Thermal Characterization of Coal Ash Powders: Heat Capacities and Minimum Sintering Temperatures
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THERMAL CHARACTERIZATION OF COAL ASH POWDERS: HEAT CAPACITIES AND MINIMUM SINTERING TEMPERATURES R. LEDESMA, P. COMPO and L. L. ISAACS Department of Chemical Engineering, The City College of CUNY, New York, New York 10031 Received 15 October, 1986; refereed ABSTRACT Ashed mineral matter from ground coals were prepared by combustion at 973 K. Five coal samples ranging in rank from lignitic to high-volatile bituminous coals were used as starting materials. The heat capacities of the coal ash materials were measured between 140 K and 900 K by differential scanning calorimetry. The minimum sintering temperatures of each of the ash powders were determined using a high temperature dilatometer. INTRODUCTION The use of coal either as a fuel for power generation or as a raw material in chemical processes is usually accompanied by generation of ash. The ash comes from the mineral content of the coal and may amount to as much as 15% of the total weight for soft coals. The ash contribution to the heat load must be accounted for in an energy balance. Most of the processes proposed for the conversion of coal to synthetic fuels (and feedstocks) require the use of fluidized bed type reactors. Since the agglomeration of ash particles in such reactors is an operating problem, the value of the minimum temperature for ash sintering becomes an important operating parameter. Because of the wide variation in the composition of coal mineral matter, it is necessary to ascertain how ash properties vary with the composition and the type of the coal. In this communication, the results of thermal characterization
experiments
on ashes
derived
from
low temperature
(973
K)
combustion of five different coals, specifically the results of heat capacity and dilatometry measurements, are reported. EXPERIMENTAL Ash Preparation A suite of five coals was obtained from the Pennsylvania State University Coal Bank. Their rank and other pertinent properties are listed in Table I. In Table II, the major mineral constituents of some similar type coals [1] are listed. The coals were ground with a ball mill and sized by sieving. The 250 mesh sieve fractions were used for the preparation of the ash powders. These were prepared by placing five gram portions of the coals into muffle furnaces which were preheated to 825 K, and then raising the temperature slowly to 973 K. The ashes were kept at 973 K for two to three hours to ensure total combustion. No indication of particle size coarsening was observed. The chemical compositions of the prepared ashes were not determined directly. Spectroscopic analysis of ashes obtained from coals according to ASTM procedures, reported as oxides, for the coals whose mineral constitution breakdown is given in Table II, indicate that they contain: 33-55 wt% Si0 2 , 10-25 wt% A12 03 , 5-15 wt% Fe 2 0 3 and 2-20 wt% CaO as their major constituents [1].
Mat. Res. Soc. Symp. Proc. Vol. 86. ý1987 Materials Research Society
128
Table I. Properties of Ashes and Coals. Total Mineral
Pyritic
Matter Content
Sulfur
Sample Ident
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