OCR Text |
Show DISCUSSION OF RESULTS The singular PSU-FCL plane flame furnace combustibility data in Tables V and VI allow the decoupling of the flame initiation and termination events in order to identify what is primarily responsible for the loss-of-combustibility upon devolatilization, and what is the extent of this loss. Table V presents the major findings from the coal ignition tests. It shows the impact of reduced volatile matter on ignition quality as characterized by an ignition delay time. As is quite evident in Table V, even a small reduction (35.0% to 29.5%) in the volatile matter content of the coal results in the development of an ignition delay time when there was none for the higher volatile (ca. > 35%) coal. Further reduction in coal volatility (29.2% to 6.5%) makes the ignition delay time substantial, i.e., a factor of 7 longer. Said another way, every percentage point loss in volatile matter resuts in an approximately 8 millisecond ignition delay, all other coal parameters equal. Although the specific candidate coal-water slurry coals listed in Table IV have not yet been tested in terms of reactivity in this laboratory, use of reactivity data 3 available in the published literature would appear to indicate that dramatic changes in reactivity would not be manifest. As Table IV does indicate, however, the volatility has undergone a dramatic change [6- fold reduction]. Thus, it should be deduced from these combustion tests that coal volatile matter content does indeed provide a useful measure of coal ignitability. The suspicion that the amount of hydrogen in a 12 coal may have an impact on its ease of ignition is not borne out by the data in Table V because the top six coals have a near-constant hydrogen content of 5 ± 0.6%. As the data in Table VI indicate, decreasing the coal volatile matter content by 80% results in a 1.6-fold increase in burnout time. Hence, coal volatility also plays a primary role in the overall coal combustion rate. Finally, decreasing the coal volatility over the stated range results in a 2.75-fold increase (206 to 567 milliseconds) in total flame residence time, which is directly proportional to the combustor volume required to contain the flame, and, hence, the capital cost of the combustor. Of the 361 millisecond increase in total flame residence time as coal 17-22 |