Plans for a large scale conversion of parts of the gas network are being tested by the H21 project in Leeds. But where does the hydrogen come from and how much energy is required to produce it? New Power Report examined the question in our September issue. How close did we get?
In two decades Leeds could be running on low-carbon hydrogen, if the ‘H21′ project, led by Northern Gas Networks, is successful. All the GDNs have joined in on this project – along with a raft of other organisations from universities to appliance suppliers – and they hope to get funding of £15 million through Ofgem in the current round of NIC applications.
Why Leeds? NGN is based in the city, and the company’s Nick Phillips gives three reasons: “The characteristics of the network means we can isolate it – it has a ring main with just a few entry points. There is an air quality issue and we have a good relationship with the city council.”
There are other reasons. It is close to sites where plans to trial carbon capture and storage had been at a fairly advanced planning stage before funding was pulled. Plans for a pipeline to transfer the carbon dioxide were also well advanced, and if it proceeded, Leeds has good access to existing infrastructure in the North Sea that could be used to sequester the carbon dioxide.
Why is that needed? Unlike natural gas, there are no reservoirs of existing hydrogen to be tapped. Instead it has to be manufactured and that may be a carbon-intensive process.
Hydrogen production at scale
There are two main routes to produce hydrogen, steam reforming and electrolysis.
Steam reforming is currently the most common. In this method the extraction and use of fossil gas is still required. The process uses a catalyst such as nickel to convert natural gas (largely methane) into hydrogen and carbon dioxide (in fact, this is a complex multi-stage process that also involves removing contaminants such as sulphur from the natural gas input).
The reaction will only happen at very high temperatures (approaching 800-1,000ºC). This requires energy input – possibly from burning methane. In this case a quarter of the entire methane input is used to provide the necessary heat. Overall, the process may use more methane than would be the case if the gas were used directly in the gas network. However, converting it in bulk and using hydrogen in the grid allows the carbon dioxide to be captured and stored or sequestered and means the gas is emissions-free at the point of use. In an article published in The Engineer last year, Mark Crowther, technical director at Kiwa Gastec, said four steam-reforming plants would be required to serve Leeds.
An alternative chemical process is available known as partial oxidation. It requires much less heat, but produces less hydrogen from the same volume of gas than steam reforming.
Electrolysis is the alternative way of producing hydrogen. In this case the raw material is water, and there are a variety of different electrolysis options that can be used to produce hydrogen, in some cases using high temperatures to reduce the power needed. The by-product is oxygen.
Electrolysis requires electricity and the process is low-carbon as long as the power source is a low-carbon one. That could mean a build-out of renewables or of nuclear power.
How much power is required to produce the necessary hydrogen?
New Power Report did a “back of the envelope” calculation, using a variety of internet sources.
Drawing on the experience from low-temperature electrolysers in the fertiliser industry, an EU estimate suggests the electrical energy inputs required for electrolysis is about 50kWh/kgH2.
Hydrogen’s energy density is 33MWh/t, and that of natural gas is 14MWh/t. (Note that although the energy content of hydrogen is higher by weight than liquid fossil fuels or natural gas, its energy density by volume is low, about a third that of natural gas, so there are gas transfer issues we have not considered here.) In Leeds the annual gas demand is 6.4TWh/year or 200Mt, which means a requirement of 10TWh for the city.
Four London Arrays might be required to produce the energy delivered by the gas grid in Leeds
This is a significant amount of power. It is about 3% of the UK’s annual power generation (336TWh in 2016). For comparison, the London Array wind farm’s net overall output for the year 2015 was 2.5TWh, which means four such wind farms might be required to provide the energy delivered by the gas grid in Leeds.
Across the country
Current discussion focuses on using different gas mixes in different areas, and switching to electricity or heat networks in others. But suppose the gas grid across the entire country were to be converted to hydrogen, as some propose?
Domestic use gives a clue: in the Leeds project, the area for conversion contains about 264,000 meter points, and a population of about 660,000 people. Across GB, the total number of domestic gas customers is close to 22 million, according to Ofgem.
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