Bunker fuel, often called heavy fuel oil, is the thick, residual petroleum product used to power the vast majority of the world’s large commercial ships. This fuel is fundamental to global trade, moving approximately 80% of all goods across the oceans. However, its widespread use comes at a steep environmental cost. Burning this fuel releases a highly polluting cocktail of substances into the air and water, establishing it as one of the dirtiest and most environmentally damaging fuels in common use today.
The Composition of Residual Fuel
Bunker fuel is essentially the leftover residue from the crude oil refining process after the more valuable, lighter fractions like gasoline and diesel have been extracted. A defining characteristic of this fuel is its extremely high sulfur content, which historically could be up to 3.5% by mass, far exceeding the limits for road transport fuels. This residual fuel also contains significant levels of heavy metals and other contaminants that pose environmental hazards upon combustion. When this viscous, tar-like substance is burned in marine engines, these concentrated impurities, such as vanadium and nickel, are released as highly toxic components of the ship’s exhaust.
Atmospheric Pollution and Acid Rain
The combustion of high-sulfur bunker fuel is a major source of atmospheric pollution, impacting air quality far beyond the open ocean. The sulfur content in the fuel converts primarily into Sulfur Oxides (\(\text{SO}_x\)), which are reactive gases. Once released, these \(\text{SO}_x\) compounds react with water vapor and other atmospheric components to form sulfuric acid, the main ingredient in acid rain. Emissions also produce substantial amounts of Nitrogen Oxides (\(\text{NO}_x\)), which contribute to the formation of ground-level ozone and smog, particularly in coastal areas. Furthermore, the global shipping fleet is a significant contributor to climate change, accounting for approximately 2% to 3% of the world’s total energy-related carbon dioxide (\(\text{CO}_2\)) emissions.
Direct Health Consequences of Emissions
The most direct consequence of burning bunker fuel is the generation of fine Particulate Matter (\(\text{PM}_{2.5}\)), a major public health hazard. \(\text{PM}_{2.5}\) refers to airborne particles smaller than 2.5 micrometers, which are small enough to bypass the body’s natural defenses when inhaled. The toxic components of the exhaust, including heavy metals and unburned hydrocarbons, are often carried on the surface of these fine particles. Once in the body, \(\text{PM}_{2.5}\) triggers oxidative stress and inflammatory responses, mechanisms linked to cardiovascular and respiratory diseases. Short-term exposure can exacerbate existing conditions like asthma, while long-term exposure is associated with increased risk of heart attacks, strokes, and premature death.
Contamination of Marine Environments
Beyond atmospheric pollution, the use of heavy bunker fuel presents two distinct threats to the marine environment: catastrophic spills and chronic operational discharges. Due to its high viscosity and density, heavy fuel oil is particularly difficult to clean up if a spill occurs, often sinking or coating shorelines and wildlife with a thick, tar-like substance that causes long-lasting ecological damage. A more chronic contamination issue arises from exhaust gas cleaning systems, known as scrubbers, which many ships install to comply with air pollution regulations. Open-loop scrubbers use seawater to “wash” sulfur compounds out of the exhaust gas, resulting in the discharge of large volumes of acidic, contaminated wastewater directly into the ocean. This washwater contains heavy metals and other toxins, impacting local water quality and harming sensitive marine life.