introduction:
Biodiesel1 comprises mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fat (or mixtures thereof). When biodiesel is blended with diesel, the designation indicates the percentage of it in the blend, e.g, B2 [2% biodiesel, 98% diesel (v/v)] and B100 (pure biodiesel). Worldwide, the feedstocks for biodiesel production in greatest supply are soybean oil, palm oil and rapeseed oil (Fig. 1). When alternative fuels are considered, their energy contents and conversion efficiencies are of particular interest, even though petroleum based fuels require more energy to produce than what they contain. Biodiesel has tremendous potential in this respect. A life cycle analysis2 concluded that biodiesel yields 3.2 units of fuel product energy for every unit of fossil fuel used to produce it; other projections go as high as 3.6.
If biodiesel is to be accepted as a fuel for diesel (compression ignition) engines, it will have to be produced and handled in such a way so that the variations in it’s properties and performance characteristics will be less than or equal to what the consumer is used to with (petroleum based) diesel fuel. A single, widely accepted, standard for biodiesel is not available. Instead, the standard ASTM D675102 was developed for the USA. and CEN 14214 for the European Union (EU).
Conversion Technologies:
Transesterification (alcoholysis), most widely accepted and almost exclusively used technology for the conversion of natural fats and oils to biodiesel, is the reaction of a fat or oil with an alcohol to form esters and glycerol. Fat or oil will depend on what is available in the region of the biodiesel production facility, their respective prices and the flexibility of processing facility to handle multiple feedstocks. Alcohol is methanol because it is low cost, easily recyclable and toxic. Because the reaction is reversible3, excess alcohol is used to shift the equilibrium to the products side. Typical mole ratios of alcohol to triglycerides range from 5.25:1 to 6.1:1. A catalyst is usually used to improve the rate of reaction and yield. The amount (0.1-0.5 % v/v) and type (base vs acid) of catalyst used depends on the quality [free fatty acid (FFA) and moisture] of triglycerides. Even though conversion efficiencies are good with conventional transesterification and feedstock costs represent 65 to 75 percent of the cost of producing biodiesel, there are significant research interests in improving the process and thereby the economics of biodiesel production. These interests include the development and evaluation of heterogeneous catalyst systems, the use of ethanol, in situ transesterification, reducing NOx emissions, and adding value to the coproducts (oilseed meals/presscakes and glycerol). Additionally, efforts in the area of standardization of biodiesel will enhance its marketability. Heterogeneous catalyst systems have major advantages over the homogeneous catalysts currently used in biodiesel production. The use of heterogeneous catalysts will eliminate the need for a water wash to remove excess catalyst. This will reduce both the capital cost of a plant and the operating cost. It also is perceived that FFAs present in feedstocks, particularly in yellow and brown greases, could be converted concurrently to alcohol esters rather than being separated out and used for some other, lower value purpose. Yet another potential advantage is that a higher quality glycerol may be obtained.
Because ethanol is produced in large quantities from readily renewable resources and because it is more environment-friendly than methanol, there is interest in using it in biodiesel production. From a reaction standpoint, ethanol will work fine albeit the reaction may be slower. The primary problem arises in the recycling of the excess alcohol. Around twice as much anhydrous alcohol is used in the process as what is required stoichiometrically. The reclaimed ethanol will not be anhydrous with simple distillation as with methanol and there will be the issue of breaking the azeotrope before it can be reused.
In situ transesterification4, a method that utilizes the original agricultural feedstock as the source of triglycerides for direct transesterification, eliminate the hexane extraction process and works with virtually any lipid bearing material. In this process, soybeans are dehulled, cracked, rolled into flakes, dried to remove moisture and incubated in a solution of methanol and sodium hydroxide to yield fatty acid methyl esters (95-100 %). Similar research has been conducted with other feedstocks including distillers dried grains with solubles from ethanol production and meat and bone meal from animal slaughter and rendering.
Reducing the combustion temperature5 can reduce the increased NOx emissions resulting from biodiesel fuel or biodiesel fuel blends. Vaporized injection, rather than spray injection, better distributes the fuel and avoids hotter areas during combustion that contribute to NOx generation. Controlling the timing of engine using injection sensors that determine the concentration of biodiesel in the fuel and adjusting the timing accordingly can reduce the combustion temperature. Exhaust gas recirculation, which also results in a lower combustion temperature; and NOx traps similar to catalytic converters that store NOx and which are regenerated by injecting more fuel in the cylinder or upstream from the trap to convert NOx to CO2 and unreactive nitrogen. Exhaust after treatment combined with engine management controls appears to provide the best NOx reduction at this time. Adding value to the coproducts of biodiesel
production, or at least maintaining their current values, will enhance the economic viability of biodiesel. The oilseed meals/press cakes are used predominately as animal feeds. Their values as feeds or feed supplements are functions of their protein content and quality, and the need for additional processing to inactivate or remove anti-growth factors such as trypsin inhibitors and glucosinolates. Opportunities for adding value to the meals include industrial uses for the proteins, such as adhesives, and the extraction of higher value materials, such as policosanols. Glycerol, a coproduct of biodiesel production, may be used to produce 1,4 propanediol, a commodity chemical and a precursor for many products.
A universally accepted standard for biodiesel, and adherence to the standard, by all producers will enhance the acceptability of biodiesel and its blends, both by the blenders and the consumers. The primary difference in the current USA and EU standards is the way stability and iodine number are handled (ASTM D6751-02 and CEN 14214). This difference is traceable to soybean oil being the oil of preference in the USA as opposed to rapeseed oil in the EU.
Biodiesel Production and Use
Global Scenario:
The 1.0 billion ton of diesel fuel used annually
worldwide or even the 188 million tons used annually in the USA6 dwarfs the current and future production capabilities of the vegetable oil and animal fat industry. Hence, petroleum based hydrocarbons will continue to be the workhorse for diesel engines. Biodiesel use has the greatest penetration in the EU7 with an estimated 1.9 million tons having been used in 2004. The use of biodiesel in the USA1 pales in comparison, with an estimated 86 thousand ton used in 2003 and 120 thousand tons used in 2004. Production capacity is expanding rapidly in the USA, with an estimated plant capacity of 770 thousand tons and considering current and planned facilities, production capacity could exceed 1.5 million tons by 20078. Vegetable oil feedstocks, worldwide, are estimated to be 100 million tons9. Currently, rapeseed oil is the feedstock of choice for biodiesel production in the EU, soybean oil is the feedstock of choice in the USA, and the use of palm oil for biodiesel production is growing in Asia. A total for animal fats, worldwide, is estimated at 15 million tons10. The vegetable oil feedstocks, if converted to biodiesel by transesterification, and assuming roughly a volume per volume conversion rate, would supplant only 10 percent of diesel used in the world. Using the total animal fats, and assuming same conversion efficiency as for vegetable oils, equivalent biodiesel production would amount to an additional 1.5 percent of the diesel use in the world.
USA Scene
Soybean oil carryover in the USA has been
between 450 to 900 thousand tons, of which only 225 to 450 thousand tons (< 0.3% of total diesel fuel use in the USA) can be diverted to biodiesel industry. Other edible oils (corn, cotton, sunflower), which account for around 1.6 million tons of total consumption, are typically higher priced than soy and unlikely to be used for biodiesel purposes. However, 450 thousand tons of soybean oil represents 5 percent of total domestic soybean oil use and most of the current carryover stocks. Animal fats and waste greases are consumed for use in soap, food, feed, industrial and export markets. The historic price discount between these feedstocks and soybean oil has varied from 25-75 percent. In 2003, the total US tallow and grease production was 3.9 million tons. At the right price and assuming no technical barriers, a significant portion of these feedstocks, say 450 thousand tons, could be bid away from their current uses. It is expected that animal fats would be the feedstock of choice as long as it is priced below soy and biodiesel demand rises. In summary, the total available feedstocks in the USA that can be readily converted to biodiesel are 450 to 900 thousand tons, which is less than 0.6 percent of the diesel fuel used in the USA.
In the USA, both vegetable oils and animal fats are imported and exported. If biodiesel demand exceeds the estimated demand benchmark of 450 to 900 thousand tons, domestic to foreign fat and oil spreads would widen and exports would decrease, imports would increase or both. Some USA based econometrics were used to predict price impacts from a demand shock. Assuming petroleum diesel fuel stays at historically high prices of around $0.32 US/l (rack untaxed) and that soybean oil is $0.51 US/kg, approx 300 thousand tons of soybean oil demand would raise the price of soybean oil $0.07 US/kg and 680 thousand tons would raise it $0.14 US/kg. This would cause B2 blends to move from a slight discount to diesel to a premium between 300 and 680 thousand tons of use. As a result, at current diesel fuel prices, biodiesel can only pull about 300 thousand tons from domestic soybean oil supplies before B2 blends move to a premium over diesel. After that, either B2 must be sold at a premium or trade flows will be impacted to keep feedstock prices in check
Future Prospects:
Future raw material availability for biodiesel production, worldwide, is significant. Additional sources include expanded oilseed production, higher oil content varieties, and substitution of higher oil content crops. In the USA, it is estimated that roughly expanding oilseed production by releasing productive land currently in government set aside programs 4 million ha and switching from lower value small export grains, 8 million ha could produce additional vegetable oil feedstocks of 2.1 million and 4.2 million tons. If the average oil yield from soybeans would improve to 20 percent versus the current 18 percent, which has been proven with several improved varieties, an additional 800 thousand tons of vegetable oil would be available, or if future improvements could increase oil yields to an average of 22 percent oil, 1.60 million tons would be available. If oil demand outpaces protein demand, soybean production could be replaced with higher yielding oil crops such as sunflower and canola, which produce approx 100 l/ha more oil than soybeans. Assuming soybean production at 29 million ha12 and a 20 percent conversion to higher yielding oil crops an additional 2.6 million tons would be available. Overall, conversion of all existing and potential feedstocks in the USA will not generate more than 12 percent of the diesel demand. Therefore, biodiesel will be consumed primarily in niche markets: 20% biodiesel blends for emission benefits and 5% or less blends as a fuel additive for lubricity benefits in ultra low sulfur diesel fuel11.
Conclusion:
The opportunities for expanded biodiesel production are bright considering the high demand for petroleum products in both industrialized and developing regions of the world. As a result, the increased awareness of the negative environmental factors associated with petroleum fuels and a desire to move to renewable fuels, biodiesel production is growing significantly. However, it represents only a small fraction of the current diesel fuel demands. Even as biodiesel production and available feedstocks expand, that growth will be challenged to keep pace with the growing demand for renewable or diesel fuel. Biodiesel feedstock availability, the price, and the resulting by-products, combined with government incentives based on economic or environmental issues, ultimately will determine the competitiveness of biodiesel as a direct substitute for petroleum diesel. It is anticipated that biodiesel will drive the industrial applications for vegetable oils and animal fats but will not displace food applications which will continue to lead vegetable oil price for most desirable oils, while lower grade oils will become industrial feedstock. Conversion technologies will continue to improve and will allow biodiesel production to remain competitive as government incentives are phased out.
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