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 to 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 resulting chars were recovered and analyzed for composition and structure, so that comparisons could be made between the effects of devolatilization with combustion and of pyrolysis in an inert atmosphere. It was found that the fractions of C and H in the char were only slightly sensitive to the type of fluidizing gas used. The amount of nitrogen in the recovered char and also the devolatilization time showed no dependence on the type of fluidizing gas, whereas BET areas were slightly larger after combustion in air. It is concluded that these effects are small relative to other errors, inherent in experiments on coal combustion, so that chars prepared in a bed fluidized by hot N-2 are very similar to those formed during coal combustion at nominally the same temperature. Equally, the overall composition of the volatile matter released 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 coal particles are also discussed in detail. It is concluded that most of 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 primary product of devolatilization, being produced in appreciable amounts when coal is heated in inert N-2. The ratio of C/N in the volatiles is found to be a constant during the latter stages of devolatilization, but beforehand at lower temperatures, carbon species are preferentially released. Overall, devolatilization of small particles (< 2.4 mm) in a fluidized bed at 900 degrees C is kinetically controlled. The activation energy is small, being 15 +/- 6 kJ/mol.