Theory

NOx: What is it? Where does it come from?

Nitrogen oxides (NO_x), are the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO₂) along with particles in the air can often be seen as a reddish-brown layer over many densely populated industrial and urban areas.
Nitrogen oxides form when fuel is burned at high temperatures, as in a combustion process in a Marine Diesel Engines. The primary sources of NO_x are ships, motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels.
Advances in thermal efficiency have directly contributed to a rise in NO_x emission. Dominating factors in the formation of NO_x are temperature and oxygen concentration – the higher the temperature and the higher the residence time at the high temperature in the cylinder the greater the amount of NO_x that will be generated. A longer combustion time span means that the low speed engines generate more NO_x than medium or high speed engines.

NO_x and the pollutants formed from NO_x can be transported over long distances, following the pattern of prevailing winds. This means that problems associated with NO_x are not confined to areas where NO_x are emitted. Therefore, controlling NO_x is often most effective if done from a regional perspective, rather than focusing on sources in one local area.
NO_x emissions are increasing and since 1970, EPA has tracked emissions of the six principal air pollutants – carbon monoxide, lead, nitrogen oxides, particulate matter, sulfur dioxide, and volatile organic compounds. Emissions of all of these pollutants have decreased significantly except for NO_x which has increased approximately 10 percent over this period.

Harmful effects of NO_x

• contributes to the formation of ground-level ozone, which can trigger serious respiratory problems.
• reacts to form nitrate particles, acid aerosols, as well as NO₂, which also cause respiratory problems.
• contributes to formation of acid rain.
• contributes to nutrient overload that deteriorates water quality.
• contributes to atmospheric particles, that cause visibility impairment most noticeable in national parks.
• reacts to form toxic chemicals.
• contributes to global warming.

Ground-level Ozone (Smog) – is formed when NO_x and volatile organic compounds (VOCs) react in the presence of heat and sunlight. Children, people with lung diseases such as asthma, and people who work or exercise outside are susceptible to adverse effects such as damage to lung tissue and reduction in lung function. Ozone can be transported by wind currents and cause health impacts far from original sources. Millions of people live in areas that do not meet the health standards for ozone. Other impacts from ozone include damaged vegetation and reduced crop yields

Acid Rain – NO_x and sulfur dioxide react with other substances in the air to form acids which fall to earth as rain, fog, snow or dry particles. Some may be carried by wind for hundreds of miles. Acid rain damages; causes deterioration of cars, buildings and historical monuments; and causes lakes and streams to become acidic and unsuitable for many fish.

Particles – NO_x reacts with ammonia, moisture, and other compounds to form nitric acid and related particles. Human health concerns include effects on breathing and the respiratory system, damage to lung tissue, and premature death. Small particles penetrate deeply into sensitive parts of the lungs and can cause or worsen respiratory disease such as emphysema and bronchitis, and aggravate existing heart disease.

Water Quality Deterioration – Increased nitrogen loading in water bodies, particularly coastal estuaries, upsets the chemical balance of nutrients used by aquatic plants and animals. Additional nitrogen accelerates “eutrophication,” which leads to oxygen depletion and reduces fish and shellfish populations. NO_x emissions in the air are one of the largest sources of nitrogen pollution in the Chesapeake Bay.

Global Warming – One member of the Nitrous oxide (NO_x), is a greenhouse gas. It accumulates in the atmosphere with other greenhouse gasses causing a gradual rise in the earth’s temperature. This will lead to increased risks to human health, a rise in the sea level, and other adverse changes to plant and animal habitat.

Toxic Chemicals – In the air, NO_x reacts readily with common organic chemicals and even ozone, to form a wide variety of toxic products, some of which may cause biological mutations. Examples of these chemicals include the nitrate radical, nitroarenes, and nitrosamines.

Visibility Impairment – Nitrate particles and nitrogen dioxide can block the transmission of light, reducing visibility in urban areas and on a regional scale in national parks.

Regulations applying to the Marine industry:

The Marpol Protocol of 1997 (Annex VI – Regulations for the Prevention of Air Pollution from Ships).  Adoption: 26 September 1997

“Subject to the provision of regulation 3 of this Annex, the operation of each diesel engine to which this regulation applies is prohibited, except when the emission of nitrogen oxides (calculated as the total weighted emission of NO₂) from the engine is within the following limits:

Limits for NO_x Emission from a Merchant Vessel

Tier 2 – From 1st Jan 2011

1. 14.4g/kWh when n is less than 130 RPM
2. 40*n(-0.23) g/kWh when n is 130 or more but less than 2000 RPM
3. 7.7 g/kWh when n is 2000 RPM or more

where n = rated engine RPM

Tier 3 (for Emission control areas only) – From 1st Jan 2016,  applies only in ECA (not in        SECA). 

1. 3.4g/kWh when n is less than 130 RPM
2.  9.0*n(-0.2) g/kWh when n is 130 or more but less than 2000 RPM
3.  2.0 g/kWh when n is 2000 RPM or more

1. Injection Retardation:  By injecting fuel later in the compression cycle of the piston, the fuel has less time to burn, creating less NO_x.  However, it still leaves the other byproducts.  This is usually combined with higher injection pressures, which cause finer fuel droplets, which burn more quickly, leaving the opposite problem of too much NO_x.  Despite this, there is a measurable drop in both levels when compared to unmodified engines.
2. Charge Air Cooling: The use of a cooling agent to cool the air before it enters the piston, lowering the burn temperature, and creating less NO_x.
3. Catalytic Converter: Due to the cost of materials, this is exclusively for small land based units and automobiles.  It utilizes rhodium as a catalyst to change nitrogen oxides back to nitrogen gas and water.
4. Direct Water Injection (Wartsila): The key element in the DWI system is the combined injection nozzle which has one needle valve for the fuel and another one for the fresh water.  Water to fuel
   Injection nozzle injects both water and fuel  in the ratio 0.4 to 0.7, coating droplets of water with fuel.  This increases atomization of the fuel, and creates a low temperature combustion which reduces NO_x emissions by 50%-60%, with unaffected or slightly improved specific fuel consumption.
5. Selective Catalytic Reduction System (Wartsila): By spraying the exhaust gases with a mist of ammonia, and passing it through a catalyst, the ammonia and NO_x react, and change to nitrogen gas and water.  85%-95% emissions reductions are possible. This equipment can be retrofitted; however it is bulky and has to be fitted before the exhaust boiler.
6. Exhaust gas recirculation
   50 to 60% reduction of NO_x is possible for 15% recirculation of exhaust gases into the inlet manifold. The exhaust gases being recirculated are cleaned and cooled before recirculation to the scavenge air side. It reduces NO_x by lowering the oxygen concentration in combustion zone.
7. Fuel water emulsion
   Adding water to the fuel dramatically sinks both soot emissions and nitrogen oxide emissions from diesel engines. This equipment can be retrofitted on all diesel engines, regardless of whether they are pre-chamber/swirl chamber engines or unit injector/common rail direct injection engines. The emulsion is produced immediately before the injection pump; no intervention in the engine or the direct injection system is required.

The MC-90 Ships model SCR Plant Description

This ship is fitted with two options for reducing NOx

1.Selective Catalytic Reduction unit which treats the exhaust gases before they enter the turbocharger. Pre-programmed quantity of Ammonia is added to the exhaust gas stream, and the mixture passed through a  catalyst at a temperature between 300°C and 400°C. Within the SCR Reactor the hot exhaust gases containing  NO_x  gases are mixed with the ammonia stream. This reduces the NO_x to N₂ and H₂O.
If the temperature of reaction is too high (above 490°C), the ammonia burns and does not react, and at low temperatures (below 250°C) the reaction rate is low and the catalyst can be damaged therefore the SCR is not used for lower engine RPMs

2. Fuel water emulsion situated between the fuel oil meter and the venting box is the Fuel-Water Emulsion Control Unit which is designed for emulsification of the fuel to reduce the NOx values in the exhaust gas from the engines.

Tabulate your data as shown below: