COMBUSTION AND GASIFICATION OF COALS IN FIXED-BEDS

Fixed-bed processes are commercially used for the combustion and conve rsion of coal for generation of power or production of gaseous or liqu id products. Coal particle sizes in fixed-bed processes are typically in the mm to cm diameter range, being much larger than in most other c oal processes. This review provides a broad treatment of the technolog y and the science related to fixed-bed systems. Commercialized and dev elopmental fixed-bed combustion and gasification processes are explore d, including countercurrent, cocurrent, and crosscurrent configuration s. Ongoing demonstrations in the U.S. Clean Coal Technology program ar e included. Physical and chemical rate processes occurring in fixed-be d combustion are summarized, with emphasis on coal devolatilization an d char oxidation. Mechanisms, rate data and models of these steps are considered with emphasis on large particles. Heat and mass transfer pr ocesses, solid flows, bed voidage, tar production and gas phase reacti ons were also considered. Modeling of fixed-bed processes is also revi ewed. Features and assumptions of a large number of one- and two-dimen sional fixed-bed combustion and gasification models are summarized whi le the details of a recent model from this laboratory are presented an d compared with data. Research needs are also discussed.