Hydrogen can be produced from the electrolysis of water without any direct CO2 emissions. However, one needs to look back up the chain to the source of the electricity. If the power consumed by the electrolyser is simply taken from the grid, then it has the associated CO2 emissions from the power generation sector. The carbon intensity of the power used will potentially be greater than the average carbon intensity of the grid, since if the power demand of the electroyser is regarded as an incremental load, then the associated carbon intensity will be that of the incremental generation plant that is be added to the grid to meet this demand. At best this would be a combined cycle gas turbine (CCGT) plant; at worst a coal-fired plant, open cycle gas turbine (OCGT) plant or diesel engine set. Clearly, none of these results in the production of decarbonised or ‘green’ hydrogen, as it is called when hydrogen is produced from zero carbon sources.

To produce green hydrogen via electrolysis it is therefore necessary to have a dedicated supply of zero carbon electricity – from either a renewable source or nuclear station. The electricity does not necessarily have to be generated adjacent to the electrolysis plant – it could be connected by private wire or via the grid, provided that the renewable supply is hypothecated to the electrolyser. This allows the electrolyser to be located adjacent to the demand centre for hydrogen. If the power supply is from an intermittent renewable source then either a battery back-up would be required to maintain operation of the electrolyser, or it must be accepted that hydrogen can only be produced when the renewable power supply is available; but these options would either add capital cost or reduce output from the facility.

Green hydrogen produced via electrolysis will facilitate conversion of some sectors with a moderate demand for hydrogen, such as rail transport and HGV road transport to zero-carbon hydrogen fuel, and facilitates the production of the hydrogen fuel at the point of demand at the depot. However, if hydrogen is to play a major role in the decarbonisation of heat and industry, electrolysis alone will not be able to provide the quantities of hydrogen required due to the massive increase in the installed power generation capacity that would be required to supply power to the electrolysis plants – potentially doubling or trebling the size of the power generating sector purely to produce hydrogen, since the demand for gas exceeds the demand for electricity in the UK. 

Traditional sources of hydrogen, from the reforming of natural gas or gasification of coal and other solid feedstocks, will be required to produce hydrogen in bulk. These processes are commercially proven, and in safe and reliable operation in facilities around the world, including several natural gas reforming plants located in the UK. However, with these technologies hydrogen is produced with associated production of CO2, which would traditionally be emitted to the atmosphere. Utilising hydrogen from such a source in place of natural gas, for example for domestic heating or to supply energy-intensive industry, would actually result in higher CO2 emissions than those associated with using natural gas directly. However, it is possible to mitigate these CO2 emissions by employing carbon capture, utilisation and storage (CCUS). Advanced natural gas reforming and coal gasification technologies lend themselves to CCUS, since the CO2 can be removed from the ‘syngas’ (synthesis gas) stream as part of the normal hydrogen purification process and then exported for utilisation or permanent storage. Indeed, this is already happening in the fertiliser industry; in ammonia/urea fertiliser complexes, the CO2 ‘captured’ from the hydrogen production unit is utilised as feedstock for urea synthesis. 

Adding CCUS will increase the cost of hydrogen production via reforming or gasification, and therefore one challenge is for governments to develop a mechanism to ‘reward’ the decarbonisation of hydrogen production.

Through CCUS, the CO2 emissions of hydrogen produced from fossil fuel sources can be reduced by around 90%. The so-called ‘blue’ hydrogen, hydrogen produced from fossil fuels in combination with carbon capture, therefore has a significantly lower carbon footprint if used to displace natural gas for heating. However, it is not zero carbon as there are still some residual CO2 emissions associated with the hydrogen production plant that cannot be directly eliminated – is there anything more that can be done?

While the hydrogen production process itself cannot be made zero carbon, there is a way to make it ‘net zero’. Applying carbon capture to CO2 emitted from the combustion of biomass has been proposed as a viable approach to remove CO2 from the environment – referred to as Bio-Energy with Carbon Capture and Storage (BECCS). CO2 is absorbed from the atmosphere during the growth of the tree (or other biomass source) and is then permanently stored following capture of the CO2 from the combustion products of the biomass; therefore, there is a transfer of CO2 from the atmosphere to storage. BECCS is currently being demonstrated at small scale at the Drax power station in North Yorkshire, with £5 million of government funding recently announced to advance the project.

The same approach can therefore be employed for hydrogen production. Hydrogen produced through the gasification of biomass with CCUS will be carbon negative, i.e. there will be an associated net reduction in atmospheric CO2 over the growth cycle and conversion/CCUS process. Established biomass gasification technologies can be applied, with CO2 captured from the syngas using the same processes applied to coal gasification. By balancing hydrogen production between natural gas, coal and biomass, a net zero carbon balance can be achieved. Thus, it is possible to match the green hydrogen credentials of electrolysis, but at the large scale required for the decarbonisation of heat.

Utilising biomass as feedstock in place of coal or natural gas increases production costs, and therefore, along with the addition of CCUS, mechanisms to financially support the production of decarbonised hydrogen through this route will be required.

Hydrogen is likely to play a major role in meeting future decarbonisation objectives. However, as part of a ‘net zero’ strategy, hydrogen production itself needs to be net zero. Fortunately, the technologies exist to achieve this – through a combination of green hydrogen produced via electrolysis using zero-carbon electricity, blue hydrogen from fossil fuel sources with CCUS and carbon-negative hydrogen via BECCS the industry has the toolkit to play its role in a net zero future. The UK should therefore embrace hydrogen as a key element of the country’s energy future, with government and industry collaborating to accelerate the implementation of a decarbonised hydrogen economy.

By Tony Alderson, Carbon Capture and Storage Technical Lead