Does a large coal particle in a hot fluidised bed lose its volatile content according to the shrinking core model?

Abstract

Near-spherical particles (diameter ∼ 14 mm ) of six different coals (ranging in rank from an anthracite to a lignite) were immersed in turn in an electrically heated bed of sand fluidised by nitrogen. Such a bubbling fluidised bed ( U / U mf = 3.0 ) was held at a fixed temperature between 750 and 950 °C. A coal particle in such a situation decomposes in the overall reaction: coal → volatile matter + char , under conditions of pyrolysis, i.e., in the absence of oxygen. Particles of coal were extracted from the hot bed and quenched in gaseous nitrogen. It was found that, apart from the lignite, a coal particle fragmented. Thus an anthracite shattered into pieces soon after entering the hot bed, but a bituminous coal fractured into small lumps of char just after devolatilisation had finished. On each occasion the particles removed from the bed were individually weighed and counted. Also, bituminous and lignite particles were withdrawn from the hot bed before any fragmentation had occurred; they were then quenched in N 2, weighed, sectioned and examined in a microscope. Apart from the final stages of devolatilisation, these particles were seen in this way to have a central core of virgin coal, surrounded by char. The boundary between these two regions was sharp, indicating a shrinking core. The velocity of the boundary was found to be in effect constant; this conclusion was confirmed by measurements of a coal particle's mass at increasing times. All these observations indicate that pyrolysis of these relatively large particles proceeds with a shrinking core. A model for the process involves heat transfer from the fluidised bed to the coal particle's exterior, followed by heat conduction through the outer layer of char to provide the enthalpy required for endothermic thermal decomposition in the moving reaction zone. This model predicts that, apart from the very beginning and end of devolatilisation, there is an almost constant velocity for the shrinking core of raw coal. The quantitative agreement of the model with the observations is not perfect in every respect, but indicates that a bituminous coal in effect loses its volatile matter at ∼ 550 ± 100 ° C . The apparent enthalpy change for the thermal decomposition of a bituminous coal is deduced to be in the range 2.4 to 3.1 MJ/kg of coal and the thermal conductivity of the resulting char is 1.7 ± 0.9 W m −1 K −1 . Also, t d , the time for complete devolatilisation, depends on a coal particle's radius, a, according to t d = constant 1 × a 2 + constant 2 × a .