In Focus
July 2011

Right blend of oxygenates may help reduce particulates in diesel fuel

After years of research, diesel oxygenates may be on the verge of making a significant market impact. Driving this development are concerns for the environment, as well as the promise of profits in established and emerging markets. For this potential to be realised, though, problems of cost and scaling up to commercial production quantities have to be solved.

Those hurdles may not be too high, based on continuing research interest and commercial activity.

"With regard to oxygenates for diesel applications, the major ones of commercial interest at present are biodiesel (methyl esters), DME (dimethyl ether) and biobutanol," said André Boehman.

A professor of fuel science at Pennsylvania State University in State College, Pennsylvania, U.S.A., Boehman has investigated diesel combustion and oxygenates for years. He noted that the French multinational oil company, Total SA, and its British rival, BP plc, are today engaged in producing diesel oxygenates, with Total working on DME and BP on biobutanol.

Boehman was part of a 2009 research project that investigated how to optimize synthetic oxygenated diesel fuels. During the research, the scientists discovered the mechanisms behind the sooty particles formed during diesel combustion. They also uncovered how best to improve the burning of the fuel through the use of oxygenates.

The key is that oxygen ties up the carbon released by combustion of the fuel. In doing that, the oxygen blocks the formation of soot precursors and, therefore, reduces soot formation. However, for the best blocking, the oxygen must be in the right place, structurally and chemically speaking.

The investigators looked at oxygenates from the main functional families: ethers, esters, aldehydes, alcohols and ketones. After working their way through 20 different structures, they found that those from the carbonyl group, either in ketones or in aldehydes, were the most effective at reducing the threshold sooting index.

Selected Diesel Oxygenate Properties

Research project member Ripudaman Malhotra, associate director of the chemical science and technology lab at SRI International in Menlo Park, California, U.S.A., said the result points the way toward better diesel oxygenates.

"The most interesting aspect was the group-additivity model that we developed. It should help design new additives," he said.

There have also been numerous other diesel oxygenate research projects. For example, scientists at Xi'an Jiaotong University in Xi'an and the Hong Kong Polytechnic University, both in China, released research this year on blends of diethyl adipate as a diesel oxygenate. They found that the right blend could reduce particulates.

Pennsylvania State's Boehman noted that DME is being used in China, Japan and Sweden on a trial basis. He added that studies have shown that a high enough oxygen content eliminates soot altogether, leaving engineers free to go after NOx, or oxides of nitrogen, and other pollutants.

Getting rid of diesel soot could bring benefits to both people and the planet. With regard to the first, airborne particulates pose a significant health burden. The problem isn't what can be seen, as those are trapped by the nose and other organs before they can do damage.

On the other hand, particulates smaller than 10 microns, or millionths of a meter, bypass these defenses and lodge within the lungs. Those smaller than 100 nanometers, or billionths of a meter, can pass through the lungs and into the blood stream. Through their ability to reach deep within the body, particulates can lead to asthma, lung cancer, heart disease, birth defects and death. Estimates in a 2004 Journal of the American Medical Association study put the number killed by particulates in the U.S. as high as 50,000 a year. In areas of the world without the same air pollution control standards, the number could be higher.

Diesel blends with higher oxygen levels produce fewer particulates. Because of that, groups concerned with improving public health have weighed in on which fuels they prefer.

"Several local chapters of the American Lung Association have endorsed biodiesel use at a 20 percent blend level because of its health effect benefits," said Jessica Robinson, director of communications for the Jefferson City, Missouri, U.S.A.-based National Biodiesel Board.

As for the planetary pluses, recent studies have shown that black carbon, the dark particulates produced by diesel fuel, are an important contributor to global warming. Such particulates are unlike carbon dioxide, the number one greenhouse gas, in one significant way. Particulates aren't airborne for long. After a short while they're gone, whereas CO2 can circulate in the air for generations.

What this means for climate change is that cutting CO2 will have no immediate effect. However, reducing sunlight-absorbing particulates will help within a few months or years. Thus, to slow down climate change in the short term, one of the most effective steps may be to reduce the black carbon particulates produced by diesel engines and other sources.

For emerging economies, oxygenating diesel could be a particularly good way to get health and environmental benefits. In the developed world, diesel particulates are controlled through the use of filters. The technology adds to the cost of a vehicle and demands certain fuel characteristics, one being the use of low sulphur diesel fuel.

A diesel oxygenate could change this picture, since particulate control would come from fuel characteristics, not a filter. There is also the possibility of making the change without modifying diesel engines, according to research published in 2010 by Laurencas Raslaviĉius,

"We obtained combustion characteristics of diesel-biodiesel-bioethanol blend very similar to the diesel fuel," said Raslaviĉius, who was at the time a graduate student at the Lithuanian University of Agriculture Institute of Agro-Engineering in Raudondvaris, Lithuania. He is now a bioenergy sector innovations engineer with Lithuania's Axis Technologies.

A final advantage of the diesel oxygenate approach is that the fuel additive can be created from local and possibly renewable resources. The result of doing so would be a cleaner burning and more environmentally sustainable diesel blend.

Making that possibility become a reality will take more research and development, Boehman said. The challenge lies in finding ways to turn laboratory-scale demonstrations into something that can be produced cost effectively for millions of vehicles.

There are some indications that this hurdle is being overcome. For example, the cost to make bioethanol runs from US$0.20 to 0.40 per liter, depending on the feedstock. A stated goal of DuPont, BP's partner in the joint biobutanol venture Butamax, is to make the fuel equivalent to the cost, on an energy density basis, of bioethanol. Thus, the cost of biobutanol would have to be, at most, US$0.60 per liter and possibly as low as US$0.25 per liter.

According to a presentation made by the company in February in Barcelona, Spain, the company has been able to reduce production costs substantially and will reach its commercial manufacturing target toward the end of this year. If this forecast proves correct, it will answer a fundamental question that all diesel oxygenates face.

As Boehman said, "Commercially, what is always the leading question is 'Can you produce these things at scale?' Scale and cost are always going to be concerns."

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