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6 Things to Remember about
Hydrogen vs Natural Gas
August 12th, 2021By: Jeff Koestner, P.E., Senior Mechanical
Engineer, POWER Engineers
Thanks to possible federal funding for research and development for hydrogen-
based energy—and the growing awareness that hydrogen could serve as a
dispatchable solution for other renewable intermittency—hydrogen has become a
trending topic in the power industry. Whether we’re discussing using pure
hydrogen or blending with natural gas, it seems like everyone is talking about the
possibilities of using hydrogen to reduce or eliminate carbon emissions into the
earth’s atmosphere.
One of the major benefits of hydrogen is that it can be used with existing
combustion turbines, including ones currently being fueled by natural gas.
Several power plants are under construction right now that will start with a low
hydrogen-to-natural gas ratio and eventually transition to entirely hydrogen,
making the plant carbon emission-free.
But there are some key differences between the two fuels that need to be
considered during design. For ease of understanding, we’ll compare pure
hydrogen to methane, which typically makes up about 90% of a natural gas
mixture.
1. Chemical formula
Let’s start with the basics: the chemical formulas. Hydrogen, a pure element, is -
H2. Methane, on the other hand, is a compound made up of carbon and hydrogen
(CH4.). The absence of carbon in hydrogen is the major driver behind the
hydrogen versus natural gas discussion. When hydrogen and oxygen combine in
combustion, the only byproduct is H 2O—water vapor. When methane burns, the
carbon within the compound combines with oxygen during combustion to create
carbon dioxide, or CO 2.
Bottom Line: Hydrogen combustion doesn’t create carbon emissions, making it a
“clean” alternative to methane.
2. Molecular Weight
The periodic table of elements is ordered by molecular weight. Hydrogen, as the
first element on the periodic table, is a very light molecule. Methane is much
heavier, with a molecular weight of 16. Practically speaking, this difference
means hydrogen is a smaller molecule, which increases the potential for leakage
when using hydrogen as a fuel source. Special consideration needs to be put into
the materials used to reduce leakages through gaskets, valves or any sealing
locations within a compressed hydrogen system.
Bottom Line: Hydrogen is smaller and lighter, meaning it can slip through
cracks methane can’t.
3. Flammability Limit
For combustion to occur, you need fuel, air (oxygen) and an ignition source. The
lower and upper flammability limits represent the percentage of fuel in a fuel and
air mixture that’s required for that mixture to ignite. For hydrogen, the lower and
upper flammability limits are 4% and 75% respectively, as compared to natural
gas at 7% and 20%.
This means that hydrogen will burn with lower amounts of air present and with
higher amounts of air present when compared to natural gas. This wide
flammability range makes controlling the combustion of hydrogen more difficult
than controlling the combustion of natural gas, which has a much narrower range.
Along with flame speed, the flammability limit is among the distinguishing
design issues that arise with the burning of hydrogen. Special considerations must
be taken to ensure your system is properly controlling combustion.
Bottom Line: Hydrogen will combust with both higher and lower concentrations
of air present, making combustion more difficult to control.
4. Flame Speed
Flame speed, in simple terms, is how fast the flame travels from a starting point
through the unburned air and fuel mixture.
Picture pouring a line of gasoline along the ground, then lighting one end. The
flame speed would be represented by how fast the flame runs across the line of
gas on the ground. As you can see from the table above, the flame speed of
hydrogen is almost 10 times that of methane.
Flame speed is one of the more significant design issues when it comes to
hydrogen combustion, as controlling the location of the combustion becomes
more challenging. When hydrogen is burned in a gas turbine combustor, the
flame tends to move upstream of the ideal combustion location, which can cause
an oscillating combustion phenomena or flashback—similar to a backfire you
may have experienced with a car or lawnmower engine. Changes in combustor
design are typically required to manage the flame speed.
Bottom Line: Quick flame speed plus a wider combustion range make hydrogen
more challenging to control.
5. Adiabatic Flame Temperature
Adiabatic flame temperature is the temperature a flame in the combustion process
emits, assuming no heat is lost in the process. Hydrogen’s adiabatic flame
temperature is approximately 500 °F hotter than natural gas. As you might
imagine, not all the equipment or components subjected to the temperatures of
the combustion may be able to withstand that increase in temperature. Therefore,
materials, heat dissipation/cooling requirements and locations of temperature-
sensitive components should be considered.
Another consequence of a higher flame temperature is the potential for an
increased amount of nitrogen oxides, or NOx emissions. During combustion,
flame temperature and the amount of nitrogen in the air are contributing factors to
NOx. Therefore, hydrogen’s increased flame temperature can increase NOx
emissions as compared to burning natural gas.
However, NOx production from combusting hydrogen can be mitigated in most
cases. Modifying the combustion by adjusting air and fuel ratios and controlling
flame hot spots, plus increasing emission treatment in the stack, such as selective
catalytic reduction systems, are a few options.
Bottom Line: Hydrogen burns hotter than natural gas, so be mindful of material
selection, heat dissipation and NOx emissions.
6. Heating Value
One last consideration is heating value. The lower heating value represents how
much energy you can get out of one pound (on a mass basis) or out of one cubic
foot (on a volumetric basis) of fuel. On a BTU/lb basis, Hydrogen has about 2.5
times the energy density of methane. So, if you burn one pound of hydrogen vs
one pound of natural gas, you will get 2.5 times the energy. Sounds great, right?
But because hydrogen is so much lighter, or less dense, you need approximately 3
times the volume of hydrogen as compared to natural gas to get the same amount
of energy. So, to get the same “bang for your buck” out of hydrogen as compared
to natural gas, you would either need to increase the pressure of the fuel supply or
increase the volumetric flow of hydrogen.
Bottom Line: Hydrogen might seem like a bargain in terms of heating value per
pound, but you’ll need to bring in much more volume to get the same amount of
energy as natural gas.
