As the US Renewable Fuel Standard (RFS2) increasingly mandates the commercialization of biofuels other than corn ethanol (US Congress, 2007a,b), second-generation biofuels must be developed to satisfy these mandates. Moreover, consensus is beginning to converge on the notion that optimal biofuels should be ‘drop-in’, or in other words, compatible with existing fuel-related infrastructure (Regalbuto, 2009; Babcock et al., 2011). As biobutanol has several unique infrastructure compatibilities and the potential to satisfy the RFS2's increasing mandates for second-generation biofuels, it is currently receiving much attention from both the research community (Ezeji et al., 2004, 2007; Atsumi et al., 2008; Dellomonaco et al., 2011; Higashide et al., 2011) and the private sector biofuels industry (Butamax Advanced Biofuels, 2011; Cobalt Technologies, 2011; Gevo, 2011).
The history of biobutanol production and commercialization
The term ‘biobutanol’ is generically used to refer to any form of butanol that is derived from naturally occurring biological feedstocks. While observations of biobutanol fermentation span back to the work of Louis Pasteur, the first major breakthrough in its efficient and economical production came with the development of the acetone-butanol-ethanol (ABE) process in 1916 (Arnold, 2008). Using the bacterium Clostridium acetobutylicum to ferment naturally occurring starches and sugars, the ABE process traditionally yields butanol, acetone, and ethanol in a ratio of 6 : 3 : 1 by weight, respectively (Gibbs, 1983). While the ABE process attained commercial deployment in the early to mid-20th century as a means of producing butanol for the chemicals industry, it fell out of favor in the late 1950s when petroleum became a cheaper feedstock for butanol production (Arnold, 2008).
In the early 1990s, research interest in the production of biobutanol was rekindled, and the yields from the ABE process were enhanced through the discovery and implementation of the mutant bacterium Clostridium beijerinckii BA101 (Wu et al., 2008). Using this bacterium and modern gas stripping techniques, the butanol, acetone, and ethanol yield ratio from the ABE fermentation process has been substantially increased to 44 : 23 : 1 by weight, respectively (Ezeji et al., 2004). Despite these advances, the ability of biobutanol produced via the ABE process to successfully enter the US fuel market remains questionable. Not only do complex issues surround the potential markets for the acetone and ethanol that are simultaneously produced by the ABE process (Wu et al., 2008), but as we will see below, the commercialization of this form of biobutanol as a transportation fuel could be hindered by its classification for purposes of the RFS2. As such, commercialization efforts for biobutanol produced through the ABE process are primarily focusing on chemical markets as opposed to transportation fuel markets (Cobalt Technologies, 2011; TetraVitae Bioscience, 2011).
Arguably the most important breakthrough in the production of biobutanol came in 2008 when the Escherichia coli bacterium was genetically modified to produce only isobutanol, the highest-octane isomer of butanol (Atsumi et al., 2008). With the advent of genetically modified microorganisms capable of producing biobutanol without significant production of co-products (i.e., acetone and ethanol), the private sector biofuels industry has taken notice, and efforts are underway to attempt to commercialize biobutanol as a transportation fuel (Butamax Advanced Biofuels, 2011; Gevo, 2011). The business model of these companies involves acquiring existing ethanol production facilities, retrofitting them to produce biobutanol, and also licensing their technologies as retrofit packages to other owners of ethanol production facilities. While first-generation biofuel feedstocks such as corn starch and Brazilian sugarcane are currently being considered the feedstocks of choice for biobutanol production, these companies claim that their proprietary biocatalysts are capable of producing biobutanol from any fermentable sugar.
The advantages of biobutanol
The recent focus on biobutanol production is substantiated by its many reported advantages as a transportation fuel. First, its biggest advantage is its relatively high energy content when compared to first-generation biofuels. Based on its low heating value, biobutanol derived from the fermentation of cornstarch has an energy content of 99 837 BTU per gallon (Wu et al., 2008). This equates to 86% of the energy content of gasoline, which possesses 116 090 BTU per gallon (Wu et al., 2008). By way of comparison, corn ethanol only possesses 76 330 BTU per gallon, which equates to an energy content roughly 66% that of gasoline (Wu et al., 2008). As such, if we put aside the effects of cost, simply account for the energy content of fuel (i.e., assume that distance traveled perfectly correlates with BTUs used regardless of fuel type), and compare a 16% biobutanol/84% gasoline blend (Bu16) with the 10% ethanol/90% gasoline blend (E10) widely available in the United States today, the widespread use of Bu16 would not only result in slightly greater (<2%) fuel economy for consumers but also would displace over twice as much hydrocarbon gasoline.
The second major advantage of biobutanol is that it reportedly has compatibilities with each stage of the existing ‘field-to-tank’ macro-level infrastructure needed to produce biofuels, bring them to market, and consume them. In regards to feedstock-related infrastructure, biobutanol can not only be produced from current first-generation biofuel feedstocks such as corn starch (Ezeji et al., 2007) and cane sugar (Butamax Advanced Biofuels, 2011), but its production from a cellulosic feedstock has also been demonstrated (Higashide et al., 2011). Additionally, the private companies currently working to commercialize biobutanol as a transportation fuel claim that it is compatible with existing biofuel production infrastructure in that their technologies allow existing corn ethanol plants to be minimally retrofitted to produce biobutanol in addition to ethanol (Butamax Advanced Biofuels, 2011; Gevo, 2011). Most importantly, it has been reported that biobutanol is compatible with current fuel distribution infrastructure since it does not produce the microbial-induced corrosion that is associated with ethanol (Wu et al., 2008). As such, it can potentially be blended with gasoline at the refinery, transported through existing fuel pipelines, and distributed at existing retail fueling stations. Furthermore, it has been reported that fuel blends containing as high as 100% biobutanol are compatible with existing spark-ignition internal combustion engines (Ramey, 2007; Szwaja & Naber, 2010).
While these reported advantages render biobutanol a prime candidate to act as a biofuel alternative to gasoline, two distinct regulatory frameworks will heavily impact its successful commercialization in the United States. First, the RFS2 will have a tremendous impact as it effectively mandates a captive market for biofuels (US Congress, 2007a,b). Second, the US Clean Air Act's (CAA) ‘substantially similar’ regulatory framework for new fuels and fuel additives effectively governs the lawful blending limits for biobutanol (US Congress, 1972; US Environmental Protection Agency, 2008). As such, this paper will first analyze the RFS2's treatment of biobutanol (both currently and prospectively) and then move on to analyze the three distinct options for the commercialization of biobutanol under the CAA, which each resulting in differing lawful blending limits. Finally, we will conclude by arguing that specific provisions of the CAA could create unjustified regulatory hurdles for the commercialization of biobutanol and suggesting unique regulatory reforms that could be implemented to rectify this situation.