Biodiesel NOx emissions

Nitrogen oxides (NOx) are a group of gases that form when fuel is burned at high temperatures. These gases contribute to ground-level ozone, acid rain, and visibility impairment. Over half of human made NOx emissions come from fuel combustion in motor vehicles.

Compared to conventional diesel fuel, use of biodiesel is generally found to reduce emissions of hydrocarbons (HC), carbon monoxide (CO), and particulate matter (PM); but to increase oxides of nitrogen (NOx) emissions.

In modern diesel engines, several factors related to fuel composition and engine control strategies are important, though no single theory provides an adequate explanation of the biodiesel NOx effect under all conditions. There is evidence to suggest that effects on injection timing, ignition delay, adiabatic flame temperature, radiative heat loss, and other combustion phenomena all play some role.

The biodiesel NOx effect can be mitigated by modifying engine control settings — particularly by retarding injection timing and increasing exhaust gas recirculation (EGR). The absolute magnitude of the biodiesel NOx effect appears to be reduced with modern engines, although there are cases where the percentage change is still substantial.

Sophisticated after-treatment systems required to achieve the diesel engine emissions standards do not appear to be significantly affected by use of biodiesel. However, longer term study is warranted, as such systems have only been in commercial use for a short time.

It is difficult to reduce thermal NOx. Reducing the combustion temperature might reduce NOx but increases PM (Particulate Matter) emissions. NOx and PM are indirectly proportional. The Nitrogen in the diesel is from the proteins/amino compounds from the raw material as such. Using feed stock that has low fuel bound nitrogen is a good option. May be a good idea to use a catalytic converter or secondary or auxiliary device at the exhaust or tail pipe to convert the NOx.

The trend of NOx emissions from use of biodiesel is still uncertain. Several studies show an increase in NOx emissions, while others show a decrease. Further research is still needed on NOx emissions from engines burning biodiesel.