Fuel oil

Fuel oil is a category of liquid fuels derived from the distillation of petroleum, also known as crude oil. These fuels are used primarily to generate heat in boilers or to provide power in engines.

While the term is broad, it specifically refers to liquids heavier than gasoline or naphtha. Fuel oil is essential for global industry, powering everything from massive cargo ships to home heating systems. It is classified into different groups based on how heavy or thick the liquid is.

To understand fuel oil, one must understand fractional distillation. This process separates crude oil into different parts by heating it.

Small molecules, such as those found in propane or kerosene, have low boiling points. These molecules evaporate quickly at the start of the process. Heavier petroleum-derived oils, like diesel, are much less volatile. They distill out more slowly because they require more heat to turn into gas. The very heaviest parts, called residues, are what remain after the lighter fractions have been removed.

Fuel oils are often categorized by number, which indicates their properties. Number 1 fuel oil is a light distillate used for jet fuel and high-performance diesel engines. Number 2 fuel oil is a common distillate used for home heating and diesel engines.

Number 4 fuel oil is a commercial heating oil used in burners without preheaters. Number 6 fuel oil is a high-viscosity residual oil. This is the heaviest type and consists of the material left after more valuable cuts have boiled off. As the fuel number increases, the boiling point and viscosity also increase, while the price generally decreases.

Residual fuel oil, often called heavy fuel oil or bunker fuel, has unique physical properties. It is extremely viscous, meaning it is very thick and sticky.

In cooler temperatures, No. 6 oil can even congeal into a tarry, semisolid mass. To make it flow through pumps and burners, it must be heated with special systems. For example, No. 6 oil often requires storage at high temperatures to remain liquid. Because this heating equipment is heavy and takes up space, residual fuel cannot be used in small, fast road vehicles. Instead, it is used by large ships and power plants.

Historically, residual fuel oil played a massive role in global transport and industry. It once powered railroad steam locomotives and large steamships.

Many industrial plants were constructed in the early to mid-20th century specifically to use this cheap, available fuel. However, usage has declined significantly over the decades. In 1973, residual fuel produced 16.8% of electricity in the United States. By 1983, that number fell to 6.2%. Today, all petroleum-based electricity production accounts for only about 3% of total production due to competition with natural gas.

Despite being the cheapest liquid fuel available, residual oil carries significant environmental and maintenance costs. It often contains high amounts of sulfur, sometimes up to 3.5% by weight. When burned, this sulfur forms sulfur dioxide, a pollutant that can cause corrosion in heating systems.

In New York City, a small group of buildings using No. 4 and No. 6 fuel oils was responsible for 86% of the city's building soot in 2011. Additionally, burning fuel oil produces higher carbon dioxide emissions than natural gas. Because of these factors, many regions are phasing out these heavy fuel grades.

The use of fuel oil also has major implications for human health. Air pollution from the shipping industry has been linked to serious respiratory and cardiovascular diseases.

Before new sulfur limits were introduced in 2020, shipping pollution was estimated to cause 400,000 premature deaths annually. It was also linked to 14 million cases of childhood asthma each year. Even after cleaner rules were implemented, it is still estimated to cause 250,000 deaths and 6.4 million asthma cases annually. Countries located on major shipping lanes, such as Taiwan and Japan, face particularly high exposure to these pollutants.