|Eco-restructuring: Implications for sustainable development (UNU, 1998, 417 pages)|
|Part I: Restructuring resource use|
|6. Fuel decarbonization for fuel cell applications and sequestration of the separated CO2|
Transport costs for new pipelines
The cost of hydrogen pipeline transport is higher than for natural gas. Pottier et al. (1988) estimate that the cost of the pipe would typically be 50 per cent higher than for natural gas transmission lines, largely because embattlement-resistant steels would be specified. Also, the optimized pipe diameter would be perhaps 20 per cent larger for hydrogen to achieve the same energy flow rate (Leeth 1979). The cost of installation would also be higher, because special care would be needed with welds. And much larger compressors would be needed to achieve the same energy flow rates, because the volumetric energy density of hydrogen is just 35 per cent of that for natural gas.
But hydrogen pipeline transport costs would not be prohibitive, for two reasons. First, the diversity of supply options implies that hydrogen would often be available from sources that are relatively close to where the hydrogen is needed (Ogdn and Nitsch 1993). Second, even where long-distance pipelines are needed, the costs involved are relatively modest compared with the total cost of hydrogen to the consumer, and the total cost per unit of service provided would typically still be less than for conventional transport fuels. For example, the transport of hydrogen 1,110 km to consumers for use in fuel cell vehicles from a production facility located at a natural gas field contributes less than one-fifth of the total cost of hydrogen to the consumer (see fig. 6.2 and table 6.8).
Transport costs for pipelines converted to hydrogen from natural gas
In many parts of the industrialized world extensive pipeline networks designed for use with natural gas are already in place. Could these pipelines be converted to hydrogen? Even neglecting considerations of embattlement, these pipelines are not optimized for use with hydrogen, whose energy density and viscosity are very different from the values for natural gas. Nevertheless, from a fluid dynamics perspective, the mismatch would not be severe. For a given pipe diameter and operating pressure, the energy flow rate for hydrogen would be about 85 per cent of that for natural gas with partially turbulent flow and 90 per cent of that for natural gas with fully turbulent flow (Christodoulou 1984). Seals, joints, and metering equipment would probably have to be replaced with conversion. Moreover, almost three times as much compressor power would be needed to obtain the same energy flow rate as for natural gas; and reciprocating rather than centrifugal compressors would be needed. Nevertheless, there could be large savings associated with the sunk costs of the pipelines themselves, if conversion were feasible.
A major concern about converting natural gas pipelines to hydrogen service is "hydrogen environment embattlement," which refers to the degradation of mechanical properties that takes place when a metal is exposed to a hydrogen environment.
The available evidence indicates that this is not an issue for existing local natural gas distribution systems, which could therefore be converted to hydrogen with only minor modifications (Ogden et al. 1995). However, because of the higher pressures (up to 70 bar or 1,000 psia) and materials used, hydrogen environment embattlement could be a serious problem for existing long-distance natural gas transmission lines that would transport pure hydrogen. The primary mechanisms are fatigue crack growth under cyclic loading and slow crack growth under stable loads near welds and other "heat-affected" zones in pipes.
Various countermeasures have been suggested, including adding gases to inhibit embattlement, pre-loading with an inert gas, coating pipelines, and selective replacement of steels susceptible to embattlement. The most promising approach seems to be gas additives. Available research indicates strong inhibition of fatigue crack growth with oxygen, at concentrations ranging from 100 ppm to 1 per cent (Ogden et al. 1995). A high priority for hydrogen research should be given to gas additives and other countermeasures for inhibiting embattlement in existing natural gas transmission lines that might be converted to hydrogen.