U. S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE Public Health Service ------- EMISSIONS FROM COAL-FIRED POWER PLANTS: A COMPREHENSIVE SUMMARY 'Stanley T. 'Cuffe and Richard W. Gerstle National Center for Air Pollution Control U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE Public Health Service Bureau of Disease Prevention and Environmental Control Cincinnati, Ohio 1967 ------- The ENVIRONMENTAL HEALTH SERIES of reports was estab- lished to report the results of scientific and engineering studies of man's environment: The community, whether urban, suburban, or rural, where he lives, works, and plays; the air, water, and earth he uses and reuses; and the wastes he produces and must dispose of in a way that preserves these natural resources. This SERIES of reports provides for professional users a central source of information on the intramural research activities of programs and Centers within the Public Health Service, and on their cooperative activities with State and local agencies, research institutions, and industrial organizations. The general subject area of each report is indicated by the two letters that appear in the publication number; the indicators are AP Air Pollution AH - Arctic Health EE - Environmental Engineering FP - Food Protection OH - Occupational Health RH - Radiological Health WP Water Supply and Pollution Control Triplicate tear-out abstract cards are provided with reports in the SERIES to facilitate information retrieval. Space is provided on the cards for the user's accession number and key words. Reports in the SERIES will be distributed to requesters, as supplies permit. Requests should be directed to the Center identi- fied on the title page, or to 5555 Ridge Avenue, Cincinnati, Ohio 45213. Public Health Service Publication No. 999-AP-35 ------- CONTENTS Page Abstract v Introduction 1 Sampling and Analytical Techniques 1 Description of Units 2 Vertically Fired Unit 2 Corner-Fired Unit 6 Front-Wall Horizontally Fired Unit 6 Horizontally Opposed Fired Unit 6 Cyclone-Fired Unit 10 Spreader-Stoker Fired Unit 11 Discussion of Results 12 Oxides of Sulfur 12 Oxides of Nitrogen 14 Solid Particulate 16 Polynuclear Hydrocarbons 19 Emissions of Trace Contaminants 21 Summary 23 References 25 iii ------- ABSTRACT The Public Health Service and the Bureau of Mines conducted a study of air pollutant emissions from the six main types of coal-burning power plants. The components tested include sulfur oxides, nitrogen oxides, polynuclear hydrocarbons, total gaseous hydrocarbons, solid particulates, formaldehyde, organic acids, arsenic, trace metals, and carbon monoxide. This report relates the effects of variables such as method of operation, type of boiler furnace and auxiliaries, reinjection of fly ash, and type of coal burned to the concentrations of gaseous and particulate pollutants in the products of combustion. ------- EMISSIONS FROM COAL-FIRED POWER PLANTS: A COMPREHENSIVE SUMMARY INTRODUCTION Total requirements for power plant energy from the combustion of coal are expected to increase from 211,000,000 tons per year in 1963 to over 600,000,000 tons per year in 2000. This 300 percent expansion is expected in spite of the increased use of nuclear and petroleum fuels. Most of this coal will be burned in large power plants with steam capacities in excess of 1 million pounds per hour. In 1962, over 60 percent of the steam generators sold in the United States had capacities of 2 million pounds of steam per hour; this trend toward large-capac- ity generators is expected to continue as the need for more economical power production increases. To assess the contribution of coal-fired power plants to the over- all air pollution burden, the Public Health Service and the Bureau of Mines conducted a joint study to determine atmospheric emissions from the main types of coal-fired power plants. The objective of the study was the assessment of a number of flue-gas-stream components of interest in air pollution. The components determined were sulfur oxides, nitrogen oxides, polynuclear.hydrocarbons, total gaseous hydrocarbons, solid particulates, formaldehyde, trace metals, carbon monoxide, carbon dioxide, and oxygen. When possible, the effects of variables such as method of operation, type of boiler furnace, type of coal burned, and reinjection of fly ash were related to the type and amount of pollutant emitted. The six typical designs of coal-fired steam generators tested were vertically fired; corner-fired; front-wall horizontally fired; horizontally opposed fired, wet-bottom; cyclone-fired wet-bottom; and traveling-grate spreader-stoker fired. Results of tests on the first four designs have been published. 2,3 This report presents comparative data for all six types of boilers. SAMPLING AND ANALYTICAL TECHNIQUES Summarized descriptions of flue-gas sampling and analytical techniques used for the power plant study have been published. 2,3,4 Standard methods were used for sulfur oxides, 5,6 nitrogen oxides, 1 ------- polynuclear hydrocarbons, 8,9,10 total particulate, formaldehyde, H arsenic, oxide, carbon dioxide, and oxygen. ;aseous hydrocarbons, solid 2 trace metals, carbon mort- Solid particulate was collected by simultaneously traversing the inlet and outlet ducts of the fly-ash collector with the sample train illustrated in Figure 1. Isokinetic sampling rates were maintained by continually measuring the velocity with a pitot tube attached to the probe. Particulate obtained from the cyclone and filters, in addition to that obtained by brushing the train and filtering the bubbler and wash water, was used for determining total particulate weight after drying at 105°C. Samples of coal entering the furnaces were collected hourly during the test periods. Proximate and sulfur analyses were made on each sample, and ultimate analyses were made on composite samples (Table 1). DESCRIPTION OF UNITS Boiler operating conditions and flue-gas data for all six types of boilers at both full and partial load are given in Table 2. Vertically Fired Unit The vertically fired dry-bottom unit (Figure 2) is rated at 1,100,000 pounds of steam per hour at 1,900 psig and 1,000°F. Coal, pulverized in two ball mills, is conveyed to 16 sets of burner ports by four exhausters. Combustion air is supplied by two 210,000-cfm forced-draft fans. The air enters the furnace through the burner ports QTHESE COMPONENTS ARE INSERTED BEHIND BUBBLERS FOR COLLECTION OF POLYNUCLEAR HYDROCARBONS HOT FILTER -THERMOMETER U-TUBES IN DRY-ICE ALCOHOL BATH THERMOMETERS BUBBLERS IN ICE-WATER BATH AIR TIGHT ORIFICE PUMP RATE METER Figure 1. Particulate sampling train. EMISSIONS FROM COAL-FIRED ------- i M Table 1. ANALYSES OF COALS BURNED Component Proximate Analysis (as -fired), % Moisture Volatile matter Fixed carbon Ash Ultimate analysis (as -fired), % Hydrogen Carbon Nitrogen Oxygen Sulfur Ash Heating value , Btu/lb Vertical Pa. 1.1 30.8 48.3 19.8 4.6 65,8 1.4 6.1 2.3 19.8 11,820 Corner Ohio 2.8 37.2 44.0 16.0 5.0 64.2 1.3 11.8 1.8 16.0 11,480 W. Va. District 8 1.8 32.9 53.6 11.7 4,8 72.1 1.4 9.0 1.0 11.7 12,645 Front-wall Ky. Strip 2.3 38.3 49.6 9.8 5.1 70.9 1.5 10.4 2.3 9.8 12,640 W. Va. Deep Mine 1.2 36.2 54.5 8.1 5.1 75.9 1.5 8.2 1.2 8.1 13,540 Spreader stoker 111. 4.1 42.9 44.8 8.2 5.5 70.0 1.4 12,4 2.5 8.2 12,650 Cyclone Pa. 1.1 37.0 54.5 7.4 5.2 77.4 1.4 6,1 2.4 7.5 13,910 Horizontally opposed m. 2.0 36.5 53.6 7.9 5. 1 73.7 1,6 9;4 2,3 7.9 13,195 ------- Table 2. SUMMARY OF BOILER AND FLUE GAS DATA Type of boiler firing Full-load tests0 Vertical Corner Front-wall Spreader-stoker Cyclone Horizontally opposed Partial-load testsd Vertical Corner Front-wall Spreader-stoker Cyclone Horizontally opposed Boiler conditions Steam rate, IbAr 1,100,000 960,000 920,000 150,000 1,332,000 149,000 815,000 696,500 628,000 119,500 1,022,000 108,000 Coal rate , ton/hr 65.6 56.1 52.2 9.2 64.4 9.6 41.1 40.8 38.8 6.9 41.3 6.6 Flue -gas volume , mscfm Be 397.4 362.9 329.0 53.9 557.6 62.2 297.2 283.8 254.0 44.0 443.5 43.8 Ae 409.9 351.0 328.0 59.6 500.8 62.2 303.8 264.2 256.0 46.7 404.1 44.6 Average flue-gas temp, °F Be 258 283 257 426 279 315 245 253 244 396 265 314 A* 268 275 255 328 256 310 235 252 244 327 240 308 Moisture , % Be 6.4 7.0 5.9 7.8 6.3 6.8 6.4 8.0 5.9 7.4 6.6 6.6 Af 5.7 8.1 6.3 7.5 5.9 6.5 6.6 7.4 6.0 7.0 6.4 6.6 C02, % Be 12.6 14. 4a 13.6 12.1 12.8 13.2 12.5 14. 9a 13.3 12.1 12.0 12.8 Af 12.2 14. 3a 13.0 11.8 12.7 13.0 12.2 14. 7a 13.0 12.0 12.2 12.4 °J> % Be 6.2 4.7 5.3 6. 6 6.4 5.9 6.6 4.2 5.6 6.9 6.8 6.0 Af 6.4 4.8 5.6 7.0 6.3 6.1 7.0 4.4 5.8 7.2 6.8 6.5 Excess air, % Be 41.0 28.6 32.9 44.5 42.6 38.6 45.0 24.2 35.0 47.8 46.0 38.4 A* 45.3 29.1 35.5 44.8 42.0 40.7 48.8 25.6 37.7 51.1 46.2 43.7 aCalculated values, based on oxygen values and fuel analysis. ^Measured at fly-ash collectors. cAverage values for three or four tests at each unit under normal steam load conditions. ^Average values for two tests at each unit. eB: Before fly-ash collector. ^A: After fly-ash collector. ------- PULVERIZER REHEAT DAMPER CONTROL mCONOMIZER LET HEADER REHEAT INLET HEADER REHEAT OUTLET HEADER SUPERHEAT OUTLET HEADER Figure 2. Boiler outline for vertically-fired unit showing sampling positions. POWER PLANTS: A SUMMARY ------- with the coal and through additional ports interspersed between the burner ports. The steam generator consists of a water-wall furnace section, a combined radiant and convection superheater, a reheater, an economizer, and an air heater. Flue gas, after passing through the regenerative air heaters, is divided into two parallel ducts and drawn through the fly-ash collector by two 300,000-cfm induced-draft fans. The clean gas is then discharged through a brick-lined steel stack. The fly-ash control system includes a cyclone-type separator followed by an electrostatic precipitator. A high-volatile bituminous coal from Pennsylvania was burned in the vertically fired unit. Three tests were run at full load and two at three-quarter load. Corner-fired Unit The corner-fired dry-bottom boiler is of the mono- tube type, i.e., without a steam drum (Figure 3). It is completely water- cooled and rated at 960,000 pounds per hour steam at 2,565 psig and 1,050°F. The 16 tangential corner-mounted burners receive pulverized coal from 4 ball mills. Two 185,000-cfm forced-draft fans supply com- bustion air. The flue gas leaving the boiler is carried through two regenerative-type air preheaters, fly-ash collectors, a low-level heat economizer, and two 275,000-cfm induced-draft fans before it is dis- charged through a stack to the atmosphere. The fly-ash collector consists of a cyclone-type separator in series with an electrostatic precipitator. The high-volatile bituminous coals burned in the corner-fired unit were obtained from West Virginia and Ohio (Table 1). Normal amounts of excess air were used during three tests at full boiler load and two tests at three-quarter load. Front-Wall Horizontally Fired Unit The front-wall-fired dry-bottom unit is illustrated in Figure 4. At full load, the unit produces 920,000 pounds of steam per hour at 1,000°F and 1,900 psig. Coal is pulverized in 4 rotary mills and conveyed to 24 front-wall burners. Combustion air from the regenerative preheaters enters the furnace as primary air with the coal and as secondary air through the annular ports of each burner. Hot gas recirculates from the economizer outlet to the bottom of the furnace. The convection heat-transfer section of the water- cooled furnace includes superheater, reheater, and economizer units. Flue gas from the two air heaters enters two parallel electrostatic precipitators for collection of fly ash. The coals burned in this unit were supplied from a Kentucky strip mine and a West Virginia deep mine. Three full-load tests and two partial-load tests were conducted at normal excess air conditions. Horizontally Opposed Fired Unit The turbo-fired wet-bottom unit (Figure 5) burns either pulverized coal or gas and is designed to re- inject all fly-ash. Steam production is rated at 150,000 pounds per hour at 1,000 psig and 835°F. The water-cooled furnace is designed for continuous drip removal of slag. The fly-ash collector consists of 6 EMISSIONS FROM COAL-FIRED ------- MECHANICAL DUST COLLECTOR ,-LOW LEVEL / ECONOMIZER TO STACK BURNERS EXHAUSTER Figure 3. Boiler outline for corner-fired unit showing sampling positions. POWER PLANTS: A SUMMARY ------- PULVERIZER Figure 4. Boiler outline for front-wall fired unit showing sampling positions. EMISSIONS FROM COAL-FIRED ------- SUPERHEAT,! OUTLET HEADER SUPERHEATER Figure 5. Boiler outline for horizontally-opposed firing unit showing sampling positions. POWER PLANTS: A SUMMARY ------- a cyclone-type separator with facilities for storage or reinjection of fly ash. Six tests were run on the turbo-fired unit to evaluate emis- sions at full and three-quarter load both with and without fly-ash reinjection. A high-volatile bituminous coal from Illinois was burned during all tests. Cyclone-Fired Unit In the cyclone-type furnace, a mixture of crushed coal and air is injected tangentially into a horizontal, cylindrical com- bustion chamber. Essentially all of the combustion takes place in this SECONDARY SUPERHEATER AND REHEAT HEADERS COAL SAMPLING POINT COAL FEEDERS COAL CRUSHERS FLUE GAS TEMPERING DUCT Figure 6. Boiler outline for cyclone type unit showing sampling positions. 10 EMISSIONS FROM COAL-FIRED ------- water-cooled chamber. A substantial amount of ash is melted in the cyclone and removed from the furnace as slag. This boiler is rated at 1,360,000 pounds of steam per hour at 2,400 psig and 1,050°F. Two forced-draft fans with a capacity of 370,000 scfm supply combustion air to the furnace and maintain positive pressure through- out the furnace-boiler system. Flue gas leaving the boiler passes through secondary and primary superheater sections, an economizer, an air preheater, and finally a fly-ash collector, as shown in Figure 6. The fly-ash collectors include two parallel electrostatic precipitators. Collected fly ash is normally reinjected into the furnace. A single type of high-volatile bituminous coal from Pennsylvania was burned during all tests. Three tests were run at approximately full load, two of which included fly-ash reinjection. Two additional tests were run at three-quarter load, both with fly-ash reinjection. All tests were conducted with normal amounts of excess air. Spreader-Stoker Fired Unit In this traveling-grate type of spreader stoker, as shown in Figure 7, crushed coal is gravity-fed to rotating blades which distribute the coal over a slowly revolving continuous grate. Collected fly ash is also reinjected at the rear of the grate. ^SUPERHEAT I Q^OUTLET HEADER COAL SAMPLING POINT Figure 7. Boiler outline for spreader stoker unit showing sampling positions. POWER PLANTS: A SUMMARY 11 ------- The coal passes slowly through the combustion zone where it is burned, and the remaining ash is discharged from the opposite end of the re- volving grate into an ash hopper. The grate has an effective area of 362 square feet and serves a boiler with a nominal capacity of 220,000 pounds of steam per hour at 875 psig and 760°F. Combustion air is supplied by both forced-draft fans and overfire- air fans. Flue gases leaving the grate pass up through a water-cooled boiler-furnace section, superheater, economizer, cyclone-type fly-ash collector, air preheater, and induced-draft fan before entering the stack. A high-volatile bituminous mixed coal from Illinois was burned during all tests. DISCUSSION OF RESULTS 1. Oxides of Sulfur The average sulfur content of coals used for electric-power production in the United States is about 2.5 percent. The sulfur content of the coals burned in the six units during testing varied from 1.6 to 2.9 percent (Table 3), with an average of 2.4 percent. The sulfur in the coal appears mainly as sulfur dioxide in the flue gas. The balance of the residual sulfur in the fuel appears as sulfur trioxide, sulfuric acid mist, or as other compounds in the fly ash or bottom slag. The sulfur content of either the fly ash or slag from five power plants tested was normally 0.1 to 0.2 percent. Since the average ash content of the coal from these units was about 10 to 12 percent, less than 1 percent of the sulfur in the coal appeared in the ash. The spreader stoker had approximately 1.0 percent sulfur in the reinjected particulate and 0.25 percent in the grate ash; this ac- counted for about 2 percent of the sulfur in the coal. The remainder of the sulfur should therefore appear as sulfur oxides in the flue gas. Other investigators have estimated that between 90 and 100 percent of sulfur entering the boiler in the coal would be expected to appear as sulfur oxides in the flue gas. 13 The concentrations of sulfur dioxide in the flue gas leaving the fly-ash collector varied from 1,000 to 1,730 ppm for all six units (Table 3). No significant changes in concentrations of sulfur dioxide were noted in full- and partial-load operation. Material balances for conversion of sulfur in the coal to sulfur dioxide in the flue gas showed conversions ranging from 91 to 105 percent. Since sulfur conversion must be below 100 percent, the errors involved in calculating theoret- ical emissions and in sampling and analysis for sulfur oxides become apparent. The change in sulfur content of coals during the sampling runs, non-uniform mixing of flue gases, and slight inaccuracies in sampling and analysis could account for errors of 5 to 10 percent. The range of conversions does not include concentrations of sulfur dioxide from the front-wall-fired unit, which averaged 120 percent of theoretical. These high values for sulfur dioxide were due probably to interference 12 EMISSIONS FROM COAL-FIRED ------- i Table 3. TEST CONDITIONS AND MAJOR POLLUTANT EMISSIONS Type of boiler firing Full-load testsa Vertical Corner Front-wall Spreader-stoker Cyclone Horizontally opposed Partial-load testsb Vertical Corner Front-wall Spreader-stoker Cyclone Horizontally opposed Coal Sulfur,0 % 2.9 1.7 2.3 2.8 2.4 2.4 2.8 1.6 1.8 2.5 2.4 2.9 Ash,d % 20.2 14.9 10.3 8.4 7.7 8.2 19.0 13.5 9.2 8.7 7.4 7.8 Emissions gr/scf6 Fly-ash BS 4.8 3.7 2.5 2.3 1.5 4.9 4.7 2.9 2.4 1.5 1.8 2.9 Ah 0.18 0.23 0.44 0.38 0.39 0.68 0.11 0.13 0.22 0.19 0.22 0.61 ppm by volume , dry basis Nitrogen oxides* BS 221 526 416 431 1204 393 161 393 500 430 742 395 Ah 310 413 606 437 1160 350 171 325 453 390 784 328 Sulfur dioxide BS 1450 1150 2120 1380 1350 1560 1700 1120 1080 1280 1380 1780 Ah 1730 1130 1680 1570 1360 1380 1640 1000 1460 1240 1370 1680 Sulfur trioxide BS 66 8 11 58 21 10 46 10 3 52 13 6 Ah 9 12 7 76 31 9 10 12 20 69 22 8 > CD aAverage values from three or four tests. b Aver age values from two tests. cMoisture- and ash-free basis. ^Moisture-free basis. eCorrected to 12% CC-2, dry basis. fReported as NO2. SBefore fly-ash collector. hAfter fly-ash collector. ------- from organic acids and/or other trace mineral acids before the an- alytical technique was modified to preclude this interference. It was theorized that the percentage conversion of sulfur in the coal to sulfur dioxide in the flue gas would be considerably lower in wet-bottom units. This theory was based on the assumptions that (1) some of the sulfur in the coal would contact the furnace bottom slag and form molten iron sulfide, particularly with low excess air, or (2) the iron content of recirculated fly ash would catalytically oxidize the sulfur dioxide in the flue gas to sulfur trioxide. Results did not sub- stantiate this theory. Between 95 and 100 percent of the sulfur that entered in the coal appeared as sulfur dioxide in the stack gases in the wet-bottom units. The concentrations of sulfur trioxide in the flue gas leaving the fly-ash collector varied between 7 and 76 ppm for all six units. The average outlet concentration of sulfur trioxide at both full and three- quarter load was 24 ppm. The total gaseous sulfur oxides found to be sulfur trioxide varied from 0.3 to 4.4 percent for measurements at the fly-ash collector inlet; the arithmetic mean was 1.7 percent. Under conditions of thermodynamic equilibrium, the percentage of sulfur trioxide should have been negligible at flame temperatures, and over 99 percent of the sulfur oxides after the air heater. 14 The measured concentrations, however, correspond to equilibrium values for con- ditions of gas temperature and oxygen content near the furnace outlet. The rapid cooling of gases in the convective-heat-transfer system near the furnace outlet appears to quench further oxidation of sulfur dioxide to sulfur trioxide. In the vertically fired unit, which produced the highest concen- trations of sulfur oxides, the sulfur trioxide concentration was reduced appreciably in passage through the fly-ash collector (Table 3). It was initially theorized that cooling of flue gases in the fly-ash collector at points of influent air leakage could result in condensation of sulfur trioxide and formation of sulfuric acid mist. The acid mist could then be adsorbed on the fly-ash particles and their removal effected by the collection of fly ash. This theory does not appear valid in all cases, because none of the other units realized significant reductions of sulfur trioxide in passage through the fly-ash collector. 2. Oxides of Nitrogen Oxides of nitrogen are formed largely by high-temperature oxida- tion of atmospheric nitrogen during combustion. Nitric oxide (NO) is the primary combustion product resulting from nitrogen fixation in the furnace. Theoretical equilibrium concentrations of nitric oxide may be as high as 1,000 ppm at 2,500°F and as low as 1 ppm or less at the lower temperatures (250-300°F) of the fly-ash collector. 15 Concentrations of nitrogen oxides measured at the inlet of the fly-ash collectors in all units except the cyclone-fired furnace ranged from 221 to 526 ppm at full load and from 161 to 500 ppm at three- 14 EMISSIONS FROM COAL-FIRED ------- quarter load (Table 4). These concentrations were generally equilib- rium levels for temperatures near the furnace outlet. The nitrogen oxides tended to decompose as the gases cooled. Apparently, the concentrations of nitrogen oxides are determined by flame temperature, incomplete decomposition as the gases flow from the flame to the furnace outlet, and rapid quenching of the decomposition reaction as the gases cool in the convective heat-transfer system. The higher temperatures encountered in the cyclone-fired furnace resulted in expected higher levels of nitrogen oxides at the fly-ash collector inlet -- 1,204 ppm at full load and 742 ppm at three- quarter load (Table 4). The wet-bottom turbo-fired furnace, which also operated with higher temperatures in the slagging zone, was also expected to produce higher concentrations of nitrogen oxides than the 395 ppm measured at the inlet of the fly-ash collector (Table 4). By concentration of combustion in the slagging zone at the bottom of the Table 4. NITROGEN OXIDE CONCENTRATIONS Full-load testsa Type of boiler firing Vertical Corner Front-wall Spreader-stoker Cyclone Horizontally opposed ppm*) Bd 221 526 416 431 1204 393 Ae 310 413 606 437 1160 350 Lb/106Btu Bd 0.38 0.95 0.68 0.65 2.5 0.65 Ae 0.55 0.71 0.95 0.76 2.2 0.59 Partial-load testsc Type of boiler firing Vertical Corner Front-wall Spreader-stoker Cyclone Horizontally opposed ppnv3 Bd 161 393 500 430 742 395 Ae 171 325 453 390 784 328 Lb/106 Btu Bd 0.28 0.73 0.82 0.73 1.9 0.66 Ae 0.31 0.57 0.74 0.68 1.8 0.56 aAverage values for three or four tests at each unit. ^Reported as NO2, at stack conditions. cAverage values for two tests at each unit. dB: Before fly-ash collector. eA: After fly-ash collector. POWER PLANTS: A SUMMARY 15 ------- furnace, however, a relatively long and gradual cooling of combustion products was provided in the upper part of the furnace and probably accounted for the decomposition of the nitrogen oxides. The result was a lower concentration of nitrogen oxides than would have been expected on the basis of combustion temperature alone. In two of the four units equipped with electrostatic-type fly-ash collectors (the vertically-fired and front-wall-fired units), the con- centrations of nitrogen oxides measured at the outlet of the fly-ash collectors were 40 to 45 percent higher than those measured at the inlet during full-load tests. This increase may be attributed to formation of ozone and atomic oxygen in the corona discharge of the electrostatic precipitator and their subsequent reaction with nitrogen to form additional nitrogen oxides. No increase in nitrogen oxide levels was found, however, at reduced loads for the vertically-fired and front-wall-fired units or at either load condition for the corner- fired or cyclone-fired boilers in passage through the electrostatic precipitators. Since the literature gives little information on this subject, additional studies are required to explain this phenomenon. 3. Solid Participate Efficient fly-ash-control equipment has enabled operators of the modern coal-burning power plant to reduce particulate emissions considerably. Ply-ash collection efficiencies of over 95 percent are not uncommon today. The three pulverized-coal-burning units, i.e., the vertically fired, the corner-fired, and the front-wall-fired boilers are operated with bituminous coals having ash contents ranging from approximately 10 to 20 percent. It has been estimated that over 75 percent of the ash in dry-bottom pulverized-coal-burning power plants leaves the furnace with the flue gas. The average grain loadings at the fly-ash collector inlet for these three units during full-load operation were 4.8, 3.7, and 2.5 grains per standard cubic foot respectively (Table 5). Ash leaving the furnace and entering the fly-ash collector amounted to about 60 percent for the vertically-fired unit, 80 percent for the corner- fired unit, and 75 percent for the front-wall-fired unit. A vertically- fired unit would be expected to retain more fly ash than a corner- or horizontally-fired unit because of the downward direction of flow resulting in fly-ash impaction on the bottom of the furnace. This condition would allow more fly ash to settle in the furnace. The lower ash retention for the corner-fired and front-wall-fired units would also be expected because their horizontal firing arrangement hinders ash dropout to the furnace floor. The combination mechanical-electrostatic fly-ash collectors for both the vertically- and corner-fired units effected average collection efficiencies of 96.4 and 93.9 percent, respectively, at full load (Table 5). The average collection efficiencies at three-quarter load increased to 97.5 percent for the vertically-fired unit and 95.7 percent for the corner-fired unit. The electrostatic fly-ash collector for the front-wall- 16 EMISSIONS FROM COAL-FIRED ------- Table 5. FLY-ASH CONCENTRATIONS AND COLLECTION EFFICIENCIES Type of boiler firing Full-load tests6 Vertical Corner Front-wall Spreader-stoker Cyclone Horizontally opposed Partial-load testsf Vertical Corner Front-wall Spreader-stoker Cyclone Horizontally opposed Ash in coal,a % 20.2 14.9 10.3 8.4 7.7 8.2 19.0 13.5 9.2 8.7 7.4 7.8 Concentrations gr/scfb BS 4.8 3.7 2.5 2.3 1.5 4.9 4.7 2.9 2.4 1.5 1.8 2.9 Ah 0.18 0.23 0.44 0.38 0.39 0.68 0.11 0.13 0.22 0.19 0.22 0.61 lbs/1000 lbc dry flue gas BS 8.8 6.9 4.6 4.2 2.8 8.9 8.7 5.5 4.4 2.8 3.1 5.1 Ah 0.27 0.42 0.82 0.66 0.62 1.27 0.21 0.21 0.41 0.35 0.36 1.1 Type of fly-ash collector^ C, E C, E E C E C C, E C, E E C E C Collector efficiency, % 96.4 93.9 83.1 83.9 74.5 83.9 97.5 95.7 91.3 87.3 86.3 77.7 aMoisture-free basis. ^Corrected to 12 percent CO2, dry volume basis. C1000 pounds of dry flue gas corrected to 50 percent excess air. dc designates cyclone; E designates electrostatic precipitator. eAverage values for either three or four tests of each unit. ^ Average values for two tests at each unit. SBefore fly-ash collector. h After fly-ash collector. ------- fired unit operated with an average collection efficiency of 83.1 percent at full load and 91.3 percent at three-quarter load. Although good reproducibility for fly-ash collection efficiency was obtained for both the vertically- and the corner-fired units, the collection efficiency varied appreciably at full-load operation of the front-wall and cyclone- fired units. No reason was apparent for this variation. The spreader-stoker unit was fired with crushed coal having an average ash content of 8.2 percent. The average grain, loading at the inlet to the mechanical fly-ash collector during normal-load operation was 2.3 .grains per standard cubic foot, which represents about 47 per- cent of the particulate entering the boiler as ash in the coal and as reinjected material. Average collection efficiency of the mechanical fly-ash collector was 83.9 percent .at normal load and 87.3 percent at partial load. The cyclone-fired unit and the horizontally opposed, downward- inclined fired unit are both wet-bottom boilers that normally operate with fly-ash reinjection. Both-were fired with bituminous coals having ash contents ranging from about 7 to 8 percent. The average grain loadings at the inlet to the fly-ash collector during full-load operation were 1.5 grains per standard cubic foot for the cyclone unit and 4.9 grains per standard cubic foot for the horizontally opposed fired unit (Table 5). The amounts of ash leaving the boiler and entering the fly-ash collector were about 50 percent for the cyclone-fired unit and 70 percent for the horizontally opposed fired unit. The electrostatic fly-ash collector for the cyclone-fired unit operated with average collection efficiencies of 74.5 percent at full load and 87.8 percent at three-quarter load. The mechanical fly-ash collector for the horizontally opposed fired unit effected average col- lection efficiencies of 83.9 percent at full load and 77.7 percent at three-quarter load. Particulate emissions from the electrostatic precipitators of the three pulverized-coal-burning units and of the cyclone unit would .meet the old American Society of Mechanical Engineers standard of 0.£5 pound particulate per 1,000 pounds of dry flue gas corrected to 50 percent excess air. 1° Particulate emissions from the mechanical collectors of the horizontally opposed fired unit would not meet this standard. Average fly-ash collector efficiencies of plants in operation a number of years were lower than those originally guaranteed by the manufacturer. Table 6 lists the guaranteed fly-ash collector efficiencies and the actual efficiencies measured during the tests. In general, operation at partial load showed higher fly-ash collector efficiencies. This indicated an over-loaded condition during normal operation. Decreases in fly-ash collector efficiencies were largely due to blockage of air passage with dust, which increased velocities; breakage and corrosion of electrostatic precipitator 18 EMISSIONS FROM COAL-FIRED ------- Table 6. COMPARISON OF FLY-ASH COLLECTOR EFFICIENCIES Type of firing Vertical Corner Front-wall Spreader-stoker Cyclone Horizontally opposed Type of collectora C & E C & E E C E C Fly-ash collector efficiency, % Guaranteed by manufacturer 98.2 97.5 95.0 93.1 95.0 89.7 Obtained in tests Normal load 96.4 93.9 83.1 83.9 74. 6b 88. 5b Partial load 97.5 95.7 91.3 87.3 86. 3b 78. Ob aC denotes a cyclone type collector; E denotes an electrostatic precipitator. blncludes only tests with fly-ash reinjection, which was normal operating procedure at these plants. electrodes; and changes in fly-ash characteristics due to variations of coal or boiler operation,, or both. The amount and composition of mineral matter in the coal largely determines the concentrations of trace metals in the fly ash. Seventeen common trace metals were determined in fly-ash samples from each of the six units (Table 7) during full-load operation. The estimated accuracy of these values is + 50 percent of the measured value. The spectrographic analyses of fly-ash samples were intended to determine any significant difference in collector efficiency for individual minerals as indicated by the content of the various metals. The average collector efficiencies of trace metals for any of the six power plants were nearly the same as the fly-ash collection efficiencies for each unit; for the spreader-stoker unit, however, metal collection efficiency was only 47 percent. 4. Polynuclear Hydrocarbons Although polynuclear hydrocarbons normally occur in minute concentrations, these compounds are of interest from an air pollution standpoint because several of them have exhibited carcinogenic properties in animal studies. 17> 18 Previous work has shown that polynuclear hydrocarbons result from the incomplete combustion of organic fuels; 19 thus, when a combustion process is poorly controlled, emissions of polynuclears may be high. Since combustion control in power plants was generally good and fuel-burning methods efficient, emissions of polynuclear hydrocarbons were low. The results are POWER PLANTS: A SUMMARY 19 ------- Table 7. METALS ANALYSIS FOR FULL-LOAD TESTS (grains/scf x 10-4)a Type of boiler firing Vertical Corner Front-wall Spreader - stoker Cyclone Horizontally opposed Avg. coll. eff. , % 89 94 86 4.7 69 QA Sam- pling point B* Ac Bb Ac Bb Ac Bb Ac Bb Ac Bb AC Cd Td Td <0.42 <0.024 0.73 0.26 Td Td 0.30 0.12 <0.97 0.58 Ba 9.5 0.34 20.8 1.4 20.4 3.4 3.65 0.94 27.2 7.5 6.8 1.1 Be 0.24 0.02 0.42 0.024 0.60 0.11 0.20 0.06 0.28 0.08 0.94 0.14 Fe 480 17 1900 102 480 . 58 1100 380 1360 380 6800 730 Pb 3.6 1.1 4.2 0.24 12.5 1.2 4.8 7.4 11.4 3.8 68 14 Cr 0.95 0.08 8.3 0.58 4.8 0.68 1.95 1.52 8.2 2.2 9.7 1.8 Cu 9.5 0.87 25 1.1 3.6 0.88 1.9 1.1 3.2 0.8 20 4.4 Sn Td 0.04 <0.42 <0.22 <0.26 0.26 Td 0.17 0.65 0.26 <0.68 0.32 Sb Td Td <4.2 <0.24 <0.8 <0.8 <2.4 <0.4 <1.4 <0.4 <6.7 <0.8 Mn 7.2 0.26 4.2 0.44 17.0 1.6 6.1 1.3 5.7 1.2 10.6 0.73 Ni 4.8 1.1 8.8 0.58 12.5 0.76 3.6 1.5 10.3 2.2 20 3.0 Mo 0.95 0.17 <1.22 0.10 3.8 0.58 0.73 0.37 1.14 0.38 10.6 2.2 V 9.5 0.88 24.8 1.4 24 2.4 6.1 1.5 13.6 4.7 42 6.6 Ti 95 3.4 420 22 460 48 48 17 136 38 540 73 Zn Td 0.34 12.2 <0.72 <24 < 2.8 <7.3 3.0 <4.2 < 1.2 42 14 Co 0.48 0.06 1.22 0.082 2.0 0.37 <0.73 0.21 2.2 0.8 5.1 0.66 As 1.4 0.11 2.6 1.6 0.66 0.30 4.0 0.54 aBased on particulate grain loading. Each value is the average of at least two tests. ^Before fly-ash collector. cAfter fly-ash collector. dlrace; blank indicates no data. ------- shown in Table 8. Figure 8 and Table 8 compare concentrations of seven polynuclears for which the analytical technique was most accurate. Concentrations of polynuclears are shown for the fly-ash-collector outlet only because early tests showed significant recovery of poly- nuclears by the fly-ash collectors. The levels of polynuclear hydro- carbon emissions for all units were well below the levels that result from the firing of coal in smaller furnaces with less precise control of the combustion process. Benzo(a)pyrene emissions from small furnaces varied from 3,800 to 400,000 micrograms per million Btu heat input; *$ average benzo(a)pyrene emission from six plants ranged from 19 to 223 micrograms per million Btu. 5. Emissions of Trace Contaminants Concentrations of carbon monoxide and gaseous hydrocarbons were low for all six units (Table 9). These low values indicated a high degree of combustion efficiency. Concentrations of these gases did not change significantly in any unit during operation at full and partial load, nor did passage of the flue gas through the fly-ash collector affect the concentrations. Formaldehyde concentrations from all units were very low, ranging between 0.03 and 0.25 ppm. The fly-ash collector reduced formaldehyde concentrations in five of the six units, with an average reduction of 45 percent. Removal of formaldehyde in the fly-ash collectors indicated adsorption of this compound by the fly-ash particles that were removed. The slight increase indicated for formaldehyde in the spreader-stoker test was probably due to an excessive amount of participate in the outlet sampling train. No Table 8. POLYNUCLEAR HYDROCARBON CONCENTRATIONS3" (micrograms /lO^ Btu heat input) Type of boiler firing Compound Fluoranthene Pyrene Benzo (a)pyr ene Benzo(e)pyrene Benzo(ghi)perylene Coronene Perylene Vertical 200 155 19 Corner 390 140 140 86 150 7 71 Front-wall 80 180 19 23 7 Spreader stoker 50 105 < 20 30 5 Cyclone 79 1025 223 395 198 6 17 Horizontally opposed 188 91 81 265 645 56 aAfter fly-ash collector during full-load operation. Average values for two tests at each unit. A blank indicates that the compound was not detected. POWER PLANTS: A SUMMARY 21 ------- I *l 8 O O M 0 1200 1000 = 900 1 800 600 500 400 TYPE OF BOILER FIRING - FULL LOAD TESTS VERTICAL II || || SPREADER- || I | HOI —1 U CORNER -J U- FRONT WALL -—4 U STOKER J U CYCLONE J U i;:g;:;| FLUORANTHENE ^3 PYRENE ^^ BENZO (a) PYRENE [^j BENZO (e) PYRENE ffi\ BENZO (ghi) PERYLENE ^| CORONENE tS^) PERYLENE HORIZONTALLY- OPPOSED - Figure 8. Polynuclear hydrocarbon concentrations at fly-ash collector outlet. ------- Table 9. SUMMARY OF TRACE GASEOUS EMISSIONS (ppm by volume, dry basis) Full-load testsa Type of boiler firing Vertical Corner Front-wall Spreader-stoker Cyclone Horizontally opposed Carbon monoxide Bd 17 11 5 29 f 44 AS 11 16 6 32 f 51 Hydrocarbons13 Bd 17 7 18 15 0 2 Ae 14 9 6 8 0 0 Formaldehyde Bd ' 0.25 0.17 0.14 0.06 0.17 0.10 Ae 0.12 0.12 0.08 0.10 0.07 Partial-load testsc Type of boiler Vertical Corner Front-wall Spreader -stoker Cyclone Horizontally opposed Carbon monoxide Bd 13 39 12 13 15 69 Ae 19 33 5 17 10 64 Hydrocarbons Bd 17 6 10 3 0 6 Ae 14 6 7 2 0 6 Formaldehyde Bd 0.26 0.11 0.14 0.03 0.15 0.11 Ae 0.07 0.04 0.06 0.06 0.11 0.09 aAverage values for three or four tests at each unit. ^Gaseous hydrocarbons at room temperature expressed as a single carbon atom hydrocarbon. °Average values for two tests at each unit. "Before fly-ash collector. eAfter fly-ash collector. %o data. significant changes in concentrations of any of these trace contaminants resulted from operation at either full or partial load. SUMMARY A series of tests of six coal-burning power plants was conducted to determine certain stack-gas components of interest in atmospheric pollution. The six units tested included three dry-bottom pulverized- coal-burning units, two wet-bottom units, and a large spreader-stoker traveling-grate unit. POWER PLANTS: A SUMMARY 23 ------- Measurements of sulfur oxides indicated that essentially 90 to 100 percent of the sulfur in the coal appeared as sulfur oxides in the stack gas. Of this amount 1 to 2 percent was in the form of sulfur trioxide and the balance was sulfur dioxide. Neither the type of furnace, the conditions of firing, nor the reinjection of fly ash affected sulfur oxide emissions significantly. Thus, concentrations of sulfur oxides are essentially determined by the amount of sulfur in the coal entering the furnace. Concentrations of nitrogen oxides varied widely, ranging from 221 ppm for the vertically fired unit to 1,204 ppm for the cyclone-type furnace. Concentrations of nitrogen oxides apparently are determined by initial flame temperatures in the firebox, decomposition in the high- temperature region of the furnace, and quenching of the decomposition reaction as the gases are cooled in the boiler section of the furnace. Control of particulate emissions varied considerably in coal- fired power plants. Combination cyclone and electrostatic-precipitator- type fly-ash collectors gave collection efficiencies of about 96 percent and an outlet grain loading of 0.20 grain per standard cubic foot at full load. Electrostatic precipitators and mechanical cyclone collectors, when used separately, gave average collection efficiencies ranging from 75 to 85 percent, with loadings at the fly-ash collector outlet varying from 0.19 to 0.68 grain per standard cubic foot. Other emissions were determined including polynuclear hydro- carbons, carbon monoxide, gaseous hydrocarbons, formaldehyde, and trace metals. None of these components were found in appreciable quantities during normal furnace-operating conditions. 24 EMISSIONS FROM COAL-FIRED ------- REFERENCES 1. Rohrman, F. A., B. J. Steigerwald. Some Potential Air Pollution Problems of the Future. Unpublished report. Division of Air Pollution, U.S. Public Health Service, R. A. Taft Sanitary Engineering Center, Cincinnati, Ohio (April 1965). 2. Cuffe, S. T., R. W. Gerstle, A. A. Orning, C. H. Schwartz. Air Pollutant Emissions from Coal-Fired Power Plants; Report No. 1. J. Air Pollution Control Assoc. 10:9 353-363 (Sept. 1964). 3. Gerstle, R. W., S. T. Cuffe, A. A. Orning, C. H. Schwartz. Air Pollutant Emissions from Coal-Fired Power Plants; Report No. 2. J. Air Pollution Control Assoc. 15:2 59-64 (Feb. 1965). 4. Cuffe, S. T. Techniques for Evaluating Air Pollutants from Power Plants. Arch. Environ. Health. 6:422-427 (March 1963). 5. Determination of Sulfur Dioxide and Sulfur Trioxide in Stack Gases. Emeryville Method Series 4S16/59a, Shell Development Company Analytical Department, Emeryville, California (1959). 6. Berk, A. A., L. R. Burdick. A Method of Test for SO2 and 803 in Flue Gases. Bureau of Mines Report of Investigations 4618 (Jan. 1950). 7. Beatty, R. L., L. B. Berger, and H. H. Schrenk. Determination of the Oxides of Nitrogen by the Phenoldisulfonic Acid Method. Bureau of Mines Report of Investigations 3687 (Feb. 1943). 8. Stenburg, R. L., D. J. Von Lehmden, and R. P. Hangebrauck". Sample Collection Techniques for Combustion Sources -- Benzopyrene Determination. Amer. Ind. Hyg. Assoc. J. 22: 271-275 (Aug. 1961). 9. Commins, B. T., P. J. Lawther. Volatility of 3, 4 Benzpyrene in Relation to the Collection of Smoke Samples. Brit. J. Cancer 12: 351-354 (Sept. 1958). 10. Sawicki, E., T. W. Stanley, T. R. Hauser, and F. T. Fox. The Detection and Determination of Polynuclear Hydrocarbons in Urban Airborne Particulates -- 1, The Benzopyrene Fraction. Intern. J. Air Pollution 2: 273-282 (1960). 11. Altshuller, A. P., D. L. Miller, and S. F. Sleva. Determination of Formaldehyde in Gas Mixtures by the Chromotropic Acid Method. Anal. Chem. 33: 621 (Apr. 1961). 12. The Silver Diethyldithiocarbamate Method of Arsenic Determination. Technical Data 142, Fisher Scientific Co. (Feb. 1960). POWER PLANTS: A SUMMARY 25 ------- 13. Committee on Air Pollution, Interim Report. Cmd. 9011, Her Majesty's Stationery Office, London (1953). 14. Orning, A. A., C. H. Schwartz, J. F. Smith. Minor Products of Combustion in Large Coal-Fired Steam Generators. Presented at the Winter Annual Meeting of the Am. Soc. Mech. Engrs., New York, New York (Nov. 29 Dec. 4, 1964) Paper No. 64-WA/FU-2. 15. Orning, A. A. Air Pollutants from Coal-Fired Electric Power Plants. National Conference on Air Pollution, Washington, D. C. PHS Publication No. 654, 159-160 (Nov. 1958). 16. Stern, A. C. Air Pollution. Academic Press, New York and London. Vol II, Chapter 37 (1962). 17. Sawicki, E., K. Cassel, (ed.). Analysis of Carcinogenic Air Pollu- tants. Natl. Cancer Inst. Monograph No. 9 (Aug. 1962). 18. Hartwell, I. L. Survey of Compounds Which Have Been Tested for Carcinogenic Activity. Second Edition, Public Health Service Publication No. 149 (1951). 19. Hangebrauck, R. P., D. J. Von Lehmden, J. E. Meeker. Emissions of Polynuclear Hydrocarbons and Other Pollutants from Heat- Generation and Incineration Processes. J. Air Pollution Control Assoc. 14: 7 267-278 (July 1964). 26 ------- BIBLIOGRAPHIC: Cuffe, S. T. and Gerstle, R. W. Emissions from coal-fired power plants: a compre- hensive summary. PHS Publ. No. 999-AP-35. 1967. 26 pp. ABSTRACT: The Public Health Service and the Bureau of Mines conducted a study of air pollutant emissions from the six main types of coal-burning power plants. The components tested include sulfur oxides, nitrogen oxides, polynuclear hydrocarbons, total gaseous hydrocarbons, solid particulates, formaldehyde, organic acids, arsenic, trace metals, and carbon monoxide. This report relates the effects of variables such as method of operation, type of boiler furnace and auxiliaries, reinjection of fly ash, and type of coal burned to the concentrations of gaseous and particulate pollutants in the products of combustion. ACCESSION NO. KEY WORDS: BIBLIOGRAPHIC: Cuffe, S. T. and Gerstle, R. W. Emissions from coal-fired power plants: a compre- hensive summary. PHS Publ. No. 999-AP-35. 1967. 26 pp. ABSTRACT: The Public Health Service and the Bureau of Mines conducted a study of air pollutant emissions from the six main types of coal-burning power plants. The components tested include sulfur oxides, nitrogen oxides, polynuclear hydrocarbons, total gaseous hydrocarbons, solid particulates, formaldehyde, organic acids, arsenic, trace metals, and carbon monoxide. This report relates the effects of variables such as method of operation, type of boiler furnace and auxiliaries, reinjection of fly ash, and type of coal burned to the concentrations of gaseous and particulate pollutants in the products of combustion. ACCESSION NO. KEY WORDS: BIBLIOGRAPHIC: Cuffe, S. T. and Gerstle, R. W. Emissions from coal-fired power plants: a compre- hensive summary. PHS Publ. No. 999-AP-35. 1967. 26 pp. ABSTRACT: The Public Health Service and the Bureau of Mines conducted a study of air pollutant emissions from the six main types of coal-burning power plants. The components tested include sulfur oxides, nitrogen oxides, polynuclear hydrocarbons, total gaseous hydrocarbons, solid particulates, formaldehyde, organic acids, arsenic, trace metals, and carbon monoxide. This report relates the effects of variables such as method of operation, type of boiler furnace and auxiliaries, reinjection of fly ash, and type of coal burned to the concentrations of gaseous and particulate pollutants in the products of combustion. ACCESSION NO. KEY WORDS: ------- |