Inhibition of steam gasification of char by volatiles in a fluidized bed under continuous feeding of a brown coal

Steam gasification of a Victorian brown coal was performed in an atmospheric bubbling fluidized-bed reactor with continuous feeding of the coal. The gasification converted no more than 28, 51 and 71% of the nascent char (on a carbon basis) at 1120, 1173 and 1223 K, respectively. The char recovered from the fluidized bed was, nonetheless, gasified toward complete conversion when exposed to steam in another reactor, in which volatiles from the pyrolysis were absent while interaction between the char and products from the gasification was minimized. Atmosphere created in the fluidized bed thus prevented the char gasification from taking place beyond upper-limit conversion. In the absence of volatiles, nascent char underwent gasification catalyzed by inherent metallic species and non-catalytic gasification in parallel. The non-catalytic gasification was greatly decelerated by the presence of H₂ in the gas phase due to its dissociative chemisorption onto free carbon sites forming H-laden carbon. H₂ was, however, not a so strong inhibitor as to terminate the gasification. It was rather suggested that much more H-laden carbon was formed through dissociative chemisorption of volatiles and/or chemisorption of hydrogen radical from thermal cracking of volatiles in the gas-phase, which resulted in prevention of the non-catalytic gasification. It seemed that the char was converted in the fluidized-bed mainly by the catalytic gasification, while the conversion was limited due to deactivation of metallic species within the char matrix and their release from the char.