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Clean Technologies and Environmental Policy (2018) 20:1685–1696 https://doi.org/10.1007/s10098-017-1480-4

ORIGINAL PAPER

Life‑cycle assessment of bioethanol production from sweet sorghum stalks cultivated in the state of Yucatan, Mexico

Patricia Aguilar‑Sánchez1 · Freddy Segundo Navarro‑Pineda2 · Julio César Sacramento‑Rivero2 · Luis Felipe Barahona‑Pérez3

Received: 10 May 2017 / Accepted: 16 December 2017 / Published online: 9 January 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018

AbstractBioethanol is being promoted in Mexico to be used in a blend with gasoline. On the north of the Yucatan peninsula, bioetha-nol could be produced from sweet sorghum, as it can grow efficiently on this land; it can be harvested 2–3 times in a year and possesses a better agronomical stability than sugarcane with low nitrogen requirements and high productivity. In this work, the potential environmental impacts and energy efficiency of bioethanol production from sweet sorghum were evalu-ated using life-cycle assessment. Four scenarios were evaluated: scenario PI considered only bioethanol production from the stalk juice; scenarios PII and PIII added cogeneration from the dry-stalk biomass in single and combined cycle, respectively. Scenario PIV considered bioethanol production from both stalk juice and dry-stalk biomass. Scenario PI demanded more fossil energy than what was generated as bioethanol, while scenarios PII and PIII were fossil energy independent. Scenario PIII showed the higher net energy ratio (1.89) and a better environmental performance in all CML-IA baseline impact cat-egories. In terms of global warming potential, the scenario PIII showed a mitigation potential of 16% with respect to the fossil reference. In the categories where the sweet sorghum scenarios represented larger emissions than the fossil reference, it was due mainly to the use of fertilizers and the conventional energy consumption in the various processing steps of the biomass. Scenario PIV showed the highest energy demand and worst environmental performance due to large demands of energy and chemicals in the bagasse pretreatment step.

Keywords Renewable energy · Biofuels · Cogeneration · Environmental impact · Energy ratio

AbbreviationsADP Abiotic depletion potentialAP Acidification potentialART Agroscope Reckenholz-Tänikon (research

station)CED Cumulative energy demandCFC Chlorofluorocarbon

CML Institute of Environmental Sciences of Lei-den University

EP Eutrophication potentialFAEP Freshwater aquatic ecotoxicity potentialGHG Greenhouse gasGWP Global warming potentialHTP Human toxicity potentialIEE Intelligent Energy EuropeLCA Life-cycle assessmentMAEP Marine aquatic ecotoxicity potentialNER Net energy ratioODP Ozone layer depletion potentialPI Scenario I—bioethanol production from

juicePII Scenario II—bioethanol and electricity pro-

duction by simple steam power plantPIII Scenario III—bioethanol and electricity pro-

duction with combination cycle power plantPIV Scenario IV—bioethanol production from

juice and bagasse

* Luis Felipe Barahona-Pérez barahona@cicy.mx

1 Instituto Nacional de Investigaciones Forestales Agrícolas y Pecuarias, Campo experimental San Martinito km 56.5, Carretera Federal México, 72000 Puebla, Mexico

2 Universidad Autónoma de Yucatán, Periférico Norte km 33.5, Tablaje Catastral No. 13615, Chuburna de Hidalgo Inn, 97203 Mérida, Yucatán, Mexico

3 Centro de Investigación Científica de Yucatán AC, Parque Científico Tecnológico de Yucatán, km 5 Carretera Sierra Papacal - Chuburná Puerto, 97302 Sierra Papacal, Yucatán, Mexico

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POCP Photochemical ozone creation potentialRSB Roundtable for sustainable biomaterialsSAGARPA Ministry of Agriculture, Rural Development,

Fishery and Food (Mexico)SSF Simultaneous saccharification and

fermentationTEP Terrestrial ecotoxicity potential

Introduction

Mexico possesses a wide range of renewable resources; how-ever, this does not necessarily mean that biofuel production is technically or economically viable (Secretaría de Energía 2015). Mexico is one of the countries that have ambitious goals regarding energy generation from non-fossil sources. The Law for the Promotion and Development of Biofuels approved in 2008 is the basis of the regulatory framework for the production of inputs for biofuels based on a variety of activities within Mexican territory, such as agriculture, for-estry, algae production, and biotechnological and enzymatic processes; this law also states that biofuel production should not jeopardize the country’s food security and sovereignty (Diario Oficial de la Federación 2008). Sustainable bioen-ergy management would offer significant advantages from socioeconomic and environmental perspectives, as it would create synergies between agriculture, forestry, energy, indus-trial, environmental and social sectors, thereby promoting sustainable development attracting investment and through the creation of employment. This has been the experience of Brazil, the USA, and other countries in Europe, where a set of strategies with a long-term view and the support of poli-cies and public resources have been implemented (Masera et al. 2011; World Bank 2014). Most of the success cases in the mentioned countries rely on the production of ethanol, which is currently the liquid biofuel with the largest market in the world.

Direct fermentation of sugars is the simplest process to produce ethanol. Crops with high yields of sugar like sug-arcane and sugar beet are extensively used for this purpose. Starch-rich crops like corn are also widely used, but they require energy-demanding processing steps before fermenta-tion, such as milling, liquefaction, and saccharification of the starchy material. Second-generation bioethanol, that is, etha-nol from lignocellulosic biomass, is currently a focal point in research and development worldwide; however, pretreat-ment of this feedstock is highly energy demanding (Hattori and Morita 2010; Zabed et al. 2017). Sugarcane and sweet sorghum are promising sources for generating bioethanol in Mexico (Chuck et al. 2011; SAGARPA 2009). Other poten-tial sources include corn and agave bagasse. However, the use of corn can create food competition issues as corn is the base of the Mexican diet. On the other hand, agave bagasse

is a year-round available lignocellulosic biomass and the production process is still under development (Barrera et al. 2016; Saucedo-Luna et al. 2011). Unlike sugarcane, sweet sorghum adapts well on arid or semiarid land and is resistant to heat, salinity, and flooding (Reddy and Reddy 2003; Cordovés et al. 2009; Dávila-Gómez et al. 2011). The potential production of sweet sorghum in Mexico is about 1,740,634 ha, with the states of Tamaulipas, Sinaloa, and Michoacán standing out with the greatest production poten-tial. Although the estimated production potential in the state of Yucatan is not among the highest, it is one of the 16 states with the greatest productivity. Also, Yucatan does not have soils suitable for cultivating sugarcane, meaning that sweet sorghum represents a unique opportunity for ethanol produc-tion (SAGARPA 2009, 2012).

Sweet sorghum is an annual, fast-growing plant with C4 metabolism. Although it is a perennial grass, in the trop-ics it can be harvested several times per year as it adapts well to arid or semiarid land at temperatures between 12 and 37 °C and resists soil salinity and flooding (Cordovés et al. 2009; Vasilakoglou et al. 2011; Dávila-Gómez et al. 2011; Monti and Zegada 2012; Rao et al. 2013; Mathur et al. 2017). It reaches a height of approximately 1–3 m, depending on the subspecies, and its use is not widespread in Mexico, although it may be used as cattle fodder. Its accumulation of sucrose is similar to that of sugarcane, but with greater agronomic stability with a shorter growing sea-son of 3–5 months, compared to sugarcane’s which is of 9–12 months (Dávila-Gómez et al. 2011; Rolz et al. 2014). This allows crop rotation or double cropping systems to be established under adequate climate conditions (IEE and Sweethanol 2011). The main attractive of sweet sorghum for ethanol production is the high soluble carbohydrate content of its stalks, which ranges from 43.6 to 58.2%, as well as its insoluble cellulose and hemicellulose content of 22.6–47.8% (Dale and Seungdo 2005). The stalk of sweet sorghum is 73% (w/w) juice, which contains sucrose, glucose, and fructose, in proportions that depend on the variety and on cultivation and harvesting conditions (Almodares and Hadi 2009; Rolz et al. 2014). Ethanol from sweet sorghum may be considered a 1.5 generation biofuel; that is, one produced with conventional technologies, but with raw materials that do not compete with food production (Wang and Liu 2009).

Biofuels production must be sustainable; that is, it must be viable in environmental, social, economic, and energetic terms. Biofuels have the potential to provide socioeconomic benefits as the presence of processing plants in rural areas (and neighboring municipalities), encourage economic dynamism, the generation of employment, and also influence other related industries (Gilio & Moraes 2016; Moraes et al. 2016). Past experiences in Mexico have reported positive impacts to local communities such as increased household income from salaries, better working conditions, access to

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jobs close to their homes, among others (Sacramento Rivero et al. 2016).

Normally, the environmental performance is assessed by estimating potential environmental impacts using a life-cycle assessment (LCA). The LCA methodology quantifies the impacts on the environment and resources that happen throughout the life cycle of the product, i.e., the production and procuring of raw materials, transformation, distribu-tion, use, and waste management processes (Finnveden et al. 2009; Singh et al. 2013). Several LCAs of biofuel produc-tion in the Pan-American region have focused on the global warming potential (GWP), resulting from greenhouse gas (GHG) emissions. Nevertheless, few studies of energy sys-tems have taken other environmental impacts into account (Shonnard et al. 2015). Furthermore, few LCAs on biofu-els production have been performed in the Mexican context (García et al. 2011; Rendon-Sagardi et al. 2014; Murillo-Alvarado et al. 2015; Sanchez et al. 2016) despite the interest of the country in developing the biofuels market.

This study aims to assess the environmental and energy performance of bioethanol production from sweet sorghum in the state of Yucatan using the LCA methodology, to answer the question of whether this crop has the potential to mitigate GHG emissions and how would other environ-mental impacts be affected.

Methods

Assessment of environmental performance: life‑cycle assessment

Goal and scope

The goal of this study is to estimate the potential environ-mental impacts of the production of ethanol from sweet sor-ghum in Yucatan, Mexico. The functional unit was defined as the amount of sweet sorghum biomass harvested and har-nessed from 1 ha of land in 1 year. The boundaries of the system were defined for a well-to-wheels analysis, which includes the stages of: sweet sorghum cultivation, transport of biomass to the industrial plant, biomass transforma-tion, use of the biofuels and the end-products, and waste management.

Sweet sorghum cultivation includes all the activities related to the cultivation of sweet sorghum, including man-ufacturing, distribution, and use of fertilizers, pesticides, fuels, and water for irrigation. The biomass transformation stage involves all the activities related to the industrial trans-formation: first, stalks are pressed to obtain the sorghum juice that will be transformed into ethanol by means of fer-mentation, distillation, and dehydration with a molecular sieve. The pressing of stalks produces a residual bagasse that

can be exploited. Figure 1 shows the options for harnessing the bagasse under four different scenarios. In scenario PI, the bagasse is not exploited and is considered waste. In scenario PII, the bagasse is burned to generate heat and electricity using a steam turbine. It was assumed that the bagasse must be dried and milled to a particle size of 8 mm before burn-ing (Miao et al. 2011). The energy derived from bagasse combustion is used internally, and the excess of electricity is sold. Scenario PIII is similar to PII, but considers heat and electricity generation using a combined cycle. Scenarios PII and PIII were defined according to the usual process in sugar refineries in Mexico, where essentially the juice is used to obtain sugar and the bagasse is burned directly to supply some of the energy requirements of the indus-trial plant. Finally, in scenario PIV the remaining sugars in the sorghum bagasse were assumed to be fermented to produce bioethanol. This requires the bagasse to be milled at a particle size of 1 mm to subsequently remove the lignin with acid treatment (chemical pretreatment). This releases monosaccharides that are transformed into ethanol through simultaneous saccharification and fermentation (SSF); the ethanol is then mixed with the ethanol stream coming from the juice fermentation and follows the same processing as in other scenarios.

Life‑cycle inventory

Table 1 shows the inputs and outputs for all scenarios. The cultivation conditions of sweet sorghum were obtained from pilot sweet sorghum plantations near the municipal-ity of Tizimin, in the state of Yucatan, Mexico, which were obtained in a previous work (Peniche 2012; Montes et al. 2009). With a plantation density of 70,000 plants ha−1, stalk and spike yields of 72,653 and 5900 kg ha−1 were assumed per crop cycle, respectively, in accordance with the results reported by Peniche (2012). Given that this is a prospective study, an average distance of 10 km from the plantations to the industrial facilities was assumed, by considering that the biorefinery would be located in the middle of the plantations (Wang et al. 2014). Greenhouse gas emissions (GHG) result-ing from direct land-use change were estimated in accord-ance with the guidelines of the Roundtable on Sustainable Biomaterials (2014), considering that sweet sorghum is cul-tivated in the climate of a tropical moist deciduous forest with a rainfall of 1000–1100 mm a−1 (Instituto Nacional de Estadística y Geografía 2016) and mineral soil (inceptisol). It was also assumed that the former land use was managed croplands. On the other hand, emissions to soil and water resulting from the use of fertilizers were determined fol-lowing the Agroscope Reckenholz-Tänikon Research Sta-tion methodology (Nemecek and Schnetzer 2012), while emissions to air were calculated by means of the RSB methodology.

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Data on the fermentation of the sweet sorghum juice and the pretreatment of the bagasse (scenario PIV) were gener-ated by Franco-Brito (2011), while that for stalk saccharifi-cation and fermentation by Equihua-Sánchez (2013), both at laboratory scale. The assumption in this study was that the inputs, conversions, and efficiencies would remain constant at the industrial scale. The distillation, dehydration, drying, and combustion processes were simulated in the software Aspen Plus v8.4. Sweet sorghum bagasse was modeled as a non-conventional solid, from the elemental analysis and

energy content listed in Table 2 (Yin et al. 2013; Negro et al. 1999). The energy consumption of the milling operations was taken as 54,054 MJ (1.63 hp) and 17,586 MJ (0.53 hp) for particle sizes of 1 and 8 mm, respectively (Miao et al. 2011). Emissions data for the combustion of natural gas and ethanol, as well as the environmental factors of the waste management (vinasses, combustion ashes, and wastewater from the chemical pretreatment of the bagasse, depending on the scenario, see Fig. 1), were taken from the Ecoinvent v3.0 database.

Fig. 1 Block diagrams of bioethanol production under different sce-narios. PI, bioethanol production from sweet sorghum juice; PII and PIII, bioethanol production from sweet sorghum and cogeneration of heat and power considering either a Rankine cycle (PII) or a com-

bined cycle (PIII); PIV, bioethanol production from sweet sorghum juice and bagasse by simultaneous saccharification and fermentation (SSF)

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The carbon neutrality assumption was considered in the calculation of the emissions of greenhouse gases (GHGs). This assumption states that the carbon dioxide released dur-ing the combustion of the biofuels will eventually be seques-tered back during the sweet sorghum cultivation, resulting in zero net emissions of GHGs.

Impact assessment

The potential environmental impacts were calculated accord-ing to the CML 2 baseline 2000 V2.05 methodology and using the software SimaPro v8. This methodology involves several environmental impact categories in addition to global

warming potential (GWP), namely the potentials of: abiotic depletion (ADP), acidification (AP), eutrophication (EP), ozone layer depletion (ODP), human toxicity (HTP), ter-restrial ecotoxicity (TEP), freshwater aquatic ecotoxicity (FAEP), marine aquatic ecotoxicity (MAEP), and photo-chemical ozone creation (POCP). This allows identifying the trade-off among the different potential environmental impacts.

Interpretation

A system expansion approach was used to compare the environmental performance of the sorghum-based products relative to the fossil-based products they aim to substitute. On the PI and PIV cases, the reference fossil system is the production of gasoline from petroleum, assuming that the bioethanol produced in these two systems is a renewable substitute for this fuel. On the PII and PIII scenarios, the fossil references are gasoline and electricity from the Mexico electricity mix (Fig. 2). The potential environmental impacts of conventional electricity generated in Mexico, and those of gasoline were obtained from the Ecoinvent 3.0 database.

Sweet sorghum stalks yield is one of the variables with the most impact on the final results of the LCA, but at the same time is one with high uncertainty as the average in this work is taken from pilot plantations, and it may vary greatly with microclimatic conditions. Also, the industrial

Table 1 Mass balance inventory Parameter Value Units ha−1 References

Land use 1 ha Peniche (2012)Fertilizer (18:46:00) 200 kg Peniche (2012)Pesticide (Arrivo 200CE)) 70.14 kg Peniche (2012)Herbicide (Glifosato) 6.8 kg Peniche (2012)Water 4.00E+03 m3 Montes et al. (2009)Residues 5.90E+03 kg Peniche (2012)Sweet sorghum yield 7.85E+04 kg Peniche (2012)Stem sorghum yield 7.26E+04 kg Peniche (2012)Stem sorghum yield 7.26E+04 kg Peniche (2012)Juice sorghum yield 4.29E+04 kg Peniche (2012)Bagasse sorghum yield 2.97E+04 kg Peniche (2012)(NH4)2SO4 5.36 kg Franco-Brito (2011)Yeast (juice) 3.57 kg Franco-Brito (2011)NaOH (pretreatment) 6.98E+03 kg Equihua-Sánchez (2013)H2O2 (pretreatment) 1.68E+04 kg Equihua-Sánchez (2013)H2SO4 (pretreatment) 85.58 kg Equihua-Sánchez (2013)Water (pretreatment) 2.34E+05 kg Equihua-Sánchez (2013)Buffer 3.40E+04 kg Equihua-Sánchez (2013)Enzyme (SFS) 2.43E+03 kg Equihua-Sánchez (2013)Yeast (SFS) 3.4 kg Equihua-Sánchez (2013)(NH4)2SO4 (SFS) 5.1 kg Equihua-Sánchez (2013)Water (SFS) 2.37E+03 L Equihua-Sánchez (2013)

Table 2 Elemental analysis of sweet sorghum bagasse (Negro et  al. 1999)

Analysis Parameter Value

Proxanal Moisture 6%Ash 4.8%

Ultanal Carbon 45.4%Hydrogen 6.1%Nitrogen 0.5%Sulfur 1.01%Oxygen 43.19%

Energy content Calorific value 17.3 MJ kg−1

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stage on scenario PIV is based on laboratory-scale data, which may change significantly during the scaling-up of the process, and the optimization of process conditions. Therefore, a sensitivity analysis was carried out to meas-ure the effect of the biomass yield and the chemical inputs to the SSF on the final results, by varying the average values by ± 10%.

Energy performance

The energy performance of the system was assessed by estimating the net energy ratio (NER), defined as the ratio between useful energy produced by the system and its fossil energy consumption in the life cycle. The latter is calculated considering the energy inputs as fuel, heat, and electricity in all life-cycle stages, including the produc-tion of raw materials. If the NER value is less than 1, the process is not energetically feasible, because it consumes more fossil energy than the renewable energy it produces. The fossil energy consumption was estimated using the SimaPro v8.0 software (as the cumulative energy con-sumption), while the energy released by the ethanol combustion was estimated from its lower heating value at 26.95 MJ·kg−1 (U.S. Department of energy 2015).

Results and discussion

Environmental performance

Energy and mass balances

Table 3 shows the estimated inputs and outputs from the mass and energy balances on all the analyzed scenarios. The bioethanol yield calculated for PI, PII, and PIII was 1940 kg ha−1 a−1, respectively. On the PII and PIII sce-narios, the electricity generation from burning the bagasse was 5751 and 17,097 kWh, respectively. This energy can supply the requirements of the industrial stage (which are dominated by the juice extraction), and still a surplus can be uploaded to the electric grid and be sold to a third party, 338.71 kWh on PII and 11,714 kWh on PIII. The electric efficiency (defined as the ratio between the elec-trical energy generated and the energy contained in the combustible material) of the combustion processes was 10.3 and 30.7% on PII and PIII, respectively; these are within the expected range of electric efficiencies for solid fuels (Franco and Giannini 2005). The bioethanol produc-tion on scenario PIV was 3129 kg ha−1 a−1, 61% higher than in the other scenarios, given the additional ethanol

Fig. 2 Life-cycle stages of the sweet sorghum and the refer-ence system

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production from the sorghum bagasse. Nevertheless, the energy requirements on this scenario were also higher due to the additional energy required for milling the bagasse down to a particle size of 1 mm.

Potential environmental impacts

Figure 3 presents the potential environmental impacts of the analyzed scenarios. Scenarios PII and PIII present the lowest impact in all categories excepting EP and TEP. This is a consequence of the lower electric energy demand com-pared to PI and PIV, due to the use of bagasse to produce electricity. In contrast, the environmental impacts of PIV are 10–15 times larger in all categories; the hydrogen per-oxide and sodium hydroxide required for the pretreatment of

the bagasse (see Fig. 1) are the main contributors to all the potential environmental impact categories on this scenario, representing between 50 and 90% of the total impacts.

On the sweet sorghum cultivation stage, emissions to air were mainly due to fertilizer production, and emissions to water were associated with the nitrogen fertilizer runoff, which can cause damage to both groundwater and soil; also important are the heavy metals, such as zinc and lead, pre-sent in fertilizers which translate to damage to soils.

Interpretation

Figure  4 shows the relative potential environmental impacts of each scenario compared to their correspond-ing fossil-based reference system. Scenario PIII shows the

Table 3 Estimated inputs and outputs on the analyzed scenarios

*Calculations made in this study; **calculations in Aspen Plus; ***cumulative energy demand—SimaPro

Parameter Value Units ha−1 Observations

Bioethanol (PI, PII, and PIII) 1.94E+03 kg *Bioethanol (PIV) 3.12E+03 kg *Vinasses (PI, PII, and PIII) 3.95E+04 kg *Vinasses (PIV) 7.82E+04 kg *CO2 (fermentation process—PI, PII, and PIII) 1.44E+03 kg *CO2 (fermentation process—PIV) 2.24E+03 kg *Water (dryer) 1.73E+04 L *Biomass loss (pretreatment) 7.21E+03 kg *Chemical residue (neutralized) 3.74E+05 kg *Effluents (from SFS) 3.03E+03 L *Exhaust gas 1.08E+05 kg *Ash (PII and PIII) 5.58E+02 kg **Steam loss (PII) 2.81E+04 kg **Steam loss (PIII) 1.17E+04 kg **Heat (dryer) 4.53E+04 MJ **Heat (distiller—PI, PII, and PIII) 2.10E+04 MJ **Heat (distiller—PIV) 3.62E+04 MJ **Energy required for milling (bagasse and juice extraction) 1.95E+03 MJ **Energy (milling—PII and PIII) 1.76E+04 MJ Miao et al. (2011)Energy (milling—PIV) 5.41E+04 MJ Miao et al. (2011)Reaction energy (pretreatment—PIII) 4.75E+04 MJ *Reaction energy (SFS—PIII) 1.70E+04 MJ *Energy (bomb dehydrator—PI, PII, and PIII) 2.64 MJ **Energy (bomb dehydrator—PIV) 4.06 MJHeat (bomb dehydrator—PI, PII, and PIII) 2.58E+03 MJ **Heat (bomb dehydrator—PIV) 4.16E+03 MJFossil energy required throughout the life-cycle (PI) 9.41E+04 MJ ***Fossil energy required throughout the life-cycle (PII and PIII) 4.99E+04 MJ ***Fossil energy required throughout the life-cycle (PIV) 9.32E+05 MJGenerated heat (PII and PIII) 2.01E+05 MJ **Heat loss (PII and PIII) 6.55E+04 MJ **Generated energy (PII) 2.08E+04 MJ **Generated energy (PIII) 6.17E+04 MJ

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Fig. 3 Comparison of the potential environmental impacts of the analyzed scenarios. Data expressed per hectare of land cultivated with sweet sorghum

Fig. 4 Environmental load of the analyzed scenarios consider-ing their fossil-based reference products. Scenario PIV was excluded as all its values are out of scale relative to the other scenarios and the fossil refer-ence system

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best environmental performance as it represents reductions in almost all impact categories that range from 15% (in the GWP category) to 75% (in the ADP and MAEP catego-ries), relative to its fossil reference system. These benefits derive from the displacement of conventional electricity present in PII, which is even larger in PIII. Scenario PIII only shows a worse performance than the fossil reference in the EP and TEP categories (188 and 99% higher, respec-tively). These environmental impacts categories are highly influenced by the use of mineral fertilizers and landfilling of the ashes from the combustion process. Other studies (Wang et al. 2014, 2015) found similar results. Thus, resid-ual biomass may provide several environmental savings depending on how it is harnessed. A similar study on the ethanol production from sweet sorghum reached a similar result (Cai et al. 2013).

Scenario PIV is excluded from Fig. 4 because its poten-tial environmental impacts resulted orders of magnitude larger than those of its fossil reference system, and the other 3 scenarios, in all the analyzed categories. The emis-sions generated were mainly associated with the produc-tion of the raw materials needed to pretreat the bagasse, specifically the large amounts of hydrogen peroxide and sodium hydroxide (see Table 1). These variations indicate

that the use of these inputs was extremely intensive, espe-cially in the ODP and the TEP impact categories.

The sensitivity analysis of the sweet sorghum yield was made over the results of scenario PIII, as it showed the best environmental performance in almost all impact categories. The result of this analysis is shown in Fig. 5. A reduction in the biomass yield has a higher effect on the final results than an increment, indicating that the relationship between the biomass yield and the environmental load is not linear. The larger effects were observed on the GWP and the MAEP (between – 8 and + 10%, respectively), followed by ADP and AP (between − 5.5 and + 6.5%, respectively). Since GWP is one of the most important impacts currently, it is obvious that maximizing yield is not only of economic inter-est, but also for environmental benefits. As the variation of the environmental load on the GWP is within the same order of magnitude than the change in biomass yield, it becomes clear that the uncertainty on this variable is crucial for the LCA conclusions concerning the GHGs mitigation potential of sweet sorghum ethanol.

Figure 6 shows a sensitive analysis of the effect of varying the input of hydrogen peroxide and sodium hydroxide on the final results, for the scenario PIV. An increase or decrease of 10% in the chemicals inputs affects the impact catego-ries in a range of 4.6–8.4%. The most sensitive categories

Fig. 5 Variation of the environ-mental load of the scenario PIII with respect to sweet sorghum biomass yield

Fig. 6 Variation of the environ-mental load on scenario PIV with respect to the amount of H2O2 and NaOH used in the bagasse pretreatment

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are HTP and ODP, followed by FWAEP, MAEP, and TEP. The least sensitive category is GWP, although it is affected significantly by the chemicals inputs (4.6% from a change in 10%). Although using the entire sweet sorghum stalk for bioethanol production (as in scenario PIV), it can be seen that it was not feasible in terms of mass and energy bal-ances, because it has greater energy requirements throughout the industrial process, with low ethanol productivity from bagasse. However, the data obtained in the laboratory by Franco-Brito (2011), Peniche (2012) and Equihua-Sánchez (2013) can still be optimized, meaning that the raw material and the energy requirements, particularly for the chemical pretreatment, could be lower.

Energy performance

Figure 7 presents the NER of the analyzed scenarios. Sce-narios PII and PIII turned out to be energetically viable due to the energy generated by the combustion of bagasse and also producing an electrical energy surplus, thus having val-ues of NER of 1.1 and 1.9, respectively. Previous research (IEE and Sweethanol 2011) mentions that the cogeneration process is essential for making any process sustainable, especially when dealing with alternative options to fossil fuels, such as biofuels, which consume large quantities of fossil energy during certain processes, such as milling and chemical pretreatments in the sweet sorghum system. After analyzing several scenarios for obtaining bioethanol from sugarcane, García et al. (2011) determined that obtaining bioethanol directly from the juice and generating heat and electricity with the bagasse resulted in an energetically prof-itable process with a NER of 4.8. On the other hand, Cai et al. (2013) assumed that the combustion of sweet sorghum bagasse provided sufficient heat and electricity to supply the process with NER values of 4.7 on a scenario of bioethanol production from juice and cogeneration from bagasse, and

3.6 for the production of bioethanol from juice and bagasse; the difference on the NER values might be due to a higher assumed ethanol yield (83.2 L of ethanol per ton of sugar-cane in García et al. (2011) against 33.84 in this study). Cai et al. (2013) proposed the use of products such as lignin and some other parts of the bagasse for cattle feed and for generating electricity, respectively.

Figure 7 also shows the distribution of the energy require-ments for the analyzed scenarios. On PI, the distillation stage requires 47,020 MJ (53% from the distillation column heat load and 67% from the vinasses treatment plant), corre-sponding to 49.9% of the total cumulative energy demand (CED). The sweet sorghum cultivation accounts for 29.4% of the CED, consisting of the energy required to produce the inputs such as fertilizers and herbicides, as well as the use of fossil diesel in agricultural machinery. These results are in line with those reported in other works (Wang et al. 2014; García et al. 2011; Acreche and Valeiro 2013; Cai et al. 2013), where a large part of the primary energy demand is due to the inputs required for the cultivation. In scenarios PII and PIII, the sweet sorghum cultivation and the distillation stage are the largest contributors to the CED, with 55.46 and 40.1%, respectively, due to the energy requirements to pro-duce raw materials and treat the vinasses, respectively. On the other hand, only 3.95, 0.28, and 0.23% corresponded to stalk transport, the fermentation process, and bagasse com-bustion, respectively. Also, on scenario PIV the chemical pretreatment process had the greatest contribution to the CED (59.12%). This was due to the production of H2O2 and NaOH required by the biomass pretreatment, as discussed before; due to the finer milling required, this process con-tributes to 16.2% of the total CED. Overall, scenario PIV requires 33,398 kWh of electricity and 85,664 MJ of heat; therefore, this option must cover 83% of its energy require-ments with fossil energy, compared to 8% on scenario PI and 4.5% on scenarios PII and PIII.

Fig. 7 Net energy ratio (left) and distribution of the required energy (right) of the analyzed scenarios

1695Life-cycle assessment of bioethanol production from sweet sorghum stalks cultivated in the…

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Conclusions

After analyzing the environmental loads and energetic per-formance of four scenarios for ethanol production from sweet sorghum cultivated in the state of Yucatan, the best options, energetically and environmentally, were scenarios PII and PIII, where the bioethanol production from the stalk juice is coupled with cogeneration using the bagasse for energy and heat. With the analysis of scenario PIV, one can conclude that increasing the production of bioethanol though SSF results in significantly increased energy demand and environmental impacts. In terms of the NER, only scenarios PII and PIII return more energy than the fossil energy they consume, the NER being at a maximum value of 1.9 when a combined cycle is used (PIII).

In scenarios PI, PII, and PIII, the cultivation of sweet sorghum was responsible for the largest emissions, mainly due to the production and use of agronomic inputs such as fertilizers and herbicides.

In all scenarios, the most energy intensive stages were the cultivation of sweet sorghum and the distillation of the bioethanol. The production of the required agronomic inputs is the main contributor of the energy requirements of the sweet sorghum cultivation. In the case of the distillation stage, the heat demand in the distillation columns and the treatment of vinasses are about the same on scenario PI; on scenarios PII and PIII, the heat demand in the columns is covered by the combustion of the residual bagasse, and therefore, the energy requirements of these scenarios are due exclusively to the treatment of the vinasses. Additionally, in scenario PIV, the chemical pretreatment process demands a similar amount of energy than these stages due to the pro-duction of large amounts of chemicals (hydrogen peroxide and sodium hydroxide). These differences in the scenarios cause that the environmental impacts in PIV are many times larger than the other three scenarios.

Acknowledgements The authors would like to thank the Bioenergy Thematic Network (Red Temática de Bioenergía, CONACYT) for its support on this work.

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