NOx and SOx – What’s it all about?

November 29, 2017

With more ships venturing to different parts of the world, the pollution caused by them is rising, leading to global con­cerns. While land-based emissions have been gradually lessening, air pollution from ships has continued to rise.

According to some estimates, if cur­rent trends continue, by 2020 shipping will be the biggest single emitter of air pollution in Europe, surpassing emis­sions from all land-based sources includ­ing automobiles and other sources.

Even more startling, European scientific studies note that air pollution from international shipping accounts for significant numbers of premature deaths around the world – particularly in places like Europe, the Far East, and parts of North and South America.

Cardiopulmonary mortality attributable to ship PM2.5 emissions worldwide.jpg

Cardiopulmonary mortality was attrib­utable to ship PM2.5 emissions worldwide.
Source: Environ­mental Science & Mortality.

The two main pollutants from ship emissions are Nitrogen oxides (NOx) and Sulfur oxides (SOx). The reason? These combustible gases, emitted into the environment in the form of smoke, can have adverse effects on the ozone layer in the troposphere, which results in the “greenhouse effect” and may contribute to global warming.

Marine fuel in an internal combustion engine is burned inside the combustion chamber by the correct mixture of fuel and air in the presence of heat or an ignition source (the compression stroke of a piston). Nitrogen reacts with oxygen under certain engine operating conditions to form Nitro­gen oxide (NOx) emissions. Contributing factors include:

  • High cylinder temperature and pressure during the combustion process
  • Improper air and fuel ratio for combustion
  • The high temperature of the intake or scavenger air inside the cylinder
  • Lower quality of fuel used for the engine

Sulfur oxide (SOx) emissions are main­ly due to the presence of sulfur compounds in the fuel. Smoke containing sulfur oxides emitted by the combustion of marine fuel will often oxidize further, forming sulfuric acid which is a major contributor to acid rain. SOx emissions also contribute to the formation of secondary inorganic aerosol gases – fine particulates that are harmful to people.

When ships operate within an emis­sion-controlled area such as the North Sea, the Baltic Sea, and portions of the North Atlantic, the sulfur content of any fuel oil must not exceed 0.1% m/m. (Outside of an emission-controlled area [ECA], the sulfur content must not exceed 3.5% m/m.) To achieve com­pliance, LNG fuel or expensive, highly refined diesel oil must be used; the alter­native is employing an expensive exhaust gas cleaning system to reduce total sulfur emission levels.

The better the grade of fuel the lower the sulfur content will be, since it is removed through the refining process. LNG is the best choice in this regard because of its clean-burning characteris­tics. It is comprised of only carbon and hydrogen (CH4). Not only are nearly all NOx and SOx emissions eliminated, but LNG will also reduce carbon dioxide (CO2) emissions by between 20% and 40%, depending on how it’s consumed.

Reducing air pollution from ships via the use of LNG is considered a very cost-effective solution to the health and environmental costs of “doing nothing.”

This blog was excerpted from Liquefied Natural Gas: The Fuel of the Future for Shipping and Transport whitepaper.