A Heated Grid Study of the Pyrolysis/Oxidation Competition During Ignition of Coal Dust

In the past as well as more recently, the hot grate method has been thoroughly used for coal combustion studies (pyrolysis, ignition, oxidation). Although not reconstituting exactly the physical conditions of coal dust firing in industrial facilities, the rather simple operation requirements and the high adaptability of this method characterize it as an interesting tool for parametric study of complexe phenomena. Typically, one of the latter is the competition between pyrolysis and subsequent volatiles oxidation and heterogeneous oxidation of the solid matrix during coal ignition ( the word "ignition" being taken here as the critical transition from slow oxidation in the kinetic regime to fast oxidation in the kinetic-diffusional regime). Amongst research people in that field, the conviction is growing that, under typical pulverized coal flame conditions, the ignition of the solid matrix may occur before pyrolysis becomes appreciable. Interesting studies of the effects of particle size and heating rate on the type of ignition (either "homogeneous" or "heterogeneous", terms which actually should be understood as "heterogeneous ignition occuring after substantial advancement of pyrolysis" , respectively "heterogeneous ignition occuring before appreciable devolatilization") have already been made e.g. by the research group of Aachen and the results have been explained at least qualitatively.