Extended exergy-based sustainability accounting of a household biogas project in rural China

Energy Policy 68 (2014) 264–272

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Extended exergy-based sustainability accounting of a household biogas project in rural China J. Yang a, B. Chen a,b,n a b

State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Beijing Normal University, Beijing 100875, China Nonlinear Analysis and Applied Mathematics Group, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

H I G H L I G H T S







Extended exergy is used to describe the sustainability level of biogas projects. A set of extended exergy based sustainability indicator is established. Biogas project has high renewability and greenhouse gas emission abatement potential. Multiple utilization of biogas digestate is a promising way to improve sustainability.

art ic l e i nf o

a b s t r a c t

Article history: Received 14 September 2013 Received in revised form 4 December 2013 Accepted 16 January 2014 Available online 10 February 2014

Biogas has been earmarked as one of the leading renewable energy sources capable of mitigating environmental emissions in rural areas. Thus, developing an accounting technique is of particular importance in coping with increasing problems related to renewable agriculture and rural energy supply. In this study, extended exergy was generalised for the sustainability evaluation of biogas projects. Furthermore, a series of extended exergy-based indicators was presented as benchmarking from the perspectives of resources, economics and greenhouse gas (GHG) emissions. The sustainability of a “Three-in-One” biogas production system in southern China was thereby evaluated based on the proposed framework. The results show that economic costs concentrate in the construction phase. GHG emissions are mainly derived from bricks and cement, with proportions of 36.23% and 34.91%, respectively. The largest resource depletion occurs during the consumption of feedstock (87.06%) in the operation phase. Compared with other renewable energy conversion systems, the biogas project has a higher renewability (0.925) and economic return on investment ratio (6.82) and a lower GHG emission intensity (0.012). With the merit of bridging thermodynamics and externality, the extended exergy-based approach presented in this study may effectively appraise the energy and environmental performance of biogas projects. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Sustainability Household biogas Extended exergy

1. Introduction Over the past decade, China has been confronted with an energy crisis characterised by a severe resource shortage and an irrational energy consumption structure. By the end of 2009, household energy consumption in rural China was 0.52 billion tce, 33.03% of which was constituted by commercial energy (coal and electricity) and 47.87% by non-commercial energy (biomass such as straw and firewood) (Chen et al., 2010a). Biomass has been and will remain a significant source of energy in rural China. Therefore, emphasis should be placed on energy structure adjustment and the innovation and application of new biomass utilisation alternatives (such as biodiesel, biomass gasification and biogas

n

Corresponding author. Tel./fax: þ 861 058 807 368. E-mail address: [email protected] (B. Chen).

0301-4215/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enpol.2014.01.017

production) to coordinate energy consumption, environmental conditions (especially GHG emission abatement) and economic development (Chen and Chen, 2011).

1.1. Biogas system in China Biogas digesters have come to symbolise access to modern energy services in rural areas and are expected to considerably improve health and sanitation and yield significant socioeconomic and environmental benefits (Srinivasan, 2008). In China, the first test of biogas fermentation was undertaken in the 1880s in Guangdong. In 1920, rectangular hydraulic digesters were invented by Luo GuoRui in Taiwan, China (Chen et al., 2010b). Large-scale construction of household biogas digesters began in the 1950s and has experienced three development stages. In the first stage (1950–1980), development was hindered and fluctuated

J. Yang, B. Chen / Energy Policy 68 (2014) 264–272

dramatically due to technical limitations. During the 1980s to 2000, there was a steady increase in the installation of household biogas digesters, with an average growth rate of 4.6%. Since 2000, the development of household biogas has entered a new stage with the launching of the “Rural Household Biogas State Debt Project,” which focuses on providing subsidies to biogas projects to mobilise the initiatives of peasants. As shown in Fig. 1, the total investment in household biogas projects amounted to 15.63 billion yuan from 2000 to 2009. In total, 14.53 million households were subsidised, constituting 41.4% of all household biogas holders. A succession of laws and policies to guarantee the completion of the household biogas project were also issued in the third stage, e.g., the “Medium and Long-Term Development Plan for Renewable Energy in China”, which highlighted the significance of biogas as a substitute for traditional energy sources National Development and Reform Commission (NDRC) (2007). Propelled by economic and policy incentives, the number of household biogas digesters increased from 8.48 million to 35.07 million from 2000 to 2009 at a growth rate of 17.1% (Fig. 1). By 2009, the proportion of total rural biogas energy consumption had risen to 1.9%, indicating that biogas has been playing an increasingly important role in rural household energy consumption.

1.2. Sustainability evaluation of biogas projects The increasing number of household biogas digesters involves a high consumption of construction materials, large transportation distances and huge feedstock inputs, which require substantial economic input and contribute to GHG emissions. Thus, it is imperative that the relative advantages and disadvantages of resource, environmental and economic perspectives of the biogas system be widely and readily available and presented in a structured, transparent and uniform manner (Nzila et al., 2012). A wide range of research has been conducted to assess the environmental performance of biogas projects. Chevalier and Meunier (2005) asserted that the environmental impact of biogas co- or tri-generation units depended on the fraction of heat (or cold) used, the distance of crop collection, the efficiencies of the unit and NOx emissions. Börjesson and Berglund (2007) analysed the overall environmental impact generated when biogas systems were introduced and replaced various reference systems for energy generation, waste management and agricultural production. A life-cycle energy and environmental assessment for a biogas–digestate utilisation system in China was also performed by Chen et al. (2012). Regarding the resource aspect, Ishikawa et al. (2006) compared an on-farm biogas plant with a centralised one from an energetic point of view. Berglund and Börjesson (2006) assessed the energy balance in biogas production and described how the net energy output from biogas systems was affected by the raw materials digested, system design and the allocation method chosen. The energy efficiencies of different biogas systems, including single and co-digestion of multiple feedstock, different biogas utilisation pathways and waste-stream management strategies, were evaluated by Pöschl et al. (2010). The cumulative energy demand (CED) and environmental (GHG emissions) profiles associated with the production of electricity derived from biogas have been identified by Bacenetti et al. (2013) from a cradle-to-grave perspective. Meanwhile, economic analysis has also been addressed in previous research. For example, Rubab and Kandpal (1996) developed a methodology for the financial evaluation of biogas technology for domestic use in India. Rajendran et al. (2013) conducted a techno-economic evaluation and sensitivity analysis for a textile-based biogas digester and concluded that a biogas project was a positive investment, unless the price of kerosene were to fall to less than 0.18 USD/L.

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Fig. 1. Government investment and total number of household biogas digesters in China. Data source: Ministry of Agriculture (1973–2005).

Although there are different approaches for embodied energy, GHG emission or economic performance evaluation of biogas projects, most work focused on the quantification of one or two parameters. For example, Wang et al. (2013) introduced life cycle assessment (LCA) into emergy evaluation to analyse each production step of a large-scale biogas project in China and compared the economic and ecological performance of the biogas production system and biogas plus electricity production system. Yabe (2013) estimated the environmental and economic benefits of centralised biogas plants running on cow manure in Hokkaido. Rehl and Müller (2013) identified the most ecological and economical feasible anaerobic digestion method by applying different energy conversion technologies based on life cycle cost (LCC) and LCA methodologies. Chen and Chen (2013) conducted a LCA of coupling biogas production to agro-industry in terms of energy, environmental and economic performance, indicating that each production stage following the biogas/digestate utilisation chain was beneficial across all three aspects. Still, there were few integrated evaluations that reflected the overall sustainability performance of biogas projects. Jury et al. (2010) discussed the resource depletion and environmental damage of methane production by mono-fermentation of cultivated crops. Nzila et al. (2012) proposed a multi-criteria sustainability assessment (MCSA) to evaluate the sustainability of biogas technologies from environmental, economic and technical perspectives. Gosens et al. (2013) assessed the comparative contribution of household-scale biogas installations to the broad set of sustainability objectives in the Chinese biogas policy framework, which targeted household budget, fuel collection workload, forest degradation, indoor air quality and health, renewable energy supply and climate change. The inclusion of many different forms of inputs in sustainability evaluations requires that they be expressed in the same or equivalent energy form so that they may be combined and compared on an equivalent basis. Thus, a uniform accounting framework that integrates environmental and resource indicators should be developed to make the sustainability evaluation of bioenergy more intuitive and transferable. Extended exergy, one type of ecological thermodynamic theory, is suggested as an effective method for assessing sustainability via the normalisation of raw material inputs, labour and capital inputs and non-energetic externalities in the resource conversion process (Milia and Sciubba, 2006; Sciubba et al., 2008). 1.3. Extended exergy analysis The concept of exergy was first proposed by Rant in 1956 and defined as the maximum amount of work that can be produced by a system or a flow of matter or energy as it comes into equilibrium

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with a reference environment (Wall, 1977). As exergy analysis provides a rational and rigorously founded thermodynamic quantification of the consumption of natural resources, environmental losses, and ecosystem degradation, it is widely used in the fields of process optimization, resource accounting and environmental impact assessment (Chen, 2005, 2006; Chen et al., 2009; Ji and Chen, 2010; Zhang et al., 2012; Shao et al., 2013). In the evolution of exergy method, Gaggioli and Wepfer (1980), Tsatsaronis (1993, 2007) and Valero et al. (1986, 2006) proposed the theory of exergoeconomics, which integrated exergy with conventional concepts from engineering economics and was designed to evaluate and optimise the performance of energy systems. Szargut et al. (1988) extended conventional exergy analysis to cumulative exergy consumption analysis (CECA), which combined exergy analysis with LCA and accounted for the sum of the primary exergy consumed in all of the links in an energy and technological network associated with the fabrication of the considered product. As a combination of CECA and exergoeconomics analysis, extended exergy analysis (EEA), which includes externalities (capital, labour and environmental remediation) measured in homogeneous units (Joules), was proposed by Sciubba (2001) to overcome the limitations of standard exergy analysis in sustainability evaluation and optimal design. EEA extends traditional exergy analysis by including socio-economic factors such as labour and capital costs in physical terms of the equivalent primary resource consumption. Thus, such an effective measure of natural–social–environmental impacts can be considered, in some sense, the real “ecological cost” of all material and energy resources, human labour, capital and environmental remediation costs related to a certain system. This theory has been revised, refined and completed through a series of publications. To make the concept of extended exergy explicit, Sciubba (2003a, 2004) presented a detailed interpretation of the features, fundamental assumptions and calculations of extended exergy, based on which two practical applications of the EEA method were presented, and further illustrated the implication and decisive meaning of EEA in assessments of complex systems such as industrial processes and nations. Extended exergy was one step further promoted by Sciubba (2007), in which the distinction between “exergoeconomics” and “thermoeconomics” was also clarified. Sciubba (2009) claimed that extended exergy is a type of ecological indicator that overcomes the limitations caused by the incompleteness of exergy, which does not measure properties such as toxicity and non-energetic externalities. The calculation of two econometric coefficients, commonly referred to as “α” and “β”, used to calculate the extended exergy equivalents of labour and capital, respectively, were revised by Sciubba (2011). EEA is, in essence, a carefully and rigorously defined extension not of the concept of exergy but of its application to measure fluxes of different types. Compared with traditional exergy analysis, EEA is advantageous in (a) addressing environmental issues, adopting their extended meaning of the ‘impact of anthropic activities on the pre-existing environment’ (Sciubba, 2003a); (b) tackling the depletion of global resources, environmental damage and sustainability issues on a consistent basis; (c) measuring the costs of increasing sustainability; and (d) proposing and exploring scenarios of reduction and improved sustainability of long-term exergetic resource consumption. EEA has been widely applied for the sustainability assessment of socio-economic systems, e.g., Nova Scotia of Canada (Bligh and Ugursal, 2012); Norwegian (Ertesva˚g, 2005); Dutch energy sector (Ptasinski et al., 2006); Siena, Italy (Sciubba et al., 2008); Turkey (Seckin et al., 2012); and China (Zhang and Chen, 2010). In terms of energy conversion systems, Sciubba (2001, 2003b) conducted EEA analyses of a gas-turbine based cogeneration process and two waste disposal solutions to assess the relative merits of each process and

examine the optimised approach. Specifically, for renewable energy systems, Corrado et al. (2006) analysed the performance of an innovative high-efficiency steam power plant by means of two life cycle approaches: LCA and EEA. Yang et al. (2009) proposed some exergy based indicators to evaluate the renewability of the corn-ethanol process. Talens Peiró et al. (2010) assessed and compared the production of 1 t of biodiesel from used cooking oil (UCOME) and rapeseed crops (RME). Because EEA provides a feasible approach that uses the principles of conservation of mass and conservation of energy together with the second law of thermodynamics for the design and analysis of energy systems and considers both the economic properties and environmental characteristics of a system, it is considered to be a powerful tool for the sustainability analysis of household biogas production systems. In this study, we conducted a detailed cost-benefit analysis in terms of resources, economics and GHG emissions for a biogas project. To integrate the socio-economic-environmental relationships, we attempted to introduce EEA into the sustainability evaluation of the biogas project. Moreover, new extended exergy-based sustainability evaluation indicators that unified GHG emissions, economic performance and resource depletion were first proposed to shed light on biomass conversion pathway planning and as goals for possible system sustainability optimisation. The rest of this paper is organized as follows: In Section 2, a new sustainability evaluation framework is introduced. Section 3 describes the studied household biogas digester in southern China. The results of the biogas digester sustainability evaluation and comparisons with other renewable energy modes are described in Section 4. The final section of the paper summarises the main conclusions and policy implications of this study.

2. Methodology 2.1. System boundary The focus of this study was on the sustainability evaluation of a biogas project using a unified framework of extended exergy analysis. The biogas project was assessed by taking into account the exergy requirements, economic costs and GHG emissions from the production of resources (cradle) to the final end-use phase (grave), i.e., including the construction, the conversion of the feedstock into biogas (operation) and the comprehensive utilisation of biogas and by-products. The system boundaries for the biogas project consisted of raw material extraction, processing, transport and construction of an 8-m3 biogas digester, biogas production and its subsequent multiutilisation of biogas and digestate. The construction phase includes construction materials inputs for biogas digester. The operation of the biogas reactor included in the system boundary is based on the use of manure and straw as feedstock. Transportation of feedstock and digestates is also considered in the operation phase. In the comprehensive utilisation phase, the substitutions of biogas for fossil fuel, residue for chemical fertiliser and slurry for pesticide and feed are considered. 2.2. Extended exergy analysis framework EEA is an extension of traditional exergy analysis used to highlight the primary production factors, including nonmaterial energy resource elements, labour production factors and economic parameters. Thus, extended exergy bridges the ‘production value’ gap between the majority of economists and energists (Chen and Chen, 2009). The intrinsic measurement of extended exergy is the amount of primary exergy homogeneously expressed in Joules that is cumulatively used during the production, operation and

J. Yang, B. Chen / Energy Policy 68 (2014) 264–272

disposal processes of a system (Dai et al., 2012b). Extended exergy is calculated by Eq. (1). EE ¼ CExC þ Ec þ Ew þ Ee

ð1Þ

where EE is the total extended exergy input of a specific system, CExC is the cumulative exergy cost, Ec represents the exergy equivalent of monetary flow, Ew represents the exergy equivalent of human labour, and Ee is specified as GHG emission abatement costs. In this paper, extended exergy costs consist of three components: (1) standard material and energy primary resource exergy used in the lifetime of a biogas project (quantified by the respective cumulative exergy content), ((2) and (3)) labour and monetary flows (two social, economic factors) and (4) GHG emission abatement costs, which are a measurement of the GHG emission burden exerted by the artificial biogas project on the atmosphere. Moreover, the energy and economic outputs and GHG emission abatement benefits gained from the biogas project are incorporated into the EEA framework for cost-benefit analysis. The benefits/outputs of biogas projects include biogas energy output, economic profits gained by the utilisation of biogas and its coproducts, and GHG emission abatement due to the substitution of biogas for traditional biomass and fossil fuels. The consumption of exergy associated with biogas production occurs not only in the biogas fermentation process but also in the processes delivering semi-finished products and raw materials for the biogas project. Cumulative exergy analysis yields the useful energy (exergy) consumed over the life cycle of an energy system, including non-energetic raw material consumption (e.g., chemical energy from ore). The unit CExC is the MJ equivalent (MJ_eq). In this research, cumulative exergy analysis was employed to indicate resource depletion of the biogas project. The economic input for the biogas project is the total investment supporting the construction and operation of the biogas digesters, including all construction, transportation and operation fees. Moreover, owing to the multiple uses of biogas digestate, economic benefits can be quantified. Economic benefits can also be attributed to the expenses saved by the substitution of biogas for conventional energy sources and the substitution of biogas digestate for feedstock inputs. In addition, human labour should also be regarded as an economic element that guarantees the functioning of the biogas project. In terms of GHG emissions that occur throughout the lifetime of the biogas project, except for the GHG emitted directly onsite during biogas production process, embodied GHG emissions generated during the production and delivery of raw materials (used as inputs of the biogas project) are taken into consideration. A tradeoff between lifetime GHG emissions and emissions avoided by substituting for conventional energy should be made in the evaluation of the environmental performance of the biogas project.

2.3. Extended exergy-based sustainability indexes Considering the resource, economic and GHG emission implications of EEA, a series of indicators derived from this concept can be proposed to reflect the conversion efficiency, renewability, carbon emission loading, economic benefits and sustainability of a biogas project. The calculations of these indicators are presented in Eqs. (2)–(6).

2.3.1. Resource depletion The conversion efficiency, or εp, can be calculated as the ratio of the useful output to the sum of the inputs that are incurred to

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produce the output (Wall, 1977) εP ¼

∑EOj CExC

ð2Þ

where ∑EOj is the sum of useful resource outputs. Renewability (R%) is defined as the ratio of renewable exergy inputs to cumulative exergy inputs, i.e., the percentage of renewable energy that drives a process. In the long run, only processes with high R% are sustainable. R% ¼ ER =CExC

ð3Þ

where ER is the renewable exergy input and CExC is the cumulative exergy input, which is the sum of renewable exergy inputs (ER) and non-renewable exergy inputs (ENR). 2.3.2. GHG emission performance GHG emission intensity (CI) is the exergy equivalent used to remove GHG emission (generated during the lifetime of the biogas digester) from the atmosphere divided by exergy output. CI can be used as a benchmark to make tradeoffs between GHG emission and energy output. CI ¼ Ee =∑EOj

ð4Þ

The economic return on investment (EROI) can be computed as the ratio of the economic profits gained from the biogas project to the sum of the economic investments delivered to produce it: EROI ¼

Yc Ec

ð5Þ

where Yc is the economic benefit gained by the multiple utilisation of biogas digestate and the substitution of traditional energy, and Ec is the exergy equivalent of monetary inflow. 2.3.3. Extended exergy-based sustainability indicator Considering both GHG emissions and economic elements in the EEA framework for biogas projects, the sustainability indicator can be represented as the ratio of economic return on investment (EROI) and the GHG emission intensity (CI). The higher the sustainability index is, the lower the level of GHG emitted by the biogas project per unit of economic activity becomes. SI ¼ EROI=CI

ð6Þ

2.4. Data sources The material inputs and economic data used in this study were obtained partly by performing a survey of households and partly collected from the agricultural statistical book of Gongcheng for the year 2009. The lifetime of a biogas digester is 15–20 years. In this study, performance during the first 10 years, i.e., the optimal utilisation period, was evaluated. The input and output were converted to exergy units by multiplying inputs and corresponding exergy coefficients that can be derived from previous research, as listed in Table 1. The GHG emission intensities of different inputs of the biogas project are provided by literature and available databases, e.g., IPCC (2006) and Hammond and Jones (2008).

3. Description of the biogas project The “Three-in-One” biogas project is the most prevalent and representative biogas utilisation mode in southern China. With the initiation of the Rural Eco-homes project, which aims to promote rural household biogas utilisation, some provinces in southern China, e.g., Anhui, Chongqing, Hubei, Hunan and Guangxi, were selected as pilots of the “Three-in-One” biogas project. Prior to the promotion of this “Three-in-One” biogas utilisation mode, its

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Table 1 Inventory analysis of the cumulative exergy demand of a household biogas digester. Stage

Item

Quantity

Unit

Exergy coefficient

Unit

Reference

Exergy (MJ)

Construction stage

Brick Stone Sand Cement Water Pipe Plastic film Steel Transportation Manure Straw Water Leakage Transportation Transportation

1320 1 3.5 950 950 2 23 8 100 1813 94 5093 4.5 10 10

Piece m3 m3 kg kg m m2 kg km kg/year kg/year kg/year m3/year km/year km/year

1.00 0.32 0.03 4.38 0.05 32.50 32.50 7.04 1.07 12.54 17.36 0.05 52.10 1.07 1.07

MJ/kg MJ/kg MJ/kg MJ/kg MJ/kg MJ/kg MJ/kg MJ/kg J/J MJ/kg MJ/kg MJ/kg MJ/kg J/J J/J

Qi et al. (2012) Finnveden and Östlund (1997) Finnveden and Östlund (1997) Finnveden and Östlund (1997) Dincer and Rosen (2007) Chen and Qi (2007) Chen and Qi (2007) Szargut (2005) Kotas (1985) Qi et al. (2012) Bösch et al. (2012) Dincer and Rosen (2007) Bösch et al. (2012) Kotas (1985) Kotas (1985)

Biogas Biogas slurry Biogas residue

450 21784 3216

m3/year kg/year kg/year

16.07 0.08 9.27

MJ/kg MJ/kg MJ/kg

Bösch et al. (2012) Qi et al. (2012) Qi et al. (2012)

3.28E þ 03 8.00E þ02 2.97E þ 02 4.16E þ 03 4.75E þ 01 3.38E þ 00 1.03E þ 03 5.63E þ 01 1.51E þ03 2.27E þ 04 1.63E þ 03 2.55E þ 02 3.02E þ 02 1.51E þ02 1.51E þ02 2.63E þ 05 9.33Eþ03 1.68E þ 03 2.98E þ 04 1.83E þ 05

Operation stage

Utilization stage Cumulative exergy input (10 years) Yield

Total yield (10 years)

used to fuel the biogas production process. The biogas digester should be refuelled once or twice per year. Except for regular refuelling every year, the fermentation feedstock should be frequently charged and discharged during routine maintenance to sustain the metabolism of methanogens. Gasoline is consumed for the transportation of fuels for the biogas digesters. To decrease transportation costs, the utilisation of local resources is given priority. In the comprehensive utilisation phase, biogas is combusted as a substitute for traditional coal and straw, whereas the biogas digestates are used for multiple purposes, including as breeding feed and organic fertiliser and for soaking seeds. Fig. 2. The “Three-in-One” biogas production system.

4. Results and discussion socio-economic-environmental impacts and sustainability should be analysed to shed light on the large-scale application of the biogas systems. The sustainability of a typical “Three-in-One” biogas project, which links a toilet, pigpen and plantation, was evaluated in this study. As shown in Fig. 2, in such a system, human and animal manure is the feedstock of biogas fermentation. Biogas generated in the digester is delivered to households for combustion. The co-products of fermentation, i.e., biogas digestate, can be used to fertilise grains and fruit trees. The construction of a biogas digester is the core of this system. In consideration of managerial convenience, gas generation rate and temperature, the biogas digester should be built to be round in shape and shallow and should exhibit a small capacity. In this study, the 8-m3 hydraulic pressure biogas digester that is commonly used in southern China was investigated. The main components of such a biogas digester include a feeding pipe, fermentation room, biogas storage room, hydraulic room and gas pipe, which are listed in detail in Table 1. The lifetime of the biogas production system includes three main stages, i.e., construction, operation and comprehensive utilisation (Fig. 3). These steps are further detailed in the life cycle inventory. In the construction phase, energy embodied in construction materials is invested in the biogas project. GHG emissions accompanied by the provision of construction materials are generated and discharged into the atmosphere. Due to the purchase of construction materials and payment for construction labour, an economic input is delivered in this phase. In the operation phase, feedstock, such as straw and animal manure, is

4.1. Resource depletion Table 1 summarizes the cumulative exergy inputs required to operate a household biogas digester. Over a 10-year lifespan, the cumulative exergy demand of a household biogas digester is 2.63E þ05 MJ, whereas the exergetic output, which involves biogas, biogas slurry and biogas residue, is calculated to be 1.83E þ05 MJ. If biogas is only used for combustion, the conversion efficiency is 35.40%. When the biogas digestates (biogas slurry and residue) are fully used, the conversion efficiency can reach 69.39%, indicating the importance of using biogas digestates to improve resource consumption efficiency. Various inflows were taken into account and weighted by multiplying by their respective conversion factors. Thus, an exergy signature can be drawn, as shown in Fig. 4. It can be observed that the feedstock (manure and straw) necessary for biogas production constitutes the largest proportion of total exergy input, followed by the resources exhausted during the construction of the biogas digester, such as brick, cement and transportation. 4.2. Economic performance Economic costs are incurred throughout the lifetime of the biogas digester, which can be divided into the construction stage, the operation stage and the utilisation stage. All expenses supporting the operation of the biogas digester are included. As an ecological engineering, the biogas project is assumed to bring about important economic benefits, e.g., biogas use can substitute

J. Yang, B. Chen / Energy Policy 68 (2014) 264–272

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Fig. 3. Scheme of the evaluation of the household biogas project.

Fig. 4. Cumulative exergy inputs of the biogas project.

for traditional energy consumption and reduce energy expenditures, biogas digestate can be used as organic fertiliser to reduce chemical and pesticide consumption in planting systems and biogas slurry can be mixed with feed to raise pigs. To investigate the economic performance of the biogas project, the economic inputs in the construction and operation stages and the economic benefits in the utilisation stage were converted to present values under a discount rate of 8%. Considering the biogas digester construction specifications and biogas production capacity, the economic costs and benefits of a biogas digester are counted and summarized in Table 2. The total economic costs are calculated to be 4.15Eþ03 yuan, 52.41%, 34.91% and 12.68% of which are constituted by construction costs, operation costs and utilisation costs, respectively. The large investment in biogas digester construction precludes farmers' enthusiasm in promoting the biogas project. The direct economic benefits for a household that participated in a biogas project is 2.83Eþ04 yuan, which is 6.82 times the investment, indicating that biogas projects are economically profitable and can contribute to improving rural quality of life. 4.3. GHG emissions The GHG emissions due to the construction, operation and utilisation phases are calculated and depicted in Fig. 5. When the GHG emissions embodied in building materials are accounted for from a life-cycle perspective, the construction stage contributes

the largest proportion of GHG emissions (87.43%), among which bricks and cement are the two largest contributors (Fig. 5). The GHG emissions in the operation stage can be attributed to biogas leakage and transportation in the refuelling process. The GHG emissions in the operation stage amount to 186.5 kgCO2-eq, constituting only 8.26% of total emissions. This finding indicates that biogas is a clean energy and has great potential to serve as a substitute for traditional energy in GHG mitigation. In the utilisation phase, the transportation of biogas digestate for multiple uses is also considered. The transportation of the digestate occupies the smallest proportion of inputs in this phase at 4.32%. Of the material inputs of the biogas project, the largest three emitters are brick, cement and transportation, which can be targeted in the low carbon management of biogas projects. As a substitute for traditional fossil fuels, biogas can avoid the GHG emissions associated with coal and direct biomass combustion. The emissions avoided by using biogas are presented in Table 3. Based on the energy structure of the rural areas in South China, the household biogas yield of 450 m3 can replace 700 kg of coal combustion, increase annual forest protection by 2700 m3 and save 1500 kg of straw that would have been used for direct combustion (Dai et al., 2012a). In Table 3, a negative value indicates carbon release and a positive value indicates carbon reduction. Assuming that the average forestry storage is 90 m3/ha and the density of wood is 1.54 kg/m3 (State Forestry Administration of China, 2010), biogas is carbon-reducing, with 3.95 t GHG avoided per household. From a life cycle perspective, the reduction in carbon emissions over 10 years by biogas utilisation is 1.74 times the amount of GHG emitted by the biogas project, demonstrating that biogas plays an important role in the mitigation of GHG concentrations. 4.4. Sustainability indicators The extended exergy-based sustainability indicators that reflect the sustainable level of the biogas project and comparisons with other renewable energy systems are presented in Table 4. The εp of the biogas project is 0.69 MJ/MJ. It can therefore be argued that 31% of exergy contents are exhausted during the biogas production process. To maximise the conversion efficiency of the biogas project, it is necessary to enhance routine maintenance by adjusting fermentation temperatures and feedstock configurations. Compared with other renewable energy systems, the εp of the biogas project is similar to that of biodiesel production but much

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Table 2 Inventory of the economic inputs of a household biogas digester. Stage

Item

Quantity

Unit

Price per unit

Construction stage

Brick Stone Sand Cement Water Pipe Plastic film Steel Transportation Labor Labor Water Transportation Transportation Fuel substitution Chemical fertilizer reduction Pesticide reduction Feed saving

1320 1 3.5 950 950 2 23 8 100

piece m3 m3 kg kg m m2 kg km

0.35 70 50 400 1.50 20 0.74 3 8.06

5093 10 10

kg/year Km/year Km/year

Operation stage

Utilization stage

Economic value (yuan) yuan/piece yuan/m3 yuan/m3 yuan/t yuan/m3 yuan/m yuan/m2 yuan/kg yuan/L

yuan/m3 yuan/L yuan/L

1.50 8.06 8.06

 4.62E þ02  7.00E þ01  1.75E þ02  3.80E þ02  1.43E þ00  4.00E þ01  1.70Eþ01  2.40E þ01  3.62E þ02  5.12E þ02  8.11E þ02  1.11E þ02  5.25E þ02  5.25E þ02 1.07Eþ04 1.45E þ04 2.90E þ03 2.56E þ02

Table 4 Comparison of different renewable energy utilisation projects based on extended exergy-based sustainability evaluation indicators. Indicators Biogas project

Wind farma

εP R% ERIO CI SI

0.028 0.751 0.973 0.777 2.800 1.380 0.023 0.006 121.739 247.660

a b

Fig. 5. Lifetime GHG emissions of the biogas project.

Table 3 GHG emission reduction potential of each biogas digester per year. Item

Value

GHG emissions or reductions (t)

Substitution of coal Substitution of firewood Substitution of straw Biogas yield Total

700 kg 352 kg 1500 kg 450 m3 –

þ 1.74 þ 0.66 þ 2.60  1.05 þ 3.95

higher than that of a wind farm, indicating that the biogas system has a relatively high technical efficiency. The R% of the biogas project is 92.50%, which is higher than the values of the other two biodiesel production systems. This high R%

0.69 0.925 6.820 0.012 568.333

Biodiesel from used cooking oilb

Biodiesel from rapeseed cropsb 0.516 0.529 5.824 0.062 94.203

Yang et al. (2012). Talens Peiró et al. (2010).

occurs because although some nonrenewable exergy inputs, such as construction materials and energy, flow into the biogas production system, these nonrenewable exergy inputs only constitute a proportion of 7.50%, illustrating that the biogas project is competitive in terms of renewability. The EROI is used to evaluate the economic efficiency of the biogas project. As shown in Table 4, the EROI of the biogas project (6.82) is much higher compared with the values of other renewable energy generation systems, such as wind power and biodiesel generated from cooking oil and rapeseed crops. The biogas production therefore creates prominent economic benefits via the comprehensive utilisation of biogas and its byproducts. CI can be considered a measure of global warming stress exerted by biogas production. The CI of the biogas project is lower than the values of the wind power generation system and biodiesel generation from rapeseed crops but higher than that of the biodiesel extracted from used cooking oil. Thus, biogas project is one of the promising renewable options for alleviating global warming stress due to the consumption of traditional fossil fuels. SI is determined by both EROI and CI. The SI value of the biogas project is higher than the values of the wind power generation system and the biodiesel production systems, due to its higher EROI and lower CI. We might expect better performance from the biogas project by improving the economic conversion efficiency as well as alleviating the environmental costs of the biogas project.

5. Conclusions In this paper, extended exergy was first introduced as a benchmark to evaluate the sustainability of a biogas production

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system by providing a unified measurement for material, energy, economic and GHG flows in different stages of a biogas digester's lifetime. As an extension of current EEA, indicators that reflect systematic performance were also proposed. These unitary thermodynamic indicators may be useful in making comparisons between biogas projects and other renewable utilisation pathways as this approach pays much attention to the relationships and integrated performance of available energy, environmental impacts and economic features. A biogas project is effective in transforming wastes (straw, manure) into useful substances (fuel, feedstock and fertiliser). To further improve the conversion efficiency, routine maintenance should be enhanced. In addition to adjusting the C:N ratio, fermentation temperature and pH value in the fermentation pool, which are the key factors that affect biogas productivity, the pretreatment of feedstock (e.g., straw pulverisation) and stirring during fermentation are effective measures for enlarging the contact surface between microorganisms and feedstock, allowing for the acceleration of the fermentation rate and thereby an increase in gas yield. The biogas production process is found to be carbon-reducing and competitive compared with other biomass utilisation methods, as can be seen in Table 3. Thus, the promotion of biogas in rural areas is consistent with the national objectives of a “lowcarbon society” and “sustainable rural development.” Moreover, the reductions in GHG emissions can finance the project; for example, Certified Emission Reductions (CERs) can be sold on the emissions trading market by participating in CDM programs. Therefore, additional revenue can be gained to compensate for the high biogas maintenance cost. The government should also take measures such as offering tax incentives to support the development of CDM programs for biogas projects. According to the results of economic analysis, although the biogas project is economically profitable, with a high economic return on investment ratio, the large investment in biogas digester construction precludes the enthusiasm of farmers in promoting biogas projects. The current prices of cement and brick, which are the main materials for biogas digester production, are more than twice prices observed in 2004, whereas the size of government subsidies has remained unchanged. The sharp increase in economic cost has aggravated the burden on poor families. Therefore, government subsidies for biogas digesters should be further promoted. In addition, an incentive mechanism may be established to attract more investments from enterprises to stimulate the industrialisation of biogas projects. Another approach that can motivate farmers' initiative in utilising biogas is the multiple uses of biogas and its coproducts. In addition to decreasing fuel costs by using biogas as a substitute for traditional fossil fuels, the utilisation of biogas residue as organic fertiliser and the use of biogas slurry as breeding feedstock, a seed-soaking material and a pesticide substitute should be promoted. In addition, the backward infrastructure and service system in many areas hinders the development of biogas projects. As subsidies are directly granted to households, no expenditure is set aside for service stations, which prevents the required technical guidance and training from being implemented. Consequently, management and service cannot catch up with the construction of biogas digesters, and biogas digesters cannot be properly maintained. As a result, many biogas digesters are abandoned, making the biogas project unsustainable. Therefore, the government should pay attention to the subsidy of service stations. For example, if the subsidy is improved from 1500 yuan/household to 2050 yuan/ household, 10% of the subsidy should be assigned for the construction of infrastructure and a service system. Moreover, the costs of biogas projects will be further lowered with the implementation of

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the “building materials to the countryside” policy launched by the China Building Materials Federation of Industry and Information Technology Ministry, which is subsidised by the government and aims to encourage the construction of energy-saving projects and housing in rural China.

Acknowledgements This study was supported by the Fund for Creative Research Groups of the National Natural Science Foundation of China (No.51121003), Specialized Research Fund for the Doctoral Program of Higher Education (20130003110027), National Natural Science Foundation of China (No. 41271543), and National Key Technology Research and Development Program (No. 2012BAK30B03).

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