Page 36 - Energize April 2021

TECHNICAL

Property Hydrogen Methane Natural gas
Specific energy (gravimetric energy density) (MJ/kg) 140 55 42 to 55
3
Volumetric energy density (MJ/m ) 10,8 35,8 36,4
Stoichiometric ratio (volume) 2,4:1 10,42:1 8,43:1
Stoichiometric ratio (mass) 34,33:1 17,14:1 10:1
Flammability range (% vol) 4 to 75 5,3 to 15 5 to 15
Minimum ignition energy (mJ) 0,02 0,29 0,3
Flame temperature in air (°C) 2045 1963 1950
Laminar Flame speed (m/s) 3,06 0,38 0,37
Auto-ignition temperature (°C) 585 580 580
Table 1: Comparative values of hydrogen and other gaseous fuels

Laminar flame speed is the speed at which a flame will propagate temperature decreases, or as the fuel to air ratio decreases. High
laminarly through a quiescent, homogeneous mixture of unburned NOx levels are the result of high temperatures of combustion and
reactants, under adiabatic conditions. Turbulent combustion speed high combustion residence time in the turbine. The higher flame
depends on the characteristics of the combustor, but bears a temperature of H 2 poses a problem for NOX control.
relationship to the laminar speed. Table 1 shows that the Laminar
flame speed of hydrogen is about ten times higher than natural gas, Outlet gas composition
which has a major impact on the design of the combustor. One of the other major concerns is the fact that the outlet gas
consists of water vapour, which has a high specific heat, increasing
Hydrogen fuel challenges the heat transfer to the body of the turbine and resulting in higher
The major problems arising from the use of H 2 as a fuel are fuel flow temperatures of the turbine components. Increased moisture
rate, flashback, combustion pressure fluctuation, and NOx emissions. content also increases the threat of hot corrosion.

Fuel flow rate Hydrogen fuelled turbine developments
The VE of hydrogen (Table 1) is 10,8 MJ/Nm3). In comparison, the The design of gas turbines makes them inherently fuel-flexible,
VED of 100% methane is 35,8 MJ/m . On a volume basis, H 2 has one and they can be configured to operate on H 2 or a mixture of H 2
3
third of the energy density of methane, and it requires a three-times and other fuels as a new unit, or be upgraded to accept H 2 fuels,
greater volume flow of hydrogen to provide the same heat (energy) even after extended service on traditional fuels. The extent of the
input as methane. Operating a gas turbine on 100% hydrogen modifications needed to achieve operation on H 2 depends on the
requires a fuel supply system configured for the required flow rates. initial configuration of the turbine and the overall balance of plant, as
well as the desired H 2 concentration in the fuel.
Leakage There are differences between traditional fuels and H 2 which
Hydrogen is a very small molecule with low viscosity, and therefore should be considered for the proper and safe use of H 2 in a gas
prone to leakage. In a confined space, leaking hydrogen can turbine. In addition to differences in the combustion properties, the
accumulate and reach a flammable concentration. 2 impact on the gas turbine system as well as the overall balance
of plant, must be considered. In a power plant hydrogen-fuelled
Flashback turbine, changes may be needed to the fuel accessories, bottoming
Flashback is the undesirable, upstream propagation of the flame front cycle components and plant safety systems.
from the combustion zone into the premixing sections of a combustor, Existing gas turbines can be modified to run on a high
because of the local turbulent flame speed exceeding the flow velocity percentage of hydrogen gas, with many of the changes affecting the
of the reactants. Flashback can lead to localised flame holding in the combustors, with minor adaptions to the balance of plant.
premixing passages, resulting in overheating and equipment damage.
Burner combustor solutions
Pressure oscillations The combustion process in a gas turbine can be classified as
Large amplitude pressure oscillations at one or more natural diffusion flame combustion or lean-premix staged combustion. In
acoustic modes of a combustor arise from resonant interaction diffusion flame combustion, both fuel and oxidiser are supplied
between oscillatory flow and unsteady heat release processes. to the reaction zone in an unmixed state (Figure 1). The fuel/air
These instabilities can lead to component vibrations, increased
heat transfer rates, flame blow-off and flashback, and can result in
system deterioration or structural damage while constraining the
operating envelope of the turbine. The short combustion time of H 2
increases the probability of pressure oscillation.

NOx levels
NOx is generated by the oxidisation of atmospheric nitrogen in
the combustor flame. Production rates rise exponentially with
increase in temperature, and fall sharply as either the combustion Figure 1: Diffusion flame combustion (GE)

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