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Batteries and Energy Storage
Releasing Hydrogen at High Pressures from Liquid Carriers: Aspects for the H Delivery to Fueling Stations 2
Karsten Müller, Kriston P. Brooks, and Tom Autrey Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01724 • Publication Date (Web): 29 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018
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Energy & Fuels
Releasing Hydrogen at High Pressures from Liquid Carriers: Aspects for the H2 Delivery to Fueling Stations Karsten Müller1,2,*, Kriston Brooks1, Tom Autrey1 1
Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99352, United States
2
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Institute of Separation Science and Technology, Egerlandstr. 3, 91058 Erlangen, Germany
*To whom correspondence should be addressed: Karsten Müller (
[email protected], Tel: +49 9131 8527455); Egerlandstr. 3, 91058 Erlangen, Germany KEYWORDS: chemical compression, hydrogen storage, system analysis, formic acid, liquid hydrogen carriers
ABSTRACT Hydrogen fueling stations require multiple stages of compression to achieve the pressure needed to refuel hydrogen fuel cell electric vehicles (FCEV) at 700 bar. The physical compression equipment constitutes a large share of the total investment cost of hydrogen fueling stations. Hydrogen carriers, i.e., materials that carry either physi-sorbed or chemi-sorbed H2 provide an alternate approach to transport and deliver higher densities of hydrogen to the fueling station at lower pressures. Additionally, some liquid phase hydrogen carriers (LPHCs) are defined by thermodynamic properties that allow H2 release at elevated pressure thus providing an opportunity to reduce the number of compressors at the fueling station. This study compares a series of LPHCs and evaluates the approach of using aqueous solutions of formic acid (FA) to deliver high volumetric densities of H2 to fueling stations and provide a first step of compression.
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While hydrogen release from most liquid carriers will provide hydrogen slightly above ambient pressure at high temperatures, hydrogen release from the decomposition of FA can provide hydrogen at pressures of several hundred bar at moderate temperatures. A challenge of formic acid is that the high pressure hydrogen is accompanied by one equivalent of carbon dioxide and thus requires subsequent separation and purification operations. Nevertheless, formic acid has the advantage of being liquid, which simplifies its handling and provides a continuous supply to a release unit. Furthermore, the energy demand for hydrogen release from FA is lower than for most alternative hydrogen carrier materials.
Introduction Fueling stations are a crucial element of the hydrogen infrastructure required for operation of fuel cell electric vehicles (FCEV). In addition to hydrogen storage on-board the FCEV, hydrogen transport across the country from production facilities to the respective fueling stations is a critical need to address if FCEVs are to make significant market penetration. The compression equipment at the fueling station is a major cost factor.1 Compression to 875 bar H2 in the storage vessels at the fueling station is required to refuel vehicles to 700 bar, in part due to the heat generated by expansion of the initial hydrogen transferred from the storage vessel to the vehicle tank warming the hydrogen. The subsequent cooling to ambient temperatures will result in an incomplete volumetric fill thus higher fill pressures are required to assure a full tank of hydrogen.2 Two conventional concepts for hydrogen provision to fueling stations are illustrated in Figure 1. a) Hydrogen is brought to the fueling station by tanker trucks, for example, tube trailers operated at about 250 bar3 or higher2 are the current state of the art used to transport 800 kg of H2. If the hydrogen is transferred from the tube trailer to the fueling station staging tanks, a compressor (or several sequential compressors with intercooling) is needed to fill even the medium pressure storage tank at the fueling station. Further compression is needed to fill the tanks of the FCEVs at 700 bar. 2 b) A similar concept delivers hydrogen to the fueling station using a pipeline. The pipeline would operate at medium pressures of about 20 to 80 bar 4. A first compressor (or set of compressors with intercoolers) is used to compress hydrogen to about 200 bar in a medium pressure storage
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Energy & Fuels
tank and a second compressor (or set of compressors with intercoolers) to 875 bar in a high pressure tank.5 Since the fueling station in this scenario would be connected to a permanent hydrogen supply the medium pressure storage tank might not be necessary. In this case the compressor series would directly transport hydrogen into the high pressure tank.
Figure 1: Schematic of different concepts for hydrogen fueling stations
A third concept, which is the topic of this study, is based on liquid phase hydrogen carriers (LPHCs), i.e., hydrogen rich liquids for delivering hydrogen to the fueling station. c) LPHCs provide the distinct advantage of using existing infrastructure, i.e., tanker trucks and pipelines that transport hydrogen at ambient temperature and pressure. One difference for the LPHC concept is the need for a release unit and a purification unit to provide hydrogen at the required purity, at the filling station where hydrogen is released and transferred into trucks, pipelines or storage vessels. There are a number of common features shared by liquid phase hydrogen carriers for transport and liquid phase hydrogen storage materials. Examples for carrier based hydrogen delivery and storage options range from the physi-sorption of hydrogen onto high surface area carbon materials 6 or metal organic frameworks7 to the chemi-sorption of hydrogen in metal hydrides8 or liquid organic hydrogen carriers (LOHC; e.g., dibenzyltoluene
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or N-ethylcarbazole10) to the
conversion into fuels like compressed natural gas, methane11 or methanol12.
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Formic acid (FA) has long been considered as a hydrogen storage material.13 It is notable that there is an ongoing project to design a fuel cell powered bus that uses a FA formulation as a hydrogen source in the Netherlands.14, 15 More recently there has been a growing interest in using formic acid as a LPHC for hydrogen delivery to filling stations.16 Below we summarize some of the relevant thermodynamic properties for a series of material based hydrogen carriers and look more closely at the attributes of FA. The goal of our current effort is to analyze the advantages and note the challenges for using FA to carry H2 in tank trucks or pipelines from distribution sites to filling stations at ambient pressure and temperatures as a liquid and avoid the first compression step at the filling station.
Liquid Carrier materials for H2 delivery Table 1 compares the thermodynamic properties of a series of liquid phase hydrogen carriers. The range of materials includes liquid organic as well as inorganic hydrogen carriers, aqueous based carriers, slurries of high density inorganic carriers and methane. Hydrogen release from liquid phase hydrogen carriers can be distinguished (in a simplified way) into two categories: negative molar Gibbs free energy of reaction ∆rG, or a positive value of ∆rG. In the first case the thermodynamic driving force for hydrogen release is large; in the latter case hydrogen release is more demanding.
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Energy & Fuels
Table 1. Thermodynamic properties of selected liquid phase hydrogen carriers (LPHCs) at 298.15 K and 1 bar.17-21 TH2 / °Ca
ρH2 / g L-1
12
>220
54
65
26
>260
56
l-NH3(d)
46
17
>300
128
(e)
-19
-42
