Coal conversion submodels for design applications at elevated pressures. Part I. devolatilization and char oxidation

Abstract

Numerous process concepts are under development worldwide that convert coal at elevated pressure. These developments rely heavily on CFD and other advanced calculation schemes that require submodels for several stages of coal chemistry, including devolatilization, volatiles combustion and reforming, char oxidation and char gasification. This paper surveys the databases of laboratory testing on devolatilization and char oxidation at elevated pressure, first, to identify the tendencies that are essential to rational design of coal utilization technology and, second, to validate two well-known reaction mechanisms for quantitative design calculations. Devolatilization at elevated pressure generates less volatile matter, especially tar. Low-rank coals are no less sensitive to pressure variations than bituminous coals; in fact, coal quality is just as important at elevated pressure as it is at atmospheric pressure. Faster heating rates do not enhance volatiles yields at the highest operating pressures. The FLASHCHAIN ® predictions for the devolatilization database depict the distinctive devolatilization behavior of individual samples, even among samples with the same nominal rank. The only sample-specific input requirements are the proximate and ultimate analyses of the coal. There were no systematic discrepancies in the predicted total and tar yields across the entire pressure range. Char oxidation rates increase for progressively higher O 2 partial pressures and gas temperatures, but are insensitive to total pressure at constant O 2 mole fraction. Char burning rates become faster with coals of progressively lower rank, although the reactivity is somewhat less sensitive to coal quality at elevated pressure than at atmospheric pressure. An expanded version of the carbon burnout kinetics model was able to represent all datasets except one within useful quantitative tolerances, provided that the initial intrinsic pre-exponential factor was adjusted for each coal sample.