Mass Flows And The Life Cycle

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The materials 'life cycle' can be characterized schematically as shown in Figure 3.1. It is obvious that the stages of the life cycle correspond to familiar economic activities, already defined as 'sectors'. At the beginning are the extractive industries, consisting of agriculture, fishing, forestry, mining, quarrying and drilling for oil and gas. Substantial quantities of waste are generated at this stage, but mostly these are left behind at or near the place where the extraction occurs, whether the farm, forest or mine.

Renewable and nonrenewable resources

Inputs from environment

Existing land forms

On On

Harvest/ extraction

Processing/ manufacturing



Recaptured flow

Exports Imports

US Economy


Recapture decisions

Additions to stock and built infrastructure

Losses/ emissions

Demolition wastes

Waste Dissipative outputs outputs

Land-form alterations

Outputs to environment

Figure 3.1 The materials life cycle

The next stage consists of primary conversion, where 'raw' materials are cleaned, sorted, separated, upgraded (or 'beneficiated', in the case of metal ores), refined and purified into finished materials. Fuels are also cleaned, refined and converted into higher quality forms of energy-carriers, ranging from clean natural gas to coke, gasoline, diesel oil and other hydrocarbon fuels, as well as petrochemical feed-stocks. Fuels are finally converted by combustion, through the agency of so-called 'prime movers' (that is, heat engines) into mechanical power. Or they produce heat that is used directly as such, either in industrial processes - such as metal ore reduction or petroleum refining - or by final consumers. A further conversion, mainly from mechanical power, generates electric power. Primary conversion processes, including combustion, account for the vast majority of material wastes.

As we will explain subsequently (Section 3.4), both the raw material inputs to, and the finished outputs of, primary conversion processes, whether material or energy carriers, can all be measured and quantified in terms of a common physical unit, namely exergy. Outputs of energy (actually exergy) conversion can all be characterized and measured as useful work - in the physical sense, not to be confused with human labor. We discuss this in more detail in Sections 3.4 through 3.6.

The third stage of the life cycle is another conversion, from finished materials and useful work - outputs of the primary conversion stage - to finished products, including infrastructure and capital goods. Wastes at this stage arise mostly from intermediate recombination, especially in the chemical industry, where many intermediate materials, such as solvents, acids and alkalis, are consumed in the conversion process and not embodied in final products. Most toxic and hazardous wastes arise from intermediate processing. The final stage, where finished products produce services, also generates wastes as the so-called final products are consumed, wear out or become obsolete in the course of providing their services to humans. This may happen almost instantly, as in the case of food and beverages, cleaning agents, paper and packaging materials, or over an extended period as in the case of appliances, vehicles, machines and structures. Recycling is essentially only applicable to paper, bottles, cans and metal scrap, which cumulatively amounts to a tiny fraction of the total materials flow.

A summary of the major mass flows in the US economy for the year 1993 is shown in Figure 3.2. (The date does not matter, for this purpose.) The units are million metric tons (MMT). We included overburden and erosion in this diagram, since estimates were available. The mass-balance principle was used in constructing Figure 3.2 to estimate a number of flows that could not be measured directly. For instance, we used the mass balance to calculate the amount of oxygen generated by photosynthesis in agriculture

From air

From earth 11819

Manure (160) plus mfg goods to extractive industries

(includes imports)


Agriculture 868 Forestry 520 Fuels 1638 Minerals 2278 Ores 773

Inorganic RM 4689 Fuel imports 399

Recycle 90 (est.)


Construction materials



Inorganic RM 4689 Fuel imports 399

Fuel Imported Egypt

To air

CO, 6173: H,0 2621: SO, 19.5: NO 21.2: CH 8.4: VOC 20

To air

O,1092: CH4+NH,23 To earth or water

Overburden 7163: Other waste 304a

Erosion 3400

To earth or water

To air

CO, 6173: H,0 2621: SO, 19.5: NO 21.2: CH 8.4: VOC 20

To earth or water


Consumables 500 Food and drink 205 s.

Other miscellaneous >(»nsumed domestically) (estimated) 95 Exports 201

Recycle 30 (estimated)


- To water and earth

Figure 3.2 US economic system as a whole from a mass flow perspective (1993 in MMT)

and forestry, the amount of atmospheric oxygen required to burn all the fossil fuels and wood, and the amount of water vapor generated by the combustion process. We used official estimates of carbon dioxide production from fuel combustion, and calculated the others as ratios, based on chemical reaction formulae. (Erosion is a special case, constituting topsoil losses from plowed fields, resulting in silting and sediment in rivers. Hence erosion 'losses' in the figure are not balanced by inputs.)

As the life-cycle perspective makes clear, economic value is added at each stage by human labor, capital services and the application of energy (exergy) services, while material and exergy wastes are discarded. Value-added is sometimes equated with embodied information that increases the order embodied in useful products. In this view, usefulness is equated with order, or orderliness. Georgescu-Roegen, in particular, has argued that each stage of the process converts low entropy (ordered) materials into high entropy (disordered) wastes. In fact, he has insisted that, thanks to the second law of thermodynamics (the 'entropy law'), this process is irreversible (Georgescu-Roegen 1971). While his view on that score was much too apocalyptic, he was the first economist to characterize the economic system as a materials processor.

The word 'useful' is potentially ambiguous. In economic terms, useful products are those outputs with a well-defined market and market price. In general, many outputs are inputs for other 'downstream' products. Yet some of the physical outputs of the system are useful without having market prices. An industrial example of this is so-called 'blast furnace gas', a mixture of carbon monoxide, carbon dioxide and nitrogen (plus other pollutants), with some heating value that makes it usable in the near vicinity of the source, but not marketable outside the firm. An agricultural example would be forage and silage fed to animals on the farm. Manure generated and recycled by grazing animals on the farm is another example; it would clearly be inappropriate to regard it as a waste (in India this material is harvested, dried and used as domestic fuel).5 A domestic example is heat for rooms, water and cooking. Finally, oxygen and water vapor - byproducts of photosynthesis - are useful. All of these are unpriced, but not unvalued intermediates.

Raw agricultural products harvested in the US in 1993 amounted to 868 MMT, of which 457 MMT was crops and the rest was silage, hay and grass. Of this, 83 MMT (net) was exported, mostly for animal feeds. Animal products amounted to 119.5 MMT. The food-processing sector converted 374 MMT of harvested inputs (dry weight) to 286 MMT of salable products, of which 203 MMT was food consumed by Americans, 66 MMT was by-products (such as starch, fats and oils), animal feeds and food exports, and 14 MMT was a variety of non-food products including natural fibers, leather, tobacco and ethanol. Evidently 500 MMT, more or less, was 'lost' en route to the consumers, mostly as water vapor and CO2, though other wastes were significant.

Consider forest products. Inputs (raw wood harvested) amounted to 520 MMT in 1993, not counting timber residues left in the forests (about 145 MMT). About 200 MMT of this weight was moisture. Finished dry wood products (lumber, plywood, particle board) weighed about 61 MMT. Finished paper products amounted to 83 MMT, which included some paper made from imported wood pulp from Canada and some recycled waste paper. The output weight also included 3.7 MMT of fillers (mainly kaolin), hydrated aluminum sulfate (alum) and other chemicals embodied in the paper. Again, the difference between inputs and output weights was very large. Quite a lot was lignin wastes from the paper mills, which are burned on-site for energy recovery, but some of the mass still ends up as pollution. About 168 MMT of harvested wood, including paper mill wastes, were burned as fuel, producing about 230 MMT of CO2 as a waste by-product.

Conceptually, it seems reasonable to mark the boundary of the extractive sector by counting the weight of finished materials, that is, materials that are embodied in products, or otherwise used, without further chemical transformation. Steel is an example. There is relatively little difference between the weight of raw steel produced (89 MMT in the US in 1993) and the weight of 'finished' steel products. The small losses of steel in the rolling, casting and machining stages of production are almost entirely captured and recycled within the steel industry.6 The same can be said of other 'finished materials', from paper and plastics to glass and Portland cement: very little or none of the finished material is lost after the last stage of production, except as consumption or demolition wastes.

What of fuels and intermediate goods like ammonia, caustic soda, chlorine and sulfuric acid? Raw fuels are refined, of course, with some losses (such as ash and sulfur dioxide) and some fuel consumption (around 10 percent in the case of petroleum) to drive the refineries. But refined fuels are converted, in the course of use, mainly to heat, mechanical power and combustion wastes. Fuels cannot be recycled. The mass of raw hydrocarbon fuel inputs was a little over 1600 MMT in 1993. It was mostly combined with atmospheric oxygen. The combustion of hydrocarbon fuels in the US, in 1993, generated around 5200 MMT of CO2, the most important 'greenhouse gas' (Organisation for Economic Co-operation and Development 1995, p. 39). This may be a slight underestimate, since some of the hydrocarbons produced by refineries do not oxidize immediately (asphalt and plastics, for instance) but, except for what is buried in landfills, all hydrocarbons oxidize eventually.

Minerals such as salt, soda ash and phosphate rock, as well as petrochemical feed-stocks, are converted to other chemicals. Some of these - mainly polymers - end in finished goods (like tires, carpets, packaging materials and pipes). Others are converted to wastes in the course of use. Examples include fuels, lubricants, acids and alkalis, cleaning agents, detergents and solvents, pesticides and fertilizers. A model scheme (and accounting system) appropriate for environmental analysis should distinguish between dissipative intermediates, such as these, and non-dissipative materials embodied in finished durable goods that might (in principle) be repaired, re-used or re-manufactured and thus kept in service for a longer period.

'Final' goods are goods sold to 'final' consumers in markets. This class of goods is reasonably well-defined. But so-called 'final goods' (except for food, beverages and medicinals) are not physically consumed. They are, in a sense, producers of services. By this test, all final outputs (not excepting food and beverages) are immaterial services and therefore weightless, the mass being discarded.7 However, it is natural to consider finished products as a category, which do have mass, as well as monetary value (counted in the GNP). In fact, this category marks the downstream boundary of the manufacturing sector.

To summarize, raw outputs of the US extractive sector, not including overburden, topsoil, air and water, amounted to 1388 MMT organic (biomass) and 4689 MMT inorganic, in 1993. All of this, plus 400 MMT of imported fuel and 90 MMT of recycled metals, paper and glass, were inputs to the concentration and conversion sectors. Manufactured 'final' outputs amounted to a little over 2700 MMT, of which 2130 MMT were for buildings and infrastructure, 82 MMT were durables (mostly producer durables) and 500 MMT were consumables, of which two-fifths were exported.

The weight of all metals produced, and consumed, in the US in 1993 was less than 100 MMT. By far the greater part, especially of steel, was used for construction purposes and motor vehicles. Except for some packaging materials (cans and foil), the metals were mainly embodied in durable goods such as infrastructure, buildings, transportation equipment and other machines and appliances. Motor vehicles accounted for about 28 MMT of mass. The weight of other consumer products is modest. For example, the weight of all textiles produced, including cotton, wool and all synthetics, amounts to around 5 MMT. Products of textiles, partly clothing and partly furnishings (including carpets) must be of the same order of magnitude.

As regards wastes, an important distinction might be made, namely between 'potentially reactive' and 'inherently inert' materials. Most metals, paper, plastics and so on are in the 'reactive' category, insofar as they can oxidize or react with other environmental components. (Most of these, especially paper and plastics, can be burned for energy recovery.) However, as a practical matter, these potentially reactive materials are vastly outweighed by the inert materials utilized in structures, such as glass, brick and tile, concrete, plaster, gravel and stone. All of the latter group of materials are chemically inert, even though some of the manufacturing processes involve heating.8 The total mass of 'finished' chemicals processed in the US economy in 1993 was about 0.5 metric tons per capita or 140 MMT, including fertilizer chemicals. Of this total, no more than 30 MMT were embodied in long-lived materials, such as plastics and synthetic rubber. The remainder was dissipated into the environment. The total mass of thermally processed building materials (cement, plaster, bricks, ceramic tiles and glass) consumed in the US in 1993 was 125 MMT. On the other hand, chemically inert structural materials (sand, gravel, stone, etc.) consumed in the US in 1993 without thermal processing amounted to about 1870 MMT.

Total consumption of extractive materials (fossil fuels, harvested biomass, construction materials, minerals and metals in the US - disregarding mine wastes) increased from about 1100 MMT in 1900 to nearly 2000 MMT in 1929, followed by a drop of over 40 percent in the Depression years. But since then there has been a steady increase to over 8100 MMT in 2004 (Figure 3.3a). Figure 3.3b shows the same consumption in terms of exergy. The exergy consumption is completely dominated by fossil fuels.

Of course population nearly tripled during that time, so the per-capita figures are more revealing. The next five figures show per capita consumption in both mass and exergy terms for fossil fuels, harvested biomass, construction materials, metals and chemicals, respectively, plus their total (Figures 3.4a-f). It is interesting to note that fossil fuels in raw form consumed per capita have almost tripled since 1900, but most of the increase was in the first three decades of the century, when consumption per capita doubled, and there has actually been a small decrease since the peak years of the early 1970s. Biomass harvested per capita has actually decreased, but most of the decrease was also in the first three decades, with a slight increase since the Depression years and a slight decrease since 1980. For construction materials, the overall per capita increase has been by a factor of five, but with major ups and downs, including a big boom in the 1920s, a very sharp drop in the early 1930s and a huge postwar boom from 1950 until the 1970s, which included the materials-intensive US national highway program. The pattern for metals consumption is similar to that for fossil fuels. Chemicals, of course, show a dramatic increase (over ten-fold since the 1930s), but that is mostly due to exploding demand for petrochemicals

8000 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0

Total mass of counted inputs to GDP Fossil fuels: coal, petroleum, natural gas, NGL Metals: iron, steel, copper, lead, zinc, aluminum

Construction: cement, gypsum, brick, lumber, sand and gravel, stone, clay Inorganic: sulfur, lime, phosphate, chlorine, ammonia Organic: ethylene, methanol, butadiene, propylene, benzene

Biomass: dry weight of harvested and non-harvested components of major non-fuel crops

1910 1920 1930 1940 1950 1960 1970

1990 2000 2010

Figure 3.3a Total major inputs to GDP (fuels, metals, construction, chemicals and biomass): in terms of mass (USA, 1900-2004)


— Total mass of counted inputs to GDP

-- Fossil fuels: coal, petroleum, natural gas, NGL

— Metals: iron, steel, copper, lead, zinc, aluminum

— • Construction: cement, gypsum, brick, lumber, sand and gravel, stone, clay _ Inorganic: sulfur, lime, phosphate, chlorine, ammonia

Organic: ethylene, methanol, butadiene, propylene, benzene O O Biomass: dry weight of harvested and non-harvested major non-fuel crops

0 qmtBqocco^eeeeyeseqeeeeqeeagQeeee^eieuiiH^

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Figure 3.3b Total major inputs to GDP (fuels, metals, construction, chemicals and biomass): in terms of exergy (USA, 1900-2004)

Exports Usa 1900 1940
Figure 3.4 a Major inputs of fossil fuels (coal, petroleum, natural gas and NGL): mass/capita andexergy/capita (USA, 1900-2004)

1.5 r

1.4 E

1.3 E

1.2 E 1.1 E

1 E 0.9 E


0.8 :




0.7 :

0.6 E

0.5 E

0.4 E

0.3 E

0.2 E

0.1 E

0 :

Inorganic: sulfur, lime, phosphate, chlorine, ammonia Organic: methylene, methanol, butadiene, propylene, benzene

Exports 1920 1930

. mass/capita ratio (left scale) exergy/capita ratio (right scale)

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

. mass/capita ratio (left scale) exergy/capita ratio (right scale)

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Figure 3.4b Major inputs of chemicals to GDP: mass/capita and exergy/ capita (USA, 1900-2004)

Gdp Sector 1910 1920
Figure 3.4c Major inputs of construction to GDP: mass/capita and exergy/capita (USA, 1900-2004)

■ mass/capita ratio (left scale) exergy/capita ratio (right scale)

■ mass/capita ratio (left scale) exergy/capita ratio (right scale)

Biomass Gravel And Stone

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Figure 3.4d Major inputs of metals to GDP: mass/capita and exergy/ capita (USA, 1900-2004)

Usa Gdp 1900 1950
Figure 3.4e Major inputs of biomass to GDP: mass/capita and exergy/ capita (USA, 1900-2004)

■ mass/capita ratio (left scale) exergy/capita ratio (right scale)

Fossil fuels: coal petroleum, natural gas, NGL Metals: iron, steel, copper, lead, zinc, aluminum

Construction: cement, gypsum, brick, lumber, sand and gravel, stone, clay Inorganic: sulfur, lime, phosphate, chlorine, ammonia Organic: ethylene, methanol, butadiene, propylene, benzene Biomass: dry weight of harvested and non-harvested components of major non-fuel crops

1900 1910 1920 1930 1940 1950

1970 1980

2000 2010

Figure 3.4f Total major inputs to GDP (fuels, metals, construction, chemicals and biomass): mass/capita and exergy/capita (USA, 1900-2004)

(which are double-counted, being derived from fossil fuels). Demand growth has slowed sharply since the 1990s.

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    Is recycled paper counted in the gdp?
    9 years ago
    Is fuel counted in the GDP?
    8 years ago

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