Air emissions in shipping explained

Today, ships transport almost 80% of the world’s goods and in terms of moving huge amounts of freight, maritime shipping is the most environmentally friendly mode of transportation.
When ships use hydrocarbon fuels to generate power, they produce air pollution that lowers the quality of the atmosphere, damages health and also exacerbates climate change.
Commercial ships generate a variety of pollutants into the atmosphere as a result of burning fuel for propulsion. Carbon dioxide (CO2), nitrogen oxides (NOx), sulphur oxides (SOx), and particulate matter (PM) are the ship-sourced pollutants most closely associated to climate change and public health concerns.
The emissions of CO2, NOx, SOx, and other pollutants are all addressed in MARPOL appendix VI, which is a part of the convention dealing with marine pollution.

Regulations for the Prevention of Air Pollution from Ship – Annex VI of Marpol 73/78. Source and Credit: Marine Online

There have been a few adjustments made in this area, and you should be aware of what they entail for you. According to latest research on EU and US enforcement of the Annex’s new provisions, we’ve also learned that enforcement is increasing. As a result, the necessity of understanding air emission elements and executing things correctly is rising as well.

In this post we will describe different air emissions, how they are generated, and in which way they can be minimized.

Carbon dioxide (CO2) is the main end product of combustion of carbonaceous fossil fuel (fuel oils) and is a colourless, odourless and non-toxic gas, a product of normal human respiration and is a natural constituent of the air.

CO2 is the major greenhouse gas contributing to the global warming of the atmosphere and its global implications are, but not reduced to: climate changes, distribution of heat/cold and precipitation on earth, rising sea level and potential impacts on the ocean current system, changes in agricultural conditions etc.

CO2 and greenhouse effect. Source and credit: PICSCanada

Practically all carbon entering the engine combustion chamber is oxidized to form CO2 which is emitted into the atmosphere with the exhaust gases. As the carbon from lubricant oxidation is negligible in this respect, the CO2 emissions from the engine are practically direct proportional to the fuel carbon content and the fuel consumption (details of fuel properties can be found in here). Due to the superior fuel efficiency of the diesel process among prime movers, also the CO2 specific emissions are lowest when comparing operation on the same fuel quality. Hence, the importance of engine’s proper tuning and adjustment that will lead to high combustion efficiency and less emissions.

Specific CO2 emissions vs. specific fuel consumption

Sulphur oxides (SOx) in form of sulphur dioxide (SO2) and sulphur trioxide (SO3) are collectively referred to as sulphur oxides. SOx is contributing to acid rain and acidification with potential detrimental effect on vegetation, human health and buildings. SOx emissions are usually regarded as negligible at sea far from coastline because of the alkalinity of sea water and short transportation distances of SOx (10 -100 km).

SOx emissions. Source and credit: Dr. Masi

All sulphur entering the engine combustion chamber is oxidized to form SOx, which is emitted into the atmosphere with the exhaust gases. As the sulphur absorbed by the alkaline lubricant is negligible in this respect, the SOx emissions from the engine are practically direct proportional to the sulphur content and fuel consumption (details of fuel properties can be found in here). Due to the superior fuel efficiency of the diesel process among prime movers, also the SOx specific emissions are lowest when comparing operation on the same fuel quality. Typically SO2/SOx is 0.95 and SO2/SOx is 0.05.

Specific SOx emissions vs. specific fuel consumption vs. fuel sulphur fuel content

Nitrogen oxides (NOx ) are referred as nitrogen monoxide (NO) and nitrogen dioxide (NO2) collectively. NO is a colourless gas and NO2 is a reddish-brown gas (gas form) and yellow brown when it start to condense. NO2 is a toxic gas and is not regarded as NOx component and its emission is typically very low and negligible from diesel engines.

NOx is contributing to acid rain and acidification, over-fertilization of soil and lakes and ozone/smog formation in the lower atmosphere especially in the highly polluted urban areas with potential damage to vegetation and human health.

NOx emissions. Source and credit: Dr. Masi

NOx emissions are relatively high from diesel engines due to high local combustion temperatures, where the main nitrogen source is the combustion air and its formation has a strong exponential temperature influence (high temperature result in high NOx formation). NOx formation is very complex and includes hundreds of different chemical reactions. Typically NO/NOx ration is 0.95 and NO2/NOx is 0.05, but in the atmosphere NO is oxidized into NO2 usually within some hours.

The fuel air mixing process during combustion produces soot particles in the fuel-rich region of the fuel spray. Some of these soot particles survive the combustion process and are not oxidized. They absorb high molecular weight hydrocarbons from the fuel and lubricating oils. The particle matter grows further by the oxidation of some sulphur in the fuel to form sulphates and also by addition of water. When heavy fuel oil (HFO) is used the fuel ash contributes substantially to particulate emissions through smoke which by definition is visible (white, grey, blue, black, brown and yellow). Typically more than 50 % of the “dry dust” particulate mass consists of ash components originating from HFO (more details can be found in here). Particulates with diameter < 4 um are regarded as invisible for the human eye. Some particulates are considered carcinogenic (can produce cancer) and particles of 0.02 – 0.2 um which are generated into all type of liquid fuels combustion processes (not only diesel) can penetrate down into the human lungs. Large particulates are of less concern to human health because they are efficiently removed by pulmonary system. As fine particles can be easily transported by air currents, their detrimental effects can be encountered at long distances from the exhaust site.

Air pollutants. Source and credit: ISO Training Institute

Hydrocarbons comprise of hundreds of individual organic compounds with a lot of different configurations of carbon, hydrogen, nitrogen, oxygen and sulphur. The diverse nature of hydrocarbon components makes it both to quantify the emissions and to identify the specific health and environmental impacts. Hydrocarbons are often divided in two categories: total hydrocarbons (THC) and non-methane hydrocarbons (NMHC)(more about marine fuels can be found in here). Volatile organic compounds (VOC) mean non-methane hydrocarbons. THC is widely used in Europe and USA.

Hydrocarbons contribute to ozone/smog formation into the lower atmosphere in urban, highly polluted areas and some of them are considered carcinogenic and contribute to greenhouse effect (especially methane). The hydrocarbon emissions from marine diesel oil are low and generally are not of major concern.

The THC and VOC formation in the combustion chamber is the result of:

  • incomplete combustion affected by local excess of air, local temperature and uniformity of the air/fuel mixture.
  • the evaporation of the unburned fuel and lubricating oil

Heavy fuel oil (HFO) tends to form less hydrocarbon emission than light fuel oil (diesel oil), as a result of less evaporation effect with HFO. In general the hydrocarbon emissions from the diesel process are lower compared with other sources, as a result of the efficient combustion process.

When operating on liquid fuel, the THC in diesel exhaust is typically the same as VOC.

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