Dr. Wen Wang
Massachusetts Institute of Technology (MIT)
With the fast development of modern civilization, energy security becomes increasingly important for countries with rather limited resources and booming urban populations. However, with excessive exploitation of the traditional energy sources (eg. coal, crude oil and natural gas), the reserves are shrinking rapidly with foreseeing exhaustion. This situation conflicts with the desire of human being for sustainability, and it must be altered immediately. In recent decades, sustainable energy sources are widely researched to meet this challenge. Among all these efforts, liquid biofuels from renewable bio-resources are one of the most attractive alternatives, with the features of sustainability and relatively easy transportation. Unlike other liquid biofuels, biodiesel does not require tedious alternation of current engines in vehicles. In addition, it provides a comparable high energy profile, and it is much cleaner than the conventional diesel in terms of the release of hazardous gases.
What is Biodiesel?
Biodiesel refers to long-chain fatty acid esters produced by esterification of triglyceride from natural resources. Biodiesel is generally biodegradable, nontoxic with an environmentally benign emission profile. It has been reported that the cetane number of biodiesel (above 47) is generally higher than petroleum diesel (above 40), which means that it has better lubricity, advanced cold-flow properties, and less sulfur content.
The first generation of biodiesel was derived from edible vegetable oil to demonstrate the feasibility for producing biodiesel. However, the price of traditional feedstock, for example, soybean oil1 (1200 US$/ton) or palm oil2 (1000 US$/ton), is too expensive to be commercialized when compared to petroleum diesel price (88 US$/ton). Even the price of used cooking oil4 (400-900 US$/ton) is still too high to be used. Besides, the competition of producing fuel with the food supply deficit makes it impossible to be realized in a large scale at current stage.
Animal fats from slaughter plants and meat manufactures are also very promising feedstock for producing biodiesel. In 2010, the EU itself produced 3.2 million tons of animal fats. Most of these animal fats are used for producing oil, which is not healthy compared with vegetable oil because of its low smoke point and saturated fat contents. The integral utilization of animal fat should be very interesting, especially to meet the energy supply deficit.
In the second generation of biodiesel production, low cost waste grease or fats, as well as other inedible triglyceride sources are used to fulfill this gap. In recent years, the proper utilization of waste grease trap oil (eg. brown grease) draws great attention from both public and researchers. In order to stop illegal reuse of grease trap oil in food industry, a solid waste pollution prevention and control act was implemented by governors in Henan province in China from January 1, 2012. They hope to improve the situation by controlling the source of the oil production. As an alternative usage approach, biodiesel production will facilitate the implementation of this act by transforming grease into biofuels. Besides, grease trap oil has a zero or negative price, and it is abundant in urban area, especially in developed countries (1.69 million tons/year in the US).
In Southeast Asia and South America, Jatropha grows naturally and in plantations. The seeds of Jatropha can be used for oil production (1 liters of oil from 4 kg input). This kind of inedible oil is a good candidate for producing high-quality biodiesel, such as jet fuel.
In recent years, lipid from microalgae is wildly researched for biodiesel production. These photosynthetic microorganisms are able to convert carbon dioxide into biomass including lipid. A pre-commercial production of marine microalgae was settled in Hawaii by Prof. Jeff Obbard from 2009. The plant produced 8 tons of algae to date for research and development purposes, and biodiesel is one of the products.
Base, Acid or Enzyme
In order to catalyze the triglyceride into biodiesel, proper catalyst is crucial. In general, three catalysts can be used in this process, which can be base, acid or enzyme catalysts. All of them have the ability to perform transesterification, but with different efficiency and requirements. The reaction rate for base catalyst is fast, however, when the amount of free fatty acid (FFA) in the feedstock is higher than 4%, saponification happens severely and it will consume too much of the base catalyst. In this case, the FFA should be removed by using either enzyme or acid catalyst. The acid catalyst can also be used for transesterification, but the reaction rate is much slower than the base catalyst. It is reported that the acid catalyst required a 0.5-1 mol % catalyst concentration in 50 hour to reach a rather high yield,7 while base catalyst could reach same level of catalysis in 30 min.
In comparison to other chemical catalysts, enzymatic methods for biodiesel production are favored when using waste oil containing high FFA as feedstock. A range of lipases and other esterases have been reported for enzymatic biodiesel synthesis.9 The enzyme could condense FFA with alcohol to form ester, when alcohol to water ratio is high. Among different enzymes, Candida antarctica lipase B (CALB) was widely researched. Besides, Candida rugosa (CRL),10 Rhizomucor miehei (RML),11 Pseudomonas cepacia (PCL)12 and Thermomyces lanuginosa (TLL)8 have also been used for this purpose. In order to facilitate the separation after reaction and decrease the cost, these enzymes were immobilized on different supporting materials before catalysis. Novozym 435 is CALB immobilized on a mesoporous structured material, while Liposyme TL IM is TLL immobilized on beads. These two immobilized enzymes are wildly used in biodiesel production. It has been reported that Nozoym 435 is quite efficient in decreasing the FFA amount in esterification reaction, while Liposyme TL IM is good at transesterification to transform triglyceride to biodiesel.
How Different between Regions
The strategy for biodiesel production in different regions varies dramatically. For developed urban areas, use of local feedstock like used cooking oil or grease trap oil is appropriate. In Singapore, BioFuel Research Pte Ltd and Alpha-Biodiesel are exploring the utilization of sewage oils. They hope to make value by recycling the waste (about 50 tons/day in Singapore) to valuable product. Another project initiated by Economic Development Board (EDB) from 2007 targeted palm oil, soya oil, and small amounts of used cooking oil to produce biodiesel. They hope to reach biodiesel production for 3 million tons by 2015. Besides, Neste Oil is currently building the world’s largest biodiesel plant in Singapore, which can produce a whopping 800,000 metric tons of fuel from vegetable oils and waste animal fat from the food industry per year. By that time, Singapore will become one of the leaders in supplying biodiesel.
For tropical regions, Jatropha fruit seed oil is a good candidate for producing biodiesel. Countries like Indonesia, Philippine, Mexico, Brazil, and Egypt are producing large amount of Jatropha oil for this purpose. As approved Jatropha plantation projects are eligible for certificates of greenhouse gas emission reduction to gain carbon credit, additional benefit will further facilitate the fast development of this technology. In Indonesia, the produced Jatropha oil is utilized locally and also shipped to nearby countries for biodiesel production. An Australian based company, Jatoil are currently shipping 20 tons of crude jatropha oil per week from Java Indonesia for producing renewable crude oil for under 65 US$ per barrel13. Locally, many companies like IndoGreen Resource Solutions P.T. are promoting the use of jatropha oil for producing the second generation biofuel. Although the Jatropha feedstock cost14 (220 US$/ton) is still high because of the low productivity and low oil extraction rate, Jatropha oil will continue to gain increasing attention in these regions in future.
For regions with marine resources and abundant sunlight, lipid from microalgae is easy to be obtained in large quantity. There are increasing numbers of companies in this field, such as Solix BioSystems, Inc. in Colorado, Cellana LLC, and Kai BioEnergy Corp in Hawaii, are all building demonstration open ponds facility with several acres to prove the concept. Based on their initial exploration, it will not be profitable if biodiesel is the only product. Other by-products produced in this process like aquaculture feed, animal feed, cosmetics, and renewable chemicals are the cash cow to sustain the business.
Future of Biodiesel
Biodiesel will be a popular substitute in the consumption of energy in future enjoying the support from government, oil companies, charities, and environmental organizations. A recent Bloomberg interview shows that the US Air Force plans to certify all of its aircraft models to burn biofuels by 201315. This most likely will not be cost-effective, but it is important to create market for biodiesel to support its development. In the meantime, billions of funds from government, venture capitals or non-profit organizations support the research, development and commercialization of biodiesel. As it could be difficult to compete with petroleum diesel in low-end market, biodiesel as jet fuel will be given great attention in the near future.
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