Defining the Alternatives

Biodiesel Magazine takes a look at the various alternative diesel fuels emerging and how they are produced.
By Ron Kotrba | February 11, 2008
The new Energy Bill signed in December didn't extend the $1-per-gallon blender's credit for bio and renewable diesel producers, nor did it address what some characterize as a loophole in the tax laws as interpreted by the Internal Revenue Service in 2007 giving eligibility of the full credit to petroleum refiners planning to coprocess biomass in their existing refining infrastructures. It did provide a renewable fuels standard large enough for many alternative diesels, including a 1 billion-gallon biodiesel-specific mandate by 2012.

Unsure of how the future may unfold for the various second-generation alternative diesels, the following is a basic look at what some of them are and how they are produced.

"In a world where every molecule counts, one of the fascinating things that the average person doesn't understand or appreciate is that these guys are making different molecules," says Michael McAdams, executive director of the Advanced Biofuels Coalition, referring to renewable diesel fuels made from emerging second-generation processes. "This is why biodiesel has its own specification-it's a completely different compound than diesel fuel. There are no oxygen molecules attached to some of these fuels produced from second-generation processes. They are just fundamentally, physically and chemically different than biodiesel. With that, it gives them advantages that biodiesel doesn't share chemically." McAdams is an advocate of technology and feedstock neutrality, and tax parity in the governmental subsidization of renewable fuels. "If you make a renewable diesel that is chemically identical to D975, and you go through the certification process with the U.S. EPA and the agency finds that the molecule is the same as what's found in petroleum diesel fuel, then they're going to certify the fuel and they've done their job-they've done the molecular composition job and they've done the job of public health and safety, which is the requirement, which would allow the fuel to be sold."

Biodiesel, or fatty acid methyl esters (FAME), is produced from vegetable oils or animal fats through a process of transesterification, and are mono alkyl esters of long-chain fatty acids. Oils and fats are comprised of triglycerides-three fatty acid molecules connected to a glycerol molecule. Transesterification occurs when triglycerides are mixed and reacted with alcohol in the presence of a catalyst such as sodium or potassium hydroxide at relatively low temperatures and pressures. The three fatty acid molecules detach from their common glycerol molecule after which each fatty acid combines with an alcohol molecule to form FAME. Unlike petroleum diesel and many other "alternative" diesel fuels, biodiesel contains oxygen-10 percent by weight.

Renewable Diesel: Thermal Depolymerization
Brian Appel, chief executive officer of Changing World Technologies Inc., tells Biodiesel Magazine he helped craft the term "thermal depolymerization" into the 2005 Energy Bill to describe the process through which renewable diesel is made. The phrase comes from words used in a patent written by Illinois microbiologist P.T. Baskis. The IRS's interpretive ruling last spring said virtually any biomass process using heat was thermal depolymerization and therefore those renewable fuels were eligible for the $1-per-gallon blender's tax credit, which some think was tailored specifically for biodiesel.

"As a chemical engineer the term 'thermal depolymerization' means something more specific to me," says National Renewable Energy Laboratory Principal Engineer Robert McCormick. "In particular, to me it means a chemical process driven exclusively by heat and pressure without the use of a catalyst." This process is considered identical or at least very similar to pyrolysis and can be done in the presence of steam, which could be called hydrothermal processing-still a form of thermal depolymerization.

Appel, whose company uses a thermal depolymerization process to convert turkey guts and waste into renewable fuel oil, says "As one of the pioneers of the renewable diesel provision [in the 2005 Energy Bill], my opinion is that the tax support was intended for technologies that dealt with waste." With thermal depolymerization, waste materials are heated up to make char and oil. According to an NREL document describing different alternative fuels and their production processes, temperatures typically needed for conversion range from 570 to 660 degrees Fahrenheit, under a pressure range of 100 to 170 atmospheres. Reaction times are 15 to 30 minutes. "My understanding is that thermal depolymerization could be conducted at even lower pressures, but may require even higher temperatures-like 570 degrees Celsius (more than 1,000 F)," McCormick says. "At the same pressure it should require a much higher temperature for thermal as opposed to catalytic" processing as used in green-diesel production discussed below.

The char byproduct from thermal depolymerization can be used for processing while the oil may be further refined or sold into the industrial heating or stationary power markets, where CWT's 8 MMgy of renewable diesel is bound. Experts say the oil is similar to pyrolysis oil or bio-oil. Appel, says CWT's fuel has characteristics of No. 2 and No. 4. "What we firmly believe is that you need to work the stationary fuels markets before you engage in meeting the quality required for transportation fuels," he says. Experts agree with Appel's approach to using waste materials and assert there is no point in using vegetable oils or animal fats in the process of thermal depolymerization-including hydrothermal (steam and heat) processing. Rather, hydroprocessing would be a more sensible use of higher-priced fats and oils for conversion to second-generation biofuels.

Green Diesel: Hydroprocessing
Many alternative diesel fuels dubbed "renewable diesel" are not or will not be produced using a chemical engineer's definition of the term thermal depolymerization, but rather through stand-alone or coprocessed hydroprocessing. The name "green diesel" has been coined for hydroprocessed fuel and connotes a specific production process. "Hydroprocessing in my opinion means something very specific, which is hydrogenation conducted on molybdenum- or tungsten-sulfide-based catalysts supported on alumina and promoted with cobalt or nickel," McCormick says. "In hydroprocessing, the goal is to catalyze reactions that use hydrogen to primarily remove sulfur [in petroleum refining] but also to remove nitrogen and oxygen." Typically, hydroprocessing requires temperatures between 600 to 700 degrees F with a lower pressure range than used for thermal depolymerization, 40 to 100 atmospheres. Reaction times vary from 10 to 60 minutes, according to NREL documents. But green diesel production requires the use of expensive, sensitive and tricky catalysts. One source tells Biodiesel Magazine that certain green-diesel technologies "are not without their catalyst problems."

ConocoPhillips Co., UOP LLC and Neste Oil are all making their debut into green diesel production (or providing technology in the case of UOP) through coprocessing at petroleum refineries or intended free-standing biorefineries. While plans for independent green-diesel production plants are advancing, oil refiners find the Neste-style add-on attractive on many levels. When fats act as dedicated feeds and are processed alone, the boiling range of the finished fuel is very narrow, and it has no aromatic hydrocarbons or sulfur, which are regulated by the EPA. Petroleum diesel aromatic content is typically 20 percent to 30 percent. Hydroprocessed green diesel possesses a cetane number of 90 to 100. These properties give the fuel good ignition and emissions characteristics. And the lack of aromatics and sulfur accompanied by a high cetane number make green diesel a premium blend-stock that could save refiners and blenders money. "If you coprocess fat with petroleum in hydroprocessing you impart some of these properties in proportion to the amount of fat included in the feedstock," McCormick says. "If not done properly there can be residual feedstock, partly converted feedstock, and any impurity present in the feedstock will be present in the product-just like with biodiesel. Green diesel may have some advantages when it's done right-but it has to be done right."

Diagram of UOP's efforts in green fuels production

Sources say ConocoPhillips is working with a company on an isomerization process to branch the hydrocarbons, thereby improving cold flow performance of its fuel. Neste plans to use palm oil in a mega refinery-sized Malaysian plant using its own catalytic isomerization process. Branching of the fuel molecules does however lower the fuel's cetane number, green-diesel tech provider UOP states in a corporate document. "[Green diesel] is produced as a high-cetane, straight-chain paraffin but its cold-flow properties can be adjusted by the appropriate level of isomerization," according to the document. "The product cetane number can reach as high as 80 or 90." The minimum cetane requirement for U.S. diesel is 40. Depending on the feedstock, the cetane number of biodiesel averages near 50.

UOP has a "dog in the fight" so its data is not objective, says McCormick, but a life cycle analysis done by the company's Tom Kalnes and Terry Marker, along with Michigan Technological University's David Shonnard, conclude that fossil energy consumption over the life cycle of green diesel is reduced by 84 percent to 90 percent when made from soybean oil or palm oil, respectively. To achieve these reductions the study indicates the hydrogen needed to facilitate catalysis must come from the soy or palm byproducts from the production process rather than from fossil fuels. "Thus, green diesel has the potential to displace more petroleum resources per energy content in the fuel compared to biodiesel," the three authors wrote. "Larger reductions in greenhouse gas emissions for green diesel relative to biodiesel were predicted by this study for soybean feedstocks, but lack of verifiable data on palm oil prevented any conclusions to be made for this feedstock." Sources privy to preliminary results of a new study expected to be released later this year say data show promising reductions in greenhouse gas emissions for green diesel hydroprocessed from soy-but the study shows greenhouse gas reductions from soy-derived biodiesel are even greater.

Ron Kotrba is a Biodiesel Magazine senior writer. Reach him at or (701) 738-4962.
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