The mobilization of sulfur can be obtained by quantifying by fuels and minerals production, concentration of sulfur in fuels and minerals, and by the transfer from producers to consumers.

Sulfur in fossil fuels is mostly in form of organic compounds that constitute the biomass. A fraction of the S in coal is also in inorganic, pyritic form. Following combustion, sulfur is oxidized to SO2 and a small fraction to SO3. The environmental impact of metabolized S begins at the mine due to acid mine drainage, and continues to the atmosphere as regional sulfurous haze. Further damage may occur following its deposition to the human lung, human-made materials, as well as aquatic and forest ecosystems. However, sulfur deposition to sulfur deficient agricultural land may induce crop growth.

Coal Production

In the United States, coal is mined in three regions: Appalachian, the Midwest (Interior), and the West. The coals in the regions differ in quality and concentration of impurities such as sulfur. Figure 2 shows the time-dependent contributions of the three regions to the national production of coal. The output of the Appalachian districts spanning Pennsylvania to Alabama, has remained at about 300 million tons/yr since about 1920. The production in the Western region was negligible until around 1970. These curves reveal that a major shift in the coal production occurred at around 1970, when the output of western coal became significant. Remarkably, within the span of a decade, low-sulfur Western coal captured a quarter of the United States coal market.

The significance of these shifts to sulfur emissions is that each coal producing district has its own range of sulfur content: a shift in the relative production rate results in a change of the average sulfur content and sulfur production. The coal production data described above define the raw material production rate Pi defined in Figure 2.

Figure 2. Coal production in the three U.S. coal-producing regions: Appalachian, Interior, and West. Data from U.S. Geological Survey Yearbooks (1880-1932); U.S. Bureau of mines, Mineral Yearbooks (1933-1980); Energy Information Administration (1983).

Coal Sulfur Content. The next parameter that will be examined is ci, the concentration of the contaminant sulfur for each coal-producing region. Knowing the production rate Pi and concentration ci permits the calculation of the mass of contaminant, Mi=ciPi, that is mobilized by each producer.

Each coal producing district has a geologically defined range for the sulfur content of its coal (Figure 3a). Western coal, for instance, is low in sulfur since it contains less than 1 percent of S. On the other hand, the districts in the Midwest produce coal ranging from 2 to 4 percent sulfur, with little production outside this range.

Figure 3. Sulfur content of coals a) Spatial distribution; b) Distribution function for each coal producing region Source: Energy Information Administration 1981.

The distribution of the sulfur that has been mobilized in coals from the three regions is shown in Figure 3b. The area under each curve represents the tonnage of sulfur mobilized in the respective regions. The data show that most of the sulfur is from Interior coals, while the sulfur contribution of the western coals is minimal.

The resulting trend of sulfur production from coal in the United States is shown in Figure 4. In the 1920s, most of the sulfur mobilization was from the Appalachian region. By the early 1980s, the mobilization of sulfur from Interior coal exceeded that of Appalachian by about 1.5 million tons of sulfur per year. Since 1970, Western coal has contributed to sulfur mobilization, but it accounts for only about a quarter of the tonnage mobilized from either the Appalachian or the Interior region and only about 12 percent of the total amount of coal sulfur mobilization. Hence, while U.S. coal production has increased in the 1970 to 1980 period from about 500 to 800 million tons/yr, the corresponding increase of the coal sulfur mobilization was only about 12 percent.

Figure 4. Trends in coal sulfur production for the Appalachian, Interior, and Western coal-producing regions.

Surface Transfer Matrix. A key link in the flow of coal sulfur is its transfer from the producers to the consumers of coal. Railroads transport more than half of the total coal shipped each year. Unit trains provide high-speed shipments of large quantities of coal to electric power plants, often hauling more than 10,000 tons per train.

The available data (Husar 1986) can provide the amount of coal that has been shipped from a given producing district to the consuming state. In effect, the data bases yield the surface transfer matrix sij, i.e. the transfer ratefrom producer i to consumer j. Example transfer matrix maps for different production regions is shown in Figure 5. As an example, a map of the resulting consumed coal sulfur content data for 1978 is given in Figure 6. The northern and mid-Atlantic states consume coal with a content about 1.4 percent sulfur. The midwestern states, as expected, consume coal with the highest sulfur content.

Figure 5. Normalized surface transfer maps of shipment to consumer states from coal producing regions. The numbers assigned to the states represent the percentages of total production consumed by the states: (a) Appalachia; (b) Interior; (c) the West.

Figure 6. Estimated average sulfur content of coal consumed in the United States in 1978.

Coal Consumption. In 1975, coal consumption was about 550 million tons/year, roughly the same as around 1920 and 1943 (Figure 7) However, since the 1930s there has been a total transformation in the economic sectors that consume coal. Before 1945, coal consumption was divided among electric utilities, railroad, residential and commercial heating, oven coke, and other industrial processes. The railroad demand was particularly high during the war years of the early to mid-1940s. Within one decade, the 1950s, coal consumption by railroads and by the residential-commercial sector essentially vanished. Currently, electric utilities constitute the main coal-consuming sector, and the trend of total coal use in the United States since 1960 has been determined by the electric utility coal demand.

Figure 7. Trend of U.S. coal consumption by consuming sector. (Husar 1986)

Oil Production

Sulfur mobilization from the combustion of oil products can be estimated from either production or consumption data. Detailed state-by-state data for oil production were not available for this report, therefore, estimates below were based on state-by-state data for oil consumption and sulfur content.

The trend in sulfur mobilization in the United States from the consumption of domestic and imported oils is shown in Figure 8a. Sulfur mobilization from domestic crude oil increased until about 1960, when it leveled off at 3 to 4 million tons/yr. About the same time the role of crude oil imports became significant. The sulfur imported with other oil products, most notably residual fuel oil, also significant. By the late 1970s, sulfur from imported oil exceeded the sulfur from domestically produced oil, but since 1978 there has been a significant reduction in sulfur in imported oil products.

As crude oil is refined a certain fraction of the sulfur is recovered as by-product, sulfuric acid. The recovered fraction has been increasing steadily since 1950. According to U.S. Bureau of Mines Mineral Yearbooks the sulfur recovered at refineries in 1980 was about 4 million tons/yr. Hence, more than half of the estimated sulfur from crude oil is now retained and recycled at the refineries.

The emitted sulfur from oil products is calculated as crude oil sulfur content minus recycled sulfur. As seen in Figure 8b, the oil sulfur emission estimated in this manner ranged between 3 and 4 million tons/yr for the period 1950 to 1978. Since then, there has been a significant decrease, caused primarily by declining imports and the increasing fraction of recycled sulfur. For 1982, emissions of sulfur from oil consumption were about 2 million tons/yr, which is less than 20 percent of the sulfur emissions from coal.

Figure 8. a.) Trend in Sulfur mobilization, before recycling, from domestic and imported oils. b.) Trends in sulfur recycling during petroleum processing and sulfur emissions from petroleum consumption in the United States (Husar 1986).

Copper and Zinc Smelting

Significant production of copper began in the United States about 1895 and reached approximately 1 million tons annually by 1920. For the next 40 years copper production fluctuated at that level, with no significant trend. During the 1960s smelter copper production again increased, reaching a peak of over 2 million tons around 1970, followed by decline in 1970s.

Virtually all copper ore is treated at concentrators near the mines. Concentrates are further processed at smelters. Production of sulfuric acid is the main process for removing sulfur oxides from smelter gases. However, acid production is practical only from converter gases. With tightly hooded converters, 50 to 70 percent of the sulfur oxides can be removed; removal of additional sulfur oxides requires scrubbing.

Zinc smelting in 1960s and 1970s was about 800,000 tons/yr. Foreign imports of zinc ores constitute a significant fraction of zinc production. Lead smelter production in the United States was about 700,000 tons/yr. However, sulfur emission from lead smelting is small compared with that from the smelting of copper and zinc and is not considered further in this report.

Sulfur emissions from metal smelting are estimated from the tonnage of sulfur mobilized by mining the ore minus the sulfur that is retained at smelters as sulfuric acid. It is evident from Figure 9a that by 1980 more than half of the sulfur in metal ore was recycled. As a result, sulfur emission (mobilized minus recycled) have fluctuated between 0.5 and 1.5 million tons/yr since the turn of the century. A particularly significant drop of emissions has occurred since 1970 (1.5 to 0.5 million tons/yr) as a result of both decline of smelter production and increased sulfur recovery (Figure 9b).

Figure 9. (a) Trend in sulfur mobilization and recycling from copper and zinc smelting and processing; (b) trend in sulfur emissions from copper and zinc smelting (Husar 1986).

Summary and Discussion of Sulfur Emission Trends

The trend of total sulfur emission for the entire U.S. is shown in Figure 10. It is evident that the S emissions have fluctuated between 8 and 16 million tons since the beginning of this century. The likely consensus of the long-term fluctuations include recessions, major wars, fuel switching, and environmental concerns. (Kissock and Husar, 1992) Over the years there was also a shift from manufacturing to power plants as the main emitters of sulfur. The aggregate U.S. emission trend graph (Figure 10) does not reveal the many dynamic changes that have occurred in the spatial and seasonal pattern of emission trends. More detailed examination revealed, for instance, that since the 1960s S emissions were reduced significantly in the northeastern states, but increased in the southeastern states. Also, since the 1960s the S emissions peaked in summer season, compared to the winter peak before the 1960s.

Figure 10. Trend of total sulfur emission for U.S.

From the point of view of industrial metabolism and sustainable development (Clark and Munn, 1986) it is significant to note that for several industrial sectors and fuel types, there has been an increase in the recycling of fuel and ore-bound sulfur. The trend of recovery estimates by Ayres (in Darmstadter et al., 1987) are given in Figure 11. The recovery of sulfur from natural gas and zinc processing is most complete, since their processing technologies allow easy separation and reuse. It is also encouraging that the recovery from copper and lead ores, as well as from oil products is approaching 50% with a likely increase in the future. Unfortunately, the S recovery from coal that is responsible for most of the sulfur mobilization, is still insignificant.

Figure 11. By-product sulfur recovery rates (Ayres in Darmstadter et al., 1987).

There are two other changes in the sulfur emissions that are significant from environmental impact point of view: power plant stack height and source displacement from urban to rural areas. The average stack height has increased over the years to minimize the near-source SO2 concentrations. Also, the major emissions, sources such as power plants were moved to rural areas, near rivers, and ponds, or close to the mines. As a consequence of these quantitative changes, the SO2 concentrations in urban/industrial areas have declined, and in rural areas have increased. For this reason, since the 1970s much of the concern about sulfur emission is due to regional air pollution as manifested by acid precipitation and regional haze.