Which of the following is not a common boiler fuel type?

The correct response indicates that hydraulic fluid is not a common type of fuel used in boilers. In typical boiler operations, fuels are categorized by their ability to produce energy through combustion or other processes that generate heat.

Natural gas is widely used in industrial and residential heating applications due to its efficiency and lower emissions compared to other fossil fuels. Pellets, particularly wood pellets, are increasingly popular as a renewable energy source and are used in controlled combustion systems. Coal has historically been one of the primary fuel sources for boilers, especially in power generation, despite a shift towards cleaner energy alternatives.

Hydraulic fluid, on the other hand, is specifically designed for use in hydraulic systems where it transmits power through fluid motion, rather than generating heat as a fuel source. Its composition and purpose do not align with the requirements for producing heat in boiler systems, making it an unsuitable choice as a fuel type. Therefore, distinguishing hydraulic fluid from these common boiler fuels clarifies its role and reinforces the understanding of proper fuel categorization in boiler operation.