Production of Biofuels and Biochemicals from Lignocellulosic Biomass

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Energy Fuels 2010, 24, 2657–2666 Published on Web 03/17/2010

: DOI:10.1021/ef901379s

Production of Biofuels and Biochemicals from Lignocellulosic Biomass: Estimation of Maximum Theoretical Yields and Efficiencies Using Matrix Algebra Francesco Cherubini* and Anders Hammer Stroe mman Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway Received November 16, 2009. Revised Manuscript Received February 4, 2010

The dependence on fossil fuels in developed countries is causing increasing concern. Global warming, issues related to peak oil, sustainability issues, and mounting concern for national energy security are the main drivers for a worldwide effort toward a reduction in fossil fuel consumption. The challenge is substantial, because fossil resources are such an integral part of our economy. However, there are many efforts to address this challenge. Development of conversion technologies fed by renewable resources is seen as a promising option. Many technologies for renewable energy are already well-developed and competitive in the market. Emerging technologies include biorefinery complexes, where biomass is used as a renewable carbon-based source for the production of bioenergy and biochemicals. The latter is perceived as a promising alternative to oil-based chemicals. Given the constraints on availability for renewable biomass supply, the importance of efficient use of biomass with a maximization of useful final products is well-acknowledged. Assessing the potentials for biochemicals can be achieved with an a priori estimation of the maximum theoretical yields, as well as a prediction of the conversion efficiencies (in terms of mass, carbon, and energy efficiency) of selected biorefinery production chains. This paper addresses this issue, providing a calculation procedure with which the theoretical yields and efficiencies of some biorefinery systems are estimated. Among the possible biomass sources, lignocellulosic biomass is selected as the raw material, because it is the most-widespread renewable source available in the world, it is locally available in many countries, and it does not compete with food and feed industries. The conversion of biomass to biofuels and chemicals requires conversion of the feedstock from a solid to a liquid state, but also the addition of hydrogen and rejection of excess oxygen, together with other undesired elements. The carbon contents of lignocellulosic biomass components (cellulose, hemicellulose, and lignin) and products are calculated with the help of mathematical equations, and then the chemical reactions for the conversion of feedstock to products are modeled using matrix algebra: the maximum amount of biofuels and/or biochemicals from biomass and the maximum mass, energy, and carbon conversion efficiency of the biorefinery pathway are determined. Following this calculation procedure, an application to some biorefinery systems is performed and discussed. Combining the best feedstock with the most promising final products, results show that up to 0.33 kg of bioethanol, 0.06 kg of furfural, and 0.17 kg of FT-diesel per kg of softwood can be produced and mass, carbon, and energy conversion efficiencies of 56%, 70%, and 82%, respectively, are achieved.

and others), the production of carbon-containing transportation fuels and chemicals can just rely on biomass, which is the only alternative carbon source to fossils on Earth. Today, the first generation biofuels and chemicals are produced from sugars, starches, and vegetable oils, but these production routes usually give rise to several issues. They compete with food and feed industries for raw materials and fertile land, their potential availability is limited by soil fertility and per-hectare yields, and the effective savings of CO2 emissions and fossil energy consumption are limited by the high energy input required for crop cultivation and conversion, which simultaneously burden other environmental impact categories such as eutrophication and acidification.3,4

1. Introduction and Aim Our global dependence on fossil resources is causing increasing concern. The main reason for this is that their production and use is associated with several environmental concerns, especially anthropogenic carbon dioxide emissions, which contribute to global warming. Furthermore, there are issues related to peak oil and security of supply, as well as price instability and global equity aspects. The utilization of biomass as raw material in a biorefinery is a promising alternative to fossil energy for the production of energy carriers and chemicals, as well as for enhancing energy security and mitigating climate change.1,2 In fact, while electricity and heat can be generated from a wide spectrum of alternatives (sun, wind, hydro, geothermal heat,

(3) Cherubini, F.; Bird, N. D.; Cowie, A.; Jungmeier, G.; Schlamadinger, B.; Woess-Gallasch, S. Energy- and greenhouse gas-based LCA of biofuel and bioenergy systems: Key issues, ranges and recommendations. Resour. Conserv. Recycl. 2009, 53 (8), 434–447. (4) Zah, R.; B€ oni, H.; Gauch, M.; Hischier, R.; Lehmann M.; W€ager, P. Life Cycle Assessment of Energy Products: Environmental Assessment of Biofuels; EMPA: Bern, Switzerland, 2007.

*Author to whom correspondence should be addressed. Tel.: þ4773598942. E-mail: [email protected]. (1) Cherubini, F.; Jungmeier, G.; Wellisch, M.; Willke, T.; Skiadas, I.; Van Ree, R.; de Jong, E. Toward a common classification approach for biorefinery systems. Biofuels, Bioprod. Biorefin. 2009, 3 (5), 534–546. (2) Cherubini, F. From oil refinery to biorefinery: production of energy and chemicals from biomass. Ital. Biol. 2009, 5, 46–61. r 2010 American Chemical Society

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Some of these limitations can be overcome by the utilization of lignocellulosic materials, such as residues from agricultural, forestry, and industry and dedicated energy crops.5-7 In fact, lignocellulosic biomass can be grown in combination with food (e.g., straw and corn stover) or on non-agricultural lands and degraded soils. Over the next 10-15 years, it is expected that lower-cost residue and waste sources of cellulosic biomass will provide the first influx of next-generation feedstocks, with cellulosic energy crops expected to begin to serve as feedstocks for bioenergy production toward the end of this time frame, then expanding substantially in the years beyond.8 The reason for this expected growth in lignocellulosic biomass supply is two-fold: on one hand, the raw material situation is favorable (widespread, relatively inexpensive, and easily available); on the other hand, their use could allow the co-production of valuable biofuels, chemical compounds, as well as electricity and heat, leading to better energy, environmental, and economic performance through the development of biorefinery concepts.9 A biorefinery is a facility that integrates biomass conversion processes and equipment to produce biofuels, power, and chemicals from biomass. The biorefinery concept is analogous to today0 s petroleum refineries, which produce multiple biofuels and products from petroleum. The efficient production of transportation biofuels can be seen as one of the most important promoting factors for the future development of biorefineries.10 The transportation sector is growing steadily, and the demand for renewable (bio)fuels, which can only be provided from biomass, grows accordingly. A large number of world countries have targets for improving the shares of biofuels in the national transportation sector. For instance, Europe aims at a share of 5.75% in 2010 and 10% in 2020 of biofuels, according to the draft directive for renewable energy, while IEA and IPCC expect a significant contribution of biofuels on transportation market in 2030 (10%-20%).11 Currently, the most common biofuels produced today in the world are bioethanol, biodiesel, and biogas (or biomethane). Already commercially available biobased products include adhesives, cleaning compounds, detergents, dielectric fluids, dyes, hydraulic fluids, inks, lubricants, and others. It should be noticed that the replacement or integration of oil refinery with biorefinery will require some breakthrough

changes in the today’s production of goods and services: biological and chemical sciences will play a leading role in the generation and design of future industries. Therefore, new synergies of biological, physical, chemical, and technical sciences are needed.9 To meet the increasing demand for a quantification of the potential for output of bio-based products, this paper aims at calculating, by means of matrix algebra, the maximum theoretical yields of biofuels and biochemicals, which can be produced from lignocellulosic biomass. Solutions are shown in a generic algebraic form and are adapted to be used in many different settings (i.e., with different feedstocks and products). Simultaneously, the conversion efficiencies of raw materials to final products are evaluated by means of mass, energy, and carbon conversion efficiencies. The aim of these theoretical results is to act as targets for the upcoming biorefinery industry and play a role of benchmark for “real” cases assessed in the literature, to point out how much the effective yields and efficiencies differ from the ideal performances. As a consequence, it becomes possible to estimate the existing technological gap of biomass conversion technologies. Furthermore, this paper illustrates that these seemingly complex issues can be addressed with simple matrix algebra. In the following sections, the investigated feedstocks and products are presented, and, after a description of the calculation procedure, the maximum theoretical yields and efficiencies of selected biorefinery examples are shown and discussed. 2. Biomass Feedstocks and Final Products 2.1. Biomass Feedstocks. The structure of biomass raw materials is totally different from that upon which the current oil refinery is based. Unlike petroleum, biomass composition is not homogeneous, because the biomass feedstocks might be made of grains, wood, grass, biological waste, and others, and the elemental composition is a mixture of carbon, hydrogen, and oxygen (plus other minor components such as nitrogen, sulfur, and other mineral compounds). The compositional variety of biomass is both an advantage and a disadvantage. An advantage is that biorefineries can produce a wider array of product classes than petroleum refineries and that they can feed on a wider range of raw materials. A disadvantage is that a relatively larger range of processing technologies is needed, and most of these technologies are still at a precommercial stage.12 To be used for the production of biofuels and biochemicals, biomass must be depolymerized and deoxygenated. Deoxygenation is required because the presence of oxygen in biofuels reduces the heat content of molecules and usually gives them high polarity, which hinders their blending with existing fossil fuels.13 Chemical applications may require much less deoxygenation, because the presence of oxygen often provides valuable physical and chemical properties to the product. In this paper, the lignocellulosic biomass raw materials considered as feedstocks are hardwood, softwood, switchgrass (a promising energy crops), corn stover, and wheat straw. Their composition is reported in Table 1.

(5) Spatari, S.; Bagley, D. M.; MacLean, H. L. Life cycle evaluation of emerging lignocellulosic ethanol conversion technologies. Bioresour. Technol. 2010, 101 (2), 654–667. (6) Gonz alez-Garcı´ a, S.; Gasol, C. M.; Gabarrell, X.; Rieradevall, J.; Moreira, M. T.; Feijoo, G. Environmental aspects of ethanol-based fuels from Brassica carinata: A case study of second generation ethanol. Renew. Sustain. Energy Rev. 2009, 13 (9), 2613–2620. (7) Cherubini, F.; Ulgiati, S. Crop residues as raw materials for biorefinery systems;A LCA case study. Appl. Energy 2010, 87 (1), 47–57. (8) Worldwatch Institute. Biofuel for Transport: Global Potential and Implications for Energy and Agriculture. Prepared by Worldwatch Institute for the German Ministry of Food, Agriculture and Consumer Protection (BMELV), in coordination with the German Agency for Technical Cooperation (GTZ) and the German Agency of Renewable Resources (FNR) ; Earthscan: London, 2006. (9) Kamm, B.; Gruber, P. R.; Kamm, M., Biorefinery systems;An overview. In Biorefineries;Industrial Processes and Products (Status Quo and Future Directions); Kamm, B., Gruber, P. R., Kamm, M., Eds.; Wiley-VCH: Weinheim, Germany, 2006; Vol. 1. (10) Cherubini, F.; Jungmeier, G. Energy and material recovery from biomass: the Biorefinery approach. Concept overview and environmental evaluation. In Energy Recovery; DuBois, E., Mercier, A., Eds.; Nova Science Publishers: New York, 2009. (11) European Biomass Statistics (EBS). A Statistical Report on the Contribution of Biomass to the Energy System in the EU 27; European Biomass Statistics: Brussels, Belgium, 2007.

(12) Dale, B. E.; Kim, S. Biomass refining global impact;The biobased economy of the 21st century. In Biorefineries;Industrial Processes and Products (Status Quo and Future Directions); Kamm, B., Gruber, P. R., Kamm, M., Eds.; Wiley-VCH: Weinheim, Germany, 2006; Vol. 1. (13) Lange, J.-P. Lignocellulose conversion: An introduction to chemistry, process and economics. Biofuels, Bioprod. Biorefin. 2007, 1 (1), 39–48.

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Table 1. Composition of the Investigated Lignocellulosic Biomass Feedstocks parameter LHV (MJ/kg) total cellulose (%) glucan (C6) (%) total hemicellulose (%) xylan (C5) (%) arabinan (C5) (%) galactan (C6) (%) mannan (C6) (%) lignin (%) acids (%) extractives (%) C (%) H (%) O (%) N (%) S (%) ash (%)

hardwood

softwood

switchgrass

corn stover

wheat straw

19.5 42.9 42.9 20.3 16.9 1.19 0.70 1.42 26.6 3.11 4.70 49.4 5.75 42.3 0.19 0.02 2.43

19.6 44.5 44.5 21.9 6.30 1.60 2.56 11.4 27.7 2.67 2.88 50.3 5.98 42.1 0.03 0.01 0.32

17.1 31.9 31.9 25.2 21.1 2.84 0.95 0.30 18.1 1.21 17.5 47.3 5.31 41.6 0.51 0.10 5.95

16.0 37.7 37.7 25.3 21.6 2.42 0.87 0.38 18.6 3.00 5.61 47.00 5.66 41.4 0.65 0.06 10.1

16.9 42.6 42.6 25.4 23.1 1.52 0.49 0.30 24.1 1.28 2.00 48.5 5.83 41.2 0.16 0.02 4.04

All types of lignocellulosic biomass are composed of three primary components: cellulose, hemicellulose, and lignin. Cellulose (having the formula (C6H10O5)n) is a strong C6-polysaccharide that is constituted by a long chain of glucose molecules (a six-carbon sugar). Hemicellulose (having the formula (C5H8O4)n) is a relatively amorphous component that is easier to break down with chemicals and/or heat than cellulose and contains a mix of C6-polysaccharides (made of galactose and mannose) and C5-polysaccharides (made of xylose and arabinose). Lignin is essentially the glue that provides the overall rigidity to the structure of plants and trees and is a network polymer comprised of multisubstituted, methoxy, aprylpropane, and hydroxylphenol units. The empirical formula for lignin is C9H10O2(OCH3)n, where n is the ratio of CH3O to C9 groups: n = 1.4, 0.94, and 1.18 for hardwood, softwood, and grasses, respectively. Lignin (15%-25% of total feedstock dry matter) is the largest noncarbohydrate fraction of lignocellulose. Different technologies for producing biofuels and biochemicals from lignocellulosic biomass components can be used. Biochemical conversion technologies focus on processing the core sugar components of cellulose and hemicellulose into ethanol, after fermentation, while chemical conversion technologies may lead to the production of levulinic acid (from C6-polysaccharides) or furfural (from C5-polysaccharides), after acid hydrolysis.14,15 The third component of lignocellulosic biomass (i.e., lignin) is not fermentable but can be burned to provide electricity and heat or undergo thermochemical treatments to produce biofuels (e.g., FTdiesel from syngas after gasification) and chemicals (e.g., phenol extraction after pyrolysis).16 Furthermore, all three main components of biomass can be gasified to syngas or pyrolyzed to bio-oil, which can then be upgraded to produce transportation biofuels or biochemicals.17,18

Figure 1. Chemical structure of the products: ethanol, furfural, levulinic acid, phenols, formic acid, and FT-diesel.

2.2. Products. Among the other possible alternative products, this paper investigates the maximum production potentials from lignocelullosic biomass of two types of transportation biofuels (ethanol and FT-diesel) and three types of chemicals (i.e., furfural, levulinic acid, and phenols) (see Figure 1). Ethanol (C2H5OH) is produced from sugar fermentation (with C5 sugars having lower conversion efficiency than C6 sugars) after a pretreatment step able to depolymerize the C6/C5 polysaccharides into their simple sugars glucose, xylose, mannose, arabinose, and galactose, which can be readily fermented by micro-organisms.19,20 Since lignin cannot be fermented, the possibility to generate ethanol via fermentation is not taken into account. By contrast, all the

(14) Hamelinck, C. N.; van Hooijdonk, G.; Faaij, A. P. C. Ethanol from lignocellulosic biomass: Techno-economic performance in short-, middle- and long-term. Biomass Bioenergy 2005, 28 (4), 384–410. (15) Cardona Alzate, C. A.; Sanchez Toro, O. J. Energy consumption analysis of integrated flowsheets for production of fuel ethanol from lignocellulosic biomass. Energy 2006, 31 (13), 2447–2459. (16) Bridgwater, A. V.; Peacocke, G. V. C. Fast pyrolysis processes for biomass. Renew. Sustain. Energy Rev. 2000, 4 (1), 1–73. (17) Li, X. T.; Grace, J. R.; Lim, C. J.; Watkinson, A. P.; Chen, H. P.; Kim, J. R. Biomass gasification in a circulating fluidized bed. Biomass Bioenergy 2004, 26 (2), 171–193. (18) Zhang, Q.; Chang, J.; Wang, T.; Xu, Y. Review of biomass pyrolysis oil properties and upgrading research. Energy Convers. Manage. 2007, 48 (1), 87–92.

(19) Bai, F. W.; Anderson, W. A.; Moo-Young, M. Ethanol fermentation technologies from sugar and starch feedstocks. Biotechnol. Adv. 2008, 26 (1), 89–105. (20) Wyman, C. E.; Spindler, D. D.; Grohmann, K. Simultaneous saccharification and fermentation of several lignocellulosic feedstocks to fuel ethanol. Biomass Bioenergy 1992, 3 (5), 301–307.

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Figure 2. Schematic overview of the conversion reactions investigated in this paper: from feedstock to products (full arrows) and from biomass components to products (dotted arrows).

three biomass components can be used to produce ethanol via syngas (after gasification).21,22 FT-diesel (assuming an average number of C atoms of 14, the chemical formula is CH3(CH2)12CH3) is produced from Fischer-Tropsch synthesis of the syngas produced by gasification.23,24 This work accounts for gasification and subsequent FT-syntesis of both the entire feedstock and the lignin fraction, while the possibility to gasify C6/C5 polysaccharides alone is not taken into account, because this option does not represent a technical and economic viable pathway (it is much more simple and convenient to convert them to ethanol via fermentation). Phenols (average chemical formula of C8.6H11.21O1.44, elaborated from the data reported in ref 18) are produced from the flash pyrolysis of biomass, especially from its lignin fraction.18,25 Levulinic acid (C5H8O3) is a product of C6 polysaccharides acid-catalyzed depolymerization and it is a very important chemical from which both biofuels (e.g., methyltetrahydrofuran (MTHF) and ethyl levulinate, among others) and chemicals (e.g., succinic acid, diphenolic acid, delta-amino levulinic acid, etc.) can be synthesized.26-28 Formic acid (CH2O2) is a co-product of the conversion of C6 sugars to levulinic acid and is an additional valuable chemical compound.

Furfural (C5H4O2) is produced from the C5 polysaccharides acid-catalyzed depolymerization and can be converted to biofuels (MTHF) and chemicals (furfuryl alcohol, fumaric acid, resins, etc.).29 In Figure 2, a schematic description of the modeled chemical reaction pathways is shown. As reported in the figure, there is a distinction between the entire feedstock and its single components (i.e., C5/C6 polysaccharides or lignin) for acting as reaction substrate. If the reaction substrate is the entire feedstock, in a wider and ideal perspective, all the products can be theoretically achieved (full arrows), via gasification and subsequent chemical synthesis. However, this case does not consider the existing physical and chemical constraints that the technological processes must face when non-thermochemical processes are applied (e.g., lignin cannot be fermented). Therefore, the dot lines of Figure 2 represent the conversion pathways in a more pragmatic perspective, i.e., considering chemical and biochemical processes can only lead to certain final products. 3. Description of the Calculation Procedure This calculation procedure can be divided in two parts. The first part involves determination of the carbon content of the different biomass feedstock, feedstock components, and final products and calculations of the carbon conversion efficiency of the biorefinery conversion pathways. The second part involves calculation of the maximum yields of biofuel or biochemical that can be produced from feedstock and feedstock components (i.e., cellulose, hemicellulose, and lignin). 3.1. First Part: Determination of the Carbon Content. Although the carbon content of the entire feedstock is known (see Table 1), problems arise for the calculation of the carbon content of its components, cellulose, hemicellulose, lignin, and others (extractives, acids, and ashes), which must be determined. The total carbon content of a lignocellulosic feedstock can be described by the following equation: ð1Þ c þ h þ l þ w ¼ TOTc

(21) Hu, J.; Wang, Y.; Cao, C.; Elliott, D. C.; Stevens, D. J.; White, J. F. Conversion of biomass-derived syngas to alcohols and C2 oxygenates using supported Rh catalysts in a microchannel reactor. Catal. Today 2007, 120 (1), 90–95. (22) Haryanto, A.; Fernando, S. D.; Pordesimo, L. O.; Adhikari, S. Upgrading of syngas derived from biomass gasification: A thermodynamic analysis. Biomass Bioenergy 2009, 33 (5), 882–889. (23) Tijmensen, M. J. A.; Faaij, A. P. C.; Hamelinck, C. N.; van Hardeveld, M. R. M. Exploration of the possibilities for production of Fischer-Tropsch liquids and power via biomass gasification. Biomass Bioenergy 2002, 23 (2), 129–152. (24) van Vliet, O. P. R.; Faaij, A. P. C.; Turkenburg, W. C. FischerTropsch diesel production in a well-to-wheel perspective: A carbon, energy flow and cost analysis. Energy Convers. Manage. 2009, 50 (4), 855–876. (25) Meister, J. J. Modification of lignin. Polym. Rev. 2002, 42 (2), 235–289. (26) Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J. Green Chemicals: A Kinetic Study on the Conversion of Glucose to Levulinic Acid. Chem. Eng. Res. Des. 2006, 84 (5), 339–349. (27) Bozell, J. J.; Moens, L.; Elliott, D. C.; Wang, Y.; Neuenscwander, G. G.; Fitzpatrick, S. W.; Bilski, R. J.; Jarnefeld, J. L. Production of levulinic acid and use as a platform chemical for derived products. Resour., Conserv. Recycl. 2000, 28 (3-4), 227–239. (28) Chang, C.; Cen, P.; Ma, X. Levulinic acid production from wheat straw. Bioresour. Technol. 2007, 98 (7), 1448–1453.

where c, h, l, and w represent the carbon content of cellulose, hemicellulose, lignin, and others, and the parameter TOTc (29) Kamm, B.; Kamm, M.; Schmidt, M.; Hirth, T.; Schulze, M. Lignocellulose based chemical products and product family trees. In Biorefineries;Industrial Processes and Products (Status Quo and Future Directions); Kamm, B., Gruber, P. R., Kamm, M., Eds.; Wiley-VCH: Weinheim, Germany, 2006; Vol. 2.

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greenhouse gas emission, because has a biological origin. This rejection also reflects the majority of the biomass treatment methods currently being used. It would be most desirable to reject oxygen as O2, but this is not a typical output of biomass conversion processes. Oxygen cannot either be rejected as H2O, because water must be added to introduce hydrogen. Even rejection as CO is not viable, because this molecule has non-negligible energy content and every mole of oxygen also removes one mole of the desired carbon, thus reducing product yields. When the product of the reaction is levulinic acid, the excess oxygen is rejected as formic acid (CH2O2), which is a co-product of the glucose hydrolysis step. Unlike the CO2, which has no market value, formic acid is a valuable chemical. (3) Rejection of other undesired elements. The undesired elements sulfur and nitrogen are rejected in their oxide forms (SO2 and NO2, respectively). This assumption has several advantages: it is analogous to the combustion process; similar to CO2, these oxides are without an energy content; and they contribute to the rejection of the excess oxygen, which saves biomass carbon. These assumptions are used to model the biomass conversion reactions and estimate the final product yields. The calculation procedure is based on an average biomass molecule (dry and ash-free), CpHqOxSyNz, where the atom numbers (p, q, x, y, and z) are determined from the elemental analysis and change together with the biomass feedstocks or components. Some parameters, normalized to one atom of carbon, are reported in Table 2. The modeled chemical reactions for producing two types of biofuels (ethanol and FT-diesel) and three types of chemicals (phenols, levulinic acid, and furfural) can be summarized by the following equation: Cp Hq Ox Sy Nz þ aH2 O f bCR Hβ Oγ þ dCO2 þ eSO2 þ f NO2

represents the total amount of carbon in the feedstock (expressed in grams). TOTc is the only known value, whereas c, h, l, and w must be calculated. Since both the amount in grams of cellulose, hemicellulose, and lignin, as well as their chemical formula, are given (Table 1), we assume that the weight fraction due to the carbon content is equal to the contribution (expressed as a percentage) of the carbon content to the molar weight of the single molecule (the ratio between the molecular carbon weight and the total molecular weight). Thus, eq 1 can be written as follows: "  # "  6C 6C Mc þ Mh6 6C þ 10H þ 5O 6C þ 10H þ 5O    5C þ Mh5 5C þ 8H þ 4O "  # 11:4C Ml þ w ¼ TOTc þ 11:4C þ 14:2H þ 3:4O where C, H, and O represent the molecular weight of carbon, hydrogen, and oxygen, respectively; Mc represents the total mass of cellulose, and Mh6 and Mh5 are, respectively, the mass of C6 and C5 sugars in hemicellulose and Ml is the total mass of lignin. As a consequence, for instance, the expression in the first square brackets is the total amount of carbon (in grams) in the mass of cellulose. All the values are known, except w (which represents the carbon content of extractives, acids, and ashes), which can be easily obtained by subtraction from TOTc of the carbon content of the other components. A detailed procedure for the derivation and the application of this equation is depicted in Appendix A. Similarly, the carbon content of products (ethanol, FTdiesel, phenols, furfural, and levulinic acid) is determined and the carbon conversion efficiency can be easily estimated with the ratio “total carbon content of products/total carbon content of feedstock”. 3.2. Second Part: Calculation of the Maximum Theoretical Yields. Biomass must be chemically converted for the production of liquid products, and chemical changes are necessary to convert biomass from a solid state to a liquid state. Fundamental is the biomass feedstock composition: it usually has few carbons (if compared to fossil fuels) and hydrogens (which must be added) and too much oxygen (which must be rejected) and other undesirable elements (such as nitrogen and sulfur, which also must be rejected).30 The calculations performed hereinafter are based on the following considerations:30 (1) Hydrogen introduction. The required hydrogen is added as water (H2O), even if this implicates an addition of extra oxygen, which must be rejected. The addition of hydrogen as H2 is underprivileged by the fact that elemental hydrogen is not present in nature and energy must be expended to produce it. (2) Oxygen rejection. Oxygen is rejected as CO2, even if every mole of elemental oxygen also removes half a mole of carbon. The rejection of CO2 seems to be the most appropriate case, because CO2 is without an energy content (in terms of its heating value) and it is not considered to be a

ð2Þ CRHβOγ is the generic final product and R, β, and γ represent the atom numbers of the product molecule (see Table 3); a, b, d, e, and f are stoichiometric coefficients that must be determined, while the atom numbers p, q, x, y, and z (for biomass components, y = z = 0) are already known and change together with the feedstock. The maximum theoretical yields of biofuels and biochemicals from lignocellulosic feedstocks are calculated by solving the linear equations arising from the atom balances of eq 2, where the stoichiometric coefficients (a, b, d, e, and f ) are the variables to be determined using matrix algebra. The use of matrix algebra to simulate chemical reactions is a procedure that has been widely applied in the scientific literature (see, for instance, refs 30-32). The resulting system of equations has the following solutions, calculated by means of matrix algebra (see Appendix B for more information): a ¼ k½2βp þ ðγ -2RÞq -βx b ¼ kð4p þ q -2xÞ d ¼ k½ðβ -2γÞp -Rq þ 2Rx (31) Alberty, R. A. Calculation of biochemical net reactions and pathways by using matrix operations. Biophys. J. 1996, 71 (1), 507–515. (32) Philips, J. C. Algebraic constructs for the graphical and computational solution to balancing chemical equations. Comput. Chem. 1998, 22 (4), 298–308.

(30) Jechura, J. Maximum Yield of Liquid Fuels from Biomass Based on Stoichiometry, Technical Memorandum; National Renewable Energy Laboratory (NREL), Golden, CO, 2006.

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Table 2. Elemental Composition of Different Biomass Feedstock and Components, Normalized to One Atom of Carbona Elemental Composition (According to Atom Number) biomass material

C (p)

H (q)

O (x)

S (y)

N (z)

hardwood softwood switchgrass corn stover wheat straw

1 1 1 1 1

1.39 1.42 1.34 1.44 1.43

0.64 0.63 0.66 0.66 0.66

0.00015 0.00007 0.00079 0.00048 0.00137

0.003 0.001 0.009 0.012 0.012

C5 polysaccharides C6 polysaccharides lignin (hardwood) lignin (softwood)

1 1 1 1

Feedstock Components 1.60 1.67 1.37 1.29

0.80 0.83 0.33 0.30

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

Feedstock

a

Source: biomass feedstock from Table 1; biomass components from the first part of this calculation procedure.

Table 3. Number of Atoms of the Final Products product

R

β

γ

ethanol FT-diesel phenols levulinic acid furfural

2 14 8.6 5 5

6 30 11.2 8 4

1 0 1.44 3 2

where k ¼

1 4R þ β -2γ

These are generic algebraic solutions that can be used for all the types of lignocellulosic feedstocks and final products, whose parameters are shown in Tables 2 and 3. Thanks to the algebraic form of the solutions, it is possible to investigate the influence of hydrogen and oxygen content of the feedstock on the final yield of the main product (CRHβOγ). If the partial derivatives of the equation for the solution of b are considered with respect to q (hydrogen in the feedstock) and x (oxygen in the feedstock), the following results are obtained: Db D½kð4p þ q -2xÞ ¼ ¼k Dq Dq

Figure 3. Relationship between the maximum yield of the main product (ethanol, in this case) and the hydrogen and oxygen content of the feedstock; the z-axis represents the values of the stoichiometric coefficient b.

Db D½kð4p þ q -2xÞ ¼ ¼ -2k Dx Dx

of ethanol, 0.57 kg of CO2, 0.0002 kg of SO2, and 0.001 kg of NO2. This means that the maximum theoretical yield of ethanol from a softwood feedstock is 68% (on a dry mass basis). Similar procedures can be applied to determine the maximum yields of the other biofuel (FT-diesel) and biochemicals (phenols, levulinic acid, and furfural) from the other types of feedstocks and feedstock components.

This means that the maximum yield of the final product is directly proportional to the hydrogen content of the raw material and simultaneously inversely proportional to the original oxygen content of the feedstock (on the condition that 4R þ β > 2γ). This relationship is clearly shown in Figure 3 (where ethanol is assumed as final product): the largest yields are obtained when the feedstock has a high hydrogen content and a low oxygen content. If the parameters corresponding to the conversion of softwood to ethanol are inserted in the above set of solutions, the following balanced chemical equation is obtained: C1 H1:42 O0:63 S0:000075 N0:00051 þ 0:33H2 O f 0:35C2 H5 OH

4. Results The first part of this calculation procedure led to the determination of the carbon content of the different biomass components and final products (see Table 4). This calculation was required by the need for the determination of the carbon conversion efficiency of biomass conversion technology from feedstocks to products. For example, the carbon content of C6 polysaccharides is ∼44% and of ethanol is ∼52%; these percentages must be multiplied by the mass in grams of the feedstock (C6 polysaccharides) and of the product (ethanol) to obtain the respective carbon contents, from which the

þ 0:31CO2 þ 0:000075SO2 þ 0:00051NO2 From this balanced chemical reaction, the mass of ethanol produced can be easily determined, because the molar composition and the stoichiometric coefficients are known. For 1 kg of biomass (softwood), 0.254 kg of water must be added (as a hydrolyzing agent, not as a solvent) to obtain 0.679 kg 2662

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maximum theoretical carbon conversion efficiency can be easily determined. In the second section, the maximum theoretical yields of biofuels and biochemicals from five types of biomass feedstocks (hardwood, softwood, miscanthus, corn stover, and wheat straw) and four biomass components (C5/C6 polysaccharides and hardwood/softwood lignin) were estimated with the generic algebraic solutions generated by means of matrix algebra. Results are reported in Table 5. The blank entries in the right-hand part of the table are due to the existing physical and technological constraints discussed in the previous section and illustrated in Figure 2 (e.g., no ethanol can be recovered from lignin or levulinic acid from C5 polysaccharides). This means that the findings in the left-hand part of Table 5 should be examined carefully, because the results imply that one or more conversion pathways will be used to convert all the biomass feedstock to products. This is a suitable assumption for a thermochemical conversion pathway where a syngas intermediate is used. In fact, with this technology, all the three biomass components (C5/C6 polysaccharides and lignin) are converted to syngas and then to ethanol or other biofuels and chemicals. Instead, current biochemical and chemical pathways (such as fermentation to ethanol or acid hydrolysis to levulinic acid and furfural) only act on selected biomass components, i.e., C6/C5 polysaccharides and sugars via

hydrolysis and fermentation. In biorefinery systems, when thermochemical processes are not applied, a biomass feedstock is usually divided into its main components, which are then autonomously used as substrates for different products; therefore, the maximum yields in the right-hand part of the table are those used in the following section, where different biorefinery systems are compared. The reader should take these aspects into account when examining the data in Table 5. 5. Discussion and Interpretation As already predicted, the results shown in the previous section pointed out the facts that the largest biofuel and biochemical yields are achieved with those feedstocks and components that have the lowest oxygen content and the highest carbon and hydrogen content, such as softwood among the feedstock and lignin among the components. As a consequence, the ethanol yield from C5/C6 polysaccharides is lower than that from the entire feedstock (which includes lignin), because sugars have a higher oxygen content. This means that the possibility to exploit lignin for biofuel production purposes strongly affects the biofuel yield. In fact, there is a great potential for the production of FT-diesel from lignin, since this is the substrate with the highest carbon content, with hydrogen acting as the limiting factor. On a mass basis, ethanol has higher production potential than FT-diesel, but this margin strongly decreases if the energy content of the product is considered (ethanol has a heating value of 27 MJ/kg, whereas FT-diesel has a heating value of 42.7 MJ/kg): the first-law energy efficiencies (defined as the ratio between the energy content of the products to the energy content of the feedstock) are ∼90% for ethanol and ∼85% for FT-diesel. This means that, although a significant mass of the feedstock is rejected (as CO2), there are limited losses on the energy content of the produced biofuel. Therefore, the biomass conversion processes can be considered to be a way to concentrate the biomass potential energy in a moreenergy-dense form. The results of Table 5 give rise to many possible interpretations. Several alternative combinations of biomass feedstocks with final products, both biofuels and biochemicals, can be identified. As an example, it is possible to preliminarily set up three types of biorefinery chains:

Table 4. Results of the First Part: Carbon Content of Biomass Components and Final Products carbon content (%) Feedstock Components C6 polysaccharides C5 polysaccharides lignin othersa

44.4 45.5 66.6 33.1 Products

ethanol FT-diesel (C14) furfural levulinic acid formic acidb phenols

52.1 84.8 62.5 51.7 26.1 75.0

a This is the only component for which the chemical formula is not fixed; instead, it varies with the feedstock. This value refers to hardwood. b Co-product of levulinic acid synthesis (fixed yield).

Table 5. Results Provided by the Second Part: Maximum Theoretical Yields of Biofuels and Biochemicals from Biomass Feedstock and Componentsa Feedstock Components Biomass Feedstock product (unit/kgfeedstock) ethanol amount (kg) energy content (MJ) FT-fuels amount (kg) energy content (MJ) phenols (kg) levulinic acid (kg) formic acidb (kg) furfural (kg) a

Polymer

Lignin

hardwood

softwood

switchgrass

corn stover

wheat straw

C5

C6

0.665 17.95

0.679 18.33

0.646 17.44

0.659 17.79

0.657 17.74

0.581 15.69

0.568 15.34

0.399 17.04 0.558 0.861 0.230 0.832

0.408 17.43 0.570 0.889 0.195 0.850

0.388 16.57 0.542 0.827 0.273 0.809

0.396 16.91 0.553 0.863 0.189 0.825

0.395 16.87 0.551 0.860 0.192 0.823

0.716 0.284 0.727

Lower heating values: 27 MJ/kg for ethanol, 42.7 MJ/kg for FT-diesel. b Co-product of levulinic acid synthesis.

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hardwood

softwood

0.583 24.90 0.815

0.600 25.63 0.839

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Figure 4. Results obtained for the selected biorefinery systems.

(1) Biofuel oriented biorefinery, where the products are ethanol (from C5 and C6 polysaccharides) and FT-diesel (from lignin). (2) Chemical oriented biorefinery, where the products are levulinic acid (from C6 polysaccharides), furfural (from C5 polysaccharides), and phenols (from lignin). (3) Biorefinery based on gasification of the entire feedstock to produce FT-fuels. Here, softwood is selected as the raw material, because it has the highest conversion yields. Its composition (according to Table 1) can be assumed as follows: 27.67% lignin, 58.5% C6 polysaccharides, 7.9% C5 polysaccharides, and 5.89% other components (such as acid, ashes, and others). From these data, it is possible to estimate the maximum theoretical product yields and the maximum theoretical mass, energy, and carbon conversion efficiency for the three investigated biorefineries (see Figure 4). The chemical-oriented biorefinery showed the highest values for the maximum theoretical mass and carbon conversion efficiency, but it does not have a direct energy product, even if both levulinic acid and furfural can be potentially converted to biofuels. The performances of other two biorefinery systems (i.e., the biofuel-oriented biorefinery and the biorefinery based on gasification) suggest that, despite almost half of the original mass of the feedstock being recovered into the products (while the remaining is released as CO2), the products maintain ∼88%-89% of the energy content of the feedstock. Therefore, it is extremely important to widen the evaluation beyond the sole mass balance to gain a complete understanding of the system potentials: the mass conversion efficiency focuses on the amount of products; the energy conversion efficiency considers their energy content (and, therefore, its application to systems with products that are not used as energy carriers is questionable); the carbon conversion efficiency is able to simultaneously account for biofuels and material products. The findings of the gasification system show that, even with the lowest mass efficiency (41%, versus 54% for the

Figure 5. Application of the algorithm to the biorefinery system producing ethanol, furfural, and FT-diesel from softwood.

biofuel-oriented biorefinery), it produces a carbon-rich product that has a much higher heating value than that of ethanol (42.7 MJ/kg against 27 MJ/kg), and, therefore, the resulting carbon and energy conversion efficiencies are higher than that for the biofuel-oriented biorefinery. An overall interpretation of these theoretical results leads to some important remarks concerning the best way in which the three biomass components of a lignocellulosic feedstock should be exploited. Regarding C5 sugars, yields of furfural are higher than those of ethanol; furthermore, C5 polysaccharides conversion to ethanol still must face non-negligible technological constraints and effective yields are lower than those of the C6 polysaccharides. Hence, regarding the biomass sugar fraction, the best treatment pathways seem to be the conversion of C6 sugars to ethanol and C5 sugars to furfural, from which a wide spectrum of biofuels and chemicals can be synthesized. Concerning the third component of a lignocellulosic biomass feedstock, the lignin, the best alternative seems to be the conversion to 2664

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FT-diesel via gasification. This biorefinery chain is depicted in Figure 5, along with its conversion efficiencies. Even if almost half of the mass of the feedstock is lost (as CO2), the final products still have 70% of the carbon and 82% of the energy content of the raw materials.

components, cellulose, hemicellulose, lignin, and others (extractives, acids, and ashes), it varies with the feedstock and must be determined. As already mentioned, the carbon content of the feedstock can be expressed by the following equation: ð1Þ c þ h þ l þ w ¼ TOTc

6. Conclusions

For each single component, we can assume that the fraction of the weight due to the carbon content is equal to the contribution (expressed as a percentage) of the carbon content to the molar weight (the ratio between the carbon molar weight and the total molecular weight). Therefore, we can write   6C Mc c ¼ 6C þ 10H þ 5O

Thanks to a set of solutions elaborated by means of matrix algebra, it is possible to estimate the maximum theoretical potentials of biofuel and biochemical production from lignocellulosic feedstocks, together with an estimation of the maximum theoretical mass, energy, and carbon conversion efficiency. In this paper, two types of transportation biofuels and three chemicals are selected as final products; however, this procedure can be similarly applied to a wider spectrum of products and feedstocks. Some biorefinery chains were investigated, and the most important aspects emerged from the results can be summarized as follows: • This paper summarizes a calculation procedure that is reproducible by the reader and can be applied to many different biomass feedstocks and products. • Lignocellulosic biomass has a large potential for producing biofuels and biochemicals in a biorefinery, thus allowing for the replacement of fossil-derived products. • The composition of the feedstock strongly affects the conversion yields and efficiencies. • A high oxygen content reduces product yields and its removal subtracts valuable carbon, while hydrogen content is the limiting factor. • Maximum biofuel yields are constrained by the stoichiometry of the conversion process. The formation and rejection of CO2 is necessary to reject excess oxygen from the biomass feedstock. • However, the formation and rejection of CO2 causes moderate energy losses, as demonstrated by the theoretical energy conversion efficiencies: even if almost half of the mass of the feedstock is rejected, the produced biofuel still has ∼90% of the energy content of the original feedstock. • This procedure allows for the determination of the most suitable type of biomass feedstock for biofuel or chemical production and the most efficient conversion pathways: the best way to exploit all the potentialities of a lignocellulosic biomass feedstock seems to be the production of ethanol from C6 polysaccharides, furfural from C5 polysaccharides, and FT-diesel from lignin. • The findings of this paper can act as a target for the upcoming biorefinery literature and are suitable for a comparison with the effective yields (e.g., in life cycle assessment studies) of biorefinery systems, to estimate the magnitude of the existing technological gap between the real and the ideal performances.

where C is the molecular weight of carbon (12 g/mol), H the molecular weight of hydrogen (1 g/mol), O the molecular weight of oxygen (16 g/mol), and Mc the total mass of cellulose. The denominator is the molecular weight of the cellulose unit formula. Considering 1 kg of hardwood, its cellulose content is Mc = 0.43 kg (from Table 1); hence, c = 0.19 kgC. Regarding hemicellulose, the equation must take in account that two different compounds are blended: C5 polysaccharides (C5H8O4, xylan and arabinan) and C6 polysaccharides (C6H10O5, galactan and mannan). Therefore, the equation is given as     6C 5C Mh6 þ Mh5 h ¼ 6C þ 10H þ 5O 5C þ 8H þ 4O where Mh6 and Mh5 are, respectively, the mass of C6 and C5 sugars in the hemicellulose. From Table 1, Mh6 = 0.02 kg and Mh5 = 0.18 kg; therefore, h = 0.09 kgC. Regarding the lignin fraction, its chemical formula is not fixed, but varies together with the feedstock: C9H10O2(OCH3)n with n = 0.94 for softwood, 1.18 for grasses, and 1.4 for hardwood. We have assumed a hardwood feedstock; therefore, the formula is C11.4H14.2O3.4 and the equation is given as   11:4C Ml l ¼ 11:4C þ 14:2H þ 3:4O where Ml is the total mass of lignin, and that allows one to determine l, which represents the mass of carbon present in the lignin (in grams). From Table 1, Ml = 0.27 kg; then, l = 0.17 kgC. Finally, eq 1 can be rewritten in explicit form as follows: "  # "  6C 6C Mc þ Mh6 6C þ 10H þ 5O 6C þ 10H þ 5O    5C Mh5 þ 5C þ 8H þ 4O "  # 11:4C Ml þ w ¼ TOTc þ 11:4C þ 14:2H þ 3:4O

Acknowledgment. The authors would like to acknowledge the Norwegian research council for funding this work through the Bio-energy Innovation Centre (CenBio).

All the values are known, except for w (which represents the carbon content of extractives, acids, and ashes), the value of which can be easily obtained by subtraction from TOTc:

Appendix A: Calculation Procedure, First Part This appendix describes the calculation procedure applied for the determination of the carbon content of the different biomass feedstock and feedstock components. In the following calculations, one kilogram of hardwood is assumed as feedstock. The carbon content of the entire feedstock is usually well-known (Table 1); however, for its

w ¼ TOTc -c -h -l ¼ 0:49 -0:19 -0:09 -0:17 ¼ 0:04 kgC With the same approach, it is possible to determine the carbon content of the final products: ethanol, FT-diesel, levulinic acid, furfural, and phenols. For example, the carbon content 2665

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of 1 kg of ethanol (C2H6O) is   2C Me ¼ 0:52  1 ¼ 0:52 kgC 2C þ 6H þ O

the feedstock (see Table 2; e.g., for softwood, p = 1, q = 1.42, x = 0.63), and the values R, β, and γ are related to the type of final product (see Table 3; e.g., for ethanol, R = 2, β = 6, and γ = 1). The solutions for variables a, b, and d can be found by solving this matrix equation, which is in the form AX = B, where A is the square matrix, the vector X is the column vector of the variables Xi (the stoichiometric coefficients, in our case), and B is the column vector of the known values (the number of atoms of the biomass feedstock (p, q, and x, in our case)). Solutions are given by X = A-1B, on the condition that A-1, which is the inverse matrix of A, exists (i.e., |A| 6¼ 0). In this case, the following condition must be valid:

The carbon conversion efficiencies of biorefinery conversion pathways from feedstock to products are then calculated by means of the ratio “carbon content of the products/carbon content of the feedstock”. Results obtained from the application of this procedure are reported in Table 4. Appendix B: Calculation Procedure, Second Part In this appendix, the calculation procedure applied to determine the maximum theoretical yields of products from biomass feedstocks and biomass components is depicted. Focusing on the carbon, hydrogen, oxygen, sulfur, and nitrogen content of the biomass, the generic chemical formula for producing a generic final product is Cp Hq Ox Sy Nz þ aH2 O f bCR Hβ Oγ þ dCO2 þ eSO2 þ f NO2

jAj ¼ 4R þ β -2γ 6¼ 0 Calculations lead to the following generic solutions:       2β γ -2R β pþ qx a ¼ 4R þ β -2γ 4R þ β -2γ 4R þ β -2γ

ð2Þ

 b ¼

The stoichiometric coefficients can be determined from the linear equations that arise from the atomic balances. For this reaction, Carbon : Hydrogen : Oxygen : Sulfur : Nitrogen :

     4 1 2 pþ qx 4R þ β -2γ 4R þ β -2γ 4R þ β -2γ

 d ¼

p ¼ Rb þ d q þ 2a ¼ γb x þ a ¼ γb þ 2d þ 2e þ 2f y ¼e z ¼f

     β -2γ R 2R pqþ x 4R þ β -2γ 4R þ β -2γ 4R þ β -2γ

Assuming that k ¼

1 1 ¼ jAj 4R þ β -2γ

the final results are

While the results for S and N are clear and can be left aside, the others are interrelated and must be calculated. These linear equations constitute a system of 3 equations with 3 variables (the stoichiometric coefficients): 8 8 > > < p ¼ Rb þ d < Rb þ d ¼ p f q þ 2a ¼ βb -2a þ βb ¼ q > > : x þ a ¼ γb þ 2d : -a þ γb þ 2d ¼ x

a ¼ k½2βp þ ðγ -2RÞq -βx b ¼ kð4p þ q -2xÞ d ¼ k½ðβ -2γÞp -Rq þ 2Rx e ¼y

This system can also be expressed in matrix notation: 2 32 3 2 3 0 R 1 a p 4 -2 β 0 54 b 5 ¼ 4 q 5 -1 γ 2 d x

f ¼z These solutions can be used to balance the chemical reaction shown in reaction 2. All the values of Table 2 and 3 can be used in the above set of solutions and the results are reported in Table 5.

The parameters p, q, and x are known and are those related to

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