| Two pulverized subbituminous coals were burned in a 200,000 BTU/hr down-fired combustor designed to produce a self-sustaining, nonrecirculating flame. Samples of char and ash were collected isokinetically after residence times of 0.07 second, 0.2 second, and 2.4 seconds. Maximum gas temperatures ranged from 1310 to 1380;TEM analysis of ultrathin sections of the coal and char showed numerous spherical inclusions present in approximately one in four coal and char particles. The inclusions fell into three distinct size groups: 3 to 5, 30 to 50 nanometers, and greater than 60 nanometers. The inclusions coalesced and flowed through char pores in later stage char particles. Two distinct types of char formed, a highly vesicular type that often had surface-associated ash globules, and a more dense char that did not have surface-associated ash implying a relatively high ash shedding rate. Only the more dense char was present in the 2.4 second samples, implying a low reactivity.;The 0.07 second samples were 30-50% burned out. Sodium, magnesium, and calcium had vaporized from organic moieties and recondensed in the smallest particles (little redistribution of these elements occurred later). Sulfur was not redistributed during this stage of combustion and CCSEM analysis showed few changes in the mineral particles in one coal but was inconclusive for the other.;The 0.2 second samples were 91-96% burned out. Sulfur was vaporized during this stage of combustion. Significant changes in size and composition of discrete mineral phases occurred, with a number of calcium compounds forming.;The samples collected after 2.4 seconds each showed greater than 99.8% burnout. Some sulfur was recaptured. Further changes in size and particle types occurred but calculations show that interactions between ash particles occurred at the surface of remaining char and not through collisions of suspended particles. |
