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Results

Methodology

Systematology

In order to establish a regional material balance, engineers, scientists, and economists have to work together, develop a common language, collect data from most different sources, and combine these data to reveal regional fluxes. Such cooperation would be facilitated if the same systems analysis approach and a common terminology were used. It is an important future task to educate experts from various fields in the techniques of materials accounting.

Fig. 6 Flow of lead through the region Unteres Bünztal, in t/yr

Fig. 7 Flow of phosphorus through the region Unteres Bünztal, in t/yr

The terms developed and the approach taken in this work are based on four steps. The basis - the first step - is a comprehensive systems analysis of the region, defining the region, the boundaries, the processes, and the link between processes by means of fluxes of goods and materials. This is followed by a rough assessment of the importance of the fluxes of goods, carried out with available or easily accessible information. On the basis of this estimation, those fluxes that have to be measured and assessed in more detail are selected. The last step consists in calculating and validating the regional material fluxes.

The Bünztal project has shown that this procedure is feasible if these four steps are taken as an iterative rather than as a consecutive process: the initial systems analysis may have to be expanded or reduced according to the first assessment of the fluxes, or because of the impossibility of balancing a process or process chain. The calculation of the final results may display a large deficit in a process, thus making it necessary to add supplementary measurements to the third step. Even with heavy expenditure, it may not be possible to balance a process (as in the case of lead in the process "river").

The experience in the Bünztal shows that methods have to be developed to take into account the uncertainty of the individual fluxes for regional material balances. These would allow one to quantify the probability that a deficit or a surplus in a regional balance is not an analytical artefact and that additional fluxes have to be looked for.

Materials accounting

The most demanding task in regional materials accounting is to reduce the very many processes and fluxes of goods to a number that is small enough for analysis and still contains the gross of the fluxes of goods and materials. To achieve this, detailed knowledge of the region is necessary. Thus the cooperation of the region itself is important, and should include the public sector as well as private institutions.

The method applied in this study yields abundant information about the flux of goods through the various sectors and branches of a region. The data about these fluxes can easily be verified by comparing the output and input fluxes of consecutive processes, or entire process chains. The method also yields satisfactory results for materials if the concentration of materials in the goods used is known. This is often not the case. The assessment of the fluxes of Pb and P. as displayed in figs. 6 and 7, requires a detailed analysis of many processes (private households, detergent manufacturing, sewage treatment, waste management, car manufacturing, scrap processing, agricultural practice, and others) and involves laborious and costly investigations. But the main obstacle for regional materials accounting is the lack of information about the composition of today's intermediate and consumer goods. While the producer of the primary raw material still knows the composition of his raw iron, zinc, or ethylene, this information is soon lost on the way to the intermediate manufacturer, and particularly when it reaches the final consumer; end-users buy goods and not materials!

For future regional materials accounting, it is indispensable that the information about the material composition of a good should flow parallel to the information about, say, the price or the weight of a good. And this information should be passed from the process of origin to the next process of destination. This appears to be the only way to collect reliable information about the material make-up of today's complex goods, such as refrigerators, motor vehicles, or houses. Technically, with the data-bank management systems now available, it should pose no problem to carry such information from its origin, through the chain of processes, to the end-user.

Regional fluxes of goods and materials

Flux of goods

The overall anthropogenic flux of goods through the Bünztal is given in figure 8. The most important single good is water, which amounts to 69 per cent of the total flux and is mainly used to transport materials and energy in households and industrial processes. Air, utilized in combustion processes such as heating and motor vehicles, comes second with 15 per cent of the total flux. Construction materials account for 8 per cent, scrap iron and junk cars for 5 per cent, and other import goods for 3 per cent.

Of all aggregated processes, private households have the largest turnover, consuming more than one-third of all goods. The branch "production of chemicals" utilizes 29 per cent of the goods, "food and drink" 9 per cent, construction business 7 per cent, metal processing 6 per cent, and the remaining branches 10 per cent.

The fraction of goods which remains in the region is comparatively small and amounts to less than 10 per cent of the import. It consists chiefly of solids to build the matrix of the anthroposphere like construction materials, and goods from processes which are specific to the Bünztal, like solid wastes from the car-shredder and the metal processing plant. Still, this 10 per cent amounts annually to 20 t/c, or 10 kg/m², or 0.6 million tonnes, for the whole region. Thus, if the future fluxes of goods remain unchanged, the accumulation of goods in the next century might surpass the 1 t/m² range.

Fig. 8 Regional flux of goods through the anthroposphere, in t/c/yr (HH = Private households, CH = Production of chemicals, CO = Construction, ME = Metal processing, FD = Processing of food and drink, R = Others)

More than 90 per cent of the goods leaving the Bünztal region (export) are waste waters and offgases. The processes "waste-water treatment" and "offgas treatment" are thus of chief importance for the quality of the environment of the neighbouring regions. The water consumption in the Bünztal amounts to one-fifth of the water input into the region, and one-tenth of the water leaving the region; the dilution potential of the surface waters is low. The ratio of geogenic to anthropogenic fluxes is much higher for the good "air"; it is around 1:500,000, thus permitting a strong dilution of offgases.

The observed flux of goods through the regional anthroposphere supports the notion of the anthroposphere as a biological organism. An important difference from the metabolism of other living things is the large amount of water, which is used to transport the excrete (anthropogenic wastes) out of the region. The activity "to clean" seems to be organized less efficiently in the anthroposphere than in natural systems. In both the biota and the anthroposphere, food, fuel, and air are goods that are important in supporting energy metabolism.

Flux of materials

In this project, the main emphasis was put on the two materials phosphorus and lead. Figs. 5 and 6 and table 2 have shown how such fluxes were determined. In the following paragraphs, it is explained how the regional balance of these materials can be used for resource management and environmental protection.

LEAD. About 340 t/yr of Pb are imported, and about 280 t/yr are exported; the difference of 60 t/yr is stored in the region (see fig. 5). The main lead flux consists of Pb contained in used cars, which are shredded in a large shredder with a capacity of more than 100,000 cars per year. The lead flux through private households is two orders of magnitude smaller; it comprises 1.6 t of Pb in leaded gasoline (which can be easily measured with high accuracy), and 5.6 t Pb in household goods (which have been determined from the Pb concentration in MSW, and thus do not represent the actual consumption of lead in households).

Much of the lead from the car-shredder is processed in a steel mill within the region, which produces iron rods for construction, filter ash from a baghouse, and furnace slag. Owing to the chemical/ physical behaviour of lead, most of the lead (200 t/yr) is concentrated in the filter ash, and some is contained in the mild steel (70 t/yr). These goods are exported and re-used; thus about 80 per cent of the lead imported into the region leaves the region again. The nonmetallic shredder residue contains about 60 tons of lead; at present, this residue is landfilled.

RESOURCE MANAGEMENT. The landfill of the non-metallic shredder residue is the largest sink for lead in the region. It can be assumed that after a decade of landfilling this stock is the most important regional reservoir of lead. Therefore, the careful management of this stock is or will become extremely important. On the one hand, the lead in the landfill poses a threat to the hydrosphere. On the other hand, it may be an important resource for the future.

Following the goals introduced in the Introduction, the shredder residue should be transformed into a good which releases sustainable fluxes only. In addition, the objectives of resource conservation require that the material be in a concentrated and re-usable form. Unfortunately, the good "shredder residue" is far from attaining these two goals, since the lead is highly diluted with organic matter and may be mobilized during landfilling. A possible solution is to treat the shredder residue thermally, thus removing the organic matter and concentrating lead in the fly ash. Further treatment is required to render the fly ash immobile; since solidification with binders like cement dilutes the potential resource of lead and makes it more difficult to re-use, other techniques such as vitrification should be attempted. The final residue should preferably be disposed of in monofills, which contain one type of mixture of concentrated materials only. (Of course, if cars were designed with sustainable development in mind, direct recycling of single car parts might become possible, which would make the shredder in the region obsolete. Direct recycling, however, seems only to be a future option.)

ENVIRONMENTAL PROTECTION. The regional lead balance allows the setting of limits for the leaching of lead from the wastes as well as for emissions from the thermal treatment of wastes and goods. The largest regional sink of lead is the landfill. The good which contains the largest fraction of lead is the residue from the car-shredder. This waste does not yet have "final storage" quality; when it is landfilled, long-term biogeochemical reactions occur, which may mobilize the lead and other materials contained in the landfill. The geogenic flux of lead through the River Bünz is about 30 kg/yr. If the landfill releases about 1 kg/yr of lead, the geogenic flux will be changed less than by its natural variations. This means that, of the total content of about 1,000 tons of Pb in the landfill (corresponding to 10-20 years of landfilling), only about 1 ppm may be mobilized per year. Thus, the future regional goal for the treatment of shredder wastes can be defined as the production of a residue that releases not more than about 10 ppm of the mass of lead when landfilled. (Of course, other materials have to be considered as well).

One technical option for producing a residue with "final storage" quality would be incineration, followed by immobilization of the incineration residues. During thermal treatment, between 40 and 60 per cent of the lead is transferred to the flue gas. Air-pollution-control techniques allow the removal of most, but not all, of the lead from the gas stream. If the lead flux in the filtered flue gas is below 5 kg/yr, the incinerator emissions will not markedly change regional lead concentrations in the soil. A load of 5 kg Pb for 1,000 years in the soil reservoir of ~400 t equals an increase of about 1 per cent, if it is assumed that 80 per cent of the lead is retained in the soil. The flux of 5 kg/yr corresponds to 0.02 mg(Pb)/Nm³. Considering the lead in the raw gas as about 30 t/yr, a removal efficiency of more than 99.98 would be required - a value that can be achieved only if the best available airpollution-control technique is applied. (A similar calculation for lead fluxes from the incineration of municipal solid wastes demonstrates that the allowable emissions are about five times higher (0.1 mg(Pb)/Nm³). The reason for this is the lower overall flux of MSW and lead when compared to the shredder residue).

PHOSPHORUS. The main import goods for P in the Bünztal region are fertilizers and feedstock, the main internal P-fluxes are the agricultural cycle soil-plant-animal-soil, and the main export pathway is the River Bünz (see fig. 7). Most of the P-input into the process industry is from cereals, which are stored temporarily in a large industrial stock. The import flux (229 t/yr) surpasses the export flux (168 t/yr); thus, about 60 tons of P are accumulated annually in the region, and the stock of P in the soil of the region is increasing.

The amount of P in the River Bünz is mainly determined by three processes. The soil, as a result of surface run-off, erosion, and leaching, contributes ~17 t/yr, the sewage-treatment plant (STP) 19 t/yr, and unknown processes such as landfills or illegal effluents 10 t/ yr. In this study, the fluxes from the third category were not investigated. The River Bünz receives ~46 t P/yr, which is about 1.6 times more than the initial load when entering the region. If the elimination of P in the sewage-treatment plant were maximized, about 13 t/yr could easily be eliminated from the river. By contrast, it is not possible to influence the fluxes due to erosion and leaching from the soil in the short term; as long as the reservoir soil is increased, the flux from this process will increase even more. Owing to the high input of P from past and current agricultural practice, the flux of P to the surface waters also will remain high in the future.

The accumulation of P in the soil cannot be detected in the short term by soil analysis (this is true for heavy metals and trace substances in general): because of the heterogeneity of the soil, as well as the limited accuracy of sampling and laboratory analysis, a change in the soil concentration becomes significant only after decades. The material balance approach, however, allows the detection of a potential accumulation before large reservoirs have been developed, and thus can serve as an early-warning system. According to the values given in figure 7, the concentration of P in the soil of the Bünztal increases by about 1 per cent per year and thus will double within roughly 70 years. (In the past, the stock of lead in the soil grew annually by 0.5-1 per cent owing to the use of leaded gasoline.)

Regional materials accounting may be used as a powerful tool for soil protection: the most efficient means of decreasing the P-load of the soil can be assessed using figure 7. The flux of P is mainly due to the two activities "to nourish" and '`to clean." It was recognized several decades ago that P can be the limiting factor for the eutrophication of surface waters. In areas where eutrophication of lakes is a serious problem, the time-span between scientific recognition of its cause and preventative action was about two decades. Most actions concerned the replacement of phosphate-based detergents, that is, processes and goods involved in the activity "to clean." In the region investigated, the turnover of P resulting from the activity "to nourish" is nearly one order of magnitude larger than that from cleaning purposes (see table 2 and fig. 7). In future it will be of great importance to reduce in time the material fluxes for the activity "to nourish" to regionally balanced levels. This suggests that regional materials accounting should be applied as an early-warning tool, and that strategies should be developed which allow a decrease in the time-lag between the recognition of a problem and the implementation of measures to control it (table 6).

Table 6 Results and consequences

Source: Own design.

Conclusions

1. If the goal of environmental protection is to conserve the quality of water, air, and soil for a long period of time, the contribution of anthropogenic materials must not change geogenic fluxes and reservoirs beyond their natural variations. This means that measures to limit the flux from the anthroposphere to the environment have to be based on regional characteristics: the area of a region, its geology, climate, reservoirs of water, air and soil, population density, the activities of the population, etc., determine which fluxes from the anthroposphere fulfil the above criteria. If the geogenic material flux is relatively small, as in the case of the water flux of the Bünztal, the admissable per capita level of anthropogenic material fluxes is much lower than in a region with abundant geogenic resources. Owing to the large population density and the high per capita turnover of the region investigated, both the sink "soil" and the relatively small conveyer belt "surface water" receive large anthropogenic inputs. The concentrations of metals and phosphorus in the soil are constantly increased; the concentrations and fluxes of many materials in the River Bünz are enlarged on their way through the valley. Regional material balances allow us to identify the most effective measures for the control of material fluxes in a region.

2. In general, the material imports and exports of modern urban regions are not in equilibrium. In the case under consideration, nearly 10 per cent of the total material flux remains in the region and is accumulated, particularly in the anthroposphere (infrastructure) and in the soil (landfills, topsoil). In fact, if imports of solid goods and fuels are considered alone, the accumulation amounts to 50 per cent of the import flux. Hence, the stock of materials is increasing. In future, this stock has the potential for becoming the region's largest waste problem, as well as serving as its major resource. As a new goal of resource management, it is suggested that materials should be kept within the anthroposphere; thus, resources should be conserved by multiple re-use, and the flux of materials to the environment (landfills, soil, and sediments) should be minimized. Engineers and designers should make sure that the material composition of a good is such that the re-use of all materials becomes possible for consecutive life cycles. Mining should be replaced by recycling. If materials are efficiently managed, the re-use of anthropogenic materials requires less energy than primary production from ores (steel, paper, glass, aluminium). However, this strategy will be successfully implemented only if the designing process is supplemented by the new objectives of a long-term sustainable metabolism of the anthroposphere.

3. In urban areas, the key processes for material fluxes are private households. They are characterized by a large turnover and a growing stock of materials. Hence, the management of wastes from households is an important part of regional material management. Recently developed integrated concepts are based on the principle that waste treatment should minimize long-term risks, and that it should produce only three kinds of residuals: (1) goods with final storage quality; (2) goods with adequate properties and markets for recycling; and (3) sustainable emissions (Brunner and Baccini, 1992).

4. A strategy for limiting inputs and outputs at the anthroposphere/environment interface, and for controlling materials within the anthroposphere itself, will replace the present practice of end-of-pipe oriented environmental protection. In order fully to exploit the potential of materials management for efficient resource conservation and environmental protection, it is essential to identify the key processes within a region and to establish their annual material balance. Study of the Untere Bünztal has shown that data on the flux of goods are abundant and readily available; but for most materials, there is not enough information to establish a regional balance. Therefore, traditional financial bookkeeping has to be supplemented by material bookkeeping on all levels, such as those of private and public households, primary production, agriculture, trade and commerce, waste-treatment facilities, etc. Such accounting is not totally new: it is already customary for materials like gold, plutonium, and morphine-based drugs, and is well established in the banking sector, the nuclear industry, and in health care. In the case of private households or small and medium-sized enterprises, it could be delegated to specialized institutions while large enterprises could do it for themselves. If the information about material fluxes from the processing of ores to the manufacturing and distribution of goods can be linked to an overall material flux, regions will have an important tool to maximize the use of their resources and to minimize environmental impacts. In the future, regions which use materials accounting for their planning and management may gain considerable economic advantages (see table 6).

References

Baccini, P., ed. 1988. The Landfill, Reactor and Final Storage. Lecture Notes in Earth Sciences, 20. Berlin, Heidelberg, and New York: Springer Verlag.

Baccini, P., and P. H. Brunner. 1991. Metabolism of the Anthroposphere. Heidelberg and New York: Springer-Verlag.

Brunner, P. H., and P. Baccini. 1992. 'Regional Materials Management and Environmental Protection." Waste Management and Research 10, no. 1.

Brockhaus, F. A. 1967. Brockhaus Enzyklopädie, vol. 2. Wiesbaden: ATF-BLIS, p. 655.

BAS Bundesamt fur Statistik. 1985. Allgemeine Systematik der Wirtschaftszweige 1985 [General Systematology for Economic Sectors 1985]. Bern: Federal Office of Statistics.

- 1987. Die eidgenössische Betriebszählung 1985 [Swiss Federal Census of Places of Work 1985]. Bern: Federal Office of Statistics.

Ciba-Geigy. 1977. Wissenschaftliche Tabellen Geigy [Scientific Tables Geigy]. Basel: CibaGeigy Ltd.

Rist, A., H. Daxbeck, and P. H. Brunner. 1989. Überblick über den Güterumsatz in Industrie und Gewerbe des 2. Sektors im Unteren Bünztal [Survey of the Material Turnover in the Production Sector of the Lower Bünz Valley]. EAWAG Project no. 300712. Dübendorf, Switzerland: EAWAG.

Settle, D. M., and C.C. Patterson. 1980. "Lead in Albacore: A Guide to Lead Pollution in Americans." Science 207: 1167-1176.

Von Steiger, B., and P. Baccini. 1990. Regionale Stoffbilanzierung von landwirtschaftlichen Böden mit messbarem Ein- und Austrag [Material Balances of Agricultural Soils]. Liebefeld-Bern, Switzerland: Nationales Forschungsprogramm "Nutzung des Bodens in der Schweiz."

9. A historical reconstruction of carbon moNoxide and methane emissions in the United States, 1880-1980

Robert U. Ayres, Leslie W. Ayres, and Joel A. Tarr

Introduction

In this chapter we attempt to reconstruct the historical emissions of two major atmospheric pollutants in the United States during the century 1880-1980: carbon moNoxide (CO) and methane (CH₄). Both are involved, directly or indirectly, in the creation of urban "smog" conditions, environmental health problems, acid rain, climate warming, or all of these. We approach the problem of historical reconstruction by identifying the main anthropogenic sources of each of the major emissions separately. This requires a discussion of a number of major industrial metabolic processes, as well as a review of some economic history. In preparing this chapter, a variety of historical and contemporary sources were used in order to produce a series of backcasts, since no direct measurement of these emissions took place until recent decades.

The methodology illustrated in this chapter (and also in chapter 11) is applicable in many other cases. Historical ex post reconstructions are of importance in calibrating more recent observational data. Such calibrations, in turn, are important for the purpose of developing viable long-term environmental forecasting models, and for a better understanding and validation of the concept of "industrial metabolism. "

The chapter is divided into two sections.

Carbon monoxide (CO)

Carbon monoxide combustion sources

Carbon moNoxide is produced by incomplete combustion of carbon-based fuels. This occurs when a carbonaceous fuel is burned in a rich mixture, i.e. in a reducing atmosphere. In recent years, by far the greatest tonnages of CO in the United States, as elsewhere, have been produced by the automobile; hence we consider this source first. Emissions depend on certain driving conditions, as shown in figures 1 and 2. In particular, uncontrolled emissions decrease with average speed. National average CO emissions for urban driving prior to the adoption of emission controls for automobiles (c. 1965) were estimated to be about 3.1 per cent by volume, or 31,000 ppm. (Hum, 1968). More recent revised (1986) EPA estimates have reduced this by 33 per cent. It is now thought that uncontrolled CO emissions from automotive vehicles amount to 2 per cent of exhaust gases by volume.

The composition of gasoline is approximately C₈H₁₈, with a molecular weight of 114. In the fuel/air mixture corresponding to average engine conditions during the pre-control era (i.e. resulting in 2 per cent CO in the exhaust), approximately 43 molecules of atmospheric nitrogen (N₂), and slightly less than 12 molecules of oxygen (O₂) combine with 1 "molecule" of gasoline. The exhaust mixture contains about 43 molecules of nitrogen, 9 molecules of water vapour (H₂O), 6.8 molecules of CO₂, and 1.2 molecules of CO, plus very small amounts of oxygen, unburned fuel, etc. Note that about 2 out of 13 of the carbon atoms in the fuel are not fully oxidized. In terms of weight, uncontrolled automotive CO emissions were equivalent to the ratio of molecular weights (1.2*28)/114 = 0.295. In other words, for each ton of gasoline consumed, nearly 0.3 tons (295 kg) of toxic CO was emitted.

Emissions of CO in 1970, from automotive vehicles, were originally estimated by EPA (1969) to be 100 teragrams, or Tg. Controls introduced in new automobiles cut motor vehicle emissions from motor vehicles by about 50 per cent per unit of fuel consumed. In absolute terms, the reduction was from 62.7 teragrams in 1970 (peak) to 52.7 teragrams in 1980 (and 48.5 in 1984). This relatively minor decrease is in sharp contrast to the 90 per cent reduction in emission that was set as a goal of air-pollution-control efforts in the late 1960s.

Fig. 1 Relationship of combustion products to air-fuel mixture (Source: Hurn, 1968)

The slowness of the change is partly due to the fact that cars remain in use for more than a decade on average. Thus changes in the emissions characteristics of new cars are not reflected in overall fleet emissions for many years. However, it is also evident that the current automotive emissions control technology is less effective on older cars.

Fig. 2 Variation in vehicle emissions with average route speed (Source: Hurn, 1968)

Motor vehicle engines are not the only combustion process where CO is generated. However, by comparison, most enclosed stationary combustion processes utilize enough excess air to reduce sharply the amount of unburned hydrocarbons and CO. Old coal-burning industrial boilers, for instance, generated apparent CO emissions in the range of 0.1 to 0.55 kg (CO) per metric ton of coal. This corresponds to an emission coefficient in the range 0.0001 to 0.00055. More recent boilers tend to be at the lower end of the range. By comparison with automobiles, however, electric power generating facilities and large industrial boilers are not significant sources of CO.

Residential uses of fuel, which are not as efficient as industrial uses, emit considerably more carbon moNoxide to the air, but still contribute much less than vehicles. EPA estimated national aggregate emissions of CO from all stationary furnaces (excluding incinerators and open fires) to be about 7.4 Tg in 1980 - up from 4.4 Tg in 1970. The increase was due apparently to temporarily decreased domestic usage of natural gas and higher domestic use of wood and oil, because of a gas shortage attributable to gas price controls that were phased out around 1980.

Carbon monoxide emissions from incineration of solid waste in 1970 were estimated by EPA to be about 6.4 Tg. This fell to 2.2 Tg in 1980, owing to phasing out of many inefficient incinerators. Specialized modern high-temperature waste-incineration plants emit very little CO. Open refuse fires, wood fires, the burning of agricultural wastes, and fires in structures are still large contributors, though they have been sharply reduced from earlier years. Unfortunately, the emission coefficient for uncontrolled fires must be regarded as somewhat uncertain.

To summarize, emissions of CO from combustion processes are primarily related to the amount of fuel burned. The production of CO is also a function of combustion efficiency: high-temperature combustion with a moderate amount of excess air yields relatively little CO (but does emit NOX). However, low-temperature fires - especially "smouldering" ones - do generate significant emissions. For this reason, it can safely be assumed that the average CO output per unit of fuel consumed was somewhat greater in the past than it is today. We justify this statement in detail later. However, the main cause was greater dependence on solid fuels (wood, then coal) for domestic and residential heating purposes.

Up until the late nineteenth century, wood was the most important type of fuel used by Americans. In 1850 it supplied over 90 per cent of fuel supplies and in 1870 it still supplied 75 per cent. The peak of physical consumption was reached with the consumption of nearly 140 million cords in the latter year. After this point, the use of wood for fuel gradually declined, reaching 2.6 per cent of total energy consumption in 1955 (Schurr and Netschert, 1960).

Wood was used preferentially as long as it was abundant, in spite of the fact that it had several disadvantages. Its preparation was relatively labour-intensive compared to coal production. Wood also has less heat value than coal; a cord of dried hardwood weighs twice as much as a ton of coal but only contains 80 per cent of its heat value. Because of its inefficient use, dependence on wood fuel contributed to energy intensiveness. In 1850 households consumed 90 per cent of all wood fuel, with 75 per cent burned for space heating in large open fireplaces that operated at low thermal efficiency. Little wood fuel was utilized to generate mechanical energy outside the transportation sector (Greenberg, 1980).

During the decades after 1850, wood was increasingly replaced as a fuel in all sectors by anthracite and then bituminous coal. Per capita consumption of fuel wood dropped from 3.51 net tons in 1850 to 2.17 tons in 1880, and then to 1.01 tons in 1900. In 1879, about 95.5 per cent of fuel wood was consumed by households and about 4.5 per cent for industrial purposes. In eastern cities such as Philadelphia, and in some western cities such as Pittsburgh with good access to coal supplies, mineral fuels began to replace wood in the 1820s. In rural areas, however, and in cities without good access to coal, wood remained the primary fuel throughout the nineteenth century. In regard to industry, railroads were the heaviest users of wood fuel in manufacturing and transportation up to about 1870. After this date, however, mineral fuels (first anthracite but later mainly bituminous coal) rapidly replaced wood. By 1880, mineral fuels, mostly bituminous coal, already constituted more than 90 per cent of locomotive fuel (Schurr and Netschert, 1960; Tarr and Koons, 1982).

For our purposes it is necessary to estimate the past emissions of CO from fuel wood and coal combustion for residential heating and cooking. Even as late as 1910, the equipment used for these purposes was very inefficient. This is easily confirmed by direct evidence. (Ashes collected in New York City were found to consist of 55 per cent carbon by weight; Hering and Greeley, 1921.) When wood or lump coal is burned in fireplaces, kitchen stoves, or domestic furnaces (as much of it was), combustion tends to be incomplete. This is because the first stage of low-temperature combustion produces mainly CO. The CO only ignites and oxidizes to CO₂ in air at temperatures above 1191-1216 °F, and at concentrations above 12.5 per cent by volume.

Thus, without mechanical fuel-air mixing (or convective mixing in large furnace volumes), the production of a significant amount of unburned CO is almost inevitable. In wood stoves, CO emissions range from 0.083 to 0.37 ton per ton of wood (0.16 average), while in open fireplaces emissions range from 0.011 to 0.04 tons/ton (0.022 average). In the case of anthracite coal, burned mostly in stoves or furnaces, we assume slightly greater combustion efficiency (higher temperature), corresponding to the low end of the range of wood (0.08 tons/ton), and we assume that the proportion of wood burned in stoves or furnaces was 20 per cent in 1800, rising to 50 per cent in 1860, then declining gradually to 10 per cent in 1950. We also assume that the proportion of wood burned in stoves had risen to 30 per cent by 1980, owing to the revival of wood-burning as a source of residential heat.

The burning of trash and refuse is still a major source of CO emissions, but emission coefficients for earlier methods are difficult to estimate. Municipal batch-type incinerators (since 1945) were mostly built to a design that resulted in CO emissions of about 0.055 per cent (520-570 ppm), compared to 12 per cent CO₂. Since each molecule occupies the same volume regardless of weight, this means that about 1 molecule of CO is created for every 21 molecules of CO₂. Assuming that dry combustible refuse - mostly paper - has a carbon content of 39 per cent by weight (similar to cellulose), we infer that about 0.0325 tons of CO are emitted per ton of refuse burned. Unfortunately, we lack statistics on the tonnage of refuse that has been incinerated over the last century.

Emissions of CO from industrial processes

The other significant sources of past and present CO emissions (see table 4) are (or were) industrial processes, primarily in the metallurgical and petrochemical industries. In particular, the reduction of iron ore to metallic iron is a process requiring the manufacture of carbon moNoxide in large amounts. This takes place in a blast furnace; the carbon is supplied from coke (nowadays supplemented by other hydrocarbons). However, until the coking process was developed, the source of carbon for smelting was charcoal, made from wood. The resulting pig iron is a solid solution of iron carbide (Fe₃C) in a matrix of iron (Fe). Further refining to pure iron or steel requires that most of this carbon be oxidized. Both smelting and refining result in the production of large quantities of carbon moNoxide.

Other metals are reduced by similar carbothermic smelting processes, also yielding CO as a by-product. In the case of aluminium, reduction takes place in an electrolytic cell, and the carbon is supplied by anodes made from petroleum coke. Again, the carbon combines with the oxygen in the alumina, yielding pure aluminium and CO.

On deeper reflection, much of the apparent complexity of the chemistry is irrelevant. Each atom of oxygen originally combined with a metal as ore is subsequently combined with an atom of carbon as CO. The amount of carbon moNoxide created in metallurgical smelting/ refining processes is thus exactly proportional to the amount of the metal that is reduced from ore. Thus, in the case of iron, hypothetical production of 50 million metric tons of pig iron (94 per cent Fe by weight) corresponds to a production of 47 million tons of pure Fe. This, in turn, implies an input of 67.14 million tons of Fe₂O₃, of which 20.14 million tons are oxygen. When the reduction processes are complete, it follows that exactly 35.24 million tons of CO must have been produced. The "bottom line" is that 0.75 tons of CO are produced within the blast furnace or steel furnace for each ton of iron (Fe) reduced from iron ore. (This relationship is universal, so it applies to all ores and all furnaces.) By a similar calculation, each ton of virgin aluminium generates 1.555 tons of CO. Under modern conditions, of course, most of this CO is either captured and utilized as fuel (called blast furnace gas) or it is flared. However, in earlier periods of industrial history this was not the case.

As noted above, the theoretical ratio of CO to Fe is 0.75. This corresponds to a C:Fe ratio of 0.43. That is, at least 0.43 tons of C are needed in principle to produce a ton of iron. Assuming that all this carbon is derived from coke, this would correspond to a blast furnace "coke rate" of 0.404 tons of coke per ton of pig iron (94 per cent Fe). As a matter of fact, according to the Office of Technology Assessment, the coke rate in Japan in 1976 was 0.43, as compared to 0.48 in the Federal Republic of Germany, 0.52 in France, and 0.60 in the United States and the United Kingdom.

As already noted, until the 1850s iron-making in the United States was based on charcoal. The making of charcoal is a very old process dating back at least 2,000 years, and remained relatively unchanged until the eighteenth century. In that century and into the nineteenth, its greatest use was in regard to the production of iron. Charcoal was an ideal furnance fuel because it was relatively free from sulphur or phosphorus impurities and because its ash had properties that were helpful in smelting the ore.

Essentially, the making of charcoal involved the controlled burning of wood in the location of an iron furnace. In the north-eastern United States, these furnaces were located on so-called "iron plantations" situated within large wooded areas. The availability of a flowing stream was also a necessity. In 1830, there were a number of iron plantations of 10,000 or more acres. Normally, the tasks of woodcutting and charcoal-making demanded more workers than any other task at the ironworks: it sometimes required as many as 12 colliers to keep a single furnace working.

Because dry weather was necessary, most charcoal was made during the late spring, summer, and early autumn. The process followed was to stack bundles of cord wood in 6- to 10-foot lengths in a cone with a base of about 25 feet in diameter, cover it with damp leaves and turf, and burn it for between three and ten days. No attempt was made to condense any of the by-product wood chemicals or impurities vented from the chimney during the charcoaling process.

As suggested already, early charcoal (and iron) furnaces were not thermodynamically efficient. Charcoal iron furnaces utilized huge amounts of fuel. Peter Temin has calculated that since one acre of timber provided an average of about 30 cords of wood and each cord of wood produced 40 bushels of charcoal, an acre of timber supplied 1,200 bushels of charcoal. In 1840, a ton of pig iron required 180 bushels of charcoal, so that the wood from one acre of land would supply fuel to make 61/2 tons of pig iron. Factors such as wood quality and labour quality could make a difference in production.

From the late eighteenth century up until the 1820s, there was little change in the technology of production in the charcoal iron industry. Production was largely a function of oven size, and furnaces stayed stable in shape until the 1820s. The typical blast furnace of the older type had constricted internal dimensions and a narrow top diameter, so that total height might be only three or four times the interior width. The furnaces were open at the top, permitting carbon moNoxide, heat, and smoke to escape. The furnace was charged with alternate layers of charcoal, ore, and limestone. As the air blast was applied, the ore melted at the air inlet (tuyere) of the furnace and dropped to the hearth, while the floating slag was drawn off from the top of the molten iron pool. When the charge was exhausted, the molten pig iron was run into a casting bed of sand about twice a day (Bining, 1973; Paskoff, 1983).

In the years after 1840 and especially after the Civil War, technological improvements resulted in a reduction of charcoal consumption per ton of iron smelted. New blast furnaces were built that reached a height of 40-45 feet in the 1840s, while after the Civil War they reached 65 feet. These new furnaces used higher temperature air blasts and were built with narrower internal dimensions and more vertical walls. They were located only in the immediate vicinity of ore beds and generally along canals and navigable rivers. This suggests that proximity to ore and transportation, rather than to timber for charcoal, were the critical factors in the location of charcoal iron furnaces (Schallenberg and Ault, 1977). Such furnaces had much higher rates of production than those in the antebellum years. One Michigan furnace produced a ton of iron with about 81.5 bushels of charcoal, or less than half the amount estimated to have been necessary in the 1840s (Schallenberg and Ault, 1977).

There were continuous improvements in the technology of charcoalmaking during the second half of the nineteenth century. Charcoal kilns were introduced in the 1850s and were adopted throughout the industry after the war. The kiln was a permanent shell built of masonry, brick, or sheet iron and had vents to control the rate of carbonization. The maximum production of kilns was between 45 and 50 bushels of charcoal per cord. A few such kilns had gas by-product collection pipes for the production of wood chemical distillates, but this was rare (Schallenberg and Ault, 1977). In the 1870s and 1880s, the retort method of making charcoal was adopted, with wood being carbonized by external heat. Output of these retorts was 60-65 bushels of charcoal per cord of wood. With this technology, the methanol, tar, and other volatile by-products were passed into an integral chemical plant for condensation and separation and eventual sale. These plants were highly mechanized and capital-intensive but had much lower labour costs than previous methods of charcoaling (Schallenberg and Ault, 1977). These improvements, however, took place in the context of a relatively static market for charcoal iron and a dramatic increase in the use of mineral fuels, especially bituminous coal and coke, in iron production.

By 1854, the first date for which comparative figures are available, charcoal fuel was used to produce 306,000 tons of pig iron, 303,000 tons of anthracite and coke, and 49,000 tons of bituminous coal. By 1870, charcoal was used in the production of 326,000 tons of pig iron, 830,000 tons of anthracite and coke, and 509,000 tons of bituminous coal and coke. By 1905, bituminous coal and coke had taken a dramatic lead, producing 20,965,000 tons, against 1,644,000 tons of anthracite and coke, and only 353,000 tons of charcoal.

"Best practice" charcoal-based iron furnaces built in the 1880s competing with less efficient coke-based technologies - achieved a conversion rate of about 1.0 (tons carbon/tons pig iron). This became the average for charcoalbased iron furnaces by 1919, when average coke-based iron still required 1.2 tons/ton. However, by that time, the best practice coke-based iron-making was as efficient as the remaining few charcoal furnaces. The coke rate is still regarded as a primary measure of efficiency. The overall average C:Fe ratio can be assumed to have declined almost linearly from about 1.9 in 1840 to its 1976 value of 0.6, as shown in figure 3.

Fig. 3 Fe reduction efficiency

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