Two or three times the amount of energy currently flows through the natural gas grid than it does through our electricity infrastructure, and mains gas is used by 83% of UK homes and by around 200,000 businesses.
Hydrogen is a serious answer to the greenhouse gas problem: when pure hydrogen is burned it produces only heat and water vapour. But the source of the hydrogen is important: so-called ‘blue’ hydrogen has been reformed from a hydrocarbon (typically methane) and CO₂ is produced in this process, but this is captured and stored rather than being released into the atmosphere. An alternative is ‘green’ hydrogen, made by electrolysing water using renewable energy such as solar or wind power – in effect it is a way of storing ‘green’ electricity, with negligible CO₂ output.
However, while hydrogen has an exceptionally high energy density per unit mass, its low density gives it only around one-third of the energy density of methane per unit of volume. This requires significantly modified end-user equipment such as boilers, gas cookers and heaters. If instead you supply a blend of NG and hydrogen, it can be used largely conventionally by equipment that consumer and business end users already have.
By the way, this concept of blending natural gas (NG) and H₂ for the end user is quite different from the concept of combining the two in distribution pipelines, to be separated (typically using a metallic or carbon-based membrane) and sent to different local networks. This idea is being explored in order to make use of existing long-distance pipelines.
BIG NEWS
The potential environmental impact is significant. The most widely suggested proportion of NG and H₂ is an 80/20 blend, measured by volume. Although that 20% volume of hydrogen only represents about 7% of the total energy content of the blend, this could have a real effect on the UK’s output of CO₂ – the typical figure is that it could save around 6 million tonnes of CO₂ per year, as much as taking 2.5 million internal-combustion cars off the road.
This is one of the reasons why the UK’s ‘hydrogen champion’ (appointed by the Department of Business) has been urging a move to a blended gas mix. In her report published at the end of March, Jane Toogood recommended that the government “make the strategic decision to support blending of hydrogen into the gas network in 2023 and confirm a ‘minded to’ position on suitable commercial arrangements to support blending.” It is possible that 20% blending will become generally allowed from as early as 2026.
There have been proposals for blending hydrogen and natural gas since the early 1970s, and in fact the ‘town gas’ that was derived from coal (and which preceded ‘natural gas’) was usually a mixture which could contain more than 50% hydrogen. Pilot studies of H₂/NG blending first took place in Germany, and in the UK there have been a number of programmes.
The two main demonstrations of 80/20 blending were part of the ongoing HyDeploy project, and took place on the Keele University campus in Staffordshire, and at Winlaton near Gateshead. The Winlaton trial took in 668 homes and ran from August 2021 to June 2022. Despite some concerns from residents, there were no real technical issues in either trial.
HyDeploy has also demonstrated a 20% blend on an industrial scale, both at Pilkington Glass and at Unilever’s Port Sunlight plant. There, a 7MW CHP steam boiler was run on the blend with “no impact on the process”, according to the site engineer.
Still, the first concern has to be the safety of hydrogen – with its public image of extreme inflammability – in the event of a leak. A major survey by the US National Renewable Energy Laboratory said: “Because hydrogen has a broader range of conditions under which it will ignite, a main concern is the potential for increased probability of ignition.” However, subsequent studies have concluded that the direct risks are not significant, and the low density of hydrogen means that it disperses rapidly.
Most gas pipes are either steel or polyethylene (PE), and there are issues with hydrogen permeation –gas passing through the walls of the pipe – and causing weakening due to chemical reactions. Hydrogen atoms are very small, so the risk of permeation is certainly higher than for methane. A test in the Netherlands concluded that at distribution pressures of 2 bar, the permeation level through standard PE pipe was around twice that of methane; other sources suggest that it could be up to five times higher than methane, but would still be acceptable. The same study also looked at the effects of hydrogen weakening on electrofusion welds in the pipe, and again concluded they could be disregarded. Steel pipes can also suffer from hydrogen embrittlement, but this is well understood, and has been examined in a number of studies. It should not be a barrier to H₂ blending.
The lower volumetric energy density of hydrogen means that even a 20% blend has a noticeably lower energy content (usually measured using the Wobbe Index) than traditional natural gas. This may not be an issue in a domestic setting, but some industrial plant (such as combined cycle gas turbines) are designed to work at a specific value on the Wobbe Index.
A related issue is keeping the proportions of the blend consistent: hydrogen is injected into the network at specific locations, but if the blend already contains 20% hydrogen at that point, no more can be added. Equally, if an end user receives gas with an excessive proportion of hydrogen, they effectively get less usable energy – so accurate metering, which takes into account the gas mix, is critical. And while current gas meters are said to work with a 20% blend, higher proportions require new techniques such as ultrasonic and Coriolis measurement (an example of the former is Pietro Fiorentini’s H2-SSM smart meter, pictured above). However, these have been developed and several are already approved for UK service.
BOX: HYDROGEN-READY APPLIANCES
Any domestic boiler or other gas appliance made in the past few years has been ready for the proposed 20% hydrogen blend, as the change in performance and burn characteristics is minor – and firms have logos to indicate that they are ‘hydrogen blend ready’.
But as the proportion is increased, and particularly if a switchover to 100% hydrogen takes place, important modifications are needed: for instance, hydrogen burns with a very pale blue flame, so a UV sensor is required to detect that the flame is burning properly. A higher flame speed means that flashback protection needs to be modified, and hydrogen creates far more water condensate than methane.
More concerning are NOx emissions: burning hydrogen can lead to higher combustion temperatures, which generate nitrogen oxides, which cause air quality issues and can lead to respiratory problems. Catalytic aftertreatment can remove the NOx, but this is an expensive solution, so appliance manufacturers would rather optimise burning conditions to prevent NOx production in the first place.
Technicians will also need to be trained in the installation and maintenance of hydrogen-burning equipment, particularly if higher-H₂ blends or a switchover to 100% hydrogen are planned. Nevertheless, boiler manufacturer Worcester Bosch says there is “an opportunity for a new generation of hydrogen-ready appliances to be introduced, initially running on natural gas, before being switched over to hydrogen once the full infrastructure is in place.”