Entropy analysis
as a tool for optimal sustainable use of biorefineries
Kasra Samiei
This thesis comprises 30 ECTS credits and is a compulsory part in the Master of Science with a Major in Chemical Engineering – waste management and resource recovery technology
No. 2/2007
Entropy analysis as a tool for optimal sustainable use of biorefineries
Kasra Samiei
Master's thesis
Subject Category: Technology Series number: 2/2007
University College of Borås School of Engineering SE-501 90 BORÅS
Telephone +46 (0)33 435 4640
Examiner: Mohammad Taherzadeh
Supervisor: Morgan Fröling, Department of Chemical and Biological Engineering, Chalmers University of Technology
Client: Department of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg
Date: 16 November 2007
Keywords: biomass conversion, biorefinery, entropy, life cycle assessment, second law analysis, thermodynamic optimization
To my parents for their love and support
Abstract
The biorefinery concept is attractive. Increasing international concerns over issues such as climate change have led to political as well as social pressures for a shift from fossil fuels to renewable resources and biomass is one abundant renewable resource.
Biomass has the potential of supplying many of the fuels and chemicals which are currently dependent on petroleum. Much development is still needed in the field of biorefineries and a systematic approach to evaluate and compare process technologies and to suggest optimizations seems necessary.
The objective of this thesis is to develop entropy analysis as a possible evaluation tool for optimization of biorefinery processes. This is a new application of entropy analysis which is rarely discussed in the literature. The scientific basis of the entropy analysis is described and the proposed methodology is explained. The position of entropy analysis among other system analysis tools such as exergy analysis and life cycle assessment is discussed along with entropy analysis earlier applications.
A case study is introduced which is the IBUS (Integrated Biomass Utilization System) project in Denmark. The idea in IBUS is to integrate the biomass plant with a power plant to utilize the surplus steam from the power plant for the internal use of the biorefinery. The suggested method of entropy analysis is applied to this case study to compare different processes for production of ethanol along with solid biofuel and animal feed from Danish wheat straw. The evaluation is a gate to gate analysis in which production of energy carriers are also included in addition to biorefining of wheat straw. A parallel life cycle assessment study with equivalent system boundaries is also carried out to compare the results with a conventional environmental systems analysis method.
The results from the entropy analysis of the IBUS case study show that fermentation of C5 and C6 sugars by yeast is the most efficient process thermodynamically while fermentation of only C6 sugars by yeast is the least efficient among the three cases studied. Integration of the biorefinery with a coal fired CHP plant is identified as a wise choice by the results of the entropy analysis method.
For the IBUS process alternatives investigated in this study, the entropy results and the LCA results (aggregated environmental load) are in correlation; entropy results are consistent with weighting results based on two different weighting methods namely Eco indicator 99 and EPS 2000. Entropy generation is also in correlation with production cost for the processes analyzed in this evaluation. Another observation is that cooling in the biorefining process contributes highly in the generation of entropy.
This potential improvement option is not surfaced by the LCA conducted.
The potential for further investigation and development of the tool is recognized reflecting on some interesting observations in the results. Improvement of the tool is highly possible for example by supplementing other implications of entropy in process design such as "waste potential entropy" concept which is developed as an eco-toxicity measure.
Contents
Abstract..………..………..………….3
Contents………...…....…………..4
Chapter 1. Biorefinery……….………..…………...5
1.1 Introduction……….………...……….….5
1.2 A hot topic…………………..………...7
1.3 Biomass………... ……….....…………...…………...8
1.4 Economy………...………..…………10
1.5 Existing biorefineries………..………..……….…………12
1.6 Biorefinery of tomorrow……….……….…………...14
1.7 Sustainability……….……….………...15
1.8 Conclusions……………….………….…….………....16
Chapter 2. Entropy analysis among other system analysis tools…...…......……….17
2.1 Introduction………………...……….……….17
2.2 Flow based measures for eco-sustainability……………….………..17
2.3 Thermodynamic background……….21
2.4 Second law analysis……….25
2.5 Entropy and sustainability……….………..……27
2.6 Entropy analysis………..28
2.7 Summary of the methodology…………………35
2.8 Conclusions……….………..36
Chapter 3. Case study…………..……….…..37
3.1 Introduction……….37
3.2 IBUS………..……….37
3.3 Goal and scope definition………...39
3.4 Data………..41
3.5 Method……….42
3.6 Entropy flows………..42
3.7 Entropy change in the system for the ethanol process………………..………44
3.8 Entropy change in the environment………………..…..45
3.9 Entropy change due to energy production……………….………...45
3.10 LCA on IBUS………......……….…50
3.11 Results……………………….…...54
3.12 Discussion……….......……….........61
3.13 Conclusions……… ………………62
Chapter 4. Overall conclusions and recommendations………....………….…63
4.1 Why entropy as sustainability indicator for biorefineries...... ....………63
4.2 Case study: the IBUS process………..………... ....…………..…....64
4.3 Entropy analysis compared to conventional life cycle assessment .…………….......…..64
4.4 Recommendations……… ....………...65
Acknowledgement....……….…… ….………...66
References……… …….………...67
Chapter 1 Biorefinery
1.1 Introduction
"Man cannot create material things. His efforts and sacrifices result in changing the form or arrangement of matter to adopt it better for the satisfaction of his wants".
Alfred Marshall [1]
The term biorefinery is analogous to petroleum refinery. Similar to a petroleum refinery, a biorefinery converts a variety of biomass feedstock to produce a spectrum of biobased products, including electricity, fuels, chemical intermediates and consumer products [2].
Biorefinery can mean many different things depending on perspective [3] because it is still a concept under development, in fact. Some definitions of biorefinery are presented in Box 1.1.
In a sense crude oil itself is originated from living organisms but this origin goes way back to millions of years earlier. So crude oil is not considered biomaterial but fossil material. A scheme of a refinery versus biorefinery concept is presented in Figure 1.1.
Figure 1.1 Scheme of a refinery versus biorefinery concept [4]
Box 1.1 Some definitions of biorefinery
Kamm et al., 2006 [12] have classified biorefinery development into three phases and accordingly discusses three generations of biorefineries. An example of the type
"generation-I biorefinery" is a dry milling ethanol plant. It uses grain as a feedstock, has a fixed processing capability, and produces a fixed amount of ethanol, feed co- products and carbon dioxide. It has almost no flexibility in processing. An example of a type "generation-II biorefinery" is a current wet milling technology. This technology uses grain feedstock, yet has the capability to produce a variety of end products depending on product demand. Third generation (generation-III) and more advanced biorefineries have not yet been built but will use agricultural or forest biomass to produce multiple product streams, for example ethanol for fuel, chemicals and plastics [12].
• A biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power and value-added products from biomass.
http://en.wikipedia.org/wiki/Biorefinery [5]
• In a biorefinery, biomass is refined in a chemical or biochemical process with the purpose of producing a high quality chemical or material.
Bioraffinaderier – vägen mot hållbar utveckling [6]
• A facility able to produce a variety of outputs such as electricity, chemicals, plastics, food and fibre in addition to biofuels.
Fulton et al., 2004 [7]
• A biorefinery is an integrated facility that uses biomass as a feedstock for conversion into a range of differentiated products such as transport fuels, bulk and fine chemicals and using waste biomass for heat or power. The approach is similar to that of petroleum refineries involving a wider range of processes and techniques.
http://www.biorenew.iastate.edu/resources/glossary-of-biorenewables-terms.html [8]
• Biorefinery is a refinery which replaces petroleum as the feedstock with renewable biomass.
T. E. Amidon, 2006, The Biorefinery in New York: Woody Biomass into Commercial Ethanol [9]
• Biorefinery is an industrial facility able to convert biomass, including lignocellulosic material such as wood chips, into a range of material, chemical, and energy products, analogous to a petroleum refinery.
Chum, H.L. and Overend, R.P., 2001. Biomass and reneawable fuels [10]
• An industrial complex that will convert renewable, lignocellulosic biomass into fibre, chemicals, fuels, power and/or products with minimal or no negative environmental impacts.
Ed Lehrburger, 2005 [11]
1.2 A hot topic
"…forests can be made to produce fifty times their present volume of end products and still remain a permanently self-renewing source for raw materials…. Only forests-no other raw material resource can yield such returns. The forest can, and so must, end the chronic scarcities of material goods that have harassed man's experience since the beginning of history."
Egon Glesinger, The Coming Age of Wood, 1949 [13]
As also seen in the quotation above, the idea of utilizing biomass as a source of products is not a new idea. Many others have shared the same vision and made attempts to bring this idea into practice. Among them were Henry Ford who was convinced that he could develop industrial organic chemicals from the oil of soybeans. He supported research into crop-based polymers that he hoped could be used for constructing the auto body, and in 1941 a "farm-grown automobile" was displayed at the Ford factory in Dearborn, Michigan [14].
Such early attempts have not been very successful in large industrial scale, however, mainly because of rapid development and dominance of petrochemical industry. But things are different now. The descent of petrochemical industry has arrived and this gives us courage to pursue what earlier innovators also believed in.
In 2005 the Swedish government announced their intention to become the first country to break their dependence on oil and other fossil raw materials by 2020 [15].
Biorefineries is one possible approach to reach this goal considering Sweden's large potential of renewable resources, especially forests.
Not only in Sweden and Europe but also worldwide, biorefinery concept is gaining more and more attention and is the focus of research and development programs.
Table 1.1 shows the year 2002 levels and future targets of the United States of America for a national bio-based industry [16].
The global increasing trend towards a bio-based economy comes as the result of several reasons including:
• Fossil starting materials are non-renewable in contrast with biomass which is renewable. This is becoming more and more important as fewer and fewer reserves are being explored. They will end one day, not very far into the future.
• As the price of crude oil increases, countries seek alternative sources of energy and material to become independent of the insecure and unstable oil supply in the global market.
• Growing environmental concerns throughout the world is another factor for the shift towards bio-economy. The main concern is climate change. The Kyoto protocol has required the signatories to reduce their greenhouse gas emissions and fossil based industry is a main emitter.
Target 2030 Target 2020
Target 2010 2002
Year
5%
5%
4%
2.8%
Biopower(Bioenergy) Biomass share of electricity and heat demand in utilities and industry
20%
10%
4%
0.5%
Biofuels
Biomass share of
demand for transportation fuels
25%
18%
12%
5.0%
Bioproducts
Share of target chemicals that are biobased
Table 1.1 The USA national vision goals for Biomass technologies by the Biomass Technical Advisory Committee [16].
• Social factors also play a role for example development of rural areas and diversifying the agricultural products are among the reasons to promote biorefineries.
• Biomass is our sole source of renewable liquid fuels and chemicals.
1.3 Biomass
"Everything that exists is the seed of that which shall come out of it".
-Marcus Aurelius (121-180 A.D.) [17]
Biomass, in technical context, refers to living and recently dead biological material which can be used as fuel or for industrial production [18]. Technically available biomass includes a wide range of products and by-products from forestry and agriculture as well as municipal and industrial waste streams. It thus includes trees, arable crops, algae and other plants, agricultural and forest residues, effluents, sewage sludge, manure, industrial by-products and the organic fraction of municipal solid waste [19].
Biomass is a renewable resource and is carbon neutral. That means its combustion does not contribute to net increase of carbon dioxide concentration in the atmosphere.
Even so biomass could still contribute to global warming indirectly, if utilized unsustainably. This happens when the natural carbon equilibrium is disturbed; for example by deforestation or urbanization of green sites. Besides, it takes some energy
(and therefore a discharge of CO2) to grow, harvest and transport the biomass for use as a fuel source. However the use of biomass fuels can result in displacement of the emissions of carbon dioxide ordinarily released using fossil fuels. This displacement will depend entirely on the efficiency with which the biomass energy can be produced and used [18, 20].
The total global supply of biomass is estimated to be 300 billion tons and nearly 50%
of it is lignocellulose [21]. Stem wood is the most abundant source of organic chemicals on earth and that explains the widespread attention towards lignocellulose as an alternative source of energy and materials. Examples of lignocellulosic biomass already used for industrial purposes are corn stover, sugarcane bagasse, wheat straw, rice straw, willow tree, Eucalyptus wood and coffee waste [22].
Amazing as it might seem to anyone who is not a chemist, just as large a range of chemicals and plastics can in theory be produced from biomass carbohydrates as can be made from petrochemical hydrocarbons [23]. In fact cellulosic material can supply many of the fuels and chemicals for which society currently depends on petrochemicals [24].
This potential has been confirmed by several other authors for example Stringer, 1996 [25] believes that plant based material, or biomass, provides a plentiful and renewable alternative source of materials for many industrial uses, but it is little used for such purposes. Or Tom Binder, 2005 from ADM [26] presenting to the first biorefinery workshop in Washington D.C. acknowledges the same potential as the motive of their company: "We believe that nature holds solutions to many of the world's problems and we will not stop until we find them. We find and create hundreds of products from nature's resources".
Table 1.2 lists several of the biomass-derived materials and their increasing share on the market between 1992 and 1996 [27]. The Table shows that in 1996 plant-derived adhesives made up 48% of the total adhesives market; 50% of surfactants on the market were plant-based; 18% of detergents were based on plant matter and vegetable oil-based inks made up 16% of the ink market.
While the costs of the plant-derived products were falling over time, there remained a price difference of 20 percent for inks, 50 percent for detergents and 100 percent for paints higher than their petro-based alternatives [28]. Such cost differentials suggest that these products will need both technical and market-scale improvements if they are to grow out of the niche markets. The following section will further explore the economic aspect of biomass refining.
product 1996 production (million tons/year)
Percent of total Market, 1992
Percent of total Market 1996
Wall paints 7.8 3.5 9.0
Specialty paints 2.4 2.0 4.5
Pigments 15.0 6.0 9.0
Dyes 4.5 6.0 15.0
Inks 3.5 7.0 16.0
Detergents 12.6 11.0 18.0 Surfactants 3.5 35.0 50.0 Adhesives 5.0 40.0 48.0 Plastics 30.0 1.8 4.3 Plasticizers 0.8 15.0 32.0 Acetic acid 2.3 17.5 28.0 Furfural 0.3 17.0 21.0 Fatty acids 2.5 40.0 55.0 Carbon black 1.5 12.0 19.0
Table1.2 Industrial products derived from biomass [27].
1.4 Economy
"The threat of the materials problem is not that we will suddenly wake up to find the last barrel of oil exhausted or the last ton of coal gone, and that economic activity has suddenly collapsed. The real and deeply serious threat is that we shall have to devote constantly increasing efforts to acquire each pound of materials from natural resources which are dwindling both in quality and quantity, thus finding ourselves running faster and faster in order to stay standing still. In short the essence of the materials problem is costs".
Paley commission report, 1952 [29]
The idea of biorefinery sounds attractive but for a bioproduct to succeed in the market its production needs to be economically feasible, just like any other product. The price of raw materials and the technologies to process them are among the main challenges.
And when it comes to substituting fossil-based products with bio-based products, the challenge is if the latter can compete with the existing products. As a matter of fact, biofuels and bioenergy are today seldom competitive with fossil alternatives without tax incentive [30].
Cost is not only a weak point for biofuels and bioenergy but also rather a disadvantage for many biopolymers. Table 1.3 shows a comparison between the price of biodegradable and petro-based polymers in Europe [31]. As seen in Table 3.1, there exists a very unfavorable ratio of 2.2 to 7.8 between average prices of similar biodegradable and petro-based polymers.
Biodegradable polymer Traditional Polymers Price ratio
Foamed starch HDPE (PE) 2.2
Starchblends (film type) LDPE (PE) 4.4
Polyesters (synthetic BDP) PP 5.8
Polylactic acid (PLA) PS 2.9
Cellulose acetate PVC 7.8
Cellophane PET 3.5
Table 1.3 Price ratio between average prices of biodegradable and traditional polymers in Europe [31]
This could change in future, however, and there are possibilities to promote the economy of biomass processing. Biorefinery is one such possibility to reduce costs.
Process integration and by-product management which are the core elements associated with biorefinery are ways to promote the efficiency and economy of the production. Finding use and market for by-products and diverting waste into usable products is particularly important in biological processes since such processes are naturally associated with considerable amount of by-products.
According to Ragauskas et al., 2006 [32] "The integration of agroenergy crops and bioenergy manufacturing technologies offers the potential for the development of sustainable biopower and biomaterial that will lead to a new manufacturing paradigm". Electricity production is a key issue, especially taking into account the potential for reducing CO2 emissions, which could be reduced to below zero if co- generated electricity displaces high emission electricity from fossil fuels [33].
An advantage of bio-based chemicals over petroleum based source materials is that the latter require oxygenization because petroleum contains little oxygen while plant based chemicals already have a high oxygen content. This creates a great opportunity by eliminating the need for several capital intensive, oxidative processes used in the petroleum industry [32, 34].
1.5 Existing biorefineries
"Our products reflect our philosophy…respect for other cultures, the past, the natural world, and our customers. It's a partnership of profits with principles".
Anita Roddick-The Body Shop promotional literature, 1990 [35]
There are existing biorefinery plants mostly in the pilot or demonstration scale in the globe. Some of the current projects in Europe are summarized in Table 1.4. The idea is to present an insight of the possible raw materials, routes and end products and to exemplify the biorefinery concept. Observe that this is only a sample collection and may not represent the most important, in any aspects, or biggest biorefineries.
In addition to the list in Table 1.4, pulp and paper mills could be examples of biorefineries where forest biomass is traditionally being converted into pulp, paper and energy. Future products are likely to be more diverse including various biofuels deriving from herbaceous and wooden biomass [39]. An illustration of pathways to possible future products from a pulp mill is presented in Figure 1.2 [40].
Chips pulp paper
Bark, Forest Residues
Suberin, Hemicellulose Lignin Extractives
Fatty Acids Phenols Pharmaceuticals Fiber Additives Carbon Fiber Antioxidants Barriers Binders Bioactive Chemicals Hydrogels Soil improvement
Solid fuel Solid/Liquid fuels Electric power Figure 1.2 Pathways to possible products from a pulp mill biorefinery [4].
Pulp mill
Website www.biogasol.dk www.chrisgas.com www.joanneum.at www.biorefinery.de www.bioethanol.info
Comment This is a biorefinery from an agricultural residue (straw) to several valuable biofuels.By integrating the bioethanol plant with a biogas digester and by making use of the byproducts, the producers claim to be able to produce ethanol in Denmark cheaper than Brazilian ethanol[36]. The hub of the integrated project is the existing 18 MW biomass-fueled pressurized air blown gasification combined cycle CHP plant in Värnamo [37]. By linking the biorefinery to an energy generation unit which is a biogas digester, the economic evaluation for pilot scale implementation shows positive results [38]. The primary biorefinery is combined with a green crop drying plant and the raw materials come from the region Havelland with 30.035 ha permanent grass land [4]. Integrated Biomass Utilization System is basically integrating a biomass plant with a combined heat and power plant. The surplus steam and electricity from the power plant supply the need of the ethanol plant and the produced biofuel will be co- combusted with fossil fuels in the CHP plant.
Process stages - Pretreatment: Wet explosion - Enzymatic hydrolysis - two-stage fermentation: SSF by yeast Xylose fermentation by thermophilic anaerobic bacteria - Anaerobic treatment of wastewater - Pelletizing - Gasification - Solid state fermentation - Press out technology - Fermentation - Thermophilic fermentation of C5 and C6 sugars
Products - Bioethanol - Biogas - Hydrogen fuel - Pellets - Methanol - Clean hydrogen-rich synthetic gas From this to fuels such as DME, Methanol,, H2, etc. - Lactic acid for ex. For solvents - Amino acids For feed or pharmaceuticals - Fibre for feed or board or insulation - Energy as heat or electricity - Biogas - Press cake for fodder and fuels - Proteins for industry and cosmetics i.e. emulsifier, foaming and gelling agents - Biogas - Bioethanol - Solid biofuel - Animal feed
Feedstock Wheat straw Biomass and waste Green biomass: Grass, Clover, etc. Green Biomass such as Alfalfa, wild mix grass Lucerne Wheat straw MSW Grains
Biorefinery Maxifuel Lyngby Denmark ChrisGas Växjö Sweden Green biorefinery Graz Austria Green biorefinery Brandenburg Germany IBUS Fynsvaerket Denmark Table 1.4 Some existing biorefinery projects in Europe.
1.6 Biorefinery of tomorrow
"The next stage will not follow. The changes required will not be realised solely by political decisions, nor by market forces in industry alone, nor by individual farmers and forest farmers who see future opportunities for profit. Not until all the positive forces in society aim for the same goals can Sweden achieve independence from oil".
Göran Persson, Prime minister of Sweden 1996-2006 [15]
The vision of the future biorefinery is an industrial complex where diverse products are produced from renewable resources with least environmental impacts. A biorefinery in this vision is very different from current industrial production plants where they are normally pictured as polluting and hazardous.
The biorefinery of tomorrow will need to use and link different scientific expertise including plant science, industrial agriculture, waste management, process technology and marketing. Table 1.5 shows what could be associated with a future biorefinery [11].
Certainly this is only one way of visualizing the future. Others have shared their vision in different ways. Kenneth Geiser, for example, in his book "Materials Matter", 2001 [34] describes an intriguing and interesting vision. "Material Factories of the future which look like large green houses with long tanks of microbially infused carbohydrates in which tiny organisms manufacture well-tailored materials from recycled organic wastes".
Plant science Production Processing End uses
Genomics wood, trees hydrolysis Fuel Enzymes grasses fermentation Power Metabolism energy crops bioconversion Plastics Composition agricultural residues chemical conversion Solvents
MSW gasification Chemical intermediates
Combustion Phenolics
pulping Adhesives
Hydraulic fluids
Fatty acids
Carbon black
Paints
Dyes, pigments
Ink Detergents
Pulp and paper
Horticultural products
Fiber boards
Plastic filler
abrasives
Table 1.5 Vision of what could be involved in a future biorefinery [11].
Whatever the vision would be, there are challenges ahead. As the world population and consumption rate is increasing, the main challenge would be to get the most out of the raw materials. A sustainable extraction of the natural resources requires us to aim for the highest possible efficiencies for our processes to utilize biomass. This could perhaps explain why professor B.K. Ahring from the maxifuel project in Denmark believes the keyword of their project is "maximum exploitation of the raw materials" and describes the developing biorefinery as "a carbon slaughterhouse", where every single carbon atom of the raw material is exploited [36].
In short optimal exploitation of the natural resources is the main challenge of the future and this underscores the importance of evaluation and optimization tools being available.
1.7 Sustainability
"Can we move nations and people in the direction of sustainability? Such a move would be a modification of society comparable in scale to only two other changes: the Agricultural Revolution of the late Neolithic and the Industrial Revolution of the past two centuries. These revolutions were gradual, spontaneous, and largely unconscious. This one will have to be a fully conscious operation, guided by the best foresight that science can provide… If we actually do it, the undertaking will be absolutely unique in humanity's stay on the earth".
William Ruckelshaus [34]
The word "bio" commonly implies greenness or sustainable or at least nature friendly to the audience. Accordingly, biorefinery, bioenergy, biofuels, biopolymers, etc. have a rather high public image and acceptance, at least as a first impression. This perception is not totally wrong but not necessarily always true. There are clear environmental advantages of bio-based products or biological processes but being bio could also mean problems in some other environmental aspects. Soil erosion, loss of biodiversity, land use, water consumption and genetic modification are some of the areas where a bio-based industry does not seem very advantageous.
Food for fuel argument is another issue where utilizing food crops to produce fuel and energy has been debated on moral grounds. In a more general level, this fits into a wider debate usually referred as land use competition. Since the quantity of land available on earth is limited, one main question arising is whether to use land for production of food, fuel, urbanization, etc.
Another environmental aspect to take into account is that a bioproduct might have utilized considerable amount of fossil fuels and energy in the production of the raw materials. A life cycle approach, therefore, is needed to analyze the sustainability issue in these cases.
In fact shifting from fossil-based production to bio-based production in large scale, if not approached scientifically, could only replace one set of environmental problems with other ones.
While the primary barrier to biorefining is generally recognized to be the lack of low- cost processing options or platforms [41], the sustainability of the issue should not be overlooked. Reflecting on the lessons learnt from petroleum industry development, mere economical viewpoint is not an enough broad basis for evaluation.
Admitting the benefits of biomass as a source of energy and materials, biorefinery could potentially contribute to sustainable development by replacing petrochemical- based products. However, Scientific, robust tools to evaluate the environmental performance of the replacing processes are needed, already in the early stage of the development of the concept, in order to realize this potential. Otherwise our today's solutions will become tomorrow's problems.
1.8 Conclusions
• A unique definition of biorefinery is difficult to decide on because the concept is still in its infancy. One definition is a facility that integrates biomass conversion processes and equipment to produce fuels, power and value-added products from biomass, analogous to a petroleum refinery.
• There is an increasing trend towards biomass worldwide as an alternative source of energy and materials due to economical, environmental, social and political reasons.
• Lignocellulosic biomass is the most abundant source of organic chemicals on earth and has the potential of supplying many of the fuels and chemicals for which society currently depends on oil.
• High cost of production is currently the main challenge of biomass conversion and an integrated biorefinery is, in fact, a solution to reduce costs.
• Optimal exploitation of biomass sources is a main challenge of future and biorefining is a solution to this. What needed are tools to evaluate and assist in selecting the best possible combination of raw materials, process routes and products between the various alternatives.
• As sustainability is being more and more in focus, there is a need for a tangible quantitative description that allows the sustainability of technological processes to be systematically evaluated, compared and improved.
Chapter 2 Entropy analysis among other system analysis tools
2.1 Introduction
Referring to the conclusions in the first chapter, optimization of biomass conversion processes in biorefineries is where more improvement is necessary and important. To achieve this, evaluation tools are needed to measure and compare different processes in terms of efficiency, environmental performance and cost reduction potentials.
In this chapter several of such tools are briefly introduced and the underlying bases for their development are discussed. This is presented in Section 2.2 and is mainly inspired from a review done by Krotscheck, 1997 [42]. Entropy analysis is the suggested tool in this thesis and is going to be elaborately discussed in Section 2.6 comprising its necessity in a biorefinery (2.6.1), the previous applications (2.6.2) and suggested methodology (2.6.3). Before that and to get to that point of discussion, some thermodynamic background is reviewed in Section 2.3; different versions of second law analysis are introduced and discussed in Section 2.4; and the link between entropy and sustainability in the scientific literature is reviewed in Section 2.5.
2.2 Flow based measures for eco-sustainability
There are number of tools available to analyze a process or a product. Christian Krotscheck, 1997 [42] has done a review and comparison of eco-sustainability measurement tools. His study reviews and compares Life Cycle Assessment (LCA), Material Input Per unit Service (MIPS), Sustainable Process Index (SPI), Appropriated Carrying Capacity (ACC), Pollution Control Costs (PCC) and Waste Potential Entropy (WPE) as different types of mass and/or energy flow based highly aggregated indicators. These eco-sustainability indicators will be briefly discussed here. However the discussion on LCA will be more elaborative since LCA will be further used in our case study to compare and analyze the entropy tool results.
2.2.1 Life Cycle Assessment (LCA)
Life Cycle Assessment (LCA) is a tool to evaluate the environmental performance of a product or service throughout its life span, from raw material extraction to disposal.
The main characteristic and the strong point of LCA is its holistic approach; it analyses a product from its cradle to grave and considers different environmental issues at a time. In this way trade-offs between different types of environmental impact and between different parts of life cycle are made evident [43, 44].
LCA procedure consists of four distinct phases namely Goal and scope definition, Inventory analysis, Impact assessment and Interpretation of results. Goal and scope definition is the first step in the analysis in which the purpose of the study and the system boundary is determined. One important decision of this stage is the choice of functional unit. In the Inventory analysis the incoming and outgoing flows to the system are determined and calculated per functional unit. Normally only environmentally relevant flows are considered. These include resource use, emission and waste streams. Impact assessment is translation of life cycle inventory results into environmental impacts [43, 44]. Flows including emissions and resource use are classified into different categories of environmental harm and the impact for each category is quantified by calculating the flows into a common unit for a specific impact. A weighting stage might follow to aggregate the results further into a single number of environmental harm. Finally, the results need to be interpreted and discussed according to the goal of the study.
There is a series of international standards for LCA, ISO 14040-14043 issued from 1997 onwards which indicates the importance and wide acceptance of LCA. The standardization of LCA became necessary when some enterprises started to perform and publish biased LCA studies in the 1990s to promote their own product [43]. There were some cases of contradictory results which led to criticism that LCA results show what the analyst had been paid to show. Although a uniform and standard LCA methodology now obstructs misusing of the tool, there are still subjective elements in LCA methodology dependent on the will of the performer.
One problem with LCA, therefore, is subjectivity. For example even though LCA outlook implies a holistic view and approach, not all the environmental impact categories could be dealt with in a study in practice [45]. Therefore, the choice of impact categories is always one decision by the commissioner or performer of LCA to specify their viewpoint. Another example of subjective elements in LCA is the weighting stage where different ready-made weighting methods measure people's values in different ways [43, 45].
It is not only lack of time and data which limits an analyst but also lack of characterization methods for some impact categories. Environmental impacts such as biodiversity loss, land use, soil erosion are very difficult to be quantitatively assessed in LCA.
Another characteristic of LCA which is its strong point in one way is that it potentially follows the fate of an emission in the environment to identify the environmental harm it would cause. Impact assessment is based on scientific models of cause-effect chains in natural systems. In this way, Life Cycle Impact Assessment is able to answer specific questions such as effect of an emission on Global Warming for instance.
This capability, however, only applies to known impacts. If an emission harms the environment in a way other than the identified environmental impacts, LCA tends to neglect it. One good example to clarify the point here is the case for CFC (Chloro Fluoro Carbon) compounds. When CFCs were first marketed, it was as a nontoxic substitute for the ammonia used in refrigeration and the chlorinated solvents used throughout many workplaces [34]. They were never identified as hazardous to human
health but they were damaging the environment in another way, unknown at that time.
The dissipation of CFCs into the atmosphere were stimulating chemical changes in the ozone layer. Only after quite considerable damage happened in the ozone layer, the crisis started to attract attention.
Krotscheck also confirms there are many outputs that are not seen as hazardous today but may emerge as hazards on our way to a more sustainable society [42]. His example is carbon dioxide which would not have entered a list of hazardous substances as recently as the 1970s, yet today is acknowledged as a major ecological risk factor. He concludes that LCA fails to reflect these potential pressures and is rather suitable for short-term evaluation.
Impacts such as climate change or stratospheric ozone depletion once happened will take a very long time to stop and manage, if at all possible. To move towards sustainability, a more extensive commitment is needed. A more precautionary approach would be to show concern for dissipation of energy and materials to the environment even when the mechanism of the environmental damage is not totally clear.
The scale of production today, the rapidity of industrial development around the world, and the kind of environmental concerns that are arising - climate change, endocrine disruption, biodiversity loss - suggest conventional approach to the management of industrial materials is inadequate [34].
2.2.2 Material Input Per unit Service (MIPS)
The Material Input Per unit of Service (MIPS) methodology was developed by Fredrich Schmidt-Bleek, 1993-94, as a measure of potential environmental impact [46, 47]. He and his colleagues in German Wuppertal Institute refer to the amount of material to be moved or processed and the amount of energy consumed per unit of finished material as the ecological rucksack or the ecological burden of each unit of material. For example, the catalytic converter used in cars which weighs less than 20 pounds had been found to carry a rucksack of some 2.5 tons, because the mining and processing of the platinum requires the movement of such large amounts of overburden and ore [47].
MIPS is very input oriented and according to Krotscheck neglects the emission side of activities [42]. Baumann and Tillman, 2004 [43] argue that MIPS is a simpler tool than LCA in the sense that environmental impact has been approximated by material usage.
2.2.3 Sustainable Process Index (SPI)
Sustainable Process Index (SPI) is based on the assumption that in a sustainable economy the only real input that can be utilized over the long term is solar exergy and that its utilization per se is bound to the surface of planet earth [42]. Furthermore, area is a limited resource in a sustainable economy because the surface is finite.
Developed by Krotscheck and Narodoslawsky, 1996 [48], SPI is the fraction of the area per inhabitant related to the delivery of a certain service and Krotscheck claims very high abilities for SPI as a measure of sustainability [42].
2.2.4 Appropriated Carrying Capacity (ACC)
The appropriated carrying capacity is proposed by Rees, 1994 [49] to be the fundamental base for demographic accounting. The concept initially was designed to make the human "footprint on the ecosphere", a concept proposed by Wackernagel et al., 1993 [50], of regions visible to their inhabitants. ACC, thus, is the area that a region uses to run its economy. Among the drawbacks of the tool, according to Krotscheck are the poor reflection of non-agricultural resources and negligence of waste side of the activities.
2.2.5 Pollution Control (abatement) Costs (PCC)
Pollution Control Costs (PCC) concept developed by Krozer, 1995 [51] shows the costs for control measures (eg. pollution control technology) to attain a specific environmental quality (carrying capacity). The underlying assumption is that coevolution of man and ecosphere requires efforts to prevent the degradation by residuals.
As an economic based measure, PCC has the advantage of direct compatibility with currently used economic evaluation methods. However, Krotscheck argue that like any full cost assessment approach, there are inherent problems. The levels to which emissions should be reduced (sustainable standard) are subjective and abatement costs depend on the state of technology. He concludes that the standardization of pollution prevention systems will discourage initiatives to invent new process alternatives or strategies [42].
2.2.6 Waste Potential Entropy (WPE)
Waste Potential Entropy (WPE) is developed by Ayres and Martinas, 1994 [52] as a thermodynamic based measure for eco-toxicity. It is argued that what is dangerous to the environment is the emission of material fluxes (i.e. waste streams) that are not in thermodynamic equilibrium with the environmental systems into which they are emitted. The most dangerous aspect of disequilibrium is argued to be differential physico-chemical composition between the waste stream and the environmental medium into which it is emitted. Such differentials constitute potential entropy increase.
Waste potential entropy is, thus, a measure of the potential of waste streams for driving uncontrolled chemical or physical processes in environmental systems; it is not an adequate predictor of the actual damage that will be done by given pollutants.
Despite this and despite the fact that Krotscheck rejects the WPE valuation in his conclusion [42], The authors, Ayres and Martinas, argue that "it is quite reasonable to postulate that the aggregated damage potential of human economic activity is proportional to the total entropy potential of aggregated waste emissions" [52].
The entropy tool to be presented in this thesis report in the following sections is different from WPE in the approach and outlook even though the scientific bases for both are the same (2nd law of thermodynamics and entropy generation as a measure of unsustainability). Although not dealt with in this thesis, further research to possibly link the two approaches is worth considering in future developments of the proposed tool.
2.3 Thermodynamic background
Before getting into the discussion of the methodology of entropy analysis tool, some background information on scientific basis of the proposed tool is presented in this section. These basics are necessary for better understanding of the evaluation and proper interpretation of the results.
Thermodynamics is essentially the science of conversion of energy and matter. This is of importance to environmental scientists as it allows us to understand and predict the extent of energy and material utilization. It is more and more recognized that thermodynamic process analysis can play a significant role in the design and evolution of future technological processes [53].
The starting point for most thermodynamic considerations are the laws of thermodynamics. The established basic laws of thermodynamics are presented in Box 2.1. Sometimes a forth law is discussed in the scientific literature which is basically an attempt to apply thermodynamics to evolution of the organic world [54]. However, that is not presented in the Box since it is yet not a solidified concept [55]. Some authors also introduce a combined law of thermodynamics which is basically a combination of first and second law [56], again not presented in the Box. The second law of thermodynamics, also known as entropy law, is the basis of the evaluation tool under study in this thesis and therefore will be more in focus in this section.
Box 2.1 Established basic Laws of thermodynamics
• Zeroth law. If two systems are separately in thermal equilibrium with a third, then they must also been in thermal equilibrium with each other [57].
• First law. Energy can neither be created nor destroyed; it can only change forms [58].
• Second law. The sum of the entropy changes of a system and its exterior can never decrease [59].
• Third law. The entropy change of a system during a reversible isothermal process tends towards zero when the thermodynamic temperature of the system tends towards zero (Nernst Principle) [60].
Entropy and second law are two of the vaguest concepts going in current science [61].
According to Bridgman, 1941 [62], "there's as many formulations of second law as there have been discussions of it". Box 2.2 presents some various interpretations of the 2nd law of thermodynamics.
Entropy is often taken as being something hard to grasp [68]. For the sake of clarity, entropy may be defined, in its cleanest form as "the quantitative measure of amount of thermal energy not available to do work in a closed system" [69, 70]. Box 2.3 presents some standard textbook descriptions of entropy.
Box 2.2 Some interpretations of second law of thermodynamics [61]
• The ratio of the heat content of a closed system to its absolute temperature always increases in any process.
Clausius 1850[62]
• There is at present in the material world a universal tendency to the dissipation of mechanical energy.
Kelvin 1852 [63]
• For any process the sum of all entropy changes occurring as a result of the process is greater than zero and approaches zero in the limit as the process becomes reversible.
Clausius 1865 [64]
• The entropy of universe tends to increase.
Bent 1965 [65]
• No process is possible whose sole result is the transfer of heat from a colder to a hotter body (Clausius statement)
Adkins 1983 [57]
• No process is possible whose sole result is the complete conversion of heat into work (Kelvin statement)
Adkins 1983 [57]
• The entropy of universe increases in a spontaneous process and remains unchanged in an equilibrium process.
Chang 1998 [66]
• No process will occur spontaneously unless it is accompanied by an increase in the entropy of universe.
Haynie 2001 [67]
• When energy does work, its quality, its exergy, diminishes.
Kay 2002 [61]
In classical thermodynamics, the change of entropy of a thermodynamic system could be mathematically defined as the ratio of a reversible heat interaction to the absolute temperature [71] as shown in Equation (2.1).
dS = δQ/T (2.1)
Standard molar entropy of a substance is then defined as the entropy content of one mole of substance under conditions of standard temperature and pressure (STP) [83].
It is shown by S0 in this report, as it is also common in thermodynamic texts and the unit is usually Joules per mole-Kelvin. Standard entropy is the same concept but the unit is normally in Joules per gram-Kelvin.
Box 2.3 Examples of standard textbook descriptions of entropy [72]
• Energy broken down in irretrievable heat [73].
• Boltzmann's constant times the logarithm of a multiplicity, where the multiplicity of macrostate is the number of microstates that correspond to the macrostate [74].
• The number of ways of arranging things in a system (times the Boltzmann's constant) [75].
• A non-conserved thermodynamic state function, measured in terms of the number of microstates a system can assume, which corresponds to a degradation in usable energy [76].
• A direct measure of the randomness of a system [77].
• A measure of energy dispersal at a specific temperature [78].
• A measure of the partial loss of the ability of a system to perform work due to the effects of irreversibility [79].
• An index of the tendency of a system towards spontaneous change [80].
• A measure of the unavailability of a system's energy to do work; also a measure of disorder; the higher the entropy the greater the disorder [81].
• A parameter representing the state of disorder of a system at the atomic, ionic and molecular level [69].
• A measure of disorder in the universe or of the availability of the energy in a system to do work [82].
Another important term in relation to the second law of thermodynamics which is discussed quite frequently especially in the domain of sustainability, is exergy. Exergy is described by Szargut, 1988 [84] as the maximum amount of work that can be done by a subsystem as it approaches thermodynamic equilibrium with its surroundings by a sequence of reversible processes. Exergy represents the portion of energy available to do work and is accepted as a measure of the quality of energy [85]. The exergy loss is proportional to the entropy increase and is the product of the entropy increment and the ambient temperature [86]. The mathematical relationship between the two concepts, Equation (2.2), also confirms this.
dB/dT = -S (2.2)
Where dB is an element of exergy, T represents temperature of the environment and S is entropy.
Closing this section, some interesting quotations around the 2nd law of thermodynamics are presented in Box 2.4, indicating the "mysterious" nature of this principle of universe; "the principle of evolution" as Perrot, 1998 [60] called it.
Box 2.4 Some quotations on 2nd law of thermodynamics
• The law that entropy always increases, holds, I think, the supreme position among the laws of nature. If someone points out to you that your pet theory of universe is in disagreement with Maxwell's equations- then so much the worse for Maxwell's equations. If it's found to be contradicted with observation-well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.
Sir Arthur Stanley Eddington [87]
• The tendency for entropy to increase in isolated systems is expressed in the second law of thermodynamics. Perhaps the most pessimistic and amoral formulation in all human thought.
Greg Hill & Kerry Thornley [88]
• Everything tends towards disorder. Any process that converts energy from one form to another must lose some as heat. Perfect efficiency is impossible. The universe is a one-way street. Entropy must always increase in the universe and in any hypothetical isolated system within it.
James Gleick [89]
• ─ What would you do to look for life on Mars?
─ I'd look for an entropy reduction, since this must be a general characteristic of life.
James Lovelock [90]
2.4 Second law analysis
Second law analysis is understandably based on the second law of thermodynamics for system analysis and is in contrast with first law analysis which is basically application of direct energy balance. There are different approaches to second law analysis. According to Rosen, 1999 [91] the main approaches discussed in the literature are exergy analysis, physical exergy analysis, exergy consumption analysis, negentropy analysis and entropy analysis. Figure 2.1 depicts different types of second law analysis in order of level of subjectivity. As the level of subjectivity increases, the environment needs to be specified in more detail but Rosen believes that the results, then, would contain more information.
Figure 2.1 Different approaches to Second Law Analysis stem from 2nd law of thermodynamics, shown in this diagram in order of increasing subjectivity (starting from the bottom), based on Rosen, 1999 [91]
Exergy Analysis
Physical Exergy Analysis
Exergy Consumption Analysis
Negentropy Analysis
Entropy Analysis
2nd law of thermodynamics
Rosen seems to prefer exergy analysis while German scientist Götz Alefeld, 1988 [92]
favors entropy analysis over exergy. Back in 1988, he wrote about the problems with exergy analysis. He believes the exergy concept is not the second law in its full general way and it is a reduced version of the second law. He explains that if the process temperatures of the system you are considering are not identical with the ambient temperature, exergetic efficiencies signalize values which are often too high and even contradicting the second law [92].
Bejan, 1982 [93] agrees that to calculate entropy generation rate and minimize it, the analyst does not need to rely on the concept of exergy.
Huang et al., 1992 [86] also argued that entropy balance was a "more rigorous solution" in their analysis than was the exergy balance at describing the system's performance. They used the entropy balance method to study thermal systems and concluded: "The entropy method yield not only the same information as is conveyed by the methods of energy and exergy analysis but it also predicts clearly the influence of irreversibilities of individual components on the coefficient of performance and its effectiveness, based on the process properties, rather than on ambient conditions".
Different authors, however, disagree over which approach is the most useful. The usefulness of the approach also depends on the application being studied.
Comparing with first law analysis (energy balance), second law analysis is more rigorous by taking into consideration the availability of energy not only its existence.
Such advantage has been discussed in several studies. For example according to Fartaj, 2004 [94] second law analysis provides an insight that the first law analysis can not. He analyzed a double-stage heat transformation cycle by energy, exergy and entropy balance methods. The results of his investigation showed that the second law could help to locate the system non-idealities that either were not identified or misevaluated by energy analysis. He believes that comparison of energy quantities from the first law (energy balance method) alone could be misleading. Kotas, 1985 [95] also argues that energy balances in general provide no information about internal losses. An energy balance for an adiabatic system could lead one to believe that these processes are free of losses of any kind. This is because the first law of thermodynamics does not distinguish between heat and work while the second law deals with quality of energy. Dealing with conservation of energy, the first law can not indicate that one form of energy can or can not be completely converted to one another. For example, it is always impossible to completely convert heat into work [94].
Entropy analysis is the focus of this report and is going to be elaborately discussed in the following sections. There have been previous applications of entropy analysis which will be briefly reviewed in section 2.6.2 but not in the field of biorefineries.
This thesis is an attempt to test this tool in a biorefinery and to suggest future directions.
2.5 Entropy and sustainability
Several authors observed and argued for a link between entropy and sustainability and some developed and proposed measures for sustainability based on the entropy law.
Some of the arguments in this debate are briefly discussed in this section. The interested reader is suggested to refer to the cited articles for more in depth discussions.
Minimum production of entropy is suggested to be a criterion of sustainability by T.M. Addiscott, 1995 [96]. He specifically elaborates this in agriculture where he suggests that the principle of Minimum Entropy Production provides a useful framework within which sustainability can be discussed. He goes on to suggest a way of assessing sustainability developed by Bertilsson, 1992 [97] known as "audit of small molecules". This oversimplified approach argues that the larger emission of small molecules in a system, the larger production of entropy; which means less sustainability.
Entropy as an indicator of sustainability in agro-ecosystems is also suggested by Steinborn and Svirezhev, 2000 [98].
Hornbogen, 2003 [99] also discusses the concept of sustainability based on the rate of entropy production and believed it to be useful as "foundation for a science of recycling". He argues that "positive entropy is not only a measure for the devaluation of heat but also for dispersing, mixing of matter on the solid surface, dissolving it in groundwater, oceans and in the atmosphere" and therefore entropy minimization describes many important aspects implied in sustainability. He postulates: "The industrial process leads to desired and undesired increases on entropy due to mixing or dispersion of matter. Material sustainability is achieved if enough energy is provided to create negative entropy (ordering force) to compensate for the undesired raise in entropy" and uses entropy for a definition of a quantitative concept for sustainability in regard to matter.
For Ayres and Martinas, 1994 [52] the criterion for sustainability from a thermodynamic perspective would seem to be fairly simple to state: " The earth system is sustainable if the average rate of entropy production does not exceed the average rate of influx of entropy potential from the sun, which is the only sustainable source". Earlier, they defined entropy potential as stock of physical information, namely the difference between the actual entropy of a system and its final entropy, after it reaches equilibrium - or becomes indistinguishable from - its local environment. They went on to introduce "waste potential entropy" as a measure of potential eco-toxicity, which was also discussed in section 2.2.6 of this report.
More recently, Ernei Jowsey, 2007 [100] proposes what he calls "a new basis for assessing the sustainability of natural resources". In his view entropy concept is an important way of measuring the quality of natural resources and for understanding the irreversible nature of environmental and resource processes.
Another recent paper by Wolfgang Winkler, 2006 [101] recommends that "reduction of entropy flows to the environment reduces primary energy consumption of
mankind". He uses the second law of thermodynamics to define the principle design rules of sustainable technologies in minimizing the irreversible entropy production.
He explores his idea by discussing reversible power generation in fuel cell hybrids, reversible transportation and reversible heat processes.
One last interesting relation in this section is the interface between entropy theory and Chaos theory, commented on by Addiscott [96]. Chaos theory describes systems that are in a state of pure disorder but in which order and even beauty can be found [102].
People like Cambel, 1993 [103] have started to explore this interface.
2.6 Entropy Analysis
Entropy analysis is a method to evaluate a process based on the 2nd law of thermodynamics. It is basically calculating entropy generation in a system and its surroundings (caused by the process or the product under study) and using it as a proxy for the evaluation of the process. The magnitude of the entropy generation calculated in this way is not useful by itself; it makes sense in comparison. The comparison could be done with the entropy generation in an alternative process (with the same goal) or with an imaginary chemically ideal process. In the latter case, a thermodynamic efficiency will be resulted as an indicator of how far the studied process is from the best thermodynamically possible. Optimization possibilities could be identified in this case. The comparison could also be done between entropy generations in different stages of production.
To conduct a fair comparison, it is obviously important to apply equivalent system boundaries and methodological choices for the different alternatives. It is also important to compare different routes to a product on a life cycle basis. This is because a claimed better process could be worse in the production of raw materials for example. In such fair conditions for comparison, a higher entropy generation for the same amount of product or service is an indicator of a less efficient process. Process optimization from chemical engineering point of view would hence correspond to minimizing the entropy generation.
Therefore, the first step in the analysis is very similar to LCA's where a clear definition of goal and scope of the study is needed.
Furthermore, it is wise to have a conceptual understanding of what being measured before applying the tool. Entropy generation calculated in this way is a thermodynamic concept. A physical translation would be dissipation in case of materials and dispersal in case of energy. When a process generates entropy in the universe - which is a fact for every real process according to the 2nd law of thermodynamics - the raw materials and input energy are being dissipated and dispersed. Therefore, entropy generation is an indicator of how efficient a process is utilizing the natural resources.
