Destruction of Tar During Oxidative and Nonoxidative Pyrolysis of Bituminous Coal in a Fluidized Bed

Abstract

The yields of tar on devolatilization of bituminous coal were measured in a small scale fluidized bed reactor, under conditions pertinent to large scale bubbling atmospheric pressure fluidized bed combustors(AFBC). The tar was collected by cooling the entire product stream from the reactor using a water-cooled quartz tube, followed by a filter for particulate matter, and a polystyrene sorbent (XAD-2) trap for recovering lighter species. The condenser, filter, and XAD-2 were extracted with dichloromethane. The extracted tar is expected to be the source of, or to contain the species which, if not destroyed within the bed and freeboard of a boiler furnace, would be present as polycyclic organic matter in the effluent gas and particles. These compounds are of special interest because of their possible adverse health effects. Silica sand, magnesium oxide, calcium sulfate, calcined Reed limestone, and partially sulfated Reed lime from a larger, continuously fed AFBC were used as bed materials. Coal and bed particle sizes, bed temperature, bed depth, oxygen concentration, and superficial velocity were all varied in the experiments, Variations in the activity of the stones with the cumulative amount of coal fed were also examimed. Under conditions of pyrolysis in a bed of partially sulfated lime in the absence of oxygen, the yields of tar were observed to decrease with increasing bed particle size and bed depth. The yield was a minimum at bed temperatures of 1000 to 1050 K. The yield was not sensitive to superficial gas velocity or coal particle size. The yields were lowest in the presence of the calcium-containing stones, and greatest in the presence of magnesium oxide and silica sand. The yield of tar decreased with increasing oxygen content of the fluidizing gas in the presence of all of the bed materials investigated.