The Combustion Properties - and Risks - of High Hydrogen Fuels

As operators look ahead at high hydrogen fuels to decarbonize combustion assets, many are beginning to consider the unique nuances of hydrogen as a fuel. From the view of combustion properties, hydrogen is a very reactive and fast burning gas compared to methane and natural gas variants. While some of these differences may impact the burner directly, others can only be monitored using flue gas analysis to ensure safe and efficient combustion control.

Some of the more notable differences in the combustion properties of hydrogen (vs. methane and other hydrocarbon-based fuels) include:

1. Very fast flame speeds: Driven by its very fast molecular speeds, hydrogen displays extremely fast flame speeds that are 10 times faster than the flame speed of methane.
2. Compact, shorter flames: In addition to its fast flame speeds, the flame envelope of hydrogen is also much shorter and hotter-burning compared to methane. Shorter flames create a higher risk of localized, internal hot spots that could cause higher NOx emissions. These hotter localized flame zones could also impact any low-temperature metal parts used within or near the burner throat.
3. Hotter flames: Hydrogen has an adiabatic flame temperature of 2,254°C, which is approximately 15% hotter than the flames of methane at 1,963°C.
4. Very reactive to minor sparks: Hydrogen has an extremely low minimum pre-ignition energy threshold that is almost 10 times lower than methane. This low threshold to ignition energy poses a risk of flashback if run at high enough concentrations, especially in premix burners.

From an operational standpoint, use of high hydrogen fuels also has combustion-related implications at the burner and in the combustion control system. In particular, the fuel flow rate of hydrogen fuel will change considerably if measuring the flow rate by mass or by volume, but the list also includes:

1. Lower volumetric heating value: If the plant measures flow rate by volume, three times more hydrogen (by volume) is needed to achieve the same heat release compared to methane at the same pressure. Piping and flow controllers should be evaluated accordingly to handle the required flow of hydrogen fuel.
2. Much higher heating value on mass basis: If the plant measures flow rate by mass, then the hydrogen mass flow rate will be three times less compared to the mass flow of methane for the same heat release (at the same pressure) because of the differences in heating value. Again, piping and flow controllers should be evaluated accordingly to handle the required flow of hydrogen fuel.
3. Risk of embrittlement: Piping and burner nozzles sizes should be evaluated to ensure they can handle hydrogen service and will avoid material embrittlement.
4. Lower air consumption: In addition, hydrogen requires 20% less combustion air compared to methane to achieve the same heat release. So, more combustion air will need to be added at the burner to pre-empt a switch from hydrogen fuel back to methane fuel.

While burner adjustments can be fixed when permanently switching to a new fuel, these differences in heating value and combustion air requirements have implications if the hydrogen content of the fuel varies over time and/or if the burner switches from natural gas to a high hydrogen fuel, as may be the case in using natural gas during light-off and then switching to hydrogen fuels during normal operation. For this reason, it is vital that operators monitor the combustion process and ensure the burner always has adequate combustion air, and flue gas analysis is one approach to safe monitoring of these burner-related parameters.

As one safeguard using flue gas analysis, combustion analyzers, such as the Thermox WDG-V, provide multiple indications of safe operation and combustion control to ensure that the combustion process is operating effectively, efficiently, and predictably. Relevant operational measurements include:

• The excess oxygen (O₂) measurement 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. Any switching between hydrogen and methane would require this measurement to ensure adequate air is always provided to the burner.
• The combustibles measurement provides ppm-level indication of the combustibles (or rather, 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. Coupling this measurement with the excess oxygen measurement allows operators to see trends in increasing levels of combustibles and allow them to make swift changes to the combustion air.
• 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 very high levels of hydrogen – possibly due to a burner leak or loss of flame.

When it comes to switching to high-hydrogen fuels, operators can leverage flue gas analysis to safeguard against the unique combustion and operational implications of hydrogen, obtain real-time process monitoring, and ensure safe operations – with less emissions.  

To learn more about the combustion of high hydrogen fuels, watch our recent webinar: Flue Gas Analyzers for Safe Combustion of High Hydrogen Fuels.