OCR Text |
Show percent, the CO level is 475 ppm, far above the desired limit of 150 to 250 ppm. Measurements taken above the excess 0~ value of 5.0 percent indicate that a baseline value of CO for this mixture approaches 200 ppm. Visible smoking of this mixture was not detected at any excess stack 0„ value. The data in Figure 5 represents one possible explanation of the curves presented in Figure 4. In this figure, the fixed carbon of the parent coals is plotted as a function of measured CO values obtained at an excess 09 value of 2.0 percent for all of the fuel mixtures. Similar curves are obtained at any other value of excess stack 0~ chosen. Note that as the fixed carbon content of the parent coal in the fuel mixture increases, so does the corresponding value of CO as measured in the stack gases. As previously discussed, this is an undesirable effect since combustion efficiency is adversly affected by increased CO loading in the stack. Certainly, from a combustion efficiency viewpoint, even though lignite has a much lower calorific heating value than the other coals studied, the fuel mixture of lignite and No. 2 fuel oil would be the best candidate alternative fuel mixture to replace conventional fossil fuels. Referring again to the data of Figure 4, it is possible to draw some further conclusions based upon the size of the coal particles used in the fuel mixture. In particular, note the two curves resulting from mixtures with the West Virginia coal; one at 15pm, the other at 44pm. The data suggests that as the coal particle size is reduced, the combustion characteristics of the fuel mixture tend to improve as is evidenced by the drop in CO at each corresponding excess stack 00 value. Considerable caution must be exercised however in making general statements regarding the optimum size of coal particles to be used in these fuel mixtures. Atomization and pumping characteristics must be considered in regard to the optimum coal sizing recommended in these luel mixtures. Note from Table 3, that as the West Virginia parent coal size is reduced from 44|jm to 15um, a corresponding fourfold increase in apparent viscosity is observed. At higher coal loadings, such an increase in viscosity may effect pumpability of the fuel mixture and atomization resulting in flow instabilities and/or decreased combustion efficiency. 15 |