*2.3. Control of CO2 Emissions*

In total, 6% (0.5 kg/kWh) of the exhaust gas emission from marine diesel engines is carbon dioxide. Although it is nontoxic itself, carbon dioxide contributes to the greenhouse effect (global warming and climate change) and hence to changes in the Earth's atmosphere. This gas is an inevitable product of combustion of all fossil fuels, but emissions from diesel engines, thanks to their thermal efficiency, are the lowest of the all heat engines. A lower fuel consumption translates to reduced carbon dioxide emissions since the amount produced is directly proportional to the volume of fuel used, and therefore to the engine or plant efficiency.

International concern over the atmospheric effect of carbon dioxide has stimulated measures and plans to curb the growth of such emissions, and the marine industry must be prepared for future legislation. There are currently no mandatory regulations on carbon dioxide emissions from shipping, but they are expected. Under international agreements, such as the Kyoto Protocol and the European Union's accord on greenhouse gases, many governments are committed to substantial reductions in the total emissions of carbon dioxide.

The Conference of Parties to the International Convention for the Prevention of Pollution from Ships, 1973, as modified by the Protocol of 1978 relating thereto, held from 15 to 26 September 1997 in conjunction with the Marine Environment Protection Committee's 40th session, adopted Conference resolution 8 on CO2 emissions from ships. The Marine Environment Protection Committee, at its 59th session (13 to 17 July 2009), agreed to circulate the guidelines for voluntary use of the ship energy efficiency operational indicator (EEOI) as set out in the annex. This document constitutes the guidelines for the use of an energy efficiency operational indicator (EEOI) for ships. It sets out:


### *2.4. Reduction of NOx Emissions*

Fuel brought to marine engine cylinders contains potential and chemical energy, which changes by a combustion process into the thermal energy. Analysis of the combustion process in the cylinder and the reactions that are involved in nitric oxide (NO) has identified three main sources of NO of which some are converted to nitrogen dioxide (NO2) to give the NOx mixture: Thermal NO, fuel source, and prompt NO. Thermal nitric oxides are produced in exhaust in temperatures higher than 1500 K. Prompt nitric oxides are produced in the flame front, in which there is deficiency of oxygen.

The range of thermal nitric oxide emissions depends on the flame temperature, partial oxygen pressure in exhaust (air excess coefficient), time of nitrogen, and oxygen particles' presence in high temperatures. The higher the temperature in which the oxidation and concentration of oxygen take place in the reaction area, the more intensive the reaction of oxidation and the oxygen concentration that occurs. During fuel combustion in the engine cylinder, the temperature exceeds 1600 K so the conditions for nitric oxide creation are perfect.

The second group comprises nitric oxides crested from nitrogen compounds included in fuel, which form nitric oxide by oxidation of combustible components, and then molecule nitrogen. The stage of nitrogen conversion from fuel into NOx depends on the fuel type and it ranges from 20% to 80%. Over 90% of oxides of NOx produced in internal combustion engines constitute NO. The oxidation of NO into NO2 takes place in the engine outlet channels, with the presence of oxygen contained in the channels and in the atmosphere. The term nitric oxides comprises both compounds.

Heavy fuel oil burnt in marine engines (HFO) contains greater amounts of nitric compounds than marine diesel oil (MDO). The nitric oxide emissions are greater in the case of heavy fuel than in MDO.

The reduction of the exhaust emission of NOx of modern marine diesel engines can be achieved through:


Primary reduction of exhaust emission takes place by influencing the fuel combustion process in the engine cylinder. The purpose of this action is to attack the problem at its source during the process of exhaust formation. In practice, in order to reduce nitric oxide emission, the following steps are taken:


Secondary measures are necessary to apply external treatment of exhaust gases after they have left the engine cylinders. In practice, the devices available for this purpose include selective catalytic reduction (SCR)—SCR converters.

The reduction of NOx emission through fuel modification is achieved, inter alia burning in the engine, by fuel/water emulsion. The combustion of alternative fuels, including diesel oil mixture with vegetable oils or their esters, may also be taken into account.
