|Industrial Metabolism: Restructuring for Sustainable Development (UNU, 1994, 376 pages)|
|Part 2: Case-studies|
|10. Sulphur and nitrogen emission trends for the United States: An application of the materials flow approach|
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.
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.)
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.
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.
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.
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.
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.
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.
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.