Heating oil is typically black, primarily due to the presence of certain compounds and impurities. During the refining process of crude oil, it is separated into different components, each with different boiling points, hence varying physical properties. Heating oil may contain more heavy oil components, such as asphalt and colloids, which usually reside at the bottom of crude oil and are gradually separated during the refining process.
The color of crude oil is mainly determined by four components: saturated hydrocarbons, aromatic hydrocarbons, colloids, and asphalt. Saturated hydrocarbons are colorless, aromatic hydrocarbons are yellow, while colloids and asphalt are brown or black. Therefore, the darker the color of crude oil, the higher the content of colloids and asphalt, typically indicating poorer quality.
During the production process of fuel oil, black powdery substances may also be added to increase its calorific value and viscosity. When fuel oil is burned, it produces many harmful gases and particulate matter, some of which are visible as black smoke.
Furthermore, the black smoke generated during the combustion of fuel oil is also due to the presence of more difficult-to-burn substances in the fuel oil, such as sulfur, alkanes, aromatics, and other chemicals, which impurities darken the fuel oil.
Therefore, heating oil is black because it contains more heavy oil components and impurities during the production process, which release black smoke when burned, resulting in the color we see in heating oil.