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GTL Diesel Fuel Is Not Alone

Opinion by Thomas F. Glenn - President, PetroTrends, Inc

Editor's Note: the following is a guest editorial that appeared in the April issue of Hart's Gas-to-Liquids News.

With the exuberance about the future prospects for GTL fuels, it would be easy to overlook that fact that there are several other alternative fuel options available in the market place. These options include dimethyl ether, liquefied petroleum gas (LPG), compressed natural gas (CNG), methanol, and biodiesel.
With the exception of biodiesel, the use of the other alternatives requires some level of mechanical modification to the diesel power platform. In addition, some of the alternatives require expensive infrastructures to deliver and fuel at the pump, carry a significant manufacturing cost burdens, or fall short in a given performance parameter or quality metric (e.g. energy density). But what about biodiesel? Is it another in a line of hopeful yet hindered solution to meet future emission standards? And if it can compete, does it have what it takes to compete with GTL diesel?

From the Ground Up

Biodiesel is a fuel derived from such vegetables as soybean, canola, corn, cottonseed, peanut, and sunflower. It can, however, also be made from rendered tallow, algae, and virtually any lipid feedstock, including recycled cooking oils. In chemical terms, biodiesel is the monoalkyl esters of long-chain fatty acids. It is viewed as an environmentally friendly fuel because it is made from a renewable resource. From well to wheels, it is considered more environmentally friendly than petroleum based diesel fuel because it contributes less to air pollution, (specifically, CO2, particulate matter, carbon monoxide, and oxides of sulfur).

Another attractive characteristic of biodiesel is that when used in blends with petroleum-based diesel there are no modifications required to the engine or its power platform.

Unlike petroleum-based diesel fuel where prices are pegged to crude oil, the price of biodiesel is often tied to soybeans. The price of soybean, one of the most common feedstocks used to make biodiesel, is volatile. Over the last five years it has moved from a high of nearly $8.50/bushel in 1997 to a low of close to $4/bushel last year.

Currently prices are about $4.60/bushel. Based on recent prices of soybeans and other biodiesel feedstocks, the price of B20 tends to range from $0.25 to $0.35 per gallon (/gal) above that of conventional No. 2 diesel fuel. It has been as much as $0.80/gal higher. The price for straight biodiesel fuel (B100) can range from $1.30 to nearly $2/gal above that of conventional diesel. It is important to note that additional scale is expected to bring prices down and favorable tax structures could also be used to close the gap in prices.

Global capacity for biodiesel is estimated at nearly 1 million tons. Europe is by far the largest producer of biodiesel at close to 85% of the total. Virtually all of the balance is produced in the U.S. Additional capacity is planned, including plants in Asia. Assuming these plans come to fruition, global capacity for biodiesel fuel could increase by three times its current level over the next three years.

But Can it Compete With GTL?

Biodiesel presents formidable competition to GTL diesel. Both biodiesel and GTL are favored as low and ultra low emissions solutions because they significantly reduce vehicle emissions; require little to no engine modifications; reduce reliance of foreign crude; GTL makes use of stranded gas and flare gas, and biodiesel is a renewable resource. Each has a positive impact on the U.S. economy because GTL could extend the economic life of the TransAlaska Pipeline, while biodiesel supports the agriculture industry.

Biodiesel and GTL each offer significant improvements over conventional petroleum based diesel fuel in a number of areas. The most significant is that both contain virtually no sulfur or aromatics. The result is a step change improvement in tailpipe emissions. Based on data generated from Southwest Research and others, the improvements in tailpipe emissions are most notable in hydrocarbon (HC) emission and carbon monoxide (CO). Biodiesel enjoys a significant edge over GTL diesel in HC emission and a slight edge in CO. They are very close in particulate matter. GTL diesel does, however, enjoy a significant edge in terms of NOx emissions.

Proponents of biodiesel suggest that although NOx is slightly higher, it is not a significant issue since it can be handled by NOx-reducing engine technology. Unlike with conventional diesel fuels, this technology will work with biodiesel because this fuel is free of sulfur.

Another important air quality issue is the reduction in the emission of polycyclic aromatic hydrocarbons (PAH) associated with the use of GTL and biodiesel fuels. Both GTL diesel and biodiesel have a very low total aromatics content and can achieve PAH emissions reduction in the range of 80% to 90% compared to conventional diesel.

Although more testing, cost analysis and time will likely be required before the differences in tailpipe emissions can be fully assessed, it appears that GTL has a slight net edge over biodiesel due to the NOx issue.

Another measure of fuel quality that distinguishes GTL and biodiesel fuel from conventional diesel is cetane number. The cetane numbers of GTL diesel and biodiesel are very high as compared to conventional diesel. GTL diesel tends to enjoy a slightly higher cetane number than that found in a typical biodiesel fuel. The higher the cetane number, the higher the ignition quality of the fuel. The high cetane value of these fuels also eliminates the need to include cetane improver additives in the final blend, resulting in an additive cost savings.

While GTL might enjoy a slight net edge on biodiesel in the areas of emissions and cetane number, biodiesel has an advantage in lubricity. It exhibits very good lubricity properties and has been tested and approved by Stanadyne Automotive Corp. as a lubricity "additive" in ultra-low-sulfur-diesel (ULSD) fuels on the road today. A blend of as little as 2% biodiesel can impart the lubricity required in rotary and distributor-type diesel fuel pumps. An over treat of biodiesel as a lubricity component is of little concern to fuel blenders, because biodiesel will not impart any negative effects that could occur if a lubricity additive were overdosed.

Biodiesel's advantage in lubricity is considered to be only marginal. Since producers of low-sulfur diesel are already accustomed to using lubricity additives that are not expensive and do not require high treat rates.

Other significant differences in the chemical and physical properties of GTL diesel and biodiesel products currently produced include flash point and cold filter plugging point (CFPP). The flash point of GTL diesel is a function of the light ends of the distillate curve. Among the current FT diesel products, flash points range from about 140° F to about 200° F, while the flash point requirements of ASTM, CARB, and EU specifications range from 125° F to 131° F. According to GTL diesel fuel developers, the flash point can be manipulated by changing the distillation curve to reduce the light end content. This manipulation is evident based upon the range of flash points published by such GTL developers as ExxonMobil, Shell, Syntroleum, Rentech, Mossgas, and SasolChevron. This exercise may result in a penalty on yields.

By comparison, the flash point of biodiesel is about 330° F, which makes it a very thermally stable fuel. This high flash point has also resulted in its use in indoor applications where fire and fuel safety are of utmost concern. GTL diesel and biodiesel flip sides when it comes to CFPP.

The CFPP of GTL diesel is a strong function of the FT process used by each manufacturer. In some cases, the CFPP can be lowered through isomerization, while others may simply opt to use a pour-point depressant. The CFPP of GTL diesel can be effectively adjusted to meet the needs of various climate conditions.

Biodiesel again loses ground on cloud point and CFPP as compared to GTL diesel. The CFPP of biodiesel generally ranges from -3° C to -15° C. For this reason, B100 grades of biodiesel are carefully selected to ensure proper cold flow characteristics in each application. Much biodiesel use in the U.S. is of a 20% blend (B20) with conventional diesel, which tends to negate the effects of the higher CFPP of biodiesel.

One final area of weakness for biodiesel when it steps up to the plate to face GTL is expected to be its stability during storage. The high oxygenate content of biodiesel make it relatively unstable during storage and prone to oxidative decomposition. This decomposition generates residue that can plug filters and injectors and compromise engine performance.

While it appears GTL has a slight net edge over biodiesel in several key performance areas, it is expected to continue to enjoy success as a blend stock to upgrade conventional No. 2 diesel fuel. Its high quality, interest in reducing reliance on foreign oil, and the interests of the agricultural community will drive this success. It is unlikely, however, that biodiesel will take the lead in the race towards clean fuel due to the cost burden it is expected to run with.