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Biofuel production

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Contents

Introduction

Biofuels comprise a class of fuels derived primarily from plant matter, whether purpose-grown or waste products. Interest in biofuels has increased rapidly since 2000, due to a combination of high prices for fossil fuels, greater cognizance of the potential environmental hazards of fossil fuel combustion, and concerns over the security of international fossil fuel energy supplies.

Classes of Biofuels

In general, biofuels fall into one of two classes, based on the chemical structure of the final fuel product. Alcohol biofuels, usually based on ethanol, are by far the market leader by use. In the United States approximately 15-20 million barrels of fuel-grade ethanol were produced every month during 2008.[1] Hydrocarbon biofuels, which resemble gasoline or diesel fuel in their chemical structure, are much less common.

Many biofuel products are not sold in their pure state. In most advanced industrial countries, biofuels like ethanol are used as blending agents to improve the emissions properties of conventional gasoline. This results in a variety of fuel blends like E85 (85% gasoline/15% ethanol). Some countries, like Brazil, have made a concerted effort to move much of their transportation fuel consumption over to ethanol.

Production Methods

First-Generation Biofuels

First-generation biofuels are those in common use today, and have changed little in a century of use. Alcohols, chiefly ethanol, dominate first-generation biofuels, and are almost entirely produced via fermentation of starches and sugars. Fermentation plants constitute the vast majority of the biofuels production capacity in the United States and around the world. Preference for starch versus sugar as a feedstock depends on local agricultural conditions; in starch-dominant agricultural regions like the American Midwest, significant acreage has been diverted from food or feedstock production into fuel production.[2] In comparison, the sugar-growing giant Brazil has made sugar-derived ethanol a significant contributor to its national energy supply.

Fuel production from starches like corn, or sugars like cane sugar, uses an anaerobic fermentation process not unlike that used to produce various forms of alcohol for human consumption. Native or bioengineered yeasts are fed simple or complex carbohydrates in an aqueous environment, which they metabolize. The metabolic byproducts include ethanol, which dissolves in the aqueous solution. Distillation of the resulting solution gives ethanol at up to 95% purity. To achieve a completely water-free ethanol for use as a fuel or chemical intermediary requires additional chemical drying.

Sugars provide the most direct pathway from carbohydrates to fuel. Sugars in cane or beet sugar are in a chemical form directly accessible to yeasts. Starch ethanol requires additional pre-processing of the starch before fermentation, due to the more complex physical and chemical structure of the starches compared with simple sugars. Typical pre-processing requires either wet- or dry-grinding of the feedstock and the subsequent partial digestion of its complex sugars using the enzymes alpha amylase and gluco amylase. This breaks down the starch into simple sugars like glucose, which are then digestible by yeasts during fermentation.

Production of biofuels from starches and sugars in this method has several drawbacks. First, it requires the conversion of prime agricultural land to fuel feedstock production. Second, there is some controversy as to whether the resulting fuel contains as much energy as all the energy that went into making it. Some published estimates suggest that starch-derived ethanol may be net energy negative. Third, current agricultural production methods for large-scale commodity crops like corn or soy use significant quantities of petroleum-derived fertilizer, which reduces the biofuel's impact on energy security. Finally, the energy required for the fermentation process significantly increases the greenhouse gas emissions footprint of the final biofuel.[3]


The benefits of producing bioethanol from starch are heavily debated. For many years it was questioned whether the production of bio ethanol from corn was saving fossil fuel. However, since the comparative study by A. Farrell et al. (ref 3) of a great variety of LCA results this is no longer questioned. Contributing is also the recent documentation of the increase in energy efficiency of new ethanol plants (ref 6). Here it is documented that the Net Energy Ratio (energy output divided with energy input) has improved from 1.2 in previous studies to 1.5-1.8 on the basis of up-dated data. Ethanol-to-petroleum ratios are likewise calculated to be between 1:11 to 13:1 for todays most typical corn-ethanol systems. In parallel with the increase in energy efficiency also the figures for reduction of green house gas emissions have improved. Green house gas emissions of between 48% and 59% have been reported for new US corn plants[4]. However, these figures do not include potential indirect land use changes. The potential contribution from such changes is at present the most heavily debated issue in relation to biofuels.

Second-Generation Biofuels

Recent interest in biofuels as part of a possible solution to both energy security and global climate change has led to intensive research on alternative methods of biofuel production from a range of feedstocks.

Cellulosic Ethanol

Cellulose comprises the majority of plant biomass, making up the physical structure of the plant and giving it its structural integrity. Cellulose is an attractive biofuel feedstock because it can be sourced from fast-growing grass crops that thrive on marginal agricultural land and can be grown with a minimum amount of water and fertilizer input.

However, cellulose has a very complicated physical and chemical structure that complicates its conversion to ethanol. In particular, the cellulose is often surrounded by layers of lignin that are resistant to most forms of biochemical or biological attack. Standard processing methods use an initial acidic processing step to attack the lignin, followed by enzymatic digestation of cellulose to produce carbohydrates that can then be digested by yeasts and turned into ethanol.

This process has several drawbacks. Its multiple steps decrease overall efficiency and raise costs. More importantly, even with the pre-processing, conversion of cellulose to fuel occurs at a very low rate.

Improvements on this process have been suggested. Possibilities include:

The enzymatic breakdown of cellulose comes with many challenges. First and foremost the cellulose fibrils are embedded in a network of hemicellulose and lignin which makes it difficult for the enzymes to get access to the cellulose. Secondly the structure of the cellulose calls for the action of many different types of enzymes. This means difficulties in process optimization and high enzyme loads. Thus production of ethanol from cellulose with wild-type enzymes takes about 100 times more protein per gallon of ethanol than to produce ethanol from starch. Thirdly the development of commercially viable processes requires the integration of processes developed by different partners along the value chain.

The number of and magnitude of these challenges of breakdown cellulose biochemically have caused many to believe that cellulosic ethanol is still far from reality. However, great progress has been made over the last 2 years to develop more efficient enzyme blends. In March 2009 Novozymes launched (ref 7) 2 new enzyme products which makes possible the degradation of cellulose from both acid and alkaline pretreatment (the latter requires hemicellulase in addition to cellulose). Based on trials with market leading partners it has been demonstrated that by use of these products the enzyme cost is no longer the highest cost and that enzyme cost in 2010, where the first commercial scale production is expected will be between 0.5 and 1.5 USD/gallon – close to the enzyme costs for starch based ethanol. http://www.bioenergy.novozymes.com/

Cellulosic Hydrocarbon Fuels

Ethanol fuels made from cellulose have problems in Production, distribution, and use. Many researchers have proposed that cellulose could instead be converted to a hydrocarbon fuel that would behave much like present-day gasoline or diesel fuel derived from oil.

Conversion of cellulose to hydrocarbon fuels can occur via one of several processes currently under development:

  • Gasification, followed by conversion of the resulting gas into diesel, gasoline, ethanol, or methanol fuels.
  • Pyrolysis: rapid heating of the cellulose to very high temperatures, causing it to break down into simpler organic molecules that can then be converted into fuels


Other Hydrocarbon Fuels

Other second-generation biofuels besides those derived from cellulose are under development.

  • Algae can transform carbon dioxide, whether in the air or from effluent from carbon fuel combustion, into oils which then can be processed to yield diesel-like fuels.
  • Diesel-like fuels can be made from natural oil-based fuels use oil crops like rapeseed or canola. The oils produced by these plants can be converted to motor vehicle fuels by processes such as:[6]
    • Transesterification, which reacts natural oils with alcohols such as methanol to produce diesel fuel
    • Hydrotreating, which reacts the oils with hydrogen in the presence of catalysts to generate diesel fuel. It is presently less expensive than transesterification.
    • Pyrolysis, in which oils are rapidly headed to 600-800 degrees Celsius, producing a mixture of organic products including both fuel products and other organics like carboxylic acids. Pyrolysis is less complicated than either transesterification or hydrotreating and requires no additional reagents, but suffers from reduced chemical selectivity for the desired fuel compounds.
  • Ether-based fuels can be made from a variety of biomass sources using processes such as gasification.

References

  1. ↑ "EIA-819 Monthly Oxygenate Report" December 2008 (Washington, DC: Energy Information Administration, 2008), at [1]. Accessed 6 January 2009.
  2. ↑ 90% of American ethanol production uses corn as the feedstock. Biomass Energy Data Book, table 2.9, "Ethanol Production by Feedstock" (Washington, DC: United States Department of Energy, 2006), at [2]. Accessed 13 January 2009.
  3. ↑ For a summary of the challenges of biofuels, see Kammen, D. M., Farrell, A. E., Plevin, R. J., Jones, A. D. and Delucchi, M. A., in OECD RESEARCH ROUND TABLE: BIOFUELS: LINKING SUPPORT TO PERFORMANCE Perkins, S., Ed. (Organization for Economic Cooperation and Development, Paris, 2007). For a survey of the greenhouse gas footprint of biofuels, see Alexander E. Farrell, Richard J. Plevin, Brian T. Turner, Andrew D. Jones, Michael O'Hare, and Daniel M. Kammen, "Ethanol Can Contribute to Energy and Environmental Goals", Science ), 27 January 2006, p506.
  4. ↑ Liska A. et al (2008), Improvements in Life Cycle Energy Efficiency and Greenhouse Gas Emissions of Corn-Ethanol, Journal of Industrial Ecology, http://www.growthenergy.org/2009/reports/2009%20JIE%20Improvements%20in%20corn%20ethanol-Liska%20et%20al.pdf
  5. ↑ See for instance Gable, Mats, Per Sassner, Anders Wingren, and Guido Zacchi. Adv Biochem Engin/Biotechnol (2007) 108: 303-327; Galbe, M, G. Zacchi. Appl Microbiol Biotechnol (2002) 59:618-628; Lynd, Lee R., Willem H. van Zyl, John E. McBride, and Mark Laser. Current Opinion in Biotechnology (2005) 16:577-583; Hahn-Hagerdal, B., M. Galbe, M.F. Gorwa-Grauslund, G. Liden, and G. Zacchi. TRENDS in Biotechnology ; Huber, George W., Sara Iborra, and Avelino Corma. (September 2006) Chem Rev. 109(9):; Younesi, Habibollah, Ghasem Najafpour, and Abdul Rahman Mohamed. Biochemical Engineering Journal (2005) 27:110-119.
  6. ↑ George W. Huber, Sara Iborra, and Avelino Corma, "Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering", Chem. Rev.
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