Laser pyrolysis of single coal particles in an electrodynamic balance

Abstract

A novel method has been developed to study the rapid devolatilization behavior of single coal particles. The objective was to establish a general understanding of the controlling mechanisms for coal devolatilization and to determin some characteristic times for volatile evolution at energy fluxes representative of heat engine combustors. Single coal particles were suspended electrodynamically and irradiated from two sides with well characterized energy pulses. Heavy volatiles (condensible) evolution during the heating process was clearly time resolved using high-speed cinematography. Experiments were performed on two particle size fractions (30 and 60 μm nominal radius) of a HVA bituminous coal. Heating times were on the order of 10 ms with heating flux intensities ranging from 0.5 to 2.5 kW/cm2. The equivalent heating fluxes (corrected for geometric and heat loss considerations) ranged from 0.2 to 0.8 kW/cm2 which translated to heating rates on the order of 5 × 104 to 2 × 105 K/s. Several characteristic times were defined to describe different phases of the devolatilization process. Important conclusions were that pyrolysis times (through completion of the heavy volatile evolution) scaled with the incident heating flux and were particle size dependent. The devolatilization process was divided into three phases. Phase I represented the time required to heat the particle to the temperature at which light volatile (noncondensible) evolution commenced. This phase was particle size and heat flux dependent with the most important consideration being the particle surface temperature. Phase II involved softening of the particle surface layer. This phase was particle size independent but was strongly dependent on the incident heating flux. Phase III was the time over which heavy volatiles (condensible tars) release occurred. This phase was obviously dominated by bubble growth and internal transport mechanisms. The heavy volatile release was not continuous but occurred in bursts, as tar vapors bubbled through the plastic coal shell. Phase III was strongly particle size dependent and was only weakly coupled to the heating intensity. These results suggest a sequential model to describe coal devolatilization which considers heat transfer, chemical kinetics, and mass transfer mechanisms whose relative importance varies depending on which phase of devolatilization is considered.