REPORT
PRACTICAL. AVAILABLE TECHNOLOGY
steam generators for utilities Pope, Evans and Robbins' unit will burn a variety of U.S. coals; continuous operation at a West Viriginia utility starts next July Fluidized beds have been used for many years in the chemical industry to enhance reaction rates, but their use in steam generators is a relatively new concept. More recent is the development of the fluidized bed combustion technique to produce utility boilers at a fraction of their present size. Such boilers would burn almost any fuel, including low-quality coals such as lignite. Although the basic principles of fludizied bed combustion of coal have been known for 50 years, little progress has been made on a commercial design. Pope, Evans and Robbins (PER), consulting engineers, will complete installation of the first boiler at the Rivesville Power Station (Fairmont, W.Va.) in the first quarter of 1975. Continuous operation of the unit is scheduled to begin in July 1975. Mr. C. G . McKay, Vice-presidentOperations of Allegheny Power Service Corp., says, "We were interested in the fluidized bed boiler project because it shows great promise of being able l o use high-sulfur coals l o meet the standards of power plant emissions that have been established by EPA and cut the overall cost of generating electricity without the use of stack gas scrubbers that still have not been perfected. Our Rivesville Station, operated by Monongahela Power Co., an Allegheny subsidiary, was selected as the site for this project because of the availability 01 local high-sulfur coal, appropriate operating and supportive facilities for this test, and steam conditions of 968
Environmental Science &Technology
1270 psig, 925'F, with no reheat, that can use existing turbine generating equipment. Also, the 300,000 Ib/hr packaged unit, the highest capacity factory-assembled boiler in transportable size, can be accommodated in space available in the present building." Fluid bed combustion has the eyes and ears of high government officials. For example, in a February 15, 1974, letter to the President of the U.S., Secretary of the Interior Rogers C. B. Morton wrote, "We believe the fluidized bed combustion boilers offer the best prospects for providing large volumes of clean energy from coal at an early date. Current pollution controls allow only limited use of highsulfur coal in conventional boilers. The early demonstration of a lowcost, pollution-free combustion technique would help alleviate this undesirable situation and stimulate the production and utilization of Eastern coal." In 1965, PER began active development work on atmospheric fluidized bed boilers in their Alexandria, Va., laboratory using a unit capable of burning 100 Ib of coal/hr. In 1967, a small boiler (5000 Ib of steam generationlhr at 300 psig) was built by PER under contract to the Office of Coal Research (OCR) for development of the start-up technique, turndown method, and pollution control. By the end of 1973, about 5000 hr of test operation were performed by PER on this unit, using crushed limestone as bed material for sulfur cap-
ture. Combustion intensity of 1.2 million Btu/hr/ft2 bed surface was achieved, equivalent to 480,000 Btu/ hr/ft3 within the oxidizing fluidized bed. In fact, two general approaches are being taken in development work and in construction and operation of such boilers. In France and Czechoslovakia, fluidized bed furnaces have been developed for steam generators in which there is no direct contact between the inert fluidized particles and the heat transfer surfaces, the boiler tubes. On the other hand, work involving direct contact heat exchange has been actively pursued in England and in the U.S. I n the boilers without direct contact, the hot off-gases generate ail the steam in conventional fashion, and no appreciable size or cost advantages are attained.
How it works Basic design for direct-contact heat transfer units involved inert granular material supported by fiuidizing air from a distribution grid. The grid contains horizontal boiler tubes immersed in the fluidized bed and surrounded by heat exchange sur-
Allegheny Power's VP McVay "to use high sulfur coals . . . without use of stackgas scrubbers"
face membrane walls. Combustion within the fluidized bed is very intense. The granular bed material is limestone or ash, and very high volumetric heat release and heat transfer rates are obtained. As a result, there is no need for a large space-wasting furnace, and the amoun! of surface required is reduced. John E. Mesko. vice-president of PER, explains that start-up of an atmospheric fluidized bed boiler requires heating a portion of the bed to 800°F, hot enough to ignite bituminous coal. After coal ignition. the temperature of the bed rises rapidly until the system achieves thermal equilibrium. At equilibrium, the energy released in the bed equals the energy absorbed by the boiler tubes, plus the energy contained in the hot gases leaving the bed. The desired bed temperature is obtained by setting the proper ratio of heat transfer surface to heat release volume.
Operating characteristics of the bed dictate an optimum design temperature range of 1500-1600°F, with excess oxygen at about 3%. At these conditions, about 50% of the heat released by the burning coal is absorbed in the immersed tubes. Coal burns rapidly in a fluidized bed, even at 15OOOF. The rate is so high that at any point the bed is composed almost entirely of the inert particles that were added before combustion began. Typically, a sample of bed material would analyze at less than 1% carbon. Based on bed volume, the heat release is 300,000-400,000 Btu/hr per ft3. Counting the open furnace space above the bed, the rate is 100,000200,000 Btu/hr/ft3. The wide range in this overall furnace release rate follows from the fact that the furnace volume is set by design considerations other than heat release-i.e., tube arrangement, access for maintenance, water circulation. In either way of calculating, the heat release rate is much higher, as much as a factor of 10 more than the release rate in conventional coal-fired furnaces. The heat transfer coefficient is also higher, 50-60 Btu/hr/ft2/'F. But the important advantage is in the uniformity of the heat flux. The peak and average tluxes are equal; there is no danger of burnout as long as the feedwater treatment is intelligently handled. A typical riser has a heat flux of 50,000 E3tu/hr/ft2. Tubing length is reduced because most of the steam is generated in tubes heated on both sides. Actually, effective projected radiant surface (EPRS) is a concept not used in designing an atmospheric fluidized bed boiler. Superheater surface requirements are also reduced because of
Fluidized bed steam generator
Fuel injection pipes
PER-Foster Wheeler steam generator
Series economizer circuit
Parallel boiler circuit
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+ superheater
Primary superheater Attemporator spray
300,000 LBIHR 1300 PSlG 925" F Steam to turbine
the high heat transfer coefficient and the fact that fireside deposits do not form. Fireside corrosion is also avoided, apparently because the sodium, potassium, and vanadium in the coal are not released, or if released, are picked up by the bed particles instead of the tubes. The ratio of bedparticle-surface area to heat-transfer-surface area is about 15-1. Condensing vapors are thus more likely to deposit on a bed particle than a tube.
detrimental way. All fuels are burned at a heat release rate equivalent to 110,000 Btu/hr/ft2 of EPRS. In a pulverized coal-fired unit, this value may be as low as 40,000 with a high alkali coal. The boiler designed and developed to date could not be used if the coal were very fine and dry, since the coal is difficult to feed uniformly, and successful operation depends on even fuel distribution. With current mining and transport methods, a '12-314in. top size is easy to get.
Unlimited coal type Unlike a pulverized coal-fired boiler, ash properties are not significant to the design of an atmospheric fluidized bed boiler. The same basic design applies for all coals. The bed temperature is too low for the ash to soften or change chemically in any
Keeping low emissions Tests performed recently bv the Bureau of Mines, Department of the Interior, on the Pope, Evans and Robbins unit in the Alexandria, Va., laboratory established that both NOx and SO2 emissions were held below EPA emission standards for new coal-fired plants. The conclusion ,reached by the Pittsburgh Energy Research Center of the Bureau of Mines indicates that "Test data obtained during the two 4-hr tests on the fluidized bed boiler showed emissions of 0.80-1.20 Ib SO2/1O6 Btu and 0.11-0.17 Ib NOx (calculated as N02) per l o 6 Btu, compared with EPA Standards of 1.20 Ib S 0 2 / 1 0 6 Btu and 0.70 Ib N02/106 Btu for new coal-fired plants," while burning coal with 4.5-4.8% sulfur content. The coal fed to a fluidized bed boiler is crushed to a 1.4-in. or 3.4in. top size, not pulverized. A good fraction of the ash stays in the fluidized bed, or if carried out with the products of combustion, is separable in a low-pressure drop cyclone dust collector. Little, if any, metal oxide or sulfate fume is formed, and the quantity of very fine particulate is relatively low. Sulfur trioxide has also not been detected when limestone is
FI ue
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Volume 8, Number 12, November 1974
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limestone as bed material, based on the following reactions:
compliance reliably
CaCO, CaO
CaSO,
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+
heat + CaO
SO,
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+
minimal maintenance. Most Systems have capability for mUltiple-pOint sampling with a single analyzer, resulting in significant cost savings for many installations. All employ the Du Pont UV-Visible Photometric Analyzer-proven in the field in over a thousand installations-as the basic detector. Systems are available for continuous monitoring of one pollutant, for sequential monitoring of pollutants, and for laboratory or survey use. If compliance with emission standards is your responsibility, DU Pont Instruments can help YOU. For full information on Du Pont Pollution Source Monitoring Systems, write Du Pont Instruments, Room 240046, Wilmington, DE 19898.
, A-99573
970
CIRCLE 10 ON READER SERVICE CARD
Environmental Science & Technology
COz
liZO, +CaSO,
CO CaO
Whether you need to monitor SO2 or NOXin your stack gas, Du Pont Instruments provides a source monitoring system that’s complete from sample probe to readout & field proven in power plants, smelter operations, refineries, pulp mills and other manufacturing plants.
+
+
SO,
+
C02
or by adding a low-cost catalyst (common salt) with the coal. Current material balances indicate that for 2000 Ib (1 ton) of 5.6% SUIfur coal, 4oo of limestone is required to remove over 90% .of the sulfur. Studies are now being undertaken by EPA to develop alternate
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achievable, inasmuch as there are relatively few micron-sized particles and there is virtually no sulfuric acid mist even at exhaust temperatures of 250°F. Building the unit
The design of the 300,000-lb/hr unit, as a cell, for a 800-MW fluidized bed steam generator was developed by Foster Wheeler Corp. (Livingston, N.J.). Their design consists of four modules, each containing seven vertically stacked fluidized beds, called cells. During the latter part of 1973, the 300,000-lb/hr capacity unit was released for fabrication. Based on competitive price bidding, the award went to Foster Wheeler. Final dimensions of the boiler are 12 ft wide, 25 ft high, and 38 ft long. The atmospheric fluidized bed boiler is seen as the necessary first step on a long path toward better use of coal, the only major fossil fuel r e source in the U S . I f and when the successful performance of the first 30-MW unit is demonstrated, then other utilities will commit funds for the installation of such boilers in the 200-800 M W capacity range.
