2.1 NOx limitation curve
The principle for establishing the NOx limit was the subject of debate within the IMO for many years. Different proposals were presented, tying the NOx limit to specific fuel consumption, engine speed or having one single NOx limit for all engines, etc. For the sake of compromise a consensus has now been reached on a NOx limit curve as a function of engine-rated speed (Fig. 1).
The compliance test was proposed according to the ISO 8178 test cycles and reference fuels (distillate fuel)[1].
2.2 A single NOx limit for all diesel engines within two five-year periods
The engine speed-related curve adopted was strongly put forward by the low-speed two-stroke engine manufacturers. They maintained that the NOx formation was mainly a function of the engine speed, and consequently the low-speed engine category could have higher NOx emissions. Research has clearly shown, however, that NOx-formation in a diesel engine is far more complicated. As a result, the IMO agreed that a single NOx limit value for all diesel engine types should be the ultimate goal. The intention is to review and modify the limit to achieve the single limit within the next two five-year periods.
The IMO NOx emission curve, however, also tends to be established as a baseline for calculation of NOx emission fee rates, which some countries are planning to apply to shipping within their national borders. For voyages to the ports of Los Angeles/Long Beach a USD 10000 fee has been proposed for each ton of NOx emitted. In following the slope of the IMO curve the emission fee will consequently be highest for low-rpm engines.
2.3 LOW NOx Combustion to comply with the IMO legislation
Nitrogen oxides in diesel exhaust originate from two sources: oxidation of the molecular nitrogen (N2) of the combustion air at high temperatures (thermal NOx), and oxidation of nitrogen compounds in fuel (fuel NOx). Thermal NOx is the main contributor to total NOx emissions. Fuel NOx accounts for only 10-20%.
The formation of thermal NOx depends on the concentration of the reactants nitrogen and oxygen, the temperature and the residence time. The process is extremely complex, including hundreds of reactions. Furthermore, local conditions are significant due to inhomogenity in the combustion chamber. The main point however is that the higher the temperature and the longer the residence time at high temperatures, the more NOx will be formed. All measures that reduce the magnitude and duration of high temperature peaks in the combustion chamber reduce NOx formation.
