About nitrogen oxide emissions

General            

• Nitrogen monoxide (NO) and nitrogen dioxide (NO₂) are grouped together under the general term nitrogen oxides (NO_x)
• NO is a colourless gas
• NO₂ is a dark brown, toxic gas
• Nitrous oxide (N₂O) is a colourless gas heavier than air and one of the greenhouse gases not regarded as a NO_x  component

Environmental impacts

• NO_x emissions generally increase acidification and eutrophication (overrich nutrients in water systems and soil)
• NO_x emissions promote the formation of ozone and smog (particulates) in the lower atmosphere in heavily populated urban areas. Ozone in the lower atmosphere is harmful to vegetation and human health.

Formation in engines

• The N₂O emissions of reciprocating engines are typically low
• The NO_x emissions of a diesel engine are relatively high owing to the high combustion temperatures in the diesel process
• The NO_x emissions of a gas engine are relatively low because this engine’s combustion process is based on premixing of the fuel and air.
• The formation of NO_x emissions in an engine is thermal; the primary source of nitrogen is the nitrogen in the combustion air
• The combustion termperature, the degree of fuel/air premixing and the duration of the fuel in the cylinder all strongly affect the formation of NO_x. NO_x formation is highest with a high combustion temperature, low degree of premixing and long fuel duration
• NO_x formation in an engine is an extremely complex process comprising hundreds of different chemical reactions and many intermediate products
• The typical NO/NO_x ratio in a diesel engine’s exhaust gases is 0.95, and for NO₂/NO_x 0.05
• After being released as exhaust into the atmosphere NO oxidizes into NO₂ typically within a few hours.

Methods of reducing emissions in the engine

• Delayed fuel injection timing, which reduces the in-cylinder duration of the combustion gases at high temperatures
• Raising the degree of premixing, and in a gas engine increasing the amount of air
• Advancing the closing time of the inlet valve to lower the final combustion temperature (“Miller valve timing”)
• Reducing the temperature and pressure of the combustion air fed into the cylinders
• Optimizing the geometry of the combustion space and the compression ratio
• Introducing water to the combustion space to reduce the temperature by humidification of the combustion air (Wetpac H) or by injecting water directly into the cylinders (Wetpac DWI) or by emulsifying water into the fuel (Wetpac E)
• Many measures aimed at reducing NO_x  emissions also increase fuel consumption and the formation of particulates. Optimization of an engine’s emission levels therefore requires that all these factors are taken into account.

Feasible emissions reduction technology

• A Selective Catalytic Reduction (SCR) catalytic converter.