Ammonia As Hydrogen Carrier to Unlock the Full Potential of Green Renewables

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Abstract

For decades, grid-scale energy storage has been used to balance load and demand within an energy generation system composed mainly of base load power sources enabling thus to large nuclear or thermal generating plant to operate at peak efficiencies. Energy storage has contributed over the time to meet peak demand and regulate frequency beside peak fossil fuel power plant who usually provided the bulk of the required energy. In the aforementioned context where inherent variability of the power generation asset was mainly a minor issue, energy storage capacity remains nevertheless limited for economic reason storing electricity during low electricity demand and releasing it back into the grid during high demand, typically over a daily cycle. In a context of a global spectacular decrease in levelized cost of renewable electricity (typically PV or onshore/offshore wind) observed in the recent years, the European Commission as a clear front runner in the fight against climate change has set as soon as the year of 2014 a committing objective of 27% for the share of renewable energy consumed in the European Union by 2030. This objective which was later confirmed in its intention in 2015 when the European commission ratified the Paris Agreement is now regarded as a conservative objective for the EU, the penetration of RE being lately advised to be further accelerated to 34% by 2030 [1].

Energy storage is acknowledged has a decisive element to insure a reliable and efficient penetration of renewable electricity in the energy system and starting from its role as initially thought, it is now expected from a portfolio of energy storage means to offer a multitude of services . Depending of the situation, energy storage technologies must have the capability to provide key services like grid services adequacy (congestion management, curtailment reduction) or ancillary services (frequency response, black start, voltage) but also the ability to react within seconds to large electrical load changes. Finally, it is projected for energy storage to shortly contribute to the decarbonization of other energy intensive sectors thanks to the sectorial integration of the power sector with transport, the industry and the heating and cooling sectors. If battery systems reveals to be decisive components of the energy management system especially for fast response services, hydrogen based energy carriers appear as one of the only solution when it comes to seasonal energy storage of large energy quantity and more specifically for all situation dealing with a large energy-to-power ratio situation. Also, in this new energy paradigm where distributed renewable generation cohabit with increasingly larger wind or PV power plant located farther away from consumption site, the ability to store energy under the form of a dispatchable energy carrier is a key element

As a versatile and flexible energy storage mean, green hydrogen produced by electrolysis through power-to gas offer compelling reasons for its breakthrough penetration into the energy system. Hydrogen has the unique capability to significantly and potentially simultaneously decarbonize several energy consuming sectors through electrification. Hydrogen can be seen as a renewable electricity vector with ability to be transported from green electricity surplus area to energy consuming one with minimum losses. This allow for instance to consider renewable electricity transport from neighboring or distant countries but also to improve profitability of offshore energy harvesting.

There is so far a great deal of discussion and research about the most efficient way to transport hydrogen according to distance and volume. So far, only a few studies focused on hydrogen transportation cost and most of them demonstrated that hydrogen transportation cost shall not be neglected. As many hydrogen transportation technologies are available, there is a critical need for techno economic evaluation in order to obtain reliable hydrogen transportation costs and address the technological challenges involved.

Amongst all means, ammonia appears to be one the most promising hydrogen carriers. Ammonia is easily liquefied by compression at 1 MPa and 25°C and shows a vapor pressure similar to propane. Ammonia presents a high hydrogen gravimetric density of 17.8 % by weight and simultaneously an impressive volumetric hydrogen density with approximately 108 kg H2/m3 embedded in liquid ammonia at 20 °C and 8.6 bars. Comparing this to advanced hydrogen storage systems, e.g. metal hydrides, which store H2 up to 25 kg/m3 or to liquefied hydrogen (1,5 time lower) the advantage of ammonia in carrying hydrogen per unit volume is significant[1]. This generally translate for instance in ammonia providing a lower cost per unit of stored energy compared to hydrogen as calculated for instance in previous study (storage over 182 days ammonia storage would cost 0.54 $/kg-H2 compared to 15 $/kg-H2 of pure hydrogen storage).

In addition to that, thanks to the large return of experience regarding ammonia chemistry, manufacturing or handling but also using ammonia existing infrastructure for storage and transport, ammonia can be used as a profitable energy carrier for hydrogen distributed generation using compact ammonia decomposition reactors. This possibility is generally promoted by the fact that ammonia decomposition has a single feed stream and is therefore accomplished in a single step which leads to significant cost advantage in consequence of reduced balance-of-plant (BOP). This contrast particularly with the multi-step process inherent for instance in steam reforming of hydrocarbons or in methanol reformaing. This provides key opportunities for decentralized power generation thanks to hydrogen fuel cells but also pave the way toward on-board ammonia decomposition for fuel cell vehicule application. Besides this, ammonia also have the capability to be used directly for centralized power generation in ammonia turbine, in distributed power generation through the use of high temperature solid oxide fuel cells or mobility application in direct combustion engine. For all considered possibilities, ammonia has the unique features to behave as a CO2-free energy storage mean unlike other hydrogen carriers like methanol, methanol, formic acid and all Fischer-Tropsch product which release carbon oxides during their use.

In this presentation, the potential of ammonia as hydrogen carrier for large scale hydrogen transportation will be discussed on the basis of both modelling and experimental work.

Thanks to the in-house HYTAC calculation tool (Hydrogen Transportation Analysis and Costing) developed within ENGIE Lab CRIGEN to compute the hydrogen transportation cost, several large scale hydrogen production and transportation scenario are analyzed for various renewable electricity sourcing and according to different demand scenario and transportation distance. The potential of ammonia, liquid organic hydrogen carrier, methanol and SNG as hydrogen carrier for the long distance transportation of green hydrogen from several areas of important renewable energy technical potential to oversea utilization point is analyzed and compared. In the presented work, the entire process value chain is considered from renewable electricity sourcing, green hydrogen production, hydrogenation process, maritime transportation, dehydrogenation and final utilization of hydrogen for mobility or industrial applications. A cost breakdown analysis is provided helping to define the main contributor to the levelized cost of delivered energy and demonstrate R&D efforts to be done to reach a economic profitability.

Also, as far as ammonia is concerned, the question of green hydrogen recovery using a centralized or a decentralized ammonia cracker is still unanswered. First, centralized fired heated ammonia cracker does not exist so far at size to produce hundred of tons of hydrogen per day. Then, considering the stringent limit in ammonia content into the hydrogen feeding a PEM fuel cell (0.1 ppm), selective dehydrogenation reactor are required in order to reduce purification cost of the forming gas produced. In this presentation, first results of experiments performed on an innovative compact decomposition reactor will be introduced. Obtained performances will be compared on the technical and economic point of view with a designed centralized cracker.

[1] A. Valera-Medina. Ammonia for large scale powergen. Paper presented at the NH3 Event, Rotterdam, 18 May 2017.

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