The Surprising Physical Properties of Hydrogen as a Combustion Fuel

As operators seek to decarbonize their combustion processes, many are looking to use hydrogen as an alternative to natural gas and high hydrogen fuel blends as a fuel source in their existing fired equipment. While hydrogen shows promise to reduce emissions, it is always important to consider safety when changing fuels in any burner application. 

Notably, hydrogen has several physical properties that make it unique and significantly different from methane and other hydrocarbon-based fuels. These properties include:

1. Much smaller mass!  Unlike methane, diatomic hydrogen is a very small molecule with 8 times less mass compared to methane.
2. Lighter and less dense!  As a result of its smaller mass, hydrogen is proportionally 8 times less dense compared to methane. Just as balloons are lighter than air, hydrogen will also rise faster than methane in unmixed processes.
3. Faster!  Because of its low molecular weight, hydrogen also exhibits very fast molecular speeds that are almost 3 times faster than methane at the same temperature. That means hydrogen diffuses nearly 3 times faster compared to methane.

4. Highly flammable!  Methane is flammable with a concentration between 5 – 15% (by volume) in a mixture of air, which is a narrow range of flammability compared to hydrogen, which has a much wider range of flammability between 4 – 75% by volume.

In essence, hydrogen is 8 times lighter, approximately 3 times faster, and more flammable compared to methane!

As fired equipment transitions to hydrogen fuels, operators can leverage flue gas analysis to ensure safe combustion control during start-up and normal operation. Inherently, a hydrogen leak due to a loss of burner flame poses a larger risk, and flue gas analysis provides the critical measurements to detect and alert for unsafe conditions. Combustion analyzers, such as the Thermox WDG-V, also offer multiple indications of safe operation and combustion control, including:

• Excess oxygen (O₂) in the flue gas, which correlates with the burner air-to-fuel ratio. Processes operating too close to 0% excess oxygen (stoichiometric conditions) can be monitored, trended, and corrected to ensure sufficient air in the combustion chamber.
• The combustibles measurement provides ppm-level indication of the combustibles (the combined CO and H₂) in the exhaust gas. These combustibles detectors will respond swiftly to any unburnt H₂ from the burner and provide ppm-level visibility for operators to ensure complete combustion of high hydrogen fuels.
• The methane/hydrocarbons measurement (of the Thermox WDG-V) also uses a catalytic-type detector, which also responds to hydrogen. Operators can use this detector for percent-level monitoring of hydrogen during start-up, light-off, and normal operation and for providing an alert for high levels of hydrogen – possibly due to a burner leak or loss of flame.

When it comes to switching fuels, hydrogen is certainly a clean-burning fuel, but is not without its risks of fast-diffusivity and high flammability.  Operators can leverage flue gas analysis to safeguard against these physical properties of hydrogen, obtain real-time process monitoring, and ensure safe operations – with less emissions.  

To learn how AMETEK Process Instruments can provide the optimal gas analysis solutions for applications that support cleaner energy, check out these resources.