Experts discuss optimizing biodiesel for future growth

By Ron Kotrba | February 08, 2011

During the National Biodiesel Conference & Expo’s breakout session called Changing the Biodiesel Paradigm: Optimized Biodiesel, Bob McCormick, a principal engineer in the fuels performance group at NREL, gave what he called a rather unusual presentation for him. Typically presenting testing data and results, McCormick gave more of a philosophical talk about what an optimal biodiesel fuel should be. “Today biodiesel is a fuel with properties of convenience,” he said. As a result, within an ASTM context, one cannot really show that more than a 20 percent blend is a fit-for-purpose fuel.

An optimized biodiesel, however, would be different depending on who is asked what optimal is. “There is uncertainty on what target properties should be, depending on where you are in the value chain,” as well as where one is geographically, he said. Methyl stearate has a high melting point but good cetane value and stability, whereas linolenic acid methyl ester has a low cetane number and a low melting point. Methyl oleate is sort of the best of both worlds, he said, with a cetane number of 55, a melting point of minus 20 degrees Celsius, and decent stability.

“In this narrow universe of C16 to C18 fuels, there’s quite a range of properties,” he said. If an ester group is put in the middle of a branched molecule like methyl palmitate, for instance, rather than at the end of the unbranched chain, there are property advantages to be had. The melting point of methyl palmitate is 29 C, but if it’s branched at the end, the melting point is lowered to 17 C. Move that up the chain and the melting point plummets to minus 13 C. “While you lose some cetane, it’s still adequate,” McCormick said.

He then spoke about esters from cellulosic biomass, such as pentyl pentanoate. Conventional lipid sources can sustain about 2 billion gallons per year of biodiesel production, but there’s 1.3 billion tons of sustainable biomass produced in the U.S. annually. “That’s pretty cheap stuff compared to lipids,” he said. There’ll be a 12 billion gallon market for biodiesel by 2050, so cellulosic biomass could be at least one answer to the feedstock limitations a growing biodiesel industry could face. To tailor properties for improved product while increasing yield and optimizing performance will take a collaboration of people who work on fuel properties and plant genetics, and is a long-term prospect, he concluded.

Timothy Jacobs with Texas A&M’s mechanical engineering department discussed low temperature combustion and how biodiesel fits in with it. Conventionally, there has always been a trade-off between NOx and particulate matter, meaning if PM is lowered, NOx is increased. But low temperature combustion is “an opportunity to defeat that trade-off,” Jacobs said. “You simultaneously decrease NOx and PM.” He noted, however, that efficiency issues and higher carbon monoxide and hydrocarbon emissions emerge. Biodiesel, he said, could be the answer to those issues.

A 4.5-liter diesel engine equipped with the latest technology advances, including high pressure common rail direct injection and variable geometry turbo, was tested with palm biodiesel. The engine incorporated high exhaust gas recirculation and the injection timing was retarded, and biodiesel did, in fact, reduce CO and HC emissions—albeit not to levels low enough to meet regulations—and it also increased efficiency of the engine during low temperature combustion.  

As far as why or how biodiesel increased the efficiency performance under these conditions, Jacobs said he’s not really sure at this point. “This will create more research opportunity out there,” he said. “It’s related to what combustion with biodiesel looks like.” Biodiesel’s peak rate of heat release is sooner, so in terms of efficiency, it is desirable for combustion to occur as close to top dead center as possible.  

Navistar’s Rodica Baranescu has been with the company for nearly three decades, and she said the OEM began looking at biodiesel in the late 80s. “Discussion of optimizing biodiesel rather than [the industry saying take] biodiesel as it is represents growth and maturity,” she said. Diesel fuel improvements are mostly attained with additives, she said, and it’s common for customers to overuse them. “Use of an additive is like use of medicine—don’t use it unless a doctor prescribes it.”

She said Navistar has been finding internal deposits in modern common rail injection systems recently. The systems are quite complex and she, along with the entire diesel world, are not sure why. She said Navistar is working on private-label detergents and trying to more fully understand this phenomenon.  The answer is still two to three years away, and the diesel industry must develop fuels formulations that will not produce these deposits.

“And we need to improve the diesel fuel standard (ASTM D975) to increase cetane, gain metals controls and improve fuel cleanliness,” she said.  D975’s cetane standard in the U.S. is 40. “It’s been so since 1948,” she said. It should at least be increased to 43.

Also the energy content for fuel is not defined, and that’s needed. In addition, metal content is not defined in the standard and that’s needed too. Lubricity standards must also to be met—most fuel comes in under the spec.

“U.S. diesel has some of the lowest cetane values in the world,” Baranescu said.

It may be necessary, eventually, to branch diesel fuel into two main types: one with a high cetane (55 or higher) for diesels or diesel-electric hybrids; the other with a max cetane number of 45 for homogeneous charge compression ignition (HCCI) combustion. “The adoption of HCCI activity will determine this,” she told the crowd. “As the diesel fuel paradigm changes, so too will biodiesel’s change.” 

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