Matthew A. Oehlschlaeger 1 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 Prospects for Biofuels: A Review 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, includ- ing 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 poten- tially 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. [DOI: 10.1115/1.4023602] Keywords: biofuels, review, lifecycle, greenhouse gas emissions, energy return on invest- ment, water, tailpipe exhaust emissions 1 Introduction Given the volatile history of energy prices, rising greenhouse gas (GHG) emissions, rising atmospheric GHG concentrations, and the inexorable link between national security and energy se- curity, biofuels have become of a topic of research, debate, and private- and public-sector investment. Biofuels, a diverse classifi- cation of fuels produced from biomass comprised of photosynthesis-fixed carbon, have, in some cases, the potential to be sustainable, secure, low carbon footprint, high-energy density transportation fuels. Over the last decade or more, significant research efforts dedicated to lifecycle analyses of biofuels and their production have been reported. Lifecycle analyses have examined GHG emissions, energy costs of biofuel production (sometimes described in terms of net energy gained (NEG) or the energy return on investment (EROI)), water and land require- ments, and tailpipe emissions. The most conclusive lifecycle stud- ies have focused on biofuels with well defined and understood feedstock growth and fuel production processes; for example, those in current production as first-generation biofuels including ethanol produced by the fermentation of sugars from corn or sug- arcane and biodiesels (alkyl esters) produced by the transesterifi- cation of oils from soybean, rapeseeds, palm, or animals fats. In light of their potential benefits, global biofuel production has dramatically increased over the last decade and in many countries biofuel production targets have been mandated. For example, in the United States the Renewable Fuel Standard (RSF) [1] states that 7.5  10 9 gallons of renewable fuel must be blended into transpor- tation fuels by 2012, or approximately 4% of 2012 U.S. gasoline and diesel consumption [2], and 36  10 9 gallons by 2022, approxi- mately 20% of projected 2022 U.S. gasoline and diesel consump- tion [2]. The European Union (EU) Renewable Energy Directive (RED) mandates a 10% renewable energy content in the transporta- tion sectors of every EU member state by 2020 with 7% from biofuels and 3% from vehicle electrification [3]. Additionally, both the U.S. and EU directives specify standards for the allowable lifecycle GHG emissions of biofuels, ensuring that the adoption of biofuels results in a net reduction in GHG emissions. Figures 1 and 2 illustrate, respectively, the growth in biofuel production over the last decade (data from Ref. [4]) and the projected increase in United States biofuel production for the next decade based on the target established by the U.S. Energy Independence and Security Act (EISA) of 2007 [5] and implemented as the U.S. Environmental Protection Agency Renewable Fuel Standard (RFS) [1]. Clearly the trend in biofuel penetration into the transportation fuels market is upward and will be for some time; however, the sustainability of biofuels and the ultimate limit in biofuel penetration into the trans- portation fuels market will be governed by research-driven develop- ments in the growth of feedstocks and production of biofuels offering substantial improvements in biofuel “performance” (GHG emissions and energy, land, and water costs). Biofuel lifecycle “performance” is a function of the feedstock growth and fuel production agricultural, energy, water, land, and other costs. Biofuels can be synthesized from a large number of feedstocks in a variety of distinct biochemical and/or thermochemi- cal processes; some selected pathways for biofuel production are outlined in Fig. 3. First-generation biofuels are commercially well- developed biofuels which account for nearly 100% of the current biofuel market and include ethanol produced via the fermentation of sugars from corn in the United States and sugarcane in Brazil (see review of fermentation synthesis by Bai et al. [6]) and biodie- sel produced via the transesterification of oils from soybeans in the United States, rapeseeds in Europe, and palm in Malaysia (see review of transesterification synthesis by Meher et al. [7]). First generation biofuels have clearly seen widespread adoption and pro- duction growth, based on the trends illustrated in Figs. 1 and 2. However, first-generation biofuel feedstocks may be unsustainable due to their high requirements for agricultural, energy, and water inputs, and land usage, as is shown in the subsequent sections. Second-generation biofuels, presently at the pilot plant and scale-up stage of commercialization, aim to increase biofuel yields in terms of agricultural, energy, water requirements, and land area usage. Second-generation biofuels are those produced from non- food crops including lignocellulosic biomass (e.g., switchgrass and Miscanthus), woody crops, agricultural waste, and nonedible oils (e.g., Jatropha). Biofuels can be synthesized from these 1 Corresponding author. Manuscript received October 15, 2012; final manuscript received February 3, 2013; published online May 17, 2013. Assoc. Editor: Alexander L. Brown. Journal of Thermal Science and Engineering Applications JUNE 2013, Vol. 5 / 021006-1 Copyright V C 2013 by ASME Downloaded From: http://thermalscienceapplication.asmedigitalcollection.asme.org/ on 06/24/2015 Terms of Use: http://asme.org/terms