The large-scale production of a renewable and environmentally sustainable alter­native fuel faces several technical challenges that need to be addressed to make biodiesel feasible and economical. The two main concerns with any renewable fuel are raw materials and the technologies used for processing. Advances in genetic modification and other biotechnologies are resulting in new or modified feedstocks that have significantly increased the yields of alternative fuels, such as genetically modified Clostridium to improve alcohol production [16]. Technological advance­ments are also being made to convert the feedstocks into fuels by improving techniques or developing completely new and environmentally friendly approaches to biofuel production.

There are many feedstocks for biodiesel production such as virgin oils, biomass, algae, and waste oils, to name a few. Feedstocks also vary with climate and location and what might be a great source in one place may not be a good source in another. A considerable amount of research has been done using edible sources of virgin oils from vegetables, like soybean, rapeseed, sunflower seed, and canola oils, to produce biodiesel. However, oil with water or high free fatty acid content can result in the formation of soap as a by-product. Therefore, additional steps must be taken to prevent soap formation, which requires the utilization of more resources.

The production of biodiesel has increased demand for soybean oil from 1.56 bil­lion pounds in 2005-2006, to 2.8 billion pounds in 2006-2007 [17]. The increasing demand for virgin vegetable oil stocks has lead to an increase in price of these oils. The profitability of biodiesel relies heavily on the cost of its feedstock. The costs of soybean oil can account for up to 75% of the final cost per gallon of biodiesel. This has resulted in crops being sold as fuel crops, reducing the food supply and leading to an increase in food prices around the world.

To help with this issue, many oil-bearing non-edible plants have been investi­gated for the production of biodiesel. These are mainly tree species that can grow in harsh environments, such as Jatropha curcas, Pongamia pinnata, Castor, Mohva, Neem, Sal, etc. Jatropha curcas has the most significant potential due to its char­acteristics and growth requirements [16, 18]. It requires very little fertilizer and water (as little as 25 cm a year), is pest resistant, and can survive in poor soil conditions such as stony, gravelly, sandy or saline soils. Most important, it is fast growing, and can bloom and produce fruit throughout the year with a high seed yield. Optimized production has been found to yield an average of more than 99% of Jatropha biodiesel [19], which has comparable fuel properties to that of diesel from petroleum. It is expected that some varieties of Jatropha can produce as much as 1,600 gal of diesel fuel per acre-year compared to the wild variety that produces about 200 gal/acre-year [20]. Jatropha trees can capture four tons of carbon dioxide per acre and the fuel emits negligible greenhouse gases.

There is a growing interest in using algae as a feedstock for biodiesel production within the United States. Algae have become an appealing feedstock due to their aquatic environment providing them an abundant supply of water, CO2, and other nutrients. This results in a photosynthetic efficiency that is significantly higher than the average land based plants [21]. However, the power required to use artificial lighting to grow an aquatic species, such as microalgae, for the production of a biofuel would greatly reduce the overall efficiency of the process [22]. As the algae convert carbohydrates into triglycerides, the reproduction rate slows down so that the higher oil storing strains of algae reproduce at a much slower rate than lower oil storing strains [23]. This was shown by the Department of Energy’s (DOE) Aquatic Species Program, which found the overall yield to decrease as the algae’s oil storage increased.

Recently, Vasudevan and Briggs [21] summarized research on biodiesel pro­duction in a review article. According to them, a crude analysis of the quantum efficiency of photosynthesis can be done without getting into the details of the Calvin cycle; rather simply by looking at the photon energy required to carry out the overall reaction, and the energy of the products. In general, eight photons must be absorbed to split 1 CO2 and 2 H2O molecules, yielding one base carbohydrate (CH2O), one O2 molecule, and one H2O (which, interestingly, is not made of the same atoms as either of the two input H2O molecules.)

With the average energy of “Photosynthetically Available Radiation” (PAR) pho­tons being roughly 217 kJ, and a single carbohydrate (CH2O) having an energy content taken to be one-sixth that of glucose ((CH2O)6), or 467 kJ/mole, we can cal­culate a rough maximum efficiency of 26.9% for converting captured solar energy into stored chemical energy. With PAR accounting for 43% of incident sunlight on earth’s surface [24], the quantum limit (based on eight photons captured per CH2O produced) on photosynthetic efficiency works out to roughly 11.6%. In real­ity, most plants fall well below this theoretical limit, with global averages estimated typically at between 1 and 2%. The reasons for such a difference generally revolve around rate limitations due to factors other than light (H2O and nutrient availabil­ity, for example), photosaturation (some plants, or portions of plants receive more sunlight than they can process while others receive less than they could process), and photorespiration due to Rubisco (the protein that serves ultimately as a catalyst for photosynthesis) also accepting atmospheric O2 (rather than CO2), resulting in photorespiration.

In the US, the average daily incident solar energy (across the entire spectrum) reaching the earth’s surface ranges from 12,000 to 22,000 kJ/m2 (varying primarily with latitude). If the maximum photosynthetic efficiency is 11.6%, then the max­imum conversion to chemical energy is around 1,400-2,550 kJ/m2/day, or 3.8 x 1012 J/acre-year in the sunniest parts of the country. Assuming the heating value of biodiesel to be 0.137 GJ/gal, the maximum possible biodiesel production in the sun­niest part of the US works out to be approximately 28,000 gal/acre-year, assuming 100% conversion of algae biomass to biodiesel, which is infeasible.

It is important to keep in mind that this is strictly a theoretical “upper limit” based on the quantum limits to photosynthetic efficiency, and does not account for factors that decrease efficiency and conversion. Based on this simple analysis though, it is clear that claims of algal biodiesel production yields in excess of 40,000 gal/acre — year or higher should be viewed with considerable skepticism. While such yields may be possible with artificial lighting, this approach would be very ill-advised, as at best only about 1% of the energy of the energy used to power the lights would ultimately be turned into a liquid fuel (clearly, one needs to look at the overall efficiency).

This upper limit also allows us to assess how truly inefficient many crops are when viewed strictly as biofuel producers. With soybeans yielding on average 60 gal of oil (and hence biodiesel) per acre-year, the actual fuel production is stag­geringly small in comparison to the amount of solar energy available. This should further make it clear that using typical biofuels for the purpose of electricity gener­ation (as opposed to the transportation sector) is an inefficient means of harnessing solar energy. Considering that photovoltaic panels currently on the market achieve net efficiencies (for solar energy to electrical energy) on the order of 15-20%, with multi-layer photovoltaics and solar thermal-electric systems achieving efficiencies of twice that in trial runs, biomass to electricity production falls far behind (con­sidering typical plant photosynthetic efficiencies of 1-2%), with conversion of that biomass energy to electrical energy dropping the net efficiency to well under 1%.

Currently, the research for algae growth for fuel production is being done using photobioreactors. Unfortunately, current designs demand a high capital cost, which makes large-scale production uneconomical until a low cost design or new method of production is discovered. Storing energy as oil rather than as carbohydrates slows the reproduction rate of any algae, so higher oil strains generally grow slower than low oil strains. The result is that an open system (such as open raceway ponds) is readily taken over by lower oil strains, despite efforts to maintain a culture of higher oil algae. Attempts to grow higher oil extremophiles, which can survive in extreme conditions (such as high salinity or alkalinity) that most other strains cannot tolerate, have yielded poor results, in terms of the net productivity of the system. While an extremophile may be able to survive in an extreme condition, that doesn’t mean it can thrive in such conditions.

Many research groups have therefore turned to using enclosed photobioreac­tors of various designs as a means of preventing culture collapse or takeover by low oil strains, as well as decreasing the vulnerability to temperature fluctuations. The significant downside is the much higher capital cost of current photobioreactor designs. While such high costs are not prohibitive when growing algae for pro­ducing high value products (specialty food supplements, colorants, pharmaceutical products, etc.), it is a significant challenge when attempting to produce a low value product such as fuel. Therefore, substantial focus must be placed on designing much lower cost photobioreactors and tying algae oil production to other products (animal feed or fertilizer from the protein) and services (growing the algae on waste stream effluent to remove eutrophying nutrients, or growing nitrogen fixing algae on power plant emissions to remove NOx emissions).

An additional challenge, when trying to maximize oil production with algae, is the unfortunate fact that higher oil concentrations are achieved only when the algae are stressed — in particular due to nutrient restrictions. Those nutrient restrictions also limit growth (thus limiting net photosynthetic efficiency, where maximizing that is a prime reason for using algae as a fuel feedstock). How to balance the desire for high growth and high oil production to the total amount of oil produced is no small task. One of the goals of DOE’s well-known Aquatic Species Program was to maximize oil production through nutrient restriction; however their study showed that while the oil concentration went up, there was a proportionally greater drop in reproduction rate, resulting in a lower overall oil yield.

One approach to balancing these issues has been successfully tested on a small commercial scale (2 ha) by Huntley and Redalje [25], using a combination of photobioreactors and open ponds. The general approach involves using large photo­bioreactors for a “growth stage”, in which an algal strain capable of high oil content (when nutrient restricted) is grown in an environment that promotes cell division (plentiful nutrients, etc.) — but which is enclosed to keep out other strains. After the growth stage, the algae enter an open raceway pond with nutrient limitations and other stressors, aimed at promoting biosynthesis of oil. The nutrient limitations discourage other strains from moving in and taking over (since they also require nutrients for cell division).

Waste oils, such as restaurant grease and spent frialator oil, can also be used in the production of biodiesel. This eliminates the “food or fuel” debate that affects virgin edible oil sources. These waste oils normally cost money for restaurants and other establishments to dispose off. This can have a negative feedstock cost which reduces the overall cost of production. However, like virgin oils, traditional pro­cesses of converting waste oils to biodiesel can result in soap formation due to the presence of water and free fatty acids. The waste oils usually contain particulates that require filtration or separation prior to processing. Demand for waste oil as a biodiesel feedstock has already resulted in companies now paying restaurants for their waste vegetable oil (WVO). Quantities of WVO are limited (it is estimated to be about 1.1 billion gallons per year in the US), but it is certainly a good option for producing biodiesel.

2.3 Comparison of Technologies

A conventional base-catalyzed reaction is used in the majority of transesterifica­tion processes to produce biodiesel. Sodium hydroxide is used as the catalyst when methanol is the acyl acceptor, and potassium hydroxide is used when ethanol is the acyl acceptor, due to solubility considerations [15]. The ethyl esters have a slightly higher energy value than the methyl esters due to the presence of the additional carbon atom, and ethanol can be more easily produced from renewable sources, such as corn. Typical reactions take place with a high molar ratio of alcohol to oil of about 6:1 with methanol, and 12:1 for ethanol [15]. The excess alcohol allows for complete conversion of the triglycerides to the fatty acid esters. An advantage of base-catalyzed transesterification is the relatively short reaction time to achieve conversion levels of 98% or greater, compared to other processes. The reaction is a direct process, needing no intermediate steps, and operates at a relatively low tem­perature and pressure of about 66°C and 1.4 atm, respectively. However, a major disadvantage of the base catalyzed process is the formation of soap when water or free fatty acids are present in the feedstock. Thus the feedstock should be anhy­drous but the process still requires a large amount of base to be added to neutralize the fatty acids [15]. Soap formation results in additional downstream separation problems combined with a reduction in the fatty acid ester yield. The process also requires two steps and uses large amounts of chemicals as catalysts.

Acid-catalyzed transesterification is a viable alternative, in which sulfuric acid is typically used. One advantage over the base-catalyzed method [26] is that it is not as susceptible to soap formation. The resulting downstream product is easily separated and produces a relatively high quality glycerol byproduct. The process also requires only one step, compared to two steps in the base-catalyzed process. However, acid-catalysis reactions are slower and result in lower yields than base- catalysis, ranging from 56.8 to 96.4% depending on the feedstock [27]. A major disadvantage to either base or acid transesterification process is the disposal of the glycerol byproduct. Glycerin is already inexpensive, easily available, and is used in a wide array of pharmaceutical formulations. The major issue is with the purity of the glycerin; the byproduct glycerin from the production of biodiesel is 80-88% while industrial grade is 98% or higher [15]. The low market value of glycerin does not make purification economical. Many researchers are investigating innovative chemical and biological processes for the conversion of glycerin into value-added products including antifreeze agents, hydrogen, and ethanol [28].

A relatively new and promising development in the production of biodiesel is via enzymatic transesterification with lipase as the catalyst. Several microbial strains of lipases have been found to have transesterification activity; Pseudomonas cepa­cia [29], Thermomyces lanuginosus [30], and Candida antarctica [31] are a few that have been reported. The products of an enzyme-catalyzed reaction can easily be collected and separated. Unlike alkali-based reactions, enzymes can be recycled since they are not used up and require much less alcohol to perform the reaction. However, enzyme reactions take much longer to complete and can have lower yields due to inhibition of the enzyme caused by glycerol formation. Methanol, the acyl acceptor, can also strip the essential water from the active site of the enzyme, result­ing in deactivation of the enzyme. Enzymes are also expensive and require treatment such as immobilization, purification, pre-treatment, and modification [32].

New technologies are being developed to produce biodiesel that do not form glycerol as a byproduct. The hydrocracking process uses hydrocracking, hydrotreat­ing, and hydrogenation reactions to convert a wide range of feedstocks to biodiesel with yields of 75-80% [15]. This process is currently being utilized in petroleum refineries and uses a conventional commercial refinery hydrotreating catalyst. However, the hydrocracking process requires hydrogen, which is primarily obtained from natural gas. To reduce the costs of hydrogen, the process could be easily integrated with a refinery.

The production of biodiesel has significantly increased over the past few years. The National Biodiesel Board reports an increase in production from 250 million gallons in 2006 to 450 million gallons in 2007, an increase of 55.6%. European countries produced 5.7 million tons of biodiesel in 2007 (~1.5 billion gallons), which is an increase of 16.8% from 2006 according to the European Biodiesel Board. Germany is the World leader in biodiesel production and produced 2.9 million tons (~790 million gallons) in 2007, which is over 50% of the European biodiesel market.