Behavior of Inherent Metallic Species as a Crucial Factor for Kinetics of Steam Gasification of Char from Coal Pyrolysis

Abstract

Four pairs of raw and acid-washed brown coals were subjected to pyrolysis and in-situ steam gasification at 900 °C in fixed-bed reactors with or without forced gas flow through the bed of char particles. The chars from the acid-washed coals were gasified obeying a first-order kinetics with respect to the fraction of unconverted char regardless of the presence/absence of forced gas flow. Its presence/absence considerably influenced the gasification kinetics of the chars from the raw coals, even altering the manner of change in the rate of gasification with time and the char conversion, such as the apparent order of reaction. It was found that alkali and alkaline earth metallic (AAEM) species were allowed to stay in/on the char in the absence of the forced gas flow but volatilized extensively in its presence. Thus, the catalysis of AAEM species diminished in the presence of forced gas flow much quicker than in its absence, in which the catalysis was lost exclusively by intraparticle deactivation of AAEM species. The char conversion was quantitatively described as a function of time by a kinetic model that considered the progress of catalytic and noncatalytic gasification in parallel and also the loss of catalysis along with the progress of gasification.

Kinetics and Mechanism of Steam Gasification of Char from Hydrothermally Treated Woody Biomass

“…An equation with the same type of function as eq 6 can be derived by another kinetic model that considers the progress of non-catalytic gasification and AAEM-catalyzed gasification in parallel. 47,48 The rate of gasification is expressed by…”

Catalytic Coal Gasification

“…At high temperatures (900e1200 C), the dramatic volatilization of Na (up to 80%) was accompanied by increases in tar yield during the pyrolysis of the Na-form samples [30], as shown in Figure 5. Kitsuka [31] also proved that forced gas flow through the bed of char particles could diminish the amount of alkali metal during steam gasification at 900 C, thereby causing completely different changes in the char conversion over time. Quyn et al [10] suggested that the high mobility of Na allows it to migrate onto the char surface to catalytically active species such as eOeNa and/or Na clusters, thus improving char reactivity.…”

Calcium-promoted catalytic activity of potassium carbonate for steam gasification of coal char: Influences of calcium species

“…Catalytic carbon gasification with external alkali and alkaline earth metals (AAEMs) or with internal AAEMs in coal or biomass has been extensively documented [22][23][24][25][26][27][28][29]. It is well known that calcium species are, in general, not catalytically remarkable in gasification of medium-to-high rank coal at such a low temperature as used for potassium carbonate [20].…”