Intermittent control svstems Controversial yet workable-all Tennessee Valley Authority sulfur dioxide elimination programs are scheduled to be operational this year Thomas L. Montgomery John W. Frey William B. Norris Tennessee Valley Authority Muscle Shoals. Alabama 35660 Intermittent control systems vary emission rates on the basis of existing or predicted meteorological conditions in order to achieve a specified degree of ambient air quality. The concept is a straightforward engineering approach that has been used in the field of engineering for many years. These programs were first used by the smelting and power industries in Canada and the U.S.Similar programs have also been developed and implemented in other countries. The enactment of clean-air legislation during the past decade on both the Federal and state levels reflects a public awareness of the need to control air pollution. This legislation has stimulated the installation of control devices or other control techniques to enable sources of emissions to comply with air quality standards. The principal techniques for control of emissions from stationary sources are intermittent and constant control systems. Intermittent control systems are methods that are designed to achieve ambient air quality standards constantly by varying emission rates in response to changing atmospheric dispersion conditions. On the other hand, constant control systems are designed to achieve fixed emission rates continuously without regard to such changes. Through consideration of the most adverse atmospheric dispersion conditions, fixed emission rates can be set at a level that should ensure that ambient standards are always met. Regardless of whether intermittent or constant controls are used, the goal is to enable the source to which they are applied to comply continuously with ambient air quality standards. Although it is true that during air stagnation alerts, adjustments of emission rates of sources within large areas, often urban areas, are made according to procedures established and enforced by air pollution regulatory agencies, for this article the concept of intermittent control systems will not be made broad enough to include such programs. Emphasis will be placed on individual sources that can identify and control their own contribution to air pollution.
Background The use of intermittent control systems for air quality control is not new; the technique was pioneered by the smelting and power industries several years ago. Since its introduction, it has become attractive to operators of large isolated point sources of SO2 who have felt that reliable constant controls were either unavailable or inappropriate. The same techniques can be applied toward the control of ambient concentrations of other pollutants, such as particulates or oxides of nitrogen. However, most applications are directed toward control of Son. 528
Environmental Science & Technology
The intermittent control system approach to air quality control has been and continues to be a controversial one. Because of the possible far-reaching effects its use could have in the areas of economics, energy conservation, and health, its legitimacy is being thoughtfully considered in the highest levels of government. For example, the present Administration recently revised its policy with regard to it, and Congress is now considering legislation that is intended to clarify its role in air quality control. The Tennessee Valley Authority (TVA), a Federal agency, has participated for several years in the development and use of intermittent control systems. The TVA program includes controls of ground-level SO2 concentrations by means of these systems at nine of its coal-fired power plants. The TVA plan for such systems is scheduled to be fully operational during 1975 and is being designed to bring all TVA coal-fired plants into full compliance with primary and secondary ambient SOn standards. The concept of an intermittent control system is a straightforward engineering approach to process control of any typean approach that has been used for a variety of purposes in such engineering fields as manufacturing, transportation, and communication. Observations or measurements are made, and the process is adjusted as necessary to produce the desired result. Details of each approach depend on the specific process that is being controlled. The basic concept of intermittent control is illustrated in Figure 1. The process for which
The intermittent control system concept -loop
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control is desired is shown with the associated levels of control that may be applied to it. When environmental processes are being considered, the transfer caused by atmospheric dynamics (which are uncontrollable and not coupled to the process itself) must be taken into account. The combined effects of the process and the subsequent atmospheric interaction produce the output, which consists of ambient air quality levels that are usually measured by ambient monitors and expressed in terms of ground-level concentrations. In most engineering approaches, the controller consists of valves or feedrate limiting devices. For environmental intermittent control systems, active control involves predictions of atmospheric dispersion, feedback control involves feedback from ambient concentration monitors, and passive control is concerned with periodic upgrading of the system. Response times for individual components such as the process or controller are implicitly taken into account. In this approach, active control (loop A) can be either continuous or semicontinuous, whereas feedback control (loop B) is ordinarily continuous and is automatic. Passive control (loop C) is ordinarily executed about once a year; then, all essential elements of the controller are adjusted to accommodate any improvements in the system that may have been determined in the preceding period.
Evolution of ICs As early as 1934, Cominco, Ltd., began utilizing a program for reducing emissions at its lead and zinc operations in Trail, British Columbia, under specified atmospheric conditions. The systems employed by the American Smelting and Refining Company (ASARCO) at its smelting plant in El Paso, Texas, and by TVA at its Paradise power plant near Drakesboro, Kentucky, were prototypes for the control of SO2 emissions from stationary sources in the U.S. The ASARCO intermittent control system (termed a closedloop system) was developed and implemented in 1967. This system controlled SOz emissions primarily by response to a feedback loop involving 18 Thomas autometer ambient SO2 monitors. Output from these monitors was fed back to a con-
trol room at the El Paso facility, where it was observed by a meteorologist who determined whether process curtailment was necessary to meet required ambient SO2 concentrations in the vicinity of the smelter, which he based on the ambient SO2 concentration trends, his knowledge of the local atmospheric dispersion conditions, and the prevailing meteorology. The TVA Paradise intermittent control system (first termed an operational control program) was developed and implemented in September 1969. This system involved only an active control loop. A network of 14 Instrument Development Corp. ambient SO2 monitors was installed near the plant only for documentation of air quality and retrospective evaluation of the system. The Paradise program was able to operate with only the active atmospheric dispersion prediction loop because critical dispersion conditions at this plant consist only of limited layer mixing or trapping-conditions that usually occur between midmorning and midafternoon and that can usually be anticipated from early morning meteorological observations and plant operating conditions. Other intermittent control systems have been or are being implemented. For example, an intermittent control system has been developed and implemented at the Dow Chemical plant at Midland, Michigan. ASARCO has additional programs such as its intermittent control system in Tacoma, Washington. TVA is developing additional programs at its coal-fired power plants. Intermittent control systems for SO2 control have also been developed recently and installed in Europe at the Ente Nazionale per I'Energia Elettrica La Spezia oil-fired power plant on the Mediterranean coast of northern Italy, and at 23 power stations of the Electricite de France. A representative list of existing or anticipated intermittent control systems is shown in Table 1. Intermittent control systems are particularly well adapted to sources that require infrequent control actions to prevent ambient SO2 problems. These systems are also better suited for isolated sources where their impact on ambient concentrations can be more easily determined. Such systems can best be designed for standards of intermediate averaging times, such as 3 and 24 hours. These periods provide sufficient time
TABLE 1
Existing or anticipated intermittent control systems" Organizationb
American Smelting and Refining Co. Bunker Hill Co. Cominco, Ltd. International Nickel Co. Kennecott Copper Corp. Magma Copper Co. Mt. Isa Mining, Ltd. Phelps Dodge Corp. Central Illinois Light Co. Commonwealth Edison Co. Dow Chemical Co. ilectricite de France
Ente Nazionale Per L'Energia Elettrica New Brunswick Electric Power Commission Ontario Hydro Tennessee Valley Authority
Plant locationsC
Smelters Hayden, Arizona; Glover, Missouri; East Helena, Montana; El Paso, Texas; Tacoma, Washington Kellogg, Idaho Trail, British Columbia, Canada Sudbury, Ontario, Canada Hayden, Arizona; McGill, Nevada; Hurley, New Mexico; Salt Lake City, Utah San Manuel, Arizona Mt. Isa, Queensland, Australia Douglas, Arizona; Morenci, Arizona Fuel-Burning Sources Bartonville, Illinois Kincaid, Illinois Midland, Michigan Beautor, Oise; Martigues, Bouche-du-Rh6ne; AmbBs, Gironde; Strasbourg, Haut-Rhin; Vitry,, Val de Marne; Cordemais, Loire-Atlantique; Nantes Che. vire, Loire-Atlantique; Vaires sur Marne; Seine et Marne; La Maxe, Moselle; Bouchain, Nord; Les Ansreuilles, Nord; Champagne sur Oise, Oise; Creil; Oise; Dunkerque, Nord; Loire-sur-RhGne, Rhane; Chalons sur SaGne; Le Havre, Seine-Maritime; Yainville, Seine-Maritime; Gennevilliers, Hauts de Seine; Saint Ouen, Val-d'Oise; Montereau, Yonne; Porcheville,Yvelines La Spezia, Liguria, Italy St. John, New Brunswick, Canada Toronto, Ontario, Canada see p 531
a Prepared by T V A this list has not been coordinated with all of the listed companies; furthermore, it should not be considered exhaustive.
Many of the orgahizations listed use or anticipate using intermittent control systems as a supplement to constant control measures. Unless otherwise indicated, plants are located in t h e U.S.
530
Environmental Science & Technology
could be developed for long-term averaging times, such as one year; however, if a source has problems with long-term average concentrations, constant emission control or a source modification, such as taller stacks, is usually a more practical solution. The N
A experience
TVA's intermittent control systems for SO2 abatement are called sulfur dioxide emission limitation (SDEL) programs. Until 1973, only the Paradise and Widows Creek plants had such programs. Early in that year, TVA decided to extend the use of intermittent control system approach to 9 of its 12 coalfired power plants. Since both primary and secondary ambient standards as well as fixed emission standards will be met at Bull Run, John Sevier, and Watts Bar during 1975, SDEL programs are not needed at these plants. For the remaining nine, TVA considers SDEL programs to be the most effective way to meet both primary and secondary SO2 ambient standards. To reduce the frequency and magnitude of SO2 emission reductions at the Kingston, Shawnee, and Widows Creek plants, new, taller stacks are being constructed. The stacks are scheduled to be in operation in 1976 for the Kingston plant and in 1977 for the Shawnee and Widows Creek plants. TVA plans to use two types of SDEL programs, called Class I and Class II programs. The less complex Class I programs are already in operation at four plants-Allen, Cumberland, Gallatin. and Paradise. /4 typical Class I program includes: nine ambient SOp monitors an instrumented meteorological tower an environmental data stat.inn rnntslininn 51 h-++ar\, nf _, minicomputers operated onsite by a staff of two. Its capital and operating costs are estimated to be either $0.3/kW and 0.06 millslkWh, respectively, or $1.8lkW and 0.08 millslkWh, depending on whether fuel-switching facilities
._.. __...-......
I
these plants are being upgraded; the upgraded version is presented here. Class I programs include both active and passive control but do not contain the feedback-control features. The more complex Class II programs are being installed at five plants-Colbert, Johnsonvilie, Kingston, Shawnee, and Widows Creek. These programs involve active, feedback, Class II and Dassive control as outlined in Fiaure - I.A tvDical .. program will include: 12 SO2 ambient rnonitors SO2 emission and generation monitors at least one instrumented meteorological - tower a rawinsonde system a battery of minicomputers -.^'^1:^^ I^ L^ 1-2 L.. - ^I^. an environmental Audta X ~ I I U I Itu V C uprateu ~ uy a staff of five. Capital and operating cix t s are estimated to be $0.7/kW and 0.27 millslkWh, respr!ctively, for Class II programs be711,111 --A ,U,. 4I 7, -:,,-,L\",. -u-~ I V ~ ~ ~ w D ~ O ~L C T~ fore modifications and $5.r,n~ wards (Table 2). Both types of programs will be supported by a TVA meteorolooicai forecast center located at Muscle Shoak, Alabama ^_^_^
The Paradise program Further details of the approach used in Class 1 programs illustrated by the Paradise program. During 1969, exten-
?
that high ambient SO2 concentrations could be expected only when preceded by a specific pattern of meteorological circumstances determinable by measurements and observations made near dawn. Efforts to mathematically model the dispersion for these select circumstances produced acceptable results that indicated the use of feedback as a secondary means of control was unnecessary. Emission monitors were not required because the plant burns coal of a reasonably constant quality.
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An interim p m e r a m l i a s been in operation a t t h i s p l a n t since 1971
Volume 9.Number 6 , June 1975
531
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After more than five years of operation the program is being extensively upgraded. The ambient monitoring network is being upgraded with 12 continuous Philips SO2 monitors that will replace the previous monitors. A new meteorological tower has been erected, and Data General Corp. minicomputers, housed in an environmental data station near the plant, are being programmed to collect and average ambient and meteorological data and to run the daily calculations for the SDEL program. The principal items of hardware and their interrelationships are shown in Figure 2. The locations of the meteorological and ambient SO2 facilities relative to the plant are shown on the second map. A staff of two will operate the program about eight hours each day of the week. Their duties will include releasing and tracking pilot balloons, making aircraft flights to determine vertical temperature profiles, and operating the SDEL program minicomputer. They will also monitor the data being telemetered to the environmental data station from the meteorological and ambient SOn facilities. The elements of the Paradise program are pictured conceptually in Figure 2. To support this and all other TVA SDEL programs, both Class I and Class II, a meteorological forecasting center will be located at Muscle Shoals, Alabama. This center will be staffed by six meteorological forecasting technicians who, for Class I programs, will provide forecasts of wind speed, wind direction, and temperature to cover the critical dispersion periods. The estimated capital and annual operating costs of the upgraded Paradise program are $0.3/kW and 0.07 mills/kWh, respectively. The corresponding costs for the typical Class I program are $0.3/kW and 0.06 mills/kWh, respectively. These and other similar estimates are given in Table 2. During its first 5-i/2 years (September 1969 through March 1975) the Paradise program has required generation reductions ranging from 9 MW to 970 MW. The durations of reduc532
Environmental Science & Technology
tions have ranged from 0.2-6.0 hr. Other operating characteristics for this time period are presented on the map. The other TVA Class I programs are similar to the Paradise program. In fact, the main conceptual elements and items of hardware, as shown in Figure 2, are the same. The principal differences involve plant configuration and operating conditions and number of monitors used in the ambient network. In addition, the dispersion models for the Allen and Gallatin programs are designed to include a more comprehensive set of atmospheric conditions than either the Paradise or Cumberland programs. The upgrading of all Class I programs is scheduled to be completed during 1975.
The Johnsonville program This program will illustrate TVA's approach to Class I programs. Located in western Tennessee, the Johnsonville plant is characterized by relatively short stacks (83 m for units 1-6 and 122 m for units 7-10). Because ambient concentrations may exceed the applicable 3- and 24-hour-average air quality standards day or night, to implement a program that operates during only a part of the daylight hours, such as the one at Paradise, would be inappropriate. Therefore, TVA is designing a program that will operate 24 hours each day. Because it will function nearly automatically, the program has requirements not found in Class I programs. For example, concentrations of SO2 in the stacks, as measured by Du Pont continuous emission monitors, and unit generation will be telemetered to a Data General Corp. minicomputer, which will process and transmit them upon request to a second minicomputer containing the operating SDEL program software. In addition, all measurements from the 11 continuous Philips SO2 monitors in the ambient network and from the recently erected meteorological tower will be telemetered to the datalogging minicomputer for processing and periodic incorporation into the operating computer program.
TABLE 2
Estimated cost of TVA SDEL programs 1975 programs Type of SDEL program
Class i
Average Class I1
Average Overall average
Laterprogrsmsa
Plant Size
Development cost
(MW)
(I/kW)
Operating costh (mills/kWh)
Allen Cumberland Gallatin Paradise
990 2,600 1,255 2,558
0.6 0.2 0.4 0.3
0.05 0.06 0.07 0.07
Colbert Johnsonville Kingston Shawnee Widows Creek
1,396 1,485 1,700 1,750 1,978
0.3 0.7 0.7 0.6 0.7 0.6 0.7 0.5
0.06 0.15 0.22 0.37 0.32 0.26 0.27 0.17
Plant
Development cost
Operating costh
($/kW)
(millslkWh)
0.6 4.2 0.4 0.3 1.8 4.7 4.7 7.1 7.5 4.3 5.7 3.8
0.05 0.11
0.07 0.07 0.08 0.19 0.23 0.17 0.18 0.13 0.17 0.13
"Considers tailer stacksfor Kingston. Shawnee. and Widows Creek' fuel-switching facilities a t Colbert Cumberland a n d Johnsonville: no difference for Ailen, Gallatin. a n d Paradise. TVA'5 SO1 ~ o n t r oplans l ais0 include a scrubber on a 55a-MW'unit w i t h a &t ol $%/kW. 'Includes annualized c a p i t a l charges a t 9.64%.
Just as for Class I programs, meteorological data will be obtained by routine releases of pilot balloons and airplane flights. However, a rawinsonde system for use during the night or during foul weather to measure wind speed, wind direction, temperature, pressure, and relative humidity as a function of altitude, will be available. The minicomputer that controls the operations of the generation and emission monitors within the plant will also serve as a means of communication from the SDEL program computer to plant operators. For instance, when action is required to meet air quality standards, a message displaying alternatives for generation reduction to be read by the shift engineer is printed on the plant-computer teletypewriter. The principal terms of hardware and their interrelationships to each other are depicted in Figure 3. The locations of the ambient SO2 and meteorological monitors are shown in the second map. The active control loop will consist of a program general enough to account for significant spatial and temporal changes in atmospheric dispersion characteristics even to the point of considering aerodynamic effects produced by nearby terrain and by the shape of the powerhouse. The passive control loop will be a time-series model developed from extensive analyses of historical ambient SO2 data collected in the vicinity of this and other TVA coal-fired plants. In order to implement the system-upgrading loop a procedure is being developed for storage of the input and of the significant output of the program. The program will be operated continuously by a staff of five in an environmental data station located within view of the piant. The support provided by the Muscle Shoals meteorological forecasting facility to Class II programs will include 8- and 16-hour forecasts updated approximately every four hours. The conceptual elements of this program and their interrelationships are shown in Figure 3. The estimated capital and annual operating costs of the Johnsonville program are $0.7/kW and 0.22 millslkWh. respectively, At a later date, a fuel-switching capability may be installed to reduce the frequency of implementing generation curtailment. When this modification is accounted for, these estimates become $4.7/kW and 0.23 millslkWh, respectively. In comparison, the average Class II program will have costs of $0.7/kW and 0.27 millslkWh, respectively, before modifications, either addition of fuel-switching facilities or tailer stacks, and $5.7ikW and 0.17 millslkwh afterwards. Table 2 summarizes the data pertaining to estimated costs. The design and operation of all Class 1 I programs will be similar. The number of monitors and meteorological towers will vary from piant to plant as will the SDEL program software that will be written for specific plant design and operation and for the topography specific to a plant site. All Class II SDEL programs are scheduled to be operational during 1975.
Additional reading Hewson, E. W., O.J. RoyMeteorol. Soc.. 71,266 (1945). Leavitt, J. M.. Carpenter, S. E., Blackwell, J. P., and Montgomery. T. L., J. AirPollut. Contr. Assoc., 21, 400 (1971). "Air Quality and Stationary Source Emission Control,'' A Report by the Commission on Natural Resources. National Academy of Sciences, National Academy of Engineering, National Research Council, prepared for the Committee on Public Works, United States Senate (1975). Carpenter, S. E., Montgomery. T. L., Leavin, J. M., Colbaugh, W. C., andThomas, F. W.. J. AirPoIIut. Contr. ASSOC.,21, 491 (1971). Montgomery. T. L.. and Frey. J. W., Min. Congr. J., 61,44 (1975). The mention of a commercial product does not constitute an endorsement by the Federal Government or the Tennessee Valley AUthority.
Thomas L. Montgomery joined the Tennessee Vaiiey Authority, Division of Environmental Planning, in 1963. After receiving his doctorate in air poiiution from the University of Pittsburgh in 1969, he returned to TVA. Now, Dr. Montgomery serves as chief of the Air Ouaiity Branch, which coordinates TVA's air quality management plans and activities by advising on plans for air quality comrois at all TVA installations including those for both existing and proposed projects. John W. Frey has had 12 years' industrial experience as a research physicist in the development of instrumentation designed for the detection of materials of interest in air pollution and industrial hygiene programs. Mr. Frey joined TVA in 1972 as Supervisor, Air Ouaiity Studies Section-that section of the Air Quality Branch which performs analyiical and special field studies in support of existing and proposed power piants and other facinties and characterizes their operational impacts. William B. Norris joined the Air Ouaiity Branch of TVA in 1971 as a Research Analyst in the Air Ouaiity Studies Section. Volume 9. Number 6, June 1975 533
