Outlook for Lignocellulose Derived Fuels

Introduction

Fig. 1: Routes to ethanol and synthetic diesel from lignocellulosic biomass.

Increasing energy costs, environmental degradation, and diminishing supplies of fossil fuels stress the importance of producing sustainable fuels from renewable sources. The depletion of cheap fossil fuels will inevitably increase their price, allowing for the emergence of other sources of energy that are currently economically infeasible. Ideal fuels would have physical and energetic properties similar to those that are currently used and could thus be dropped into existing markets; many biofuels fit this profile.

Much of the first generation effort to generate biofuels focused on the fermentation of ethanol from corn and sugar. Two challenges exist with using only corn and sugar as a source of biofuels: (1) corn and sugar alone cannot replace the 5.4 × 10²⁰ J consumed worldwide every year, and (2) producing large quantities of biofuels from food crops necessarily links food and fuel prices. [1] However, this is not to say that plants do not provide a means to reduce dependence on fossil fuels; on the contrary, plants represent a ubiquitous, renewable resource that are particularly well suited to provide for humanity's energy demands.

The most abundant form of biomass on the planet, lignocellulose, is particularly suitable to be an energy source. Composed primarily of cellulose (30-50%), hemicellulose (15-35%), and lignin (10-20%), lignocellulosic biomass is a nonfood form of plant biomass including wood, agricultural residues, and municipal and industrial waste. [2]

Pretreatment and Conversion to Fuel

While lignin and cellulose are the two most abundant polymers on Earth, the degradation of these molecules into their monomeric subunits represents a significant challenge in utilizing lignocellulose as an energy source. Decades of research have resulted in the discovery and evolution of enzymes called cellulases that are capable of digesting cellulose into monomeric glucose at up to 95% yield over several days. [3] In contrast, lignin degradation is a substantially slower process, often taking weeks or months. [4] If lignin bonds to cellulose or hemicellulose, which is often the case in lignocellulosic biomass, the resulting lignin carbohydrate complexes (LCCs) are particularly resilient to enzyme degradation to the point where cellulases lose much of their efficacy. [5] As a result, lignocellulosic biomass requires some form of pretreatment to overcome recalcitrance.

Pretreatment involves mechanical steps followed by the extraction of the celluloses and hemicelluloses with acid or ammonia in an energy intensive process, fermentation of these sugars, and distillation to yield bioethanol. [6] Alternatively, lignocelluloses can be oxidized to syngas (H₂ and CO) or biooils via gasification or pyrolysis, respectively, from which liquid fuels can be formed by chemical or microbial means (Figure 1). The advantages of biochemical conversion are high selectivity in deconstructing biomass to desired end products, while thermochemical conversion provides low residence time and the ability to handle various feedstocks in a continuous, nonselective manner. [5] Combustion is another means to extract energy from lignocellulose. For example, much of the pulp industry combusts pulping black liquor to recover heat and power. [7]

Supply Limitations

It is worth gaining perspective on the ability of lignocellulosic biomass to replace current US energy use. In 2005, the US DOE and USDA performed a study indicating the annual biomass resource potential was 1.3 billion dry tons per year of biomass, 368 million dry tons from forestlands and 998 million dry tons from agricultural lands. [8] If the US consumes about 9.98 × 10¹⁹ J/year and the density of biomass is about equal to the density of cellulose, or about 1.36 × 10¹⁰ J/ton, this biomass could sustain [1]

of the US total energy use per year, assuming no inefficiencies. Moreover, if the US consumes 1.91 × 10⁷ barrels of petroleum per day, there are 0.1504 tons per barrel, and the density of petroleum is about 4 × 10¹⁰ J/ton, biomass could replace [1,9]

of the US oil consumption per year assuming no inefficiencies.

It is not reasonable, however, to assume these fuel sources are interconvertible - significant inefficiencies exist. For example, suppose you decide to convert all of the biomass into ethanol for use as transportation fuel. Assuming 70% of the biomass is composed of cellulose and hemicellulose, 90% of these sugars can be recovered, 90% of these sugars can be converted into ethanol (fermentative yield), and the maximum yield of ethanol from glucose is 0.51 g/g, we can determine the percent of energy that can be converted from biomass into ethanol: [10]

This calculation suggests converting to ethanol allows biomass to sustain up to 25 and 10 percent of US petroleum and total energy consumption, respectively. There are three critical points worth making: (1) the DOE and USDA assumption that 1.3 billion dry tons per year of biomass are available is roughly seven-fold higher than the current level of biomass consumption in the US; (2) studies have yet to demonstrate that small-scale conversion of lignocellulosic biomass to ethanol are representative of large scale operations; (3) this calculation does not account for the energy content of lignin that could be extracted by other means.

Outlook

It is unlikely that lignocellulosic biomass will constitute a substantial source of energy in the current fossil fuel-dominated economy. As a fuel source, the most significant current limitations of lignocellulose are the limited feedstock availability, rudimentary supply-chain logistics, and slow enzyme kinetics to generate monomeric sugars. [5] Technological innovation will reduce a number of these limitations in the future. However, it is unlikely that the processes necessary to convert lignocellulose into useful energy will be commercialized in high volume until they are unsubsidized, cost-effective, and cost-competitive. While lignocellulosic biomass will not replace total US or world energy consumption alone, it will likely provide a substantial, renewable portion of this energy as fossil fuels deplete.

© Nicholas Plugis. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] BP Statistical Review of World Energy," British Petroleum, June 2012.

[2] A. Limayem and S. C. Ricke, "Lignocellulosic Biomass for Bioethanol Production: Current Perspectives, Potential Issues and Future Prospects," Prog. Energy Combustion Sci. 38, 449 (2012).

[3] X. Wu et al., "Biofuels from Lignocellulosic Biomass," in Sustainable Biotechnology, ed. by O. V. Singh and S. P Harvey (Springer, 2009), p. 19.

[4] M. Saritha, A. Arora and Lata, "Biological Pretreatment of Lignocellulosic Substrates for Enhanced Delignification and Enzymatic Digestibility," Indian J. Microbiol. 52, 122 (2012).

[5] S. P. S. Chundawat et al., "Deconstruction of Lignocellulosic Biomass to Fuels and Chemicals," Annu. Rev. Chem. Biomol. Eng. 2, 121 (2011).

[6] "Bioenergy - Chances and Limits," German Academy of Sciences Leopoldina, August 2012.

[7] X. Zhang, M. Tu and M. G. Paice, "Routes to Potential Bioproducts from Lignocellulosic Biomass Lignin and Hemicelluloses," Bioenergy Res. 4, 246 (2011).

[8] P. D. Perlack et al., "Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technological Feasibility of a Billion-Ton Annual Supply," Oak Ridge National Laboratory, U.S. Department of Energy, DOE/GO-102005-2135, April 2005.

[9] H. Marzoughi and P. L. Kennedy, "The Impact of Ethanol Production on the U.S. Gasoline Market," 2012 Southern Agricultural Economics Association Annual Meeting, 4 Feb 12.

[10] W. E. Mabee et al., "Updates on Softwood-to-Ethanol Process Development," Appl. Biochem. Biotechnol. 129, 55 (2006).