Fossil Fuels Utilization - ACS Publications - American Chemical Society

1 Advanced Physical Coal Cleaning A Strategy for Controlling Acid Rain Precursor Emissions Thomas J. Feeley, III, and Bernard D. Blaustein Downloaded by 194.50.116.180 on August 24, 2016 | http://pubs.acs.org Publication Date: September 18, 1986 | doi: 10.1021/bk-1986-0319.ch001

Pittsburgh Energy Technology Center, U.S. Department of Energy, Pittsburgh, PA 15236 The United States has made impressive s t r i d e s i n reducing atmospheric emissions o f s u l f u r dioxide since passage o f the Clean A i r Act Amendments o f 1970. However, s i g n i f i c a n t amounts o f SO2 continue to be emitted and these emissions may increase. A large percentage o f these emissions are produced by coal-fired electric utilities, and these sources have been targeted f o r further SO2 reductions i n numerous acid r a i n c o n t r o l proposals. Advanced physical f i n e -coal cleaning t o remove p y r i t e may provide a v i a b l e strategy f o r reducing future SO emissions. There i s a large research e f f o r t t o develop advanced coal-cleaning technologies as a c o s t - e f f e c t i v e means to control acid r a i n precursor emissions from c o a l - f i r e d power plants and large i n d u s t r i a l sources. 2

In recent years there has been increased concern about emissions o f s u l f u r dioxide ( S O 2 ) and nitrogen oxides (N0 ) from f o s s i l f u e l combustion and about the r o l e that SO2 and N0 play i n the format i o n o f acid p r e c i p i t a t i o n . With the passage o f the Clean A i r Act Amendments i n 1970 and 1977, regulations were enacted to control the major emitters o f SO2 and N0 (1,2). New Source Performance Standards (NSPS) were established i n " 1971 to l i m i t SO2 emissions from c o a l - f i r e d power plants. The NSPS were l a t e r revised i n 1979 to include a percentage reduction requirement. These regulations proved t o be an e f f e c t i v e t o o l i n protecting a i r q u a l i t y . After reaching a peak i n 1973, national annual emissions o f SO2 declined by 28 percent between 1973 and 1983. This decline i s a l l the more impressive because e l e c t r i c u t i l i t y coal consumption increased by about 60 percent during the same period. R e l a t i v e l y large amounts of pollutants continue to be emitted, however, and emissions are projected to increase. Table I gives the National Acid P r e c i p i t a t i o n Assessment Program's 1980 emissions estimates f o r SO2 and N0 by source category ( J ) . As shown, sources i n the United States emitted 24.6 teragrams o f S 0 and 21.5 teragrams o f N0 i n 1980. Table I I presents projected U.S. emissions estimates ( 4 ) . The data X

X

X

X

2

X

This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

1. FEELEY AND BLAUSTEIN Table I.

Advanced Physical Coal Cleaning

3

Pollutant Emissions i n 1980 by Source Category Pollutant (Tg/Yr)

Downloaded by 194.50.116.180 on August 24, 2016 | http://pubs.acs.org Publication Date: September 18, 1986 | doi: 10.1021/bk-1986-0319.ch001

Source Category

SO

Electric Utilities I n d u s t r i a l Combustion Residential/Commercial Combustion Non-ferrous Smelters Other I n d u s t r i a l Processes Transportation Miscellaneous Total

2

N0

7.3 4.1 0.6 Neg. 0.9 8.3 0.3 21.5

15.7 3.4 0.8 1.1 2.7 0.8 0.1 2 θ

Data taken from reference 3, page 34.

Note change i n u n i t s .

Table I I . U.S. Current and Projected SO2 and NO Current 1980 (Tg/Yr) Source Category Electric Utilities I n d u s t r i a l B o i l e r s and Process Heaters Nonferrous Smelters Residential/Commercial Other I n d u s t r i a l Processes Transportation TOTALS Source:

SO

2

N0

X

Projected 1990 (Tg/Yr) SO

X

2

N0

X

x

Emissions Projected 2000 (Tg/Yr) SO

2

N0

X

15.0 2.4

5.6 3.5

15.9 3.4

7.2 3.0

16.2 6.5

8.7 4.0

1.4 0.8 2.9

0.7 0.7

0.5 1.0 1.2

0.7 0.8

0.5 0.9 1.5

0.6 1.1

0.8 24.1

8.5 19.0

0.8 22.8

7.8 19.5

1.0 2 θ

ΛΑ 24.1

Reference 4, page 2-94.

c i t e d by Homolya and Robinson (page 2-94 i n reference 4) show an increase i n both SO 2 and N0 f o r the year 2000 as compared to 1980. X

Acid Deposition The l e v e l of emissions of SO2 and N0 , and t h e i r e f f e c t s i n the United States and i n other i n d u s t r i a l i z e d countries have been the target of considerable debate. When SO2 and N0 are emitted into the atmosphere, a large f r a c t i o n of these p o l l u t a n t s can be oxidized to s u l f a t e and n i t r a t e during atmospheric transport, and then deposited as a c i d i c compounds (4-8). Acid r a i n i s the popular term to describe t h i s complex phenomenon. A c i d i c compounds can be deposited i n both wet and dry forms, and t h i s process i s more properly referred to as acid deposition or acid p r e c i p i t a t i o n . The wet forms of a c i d deposition include r a i n , snow, fog, and dew. Dry deposition occurs v i a absorption of SO2 and N0 on surfaces and v i a X

X

X


FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS


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the g r a v i t a t i o n a l s e t t l i n g (deposition) of f i n e particulate aerosols, some of which are a c i d i c . For a discussion of dry deposition, see the chapter by Hicks i n reference 4. The question of what to do about a c i d deposition i s a very complex and c o n t r o v e r s i a l one. The controversy surrounding acid deposition i s fueled by a less-than-perfect understanding of i t s sources, chemistry, transport, e f f e c t s , and m i t i g a t i o n . Numerous overviews on a c i d deposition have been published (3-13) and much research i s c u r r e n t l y being c a r r i e d out (V4,.15). F i e l d studies have indicated that large portions of the eastern United States and southeastern Canada are subject to deposition of environmentally s i g n i f i c a n t amounts of s u l f a t e and n i t r a t e , as shown i n Figures 1 and 2. In some l o c a t i o n s , these a c i d i c compounds have contributed to an average annual p r e c i p i t a t i o n pH as low as 4.2 (see Figure 3). (Figures 1 to 3 are adapted from Figures 8-14, 8-15, and 8-17 i n reference 4.) In the eastern United States, s u l f u r compounds are responsible f o r about two-thirds of the a c i d i t y of p r e c i p i t a t i o n , with nitrogen compounds accounting f o r the remaining one-third. This r e l a t i v e c o n t r i b u t i o n to acid p r e c i p i t a t i o n i s e s s e n t i a l l y reversed i n the western United States. Many of the areas c u r r e n t l y r e c e i v i n g high l e v e l s of a c i d deposition contain aquatic and t e r r e s t r i a l ecosystems susceptible to a c i d i f i c a t i o n (16-21). The adverse e f f e c t of acid deposition has been most c l e a r l y documented i n the a c i d i f i c a t i o n of lakes and streams located i n these s e n s i t i v e areas. A c i d i f i c a t i o n can a l t e r b i o l o g i c a l populations and communities, as w e l l as reduce the number of species w i t h i n them. Often, a c i d i f i e d waters are found i n regions that have l i t t l e or no n a t u r a l capacity to n e u t r a l i z e acidity. The i n s u f f i c i e n t b u f f e r i n g capacity of these lakes, streams, and surrounding s o i l s serves to exacerbate the problem. There i s further concern about impacts on f o r e s t s , although the extent to which a c i d deposition contributes to f o r e s t damage and decline i s not at a l l c l e a r . Our incomplete understanding of acid deposition i s underscored by our lack of knowledge of the sequence of events involved i n i t s formation. I t i s generally agreed that SO 2 and N0 emitted from human a c t i v i t i e s are the primary precursors of a c i d deposition i n the United States. Once i n the atmosphere, these p o l l u t a n t s undergo a complex s e r i e s of chemical reactions as they are converted to t h e i r a c i d i c forms. During t h i s conversion, SO2 and N0 can be transported long distances, so that for a given receptor s i t e , remote sources can add s i g n i f i c a n t l y to the c o n t r i b u t i o n of l o c a l sources (22). Therefore, emissions from one region of the United States can contribute to deposition of a c i d i c materials i n another region hundreds of kilometers downwind. However, because of the complex chemical transformation and atmospheric transport processes involved, there i s as yet no accepted method to e s t a b l i s h with c e r t a i n t y the s p e c i f i c sources that give r i s e to acid deposition i n a given receptor area. Only a few papers i n t h i s symposium d i r e c t l y address the issue of a c i d deposition. Almost a l l , however, are related to some aspect of the problem. The chemistry and e f f e c t s of combustionderived a i r p o l l u t a n t s , as w e l l as methods for c h a r a c t e r i z i n g these emissions, are reported. In a d d i t i o n , a number of papers focus on the three categories of c o n t r o l options a v a i l a b l e f o r reducing SO2 and N0 : precombustion, during combustion, and postcombustion. X

X

X



Figure 1. P r e c i p i t a t i o n amount - weighted mean sulphate ion deposition f o r 1980 (m moles per square metre).



Figure 2. P r e c i p i t a t i o n amount - weighted mean n i t r a t e ion deposition f o r 1980 (m moles per square metre).




1. FEELEY AND BLAUSTEIN

Figure 3. P r e c i p i t a t i o n amount - weighted mean annual pH i n North America f o r the calendar year 1980.


7

FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS

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Precombustion c o n t r o l involves removal of s u l f u r compounds from f u e l p r i o r to combustion. Control during combustion employs techniques to minimize the formation and/or release of SO2 and N0 during the combustion process. F i n a l l y , SO2 and N0 can be removed from the combustion f l u e gas using various postcombustion c o n t r o l methods. This chapter discusses the p o t e n t i a l of m i t i g a t i n g a c i d deposition through precombustion cleaning of coal to remove s u l f u r compounds. X

X

Sulfur Dioxide Emissions Although both SO2 and N0 are precursors to a c i d deposition, SO2 has been targeted most often f o r reduction. Of the approximately 25 teragrams (27 m i l l i o n tons) of SO2 emitted i n 1980, coal-burning electric utilities were responsible f o r nearly two-thirds. As mentioned, the 1971 and revised-1979 NSPS are designed to curb emissions of SO2 from c o a l - f i r e d utilities. These performance requirements have been quite successful i n c o n t r o l l i n g SO2 emissions from new coal-burning u n i t s brought on l i n e since 1971. However, i t i s the r e l a t i v e l y uncontrolled emissions from older c o a l - f i r e d power plants that have c a l l e d attention to these b o i l e r s for further c o n t r o l measures. Numerous l e g i s l a t i o n has been offered at both the s t a t e and f e d e r a l l e v e l to achieve a reduction in S O 2 , much of which w i l l come from further r e s t r i c t i o n s on c o a l f i r e d u t i l i t y emissions. Some b i l l s c a l l for as much as an 8-12 m i l l i o n ton reduction i n annual SO2 emissions from 1980 l e v e l s . There are a number of options a v a i l a b l e f o r c o n t r o l l i n g SO2 from c o a l - f i r e d power plants. S e l e c t i o n of a c o n t r o l strategy should further reductions i n SO2 be required w i l l be contingent on a number of factors s p e c i f i c to each u t i l i t y . Not the l e a s t important are the cost and reduction e f f i c i e n c i e s of the a v a i l a b l e options. Table I I I presents cost and performance estimates f o r various SO2 c o n t r o l technologies from a recent O f f i c e of Technology Assessment (OTA) report (8). As shown, with the possible exception of switching to a lower s u l f u r c o a l , there i s a general increase i n cost ( m i l l s per kilowatt hour) as reduction e f f i c i e n c i e s increase. The r e l a t i o n s h i p between SO2 reduction and cost i s also i l l u s t r a t e d i n Figure 4 from the OTA report, which shows u t i l i t y SO2 c o n t r o l costs f o r various reduction scenarios. There i s considerable s c a t t e r i n the estimates, but the eight studies c i t e d agree s u f f i c i e n t l y to show that for SO2 reductions of several m i l l i o n tons per year, t o t a l annualized costs w i l l be i n the b i l l i o n s of d o l l a r s . The p o t e n t i a l l y high cost of reducing current SO2 emissions has prompted continued development of pollution control technologies. One p r a c t i c a l method for c o n t r o l l i n g SO2 emissions from c o a l - f i r e d power plants i s physical coal cleaning (23-26). Although greatly c o a l - s p e c i f i c , coal cleaning can i n some cases produce coal that i s i n compliance with the 1971 NSPS of 1.2 pounds of S 0 emitted per m i l l i o n Btu heat input (24). The 1971 and 1979 NSPS f o r SO2 f o r coal-burning e l e c t r i c utilities, and the percentage reduction requirements used to c l a s s i f y coals i n t o four categories, each subject to a d i f f e r e n t l e v e l of SO2 reduction, are shown i n Table IV. Allowable s u l f u r dioxide emissions to the atmosphere range from a high of 1.2 lb/10 Btu heat input to a low of 0.6 lb/10 Btu heat input or l e s s , depending on the amount of


X

2

6

6


1.

FEELEY AND BLAUSTEIN


9

Table I I I . Overview o f Control Technologies f o r S u l f u r Dioxide


Control Technology

Reduction Efficiencies (percent)

Revenue Requirements (mills/kWh)

Stage o f Development

30-90 5-40

0-7 1-5

In use In use

60-85

NA

Emerging

70-95

10-17

In use

40-90

9-15

In use

70-90

12-25

Available

Fuel switching Physical coal cleaning Chemical coal cleaning Wet f l u e gas desulfurization Dry f l u e gas desulfurization Regenerable f l u e gas desulfurization Source:

Reference 8, page 157.

Table IV. New Source Performance Standards f o r SO 2 , 1971 and 197S

Source

P o t e n t i a l SO2 i n Raw Coal, l b / S 0 / 1 0 Btu 6

2

—

Coal-Fired B o i l e r s , >250 m i l l i o n Btu/hr, for which construct i o n commenced a f t e r 08/17/71 Coal-Fired U t i l i t y B o i l e r s , >250 m i l l i o n Btu/hr, f o r which construction cornmenced a f t e r 09/19/78

12

SO 2 Reduction Required, %

—

70 70 to 90 90 >90

Sulfur Dioxide Emission Limits lb SO2/IO

6

Btu

1.2

0.6 0.6 1.2 1.2

s u l f u r i n the raw c o a l . For low-sulfur coals, only a 70 percent reduction i n p o t e n t i a l SO2 emissions i s required when actual emissions are l e s s than 0.6 l b / 1 0 B t u heat input. The 1979 NSPS permit a s l i d i n g scale o f 70 percent to 90 percent SO2 reduction f o r intermediate-sulfur coals (see Table IV). At present, as shown i n Table I I I , f l u e gas d e s u l f u r i z a t i o n i s the only technology a v a i l able f o r achieving 70 to 90 percent SO2 reductions. Coal cleaning followed by f l u e gas d e s u l f u r i z a t i o n i s one possible strategy f o r meeting the 1979 NSPS s more stringent percentage reduction requirements (25). 6

1



o ο

S02

4

*•

I

8

•

ο*

+ο

10

reductions (million tons/year, after SIP compliance)

6

DOE PEDCO Peabody coal Teknekron - EPA/DOE ICF-EEI ICF-EPA/DOE Ο ICF - NWF * OTA

X

+ • Ο • •

12

2

Figure 4· Comparisons of u t i l i t y S0 control costs — estimates of 31-State aggregate control costs made by OTA and other groups.

ih-

2h

3r-

61-


14

C/î

z

r Π Ο Ζ π m

z

m

ο z

z

Fossil Fuels Utilization - ACS Publications - American Chemical Society

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