THE DEVOLATILIZATION OF COAL AND A COMPARISON OF CHARS PRODUCED IN OXIDIZING AND INERT ATMOSPHERES IN FLUIDIZED-BEDS

This is a study of the devolatilization of coal in a laboratory-scale bed of silica sand, fluidized with either air or N-2 and electrically heated to 750 or 900 degrees C. Coal particles (diameter 1.4-1.7 or 2. 0-2.36 mm) were fed in batches to the surface of the bed and allowed t o devolatilize in either an oxidizing atmosphere of air or inert N-2. In the first case, combustion of the volatiles occurred, but there was only thermal decomposition (pyrolysis) in the second situation. The r esulting chars were recovered and analyzed for composition and structu re, so that comparisons could be made between the effects of devolatil ization with combustion and of pyrolysis in an inert atmosphere. It wa s found that the fractions of C and H in the char were only slightly s ensitive to the type of fluidizing gas used. The amount of nitrogen in the recovered char and also the devolatilization time showed no depen dence on the type of fluidizing gas, whereas BET areas were slightly l arger after combustion in air. It is concluded that these effects are small relative to other errors, inherent in experiments on coal combus tion, so that chars prepared in a bed fluidized by hot N-2 are very si milar to those formed during coal combustion at nominally the same tem perature. Equally, the overall composition of the volatile matter rele ased during combustion in a fluidized bed is the same as in pyrolysis in nitrogen. The effects of other parameters, such as the temperature of the bed, the flow rate of the fluidizing gas and the size of the co al particles are also discussed in detail. It is concluded that most o f the volatiles burn in a fluidized bed (at or less than 900 degrees C ) far away from the original coal particle. Also, NOx is in effect a p rimary product of devolatilization, being produced in appreciable amou nts when coal is heated in inert N-2. The ratio of C/N in the volatile s is found to be a constant during the latter stages of devolatilizati on, but beforehand at lower temperatures, carbon species are preferent ially released. Overall, devolatilization of small particles (< 2.4 mm ) in a fluidized bed at 900 degrees C is kinetically controlled. The a ctivation energy is small, being 15 +/- 6 kJ/mol.