Sustainable ethanol production from lignocellulosic biomass – Application of exergy analysis

Energy 36 (2011) 2119e2128

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Sustainable ethanol production from lignocellulosic biomass e Application of exergy analysis Karina Ojeda, Eduardo Sánchez, Viatcheslav Kafarov* Center for Sustainable Development in Industry and Energy, Industrial University of Santander, Cra 27 Cl 9, Bucaramanga, Colombia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 December 2009 Received in revised form 9 August 2010 Accepted 11 August 2010 Available online 6 October 2010

The sustainability of the second-generation biofuels requests to confirm that the energy produced from lignocellulosic biomass is significantly greater than the energy consumed in the process. As lignocellulosic biomass does not affect the food supply, sugarcane bagasse was analyzed as a raw material for second-generation biofuels production. Exergy analysis serves as a unified and effective tool to evaluate the global process efficiency. Exergy analysis evaluates the performance of sugarcane bagasse and its sustainability in the bioethanol production process. In this work, four ethanol production topologies using the typical daily amount of residual biomass produced by the sugar industry were compared. The exergy analysis concept is effective in screening design alternatives with the lowest environmental impact for second-generation bioethanol fuel production from renewable resources. This study was executed by the use of the Aspen PlusÒ program and other software developed by the authors. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Exergy Sustainability Second-generation bioethanol

1. Introduction The fast development of the world’s bioethanol industry has contributed to the debate on “food versus fuel” and the industry’s environmental impact. One of the largest potential feedstock for ethanol is lignocellulosic biomass which does not compete with food crops, specifically agricultural residues (e.g., sugarcane bagasse, crop straws, and corn stover), herbaceous crops (e.g., alfalfa, switchgrass), forestry wastes, wood, wastepaper, and municipal waste. Other point to be considered in biofuels production is “energy consumption vs. energy content in produced ethanol.” The processing of a renewable energy source usually involves the consumption of non-renewable resources (NRR). When the exergy content of an NRR is altered through an irreversible process, the environment is also considered altered. Hence, much research [1e3] has been undertaken on the exergy accounting of NRR consumption in order to measure the environmental impact of many manufacturing processes. According to WORC [4], the biomass energy should be grown or produced in a sustainable way that provides net environmental benefits. Biomass energy crops should be grown and harvested in a way that embodies best stewardship practices to maintain or improve air, water and soil quality. Among the criteria for judging sustainable biomass energy production, can be remarked the Net energy

* Corresponding author. Tel.: þ5776344746; fax: þ5776344684. E-mail address: [email protected] (V. Kafarov). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.08.017

balance, which mentions that more energy should be released through biomass energy use than consumed in producing it (over its lifecycle). This includes energy consumed from planting, cultivating, and fertilizer or pesticide application, harvesting and transporting to market. Consequently, the second-generation biofuels represents a great alternative to reach the sustainability of this industry. In this sense, the global energy consumption in ethanol production from lignocellulosic residues, like bagasse, is lower than that of non residual energy crops because the residual biomass from existing sugar industry is used. As such the energy consumption for all farming stages of sugarcane is assumed by the traditional sugar production chain. In accordance with Dincer and Rosen [5], sustainable development requires sustainable energy resources and the efficient use of such residues. Measuring the renewability of an energy resource using traditional energy accounting methods is also questionable since these methods are based on the first law of thermodynamics, which includes the principle of energy conservation. Moreover, a meaningful energy-related yield calculation, which would indicate if there is some type of actual net gain or loss during the utilization of an energy resource, should take into account the differences in all forms of energy (including the chemical energy of all materials) and the second law of thermodynamics, which recognizes changes in energy quality or usefulness [6]. Exergy analysis is a thermodynamic analysis technique based on the second law of thermodynamics which provides an alternative and illuminating means of assessing and comparing processes and systems rationally and meaningfully. In particular, exergy analysis

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Nomenclature E S V n N M T p H LHV DHc DGF b wi A

Exergy (kJ/mol), (MJ/h) entropy (kJ/mol K), (MJ/kgK) volume (m3) moles of substance (mol) molar flow (mol/h) mass flow (kg/h) temperature (K) pressure (kPa) enthalpy (kJ/mol), (MJ/kg) lower heating value (kJ/kg) entalphy of combustion (kJ/mol) standard Gibbs free energy of formation (kJ/kg) chemical exergy of the pure element of the substance (kJ/kg) mass fraction (kg/kg) functional groups contributions (kJ)

yields efficiencies which provide a true measure of how nearly actual performance approaches the ideal, and identifies more clearly than energy analysis the causes and locations of thermodynamic losses and the impact on the natural environment. Consequently, exergy analysis can assist in improving and optimizing designs [7]. Exergy analysis is linked to sustainability because to increase the sustainability of energy use, we must be concerned not only with loss of energy, but also loss of energy quality (or exergy) [5]. One principal advantage of exergy analysis over energy analysis is that the exergy content of a process flows

B CS

functional groups contributions (kJ/K) calorific value of sulphur (kJ/kg)

Subscript i o deval ch ph org

substance or element state conditions of the thermodynamic environment devaluation chemical exergy physical exergy organic substance

Greek letters chemical potential (kJ/mol2) chemical exergy (kJ/kg) ratio of chemical exergy to the LHV or dry organic substances (dimensionless) s process exergy efficiency

m 3 b

a better valuation of the flow than the energy content, since the exergy indicates the fraction of energy that is likely useful and thus utilizable. Application of exergy analysis to a component, process or sector can lead to insights into how to improve the sustainability of the activities comprising the system by reducing exergy losses. Thus, to justify the production of second-generation biofuels it is necessary to confirm that the energy produced from the lignocellulosic biomass is greater than the energy consumed in the ethanol production through exergy analysis. The main objective of this paper is to apply exergy analysis concept for the evaluation of the

Fig. 1. Generic block diagram of fuel ethanol production from lignocellulosic biomass.

K. Ojeda et al. / Energy 36 (2011) 2119e2128

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Fig. 2. General diagram of fuel ethanol production from lignocellulosic biomass of selected case # 1.

transformation of lignocellulosic biomass to bioethanol and to approach the sustainability of second-generation biofuels production. In the first part of the paper, the different processes involved in ethanol production from lignocellulosic biomass and the exergy analysis concept are described. In the second part, several case studies about lignocellulosic biomass transformation were selected and simulated using the typical daily amount of residual biomass produced by the sugar industry (1200 tonnes). Finally, the exergy analysis methodology was applied to evaluate of sustainability development of second-generation ethanol production. This methodology requires an analysis of each stage of the production

process and the global evaluation of several scenarios of production to verify the sustainable development of the biofuels industry using lignocellulosic biomass. 2. Production of second-generation ethanol The ethanol production from lignocellulosic biomass includes five main steps: biomass pre-treatment, cellulose hydrolysis, hexoses fermentation, separation and effluent treatment. Furthermore, detoxification and fermentation of pentoses released during the pre-treatment step can be carried out [8]. The sequential

Table 1 Description of main stream e Case 1. Stream

1

Mass Flow (kg/h) 50,000 Temperature (K) 298.15 Pressure (atm) 1 Component Mole Fraction Water 0.8469 Lignin 0.0336 Cellulose 0.0433 Hemicellulose 0.0254 Ash 0.0508 Xylose 0.0000 Ethanol 0.0000 0.0000 CO2 NH3 0.0000 Glucose 0.0000 Furfural 0.0000 0.0000 H2SO4 Lime 0.0000 Glycerol 0.0000 Acetic acid 0.0000 Lactic acid 0.0000 Succinic acid 0.0000

2

4

5

24

29

32

34

5000 298.15 1

17,612.5 483.15 22.9

72,612.49 461.15 6.09

95,137.45 306.04 1

93,637.45 299.82 1.29

6604.33 350.87 1

5858.51 298.15 1

0.9958 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0042 0.0000 0.0000 0.0000 0.0000 0.0000

1.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.9120 0.0192 0.0245 0.0036 0.0291 0.0102 0.0000 0.0000 0.0000 0.0002 0.0007 0.0004 0.0000 0.0000 0.0000 0.0000 0.0000

0.9534 0.0000 0.0000 0.0000 0.0002 0.0004 0.0310 0.0130 0.0002 0.0005 0.0000 0.0000 0.0000 0.0001 0.0008 0.0001 0.0002

0.9694 0.0000 0.0000 0.0000 0.0002 0.0004 0.0282 0.0000 0.0000 0.0005 0.0000 0.0000 0.0000 0.0001 0.0008 0.0001 0.0002

0.1391 0.0000 0.0000 0.0000 0.0000 0.0000 0.8417 0.0000 0.0192 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.0050 0.0000 0.0000 0.0000 0.0000 0.0000 0.9950 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

Based on Fig. 2. 1: Biomass, 2: acid, 4: steam, 5: pretreated biomass, 24: fermented liquor, 29: liquor after CO2 absortion, 32: to molecular sieves, 34: ethanol.

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Fig. 3. General diagram of fuel ethanol production from lignocellulosic biomass of selected case # 2.

configuration employed to obtain cellulosic ethanol implies that the solid fraction of pretreated lignocellulosic material undergoes hydrolysis (saccharification); this fraction contains the cellulose in an accessible form to acids or enzymes. Once hydrolysis is completed, the resulting cellulose hydrolyzate is fermented and converted into ethanol. The significant variety of pre-treatment methods of biomass has led to the development of many flowsheet options for ethanol production. Pre-treatment is currently one of the most expensive stages in second-generation technologies. Pretreatment is, however, crucial for ensuring good ultimate yields of sugars from both polysaccharides, yields from enzymatic hydrolysis without pre-treatment are usually less than 20% whereas with pre-

treatment yields can rise to over 90% [9]. According to Claassen et al. [10], one of the main problems in bioethanol production from lignocellulosics is that Saccharomyces cerevisiae can ferment only certain mono and disaccharides like glucose, fructose, maltose and sucrose. This microorganism is not able to assimilate cellulose and hemicellulose directly. In addition, pentoses obtained during hemicellulose hydrolysis (mainly xylose) cannot be assimilated by this yeast. Pentose fermentation, when it is carried out, is accomplished in an independent unit. The need for separate fermentations is due to the fact that pentose utilizing microorganisms ferment pentoses and hexoses slower than microorganisms that only assimilate hexoses. Moreover, the former microorganisms are

Table 2 Description of main stream e Case 2. Stream

1

2

5

12

18

24

32

34

Mass Flow (kg/h) Temperature (K) Pressure (atm) Component Mole Fraction Water Lignin Cellulose Hemicellulose Ash Xylose Ethanol CO2 NH3 Glucose Furfural H2SO4 Lime Glycerol Acetic acid Lactic acid Succinic acid

50,000 298.15 1

15,000 298.15 1

245,000 438.15 3.95

181.44 323.15 1

7040.84 303.15 1

96,104.67 306.21 1

7003.62 350.95 1

6223.11 298.15 1

0.8469 0.0044 0.0433 0.0254 0.0508 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.9972 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0028 0.0000 0.0000 0.0000 0.0000 0.0000

0.9796 0.0044 0.0053 0.0002 0.0067 0.0030 0.0000 0.0000 0.0000 0.0004 0.0002 0.0002 0.0000 0.0000 0.0000 0.0000 0.0000

0.5780 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.4220 0.0000 0.0000 0.0000 0.0000

0.5585 0.0001 0.0000 0.0000 0.0090 0.4043 0.0000 0.0000 0.0000 0.0281 0.0002 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.9512 0.0000 0.0000 0.0000 0.0002 0.0006 0.0326 0.0130 0.0001 0.0009 0.0000 0.0000 0.0000 0.0001 0.0008 0.0001 0.0003

0.1390 0.0000 0.0000 0.0000 0.0000 0.0000 0.8451 0.0000 0.0158 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.0050 0.0000 0.0000 0.0000 0.0000 0.0000 0.9951 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

Based on Fig. 3. 1: Biomass, 2: acid, 5: pretreated biomass, 12: lime, 18: liquor to pentose fermentation, 24: fermented liquor, 32: to molecular sieves, 34: ethanol.

K. Ojeda et al. / Energy 36 (2011) 2119e2128

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Fig. 4. General diagram of fuel ethanol production from lignocellulosic biomass of selected case # 3.

Table 3 Description of main stream e Case 3. Stream

1

Mass Flow (kg/h) 50,000 Temperature (K) 298.15 Pressure (atm) 1 Component Mole Fraction Water 0.8469 Lignin 0.0336 Cellulose 0.0433 Hemicellulose 0.0254 Ash 0.0508 Xylose 0.0000 Ethanol 0.0000 0.0000 CO2 0.0000 NH3 Glucose 0.0000 Furfural 0.0000 Glycerol 0.0000 Acetic acid 0.0000 Lactic acid 0.0000 Succinic acid 0.0000

2

5A

18

24

29

32

34

77,110.7 298.15 1

127,075.8 367.36 1.5

6763.54 303.15 1

96,009.98 306.22 1

94,509.98 299.82 1.29

7055.41 350.94 1

6268.84 298.15 1

0.6303 0.0000 0.0000 0.0000 0.0000 0.0000 0.3697 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.7095 0.0127 0.0156 0.0010 0.0193 0.0082 0.2324 0.0000 0.0000 0.0008 0.0005 0.0000 0.0000 0.0000 0.0000

0.5469 0.0051 0.0000 0.0000 0.0090 0.3858 0.0319 0.0000 0.0000 0.0203 0.0009 0.0000 0.0000 0.0000 0.0000

0.9511 0.0001 0.0000 0.0000 0.0002 0.0005 0.0329 0.0130 0.0001 0.0008 0.0000 0.0001 0.0008 0.0001 0.0003

0.9671 0.0001 0.0000 0.0000 0.0002 0.0005 0.0300 0.0000 0.0000 0.0008 0.0000 0.0001 0.0008 0.0001 0.0003

0.1388 0.0000 0.0000 0.0000 0.0000 0.0000 0.8451 0.0000 0.0161 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.0050 0.0000 0.0000 0.0000 0.0000 0.0000 0.9950 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

Based on Fig. 3. 1: Biomass. 2: solvent. 5A: pretreated biomass. 18: liquor to pentose fermentation. 24: fermented liquor. 29: liquor after CO2 absortion. 32: to molecular sieves. 34: ethanol.

more sensitive to the inhibitors and to the produced ethanol; for this reason, the hemicellulose hydrolyzate resulting from pretreatment should be detoxified. The sequential configuration employed to obtain cellulosic ethanol includes the hydrolysis of a solid fraction of pretreated lignocellulosic that contains cellulose easily accessible to acids or enzymes. Once the hydrolysis has been completed, the resulting cellulose hydrolyzate is fermented and converted into ethanol [8]. 3. Exergy analysis e Formulations Exergy is the maximum amount of useful work that can be extracted from a physical system by exchanging matter and energy with large reservoirs through complete reversible process (heat

Table 4 Specific chemical exergies of chemical species involved in the process. Component

Specific chemical exergy (MJ/kg)

Component

Specific chemical exergy (MJ/kg)

Ethanol Water (l) Water (g) CO2 Glucose Sugarcane bagasse Calcium Sulphate

27.154 0.05 0.527 0.434 15.504 18.879 0.0499

Xylose Cellulose H2SO4 Hemicellulose Lignin CaO

12.224 16.96 1.107 14.595 28.161 1.965

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Table 5 Exergy Flows for main streams e Case 1. Stream

1

2

4

5

24

29

32

34

Ni (kmol/h) Mi (kg/h) hi (MJ/kg) ho (MJ/kg) si (MJ/kgK) so (MJ/kgK) Exch (MJ/h) Exph (MJ/h) Ex (MJ/h)

1638.63 50,000 23.4408 23.4408 0.0350 0.0350 548,502.30 0 548,502.30

272.44 5000 15.6830 15.6830 0.0089 0.0089 350.76 0 350.76

977.64 17,612.5 14.9711 15.8578 0.0068 0.0090 9279.05 4075.82 13,354.86

2863.03 72,612.49 20.1641 22.7600 0.0296 0.0360 519,060.70 50,264.79 569,325.50

4888.37 95,137.45 14.7650 14.7975 0.0085 0.0095 213,117.25 26,560.08 186,557.17

4917.52 93,637.45 15.0430 15.0492 0.0088 0.0100 196,132.57 32,631.61 163,500.96

158.72 6604.33 5.5015 6.5971 0.0044 0.0074 168,173.53 1344.57 169,518.10

127.56 5858.51 6.0426 6.0426 0.0075 0.0075 158,765.58 0 158,765.58

transfer process, mass transfer process, chemical reaction etc.) in a reference stated by Shukuya and Hammache [11]. The energy and exergy balances for a flow process in a system during a finite time interval may be written as:

Turbaev [14], the chemical and thermomechanical exergies can be calculated from thermodynamic data of substances. Chemical exergy at p ¼ p0 is expressed through thermodynamic characteristics of devaluation process [15] as

Energy input  Energy output ¼ Energy accumulation

30 ¼ Ddeval H0 þ T0 Ddeval S0

(1)

Exergy input  Exergy output  Exergy consumption ¼ Exergy accumulation

(2)

Exergy consumed, is the product of entropy generated and the environmental temperature [12].

ðExergy consumedÞ ¼ ðEnvironmental temperatureÞ  ðEntropy generatedÞ

(3)

According to Wall [13], the exergy E of a system may be written as

E ¼ SðT  T0 Þ  Vðp  p0 Þ þ

X

ni ðmi  mi0 Þ

(4)

i

where the extensive parameters are entropy S, volume V, and number of moles of substance i ni, and the intensive parameters are temperature T, pressure p, and chemical potential of substance i mi for the system. The subscript o describes the state when thermodynamic equilibrium with the reference environment is established. The exergy of a flow can be written as

E ¼ H  H0  T0 ðS  S0 Þ þ where H is the enthalpy.

bLHV ¼



X

mi0 ðni  ni0 Þ

(5)

i

(6)

To evaluate the chemical exergy of a mixture, the knowledge of its enthalpy of combustion, elemental composition and absolute entropy is necessary. When some of these data are absent, the methods of estimation of energy are used. If the heat of combustion only is known, the Rant’s equation [16] gives

30 ¼ 0:975DHc0

(7)

If the elemental composition is known, the chemical exergy of various fuels can be evaluated accurately using the Szargut’s correlations [17]. Chemical exergy of biomass in calculated from the correlations for technical fuels using LHV, lower heating (net calorific) value, and mass fractions of organic material, sulphur, water and ash in the biomass [18]









3ch;total ¼ worg bLHVorg þwS 3ch;S CS þwwater 3ch;water þwash 3ch;ash

The factor b is the ratio of the chemical exergy to the LHV of organic fraction of biomass. This factor is calculated from statistical correlations developed by Szargut and Styrylska [19]. In addition, Govin et al. [15] developmed a correlation formula to estimate chemical exergies of oil fractions and fuel mixtures from enthalpy of combustion and atomic composition (Eq. (9)) [20].

        

1:0412 þ0:2160 wH2 =wC  0:2499 wO2 =wC 1 þ 0:7884 wH2 =wC 1  0:3035 wO2 =wC þ0:0450 wN2 =wC

Chemical exergy of a substance is the maximal possible useful work that may be produced by process of physical and chemical equilibration of the substance with the ambient. According to

(8)

(9)

Govin et al. [15] have restricted their investigations by C, H, O, N, S elements which are the most important for fuel mixtures. Alternatively, Hepbasli [20] reported that specific chemical exergy of

Table 6 Net Exergy Flows and efficiency for stage e Case 1. Stage

Pre-treatment

Neutralization þ Pentose fermentation

SSF

Purification

Total exergy flow input (MJ/h) Total exergy flow output (MJ/h) Total exergy Q output (MJ/h) Total exergy Q input (MJ/h) Total Irreversibilities (MJ/kg ethanol) Total Exergy Emissions (MJ/kg ethanol) Efficiencya (%)

562,307.8 534,659.8 23,059.7 40,533.9 7.701 1.192 87.53

59,972.2 55,546.8 361.3 28.22 0.607 2.819 65.04

324,683.3 310,638.3 2121.8 507.04 2.122 46.086 48.00

191,428.4 192,750.1 26,509.3 41,288.53 2.297 5.801 68.22

a

Effective exergy output/total exergy input.

K. Ojeda et al. / Energy 36 (2011) 2119e2128

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Table 7 Exergy Flows for main streams e Case 2. Stream

1

2

5

12

18

24

31

34

Ni (kmol/h) Mi (kg/h) hi (MJ/kg) ho (MJ/kg) si (MJ/kgK) so (MJ/kgK) Exch (MJ/h) Exph (MJ/h) Ex (MJ/h)

1638.63 50,000 23.4408 23.4408 0.0350 0.0350 548,502.30 0 548,502.30

822.43 15,000 15.7413 15.7413 0.0089 0.0089 948.10 0 948.10

12414.31 245,000 21.2696 24.0468 0.0307 0.0362 516,193.55 279,870.39 796,063.94

4.36 181.44 29.0376 29.1666 0.0126 0.0130 115.37 1.06 116.43

92.69 7040.84 3.1543 3.1569 0.0046 0.0046 75,976.78 7.20 75,983.98

4904.45 96,104.67 14.6845 14.7175 0.0084 0.0096 230,697.02 30673.24 200,023.78

167.92 7003.62 5.5031 6.5986 0.0044 0.0074 178,446.89 1428.82 179,875.71

135.49 6223.11 6.0426 6.0426 0.0075 0.0075 168,647.80 0 168,647.80

structurally complicated materials, e.g. biomass, can be estimated from their elemental compositions as given below

3Ch ¼ 4:1868



8177:79½C þ 5:25½N þ 27; 892:63½H  3173:66½O þ0:15½Oð7837:677½C þ 33; 888:889½H  4236:1½OÞ

The standard exergy of many compounds can be found in the literature [21]. When not available, the chemical exergy content of any pure substance can be computed approximately by the Eq. (11) [22].

3ch ¼ DGFo þ

X

wi bi

(11)

process, the amount of exergy loss reflects thermodynamic efficiency of the process. In addition, there is still some exterior exergy

 (10)

loss in practical processes, such as exergy discharged into environment with material flows and work loss etc. For measuring extent of resource utilization and thermodynamic characteristics of a process, process exergy efficiency (s) as an evaluation indicator for degree of exergy utilization is defined:

i

where DGFo signifies the standard Gibbs free energy of formation of the substance [J/kg]; bi, chemical exergy of the ith pure element of the substance [J/kg]; and Ni, mass fraction of the ith pure element of the compound. The Gibbs free energy of formation is available for most chemical compounds in standard reference sources for a large number of chemicals. If not available, it is determined by methodologies as the Van KreveleneChermin equation [23].

DGFo ¼ A þ BT

(12)

where T, temperature [K], DGfo, [kJ] and A.B: functional groups contributions. The exergy of a heat stream Q is given with the help of the Carnot factor

 T EQ ¼ Q 1  0 T

(13)

where T is the temperature at which Q is available. According to Yang et al. [11], resource utilization efficiency in any processes can be measured with exergy transform efficiency because the essence of material or energy utilization is its exergy consumed. There is an exergy loss in practical operation processes because of its irreversibility. In a given production or consumption

s¼

effective exergy obtained exergy consumed in resource

(14)

According to Sorin et al. [24] it is possible to compute the exergy contents of all in-coming and out-going streams to and from a system and to establish an overall exergy balance over any system. The total exergy input of a real system is always higher than its exergy output, because a certain amount of exergy is irreversibly destroyed within the system. This exergy, generally referred to as the internal exergy losses, is directly linked to the thermodynamic irreversibilities in the system. As reported by Talens et al. [25], Exergy Flow Analysis provides a way for process assessment that can be used as a tool for identifying material wastes and energy loss, detecting areas needing technological improvements by calculating the exergetic efficiency. Exergy is also a useful indicator for measuring material potential reactivity and quality, comparing different production processes of product substitutes that are especially useful in comparing renewable sources of energy. Additionally, the maximum improvement in the exergy efficiency for a process or system is obviously achieved when the exergy loss or irreversibility (Exin e Exout) is minimized. Consequently, Hepbasli [20] reported that it is useful to employ the concept of an exergetic ‘‘improvement potential’’ when analyzing different processes or sectors of the economy.

Table 8 Net Exergy Flows and efficiency for stage e Case 2. Stage

Pre-treatment

Neutralization þ Pentose fermentation

SSF

Purification

Total exergy flow input (MJ/h) Total exergy flow output (MJ/h) Total exergy Q output (MJ/h) Total exergy Q input (MJ/h) Total Irreversibilities (MJ/kg ethanol) Total Exergy Emissions (MJ/kg ethanol) Efficiencya (%)

807,523.09 579,268.32 107,883.16 36,029.63 25.132 19.299 54.43

83,383.77 74,143.94 444.58 34.42 1.415 3.763 60.81

382,764.08 323,620.72 2035.81 488.58 9.255 27.560 49.48

202,931.45 208,164.20 27,846.72 42,849.42 1.570 6.350 68.62

a

Effective exergy output/total exergy input.

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Table 9 Exergy Flows for main streams e Case 3. Stream

1

2

5A

18

24

29

32

34

Ni (kmol/h) Mi (kg/h) hi (MJ/kg) ho (MJ/kg) si (MJ/kgK) so (MJ/kgK) Exch (MJ/h) Exph (MJ/h) Ex (MJ/h)

1638.63 50.000 23.4408 23.4408 0.0350 0.0350 548.502.2 0 548.502.2

2716.4 77.110.7 9.9665 9.9665 0.0081 0.0081 1.253.444.7 0 1.253.444.7

4319.25 127,075.8 17.2996 17.6253 0.0306 0.0336 1,761,164.438 71,650.2 1,689,514.2

91.72 6763.54 3.3015 3.3134 0.0040 0.0040 75.456.8 106.4 75.563.3

4902.29 96.009.98 14.6935 14.7286 0.0085 0.0092 231.281.4 17809.0 213.472.4

4931.52 94.509.98 14.9686 14.9769 0.0088 0.0095 214.366.1 17583.9 196.782.1

169.17 7055.41 5.5022 6.6004 0.0044 0.0074 79.783.5 1396.1 181.179.7

136.49 6268.84 6.0426 6.0426 0.0075 0.0075 169.886.2 0 169.886.2

4. Selection of process topologies

4.1. Process description

Many flowsheet configurations have been proposed for ethanol production from lignocellulosic biomass, which incorporated pretreatment, hydrolysis and fermentation steps [26,27]. Numerous pre-treatment methods or combinations of pre-treatment methods are thus available, all having their specific advantages and disadvantages. The choice for a pre-treatment technology heavily influences cost and performance in subsequent hydrolysis and fermentation. To evaluate the second-generation biofuels production it was necessary to establish a methodology for selected several technological routes through of to estimate the effects of parameters as: yield, operation conditions, energy consumption, current available technology and environmental impact. In this work, a qualitative valuation was made to classify the effects of each parameter on based of methodologies proposed by Mosier et al. [28] and literature data [29,30]. Thus, steam explosion, diluted acid and organosolv process were selected for pre-treatment stage of sugarcane bagasse. The enzymatic hydrolysis technologies are object of this study. Where enzymatic hydrolysis is applied, different levels of process integration are possible (Fig. 1) [8]. Simultaneous Saccharification and Fermentation (SSF) consolidates hydrolyzes of cellulose with the direct fermentation of the produced glucose. This reduces the number of reactors involved by eliminating the separate hydrolysis reactor and, more importantly avoiding the problem of product inhibition associated with enzymes: the presence of glucose inhibits the hydrolysis [30]. The SSF was selected in this work according to the comparison of enzymatic process configuration described by Hamelinck et al. [9] and literature data [31e33]. Based on selected stages, in this paper three technological routes have been considered for the second-generation ethanol production, which were simulated and analyzed by the use of the ASPEN PlusÒ program and other software developed by the authors.

The three case studies selected by ethanol production process from sugarcane bagasse analyzed in this paper were divided into the stages described below for separate evaluation and identification of irreversibility generation in each.

Table 10 Net Exergy Flows and efficiency for stage e Case 3. Stage

Pre-treatment Pentose SSF fermentation

Total exergy flow input (MJ/h) Total exergy flow output (MJ/h) Total exergy Q output (MJ/h) Total exergy Q input (MJ/h) Total Irreversibilities (MJ/kg ethanol) Total Exergy Emissions (MJ/kg ethanol) Efficiencya (%)

1,801,947.04

109,323.30

285,514.07 214,632.43

1,690,327.06

54,516.93

252,851.91 217,617.73

190,930.58

728.51

1536.47

27,935.93

229,198.82

33.36

342.20

42,940.89

23.944

8.664

5.039

1.925

0.130

2.530

14.353

7.643

83.13

49.85

57.10

65.96

a

Effective exergy output/total exergy input.

Purification

4.1.1. Case 1 The bagasse [mass fractions: w(cellulose) ¼ 23%, w (hemicellulose) ¼ 11%, w(lignin) ¼ 13.5%, w(ash) ¼ 2.25% and w (water) ¼ 50%] was pretreated with acid catalyzed steam explosion; in this method, the bagasse was treated with high-pressure saturated steam (461.15 K, 0.6 MPa, w(H2SO4) ¼ 2.25%) and then the pressure was swiftly reduced. The process causes hemicellulose degradation of 70% and lignin was not solubilized. The cellulose was sent to SSF (hydrolysis yield 80%) using S. cerevisiae and an enzyme concentration of 20 filter paper units FPU/g cellulose. The SSF operated at 308.15 K. The fermentation yield was 85%. The CO2 obtained in the fermentation stage was sent to an absorption tower. Hydrous ethanol was obtained by the stripping and rectification stages. In order to remove the remaining water and obtain anhydrous ethanol, dehydration was required, for this, molecular sieves were used. Fig. 2 shows a simplified diagram of ethanol production in this case. Main compositions and operation parameters are shown in Table 1. 4.1.2. Case 2 The lignocellulosic biomass [mass fractions: w(cellulose) ¼ 23%, w(hemicellulose) ¼ 11%, w(lignin) ¼ 13.5%, w(ash) ¼ 2.25% and w (water) ¼ 50%] was pretreated with diluted acid (w(H2SO4) ¼ 1.5%, 438 K). The process caused hemicellulose degradation of 90%. The recovered solution (xylose and lignin) was neutralized with lime and the lignin was precipitated. The cellulose was sent to SSF (hydrolysis yield 80%) using S. cerevisiae and an enzyme concentration of 20 FPU/g cellulose. The SSF operated at 305 K. The fermentation yield was 92%. The CO2 obtained in the fermentation stage was sent to absorption tower. Hydrous ethanol was obtained by the stripping and rectification stages. In order to remove the remaining water and obtain anhydrous ethanol, dehydration was required, for this, molecular sieves were used. Fig. 3 shows a simplified diagram for this case. Main compositions and operation parameters are shown in Table 2. 4.1.3. Case 3 The sugarcane bagasse [mass fractions: w(cellulose) ¼ 23%, w (hemicellulose) ¼ 11%, w(lignin) ¼ 13.5%, w(ash) ¼ 2.25% and w (water) ¼ 50%] was pretreated with organosolv solution [w (ethanol) ¼ 34% and w(water) ¼ 66%, 458 K, 2 MPa]. The cellulose was sent to SSF (hydrolysis yield 80%) using S. cerevisiae and an enzyme concentration of 20 FPU/g cellulose. The fermentation yield was 90%. The CO2 obtained in the fermentation stage was sent to an absorption tower. Hydrous ethanol was obtained by the stripping

K. Ojeda et al. / Energy 36 (2011) 2119e2128

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Table 11 Comparison of cases simulated.

Case 1 Case 2 Case 3

Exergy Efficiency (%)

Renewability

Sustainability index(SI)

Exergetic Improvement Potential(IP)

Pre-treatment Exergy Efficiency (%)

SSF Exergy Efficiency (%)

Purification Exergy Efficiency (%)

79.58 73.98 78.37

1.899 2.093 0.503

4.897 3.843 4.621

27170.1 60515.7 624113.1

87.53 54.43 83.13

48.0 49.48 57.1

68.22 68.62 65.96

Exergy efficiency ¼ 1 e Ex des/Ex input, Renewability ¼ net bioenergy outputs/net fossil energy inputs, SI ¼ 1/Dp; Dp ¼ Ex des/Ex input, IP ¼ (1 e s)(Exin e Exout).

and rectification stages. In order to remove the remaining water and obtain anhydrous ethanol, dehydration was required, for this, molecular sieves were used. Fig. 4 shows a diagram for this process. Main compositions and operation parameters for this case are shown in Table 3. 5. Results and discussion Using Eqs. (1)e(13), exergy was calculated for all material streams in the process. The reference temperature was 25  C, the relative humidity of atmospheric air was 70% and the chemical exergy of ash was neglected. The specific chemical exergies of the different components are listed in Table 4, based on values reported by Szargut et al. [17] and the application of equation (10). The values in Table 4 can be used to evaluate the chemical exergy of matter at various stages of the process. Exergy flows for main streams in each case are shown in Tables 5, 7 and 9. Net exergy flows and efficiency of process are shown in Tables 6, 8 and 10. In the exergy analysis performed in this paper, the irreversibility generation and the exergetic efficiency were calculated for each case. A comparison of all cases is reported in Table 11. Highest exergy efficiency was reported on case 1. For this case, the greatest irreversibility was observed on pre-treatment stage. In contrast, lowest exergy efficiency was observed on case 2. Irreversibility represents the loss of quality of materials and energy due to dissipation. For all cases, SSF was the most improvement potential stage. These improvements include reduction of process steam requirements, adjustment of cogeneration systems or design of heat network for thermal integration of the process. According to Rosen et al. [34] the sustainability of the fuel resource can be expressed by sustainability index (SI) as the inverse of the depletion number. Depletion factor (Dp) is the relation between the exergy destruction and the exergy input. Thus, the sustainability index allows to analyse the relationships between environmental impact and sustainability versus exergy efficiency indirectly, applying to energy systems using renewable energy sources. In this study, the case 1 had the highest sustainability index, for this reason lowest environmental impacts were anticipated. Additionally, renewability index (net bioenergy outputs/net fossil energy inputs) higher than 1 is a minimum requirement to indicate that biofuel system can help to reduce dependency on fossil energy. This index was higher than 1 for cases 1 and 2. Case 3 reported renewability index lower than 1 because pre-treatment stage has higher energy consumption due to recovery solvent system. However, the solvent recycle strategy employ the exergy content in the solvent, therefore, the external exergy input is reduced and the exergy efficiency increases in the pre-treatment stage. 6. Conclusions In this study, three different topologies for second-generation ethanol production from sugarcane bagasse were simulated and analyzed. The results show highest exergy efficiency in case 1 (Steam

Explosion Pre-treatment þ SSF þ Dehydration) reaching 79.58%. The Case 2 (Acid diluted Pre-treatment þ SSF þ Dehydration) was showing lowest exergy efficiency (73.98%). For all cases, improvements in pre-treatment and SSF stages and design of heat network for thermal integration of the process are suggested to reach the sustainable development of this biofuels using these technological routes. The relationship between exergy analysis and sustainability was observed. Exergy methods can be used to improve sustainability. Exergy loss was observed in all process; the study suggests that these occurred particularly due to the use of non-renewable energy forms, should be minimized to obtain sustainable development. Detailed application of exergy analysis to all stages of secondgeneration bioethanol production using several technological routes will provide a powerful tool to respond to the “energy consumption vs. energy content in produced ethanol” and to verify the sustainable development of the biofuels industry using lignocellulosic biomass. The reduction of dangerous emissions associated with biofuels production processes and the integrated use of resources constitute the main objective of guidelines to be followed for sustainable development. For this purpose it is of great importance to develop an exergy analysis for accounting both for materials use and waste residuals. The exergetic analysis presented in this work confirmed to be very useful for the identification of irreversibility generation in second-generation ethanol processes. Exergy methods are essential in improving efficiency, which allows maximizing the benefits it derives from its resources while minimizing the negative impacts (such as environmental damage). Acknowledgements The authors acknowledge the support provided by the Colombian Institute for Development of Science and Technology “Francisco Jose de Caldas” (COLCIENCIAS), Contract No 336-2007 “Optimization of joint production (sugar-alcohol) and development of new bioethanol production process” and the Ibero-American Program on Science and Technology for Development (CYTED), Project 306RTO279 “New technologies for biofuels production” UNESCO code: 330303, 332205, 530603, 330999. References [1] Neelis ML, Van der Kooi HJ, Geerlings JC. Exergetic life cycle analysis of hydrogen production and storage systems for automotive applications. Int J Hydrogen Energ 2004;29:537e45. [2] Tonon S, Brown MT, Luchi F, Mirandola A, Stoppato A, Ulgiatic S. An integrated assessment of energy conversion processes by means of thermodynamic, economic and environmental parameters. Energy 2006;31:149e63. [3] Yang Q, Chen B, Ji X, He YF, Chen GQ. Exergetic evaluation of corn-ethanol production in China. Commun Nonlinear Sci Numer Simul; 2007;. doi:10.1016/j.cnsns.2007.08.011. [4] WORC. Biofuels sustainable criteria. 2006. See also: http://www.worc.org/ userfiles/Biofuels%20Sustainability%20Criteria.pdf. [5] Dincer I, Rosen MA. Exergy, energy, environment and sustainable development. Elsevier; 2007. [6] Berthiaume R, Bouchard C, Rosen MA. Exergetic evaluation of the renewability of a biofuel. Exergy Int J 2001;1(4):256e68.

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