Biodiesel fuel (BDF) produced by alcoholysis of vegetables oils or fats is viewed as a promising renewable fuel source. Diminishing petroleum reserves and increasing environmental regulations have made the search for renewable fuel options a pertinent challenge. In this article, current technological methods for the production of BDF are discussed, including both chemical and enzymatic processes.
by Professor Hideki Fukuda

Growing environmental concern about climate change and the depletion of oil reserves has resulted in government-led research into environmentally friendly and sustainable biofuels. Attractive features of biodiesel fuel are: (i) it is plant derived and as such its combustion does not increase current net atmospheric levels of CO2 (ii) it can be domestically produced, offering the possibility of reducing dependence on petroleum imports (iii) it is biodegradable and (iv) relative to conventional diesel fuel, its combustion products have reduced level of carbon monoxide and nitrogen oxides. The overall life cycle emissions of CO2 from 100% biodiesel fuel are 78.45% lower than those of petroleum diesel, and a blend with 20% biodiesel fuel reduces net CO2 emissions by over 15%. Substituting 100% biodiesel for petroleum diesel in buses reduces the life cycle consumption of petroleum by 95%, while a 20% blend of biodiesel fuel causes the life cycle consumption of petroleum to drop 19%.

Technologies for biodiesel fuel production
A number of studies have presented promising methods that make use of triglycerides as an alternative fuel for diesel engines. However, the direct use of vegetable oils or oil blends is generally considered impractical because of the high viscosity, acid composition and free fatty-acid content of the triglycerides. Three main processes have been investigated in an attempt to overcome these drawbacks and allow vegetable oils and oil waste to be utilised as a viable alternative fuel: microemulsions, thermal cracking (pyrolysis) and transesterification. The use of microemulsions with solvents such as methanol, ethanol, and 1-butanol has also been studied as a means of solving the problem of high viscosity of vegetable oils. However, several drawbacks like heavy carbon deposits, incomplete combustion and increase of lubricating oil viscosities were noticed in laboratory screening endurance tests. Another method, pyrolysis, refers to chemical change caused by the application of thermal energy in the presence of air or nitrogen. Thermal decomposition of triglycerides produces several compounds including alkanes, alkenes, alkadienes, aromatics and carboxylic acids. Pyrolysed soybean oil, for instance, contains 79% carbon and 12% hydrogen. Even though it has low viscosity and a high cetane number compared to pure vegetable oils, the removal of oxygen during thermal processing eliminates any environmental benefits of using an oxygenated fuel.

To address these issues, transesterification, also called alcoholysis, has been employed to reduce viscosity and to improve the physical properties of
biodiesel fuel. Transesterification involves the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis, except that the alcohol is used as an acyl acceptor. Methanol and ethanol are utilised most frequently, especially methanol because of its low cost and its physical and chemical advantages. This process has been widely used to reduce the viscosity of triglycerides, thereby enhancing the physical properties of renewable fuels to improve engine performance. Thus, fatty acid methyl esters (FAME) obtained by transesterification can be used as an alternative fuel for diesel engines. Transesterification can be carried out by using acid, alkali and enzyme catalysis methods.

Alkali and acid catalysed transesterification
Alkalis used for transesterification include NaOH, KOH, carbonates and alkoxides such as sodium methoxide, sodium ethoxide, sodium propoxide and sodium butoxide. Alkali catalysed transesterification proceeds very much faster than acid catalysis and is thus most often used commercially. For alkali catalysed transesterification, the glycerides and alcohol must be highly anhydrous because water causes a partial saponification reaction, which produces soap. The soap consumes the catalyst and reduces the catalytic efficiency, as well as causing an increase in viscosity, the formation of gels and difficulty in achieving separation of the by-product, glycerol. Acids used for transesterification include sulphuric, phosphoric, hydrochloric and organic sulphonic acids. Although transesterification by acid catalysis is much slower than that of alkali catalysis, acid catalysed transesterification is more suitable for glycerides that have a relatively high content of free fatty acids. In general, acid catalysed reactions are performed at high molar ratios of alcohol to oil, and low to
moderate temperature and pressure.

Enzymatic transesterification
Although chemical transesterification using an alkali catalysis process gives high conversion levels of triglycerides to their corresponding methyl esters, with short reaction times, the process has several drawbacks. The reaction is energy intensive, recovery of glycerol is difficult, acidic or alkaline catalysts have to be removed from the product, alkaline waste water requires treatment, and free fatty acids and water interfere with the reaction. Enzymatic catalysis using extracellular and intracellular (whole-cell biocatalysts) lipases are able to catalyse transesterification of triglycerides effectively in either aqueous or non-aqueous systems. In particular, it should be noted that the by-product, glycerol, can be easily recovered without any complex process, and also that free fatty acids contained in waste oil and fats can be completely converted to methyl esters. The main obstacle of the enzyme catalysed process is the high cost of lipase. In order to reduce the cost, immobilised enzymes and whole-cell biocatalysts are introduced for ease of recovery and reuse. Among the commercial lipases, Candida antarctica lipase B (Novozym 435) immobilised on acrylic resin is the most effective lipase for methanolysis, a transesterification reaction where methanol is used as an acyl acceptor. Lipase deactivation by methanol can be overcome by the stepwise addition of methanol. Bench and pilot scale experiments that used Novozyme 435 led to a conversion of 95% methyl ester (ME) and were maintained for over 50 reaction cycles.

The cost of lipase enzymes significantly limits their applicability for the bulk production of fuels and chemicals. This has prompted research into the potential use of microorganisms, such as filamentous fungi, which could serve as whole-cell biocatalysts based on the ease with which they can be immobilised. Furthermore, the relative ease of process scale-up when using immobilised fungi renders them more practical whole-cell biocatalysts with several commercial advantages. It has been demonstrated that immobilised mycelium of Rhizopus oryzae, within biomass support particles (BSPs) made out of polyurethane foam, is a potential whole cell biocatalyst with a conversion of over 85% ME with step-wise addition of methanol.

Future prospects
In biodiesel fuel production, most of the problems associated with alkali catalysis, such as glycerol separation, can be alleviated by enzymatic transesterification. Enzyme and whole-cell biocatalysts can provide advantages over conventional chemical processes with regard to ease in downstream processing. Further improvements in the cost and reaction rate of enzyme and whole-cell catalysed biodiesel fuel production will be necessary before industrial applications are practical. These limitations might be resolved by using packed bed reactors that contain whole-cell biocatalysts under
continuous operation conditions.

The author
Professor Hideki Fukuda
President, Kobe University
1-1 Rokkodai-cho • Nada-ku
Kobe 657-8501 • Japan
email: fukuda@kobe-u.ac.jp