Page 35 - Energize April 2021

TECHNICAL

Gas to power: Hydrogen as a fuel

for gas turbines

by Mike Rycroft, Now Media features writer

There is an increasing interest in the use of hydrogen (H 2 ) for power generation, the use of which
can reduce carbon emissions from fossil fuels in existing gas generation plant and provide an energy

storage solution for power-to-gas-to-power schemes, which would complement the increasing share
of non-dispatchable renewable energy resources.

ydrogen is seen as a means of storing “surplus electricity” accommodate up to 100% hydrogen fuel. Current commercial
from renewable energy sources. Current thinking is that, as versions can handle H 2 up to 50%.
Han alternative to other forms of energy storage, H 2 could be Let us now consider the properties of H 2 and how these differ
produced when surplus wind or solar power is available, and then from those of conventional gas turbine fuels. Table 1 compares
used to generate electricity when needed. significant properties of H 2 and other common gaseous fuels.
The use of H 2 as a fuel has held a fascination for many years,
mainly because the resultant gas from hydrogen combustion • Energy density refers to the energy contained in a unit of
is water, a “clean” substance. H 2 is also storable in bulk, can the gas.
be transported in liquid or compressed gas form, or piped to • Volumetric energy density (VE) refers to the amount of
3
consumers. The aim of using H 2 as a fuel for gas turbines is to energy per unit of volume and is expressed in MJ/m .
reduce CO 2 emissions in the electricity generation industry. • Gravimetric energy density or specific energy (SE) refers
to the amount of energy per unit mass and is expressed in
Power-to-gas-to-power (PGP) MJ/kg.
Perhaps the best place to start would be to put the “green
hydrogen” cycle using gas turbines in context. Table 1 shows the Table 1 shows that H 2 has a higher SE density but a much lower VE
requirements and output of three different sizes of open cycle gas when compared to other gaseous fuels. This affects the flow rate
turbine operating on 100% green hydrogen fuel for a period of 8000 required in the gas turbine.
hr/year, using current electrolyser efficiency figures.
The PGP storage process using green hydrogen and open cycle Stoichiometric ratio (SR) is the exact ratio between air and
gas turbines has a round-trip efficiency of approximately 20%. flammable gas at which complete combustion takes place. A lean
mixture leaves excess oxygen in the exhaust (dilution). A rich mixture
Hydrogen fuelled turbines: Industry experience leaves unburnt gas in the exhaust. Table 1 shows there is a marked
Turbines designed to run on industry off-gases which contain difference between the SR of hydrogen and that of other common
hydrogen have been used successfully for years. The question gaseous fuels. This has an obvious impact on the design of the
that arises with PGP is whether existing turbines can be modified combustion chamber and the controls.
to accept a high percentage of H 2 in the fuel. In addition to pure
hydrogen, the industry is investigating H 2 carrier fuels, such as Flammability range of a gas is the upper and lower limits of the
ammonia, as an alternative fuel. concentration of the gas in an air/gas mixture that will support
All of the major gas turbine manufacturers, including Siemens, combustion. Hydrogen has a very wide flammability range compared
Hitachi, GE, and Ansaldo Energia have set goals of producing to other fuel gases which makes it possible to burn from a very lean
new turbines, or upgrades of existing turbines, which could mixture to a very rich mixture.

Turbine size Yearly output Hydrogen flow rate Electrolysis water Electrolysis surplus (curtailed)
3
(MW) (GWh) (m /h) requirement (m /h) energy requirement (GWh)
3
11 88 11 700 10 500
87 696 78 000 68 3600
240 1920 200 000 174 9400
550 4400 415 000 361 19 500
Table 1: PGP cycle using gas turbines (GE)

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