What is a common type of fuel used in boilers?

Boilers can utilize various types of fuel, making it essential to understand the common options available. Natural gas is widely used due to its efficiency and cleanliness compared to solid fuels, providing a reliable heat source with lower emissions. Coal has been a traditional fuel, especially in industrial settings, and while its usage has decreased with the rise of cleaner alternatives, it is still relevant in some applications. Electricity is also a viable option; electric boilers are clean, efficient, and preferred in settings where other fuel sources may not be accessible or practical.

Given that all three fuel types—natural gas, coal, and electricity—are actively used in boiler applications, it is accurate to conclude that the correct answer encompasses all these options. Each fuel type has distinct advantages and is selected based on specific requirements such as availability, cost, efficiency, and environmental considerations. This versatility highlights the broad range of fuel types that boilers can operate on, making "all of the above" the correct choice.