Combustion of a droplet of coal-water mixture heated by laser beams

Abstract

A heating system using a CW argon ion laser beam has been used simultaneously with a two color pyrometer and a high speed camara to study different steps occurring during the combustion of a coal water mixture (CWM) droplet, positioned around a microthermocouple junction. Experiments have been performed using by-products of coal processing (schlamms) from Freyming-France coal mixed with 38% water. Four distinct steps have been characterized including: water evaporation, devolatilization, volatile combustion and heterogeneous char combustion. A detailed parametric study has been made. The main results are as follows: the duration of heterogeneous combustion does not depend on the heating power, but surprisingly, this duration is increased when the particle size of droplet constitutive coal decreases. This is attributed either to ash effect or to porosity evolution. Ash does not influence the evaporation and total heterogeneous combustion durations. A computer model has been developed to describe the different steps. The water evaporation and fixed carbon combustion are assumed to be mass diffusion controlled, the combustion following a shrinking core model. Good agreement is obtained between computed and measured parameters.