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.
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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.
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