Shelton Ehrlich Pope, Evans, and Robbins Arlington, Virginia
Air pollution control through
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n the next 15 years, as much coal and oil-fired power generating capacity will be built by U.S. power companies as exists today. Depending on the rate of breeder reactor development, fossil-fuel plant capacity may then double again before the year 2000. The need for methods to reduce sulfur oxide emissions from these as yet unbuilt boilers is, therefore, quite pressing. Nevertheless, in 1970, more than 6000 Mw. of new capacity will be brought on line without any gaseous emission control systems added or even allowed for in the design A number of flue gas cleaning processes have been proposed, all of which suffer from the common liability of having to treat enormous gas volumes that contain very low concentrations of sulfur oxides. For example, a hot gas treating system-such as the catalytic oxidation, alkalyzed alumina, or molten carbonate process-appended to a 480 Mw. boiler might have to handle 2,000,000 cubic feet of flue gas per minute containing as little as 0.2% SOz. Successful commercial development of systems that can cope with these liabilities is a formidable task which appears, as of this date, to have eluded some of the best talent in American industry. Removal of the sulfur from fuel before it is burned also appears difficult and costly in the case of coal. For oil, desulfurization is technically simpler but still costly. Possibly, an economical solution lies in changing the combustion system and boilers. Two radical departures in steam generation technology look promising for making power generation from fossil fuels cleaner and cheaper. Supercharged
The new technology suggested here for oil involves combustion at an elevated pressure. Although the basic idea is an old one (the Velox cycle 396 Environmental Science & Technology
was suggested in the 1920’s), it has been updated and improved by Gorzegno and Zoschak of the Foster Wheeler Corp. This concept has actually been applied with some success to propulsion units for several U S . naval vessels. In one of Gorzegno and Zoschak’s design concepts for a 480 Mw. power station, the fuel would be burned at a pressure of 8 atmospheres. This supercharged boiler system would actually cost less to build, and would operate more efficiently than a one atmosphere boiler. The reason for the capital cost improvement is that fewer boiler tubes are needed to absorb heat from high pressure gas than from a hot gas at atmospheric pressure. The efficiency improvement arises from the power generated by the gas turbine in excess of that required to compress the combustion air. Gas volumes
A gas cleaning process added to Gorzegno and Zoschak’s boiler design would have one major advantage over those added to conventional boilers. The sulfur oxide removal process for such a high pressure boiler would handle only one eighth the volume of gas of an equivalent atmospheric pressure boiler. And since the gas cleaning process handles a far smaller gas volume, it can treat it more intensively. For example, instead of a typical pressure drop measured in inches of water column for a conventional system, it may be measured in pounds per square inch for the pressurized boiler unit and still be feasible. Any of the proposed gas cleaning processes which recover elemental sulfur may then succeed in the high pressure system, while failing economically in the one atmosphere unit. Based on the successful application to ships, it appears that the first power boiler of this advanced design could be on line for power genera-
tion in less than five years and in such a system sulfur recovery might be profitable. Although residual fuel oil is of increasing importance for power generation, it is still far less important than coal. Coal still remains-and will remain for some time-the least costly fuel for power production on a national scale. For coal, supercharging does not appear to be an immediate prospect; however, work at the U.S. Bureau of Mines and the British Coal Utilization Research Association (now BCURA Industrial Laboratories) may make it a strong contender for the 1980’s. On the other hand, fluidized-bed combustion of coal at atmospheric pressure and the removal of sulfur oxides before or as they are formed does have immediate potential for power generation without air pollution. Fluidization technology has been well documented and will not be described here. But, it is appropriate to note that fluidized-bed combustion of coal was invented by Winkler in 1921 before the term fluidization was coined. There are also very important differences between the fluidization techniques of the chemical and petroleum industries and those practiced by the steam-power engineer, The differences are sufficiently profound to cause a number of patents on fluidized-bed steam raising based on refinery art to be commercially worthless. A coal-burning, fluidized-bed boiler has several economic advantages over conventional steam generators. The combustion rate, measured in pounds of fuel per hour per cubic foot, may be five times the value of that in a conventional boiler. Therefore, the furnace could be smaller. The effective average heat flux into the boiler tubes may also be five times as high as in the conventional boiler, and fewer boiler tubes would be required. Combustion takes
feature
new combustion processes place at temperatures low enough (1 600’ F.) to preclude the formation of corrosive ash compounds. Therefore, the tube metals may be less costly. The peak heat flux may be two thirds the value of that found in a conventional boiler, so thinner tubes of lesser alloys would be required. Finally, one of the major economic advantages is the ability of the fluidizedbed furnace to burn fuels having a very wide range of characteristics. Foreign technology
In order to exploit these potential economic advantages, research and hardware development activities have been undertaken in a number of countries. including France, Czechoslo-
vakia, England, and the United States. The work in France and Czechoslovakia has not been aimed at low cost boilers as described above, but at the development of fluidized-bed furnaces which could burn low grade fuels and even mine wastes. In France, Godel developed and commercialized the Ignifluid boiler which was originally intended for combustion of anthracite fines. Ignifluid is not a fluidized-bed boiler as described earlier, since no boiler tubes are “wetted” by the fluidized bed. However, it is a very effective fluidizedbed combustor and has been applied to coals so poor that no combustion system then available could use them. Because most of the ash is fused in
the process, Godel’s Ignifluid boiler incorporated a moving grate for ash removal. Improvements
Godel’s prolific mind has conceived a number of promising improvements to his already successful design. He has French and U.S. patents on a scheme for integrating sulfur oxide removal, heat transfer, and carbon fines combustion. One of Godel’s recent schemes involves burning a high sulfur coal in a lower, agglomerating, fluidized bed and removing sulfur oxides in a second bed which also burns the carbon fines and acts as a heat exchanger. There are some problems to be solved before the scheme could be
Supercharged oil combustion
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successfully applied, but, though serious, these problems are not insurmountable, given Godel's talent for making things work. In Czechoslovakia, Novotny and Koutnik at the Fuel Research Institute also have attempted to develop a system that could burn very poor coals in a compact unit. They sought to eliminate the necessity for a chain grate by avoiding fusing of the ash. They achieved this by burning in two stages. The first stage is a fluidizedbed furnace (with no boiler tubes) operating under reducing conditions; a second stage operates much like a cyclone furnace. In the first stage, much of the sulfur could be taken up by calcium oxide as calcium sulfide, which could then be roasted to produce SOz. In the particular application of Novotny and Koutnik's design to mine washings and extremely high ash coals, separation of ash from the calcium oxide presents a problem. However, the ash left in most American coals that have been cleaned of stones is so fine that it is washed from the fluidized bed by the flue gases as the coal is burned. The goal of the English work was not the combustion of low grade coals but the development of low cost boilers to compete with gas and oil in industrial size units and with nuclear power in the utility industry. The work that is furthest along is that by Thurlow and Wright (now under the direction of Hoy) of BCURA. They have developed an operational fire tube boiler capable of producing 8000 lb./ hr. of steam. A boiler of this size would be suitable for British laundries, apartment houses, and applications of similar size. Unfortunately, control of sulfur oxides in this size range is not practical by any known means. However, these men were the first to show that it was possible to burn coal at practical intensities at temperatures as low as 1500° F. With a reactor operating at that temperature, a number of schemes for removing sulfur oxides can be conceived that might be practical for boilers 10, 100, and 1000 times the capacity of their unit. The British program in fluidizedbed combustion, now being directed by the National Coal Board, has recently taken on high priority. It may well be that the next coal-fired, central station boiler built in Britain will be a unit using this new combustion concept. 398 Environmental Science & Technology
lgnifluid coal combustor b Flue
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gas to stack
Dust collectors
T i d i z e d bed economizer
Gas distributor
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o sulfur recovery system
SOZ sorbent
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Combustion
Fluidized bed superheater Superheated steam to turbine
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Stateside prospects
Despite their promise, the three fluidized-bed designs described above will probably not be applied in the United States. Neither the Ignifluid nor the Czechoslovakian design would enjoy a capital cost advantage over existing boiler technology. Neither exploits the heat transfer advantage of a fluidized bed. Also, Ignifluid loses some fuel when carbon is fused into discharged ash. BCURA'S fire tube or shell boiler, which is a true fluidized-
bed boiler, would also not find a U.S. market since gas and oil predominate in boilers of that type. However, there is reason to hope that the current British effort will result in superior designs for power boilers which U.S. utilities might request from American boiler manufacturers. I n 1965, the Department of Interior's Office of Coal Research began a very modest program for the development of coal-fired, fluidized-bed boilers. The agency chose as a first goal the smallest and simplest system that
could compete in the U S . marketplace-a shop-assembled, rail-transportable boiler capable of producing 250,000 pounds of steam per hour. Since coal-fired boilers above 50,000 lb./hr. could not be shop assembled, the goal established-a fivefold increase in capacity-was modest only in the funding level. Based on an idea of Michael Pope, this effort led to the design by my colleague, John Bishop, of a totally unconventional steam generator in which high levels of air preheat were essential. The design for this boiler, the Pope-Bishop boiler, is the most compact of the four systems described in this ,article. It initially presented the problem typical of fluidized-bed combustion: low efficiency due to unburned carbon being blown from the unit. Reinjection of collected fly ash as a means of solving the problem was ineffective, since once-fired carbon is a rather unreactive substance especially when competing for oxygen with fresh coal. This led to the invention of a carbon burnup cell, which appears to have solved the problem. Another feature of the design is the manner in which a very large watercooled fluidized bed is brought to op-
Fluidized bed furnace for mine wastes
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Low carbon ash
Prototype BCURAfluid bed boiler Steam outlet
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England. Fluidized bed unit installed at BCURA’s labs Volume 4, Number 5, May 1970 399
Pope-Bishop boiler Main steam header
Coal and SO2 sorbent
Cross headers
Air distribution grid
erating temperature. A prototype section of such a boiler is now operational at our laboratory. SOacleanup
In Thurlow and Wright’s boiler and in the Pope-Bishop boiler, conditions are ideal for the capture of SO, with limestone. For this reason, the National Air Pollution Control Administration (NAPCA) of the Depairtment of IC-_L^_ Health, Education, and Weu.uc sponsored work in this area. Since funds were not available in 1969 to the Office of Coal Research for continuing development of the boiler, NAPCA has undertaken to conduct some studies on boiler development in addition to their work on the air pollution control potential or fluidized-bed combustion. The Pope-Bishop boiler wuld be ready for market in two years or less. If it is a technical success, US. boiler manufacturers should he able to readily extrapolate the concept to very large utility boilers, certainly before 1980. Cost estimates indicate that the 250,000 Ib./hr. boiler would enjoy a 50% capital w s t advantage. The advantage for larger utility boilers would probably be somewhat less, though still substantial. It also seems quite likely that, given a “reactor” operable in the range of 1450O to 2150° F., sorbents other than limestone will be 400 Environmental Science &Technology
found for economical control of sulfur oxides. The boiler designs that have evolved over almost one-half century for burning pulverized coal, and which are essentially the same for gas and oil., are tremendously successful in every respect except control of sulfur oxide emissions. If it were not for this flaw, there would be little hope that industries as unde rstandibly conservaL-:l.LLVC db u w u - m m c r s and utilities could be enticed to try superchars:ed oil combustion or fluidized-bed ccial combustion. This is true despite the cost advantage of the new systems;. However, if attempts at control tlirough the cleaning of flue gas continue to be frustrated by high costs or lack of effectiveness, a change in thi? way highsulfur oil and coal are bunled may be justified. ~
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ward clean air, they are nor insrant solutions. While the search continues for the instant solutions, a commitment of resources to combustion alterations appears justified. Although unreported in the U S . press, such a commitment has been made in England. It appears that a similar effort may also be under consideration in the
us.
No instant solution
To bring off such a basic change will involve development cos& measured in millions of dollars and the best talent in the boiler and utility industries, as well as by researchers and consulting engineers. It takes money, manpower, and, unfortunately, a great deal of time to make fundamental changes in a large and well developed industry. While supercharged combustion of oil and fluidized combustion of coal hold the promise of a route to-
Shelton Ehrlich is assistant program manager f o r fluidized combustion projects at Pope, Evans, and Robbins’ Arlington (Va.), laboratory. He received his B.S. f r o m the University of Missouri in 1957 and an M.S. from the University of California. H e first joined Pope, Evans, and Robbins in 1959.
