Oxy-coal combustion is a promising clean-coal technology that may reduce global warming. Coal is likely to remain a vital source in any conceivable future energy scenario despite the importance of exploring renewable sources of energy. To mitigate global warming, CO2 emissions should be constrained by improving efficiency of power generation, transmission, distribution, and use, as well as through CO2 capture and sequestration. The low, nitrogen-diluted CO2 concentration in the exhaust gas is a major driving force for high costs of capturing CO2 from conventional air-blown pulverized-coal combustion. Burning coal in pure oxygen or in oxygen blended with recycled flue gases can produce a CO2-rich effluent, thereby lowering the costs of the direct compression and storage of the flue gas.
For technology development in oxy-coal combustion to reach its full potential, this study provides a fundamental understanding of the process using single coal particles, under pertinent conditions. Combustion kinetics of coal in an O2/CO2 atmosphere rather than O2/N2 is important because CO2 can (1) react with coal, unlike N2, (2) influence temperature through the differences in its transport properties and heat capacity relative to nitrogen, and (3) inhibit vaporization of refractory oxides of ash through the reversible reaction MO + CO = M + CO2, where M might be Fe, Mg, or SiO. This work applies all of the experimental and analytical tools developed over years of study in conventional coal/air combustion to the domain of oxy-coal combustion, and addresses important fundamental questions: Do coal particles burn hotter and faster in oxy-coal combustion Are pollutant emissions enhanced or reduced. Burnout histories of individual coal particles are observed, with multi-color optical pyrometry, and combustion temperatures and reaction rates are deduced. Because maximum coal-particle temperatures can exceed maximum gas temperatures by as much as several hundred degrees, knowledge of particle temperatures is important for evaluation of overall furnace efficiencies, furnace exit gas temperatures, as well as slugging and fouling tendencies of ash in a boiler. Furthermore, in this work, combustion-generated particulates and other pollutants are monitored in the effluent gases of oxy-coal combustion and are compared in character to those from the conventional combustion process. Because the strategies for handling pollutant emissions from oxy-coal plants are still in their formative stages, these data will be especially valuable.
