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 April 2007
Feature
RALPH A. CHERRILLO is secretary of ASTM International Committee D02 on Petroleum Products and Lubricants. He is currently project leader for GTL fuel in the U.S. with Shell Global Solutions (US), Inc. He is also a member of the American Chemical Society, the American Institute of Chemical Engineers, Coordinated Research Council, the International Association for the Stability, Handling and Use of Liquid Fuels, and SAE International. He can be reached at ralph.cherrillo@shell.com.

Synthetic Diesel Fuels Extending the Alternative Fuels Journey in Committee D02

Alternative and synthetic diesel fuels are currently available in the marketplace, and the standardization community is working feverishly to reflect this reality. Automotive diesel fuels fall under the jurisdiction of ASTM International Subcommittee D02.E0 on Burner, Diesel, Non-Aviation Gas Turbine and Marine Fuels, which is responsible for ASTM standard D 975, Specification for Diesel Fuel Oils. The issue facing Committee D02 on Petroleum Products and Lubricants is how to transition this specification toward covering synthetic fuels while continuing to address alternative diesel fuels.

ASTM Committee D02 has a long and distinguished history of developing test methods and specifications for automotive fuels. These activities must ensure that the needs and interests of all stakeholder groups are considered and delivered. Stakeholders strive to develop specifications and tests for fuels that work in real-world engines, in addition to meeting all applicable regulatory and safety requirements. While consumers, regulatory agencies, and engine, fuel and performance additive manufacturers are at the top of the stakeholder list, the interests of test equipment manufacturers, multiproduct pipelines, fuel terminal operators, and fuel quality certifying agencies are also critical to the process.

Alternative Diesel Fuel Challenges

Historically, Subcommittee D02.E0.02 on Diesel Fuel Oils has concentrated its activities on diesel fuels from conventional petroleum or mineral oil sources. In the last decade, these activities have been expanded to include the consideration of alternative diesel fuel sources to reduce dependence on imported crude sources for energy security, and to make exhaust emissions more favorable environmentally. The pursuit of alternative, low emission diesel fuels is a prime example of sustainable development in action.

Figure 1—Synthetic diesel fuels are typically clear, clean products with virtually no sulfur or aromatics.

Alternative diesel fuel options include investigations of vegetable and animal oil options as well as a water-emulsified diesel fuel. The prime vegetable oil options are biodiesel from fatty acid methyl esters, and ethanol-emulsified diesel fuels. The prime vegetable oil options are biodiesel from fatty acid methyl esters (FAMEs), and ethanol-emulsified diesel fuels. ASTM D 6751 is the Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels. Specification activities that address blends of up to twenty percent biodiesel component (B1 - B20) in conventional diesel fuels are nearing completion. However, animal oil-derived options are far less well-defined at the moment.

As alternative diesel fuel sources are considered, there is a fundamental need to understand the “what,” “why” and “how” issues related to the construction of the existing test methods and specifications for conventional fuels. From that understanding, we must seek to evaluate how to extend them to apply to something different and new while maintaining continuity.

While the evolution toward alternative diesel fuels is happening, the advent of synthetic diesel fuels in the market has made the area more complex. At its fall meeting last December, Subcommittee D02.E0.02 chartered a new task force to investigate synthetic diesel fuels, or XTL. (XTL is the naming convention that refers to the manufacture of synthetic liquids through an X-to-liquids Fischer-Tropsch process, where X refers to the feed stream source. X can include natural gas, biomass or coal, but more about that later.)

Synthetic Diesel Fuels

A review of conventional fuel conventions is needed before the approach to synthetic fuels can be understood. Conventional fuels are made from crude mineral oil. Petroleum crude is distilled, or fractionated, into a number of cuts, which is a physical separation process. Each of these cuts is further treated with various chemical processes that serve to modify molecular structures and remove undesirable components and impurities. Therefore conventional petroleum processing is refining — separating complex mixtures of hydrocarbons, collecting desirable components and adjusting properties to suit needs.

By contrast, synthetic fuels are made by using simple raw materials to selectively build desirable chemical structures. Usually, this tailor-made approach yields high quality products without the need to deal with impurities or contaminants. Synthetic diesel fuels exhibit superior properties, including low sulfur and nitrogen contents, extremely high cetane numbers and very low aromaticity. The ranges of these properties in synthetic diesel fuels produces the highly desirable effects of lowering engine exhaust emissions. In addition, these new fuels are fully compatible with the existing fuel infrastructure.

Synthetic fuels can be manufactured by a number of processing routes, but those that are attracting a great deal of attention in the industry are the variants of the Fischer-Tropsch (FT) process.

The basic technology was developed in Germany in the 1920s, and is widely known as the Fischer-Tropsch process after its inventors. In essence, the FT process uses specific catalytic reactions to synthesize complex liquid hydrocarbons from simple chemicals. As crude oil prices rise, the economic incentive to make synthetic fuels using the FT process becomes more attractive.

Feedstreams are first gasified to produce carbon monoxide and hydrogen, which is called synthesis gas. The synthesis gas of pure chemical building blocks is fed into the FT process reactors, where it is converted into liquid hydrocarbons of extremely long carbon chain lengths. These are liquids at processing temperatures that would solidify at ambient temperatures since they are predominantly straight-chained paraffins. The liquid hydrocarbons are further processed and distilled into high quality products, which are predominantly mixtures of normal- and iso-paraffins. There are very few aromatics and olefins produced, which makes them ideal for diesel fuel.

The FT process is quite flexible, in that it can be applied to a variety of feedstreams, and can make a remarkable selection of liquid products — feedstocks for the chemicals industries, fuels and lubricating base oils. In addition, the synthetic product slate is rather independent of the feedstream.

Naming and Terminology Challenges

The naming convention for specific FT processes links the product to the starting raw material. Natural gas, biomass and coal are the most commonly used feedstreams. When starting with natural gas, GTL, or gas-to-liquids, products are produced. Starting with biomass yields BTL products, and of course, starting with coal yields CTL products. As a result, when the FT process is spoken about generically, it is often called XTL, where the X can be assigned as G, B or C.

Figure 2—Synthetic diesel fuel combustion (shown on the right) is immensely cleaner than conventional diesel combustion.

For more information on the GTL process and diesel fuel products, please refer to the references listed at the end of this article. Typical properties for a commercially available GTL diesel fuel are presented in Table 1, which also illustrates fuel specifications excerpted from D 975 for comparison.

Synthetic diesel fuels represent a significant departure from terminology in the existing diesel fuel specification, which represents the “what” and “how” of what exists to guide us. Frankly, what is in place today is only a fair start when considering the broad range of alternative diesel fuels being developed for the market. Therefore, a large component of synthetic diesel fuel standardization efforts will be directed at creating a system of terminology that effectively deals with conventional mineral oil-derived fuels as well as vegetable and animal oil-derived alternate fuels, extending ultimately to synthetic fuels. The struggle will be to find a place for each of these materials on a diesel fuel continuum, one that meets all stakeholder needs so that all speak a shared language and understand the implications.

Test Method Challenges

There are quite a number of standard test methods for diesel fuel properties of interest that have evolved over the last 60+ years. Each of these test methods needs to be reviewed to determine whether synthetic diesel fuels are (1) appropriately addressed in the method scope statement, (2) considered safe when tested by the procedure or test instrument, and (3) adequately addressed in the precision and bias statement through an applicable round robin.

When discontinuities are found, a reasonable resolution must be sought. Remember that each property test method is under the jurisdiction of one of the 14 property subcommittees of D02, so the task can be rather complex and time consuming.

For example, consider the cetane number, which is a critical combustion property. D 613, Test Method for Cetane Number of Diesel Fuel Oil, which is the reference method, clearly states in the scope section that typical testing is in the range of 30 to 65, with test laboratories usually reporting the upper limit to be >76. Synthetic diesel fuels exhibit cetane numbers in the range of 80 – 100. Therefore, considerations for how to set specification limits for XTL fuels must be made in a manner that conveys the nature of its high natural cetane number. Consistent treatment must be applied to other cetane test methods (which include D 4737, Test Method for Calculated Cetane Index by Four Variable Equation, D 6890, Test Method for Determination of Ignition Delay and Derived Cetane Number (DCN) of Diesel Fuel Oils by Combustion in a Constant Volume Chamber, and D 7170, Test Method for Determination of Derived Cetane Number (DCN) of Diesel Fuel Oils-Fixed Range Injection Period, Constant Volume Combustion Chamber Method).

Specification Challenges

Once the terminology and test method issues have been resolved, the focus can be shifted to dealing with specification challenges, of which there are two.

First, the determination must be made for whether synthetic diesel fuels are covered under D 975 today, either as a fuel blend component or as a neat fuel. The broadest definition of what synthetic fuels on the horizon are likely to be will be considered. These deliberations are by no means a one-time effort, since specifications are living documents subject to frequent revisions. Therefore, the situation at hand must be adequately dealt with, while recognizing that it must be revisited every time D 975 is subsequently revised.

Second, if the previous effort becomes futile, the creation of a stand-alone synthetic diesel fuel specification is required to allow meaningful buyer-seller discussions. Here again, the broadest definition range of synthetic fuels to accommodate new technologies and products would have to be reviewed.

Figure 3—Alliance for Synthetic Fuels in Europe (ASFE) demonstration display featuring three diesel passenger cars from different manufacturers surrounding a see-through dispensing pump (left), which illustrates the clear fuel.

Final Thoughts

As all these challenges are addressed, awareness of stakeholder interests will be maintained and every effort will be made to be thoughtful, timely and fair. The task force’s plan is to develop, ballot and adopt the required revisions to standards through the various committees and agree on new standards by the third quarter of 2008, which is an ambitious target.

References

[Reference materials may be obtained from the author upon request.]

Future Fuels and Lubricant Base Oils From Shell Gas to Liquids (GTL) Technology, SAE Technical Paper Series 2005-01-2191, Clark, R.H., Wedlock, D.J., and Cherrillo, R.A.

Shell Gas to Liquids in the Context of Future Engines and Future Fuels, Proceedings of the 5th International Colloquium “Fuels,” Tech. Akad. Esslingen, Ostfildern, Germany, January 2005, Clark, R.H. Juergens, J.J.J., and Stradling, R.J.

Emissions from Fischer-Tropsch Diesel Fuels, SAE Technical Paper Series 2001-01-3518, Johnson, J.W., Berlowitz, P.J., Ryan, D.F., Wittenbrink, R.J., Genetti, W.B., Ansell, A.A., Kwon, Y., and Rickeard, D.J.

Comparative Emissions Performance of Sasol Fischer-Tropsch Diesel Fuel in Current and Older Technology Heavy-Duty Engines, SAE Technical Paper Series 2000-01-1912, Schaberg, P.W., Mybergh, I.S., Botha, J.J., and Khalek, I.A.