The hydrogen economy could take a step towards reality with an approach of delivering hydrogen by blending it with natural gas in natural gas pipelines. One of the key problems in the hydrogen economy is the lack of delivery infrastructure for hydrogen. At the same time the U.S. has an extensive network of natural gas pipelines and one supposes these could be used for hydrogen delivery. A paper published by the U.S. Department of Energy’s Fuel Cell Technologies Office today goes over the possibility of hydrogen delivery by blending it with natural gas.
The advantages include a distribution system hydrogen electrolyzed from water, powered by intermittent renewable electricity sources (wind, solar). Intermittent renewable electricity sources are seen as unsuitable for mass-scale use to power cities etc because they cannot be trusted to supply base load electricity. But, if they could electrolyze hydrogen, store the hydrogen into tanks, then deliver that hydrogen through some means, that could make intermittent renewable energy sources more feasible.
The idea is to mix hydrogen with natural gas, delivering both through the same pipelines. That would give us a delivery system for hydrogen, where no practical system exists today.
There are approximately two ways this would be used:
- The hydrogen will be extracted at a downstream location, and used as pure hydrogen
- The blended hydrogen+natural gas will be burned in existing natural gas gizmos
The study claims hydrogen can be mixed with natural gas at concentrations from 5-20% and the blended gas can still be burned in today’s natural gas gadgets. These are called “end-use systems” and are gas stoves, gas heaters, and industrial gas systems. The concern is whether any modifications are required to end-use systems to successfully use blended hydrogen+natural gas, and what are the costs of modifying existing end-use systems. The report suggests “we” should start with low concentrations of hydrogen and inch it upwards as experience warrants.
Hydrogen has a broader range of conditions under which it will ignite. Hurm.
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| Natural Gas Pipelines in the U.S. |
Metal pipes can degrade when exposed to hydrogen over long periods, particularly at high concentrations and pressures. The paper notes the U.S. pipeline network is primarily made of low-strength steel, typically API 5L A, B, X42, and X46, and these are generally not susceptible to hydrogen-induced embrittlement under normal operating conditions. Hence the paper suggests this isn’t a big risk, in the U.S. The transmission pipelines in the U.S. operate at high pressures, 2,000 psi or more. The paper thinks there are relatively simple adaptations that can be made in the transmission network allowing hydrogen concentration up to 50%.
Hydrogen blends can also affect the accuracy of natural gas meters.
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Hydrogen can more easily leak, especially in the sorts of pipes used in the distribution system rather than the steel pipes used in the transmission system. In this area the paper talks about hydrogen leaking through plastic pipe walls, because of the typical polymer pipes used for natural gas distribution.
The three mechanisms for separating hydrogen from a blended gas are:
- pressure swing adsorption (PSA),
- membrane separation,
- electrochemical hydrogen separation (EHS, or hydrogen pumping).
PSA units are best at the boundary where high pressure natural gas transmission lines connect with lower pressure distribution pipes. PSA is the most commercially ready.
Membrane separation technologies work very efficiently with relatively high concentrations of hydrogen. Some membrane technologies can extract near 100% pure hydrogen.
Electrochemical separation (also known as hydrogen pumping) is a more elaborate method for bulk hydrogen recovery. One uses a Nafion-based membrane system.
See: http://www1.eere.energy.gov/hydrogenandfuelcells/hydrogen_publications.html#h2_delivery
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