American Chemists look for ways to reduce the biodiesel NOx problem.

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Using biodiesel instead of conventional diesel reduces emissions such as the overall life cycle of carbon dioxide(CO2) emissions, particulate matter, carbon monoxide, sulfur oxides(SOx), volatile organic compounds (VOCs), and unburned hydrocarbons. However, while reducing the aforementioned types of emissions, nitrogen oxides (NOx) emissions, mostly NO and NO2, are increased.8 Figure 2 shows the impact that biodiesel-diesel blends have on emissions.

2. NOx Problem
Nitrogen gas (N2) makes up approximately 78% of the Earth’s atmosphere. The partial pressure of N2 ranges from 0.55 atm at 10 000 feet to 0.78 atm at sea level, while the volume fraction is 0.78 at both altitudes. The reactivity of N2 is low because it possesses a strong triple bond that requires 941 kJ/mol to break. Because of this strong triple bond, only combustion, lightening, and nitrogen fixing organisms can remove N2 from the atmosphere. The atmospheric lifetime of nitrogen gas is approximately 107 years, while the atmospheric lifetime of NOx gases range from 1 to 8 days.9 A complete overview of atmospheric nitrogen can be found elsewhere.9 NOx are produced during the combustion of many fuels. The two most common NOx are nitric oxide (NO) and nitrogen dioxide (NO2). NOx emissions pose serious problems for both the public health and the environment. NOx is a major contributor to ground-level ozone. While atmospheric ozone is beneficial in reducing harmful solar radiation, ironically, groundlevel ozone can cause respiratory irritation, a reduction of lung function, asthma attacks, and permanent lung damage.10 Ozone can also damage vegetation and reduce crop yields...

... There are several physical and chemical properties of biodiesel that are closely interrelated to the emission of NOx. From three main types of NOx formation: (i) thermal NOx, (ii) prompt NOx, and (iii) fuel NOx, the main contributor to NOx emissions from biodiesel is thermal NOx. This type of NOx formation is highly dependent upon temperature. For biodiesel, there are two effective ways of reducing this temperature. The first is by using water injection or an emulsion of biodiesel and water. The second method is by retarding the ignition timing in a biodiesel-powered engine. When ignition timing is retarded, the injection of the diesel fuel is moved from before the top of the compression stroke to very close to the top or after the top of the compression stroke. This would reduce the residence time and maximum temperature developed in the combustion chamber. The most effective way to reduce NOx emissions through combustion modification from a biodiesel-powered engine is by using a combination of water injection and retarded injection timings. However, some problems have been found affecting the performance of the engine with such measures. Postcombustion catalytic and noncatalytic reductions of NOx are also the effective means....

From the Energy and Fuels ASAP articles. Abstract: http://pubs.acs.org/cgi-bin/abstract.cgi/enfuem/asap/abs/ef050202m.html Subscription required for full article.

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1. Yet another reason...

For diesel hybrids. They operate the diesel engine in the sweet spot range where there are low NOx emissions.