Power-To-Gas: Admixture of hydrogen from renewable energies

Introduction

In addition to the requirements for a secure, reliable and affordable power supply, the idea of sustainability within the context of the energy revolution is coming into focus. Renewable energy sources such as wind, water or solar have an important role to play in this energy mix. Since the electricity generated from these upcoming but highly fluctuating energy sources cannot be transported or consumed in a way that allows for grid compensation, it must be stored. One possibility is to store the energy as gas in the existing natural gas network. For years, there have been developments towards converting electrical energy into storable gases such as hydrogen (H₂) or synthetic natural gas. The process of converting electricity into gas by electrolysis is known as power-to-gas (also PtG or P2G). The hydrogen produced can be fed into the existing natural gas network, stored there, transported and consumed as required. In numerous countries of the European Union, research projects have been running since about 2010 looking at the question of how much hydrogen the existing natural gas network is able to absorb without the gas consumption points being negatively affected. In industry, very different limit values for the admixture of H₂ with natural gas are currently mentioned. Values typically range from 5 to 25% by volume. However, what seems clear is that the proportion will increase steadily over the coming years. How quickly this happens will certainly depend on the speed of investment and the progress made with developing power-to-gas technologies. The question of what effects the admixture of hydrogen into natural gas has on the infrastructure installed today is of increasing concern to the industry.

Technical Guideline G19 (TR G 19)

In December 2014, the Physikalisch Technische Bundesanstalt (PTB) issued the Technical Guideline TR G 191, which regulates “feeding hydrogen into the natural gas network” for “measuring instruments for gas.” The Guideline declares the use of gas measurement devices “of any technologies” shall be safe, provided that the hydrogen content of the natural gas is less than 5% by volume. The use of meters is permitted for a proportion between 5 and 10% by volume of hydrogen, provided the manufacturer explicitly permits this. For the use of meters with natural gas containing > 10% hydrogen by volume, a manufacturer’s declaration as well as a PTB declaration of clearance must be submitted in addition to the manufacturer’s declaration. The FLOWSIC600 and FLOWSIC600-XT gas meters installed today can be used in applications with up to 10% hydrogen by volume in natural gas; this is possible within the calibration error limits and without the need for a new metrological test. SICK has published a corresponding manufacturer’s declaration in accordance with TR G 19.2.

Effect of the admixture of hydrogen on measuring capability

The addition of hydrogen has an effect on the characteristic curve behavior and thus on the measuring uncertainty of the devices. A measuring capability does not amount to the same thing as an unchanged measurement accuracy. The latest test results of an ultrasonic gas meter calibrated with natural gas show the relative error (measurement deviation) on the measurement result (Fig. 1 and Fig. 2) caused by a hydrogen admixture of 10% and 25% by volume, respectively.

Image caption: Fig.1 (left): Influence of the H₂ content on the measurement error of a DN100 FLOWSIC600-XT after application of the linearisation correction, on the basis of pure natural gas data. Fig 2 (right): Influence of the H2 content on the measurement error of a DN200 FLOWSIC600-XT after application of the linearisation correction, on the basis of pure natural gas data.

The relative error is about 0.1% with a proportion of 10% hydrogen by volume in the natural gas in the lower flow rate range (Qmin). This error lies far within the transport error limits for natural gas measurements subject to calibration. Similar data was published in a technical report by gwf-Gas in May 2013. A FLOWSIC600 DN80 was used for the investigations. The report concludes, “Up to 10% H2 content by volume, no influence on the ultrasonic gas meter can be detected if the hydrogen is well mixed with the natural gas”. SICK ultrasonic gas meters are able to measure natural gas containing hydrogen. A recalibration is not necessary if up to 10% by volume of hydrogen is fed in.

Effect of the admixture of hydrogen on material compatibility

The Federal Institute for Materials Research and Testing (BAM), in its report entitled Resilience assessments of metallic container materials and polymeric sealing/coating and lining materials of January 2015, examined the material resilience of certain materials for use with natural gas containing hydrogen. This shows that the gas flow meters made of the usual material alloys (steels) and all other parts in contact with the medium, such as ultrasonic probes and sealing rings, are resistant to natural gas containing hydrogen.

Effect of the admixture of hydrogen on explosion protection

Hydrogen has a different specific ignition capability from that of natural gas. Taking account of purely hydrogen flow rate measurements, the applicable explosion group under explosion protection regulations is IIC. This defines higher requirements for the equipment with regard to ignition gap dimensions and energy inputs than for natural gas. Explosion group IIA is sufficient for a natural gas measurement. In September 2016, the Federal Institute for Materials Research and Testing (BAM) published its report which shows that the explosion pressure changes only slightly up to an H2 proportion of 25% by volume. Likewise, a 10% by volume admixture of hydrogen has no significant influence on the standard gap width for the gas group IIA (Fig. 3). The results lead to the conclusion that a 25% admixture of hydrogen by volume, in all likelihood, does not inadmissibly reduce the standard gap width for the gas group IIA.

Conclusion

Gas flow meters of the SICK FLOWSIC600 and FLOWSIC600-XT families, due to their ultrasonic technology, are suitable today for measuring natural gases containing proportions of hydrogen up to 10% by volume within the scope transport according to the laws of calibration. The reliability and quality of the measurement results are not affected by changes in density, flow velocity or speed of sound.

Image caption: iStock.com/peterschreiber.media