Environmental Aspects of the Combustion of Sulfur-Bearing Fuels

Chapter 3

Environmental Aspects of the Combustion of Sulfur-Bearing Fuels

Downloaded by UNIV OF SOUTHERN CALIFORNIA on February 25, 2013 | http://pubs.acs.org Publication Date: June 29, 1990 | doi: 10.1021/bk-1990-0429.ch003

B. Manowitz and F. W. Lipfert Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973

This paper describes the origins of sulfur in fossil fuels and the consequences of its release into the environment after combustion, with emphasis on the United States. Typical sulfur contents of fuels are given, together with fuel uses and the resulting air concentrations of sulfur air pollutants. Atmospheric transformation and pollutant removal processes are described, as they affect the pathways of sulfur through the environment. The environmental effects discussed include impacts on human health, degradation of materials, acidification of ecosystems, and effects on vegetation and atmospheric visibility. The paper concludes with a recommendation for the use of risk assessment to assess the need for regulations which may require the removal of sulfur from fuels or their combustion products. Sulfur species are found i n ambient a i r i n most parts of North America and i n most i n d u s t r i a l countries. Their sources include natural emissions (biogenic and volcanic), smelting of ores and other i n d u s t r i a l r e f i n i n g processes, and combustion of sulfurbearing fuels. This paper w i l l focus on the combustion sources i n the United States and some of the effects of t h e i r sulfur emissions. The environmental effects of sulfur i n the environment have been of interest for many years and much of the information presented here has been drawn from the various conference proceedings and assessment documents that have been published i n recent years (1-11). When s p e c i f i c references are not l i s t e d i n the text, the information represents a consensus from these various sources. Sources of Sulfur Emissions Types of Fuels and Their Uses. Fuel combustion i s the overwhelming source of energy i n the United States, providing heating, e l e c t r i c 0O97-6156/90/0429-O053$O6.00/0 © 1990 AmericanChemicalSociety

In Geochemistry of Sulfur in Fossil Fuels; Orr, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


54

GEOChemISTRY OF SULFUR IN FOSSIL FUELS

power, transportation, and energy for manufacturing processes. The fuels used vary by application, the major categories being natural gas, gasoline, d i s t i l l a t e f u e l o i l , heavy and residual f u e l o i l s , coke and coal, i n approximate increasing order of t h e i r sulfur contents. This i s also roughly the order of increasing d i f f i c u l t y i n f u e l handling and combustion, so that the cleaner fuels tend to be used i n the smaller, more widely dispersed f a c i l i t i e s , and the d i r t i e r fuels i n larger, centralized f a c i l i t i e s ( r e s i d e n t i a l firewood i s an exception). The effects of fuel combustion on the environment ( a i r quality) w i l l depend on the impurities i n the f u e l ( i . e . , sulfur and ash), the e f f i c i e n c y of combustion, the height of the stack through which combustion gases are discharged, and the dispersive capacity of the atmosphere. Typical ranges of sulfur contents for various f u e l types and rates of sulfur emission per m i l l i o n Btu released are given i n Table I, as well as the estimated t o t a l national s u l f u r emissions. The 18 m i l l i o n metric tons of S0 emitted from f u e l combustion i n 1985 i s down about 20% from the peak, which occurred about 1970. Table II gives the national d i s t r i b u t i o n of r e s i d e n t i a l heating fuels as of 1983; natural gas i s the f u e l of choice everywhere except i n the Northeast, where i t i s evenly divided with o i l . Note that e l e c t r i c heating tends i n e f f e c t to be heating by means of coal combustion except i n the Northwest, where hydro and nuclear have larger shares of e l e c t r i c generation. 2

Table I.

Types of Fuels and Associated Sulfur Emissions

Fuel Type

S Content lb/10 Btu

U.S. S0 Emission* 10 metric tons/yr

1 - 8 1 1.2 - 3.0 0.3 - 3.0 0.1 - 0.3 0.0006 0.03 0 - 0.1 >0.15

14,930 38 616 1374 329 71 252 392 26

6

BituM. Coal Anthracite Lignite Resid. O i l Dist. O i l Natural Gas Gasoline Diesel Fuel Wood * Source: Ref.

(1985)

2

3

12

Trends. This picture has changed considerably from that which existed p r i o r to and during the decade after World War I I , the time at which many adverse environmental effects may have peaked. Prior to the development of transcontinental natural gas pipelines, (local) coal and coke were widely used for r e s i d e n t i a l and commercial space heating, with severe effects on l o c a l a i r quality. In the 1960s, high-sulfur residual fuel o i l was heavily used by commercial and i n d u s t r i a l sources along the Eastern seaboard. Following the passage of the 1970 Clean A i r Act, f u e l sulfur l i m i t a t i o n regulations were implemented throughout the U.S. as necessary to meet ambient a i r


3.

MANOWTTZ & LIPFERT


Table I I .

55

Combustion of Sulfur-Bearing Fuels

D i s t r i b u t i o n of Residential Space Heating Fuels (% of Housing Units)

Region: Northeast housing: t o t a l urban f u e l type

Midwest t o t a l urban

South t o t a l urban

e l e c t r i c * 7,.2 coal 1,.2 oil 44,.5 gas 42..5 wood 3,.0

10.,0 0.,3 8..1 77..8 3..4

31..0 1.,1 8..2 51..8 6.,7

5.,5 0. 6 44..5 47..9 0..5

7.,6 0.,1 4..3 87..0 0..6

30..0 0..1 9..4 59,.2 1,.2

West t o t a l urban

21..5 0..1 2,.7 67,.3 5,.4

19..0 2..5 72,.0 2,.0

* The fuels used to generate e l e c t r i c i t y w i l l vary l o c a l l y and by region. Source: Ref. 3

q u a l i t y standards. In addition, newly constructed sources were required to emit no more than 1.2 pounds of S0 per m i l l i o n Btu consumed, even i f this required i n s t a l l a t i o n of gas cleaning devices ("scrubbers"). In 1977, regulations were further tightened to l i m i t the use of excessively t a l l stacks as a means of achieving acceptable ground-level a i r quality, to prevent worsening of a i r quality i n locations with a i r quality better than the standards, and to require at least 70% removal of sulfur for large sources. This l a s t provision resulted i n a p r a c t i c a l S0 emission l i m i t of 0.6 lb/10 Btu. Sulfur removal devices or scrubbers have t h e i r own secondary environmental impacts by v i r t u e of the need for land disposal of the sludge. 2

2

6

Sources of Sulfur i n Coal and O i l . The major coal beds of eastern North America are of Pennsylvanian age. During that time, there was a constantly fluctuating sea l e v e l across f l a t lowlands over the North American i n t e r i o r . Coal was formed j u s t before the onset of marine conditions, so that coal swamp forests occurred on broad lands along or near the sea shore. Thicker sections accumulated on the more rapidly subsiding I l l i n o i s and Forest City basins and i n the Appalachia fIreland basin (14). Western coals were formed during the Cretacious and T e r t i a r y . During those time periods, the region was emergent land, so the coal swamps were i n contact with fresh rather than saline waters. The f i r s t step i n the formation of coal i s the a l t e r a t i o n of plant material into peat by bioChemical ( i . e . , microbial) processes. The o v e r a l l properties and composition of a given peat depend upon the environmental conditions that existed during the a l t e r a t i o n period after b u r i a l . During the bioChemical stage, microorganisms can reduce the sulfate i n saline pore water to an active form of sulfur, e.g., H S. H S w i l l react with components of the sediment, forming p y r i t e when iron i s present and forming 2

2



56

GEOChemISTRY OF SULFUR IN FOSSIL FUELS

organic sulfur compounds with the lignin-cellulose-humic matter present. Plants with a d i f f e r e n t bioChemistry are found i n fresh water environments, as compared to marine environments. They decay to d i f f e r e n t products and the resultant peat w i l l have a lower sulfur content. However, i t i s the fresh water contact that provides the general explanation of the low sulfur content of Western U.S. coals, and the saline water contact that provides the general explanation of the high sulfur content of the Eastern coals (Figure 1). Further evidence of this phenomenon i s that those Eastern coal beds that have high sulfur content are overlain with marine shale or limestone, whereas the low sulfur coal beds are not (3, 14). The sulfur content of crude o i l s varies from a f r a c t i o n of a percent to more than ten percent. The general explanation that the source of the sulfur arises from b i o l o g i c a l reactions on marine sulfate holds for o i l as well as for coal. However, the s i t u a t i o n i s more complex because during maturation, migration, and reservoir retention, processes can occur which w i l l a l t e r the d i s t r i b u t i o n of sulfur compounds. Thermal a l t e r a t i o n processes taking place at great depths result i n gradual d e s u l f u r i z a t i o n of o i l s . The p r e c i p i t a t i o n of asphaltenes by l i g h t organic fractions generally results i n desulfurization. Biodégradation of o i l s i n the reservoir generally results i n a concentration of sulfur compounds i n the non-biodegraded f r a c t i o n , the portion extracted. Thus the sulfur content of f u e l o i l s depends to some extent upon the biogeochemical c h a r a c t e r i s t i c s of oil-bearing deposits. Removal of Sulfur from F o s s i l Fuels. In the case of coal i t becomes important to quantify the organic sulfur f r a c t i o n versus the inorganic, for various grades of coal, and to i d e n t i f y the sulfur compounds i n the organic f r a c t i o n . Inorganic sulfur may often be removed p r i o r to combustion through physical cleaning of the coal, which can also remove a portion of the mineral matter. However, Chemical cleaning, a more complex and expensive process, i s required to remove the organically bound sulfur. The alternative to removing sulfur p r i o r to combustion i s to process the combustion gases through any of a number of Chemical processes, often referred to c o l l e c t i v e l y as "scrubbers." These processes add s u b s t a n t i a l l y to the cost of the f a c i l i t y and tap o f f a part of the energy produced, as well as producing a waste stream that must be disposed of. Removal of sulfur compounds from f u e l o i l s i s usually accomplished at the refinery. Although most of the sulfur i n o i l i s i n the high b o i l i n g f r a c t i o n , some middle d i s t i l l a t e s have high s u l f u r contents. A better understanding of the Chemistry of sulfur compounds i n o i l w i l l also f a c i l i t a t e t h e i r e f f i c i e n t removal. Fate of Sulfur Emissions It i s commonly assumed that a l l fuel sulfur i s oxidized to S0 during the combustion process, regardless of i t s Chemical form i n the f u e l ; a small percentage i s emitted as s u l f u r i c acid or metal sulfate p a r t i c l e s ( t y p i c a l l y

Environmental Aspects of the Combustion of Sulfur-Bearing Fuels

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