Research Papers

Prospects for Biofuels: A Review

[+] Author and Article Information
Matthew A. Oehlschlaeger

Department of Mechanical, Aerospace, and Nuclear Engineering,
Rensselaer Polytechnic Institute,
Troy, NY 12180
e-mail: oehlsm@rpi.edu

Haowei Wang

Department of Mechanical Engineering,
California State University Fullerton,
Fullerton, CA 92831

Mitra N. Sexton

Knolls Atomic Power Laboratory,
Niskayuna, NY 12309

References cited in fig. 6 are [42-44].

1Corresponding author.

Manuscript received October 15, 2012; final manuscript received February 3, 2013; published online May 17, 2013. Assoc. Editor: Alexander L. Brown.

J. Thermal Sci. Eng. Appl 5(2), 021006 (May 17, 2013) (9 pages) Paper No: TSEA-12-1174; doi: 10.1115/1.4023602 History: Received October 15, 2012; Revised February 03, 2013

Biofuels have the potential to be sustainable, secure, low carbon footprint transportation fuels. Primarily due to government mandates, biofuels have become increasingly adopted as transportation fuels over the last decade and are projected to steadily increase in production. Here the prospects of biofuels are summarized in terms of several important performance measures, including: lifecycle greenhouse gas (GHG) emissions, energy return on investment (EROI), land and water requirements, and tailpipe emissions. A review of the literature leads to the conclusion that most first-generation biofuels, including corn ethanol and soybean biodiesel produced in the United States, reduce tailpipe pollutant emissions and GHG emissions—provided their feedstocks do not replace large quantities of fixed carbon. However, their production is perhaps unsustainable due to low EROI and significant land-use and water requirements. Second-generation biofuels; for example ethanol produced from lignocellulosic biomass, have the potential for larger reductions in GHG emissions and can provide sustainable EROI with reasonable land area usage; however, they require water inputs several orders-of-magnitude greater than required by petroleum fuels. Advanced biofuels from algal oils and synthetic biological processes are further from commercial reality and require more assessment but potentially offer better performance due to their orders-of-magnitude greater yields per land area and lower water requirements; at present, the energy costs of such biofuels are uncertain.

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Fig. 1

Biofuel production by region for 2001–2011 [4]. For comparison, in 2011 petroleum oil production was 83.6 × 106 barrels per day worldwide and 14.3 × 106 barrels per day in North America.

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Fig. 2

Past and projected United States biofuel production for 2001–2022 [4]. Projection to 2022 based on Energy Independence and Security Act target [5]. For comparison, U.S. petroleum oil production and consumption in 2011 was 7.8 × 106 and 18.8 × 106 barrels per day, respectively.

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Fig. 3

Pathways for biofuel synthesis from various feedstocks

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Fig. 4

Lifecycle GHG emissions as a percentage of those produced by conventional petroleum gasoline or diesel, data from Ref. [35]. The negative result for switchgrass ethanol from a biochemical (fermentation) process indicates that this fuel results in negative net GHG emissions (i.e., carbon capture is greater than GHG emissions).

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Fig. 5

The carbon debt incurred by the replacement of an ecosystem with a biofuel crop (in years of biofuel production required to repay the GHG emissions resulting from displacing a natural ecosystem), adapted from Ref. [36]

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Fig. 6

Ethanol and biodiesel yields for different feedstocks, ethanol yields [42], and biodiesel yields [43,44]

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Fig. 7

The energy return on investment (EROI) for selected biofuels and fossil resources; sources of EROI data: oil, natural gas, shale oil, and bitumen from tar sands [47], sugarcane ethanol [51], cellulosic ethanol [52], corn ethanol [31], and soybean biodiesel [53]

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Fig. 8

The energy supplied to society (gray area) and the energy use to procure that energy (white) as a function of EROI, adapted from Murphy and Hall [47]

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Fig. 9

Total water requirements for biofuels [63] with comparison to petroleum [65]

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Fig. 10

Tailpipe emissions for biofuels, data for B100 and B20 from the U.S. EPA [67] and E85 from Yanowitz and McCormick [68]




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