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net-zero carbon emissions.

“For the direct combustion approach, hydro-
gen is injected and ignited in a modified
turbofan’s combustor to generate thrust – a
process which is identical to that in tradi-
tional jet engines. With hydrogen replacing
kerosene, the direct combustion products
are water vapour (H2O) + energy, plus some
trace gases. The upside is that CO2 and the
majority of soot emissions are eliminated,”
said Llewellyn.

NOx of the Matter
The challenge however is to significantly hydrogen combustion by-products to increase the certainty of
reduce NOx and persistent contrails and to these predictions.
this end, Airbus is working with its technology
partners to eliminate this potential climate
impact of hydrogen combustion. Final Effect

On water vapour, IATA says “although the But the critical question is what will be the overall impact
radiative forcing (difference between the on reducing emissions to zero by 2050? In a white paper,
energy absorbed through the Earth’s atmo- Performance Analysis of Evolutionary Hydrogen-Powered Aircraft
sphere compared to the energy that is by Jayant Mukhopadhaya, Ph.D. and Dan Rutherford, Ph.D. for
reflected back into space) is lower than that the International Council on Clean Transportation (ICCT), it was
of CO2, it still contributes towards global found that overall LH2-powered aircraft entering service in 2035
warming. Hydrogen combustion would pro- could contribute
duce about 2.6 times more water vapor than to aviation’s 2050 climate goals but with performance penalties
kerosene fuel.” relative to fossil-fuel aircraft.
IATA quotes a study entitled “Contrails in The authors said: “Compared to fossil-fuel aircraft, LH2-powered
a comprehensive global climate model: aircraft will be heavier, with an increased maximum take-off mass
Parameterization and radiative forc- (MTOM), and less efficient, with a higher energy requirement per
ing results” by Michael Ponater, Susanne revenue-passenger-kilometre (MJ/RPK). They will also have a
Marquart, Robert Sausen, which found that shorter range than fossil-fuel aircraft.”
“the positive effects of a zero-CO2 combus-
tion, would offset the drawback of increased Nevertheless, they added “we estimate that evolutionary LH2-
water vapor exhaust. Moreover, CO2 has a powered narrow-body aircraft could transport 165 passengers up
lifetime in the atmosphere of up to 100 years, to 3,400 km and LH2-powered turboprop aircraft could transport
while that of water vapor can go from a few 70 passengers up to 1,400 km. Together, they could service about
days up to 1 year. Regarding condensation one-third (31 to 38 per cent) of all passenger aviation traffic,
trails, due to the absence in solid particles at as measured by revenue passenger kilometres. This represents
the exhaust of the engine when burning hydro- 57 per cent to 71 percent of all RPKs serviced by narrow-body
gen, ice crystals have no-where to nucleate, aircraft and 89 per cent to 97 per cent of all RPKs serviced by
so the number of water crystals formed at the turboprops.”
exhaust would decrease. Nevertheless, due to
the increased amount of water vapor exhaust, The white paper found that under the most optimistic fuel and
the crystals that do nucleate, would have a fleet turnover assumptions, evolutionary LH2-powered aircraft
larger size. The overall effect is expected to could cap, but not absolutely reduce, passenger aviation CO2
decrease the radiative forcing effect of con- compared to 2035 levels. It said that this would require all
trails. The end result, according to the study replaceable missions in 2050 to be serviced by LH2-powered
would mean that the radiative forcing from aircraft using green hydrogen and would result in mitigation of
aviation could be 20 to 30 per cent lower by 628 Mt-CO2e in 2050, representing 31 per cent of passenger
2050 and 50 to 60 per cent by 2100 if LH2 aviation’s CO2e emissions.
aircraft were introduced.” However, it warned that modelling suggests that “a 20 per cent

However, it warns that the effects of cirrus-in- to 40 per cent adoption rate is realistically achievable
duced clouds will need careful analysis, and and would mitigate 126 to 251 Mt-CO2e in 2050, representing 6
more research into the radiative forcing of per cent to 12 per cent of passenger aviation’s CO2e emissions.”

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