Bearing in mind the trade-offs required, it is not immediately clear how the natural gas grid can be adapted to integrate hydrogen. There are four distinct strategies to tackle this:
1. Reform natural gas downstream to produce hydrogen
This is the most obvious approach to delivering hydrogen using the gas grid. Hydrogen produced from natural gas through a process of steam methane reforming (SMR) is injected into the low-pressure distribution network, eliminating the need to adapt either the upstream transmission pipeline or the downstream distribution network. However, small-scale SMR is economically challenged, losing much of its benefits from economies of scale that currently make it the low-cost, dominant form of hydrogen production. To exacerbate the issue, carbon capture and sequestration costs are also considerable on a small scale if a zero-carbon hydrogen stream is desired.
2. Build dedicated new hydrogen pipelines
There are currently around 4,500 km of dedicated hydrogen pipelines around the world, though nearly all are short-distance pipelines. To meet the needs of long-distance hydrogen delivery, new hydrogen pipelines would need to be built with the appropriate steel grades and compressor station technologies. While new pipelines address the compatibility question, they come at a high price.
3. Retrofit compressor stations for hydrogen
Pipelines themselves are very expensive, costing millions of US dollars per kilometer. Instead of building a brand-new pipeline, an existing natural gas pipeline could be switched to hydrogen use and the compressor stations that maintain gas flow upgraded to hydrogen compressors. Depending on steel grade, the pipeline operating pressure may need to be reduced to lessen the risk of hydrogen embrittlement.
4. Blend hydrogen with natural gas
This is a near-term step that gas system operators can take to introduce hydrogen and decarbonise the gas grid. Hydrogen-natural gas blends can be reasonably tolerated up to 10% or 20% of capacity, which – given the sheer size of the natural gas system – is an extremely high limit from a hydrogen supply perspective. A typical natural gas transmission pipeline will require hundreds of megawatts of electrolysis to produce enough hydrogen to reach that limit. Should pure hydrogen be required downstream, pressure swing adsorption (PSA) technology can be used to separate the hydrogen from the natural gas feed, yielding a high-purity hydrogen stream and a separate natural gas stream.