Reduction Technologies for Nitrogen Oxide Emissions

Nitrogen oxide (NOX) is considered as one of the pollutants released during combustion. It is composed of nitric acid, nitrogen dioxide, and nitrous oxide. Nitrogen oxide emissions are believed to aggravate asthmatic conditions and when it reacts with oxygen in the air, it can also produce ozone which is also considered as an irritant. When nitrogen oxide emissions coming from industrial and mobile sources are dissolved in atmospheric moisture, the result is Acid Rain which can damage trees and entire forest ecosystems.

Diesel combustion nitrogen oxide emissions

Diesel engines differ from gasoline engines since they utilize compression to ignite air/fuel mixture as opposed to a spark source. Compression results into much higher temperature in the area of combustion and as consequence oxygen and nitrogen in the intake air combine to form nitrogen oxide. It is typical for diesel engines to have nitrogen oxide emissions that are 1.5 or 2 times higher than those from gasoline fueled vehicles. Diesel engines are undergoing considerable design changes including higher fuel injection pressures, increased use of turbo-charging, retarded injection timing, electronic engine controls revised combustion chamber design, improved lube oil control and exhaust after treatment devices.

NOX reduction technologies

Selective catalytic reduction

Nitrogen oxide emissions from motor vehicles can be controlled through selective catalytic reduction (SCR) for diesel engines which is considered as one of the cost effective and fuel efficient technology available. The three main components of SCR are: diesel exhaust fluid (DEF), hot exhaust and catalytic converter. Vaporized DEF is injected into the exhaust stream of the diesel engine. When the hot exhaust combines with DEF in the SCR catalyst, it is broken down into two components, pure nitrogen and water vapor. 

Results of using SCR technology for reduction of nitrogen oxide emissions

-        90% reduction in the levels of nitrogen oxide

-        3 – 5% reduction in carbon dioxide emissions from Class 8 trucks due to reduced fuel consumption

-        30 to 50% reduction of black carbon levels or otherwise known as particulate matter

Selective non-catalytic reduction (SNCR) of NOx

SNCR is based on a chemical reduction of nitrogen oxide molecule into molecular nitrogen and water vapor through the injection of a reducing agent such as ammonia or urea into the post combustion flue gas. This reduction in nitrogen oxide emissions is favored over other chemical reaction processes at temperatures ranging from 1600^oF and 2100^oF making it a selective chemical process.

Exhaust gas recirculation (EGR)

This technique for reducing nitrogen oxide emissions is used in spark ignited machines for years. EGR controls NOX emissions by keeping combustion temperature below that at which nitrogen oxide is formed. When nitrogen is exposed to very high temperatures and high pressures in the combustion chamber it combines with oxygen to form nitrogen oxide. In order to keep combustion temperature low a small amount of exhaust gas is rerouted into the intake cycle to dilute intake air, reducing the oxygen content of the combustion mixture. However, the quantity of exhaust gas must be carefully controlled as it can result into increased particulate and carbon monoxide emissions due to insufficient air to complete the combustion cycle.