The current biofuel market is largely dominated by ethanol, which ac­counts for 90% of world biofuel production. [19] Indeed, the rate of etha­nol production around the world is increasing rapidly, from 13 billion gal­lons in 2007 to the current level of almost 20 billion gallons in 2009, [20] with the US (55%, corn-derived) and Brazil (33%, sugar cane-derived) being the main producers. [21] The ethanol industry has benefited from a mature and simple technology in which selected microorganisms (e. g., yeast, bacteria, and mold) transform aqueous sugars to the desired final ethanol product. Ethanol is added to gasoline to increase the octane num­ber of the mixture and, with it, improve the combustion characteristics of the fuel. The oxygen in ethanol allows low-temperature combustion with the subsequent reduction of pollutants such as CO and NOx. [19] Addi­tionally, apart from CO2 emission savings, blending gasoline with ethanol helps to reduce SOx emissions to the atmosphere, because ethanol contains a negligible amount of sulfur compared to petroleum. [18]

A key aspect that is responsible for expansion of the ethanol industry in recent years is the compatibility of ethanol with the existing infrastructure for gasoline. Thus, ethanol blended with conventional gasoline is currently used in many countries as a renewable fuel in existing spark-ignition en­gines. This compatibility, however, is not complete and the use of etha­nol is presently limited to low-concentration blends (5-10% by volume), namely E5-E10. Ethanol-enriched mixtures such as E85 require cars with specially designed engines, designated as flexible-fuel vehicles (FFVs), which are commonly used only in a few countries like Brazil and Sweden. E85 mixtures are not tolerated by conventional vehicles, because ethanol, especially in high-concentration blends, can cause corrosion of some me­tallic components in tanks and deterioration of rubbers and plastics used in internal combustion engines. [21] This constraint in ethanol blending represents the main issue of the growing ethanol industry. As outlined in Fig. 1, projections indicate that the US ethanol industry will approach the blending wall (i. e., the point at which blending 10% of ethanol in each gallon of gasoline will not be able to accommodate the rate of ethanol production) in 2010. [22] Furthermore, experts predict that the number of E85 fuelling stations and flexi-fuel vehicles will not grow sufficiently fast to accommodate the growing volumes of ethanol produced in the US. [23] A potential solution to overcome the blending wall is to raise the amount the ethanol allowed in gasoline to beyond 10% by commercializing inter­mediate ethanol blends (i. e., E15- E20) (Fig. 1). However, there are seri­ous issues in using blends with higher ethanol concentrations. In countries like the US, the utilization of E15-E20 blends in regular vehicles is still not authorized, since the effect of these mixtures on pollutant emissions, driving performance and materials compatibility (e. g., tanks, pipelines, dispensers) is not fully understood, [23] and current European standards allow only for E5 blends. [18]

Apart from the aforementioned blending issues, ethanol presents an­other important limitation as a transportation fuel. Ethanol contains less energy per volume (i. e., energy density) than conventional gasoline, which ultimately reduces the fuel mileage of the vehicles. In this sense, it has been estimated that cars running on ethanol rich mixtures like E85 operate with 30% lower fuel mileage. [24] This fact, along with the small price differential between E85 and regular gasoline, has discouraged drivers to purchase E85 cars or fuel so far. [22]

Conventional transportation fuels are composed of liquid hydrocar­bons with different molecular weights (e. g., C5-C12 for gasoline, C9-C16 for jet fuel, and C10-C20 for diesel applications) and chemical structures (e. g., branched for gasoline, linear for diesel). The entire transportation infrastructure (including engines, fueling stations, distribution networks, and storage tanks) has been developed to take advantage of the excellent properties of these compounds as fuels. Thus, the special composition of hydrocarbons fuels, based only on carbon and hydrogen, provides them with high energy-density and stability (allowing efficient storage at ambi­ent conditions) and superior combustion characteristics, properties highly desired for transportation liquids. Thus, instead of using biomass to pro­duce oxygenated fuels (such as ethanol) with new compositions, an attrac­tive alternative would be to utilize biomass to generate liquid fuels chemi­cally similar to those being used today derived from oil. [17,25] These new fuels would be denoted as green gasoline, green diesel and green jet fuel, and they would be essentially the same as those currently used in the transportation fleet, except that they would be synthesized from biomass instead of petroleum. When compared with ethanol, the production of hy­drocarbon fuels from biomass has important advantages. The main benefit would include full compatibility with the existing energy system. Since green hydrocarbon fuels would be essentially the same as those currently obtained from petroleum, it would not be necessary to modify engines, pumps or distribution networks to accommodate the new renewable liq­uids in the transportation sector.

Unlike ethanol, biomass-based hydrocarbons fuels are energy equiva­lent to fuels derived from petroleum. The heating value (i. e., the heat re­leased when a known quantity of fuel is burned under specific conditions) of ethanol is only two-thirds that of gasoline, which, as indicated above, penalizes the fuel mileage of the vehicles running on gasoline-ethanol mixtures. The use of renewable hydrocarbon fuels would additionally help to meet the increased standards of fuel economy imposed by governments to the automobile industry. In the case of the US, these standards establish a mandatory increase in average fuel economy from the current 25 miles per gallon (mpg) to 35 mpg by 2022. [26]

The addition of oxygenated components to conventional fuels in­creases the water solubility of the mixture. This increase is particularly marked in the case of gasoline-ethanol blends, since pure ethanol is high­ly hygroscopic and completely miscible in water. Thus, adding 10% of ethanol to regular gasoline raises the water solubility of the blend more than 30 times (from 150 ppm v/v of regular gasoline to 5000 ppm for E10). [27] Once the water contamination reaches the saturation level, additional water separates from the mixture, removing the ethanol from gasoline and leading to phase separation. In fact, when phase separation occurs in the storage tank, the ethanol-water layer may combust in the engine at higher temperatures causing damage to it. [27] The water tol­erance of a gasoline-ethanol blend (i. e., fraction of water that the mix­ture can contain without phase separation) decreases with temperature and increases with ethanol content (Fig. 2). Consequently, phase separa­tion is an important concern in countries with cooler climates and when low-concentration blends such as E5 are used. Water can be absorbed by the ethanol-gasoline mixture from the atmosphere (in the form of moisture), from the air trapped in the tank (by condensation of water when temperature decreases), or even from the ethanol itself which typ­ically carries traces of water when delivered from the biorefinery. In this respect, many countries have regulated the maximum amount of water allowed in fuel-grade ethanol to the level of 1% (v/v), to avoid phase separation issues. [28] Ethanol affinity for water has important implica­tions for distribution logistics as well. Pipelines, considered to be the least expensive means of safely transporting bulk fuel shipments, [23] are not suited to transport ethanol or gasoline-ethanol blends on a com­mercial scale, because apart from corrosion issues, ethanol can pick up water in the pipeline with the potential result of phase-separation. Con­sequently, ethanol has to be distributed by other fossil fuel-consuming transportation modes such as rail, truck and barge. The hydrophobic character of biomass-derived hydrocarbons eliminates these problems, since these molecules are immiscible in water. Additionally, the ability of liquid hydrocarbons to self-separate from water, as represented in Fig. 3, is highly beneficial in that it eliminates the need for expensive and energy-consuming distillation steps required in the ethanol purifica­tion process. In particular, ethanol is initially obtained in form of a di­lute aqueous solution (5-12% v/v), which is subsequently concentrated to 96-99% by distillation. It is estimated that this intense water removal step is responsible for 35-40% of the total energy required for ethanol

production, [29] and this energy is typically supplied by combustion of fossil fuels such as natural gas.

Any technology envisaged to convert biomass feedstocks into liquid fuels must address one important limitation of this resource: the low en­ergy-density of biomass compared to fossil fuels. Although the energy — density of biomass varies considerably depending on the source, an aver­age value for biomass (15-20 MJ kg-1) is well below that of crude oil (42 MJ kg-1). [30] Large amounts of biomass will thus be required to produce liquid fuels, leading to high costs for transporting the biomass source to the processing location. [31] Furthermore, if biomass transportation in­volves utilization of fossil fuels, then the overall CO2 emission savings of the bioprocess would be penalized. Consequently, for biomass conversion technologies to be cost-competitive and truly carbon-neutral, it is neces­sary to develop efficient processing units at small scale that can be distrib­uted close to the biomass source. [17] Even though ethanol plants achieve a significant size reduction compared to petrochemical refineries, the mild conditions employed and the low levels of ethanol achieved for bacterial fermentation (e. g., 30-50 oC, ethanol concentrations lower than 15% v/v) significantly limit the reaction rates and require reactors with a size large enough to make the process economically feasible. As will be described in following sections, biomass-based hydrocarbon fuels, in contrast, can be produced at high temperatures and using concentrated water solutions, [14] which allow for faster conversions in smaller reactors.

The utilization of edible biomass (such as corn or cane sugar) for the large-scale production of fuels can produce competition with food for land use. The so-called food-versus-fuel debate has arisen in many countries as a response to the sharp increase in food prices during 2007 and 2008. Although it has been pointed out that this rise in price was the result of several linked worldwide events, [22] some authors indicate that the increased demand for corn to produce ethanol had a direct impact on food prices, especially in food- insecure areas of the world where food is based on grain consumption. [32] These issues have driven researchers around the world to develop technolo­gies to process non-edible biomass (e. g., lignocellulosic biomass), thereby permitting sustainable production of a new generation of biofuels (so-called second generation of fuels), without affecting food supplies. Lignocellulosic biomass has two important advantages over edible biomass feedstocks: it is more abundant and can be grown faster and with lower costs. [33] In this respect, it is estimated that the US could sustainably produce more than 1 billion tons of non-edible biomass per year by 2050 with relatively modest changes in land use and agricultural and forestry practices. Once converted into biofuels, these lignocellulosic resources would have the potential to dis­place more than one third of the petroleum currently consumed by the trans­portation sector.34 Lignocellulosic feedstocks ($3 per GJ) are slightly less expensive than edible biomass (5$ per GJ), and potentially more economical than crude oil (10-15 $ per GJ) and vegetable oils (18-20 $ per GJ); however, due to its recalcitrant nature, lignocellulose is more difficult to convert and, consequently, processing costs increase for this resource. [35] Thus, accord­ing to recent analyses, the cost of producing lignocellulosic ethanol would be almost double that of corn-derived ethanol. [19] This fact represents the main limitation of lignocellulose as a renewable resource, and the lack of cost-competitive technologies for the generation of liquid fuels from non­edible sources has been identified by experts as the key bottleneck for the large-scale implementation of lignocellulose — derived biofuel industry. [36] Recalcitrance of lignocellulosic biomass can be explained in terms of its chemical structure, comprised of three major units: cellulose, hemi — cellulose and lignin. [13,37,38] Cellulose (40-50%) is a high molecular weight polymer of glucose units connected linearly via b-1,4-glycoside linkages. This arrangement allows for extensive hydrogen bonding be­tween cellulose chains, which confers this material with rigid crystallinity and, thus, high resistance to deconstruction. [39] Cellulose bundles are additionally attached together by hemicellulose (15-20%), an amorphous (and consequently more readily deconstructed) polymer of five different C5 and C6 sugars. Cellulose and hemicellulose, the carbohydrate frac­tion of lignocellulose, are protected by a surrounding three-dimensional polymer of propyl-phenol called lignin (15-25%), which provides extra rigidity to the lignocellulose structure. To overcome lignocellulose recal­citrance, a variety of physical and chemical methods have been developed, and a comprehensive description of such technologies can be found else­where. [40-42] The approach most commonly used involves pretreatment of lignocellulose (with the aim of breaking/ weakening the lignin protec­tion and increasing the susceptibility of crystalline cellulose to degrada­tion), followed by hydrolysis to depolymerize hemicellulose and cellulose and, thus, isolate the sugars from the lignin fraction.