Extended exergy accounting applied to biodiesel production

Energy 35 (2010) 2861e2869

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Extended exergy accounting applied to biodiesel production L. Talens Peiró a, *, G. Villalba Méndez a, b, E. Sciubba c, X. Gabarrell i Durany a, b a

SosteniPrA (UAB-IRTA), Institute of Environmental Science and Technology (ICTA), Edifici Q-ETSE, Room QC 3101, Universitat Autònoma de Barcelona (UAB), E-08193 Bellaterra (Cerdanyola del Vallès), Barcelona, Spain b Department of Chemical Engineering, Edifici Q, Universitat Autònoma de Barcelona (UAB), E- 08193, Bellaterra (Cerdanyola del Vallès), Barcelona, Spain c Department of Mechanical and Aeronautical Engineering, University of Roma 1 “La Sapienza”, Via Eudossiana 18, I-00184 Roma, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 October 2009 Received in revised form 9 March 2010 Accepted 9 March 2010 Available online 10 April 2010

When evaluating the production of renewable energies such as biofuels, it is necessary to include in the assessment the resource inputs, capital, labor investment and environmental remediation costs. Extended Exergy Accounting (EEA) is a system analysis method that calculates, on the basis of detailed mass and exergy balances, the total amount of primary exergy resources necessary to obtain a product or service. The conceptual novelty of EEA is represented by the fact that it also includes externalities (capital, labor and environmental impact) measured in homogeneous units (Joules). As an illustration of EEA, we assess and compare the production of 1 ton of biodiesel from used cooking oil (UCOME) and rapeseed crops (RME). The extended exergy “content” of UCOME and RME are 51.90 GJ and 77.05 GJ respectively. The production of UCOME uses 25.15 GJ less resources (materials and energy) and requires lower total investments and environmental remediation costs than that of RME. On the other hand, UCOME requires 35% more workhours. In summary, the extended exergy of UCOME is about 1.5 the extended exergy content of RME. Thus, we can conclude that biodiesel production from UCO is less resource use intensive than the production from RME. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Exergy analysis Biodiesel Used cooking oil Rapeseed

1. Introduction Energy supply and energy use are key issues in our present society. In the European Union, 30% of the energy supply is used in road transport [1], which is mainly based on liquid fuel running engines: thus, an efficient and environmentally benign production of liquid biofuels plays a relevant role in the improvement of the level of sustainability of the transport sector and energy efficiency of our society [2,3]. Responsible use of biofuels should address the issues of resource availability and use, economic investment and environmental impact. Present studies generally discuss these issues separately and measure them in different units. Resource use (materials and energy) and environmental impact studies are accounted for in physical or equivalent units (tons, Joules, kg equivalent of CO2, etc.) whereas economic investments are accounted for in monetary units (euro or dollar) [4]. Integrated assessment tools fail in integrating the analytical results into an aggregated indicator of the physical and economical inputs/outputs functional relation.

* Corresponding author. Tel.: þ34 93 581 3760; fax: þ93 586 8008. E-mail address: [email protected] (L. Talens Peiró). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.03.015

Extended Exergy Accounting (EEA) is a system analysis method that adopts a single quantifier (exergy, expressed in Joules) to account for materials, energy, labor and capital and to compute a presumed environmental impact based on remediation costs. Exergy is defined as the maximum amount of useful work obtained when a system is brought to equilibrium with its surroundings through a series of reversible processes, in which the system interacts only with its reference environment [5,6]. Exergy is based (and computed by) both laws of thermodynamics: energy conservation (first law) and entropy generation (second law). Exergy accounts for the energy embodied in the system plus the energy loss due to the generation of entropy in the system, so it can be rightly said to express the work potential of a system. Exergy is widely used in engineering to identify and locate losses and to account for efficiencies and wastes in the thermo-mechanical and chemical energy conversion systems [7e10]. In recent years, exergy is being increasingly employed in environmental, thermoeconomic, and even sustainability analyses of industrial systems [11]. Exergy accounting is used to study the material and energy flows representing the production and resource use patterns of countries [12e16]. Its application leads to a better understanding of the relation between economic sectors and to a more accurate calculation of resource use and efficiency. In 1998, one of the authors (E. Sciubba) proposed to include in Exergy Accounting properly

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Nomenclature CExC CO2 efs;0 Ein;Society EEA eek eeL EK EL HDI HDI0

cumulative exergy consumption, MJ/ton carbon dioxide exergy use for mere survival, 1.05  107 J/person$day total exergy flow, MJ extended exergy accounting exergetic equivalent of capital, MJ/V exergetic equivalent of labor, MJ/hour exergy of capital, MJ exergy of labor, MJ Human Development Index Human Development Index of a primitive society, 0.055

computed equivalent exergetic values for labor and monetary flows, originating the so-called Extended Exergy Accounting (EEA) [17]. EEA is based on the standard economic convention that the cost of a product is given by the sum of capital, labor, and fuel, plus costs associated with the environmental effects of effluents and pollutants, and on the novel assumption that the non-energetic inputs can be made homogeneous with the energetic ones. This paper applies the EEA method to compare two diesel fuel alternatives: biodiesel from used cooking oil (UCOME) and biodiesel from rapeseed crops (RME). Even though the number of stages involved in each life cycle is different, each biodiesel competes in the market at the same price and tax reduction. Thus, it is interesting to evaluate the total resource use of each biodiesel to know if recycling UCO has further benefits apart from reducing a waste stream. The results from EEA give an aggregated value of the total exergy required for producing biodiesels: this quantifier, called extended exergy content or extended exergy “cost”, represents the primary exergy globally embodied in the product, and therefore constitutes a convenient and uniform basis of comparison for the two different final products. 2. Methodological issues In this study, EEA is used to account for the total amount of exergy resources used in the biodiesel produced from UCO and rapeseed crops in Catalonia (Southern Europe). The method requires a highly disaggregated analysis of the mass and exergy fluxes within a Society and “builds up” the “extended exergy” content of a product/service by adding up successive contributions to its formation process. The exergy of a product is the sum of the exergy content of all materials and energy fluxes required for its production [5]. As stated above, EEA also includes Labor, Capital and Environmental impact. These are incorporated in the global exergetic budget of the process by calculating the equivalent exergy content of the workhours generated in the Society and the monetary circulation therein [18]. Thus, the “extended exergy cost” of a commodity is expressed in kJ/unit (kg, kJ, unit service) and represents the cradle-to-grave exergetic burden that the Society must bear to produce that commodity (product or service). Exergetic equivalents are calculated for monetary flows, labor and environmental remediation expenses using published socioeconomical, industrial cost data and the environmental legislation of Catalonia [19,20]. 2.1. Material and energy fluxes The material input to a system is calculated using the Cumulative Exergy Consumption (CExC). The CExC is the sum of the

global monetary indicator, V/year M2 number of inhabitants, persons Nh number of workers, persons Nw Nworkhours cumulative number of work-hours in a year, hour RME biodiesel from rapeseed oil s salary, V/hour UCOME biodiesel from used cooking oil wh number of work-hours, hour Greek letters numerical factor numerical factor

a b

embodied exergy of the material and the exergy of all the materials and energy a commodity “consumes” during its lifetime cycle. In other terms, it measures the cradle-to-grave resource use of a commodity. The accounting for the exergy “consumed” in the lifespan of a product begins at the final stage (end of life, including recycling and disposal) and goes back through the processes of producing semi-finished products, down to the extraction of raw materials from natural resources required for their production. Szargut et al. published CExC values for a number of chemicals and fuels [5]. In all cases in which such values are not available, it is necessary to have data of the composition of the inputs supplied for its production [21]. The exergy embodied in a material is calculated using the reference state defined by Szargut et al. that considers a reference temperature (298 K), pressure (1 atm) and an average conventional composition of the Earth’s litho-, hydro- and atmosphere. For most organic compounds exergy values are though not available. In such cases, exergy is calculated here by adding group contributions of standard chemical exergy data or by using compositional data and lower heating values, as suggested by Styryslka and Szargut [5]. Electronic annex 1 shows how exergy and CExC of inputs are calculated. The energy input to a system is the sum of the energy fluxes in each stage of the system. Energy is usually supplied as solar irradiation (direct and indirect), electricity, natural gas and diesel. Electricity is produced from coal, natural gas, hydroelectric and petroleum and smaller amounts from solar, wind power and biomass. For electrical energy, the exergy content is equal to the energy content. Thus, 1 MJ of electrical energy equals 1 MJ of exergy [15,22]. For natural gas and diesel, the exergy is calculated using exergy factors estimated using composition and the lower heating values, as proposed by Szargut [5]. The exergy factors for natural gas and diesel are 1.04 and 1.07 respectively [23]. 2.2. Externalities: labor, capital and environmental impact Including externalities is especially important when evaluating renewable energies. All existing methods calculate the “cost” balance by quantifying energy and material resources in terms of either a physical (mass, energy or exergy) or a monetary proxy, but fail to include capital, labor and environmental costs in a homogeneous and coherent way. Presently, there are many technologies available to produce alternative energy sources, and in some cases is not clear whether they are environmentally feasible [24,25]. EEA accounts for these “externalities” in terms of exergy [18]. The energy and material fluxes (incoming resources) into a society can be computed as the exergy resource input into society (Ein,Society), defined as the sum of the materials and energy resources, the solar radiation plus the exergy imported to the

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area/country under study [14,19]. Electronic annex 2 includes a detailed list the inputs accounted for as materials and energy resources to Catalonia in 2006 [26,27]. Sciubba proposed a method to compute the exergetic equivalent for labor, capital and environmental remediation costs [18]. The exergetic equivalent for labor is defined as the exergy use in one work-hour (eeL) and is calculated as the ratio of the exergy input of that society to the total number of work-hours it supports.

eeL ¼

a  Ein;Society Nworkhours

(1)

where eeL, exergetic equivalent of labor, MJ/hour; a, numerical factor; Ein;Society , total exergy flow, MJ; Nworkhours , cumulative number of work-hours in a year, hour. The exergetic equivalent of capital is defined as the exergy use in one monetary unit (eeK) and is calculated by dividing the total exergy flux that sustains Capital circulation in the society by a proper indicator of cumulative monetary circulation maintained in the period of time.

eeK ¼

a  b  Ein;Society M2

(2)

where eeK, exergetic equivalent of capital, MJ/V; a, numerical factor; b, numerical factor; Ein;Society , total exergy flow, MJ; M2, global monetary indicator, V/year. a and b are numerical factors that depend on the type of societal organization, the historical period, the technological level and the geographic location of the society. The numerical factor a is calculated by using the following equation:

a ¼

365Nh HDIefs;0 EL ¼ Ein;Society HDI0 Ein;Society

(3)

where EL, exergy of labor, MJ; Ein;Society , total exergy flow, MJ; Nh, number of inhabitants, persons; HDI, Human Development Index [28]; efs;0 , exergy use for mere survival, 1.05  107 J/person$day [29]; HDI0, Human Development Index of a primitive society, 0.055. The numerical factor b is calculated using equation (4):

b¼

EK

aEin;Society

¼

M2 Nw whs

(4)

where EK, exergy of capital, MJ; Ein;Society , total exergy flow, MJ; M2, global monetary indicator, V/year; Nw, number of workers, persons; wh, number of work-hours, hour; s, salary, V/hour. Environmental remediation costs are calculated as the monetary cost companies pay to manage and treat the wastes they generate. Waste management costs include direct and indirect cost of the treating facility (in Euros). The extended exergy of labor (EEL) is calculated based on the exergetic equivalent eeL and the total workhours required for the system. The extended exergy of capital (EEK) and environmental remediation (EEEnv) are calculated using the exergetic equivalent eeK and the economic investment and environmental remediation costs that the system under study requires. The extended exergy of a commodity is the sum of its CExC plus the additional extended exergy calculated above. Electronic annex 3 includes explanatory notes showing how numerical factors (a and b) and exergetic equivalent are calculated for several countries. Fig. 1 illustrates the EEA of a final product. As shown, EEA is the sum of materials, energies, labor, capital and environmental remediation expenses, all expressed in homogeneous terms (Joules/kg). The exergy of materials is measured by the respective Cumulative Exergy Consumption (CExC). Labor, capital and environmental impact depend on the exergy flow in society, the

Fig. 1. Details of the items accounted for CExC and EEA of Catalonia.

monetary circulation (M2), the yearly work-hours and the above mentioned econometric factors. Labor, capital and environmental impact are accounted for as a function of the inputs to the area/ country under study, in our case Catalonia. 3. Case studies 3.1. Used cooking oil methyl ester (UCOME) For calculating the extended exergy of UCOME, we need disaggregated data of material and energy inputs, capital and labor and additional information about the remediation costs linked to its environmental impact. Material and energy inputs are quantified based on data presented by Talens Peiró et al., and referred to a 2-step transesterification process [30e32]. The available balances are somewhat approximate; the global mass balance shows a discrepancy of about 3.5%. The production of biodiesel from used cooking oil (UCO) includes the collection of UCO, the pre-treatment

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and the transesterification, also the transport stages involved in delivering the oil to the pre-treatment and production facility (as described by real life biodiesel producers in Catalonia). UCO is a waste flow with zero investment cost as defined by the common accounting practices (both monetary and thermoeconomic). As a result, the production of fresh vegetable oil before its use in cooking is not considered in this study. The inputs are UCO, chemicals (methanol, sodium hydroxide and sulphuric acid), utilities (electricity, natural gas, fuels) and water. Outputs are UCOME, glycerol and fertiliser (potassium sulphate). The production of 1 ton of UCOME requires 1.17 ton of UCO and produces 190 kg of process glycerol and 22 kg of fertiliser as by-products. Fig. 2 illustrates the exergy inputs required for production of UCOME from used cooking oil. Economic data is obtained from companies (COPREOIL and BIOCAT) located in Catalonia. The number of vans required for the production of UCOME is calculated based on the amount of UCO collected per van annually [31]. The cost of the van is calculated including the fixed costs (obsolescence, interests, insurance and

warehouse) and also the variable costs (maintenance) [33,34]. An operational lifetime of 5 years and 1875 working hours per year are assumed. The total cost of a van is 1.57 V per workhour. The investment required for the pre-treatment and transesterification facilities is also based on data provided by COPREOIL and BIOCAT. Costs include direct cost (purchase of equipment cost and installation, piping, instrumentation and control, civil work, electrical equipment and materials and service facilities), indirect costs (construction and contingencies) plus maintenance [35,36]. The land cost for the UCOME system is also included in capital costs based on the price per hectare in industrial parks in Catalonia [37]. Labor is calculated based on the personnel needed for the collection and biodiesel facility. For the collection of UCO, labor is calculated based on the number of vans calculated in the previous section [30]. At the pre-treatment facility, 3 workers (including 1 supervisor) per shift and 1 shift per day are required. The biodiesel facility requires 2 workers (including 1 supervisor) per shift and 3 shifts per day. Table 1 shows the total investment cost and workhours necessary for producing 30,000 tons of UCOME.

Fig. 2. Exergy inputs and outputs in the production of 1 tonne of UCOME.

L. Talens Peiró et al. / Energy 35 (2010) 2861e2869 Table 1 Total investment cost and work-hours for the production of 30,000 tons of UCOME in Catalonia. Total investment cost (V) Work-hours (hours) Collection of UCO 412,000 Pretreatment of UCO 1,260,000 Transesterification to UCOME 15,300,000 3,060,000 Land usea

217,573 5627 13,130 e

Total

236,330

20,032,000

a

Land use is calculated based on the cost of industrial land and the land presently used by local companies. One hectare of industrial land costs 165,000 V [36]. The pretreatment facility requires 13.87 ha (includes a parking for the collecting vans) and the transesterification requires 4.68 ha.

The environmental impact of the system is calculated based on the cost of emissions, the wastewater treatment and organic waste management. When companies do not give details of the emissions generated in their processes, they are calculated together as exergy loss by an exergy balance of the process [31,38]. In this paper, emissions of UCOME are calculated based on the emissions generated by vans in the collection stage and heavy duty vehicles in the delivery of UCO to the biodiesel facility. The cost for emission capture and storage in open air is still under development [37]. Due to this limitation, the environmental cost of emissions is estimated using the current price of CO2 in the market [39]. Electronic annex 4 shows the emissions generated by the UCOME system. The cleaning of the containers at the pretreament stage generates wastewater. Such wastewater contains oils which can exist in different forms depending on the drop size and also atmospheric conditions: free (up to 150 mm), dispersed (20e150 mm) or emulsified (less than 20 mm) [40]. In the pre-treatment facility used oil exists as emulsified, thus primary and secondary treatments are required. When accounting for the cost of treating oily wastewater, only the cost of secondary treatment (Ultrafiltration process) is taken into account since there is no published data on the cost of the primary treatment for oils. At the pretreament, organic waste generally food waste is also separated and sent to landfill. Presently, the cost of landfill is 10 V per ton [41], however the price is expected to rise due to European Union directives seeking to minimise the waste to landfill and increase strategies for energy recovery. The European Union suggests technologies such as biochemical and thermochemical conversions to recover energy and reduce waste [2]. Such possibilities are further studied in the results and discussion section. Table 2 summarises the environmental costs of emissions, wastewater treatment and organic waste management for the production of 30,000 tons of UCOME. 3.2. Rapeseed methyl ester (RME) As explained in section 2, the aspects considered in the extended exergy assessment are the material and energy flows plus Table 2 Amount of wastes generated and environmental costs of the production 30,000 tons of UCOME.

CO2 emissions Wastewatera Organic waste Total a

Amount generated (tons/ton UCOME)

Environmental remediation costs (V)

0.01 0.25 0.11

4900 100,700 32,000 137,600

The cost of treating wastewater by ultrafiltration is $2.65 m3 [39]. The future value is calculated using the methodology proposed by Bejan et al. and assuming a yearly interest increase of 5%. The calculated cost is 13.42 V/m3 [34].

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externalities: labor, capital and environmental costs. Material and energy inputs are taken from references [42,43]. The inputs to the production system are seeds, fertiliser (nitroammonium sulphate and Nitrogen Phosphorous Sodium fertiliser), insecticides (chlorpyrifos), pesticide (trifluraline) and fuel consumption for the operation of agricultural machinery and transport. The outputs from this stage are rapeseeds and agricultural waste (straw). Rapeseeds are input to the production stage, which has four stages extraction, pre-treatment, delivery and transesterification. The inputs are chemicals (hexane, bentonite, phosphoric acid, caustic soda, nitric acid, aluminium sulphate, nitroammonium sulphate, methanol, hydrochloric acid, ammonium nitrate, ethylene glycol and vitamin D), utilities (electricity, natural gas, fuels) and water. Outputs are RME, glycerol and agricultural waste from rapeseeds. The production of 1 ton of RME requires 3.20 tons of rapeseeds and produces 96 kg glycerol. Fig. 3 summarises the exergy inputs required for the production of RME. The input from solar radiation to the crops is calculated using the average solar radiation in Catalonia and the period of time estimated to grow the crops [44,45]. Rapeseed oil is used as an input for biodiesel production. Thus for accounting for the total resource use for RME, it is necessary to also include rapeseed oil as an input to the system. Economic data for the crop production are based on 1000 workhours per year [33]. Electronic annex 5 includes the cost per hour for all the machinery required for the crop production [43]. For the agricultural stage, the total investment cost is 77 V per ha, such cost includes the fixed costs (obsolescence, interests, insurance and warehouse) and also the variable costs (wear and reparation and maintenance). Economic data for the extraction, pre-treatment and transesterification facilities is calculated based on the purchase equipment cost and using the methodology proposed by Bejan et al. to calculate the total capital investment [35,46]. The total investment cost includes direct cost (purchase of equipment cost and installation, piping, instrumentation and control, civil work, electrical equipment and materials and service facilities), indirect costs (construction and contingencies) plus maintenance. The cost of land is calculated separately based on the price of the agricultural land for dry farming and the cost for industrial parks in Catalonia [20,37]. Labor is calculated separately for the crop production and the transesterification of RME. Labor for the crop production is calculated based on the work-hours of the machinery used per ha [43]. Labor for the extraction, pre-treatment and transesterification is calculated using data provided by Westfalia AG, an experienced company building biodiesel facilities in Spain [46]. The extraction and pre-treatment of rapeseed oil is done in the same facility and require 2 workers per shift (including one supervisor) and 3 shifts per day. The transesterification facility requires 3 workers (including one supervisor) per shift and 3 shifts per day [36]. Table 3 summarises the total investment cost and work-hours necessary for producing 30,000 tons of RME. Environmental remediation costs of RME are calculated based on the costs of emissions, wastewater treatment and agricultural waste management. Emissions to air are only estimated based on the amounts generated during crop production and transport stages (as done in the UCOME system). Emissions are generated when nitrogen (NPK and nitroammonia sulphate) is added to crops. The factors to consider when accounting for emissions are the ammonium volatilization, nitrous oxide, oxides of nitrogen and nutrient leaching. Such emissions are calculated using the emission factors proposed by several authors [43,47,48]. Emissions due to transport are accounted for by using data published for heavy duty vehicles [49] and the current price of CO2 companies are paying [39]. Electronic annex 4 shows the emissions generated by the RME system. Emissions to soil are not included due to the

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Fig. 3. Exergy inputs and outputs in the production of 1 tonne of RME.

lack of information on the composition of commercial synthetic fertilisers [47]. Wastewater is generated during the extraction of rapeseed oil at the extraction stage. The cost of wastewater treatment only includes the ultrafiltration cost, as done for the UCOME [40]. Agricultural waste is generated during the crop production and the extraction of rapeseed oil. Agricultural waste is sent to landfill at the cost of 10 V per ton [41]. Table 4 summarises the environmental remediation costs for producing 30,000 tons of RME.

Table 3 Total investment cost and work-hours for the production of 30,000 tons of RME in Catalonia. Total investment cost (V)

Work-hours (hours)

Crop production Extraction and pre-treatment Transesterification to RME Land usea

2,224,000 2,343,000 16,400,000 4,100,000

133,330 7500 13,130 e

Total

25,067,000

153,960

a

Land use is calculated based on the cost of industrial land and the land presently used by local companies. One hectare of industrial land costs 165,000 V [36]. The pretreatment facility requires 13.87 ha (includes a parking for the collecting vans) and the transesterification requires 4.68 ha.

4. Results and discussion 4.1. EEA of UCOME and RME A comparison based on the extended exergy of UCOME and RME is performed to identify which biodiesel production process requires the lower resource input (including externalities). The major exergy input to RME is solar radiation and is not included for comparing with UCO since it is so large that belittles the total exergy input by materials and energy, as referred by other authors [50]. Rapeseed oil is an input for the production of RME and

Table 4 Environmental costs of the production 30,000 tons of RME. Amount (tons/ton of RME) Equivalent CO2 emission Wastewatera Organic waste Total a

Environmental remediation costs (V)

0.03

11,330

2.11 1.54

850,000 462,000 1,323,330

The cost of treating wastewater by ultrafiltration is $2.65 m3 [39]. The future value is calculated using the methodology proposed by Bejan et al. and assuming a yearly interest increase of 5%. The calculated cost is 13.42 V/m3 [34].

L. Talens Peiró et al. / Energy 35 (2010) 2861e2869

therefore its exergy is included in total exergy input accounting. The exergy inputs for UCOME and RME are 47.77 GJ/ton and 77.05 GJ/ton respectively. Table 5 summarises the exergy inputs from the oils, the materials and energies. The exergy embodied in rapeseed oil is greater than the exergy of UCO due to the higher content of oleic and linoleic acids in rapeseeds [51]. The exergy input of materials is 2.5 times higher for RME, as shown in Table 5. The major exergy input for UCOME is methanol (2.46 GJ) as shown in Fig. 2. In the production of RME, the major exergy inputs are synthetic fertilizers (8.78 GJ), methanol (6.48 GJ) and ethylene glycol (2.88 GJ) as shown in Fig. 3. The exergy of methanol is lower in the UCOME production because during the transesterification, almost the half of the input is recovered and supplied in-process. The exergy of energy input is almost 6 times higher for the production of RME (without considering the exergy from solar radiation). The major energy input to UCOME is diesel (1.00 GJ), mainly consumed during the collection of UCO. In the RME production, the major energy input is natural gas (8.03 GJ) followed by diesel (3.55 GJ) and electricity (1.53 GJ). In summary, RME requires almost 3 times the exergy inputs than UCOME. In the case study of biodiesel in Catalonia, because of data limitations, we have considered labor, capital and environmental remediation exergy flows only for the stages illustrated by Figs. 2 and 3. Socio-economic data regarding the exergy input to society and cumulative number of work-hours in a year (2006) is taken from references [20]. The exergy input to society in Catalonia (Ein;Society ) is 6.38  1019 J/year (see electronic annex 2). The cumulative number of work-hours in a year is 6.15  109 workhours. The global monetary circulation for Catalonia is 1.71 1011 V, calculated as the contribution of Catalonia to the Spanish economy and using the global circulation for Spain (M2) [52,53]. The numerical factors a and b for Catalonia are 0.007 and 1.875 respectively (see electronic annex 3). UCOME and RME require 236 kwork-hours for UCOME and 154 kwork-hours respectively. The equivalent exergetic factor of labor (eeL) for Catalonia of 0.076 GJ/hour, the exergy of labor for UCOME and RME is 0.59 GJ and 0.39 GJ respectively (see electronic annex 6). RME uses an integrated technology for the extraction and pre-treatment of rapeseeds, thus the labor and human services required are lower than for UCOME. The total investment cost for the production of 30,000 tons of UCOME and RME is 20.03 million V and 25.07 million V, respectively. The equivalent exergetic factor of capital (eeK) for Catalonia is 0.005 GJ/V. The exergy of capital for UCOME is 3.51 GJ and for RME is 4.39 GJ. The RME requires higher investment on equipment at the crop production (tractors) and at the pretreatment and extraction stages, as shown in Table 3. In the RME, even though the price per hectare for agricultural land is lower, the land surface Table 5 Exergy inputs and outputs required to produce 1 ton of biodiesel.

Inputs (GJ) Oil Materials Energy

UCOME

RME

37.13 8.41 2.23

38.15 20.70 13.19

Sum inputs

47.77

72.04

Outputs (GJ) Biodiesel Glycerol Agricultural waste Organic waste Water

31.35 1.55 e 0.17 0.01

37.20 1.98 27.78 e 0.16

Sum outputs (GJ)

33.05

67.10

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required is greater and the investment cost increases 4% compared to UCOME. The environmental costs of the emissions and wastes generated by the production of UCOME and RME costs 137,600V and 1,323,330V, respectively. For the RME system, the major emissions are ammonia from NPK fertilizer and nitroammonium sulphate (69%), and carbon dioxide (22%) all of them emitted at the crop production. The environmental impact of RME due to emissions is almost 10 times greater than for UCOME. Even though the equipment for the transesterification of UCO and rapeseed oil does not have any particular difference, the technologies (working parameters) used are different and as a consequence the production of RME requires higher water input than UCOME. As consequence, the amount of wastewater and its treatment cost are greater. The environmental cost for waste treatment is calculated by using the exergetic equivalent factor of capital (eeK) for Catalonia. The exergy of environmental cost for UCOME is 0.02 GJ and for RME is 0.23 GJ. Annex 6 reports the method for calculating the econometric factors (a and b) and the exergetic equivalents (eeL, eeK and eeEnv). Table 6 summarizes the resource input and externalities required for the production of 1 ton of UCOME and RME. 4.2. Introducing by-product recovery A part of a comparison, EEA helps find ways of optimising, i.e. reducing exergy expenditure. In this case study, EEA can help identify ways of reusing the organic waste thereby reducing the overall exergy input and increasing efficiency. UCOME and RME generate organic and agricultural wastes that can be converted to compost, electricity or methanol all of which can be reused in the system. Organic and agricultural wastes can be transformed to products/energy by biochemical and thermochemical processes. Biochemical processes use enzymes of bacteria and other microorganisms to break down biomass and produce other energies/ products [24]. Biochemical processes occur under aerobic (atmospheric) or anaerobic conditions. The aerobic digestion of organic wastes produces compost, which can be used as fertiliser. The anaerobic digestion of wastes produces biogas. Biogas is gas mixture containing mainly methane (CH4) and carbon dioxide (CO2) that can be used to produce electricity or as a substitute of natural gas for methanol production. By thermochemical processes, solid waste undergoes a heat treatment to convert it into gaseous or liquid fuel [24]. The exergy requirements to produce compost, electricity and methanol from organic and agricultural waste are compared: compost consumes 0.31 GJ/ton [54]; the anaerobic digestion to produce biogas consumes 3.72 GJ/ton [55]; and the exergy input to produce electricity and methanol is 15.21 GJ/ton [21]. Methanol can be also produced by gasification of the organic and agricultural wastes, but there is no data published on this production process. The recovery of organic/agricultural waste as compost requires the

Table 6 Extended Exergy Content of 1 ton of UCOME and 1 ton of RME. Items

UCOME

RME

Resource Input (GJ) Oil (GJ) Materials (GJ) Energy (utilities) (GJ)

37.13 8.41 2.23

38.15 20.70 13.19

0.59 3.51 0.02

0.39 4.39 0.23

51.90

77.05

Externalities Labor and human services (GJ) Capital Cost (GJ) Environmental remediation (GJ) Extended Exergy (GJ)

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Table 7 Exergy input and exergy recovered of organic/agricultural waste generated by the production of 1 ton of UCOME/RME. Modified from [54e57]. UCOME

Compost Biogas Electricity Methanol using biogas Methanol via gasification

RME

Exinput (GJ)

Exrecovered (GJ)

Exinput (GJ)

Exrecovered (GJ)

e 0.60 0.52 0.25

0.21 0.26 0.18 0.10 0.56

8.77 8.03 1.53 6.50

2.60 5.22 3.16 1.72 10.11

lowest exergy input. Compost generated from organic waste of UCOME is not used in-process, thus a better option is to produce electricity or methanol to be supplied in the pretreatment and transesterification. Table 7 summarises the exergy input required to recover the organic/agricultural waste and the potential exergy recovered to compost, electricity and methanol. In the UCOME process, methanol obtained by the gasification can substitute 100% the methanol input. Methanol from agricultural waste can substitute 65% the methanol required initially in the RME system. Alternatively, the electricity obtained from agricultural wastes can substitute 100% the electricity initially required by the production of RME. The recovery of organic and agricultural wastes as methanol and electricity helps closing the material cycle of wastes generated in the system and its dependency from external sources. To conclude which by-products offers better benefits in the overall UCOME and RME systems, it is necessary to include data regarding capital, labor and environmental costs. Unfortunately, some of the technologies proposed above are still under development at industrial scale and details of the investment and social costs are not available. Including the economic, social and environmental impact of the recovery of by-products allows accounting for their extended exergy and better assesses present and future renewable energies. 5. Conclusion In conclusion, UCOME requires 1.5 the resources, investment and environmental remediation costs used for RME. The greatest difference is due to the inputs and externalities required during the crop production stage and the difference in the technologies used for the transesterification of oils. UCOME requires greater labor, basically at the collection of UCO, however such collection can be further optimized to require less transport (diesel consumption) and labor. As illustrated in this paper, the extended exergy help quantify the resources (physical and economical) required for obtaining a product which allow more complete assessment and comparison with other products. Quantifying by one exergetic value all the inputs (resources and externalities) of a commodity leads also to a deeper discussion on the definition of exergy when applied at macroscale. Ecological and economic systems are not consistent with thermodynamics of equilibrium since they have no homogeneous state and events are never reversible [58]. Therefore further research is needed to thermodynamics of non-equilibrium systems to better the accounting of labor and capital, particularly if extended exergy values are aimed for comparing products of different geographical location and economic context. Appendix. Supplementary data Supplementary information associated with this article can be found in the online version, at doi:10.1016/j.energy.2010.03.015.

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