OCR Text |
Show distribution into the furnace. Typical field simulation runs with approximately 15 percent fines produced particulate loadings of approximately 1.1 to 1.3 pounds per million Btu. Higher levels of oxygen in the suspension phase produced slightly lower emissions probably due to more efficient carbon utilization in the suspension phase. Slightly higher emissions were produced at higher firing rates due to the higher gas velocities in the suspension zone. As expected, the lowest particulate emissions occur when the fines are removed from the coal feed; when the coal supply was limited to only 1.2 percent fines ( < .093 inches) the particulate emissions dropped to only 0.23 pounds per million Btu, a reduction over the base case of 81 percent. Figure 8B shows the reduction in particulate emissions possible utilizing staged combustion and/or fines removal for simultaneously controlling NO emissions, the particulate emissions under the base, commercial-practice case are compared to emissions obtained under optimum furnace stoichiometry distribution and 0, 15, and 30 percent fines content of the fuel. As expected, the particulate carry-over increases in proportion to the fines percentage, but the concepts for NO control also led to reductions in particulate carry-over as high as 67 percent for identical fines percentages. In order to evaluate the extent of carbon utilization in the suspension phase, several of the particulate samples collected were characterized in terms of their chemical constituents, the results of these chemical analyses indicate that hydrogen evolution is essentially complete in all the coal particulates. The levels of nitrogen in the particulates indicate that nitrogen evolution rates have greatly lagged hydrogen evolution but slightly exceeded carbon evolution rates. These results are in qualitative agreement with the work of Starley et al. (6), in which fuel utilization in the suspension phase was studied in a vertical tube furnace. The data also indicate that carbon content generally increased with increasing particle size, indicating that slower rates of heating and combustion in the large particles outweighed the longer suspension phase residence times which the larger particles would have experienced. Staging the fuel bed to produce fuel-rich conditions did not have a significant effect on carbon content in the particulates. 25-20 |