The gas quality in a hydrogen distribution grid

Abstract

Although the Dutch energy supply is gradually progressing from fossil fuels to renewable energy sources, the consequences for the grid are becoming increasingly evident. Meanwhile, the gas extraction from the Groningen natural gas reserve is de- clining as the induced earthquakes in the northern Netherlands persist. Hydrogen, as a flexible carbon-free molecule, offers a potential solution to the overcapacity of the electricity grid and thus has the potential to fulfill an important role in the fu- ture energy supply. It has recently been proved that by making few adjustments in the grid assets, the existing gas grid can be made compatible with hydrogen. A relating issue to hydrogen distribution through the gas grid is the resulting hydrogen qual- ity. Similarly to natural gas, the roll-out of a national hydrogen grid needs a national quality standard. There will be a variety of end-use applications, such as hydrogen boilers and fuel cell technologies that have different quality requirements.
This report researches the influences of the distribution of hydrogen through the existing gas grid. There are five sources of contamination: odorant, inward permeation of air through polymer pipelines, particles circulating in the grid, leaks causing an entrance for contaminants and byproducts from hydrogen production technologies. All five sources are considered, but the focus of this report is on inward permeation of air through low pressure polymer pipelines. There are three significant risks which are linked to the permeation of air: feed dilution, explosion risk and damage to fuel cells. Fick’s laws for diffusion were used to create a computation model, from which relationships were found between seven variables and the amount of contamination. Correlations were found between the amount of permeated air and the pipeline material, pressure, inner diameter, wall thickness, flow velocity, temperature and soil type. Pure hydrogen was modelled to be distributed through the low pressure grid at different conditions. After traveling 100 meters through an MDPE pipeline with 26 mm inner diameter and wall thickness of 3 mm at a flow velocity of 1 m/s, 1.4 mg oxygen and 1.9 mg of nitrogen per m3 hydrogen will have diffused into the pipeline. The results have been implemented in a case study in Stad aan ’t Haringvliet in Goeree-Overvlakkee, and the contamination for the farthest distance in the grid was found to be 0.057 ppm oxygen and 0.085 ppm nitrogen at a flow velocity of 1 m/s. Following the research set out above, no issues were found in connecting hydrogen boilers to the existing grid. Low temperature PEM fuel cells are more sensitive to impurities than boilers and some concerns were found under certain specific conditions with exceeding the current hydrogen fuel contamination limit for oxygen stated in ISO 14687-2. After traveling 529 m through MDPE and 5.8 km through HDPE the oxygen limit was exceeded. This is not considered as a constraint for the development of a future hydrogen grid, as this upper limit was set for the fuel requirement of metal hydride storage, and existing hydrogen road vehicles have another means of storage. A positive side effect of the presence of oxygen in the hydrogen feed is that it reacts with carbon monoxide, thereby decreasing fuel cell poisoning effects. Nitrogen contamination of the hydrogen feed can dilute the fuel and only at high concentrations increase fuel cell cathode poisoning caused by carbon monoxide. These high concentrations are not expected to be achieved as a result of inward permeation of nitrogen. Consequently, through the obtained results in this research it is believed that a sufficiently high purity hydrogen can be achieved in the existing distribution grid.