It wouldn't be difficult to insert a snappy and clever introduction to this article, grabbing the sensationalist curiosity in everyone. But air pollution is a sobering issue, and it's getting graver with time.

As populations increase, more and more people worldwide strive to attain vehicular mobility-and progressively, that mobility is being achieved. Logically so, increased internal combustion travel brings with it apparently inseparable baggage, or amplified levels of air pollution.
The challenge then is to effectively mitigate some of the damage done by such widespread use of polluting forms of transportation. This is where the proliferation of both biodiesel as a fuel and the diesel engine as an efficient mode of transportation become imperative.

U.S. EPA targets diesel exhaust
Data produced by the U.S. EPA suggests transportation emissions play a major role in air quality degradation in the United States. Of course, diesel vehicles have been contributing to the problem, albeit not in a way many people think.

Nitrogen oxides (NOx) and particulate matter (PM), and to a much lesser degree, carbon monoxide (CO) and hydrocarbons (HC) are the four main pollutants emitted from diesel tailpipes. In 2007, the EPA will begin more stringent regulations of PM and NOx emissions, with the most severe regulations coming in 2010. To meet these requirements, many new diesel vehicles, trucks and buses will be outfitted with emissions control technologies, starting in 2007 with particulate filters and climaxing with both particulate filters and NOx abatement devices three years later.

Even though gasoline vehicles have been sporting noxious pollutant-neutralizing catalytic converters for approximately 30 years, the sheer number of gas-powered automobiles plus an increased number of miles traveled, together place gas, rather than diesel, vehicles at the top of the "major transportation air polluter" list.

"From a carbon monoxide and hydrocarbon standpoint, we look at what's called the 'inventory,' which is all the carbon monoxide and hydrocarbons that are emitted in the air," said Robert McCormick, senior fuels engineer at Golden, Colo.-based NREL. "Diesel is a really small contributor to the inventory. Gasoline automobiles are by far the largest contributors to the carbon monoxide and hydrocarbon inventories."

As experts like McCormick know, diesels are quite fuel efficient and much lower in some emissions like CO and HC (sometimes called VOCs) than gas engines. This efficiency comes from the diesel engine's lean-burning high air-to-fuel ratio, and it's also what gives diesels lower emissions of these two pollutants.

Jon Van Gerpen, the agricultural and biological sciences department head at the University of Idaho, said untreated diesel vehicles are in the same category-they release comparable emissions-as gas-powered cars with fully functioning catalytic converters. Van Gerpen, an expert in the mechanics of compression ignition engines and their emissions, said that by putting an oxidation catalyst on a diesel engine, CO and HC emissions actually fall below measurement thresholds.

Gasoline powered vehicles have significantly lower NOx emissions as a result of their functioning catalytic converters. Untreated diesels burning conventional diesel, despite being very low in CO and HC emissions, heavily contribute to the NOx and PM inventory. And diesels are visible polluters, spewing black soot over everything in and around their paths, not only as they're truckin' on down the interstate but also as they make their way from one city bus stop to the next. In fact, vehicle-to-vehicle comparisons show that diesel vehicles definitely do emit much more NOx and PM than gasoline vehicles. These factors all contribute to the EPA's reasons for requiring diesel exhaust treatment.

The unpolished truth about biodiesel's benefits
With the overall emissions profile of diesel vehicles gleaming in comparison to that of their gasoline counterparts, the question becomes: To what degree can the use of biodiesel improve diesel emissions, especially for the older fleet vehicles that may continue operation without exhaust treatment? "I am not aware of any retrofit technology that is going to be required for over-the-road trucks," Van Gerpen said, although he acknowledged there might be city bus retrofit requirements in some parts of the United States.

Thus, from an emissions perspective, biodiesel may be an ideal transitional technology as the diesel industry slowly moves from the engines and vehicles of the past toward the finely tuned, high-tech and clean models of 2007 and beyond. "There's quite a few years here where there's going to be some significant benefits from using biodiesel in older fleets-'older' as in anything before 2007," Van Gerpen said. "As those fleets age, there's going to be a lot [of them] that could benefit from biodiesel."

Biodiesel provides many emissions benefits, but some results can hinge on a choice between two commonly administered types of exhaust analysis. That is, whether a researcher relies upon engine dynamometer or vehicle chassis dynamometer testing. Putting that variable aside for now, three out of four major pollutants-PM, HC and CO-are significantly reduced when B100 or biodiesel blends are tested under either method. The higher the biodiesel blend, the higher the reductions are. "It's approximately linear," McCormick said, referring to the positive relationship between increased biodiesel blend percentages and corresponding gains in emissions reductions seen in the lab.

According to McCormick, the emissions benefits of biodiesel come in part from it being an oxygenate, which gives the fuel a more complete combustion. "That means it lowers the products of partial combustion, which are particulate matter, carbon monoxide and hydrocarbons," he said. "For [B100] ... particulate matter reductions can be in the range of 30 [percent] to 50 percent." Similar CO and HC reductions can result in testing [B100], McCormick said.

The phrase "up to 50 percent reduction" usually sounds good when talking about reducing something that's bad or undesired, but when a percentage is tossed around independent of the volumes or quantities it is derived from, one should always be critical and ask, "What are the numbers that constitute the percentage?" Asking this question-and getting the answer-can only arm the discerning biodiesel advocate with the appropriate tools to tackle oppositional arguments.

McCormick shared a complete data set with Biodiesel Magazine from an NREL vehicle chassis dynamometer test using B20 in transit buses powered by 2000 model year Cummins ISM engines. "For carbon monoxide ... we saw a 32 percent reduction in CO, and that's from about 3.5 grams per mile [of CO from conventional diesel] down to maybe 2.7 grams per mile [of CO using B20]," McCormick said. "For hydrocarbons ... we saw a [25] percent reduction for B20, from about 0.8 to 0.6 grams per mile."

And there lie the facts. With petroleum diesel only producing 0.8 grams per mile of HC, 25 percent HC reductions may be somewhat negligible. In other words, with such a low initial base number from petroleum diesel, it seems prudent to focus on the largest volume of emissions reductions from using biodiesel: the lessening of PM.

Those particulates matter
PM emissions from diesel vehicles burning conventional diesel are quite high and very noticeable. And that's where using biodiesel can make the biggest dent in air pollution, McCormick said. "You might get a higher percent reduction for hydrocarbons [using biodiesel blends], but nobody's really that concerned about hydrocarbons from diesel because it's such a small part of the inventory," he said. "PM reduction [from using biodiesel and biodiesel blends] is pretty significant. In an urban area, if you could get a 15 percent reduction in PM across the board-burn B20 in every vehicle-I think that would have a noticeable effect. I think that would be significant."

The role of biodiesel as a transitional technology that allows older fleets to reduce PM carries considerable weight in that it would contribute to improved air quality and, more concretely, reduced PM-linked health risks. PM is a carcinogen, and the smaller the particles, the bigger the health risk. "The fact that biodiesel is a non-synthetic chemical gives it a number of advantages in particle formation," said Dr. Bailus Walker, a professor at Howard University and a vice president of the American Public Health Association. "Biodiesel is not a major cause of fine or ultra-fine particle matter, which penetrates the respiratory tract. It poses no problem, and it actually helps reduce the overall particle load on communities." According to Walker, PM from petroleum diesel adds to the total "body burden of particles," which he said adversely affects the respiratory system. The respiratory system is closely related to the heart, or cardio, system, so when people are exposed to diesel PM, it not only causes such things as asthma and lung disease, but it also exacerbates heart problems as well, Walker said.

It's not just an urban problem either. "Pollution does not respect geographic or economic boundaries, so no community is safe from it," Walker told Biodiesel Magazine. "It's an urban problem, it's a suburban problem, and it's a rural problem. It's a problem with the whole country, and as we've seen, pollution even crosses international boundaries."

The rest of the story
For a comprehensive look at NOx emissions, see the June 2005 issue of Biodiesel Magazine. The long story on NOx made short is that some tests show biodiesel increases NOx slightly while other tests show a neutral comparison with petroleum diesel or maybe even a slight reduction. Both Van Gerpen and McCormick agree that the results obtained from NOx testing depend on what type of test is run-engine tests versus vehicle tests-along with what application the tested vehicle operates under in the real world. Long haul trucks are going to be hitting a particular speed and load cycle that city transit buses rarely see. Therefore, city transit buses may find their "emissions sweet spot" on a vehicle chassis dynamometer, as McCormick said, while an over-the-road truck is better represented by an engine dynamometer test.

To better capture the complex totality of diesel emissions, different tests are needed for different applications.

Michael Hass and Melissa Haas of the USDA Agricultural Research Service in Pennsylvania have done some work with butylated hydroxianisole (BHA), an antioxidant. Hess told Biodiesel Magazine that their tests showed a 4 percent to 5 percent NOx reduction when 1,000 parts per million of BHA was used. But due to the expense of BHA, which would add 14 cents per gallon to the price of B100, much work has stalled on this. But if one could find an antioxidant that lengthens the shelf life of the fuel and reduces NOx while remaining economical, a noteworthy find would be made.

Final important thoughts
"I would advocate a policy that favored 'dieselization' for energy security benefits and ozone benefits," McCormick said, speaking on the diesel engine's efficiency and reduction of emissions. "Of course, you want to use biodiesel … to get further energy benefits and PM reductions."

Dieselization, or the switch from predominantly gas-powered mobility to diesel, coupled with the use of biodiesel would have a mitigating effect on global warming, Van Gerpen said. "I think [the global warming argument] is an argument we need to emphasize more and more in the future," he asserted. "We're not releasing fossil CO2, or fossil carbon, using biodiesel." He cited the NREL study done a few years ago, which stated that biodiesel reduces fossil carbon emissions by approximately 78 percent. "That number is really far better than any other fuel out there," Van Gerpen said. "So, as we look to global warming, I think it'll become an increasingly serious issue as time goes on. We have a tremendous benefit there with biodiesel."

Ron Kotrba is a Biodiesel Magazine staff writer. Reach him by e-mail at rkotrba@bbibiofuels.com or by phone at (701) 746-8385.