A key review on emergy analysis and assessment of biomass resources for a sustainable future

ARTICLE IN PRESS Energy Policy 38 (2010) 2948–2955

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A key review on emergy analysis and assessment of biomass resources for a sustainable future Gaijing Zhang a,n, Weiding Long b a b

College of Mechanical Engineering, Tongji University, Shanghai 201804, China Sino-German College of Applied Sciences, Tongji University, Shanghai 201804, China

a r t i c l e in f o

a b s t r a c t

Article history: Received 23 June 2009 Accepted 18 January 2010 Available online 21 February 2010

The present study comprehensively reviews emergy analysis and performance evaluation of biomass energy. Biomass resources utilization technologies include (a) bioethanol production, (b) biomass for bio-oil, (c) biodiesel production, (d) straw as fuel in district heating plants, (e) electricity from Municipal Solid Waste (MSW) incineration power plant, (f) electricity from waste landfill gas. Systems diagrams of biomass, which are to conduct a critical inventory of processes, storage, and flows that are important to the system under consideration and are therefore necessary to evaluate, for biomasses are given. Emergy indicators, such as percent renewable (PR), emergy yield ratio (EYR), environmental load ratio (ELR) and environmental sustainability index (ESI) are shown to evaluate the environmental load and local sustainability of the biomass energy. The emergy indicators show that bio-fuels from crop are not sustainable and waste management for fuels provides an emergy recovery even lower than mining fossil fuel. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Emergy Biomass Sustainability

1. Introduction Energy is essential to economic and social development and welfare enhancing. Unfortunately, the greatest part of the world’s energy is currently produced and consumed in unsustainable ways since fossil fuels provide more than 90% of the world’s total commercial energy needs, with oil the leading source in the global energy mix (Yuksel, 2008). Achieving solution to environmental problems that we face today requires long-term potential actions for sustainable development. In this regard, renewable energy resources appear to be one of the most efficient and effective solutions (Kaya, 2006). Among renewable energy sources, biomass offers good future potential as an energy source since it can reduce carbon dioxide emission and replace fossil fuels directly. Biomass is very diverse and includes wood residues, organic wastes, crops residues, crops grown specifically for energy production, animal wastes, black liquor (black liquor is a byproduct of the kraft process, one of the processes used by pulp mills during the production of paper pulp) and municipal solid waste (MSW). While, crops production for energy must be balanced against the need for food, fiber, animal feed, biochemical and soil carbon and forest sinks due to the limited availability of land biomass production. Biomass energy plays more and more important role in economic development and environmental protection. When

n

Corresponding author. Tel.: + 86 21 6958 3735. E-mail address: [email protected] (G. Zhang).

0301-4215/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.enpol.2010.01.032

seeking an alternative source of energy, one must evaluate the whole production chain to correctly evaluate potential environmental benefits and disadvantages. However, classic methods foresee the carrying out of energy or economic assessment. Nonetheless, placing these valuations in the broader context of environmental sustainability requires more integrated analysis. For this purpose, emergy analysis is considered as a valid approach to quantify both environmental and economical costs. Emergy analysis is introduced during the 1980s by Howard Odum. Emergy analysis assesses all the inputs that supply a system, especially those that are usually neglected by classic economic accounting methods, by means of a thermodynamicsbased measure, giving an appraisal of the actual environmental cost of any class of resource which is not merely limited to its economic price or energetic content (Pulselli et al., 2006). Emergy analysis has been used to evaluate the natural and human contributions required in many fields. In this study, the emergy analysis used in assessment of biomass utilization technologies was reviewed.

2. Emergy analysis 2.1. Emergy and transformity Emergy is regarded as ‘energy memory’ and can be defined as the available energy that was used previously in the direct and indirect work of making a product or service and expressed in units of one type of energy(Odum, 1996). Emergy of a product is

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not a state function, since it depends on the pathway that is produced. This methodology allows the evaluation of a process on a common basis, and the solar of emergy evaluation is the conversion of all process inputs, including energy of different types and energy inherent in materials and services, into emergy by means of a conversion factor called transformity. Transformity is an intensive quantity and is measured in sej/J (emergy per unit energy). It represents the inverse of an efficiency comparing two similar processes; a higher transformity means that more emergy in needed to produce the same amount of output. Therefore, the transformity is a measure of hierarchical position in energy transformation chains. The relationship between the emergy (Bi) of fuel i and its transformity (Tri) is: Bi ¼ Tri Ei where, Ei denotes the energy content. Transformity is a very central problem in emergy analysis, and the most appropriate one should be chosen; for example,

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transformity of electricity (such as 1.59E +05 (Odum, 1996), 2.00E+ 05 (Ulgiati et al., 1994), 3.36E + 05 (Xiaobin et al., 2008)) is very different from one country to another as it depends on the production process. In the paper, the most appropriate and available transformities are used, which are taken from the different literatures (listed in Table 1). 2.2. Emergy indicators Emergy indicators are used to evaluate eco-technological processes and the whole economies. Emergy indicators for a given system are shown to be functions of renewable, nonrenewable and purchased emergy inflows. The systems diagram in Fig. 1 shows nonrenewable environmental contributions (N), renewable environmental inputs (R), and inputs from the economy as purchased (F) goods and services. Y is yield. Several indicators are given in Table 2 to evaluate the global performance of a process (Mark and Sergio, 2004).

Table 1 Transformity for biomass emergy analysis. Item

Unit

Transformity

Item

Unit

Transformity

Air Coal for hot water/ steam generation Ammonia Biodiesel Combined cycle cooling water Combustion and cooling air Constructions costs Fiber glass Citric acid Ceramic material Charcoal Cement Diesel Electricity generated Hydroelectricity Themoelectricity Ethanol produced, with services Ethanol produced, without services Earth cycle Glycerin Heat Hydrochloric acid Incineration and power generation services ISOPAR Herbicides Insecticides pesticides Landfill service Lubricants Labor Lime

sej/g sej/J sej/g sej/J sej/J sej/J sej/g sej/g sej/g sej/kg sej/J sej/g sej/J sej/J sej/J sej/J sej/J sej/J sej/J sej/g sej/J sej/g sej/$ sej/kg sej/g sej/g sej/$ sej/J sej/$ sej/g

5.16E + 07a 6.72E + 04b 6.38E + 09b 8.60E +05c 6.07E +04a 1.50E +03d 2.39E + 08e 3.00E + 09f 2.65E + 09g 3.30E +13h 1.07E +05h 3.48E + 09i 6.60E +04j 1.59E + 05d 8.00E + 04d 1.59E + 05d 1.73E + 05k 1.28E + 05k 1.02E +04b 1.80E +12c 1.00E + 05l 2.65E + 09g 4.00E + 12m 3.80E +12h 2.49E + 10l 2.49E + 10n 4.00E + 12o 6.60E +04p 3.46E + 12a 1.68E + 09b

Materials MSW Machinery Machinery(mainly steel) Natural gas Nitrogen Nitrogen fertilizer Organic matter in topsoil used up Process and cooling water Phosphoric acid Phosphate fertilizer Quench Reactor Rain, geppot Rain chemical Sodium methyl ate Sodium hydroxide Softened water Sand Sugarcane biomass Seeds of corn Seeds of wheat Sunlight Vegetable oil Vessels Water Wind,kinetic Workers Water for irrigation Wind

sej/g sej/kg sej/g sej/g sej/g sej/g sej/g sej/J sej/J sej/g sej/g sej/kg sej/kg sej/J sej/J sej/g sej/g sej/g sej/J sej/J sej/g sej/g sej/J sej/J sej/kg sej/g sej/J sej/J sej/J sej/J

1.00E + 09rp 3.34E +11g 1.40E+ 09rq 1.13E +10l 4.84E +04rr 4.19E +09q 6.38E +09b 1.24E +05b 6.89E +04i 2.65E +09g 6.55E +09b 1.80E+ 13h 1.80E+ 13rh 1.04E+ 04j 1.82E +04j 2.65E +09g 2.69E +09g 6.64E +05a 2.00E + 07s 2.46E +04h 5.88E +04k 7.91E +04t 1i 1.30E+ 06q 1.65E +05p 2.03E+ 05i 1.50E+ 03j 4.00E + 05q 6.89E +04g 2.52E +03b

a

(Wang and Zhang, 2004). (Bargigli and Ulgiati, 2003). c (Liu Sheng et al., 2007). d (Lan S.F. et al., 2002). e (Daniel, 2005). f (Earth trends, 2007). g (Lapp, 1991). h (Alonso-pippo et al., 2004). i (Odum, 1996). j (Brown and Buranakarn, 2003). k (Brown Mark and Ulgiati, 2004). l (Nilsson, 1997). m (Marchettini et al., 2007). n (Sui Chunhua and Lan, 2001). o (AREA, 2000). p (Tiezzi.2001). q (Ulgiati and Brown, 2002). r (Brown and Ulgiati, 1997). s (UFL,2007). t (Xiao bin et al., 2008). b

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purchased Resources F Services

Local Nonrenewable Resources N Local Renewable Resources

Environmental R system

Economic Use

Yield

Fig. 1. Diagram showing the principles of emergy accounting.

Table 2 Emergy metrics and indicators calculation formulas. Name and abbreviation

formula

Yield (Y) Percent Renewable (PR) Emergy yield ratio (EYR) Emergy investment ratio (EIR) Environmental loading ratio (ELR) Environmental sustainability index (ESI)

Y= R +N + F R/Y Y/F F/(R+ N) (F+ N)/R (EYR)/ (ELR)

Among the existing emergy-based indicators, PR represents the first measure of system sustainability: the lower the fraction of renewable emergy used, the higher the pressure on the environment. In the long run, only processes with high values of this index are sustainable (Brown and Ulgiati, 1997). EYR is the ratio of the emergy yield from a process to the emergy costs. The ratio is a measure of how much a process will contribute to the economy. And EYR indicates the efficiency of the system using purchased inputs (Ortega et al., 2005). ELR is given by the ratio between nonrenewable and imported emergy used to renewable emergy used. It is an indicator of the pressure of a transformation process on the environment and can be considered as a measure of ecosystem stress due to a production (Ulgiati and Browm, 1998). EIR is the ratio of emergy fed back from the outside of a system to the indigenous emergy input (both renewable and non-renewable). It evaluates whether a process is an economical user of the emergy that is invested in comparison with alternatives (Brown and Ulgiati, 1997). ESI is the ratio of the emergy yield ratio to the environmental loading ratio. It measures the potential contribution of a resource or process to the economy per unit of environmental loading. 2.3. Emergy evaluation procedure Emergy accounting uses the thermodynamic basis of all forms of energy, materials, and human services but converts them into equivalents of one form of energy. Emergy accounting is organized as a top down approach leading to the conversion of all inputs to a system into their solar energy content (Ulgiati and Brown, 2002). There are five main steps required to complete an emergy evaluation. First, a detailed systems diagram is completed. The second step is to translate this knowledge into an aggregated diagram of the system addressing specific questions. Third, descriptions of the pathways in the aggregated diagram are

transferred to emergy analysis tables where the calculations needed to quantitatively evaluate these pathways are compiled. The fourth step in the method is to gather the raw data needed to complete the emergy analysis tables along with the conversion factors (energy contents, transformities, etc.) that are needed to change the raw data into emergy units. Finally, after the raw data has been converted into emergy units, indices are calculated from subsets of the data. These five steps are discussed in more detail in the following sections (Daniel, 2005).

3. Emergy evaluation of the biomass energy 3.1. Bioethanol production Bioethanol is an alternative fuel for gasoline and diesel fuel, which is receiving great attention worldwide. The bioethanol has developed rapidly since 2004. The annual world total production of bioethanol is estimated to be 45 billion liters in 2007, while the annual production of biodiesel is approximately 5 billion liters (Earth trends, 2007).The processes of bioethanol production from crops include crop production, harvesting and transport to plant and industrial processing. Fig. 2 describes the main steps of the process. Mark and Sergio (2004) evaluated bioethanol production from corn by emergy analysis. The corn production data refers to Italian agricultural standards, while corn-to-ethanol industrial conversion refers to average available conversion technologies. Xiao bin et al. (2008) performed an emergy analysis of the bioethanol production from wheat in China and compared it with the bioethanol production from corn in Italy. Consuelo L.F. and Ortega (2010) assessed the sustainability of large-scale ethanol produced from sugarcane in Brazil. Their results are listed in Table 3. The main conclusion drawn from the results of their study is as follows: bioethanol from wheat or corn is not a sustainable source of fuel. 3.2. Bio-oil production Fast pyrolysis can directly produce a liquid fuel from biomass named bio-oil. It can be a substitute for fuel oil in any static heating or electricity generation application and can also be used to produce a range of commodities and chemicals, such as phenol and its derivates. Though many types of biomass to bio-oil production have been investigated, the sugarcane trash is a better choice from the economical point of view (Alonso-pippo et al., 2004). The energy flow diagram for the bio-oil production plant is

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shown in Fig. 3, which makes an emergy analysis of production of bio-oil from sugarcane biomass. This diagram is made based on the production scheme for charcoal and bio-oil as main products of the fast pyrolysis process used in this plant (Alonso-pippo et al., 2004). Their results are listed in Table 3.

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straw as fuel in district heating plants. He used the data of Sweden. Their results are listed in Table 3. 3.5. Electricity from municipal solid waste (MSW) 3.5.1. Electricity from MSW incineration MSW incineration power plant is profit in waste minimization and resource regeneration, which is divided into three different phases: (1) collection, (2) specific waste treatment, (3) the disposal of solid and liquid residues from plants. The characteristics of each process are shown in Fig. 6, with the energy diagram of MSW incineration power plant. The emergy benefits are shown and represented by the arrow to the market, which are electricity and heat (Marchettini et al., 2007). The analysis of incineration shows that the greatest emergy investment is concentrated in the disposal phase (47.14%) and treatment phase (44.74%). At the same time, Lou Bo (Lou B, 2004) also performed an emergy analysis of an electric power generation with waste incineration. Their results are listed in Table 4.

3.3. Biodiesel production Biodiesel is renewable energy made from oil plants (soybean, peanut, rapeseed, sea cabbage, corn, cottonseed, sunflower seeds, etc), animal fats and waste cooking oil. The technology of biodiesel production is immature in China, so it needs government subsidy. Fig. 4 describes the main steps of the biodiesel production process (Liu Sheng et al., 2007). Liu Sheng et al. evaluated the biodiesel production from vegetable oil by emergy analysis. Ota’vio Cavalett and Enrique Ortega assessed the biodiesel production from soybean in Brazil (Cavalett and Ortega, 2009). Their results are listed in Table 3. 3.4. Straw as fuel in district heating plants

3.5.2. Electricity production from waste landfill gas Landfill is generally recognized as the final destination of the refuse that cannot be further segregated or recovered in any other way. Nowadays, refuse is no longer simply considered as ‘‘waste’’, but rather something that must be recovered or re-used as

The processes of straw as fuel in the district heating plants include cereal production, field drying, harvesting /handling, heat plant. An emergy flow diagram for straw fuels is shown in Fig. 5 (Nilsson, 1997). Daniel Nilsson performed an emergy analysis of

Surface Water

Geologic Processes

Goods& Machinery

Fuels & Fertilizers

Rain

Labor& Services

Water

$

Wind Assets

Soils

$

$ Crop production

Sun

Transport

Processing Bioethanol Wastes

Fig. 2. Energy systems’ diagram of bioethanol production from crop.

Table 3 Emergy indicators for nonelectric energy carriers. Item Bio-ethanol

Bio-oil Biodiesel Straw for fuel

Wheat Corn sugarcane Vegetable oil Soybean

Tr(sej/J)

EYR

ELR

2.77E +05 1.89E +05 4.87E +04 6.97E +04 8.60E+ 05 3.90E+ 05 1.0E+ 05

1.24 1.14 1.57 3.36 3.68 1.62 1.1

4.05 7.84 2.23 0.45 3.55 2.26

EIR

0.42 3.57 11

PR

ESI

19.81% 11.31% 30.9% 68% 6% 31%

0.31 0.15 0.71 7.46 1.04 0.72

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Steel& Equipments

Chemicals Products

Labor

Charcoal

Sand

Biooil

Water Non condensable gas

Combustion Chamber Ceramic Material

Vapor organic

Power Charcoal

Power Charcoal

Biooil

Quenching System

Biomass

Ashes Ashes

Feeder

Reactor

Electricity

Fig. 3. Energy systems’ diagram of bio-oil production from sugarcane trash.

Water

Air

Chemical raw material

Auxiliary energy

Labour Government subsidy

Raw oil

$ Pretreatment

Methylester treatment

Biodiesel and byproduct

Fig. 4. Energy systems’ diagram of biodiesel production from raw oil.

a potential resource (Korhonen et al., 2004; Dijkema et al., 2000). The system of electricity generation from waste landfill gas is divided into three different phases: (1) collection, (2) specific waste treatment: landfill, (3) the disposal of solid and liquid residues from plants. The energy diagram of landfill is shown in Fig. 7. The emergy benefit of managed landfill shown and represented by the arrow to the market is electricity. As traditional

emergy analysis does not account for polluting emission, the emergy cost of emission abatement and leachate treatment are included in the waste treatment. The analysis of the managed landfill shows that the treatment phase requires the greatest emergy investment (95.55% of the total emergy investment) while disposal is almost negligible (1.06%) and collection represents only 3.39% of the total emergy cost. At the same time, Lou Bo

ARTICLE IN PRESS G. Zhang, W. Long / Energy Policy 38 (2010) 2948–2955

Diesel

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Machines

Nitrogen

Electricity

Wind Goods services Ash Field drying

Rain

Harvesting handling

Heat

Heat plant

Grain

Cereal production

Sun

Fig. 5. Energy system’ diagram of straw fuels

Fuel & lubricants

Natural gas

Electricity

Water

Labor

Construc -tion costs

Materials

Chemicals Machinery

Plant cost

Market Ash treatment

MSW Collection

Incinerator Managed landfill

Fig. 6. Energy system’ diagram of electricity from MSW incineration.

4. Results and discussion

Table 4 Emergy indicators for electricity production. Item

Tr(sej/J)

EYR

Electricity production from MSW incineration Italy 1.48E+ 05 China 1.59E+ 05

3.2 1.00

Electricity production from waste landfill gas Italy China

1.48E+ 05 1.59E+ 05

0.19 1.36

Electricity production from coal

1.71E+ 05

5.48

(Lou B, 2004) also performed an emergy analysis of an electric power generation with waste landfill gas in China. Their results are listed in Table 4.

4.1. Emergy analysis of nonelectric energy carriers from biomass A series of emergy performance indicators for nonelectric energy carriers from biomass are given in Table 3. Transformities can be used to compare different production systems that generate the same product, helping to choose a better alternative. The values of transformity of bioethanol, biodiesel and straw for fuel, compared to transformity of fossil fuels (coal, 6.71E+04sej/J; natural gas, 8.05E+04sej/J; crude oil, 9.07E+04sej/J; Odum, 1996), demand a similar or higher amount of environmental support than fossil fuels. Since transformities are efficiency measures on the space and time scales of the biosphere, this result simply means that natural work in making fossil fuels has been more efficient than our work of cropping and converting cereals. EYRs of bioethanol, biodiesel from soybean

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Fuel

Water

Electricity

Refuses from RDF

Labor

Materials for constr -uction

Chemicals

Materials for manage -ment

Machinery

Plant cost

Managed Landfill Waste

Collection

Waste collected

Biogas

Market

Biogas combustion

Waste buried

Disposal

Leachate

Fig. 7. Energy system’ diagram of electricity from waste landfill gas.

and straw for fuel are very low compared to fossil resources (coal, 8 10.5; natural gas, 6.8 10.3; crude oil, 3.2 11.1; Odum, 1996). This means that cropping for fuel provides an emergy return and a contribution to the economy-even lower than mining or extracting nonrenewable resources. But the EYRs of bio-oil and biodiesel from vegetable oil are higher than other bio-fuels. Maybe it is because the two systems do not consider the process of the raw materials cropping. From the past experience gained in previous case studies investigated, it appears that low ELRs (around two or less) indicate relatively low environmental impacts ( or process that can use large areas of a local environment to ‘dilute impacts’). ELRs between three and ten indicate moderate environmental impacts, while ELRs ranging from ten to extremely high values indicate much higher environmental impacts due to large flows of concentrated nonrenewable emergy in a relatively small local environment [Brown Mark and Ulgiati, 2004]. The ELRs of bio-fuels listed in Table 3 are between two and ten except bio-oil, which indicate moderate environmental impacts. The PRs of bioethanol are lower than 30% and the PRs of biodiesel are similar to 30%. From the point of view of renewability of resources, the nonrenewable percentage of the emergy flow to the bioethanol or biodiesel production system is about 70%. This means that the bioethanol or biodiesel from crop is less than one third renewable. However, such a result is still better than fossil fuels that are considered as totally nonrenewable resources. ESI is usefully applicable to measure openness and loading changes occurring over time in both technological process and economies. ESI value is lower than one appears to be indicative of consumer products or processes and those greater than one indicative of products that have net contributions to society without heavily affecting its environmental equilibrium. The ESI values of bioethanol and biodiesel from soybean are lower than one which indicates that the output of these products is lower than input. The ESI of biodiesel from vegetable oil is slightly greater than 1, but it is still low. If we want to have a more sustainable process to produce biofuel from crops, it is fundamental to find other procedures that allow increasing the system’s renewability. Transformity of bio-oil is 6.97E+ 04sej/J, which is similar or lower amount of environmental support than fossil fuels. EYR of bio-oil production was 3.36. The ELR and PR of bio-oil are 0.45 and 68%, respectively, which indicate that the system produces low load on the environment. ESI is 7.46, which indicates that the

system has net contributions to society without heavily affecting its environmental equilibrium. Their result is that the bio-oil production process is sustainable and its application as a substitute for oil byproducts is a very promising choice. But comparing with other bio-fuels, the emergy analysis of bio-oil production system only includes the process of sugarcane trash to bio-oil conversion without other processes such as sugarcane cropping, transport to factory. This may be the reason that leads to satisfactory results of bio-oil emergy analysis. 4.2. Emergy analysis of electricity production from biomass The emergy indicators for electricity production are listed in Table 4. The EYRs of electricity production from MSW incineration and waste landfill gas are lower compared with electricity production from coal. This means that waste management for fuels provides an emergy recovery even lower than mining fossil fuel. EYRs of electricity production from MSW incineration in Italy and in China are 3.2 and 1.0, respectively. The reason may be that the emergy benefit for MSW incineration in Italy is electricity and heat, for MSW incineration in China is only electricity. When the emergy investment is constant, the emergy recovery is higher and the EYR is higher too. In a word, the results of MSW incineration in Italy and China are different, which may be caused by different consideration by each system. The EYRs of electricity production from waste landfill in Italy and China are 0.19 and 1.36, respectively. The emergy benefits for the two systems are the same. The difference is that the former considers the emergy cost of emissions abatement and leachate treatment included in the waste treatment and the latter does not consider it. When the emergy recovery is the same, the investment emergy is higher and the EYR is lower. The results of waste landfill in Italy and China are different, which may be caused by different consideration by each system.

5. Conclusions Emergy analysis of biomass is comprehensively presented in this study. Some concluding remarks which can be extracted from

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this study are as follows:

1) Emergy is an expression in one type of energy (solar energy) of all the available energy used in the work process, directly and indirectly generates a product or service. In this regard, emergy analysis is a powerful tool, which can be successfully used in the evaluation of bio-fuels production system. 2) Concerning the emergy indicators listed in Table 3, the processes of bio-fuels from crop do not provide a sustainable product because its demand for environmental support is huge and mainly based on the indirect support embodied in nonrenewable resources. 3) Concerning the emergy indicators listed in Table 4, EYRs of the electricity production from waste landfill gas and the combustion of waste are lower than fossil fuels. This is why the application of these methods needs the support from government and the application of new technologies at present. 4) Transformities of electricity production from waste landfill gas and MSW incineration are lower than transformities of bioethanol and biodiesel. This means that the bioethanol and biodiesel demand higher amount of environmental support than electricity production from waste management. 5) Although emergy analysis can assess the contributions of natural environmental resources to bio-fuels production system and can overcome many shortcomings of traditional energy analysis and economic analysis, there are also shortcomings in emergy analysis, such as the confirmation of emergy transformity and the threshold of sustainability. Confirmation of emergy transformity is the key and difficult point of emergy analysis. Transformity values calculated by Odum and his colleagues are often used directly in existing research. It is appropriate to use these transformity values to conduct emergy analysis for large region and system, but it is questionable for small system, such as biofuels production, to use these transformity values for emergy analysis. The biggest challenge for emergy analysis is that it is hard to table sustainability threshold. Therefore, it is unable to determine whether the system is sustainability or not. It is only to determine the sustainability change of system by comparison from a cross-quality perspective based on different spatial units and parallel-quality perspective based on different periods.

Policy decisions concerning energy use and investments in energy technology require that decision-makers have the ability to compare wholistically net yields, environmental impacts, and sustainability. The emergy indicators and emergy analysis method are tested only on some examples of bio-fuels production systems. This means that the results are not statistically significant as they would be if obtained considering different location. However, this does not affect the value of the method proposed that has a general validity. The author expects that the analyses reported here will provide people knowledge about how effective to use biomass energy.

Acknowledgment This work was supported by Science and Technology Commission of Shanghai Municipality-Technical guideline for the efficient utilization of sustainable energy (08DZ1205705).

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