3
Alexander Weir, Jr. Southern California Edison Co., Rosemead, Calif. 9 7770
Choose your side. cientists of the federal gover and one electric utility remain at loggerheads on opacity
This is in response to the letter by Don R. Goodwin, EPA’s Director of Emission Standards and Engineering(€SAT, January 1977,p lo),stating that “Weir et al . . . misunderstoodthe purpose and use of opacity standards,” and “we consider the article’s conclusions to be incorrect.” Mr. Goodwin was referring to our paper “Factors Influencing Plume Opacity” (€SAT, June 1976,p 539),where we indicated that previously unrecognized variables related to the altitude and azimuth of the sun could result in a power plant meeting New Source Performance Standards for particulate emissions meeting EPA opacity standards in the winter but not the summer, or having a visual opacity of 18% at sunrise, increasing to 85% at noon, as measured in accordance with EPA Method 9. Mr. Goodwin also stated “However, the results of field studies conducted by EPA (ref. 3) show the positive errors caused by environmental lighting and background contrast are much smaller than suggested by Weir et al. The field studies showed that qualified observers are able to consistently read opacities of black and white plumes with positive errors not exceeding 7.5% opacity based on sets of 25 consecutive readings (ref. 3). The positive errors of this magnitude were infrequent and do not reflect a positive bias in Method 9.” EPA field data The data from the field studies referenced by Mr. Goodwin do not support these statements. Table 1 indicates that half the certified observers could only read within 7.5% of the meter reading one time out of ten, and two-thirds of the time the observers differed from the meter by more than 10 % opacity. These EPA data do not indicate to the writer that qualified observers are able to consistently read opacities of plumes within 7.5% opacity based on sets of 25 consecutive readings, at least on power plant stacks. The EPA report cited by Mr. Goodwin (ref. 7) ironically offers some confirmation of our thesis that visual opacity is greatly influenced by the position of the sun, both its altitude as well as its azimuth. In the discussion of the “accuracy” of the data from Steam Station Test No. 3,performed September 30,1974,on Boiler No. 9,and October 1, 1974,on Boiler No. 10 of Duke Power Company’s Riverbend Steam Station, the following statement is made on page 28 . . the deviations from the meter are calculated for the experimental group as well. The three cases, 45’, 90°,and 0’ are treated separately and regression lines calculated for each case . . . The first line does not have a significant slope while the second two do. However, it is easy to see that the two slopes are different and thus that the observer’s determinationsdiffered when the manner and position of observation differed. It is interesting to note that the most accurate readings were made when the group was at an approximate 45’ angle to the sun.” Our data (ref. 2) on the azimuth effect obtained at about the same latitude (35ON) but at a higher opacity level (the maximum observed opacity in this series of 24 EPA tests was 40.8% opacity) indicate a negligible increase in opacity at noon on the summer solstice (June 21)but a very pronounced influence at noon on the winter solstice (December 21)as shown in Table 2. It is postulated that i f the EPA observations had been made at a higher opacity level or nearer to the winter solstice (December 21)than the autumnal equinox (September 21),a larger effect of azimuth would have been observed. However, since the data were obtained at different times of the day as well as on two different boilers, it is not surprising that the azimuth effect was not quantified in the EPA report. It is perhaps of interest that one EPA observer’s set of 25 readings at 9:45a.m. on October 1 average 18.1% opacity when the transmissometer reading was 36.0% opacity, but this same observer’s set of readings average 31.8% at 1 p.m. when the transmissometerreading was ‘I.
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Three basics affecting visual opacity, according to Weir At noon
Altitude.Due to sun light scattering, visual opacity increases as the sun's altitude increases. In summer, the altitude of the sun at noon is 79'; in winter, 33"; spring 56" and fall 34".
N
Noon Azimuth. At North latitudes greater than 23.5", the sun is always due south at noon and is at its maximum altitude. At sunrise and sunset the sun's altitude is zero.
U
3-ft diameter
30-ft diameter
Stack diameter. Visual opacity increases as stack diameter increases even though the grain loading remains constant. Observers trained on small stacks have difficulty with larger ones. ~~
562
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Environmental Science & Technology
These EPA data illustrate the effect of increasing the sun's altitude about 20' on visual opacity. However, since the observer's offset angle was 30' in the mornirig and 45' at 1 p.m., some of the increase in observed opacity must be attributed to the azimuth effect. Perhaps the most significant conclusion in the EPA report is that "the observer's determinations differed when the manner and position of observation differed."
The conflict Thus the EPA field studies cited by Mr. Goodwin do not support his statement, "The effects of geographic location, time of day and other illumination variables do not significantly affect the ability of trained observers to accurately evaluate plume opacities." Mr. Goodwin's statement that "the field studies were done by certified observers who were trained at different geographic locations . . ." illustrates a complete lack of understanding of the influence of light scattering angle and thus the altitude of the sun on what a visuat observer would see. It is not the geographical location where the observer was trained but the geographic location of the stack that is important. Like our studies on the effect of sun azimuth, the effect of latitude is more pronounced in the winter than in the summer. A decrease in North latitude of the stack from 56 O N to 24 ON results in an increase in plume opacity from 30% opacity to 65 % opacity at noon on December 21, but the same plume at noon on June 2 1 would only increase from 75 % opacity to 88 % opacity for the same latitude decrease. Thus a power plant emitting the
same amount of particulate matter of the same particle size would have a more difficult time meeting opacity standards in Florida than in Alaska. However, this is not just an academic argument concerning the influence of the sun’s altitude on plume opacity. The EPA viewpoint can result in the expenditure of hundreds of millions of dollars by industry to meet opacity standards that are more restrictive than mass emission standards and result in raising the price of concrete, steel, and electricity to consumers. Mr. Goodwin stated, “Another point of concern to EPA is that Weir et al. apparently misunderstood the purpose and use of opacity standards . . . opacity standards are set at levels that are not more restrictive than the corresponding mass-emission standard.”
The HydrogenGenerator
Weir’s position
Our data indicate that a new coal-fired power plant could meet EPA’s New Source Performance Standards for emission of particulate matter with a grain loading of about 0.046 grains per standard cubic foot. However, this concentration would have a plume opacity of about 85 % in a 32.54 diameter stack at noon on June 21, assuming a mean particle diameter of 2.6 microns. In order to meet EPA’s 20 % opacity standard the year round, the grain loading would have to be reduced from 0.046 to about 0.007grains per standard cubic foot if the mean particle size was unchanged. However, since additional control equipment would result in removing more of the larger particles, the mean particle size would change, and thus a grain loading of about 0.003-0.005 grains per standard cubic foot would be required to meet 20% opacity, which is about one-tenth the mass emission required by EPA’s New Source Performance Standards. In conclusion, we have examined the report of the field studies referenced by Mr. Goodwin and do not find any EPA data that invalidate the conclusions of our article, and, as a matter of fact, some of the EPA data taken at a power plant support our thesis on the influence of the sun’s position on visual plume opacity. On a constructive note, the change in the EPA position (ref. 3) on the use of transmissometers to measure opacity is a welcome one as is the statement that Ringelman number is no longer considered by EPA to be equivalent to opacity.
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Additional reading 1. Hamil, rl. E., Thomas, R. E., and Swynnerton, N. F., “Evaluation and Collaborative Study of Method for Visual Determination of Opacity of Emissions from Stationary Sources” Environmental Protection Agency, Research Triangle Park, North Carolina, EPA Report No. 650/4-75-009, January 1975, 70 pp. 2. Weir, A., Jr., Jones, D. G., Papay, L. T., Calvert, S., and Yung, S., “Measurement of Particle Size and Other Factors Influencing Plume Opacity” presented at International Conference on Environmental Sensing and Assessment, September 14-19, 1975, Las Vegas, Nevada. Sponsored by U S . World Health Organization, U.S. EPA, American Chemical Society, and others. 3. Goodwin, D. R . , “Public Comment Summary: Opacity Provisions under Standards of Performance for New Stationary Sources of Air Pollution” U S .EPA, Office of Air Quality Planning and Standards, Research Triangle Park, N.C. 2771 1, August 1975, p 24.
Alexander Weir, Jr. is the inventor of the WEIR Scrubber (€S&T, June, 1976, p 534),which was the first SO2 scrubber in the US. to meet the National Academy of Engineering’scriteria of “successful operation for over a year at a size larger than 100 megawatts.” He has been a member of the American Chemical Society for 33 years and received his Ph.D. in chemical engineering from the University of Michigan. He is the Manager, Chemical Systems Research and Development, for SCE.
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