Hydrogen is not a new fuel source: its characteristics as a potent and light energy carrier are well-established. Even before the first 19th century power grids, hydrogen was used for small scale lighting and early aeronautics.
Today, hydrogen is used widely as a feedstock for ammonia production (mainly for fertilizers), for refining and for other chemical processes. In 2018, the global market for pure hydrogen was app. 74Mt. Citation IEA 2019, The Future of Hydrogen However, with future demand for hydrogen and hydrogen-based e-fuels to replace fossils, this is projected to increase by a factor of 10 or more towards 2050. Citation Energy Transitions Commission 2020, Making Mission Possible: Delivering a Net-Zero Economy Current production of hydrogen is emissions-intensive as it derives from fossil fuels. Steam reforming of natural gas, for example, involves producing hydrogen from methane while the carbon is released as CO2. At present, fossil hydrogen production alone accounts for about 6% of global natural gas and 2% of global coal consumption, and approx. 2% of all global energy related GHG emissions is from production of hydrogen.
Renewable hydrogen is completely carbon free. It is produced by using renewable energy to split water into its elements in an electrolyser, with no GHG emissions as a byproduct.
Therefore, it can be swapped on a like-for-like basis with fossil hydrogen to deliver large emission reductions in chemical and other heavy industries in the immediate future.
Hydrogen has low volumetric energy density which makes it a challenging energy carrier for transportation – but in combination with either carbon or nitrogen, it can form e-fuels.
The density of e-fuels make them more suited to decarbonizing heavy transport – but how can they be made?
Renewable hydrogen, combined with carbon produces renewable methanol or kerosene. Sustainable, i.e. nonfossil, carbon can be sourced through carbon capture from a high concentration renewable source e.g. from gasification or burning of sustainably sourced biomass, bio-waste or biogas production. It could also be obtained from atmospheric CO2 through direct air capture, but this technology is still experimental and too costly in the nearterm future.
Combined with nitrogen, which makes up about 78% of our atmosphere, renewable hydrogen can react to form renewable ammonia. These energy carriers offer significantly higher volumetric energy density compared to pure hydrogen.
As costs are reduced, synthesised e-fuels is a possible pathway for net-zero emissions deep-sea shipping and aviation, based on engines and fuel infrastructure very similar to what is used today.
E-fuels are less energy efficient than hydrogen which, in turn, is less energy efficient than electrification: so decarbonization pathways must focus on electrification first, renewable hydrogen second and thirdly using e-fuels in those sectors which cannot be directly electrified.
Examples of hydrogen production methods
There are many methods for producing hydrogen at industrial scale. Most common today is steam reforming of fossil feedstocks, where natural gas reacts with steam under high temperatures and pressure, ultimately yielding hydrogen and CO2.
Another way is through high-temperature pyrolysis in the absence of oxygen, splitting hydrocarbons, e.g. methane, to produce hydrogen and solid carbon. Hydrogen can also be produced through electrolysis, where direct electrical current is used to split water into its elements, hydrogen and oxygen.
Coal gasification today contributes with approx. 23% of global hydrogen production. As with steam methane reforming, this is a highly carbon-intensive method, estimated at around 19 kg CO2e per kg hydrogen. Citation IEA, The Future of Hydrogen, 2019.
Steam reforming of natural gas is the most common production method for hydrogen, making up approx. 75% of global hydrogen supply. As the process is both highly energy intensive, and as fossil natural gas is used as feedstock, steam reforming of natural gas is estimated to emit 9 kg CO2e per kg hydrogen. Citation EU Commission, Hydrogen Strategy, 2020.
Fossil hydrogen with carbon capture and storage (CCS) e.g. steam reforming with CCS, is proposed as a future bridging technology for low carbon hydrogen at scale. CCS can generally be expected to catch up to 85% of CO2 emissions, with 1.5-4 kg CO2e released to the atmosphere per kg hydrogen, not indcluding fugtive emissions of methane in the supply chain. Citation At 60% (for CCS only at process stream) and 85% (for both process and energy stream) capture rates. Cf C.E. Delft, 2019. CCS is still an immature technology at scale, and the cost advantage to other technologies is unproven. Furthermore, as steam methane reforming still requires natural gas, in a European context, large investments in CCS would uphold or even increase import dependence.
Hydrogen from electrolysis makes up some 2% of worldwide hydrogen supply. As it typically takes about 50 kWh of power to produce 1 kg of hydrogen, the carbon footprint is very dependent on the electricity used. Coalbased power, for instance, would result in extremely carbon intensive hydrogen. Using an average European electricity mix would emit app. 14 kg CO2e per kg hydrogen, whereas electrolysis powered by renewable energy comes with no direct emissions. Citation EU Commission, Hydrogen Strategy, 2020.
Pyrolysis is the process of breaking hydrocarbons apart using high temperature and pressure in a non-oxidative environment. This can be done with e.g. fossil methane or with biomass, yielding hydrogen and solid carbon. Pyrolysis adds the benefit of allowing further use of both carbon and hydrogen from the feedstock. Even so, as the process is energy intensive and comes with fugitive emissions, pyrolysis of methane is estimated to emit about 4 kg CO2e. per kg hydrogen. Citation Parkinson et. al., Levelized cost of CO2 mitigation from hydrogen, 2019 production routes