Challenges of integration of sustainable fuels into the energy infrastructure

Gasunie Engineering and Technology - www.getgasunie.nl

CHALLENGE: to accelerate the use of sustainable fuels.
The rapid introduction of carbon-neutral fuels into the energy supply will assist both the transition to a sustainable energy infrastructure and reduction of the atmospheric carbon burden.

How can this introduction be facilitated?
For example, the addition of sustainable hydrogen, from wind, solar energy or biomass, to natural gas has been suggested in the US and Europe as a promising scenario for the transition. If one also considers the addition of biogas, sustainable fuels can be introduced into the energy supply with minimal investment both in end-user equipment and pipeline transport capacity. However, this advantage also presents a number of challenges for implementation. The amount of hydrogen/biogas that can be added to natural gas is limited by the specific properties of the infrastructure and end-user systems. For example, the structural integrity of many types of steel commonly used in high-pressure pipeline grids can be threatened by "hydrogen cracking", and the potential shortening of the lifetime of the grid is a powerful argument for limiting the amount of hydrogen added to natural gas. Further, since most sustainable gases have a lower volumetric calorific value than natural gas, addition of these gases to pipelines without increasing the gas pressure reduces the net energy transport. A more immediate hurdle is the response of end-user equipment to hydrogen/biogas-enriched natural gas; because the combustion properties of these gases are as a rule significantly different than those of natural gas, their mixtures could have properties that interfere with the safe and effective functioning of combustion equipment. One possible means of circumventing this problem is to produce synthetic natural gas, for example by removing excess CO2 from biogas derived from fermentation of biomass until the gas "quality" is compatible with the pipeline gas it replaces. Of course, all gas treatment reduces the energy efficiency of the production process and increases the price, substantially.

These and other CHALLENGES must be met to accelerate the use of sustainable fuels.

At Gasunie Engineering and Technology, we are leading a number of research and demonstration projects aimed at overcoming these barriers:

	Determining the amount of H2/biogas added to natural gas acceptable for both pipeline integrity and the current end-user equipment. Recent results from our laboratory show that, in the Netherlands, a maximum of 5% H2 may be present in the natural gas for the household market, and still guarantee the performance of domestic appliances. The results also show that the wider the range of Wobbe Index of the distributed gas, the more hydrogen can be accepted. One caveat is that the maximum hydrogen concentration can only be realized at the maximum of the Wobbe range, and decreases with decreasing Wobbe Index. Thus, any system for admixing H2 into natural gas for this market must mix according to Wobbe Index. On the other hand, many combustion systems for use in industrial processes seem more robust than domestic appliances, and can accommodate a substantially higher percentage of hydrogen in natural gas without malfunction. However, experiments showed that hydrogen addition resulted in a substantial increase in NOx emissions. Since this increase arises through the higher flame temperature caused by the presence of hydrogen, the potential decrease in CO2 emissions is intrinsically counterbalanced by an increase in NOx emissions. New combustion technology must be developed to compensate for this deleterious effect.
	Investigating methods to circumvent the limitations found (e.g., new combustion equipment, decentralized gas addition, i.e., adding gas either in the local distribution network or even at individual end users). The results described above suggest that, in the short term, sustainable hydrogen can be introduced into the existing natural gas infrastructure on a larger scale by restricting hydrogen-natural gas cofiring to industrial clusters. This option could also substantially avoid the risks for the integrity of the high-pressure transmission pipelines caused by hydrogen embrittlement.

Finally, because hydrogen addition tends to reduce the volumetric calorific value of the fuel, a larger volume of fuel must be consumed to satisfy the energy demand; the net result is that, per unit of energy used, the CO2 reduction achieved is only half of the hydrogen reduction. This effect, together with the NOx penalty mentioned above, must be incorporated into the policy debate on this mode of hydrogen introduction. Through scientific and technical expertise, combined with an eye for seeing the technical challenges that lie behind decisions regarding energy policy, Gasunie Engineering and Technology is helping to clear the path towards sustainability, using natural gas as the bridge.

For more information: H.B. Levinsky (h.levinsky@gasunie.nl) and W.J.Th. van Gemert (w.j.t.van.gemert@gasunie.nl)