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Fig 9. Wood/Coal mix on belt

Fig 8. Transporting fuel to the reclaim

fuel mix a simple calculation was done to determine what coal flow rate was required at a total 400 tons/hr mixed fuel rate to give the desired percentage of wood to coal. For example, to achieve 10% biomass mix, the desired amount of coal is 0.90 X 400 to give 360 tons/hr. The coal would be adjusted to achieve this rate. Then the wood was introduced and the wood rate increased until the belt scales read a total of 400 tons/hr. This procedure worked very smoothly, and sprinkled the wood nicely on top of the coal along the length of the belt to help make a uniform mixture.

Figures 8 and 9 show wood handling and the blend on the belt during the tests. A bucket loader used in initial tests was found to be unnecessary. In later tests, the bulldozer proved sufficient to feed the wood reclaim hopper.

The only drawback seen with this procedure using Gadsden’s existing coal reclaim equipment was that the wood flow would not feed below a certain minimum. For the Gadsden tests, this meant that the lowest wood concentration that could be

created with this system was about 8% by mass. The problem could be addressed in the future by equipping the reclaim hoppers with vibrators.

A quick check during the tests was made by a manual separation of wood from coal of a mixed 15% sample taken at the feeders. This sample showed wood weight percentage of 14.9%. This agreement is probably better than deserved, given the rudimentary nature of this measurement. However, during the tests, it served to verify that the wood input percentage to the unit was known .

Fuel samples were also taken in each test for later lab analysis, including higher heating value (HHV). Figure 10 shows a plot of actual fuel mix HHV compared with estimates calculated from coal and wood properties for different percentages of wood. There is considerable scatter in just the coal HHV, but overall the agreement between actual and predicted HHV is encouraging.

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	13000
	12500
	12000
Btu	11500
Fuel			Theoretical
	11000
	10500
	10000
	0	2	4	6	8	10	12	14	16

% wood

Fig: 10: Comparison of actual and theoretical fuel HHV

Feeders

Fuel flow rate to the Gadsden unit’s coal pulverizers is controlled with Stamet feeders, and are shown in the boiler diagram in Figure 11. These feeders are designed to handle a variety of difficult materials, but they control on fuel volume rather than fuel weight. The low density of the wood, together with the reduced HHV of the fuel mix compared with coal alone, therefore required an increased feeder speed.

This can be seen in a plot of normalized feeder rate with per cent wood co-fired, as shown below in Figure 12. The feeder rate is a relative measure of the fuel volume entering the unit, and in the figure it is normalized for load change by dividing by the steam flow rate. It is also normalized by comparison with volumetric flow at full load with zero percent wood. The curves for whole tree and clean blends are fairly linear, with the whole tree blends having a lower density. The curves show that at a constant unit load, increasing the wood percentage from zero to 15% increases the fuel volume by

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Bunker

Furnace/

Boiler

Feeder

Burners

Mill

Fig 11: Plant Gadsden Boiler

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about 23% with clean chips and by about 31% with whole tree. This measurement is reasonably consistent with the discussion of fuel volumes above.

These results indicate that because of the low energy density of the wood, coal feeders used for co-firing need to have sufficient capacity to move the additional volume of fuel. When considering co-firing, thought needs to be given to the fuel feeder maximum rate. Load limitations will occur if the feeders are unable to meter enough fuel volume to the mill. Whole tree chips will require a greater feeder capacity the clean chips.

Figure 12: Relative feeder rate versus wood concentration.

Bunkers

The Plant Gadsden coal bunkers are large storage bins inside the plant that store coal for immediate use in the furnace. Coal is loaded into the bunker from the top, and is retrieved from the bunker through three inverted pyramidal ports at the bottom.

The feeder rate measurements give a measure of the volumes of fuel blend required. The volume of the bunkers is fixed. Therefore, the reduction in energy density resulting from adding wood to the fuel increases the frequency of bunker refilling. The feeder rate suggests that for 15% blend, a reduction in effective bunker volume of 23% to 31% occurs with clean and whole tree chips respectively.

The other issue for co-firing is that the presence of wood mixed with coal increases the tendency of the fuel to bridge, to hang on the walls, and to rat-hole. In general, the higher the concentration of wood chips, the higher the tendency to resist flow. The largest effect seemed to be associated with pine needles in the fuel. Clean chips which contained little

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or no needles appeared to cause little problem at 10% or 15% mix. The whole tree chips were somewhat more difficult, requiring occasional air cannon and hopper pneumatic vibrator usage. The production chips were the most difficult to handle in the bunker; at 15% biomass concentration, air hoses and air lances were deployed to break up agglomerations of the fuel in the bunker. The presence and length of pine needles are believed to be the biggest factors in impeding bunker fuel flow. Based on discussions with fuel handling and operations personnel, however, it is believed that with the exception of the 15% production chips, the bunker flow, while difficult, was acceptable. It should also be noted that once the material reached the mill feeders, there were no problems observed transporting any of the biomass blends to the pulverizers and into the boiler.

Pulverizers – Milling Power

In previous tests, co-milling wood in the pulverizer (or mill) consistently results in higher mill power requirements, and this was observed in these tests. Figure 13 is a plot of average mill amps versus load, here presented with boiler steam flow. Ten percent wood addition by mass results in about 10 to 14% higher mill amps than with coal alone. As might be expected, tests at 8% wood resulted in somewhat lower power use. Mill amps at 15% wood co-milling were between 13 to 17% higher. These results are a little higher but relatively consistent with results obtained in other co-milling tests.

	320
		0% wood
	300	10% wood
		8% wood
Amps		15% wood
		Linear (0% wood)
	280	Linear (10% wood)
		Linear (15% wood)
Mill Total	260
	240
3
	220
	200
	300	350	400	450	500	550	600

Steam Flow (Klb/hr)

Figure 13 – Mill amps versus unit steam flow (3 mill operation)

The higher mill amps are due to a layer of wood material building up on the side of the pulverizer bowl. The mills in these tests are bowl mills, in which coal falls into a spinning bowl near the mill bottom. Three large spring-loaded rollers are located close to, but not touching, the inner walls of the bowl. Coal introduced into the bowl is forced

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between the rollers and the bowl, where it fractures into finer pieces. The wood, by its nature, does not fracture, but instead begins to line the bowl itself. This layer of material causes the rollers to push out against their springs, pushing the coal into the woody mass. It is believed that the coal pushing hard into the wood cuts the wood into fine particles for combustion.

Roller

Bowl

Wood

Deposit

The data from the mill linear potentiometers demonstrate that the bowl buildup is probably responsible for the mill amp increase with cofiring. These linear potentiometers were mounted on the spring-mounted roller arms, and recorded a measure of roller displacement and consequent spring compression in each test. The relative displacement results plotted together with mill amps in Figure 15, show a fairly linear relationship. The compression of the mill springs by the wood lining in the bowl increases the power required of the mill motors to turn the bowl.

Fig 14 Mill bowl with wood lining

	350.0
	330.0	0% Wood
	310.0	8% Wood
		10% Wood
amps	290.0	15% Wood
	270.0
mill	250.0
Total	230.0
	210.0
	190.0
	170.0
	150.0
	3.9	4	4.1	4.2	4.3	4.4	4.5	4.6	4.7

Relative Avg Roller Displacement

Fig 15. Relationship of roller displacement with mill amps

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The type and size of the wood chips at full load 10% wood did not seem to have a consistent effect on mill amps, as can be seen in Figure 16 below. At full load, the short fiber whole tree chips (including needles) required perhaps 5 percent less amps than the long fiber whole tree chips, which could reflect additional size reduction to be done on the wood chips. The amps with short fiber clean chips (without needles) were also about 5 percent more than the short fiber whole tree chips, suggesting that the needles are easier to process than the solid wood chips. At low load, however, the effects are reversed with short fiber whole tree chips having the highest mill amps. Given the scatter in some of the coal alone data, it is as likely that the difference seen here between types of wood are actually due to day to day variation in unit operating personnel and in coal properties.

	100
	95
	90
per mill	85
	80						coal
amps	75						short fiber
							whole tree
	70						long fiber
	65						whole tree
							short fiber
	60						clean chip
	250	300	350	400	450	500	550	600
				Steam Flow (klb/hr)

Fig 16: Mill amps – 10% wood

Pulverizer Fineness and Performance

Detailed pulverizer fineness measurements were conducted by Innovative Combustion Technologies (ITC) in support of these tests. Each day a full load fineness test was performed in an effort to determine the impact of wood in the fuel mix on mill outlet fineness and on mill operating parameters. The details of their study are reported in Innovative Combustion Technologies Report # 2k7-130 (Sharer).

ITC’s overall findings included:

1.“There was no indication that the wood chips or needles increased the potential for riffle or burner line pluggage.” This was an important finding, because early wood chip tests had produced fibers that caused severe fuel line blockage at the riffles. It appears that a major goal of this project has been achieved of reducing the size of the chips so that long fibers would not impede fuel flow.

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2.“Fineness on Pulverizer no. 6 varied significantly with feed rate/wood chip content. At the higher feed rates, passing 200 mesh decreased by approximately 12% whereas fineness on Pulverizers 4 and 5 changed by only 2% to 3% across the same range.” All the mills were recently rebuilt, and settings for each mill were the same. Why one mill should perform so differently is not known. There is no indication in mill amps or other data that mill number 6 was more heavily loaded with wood than the others.

Figure 17 Close Spacing of Riffle Distributor plates

3.“Dirty air balance was virtually unchanged test to test.” This implies that addition of wood does not affect the distribution of flow to the four burners served by each mill. It is also an indication that the riffles were not substantially plugged in the testing.

4.“Unit no 2 operators had problems during certain tests maintaining mill outlet temperatures while still maintaining mill inlet suction.” In particular, full load tests on November 8 (10% long fiber whole tree) and November 9 (15% production chip whole tree) experienced mill temperatures well below set point and mill pressures that bounced positive.

The problem of mill temperature is important for removing moisture from the fuel in the mill. Low moisture is required for better grinding and for better mixing of the fine particles with combustion air in the furnace. However, moisture content of the fuel increases sharply with the addition of wood. The coal itself had about 6% moisture. However, when blended with wood at 10% and 15% by weight, the resulting fuel mixes had 9% and 12% moisture respectively.

Evaporation of the coal moisture in the mill reduces the temperature there, and the hot air dampers are increased to boost the mill temperature. Unfortunately, the increased percentage of wood in the furnace reduces air heater temperatures, including mill hot air

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temperature. Once the hot air system is maxed out, then the temperature in the mill will decrease. If it reaches unacceptable levels, the load must be reduced. Maximum unit load in these tests is discussed in the next section, below.

It is important that negative underbowl pressure be maintained in the mill. This is to prevent coal dust from blowing out of the mill into the plant and to prevent contamination of the mill gearbox bearings.

However, increased (less negative) mill pressures were encountered with wood addition during testing. Initially it was thought that the increased volume of fuel material entering the mill was responsible for the pressure increase. That fuel volume increase is substantial. However, in later tests with coal alone, the unit was run at full load on two mills with even higher mass and volumetric loading on each operating mill without adverse pressure effect. There, the increased volume alone cannot be responsible.

Another major fuel factor contributing to the mill pressures with the addition of wood is the increased fuel moisture. Figure 18 shows a plot of mill under-bowl pressure with the rate of moisture entering the mill. The plot shows a steady increase of pressure with moisture flow. In this case, the two mill coal only test falls squarely in the data band. It is believed that the increased moisture from the fuel overwhelms the drying system. Since the fuel is not properly dried, more of it recycles into the mill and the mill inventory increases. As the inventory increases, the mill pressure increases to overcome the resistance of the material. In essence, the mill is being choked with fuel.

	1
(" H2O)	0.5
	0
Press
	0	Coal	0.5	1	1.5	2	2.5	3
	-0.5	8% WT
Underbowl
		10% SWT
	-1	10% LWT
		15% LWT
							2 Mill Coal Test
Mill
		15% PC
	-1.5
		10% clean
		15% clean
	-2

Mill Moisture Flow (klb/hr)

Fig. 18 Mill Under-bowl Pressure versus Mill Moisture Flow

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These results are consistent with the mill performance curves that show the effect of moisture on the mill maximum output. Figure 19 shows a portion of the curve for this Raymond RB-593 deep bowl mill. For the Gadsden mill, which produces 70% through 200 mesh screen at 26,000 lb per hour and 6% moisture fuel, the output should be

Fig 19: Section of Pulverizer Curve showing Performance Degradation with Fuel Moisture

essentially unchanged

when the moisture is increased to 9%. However, after that point, further fuel moisture increases can be expected to reduce mill capacity as the operating curve follows the bend in the moisture curve. For 12% moisture, which is the typical moisture with 15% wood, the curves predict a maximum fuel throughput of 24,800 lb per hour. This should be reflected in load derate at these conditions, which is described in the next section, below.

A copy of the mill performance curve will be found in the appendix.

Maximum Unit Load

Each test day, the operators attempted to set the unit operation at full load, which was at about 550,000 lb/hr steam flow. However, on many days when co-firing wood, full load had to be set at a lower load. Unit stability was a factor, but the main reason was because of problems with mill pressure and mill temperature. The effect of wood percentage on maximum unit load is shown in Figure 20 below. Maximum load was fairly unchanged

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