Effects of Pyrolysis Conditions on Internal Surface Areas and Densities of Coal Chars Prepared at High Heating Rates in Reactive and Nonreactive Atmospheres

Abstract

Concern about comparability and validity of different methods for producing coal chars for research has motivated this investigation of the effects of devolatilization conditions on the physical properties of coal chars. It is evident from the findings of this study that care must be taken to prepare chars under conditions similar to those of full scale coal combustion boilers prior to performing char oxidation studies. Two different entrained flow reactors were used to prepare chars under a variety of different pyrolysis conditions at maximum particle temperatures between 840 and 1627 K and heating rates between 1O 4 and 2 × 1O 5 K/s. Under these conditions micropore (CO 2 ) surface area generally increases with residence time and mass release for lignite and bituminous coals, as does true density. Micropore surface area also increases somewhat with increasing maximum particle temperature and heating rate. Mesopore (N 2 ) surface area is most affected by reactive gas atmospheres (carbon activation). The presence of steam in the postflame gases of methane/air flat flame burners is a significant factor in increasing mesopore surface area of chars prepared in such burners, even though the mass conversion by steam gasification is small. Partial char oxidation with O 2 significantly affects char N 2 and CO 2 surface area at these heating rates and residence times (50-1000 ms), sometimes increasing and sometimes decreasing internal surface area. Low-rank lignite and subbituminous coals have higher potentials for forming chars with increased N 2 surface area than bituminous coals. The moisture 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 CO 2 surface area