Ind. Eng. Chem. Prod. Res. Dev. 1986, 25,348-355
348
and Space Administration under Contract NAS3-22510. We are indebted to Drs. Ed Wong and David Bittker of NASA for advice and consultation and to Dr. S. E. Buttrill, Jr., G . A. St. John, Dr. R. Malhotra, and Dr. Doris Tse of SRI International for the FIMS data. David Dulin did the first oxidation experiments. Registry No. DOD, 112-40-3;TET, 119-64-2;BCH, 92-51-3; 1-PH, 1077-16-3;PCH, 827-52-1;EtN, 939-27-5;IND, 95-13-6; NMP, 96-54-8; t-BuNH,, 75-64-9; HC‘02H,64-18-6; pyridine, 110-86-1.
Literature Cited Alagy, J.; Clement, G.; Balaceanu, J.-C. Bull. SOC.Chim. 1961, 28, 1792. Bowden, J. N.; Brinkman, D. W. “Stablllty Characterlstics of Some Shale and Coal Liquids”; Final Report to U.S. Department of Energy on Contract No. EY-77-A-02-4162, 1960. Buttrill, S. E., Jr.; Mayo, F. R.; Lan, 6.; St. John, G. A.; Dulin, D. NASA Contractor Report 165534, Jan 1982. Cernansky, N. P.; Cohen, R. S.;Reddy, K. T. Final Report on NASA Grant Award NAG3-183, June 1965. CRC Report No. 509 Coordinating Research Council, Atlanta, 1979. CRC Report No. 509, p 53. CRC Report No. 509, pp 59, 76. Frankenfeld, J. W.; Taylor, W. F.; Brinkman, D. W. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 606, 615, 622. Goetzinger, J. W.; Thompson, C. J.; Brlnkman, D. W. A Review of Storage Stability Characteristics of Hydrocarbon Fuels, 1952- 1982; Bartlesvlile Energy Technology Center, L‘ ?. Department of Energy: Bartlesville, OK, 1983.
Hazlett, R. N.; Hail, J. M.; Matson, M. Ind. Eng. Chem. Prod. Res. Dev. 1977, 16, 171. Howard, J. A. Adv. Free-RadicalChem. 1972, 4 , 49. Malhotra, R.; ,C?ggiola, J.; Young, S. E.; Tse. D.; Buttrlll, S. E., Jr. “Analysis of Middle Distillate Fuels by High Resolution Field Ionlzation Mass Spectrometry” Prepr. Pap .-Am. Chem. Soc ., Div. Fuel Chem. 1985, 30(1), 192. Mayo, F. R.; Miller, A. A. J. Am. Chem. SOC.1956, 78, 1023. Mayo, F. R.; Syz, M. C.; Mill, T.; Castleman, J. K. Adv. Chem. Ser. 1988, 7 5 , 38. Mayo, F. R.; Richardson, H.; Mayorga, G. D. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 1978, 20(1), 38. Nixon, A. C. I n Autoxidation and Antloxidsnts; Lundberg, W . 0.. Ed.; Interscience: New York, 1962; Vol. 2. Russell, G. A. J. Am. Chem. SOC.1955, 77, 4583. Russell, 0. A. J . Am. Chem. SOC.1956, 78, 1035, 1041. St. John, G. A.; Buttrill, S. E., Jr.; Anbar, M. ACS Symp. Ser. 1978, 71, 223. Smith, H. M.; Ward, C. C.; Schwartz, F. G. et al. Distillate Fuel Storage Stability; Western Petroleum Refiners Association: Tulsa, OK; US. Bureau of Mines: Bartlesville, OK, 1958. Stavinoha, L. L.; Brinkman, D. W. “Accelerated Stability Test Techniques for Diesel Fuels”; Report to US. Department of Energy under Contract DEAC19-79BC10043, 1980; p 36 ff. Stavlnoha, L. L.; Brinkman, D. W. “Optimization of Accelerated Stability Test Techniques for Diesel Fuels”; Final Report to U.S. Department of Energy on Contract No. DE-AC19-79BC-10043, 1981. Taylor, W. F. Ind. Eng. Chem. Prod. Res. Dev. 1974, 13, 133.
Received f o r review May 23, 1985 Revised manuscript received October 28, 1985 Accepted December 11, 1985
Byproduct Gypsum from Flue Gas Desulfurization Processes Harvey S. Rosenberg Battelle Columbus Laboratories, Columbus, Ohio 4320 1
Byproduct FGD gypsum is produced in the United States, Japan, and West Germany. Numerous FGD process variations have been developed for gypsum production. The gypsum quality depends upon the type of process and the operating conditions. The ability to market byproduct FGD gypsum, mainly for use in wallboard or cement, depends upon the supply-demand situation and the gypsum quality. The major reason for FGD gypsum production in the US. is to make the disposal of the solid waste less difficult. I n Japan, all of the FGD byproduct gypsum is used to satisfy 40% of the gypsum demand. I n West Germany, the plan is to market all of the byproduct gypsum, but it is likely that the supply will exceed the demand as more FGD systems come on line.
Introduction Gypsum can be produced from flue gas desulfurization (FGD) processes that use lime or limestone directly for wet scrubbing or indirectly for precipitation of calcium salts and regeneration of the sorbent. The indirect processes were developed mainly to avoid scaling problems in the SO2 absorber and are not as widely used as the direct processes. Therefore, this discussion is focused mainly on the direct processes. In these processes, forced oxidation is required to convert the calcium sulfite to sulfate. There is some natural oxidation of sulfite to sulfate in wetscrubbing processes because of slurry exposure to flue gas oxygen and NO, in the prescrubber and/or absorber and to air in the recycle tank. The level of natural oxidation depends upon several factors including the sulfur content of the fuel, the amount of excess air to the boiler, the presence of oxidation catalysts (fly ash and/or corrosion products) or inhibitors (organic compounds, thiosulfate) in the slurry, the ratio of NO to NO2 in the flue gas, the design of the recycle tank (open vs. covered), etc. Forced oxidation is carried out (1) to prevent scaling in the absorbers by providing abundant gypsum crystals in the slurry for preferential nucleation, (2) to aid in sludge
dewatering for disposal purposes, or (3) to produce gypsum for sale. The forced oxidation can be carried out in situ in the scrubbing loop or on a bleed stream from the absorber. In situ oxidation is usually not practiced with wet lime scrubbing because the pH of the scrubbing liquor is too high. Bleed-stream oxidation has the advantage of separating the forced-oxidation process completely from the S02-absorption process. This is important in producing salable gypsum because the oxidation tank can be designed to promote the growth of the large crystals desired for wallboard. The main drawback is that any excess reagent left in the absorber slurry increases the pH of the bleed stream when the latter is withdrawn from the scrubbing loop. Sulfuric acid can be added to the bleed stream to react with excess lime or limestone and lower the pH to promote oxidation. Because sulfuric acid is expensive, additives such as MgO are sometimes used to promote oxidation (in situ as well as bleed stream) as well as increase the SO2 removal efficiency. If the forced-oxidation system is installed to provide gypsum seed crystals for the scrubbing slurry and/or to reduce dewatering cost and improve the disposal operation, the design can be relatively simple because the product
0196-4321/86/1225-0346$01.50/0 0 1966 American Chemical Society
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does not have to be as pure as when it is made for sale. Air can be sparged into the recycle or reaction tank without any need to use levels of FGD system operating variables any different from those considered optimum for operation without forced oxidation. Even the pH does not have to be changed. Although use of the same tank for oxidation and limestone addition seems to be feasible, the increase in pH resulting from the limestone addition is clearly not conductive to oxidation. Better results can be obtained by using two tanks with oxidation in the first one (on slurry directly from the absorber) and limestone addition to the second. If forced oxidation is used with wet scrubbing with the objective of making salable gypsum, consideration must be given to the product purity required. Gypsum has two major uses: (1) the production of stucco, from which plasters and prefabricated construction materials are made, and (2) as a minor ingredient in portland cement to retard the setting rate. Most of the stucco is used to manufacture wallboard for interior surfacing material, which has largely replaced gypsum plasters once used for the same purpose. There are stringent demands on the properties of gypsum used for wallboard manufacture. Factors that affect the properties of the casting slurry or the finished board must be carefully controlled. Among these are the calcining characteristics that affect slurry properties such as flow characteristics and setting rate and impurities that could cause poor bonding of the paper, reduced strengths, and efflorescence and discoloration. Important properties of the gypsum include the particle size, which determines the slurrying properties, and the presence of soluble salts, even at low levels. Wallboard manufacturers control raw gypsum properties to some extent by selective mining and blending. They also have extensive experience in the use of additives to modify the effects of gypsum properties. The gypsum specifications in the cement industry are generally not as strict with regard to the level of impurities allowed. The level of chlorides and other impurities in the fuel, makeup water to the FGD system, and lime or limestone affect the purity of the byproduct gypsum. In order to obtain high-purity gypsum, there must be an efficient particulate collection device ahead of the FGD system. Also, there should be a separate prescrubber loop to remove impurities, and/or the gypsum should be washed. A separate prescrubber loop allows the particulate collection device to be less efficient because removal of residual particulates can be accomplished in the prescrubber. However, a bleed stream from the prescrubber must be discharged to the environment. If the gypsum is washed and the FGD system is operated with a closed water loop (no discharge), then the impurities build up in the scrubbing slurry. A t some point, it is necessary to blow down the system to keep the impurity concentration within limits. (Chloride is a particular problem with regard to its effect on both SO2removal efficiency and corrosion.) Permits to discharge either a prescrubber bleed stream or an FGD blowdown may be difficult to obtain in some locations. If forced oxidation is carried out in the scrubbing loop, two-stage scrubbing is a method for achieving the very high limestone utilization needed to meet wallboard specifications. Either cocurrent or countercurrent scrubbing is used in the first stage, but countercurrent scrubbing is typical for the second stage. If single-stage scrubbing is used (cocurrent or countercurrent),high utilization may require separate tanks for oxidation and limestone addition, the use of an additive such as adipic acid (increases limestone
utilization), and the use of hydroclones (liquid cyclones) for dewatering. However, insufficient data are available to assure that these measures would be adequate. With either in situ or bleed-stream oxidation, the oxidizer must be designed to enhance mixing for improved air/slurry contact to increase oxygen diffusion. For in situ oxidation, this is usually done by distributing compressed air into a sparger located in the bottom of the oxidation tank. Some variations include the use of a flue gas sparging system for SO2 absorption and sulfite oxidation or air injection below the SO2-absorption zone but above the liquid level in the absorber. For bleed-stream oxidation, an oxidation tower with compressed air fed from the bottom through a rotary atomizer is the typical design. FGD byproduct gypsum can be dewatered in continuous vacuum belt filters (rotary or horizontal) or continuous or batch centrifuges. Belt filters allow for easy washing of the gypsum but require primary dewatering in a thickener or hydroclone. Continuous centrifuges may or may not require primary dewatering. Batch centrifuges do not require primary dewatering and yield a product with the least amount of surface moisture. Further drying of the gypsum may increase its marketability because of reduced handling problems and shipping costs. However, the drying step can add significantly to the processing cost. FGD byproduct gypsum is usually the dihydrate form of calcium sulfate (CaSO4.2Hz0). Gypsum users may prefer different chemical forms such as a or 0hemihydrate (CaS04J/2H20)crystals, called plaster of Paris, that can be produced by calcining the dihydrate. The 0form can be produced only by thermal calcination and is the main raw material for wallboard. The a form, which has special uses, can be produced under less energy intensive conditions by heating gypsum in water or in a steam atmosphere. It has been suggested to use the a hemihydrate to control mine subsidence. In addition to the technical difficulties, there is the problem of developing a dependable market and of continuing to produce an acceptable gypsum after the market is developed. If selling the gypsum would eliminate the cost and operating problems of solid-waste disposal, the situation would be different. However, there would still be the ash to dispose of, for which the tonnage is usually at least equal to that of the sludge and sometimes several times greater. Moreover, it would be necessary to provide sludge disposal facilities even if the gypsum were sold-so that power production could continue if the market failed. Thus far, FGD processes that produce gypsum have been developed and/or are used in mainly three countries, the United States, Japan, and the Federal Republic of Germany. The situation in each country with regard to production and use of FGD byproduct gypsum is quite different as discussed below.
Situation in the United States Interest in gypsum-producing FGD processes was slow to develop in the United States. There was little incentive to produce gypsum for sale, the prospects for which were consistently regarded as poor. The first forced-oxidation systems were designed to provide gypsum seed crystals to control scaling in the absorbers. By the mid-l970s, interest in forced oxidation for waste disposal purposes was growing. During this period, most suppliers of wet lime and limestone FGD systems developed forced-oxidation versions of their processes. There are 28 known FGD installations with forced oxidation in operation or planned at 21 sites in the US. (Melia et al., 1985; Smith, 1984). Practically all of the installations utilize limestone as the reagent, and two of the sites utilize processes supplied or
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Flue I I
I
I Air
Air
US. Type 2
U.S. Type 1
,
Flue
Flue I Gas
I
Gas;
L
I I
--
t
Ellison and Kutemeyer (1984).
I
Air
U S . Type 3
?
CaC03,HzO
..
c
I
I Air
U S . Type 5
U S . Type 4
_ _ _ -w A Absorber
Table I. Tentative Specifications for FGD Byproduct Gypsum in the U.S." w m u m user United National Georgia States Gypsum Pacific Gypsum specification co. Corp. co. purity, % 94.0 90 calcium sulfite content, % 0.5 sodium, max ppm 500 200 75 800 200 I5 chloride, ppm 500 50 magnesium, ppm 15 max 10 12 max free water, % 6.0-8.0 3.0-9.0 6.5-8.0 PH particle size, pm 20-40
;as
-
OX: Oxidlzer
Liquid or Slurry
RT: Recycle Tank
Figure 1. U.S.wet lime/limestone FGD processes with forced oxidation.
developed by Japanese companies. The total utility FGD capacity currently in operation with forced oxidation is about 9700 MW. This represents about 20% of all the utility FGD capacity and about 40% of the wet limestone FGD capacity currently in operation. Since the trend is away from lime and toward limestone as the reagent for wet scrubbing because of the cost differential between the reagents, the percentage of FGD capacity employing forced oxidation may increase in the future. A complicating factor is the developmentof spray dry scrubbing with lime, which is competitive with wet scrubbing for low-sulfur coal. The U.S. FGD installations with forced oxidation can be classified into six process types as follows: U.S. Type 1, single-stage scrubbing with in situ oxidation in recycle tank. U.S. Qpe 2, single-stage scrubbing with in situ oxidation in oxidation tank ahead of recycle tank. US.Type 3, same as US.Type 2 except oxidation tank is in bottom of absorbers and one absorber is operated at a lower pH to complete oxidation. U.S. Type 4, two-stage scrubbing with in situ oxidation in first-stage recycle tank. U.S. Type 5, single-stage scrubbing with bleed-stream oxidation and MgO added to absorber loop (a similar process has been developed in Japan). US. Type 6, two-stage scrubbing with bleed-stream oxidation and H2S04added to bleed stream (developed in Japan and described later as Japan Type 2). Schematics of the first five of the above process types are shown in Figure 1. Most of the newer installations utilize in situ oxidation for sludge dewatering rather than
for control of gypsum scaling as at the older installations. Only four sites, St. Johns River (Jacksonville, FL), Muscatine (Muscatine, IA), Big Bend (Tampa, FL), and Pasadena (Pasadena, TX), plan to produce gypsum for sale. The former three will utilize in situ oxidation (U.S. Type 4) and the latter will utilize bleed-stream oxidation (U.S. Type 6/Japan Type 2). St. Johns River is not scheduled for start-up until 1987. The FGD system at Muscatine is designed to produce wallboard-gradegypsum, but this is not guaranteed by the supplier. Although start-up was in June 1983, there has been no sale of gypsum yet; the waste is being used as landfill. The gypsum quality was good at first but has deteriorated, possibly because too much limestone is fed to the first stage. This causes poor oxidation and high (17%) CaC03 in the dewatered sludge. The problems can be attributed to the high SO2 removal efficiency (96%) required and the high-sulfur (3.2%) coal. The FGD system at Big Bend is currently undergoing start-up. The utility people do not have a contract to sell the gypsum yet, but they have several prospects. They intend to sell the gypsum to an intermediary company, which will make pellets and market the product to the cement industry. Meanwhile, the waste is being used as landfill and is not yet of sufficient quality for recovery. The planned cogeneration system in Pasadena, TX, is unique in that petroleum coke (5%-7% sulfur) will be used as the fuel (Smith, 1984). The limestone FGD system will have a wet electrostatic precipitator for mist elimination and will utilize bleed-stream oxidation with sulfuric acid addition to produce gypsum for wallboard. The process is based on Japanese technology. Start-up is scheduled for June 1986. Tentative specifications for FGD byproduct gypsum have been offered by three large gypsum users in the U.S. and are shown in Table I (Ellison and Kutemeyer, 1983). A recent Tennessee Valley Authority (TVA) report (O'Brien et al., 1984) discusses the 1985 marketing potential for FGD byproduct gypsum for the eastern twothirds of the U.S. The 114 cement plants and 52 wallboard plants in this area were assumed to be the potential market for FGD gypsum sales from 14 selected power plants. According to this study, the power plants could market 4.35 million tonslyear of gypsum (92% of their production), filling 63% of the cement-plant requirements and 20% of the wallboard-plant requirements. Cement plants are a geographically disperse market available to most power plants, but able to absorb the production of only a few power plants; wallboard plants are a larger market, but for them power-plant location is a more important marketing factor. Other variations on the marketing model indicated that (1)drying and briquetting had little effect
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
on the marketing potential, (2) sales were reduced 25% when the savings in FGD cost were not used to offset freight costs, and (3) relocation of wallboard plants to sources of byproduct gypsum appeared economicallyfeasible in some cases. Situation in Japan FGD efforts in Japan have been oriented toward processes that yield salable byproducts because Japan is subject to limitations in domestic supply of natural sulfur and its compounds and to limitations in land space available for solid-waste disposal. In addition, most of the boilers in Japan are oil-fired (very little ash is produced), the fuel is low in sulfur (easier to achieve reliable FGD operation; less byproduct produced per MW), and the industrial areas are located mainly on the southwest coast (an FGD purge stream can be discharged to the ocean after appropriate treatment). The number of FGD installations larger than 10000 (Nm3)/h (equivalent to 3 MW) in operation in Japan as of the end of 1983 totals 470, and the combined capacity is equivalent to about 36000 MW (Ando, 1983b). About 60% of the capacity is accounted for by utility boilers, including 6200 MW for coal-fired boilers. About 6890 of the capacity utilize processes that produce gypsum, 50 % by direct lime/limestone processes and 18% by indirect or modified lime/limestone processes. Gypsum-producing processes predominate because they generally cost less than FGD processes that produce other byproducts. There are nearly 900 FGD installations smaller than loo00 (Nm3)/h that produce mainly sodium sulfite waste liquor for disposal; however, the total capacity of these installations is less than 8% of the total capacity of the installations larger than 10000 (Nm3)/h. A wide variety of FGD processes with gypsum production has been developed in Japan (Ando, 1978). The indirect processes utilize various solutions for SOz absorption and lime or limestone for precipitation. The scrubbing liquors arranged in order of decreasing pH include sodium sulfite, ammonium sulfite, sodium acetate, ammonium sulfate, aluminum sulfate, and dilute sulfuric acid with ferric sulfate catalyst. Lower pH yields smaller SOz-absorption capacity, better oxidation of the scrubbing liquor, and easier reaction with limestone. In cases where limestone reacts very slowly, lime can be used instead. Sodium sulfate poses a special problem because it is difficult to react, even with lime, but has to be decomposed or purged from the system because it does not absorb SO2. With sodium sulfite and ammonium sulfite scrubbing, limestone or lime is added to the absorber bleed stream before oxidation to precipitate calcium sulfite. The calcium sulfite is then oxidized (with sulfuric acid addition) to produce gypsum. With the other scrubbing liquors, the bleed stream is f i s t oxidized to convert sulfite to sulfate (without acid addition) and limestone or lime is then added to precipitate gypsum. Gypsum usually grows in larger crystals in the indirect processes than in the direct processes. The liquor from the gypsum centrifuge is mainly returned to the FGD system, but a small portion is usually purged to control the concentrations of soluble species such as chlorine and magnesium. The indirect processes are fairly complex and are in limited use, mainly on industrial boilers. The aluminum sulfate and dilute sulfuric acid processes have been evaluated at the prototype scale on coal-fired utility boilers in the US. Modified lime/ limestone processes aimed at scale prevention include the use of lime dissolved in a 30% solution of calcium chloride or the use of a magnesium hydroxide slurry containing lime or limestone as the scrubbing liquor. In both cases, bleed-stream oxidation is used to produce
351
gypsum. In the former process, sulfuric acid is added to the bleed stream to enhance oxidation, while in the latter process, the presence of magnesium ions promotes oxidation. Magnesium tends to hinder the crystal growth of gypsum. However, with proper design of the reactor or crystallizer, good-quality gypsum can be obtained even with the presence of a high concentration of magnesium. The magnesium hydroxide modified lime process is in use on coal-fired utility boilers in Japan and is discussed in more detail later. An activated carbon process that produces gypsum has been developed in Japan. The activated carbon adsorbs SO2, H20, and oxygen from the flue gas to form sulfuric acid. The carbon is regenerated by washing with water, which produces dilute sulfuric acid. The dilute acid is reacted with limestone to produce good-quality gypsum. The process is costly in comparison with other FGD processes. Most of the FGD installations in Japan were constructed between 1972 and 1977. The growth rate of FGD capacity has decreased since 1977 because ambient SO2 concentrations have been lowered and not many new boilers have been built because of a recession. Virtually all of the FGD systems constructed since 1978 produce gypsum. These include 18 plants using wet lime/limestone scrubbing with a total capacity equivalent to 4800 MW (mainly limestone scrubbing for coal-fired boilers) and 19 plants using indirect or modified lime/limestone scrubbing with a total capacity equivalent to 1900 MW. Most utility companies in Japan switched from coal to oil as a fuel between 1960 and 1974. An exception was the Electric Power Development Co. (EPDC) which was formed by the government in conjunction with major power companies to use domestic coal. However, the large increase in oil and gas prices has caused power companies to begin to build new coal-fired boilers that will use mainly imported coal. The total capacity of coal-fired boilers is now about 8500 MW (5% of total utility capacity and 9% of thermal utility capacity) and is expected to increase in the future. There are 31 coal-fired boilers with FGD systems in Japan that produce gypsum (Ando, 1983b). There is one 156-MW coal-fired boiler on Kyushu with a wet lime FGD system that produces sludge for waste disposal and several small ones on Hokkaido that do not have FGD systems. All but one of the installations that produce gypsum utilize limestone as the reagent. The FGD installations on coal-fired boilers (excluding the one that produces sludge) can be classified into five process types as follows: Japan Type 1,two-stage scrubbing with a third absorber to lower bleed-stream pH; bleed-stream oxidation with catalyst added to absorber loop. Japan Type 2, two-stage scrubbing with bleed-stream oxidation and H2S04added to bleed stream (same as US. Type 6). Japan Type 3, separate prescrubber loop with purge stream; bleed-stream oxidation with H2S04added to bleed stream. Japan Type 4, separate prescrubber loop with purge stream; in situ oxidation in specially designed absorber. Japan Type 5, separate prescrubber loop with purge stream; bleed-stream oxidation with MgO added to absorber loop (similar to US.Type 5). Schematics of the above five processes are shown in Figure 2 (Ando, 1983b). Most of the new FGD installations use the Japan Type 3 process, which has a prescrubber with a separate liquor loop. The prescrubber serves to remove impurities such as fluorine, chlorine, and fly ash
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Flue
Reaction Zone
Gypsum Slurry
d
Figure 3. Jet bubbling reactor. Reprinted with permission from Clasen (1983). Copyright 1983 Canadian Electrical Association.
dill
JapanType4
Japan Type 5
+ Liquid or Slurry
Figure 2. Japanese wet lime/limestone FGD processes with forced oxidation (Ando, 1983b).
and cools the gas to help prevent scaling and protect the lining in the absorber vessel. However, the prescrubber increases the capital cost of the FGD system and the amount of wastewater produced. It may be possible to eliminate the prescrubber for coals with low fluorine and chlorine or by using additives to counteract the effect of fluorides and chlorides on SO2 removal efficiency. Most of the new installations add a small amount of sulfuric acid to the bleed stream to reduce the pH of the calcium sulfite slurry to 4.0-4.5. This is done to react with excess limestone and to achieve complete oxidation to produce good-quality gypsum. However, the Japan Type 4 and Type 5 processes can achieve complete oxidation without sulfuric acid addition. The former utilizes a jet bubbling reactor, shown in Figure 3 (Clasen, 19831, in which the flue gas is bubbled down into the limestone slurry through a large number of sparger tubes. Air is fed to the bottom of the reactor for in situ oxidation to produce gypsum. The pH is held a t about 3.5 to obtain a high degree of limestone utilization and sulfite oxidation. The process was developed as an improvement to the indirect process that utilizes dilute sulfuric acid with ferric sulfate catalyst for the scrubbing liquor. In the Japan Type 5 process, magnesium oxide is fed to the absorber together with lime to promote SO2removal. The magnesium also promotes oxidation and negates the detrimental effects of chloride on SO2 removal. A unique feature of the process is treating the feed lime with part of the oxidized slurry before introduction into the absorber. The purpose is to convert the magnesium sulfate formed in the oxidizer back to magnesium hydroxide to serve as a reactant for SOz. The absorbent ratio is one part of magnesium hydroxide to at least three to four parts of lime. It is not clear why the absorbent regeneration step is necessary because in the U.S. high-magnesium lime is used without any such regeneration (US.Type 5 process) and with good results even for high-sulfur coal. Perhaps the regeneration step aids in preventing scale formation in the absorber. In Japan, wastewater from the prescrubber and/or absorber loop is treated with lime to precipitate heavy metals
and fluoride prior to filtration (Ando, 1983a). The filtrate is then treated with an ion-exchange resin to remove dithionate ( S 2 0 t - ) ,which forms in the absorber and gives rise to a chemical oxygen demand (COD). The small amount of concentrated dithionate solution obtained by desorption from the resin is either decomposed with an acid and oxidizing agent or incinerated. The treated water usually contains less than 10-15 ppm each COD and suspended solids, less than 15 ppm fluoride, and less than 1 ppm oily material. The pH ranges from 6.5 to 8.5 and thus satisfies wastewater discharge regulations. In Japan, there are no regulations for the concentration of soluble salts such as calcium and magnesium chlorides, since the treated FGD wastewater is discharged into the ocean. If soluble salts must be controlled, as for example in West Germany, then it is important to minimize the amount of water that must be removed from the FGD system. Since Japan has little natural gypsum, there has been a market for virtually all of the FGD byproduct gypsum. In 1980, the annual gypsum demand was about 6.6 million tons including about 2.8 million tons for cement retarder and about 2.4 million tons for wallboard. The supply included about 2.9 million tons of byproduct from phosphoric acid production (phosphogypsum) and about 2.2 million tons from FGD (Ando, 1983a). About 70% of the FGD byproduct gypsum in Japan is said to be used in cement manufacture. The price of byproduct gypsum fluctuates with supply and demand. In 1972, the supply was limited and the price reached 3000 yen/ton. In 1974, there was a surplus because of the rapid increase in FGD byproduct gypsum production and the decrease in demand due to a recession. These factors caused the price of byproduct gypsum to decrease to practically zero. The price has increased since 1976 and again reached 3000 yen/ton in 1980. The return from gypsum sales offsets only about 5% of the annualized FGD cost; however, the cost of waste disposal is avoided. Phosphogypsum in Japan is of higher quality than in the U S . because of modifications made to the process for phosphoric acid production. However, it still contains phosphoric acid impurity which is not present in FGD byproduct gypsum. The major impurities found in the latter are (1) unreacted lime or limestone, (2) calcium sulfite, (3) fly ash and unburned carbons, (4)chloride and fluoride, and (5) magnesium, ammonium, and sodium. These have been discussed previously except for ammonium, which can come from an upstream selective catalytic reduction process for NO, control or an indirect FGD
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 353 Table 11. Gypsum Quality and Specifications in Japann quality of gypsum from FGD processes Japan Types 1 and 2 Japan Type 3 Japan Type 4 90-97 99 99.2 purity, % composition 0.1-0.6 0.5-1.0 0.1 max CaC03, % 45.31 46.20 46.1 SO3, % not reDorted 2.0 max 0.2 max ash. % 300 170 not reported Mgb, ppm not reported 400 40 NazO: PPm 300 max 20 max not reported chloride, ppm 11-12 6-8 6 free water, % 6.0-8.0 6.8 6.5-7.5 PH 50 50-150 50 particle size, wm not reported wet tensile strength, kg/cm2 12-13 13
gypsum specifications cement wallboard 90 min 95 min 2 max 2 max
1 2 max
50 min
0.8 max 44 min 2 max 800 max 400 max 300 max 12 max 5.5-7.5 50 min 8 min
"EPDC (1982) and Clasen (1983).
process using ammonium sulfite solution for scrubbing, and sodium, which can come from an indirect scrubbing process using sodium sulfite solution. A small amount of ammonium sulfate generally does not affect gypsum quality, but 0.3% sodium sulfate shortens the setting time and reduces the strength. In general, the impurities in byproduct gypsum produced from wet limestone FGD processes are found in small amounts and do not affect quality. When byproduct gypsum is in oversupply, the users tend to set strict specifications on the impurity content, but when gypsum demand is high, they readily use lower grade material (Ando, 1983a). The presence of a considerable amount of impurities may change the properties of gypsum appreciably. However, most impure gypsum is still useful as long as the amount of impurities remains fairly constant. A comparison of the quality of gypsum from several types of wet limestone FGD processes with the general requirements for the cement and wallboard industries is shown in Table I1 (EPDC, 1982; Clasen, 1983). Situation in West Germany Prior to July 1983, SOz emissions in West Germany were regulated by the individual states. State pressures resulted in the installation of partial-capacity FGD systems on several utility boilers with additional capacity for these units and additional systems for other units on order by that time. All but one of these systems are designed to produce byproduct gypsum for sale (Goldschmidt, 1983). The exception is an installation of the Walther ammonia scrubbing process on a 475-MW boiler in Mannheim. This process produces ammonium sulfate for fertilizer use. Because of perceived damage to German forests, the federal parliament ratified strict emission regulations for boilers in July 1983. The SOz-emission regulation for coal-fired boilers is 400 mg/(N m3) (about 140 ppm) and at least 85% removal for new units and existing units with a remaining service life of more than 30000 h, both larger than about 100 MW. The regulations are less stringent for smaller boilers and/or existing boilers with less remaining service life. The law requires compliance by July 1, 1988, and even earlier for the largest SOz emitters. As a result, about 150-200 of the approximately 1500 oil- or coal-fired boilers in West Germany must be retrofitted with FGD systems (Siegfriedt and Ludwig, 1984). FGD sludge disposal is not considered a viable option in West Germany, mainly because of pressure from environmental groups. In addition, there is a lack of mineable gypsum coupled with a high demand for gypsum in the construction industry. Since the sulfur content of German fuels is moderate, a large portion of the boilers requiring FGD systems can probably produce gypsum for sale.
Much of the FGD technology in Germany has been transferred from the U.S.and Japan through licensing agreements. Two FGD processes that produce gypsum have been developed entirely by German companies. Bischoff originally developed a wet lime scrubbing process that produced sludge for disposal. However, the process had the capability of bleed-stream oxidation to produce gypsum. Bischoff has developed an improved process using limestone or lime in single-stage scrubbing with in situ oxidation in the bottom of the scrubber, which serves as the recycle tank. Hydroclones are used to segregate the slurry into two streams-one of higher pH (rich in limestone or lime) fed to the spray nozzles for SO2absorption and one of lower pH (rich in reaction products) fed to the recycle tank for oxidation. The original system was installed as the first phase at a power plant in Wilhelmshaven, and sludge is ponded. Part of the thickener overflow is discharged to the sea. The new system was installed as the final phase. The gypsum is dewatered on a belt filter so that it can be easily washed to remove chlorides. The chloride concentration is high because sea water is used for makeup to the FGD system. It is claimed that the gypsum made by in situ oxidation is better than that made by others with bleed-stream oxidation. Saarberg-Holter developed a gypsum producing process that uses clear thickener overflow for SO2 absorption, oxidizes the scrubber effluent in a separate tank, and adds lime just ahead of the thickener. Hydrochloric acid and formic acid are used as additives, also ahead of the thickener. In the first installation at the Weiher plant in Saarbrucken, the chloride content of the gypsum (as much as 1.3%) was of concern. This process has also been improved to use limestone with in situ oxidation. Hydrochloric acid is not added because there is usually sufficient chloride in the coal; there is about 5% calcium chloride in the scrubbing liquor. A purge stream is mixed with cooling-tower blowdown and discharged to the river. There are no limits on chloride concentration in the wastewater, but this may change in the future. If so, the calcium chloride would have to be crystallized out of the wastewater. Sodium chloride is used to melt ice on roads in Germany, and the use of calcium chloride for this purpose is under discussion. The gypsum-producing FGD processes developed in Germany can be classified as follows: Germany Type 1, single-stage scrubbing with in situ oxidation in recycle tank (bottom of absorber); hydroclones for slurry segregation (similar to US. Type 1). Germany Type 2, two-stage scrubbing with in situ oxidation in both recycle tanks (bottom of absorbers); carboxylic acid added to first stage. Schematics of these processes are shown in Figure 4.
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
4
Table 111. Patterns of Gypsum Supply and Demand in 1983"
I I
gypsum,
lo6 short tons/year total consumption total domestic production total byproduct production FGD byproduct production potential FGD byproduct production from steam-electric stationsd potential production/total consumption, dimensionless
I
I Air
Germany Type 1
W. U S . Japan Germany 21.8 7.3 5.5 13.7 7.3 2.0 0.8 7.3 0.5* 0 2.gC 0.3 86.1 3.9' 5.0 3.9
0.5
0.9
" Based on information from Ando, 1983a,b; Goldschmidt, 1983, 1985; Melia et al., 1985; U.S. Bureau of Mines, 1984. *Assumes 0.2 X IO6 tons/year of non-FGD byproduct gypsum. cIncludes 1.2 X lo6 tons/year from FGD systems on nonutility sources. dUnder actual fuel and load conditions for 1983.
A
- - - - - .--
Gypsum b
I I
I I
A
Air
I
Air
Germany Type 2
--__
4
Gas
-
Liquid or Slurry
A: Absorber
Figure 4. West German wet lime/limestone FGD processes with forced oxidation.
Four other FGD system suppliers in West Germany have licensed foreign technology. Steinmuller has licensed the Chemic0 (now General Electric Environmental Services) lime-scrubbing process and the Mitsui forced-oxidation system (bleed-stream oxidation with sulfuric acid addition in a two-stage reactor). The gypsum is washed with fresh water in the centrifuge to reduce the chlorides to less than 100 ppm on a dry basis. A purge stream is taken from the thickener overflow and sent to wastewater treatment. Steinmuller has now changed to in situ oxidation in the absorber vessel. Thyssen has licensed the Mitsubishi lime/gypsum process (Japan Type 3), and Deutsche Babcock has licensed the Kawasaki process (Japan Type 5 ) . Knauf and EVT have joined with Research-Cottrell to supply the latter's FGD process (U.S. Type 4) in Germany. Two other gypsum processes are offered on the German market. K-H-D Engineering is the licensee of Peabody Process Systems (U.S. Type 2), and Uhde is licensed to sell the Kobe steel process (absorption with lime dissolved in a 30% solution of calcium chloride with bleed-stream oxidation and sulfuric acid addition). Wastewater from the latter process, which is minimized because of the high chloride concentration in the scrubbing liquor, is concentrated in an evaporator. Knauf and Research-Cottrell have developed a technique that uses flue gas for the drying of FGD byproduct gypsum (Byme, 1983). Gypsum from the vacuum filters, with about 10%-20% moisture, is fed to a paddle dryer where it is contacted cocurrently with flue gas at an inlet temperature up to 400 OF. The dryer outlet temperature
must be above 105-140 O F . Most of the gypsum exits the dryer with the flue gas and is separated in cyclones. With a properly designed dryer, it is possible to get the moisture content below 0.5%. The dry gypsum is not hygroscopic and can be briquetted easily. The gypsum does absorb HC1 from the flue gas (about a 500 ppm increse in the gypsum for coal with 0.1% chlorine) but still meets the specifications for cement and stucco, which are the predominant uses of gypsum in Germany. Drying with natural gas is required to produce wallboard-grade gypsum. The big question in Germany relates to the supply-demand situation for gypsum. Currently, there is strong resistance to throwaway FGD processes, but when the gypsum supply begins to approach the demand, the lower cost of throwaway processes, especially systems that use a spray dryer absorber, may become attractive. It has been suggested that the waste be transported to the coal strip mine on the return haul to help restore the original topography. However, it is more likely that government pressure will be brought to bear so that all of the FGD byproduct gypsum is put to use (Siegfriedt and Ludwig, 1984).
Concluding Remarks Factors that appear to affect the marketability of FGD byproduct gypsum include supply vs. demand, gypsum quality, distance from source to user (shipping cost), and ability to dispose of sludge. Not all of these factors are mutually independent. For example, it was previously stated that in Japan the specifications set by the users depend upon the supply-demand situation. In an oversupply situation, the specifications are strict, but when demand is high, lower grade material is acceptable. Also, the ability to dispose of sludge is somewhat related to the supply-demand situation. The supply-demand situation for gypsum in the US., Japan, and West Germany in 1983 is outlined in Table 111. In the U S . the demand is met by mined gypsum, both domestic and imported. Virtually no byproduct FGD system has been marketed yet, but several plants have plans to do so. If all coal- and oil-fired steam-electric capacity were equipped with a gypsum-producing FGD system, the supply of this byproduct would equal almost 4 times the demand. Since FGD capacity will increase in the future as old boilers are retired and new ones are built, it is obvious that most of the byproduct will have to go to waste disposal. The situation in Japan is quite different. All of the demand is satisfied by byproduct gypsum, 40% of which comes from FGD systems. The steam-electric stations and
Ind. Eng. Chem. Prod. Res. Dev. 1886, 25, 355-360
industrial boilers currently in operation without FGD systems burn low-sulfur oil, and there are no plans to retrofit them with FGD. The current potential for byproduct gypsum production from FGD systems is about one-half of the demand. However, new coal-fired boilers are being built with FGD systems, and as they are brought on line the byproduct FGD production will increase considerably. It remains to be seen if the market can absorb this increase. At present, there is virtually no FGD sludge disposal in Japan. In Germany, FGD systems are just beginning to come on line. The supply of byproduct gypsum would almost equal the demand if all of the coal- and oil-fired electricgenerating capacity were equipped with FGD systems producing gypsum. In light of the recently enacted strict emission regulations for SOz, it is likely that the supply will exceed the demand at some point in the future. If and when this happens, new uses for gypsum will have to be developed, gypsum will have to be sold as an export, and/or FGD waste disposal will have to be allowed. Another possibility is to use FGD processes that produce other byproducts such as ammonium sulfate, sulfuric acid, or elemental sulfur. However, these processes are usually more expensive, and there is also a supply-demand situation for other byproducts. The type of FGD process to select for byproduct gypsum production depends upon the major objective beyond S02-emissionscontrol. About 12 process variations have been discussed previously. If the major objective is to produce high-quality gypsum, the best process is probably one with a separate prescrubber loop with a purge stream and bleed-stream oxidation with sulfuric acid added upstream of the oxidizer (Japan Type 3). It is interesting to note that while this process type is most prevalent in Japan, about 70% of the FGD byproduct gypsum is used for cement. I t is not known whether this occurs because the gypsum specifications for cement are less stringent than for wallboard or because the cement plants are more geographically dispersed than the wallboard plants so that shipping costs to the former plants are less.
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If the major objective is to minimize the quantity of wastewater produced because of strict regulations requiring zero discharge, then a process with a high chloride concentration in the scrubbing liquor may be the best. This type includes the modified process developed in Japan where the scrubbing liquor consists of lime dissolved in a 30% solution of calcium chloride or the high-chloride process developed in Germany (Germany Type 2). If the major objective is to minimize process cost, then the best process is probably the simplest one-single-stage scrubbing with in situ oxidation (U.S. Type 1 or Germany Type 1). The process with in situ oxidation in a specially designed absorber (Japan Type 4) may offer the best compromise between FGD cost and gypsum quality. However, each application of FGD is site specific and has to be evaluated under its own criteria.
Literature Cited Ando, J. “SOp Abatement for Stationary Sources in Japan”; report for the US. EPA, EPA-600/7-78-210: Washington, DC, 1978. Ando, J. “SOpAbatement for Coal-Fired Boilers in Japan”; report for the US. EPA, EPA-600/7-83-028: Washington, DC, May 1983a. Ando, J. “Proceedings, Eighth Symposium on Flue Gas Desulfurization”; EPA/EPRI, New Orleans, LA, Nov 1983b. Byrne, R., presented at the Pacific Coast Electrical Association Conference, Los Angeles, 1983. Ciasen, D. D. “Flue Gas Desuiphurization Symposium Record”; Canadian Electrical Association: Ottawa, 1983. Ellison, W.; Kutemeyer, P. M. Power 1983, 127(2), 43-45. EPDC (Electric Power Development Co.) “Design and Operating Experience of Flue Gas Desulfurization Systems for Large Capacity Coal Fired Boilers by EPDC“; EPDC: Tokyo, 1982. Goldschmat, K. VIK-Mitt. 1983, 4 ( 5 ) , 79-87. Goldschmidt, K. VEBA Kraftwerke Ruhr AG, personal communication, 1985. Melia, M. F.; McKibben, R. S.; Pelsor, B. W. “ U t i l i FGD Survey, October 1983-September 1984”; PEI Associates, Inc. report for the Electric Power Research Institute: Palo ARo, CA, 1985. O’Brien, W. E.; Anders, W. L.; Dotson, R. L.; Veitch, J. D. “Marketing Byproduct Gypsum from Flue Gas Desulfurization”; TVA report for the U.S. EPA, EPA-600/7-84-019: Washington, DC, 1984. Siegfriedt, W. E.; Ludwig, M., presented at the American Power Conference, Illinois InstRute of Technology, Chicago, 1984. Smith, D. J. Power Eng. lS84, 88(10), 26-34. U.S. Bureau of Mines ”Minerals Yearbook 1983”; U S . Government Printing Office: Washington, DC, 1984; pp 425-434.
Received for review July 29, 1985 Accepted December 20, 1985
Stability Study for a Western Crude Shale Oil Paul F. Meler,’ Rlsdon W. Hanklnson, Donald K. Petree, Nancy K. Phllllps, Wllllam R. Parrlsh, and Gregory C. Allred Phillips Petroleum Company, Bartlesville, Oklahoma 74004
The storage stability of a crude shale oil, obtained by retorting shale from the Green River formation in Colorado by the Union B process, was studied by monitoring five physical properties over an 18-week period. There were no significant changes in properties with storage time, indicating the oil was stable during this time period. The time period chosen for the study is expected to be typical of the time between retorting and refining a crude shale oil. The shale oil samples were collected immediately after retorting and stored at room temperature under a nitrogen blanket in the absence of light. Samples were stored In both glass and metal containers. The properties measured for each sample were density, heat capacity, molecular weight, UV spectroscopy, and a simulated distillation GC pattern.
Introduction The storage stability of shale oil is important since shale retorting and shale oil upgrading will probably be done in different locations. Sedimentation, caused by protracted 0196-4321/86/1225-0355$01.50/0
storage, could increase the cost and difficulty in processing and upgrading the oil. Frankenfeld et al. have shown that in accelerated storage or “aging” studies sedimentation is possible for shale oil, 0 1986 American Chemical Society
