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The shape of the combustible loss curve for low load (Figure 32) is similar to the full load curve shape. The 8% and 10% wood co-firing for both clean and whole tree chips are essentially identical to the coal alone results. At 15% wood co-firing, losses are about

	6
								0 low
(%)	5							10% low
								8% low
Loss	4							15% low
								10% clean
Carbon
	3							15% clean
Unburned	2
	1
	0
	3	3.5	4	4.5	5	5.5	6	6.5	7

Economizer Out O2 (%)

Fig 32: Unburned carbon loss – low load for whole tree (WT) and clean chips

0.5 to 1.5 percentage points higher than for coal alone for both whole tree chips and clean chips. Commercial low load operation is performed at an economizer outlet O2 of about 6.0%, equivalent to a plant exit O2 reading of about 5.0%.

Efficiency – Moisture

Moisture losses are associated with the energy required to evaporate liquid moisture in the fuel. Because the biomass in these tests has much higher moisture content than the test coal, particularly on the basis of fuel energy, higher percentages of wood chip cofiring are associated with higher moisture losses. Wood chips moisture contents of 50% to 67% were measured. This is in comparison with coal moisture of about 6%. On a per energy basis, the wood has about 10 to 20 times the moisture as coal. This is reflected in increased losses.

A plot of moisture loss for full and low load is shown in Figure 33. Moisture loss for coal alone is about 0.6% of the fuel input. With wood co-firing, 10% wood averaged about 0.94% moisture loss with both whole tree and clean chips. Moisture loss with 15% whole tree and clean chips averaged about 1.2% and 1.1% moisture loss. Overall this appears like a small effect. However, if only energy from the wood itself were used to evaporate the water in the fuel wood, the moisture loss represents roughly about 10% of the wood input energy.

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	1.6
	1.4
	1.2
(%)	1
Loss
	0.8
Moisture
	0.6
	0.4
	0.2
	0
	3	3.5	4	4.5	5	5.5	6	6.5	7
				Economizer O2 (%)

0 full

8-10% full

10% clean

15% full 15% clean

Figure 33: Moisture loss – full and low load for whole tree (WT) and clean chips

Efficiency – Hydrogen Losses

When the high heating value of a fuel is measured, the water vapor in the combustion products from any hydrogen in the fuel is allowed to condense. The energy released as the vapor condenses is assumed to be available as part of the fuel’s energy. However, in most plant applications, the hydrogen-derived moisture in the combustion products exits the stack as a vapor. As a result, the energy of condensation is counted as a loss. However, there was no discernable difference in hydrogen loss calculation for the different fuels these tests.

Although the difference in hydrogen loss was not measurable for coal alone and coal/wood mixtures, it can be estimated from a theoretical basis. Calculations confirm that the effect is small. Addition of 10% and 15% wood increases the hydrogen losses by about 0.06 and 0.11 percentage points respectively.

Efficiency - Overall

The overall boiler efficiency as determined in these tests is shown in the results table. The overall efficiency includes the influence of the major losses noted above. In addition, estimates for minor losses for boiler radiation, inlet air moisture, and

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unmeasured losses were included. The boiler efficiency then is the difference between the sum of all the losses measured or calculated and 100%.

Figure 34 compares the full load boiler efficiency results plotted against furnace exit O2. The black line marks a curve for boiler efficiency with coal alone, and is a typical shape. The boiler efficiency in relation to combustion air is basically a balance between unburned carbon losses, which dominate at low O2, and dry gas losses, which increase with increased O2. Maximum efficiency in these tests with coal alone was obtained in the range of 4.7 to 5.2% economizer outlet O2.

	86
	85
(%)	84
Efficiency
	83				0 full
					10% full
Boiler
	82				8% full
					15% full
	81				10% clean
					15% clean
	80
	3	4	5	6	7

Economizer Out O2 (%)

Fig 34: Boiler efficiency versus O2 - full load co-firing for whole tree (WT) and clean chips

Boiler efficiency co-firing 8 - 10% green wood essentially followed the same curve. Nearly all the points fell within a tenth or two of the coal alone curve. The boiler efficiencies measured co-firing 15% wood were close to the curve but in general a little lower, with efficiencies between 0.0% and 0.7% less than with coal alone.

This difference seems fairly small. However, because the change occurs due to the addition of wood, it is reasonable to attribute the difference in loss entirely to the wood added to the fuel. For the case of 15% wood co-firing and an average of 0.35% loss in total boiler efficiency, this represents a loss of about 6% of the wood at full load. This loss should be accounted for in any assessment of green wood co-firing economics. Note that for 10% co-firing, no penalty need be assessed on the wood for efficiency loss.

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This result was surprising because the extraordinary moisture content of the wood chips in the co-firing fuel was expected to reduce boiler efficiency substantially. Unburned carbon in ash was expected to be higher due to the green nature of the wood as well. The moisture loss is basically calculated based on lab tests, and gave expected results. However, unburned carbon in ash was about the same for 8-10% co-firing, and only slightly less for 15% wood. The lower exit temperature in the stack and the resulting reduced dry gas losses essentially offset (for 8-10% wood) or nearly offset (15% wood) the cost of the other losses.

Fewer tests were run at low load, and the resulting data are much more scattered than for full load. The low load results are shown in Figure 35 and, in an attempt to understand the results, a curve based on the principles of the full load curve was superimposed. It appears from these results that boiler efficiency while co-firing 8-% to 10% is basically the same as for coal alone. The lowest points on the plot are tended to be 15% wood tests. These results suggest that the boiler efficiency for 15% wood would be between 0% and 1.2% less than for coal alone. If an average 0.6% loss is assumed this is equivalent to about 10% of the wood being lost at 15% co-firing due to reduced boiler efficiency.

	87
	86
(%)	85
Efficiency	84
	83	0 low
Boiler
		10% low
	82	8% low
	81	15% low
		10% clean
	80	15% clean
	3	3.5	4	4.5	5	5.5	6	6.5	7

Economizer Out O2 (%)

Fig 35: Boiler efficiency versus O2 - low load co-firing for whole tree (WT) and clean chips

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EMISSIONS

NOx

Overall, NOx values were about the same when co-firing wood, compared with coal alone. NOx measurements for each test are presented below in Figures 36 – 39, plotted with economizer outlet O2. As expected, NOx values rise smoothly with increased excess air in the furnace.

At full load with 8-10% wood, there seemed to be a small but consistent reduction of about 0.01 to 0.02 lb/MMBtu. There is considerably more scatter and fewer tests in the 15% wood results for full load, but the points fall fairly close and on both sides of the coal-alone curve. In both cases, the lowest NOx values were obtained with whole tree chips.

At low load with 8-10% wood, the co-firing points again follow the coal alone curve closely, with whole tree and clean chips having similar results. At 15% wood co-firing and low load, the NOx results are both more limited, and more scattered. The data for the whole tree chips follow the coal alone curve fairly well, while the clean chips fall slightly above it.

Many investigators have reported decreases in NOx with the addition of biomass to a coal-fired pulverized coal furnace. The contribution of energy from the wood chips in these tests, however, is a relatively small fraction, approximately 3-5%, of the energy in the furnace. Further, the measured nitrogen content of the wood tested here (0.97 to 1.67 lb N/ MMBtu) is close to that for this coal (1.23 lb N/MMBtu). The increased moisture content of the wood-coal mixtures reduced flue gas temperatures in the unit. However, based on discussions with Southern Company NOx experts, the reduction in thermal NOx due to increased moisture is believed small.

Therefore, based on these tests, any contribution to NOx changes due to co-firing green wood chips can be expected to be small. Given the scatter in the data that results from testing on different days with different operators and coals property variation, it is believed that the NOx emissions in these tests are about the same as for coal alone. There are indications that co-firing whole tree chips may have slightly reduced NOx emissions compared with co-firing clean chips. Some further small NOx reduction may be obtained when operational stability problems are resolved.

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	1
	0.9
(lb/MMBtu)	0.8								Coal
	0.7								10% WT
									8% WT
	0.6								10% clean
NOx
	0.5								Linear (Coal)
	0.4
	0.3
	3	3.5	4	4.5	5	5.5	6	6.5	7
				Economizer Outlet O2

Fig. 36 Full load NOx emissions, 8 – 10% wood

	1
	0.9
(lb/MMBtu)	0.8
	0.7								Coal
									15% WT
	0.6								15% clean
NOx									Linear (Coal)
	0.5
	0.4
	0.3
	3	3.5	4	4.5	5	5.5	6	6.5	7
				Economizer Outlet O2

Fig. 37 Full load NOx emissions, 15% wood

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	0.9
	0.8
NOx (lb/MMBtu)	0.7
	0.6
	0.5
	0.4
	0.3
	3	3.5	4	4.5	5	5.5	6	6.5	7

Economizer Outlet O2

Coal 10% WT 8% WT

10% clean Linear (Coal)

Fig.38 Low load NOx emissions, 8 – 10% wood

	0.9
	0.8
NOx (lb/MMBtu)	0.7
	0.6
	0.5
	0.4
	0.3
	3	3.5	4	4.5	5	5.5	6	6.5	7

Economizer Outlet O2

Coal 15% WT

15% clean Linear (Coal)

Fig. 39 Low load NOx emissions, 15% wood

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Carbon Monoxide

Carbon monoxide results are presented below for full and low loads in Figures 40 and 41. For most of the test range of economizer outlet O2, full and low load carbon monoxide measurements were low for both co-firing and coal alone operation. However, at the lowest tested O2 conditions, the CO began to rise, but the test data in this area are very limited. Based on this limited data, there were indications that at the lowest O2 conditions, CO may rise more rapidly with co-firing than with coal alone.

	1400
	1200										0 full
	1000										10% full
											8% full
CO (ppm)	800										15% full
	600
	400
	200
	0
	0	0.5	1	1.5	2	2.5	3	3.5	4	4.5	5	5.5	6	6.5	7
						Economizer Outlet O2 (%)

Fig. 40: CO plotted with Economizer outlet O2 – full load

	2 5 0 0
	2 0 0 0					0 low
						10% low
						8% low
CO (ppm)	1 5 0 0					15% low
	1 0 0 0
	5 0 0
	0
	0	1	2	3	4	5	6	7
				Economiz e r O utle t O 2 (% )

Fig. 41: CO plotted with Economizer outlet O2 – low load

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Some of the performance tests allowed for closer examination of the CO data. In some of these tests, point by point measurements were made of gas analysis, including CO, sequentially sampling each point in the measurement grid once during the test period. In some of the tests, however, the gas measurement grid samples were combined and the CO data of the combination sample were recorded at 10 second time intervals during the test. During any test, the boiler test conditions swing a small amount, giving a natural variation in O2 and CO for the unit. As a result, in these combined sample tests, the test as a whole could be broken into small sub tests. Results of this analysis for full load and low load data are shown below in Figure 42 and 43.

	6 0 0
	5 0 0
	4 0 0
							C o a l
CO	3 0 0						8 % S F W T
							1 0 % S F W T
							8 % S F W T
	2 0 0
	1 0 0
	0
	0	1	2	3		4	5
			Eco n o m iz e r O u tle t O 2
			\Fig. 42. CO sub-test results for full load
	3000
	2500
	2000						Coal
							10% S FW T
CO	1500						8% S FW T
							15% P W CT
							10% S FCTC
	1000						15% S FCTC
	500
	0
	0	1	2	3	4	5	6
			Econom ize r Outle t O2
			Fig 43. CO sub-test results for low load
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These plots suggest that CO with wood-coal fuel blends tended to be at least the same and generally higher than for coal alone. There is not a lot of consistency in these results regarding influence of wood chip type or percentage of wood in the mix. However, low load data for the relatively dryer clean chips suggest that in general the drier chips showed less CO increase than did the wetter whole tree chips. Unfortunately, no data were available on the effect of grind size on the CO.

Based on discussions with operations personnel, the unit is typically operated at about 4.5% O2 at full load and at about 5.0% O2 at low load. This matches well with the region of optimum boiler efficiency shown in Figures 34 and 35 above. It also means that the boiler is currently operated in an O2 condition in which the co-firing of wood will have little or no effect on CO emissions. Note that O2 in these discussions and those of the other sections was measured at the economizer outlet, and due to air leakage in the ductwork is probably higher than boiler combustion O2.

Particulate

Opacity was very low in these tests. The unit has an opacity limit of 20%. However, it is equipped with two electrostatic precipitators that, when firing good coal, keep the opacity near zero.

A plot of opacity readings from these tests is shown in Figure 44. The plot shows that opacity with and without wood co-firing was about the same. There is no indication of opacity change with co-firing.

This result is supported by particulate loading tests that were conducted by Sanders Engineering & Analytical Services, Inc. during one of the co-firing tests. A copy of their report is included in the appendix. When the unit was co-firing 10% long fiber whole tree chips, Saunders measured an ash particulate loading of 0.024 lb/MMBtu. This was less than but comparable to measurements taken earlier in the year with coal alone (0.041 lb/MMBtu.) Both measurements are well below the permit limit of 0.12 lb/MMBtu.

Other co-firing tests by Southern Company and others have sometimes shown a small increase in opacity with wood addition. This has typically been attributed to either fine particulate size or change in the color of the ash. However, measured particulate loadings have not been shown to increase. Ash content of the wood is very low, at only a tenth the ash loading of coal on a per MMBtu basis.

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