Second-generation bioethanol from industrial wood waste of South American species

substitution of a percentage of gasoline by ethanol. Pines and eucalyptus are the usually forested plants in these countries, and their industrial wastes, as chips and sawdust, could serve as promising raw materials to produce second-generation bioethanol in the context of a forest biorefinery. The process to convert woody biomass involves three stages: pretreatment, enzymatic saccharification, and fermentation. The operational conditions of the pretreatment method used are generally defined according to the physical and chemical characteristics of the raw materials and subsequently determine the characteristics of the treated substrates. This article also reviews and discusses the available pretreatment technologies for eucalyptus and pines applicable to South-American industrial wood wastes, their enzymatic hydrolysis yields, and the feasibility of implementing such processes in the mentioned countries in the frame of a biorefinery.


Introduction
The global use and production of biofuels have grown significantly in the last decade.The prominence of biofuels is evidenced by the increase of their production from 46 million L in 2006 to 118 million L in 2013 (Zaman et al., 2016).At commercial scale, ethanol is combined with gasoline, and biodiesel is blended with diesel.The main interest in biofuel production is attributed to the reductions achieved in the emission of greenhouse gasses (GHG) produced by fossil fuels.Nowadays, about 60% of the global bioethanol production is based on sugarcane while the rest is obtained mainly from corn and other crops.However, the use of edible crops for biofuels production has led to significant pressure on arable land originally used for the production of food crops and hence, investigation of other carbohydrate sources of no food/feed value is a necessity (Solomon and Bailis, 2014;Cremonez et al., 2015).
The development of bioethanol production responds more to social mandates rather than to economic issues.In fact, national and international policies including subsidies and tax exemptions, as well as biofuel blending mandate strongly encourage the production of biofuels (Laaksonen-Craig, 2008; Willem van Gelder et al., 2012; Cremonez et al., 2015).Without regulations, generally propped up by local producers or NGOs, bioethanol production would not have stood a chance to develop due to the fierce competition with the oil industry.For example, some of the recent regulations about biofuels in the European Union (i.e., Renewable Energy Directive 2009/28/EC, Fuel Quality Directive 2009/30/EC, and Directive to reduce indirect land use change for biofuels and bioliquids (EU)2015/1513), as well as in the United States (i.e., Energy Policy Act (EPAct) 2005 and 2013 Cellulosic Biofuel Standard, Final Rule, U.S. EPA, Vol.79, No. 85, May 2, 2014) have targeted increased inclusion percentage of renewable fuels in gasoline with a focus on the use of cellulosic and lignocellulosic materials as raw materials (EBTP, 2009;Duffield et al., 2015).The United States has set forth plans to produce 60 billion L of second-generation biofuel, i.e., about 20% of its liquid transportation fuel, by the year 2022.These biofuel policies have driven the second-generation biofuel development (Eisentraut, 2010;Balan, 2014).North America, especially the United States, has been an outstanding leader in cellulosic ethanol production (Griffin et al., 2016).Extensive information about biofuels policies and regulations can be found in the literature (Solomon et al., 2007;Sorda et al., 2010;Solomon and Bailis, 2014).
In Brazil, Proalcool (National Ethanol Program) Decree n. 76.593, 1975, first mandated the addition of ethanol to gasoline for use in motor vehicles, initiating a great expansion of the bioethanol industry in the country.Later, Law 10.464 / 02, 2002, requested for a mandatory blend of between 20 and 25% (Cassuto and Gueiros, 2012).Pilot plants of second-generation bioethanol are already operating in Brazil but the financing needed for commercial plants of such is around of USD 125-250 million (Eisentraut, 2010).In Argentina, the Law 26.093, Regime of Regulation and Promotion for the Sustainable Production and Use of Biofuels, 2006, stipulated 5% fuel bioethanol in the gasoline mix (Diputados, Senado y Cámara de Argentina, 2006).In line with that, Argentina produced around 350,000 m 3 of first-generation bioethanol from sugarcane and corn in the year 2010 (García et al., 2011) but the mandate was never fulfilled (Biofuels-Digest, 2016).Nevertheless, recent regulations have raised the proportion of bioethanol in gasoline to 10% (Lemos and Mesquita, 2016).Chile has also announced the intention of developing secondgeneration biofuels but it lacks available biomass resources (Eisentraut, 2010).
The global production of bioethanol in the year 2015 was 90 billion L (Araújo, 2016) out of which the United States and Brazil accounted for more than 70% (Achinas and Euverink, 2016).The liquid biofuels production also resulted for about 1.8 million jobs created worldwide in the year 2014, 47% of which took place in Brazil (Araújo, 2016).Latin America and the Caribbean reached a 27% of the worldwide biofuel production in the year 2012, mostly in the form of bioethanol and biodiesel (Solomon and Bailis, 2014).Brazil is the largest producer of ethanol in this region since the year 1960 and one of the main producers on a global scale.The market for first-generation ethanol is already established in Brazil, Colombia, and Argentina whilst it is in its developing stages in Paraguay and Bolivia as well (Janssen and Rutz, 2011).
The high volumes of wood industrialized in South American countries coupled with their intention to turn to renewable energies, make the secondgeneration bioethanol production a viable option to valorize the residues of the forest industry.It should be noted that the forest industry is in general relying increasingly on forests located in South America, Africa, and Asia (Toppinen et al., 2010).For example, pulp trade increased by 3% in 2015 due to the startup of new pulp mills in Brazil and Uruguay (FAO, 2015).Uruguay has encouraged forestry in the last decades, and consequently, the new forest sector has grown rapidly, attracting foreign investments due to the attractive cost-benefit ratios (Olmos and Siry, 2009).The main forest cultures in South America are pines and eucalyptus.These species are globally considered as good raw materials for papermaking and wood products and are mainly industrialized in Brazil, Chile, Uruguay, and Argentina (FAO, 2006).
The use of the lignocellulosic biomass is considered as a sustainable pathway for biofuels production as substitution for fossil fuels.Life cycle assessment studies evaluating the environmental implications of the production of ethanol from fast-growing wood crops in comparison with conventional gasoline have shown reductions in almost all impact categories under assessment when shifting to ethanol-based fuels (González and García, 2015).Another advantage of or second-generation ethanol (cellulosic bioethanol) over the technologically mature firstgeneration ethanol is the ability to use different types of lignocellulosic materials as a source of glucose.In this context, lignocellulosic materials are being intensely studied as feedstocks for bioethanol production, while focusing on improving the technological processes involved in order to reduce the production cost of fermentable sugars and their fermentation to ethanol (Alvira et Zabed et al., 2017).However, among sixteen commercial-scale cellulosic ethanol projects using sugar platform in the world in the year 2012, only one has been based on wood as raw material (Araújo, 2016).
The present manuscript briefly reviews forestry and wood industry in South America and the potentials for biofuels and particularly bioethanol production.It also presents a short description of the main regional forest resources and their characteristics, including the availability of wood residues that could be potential sources for bioethanol production.Subsequently, available pretreatment technologies and their enzymatic hydrolysis (EH) yield for the main feedstocks of the South-American wood industry, i.e., eucalyptus and pines wood wastes were discussed.Finally, the feasibility of implementing relevant processes in the frame of a biorefinery for Argentina, Brazil, Chile, and Uruguay was presented.

Forestry and wood wastes in South America
Forest plantations in South America consist almost exclusively of fastgrowing exotic species.For example, Eucalyptus and Pinus in Brazil have rotation lengths of 8-10 and 16-25 years and mean annual increments of 18-20 and 15-25m³ ha -¹ yr -¹, respectively (FAO, 2001).Much progress has been made in the improvement of the yield and in the silviculture of fastgrowing species, since in Argentina, for example, the growth is almost two folds that of two decades ago.In addition to the geographical advantages, this is due to the permanent development of silvicultural techniques oriented to improve the productivity and sustainability of the plantations.
Please cite this article as: Vallejos M.E., Kruyeniski J., Area M.C.Second-generation bioethanol from industrial wood waste of South American species.Biofuel Research Journal 15 (2017) .
Plantations have been observed in recent times due to their alleged negative effects on water resources, soil, and biodiversity.Hence, efforts have been intensified to design plantations adapted to specific conditions of each region, and site, with the purpose of minimizing or totally avoiding such effects, and on the contrary to increase the production of environmental services such as watershed regulation, carbon capture, and soil stabilization (Idígoras, 2016).
The most relevant species and the most suitable lignocellulosic wastes potentially available as feedstock for the production of ethanol in South America are shown in Table 1.The advantages of wood wastes compared with agro -industrial wastes are related to the harvesting, storage, and transportation.Wood wastes are in general unexploited resources with great potential for ethanol production (220-285 L/ton of wood).They have low ash content and their transport cost is low because of their high density, as compared with agro-industrial wastes (Zabed et al., 2017).
Most wastes from sawmills in Argentina and Brazil are scarcely utilized and are usually burned for energy production.In Brazil and based on the data recorded in the year 2007, about 30% of the forestry processing residues (5,500,000 tons of dry matter per year) were unused (Kline et al., 2008).The projections of the wastes and biomass associated with current .
forestry activities which are potentially available for bioethanol production between the years 2017 and 2027 in Brazil and Argentina could stand at 7,800,000 and 500,000 of metric tons, respectively.Biomass wastes derived from the lumber industry (sawdust, bark, and harvesting residues) can reach 1.8 million of m 3 in Chile between 2019 and 2021.These wastes are used as industrial fuel to generate heat and electricity (Berg and Segura, 2016).A current ongoing initiative is the consortium BIOENERCEL S.A., which was created to develop technologies and human resources for the conversion of lignocellulosic biomass to ethanol and biodiesel (García et al., 2011).
The estimation of the available volumes of current and potential lignocellulosic materials from wood wastes is tough to quantify due to the social and environmental complexity of the scenarios.However, the potential growth of the forest surplus in the year 2050, in a scenario of average plantations and demands, is estimated at 6.4 Gm 3 (74 EJ yr -1 ) where 40% is expected to end up as wastes.Figure 1 shows the proportion of forest residues and wastes potentially available, estimated from a study on the global bioenergy potentials towards 2050.
Wood harvest residues are twigs, branches, and stumps.Industrial process residues are residues generated during the processing of wood into final products.Most wood processing residues are sawdust and wood chips.Wood waste is discarded wood products, such as waste paper and demolition wood.The estimated bioenergy potential of wood wastes based on their energy content on dry basis ranges from 17 to 21 MJ/kg are (average 19 MJ/kg) and accordingly, bioenergy production potentials at global scale will be: wood harvest residues 8 EJ yr -1 , wood process residues 11 EJ yr -1 , and wood waste 11 EJ yr -1 , 1 EJ = 10 18 J (Smeets et al., 2007; González and González, 2015).
The wood of the genus Eucalyptus has a similar structure, indistinctly of the species.They have libriform fibers for mechanical support (between 40 and 80% of the tissue), fiber-tracheids, and vasicentric tracheids for both transport and support, and vessel cells with tubular form elements that are interconnected to form long vessels for liquids transport.Despite this, there is a great variation in density and durability among species.For example, the specific gravity of E. grandis varies between 0.48 and 0.64, whereas that of E. globulus is 0.68-0.82and for E. camaldulensis (industrialized wood) is 0.67-0.87(Meier, 2015).
On the contrary, ninety percent of the wood structure of conifers is formed by a single kind of longitudinal cells (tracheids) which perform both liquid transport and support (Area and Popa, 2014).Loblolly pine has greater growth rates and is more suitable for the pulp industry and some uses of wood whereas slash pine is a rustic species which produces resin, sometimes commercialized as a by-product.Both loblolly and slash pines are harder, denser (specific gravities: 0.41-0.51P. radiata; 0.47-0.57P. taeda; 0.54-0.66P. elliotti), and possess better strength-to-weight ratio than radiata pine (Meier, 2015).
Cellulose, hemicelluloses, and lignin are the main components of wood in all trees.Lignin is formed by random copolymers deriving from unsaturated alcohol derivatives of phenyl-propane, having several functional groups as methoxyl, phenyl hydroxyl, benzyl alcohol, and carbonyl groups.Hemicelluloses are composed mostly of glucuronoxylan, glucomannan, galactoglucomannans, arabinoxylans, and glucuronoarabinoxylans in hardwoods and gramineous plants, whereas they are formed mainly of galactoglucomannans, arabinoglucuronoxylan, and arabinogalactan in softwoods.Cellulose is formed by linear polymers of β (1-4) D-glucopyranosyl units, mostly aggregate in crystalline, highly ordered structural entities.Hardwoods and softwoods also have minor but varying amounts of extractives as fats, waxes, alkaloids, proteins, gums, resins, starches, and ash (Vallejos et al., 2017).Lignin is not bound directly to cellulose, but it is covalently bound to hemicelluloses, which is in direct relation with the swelling capacity of wood (Salmén and Burgert, 2009).
A comparison of the chemical composition of regional (South American) woods is shown in Table 2.
The data presented in the table reveal the great variations in chemical composition due to the different species and ages of the trees.For example, old trees of E. camaldulensis (red eucalyptus), widely used in sawmills, show a composition totally different from E. grandis used for pulp manufacture, proving that the processes of conversion of raw materials into sugars for bioethanol production must be optimized in each case.

Pine and eucalyptus pretreatments for bioethanol production
A pretreatment is essential to make cellulose more accessible to the enzymatic attack for the production of second generation bioethanol.The requirements that an effective pretreatment should meet are (Bengoechea et al., 2012): -Reduction of the crystallinity of cellulose.
-Elimination of acetyl groups from hemicelluloses.
-Elimination of the bonds between hemicelluloses and lignin, with the consequent separation of lignin.
-Increase in the surface area of the material.
-Minimal formation of toxic degradation products to avoid or simplify the detoxification stage.
-Low energy consumption and investment cost.
-Use of cheap and easily recoverable reagents.
The most promising strategy is to integrate ethanol production within a biorefinery scheme in which lignin, hemicelluloses, and extractives from the lignocellulosic biomass would be converted into high-value coproducts. .This would assist with compensating for the costs associated with pretreatment and enzymes used for the hydrolysis of cellulose.The biorefinery thereby extends the concept of pretreatment to a fractionation of the material, obtaining fractions as pure as possible for their use and transformation into high-value products.
The most studied pretreatments and their effects are: ).Literature about bioethanol production from pines is limited, and specifically about South-American pines is non-existent.A summary of pretreatments atempted in the last years to increase enzymatic digestibility of pine substrates is shown in Table 3.
Most relevant pretreatments studied to increase enzymatic digestibility of eucalyptus substrates are shown in Table 4.
For a better visualization of the relationship between processes and EH, the pretreatments referenced in Table 3 and 4 were grouped in order of EH as low, medium, and high, and were schematized as shown in Figures 2, 3, and 4 (pine) and Figures 5 , 6, and 7 (eucalyptus).Since references were heterogeneous in the form to express EH yields, the non-comparable schemes were excluded.
General studies have stated that dilute acid hydrolysis followed by EH is a promising technology for all raw materials, including pines (Galbe and Zacchi, 2002;Chandel et al., 2007).Nevertheless, the achieved digestibility is rather low in spite of the increasingly complex quantity and the variety of pre-treatments tested for pine in the last two years (Tian et al., 2016;Rajagopalan et al., 2017; and others listed down in Table 3).
Most studied processes for pines include mechanical and acid pretreatments followed by alkaline or organosolv delignification.Results of EH are generally poor, with few cases above 90%.Best digestibilities (EH > 90%) were obtained on sawdust, using alkaline treatments without a previous additional stage.The only exception was a case of radiata pine but it may be ascribed to its comparatively low density, as mentioned in Section 2. The operational conditions of the pretreatment methods are defined according to the physical and chemical characteristics of the raw material and determine the characteristics of the treated substrate.The digestibility of pine with high lignin content is low, so some lignin must be extracted (Meier, 2015;Kruyeniski et al., 2016a).The intention of acid pretreatments is to extract hemicelluloses to increase porosity, but it does not result in an improvement in the EH of pines because of lignin condensation, being harmful to its final form and use (Sannigrahi et al., 2008;Stoffel et al., 2014).In conclusion, new processes must be developed and optimized to improve the digestibility of resinous pines.
Like for pines, the best results for eucalyptus EH (between 90% and 100%) could be obtained by delignification treatments.Nevertheless, in this case, the inclusion of a previous additional stage involving hot water with or without acid (acid hydrolysis or autohydrolysis generated by the deacetylation of xylans) could lead to enhanced EH.Unlike pines, it is possible to apply a combination of acids, alkalis, and solvents when handling eucalyptus without producing a significant condensation of lignin, to obtain high EH yields and byproducts (Figures 3, 4, 6 and 7).This allows the extraction and exploitation of hemicelluloses and lignin in the biorefinery context.However, the recovery of chemicals and the scale of production should also be taken into account when delignifying processes such as kraft or sulfite are included.On the contrary, the application of treatments such as ionic liquids or complex solvents seems not to be so effective in enhancing the EH of these hardwoods.It is also clear that high enzyme charges would not be needed to increase digestibility if the correct pretreatment would be applied.
Comparing the EH data in the different schemes, it is evident that eucalyptus wood is less recalcitrant to EH than pine wood, and that there is already a sufficient study background to define applicable pretreatment technologies with smaller adjustments.
Almost all reports on pines and eucalyptus indicate that pretreatments should involve mechanical treatment for size reduction of the raw material as well.This is unnecessary when using wood wastes like sawdust, which is a basic advantage reducing costs and technological complexity.However, studies on pretreatments for bioethanol production from both      Ethanol yields are usually expressed in comparison with the theoretical yield, i.e., 0.511 g of ethanol/g hexose.Like in the case of the EH, ethanol yields are influenced by the raw material, the pretreatment, and the fermentation process, which can be Separate Hydrolysis and Fermentation (SHF) or Simultaneous Saccharification and Fermentation (SSF).In several cases, it is also necessary to apply a detoxification process to eliminate  fermentation inhibitors such as 5-hydromethyl-furfural (HMF), furfural, or acetic acid among others (Hahn-Hägerdal et al., 2006).Reported ethanol yields obtained from pretreated pines with methods resulting in low to medium saccharification (Table 3 and Figures 2 and 3) were generally low, in the range of 28% (Bahmani et al., 2016) to 46.6% (Tain et al., 2016), using an SHF process.On the contrary, delignified materials present high ethanol yields, either using SHF or SSF, for example, 80.42% with an SSF process (Valenzuela et al., 2016), 88% with an SHF (Fárias-Sánchez et al., 2017), and about 90% applying an SHF process (Kruyeniski et al., 2016b).Ethanol yields in the case of eucalyptus are usually high, without differences among materials subjected to acid hydrolysis or delignification.Based on the data reported in the selected works tabulated in Table 4, an ethanol yield of 86.4% was obtained by the SSF of a hydro-thermally pretreated material at 230°C (Romaní et al., 2010) and 92% was obtained with an SHF process using a material pretreated by diluted acid (McIntosh et al., 2012).Furthermore, the ethanol yield of the SSF of a kraft pulp was 96% (Monrroy et al., 2012) while that of an organosolv-pretreated pulp stood at 91.1% (Romaní et al., 2011).Nevertheless, the ethanol yield of the SSF of wood pretreated by an ionic liquid resulted in only 38% (Lienqueo et al., 2015), while that of the pre-saccharification simultaneous saccharification fermentation (PSSF) of eucalyptus wood pretreated by diluted acid and steam explosion at pilot scale was 42% (McIntosh et al., 2016).This conforms that certain pretreatments lead to physicochemical changes in the materials used which may not be positive for the EH nor for fermentation.

Technical and economic aspects of bioethanol production
As mentioned earlier, second-generation bioethanol is still under development at pilot and pre-commercial scales.In better words, its economic feasibility at large scale has not yet been justified because of its high costs, which are two to three times more expensive than petroleum fuels considering an equivalent energy basis ( Lynd et  Nevertheless, the production cost can be decreased by: (i) improvement in feedstock production and logistics, (ii) increase in energy efficiency of the processes involved (i.e., pretreatment, saccharification, and fermentation) and (iii) the production of multiple products (Carriquiry et al., 2011;Melin et al., 2011).On the other hand, in most cases, energy cost determines the global process cost.Therefore, energy savings by optimizing the operating conditions of the different systems are essential to increase profit margin and reduce emissions (Kemp, 2007).
Overall, developing suitable pretreatments to minimize energy consumption and to improve enzymatic saccharification and fermentation are key to achieve high sugars and ethanol yields.Any given pretreatment process can be evaluated through its energy efficiency and the attainable sugars yield (Zhu and Pan, 2010;Walker, 2011;Kang et al., 2014).In biorefineries, the pretreatments implemented could affect the downstream processes, the scale-up, and the technological scheme.The type of pretreatment could also determine the chemical recovery processes and the wastewater treatment.In general, scale-up is a technological challenge that involves high capital investments, as well as detailed research and development (Naik et al., 2010;Aditiya et al., 2016;Muktham et al., 2016).
As mentioned in the previous section, size reduction (increasing surface area) of wood chips by milling could improve the enzyme accessibility to cellulose.Unlike agricultural wastes, wood chips milling requires high electric-mechanical energy, approximately 500 to 800 kWh/ ton, which is equivalent to 25 -40% of the thermal energy produced by the ethanol.Therefore, pretreatments that need a prior size reduction, e.g., those with ionic liquids (IL), should take into account this energy demand (Zhu et al., 2010).An alternative to size reduction is the use as feedstock of sawmills wastes such as sawdust because these lignocellulosic materials do not require size reduction.
On the other hand, by performing size reduction of wood chips after chemical treatment, an energy savings of about 80% could be achieved.So, the cellulosic pulping industry has a high potential for bioethanol production, and its treatments, processes, and equipment are technologically exchangeable.Other benefits of size reduction after a chemical treatment are a better separation of the pretreated solids from the liquid, energy savings in mixing with respect to the pretreatment of .fiberized or pulverized materials, and a reduction of thermal energy in the chemical treatment due to the use of low liquid-to-solid ratios (LSR).Fiberized materials require high LSR because they have much more water intake than wood chips, and consequently, they need a greater thermal energy for heating up the water or liquor (Zhu et al., 2010;Vallejos et al., 2012Vallejos et al., , 2015Vallejos et al., and 2017)).
Temperature and LSR mainly govern the thermal energy consumption in chemical pretreatments, so the reduction of these parameters is critical for the increase of their energy efficiency (Balan, 2014;Kang et al., 2014).The pretreatments with IL are carried out at temperatures below 100°C but require LSR of 10 to 20 (Tian et al., 2016).Although the temperature is low, high LSRs increase the thermal energy consumption to values greater than the thermal energy of aqueous thermochemical pretreatments.For example, the required energy at 75°C and LSR of 10 is 18% more than that required at 180°C and LSR of 3 (Zhu et al., 2010).The performance of different pretreatments at varied LSR was studied in several works (Saska and Ozer, 1995 The production of multiple products is key to have competitive production cost against the first-generation biofuels and that depends on the pretreatment processes.Several high-value products can be produced through the secondgeneration biorefineries to reduce the overall processing cost of biofuels (Stephen et al., 2012;Balan, 2014).Lignin is an aromatic polymer usually used in the pulp and paper industry to generate energy (heat and power).Regional pines can be a valuable source of vanillin, which is produced from lignin at large commercial scale and competes with vanillin based on guaiacol derived from petroleum (Pinto et al., 2013).On a smaller scale, bakelite, resins, and plastic filler materials can also be produced and new byproducts will surely be obtained from lignin based on catalytic processes in the following years.The type of recovered lignin depends on the treatments used to remove it from the biomass.For example, high purity isolated organosolv lignin can be used for producing high valued byproducts (Hubbe, 2015).Isolated lignin from a steam explosion or dilute acid pretreatments is highly condensed and can be used for energy generation through producing products like pellets or brickets (Stephen et al., 2012).
Important aspects to take into account when starting a business of this type are the costs of feedstock, enzymes, and capital since they are crucial in defining the costs of second-generation bioethanol.According to a calculation made in the year 2016, the contributions of the costs of each one of these factors to bioethanol production cost was USD 0.26/L for feedstock, USD 0.26-0.40/Lfor enzymes, and USD 1.85/L for capital investment, including on-site enzyme production (Araújo, 2016).
Bioethanol production costs depend on the biomass source and only a few kinds of biomass having prices close to fossil fuels can be competitive.For bioethanol to compete economically with petrol, production costs should be no greater than EUR 0.2/L approximately.Some bioethanol production costs from wood are EUR 0.44 -0.63/L for spruce (softwood), EUR 0.48 -0.71/L for willow (hardwood), and EUR 0.11 -0.32/L for wood wastes (Walker, 2011).The great advantage of using low-cost sawing waste is accordingly very clear.
Estimations for Brazil in the year 2020 show that eucalyptus production costs, specifically as raw material for bioethanol, will be 2.4-3.3USD/GJ generated, involving mainly fertilizers and harvesting costs.The technological scheme proposes a pretreatment sequence including mechanical and acid treatments followed by enzymatic saccharification of the residual solids and fermentation of sugars.The investment costs for a capacity of 400 MWth would be USD 374 million (van Eijck et al., 2014).
Moreover, a techno-economic analysis on the production of secondgeneration bioethanol concluded that high-performance enzymes at a price less than USD 18.2/L of ethanol would be required and that higher ethanol concentration in the fermentor would be needed to be competitive (Kazi et al., 2010).The reduction in the hydrolysis time while maintaining the same yield would could also result in a reduction of capital cost.Improvements could also be achieve through the implementation of the SSF.Further research is still needed in this domain to achieved significantly higher level of optimization of the processes involved.
Energy integration, chemicals recovery, higher capacities, and integration of the ethanol plant with already existing facilities, could also reduce the ethanol production cost (Von Sivers and Zacchi, 1995).As examples, the combination of enzymes recycling and decreases in hydrolysis time led to decreased ethanol production cost by 27% for hardwoods and 38% for softwood feedstocks (Gregg et al., 1998).The co-location of the bioethanol plant into a softwood kraft-pulping mill, using the kraft process plus oxygen delignification as pretreatment were also shown to result in economic production of bioethanol (Wu et al., 2014).More specifically, an economic analysis showed that through such implementations, an ethanol yield of 285 L/ton of dry wood with a total production cost of USD 0.55/L could be obtained (Wu et al., 2014).In this sense, it would be interesting to evaluate how the incorporation of a different pretreatment to a different raw material (like pin-chips or sawdust) would work, taking advantage of the existing infrastructure of the mill.

Conclusions
Eucalyptus and pines are the most important woody raw materials in South America.Most processes applied as pretreatments to pine begin with a reduction in the size of the materials by grinding, except in the case of delignifying processes that, in general, work directly with chips.Treatments with diluted acid, steam explosion or supercritical CO2, aimed at the extraction of hemicelluloses, slightly affect the digestibility of the material.If acid-treated pine wood is delignified, the yields of EH increase but to a less extent than when the delignification treatments are applied to the untreated materials.Materials treated under mild conditions (slightly acidic or alkaline processes, including organosolv delignification), generally have lower EH yields than either medium or highly alkaline treatments.The above are conclusions generally drawn, a few cases have been mentioned in which treatments with diluted acids and diluted alkalis led to high EH yields though.This could be ascribed to the variations in the materials used affecting the subsequent delignification or in better words, to the particular characteristics of the raw materials (pine).
Eucalyptus wood is less recalcitrant to EH than pine wood, so autohydrolysis and alkaline pretreatments are effective options.Novel and more complex treatments or treatment combinations are being studied, but EH yields do not exceed that of alkaline delignification.
Like in the case of the EH, ethanol yields are influenced by the raw materials, the pretreatment, and the fermentation process.The differences between pines and eucalyptus observed in the EH are reflected in the yields of ethanol obtainable from pretreated materials.
Overall, the success of the second-generation bioethanol depends on its technical, environmental, and economic feasibility.The price of the lignocellulosic raw material is one of the most important items in the cost distribution of second-generation bioethanol, whence it is clear the great advantage of using low-cost sawing waste.Biorefineries can also contribute to the reduction of the overall processing cost of bioethanol production by processing wood wastes, using energy and cost-effective technologies, and simultaneous production of high added-value products.
Second-generation bioethanol could generally be regraded as a viable option to valorize the residues of the forest industry in South America.This could be well explained by the highly economically available and unexploited wood residues generated by the fast-growing plantations in Argentina, Brazil, Chile, and Uruguay on one hand and the growing interest in further development of renewable energies in the region on the ither hand.

Fig. 1 .
Fig.1.Proportion of forest residues and wastes potentially available, estimated from a study of the global bio-energy potentials to 2050 (adapted from Smeets et al., 2007).* n.d.: not defined.

Table 1 .
Forest resources and lignocellulosic wastes available as feedstock for the production of 2 nd generation bioethanol in South America.

ha) Plant type or waste Area (%) Amount of waste (ton/y) Ethanol production potential Refference
Please cite this article as: Vallejos M.E., Kruyeniski J., Area M.C.Second-generation bioethanol from industrial wood waste of South American species.Biofuel Research Journal15 (2017)

Table 2 .
Comparative chemical composition of regional Pinus and Eucalyptus.

Table 3 .
Summary of pretreatments methods used for pine and their corresponding enzymatic digestibility.

Table 4 .
Summary of eucalyptus pretreatments and their corresponding enzymatic digestibility.