Adapting existing burners, boilers and turbines to hydrogen or hydrogen blends has become a common engineering question. Hydrogen differs markedly from natural gas in a few critical properties, and those differences act directly on design assumptions.
The flame speed is much higher
Hydrogen's laminar flame speed is several times that of methane. The direct consequence:
Flashback risk. In a premixed burner the flame can travel upstream against the flow and enter the mixing region. A mixing-duct velocity that is safe for natural gas may not be safe for hydrogen. This is the most concrete constraint encountered in converting to hydrogen.
The flame temperature is higher
A higher stoichiometric flame temperature means two things: wall heat fluxes rise, and thermal NOx rises exponentially. Leaning out to bring the temperature down is the remedy, but that narrows the stability margin. See the NOx note and the flame stability note.
The density is very low
Carrying the same energy requires a much larger volumetric flow. Existing fuel lines, valves and injectors are sized for natural gas volumes; with hydrogen the same lines are undersized or the pressure loss becomes unacceptable.
Low density also means that in a leak the gas rises rapidly and accumulates at the ceiling — ventilation design has to account for that, and it is a natural convection and dispersion problem.
The radiation character is different
A hydrogen flame produces no soot and water vapour is the dominant emitter. The radiation reaching the wall has a different spectral structure from that of a hydrocarbon flame; a grey-gas approach calibrated for hydrocarbons produces systematic error with hydrogen [1]. See the radiation note.
A modelling warning
Hydrogen's high diffusivity creates preferential diffusion effects: the local mixture fraction departs from what a simple mixture-fraction assumption predicts. Mixing-based combustion models therefore need more care with hydrogen.
References
- Magnussen, B. F. & Hjertager, B. H. (1977). On mathematical modeling of turbulent combustion with special emphasis on soot formation and combustion. Symposium (International) on Combustion, 16(1), 719–729. DOI: 10.1016/S0082-0784(77)80366-4
- Modest, M. F. & Mazumder, S. (2021). Radiative Heat Transfer, 4th ed. Academic Press. ISBN 978-0-323-98406-5.