Transient high-temperature thermal deactivation of monomaceral-rich coal chars

Study of Activation of Coal Char

“…It cannot be stated with certainty whether the utility chars burned under Zone II or Zone III (external mass transport-control) conditions in the boiler, though the magnitude of the surface areas suggest the former-they are higher than those of fresh pyrolysis chars, and the very low areas of fresh pyrolysis chars would be maintained under Zone III conditions because oxygen cannot access any porosity. It has also been previously concluded, based upon other evidence, that Zone II is likely the dominant mode in pulverized coal combustion [1,21]. This char had been burned off to 11.9% extent, and its content of microporosity is lower than that for the 2.9% burn-off Zone I laboratory char, despite its considerably higher burn-off.…”

“…This is why use of oxygen in a combustion-type process is of limited commercial interest. Practical char combustion processes can be at least partially pore-transport controlled, even in pulverized coal firing [1]. Much of the practical-scale pulverized combustion that takes place in utility boilers is almost completely mass transfer controlled, meaning that the oxidizing gas (oxygen in that case) reacts as soon as it finds the surface of a coal/char particle and has no opportunity to penetrate to the interior and open up porosity there.…”

Thermal Annealing of Chars from Diverse Organic Precursors under Combustion-like Conditions

“…Char reactivity is influenced by precursor, degree of heat treatment, degree of oxidation, and the initial formation conditions including heating rate and gas environment. Extensive studies have documented the strong dependence of char reactivity on heat treatment history, but there have been relatively few studies focusing on the short times of relevance to combustion processes [1,2]. This paper presents measurements of reactivity loss in coal and alternate fuel chars as a function of heat treatment under combustion-like conditions.…”

Study of Activation of Coal Char

“…There is evidence that there is a transition to a partially pore-transport controlled regime, even in pulverized combustion in which external transport control dominates early on. Intrinsic reaction rate shows itself as a factor in determining final char burnout kinetics [1]. Though still debated, the majority opinion remains that the correct way to express char reactivities is on a surface area basis [2].…”