Liquid Hydrogen and Carbon Emission

The emission of GreenHouse Gases (GHG) has been a key concern in the aviation industry due to the staggering amount of CO2 emission- 915 million tons of CO2 in 2019. 

Hydrocarbon fuels such as kerosene emit water and CO2 as by-products. Any type of hydrocarbon fuel gives out CO2 to the atmosphere, and burning less fuel will only reduce the emission. Modern gas turbine engines are significantly fuel-efficient and burn 80% less fuel compared to the engines in the 60s. It is questionable and less practical to reduce CO2 emission by improving the fuel efficiency, as it could only be an improvement from a few percent no matter how hard the engine is optimized. 

With that said, tackling the CO2 emission solely by efficient engines loses its position in the viable list. Meanwhile, Sustainable Aviation Fuel (SAF) and Liquid Hydrogen (LH2) top the list, as they seem to be the future of aviation fuel. 

Liquid Hydrogen

Here are some plus points for LH2.

• Contains 2.5 times energy in one kilogram compared to the kerosene. 
• As LH2 is not a hydrocarbon fuel with no carbons, zero CO2 emissions! 
• A 90% reduction of emitting nitrogen oxides, which are harmful to the environment. 
• The emission of Sulfur, particulate matter, and oxides of nitrogen will drop drastically. 

Even LH2 comes with some challenges, 

• The volumetric density of LH2 is four times lesser than kerosene fuel. In simple words, one liter of LH2 occupies more space than one liter of kerosene. This requires redesigning of fuel tanks. 
• GreenHouse Gasses have not fled the scene yet. Water vapor, the major by-product of LH2 is a GreenHouse Gas. However, water vapor inherits a lower radiative forcing compared to CO2, which adds some extra points to the LH2 camp. 
• Water vapor emission from LH2 is considered 2.6 times more than the CO2 emission by kerosene fuel. CO2 stays for almost 100 years in the atmosphere and water vapor would only last from a few days to a year. 

Will the Contrails Disappear? 

As LH2 does not emit a great number of solid particles, water vapor in the atmosphere cannot find a host to nucleate. Hence, there will be a reduction of contrails. Due to the abundance of water vapor, rarely available solid particles will turn into large ice crystals, which will have less radiative forcing characteristics. 

Apart From SAF and Hydrogen Fuel

Unarguably, SAF, fuel-efficient engines, and hydrogen fuel will not resolve the matter in near future. These viable options should be accompanied along with other aspects when achieving this common goal. 

Many airlines, including Singapore, initiated carbon-offset programs as passengers are offered the opportunity to nullify their carbon footprints by paying for environmental projects.

ICAO shouldering the responsibility as a regulatory body came up with Doc 9988 assisting its member states with their action plan on reducing carbon emissions.  

LH2 is a viable option as aviation fuel. As there is no such thing as a free lunch, LH2 usage comes with some challenges that require considerable attention by industry experts. Low volumetric density, keeping the production costs affordable, transportation and storing means, and risk associated with LH2 usage will be the key areas that should be addressed in the journey of making liquid hydrogen a standard aviation fuel.