Near-extinction and final burnout in coal combustion

Abstract

The late stages of char combustion have a special technological significance, as carbon conversionsof 99% or greater are typically required for the economic operation of pulverized coal-fired boilers. In the present article, two independent optical techniques are used to investigate near-extinction and final burnout phenomena for Illinois #6 and Pittsburgh #8 bituminous coals. Captive-particle image sequences, combined with in situ optical measurements on entrained particles, provide dramatic illustration of the asymptotic nature of the char burnout process. Single-particle combustion to complete burnout is seen to comprise two distinct stages: (1) a rapid high-temperature combustion stage, consuming about 70% of the char carbon and ending with near-extinction of the heterogeneous reactions due to a loss of global particle reactivity, and (2) a final burnout stage occurring slowly and at lower temperatures. For particles containing mineral matter, the second stage can be further subdivided into (2a) late char combustion, which begins after the near-extinction event and converts carbon-rich particles to mixed particle types at a lower temperature and a slower rate, and (2b) decarburization of ash—the removal of residual carbon inclusions from inorganic (ash) frameworks in the very late stages of combustion. This latter process can be extremely slow, requiring over an order of magnitude more time than the primary rapid combustion stage. For particles with very little ash, the loss of global reactivity leading to early near-extinction is believed to be related to changes in the carbonaceous char matrix, which evolves over the course of combustion as a result of simultaneous oxidation and heat treatment. More realistic models are needed to predict the asymptotic nature of char combustion and to make accurate predictions in the range of industrial interest.