The Sustainable Process Index a new dimension in ecological evaluation

ECOLOGICAL ENGINEERING ELSEVIER

Ecological Engineering 6 (1996) 241-258

The Sustainable Process Index A new dimension in ecological evaluation Christian Krotscheck *, Michael Narodoslawsky Institute c?fChemical Engineering, Graz University of Technology, A-8010 Graz, lnffeldgasse 25, Austria Accepted 23 October 1995

Abstract The Sustainable Process Index (SPI) is a measure developed to evaluate the viability of processes under sustainable economic conditions. Its advantages are its universal applicability, its scientific basis, the possibility of adoption in process analyses and syntheses, the high sensitivity for sustainable qualities, and the capability of aggregation to one measure. It has proved to he useful in industrial strategic planning. The concept of the SPI is based on the assumption that in a truly sustainable society the basis of economy is the sustainable flow of solar exergy. The conversion of the solar exergy to services needs area. Thus, area becomes the limiting factor of a sustainable economy. The SPI evaluates the areas needed to provide the raw materials and energy demands and to accommodate by-product flows from a process in a sustainable way. It relates these areas to the area available to a citizen in a given geographical (from regional to global) context. The data necessary to calculate the SPI are usually known at an early stage in process development. The result of the computation is the ratio between the area needed to supply a citizen with a given service and the area needed to supply a citizen with all possible services. Thus, it is a measure of the expense of this service in an economy oriented towards sustainability. Keywords: Ecological evaluation; Sustainable development; Sustainable Process Index; Ecosphere and industrial metabolism; Ecological process engineering; Exergy

1. Introduction W e are living in a transitional time. E c o l o g i c a l concerns are steadily gaining m o r e interest in the public as well as in politics. Y e t up to n o w they h a v e not penetrated

Corresponding author. Elsevier Science B.V. SSDI 0925-8574(95)00060-7

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economical considerations to a large extent. Thus, although there are plenty of reasons to tailor technologies according to ecological requirements, market forces continue to lead in a different direction. Therefore, new or additional objectives are needed to achieve ecological engineering. The most general definition of these objectives is to minimise the global (anthropogenic) impact potential on the environment. This asks for a life-cycle process analyses and evaluation (assessment) concept. Usually a process is defined as what goes on in a plant, limited by the physical installations of the plant. In this concept, engineers take responsibility for the safety within the plant and the quality of the products leaving it. The process is linked to the 'outside world' by authority regulations and, above all, prices for raw materials, labour, energy, waste and products. This process model is highly insufficient for ecological process engineering. It separates the process from the environment and leaves the optimization of a process dependent on predominantly economic signals and on the principle of maximising profit. In the same manner as prices fail to reflect environmental realities, processes fail to be ecologically safe. For ecological process engineering a process has to be defined as providing a certain service, taking into account the whole chain from raw material generation, production and distribution to taking care of the products (which provide the service wanted) after their use and of the by-products (which are not wanted but have originated from the process). By defining a process more widely ecological process engineering can deal with process design and optimization much more consistently with respect to ecological problems, albeit at the cost of increased methodological complexness (Fig. 1). To calculate the ecological impact potential of specific processes, we developed a method that is able to aggregate the life-cycle wide environmental burden on the base of mass and energy flows. With this measure the evaluation in terms of ecological process engineering relies on a new economic guideline: the principle of sustainability.

/

~

Anthropogenic

/ i -i

°r°°°""

I r a w materials,

I energy

,

generation ',

q

. ".--~

/ '

s~ ~ - ~

x

\

I

a

I

products |

t

degeneration

Ecosphere

//

I

~ Limits for embedding processes ~ "

Fig. 1. The limits for embedding anthropogenic processes into ecosphere have to include both: ecosphere and anthroposhere. This is the basis of the life-cycle assessment strategy in the SPI concept.

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243

2. The principle of sustainability Sustainability has been defined as a key to the solution of current ecological. economic and developmental problems. This concept has been broadly discussed since it was brought to public attention by the Brundtland Report (World Commission on Environment and Development, 1987) and it has since been developed into a blueprint for reconciling economic and ecological necessities. Among others (Brown et al., 1988; Turner, 1988; Lowrance, 1990; Svedin, 1992) Daly (1991) and Moser et al. (1993) contributed to make this concept scientifically acceptable so that it is possible to take it as a yard stick for strategic planning. The concept of sustainability can be considered to be the base of strategic planning in two ways: It is an inherent long-term concept. Thus, it allows long-term planning in the face of changing boundary conditions. • The reference systems are natural flows or states and their variations. Hence planning on this basis is independent of social conventions like threshold values which may vary with time. 2.1. Operationalising the principle of sustainabili~' The task group 'Ecologic Bioprocessing of the European Federation of Biotechnology' (Moser et al., 1993) has developed the most operational working hypothesis of sustainability. According to this definition, sustainability requires the following four criteria: 2.1. I. Anthropogenic material flows must not exceed the local assimilation capaci~ and should be smaller than natural fluctuations in geogenic .flows This requirement maintains the quality of the material base for ecosystems (soil, aquifers, atmosphere, etc.). It is based on the assumption that geogenic flows are subject to fluctuations, which do not jeopardise evolution and that the local assimilation capacity is a measure of the rate with which ecosystems accept input streams without losing their evolutionary potential. This capacity changes with geography and to some extent with time, too. Another assumption is that the rate of acceptance of input streams to the supporting ecosystems is clearly more restrictive than any rate of depletion of natural resources. We are facing a 'waste crunch' in contrast to a 'resource crunch', a fact that has been accepted quite widely during the last few years (Meadows et al,, 1992). 2.1.2. Anthropogenic material flows must not alter the quali~ and the quanti~ o( global material cycles Most of the dominant global material cycles (like the carbon, nitrogen or water cycle) have natural buffer stocks. In some cases these stocks are exploitable deposits, in other cases they are unusable storage systems. Today the deposits are mined and exploited very fast, but the knowledge of the environmental impacts of exploitation is rather insufficient. This requirement does not totally rule out the use of materials from these natural

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buffer stocks (like aquifers and fossile raw material deposits) but defines the input streams for industrial systems. It links the rate of exploitation to the rate of replenishment of these natural systems. In some cases even the quality might change, e.g. like for fossile raw materials. Here the main deposition of organic matter occurs by oceanic sedimentation. In this context the most important aspect is to keep the carbon concentration in the global cycles roughly constant. At least at the first glance, the form of carbon storage seems to be less important. The importance of the quality aspect can be illustrated by existing aquifers: If we contaminate these stocks the future utilisation is endangered. 2.1.3, Renewable resources can only be extracted at a rate that does not exceed the local fertili~'

This requirement again defines the input streams for industrial systems. In order to fulfil this requirement a locally adapted agriculture is called for which guarantees long-term preservation of the fertility of land. Thus, erosions must be stopped as well as soil contamination and salination. 2.1.4. The natural cariet 3, o f ~wecies and landscapes must be sustained or improced

This is a very far-reaching requirement. It calls for maintaining the important interaction between man and nature at a physical as well as a psychological level and for the use of nature's resources under the boundary conditions of aesthetics. Beauty is an intrinsic property of sustainability. Only if we maintain a sufficiently comfortable environment by accepting the rules of natural landscape we can ensure that man will evolve in this system. This can also be seen from a very pragmatic point of view, since land as well as species are factors of the utmost importance in a society pursuing sustainable development. Degrading these factors irreversibly will impede our own chance to improve our quality of life and it will deprive future generations of an important basis for living. 2.2. The need )Cor sustainabili~ measures

As traditional economic evaluation methods are not able to account for sustainability (Daly and Cobb, 1989; Constanza, 1991; Schleicher, 1993) and do not measure ecological aspects within the enlarged task of ecological process engineering, supplementary measures are necessary. This is especially important since today's technological decisions face (at least partly) new economic realities that come closer to the principle of sustainability. Strategic measures evaluate the viability of processes under new constraints of competition. A number of interesting approaches to this problem already exist. For example, LCA or Eco-balances (Abel et al., 1990; Guin6e et al., 1993) are widely applied in order to relate flows to and from processes to ecologically critical flows. The Degree of Invasiveness of a technology can be evaluated by a method of Von Gleich and Grimme (1991). The Material Intensity Per Unit-Service evaluates the mass of material necessary to be moved in order to provide a certain service (Schmidt-Bleek, 1993). Other indicators and measures were suggested and tested (Clark, 1986; Kuik and Verbruggen,

c. Krotscheck, M. Narodoslawsky / Ecological Engineering 6 (1996) 24 l-258

245

1991; Toman and Crosson, 1991; Hoimberg and Karlsson, 1992; Rees and Wackernagel, 1992; Swaminathan and Saleth, 1992; Wackernagel et al., 1993) and sustainability standards were fixed (Hueting, 1991) as well. It is important to distinguish indicators from standards and measures of sustainability Indicators describe a certain appearance, like desertification, population growth, GNP~ infant mortality or birth rates. Therefore, they cannot be used as a strategic measure. Standards fix the targets of development. However, a standard is always defined from the present point of view. We can use standards to reach strategic goals but a comprehensive definition adapted to development seems impossible. On the other hand, the orientation of a standard can only be given by a social consensus. Thus, standards are per se not applicable as strategic measures which are supposed to give an orientation to our common future. Measures are the key to strategic planning because they can give orientation. Measures do not only make a process (like the embedding of human activities into the biosphere) more understandable, they also make the choice between alternatives more amenable to rational discussion in society. It is in this respect that we look at evaluation criteria for sustainability. The requirements for effective evaluation criteria are the following: • objectify the inherent long-term principle of sustainability be as generally applicable as possible to products and services in order to minimise the number of methods needed to provide guidance towards sustainability; use physically measurable data as input; be solely based on scientific information and not on emission or immission threshold values, which may vary with time and place: be based on a limiting factor since competitiveness is linked to the shortage of a resource almost by definition; include limits for resources and natural capacities; aggregate the quantity and the quality of material and energy flows within one measure; be adaptable to individual regions and also to import and export; be internationally acceptable and applicable; be intelligible and obvious in order to enable a rational discussion of the implications of different evaluated alternatives. Any criterion has to be inter-subjective, plausible, comprehensible and educationally valuable. The final result should be understood by everybody. In addition, the shortage of a resource on which a measure is based must have a big potential to indicate the competitiveness of each technology and organisation. Finally, we have to be aware that every measure is designed for a special field of application.

3. The basic concept of the SPI The first consideration concerns the ultimate dimensional unit of a measurement for sustainability. Due to the requirements listed above we have chosen the factor 'area' as the basic unit for the computation of the SPI. The reason behind this choice is that in a sustainable economy the only real input that can be utilised in the long term is solar

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exergy. Its utilisation per se is bound to the surface area. Furthermore, area is a limited resource in a sustainable economy because the surface of our planet is finite. However, area is not only a quantitative measure. In order to generate raw materials in a sustainable way, top soil quality has to be preserved to guarantee fertility. The preservation of soil as a production factor requires that material flows from and to this medium must be limited to sustainable levels. This is also true of material flows from and to the compartments of water and air, which form the other important bases of agriculture. Thus, any flow to soil, water or air requires a certain area if it is to be accommodated without endangering long-term fertility. By taking into account the dual function of area as a recipient of solar energy and as a production factor, the SPI can measure and relate the ecological impact of a process with respect to the quantity and the quality of the energy and mass flows it induces. Processes needing more area for the same product or service are less competitive under sustainable economic conditions.

3.1. Computation of the SPI In order to get the SPI, streams (either material or energy) entering or leaving a process are identified by an area. Due to the historic development of process engineering, the necessary installations (equipment) and staff for converting raw materials into services (products) are calculated separately. The calculation of the SPI starts with the computation of the total area A~ot assigned to embed a process sustainably in the ecosphere. Ato t -= A R -t- A E "4- A l + A s + A p m 2

( 1)

where A R is the area requirement to produce raw materials, A E is the area necessary to provide process energy, AI is the area to provide the installations for the process, A s is the area required for the staff and Ap is the area to accommodate products and by-products. These areas will be computed for one year of operation (reference period). Within this year, a number Stot of unit-services (e.g. product units) will be supplied by the process in question (Fig. 2). The area ato t is called the specific (sustainable) service area (the specific yield Ytot is the inverse specific service area). ato t

Ato t

Stot

l

--

Yt,,t

m 2 yrunit

This specific area is already order to make this measure more the region being relevant to the area for the supply of goods and SPI =

atot

cap/unit

I

(2)

a possible comparative measure of sustainability. In striking, it is divided by the area per inhabitant (ai~) in process. This area is the theoretical (mean) available energy for each person (per capita). (3)

Oin

The SPI is a number, which is based on the ratio of two areas in a given time period (usually per year) in order to provide one inhabitant with a certain service or product. One area is needed to embed the process sustainably into the ecosphere. The other is the

C. Krotseheck, M. Narodoslawsky / Ecological Engineering 6 (1996) 241-258

SPIEtOFI ~

....

•

I

""

- - - - - -

247

Clothing

...

Mobility

Food

Hobby ..

Housing ".

[

Others

Fig. 2. The SPI of ethanol for use as fuel. The SPI is the fraction of two areas: the area ato t needed to embed a process (product or service) sustainably into the ecosphere and the area ain available for one inhabitant.

area available (on a statistical base) for every inhabitant. The SPI, thus, is the fraction of the area per inhabitant related to the delivery of a certain product or service unit. Its physical meaning is, how much of the area theoretically available for a person to guarantee its sustainable subsistence, is used up to produce the service or product unit in question. 3.2. The raw material a r e a A R

The raw material production (generation) area consists of two segments: the area fi~r renewable ARR and non-renewable raw materials ARN. Both parts have to be treated differently. A R = ARR 4- ARN m 2

(4)

3.2.1. The renewable raw material area ARR

Renewable raw materials take part in at least one global cycle. For this category of materials, the area required for production is the area needed to convert the building blocks available in the biosphere into biomass which is subsequently fed into the process. Thus, the specific yield YR kg m -2 yr-~ and the feed F R k g / y r of a processed resource are the basis of computation: ARR

FR =

-

-

m2

(5)

YR

Table 1 lists some average yields of renewable resources in Central Europe. It is very important to note that in this case the area also accounts for the ultimate disposal of the product since it embeds the raw material generation into global material cycles. From this point of view, the same treatment can be applied to fossile organic raw materials. So they can be treated as renewable resources albeit with a low rate of regeneration. The

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248

Table 1 Fresh harvest yields of different renewable raw materials in Central Europe Raw material

kg m 2 y r - i

Winter wheat Summer wheat Rye Winter dredge grain Summer dredge grain Barley Oats Corn Rape-seed Potatoes Sugar beets Mangel-wurzel Green and silo corn

0,515 0.414 0.412 0.416 0.389 0.481 0.369 0.848 0.284 2.364 4.903 4.570 3.985

reason for this is that at any given time there is a stream from the global carbon cycle into a long-term storage compartment. This is mainly realised by sedimentation into the beds of the oceans. When fossile raw materials are used at the same rate at which carbon is removed into long-term storage, there is no alteration of the global cycle itself and no unsustainable accumulation occurs (e.g. in the atmosphere). However, the area to 'produce' one kilogram of organic sediment per year is about 500 m 2 (Bolin and Cook, 1983; Wolf-Gladrow, 1992), which means a yield of 0.002 kg m 2 yr i 3.2.2. The non-renewable raw material area ARN Non-renewable raw materials cannot be embedded into global material cycles. Their use is inherently dissipative, which has to be taken into account when computing the area necessary to accommodate products and by-products into the biosphere. However, their generation needs energy and other expenditures. In order to get a rough estimate for the ARN of substances, only the energy demand is considered. Usually the precise energy content per mass unit is not available. In these cases the following procedure is proposed, where the price of the raw material (which is well-known) is the base of computation:

C N • 0.95 ED

CE

kWh/kg

(6)

In this formula, E• is the energy demand to supply one kilogram of the material in question. C N is the price of this material (world market price, taxes excluded) and C E is the price of one kilowatt-hour (kWh) of energy (industrial price, taxes excluded). This relation is based on the assumption that energy almost exclusively defines the prices of basic raw materials. Although this seems to be a very crude estimate, it is true for a large number of staple products with only minor deviations from the factor 0.95. In order to obtain an area for the generation of non-renewable raw materials, it is necessary to calculate an area per energy unit. As the conversion and upgrading of primary raw materials (like ores) is almost exclusively performed on industrial scale,

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249

Table 2 Service energy yields from one square meter of some energy conversion technologies in Central Europe (all values in kWh m - 2 yr-I) Type

Heat

Electricity

Mobility

Solar-collectors Photovoltaics Waterpower-station Energy from woody plants Ethanol from sugar beets

100 10 35 3.7 1.8

12 43 1.6 0.8

7.5 26 1.2 0.5

this calls for the definition of a mean industrial energy supply density (or mean industrial energy yield) YEI kWh m -2 yr-~ under sustainable conditions. This yield takes into account the energy mix (process heat, electricity, mechanical power, etc.) used in a sustainable industry. FR •ED ARN

-

-

m2

(7)

YEI The YEI may vary with the geographic context and with the technologies used in the technology mix to supply the energy. However, as a result of numerous case studies carried out by the authors in this field, it may be stated that this variation is confined to a relatively small range between 2 and 12 kWh m -2 yr -L . 3.3. The energy supply area A E

The area needed to supply one kilowatt-hour of service energy under sustainable conditions varies considerably with the quality of the energy needed (different temperature levels of process heat, electricity or mechanical power, liquefied fuel etc.; see Table 2) and the transformation technology. As a rough guideline we can say that the higher the quality of the energy service the higher the area required for supplying it. Besides, storage of energy (e.g. in liquid fuels) is costly in terms of sustainable area requirement. The energy yields in Table 2 also include the infrastructural needs for the generation of energy (see below). There are large differences in the ratio between the area needed to provide the energy (which is a function of the effectiveness of the transformation of solar energy into the energy form needed) and the area needed to establish the infrastructure for this transformation (which depends on the complexity of the technology and on the necessary technical effort). The energy supply area is calculated using the energy yield YE kWh m -2 yr-~ and the energy F E k W h / y r utilised in the process: Ae

= FEm 2 YE

(8)

3.4. The area f o r process installation A 1

The area needed to provide the installation for a process is divided into two segments: the 'direct' use of land (i.e. the land occupied by the process installation itself) and the

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250

'indirect' use of land (i.e. the area to provide and maintain the installation in a sustainable way). These two segments have to be treated differently: A~ = AID -+- All m 2

(9)

3.4.1. The direct use o f land AID

Land used directly is an area which is occupied while the process is operating. Therefore, it has to be included into the total area in the same way as the raw material area or the energy supply area. 3.4.2. Indirect use o f land A n

The area to establish the infrastructure for a process is needed only once in the life-span of a plant. In order to be comparable to the other areas, which are based on the consumption per year, this area has to be 'depreciated' like an investment over the life-span of a plant. Most installations are made of non-renewable raw materials. However, unlike raw materials used in a process, most of the materials forming the process infrastructure can be reclaimed after their use in the process and can be recycled. Therefore, their use is not dissipative. The ecological impact is merely caused by the creation of the installation. Similar to the assumption and the evaluation of the area for non-renewable raw materials, the energy demand for this procedure will be estimated and converted into an area required. Subsequently, the resulting 'depreciated area' (over the life-span LS years) will be included in the A n. This energy demand can roughly be estimated from the total costs C l $US (industrial price, taxes excluded) following the equations C~" 0.54 ED

-

-

C E • LS

kWh/yr

(10)

Et~ All = - m 2 YEI

( 11 )

or by combining a life-span of 15 years, an industrial energy supply density of 6 kWh m -2 yr -1 and energy cost of 0.02 $ U S / k W h to one factor: All = 0 . 3 0 " C E m 2

(12)

As a matter of fact, areas representing the installation of processes do not usually dominate the SPI. Notable exceptions are energy conversion technologies and agriculture, where direct use of land is important. 3.5. The staff area A s

The number of workers N s c a p / y r in a factory is allocated to an area. The more staff a process requires the bigger the impact on the environment. Consequently, personnel

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251

needed for farming, distribution and running the plants must also be included in the calculation. A s = Ns

Ns m 2

(13')

• ain = - -

YS To keep consistent with the factor yield, let Ys cap m -z yr-1 be the inverse area ain. This may help us to recognise that we are as dependent on areas physically as any other creature. This area allows us to relate labour and matter a n d / o r energy intensive processes to one another.

3.6. The area for sustainable dissipation of products Ap In the SPI concept, any stream leaving a process is considered to be a product stream, regardless of whether it can be sold or whether it is economically worthless. It is furthermore supposed that all products including the by-products are eventually dispersed into the environment. Therefore, their quantity and quality have to be considered. In order to estimate the area allocated to dissipation, the following reasoning is applied: If there is a rate at which the content of a given environmental compartment (soil, water, air) is renewed, any product stream can be 'diluted' by the newly added mass, until this mass reaches qualities (concentrations) that are equal to the quality of the initial compartment. Therefore, it is necessary to know the rate of renewal of a certain environmental compartment and the actual concentration of different components (e.g. heavy metals, sulphur, chloride etc.) in this compartment. The rate of renewal of the compartment water is calculated via precipitation and seeping ratio. In Austria a precipitation of 1200 kg m -2 y r - ~ and a seeping ration of about 30% accounts to a rate of renewable of 360 kg m -2 y r - i (Table 3, Fig. 3). For the compartment soil the compost generation rate is used to get the rate of renewal. In Austria it is about 0.42 kg m -2 yr -1 for grassland. This is a typical value for Central Europe, where there is a loss in mass during composting of about 56% (Table 4). The case is different for the compartment air. Here the degeneration potential for

Table 3 Allowable yearly dissipation into the compartment "water' with a precipitation of 1200 kg m -~ yr-L and a seeping ratio of 30% Heavy metal

mg m-2 y r - t

Heavy metal

mg m 2 yr- i

Organic substance

mg m - 2 yr- i

As B Be Cd Co Cu Fe Hg K

14.4 180 1.8 1.8 18 36 36 0.36 3600

Mn Ni Pb Sb Se Sn TI V Zn

18 10.8 14.4 3.6 3.6 18 0.22 18 1800

HC total PAC AOX BTX PCB COD DDT. TDE Lindane Vinyl chloride

36 0.072 10.8 10.8 0.036 7200 0.036 1.08 0.11

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252

IIIIIB ~ ;!i~?iQ ¸ .......

+

Products

Fig. 3. Compartment water: if there is a rate of renewal for the ground water (eluviation), any product stream can be diluted by the newly added mass, until this mass reaches qualities (concentrations) that are equal to the quality of the initial compartment.

certain substances is calculated via natural emissions (Table 5), e.g. an ecosystem of a certain size emits sulphur dioxide. Then the amount is related to the area of the ecosystem. This directly results in a dissipation potential (see Eq. 14). In contrast to the definition of yields, dissipation is linked to sinks ('sinks' in contrast to 'sources'). These sinks (s) are described by the rate of renewal and the actual concentration: Sc. m =

(14)

Rc • c ..... kg m m -2 yr 1

The product area can be calculated using the rate of renewal R c kg m 2 yr ~ of the environmental compartment, the actual concentration of the substance co,m k g m / k g in the compartment and the product flow Fp.c, m k g m / y r to this compartment (index m

Table 4 Allowable yearly dissipation into the compartment "soil' with a compost generation rate of 0.42 kg m -~ yr i Heavy metal

mg m - 2 yr- ~

Heavy metal

mg m - 2 yr- ~

Heavy metal

mg m - 2 yr- ~

Organic substance

mg m - 2 yr- t

As B Be Cd Co Cu Hg Li Pb

8.4 10.5 4.2 0.42 21 42 0.42 25.2 42

Mn Mo Ni Pb Sb Se Sn Th TI

840 2.1 25.2 42 2.1 2.1 8.4 6.3 0.42

U V Zn Zr

2.1 21 126 126

PAC PCDD PCDF PCB

0.13 2 . 1 . 1 0 -6 2.1.1() -~ 0.042

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Table 5 Allowable yearly dissipation into the compartment 'air' (global values) Substance mg m - 2 yr- f Substance CO VOC (total)

9800 6500 4500 1280 255

CH 4

Sulphur (total) SO:

253

mg m-2 yr- t

H 2S CS? Phosphorus (total) NO~ NH 3

390 2.0 28 131 200

describes a certain substance). The area for dissipating a single component of a certain product flow to a given compartment is: Fp,c ,m

Ap ....

S

c,

me

(15)

m

This calculation must be made for all product flows leaving the process in question. The area Aps assigned to the dissipation of a certain product s t r e a m ' S ' is the largest area A , ..... computed for this stream (either in the compartment water, soil or air). Aps,c = maxm( Ap . . . .

)

m2

(16)

The product dissipation area A . is calculated as the sum of the individual dissipation areas of each compartment: ae = Zaps,c m 2

(17)

C

4. Possibilities of reducing the SPI There are three possible ways which reduce the ecological impact of a process and subsequently lead to a reduction of the SPI: • recycling of material(s) • using energy and material cascades • multiple use of area(s) In the case of recycling (Fig. 4), the process is extended to include the original production process and the recycling process. Thus, recycling is ecologically advanta-

input

Process Limits j I ]PROCESS

services

•

I

L,

minus AREA for secondaryinput

AREA for recycling demand added

AREA for by-product dissipation cancelled

Fig. 4. Process scheme and process limits to include recycling into ecological optimization.

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geous as long as the energy, the raw material, the infrastructural and the dissipation requirements of the recycling process (expressed in the respective areas) are smaller than what is saved in terms of raw material and dissipative areas of the process itself. In the case of material cascades, the resource and the dissipation requirements are reduced. At present, energy cascades are optimised by means of the Pinch Technology (Smith and Linnhoff, 1988) via cost and energy savings. In ecological engineering, the saved areas must be bigger than the area requirement of the installed cascade. The multiple use of areas can only be integrated if we know the on-site (eco)systems. It can only be applied at the regional level. An example of the multiple use of an area is the ground water utilisation, biodegradation and the variety of species in areas where energy plants grow. During the application of the SPI to case studies, some facts became obvious: The use of fossile organic materials is very crucial since their yield is very small and the dissipation areas grow immensely if the products consist of toxic or stable synthetic substances.

4.1. Case study To demonstrate the applicability of the SP,I a case study on 'Energy from liquid fuel by synthesising ethanol from sugar beets' is enclosed. The service in question is the energy from liquid fuel that can be derived from ethanol via combustion. In the first step we have to define the process which is used to put the service at our disposal. The resources are sugar beets, cyclohexane as a separation agent and different types of energy. The fermentation is done in batch reactors, down-stream processing is continuous. The capacity of the synthesising plant is 10000 ton ethanol/yr, its operating hours 7900 h / y r , the service-life of the plant 10 yr and the reference period one year. Fig. 5 gives a structural view of the process. The overall process to produce energy from liquid fuel comprises the cultivation of sugar beets, the fertilising of the arable

'

~ ~

Arable Land: Biomass

- Solar

fertilizer biocides I

~'llhergy

I Enerly

biomass waterCarb°n dioxide

'~

I Arable Land: '_ energy ' Sugar Beet 1I carbon dioxide dioxide

Energy Conversion

}

=.a.o,

I

]

I

Combustion of L ethanol i Synthesizing Plant Ethanol [ Liquid Fuel

Fig. 5. The overall process of the case study: Energy from liquid fuel by synthesising ethanol from sugar beets.

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255

Table 6 Input and output streams of the ethanol synthesising plant described in the case study Feed

Product

By-product

Substance

ton/yr

ton/yr

ton/yr

Glucose

30 254 19 200 359 105 21.2 21.5 21.1

CO,

Ethanol Methanol Propanol Butanol Methylbutanol Acetaldehyde Cyclohexane

18.4

Energy

GWh/yr

mp-steam Electricity

14,0 0,05

10020 57.7

1.4.10 -~

10.4 18.4

land, harvest, transportation, the provision of installations and the energy for the synthesising plant. All these activities are necessary for the final energy service and are covered by the SPI. In Table 6, all inputs and outputs of the synthesising plant are listed. They define the streams of the process. The total area Atot is computed (see Table 7) and related to the number of unit-services (Eq. 2) which are 75.2 G W h / y r of primary energy from liquid fuel. ato t =

153.19km2/75.2

- 10 6 kWh/yr

= 2.04m:

yrkWh -1

SPI = 2 . 0 4 m 2 y r k W h - l / 1 9 0 0 0 m: yrcap- J = 1.07 • 10 4 cap/kWh For one inhabitant, 9320 kWh of primary energy from liquid fuel can be produced in Europe. The SPI points out the fraction of the (yearly) area per inhabitant which is used for one kilowatt-hour of primary energy supplied by liquid fuel. Table 7 summarises the contribution of each partial area. It can be seen clearly that the lion's share of the total area are the arable land for the sugar beets (20%) and the sustainable dissipation area of the by-products (63%). What are the possibilities of optimising the process? • Installing a biological waste water treatment plant to reduce the COD would increase A t and A E but the decrease of Ap is much more important. • Using the arable land for depositing the by-products after the waste water treatment. The by-products and cells could be used as fertilisers on sugar beets farms. The N, P, K, Ca, etc. cycles could be closed in this way and other fertilisers would no longer be necessary. • Producing the amount of cyclohexane sustainably from renewable resources. If we assume that these possibilities come true, the process efficiency is tripled, thus allowing the inhabitant in this scenario to use three times more kilowatt-hours with the same area.

256

C. Krotscheck. M. Narodoslaws~3,/ Ecological Engineering 6 (1996) 241 258

Table 7 Calculation of the partial areas in the case study Raw material Glucose Other cultivation requirements Cyclohexane Exploitation and extraction losses Energy mp-steam Electricity Installation Total costs Staff Plant staff Dissipation COD t" HC total ~

F R ton/yr

YR kg m 2 yr- i

30254

Factor (%) 0.98 ~

Equation (5)

30.87 3.09

(5)

9.20 1.84

(8) (8)

3.78 <

( 10,1 I )

7.79

(13)

0.23

(15) (15)

96.39 <

(I)

153.19

+ 10 b 18.4

0.002 + 20 ~

FE GWh/yr

YF. kWh m ~- yr-" i

14.0 0.05 C l Mio. $US

3.7 d 12 t. 3'1 $US m -2 yr- i

l 7.3 N s cap/yr

0.12 Ys cap m -2 yr I

12 Fp k g / y r

km 2

5.26.10- 5 sp kg m -2 yr ]

694000 0.14

Atot

0.0072 3.6.10 ~

Assuming 5 kg of sugar beets for 1 kg of glucose and a yield of 49 ton/ha in Central Europe (see Table l ). b Empirical value including fertilisers (e.g. Hydro, 1991), pest control (e.g. Dielacher, 1993), staff, energy and infrastructure (e.g. Kosaric et al., 1981; Austmeyer and R~Jver, 1989) for sugar beet farming. Empirical value (e.g. Guine& 1993). d See Table 2: energy from woody plants. e See Table 2: photovoltaics. f See Table 3: all alcohol in the by-product stream account for a load of 694 ton of chemical oxygen demand (COD) per year. g See Table 3: acetaldehyde is caulcated as 'total HC'.

A s h o r t d i g r e s s i o n s h o u l d i l l u s t r a t e t h e S P I b y a p p l y i n g t h e r e s u l t s to t h e m o b i l i t y s e r v i c e o f p r i v a t e cars: I f w e a s s u m e that it t a k e s 80 k W h / 1 0 0

k m to p o w e r an i n d i v i d u a l c a r a n d 2 . 0 4 m ~-

y r k W h - ~ to s y n t h e s i s e e t h a n o l a n d i f w e take into a c c o u n t t h e w o r l d - w i d e a r e a p e r inhabitant of 24000 m 2 yr cap-l, a person can go 14700 km. To reach the mean m o b i l i t y o f 5 5 0 0 k m o f a C e n t r a l E u r o p e a n c i t i z e n , 3 7 % o f ' h i s ' a r e a w o u l d b e u s e d to p o w e r t h e p r i v a t e c a r w i t h e t h a n o l . I f e v e r y h u m a n b e i n g a g r e e d to this w a y o f life, 3 7 % o f t h e e a r t h ' s l a n d w o u l d b e u t i l i s e d to s u s t a i n a l i q u i d fuel p o w e r e d m o b i l i t y . I f w e r u n o u r c a r s w i t h f o s s i l f u e l s ( 1 0 k W h / k g a n d 5 0 0 m 2 y r k g ~, w h i c h r e s u l t s in 4 0 m 2 y r k m - ~), w e c o u l d g o 6 0 0 k m p e r i n h a b i t a n t . T o s u s t a i n this k i n d o f m o b i l i t y f o r e v e r y h u m a n b e i n g , w e n e e d 20 a d d i t i o n a l p h a n t o m p l a n e t s ' e a r t h ' to d e l i v e r t h e f o s s i l e fuel.

C. Krotscheck, M. Narodoslawsky / Ecological Engineering 6 (1996) 241-258

257

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