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CO-MILLING GREEN WOOD CHIPS AT

ALABAMA POWER COMPANY’S PLANT GADSDEN UNIT 2

Doug Boylan

Keith Roberts

Bill Zemo

Tom Johnson

EXECUTIVE SUMMARY

Biomass co-firing is the burning of biomass together with coal in an existing coal-fired power plant. Burning biomass is generally considered carbon neutral, and the co-firing technology may represent one of the lowest cost options for generating renewable energy in the Southeast United States.

Therefore Southern Company has been conducting a series of performance and operational tests on several co-firing technologies and fuels. This report describes tests on green wood chips processed from whole and from approximately needle free pine trees. Partners with Southern Company in this project include the USDA Forest Service, National Forests in Alabama, Forest Products Lab and Southern Research Station; Auburn University, Forest Products Development Center; Precision Husky; CAWACO RC&D; Forest Based Economic Development Services;, Center of Economic Development, University of Alabama; Reynolds Wood Products; and Temple Inland. The goal was to determine what level of processing of whole trees is required to successfully co-fire the chips in a boiler, and to evaluate the benefits and costs of wood processing compared with the effect of the fuel on the power plant.

Green wood chips were successfully co-fired in blends with coal between 8% and 15% wood by weight. Of the input fuel energy, 2.8% and 5.5% was from wood, and represented about 2.0 MW to 3.8 MW biomass power.

Sulfur emissions were reduced, and particulates were no higher with co-firing. There are questions yet to be resolved regarding the effect of co-firing on carbon monoxide. The measured impact on NOx emissions was small.

With 10% co-firing, boiler efficiency was about the same as coal alone, while there was a slight reduction in efficiency with 15% wood. This result was unexpected because of the high moisture content of the wood chips (50%-67%). However, higher moisture losses were offset or nearly offset by reduced dry gas losses up the stack.

There were operational problems which will require further assessment. High moisture of the wood coal mixtures resulted in low mill temperatures and high mill bowl under pressures. Consequently, operation at 15% wood co-firing with these fairly coarse and very wet wood chips resulted in a 5 % load derate. However, the most difficult problem was unit operating stability. The coal wood mixtures had much lower fuel energy to volume ratio than the baseline coal. The issues appeared to have been related to the

installation of new automatic controls. Stability issues were corrected in subsequent tests by tuning the fuel-steam pressure automatic controls.

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INTRODUCTION

Forest managers often need to clear whole trees from their lands, either for thinning, for removal of diseased trees, to reduce fire hazard, or to replace the type of trees growing there. This harvesting can result in large quantities of wood that often find no market for producing either lumber or paper mill chips.

Recently consideration has been given to using whole trees as fuel, particularly for generating electricity. If a market as fuel could be found for the trees, it could offer economic return to the forest owners. Since combustion of the wood would result in essentially no net CO2 addition to the atmosphere and based on a renewable resource, that energy so produced could be considered renewable. This could offer benefits to the power generating company.

A project was initiated by Southern Company, USDA Forest Service, Forest Based Economic Development Services, University of Alabama, and Auburn University to investigate the effect of wood processing on co-milling forest harvested materials in a coal fired power plant. The goal of this project is to evaluate the overall costs and benefits of harvesting forest trees for use as fuel to make renewable energy in existing coal fired power plants. Auburn University and the Forestry Department assessed the harvesting and processing of the wood. Precision Husky Corporation of Leeds, AL designed and constructed a wood chipper for this project to make relatively fine green wood chips at commercial scale rates. Pine trees were harvested from a plot in the Talladega National Forest, and were chipped in a number of ways to provide five types of fuels. Southern Company and Alabama Power evaluated the most economical means of generating electricity using these wood chips with the technology of co-firing.

In co-firing, a percentage of biomass is introduced as fuel into an existing coal fired boiler, often directly blended with the coal itself. The high capital cost of building a stand-alone power plant is avoided, and the technology offers the potential for high efficiency. It is a renewable technology most likely to directly displace coal.

The units, however, are not designed for this mixture of materials. The properties of biomass are quite different from those of coal. The wood density and heating value of the mixture are both less than in coal. Wood has a tendency to break into fibers than can affect material handling. Overall, the extent to which the fuels differ can impact the operations and performance of a power plant. Extensive studies have been conducted by Southern Company and others to quantify the costs and benefits related to co-firing various materials. Generally the drier and the finer the wood fuel, the easier it has been to co-fire.

Wood chips from forestry applications, however, would have larger size, higher moisture, and also include pine needles. These chips could be reduced to fine size and low moisture, but such processing is expensive. Therefore, minimizing the amount of processing would help to reduce final product costs, but the amount of required processing is not well known. The amount of size reduction required for co-firing, the

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effect of high moisture wood, and the maximum amount a typical coal-fired power plant could co-fire required research.

Therefore, as part of the forestry project, Southern Company and Alabama Power conducted co-firing tests of several wood chip grind sizes and compositions (i.e. with and without needles). About 1100 tons of wood chips from the Talladega National Forest Shoal Creek District were delivered to Plant Gadsden, an Alabama Power Company pulverized coal plant located in Gadsden, Alabama, The plant is served by two nominally 60 MWe Combustion Engineering tangentially fired units.

During October and November of 2007, the chips were co-fired in the Unit 2 boiler, and fifty-five performance, emissions, and operations tests were performed over a period of 3 weeks. This report presents the results of those tests.

OBJECTIVES

The objectives of this study were to determine under what conditions of size, composition, and moisture content that green wood chips can be successfully co-fired with coal, and how co-milling green wood chips affects power plant operations, materials handling, boiler efficiency, and boiler emissions. Test parameters included in the investigation include:

•Chip size of wood product

•Presence of pine needles

•Percentage of wood for co-firing

•Unit load limitations

•Combustion air rate

WOOD

Wood for these tests was harvested as part of a National Forestry Service Project from a field in the Talladega National Forest near Heflin, AL. The trees were nearly all whole loblolly pines of about 8” or less trunk diameter. For some of the tests, whole trees were used (“Whole Tree Chips.”) For several of the tests, most of the pine needles were removed from each tree by pushing the trees top-forward into a small stand of other pine trees. Chips made from pine trees with reduced needles were referred to as “Clean Chips.”

The trees were all chipped with a Precision Husky PH3045 Horizontal Grinder, powered with a 500 hp diesel engine. The machine blades could be adjusted to change the size of the chips produced. In any sample, there is a great variety of chip length, but in essence “Short fiber” chips were roughly 3/8” to ½” minus. Because producing smaller sized chips reduces the machine production and hence increases cost, two other grinder settings

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were also tried. One setting produced “long fiber” chips of roughly ½” to ¾” minus. In both of these cases, the pine needles when included were sized similarly to the chips. The third chipper setting allowed rapid production of the finer wood chips, but the pine needle pieces were much longer. A limited amount of these “Production Chips” was produced.

Representative samples are shown in Figure 1 to give a rough estimate of the relative chip size for each setting

Wood samples were analyzed for fuel properties at the Alabama Power Company Labs in Calera, AL. Moisture content measurements of chip samples immediately from the field were all within 2% of 50% moisture, including a sample of pure needles. Sample analyses including some density measurements are included in the appendix.

Figure 1 Comparison of wood chip sizes. (Sharer)

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INSTRUMENTATION AND MEASUREMENTS

Most of the boiler performance and emissions test data were measured and recorded by a test team from Southern Company’s Generating Plant Performance group. The boiler was instrumented with thermocouple grids and gas extraction grids upstream and

downstream of both air preheaters. Temperatures and gas samples were taken three depths on four ports (24 test points). At each of the two air preheater gas outlet ducts, temperatures and gas samples were taken at two depths on four ports (16 test points).

Gas samples were analyzed with continuous analyzers for NOx, CO, CO2 and O2. Gas analysis and temperature data were

Figure 2 Air heater outlet test – fly ash measurement automatically recorded with a

data acquisition system.

Linear potentiometers were installed on each mill to measure the relative deflection of the mill journal with the various fuels. These instruments monitored the bed of fuel in the pulverizer bowls, and were recorded for each test. The mill 6 potentiometer system failed soon after testing started, but the other two provided good information.

	Fig 3 Mill showing roller springs	Fig 4 Linear potentiometer for monitoring
		roller displacement

Standard plant operations data were also recorded

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on the plant’s computerized Operator Information System (OIS) during the test period. Control room data recorded manually included pulverizer pressures, pulverizer control settings, and other relevant data not taken on the plant OIS system. The plant continuous emissions monitor (CEMS) recorded CO2, NOx, SO2, opacity, and gas flow at the stack. CEMS data were recorded on OIS.

Fuel samples were taken at the feeder of each of the three mills at the beginning, middle, and end of each 30 to 40 minute test. Bottom ash samples were collected from the furnace bottom at the conclusion of each test. Fly ash samples were taken each test from both air heater gas outlet ducts by traversing in a four port by three depth grid with a high volume fly ash probe.

RESULTS

In all, 55 performance and emissions tests were performed. Data were collected bearing on boiler efficiency, NOx and CO, emissions, and on unit operations.

Results of the testing at the power plant are summarized in Table 1

Analysis was performed on the data relating operating parameters. It should be noted that in these tests, excess oxygen (O2) was measured at the economizer outlet. Air leakage in the ductwork probably will cause the reported values to be somewhat higher than typical expected operating boiler O2 levels.

DISCUSSION

Wood Storage

Between August 6 and August 12, 2007, wood chips were delivered in self unloading trailers to a section of the existing coal pile at Plant Gadsden. Each type of wood chip was stored uncovered, segregated in separate and well marked piles.

At the time of delivery, the wood had a very consistent moisture content of 50%. However, in September, 2007, despite a severe ongoing drought, the Gadsden area experienced several days of intense rain which thoroughly soaked the wood piles. Subsequent spot samples tested by the plant showed that many of the piles had a moisture

Fig 5 Wood chip delivery at Plant Gadsden7

Table 1: Test Results Summary

Test	Wood	Fuel Feeder	Wood	Est.	Steam Flow		CO	NOx	Opaci4ty	AH Gas In	AH Gas	Econ out
#	Chip	Rate	Wt	Wood	(Klb/hr)		(ppm)	(lb/MMBtu)		Temp (F)	Out	O2
	Type		%	Btu %							Temp (F)	(% dry)
1	Coal	53.57	0	0	564.2		57	0.54	0.85	713.8	387.9	4.93
2	Coal	54.16	0	0	562.4		274	0.45	0.60	678.5	382.7	3.76
3	Coal	53.23	0	0	565.3		40	0.70	2.30	710.4	387.2	6.06
4	Coal	33.54	0	0	340.2		894	0.36	-1.20	560.7	359.9	3.91
5	Coal	34.06	0	0	338.0		53	0.46	-1.14	582.1	355.9	5.00
6	Coal	34.03	0	0	344.6		38	0.58	-0.95	601.6	355.0	6.15
7	SFWT	63.45	10	2.9	558.3		450	0.48	0.60	682.1	367.0	4.15
8	SFWT	58.05	10	2.9	515.6		214	0.56	-0.20	674.2	371.6	5.29
9	SFWT	57.02	10	2.9	510.9		1257	0.45	-0.35	648.1	370.8	4.06
10	SFWT	55.15	10	2.9	513.9		123	0.78	3.58	696.8	378.0	7.20
11	SFWT	37.60	10	2.9	335.6		2098	0.35	-1.15	550.8	358.2	4.12
12	SFWT	35.68	10	2.9	335.3		51	0.71	-1.00	606.3	351.9	7.25
13	SFWT	58.99	8	2.2	537.4		208	0.50	-	657.5	367.7	4.56
14	SFWT	60.58	8	2.2	544.5		124	0.85	-	698.6	360.9	7.06
15	SFWT	60.22	8	2.2	544.0		87	0.59	-	675.2	358.1	5.58
16	SFWT	64.29	8	2.2	569.5		111	0.50	-	668.2	358.6	4.26
17	SFWT	64.85	8	2.2	568.7		92	0.73	-	703.2	365.8	6.14
18	SFWT	63.23	8	2.2	570.0		162	0.50	1.90	668.4	357.6	4.47
19	SFWT	63.41	8	2.2	549.1		23	0.75	1.70	695.9	366.8	6.16
20	SFWT	62.70	8	2.2	546.1		26	0.73	2.13	696.6	367.6	6.10
21	SFWT	38.24	8	2.2	330.7		1085	0.36	-0.78	547.7	346.4	4.02
22	LFWT	61.61	15	4.4	495.6		44	0.56	1.45	660.0	352.7	4.97
23	LFWT	61.53	15	4.4	498.9		57	0.68	1.50	692.8	357.8	6.28
24	LFWT	61.56	15	4.4	495.3		84	0.82	1.80	711.1	363.5	7.41
25	LFWT	42.70	15	4.4	333.0		33	0.77	-0.50	607.7	329.6	7.49
26	LFWT	43.36	15	4.4	328.4		33	0.65	-0.21	586.4	330.0	6.48
27	Coal	51.58	0	0	565.3		114	0.52	0.38	694.0	372.9	4.50
28	Coal	52.02	0	0	567.3		128	0.52	0.80	695.3	374.6	4.44
29	Coal	52.20	0	0	566.0		27	0.76	1.65	710.6	373.5	5.79
30	LFWT	60.90	10	2.9	540.8		34	0.56	0.40	680.9	357.1	5.12
31	LFWT	60.99	10	2.9	532.7		27	0.56	-0.39	672.9	355.7	5.02
32	LFWT	62.70	10	2.9	537.3		55	0.81	1.15	712.6	361.7	7.12
33	LFWT	42.83	10	2.9	345.4		1554	0.40	-1.60	557.0	357.8	4.23
34	LFWT	43.07	10	2.9	347.9		41	0.66	-1.24	602.8	353.6	6.30
35	PWCT	66.93	15	4.4	528.2		101	0.66	-0.25	676.0	346.3	4.88
36	PWCT	51.06	15	4.4	393.3		695	0.57	-1.79	608.1	346.5	5.50
37	PWCT	50.93	15	4.4	395.7		124	0.61	-2.07	600.5	347.0	5.13
38	PWCT	43.67	15	4.4	333.6		315	0.51	-2.15	566.6	355.2	4.88
39	PWCT	45.14	15	4.4	330.8		85	0.68	-1.91	595.3	351.9	6.52
40	SFCTC	59.11	10	3.5	536.6		127	0.52	-0.58	675.2	360.3	4.55
41	SFCTC	59.43	10	3.5	535.4		61	0.63	-0.85	685.8	360.9	5.48
42	SFCTC	59.48	10	3.5	537.4		71	0.69	1.88	692.1	362.0	5.92
43	SFCTC	39.08	10	3.5	327.2		147	0.50	-0.28	556.1	356.4	5.13
44	SFCTC	39.24	10	3.5	332.9		32	0.70	-2.24	589.8	353.1	6.78
45	SFCTC	63.60	15	5.5	537.9		42	0.54	-0.48	669.2	352.6	4.38
46	SFCTC	63.34	15	5.5	537.6		73	0.65	0.13	685.4	358.2	5.37
47	SFCTC	63.66	15	5.5	537.9		104	0.78	1.00	699.3	359.4	6.66
48	SFCTC	41.48	15	5.5	335.9		26	0.72	-1.93	583.6	346.0	6.62
49	SFCTC	43.63	15	5.5	341.6		44	0.54	-2.04	559.5	348.3	4.89
50	Coal	51.83	0	0	550.4		144	0.50	-1.05	671.9	363.7	4.35
51	Coal	51.65	0	0	555.0		93	0.60	-1.06	693.1	368.6	5.50
52	Coal	52.05	0	0	545.9		146	0.73	-0.73	719.7	375.6	6.49
53	Coal	31.53	0	0	319.7		42	0.67		577.1	375.0	6.01
54	Coal	30.98	0	0	304.7	8	36	0.67	-1.94	575.6	375.3	6.05
55	Coal	31.24	0	0	311.5		51	0.59	-2.07	566.0	381.1	5.46

Table 1: Test Results Summary (Continued)

Test #	O2Wet	Mill amps	Mill Roller	Amb Air	Dry Gas	Moisture	Hydrogen	Combust.	Boiler Eff.	Fuel HHV
		total	meas (“)	Temp (F)	Loss (%)	Loss (%)	Loss (%)	Loss (%)	(%)	AR (Btu/lb)
1	4.34	255.4	4.160	90.9	8.31	0.54	3.98	1.58	84.91	11968
2	3.31	254.9	4.141	94.4	7.50	0.67	3.95	1.92	85.27	12111
3	5.33	253.8	4.150	95.4	8.85	0.57	3.96	1.20	84.71	12278
4	3.44	159.6	4.104	101.1	6.36	0.80	3.86	4.15	84.15	11557
5	4.40	157.5	4.085	100.3	6.84	0.65	3.80	2.32	85.75	11975
6	5.41	156.9	4.084	99.9	7.49	0.61	3.84	1.91	85.48	11920
7	3.66	284.9	4.474	87.1	7.18	1.03	3.99	3.43	83.69	11184
8	4.65	272.2	4.439	90.5	7.88	0.93	4.03	1.71	84.76	11898
9	3.57	270.7	4.430	93.9	6.92	1.08	4.01	2.76	84.48	11585
10	6.34	268.0	4.365	96.0	9.13	0.82	4.03	0.94	84.33	12112
11	3.63	174.1	4.346	102.7	6.13	0.90	3.99	4.21	84.02	11541
12	6.38	171.3	4.355	101.3	7.89	0.63	3.87	1.02	85.88	12461
13	4.02	263.7	4.378	91.0	7.31	1.01	3.92	2.15	84.93	11664
14	6.21	269.8	4.401	95.8	8.32	1.12	4.03	1.23	84.53	11258
15	4.91	272.2	4.425	102.0	7.25	1.00	3.95	1.94	85.10	11303
16	3.75	279.4	4.457	104.4	6.55	1.11	3.88	2.71	85.00	11358
17	5.40	283.7	4.496	106.3	7.63	1.09	3.91	1.77	84.79	11270
18	3.93	290.3	4.570	94.3	6.82	0.91	4.01	2.57	84.98	11934
19	5.42	290.4	4.612	103.3	7.79	1.06	3.89	1.32	85.15	11731
20	5.37	285.7	4.551	104.2	7.65	1.06	3.85	1.54	85.11	11564
21	3.53	170.1	4.427	108.0	5.57	0.92	3.76	4.76	84.23	11819
22	4.37	282.1	4.587	92.4	7.02	1.13	4.03	1.92	85.23	11420
23	5.53	287.9	4.650	91.8	7.87	1.37	4.01	1.84	84.19	11193
24	6.52	287.1	4.629	95.2	8.53	1.22	4.02	1.21	84.24	11348
25	6.59	191.9	4.587	102.8	7.23	1.21	4.01	1.49	85.31	11209
26	5.70	185.8	4.542	106.8	6.46	1.37	3.87	2.67	84.90	11015
27	3.96	260.8	4.195	83.8	7.47	0.44	3.92	3.16	84.39	11955
28	3.90	260.5	4.172	87.5	7.59	0.50	3.96	1.88	85.45	12390
29	5.10	258.8	4.158	90.4	8.21	0.51	3.94	1.28	85.45	12558
30	4.51	287.8	4.484	81.3	7.48	0.92	4.00	1.78	85.21	11843
31	4.42	294.4	4.522	84.8	7.32	0.99	3.98	1.83	85.27	11733
32	6.27	298.8	4.555	87.9	8.57	0.92	3.94	1.02	84.91	11851
33	3.72	255.6	4.407	92.1	6.29	0.88	4.01	2.73	85.38	11826
34	5.55	255.4	4.418	90.4	7.54	0.82	3.97	1.29	85.77	12109
35	4.30	301.2	4.540	79.4	7.18	1.07	4.04	2.18	84.85	11407
36	4.84	270.9	4.484	90.8	6.75	1.49	4.04	4.26	82.67	10312
37	4.51	267.3	4.419	93.2	6.54	1.39	4.00	2.84	84.57	10558
38	4.30	254.4	4.395	97.9	6.37	1.24	3.92	3.46	84.33	11010
39	5.74	247.0	4.203	97.4	7.19	1.35	4.06	3.12	83.56	10821
40	4.00	291.3	4.564	88.3	7.03	0.91	3.96	2.22	85.21	11622
41	4.82	290.5	4.562	92.3	7.50	0.98	4.00	1.78	85.03	11426
42	5.21	291.9	4.560	94.9	7.77	1.03	3.97	1.53	84.97	11385
43	4.51	239.1	4.365	105.3	6.33	0.99	3.89	2.64	85.47	11496
44	5.96	239.2	4.371	104.2	7.28	0.84	3.87	1.78	85.54	11819
45	3.86	293.7	4.628	92.9	6.69	0.88	3.89	3.42	84.43	11491
46	4.72	291.9	4.595	96.4	7.28	0.82	3.86	2.17	85.10	11851
47	5.86	291.1	4.581	100.1	7.89	1.30	3.92	1.94	84.14	11128
48	5.82	246.3	4.442	105.9	6.85	1.23	3.87	3.52	83.76	10665
49	4.30	249.4	4.456	107.1	6.34	1.23	3.88	2.77	85.04	11257
50	3.82	254.0	4.174	82.4	7.46	0.77	4.03	2.79	84.23	11701
51	4.84	254.8	4.170	83.6	8.18	0.62	3.92	1.66	84.83	11975
52	5.71	254.5	4.146	84.7	8.65	0.64	3.88	1.35	84.65	12035
53	5.29	218.7	4.019	93.7						-
54	5.32	217.7	4.017	94.5	7.76	0.60	3.86	1.60	85.37	11966
55	4.80	217.8	4.016	96.8	7.60	0.68	3.90	2.70	84.33	11768

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content between 60 and 70%. The exception was a clean wood chip sample, which was

	in the vicinity of 47%.
	This suggested that the pine
	needles rather than the wood
	chips might be responsible for
	accumulating rain moisture.
	Two samples of whole wood
	chips were sorted by hand to
	separate needles and chips,
	which were then all tested for
	moisture content. The results,
	shown below, indicated that the
	needles absorb appreciably
	more of their weight in water
Fig 6. Some of the wood chip storage piles during test	than the wood chips.

	Sample 1	Sample 2
Mixed Sample	64.1	56.17
Chips Component	59	53.09
Needles Component	69.73	63.85

Fig 7: Moisture Measurements of Rain Soaked Whole Tree Chips

Per cent wood

It was important in this study to know and control the concentration of wood in each test. Typically, in the past at other plants, these blends have been made by estimating volumes of coal and wood with a bulldozer. However, the Fuel Handling personnel at the plant developed a clever way of producing accurate and reproducible mixtures, using the two available coal reclaim feeders. A reclaim is basically a pit or hopper in the ground into which fuel is pushed from the fuel storage pile, where it is metered onto underground traveling conveyor belts. The amount of coal that is fed onto the belts is adjusted by a slide gate on the feeders. At the Gadsden Plant there are two reclaim feeders under the coal storage yard. For the biomass testing, one of the Gadsden reclaim feeder discharge gates was adjusted to allow wood flow to be controlled independently of the coal flow on the other feeder. The two reclaim feeders provide material flow to a single conveyor belt. Coal was initially loaded on the belt. As the belt then passed the wood reclaim feeder, wood was deposited on top of the coal on the belt.

The maximum rate of the belt was 400 tons/hr. The conveyor system is equipped with a belt scale to measure total weight on the belt at all times. When determining the desired

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