Hydrogen is a key part of aviation’s lower-carbon future, with much of the industry counting on it to play a critical role in decarbonising the sector in coming years. But the carbon impact of hydrogen can vary wildly, with the higher-carbon methods of production currently in use producing more than five times the carbon of fossil jet kerosene. Delving into the science, we sat down with Jeremy Leriche, technical director of Engie Solutions Hydrogen, a partner in the HyPort hydrogen station at Airbus’ Toulouse Blagnac site.
“There are two ways to use hydrogen,” Leriche tells us: “burning it, just like kerosene or natural gas, [or] transforming it to electricity, with a fuel cell. In any case, you will not emit any carbon dioxides, but in the case you burn it with air, you will still emit NOx or SOx.”
NOx refers to oxides of nitrogen, such as nitric oxide (NO) and nitrogen dioxide (NO2), while SOx refers to sulphur (sulfur) oxides, with sulphur dioxide (SO2), sulphur trioxide (SO3) and sulphur monoxide (SO) the principal contributors to warming from emissions.
(This article, intended for digital publication, does not use the scientific subscript notation for abbreviations such as NOx for oxides of nitrogen or CO2 for carbon dioxide in order to aim for cross-platform digital formatting compatibility.)
Furthermore, the actual emissions impact of hydrogen depends on the day-to-day industrial processes — energy and feedstocks — used to create it:
- Black hydrogen is produced using bituminous coal, a middle-grade coal currently used in electricity generation as well as steel making.
- Brown hydrogen, meanwhile, uses lignite coal, the lowest-grade of coal with the lowest concentration of carbon, used primarily for electricity generation. (Somewhat confusingly, hydrogen produced through gasification of fossil fuels is sometimes referred to as black or brown hydrogen.)
- Grey hydrogen is fabricated from natural gas using steam methane reformation, and notably its byproduct greenhouse gases (carbon dioxide) are not captured during its manufacture.
- Blue hydrogen, by contrast, does capture the carbon dioxide in steam methane reformation, although critically it requires carbon capture and storage, which will have inherent carbon sunk costs for processing, storage and infrastructure.
- Green hydrogen is essentially the holy grail of hydrogen production, using clean electricity from renewable sources — solar, wind, and so on — to electrolyse water into hydrogen and oxygen. This process is zero-carbon, although there are sunk carbon costs for the infrastructure and activities required to do so.
For the actual day-to-day production processes, Lariche tells us that, measured in kgCO2eq/kgH2 — kilograms of carbon dioxide equivalent per kilogram of hydrogen produced — the various currently used modes create:
- Black: 20 kgCO2eq/kgH2
- Brown: 20 kgCO2eq/kgH2
- Grey: 11 kgCO2eq/kgH2
- Blue: 0 kgCO2eq/kgH2
- Green: 0 kgCO2eq/kgH2
By contrast, for jet A or A1 kerosene, France’s Agency for Ecological Transition, one of the country’s public bodies responsible for environmental statistics, cites the emissions factor per unit mass — the total emissions from upstream production and combustion — as 3.81 kg CO2e, or kilograms per carbon dioxide equivalent.
Other options exist across the hydrogen production spectrum, or will soon: pink hydrogen using nuclear-powered electrolysis, yellow hydrogen using solar-powered electrolysis, or turquoise hydrogen using new and as-yet-unscaled methane pyrolysis methods.
“On the Toulouse Blagnac airport station, the technology is alkaline water electrolysis powered by renewable energy,” Lariche says. “We only produce green hydrogen. We are looking into polymer electrolyte membrane electrolysis and high-temperature electrolysis: that will enhance the performance of electrolysis and produce more hydrogen with the same power.”
Technically, of course, no fuel is entirely zero-emissions when considering its upstream production costs: even the greenest of energy has sunk carbon in the building and maintenance of (say) wind turbines or solar panels, transmission lines, access roads, and downscope emissions, but when operating at scale these impacts are amortised to a point where they are not material to calculations.
In terms of overall impact for aviation, however, Leriche concludes, “the first step is to move towards carbon neutrality by removing the use of fossil energy. You can switch from kerosene to e-kerosene for example. Then you move to zero-emissions by using alternative energy like hydrogen.”
Author: John Walton
Published 30th March 2023
;
