utilization of solid waste fuels through fluidized bed ......utilization of solid waste fuels...
TRANSCRIPT
UTILIZATION OF SOLID WASTE FUELS THROUGH FLUIDIZED BED COMBUSTION
HENRY S. KWON Dorr-Oliver, Inc_
Stamford, Connecticut
JOHN F. LAUKAITIS Keeler/Dorr-Ol iver
Williamsport, Pennsylvania
JEAN-PAUL LEG LISE OTV
Paris, France
ABSTRACT
Low grade anthracite coal waste has been successfully burned in a fluidized bed combustor for steam generation_ Fluidized bed cl'mbustors capable of burning either wood waste or coal are being designed and built to generate hot water for district heating_ Emissions of air pollutants from these fuels are low.
IN TRODUCTION
Fluidized bed combustion offers an environmentally acceptable means of utilizing various solid waste fuels for the generation of heat for industrial use. Anthracite coal waste called culm has been accumulating above ground in Pennsylvania for over the last 100 years, polluting the environment and contributing to generally depressed conditions. Although its potential as an energy source has long been recognized, actual use of this fuel has not materialized because conventional combustion devices such as stokers cannot effectively burn the fuel due to its extremely high ash content. In 1978 the Department of Energy awarded a contract to the Shamokin Area Industrial Corporation ( S A IC) of Shamokin, Pennsylvania to design, construct and operate a fluidized bed boiler using the culm waste as fueL The boiler was designed and built for S A IC through a joint effort of a fluidized bed supplier, Dorr-Oliver, Inc. and a boiler manufacturer, E. Keeler Company (now the Keeler/Dorr-Oliver Boiler Company). In the summer of 1981, the boiler was started-up and burned this waste fuel for 10, 000 hr generating steam until April 1983 when the plant was shut down due to the lack of funding. Sulfur capture was better than expected and emissions of C O and NOx were found to be very low.
OTV, the French licensee of Dorr-Oliver, Inc. was awarded a contract to design and construct two fluidized bed boilers for the Cite de ClairVivre to generate hot water for district heating. The boilers are designed to burn sawdust and wood chips and alternatively to burn bituminous coal, when wood wastes are not available. The Cite is a public institution with a population of over 1000, including professional and administrative staffs. Current heating systems consist of small oil burning boilers and stoves located in various parts of the institution. Annual oil consumption is approximately 700,000 gal (2200 t). The institution is located in the middle of a forest land where wood wastes have been accumulated over the years. The institution realized that the wastes are a potential source of energy for heating. At the same time, it hopes to reduce the impact of the wastes on the surrounding environment. The criteria for the selection of boiler system was not only economic justification, but energy efficiency and multifuel burning capability. The dispersed heating systems are currently being centralized and the two fluidized bed boilers are designed to supply heat to the new central heating system. The boilers are scheduled to start up early in 1984.
282
SHAMOKIN CULM BURN IN G BOILER
DESIGN CRITERIA
The boiler was designed to generate 23,400 lb/hr (10,620 kg/h) of 200 psig (1,380 kpa) saturated steam with a turndown capability of 2.5 to 1. The steam generated was intended for consumption at a nearby paper manufacturer 24 hr/day.
The culm supplied to the boiler plant is mine breaker residue rejects from an anthracite coal mine sized to
minus 4 in. (100 mm). The specified fuel had heating values ranging from 3000 to 5000 Btu/lb (5975 to 11,625 kJ/kg) with approximately 0.6 percent sulfur content. The typical fuel analysis is given in Table 1. The design parameters for the fluidized bed boiler are summarized in Table 2.
PLANT DESCRIPTION
Plant design was based on laboratory combustion tests using culm. Test objectives were to determine optimal feed size, bed temperature, calcium to sulfur ratio, and the effects of ash reinjection.
CULM AND LIMESTONE FEED
The laboratory test results showed that the culm had to be sized to minus 4 mesh in order to achieve good com� bustion. This requirement necessitated the installation of a culm preparation system consisting of culm crushing and screening prior to feeding the culm to the boiler.
The limestone supplied was from a local limestone quarry and was mainly mine rejects with no commercial value. The combustion tests also showed that the limestone should be of minus 1/4 in. size. Schematics of feed preparation and storage are shown in Fig. 1 .
Both culm and limestone were fed to the boiler with screw feeders. Two 9 in. (230 mm) feeders were used for the culm and one 4 in. (100 mm) for the limestone. Variable speed motor drives were used on each feeder to allow culm feed rate to follow steam demand and combustion temperature and to permit limestone feed rate to respond to the S02 emission control requirements.
FLUIDIZED BED BOILER SYSTEM
The primary objective of the project was to demonstrate the feasibility of industrial boiler operation using waste culm as a sole fuel. Emphasis was placed on the development of a boiler design which would offer availability, reliability, Simplified operation, and ruggedness required for commercial use. Figure 2 shows schematic of the boiler system. The boiler is of water wall design with in-bed vertical tubes and a convection section. It was designed to generate approximately half of the steam in the in-bed tubes and the waterwall tubes and the other half in the convection section by recovering heat from the combustion gases.
Combustion air is introduced at the bottom of fluidized bed through air distributing tuyeres. The windbo"x is divided into three zones to aid in start-up and turndown
operation. Each zone has its own air supply line leading from a common combustion air blower. One zone covering 64 percent of the bed area is the first bed area to be fluidized at start-up and remains fluidized during operation. Each of the remaining two zones covers 18 percent of the bed area and contains four in-bed tubes. Either or both of these sections can be defluidized during turndown conditions. The boiler also contains an internal convection bank dust hopper with a flapper valve for recycling collected dust.
Further ash recycling and particulate control are accomplished through a two stage cyclone system where ashes collected in the primary cyclone are recycled back to the boiler.
An oil fired burner is also included to serve as a preheat bu�ner during start-up. The burner raises the bed temperature up to the combustion temperature of bituminous coal. BituminouS- coal then brings the bed temperature further up to the culm combustion temperature.
AIR PREHEATER
Combustion gases leave the cyclones at 660°F (349°C). To improve the overall efficiency of the boiler system, heat is recovered by indirect gas to air heat exchange in the air preheater. It is a conventional shell and tube type heat exchanger. The combustion gases cool down to 350°F (177°C) and the combustion air is heated to 425°F (218°C).
ASH COOLER AND ASH CONVEYING
Ashes from the fluidized bed and the cyclones are cooled to approximately 250°F (121 0c) in the ash cooler. The ash cooler is a fluidized bed cooling device containing in-bed cooling tubes for more efficient cooling.
The water in the cooling coils is boiler feedwater. This system recovers some of the sensible heat from hot ash and transmits it to the feed water. The ash cooler is equipped with its own fluidizing blower and cyclone. The ash is discharged through rotary valves to the pneumatic ash conveying system which transports the ash to the ash silo.
BAG FILTER
A conventional pulse-jet bag filter is used as the final particulate control device. The bag fllter has an air-to-cloth ratio of 6 to 1 and the bag material is Nomex felt.
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OPERATING EXPERIENCE
Plant start-up and shakedown lasted three months. During this period many mechanical bugs were corrected and computer control systems were adjusted. The start-up procedure was also revised as required. After this period, the boiler was in continuous operation until the project was terminated, except for several scheduled shutdowns.
START-UP AND SHA KEDOWN
During the start-up it was found that the preheat burner did not have enough capacity to raise the bed temperature to the culm combustion temperature. As a result, bituminous coal was added to obtain a bed temperature at which culm combustion could be sustained. After attaining the culm combustion temperature it was also learned that the bed temperature could not be maintained with only culm. Spot check of culm samples taken during this period indica ted that the culm had only 3100 Btu/lb (7208 kJ /kg) heating value and 72 percent ash. For this low heating value, bed heat transfer was simply too excessive.
In order to balance the lower heating value of the culm with the culm combustion temperature in-bed heat transfer was artificially reduced by adding refractory to parts of the in-bed water walls. The combustor was then able to sustain the culm combustion temperature with culm only.
One recurring problem was in the culm crushing and screening operation. Heavy rains or cold weather coupled with high moisture content, which sometimes exceeded 10 percent, caused screen blinding which limited fuel feeding capacity. Once the culm was crushed and screened, wetness of the culm did not affect culm combustion in the fluidized bed_
BOILER PERFORMANCE
Culm fuel composition fluctuated a great deal and accordingly cUl-':fl feed rate was frequently adjusted to maintain bed temperature. Table 3 shows the actual fuel analysis during the parametric tests. Often the culm exhibited a heating value of around 3000 Btu/lb (6975 kJ/kg), which required culm feeding in excess of the design feed rate. In spite of the wide variation in fuel properties, steam output remained essentially a linear function of culm feed rate (Fig. 3). Combustion efficiencies expressed as carbon burn-up, generally remained around 90 percent as shown in Fig. 4. The figure also shows that culm burn-up is lower, when burning culm at lower bed temperatures.
286
BOILER TUBES
As mentioned earlier, the boiler is of waterwall design with 8 vertical in-bed tubes. Although pilot plant tests showed no sign of tube metal wastage, tubes were preselected for periodic measurements of tube thickness. The preselected tubes are 2 in-bed tubes, 3 sidewall tubes, 2 convection screen tubes, and 4 convection outlet tubes. Point locations for measurements were selected where erosion would be most likely. Ultrasonic technique was used for the measurements. After 10,224 hr of operation the measurements indicated that there was virtually no erosion in any of the tubes tested. The in-bed tubes and the sidewall tubes were polished in the areas which were affected by turbulent action of the bed. However, actual measurements showed minimal amount of change in thickness, as indicated in Table 5. These tubes had been suspected of being most likely candidates for erosion. The surfaces of the convection screen tubes and the convection outlet tubes appeared to maintain their original state.
EMISSIONS
In spite of the low combustion efficiencies experienced with the culm, CO emissions did not exceed 125 ppm .during the 10,000 hr of operation. Low volatile content of the culm may have attributed to the low CO emissions. S02 emissions were maintained at 100 ppm except when sulfur capture was studied as a function of calcium to sulfur ratio. S02 removal in excess of 90 percent was readily attainable at a calcium to sulfur ratio greater than 2.5. S02 removal efficiency as a function of Ca/S ratio is shown in Fig. 5. N Ox emissions ranged from 50 to 300 ppm. Figure 6 shows that NOx emissions tend to increase with excess air. Lower excess air means higher CO emission, which in turn contributes to lower NOx emission. Particulate emission tests were run in April 1983 according to the EPA Method 5. The results showed an average 0.079 Ib/million Btu (34 ng/J) of particulate emission at the stack.
CLAIRVIVRE WOOD/COAL BURN IN G BOILERS
ECONOMIC CONSIDERATION
In 1982 the annual fuel bill in the district amounted to over 6 million francs ($705,800) at the current rate of heating oil consumption. Wood wastes can be obtained at a cost of 90 francs ($ 10.5) per metric ton (t) including transportation, which means an annual fuel cost of only 1 million francs ($ 117,600) with wood wastes_ Therefore, an annual saving of 5 million francs ($588,200) can be realized from fuel alone.
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DESIGN CRITERIA
Two boilers are being designed and installed to generate, during the period of peak demand for heat, hot water equivalent to 12 million Btu/hr (3.5 MW) and 20 million Btu/hr (5.8 MW) each. Heat demand varies depending on weather conditions and each boiler is capable of operating at a turndown ratio of 4. Hot water is to be distributed to the various buildings and shops through a central heating network.
The wood wastes are rejects from wood harvesting and processing. Moisture content is found to be up to 50 percent by weight and heating value to be 4200 Btu/Ib (9765 kJ/kg) on as-received basis. Table 6 shows the typical fuel analysis for wood wastes and coal. The design parameters for the 20 million Btu/hr (5.8 MW) unit are summarized in Table 7.
PLANT DESCR IPTION
The plant consists of fuel preparation for wood wastes, fluidized bed boilers, and gas clean-up equipment. A common feed system can handle both fuels and change-over
. offuel can be made during operation without interruption of heat supply.
FUEL PREPARATION AND STORAGE
Wood wastes are delivered unprocessed by truck to the site. After weighing, the wastes are stored on the open ground. A front end loader moves the material to a transfer hopper. It is then conveyed to a screen where material larger than 1 � in. (30 mm) is screened out for size reduction. Feed material passing through the screen is then stored in two silos. The screen rejects are recycled to a hammer crusher where the size is reduced.
Coal is delivered presized to minus 1 � in. (30 mm) and stored on the open ground adjacent to the storage silos. When needed, the coal is loaded into the silo. Fuel preparation and storage systems are shown in Fig. 7.
FUEL FEED
Feed material is extracted from the storage silos and conveyed to a feed hopper. A short screw conveyor then feeds the material into the feed chute of each boiler. The fuel is gravity fed into the fluidized bed.
289
FLUIDIZED BED BOILER SYSTEM
The boilers are required to operat� continuously supplying hot water, as demanded, without interruption even during change-over of fuel. In addition, reliability, ruggedness, and simplified operation are also required. Figure 8 shows schematic of the boiler system. The boiler itself is comprised of a fluidized bed furnace and a convection bank. The furnace is designed to operate at bed temperatures ranging from 1300°F (700°C) to 1650°F (900°e) depending on type of fuel. Cooling coils in the bed control the bed temperature by removing excess heat. The cooling tubes are vertically arranged between the lower and the upper water headers along the combustion walls. Depending on heat input, bed height is controlled to maintain the bed temperature at design conditions. The con-o vection bank recovers sensible heat from combustion gases. The water circuits in the boiler are of forced circulation type and recycled water from the heating system is boiler feed water . The temperature of gases leaving the bank is sufficiently low for the baghouse.
Combustion air is first distributed through sparge pipes which are located under the bed. The air is further distributed into multiple tuyeres attached to each sparge pipe. The sparge pipes are equipped with shut-off valves, which enable partial slumping of the bed for turndown operations. Secondary air is introduced, as necessary, over the bed to enhance freeboard burning. Elutriated ashes are partially collected in the convection bank hopper and are recycled back to the bed.
A burner is included to serve as a preheat burner during start-up. The burner heats the initial bed of sand up to the combustion temperature of the fuel. When fuel is fed into the bed, the burner is turned off.
ASH COOLIN G
When ash is excessively built up in the bed or it is necessary to lower the bed height, bed ash is drained out by gravity and cooled for disposal or reuse at a later date.
GAS CLEAN-UP
Combustion gases are initially cleaned in a cyclone. Collected ashes can be recycled back to the bed if necessary. Final particulate emission control is accomplished by a conventional bag filter 0 OPERATING EXPERIEN CE
The boilers are scheduled to start up early in 1984 and operating experience will be presented during the June 1984 conference.
REFEREN CES ceedings of the 7th International Conference on Fluidized Bed Combustion, Volume 1, p. 567, October 25-27,1982.
[1] D. G. Chiplunker and H. S. Kwon, "Performance of a
Fluidized Bed Steam Generator Burning Anthracite Culm," Pro-[2] Dorr-oliver Internal Memo, "Final Report on Shamokin
Boiler Tube Metal Thickness Measurement," September IS, 1983 .
,
• Key Words: Boiler . Emission. Energy. Fluidized Bed • Solid. Waste Control. Wood
TABLE 1 ULTIMATE ANALYSIS OF CULM (DESIGN BASIS, % WT)
Ca rbon
Hydrogen
Nitrogen
Oxygen
Sulfur
Ash
Heating Value
HHV
27.02
1.42
0.66
3.48
0.57
66.85
4,198 Btu/lb
(9,760 kj/kg)
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TABLE 2 DESIGN PARAMETERS (SHAMOKIN AFB BOILER) -------------------------------------------------------------------BOILER
Steam Flow
Steam Condition
Culm Feed Rate
Culm Feed Size
Culm Heating Value
Limestone Feed Rate
Limestone Feed Size
Bed Temperature
Bed Height
Excess Air
Turndown (Steam)
Fluidizing Velocity
EMISSIONS
Particulates (State)
S02
295
23,400 lb/hr (10,620 kg/h r)
200 psig sat. (1,380 kpa)
8,188 lb/hr (3,714 kg/h r)
Minus 4 mesh
4,198 Btu/lb (9,7 60 kj/kg)
983 lb/hr "(446 kg/h r)
Minus 1/4 inch (6.3mm)
1,450-1,650°F (788-900°C)
3-6 ft (0.91-1.82m)
20-50%
2.5: 1
3.5-6 fps (1.07-1.82 m/s)
0.4 lb/million Btu (172 ng/j)
0.33 lb/million Btu (142 ng/j)
Equivalent to 88% capture
140 ppm
TABLE 3 ACTUAL COMPOSITION OF CULM (% WTI
Carbon
Hydrogen
Nitrogen
Oxygen
Sulfur
Ash
Heating Value
HHV
296
24.15 - 26.59
0.89 - 1.04
0.47 - 0.56
3.06 - 5.33
0.73 - 0.94
67.3 - 69.3
3,918 - 4,164 Btu/lb
(9,109 - 9,680 kj/kg)
TABLE 4 ACTUAL BOILER OPERATIN G CONDITIONS (SHAMOKIN AFB BOILER)
Steam Flow
Turndown (Steam)
Steam Condition
Culm Feed Rate
Culm Heating Value
Bed Temperature
Bed Height
Fluidizing Velocity
5,500 - 24,500 lb/hr (2,500-11,120
kg/hr)
4.5: 1
200 psig sat (1,380 kpa)
Up to 11,000 lb/hr (4,990 kg/hr)
3,918 - 4,164 Btu/lb (9,109-9,680 kj/kg)
3 - 5 ft (0.91 - 1.52m)
3.5 - 5.3 fps (1.07 - 1.62 m/s)
297
Operating Hours
2,629
4,493
7,470
10,224
Operating Hours
2,629
4,493
7,470
10,224
TABLE 5 TUBE METAL THICKNESS AS MEASURED
inch
.175
.184
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.167
IN-BED TUBES
inch
.3742
.3775
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A
(mm)
(9.504)
(9.588)
(9.555)
(9.517)
inch
.3775
.3770
.3776
.3743
B
(mm)
(9.588)
(9.575)
(9.591)
(9.506)
SIDEWALL TUBES
A
(mm) inch
(4.445) .169
(4.67 3) .17 3
(4.394) .162
(4.242) .154
B
(mm) inch
(4.292) .172
(4.394) .178
(4.115) .170
(3.911) .170
c
(mm)
(4.369)
(4.521)
(4.318)
(4.318)
Note: 1. Nominal thickness of in-bed tubes, 0.375 inch (9.525 mm)
2. Nominal thick ness of sidewall tubes, 0.180 inch (4.572 mm)
298
TABLE 6 ULTIMATE ANALYSIS OF FUEL (elAI RVIVRE AFB BOI lER, DESIGN BASES, % wTl
Carbon
Hydrogen
Nitrogen
Oxygen
Sulfur
Ash
Moisture
Heating Value
HHV
299
Wood Waste
25.0
2.7
•
0.1
19.55
0.05
2.6
50.0
4,200 Btu/lb
(9,7 65 kj/k g)
Coal
72.8
4.9
1.4
6.5
0.5
8.9
5.0
12,735 Btu/lb
(29,610 k j/kg)
TABLE 7 DESIGN PARAMETERS (CLAIRVIVRE AFT BOI LER, 5.8 MW) -----------------------------_._-_ . . _ . _ _ ._---•. _-
Boiler
Heat Absorbed
Hot Water Condition
Woodwaste Feed
Woodwaste Feed Size
Feed Heating Value
Bed Temperature
Bed Height
Excess Air
Turndown
Furnace Cross Section
Emissions
Particulates
S02
NOx (for wood)
300
20x106 Btu/hr (5.8 MW)
230° F (llOOC)
7,280 lb/hr (3,300 kg/h r)
Minus Ii inch (30mm)
4,200 Btu/lb (9,765 kj/k g)
1300 - 1650 of (700-900°C)
2 - 4 ft (0.61-1.22m)
40 - 60%
4: 1
80.5 ft2 (7.5 m2)
0.1 lb/million Btu (43 ng/j)
1.2 lb/million Btu (520 ng/j)
not required (Expected to be less
th an 0.3 lb/million Btu or 130
ng/ j)