The impact of particle size and maceral segregation on char formation in a packed bed combustion unit

Abstract

Highveld parent coal was crushed into three size fractions, namely: 5 mm–75 mm, 5 mm–53 mm, and 5–37.5 mm. The crushed samples were subjected as feed coals to heating in a packed-bed reactor to investigate the influence of particle size reduction on char formation and reactivity. Coal petrography was utilized to assess the maceral and char formation distribution of the feed coal samples and their packed-bed combustion unit’s products. The maceral distribution of the feed coal fractions differed from the typical run-of-mine Highveld coal petrographic composition; the smallest size fractions ( 53 mm and 37.5 mm) having the highest vitrinite content. Maceral distribution was further divided into total reactive maceral particles, total inert maceral particles, and total inertinite particles. The 53 mm and 37.5 mm feed coal samples had the highest total reactive maceral particle content. Inert char particles dominated in the packed-bed combustion unit samples due to high inertinite maceral group content of the Highveld coals. Unexpectedly, the 53 mm feed coal sample had higher content of total reactive maceral particles and lower content of total inert maceral particles; whereas the 37.5 mm feed coal sample had high content of reactive maceral particles and high content of total inert maceral particles. This variation in maceral group content lead to the 53 mm feed coal sample being more reactive (producing more devolatilized and porous chars and thus reacting faster with reactant gases) than the 37.5 mm feed coal sample. This was due to inert maceral particles restricting the 37.5 mm feed coal sample from fully softening and reacting with reactant gas. This was also this was attributed to variation in volatile propagation of the three particle sizes. This confirms that a feed coal with smaller particle sizes results in different reactivity, char formation, and better heat transfer during combustion than the feed coal with large particle size range. Another important factor that plays a role in combustion is maceral association; it was observed that maceral distribution has a great influence on the char formation and its reactivity more than coal particle size.