Over the past three decades interest in the reduction of air pollution, and in the development of domestic energy sources, has triggered research in many countries on the development of non-petroleum fuels for internal combustion engines. For compression ignition (diesel) engines, it has been shown that the simple alcohol esters of fatty acids (biodiesel) are acceptable alternative diesel fuels. Biodiesel has a higher oxygen content than petroleum diesel, and therefore reduces emissions of particulate matter, hydrocarbons, and carbon monoxide, while also reducing sulfur emissions due to a low sulfur content.
For spark ignition (gasoline) engines, ethanol, produced by fermentation of simple sugars generated from corn starch, can be blended with petroleum gasoline to substitute petroleum content with renewable content fuel, reduce dependence on foreign oil, reduce carbon dioxide emissions, and improve octane in the blended fuel. Since both ethanol and biodiesel are made from agricultural materials, which are produced via photosynthetic carbon fixation (e.g., by plants and by animals that consume plants), the combustion of biodiesel and ethanol does not contribute to net atmospheric carbon levels.
Initial efforts at the production, testing, and use of biodiesel employed refined edible vegetable oils (e.g. soybean oil, canola oil), used cooking oils (e.g. spent fryer oils) and animal fats (e.g., beef tallow) as feedstocks for fuel synthesis (Krawczyk, T., INFORM, 7: 800-815 (1996); Peterson, C. L., et al., Applied Engineering in Agriculture, 13: 71-79 (1997); Holmberg, W. C., and J. E. Peeples, Biodiesel: A Technology, Performance, and Regulatory Overview, National Soy Diesel Development Board, Jefferson City, Mo. (1994)).
Simple alkali-catalyzed transesterification technology (Freedman, B., et al., J. Am. Oil Chem. Soc., 61(10): 1638-1643 (1984)) is efficient at esterifying the acylglycerol-linked fatty acids of such feedstocks and is employed in making these fuels. More recently, methods have been developed to produce fatty acid methyl esters (FAME) from cheaper, less highly refined lipid feedstocks such as spent restaurant grease (Mittelbach, M., and P. Tritthart, J. Am. Oil Chem. Soc., 65(7):1185-1187 (1988); Graboski, M. S., et al., The Effect of Biodiesel Composition on Engine Emissions from a DDC Series 60 Diesel Engine, Final Report to USDOE/National Renewable Energy Laboratory, Contract No. ACG-8-17106-02 (2000); Haas, M. J., et al., Enzymatic Approaches to the Production of Biodiesel Fuels, in Kuo, T. M. and Gardner, H. W. (Eds.), Lipid Biotechnology, Marcel Dekker, Inc., New York, (2002), pp. 587-598).
In addition to acylglycerols, less highly refined lipid feedstocks can contain substantial levels of free fatty acids (FFA) and other nonglyceride materials. Biodiesel synthesis from these feedstocks can be accomplished by conventional alkaline catalysis, which then requires an excess of alkali since the FFA (which are not esterified by this method) are converted to their alkali salts. These alkali salts can cause difficulties during product washing due to their ready action as emulsifiers. Ultimately, the alkali salts are removed and discarded. This approach thus involves a loss of potential product, increases catalyst expenses, and can entail a disposal cost.
Further, with higher FFA levels, i.e. typically in excess of 2%, a general approach is to utilize an acid esterification step, since at higher FFA values the extent of soap formation with a single stage, transesterification process is excessive and renders the process uneconomical and potentially unworkable. To handle the higher FFA content, a two step process involving first acid-catalyzed esterification of the free fatty acids and then alkali-catalyzed transesterification of glyceride-linked fatty acids can be employed to achieve conversion of mixed, heterogeneous feedstocks (Canakci, M., and J. Van Gerpen, Biodiesel Production from Oils and Fats with High Free Fatty Acids, Abstracts of the 92.sup.nd American Oil Chemists' Society Annual Meeting & Expo, p. S74 (2001); U.S. Pat. Nos. 2,383,601; 2,494,366; 4,695,411; 4,698,186; 4,164,506). However, these methods can require multiple acid-catalyzed esterification steps to reduce the concentration of free fatty acids to acceptably low levels. In addition, high separation efficiency is required between the two stages to minimize the potential for acid catalyst transfer into the base catalyst section.
The feedstocks used for current biodiesel production are conventional commodity materials, thus they have other established markets which basically set the minimum commodity prices. As a result, the bulk of the biodiesel production cost relates to the feedstock cost. While there are a number of established process technologies in the biodiesel industry, as a result of the feedstock cost being such a high factor (i.e. 75% to 80%) there is a surprisingly small difference between the various processes in overall operational costs (due to this feedstock factor).
The production of ethanol for fuel use is well established and the growth in this industry over the past 2 decades has been significant. Fermentation is an (obviously) old process going back literally thousands of years to early wine and beer making. The basic techniques remain the same, however in the modern ethanol production process highly efficient enzymes and yeasts have been developed to provide for more efficient conversion of the fermentable materials. Further, the process technology associated with fuel grade ethanol production has also advanced over the years, e.g. energy recovery, so that current technology has a high degree of efficiency.
The primary feedstocks for current commercial ethanol production are corn (primarily in the United States) and sugar (especially in Brazil). As in the biodiesel case, these materials are “conventional” agricultural commodities and have historically had various markets associated with them, i.e. food sources and the like. It is also apparent that since these are commodity products, there are various non-fuel market pressures that dictate price. As such, for ethanol production, as in the case of biodiesel, the feedstock represents the vast majority of the operating cost (i.e. as much as 80%).
For both the biodiesel and ethanol fuel markets and for the large-scale expansion of the renewable fuels industries, it is apparent that development of a potentially large scale, lower cost feedstock source would be advantageous. Recently, significant advances have been made in carbon dioxide sequestering technology (aquatic species program reference, NREL, GFT, a U.S. company) using various species of algae to provide photosynthetic carbon fixation. This technology has tremendous value when applied to industrial sources of carbon dioxide such as; coal fired power generation, natural gas fired power generation, petroleum fired power generation, industrial gas generation, cement manufacturing, industrial fermentation, as well as various additional industries that are significant emitters of carbon dioxide. The algae resulting from the photosynthetic carbon fixation represents an opportunity for the production of transportation fuels as well as various value added chemical products. The volume of algae produced per acre, in a designed pond or “farming” system, is estimated at between 200,000 pounds to 600,000 pounds per year of algae on a dry basis; and is substantially greater, in terms of oil content and fermentable material content, than the volume of soybeans or corn produced per acre at 2,500 pounds to 10,000 lbs per year. The volume of algae produced using the above method allows for a far greater production density versus corn or soybeans with a relatively small geographic footprint. In addition, the algae selected comprise free fatty acids (FFA), triglycerides, polysaccharides, cellulose, hemicellulose and/or lignocellulose. However, the economical processing of the selected algae provides significant challenges for conventional biofuel processing techniques.
For the algae scenario, a significant degree of pretreatment of the sludge is required to prepare the material for the more traditional solvent extraction methods to recover the contained oil. This front-end pretreatment would then need to be combined with multi-stage esterification, (for free fatty acid esterification) and transesterification (for triglyceride conversion), and a completely separate process would be required for acid hydrolysis of the lipid depleted algae pulp to produce monosaccharides, disaccharides, trisaccharides or polysaccharides for production of ethanol by fermentation. This series of processing steps would add significant cost to the resulting materials to be produced from algae. Therefore, there is a need for further development of simplified processing routes for the production of fatty acid alkyl esters (i.e. FAME), monosaccharides, disaccharides, trisaccharides or polysaccharides in a simplified, direct process.
In addition, the current growth in biofuel production from food commodities is generating a substantial increase in co-products such as corn distillers grains, sorghum distillers grains, and rice bran meal. These co-products have underutilized value from the cellulosic content (45-55% by mass) and oil content (7-22% by mass) which represent an opportunity to increase the supply of biofuels to market by simply increasing the processing efficiency of current methods.
Again, the interest in cellulosic feeds for ethanol has increased considerably over the past several years, however some of the same issues apply to this source as to feeds such as algae. For example, with cellulosic feeds the typical approaches include enzyme treatment followed by yeasts which convert the cellulosic materials to sugars and subsequent alcohol, but has little effect on any contained oil content. For example distillers grains have both cellulosic content as well as contained oil values, both of which could be useful for conversion to biofuels.
Thus, there remains a significant need in the art to develop a simple and efficient method for the production of biofuels and ethanol from renewable energy sources.