H2Tech - Q3 2021 - 46

MEASUREMENT AND INSTRUMENTATION
CCUS measurement for low-carbon
H2
production
D. ANDERSON, TÜV SÜD National Engineering Laboratory, Glasgow, Scotland, UK
The UK has committed to drastically reducing its emissions
of carbon dioxide (CO2
) and other greenhouse gases and
achieving net zero emissions by 2050. To meet this target, substantial
changes to the ways in which energy is generated, stored,
transported and consumed are required. One potential route to
decarbonization involves the use of hydrogen (H2
vector, to power low-emissions vehicles, for heating homes and
buildings, and for industrial applications.
Decarbonization with H2
and CCUS. The H2
pathway, as it
is cleanis
described in the UK Government's Clean Growth Strategy,
is attractive for several reasons. First and foremost, H2
burning, producing only water as a byproduct, whether it is used
in direct combustion or with fuel cells to generate electricity. It
should be noted that H2
itself is not a primary energy source
(i.e., naturally occurring) and must be produced; however, it
can be considered as a means of storing energy. For the overall
process to be climate neutral, the H2
must be produced with net
zero emissions. This can be achieved in several ways, including
the electrolysis of water, provided the electricity used is generated
from clean sources such as solar or wind. The H2
in this way is often referred to as " green " H2
produced
.
It could be argued that using the renewable electricity direct,
to subsequently convert it back into water. In many cases this
ly would be more energy efficient, rather than producing green
H2
is true, but it is important to consider that the supply of renewable
electricity is generally intermittent. Solar and wind power
are both subject to diurnal and annual fluctuations. At times, the
10,000
1,000
100
10
0.001
0.01
0.1
1
-140
-100
FIG. 1. CO2 phase envelope.
46 Q3 2021 | H2-Tech.com
Sublimation point
at -78.5°C and 1 atm
Critical point
at 31.0°C and 72.8 atm
Triple point at
-56.6°C and 5.11 atm
CO2 gas
-60
-20
Temperature, °C
20
60
100
CO2 solid
CO2 liquid
Supercritical
fluid
) as an energy
supply will be insufficient to meet demand, and during periods
of peak generation there can be an excess, leading to production
being curtailed. Producing H2
via electrolysis would provide an
energy buffer for when demand cannot be met and could even
act as a long-term storage mechanism for energy.
In some processes, it may be preferable to use H2
produced
from renewable electricity rather than to use electricity directly.
Electric battery vehicles are increasingly common and represent
a major step toward the decarbonization of transport. This works
well for light-duty vehicles, but the weight and charging times of
lithium ion batteries are prohibitive for use with heavy-duty vehicles
and long-distance transportation. H2
has a gravimetric energy
density of 140 MJ/kg, which is higher than natural gas (53.6
MJ/kg) and diesel (45.6 MJ/kg), and much higher than lithium
ion batteries (< 5 MJ/kg). However, in volumetric terms, H2
is
the least-dense gas and takes up more space than both natural gas
and diesel. When stored as a compressed gas, the volumetric energy
density of H2
(2.7 MJ/L at 350 bar or 4.7 MJ/L at 700 bar) is
still greater than that of a lithium ion battery (2.2 MJ/L), making
it a serious contender for use with larger vehicles, such as HGVs.
Similarly, for the decarbonization of domestic heating, one
option is to use heat pumps and electric cooking appliances.
However, the replacement of natural gas with H2
in the gas grids
is also being considered in many countries, including the UK.
This could potentially minimize disruption to end users and allow
the gas network infrastructure, and the cumulative skills and
experience of its work force, to be repurposed. It would also negate
the need to upgrade the UK electricity grid to accommodate
the increased electricity generation required. The Clean Growth
Strategy estimates that under the electricity pathway, 647 TWh/
yr of electricity would need to be generated, a 93% increase compared
to the 335 TWh generated in 2018. Under the H2
pathway,
the annual electricity requirement would be similar to today, at
339 TWh. The use of H2
could also decarbonize industrial direct
flame applications, which are essential to provide many chemical
products but cannot be replaced with an electrical equivalent.
A strong case can be made for the use of green H2
bonized energy supply of the future. However, at present, most
H2
in the decaris
not produced from electrolysis but from a chemical process
called reforming, typically either steam methane reforming
(SMR) or autothermal reforming (ATR). In these processes,
methane reacts with high-temperature steam in the presence of
a catalyst and at elevated pressures. Syngas is produced, which is
a mixture of H2
shift reaction is used to convert the CO into CO2
and carbon monoxide (CO), before a water-gas
and more H2
.
Pressure, atm
Solid + gas
Solid + liquid
Liquid + gas
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