The development of fluidized bed combustion John Highley National Coal Board Coal Research Establishment Cheltenham, Gloucestershire GL.52 4 R Z UK During the 196Os, most energy planners in the U.K. believed that the only major future use for coal would be in power generation, and that even this use would be limited by the planned increase in nuclear power and the availability of cheap oil. Despite the discouraging outlook, the U.K. N a tional Coal Board (NCB) took the initiative during this period to develop a new coal-burning technology, fluidized bed combustion (FBC). The main objectives were to reduce the capital cost of coal-burning power stations and to facilitate the efficient use of lower grade coal with high or variable ash content. It was soon found, however, that fluidized combustion would also allow a substantial reduction in the emission of sulfur dioxide, simply by feeding crushed limestone with the coal. The potentially low environmental impact of this new technology created considerable interest in the U S . , where, by 1970, it was becoming apparent that coal would have to be used to supplement diminishing oil and gas reserves. Interest in the U.K., however, remained low; oil was becoming available from the North Sea, and there was no legislation limiting sulfur dioxide emissions. From 1970 to 1973, most research and development of FBC in the U.K. was funded by U S . agencies. In 1974 the situation changed. Following a major study of the U.K. coal industry, the government authorized the N C B to implement a “Plan for Coal” that will increase annual output to 150 million tons. The additional coal is expected to displace oil and gas in industrial and commercial applications. 270
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This new technology holds the potential f o r clean, efficient burning of coal in applications ranging f r o m heating to electric generation. A British expert examines its development and current status Coal crushing eliminated In anticipation of this planned increase in coal use, the N C B gave fresh impetus to its development of FBC, concentrating on the industrial market. Although conventional coal-burning equipment is efficient and proven, it is expensive and offers a low level of amenity in comparison with oil and gas systems. Developing a modern coalburning system which can be installed at a low capital cost is thus an essential step in increasing the direct use of coal; the alternative is the relatively inefficient production of synthetic liquid or gaseous fuel from coal.
Prior to 1973, the target of the FBC program had been coal-fired power station boilers. The fluidized combustion concept which evolved appeared attractive for these very large boilers, and subsequent progress in the U.K., U S . , and elsewhere is confirming its potential. The system is inappropriate for industrial boilers, however, because it requires crushing and drying the coal before firing, and use of a tall combustion chamber (about 4 m) and a deep fluidized bed-thereby necessitating a high-power air fan. Analysis of the standard FBC system showed that its disadvantages were linked: The deep fluidized bed and tall combustion chamber were necessary to ensure efficient combustion of the finer coal particles inevitably produced by crushing. It was clear that elimination of coal crushing would lead to a more attractive system for industry and that the reasons for crushing had to be reconsidered. The main reason for firing crushed coal is the need to maintain an effective fluidized bed in the combustion chamber. The burning coal, ash, and added limestone must remain suspended in the upward air flow, forming a violently churning layer of particles. During operation, the fluidized bed contains very little coal ( < 5 % ) and consists almost entirely of ash and limestone particles, which must be below about 5 mm in size for effective fluidization. Coal supplied to U.K. power stations contains stone from bands within the coal seams, having diameters up to the maximum coal size of 55 mm. At an early stage in the development of fluidized combustion, it was decided that crushing offered the simplest means of ensuring the correct size of ash (stone) particles in the fluidized bed. Crushing also ensured that the burning coal particles could be fluidized and thus well distributed through the bed. But because the stone mined with coal has a higher density, it is relatively
0013-936X/80/0914-0270$01.OO/O @ 1980 American Chemical Society
simple to separate by a process referred to as “washing.” Coal washing in fact reduces transport costs and minimizes the quantity of ash for disposal; the resulting savings have made the use of washed coal economic for almost all industrial users. The washed coal, w’ith an average sulfur content of only 1.35% and an ash content between 3% and IO%, is marketed as “singles” (25- 13 mm) and “smalls” (1 3-0 mm). With the recognition that, unlike power station coal, these standard U.K. industrial coal grades contain few discrete stone particles, came the realization that the main reason for coal
crushing no longer applied. Combustion tests confirmed that the commercial coal grades could be burned by simply feeding, as supplied, to the surface of a fluidized bed of silica sand. As expected, the elimination of crushing brought about a substantial improvement in combustion: the target of 97% combustion efficiency was achieved when burning the singles grade coal in a fluidized bed 150-mm deep in a combustion chamber only 900 mm in height, without the usual complication of a secondary combustion stage. With the smalls grade coal, the performance in this compact
high-intensity system was similar to that previously obtained with crushed coal in a deep bed and tall combustion chamber. This variant of fluidized combustion is being used in a new generation of compact industrial boilers and furnaces. i n the U.K., several demonstration/prototype boilers and hot gas furnaces are in daily use a t industrial sites: within the next year, most larger boiler-manufacturing companies are expected to offer package boilers on a standard commercial basis. I n the U.S., the Johnston Boiler Co., Ferrysburg, Mich., is already selling
\ ( FBC boiler conversion at Renfrew, Scotland
+
Ash removal
refiring
FBC in large-scale demonstration The first large-scale demonstration of FBC was in the conversion of an existing water-tube boiler, using a design suitable for utilities and highoutput industrial boilers. The conversion was carried out by Babcock Power Ltd. (BPL) at its Renfrew, Scotland, works. The technology was developed by the National Coal Board and British Petroleum, and was Iicensed by their jointly owned company, Combustion Systems Ltd. The boiler is a cross-tube type, rated at 13.5 MW of saturated 25-bar steam when fired with a spreader stoker. The FBC system consists of a bed 3.1 m by 3.1 m, with horizontal immersed hairpin tubes designed for evaporation with forced water circulation. The boiler was commissioned in 1975 and has operated satisfactorily
for more than 5000 hours. Studies of light-up; load control; uniformity of bed temperature; effect of fuel size, number of feed points, and fluidizing velocity on unburned carbon carry-over; heat transfer to immersed tubes; corrosion and erosion of tubes; sulfur retention by limestone; and NO, emissions have been carried out on this unit. A wide range of coals have been burned with efficiencies at least as good as those obtained with the original stoker. Retention of 90% of the sulfur was attained when burning a 3.5Yo-sulfur coal. NO, emissions were found to be well within present limits for new coal-fired boilers in the U S . The first commercial application of the FBC system that was demonstrated at Renfrew is at the Central Ohio Psychiatric Hospital in Columbus.
Babcock Contractors Inc. (BCI) ( Pittsburgh (a subsidiary of Babcoc Power Ltd.), in collaboration with th Riley Stoker Corp., converted an el isting 20-MW boiler. The unit, whic will be commissioned shortly, will bur high-sulfur Ohio coal. Currently, Fluidized Combustio Contractors Ltd., a company recent formed by Babcock Power Ltd. ar British Petroleum, is the principal U.1 organization for engineering FBI systems for utility and large industri; applications. They will be closely ir volved in carrying out a contract rc cently awarded to BCI by the Tenne: see Valley Authority for the detaile design of a 200-MW (electric outpu boiler to burn West Kentucky cot containing 4 . 5 % sulfur with 909 sulfur retention.
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boilers incorporating this technology under license. The recently commissioned 30-MW boiler at Georgetown University, designed and manufactured by Foster-Wheeler, also makes use of the advantages offered by eliminating coal crushing.
Boiler design for FBC Fluidized combustion is more than just a new technique for burning coal wit!i minimum environmental impact. It also provides boiler designers with an opportunity to make significant cost savings. Steam-raising tubes can be immersed within the bed of turbulent red-hot particles to obtain high heattransfer rates. Extensive trials have demonstrated that, contrary to early fears, corrosion and erosion of these tubes are minimal. However, a boiler with tubes immersed in the fluidized bed presents novel problems of start-up and control, and these must be solved by a combination of boiler design and control strategy. Various approaches are possible and unfortunately there is no obviously “best” design; consequently, there is considerable variety in the designs of first-generation FBC boilers. The air supplied to the fluidized bed is heated to the operating temperature of about 900 OC. The heat required for this is approximately 40% of the heat release which would be obtained if coal were burned at the theoretical maximum rate for the air supply. The burning rate is limited to this 40% level unless tubes are immersed in the bed to remove additional heat. The optimum rate, however, is determined by the need to ensure efficient combustion and prevent unacceptable emissions of unburnt volatiles and carbon monoxide. This is normally about 80% of the theoretical maximum for the air supply; in other words, efficient combustion requires 25% excess air. At this optimum rate, about half of the heat release is transferred to the tubes and the remainder is removed by the hot combustion gases to be recovered in the convection section of the boiler. Having too few tubes in the bed would necessitate a lower burning rate, which would reduce total power. output; having to heat a greater proportion of excess, unused air would at the same time reduce efficiency. On the other hand, having too many tubes would necessitate a burning rate greater than optimal, causing incomplete combustion. These considerations apply throughout the output range of a boiler, and it is necessary to incorporate design features which ensure that 272
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the direct heat transfer to tubes in the bed is maintained close to, but does not exceed, 50% of the heat release. This is usually achieved by varying the bed temperature within its operational range and by making provision to reduce the area of tubing in the active bed as output is lowered. Start-up of the FBC boiler involves heating the inert bed to about 500 OC, using a premium fuel, after which coal feeding can be initiated. The amount of premium fuel required can be minimized by limiting heat transfer to the tubes in the bed during start-up. Heating techniques include firing oil burners above the bed, passing premixed air and gas through the bed to give a flame at the surface, and fluidizing with hot gas from an external burner.
Oil-to-coal conversion Developments in oil firing over the past 20 years have led to substantially more compact boilers than those for conventional coal stokers. Also, in the U.K., horizontal fire-tube construction has almost entirely replaced the more expensive water-tube construction in boilers with outputs up to 20 M W (60 000 lb/h of steam). In the early stages of the N C B program, it was considered that there would be considerable market potential for an FBC firing system for fire tube boilers, particularly if this would permit the conversion ‘of existing boilers from oil to coal with minimum reduction in output. In 1975, the NCB started a collaborative development towards this objective with Parkinson Cowan G W B Ltd., using one of their Vekos Powermaster boilers, rated at 1.2 M W . The fire tube of a conventional boiler is less than ideal for fluidized combustion since it is intended for horizontal gas flow rather than vertical, but, following a number of modifications, a unit was operated satisfactorily for extended periods. The output achieved was similar to that of a conventional stoker; work is continuing on alternative designs for higher output. However, recent design studies for fire-tube boiler conversions from oil indicate that the market potential is less than expected because of the difficulty of installing coal storage and handling systems, grit arrestors, and ash systems into existing boiler houses. Consequently, the main market potential for FBC systems in fire tube boilers is now seen to be in new boilers, initially for the same output range as stoker-fired units, and in existing boilers originally installed for coal but since converted to oil. I n an independent program,
Northern Engineering Industries Ltd. has developed an FBC system with an output similar to that of a stoker, using a I-MW boiler. This company, in which the U.S. firm Combustion Engineering is a shareholder, is the b . K . market leader in fire tube boilers through its subsidiary N.E.I. Thompson Cochran Ltd. A 5-MW fire tube boiler has recently been commissioned and will be used to prove the FBC system prior to commercial manufacture of a range of boilers up to 6 - M W output. After initial investigations supported by the NCB, Energy Equipment Ltd. has developed a system which enhances combustion above the fluidized bed by recirculating flue gas through the bed and secondary air injection. Although intended primarily for fire tube boilers, the system has been used to convert a 16.7-MW water tube boiler from stoker firing. Energy Equipment has also been given a contract by the NCB to provide FBC boilers for a new mine.
New FBC boilers The development of new purposedesigned boilers for FBC offers the major advantage that the combustion chamber can be arranged for vertical gas flow and the correct amount of tubing can be installed in the bed in a configuration suited to achieving simple start-up and effective load control. Although this is readily achieved with water tube boilers, they are expensive to manufacture, and so there is a strong incentive to use shell-type boilers in order to compete more effectively with low-cost oil-fired package units. For smaller outputs, up to about 5 MW, vertical shell boilers are potentially attractive, but for higher outputs it is necessary to devise some method of incorporating the vertical combustion chamber into a horizontal boiler shell. Two vertical shell boilers, one for steam and the other for hot water, have been designed by the NCB and operated at commercial sites. Both were installed in 1977, were operated through the 1977-78 and 1978-79 heating seasons with NCB supervision, and are expected to continue in commercial operation. The steam boiler, at a factory in Lancashire (see photo), incorporates automatic start-up and load-following systems. It successfully met the heat demand of up to 2 M W throughout the 1978-79 heating season, during which it was the only operational boiler at the site. The hot water boiler, at a market garden in Hereford, has a rated output of 3 MW, but has only occasionally been re-
quired to operate above 1 MW. It has been modified to facilitate control at low outputs. With the experience gained from these two prototypes, the N C B is collaborating with Vosper Thornycroft (U.K.) Ltd. to design a range of vertical shell boilers. The first of these boilers, rated at 4.5 M W , is being installed at a district heating scheme near London and will be commissioned early in 1980. Five similar boilers are to be installed at various sites during 1980. The traditional method of incorporating a vertical combustion chamber into a horizontal shell boiler is to use a locomotive boiler configuration. To demonstrate this concept for FBC packaged boilers, the NCB has designed and installed, at a factory in the Midlands, a IO-MW unit. This is basically a development boiler, with several novel features in the FBC system, but it is now reaching the stage where it will reliably meet the factory load. So far there is no commercial manufacturer of this type of boiler in the U.K., but in the U S . , the Johnston Boiler Company (JBC) has adopted a locomotive-type design for its FBC boiler range (Figure 1). Using U.K.
Vertical shell boiler. This prototype 2 - M W steam unit has operated since I977 at a factory in Lancashire. U.K.
technology licensed through CSL, J BC designed and manufactured a 3-MW ( I O 000 Ib/h) prototype, which was put into operation in September 1977. This has successfully fired natural gas, distillate and residual fuel oil, and numerous bituminous coals, as well as waste materials including logged wood, sawdust, shredded rubber, waste oil, and paper sludge. J B C is now offering its “Fluidfire” multifuel boilers for outputs up to 15 M W on a standard commercial basis. The first production unit, a boiler of 40 OOO-lb/h output for Central Soya, has been shop tested and should be in operation by February 1980. A second unit of 20 OOO-lb/h output is scheduled for installation at another site a month later. The alternative approach of developing packaged water-tube boilers for FBC has not been ignored in the U.K. and is being followed by Stone-Platt Fluidfire. This company, which pioneered the use of gas-fired fluidized beds for metallurgical heat treatment, has developed a fluidized combustion method for coal and other solid fuels which incorporates circulation of the bed particles to improve combustion and sulfur retention. They have provided small (300 kW) boilers for in-
Locom
Limestone bunker
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used, to the incineration of waste with a low heat content. I n the U.K., the N C B has demonstrated the use of FBC to dispose of coal washery tailings, which is a slime comprising 50% water, 35% ash, and 15% coal. This technology has been licensed through C S L to Heenan Environmental Systems, which designed and built an incinerator for sewage sludge at Caernarvon, North Wales, that has been in operation for about two years.
Grass dryer. FBC furnaces can be used to supply hot gas f o r dryingplants, such as this grass dryer producing cattle feed
vestigation by Virginia Polytechnic Institute and General Motors, and will shortly be supplying a 3.75-MW u n i t to a factory in northern England with U.K. government support. Stone-Platt has recently acquired a majority interest in the Johnston Boiler Company, strengthening the FBC expertise and marketing potential of both organizations.
New 30-MW boiler The National Coal Board, in cooperation with M E Boilers Ltd., has designed and built a 30-MW FBC coiltype boiler to produce superheated steam at 43 bar. This is at the steel works in Sheffield and will burn commercially available coal sized 18-0 mm, with start-up by premixed air and coke oven gas. It is to achieve a turndown ratio of 6:l by having a circular bed divided into six independent compartments. The requirement to provide a rapid increase in steam supply from low output will be met by maintaining at least three of the compartments on stand-by at a temperature from which coal combustion can be ignited, the beds expanding into the tube bank on fluidization. A single compartment has already been fired on gas and it is planned to bring the boiler into operation in early 1980. Other applications Drying processes are major users of heat in industry and agriculture, and the firing of dryers was identified as a 274
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potential application for FBC in 1973. Development work has so far concentrated on applications where the hot combustion gases are in direct contact with the product. This restricts coal firing to products in which a small amount of ash contamination is acceptable, but there are many of these. In the FBC furnace, all of the heat released by combustion is removed from the bed by the combustion gases (plus substantial air), giving hot gas at up to 950 "C. The commercial application of FBC to the firing of grass dryers for producing cattle feed was first demonstrated in 1974; there are now five 5-MW FBC grass dryers in commercial operation, and further units are to be installed in 1980. The manufacturer, G. P. Worsley Ltd., is collaborating with the NCB to extend the output range and apply the furnaces to other processes. A 15-MW furnace has recently been commissioned to dry clay in cement manufacture, and a unit has been designed for drying and heating roadstone. Work is now being initiated to investigate methods of adopting FBC technology to produce dust-free and uncontaminated hot gas so that almost any product may be dried with hot gas from coal. Another potential use of FBC is in the incineration of waste products. Such applications range from combustion of high calorific value materials such as wood waste, for which almost standard FBC boilers can be
Improving generation efficiency Energy research has for many years been concerned with improving the efficiency of electricity production. At present only about 38% of the fossil fuel energy consumed by a power plant is converted to electricity; the remainder is discharged to the atmosphere through cooling towers. An accepted route to higher power-generating efficiency is to incorporate a gas turbine with a steam turbine in a combined thermodynamic cycle; but this route has been precluded until now by the high cost of premium fuels required by gas turbines. It was recognized in the late 1960s, however, that FBC could be used for a coal-fired gas turbine that would achieve high efficiency. The relatively low combustion temperature of FBC means that the alkali compounds harmful to turbine blades are not volatilized; also, the ash particles in the gases are not vitrified, and are consequently less erosive than those from other coal combustion systems. The first stage in the development of this advanced concept was to demonstrate that a coal-burning FBC unit could operate at elevated pressure. Work, started in 1968 at the British Coal Utilization Research Association (now the NCB's Coal Utilization Research Laboratories), and in 1969 a 2-MW pilot plant was commissioned. In a major research program over 10 years (with funding from the U S . Department of Energy since 1972), this plant has shown not only that the gas quality is likely to be acceptable for turbines, but also that operation at elevated pressure improves combustion efficiency and sulfur retention while lowering emission of NO,. The next major step will be the commissioning in 1980 of the International Energy Agency 80-MW (thermal) test facility at Grimethorpe in the U.K. This facility, with a bed cross section of 2 m by 2 m and the capability of operating at pressures up to 12 bar, is jointly financed by the U S . , the U.K., and West Germany. Although tests with static turbine blade cascades continue to give fa-
Cas turbine system. The lower combustion temperatures possible with FBC result in reduced eniissions of compounds harmful t o turbine blades This 2 - M W p i l o t combustor, shown aboce its pressure cessel, has been used in a IO-year program aimed at demonstrating the feasibility of combining an FBC boiler with a gas turbine to improce electric generation efficiency
vorable predictions of acceptable blade life, there will be no certainty of success until an industrial-sized gas turbine has run for many thousands of hours. However, the concept promises a fuel saving of at least 6% for utility power generation and is also attractive for the large-scale industrial market where heat and power are required.
Environmental impact The U.K. is fortunate in that its coal reserves have a low sulfur content, with present production averaging only about 1.5% sulfur. Use of coal in place of oil will thus have little impact on the presently acceptable atmospheric SO2 concentrations in urban and rural areas, which have been achieved through a policy of dispersal which relates stack height to fuel sulfur content and thermal output. It is expected that air quality standards which are at present being drafted by the European Economic Community (EEC) will not require a change in U.K. policy, although sulfur retention legislation will be required and is being introduced in countries where the coal has a higher sulfur content, notably Germany. It appears, however. that SO2 concentrations which are acceptable in the EEC countries might be causing a gradual increase in acidity of the already acid soil and inland waters of Scandanavian countries. I f this is substantiated, there could be pressures to restrict SO2 emission throughout Europe, but only in this case is it likely
that the potential of FBC to retain sulfur will be used in the U.K.
Matching the application Over the past decade, FBC has evolved from a design concept for utility boilers to a system which can be considered for a wide variety of applications. Already, a range of boilers, firing units for dryers, and incinerators are commercially available in the U.K. The development of FBC at elevated pressure is nearing the point at which a large gas turbine can demonstrate the high efficiency achievable in combined cycle power generation. Coupled with this variety of applications is the wide range of fuels which can be used (including coal of every type and quality as well as low-grade solid, liquid, and gaseous wastes), and the potential for meeting the wide variety of national environmental regulations. The range of FBC designs and operating concepts is equally wide, and it may not be possible to transfer a design developed for a specific application to other situations in another country. However, to select the best design approach for any application, it is essential to take account of the expertise available in every country. I n particular, the progress made in the U.K., operating coal-burning boilers and furnaces in a commercial environment, complements the work in the U S . , where the use of high-sulfur coal necessitates greater attention to emission control.
Acknowledgments The author wishes to thank the National Coal Board for permission to publish this article, but the views expressed are his own and do not necessarily reflect those of the U C B . Acknohledgment is made of the comments and suggestions made on the first draft by the author’s colleague, Mr. D. C . Davidson.
Additional reading H. R. Hoy and W. G . Kaye, “Work by the N C B on the development of atmospheric and pressurized fluidized bed combustion,” J . Inst. of Energy 52, no. 41 1 (June 1979) and other papers in this issue. D. B. Henschel, “Emissions from FBC boilers,” E S & T 12, 534 (May 1979). K. S. Murthy et al., “Emissions from pressurized FBC processes,” ES& 7 13, 197 (February 1979).
John Highley is responsible f o r industrial coal burning and ancillary equipment projects at the ,Vational Coal Board’s Coal Research Establishment. He was a niember of the iVCB research team that pioneered the decelopnient of FBCfor utility boilers f r o m I967 t o 1972. He receiced a B S c . degree in chemical engineerrngfroni Imperial College, London, in 1964. Volume 14, Number 3, March 1980
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