EFFECTS OF PYROLYSIS CONDITIONS ON INTERNAL SURFACE-AREAS AND DENSITIES OF COAL CHARS PREPARED AT HIGH HEATING RATES IN REACTIVE AND NONREACTIVE ATMOSPHERES

Concern about comparability and validity of different methods for prod ucing coal chars for research has motivated this investigation of the effects of devolatilization conditions on the physical properties of c oal chars. It is evident from the findings of this study that care mus t be taken to prepare chars under conditions similar to those of full scale deal combustion boilers prior to performing char oxidation studi es. Two different entrained flow reactors were used to prepare chars u nder a variety of different pyrolysis conditions at maximum particle t emperatures between 840 and 1627 K and heating rates between 10(4) and 2 x 10(5) K/s. Under these conditions micropore (CO2) surface area ge nerally increases with residence time and mass release for lignite and bituminous coals, as does true density. Micropore surface area also i ncreases somewhat with increasing maximum particle temperature and hea ting rate. Mesopore (N-2) surface area is most affected by reactive ga s atmospheres (carbon activation). The presence of steam in the postfl ame gases of methane/air flat flame burners is a significant factor in increasing mesopore surface area of chars prepared in such burners, e ven though the mass conversion by steam gasification is small. Partial char oxidation with O-2 significantly affects char N-2 and CO2 surfac e area at these heating rates and residence times (50-1000 ms), someti mes increasing and sometimes decreasing internal surface area. Low-ran k lignite and subbituminous coals have higher potentials for forming c hars with increased N-2 surface area than bituminous coals. The moistu re content of low-rank coals may be more important than rank. Lignite with a high moisture content yields char with a significantly higher N -2 surface area than char prepared from lower moisture content lignite . However, initial coal moisture content has less effect on CO2 surfac e area.