Certified Fire Protection Specialist (CFPS) Practice

Hydrogen is the fuel gas used in hot work processes that has the lowest neutral flame temperature. The flame temperature of hydrogen, when combusted in air, is considerably lower than that of other common fuel gases such as acetylene, propane, and natural gas.

When using hydrogen as a fuel gas, its combustion characteristics are distinct; while it can produce a very hot flame, the neutral flame temperature, which is defined as the temperature at which the combustion is balanced with equal parts of oxygen and fuel gas, does not reach the same high temperatures achieved with acetylene or propane. This lower neutral flame temperature makes hydrogen less effective for certain applications that require high heat, while still being useful in other applications where a lower flame temperature is sufficient.

In contrast, acetylene is known for its high flame temperature and is ideal for welding and cutting applications. Propane, while also effective, has a higher neutral flame temperature than hydrogen. Natural gas, primarily composed of methane, has a flame temperature that is generally higher than hydrogen as well. Therefore, the distinct properties of hydrogen made it the correct choice for the lowest neutral flame temperature among the listed fuel gases.