Exhaust Gases from Marine Engines and Atmospheric Pollutio
Common Pollutants
Marine diesel engines typically use heavy fuel oil (bunker fuel) with high sulfur content. The combustion gases of this fuel include sulfur dioxide, nitrogen oxides, particulate matter, carbon monoxide, carbon dioxide, and other hydrocarbons, which, through secondary reactions, lead to the formation of aerosols, secondary chemical substances, formaldehyde (HCHO), and others in the atmosphere. The United States Environmental Protection Agency (EPA) has classified diesel engine emissions as carcinogenic to humans.
Of the total atmospheric pollution emissions, shipping accounts for 18 to 30 percent of nitrogen oxide emissions and 9 percent of sulfur oxide emissions. Sulfur in the air causes acid rain, which damages crops and buildings. Inhaling sulfur can also lead to respiratory problems and even increase the risk of heart attacks. According to Irene Blooming, spokesperson for the European Environmental Coalition for Seas at Risk, a ship produces approximately 50 times more sulfur pollution per ton of transported cargo than a truck. Increased trade between the United States and China leads to a rise in ocean-going vessels, exacerbating various environmental problems.
Unlike sulfur emissions (which depend on the fuel used), nitrogen oxide emissions primarily depend on combustion temperature. Since air contains more than 70 percent nitrogen by volume, some of it reacts with oxygen during combustion. Given that these reactions are endothermic, higher combustion temperatures result in greater nitrogen oxide production. However, other pollutants, particularly unburned or partially burned hydrocarbons (also known as ultrafine particles or soot), are produced more at lower combustion temperatures.
One method to reduce nitrogen oxide emissions is passing exhaust gases through a catalytic converter and/or treating diesel exhaust fluid, where an aqueous urea solution reacts with the exhaust gases, converting nitrogen oxides into carbon dioxide and water.
Another way to reduce pollution is through wet scrubbers, which essentially spray seawater into the exhaust column. With precise design and engineering, these scrubbers can remove sulfur oxides, soot, and nitrogen oxides from engine exhaust gases. The water solution, after contact with the gas, turns into a residue containing soot and various acidic compounds. This residue can either be treated onboard the vessel (closed-loop system) or discharged from the vessel (open-loop system). However, discharging these substances into the sea, especially in coastal environments, can be harmful to marine ecosystems.
Greenhouse Gas Emissions
The continuous growth of the global population and the increasing number of developing countries have led to a rising dependence of the global economy on international trade. In 2007, the shipping industry emitted approximately 1,046 million tons of carbon dioxide, accounting for 3.3 percent of total global CO₂ emissions that year. By 2050, a threefold increase in CO₂ emissions from maritime transport is predicted. Therefore, reducing greenhouse gas (GHG) emissions to acceptable levels in the future requires optimizing fuel consumption in the maritime transport industry.
The first international IMO conference on greenhouse gas emissions control was held in Oslo, Norway, in June 2008. To develop effective mechanisms for reducing GHG emissions from shipping, the IMO’s Marine Environment Protection Committee (MEPC) was established. The International Maritime Organization (IMO) introduced the first maritime energy efficiency regulations, which came into effect in January 2013. The goal of these regulations is to reduce carbon emissions by decreasing fuel consumption in vessels through various methods. These regulations mandate that all newly built and operational ships over 400GT implement an energy efficiency management plan.
Some methods to reduce GHG emissions in the shipping industry include reducing transport speed (which can potentially be problematic for perishable goods), optimizing ship design, adopting new energy-efficient technologies, improving ship performance, and making changes to fuel standards. In 2019, international shipping organizations, including the International Chamber of Shipping, proposed the establishment of a $5 billion fund to support research and technology necessary for reducing GHG emissions.