HOW TO REDUCE & CONTROL NOX EMISSIONS ? - Complying with MARPOL Annex VI Regulation 13: NITROGEN OXIDES

Posted by : SuperSailor 02 July 2013

The diagram below shows the typical exhaust emissions from a slow speed diesel engine. The components that concern us, are the Nitrogen Oxides (NOx), Sulphur Oxides (SOx), Carbon Monoxide, Hydrocarbons, and Particular Matter.

A draft protocol has been compiled by the IMO organisation to reduce the effects of vessel emissions on overall air pollution. This protocol forms Annex VI of the MARPOL 73/78 Regulations. Applies to every ship of 400 gross tons and above. Entered into force 19th May 2005. The main parts of the protocol which affect vessel operation are regulations 12 to 18, namely:

Regulation 12 Ozone Depleting Substances
Regulation 13 Nitrogen Oxides
Regulation 14 Sulphur Oxides
Regulation 15 Volatile Organic Compounds
Regulation 16 Shipboard Incinerators
Regulation 17 Reception Facilities
Regulation 18 Fuel Oil Quality

The vessel complying with these new regulations will be issued with an IAPP certificate, similar to the present IOPP for oil pollution.

Regulation 13 Nitrogen Oxides

The main thrust of this regulation is to reduce and control NOx emissions from diesel engines. The regulation is for new or converted engines of over 130kW built after 1/1/2000. Although the type of fuel plays a major part in the composition of the emissions, the major factor that determines the amount of Nox is engine speed. For the engines that fall under this criterion, the engine must have limits of NO2 from the engine of:-
17 g/kWh for engines under 130 rpm
45n-0.2 g/kWh for engines between 130 and 2000 rpm (where n = rpm)
9.8 g/kWh for engines over 2000 rpm

These emissions contribute to `smog' formation by increasing ozone concentrations in highly inhabited areas, affecting the respiration of humans and plants, and as NO2 is soluble in water it will be absorbed by rain to produce acidic precipitation.
These oxides are formed during the combustion process when the normally inert nitrogen reacts with the plentiful oxygen present, to form nitrogen oxides. The initial reaction is the formation of Nitric Oxide (NO), which is later converted to form Nitrogen Dioxide (NO2, visible as a yellow/brown gas) and Nitrus Oxide (N2O), typically 5% and 1% of the original NO quantity.
The nitrogen comes from:-
a) the fuel (fuel NOx, which is totally converted),
b) the air (thermal NOx, the amount converted depends on how long and at what temperature the reactants are held at).
Large bore slow speed engines inherently produce larger quantities of NOx emissions, as the slower speeds and larger bores both result in higher gas temperatures.
The controlling factors of how much NOx will be produced depends upon the concentration of oxygen, and the temperature and duration of combustion (increases x3 for every 100°c),
To reduce NOx emissions we can use:-
Primary methods – denitration of fuel, alternative fuels (LPG) or affecting combustion,
Secondary methods – during exhaust.

1. Primary Methods

Reduce cylinder temperatures by:-
1. Delay the point of fuel injection. This will reduce the max pressure and temperature produced, (increases fuel consumption).

2. Modified injectors. Mini-sac and slide valve type fuel injectors (MAN B&W) improve combustion by preventing the effects of dribbling. Increasing the number of holes and changing the orientation of the spray pattern (Sulzer) improves combustion. (increases fuel consumption)
3. Double fuel injection or pilot injection. These could be used on engines fitted with the new electronic-hydraulic MX engines. This would allow some of the fuel to be injected as a pilot charge, to preheat the air and reduce the rapid rise in pressure and temperature when the main charge ignites. (increases fuel consumption).

4. Using fuel/water emulsions (MAN B&W), injecting the water into the scavenge air (HAM) or directly into the cylinder (Wartsila/NSD). The water present will absorb the heat generated during combustion, which will reduce the temperatures, (increases fuel consumption).
5. Raise the scavenge pressure so that a greater quantity of air is present in the cylinder. This will both dilute the NOx formed, and reduce the cylinder temperatures reached, as a greater mass of air is present to be heated up. This method, which may be combined with increasing the compression ratios, will slightly improve the SFC, whilst avoiding an increase in NOx emissions.

Reduce the quantity of oxygen present by:-

1. If we reduce the quantity of oxygen present, then this will reduce the NOx formed. This is carried out by recirculating the exhaust gas back into the scavenge air. The gases present in the exhaust gas (CO2 and H2O) also absorb some of the combustion heat as their specific heat capacities are higher than air, reducing cylinder temperatures and NOx levels even further. However by doing this we will increase the thermal loading on the engine, smoke and particulate levels, and the exhaust gas must be cleaned and cooled requiring further equipment.

2. Secondary methods

Cleaning of the exhaust gas by chemical conversion using a selective catalytic reducer (SCR). Ammonia is added to the gas stream, and the mixture then passes through a special catalyst at a temperature between 300 and 400 degC. This converts the NOx to N2 and H2O, as detailed:-

4NO + 4NH₃ + O₂ = 4N₂ + 6H₂O

6NO₂ + 8NH₃ = 7N₂ + 12H₂O

If the temperature of reaction is too high, the ammonia burns and does not react, and at low temperatures the reaction rate is low and the catalyst can be damaged.

The ammonia can be supplied as either a pressurised water free ammonia feed, or an aqueous ammonia solution, or a dry urea which is dissolved in water before use. All processes must be contained within a safety area, as ammonia is combustible. Thus lines are double walled, and leak detection and appropriate venting of the storage and process areas must take place.

Obviously this method involves a large amount of additional plant, and does affect the turbocharger operation. This method has been tested on engine plants and large reductions in emissions are possible. However MAN B&W anticipate that only stationary power stations, and certain special sea area (restricted craft) would need to be fitted with this unit.