The vast majority of hydrogen produced at present is not used for fuel, but in industrial processes. However, the Paris Climate Agreement and national policies are driving efforts to find alternatives to fossil fuels.
Hydrogen is sometimes characterised as ‘grey’, ‘blue’ or ‘green’, depending on whether it has been produced using a CO2-emitting method (‘grey’), involving carbon capture (‘blue’) or a zero-carbon method (‘green’). Some intermediate methods such as methane pyrolysis are even said to produce ‘turquoise’ hydrogen!
Very recently, some natural underground deposits of hydrogen have been found in Mali, and used to power a local village; similar sources are being sought around the world, as this would be the cheapest method of producing hydrogen.
The dominant ‘blue’ method of making hydrogen has been Steam Methane Reforming (SMR), in which steam turns methane into hydrogen and carbon dioxide:
CH4 + 3H2O > 4H2 + CO + H2O > 5H2 + CO2
The technique (and other, similar reforming methods for generating hydrogen) is well-known and produces high-quality hydrogen — but it also produces CO2. Carbon capture and storage/sequestration (CCS) is needed to mitigate the production of CO2, but the economics are tricky: as one related project puts it, “The costs are immediate, but the benefits are long term”.
The most common method is to capture the CO2 at source, transport it and inject it into an underground storage volume — usually a worked-out well or gas field. The scale of these operations is vast: one estimate is that the EU will have to store 1,400 million tonnes of CO2 each year by 2050. Fortunately, the Norwegian Continental Shelf alone has enough storage capacity for 50 years at that rate.
But electrolysis is the main area of interest, because it makes hydrogen the key to a zero-carbon economy: sustainable electricity from wind, solar and wave power (getting cheaper by the year) can create hydrogen from water, effectively storing energy to be used when sustainable sources are generating less.
‘Green’ hydrogen manufacturing capacity is increasing by leaps and bounds: in 2018 it was around 0.2GW globally; it is now around 6.3GW, with 1.3GW added in the first three months of 2021 alone. Oil & gas research consultancy Wood Mackenzie estimates that ‘green’ hydrogen will be competitive on price with fossil fuels by 2028 to 2033.
The current state of the art is the PEM (polymer electrolyte membrane) electrolyser: this has high efficiency and the ability to cope with spikes in current — ideal for variable sources such as wind turbines. These are now into full-scale production: products such as Siemens’ Silyzer 300 can generate up to 2 tonnes of hydrogen per hour at a claimed efficiency of around 75%.
PEM efficiency figures are getting better every year, with suggestions that they may exceed 85% by 2030. But alternative methods of turning water into hydrogen are on the way, including photoelectrochemical water splitting (a sort of artificial photosynthesis), photoelectric catalysis and high-pressure or high-temperature electrolysis (which could use the heat from a power plant such as a nuclear reactor directly).
Microbial electrolysis cells (MEC) use microorganisms to generate hydrogen (or methane) from organic material such as the sludge from wastewater. A catalyst and additional electrical energy are needed to complete the conversion, but the overall efficiency of the system is much greater than conventional electrolysis. MEC is still at the experimental stage, but the possibilities are highly encouraging.
Methane pyrolysis is a promising technique for producing hydrogen directly from methane, without generating any greenhouse gas. In pyrolysis a material is made to decompose by heating it strongly in the absence of oxygen; here, methane is passed over a catalyst (or bubbled through a molten metal) at around 1,000ºC, separating it into hydrogen and carbon:
CH4 > 2H2 + C
The solid graphite granules produced are pure enough to be used for manufacturing carbon fibres. The process does have the energy cost of the heat input, but this is around half the amount needed for SMR. Another advantage over SMR and electrolysis is that the process does not consume water. Efforts are being made to use different catalysts to lower the reaction temperature. Methane pyrolysis is still at the pilot stage: Dutch research organisation TNO and German chemicals firm BASF both reckon it will be economically viable by 2030-2035.
There are several biological methods for producing hydrogen, including dark fermentation, biomethanation, photofermentation and bioelectrochemical systems.
In practice, methods are likely to be combined into ‘biorefineries’, in which a series of different processes turn waste — from food production, a paper mill, domestic sewage or another source — into usable products including hydrogen, methane and industrial feedstocks. A more immediately achievable goal is to produce ‘biohythane’ — using bacteria or algae in a bioreactor to generate a blend of methane and hydrogen.
Some biorefineries are producing synthetic fuels on an industrial scale: in Reno, Nevada, Fulcrum Biotechnology’s refinery will turn 175,000 tons of municipal solid waste (MSW) each year into around 40 million litres of synthetic crude oil (‘syncrude’) which can then be refined further. The same firm is planning a waste-to-fuel biorefinery which will produce 120 million litres per year of sustainable aviation fuel (SAF) near Liverpool, starting from 2025.
BOX: HYDROGEN AS A FUEL
As a fuel, hydrogen has two main uses generating electricity, or generating heat. It is used in fuel cells to generate electricity, as an alternative to batteries — commercialised in cars and trucks by Hyundai and Toyota. Other firms are investigating its use as a fuel directly in internal-combustion engines.
Another way to use hydrogen as a fuel is to blend it with natural gas, giving a mixture (sometimes known as hythane) with less carbon output than pure methane. Up to 10% (by volume) of hydrogen could be injected into the natural gas grid, with most appliances such as boilers and gas cookers able to use the mixture without modification.
However, most natural gas consumers are billed on a volume basis (eg per cubic metre of gas), and the volumetric energy density of hydrogen is much lower than that of methane. As the blend of gases is liable to separate as it passes through the network, some customers could be short-changed, or find that they are receiving a mixture with too much hydrogen for efficient burning.
A methane/hydrogen blend can also be used in gas-powered vehicles such as city buses: here the proportion of hydrogen can be 20-30% by volume if the fuel and ignition systems are suitably modified.
Another way to use hydrogen as a low-carbon fuel is to make ammonia (NH4), which burns in air to produce water and oxides of nitrogen (which can be reduced to nitrogen and oxygen in an exhaust catalyst):
NH4 + 2O2 > H2O + NO2
2NO2 > N2 + 2O2
This is being considered for shipping, where refuelling can be done under controlled conditions — an ammonia spill is unpleasant, to say the least. Marine engine manufacturer MAN says that it will have a retrofit solution available for large ships within four years.