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Is Hydrogen The Best Option To
Replace Natural Gas In The Home? Looking At The Numbers
There’s been a lot of talk recently about hydrogen as a replacement
for natural gas. The scheme is to gradually add H2 to the natural gas
grid, with the H2 being made from water using “excess” renewable
electricity when it’s available.
By
Paul Martin,
Chemical process development expert
There’s been a lot of talk recently about
hydrogen as a replacement for natural gas. The scheme is to gradually
add H2 to the natural gas grid, with the H2 being made from water
using “excess” renewable electricity when it’s available. But,
ultimately, there are people who think we should have pure hydrogen
supplied to our homes instead of natural gas, using the same piping
and distribution network that we have now. In their minds, all we’d
have to do is to re-jet all our boilers, furnaces, stove cooktops, and
ovens and we’ll be away to the races. No need to abandon all that
expensive capital — we’ll just change the fuel! We’ll be burning
colourless, odourless hydrogen, making only water vapour, and global
warming will be one step closer to being solved.
Sounds great! Where do I sign?
Hold on — not so fast!
Replacing Gas With Hydrogen is An Inefficient Use of Energy
The first and most obvious criticism of
this scheme is efficiency. It doesn’t matter if you start with natural
gas or electricity, the best you can do is to convert about 70% of the
feed energy — lower heating value (LHV) of methane, or kWh of
electricity — into LHV of product hydrogen. Best case. If the
alternative is to use natural gas or electricity directly, hydrogen
brings nothing but loss to that equation.
Obviously, the whole idea here is to
eliminate the fossil greenhouse gas (GHG) emissions associated with
the burning that’s happening at your end of their pipe. Hydrogen
offers the option to do that. You can start with bio-methane from
anaerobic digestion, so the CO2 you emit when you make hydrogen is
just part of the natural carbon cycle. Or you can capture all or part
of the CO2 produced when making hydrogen from fossil natural gas at
the hydrogen plant, or by pyrolyzing the methane and selling carbon as
a byproduct for uses other than burning, or you can avoid the CO2
entirely by making the electricity you feed your electrolyzer from
wind or solar, nuclear, hydro, geothermal etc. These are all ways by
which you could end up with a fossil GHG emission-free fuel for your
burner — ideally that is, assuming you could afford it.
You could of course feed the grid with
methane from biogas instead — but while I’m convinced biogas will be
an important fuel for those fuel uses we really do need in a
post-fossil future, nobody should try to convince you that there will
be enough biogas EVER to just replace existing natural gas supplies —
or even a small fraction of those supplies. So if you want to keep
your burners, and not emit fossil GHGs, hydrogen seems like your only
option. And that’s exactly what the natural gas industry is telling
governments all over the world.
Of course, these gas companies and
electrolyzer suppliers are not giving their advice without
self-interest in mind. They are starting from the position that they
need to stay in business, and you need to keep your burners — fair
enough! The obvious alternative is to replace your burners directly
with electricity and cut out the lossy hydrogen middleman, but that
would leave them out of business. For home heating, and even for
domestic hot water, a heat pump will not only save you the 30%
conversion loss to hydrogen, it will also give you about 3 kWh worth
of heat for every kWh worth of electricity you feed. Far, far more
efficient. But not cheap — the heat pump is going to cost you quite a
few dollars — and while renewable electricity is getting cheaper by
the day, grid electricity still sells at a large multiple of the cost
of natural gas per unit of energy — because carbon taxes are
inadequate, and because in some places, fossil fuels still power the
grid.
For your cooktop, an
induction heater will give you even
better performance than a flame — you may have to throw out a few
of your old aluminum pots and pans, but otherwise you’ll likely be
very happy with that change. And your oven will do nicely with a plain
old resistance heater — with much better temperature control.
Remind me what we need a fuel gas for
again, exactly? I know only one answer to that — right now, natural
gas is a very, very cheap fuel IF you ignore the fossil GHG emissions
from both its production and distribution and burning. Displacing
natural gas use from home heating is going to be a tough struggle
regardless of how we do it — because the alternatives are going to
cost more, at least initially.
Hydrogen, on the other hand, isn’t a
cheap fuel, period. And it should be obvious that it can NEVER be as
cheap as either the natural gas or the electricity from which it is
made.
Hydrogen Distribution is Lossy and Expensive
Even assuming that you were so
nostalgically attached to your gas appliances that you couldn’t part
with them, the gas industry would still need to overcome some serious
problems that aren’t being discussed before hydrogen starts flowing
through the natural gas grid.
If we’re going to make hydrogen, whether
it’s “blue” hydrogen made from natural gas with carbon capture and
storage, or “green” hydrogen made from water using renewable
electricity, it still has to get from where it’s made to your house.
And it’s not as simple as just changing what flows through the pipes.
Compression — the Deal Killer
To move any gas economically, it needs to
be compressed. And it turns out this is the big problem with hydrogen
distribution — it’s the reason that 85% of hydrogen produced in
Europe, for instance, travels basically no distance to where it’s
consumed, because it’s made right on the same site or right next door.
Natural gas is about 8.5 times as dense
as hydrogen, and dense gases are easier (more energy efficient) to
move than less dense ones. Hydrogen partially makes up for that fact
by being more energy dense per unit mass — about 3 times as much as
natural gas. But, sadly, the work (mechanical energy) needed to drive
a compressor is related linearly to the number of moles of gas we
compress, rather than to their mass or volume per se. It also depends,
more weakly and in a more complex way, on the ratio of specific heats
of the gas — which, as it turns out, makes a minor difference (in
favour of natural gas), which increases with increasing compression
ratio. But when we compare the LHV of hydrogen per mole to the LHV of
natural gas per mole, we find that natural gas is about 2.9 times as
energy dense in molar units. Another way to put it is that it takes
about three times as much energy to compress
a MJ’s worth of heat energy if you supply it as hydrogen than if you
supply it as natural gas. And this, folks, at least in part, explains
why we don’t move hydrogen around much by pipeline. Instead, we move
natural gas to where hydrogen is needed, and build a hydrogen plant
there. (See the end of the article for the proof.)
That 3× increase in the work of
compression not only costs energy, it would also cost a gas utility
big money, since it would mean that every compressor in their network
would need to be replaced with a new unit with 3× as much power, and
also physically larger — with 3× the suction displacement. And since
hydrogen is so notoriously leaky, the hydrogen volumetric flowrate is
higher for a given heat flow in the pipeline etc., the compressors
would need to be totally different machines — considerably more
expensive ones.
Hydrogen is, already, round numbers,
about 37% best case in cycle efficiency when starting and ending with
electricity. Whereas natural gas and electricity are roughly the same
cost and efficiency to distribute on a per unit energy basis, hydrogen
is going to cost about 3× what natural gas costs in lost energy, just
to move the gas. And since the downstream equipment is only 50–60%
efficient at producing electricity again, you’re going to have to move
roughly twice as much hydrogen energy to destination to do
the same job as if you moved electricity instead. That’s forgetting
about the extra capital cost that would also need to be spent.
Pressure Drop in Piping — A Wash
You’d think that you’d suffer an
additional penalty moving hydrogen through piping once you’d gotten it
up to the desired pressure — that was certainly my first impression.
But as it turns out, the answer to that question is quite complex, and
it depends on what conditions you run the calculations at. Hydrogen is
less dense, less viscous, and more energy dense per unit mass than
natural gas. But when you run the pressure drop calculations at the
sorts of velocities and pressure drops used in pipelines which carry
gases long distances (where pressure drops are on the order of 5 psi
per mile of pipe, rather than the 5 psi per 100 ft of pipe that might
be typical in a chemical plant’s piping), hydrogen and natural gas
come out nearly even at a given rate of LHV heat delivered per hour
down a pipe of given size.
That does change at different points in
the distribution system, and to a 1st approximation, the average works
out to an existing gas pipe being able to carry about 90% of
the energy in the form of hydrogen that it could carry if it were fed
the average natural gas it was designed for. The velocity
will be about three times higher, but the density is 1/8.5× as much,
and together with the modestly lower viscosity, the factors nearly
cancel one another out. However, since every kWh of energy lost due to
friction in the pipeline has to come from a compressor, that still
means that hydrogen costs about 3× as much per unit of energy to move
from source to destination in a pipeline.
“Line Pack” — What’s That? Another Problem…
As I promise my readers, I edit my
articles when they teach me new things or point out my mistakes. And a
knowledgeable connection brought to my attention this rather major
problem that is a result of hydrogen’s lower energy density per unit
volume. “Line pack” is the name given to the amount of natural gas
stored in the piping distribution system itself. And unless we
increase the pressure of the distribution system — which we cannot do
without new pipe — we will lose that storage. A typical gas system
apparently can handle about 3–4 hours of average demand just using
stored gas in the lines. Pure hydrogen, being 1/3 as dense in energy
per unit volume, would reduce that to ~1 hour. That could mean a giant
difference in distribution system reliability, the frequency and
duration of outages, and the ability of the grid as it exists to
handle variations in demand — the big spike when everybody gets home,
cranks up their furnaces or boilers, and turns on their cooktops for
instance.
I’m already aware that sometimes
subdivisions outgrow the rate at which the gas utilities can install
new lines to them. Accordingly, some utilities evaporate liquid
natural gas from tanks into points downstream of the “bottleneck” in
order to keep the furnaces and cooktops humming through peak hours.
Doing that with hydrogen would be very expensive and very dangerous,
given that liquid hydrogen takes about 40% of the energy in the
hydrogen just to liquefy it, boils at 24 Kelvin (24 degrees above
absolute zero — liquid methane boils at a balmy 112 Kelvin or -161°C)
— and as a liquid it is still only 71 kg/m3 — methane is about 600
kg/m3 in comparison as a liquid.
Piping and Equipment
If you don’t heat it up too much,
hydrogen is quite safe to carry in mild steel piping
— even up to fairly significant pressures. The much talked about
“hydrogen embrittlement” isn’t a factor for soft mild steel or low
alloy steel piping, such as what is used in most chemical plant
piping.
However, natural gas pipelines —
particularly the pipelines carrying natural gas long distances or
underwater — are not made from mild steels. They’re made from harder,
strong steels — and those steels are, according to many reports,
susceptible to hydrogen embrittlement or other hydrogen-related damage
mechanisms, particularly in their welds and heat-affected zones — even
at fairly modest pressures and temperatures.
According to credible reports written by
natural gas distribution utilities themselves, such as
this excellent one, most of the high and medium pressure natural
gas distribution system would need to be totally replaced to handle
pure hydrogen. (See p.12 of that reference, where it says this in as
many words — and these guys, who own the pipes, should know best!)
That’s a massive cost — especially to spend on a change to a fuel
which might be better replaced with electricity anyway.
Note that hydrogen damage and hydrogen
embrittlement are complex metallurgical topics, and that nascent
hydrogen (hydrogen atoms generated by electrochemical action such as
during corrosion) causes damage that molecular hydrogen cannot until a
combination of high pressure and high temperature make that possible.
But the reports about H2 compatibility problems with pipelines used
for natural gas is quite well demonstrated, by people who know this
issue far better than I do.
The low pressure distribution system is
mostly made up of low carbon steel and HDPE pipe, and you can run
hydrogen through that easily enough. However, piping designed not to
leak natural gas can leak a lot of hydrogen due to hydrogen’s low
density and high diffusivity. And, sadly, stenching agents such as the
thiols (mercaptans) used in natural gas to help detect leaks are not
compatible with hydrogen, and especially not with hydrogen to be used
to feed PEM fuel cells such as those used in vehicles. The catalysts
in those fuel cells are extremely sensitive to sulphur compounds like
that. Given hydrogen’s extremely wide explosive range — any mixture
between 4% and 75% hydrogen in air is explosive — the lack of a
stenching agent to help you detect leaks seems a very challenging
problem for distribution of this fuel to homes and businesses.
Hydrogen/Natural Gas Mixtures
The initial projects all try to smooth
over these problems by mixing a little H2 into natural gas instead of
making the big leap to pure hydrogen. And when you hear about
“replacing 20% of natural gas with hydrogen,” you’d think that would
make a big difference!
Think again.
A 20% mixture of H2 in natural gas is a
20% mixture by volume. That mixture has only 86% of the energy of an
average natural gas, meaning that you’d have to burn 14% more
volume of gas to make the same number of joules or BTU
of heat. The savings in GHG emissions are nowhere nearly 20% — they’re
closer to 6% just looking at the burning, and less than that when you
consider the compression and pressure loss noted above. Such a
reduction would already cause heat content sensitive users to scream,
so forget about going to 30% H2! For a given amount of energy
delivered, a 20% mixture of hydrogen in natural gas would take 13%
more energy to compress and would lose about 10% more pressure per
unit length of pipe than if you were to stick with natural gas —
because the gas has to flow faster, and yet isn’t sufficiently lower
in density to compensate. Those factors would eat some of your GHG
emission savings. And while industrial users would be protected — they
pay per BTU or joule of LHV or HHV they are delivered by the gas
company — some users could be shortchanged since they pay per unit
volume instead.
But What About “Hard to Decarbonize Industries?”
Another excuse we hear for the need for
hydrogen to replace natural gas is for “high temperature industrial
heating.” For some reason, people just seem to assume that because we
run some equipment right now by burning fuels, we cannot instead use
electricity. The examples of steel and cement-making are frequently
brought up, but there are many others.
Here I have to bring in what I do for a
living. I design and build pilot plants, which are prototype units to
test new chemical processes. These plants can vary from tiny lab units
to quite large facilities that would look to the average person like
any other real chemical plant. But the one thing that a pilot plant
will almost entirely without exception be missing is any
fired equipment. There are exceptions, but aside from
the function of disposing of waste streams of combustible materials,
every function that is accomplished on a commercial chemical plant
using fired equipment is done using electricity instead on a pilot
plant. Why is that? Many reasons:
1)
Electricity is far safer and easier to control than fire, particularly
at the small scale. Electric heating provides rapid, accurate control
and reduces hot spots, reduces risks to materials of construction,
etc.
2)
Electricity costs more than fuel as a heat source, but the energy cost
of a pilot plant is seldom the most important factor to its operators.
3) Fired
heaters generally need air emissions permits and may require stack gas
testing — costs which the pilot plant avoids by using electric
heating.
4) To heat a
stream to high temperatures using a burner, you are left with a high
temperature flue gas exiting the unit. Chemical plants make use of
that hot flue gas to heat up numerous other streams to keep it from
going to waste — or use it to make steam to drive equipment or keep
things hot. On a pilot plant, it is just not worth the trouble of
doing that kind of heat integration
5) Fired
equipment is more expensive than electrically heated equipment
6) When you
need the highest temperatures, sometimes electric heating is the only
feasible option.
In steelmaking, the real need for
hydrogen isn’t for heating at all — electric arc furnaces for
steelmaking are already quite popular. Hydrogen is needed to replace
the chemical reductant carbon monoxide made from coal coke, which is
used to reduce iron oxide to iron metal. There are direct
electrochemical reduction methods also under development, so it’s
possible we could also make steel without using hydrogen at all.
In many other applications, electric
heating could easily be used to eliminate the need to burn fuels. It
would, however, require modification to major pieces of equipment,
which might have a considerable cost. But if the alternative is to
spend a multiple of that cost on hydrogen made from electricity, that
savings can pay for quite a bit of capital.
In fact, if approached with a fresh sheet
of paper and without a firebox on your head, most applications in
industrial heating currently served with fire for cost reasons
(because fuels are cheaper, as long as you can dump fossil CO2 to the
atmosphere) could easily be converted to electric heating instead.
All we really need is to price fossil
carbon emissions at a rate high enough — and durably enough — to make
the associated capital investments worthwhile in economic terms for
the affected industries.
Hydrogen for Seasonal Energy Storage
Another argument that I frequently hear
is that because of the double whammy of greater energy need for
heating and lower solar power production in winter, we’ll need
hydrogen to make up the shortfall. We’ll need to make vast quantities
of hydrogen in summer, and store it in salt caverns until winter.
While stored fuels of some kind are likely a useful part of an
emergency response plan in any post-fossil fuelled future, it is to me
a non sequitur that just because it’s possible to use hydrogen for
this purpose that doing so would actually make energetic or economic
sense. Methane, whether from biogas or even fossil natural gas, seems
a more logical choice as a gas to store, given that we already have
strategic and emergency stores of natural gas in place. And we could
just as easily store up a year’s worth of biogas methane as we could
find a way to make hydrogen in excess in summer.
Green hydrogen’s chief economic problem
as an energy storage medium is the cost of electrolyzers and storage
equipment — and as we’ve seen in this paper, distribution cost isn’t
going to be as low as some expect either. Multiplying the low capacity
factor of a wind or solar production unit by another seasonal capacity
factor of say 0.5 or less, doesn’t add up to a low capital cost per kg
of hydrogen stored. This stored fuel would be very expensive indeed,
even if the power itself were quite cheap.
Why Are We Doing This Again?
In summary, it seems to me quite clear
that hydrogen’s role as a replacement for natural gas has more to do
with a need for gas production and distribution companies to stay in
business by having something to sell than any real GHG emissions
benefit or significant technical need. And if they want to make the
necessary investments entirely on their own nickel, to provide truly
green or even “blue” hydrogen via an upgraded network to replace
natural gas, perhaps that’s okay with me. Sadly, it seems quite clear
that their caps are in hand, reaching out to the public sector to fund
the necessary infrastructure investments. Personally, my thinking is
that this would be throwing good money after bad.
DISCLAIMER: These are my personal
opinions, informed by my knowledge and practice of chemical
engineering over the past 30 yrs. My opinions are my own, and are not
to be confused with those of my employer Zeton Inc. nor of its
customers. They are motivated only by a sincere desire to get us off
fossil fuels, and by so doing, eliminate fossil GHG and toxic
emissions associated with burning them, for as low a cost and impact
on society as we can manage. My comments are not motivated in any way
on behalf of personal financial interests on my part or on the part of
my employer or its customers. Every article I write is likely to make
one or another of my customers angry — you can rest assured of that!
I have made my best effort to be accurate
in what I’ve said, doing my own confirmatory calculations. I can
provide background on those to anyone who asks. But I’m human, and
hence prone to error. I also don’t for a moment claim to know
everything there is to know about this subject matter, which is where
some people have spent their entire careers. If you can show me where
I’ve gone wrong in my analysis or calculations, with references or
dependable examples, I’ll gratefully edit my piece to reflect these
new learnings on my part.
Here are a few of my other articles that
you may find relevant and interesting:
Hydrogen From Renewable Electricity — Our Future?
Mirai FCEV vs Model 3 BEV
APPENDIX:
Here’s the abbreviated logic behind why
it takes 3× as much compressor energy to move a given amount of H2 LHV
as to move the same number of J or BTU of natural gas LHV.
Where a and b are constants, different
for each gas, but only a little different between H2 and natural gas,
and r is the compression ratio i.e. P2/P1, P1 is the initial absolute
pressure and V1 is the initial volume, the work of adiabatic
compression is given by a formula of the following form:
W = a P1V1
(1-r(1/r)^b)
Per the ideal gas law, P1V1 = nRT1, where
n is the number of moles of gas, R is the ideal gas constant, and T1
is the initial temperature.
Taking gases 1 and 2 of nearly equal
values of a and b (to avoid getting results which vary with r), and
taking them at the same initial pressure, volume and temperature, it
can be shown that:
W1/W2 = ~ n1/n2
Hydrogen has a molar LHV of 240 kJ/mol,
and a middle of the road natural gas might have a LHV of 695 kJ/mol.
The work ratio is therefore ~2.9:1 for hydrogen versus natural gas, if
we were to move a constant number of kJ of LHV per compression stroke,
or per unit time.
The actual values of a and b (related to
the Cp/Cv ratio) for H2 and natural gas at commercially significant
compression ratios adjust this 2.9:1 ratio to about 3:1.
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