Which types of fuel can be used in boilers?

Boilers can utilize a variety of fuels to generate heat, and the correct response identifies a range of conventional and commonly used sources. Natural gas is widely preferred in many boiler systems due to its efficiency and relatively lower emissions. Oil is another significant option, used in areas where natural gas is not available. Coal has historically been a major fuel source for boilers, particularly in industrial settings, although its use is more regulated today due to environmental concerns. Lastly, biomass represents an increasingly popular renewable energy source, as it can include various organic materials such as wood chips and agricultural waste.

The combination of these fuels reflects versatility in boiler applications, as they can be selected based on availability, cost, and environmental impact. Other choices may include some less conventional or impractical fuels for standard boiler operations, which reduces their viability as correct answers for this scenario.