K 2 CO 3 ‑Catalyzed CO 2 Gasification of Ash-Free Coal: Kinetic Study Jan Kopyscinski, † Rozita Habibi, † Charles A. Mims, ‡ and Josephine M. Hill* ,† † Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive Northwest, Calgary, Alberta T2N 1N4, Canada ‡ Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada * S Supporting Information ABSTRACT: The kinetics of K 2 CO 3 -catalyzed CO 2 gasification of ash-free coal was investigated with a thermogravimetric analyzer and compared to raw coal and uncatalyzed ash-free coal. At 750 °C, the gasification of ash-free coal dry mixed with 20 wt %K 2 CO 3 was approximately 3 and 60 times faster than the raw coal and ash-free coal without catalyst, respectively. Increasing the amount of catalyst from 20 to 45 wt % increased the gasification rate 3-fold. The gasification rate of ash-free coal containing potassium catalyst strongly depended upon the pretreatment (i.e., heating gas atmosphere and heating time) because it directly affected the degree of catalyst reduction. The catalytic gasification behavior could only be predicted with the extended random pore model, whereas the random pore model and integrated model were essentially equal for fitting the gasification rate for raw and ash-free coal. The activation energy for the catalyzed ash-free coal gasification was approximately 100 kJ mol -1 larger than for raw coal and the uncatalyzed ash-free coal. This increase might be due to the energy required for the potassium (i.e., catalyst) transfer to a new carbon site or caused by the pyrolysis process, because the formed char might have different properties. 1. INTRODUCTION Since early 2000, research on catalytic gasification has again become prominent especially for coal, 1-3 petroleum coke, 4-7 biomass, and their mixtures 8 for the production of hydrogen, methane, and/or synthesis gas. With the oil crisis in the 1970s and 1980s, much work has been done on catalytic coal gasification. 9 A few pilot plants were constructed during this time, but no commercial catalytic gasifier was ever build. 10 The main reasons might be of political-economic nature as the oil crisis ended, but technical issues, such as catalyst deactivation, might also have played a role. Alkali (e.g., potassium and sodium) and alkali earth (e.g., magnesium and calcium) metals, nickel, iron, and other metals have been used as catalysts to promote the gasification reaction of coal and other carbon sources. 9 Today, special attention has been given to co-feeding biomass species, such as switchgrass, which are rich in alkali and alkaline earth metals. Here, potassium naturally present in the switchgrass ash catalyzes the gasification of coal and/or petcoke. 11 However, these catalysts, especially potassium and calcium, deactivate during the process as these components react with alumina- and silica-containing mineral matter from the coal ash to form stable potassium or calcium aluminosilicates. 11-13 Thus, when the ash content (<1 wt % dry basis) of the coal is reduced prior to gasification, this deactivation can be reduced or even avoided. 3,14 The active catalyst would stay in the gasifier, while new “beneficiated coal” would be fed to the reactor. By doing so, the amount of catalyst needed could be reduced significantly. Several research projects in Japan (hyper-coal), 15 Australia (ultra-clean coal), 16 and Canada [ash-free coal (AFC)] 14,17,18 are underway to study the production of the beneficiated coal and its combustion and gasification behavior. During catalytic gasification, the catalyst undergoes an oxygen transfer cycle, in which the catalyst is reduced and oxidized. 9,19,20 The catalyst, potassium in the present case, takes oxygen from the reaction gas (in this case, CO 2 ) (eq 1) and transfers it to the surface where oxygen reacts with carbon to form carbon monoxide (eq 2). + ↔ − + KC CO KC O CO (site) 2 (1) − → + KC O K(s) CO (2) + → K(s) C KC (site) (3) KC represents a generalized site with proper potassium-carbon contact, such as a -COK complex, with an unknown stoichiometry. The third step (eq 3) symbolizes site regeneration, which requires a certain potassium mobility, designated here non-specifically as K(s). Despite numerous studies in this area, the interaction between the catalyst and carbon and the type of reactive surface intermediate are still debatable. 20 Phenoxide type 21 and K-oxide clusters 22 are the two favored surface intermediates. Moulijn and Kapteijn 22 and Freund 23,24 suggested that the catalyst accelerates the gas- ification reaction by increasing the number of surface oxygen and active sites at the carbon surface without changing the kinetic network and the activation energy dramatically. The studies dealing with hyper- and ultra-clean coals focused on gasification behavior only. No kinetic data for the catalyzed gasification of these coals have been published. Therefore, in this study, we determined the CO 2 gasification kinetics of ash- free coal with and without potassium catalyst and compared it to the corresponding raw coal. Besides the influence of the temperature and catalyst loading, the influences of the heating Received: March 28, 2013 Revised: July 2, 2013 Published: July 3, 2013 Article pubs.acs.org/EF © 2013 American Chemical Society 4875 dx.doi.org/10.1021/ef400552q | Energy Fuels 2013, 27, 4875-4883