Knocking out NOx

Biodiesel's increased NOx emissions continue to be the mainstay of critics' arguments against the widespread use of the renewable fuel. But with technological advancements driven by compliance with federal regulations aiming to virtually eliminate diesel NOx emissions, the strength of these arguments is equivalent to that of a house of cards.
By Ron Kotrba | June 01, 2005
An overwhelming number of studies show that using neat biodiesel and biodiesel blends in compression ignition engines slightly increases the generation of nitric oxide and nitrogen dioxide-any combination of which constitutes oxides of nitrogen (NOx). Without dwelling on technicalities, NOx increases can range anywhere from less than 1 percent to 15 percent higher than petro-diesel, with a long list of "depending on" factors. Some of the many variables contributing to the wide range are the type of test engines used, the test equipment itself, the feedstock source of the biodiesel tested, the process by which the biodiesel was transesterified, the blend of biodiesel in the test fuel-the list goes on.

Why is NOx an issue?
NOx emissions are thought to be a major contributor to ambient air quality degradation. NOx and volatile organic compounds (VOCs) react

with heat and sunlight to create a chain reaction of chemical conversions, one of which is the formation of ground-level ozone. Ozone is a substantial component in smog, and exposure to high levels of smog for extended bouts of time can have irreparable consequences on people's health.

Come 2007, federal law begins tightening the belt on noxious emissions, but in anticipation of that, Texas has developed its own set of regulatory standards governing diesel sales in 110 counties at risk of severe air pollution. The ruling is TxLED, and effective Oct. 1, 110 counties in Texas will be required to replace standard diesel fuel with low emissions diesel (LED). LED is a special diesel fuel containing less than 10 percent aromatic hydrocarbons by volume per gallon, and a minimum cetane number of 48. A higher cetane diesel fuel has better ignition quality, shorter ignition delay and lower temperatures during combustion, meaning less NOx generation.
Morris Brown is the TxLED program coordinator. "This ruling is an ozone control strategy to reduce NOx emissions," he said. "A decision has been made by the executive director of the agency not to accept biodiesel blends under this new ruling." Biodiesel has higher cetane numbers than petroleum diesel, and the U.S. EPA reports that it contains fewer aromatic hydrocarbons, yet the blending of biodiesel in 110 counties-as the ruling stands-will be outlawed after Oct. 1.

What is it about biodiesel that tends to increase NOx? Cetane numbers are higher, which should reduce NOx, and aromatic hydrocarbons are lower. Some say that the oxygen content in biodiesel is cause for higher NOx emissions. "One reason for increased NOx could be the molecular weight of the methyl ester," said M. Lie Ken Jie of the University of Hong Kong. According to Randall von Wedel, technical liaison for North Texas Bio-Energy LLC, the presence of more oxygen in biodiesel esters gives the fuel a higher bulk modulus and, therefore, a propensity to advance the timing.

Jie, like many others, also believes the combustion temperature of biodiesel needs to be cooled. Jie said this would accomplish "mopping up of some of the free radicals to tone down the process of violent burning." An NREL report titled "NOx Solutions for Biodiesel" states that the content of saturation in the fatty acid methyl esters is related to NOx outputs-higher saturation leads to lower NOx.
Although the causes of increased NOx are being heavily researched, two particular studies refute conventional research. Two B20-burning buses were recently tested on a chassis dynamometer at NREL's Golden, Colo., facility, and the outcome showed that emissions results from the biodiesel blend were consistent with results from the petro-diesel. In other words, the study showed that no NOx increases were emitted from burning biodiesel compared to standard diesel.

A different study-a U.S. DOE-funded collaboration called the "Weekend Ozone Effect"-finds that weekend ozone levels are just as high as weekday concentrations, which is irregular considering ozone precursors like VOCs and NOx are much more prevalent on weekdays than on weekends. This suggests that NOx might not be as big of a contributor to ozone formation and smog as was once thought.
Nonetheless, a vast majority of studies indicate higher NOx outputs from biodiesel. To combat this problem, advanced technology offers at least three broad means to control NOx.

Additives/blending stocks
Various additives or blending stocks can be used to reduce NOx from biodiesel emissions. A majority of the additives on the market aim to reduce the combustion temperature by increasing the cetane number of biodiesel. At the American Oil Chemists' Society (AOCS) conference in May, John Van Gerpen of the University of Idaho said, stoichiometrically, diesel combustion is 60 percent to 70 percent air, "which has implications in emissions." Air is comprised of nitrogen and oxygen. Under extreme pressures in the cylinders, the compressed air gets very hot. Simply put, the fuel is injected into the cylinder during cylinder air compression, and upon ignition and combustion, the even higher temperatures from the flame front (or uneven spread of combustion concentrated on the center of the piston due to fuel injector spray inconsistencies) oxidize the nitrogen. Many additives look to reduce the combustion temperature of the biodiesel in order to reduce the oxidation of nitrogen upon the fuel's ignition. Lowering the combustion temperature could be problematic, however, because doing so oftentimes increases particulate matter (PM) emissions. There is seemingly a trade-off between the two: lower NOx and higher PM.

NREL's "NOx Solutions for Biodiesel" report finds that di-tertiary butyl peroxide and ethylhexylnitrate, which are both cetane improvers, effectively reduce NOx without a PM trade-off. Also, NREL reports that tertiary butylhydroquinone, an antioxidant, is an effective NOx-reducing agent, with slight PM increases. The report notes that oxidation inhibitors are of interest for further studies.

Additives are generally blended at a concentration of 1 percent or less by volume, while NOx reducing blending stocks are blended at much higher ratios. One blending stock in particular is of interest because of its familiar feedstock source: Bio-Cetane Plus is a high cetane (around 100), NOx reducing blending stock made from vegetable oils or animal fats and licensed for production, distribution and marketing by North Texas Bio-Energy. According to von Wedel, the technology behind Bio-Cetane Plus was developed by Natural Resources Canada. He said the process to create the blending stock involves the breaking down of larger molecules, hydrotreating and hydrogenation to remove the oxygen and double bonds, which creates a very high-cetane hydrocarbon product.

"The more you add of Bio-Cetane Plus, the higher the cetane numbers go and the lower the NOx emissions," said Dan Foster, North Texas Bio-Energy managing director and founder. He said the Bio-Cetane Plus blending stock is added to B20 typically at 20 percent by volume, virtually creating a combined fuel product that consists of 40 percent renewables.

Product emissions testing is ongoing at Southwest Research Institute in San Antonio, Texas, and Foster said he is hopeful the results will convince the state to allow the sale of Bio-Cetane Plus-blended biodiesel after Oct. 1. "We've got a technical alternative coming up here," Foster said, meaning that his test results may change state officials' minds. Foster said the cost to produce Bio-Cetane Plus is the same as biodiesel. Although von Wedel said this product isn't in competition with biodiesel, Foster said, "We can certify as an alternative fuel."
Engine management

Another treatment for NOx reduction involves engine management. Homogeneous Charge Compression Ignition systems help reduce the uneven injection of fuel into the cylinders, which causes the flame front to burn hotter in areas of higher fuel concentration resulting in higher NOx generation. The fuel injection is more fully managed through a technique of vaporization injection rather than spray injection.

Also, adjusting diesel engine timing and the addition of injection sensors can aid in NOx reduction. Researchers at the University of Illinois have developed a three-dimensional combustion model to better understand how biodiesel combustion causes NOx increases. Professor Al Hansen and graduate student Wenqiao Yuan have developed a program that accounts for fatty acid content in the fuel. The two advocate the use of sensors to determine the percentage of biodiesel in the fuel, which then relays a message to the injector control unit indicating when to inject the fuel into the cylinder. Hansen said changing the timing shifts the process of combustion to a point at which temperatures are not so high, resulting in a lower occurrence of nitrogen oxidation. Van Gerpen's words at the AOCS conference corroborated this; he said a two-degree retardation in timing would satisfy considerable NOx reduction for B100. Even with engine management controls on timing and other areas, these reductions will not alone satisfy the 2010 federal NOx reduction mandate.

Exhaust aftertreatment
The EPA's federal regulation begins stringent NOx emissions restrictions in 2007, with 100 percent compliance by 2010. Similarly, sulfur content in diesel fuel will be restricted to 15 parts per million. Like the generation of NOx upon combustion, sulfur oxidizes to create SOx, which heavily contributes to the formation of acid rain. The majority of SOx in the environment comes from burning coal at electric power plants, not from diesel highway engines. A major factor in the EPA's restrictions of sulfur in diesel fuel is that, with more than 15 parts per million, aftertreatment catalysts will not function properly to meet 2010 federal regulations governing NOx, PM and other diesel emissions.

The 2010 emissions standards will mandate a more stringent control of diesel exhaust systems, which in most cases means-in addition to engine management controls-diesel particulate filters and NOx-controlling devices, including exhaust gas recirculation and most commonly either NOx traps (also called NOx adsorbers) or selective catalytic reduction.

Exhaust gas recirculation systems have been used since 2004 to meet that year's target NOx reduction. A portion of the exhaust gases from downstream is rerouted back into the engine's air intake into the cylinders, displacing fresh air with inert gases. The inert gases have less energy, which means they burn cooler to generate less NOx. Again, cooler combustion temperatures lead to more PM, so manufacturers installed diesel oxidation catalysts to marginally control the PM. In 2007, only a part of the ultimate 2010 NOx reduction regulations need to be met, so to satisfy that, fleet manufacturers are cranking up the exhaust gas recirculation flow to cool down combustion temperatures even more, which will result in higher PM. Thus, diesel particulate filters will be used starting in 2007 to combat higher PM levels, but in order to meet 2010's 100 percent compliance regulation, diesel particulate filters and NOx aftertreatment technologies will both be required to meet emissions standards.

NOx traps look to be the devices of choice in the United States for downstream NOx abatement. The inner substrate of a NOx trap looks similar to that of a catalytic converter. It consists of a "brick" with a cell-structure design through which exhaust can flow with minimal backpressure. The design maximizes surface area, which is needed for successful conversion. The cell count can average approximately 400 cells per square inch. This brick is coated with a wash of catalytic and adsorbent materials, like rhodium and barium carbonate, respectively; platinum could also be in the washcoat if a diesel oxidation catalyst isn't present upstream.

The NOx gets stored in the washcoat along the cell walls under lean conditions, and occasionally regeneration must occur under rich conditions to convert the NOx to carbon dioxide and unreactive nitrogen. To cause a rich running condition-which initiates the conversion-more fuel must be added either in the cylinder or directly upstream of the trap. Biodiesel blends are optimal for this type of NOx abatement, due to biodiesel's virtual nil-sulfur content.

Selective catalytic reduction systems are common NOx reducing devices in Europe. Selective catalytic reductions work by injecting specified amounts of reagent urea into the exhaust stream. The urea breaks down to ammonia, and in the presence of a catalyzed brick-similar to the NOx trap brick in physical structure but not necessarily in washcoat makeup-the ammonia reacts with the NOx to produce elemental nitrogen and water vapor.

Exhaust aftertreatment combined with engine management controls appear to be the winning combination for NOx reduction. All major OEM exhaust suppliers are pushing forward in developing optimal configurations of design for these devices. Emissions testing for both NOx traps and selective catalytic reductions have shown up to 90 percent NOx reduction.
So, to those critics who discard biodiesel on the grounds of increased NOx, get ready to start looking for a new angle. As Van Gerpen said, these emissions arguments about biodiesel-good and bad-will be irrelevant to vehicles of model years 2007 and up. n

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