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close this bookIndustrial Metabolism: Restructuring for Sustainable Development (UNU, 1994, 376 pages)
close this folderPart 2: Case-studies
close this folder10. Sulphur and nitrogen emission trends for the United States: An application of the materials flow approach
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View the documentIntroduction
View the documentSulphur emissions
View the documentNitrogen oxides emissions
View the documentConclusion
View the documentReferences


Rudolf B. Husar


Among the substances metabolized by industrial activities, fossil fuels are the most significant, both in quantity and by the variety of chemicals that are mobilized. Industrial consumers take fuels as inputs and exhaust residue products to air, land, and water. Sulphur and nitrogen compounds are major fossil fuel residues that are released primarily into the air and subsequently deposited to land and water bodies.

The elements redistributed by the "industrial metabolism" of fossil fuels are carbon, sulphur, and nitrogen, as well as crustal and trace metals. In fact, these are the main chemicals of which living matter is composed. As such, they are the key nutrients for the plants on earth. However, these substances may be either beneficial or harmful to the receiving ecosystem, depending on their quantity, rate, and chemical form.

In order to assess the possible harm or benefit of fossil fuel residues and the possible remedying actions, it is helpful to construct a complete material flow scheme that describes the end-to-end transfer of these materials as they pass from one long-term geochemical reservoir to another.

In order to construct such a flow model, it is helpful to utilize the ecosystem analogue for the human-induced material flow, which has producers, consumers, and receptors (recyclers) as the key players. A general description and mathematical formulation of the ecological analogue is given elsewhere (Husar, 1986; also see chapter 2 of this volume). The purpose of this chapter is to illustrate the application of the producer, consumer, receptor method to the construction of a sulphur and nitrogen flow scheme for the United States. The essence of the approach is that one follows the path of the fuel or nitrogen from "production," i.e. mining, through the consumers to the environmental receptors. (More details on the methodology can be found in Husar, 1986.) In constructing the materials flow model emphasis was placed on obtaining the relevant data from the measurement records of various US agencies. Also, data were sought for the term trends in order to illuminate the dynamics of the producer-consumer-receptor system.

Fig. 1 Trend for US fossil fuel consumption since 1850: (a) consumption by fuel type; (b) fraction of total energy by fuel type (Source: Husar, 1986)

In the following, the presentation of the fuel production and consumption data sets is accompanied by a discussion of the technological trends for different industrial sectors; the changes in technology were taken mostly from Darmstadter et al. (1987).

Combustion of coal and oil products, along with the smelting of metals, produces the bulk of the anthropogenic sulphur and nitrogen emissions to the atmosphere. The driving force for fuel production is the consumption of energy by different sectors.

From the turn of the century to the 1970s, US energy consumption has been characterized by a steady increase in total consumption and shifts from one fuel to another (fig. 1). From 1850 to about 1880, wood was the primary energy source. By 1900, and during the first quarter of this century, rising energy demand was matched by the increasing use of coal. The depression years of the early 1930s are reflected in a sharp drop in coal consumption, which increased again during the war years in the early to mid 1940s. Coal consumption declined to another minimum in 1960, because the increasing energy demands were supplied by cleaner fuels, natural gas and petroleum. Accelerated oil and gas consumption began in the late 1930s and 1940s, such that by 1950 the energy supplied by oil exceeded that of coal and maintained its rise up to the early 1970s. By 1960, natural gas surpassed coal as an energy source.

Sulphur emissions

Figures for the mobilization of sulphur(s) can be derived by quantifying fuels and minerals production, the concentration of sulphur in fuels and minerals, and the transfer from producers to consumers.

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

However, sulphur deposition to sulphur-deficient agricultural land may induce crop growth.

Coal production

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

The significance of these shifts for sulphur emissions is that each coal-producing district has its own range of sulphur content: a shift in the relative production rate thus results in a change of the average sulphur content and sulphur production. (The coal-production data described above define the raw material production rate Pi defined in figure 2.)

Fig. 2 Coal production in the three US coal-producing regions: Appalachian, Interior, and West. (Sources: US Geological Survey Yearbooks, 1880-1932; US Bureau of Mines, Mineral Yearbooks, 1933-1980; Energy Information Administration, 1983)

Coal sulphur content

The next parameter that will be examined is c;, the concentration of the contaminant sulphur for each coal-producing region. Knowing the production rate Pj and concentration c; permits the calculation of the mass of contaminant, Mi = ciPi that is mobilized by each producer.

Fig. 3 Sulphur content of coals: (a) spatial distribution; (b) distribution function for each coal-producing region (Source: Energy Information Administration, 1981)

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

The distribution of the sulphur that has been mobilized in coals from the three regions is shown in figure 3b. The area under each curve represents the tonnage of sulphur mobilized in the respective regions. The data show that most of the sulphur is from Midwest (Interior) coals, while the sulphur contribution of the Western coals is minimal.

The resulting trend in sulphur emissions from coal in the United States is shown in figure 4. In the 1920s, most of the sulphur mobilization was from the Appalachian region. By the early 1980s, the mobilization of sulphur from Interior coal exceeded that of Appalachian coal by about 1.5 million tons per year. Since 1970, Western coal has contributed to sulphur 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 per cent of the total amount of coal sulphur mobilization. Hence, while US coal production has increased in the 1970-1980 period from about 500 to 800 million tons/yr, the corresponding increase in coal sulphur mobilization was only about 12 per cent.

Fig. 4 Trends in coal sulphur emissions for the Appalachian, Interior and Western coalproducing regions

Fig. 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

Surface transfer matrix

A key link in the flow of coal sulphur 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 databases yield the surface transfer matrix Sij, i.e. the transfer rate from producer i to consumer j.

Transfer matrix maps for different production regions are shown in figure 5. As an example, a map of the resulting consumed coal sulphur content data for 1978 is given in figure 6. The northern and midAtlantic states consume coal with a content of about 1.4 per cent sulphur. The Midwestern states, as expected, consume coal with the highest sulphur content.

Fig. 6 Estimated average sulphur content of coal consumed in the United States, 1978

Fig. 7 Trend of US coal consumption by consuming sector (Source: Husar, 1986)

Coal consumption

In 1975, coal consumption was about 550 million tons/yr, roughly the same as in around 1920 and 1943 (fig. 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.

Oil production

Sulphur 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, the estimates below were based on state-by-state data for oil consumption and sulphur content.

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

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

The emitted sulphur from oil products is calculated as crude oil sulphur content minus recycled sulphur. As shown in figure 8b, the oil sulphur emissions 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 sulphur. For 1982, emissions of sulphur from oil consumption were about 2 million tons/yr, which is less than 20 per cent of the sulphur emissions from coal.

Copper and zinc smelting

Significant production of copper began in the United States in 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 in around 1970, followed by a decline in the 1970s.

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

Fig. 8 (a) Trend in sulphur mobilization, before recycling, from domestic and imported oils; (b) trends in sulphur recycling during petroleum processing and sulphur emissions from petroleum consumption in the United States (Source: Husar, 1986)

Zinc smelting in the 1960s and 1970s was about 800,000 tons/yr. Foreign imports of zinc ores constitute a significant fraction of zinc consumption. Lead smelter production in the United States was about 700,000 tons/yr. However, sulphur emissions from lead smelting are small compared with those from the smelting of copper and zinc.

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

Summary and discussion of sulphur emission trends

The trend in total sulphur emissions for the entire US 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 sulphur.

The aggregate US emission trend graph (fig. 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 have been significantly reduced in the north-eastern states, but increased in the south-eastern states. Also, since the 1960s the S emissions have peaked in the summer season, compared to the winter peak before the 1960s.

From the point of view of "industrial metabolism" or "sustainable development" (Clark and Munn, 1986) it is significant that, for several industrial sectors and fuel types, there has been an increase in the recycling of fuel and ore-bound sulphur. The trend in recovery estimates is given in figure 11. The recovery of sulphur from natural gas and zinc processing is most complete, since their processing technologies allow easy separation and re-use. It is also encouraging that the recovery from copper and lead ores, as well as from oil products, is approaching 50 per cent, with a likely increase in the future. Unfortunately, the S recovery from coal, which is responsible for most of the sulphur mobilization in the United States, is still very low.

Fig. 9 (a) Trend in sulphur mobilization and recycling from copper and zinc smelting and processing; (b) trend in sulphur emissions from copper and zinc smelting (Source: Husar, 1986)

Fig. 10 Trend of total sulphur emission for the US

Fig. 11 By-product sulphur recovery rates (Source: Ayres, in Darmstadter et al., 1987)

There are two other changes in sulphur emissions that are significant from an 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 nearsource SO2 concentrations. Also, major emission sources, such as power plants, have been 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 sulphur emissions centres on regional air pollution, as manifested by acid precipitation and regional haze.

Nitrogen oxides emissions

Nitrogen is a constituent of both the natural atmosphere and of the biosphere. When industrial metabolism releases nitrogen to the environment it is considered a "pollutant" because of its chemical form: NO, NO2, and N2O. These oxides of nitrogen can be toxic to humans and to biota, and they also perturb the chemistry of the global atmosphere.

In the 1980s, United States NOX emissions from the major sectors were as follows: transportation sector 45 per cent, power plants 35 per cent, and industrial sources 25 per cent (USEPA, 1986). In the transportation sector, the NOX emissions result from internal combustion engines. In power plants and industrial sources, NOX is produced in boilers. The overwhelming fraction of nitrogen oxide emissions arises from the high-temperature combustion of fossil fuels, while emissions from metal-processing plants and open-air burning of biomass are relatively low.

In internal combustion engines the main parameter that determines NO production is the combustion temperature, which in turn depends on the air/fuel ratio. In industrial boilers the combustion temperature is also the main factor. In addition, fuel-bound nitrogen in coal and residual oil also contributes about 20 per cent to the NOX emissions (Darmstadter et al., 1987). For this reason, combustion technology plays a significant role in the quantity of NOX emissions. In this sense, NOX emission estimates, as well as the suitable control strategies, are significantly different from those of sulphur.

Estimating historical emission trends of nitrogen oxides is difficult because most of the nitrogen oxide is formed by the fixation of atmospheric nitrogen at high temperatures of combustion rather than by oxidation of the nitrogen contained in the fuel. Thus, nitrogen oxide emissions depend primarily on the combustion temperature and, to a lesser degree, on the fuel properties. Since combustion processes in internal combustion engines and boilers have undoubtedly changed since the turn of the century, it is likely that nitrogen oxide emission factors have also changed. Because combustion parameters can vary over a wide range, and because information on historical combustion processes is generally lacking, assumptions concerning changes in emission factors over time constitute the major source of uncertainty in developing trends in nitrogen oxide emissions.

NOx emissions can be obtained from fuel consumption data weighted by an appropriate emission factor. For a given source of combustion, this factor is the quantity of nitrogen oxide emitted per unit of fuel consumed. The emission factors used here were derived from extensive inventories that list nitrogen oxide emission factors according to source type of combustion (USEPA, 1977, 1978). The numerous emission factors listed in these compilations were aggregated into four weighted-average emission factors by fuel type: coal, gasoline, natural gas, and other petroleum products. The emission factors before 1970 were estimated to reflect the fact that the average combustion temperature, and hence the production of nitrogen oxides per unit of fuel consumed, was lower, especially for coal combustion, over the past 100 years (fig. 12). A simple linear trend was assumed for all emission factors. For coal combustion, the emission factor was assumed to have increased fivefold from 1880 to 1970. For combustion of gasoline and natural gas, the emission factors were assumed to have increased by 50 and 100 per cent, respectively. The emission factor for other petroleum products was assumed to be constant over time.

Fig. 12 Trends in emission factors of nitrogen oxide by fuel type (Sources: Emission factors for the period 1970-1980 were derived from data presented by the US Environmental Protection Agency (1977, 1978). For the period 1880-1970, trends of historical emission factors were assumed to be linear, with slopes varying by fuel type)

Fig. 13 Trends in emission of nitrogen oxides in the eastern United States

On the basis of these estimates of emission factors and data on fuel consumption, national emission trends were calculated as shown in figure 13. It is evident that there was a monotonic increase in NOX emissions in the United States from the turn of the century to about 1970; since then the emissions have remained roughly constant.


A comparison of the sulphur and nitrogen emissions reveals significant differences:

1. National sulphur emissions have fluctuated between 8 and 16 million tons a year since the turn of the century; nitrogen oxides, on the other hand, monotonically increased until the 1970s.

2. Since the 1970s, the main source of sulphur oxides has been coal combustion in power plants, while nitrogen oxides are contributed primarily by internal combustion engines.

3. Sulphur emissions result from oxidation of the sulphur impurity contained in fossil fuels and metal ores; nitrogen oxides are formed primarily by fixation of atmospheric nitrogen at high temperatures, and to a lesser degree by oxidation of fuel-bound nitrogen.

4. The control of sulphur oxides will have to rely on the removal of sulphur from the fuel or flue gases. Nitrogen oxides controls can be accomplished by technological changes in the combustion itself.


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