Large commercial vessels primarily burn what’s called Heavy Fuel Oil when out at sea. This fuel is not heavily refined, has high sulfur content and produces a lot of sulfur oxide and nitrogen oxide compounds when it is burned.
Cars burn a highly refined gasoline which produces almost no sulfur oxides or nitrogen oxides. Even trucks burn a highly refined highway diesel which must contain less than 15 parts per million of sulfur, so they too produce very few sulfur oxides and nitrogen oxides. Cars and trucks do produce quite a bit of carbon dioxide and other pollutants.
So yes, the 15 biggest ships produce more sulfur oxide pollutants than all the cars in the world, because they run on completely different fuels. A ship produces more carbon dioxide emission per mile and per gallon of fuel than a car. Ships in general, however, have the lowest emission levels of any other method of cargo transport , producing fewer emissions per ton of freight per mile than barges, trains or trucks.
To do a true comparison of ships and cars, the easiest method would be to look at a Pure Car and Truck Carrier, more affectionately known as a RO/RO (Roll On Roll Off). These ships carry thousands of cars to the U.S., Europe, Asia and elsewhere. A single RO/RO can carry an average of 8,000 cars, trucks and other equipment. Let’s take an average RO/RO shipping 8,000 cars from Hong Kong to Los Angeles. This is a distance of about 7,286 miles. We’ll use an average vehicle fuel economy for the cars of 25 miles per gallon, an average fuel economy of 42 Metric Tonnes per day and an average speed of 25 knots for the RO/RO. Using these rates it would take 2,331,520 gallons or 6507.3 Metric Tonnes of fuel to move those vehicles if you simply drove them (and this doesn’t include the cost and pollution created by 8,000 drivers). The RO/RO however only requires 510 Metric Tonnes of fuel to move all of those vehicles and itself from Hong Kong to Los Angeles. This means the ship only used 7.8% of the total fuel required by all those vehicles for the same distance. I’d say that’s a pretty good savings.
World’s Largest RO/RO Ship, M/V Tønsberg
Several years ago, regulations came into effect limiting the sulfur content of the fuel that commercial ships could burn within an area designated as an Emission Control Area to less then 1% sulfur. At the beginning of 2015, those regulations became even more strict and limited that sulfur content to less than 0.1% sulfur. The entirety of Europe and the United States and their waters out to 200 nautical miles were designated Emission Control Areas as well as several other areas around the world. This means before ships enter the Exclusive Economic Zone of the US or any European countries, they must switch from burning the Heavy Fuel Oil to burning an Ultra Low Sulfur Marine Gas Oil, which is very similar to highway diesel fuel.
Heavy Fuel Oil is very cheap compared to Ultra Low Sulfur Marine Gas Oil and many engines are not designed to handle the ULSMGO because it is so much thinner than HFO it does not have the lubrication properties of the HFO. Companies are using various workarounds to make it work, such as chilling the fuel to increase the viscosity or injecting extra lubricant into certain parts of the engine. Due to the extra costs and possible mechanical issues, these regulations are continuously reevaluated and phased approaches are used for implementation.
The International Maritime Organization, a division of the United Nations which oversees and regulates all international shipping, is continuously pushing commercial shipping to improve and creating stricter environmental laws. Plans are place to reduce the sulfur content allowed in fuels to below the levels required in Emission Control Areas just last year (2014), but this is years away because current technology would make that cost prohibitive for many shipping companies. Currently, sulfur content standards for fuel used in international shipping is set at no more than 3.5%. In 2020, that will drop to no more than 0.5%. All of these regulations are contained in the Convention on Marine Pollution (MARPOL), Annex VI, which sets the regulations for Air Pollution in the Maritime Industry.
Some companies are part of the push, however, and the first two container ships powered exclusively by Liquefied Natural Gas were just launched. These ships will produce fewer emissions, of any compound, than any other vessel currently in service. The entire shipping industry is looking at conversions to natural gas or other fuels, and engine manufacturers are designing engines that can handle a variety of fuels.