Devolatilization and Combustion of Coarse-Sized Coal Particles in Oxy–Fuel Conditions: Experimental and Modeling Studies

Abstract

A generalized unsteady-state kinetic model, coupled with all modes of heat transfer, was developed to describe the combined coal devolatilization and the subsequent combustion of the residual char under oxy−fuel condition in both O 2 −CO 2 and O 2 −N 2 environments. Experiments were conducted to validate the model, which was also found to predict the experimental data published in the literature well. The effect of coal particle diameter, temperature of the reactor, and oxygen concentration on devolatilization time was investigated. Peaks in devolatilization and char combustion rates and particle center temperature were studied, and the effect of different parameters assessed. Higher reaction time was observed in an O 2 −CO 2 environment compared to that in an O 2 −N 2 environment due to lower particle temperatures resulting from endothermic gasification reaction and the difference in thermo-physical properties. Simulation studies were carried out to generate temperature, carbon, O 2 , CO, and CO 2 contours to understand the char combustion reaction mechanism. The reaction started at the external surface of the particle, following unreacted shrinking core model with two zones; the solid product layer and the unreacted shrinking core, separated by a thin reaction front. Gradually, the reaction front thickness increases, leading to the shrinking reactive core model, consisting of three zones: completely reacted ash layer, partially burnt reacting char called reaction zone, and unreacted zone or slightly reacted char core. At the outset, O 2 cannot penetrate into the particle and CO produced near the surface diffuses out to the boundary layer, forming a thin flame. Subsequently, O 2 diffuses through porous ash layer into the char, and CO burns within the pores, with hardly any CO detected in the boundary layer as the particle temperature increases.

Atomistic Simulation of Coal Char Oxy-Fuel Combustion: Quantifying the Influences of CO2 to Char Reactivity

“…3,4 For example, the devolatilization atmosphere impacts the resulting char characteristics (morphology, specific surface area, chemical structure, and reactivity). 5,6 Char combustion is the rate limiting step for entrained pulverized combustion and is in the zone II regime (chemical/diffusion limitation) for the most practical systems. The char kinetics and gas (internal/ external) diffusion are also influenced by the atmosphere change, and quantifying these differences will aid boiler design.…”

“…Oxy-fuel combustion is an emerging clean coal technology. , By using O 2 with recycled flue gas (mainly CO 2 , needed to lower the furnace temperature for material protection), the CO 2 sequestration readiness increases with lower NO x and SO x emissions. The change in the combustion atmosphere influences devolatilization, heat transfer, and char consumption. , For example, the devolatilization atmosphere impacts the resulting char characteristics (morphology, specific surface area, chemical structure, and reactivity). , Char combustion is the rate limiting step for entrained pulverized combustion and is in the zone II regime (chemical/diffusion limitation) for the most practical systems. The char kinetics and gas (internal/external) diffusion are also influenced by the atmosphere change, and quantifying these differences will aid boiler design.…”