Heterogeneous Kinetics of Coal Char Gasification and Combustion

A brief description is given of devolatilization of raw coal and combustion of the residualchar. Major achievements in the study of devolatilization and important areas for further experimentation are noted; however, attention is focussed on the rate processes involved in char combustion. Means of calculating reaction rates are shown which account for temperatures of particles, mass transfer and diffusion of oxygen into the particle's pore structure, and reaction on the pore walls. Theoretical consideration is then given to the changes in the size, density, and pore structure of chars as they burn. Experimental data are shown for observed reactivities of coal chars corrected for external mass transfer effects, and intrinsic reactivities are reported for various coal chars and other carbons corrected for pore diffusion effects. Measured data on mass-transfer rates and changes in particle structure during combustion are given. Recommendations are made for further research on char reactivity and the manner in which pore structure develops during combustion. The need for combustion-oriented kinetics studies of coal develatilization is outlined.

The combustion rates of coal chars: A review

Determination of the soot absorption function and thermal accommodation coefficient using low-fluence LII in a laminar coflow ethylene diffusion flame

The ratio CO2/CO from oxidation of Spherocarb char has been measured over a wide temperature range making use of the electrodynamic balance where single particles are heated by laser irradiation but are immersed in room temperature gas. This has allowed measurement of the CO2/CO ratio formed by the heterogeneous reaction on the char surface for temperatures up to 1670 K. For these conditions, an exponential decrease with a temperature coefficient of 3100/K is found. The CO2/CO ratio is proportional to the oxygen partial pressure raised to a power of 0.21. These results are in substantial agreement with work reported at lower temperatures. At normal combustion temperature the CO2/CO ratio from heterogeneous reaction is less than 0.1. Gas phase oxidation of CO to CO2 near the char surface, however, can become important at high char temperatures, even in a gas maintained at room temperature, and can have an important impact on surface temperature. The temperature at which gas phase reactions begin to contribute to CO2 formation and surface temperature was found to be reduced by the presence of water vapor.

The products of the high temperature oxidation of a single char particle in an electrodynamic balance

The rate of burning of small concentrations of fine graphite powder has been measured in a flat, laminar weak methane-air flame at sub-atmospheric pressure. Measurements were obtained with a laser doppler system, that not only measured particle velocities, but also particle concentrations and size distributions through the flame. Particle temperatures were measured by the two colour method and gas temperatures with thermocouples. An appreciable increase in the oxidation rate was observed in the flame reaction zone, attributable to the reaction zone transient species, O, H, and OH. A computational study of the flame, with comprehensive chemical kinetics and detailed representation of the transport fluxes, yielded the concentrations of all species through the flame. The observed rates of graphite oxidation are kinetically explained in terms of rates of reaction of all species with graphite. The observed elevations of particle above gas temperature in the reaction zone are higher than would be expected if the heating were due only to reactions in which the carbon surface was attacked. Amongst other possible explanations there is that of heating by exothermic radical recombination on the carbon surface.

The oxidation of graphite powder in flame reaction zones

Measured temperatures of burning pulverized-fuel particles, and the nature of the primary reaction product

One of the key challenges with Space Solar Power (SSP) using low-aerial-mass-density sheet-like structures is the power matching of the photovoltaic (PV) power to the optimal power point for microwave signal synthesis and power generation for wireless transmission. This work demonstrates a power tracking technique that can be readily integrated within the Power Amplifier (PA) operation that concurrently maximizes the power extracted from the PV and the transmitted power. This is done by dynamically duty-cycling the PAs to operate them close to peak efficiency and modulating their supply-impedance presented to the PV. The system does not use inductors and adds no additional mass, which is critical to the practical and economic viability of SSP. It offers higher conversion efficiencies, reduced system complexity, and greater programmability over traditional DC-DC converter solutions.

A.B. Ayling

Papers

Measured temperatures of burning pulverized-fuel particles, and the nature of the primary reaction product

Mutual Power Optimization of Photovoltaics and Wireless Power Transfer for Space Based Solar Power