The ignition temperature of lignite char in a fluidized bed combustor

Abstract

This paper summarises the experimental and modelling work carried out for the variation of bed ignition temperature of a fluidized bed combustor with the char particle diameter and the fluidizing velocity. A lignite char was used and its reactivity was represented using data from Field (1967) and Tumbull and Davidson (1984). The modelling involved solving the steady state heat balance around the fluidized bed combustor at the ignition temperature. A correlation of the total area of char ignited per unit bed mass was determined as a function of the char particle diameter and the fluidizing velocity. This correlation was used to determine the ignition temperature of the fluidized bed combustor operating at different conditions. The fluidized bed combustor heat balance was then solved for the bed ignition temperature which was influenced by both the rate of heat loss from the bed and the reactivity of the char. A sensitivity analysis suggests that the chemical rate reaction coefficient is the most prominent variable when determining the ignition temperature of a fluidized bed combustor.On presente dans cet article un travail experimental et de modelisation realise pour determiner la variation de la temperature d'allumage du lit dans un combusteur a lit fluidisee en fonction du diametre de particules du produit de carbonisation et de la vitesse de fluidisation. On a utilise un charbon de lignite, dont la reactivite a ete representee a I'aide des donnees de Field (1967) et de Tumbill et Davidson (1984). La modelisation necessite la resolution du bilan de chaleur en regime permanent autour du combusteur a lit fluidise a la temperature d'allumage. La surface totale de produit de carbonisation enflammee par matiere de lit unitaire a ete correlee en fonction du diametre de particules du produit de carbonisation et de la vitesse de fluidisation. Cette correlation a permis de determiner la temperature d'allumage du combusteur a lit fluidise pour differentes conditions. Le bilan de chaleur du combusteur a lit fluidisee a ensuite ete resolu pour determiner la temperature d'allumage du lit, qui etait influencee par le taux de perte de chaleur du lit et la reactivite du produit de carbonisation. Une analyse de sensibilite suggere que le coefficient de cinetique chimique est la variable la plus importante lorsqu'on determine la temperature d'allumage d'un combusteur a lit fluidise.Keywords: fluidized bed combustor, lignite char, ignition. luidized bed combustion is one of the three major methods F of burning a solid fuel in order to raise steam (Hoy and Gill, 1987). Fluidized bed combustors have numerous advantages over pulverized fuel and stoker fired combustors, such as lower NOx emissions, control of SOX emissions by the addition of limestone to the bed, and ease of fuel adaptability (Hoy and Gill, 1987; Prins et al., 1989). One of the main drawbacks of fluidized bed combustion is the large thermal mass required of inert material required by the bed. Since the bed must be preheated to the ignition temperature of the char u...