Converting the gas grid to hydrogen, is it really that simple?

Gasunie has plans to gradually convert the natural gas network into a hydrogen network. After all, having a sustainable energy system means that we must move away from natural gas as a fossil fuel. The expectation is that hydrogen will take its place. But is converting the natural gas grid to hydrogen economically and technically feasible? Does it deliver climate benefits? And, first and foremost, is it safe?

Cars, trucks and motorcycles can drive down the motorway. Some vehicles cause the asphalt surface to wear slightly faster than others, but basically everyone can use the motorway regardless of their vehicle. In essence, the same is true of pipelines; in principle, they can be used to transport natural gas, CO₂ or hydrogen, or any other gas. However, just as trucks subject the asphalt to greater loads, the way gases behave in a pipeline depends on their composition, and we need to take that into account.

Why is that? The hydrogen molecules (H₂) themselves are not so much the problem. It’s mainly the hydrogen atoms (H) that cause the headaches. Hydrogen molecules are inherently stable; when transported, they only break down into individual atoms in the presence of a catalyst. Iron is one of those catalysts. Steel natural gas pipelines are protected on the inside by a layer of oxide, but if that layer is degraded by welding faults, for example, exposing a clean iron surface, hydrogen molecules can decay into hydrogen atoms at that point, penetrating the steel.

The ’initial defect’, as Huising calls it, then grows as the hydrogen atoms penetrate further into the material.  This is called ‘crack initiation’. Hydrogen can cause cracks to grow twenty to fifty times faster than other gases. This happens particularly when there are large pressure changes in the pipeline. In the HyWay 27 report, PwC concluded in 2021 that without additional measures, this embrittlement effect could lead to ‘minor leaks’ in the longer term (20 to 25 years).

Leaks are obviously undesirable. They can lead to dangerous situations because hydrogen is highly flammable. Leaks are also a form of product loss. Every cubic meter of hydrogen that leaks out can no longer be sold. Furthermore, leaks lead to climate damage because more hydrogen in the atmosphere would slow the breakdown of the greenhouse gas methane.

Besides normal combustion, hydrogen can also burn at an accelerated rate, resulting in a rapid build-up of pressure – an explosion. Hydrogen can explode under certain conditions when its concentration in air is at least 10%. However, because hydrogen is so light and volatile, that 10% threshold is not easily reached. ‘If a hydrogen pipeline in an open field develops a leak, you won’t reach that threshold because of the lightness and volatility of hydrogen’, says his colleague Huising. ‘In Norway, a firm of consulting engineers, DNV, conducted a test at a hydrogen refuelling station. After creating a leak, they only reached a maximum concentration of 8%, which is not an explosive gas mixture.’

Even though the explosion hazard is limited, Gasunie is still keen to prevent leaks. So once a crack develops, action is needed to slow down the effect of accelerated crack growth. To achieve that, a crack may ‘grow by no more than 0.25 mm over an operating period of 100 years’ according to Huising. Since pressure changes in the pipeline determine the growth rate of cracks, they should be avoided as much as possible. Based on metal fatigue calculations, Huising can determine what the maximum pressure changes in the hydrogen network may be and how often they may occur. 

‘In the case of natural gas, we mainly control pressure based on customer demand. And when the pressure in the pipeline network fluctuates, it doesn’t really matter’, Huising says. ‘In the case of hydrogen, we will also be controlling pressure changes in the network.’ To keep the pressure within a limited range, we will use large salt caverns to store hydrogen. These act like an expansion vessel in a domestic central heating system, allowing us to accommodate pressure changes in the system.

Further growth of the hydrogen grid will also help. ‘In a nationwide hydrogen network of many hundreds of kilometres, you need a lot of hydrogen to increase pressure by even as little as 0.1 bar’, Huising explains. ‘The larger the network, the smaller the pressure fluctuations.’ 

If the requested volumes or capacities increase in the future, ‘you simply add a second pipeline instead of using large and expensive compressor stations’, Van Agteren explains. ‘So the natural gas network will slowly but surely switch over to hydrogen.’ Gasunie’s network expert does not see a need to build hydrogen network compressor stations before 2035.

Given the limited experience with hydrogen transmission through pipelines, numerous research groups are conducting tests. For example, Huising, with several peers from other gas transmission system operators (TSOs), is actively involved in research projects initiated by the European Pipeline Research Group and the Pipeline Research Council International. ‘We are going to run tests with damaged pipes, for example. Suppose an excavator causes a dent, how does the pipe behave in that situation? We have models to explain this for natural gas. And we need a similar tool for hydrogen’, he says. Damage to pipes caused by excavation works occurs once every five years on average, according to Van Agteren. 

Huising emphasises that the Dutch and European gas network was built in the 1960s and 1970s. The quality of the steel was much higher than that used in the United States, where parts of the main network were built as early as the 1940s. ‘They face much greater challenges in converting the gas grid to hydrogen over there. You can’t simply take the reports that warn about the problems in America and project them onto the transition in Europe and the Netherlands.’

When the natural gas network is converted to a hydrogen grid, only the pipelines themselves will be left in place. Everything else will be replaced. This mainly involves the valves. These are the points in the network where, for example, branch lines are installed to connect customers. ‘Many of those valves date back to the 1960s’, Huising says. ‘If you’re going to build a new network, it makes sense to replace them.’

A valve is like a tap in the water supply. In a natural gas pipeline, it is a sort of ball with a hole in it. Turning the ball a quarter turn allows gas to flow through the valve; turning it another quarter turn stops the flow of gas. Gasunie is considering using fewer valves in the hydrogen network, Van Agteren says. The current gas grid has a valve every 15 kilometres. ‘That approach ensures that we can limit nuisance to local communities to no more than one hour, even in the event of a major leak.’ 

With natural gas, it takes an hour to empty a 15-kilometre length of pipe. ‘Because hydrogen is much more volatile, a pipeline length of 50 or maybe 80 kilometres would empty in an hour. That’s why we would need fewer valves. There are moving parts in valves, which are associated with a greater risk of leakage than the steel pipe.’ 

Before pipelines are put into service, they are inspected internally. There are all kinds of tools for this. For example, there is a tool that can detect cracks and other defects by generating a resonant vibration in the steel. 

In emergencies, the valves close. Although 100%‘ pure hydrogen burns with a colourless flame, there are always small amounts of contaminant in the gas, as well as in the air drawn in that is needed for combustion, which make the flame visible. Technicians and inspectors have all kinds of equipment to locate damage and leaks and also to protect themselves, because Gasunie assumes as a matter of course that the hydrogen will burn with an invisible flame. The technicians’ equipment also includes cameras that detect heat radiation, gas detection cabinets and portable ‘gas sniffers’.

Because existing pipelines have to be made suitable for transporting hydrogen, some fear that the conversion, while technically possible, will be far too expensive. According to Van Agteren, this point has also been given extensive consideration. ‘When developing hydrogen infrastructure in Europe, joint studies have also looked at this aspect. The outcome is that repurposing an existing pipeline costs just 20% of building a new gas pipeline. Hynetwork, a Gasunie subsidiary, which is building the national hydrogen network in the Netherlands, has calculated that the cost of repurposing will be four times lower than new construction.’ 

In addition to lower costs, Gasunie sees another important advantage. ‘Repurposing has much less impact on the local community than installing a new gas pipeline in the ground. The Netherlands is pretty full, so from that perspective, too, reusing a pipeline that is already there is the preferred option’, Van Agteren points out.

In the United States and also European countries with a limited natural gas network, the strategy focuses on blending hydrogen with natural gas. A blend of 80% natural gas and 20% green hydrogen leads to a reduction in carbon emissions of only 7% because of hydrogen’s lower energy density. Why would you invest so much money in the network when the climate benefits are so limited, critics ask. 

Although hydrogen blending may be a first phase in many cases, the strategy of Gasunie and other European gas transmission companies is to eventually transport (almost) 100% hydrogen. The reduction in emissions is then correspondingly greater.

Back to the headline at the top of this article: ‘Converting the gas grid to hydrogen, is it really that simple?’ The answer is: yes, it can be done, but it’s not that simple. As the level of demand for hydrogen from industry, the chemicals sector and transport sector increases, infrastructure will be needed to get it to the customers.

Gas TSOs have been looking into whether and how natural gas pipelines can be used to transport hydrogen for twenty years already. A report on this issue was first published in 2004 under the banner of the European Gas Research Group. Forty European partners investigated how the transition to hydrogen could be facilitated by using existing gas infrastructure to transport a blend of natural gas and hydrogen. The project, known as NaturalHy, received funding from the European Commission.

To this day, researchers in Europe, the USA and elsewhere are investigating hydrogen transmission. This work provides new insights and leads to technological innovations that will increasingly simplify the deployment of hydrogen. Experts such as Martin van Agteren and Otto Jan Huising attend conferences on the subject, conduct field experiments, and share knowledge with researchers and specialists in other countries.

So it’s not that simple. However, with the right investments and precautions, transporting hydrogen through natural gas pipelines is safe and economical, and also delivers climate benefits.

Read more about safe and sustainable hydrogen through natural gas pipelines.