Suitability of the natural gas distribution system for operation with hydrogen

There has been a growing interest over the last years about how to implement the hydrogen society. In this connection the issue of how - and if at all - hydrogen can be transported via the natural gas grid is very important. DGC carried out a literature study covering previous investigations in this area. The literature study showed that quite a few theoretical investigations, but only very few practical tests had been carried out. It was, in fact, only possible to identify two practical projects: An American project from the late 1970's and a German project from the mid-1980's. The need for up-to-date knowledge about the Danish situation was obvious. The purpose of this project was to reveal the possibilities of transport of hydrogen via the existing Danish natural gas grid. The investigation only covers the distribution grid. For this purpose, a standalone 'miniature gas grid' was built. DGC developed the facilities for distribution of hydrogen and test of gas meters together with the gas industry, and systems for control, regulation and monitoring of these facilities were developed by DGC. The tested steel and plastic pipes from the gas distribution grid have been adapted and buried according to existing standards and authority regulations by HNG, the regional natural gas company. Tests were made for almost the entire year 2003; resulting in a total of approx. 340 operation days. Generally, there were only slight operational problems. Leaks were detected upon initialization after hydrogen filling; otherwise only at months' interval. However, a slight leakage from one of the gas meters was detected throughout the entire test period. Analyses of test data and subsequent material analyses showed that: 1) 19 bar steel distribution pipes were tight through the entire test period, apart from a few times when a threaded joint or a valve has become leaky. 2) The steel material and welds in this material have been completely unaffected by the hydrogen exposure. 3) The tensile strength is unchanged, for basic material as well as for welding zones. Similar analyses of test data and subsequent material analyses showed that: 1) 4 bar plastic (PE) distribution pipes have been leaking at a constant leak rate throughout the entire test period, corresponding to a loss of 1 bar per year. The calculated permeability of hydrogen through the pipe wall amounts to approx. 20% of the measured leak. 2) Practical welding tests have proven an unchanged weldability. 3) The melting index for three out of four hydrogen exposed PE80 pipes dropped by 5-10%. This could indicate incipient changes to the material. No changes were found in the PEl00 samples after the hydrogen exposure. 4) OIT (oxygen induction) tests showed that the efficiency of the added antioxidant was reduced and that a few years of continued hydrogen exposure could reduce weldability to an unacceptable level for high-strength PE 100 pipes. 5) The tensile strength is unchanged, for basic material as well as for welding zones. 6)Infrared spectroscopy has shown that the basic material did not change its chemical composition. Industrial gas meters and the domestic gas meter were not damaged by the test operation. 1) Recalibration at various factories has shown that all meters complied with specifications. 2) Detailed investigations of sensitive components such as rubber gaskets etc. show no signs of abnormal wear or incipient decomposition. 3) During the test period a constant minor leak from the turbine gas meter was detected. The above detailed investigations showed no defects in the leaking components. I.e. a change of design is necessary when converting to hydrogen. Other components such as valves and filters were taken apart for investigation after the test period. Nothing seems to indicate damages caused by hydrogen exposure. The overall conclusion is that test results indicate possibilities for hydrogen transport via the 19 bar steel distribution grid as well as via the 4 bar plastic distribution grid. The plastic grid requires additional investigations of the tendency towards changes in melting index and reduced resistance against oxidation alter hydrogen exposure. Also, the tendency towards increased rigidity of PEM plastic and reduced rigidity for PE100 plastic needs to be investigated further. The project showed that all joints, components and fixtures of the gas grid should be checked for leakages at regular intervals. Certain components should be modified in order to be hydrogen tight