Tables 9 through 11 compare mill amps, mill outlet temperatures, and feeder speeds, for the seven tests conducted at full load, with three mills in operation and normal excess O2. Table No. 9 shows mill amps, Table No. 10 shows mill outlet temperatures, and Table No. 11 shows feed speeds.
Table 9. Comparison of Full Load 3 Mill 2.2 – 2.3% O2 Tests: Mill Amps
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Note from these tests that the composite fuel/coal blend material handled as a very wet coal. Regarding mill outlet temperatures, shown in Table 11, the B mill on the composite fuel averaged 131° during Test 18 and 134° during Test 24.
Mill outlet temperatures on Mill C were similarly depressed when firing composite fuel/coal blend. This is a condition consistent with wet coal. During the conduct of Te st 18, outlet temperatures deteriorated sufficiently that there was serious consideration of abandoning the test and going to four mills to raise mill outlet temperatures.
Mill feeder speeds similarly show significant differences between normal coal and composite fuel/coal blend, again with emphasis on the B and C mills. Note that the Test 18 showed the highest mill feeder speeds for the B and C mills although the baseline coal during Test 9 showed the highest mill feeder speed for D Mill.
Again, the higher speeder feeds associated with the composite fuel/coal blend indicate that the material handled as a wet fuel.
These results on mill outlet temperatures and mill feeder speeds are consistent with similar results that were obtained in 1994 when blending sawdust and hybrid poplar, at 3% biomass, with coal at the Shawville Generating Station. The influence of biomass in a blend making the fuel appear very wet is not inconsistent. It should be noted that during the entire test period significant
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thunderstorms did occur creating wetter than average fuel. This, however, is not an uncommon situation.
Tables 12, 13 and 14 compare mill amps, mill outlet temperatures and mill feeder speeds for the mid- load tests. Tables 15, 16 and 17 compare mill amps, outlet temperatures and feeder speeds for the low load or minimum load tests. Note the similar trends occur, although the outlet temperatures are sufficiently higher that no concern existed regarding the operability of the unit, as was the case when firing at 100% load and with three mills.
Mill Feeder Speeds
Test Fuel Mill Feeder Speeds (%)
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Tables 18, 19 and 20 compare mill amps, outlet temperatures and feeder speeds firing at full load but with four mills in operation. In Table 18 through 20 one coal pile test is compared to two composite fuel/coal blend tests. Note again that the mill outlet temperatures are lower when firing the composite fuel /coal blend.
This is particularly shown with B mill during Test 19. Note that mill feeder speeds are typically higher with the composite fuel/coal blend than with the coal pile coal, although again the situation is not comparable to operating with three mills, when the operability of the unit was somewhat questionable over a long period of time.
Test Mill Outlet Temperature (oF)
A Mill B Mill C Mill D Mill
11 Coal Pile 165.52 160.22 159.12 160.60
19 Composite Fuel 164.69 140.39 155.19 162.02
25 Composite Fuel 153.52 154.26 155.23 163.64
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Tables 21 through 27 show mill fineness test results when comparing full load testing at both three mill and four mill operation. Note that all tests showed excellent mill performance. The tests for the coal pile coal showed slightly lower performance, particularly with respect to the amount of material passing the 200 mesh screen, when compared to the PBS baseline coal or to the composite fuel/coal blend. This is a direct result of the lower Hardgrove Grindability Index associated with the coal pile coal. The conclusion regarding material handling and fuel preparation is that the composite fuel/coal blend handle “wetter” than normal wet coal. Wet coal is experienced by all fuels during this period of time.
The weather was, perhaps, more favorable to the composite fuel/coal blend than to the two coals. However not only did the composite fuel/coal blend exhibit slightly more moisture in the fuel blend, but it also exhibited handling characteristics of lower mill outlet temperatures and higher feeder speeds associated with wetter fuels. Beyond those measures, there was anecdotally more evidence of plugging during the composite fuel/coal blend testing than during the other fuel testing, particularly the coal pile coal testing. This is consistent with the fact that the composite fuel/coal blend had lumps only supplied by the coal pile coal to facilitate movement of the fuel. It is also consistent with the fact that the fuel behaves as a “wetter” fuel.
This “wet” condition is not unexpected. The pulp mill sludge consists largely of very fine short fibers. These fibers will attract and hold moisture. They create a condition where moisture characteristics are significantly exhibited.
Table 21. Mill Fineness Test, Test #1
Test #PBSBL-082703-1200-1500 Date: 8/27/03
Sample time: 13:15 - 13:29
Mill # B C D
Mill Draft -0.49 -0.64 -0.61 Exhauster Discharge 12.17 14.82 15.31 Mill Current (Amps) 41.53 38.43 44.61 Coal Air Temperature 142.56 151.47 154.04
Feeder Speed 78.04 78.74 89.39 Classifier Setting 3.00 3.00 3.00
% on 50 Mesh Sieve 0.40 0.40 0.80
% thru 100 Mesh Sieve 95.00 94.20 93.20
% thru 200 Mesh Sieve 78.20 77.80 76.80
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24 Table 22. Mill Fineness Test, Test #9