Pipelines

Many advocates of the hydrogen economy have suggested that the solution to the problem of hydrogen transport lies in simply converting existing natural gas pipelines into hydrogen pipelines. For example, one study showed that the cost of shipping energy in the form of a hydrogen pipeline is comparable to electricity costs over long distances through high-tension power lines. The energy costs of hydrogen fuel in Chicago, derived from wind turbines on the northern plains, would be approximately the same as gasoline costing $2-$2.50 per gallon, if it were supplied via a pipeline built for hydrogen transmission.

However, research indicates that simply transforming our current network of natural gas pipelines into hydrogen pipelines is not feasible. For example, the lower energy density of hydrogen requires that the pipeline be operated with greater velocity if it is to maintain the same energy rate, which means it needs to handle higher pressures.

Perhaps the most significant concern with hydrogen pipelines is the weakening of the pipeline materials due to hydrogen embrittlement. While not totally understood, embrittlement is believed to occur through the disassociation of hydrogen atoms that diffuse through the steel lattice, reacting with carbon atoms to form methane:

2H2 + Fe3C = 3Fe(s) + CH4(g)

The methane gas can build up in microcracks within the metal, producing significant pressure intensities that lead to metal failure. In addition, the presence of hydrogen atoms along a displacement boundary within the metal lattice may lower the reaction energy needed for displacement movement, which leads to greater plasticity and eventual reduction in strength for metals.

Research on hydrogen embrittlement is focusing on the use of epoxies to coat the interior of the pipeline so that it is less permeable to atomic hydrogen diffusion.

Source: http://www.ases.org/hydrogen_forum03/Leighty.pdf, http://en.wikipedia.org/wiki/Hydrogen_embrittlement