Air pollution from ships is on the rise and global emission standards are getting stringent every year. MARPOL Annex VI limits the main air pollutants contained in ships exhaust gas, including sulphur oxides (SOx) and nitrous oxides (NOx), and prohibits deliberate emissions of ozone depleting substances.
What is NOx?
NOx is a term for mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide). They are produced from the reaction of nitrogen and oxygen gases in the air during engine combustion, in other mean, NOx formed when diesel fuel is burned with excess air-oxygen intake.
Nitrogen is a natural element in the atmosphere and is also found in the chemical structure of some fuels. During the fuel combustion process, NOx is formed in the cylinder in three ways:
– thermal formation, as a result of the reaction between atmospheric nitrogen and oxygen at high temperatures
– fuel formation, as a result of the reaction between nitrogen in the fuel and oxygen
– prompt formation, as a result of complex reactions with hydrocarbons and atmospheric nitrogen.
During the combustion process of an engine (either spark ignition type or compression-ignition type), various by-products are produced. Apart from the energy released from combustion, carbon monoxide (CO), hydrocarbons (HC), nitrous oxides (NOx ), sulphates (SOx ), etc. are also produced.
Environmental Effect of NOx:
• Acidifies the environment
• Lung and heart disease
• Economic costs
• Medical care
• Destroy the Ozone layer
• Cost of pain / ill health / premature death
What is Tier?
To minimize global NOx emissions, IMO Regulation 13 sets NOx emission limitations on installed marine diesel engines (>130kW) and varied by RPM. The NOx limitations for diesel engines are regulated under 3-tier system, i.e. under Tier I, II and III
Where’s NOx Tier status Applied?
The main changes to MARPOL Annex VI are a progressive reduction globally in emissions of SOx, NOx and particulate matter and the introduction of emission control areas (ECAs) – NOx ECA / SOx ECA to reduce emissions of those air pollutants further in designated sea areas.
|Emissions||In effect from|
|Baltic Sea||SOx, NOx||19 May 2006, Jan 2021|
|North Sea||SOx, NOx||22 Nov 2007, Jan 2021|
|North American||SOx, NOx||1 August 2012|
|US Caribbean Sea ECA||SOx, NOx||1 January 2014|
How to reduce the NOx content?
1 – Primary compliance techniques for NOx
- Exhaust gas recirculation (EGR)
Exhaust gas is fed back into the cylinder air intake, lowering oxygen and increasing CO2, which has a higher specific heat capacity than air. This slows combustion and reduces temperature, lowering NOx. The EGR fan can adjust the amount of exhaust gas that is recirculated (the EGR ratio).
- Gas as Fuel
Fuels such as natural gas (stored on-board as LNG, MGO or potentially compressed natural gas) can help the engine achieve the Tier III. However, this depends on the engine design; not all gas engines can achieve Tier III. Some of these design considerations include:
Thermal cycle – 4 strokes engine
NOx performance is linked to the thermal cycle of the engine. A pure gas Otto or Miller cycle engine can achieve Tier III emissions levels relatively easily.
Dual fuel engines
Under the NOx Technical Code, dual fuel engines (engines that use liquid fuel to ignite the gas) and engines that use liquid fuel are required to be tested and certified at the highest liquid-to-gas fuel ratio. It will be Tier III compliant when operating on gas, but not when operating on liquid fuel. The operator will only be able to use liquid fuel during an emergency or failure, reducing operational flexibility
2 – Secondary compliance techniques for NOx
- Selective catalytic reduction – SCR
In this system, urea or ammonia is injected in the exhaust gas before passing it through a unit, which consists of special catalyst layer, at a temperature between 300 and 400 Deg C. Chemical reaction between Urea/ammonia and NOx in exhaust gases reduces NOx (NO and NO2) to N2. SCR unit is installed between the exhaust manifold/receiver and the turbocharger.
High efficiency turbocharger is required for this system as there is pressure drop across SCR Reactor. Engine load should be 40% and above, as NOx is reduced to N2 within specific temperature window ( 300-400 Deg C).
If temperature is above 400 Deg C, ammonia will burn rather than reacting with NOx which will lead the system to be ineffective. If the temperature is below 270 Deg C, the reaction rate will be low and the ammonium sulphates formed will destroy the catalyst.
- Charge air humidification
Air from the turbocharger, after passing through the compressor, has high temperature. Seawater is injected to this high temperature air for cooling and making it saturated. Distillation process makes it possible to use sea water instead of fresh water.
Humidification of air is controlled by maintaining scavenge air temperature between 40-70 Deg C. Water in saturated air reduces the peak temperature as water has higher heat carrying capacity than air.
Around 60% NOx reduction is achieved by this method. By using combination of other technologies such as EGR with Scavange Air Moisturizing, NOx Tier III standards can be achieved.