting step. Biomass fuels contain comparatively large amounts of water, and water removal before combustion is seldom done commercially. The moisture has a significant delaying effect on the release of volatiles. Inside a large particle the drying and the devolatilization take place locally in successice time periods, but when considering the particle as a whole, the drying and the devolatilization occur in overlapping time periods. The water content of biomass fuels feeded in the combustor usually varies with time, which complicates the control of combustion. Various mechanisms of drying have been reported in the literature /8/,/12/, a literature review has recently been published /12/. There seems to be lack of reported research on the simultaneous drying and pyrolysis even for coal /12/. The shape of the biomass fuel particles varies (wood chips, sod peat, biomass briquettes, pellets) depending on the way of production. The effect of the shape is considered in this paper. |
