The transition of the transport sector from fossil fuels to carbon dioxide free technologies is an enormous challenge. Liquid organic hydrogen carrier (LOHC) fueled trains are an attractive option for all non-electrified railway lines currently operated by diesel trains. The driving unit of a LOHC fueled train consists of LOHC storage tanks, a hydrogen release unit, a heating unit, a fuel cell and a battery. In our study, the LOHC system dibenzyl toluene/perhydro-dibenzyl toluene is selected. The dimensioning of the individual units is not straightforward as a smaller battery would require a larger fuel cell and the choice of a fuel cell influences the size of the heating unit, as the waste heat from a solid oxide fuel cell (SOFC) can be used for the dehydrogenation process. However, a SOFC of the same power class is larger and heavier than a proton exchange membrane fuel cell (PEMFC) and reduction of the size of the heating unit can result in an increase of the fuel cell size. By using a genetic optimization algorithm, we can minimize volume, mass and total driving costs of the model-based driving unit. The optimized driving unit contains a 722 kW fuel cell, a 820 kWLHV-H2 hydrogen release unit and a 523 kWh battery. This configuration results in a volume around 45 m3, a total mass around 27 t and driving costs around 3 EUR/km for the SOFC scenario and 60 m3, 26 t and 2.4 EUR/km for the PEMFC scenario.

Hydrogen storage and transportation using liquid organic hydrogen carrier (LOHC) systems is a promising way to make use of the existing infrastructure for fuels (tank farms, tank vehicles) for the ramp-up of a future hydrogen economy. For a cost-efficient implementation of LOHC-based technologies the stability of both the hydrogen carrier compounds and the applied hydrogenation/dehydrogenation catalysts is of critical relevance. Herein, we show that catalyst stability in the continuous dehydrogenation of the LOHC compound perhydro benzyltoluene (H12-BT) benefits strongly from a combination of catalyst material pretreatment (drying and reduction) and LOHC purification (removal of volatiles and selective adsorption of oxygenated impurities). This has been demonstrated by using a pellet string reactor with only 30 catalyst pellets (commercial Elemax D101, Clariant), in which only one pellet fills the cross-section of the reactor that is hardly wider than the diameter of the applied catalyst pellets. With this specific reactor the ratio of feed flow to catalyst mass is 20-60 times higher than in a typical technical dehydrogenation reactor allowing us to quantify detrimental effects of impurities on catalyst stability faster and more accurately. Our study demonstrates that all impurities leading to CO formation in the reactor have a significant negative influence on the catalyst stability. Removing such impurities from the catalyst and the LOHC feed enables almost stable dehydrogenation performance in the string reactor for 50 h time-onstream.

In reactions that release gaseous products from liquids, heterogeneous, porous catalysts can be in an active or nucleation-inhibited state as has recently been shown by our group for batch dehydrogenation reactions. This paper investigates this practically highly relevant phenomenon now in a continuous tube reactor for the example of liquid organic hydrogen carrier (LOHC) dehydrogenation. A mechanical stimulus, increase in reaction temperature or decrease in residence time reactivates the inhibited catalyst bed. Furthermore, the experimental results indicate that the dehydrogenation reaction turns into a hydrogenation reaction for thermodynamic reasons as the catalyst bed cools down. Hydrogenation is responsible for the consumption of the gas phase and thus a liquid filling of the pellets, which causes the nucleation-inhibition. Our work reveals new aspects of nucleation-inhibition on catalyst beds and provides insights into the efficient operation of heterogeneous catalytic gas release reactions in which this phenomenon occurs.

A typical feature that the LOHC technology shares with other types of chemical hydrogen storage is that the release of hydrogen from the carrier requires an input of heat. In many use cases where waste heat from external sources is not available (e.g. in heavy-duty mobility), this is seen as a major drawback. In this paper, we show that autothermal LOHC dehydrogenation offers a very attractive way to overcome this drawback. In detail, we demonstrate autothermal hydrogen release from dicyclohexylmethane using diphenylmethane oxidation to benzophenone as source of heat. The full storage cycle involves benzophenone hydrodeoxygenation to dicyclohexylmethane, dicyclohexylmethane dehydrogenation to diphenylmethane, and diphenylmethane oxidation to benzophenone. We studied both the individual reaction steps using pure feedstocks and the integral cycle in which the intermediates and by-products of each reaction remain in the system for the subsequent reaction step. Although no efforts have yet been made to develop special catalyst materials for this purpose, the results with the applied commercial hydrodeoxygenation (Pd/C), dehydrogenation (Pt on alumina) and partial oxidation (VOx/TiO2) catalysts are already very promising. The storage cycle can be closed with high selectivity and with only minor total oxidation losses. The proposed concept of autothermal LOHC dehydrogenation offers the potential to increase the amount of useable hydrogen from a given amount of charged hydrogen carrier by up to 30%.

The release of hydrogen from liquid organic hydrogen carriers (LOHC) takes place in an endothermal dehydrogenation reaction that is accompanied by a strong volume expansion. This leads to complex hydrodynamic properties that change drastically along the reactor axis due to product gas evolution. Consequently, heat transfer into the catalytic fixed-bed exhibits a pronounced local dependency. For a better understanding of such multiphase dehydrogenation systems, we have performed heat transport measurements in the presence of the chemical reaction, namely during the dehydrogenation of perhydro benzyltoluene (H12-BT) and perhydro dibenzyltoluene (H18-DBT). The results reveal that overall heat transfer coefficients show a clear local dependency on the axial coordinate. Moreover, the two carriers were found to differ significantly in their thermal behavior. Based on a global analysis, two main regimes can be distinguished in the dehydrogenation reactor: 1.) With the LOHC mixture being primarily in the liquid phase, heat transport is dominated and intensified by the hydrogen release; 2.) With an increasing proportion of LOHC vapor in the reactor, the heat transport is dominated by the gas phase, resulting in significantly lower thermal parameters.

The benzyltoluene- and dibenzyltoluene-based liquid organic hydrogen carrier (LOHC) technology is approaching a level of maturity that enables its large-scale implementation for hydrogen storage and transport applications. To support this progress, our study investigates the continuous dehydrogenation of both LOHC systems in a packed bed tubular reactor equipped with high-resolution temperature measurement. Our experiments cover a wide range of reaction conditions resulting in various degrees of dehydrogenation (DoD). In particular, our study highlights the relevance of LOHC evaporation in the dehydrogenation reactor caused by the large amounts of hydrogen gas formed and consequently the low LOHC partial pressure in the reactor. Evaporation of the LOHC compounds is more pronounced for the benzyltoluene-than for dibenzyltoluene-based system. This leads to a shortening of the residence time in the reactor and less favourable heat transfer properties. Interestingly, these seemingly unfavourable properties of the benzyltoluene-based system are compensated by faster gas phase kinetics and lower convective heat removal from the reactor, resulting in very similar heat consumption values for both systems under continuous operating conditions.

The use of the dibenzyltoluene/perhydro‐dibenzyltoluene (H18–DBT) system as a liquid organic hydrogen carrier (LOHC) enables the safe and loss‐free storage of hydrogen. The release of hydrogen from the LOHC is a catalytic reaction and requires ≈27% of the lower heating value of the released hydrogen in the form of heat neglecting heat losses. The high heat requirement makes it necessary to design chemical conversion units that both provide good heat input and accommodate the high gas release. Up to 1200 L of hydrogen is released from 1 L of LOHC under reaction conditions. In this work, a cuboid reactor for the release of hydrogen from H18–DBT is demonstrated. In the experiments, it is shown that evaporation has a significant effect on the reaction rate and thus the amount of hydrogen releases. Therefore, a kinetic model capable of accounting for the release rate and evaporation in the reactor is developed. The model is successfully validated and shows deviations of less than 15% between measured and modeled hydrogen flow in the entire range considered. Since the model considers this important interaction between evaporation and hydrogen release for the first time, it is suitable for optimizing the reactor used.

Liquid organic hydrogen carriers (LOHCs) are a promising option for hydrogen storage, but a high efficiency of the LOHC cycle is essential in order to serve as an attractive technology in the context of decarbonization. This paper presents different methods to optimize steady-state operating points of a LOHC dehydrogenation (DH) reactor. For this purpose, an analytical model of a DH reactor is described, which is extended to a hybrid model (HM) to achieve sufficient model accuracy. The model quality is subsequently validated by measurements. For the optimization of the steady-state operation point, a Bayesian optimization framework is presented and compared to classical model-based optimization. The methods are evaluated with the HM as well as with experimental results.

Hydrogen storage and transportation in form of charged Liquid Organic Hydrogen Carrier (LOHC) systems is attractive as these hydrocarbon-based carrier molecules can provide hydrogen using the existing infrastructure for fossil fuels. For hydrogen release on board of heavy-duty vehicles, however, the limited volumetric power density of the hydrogen release units was so far seen as a critical factor. Herein, we show that the power density achieved in perhydro benzyltoluene dehydrogenation in a classical fixed-bed dehydrogenation reactor can be doubled by applying an inverted multi-tubular reactor with upstream LOHC flow and hydrogen release in the reactor housing and crossflow heating through perpendicular heating tubes. The resulting power densities of up to 0.76kW(H2-LHV) L-reactor-outside(-1) (with respect to the total reactor housing), and 2.34kW(H2-LHV) L-reactor-inside(-1) (with respect to inner reactor volume) bring on-board hydrogen release of LOHC-bound hydrogen much closer to technical reality. This very impressive increase in power density is mainly due to the fact, that the inverted arrangement of catalyst bed and heat transfer tubes offers a much higher catalyst volume per reactor volume compared to a classical fixed-bed reactor.

This short review discusses recent developments related to the storage and release of hydrogen from liquid organic hydrogen carriers (LOHCs). It focusses on three areas of recent literature: the application and development of novel, alternative LOHC systems, process development and process integration in the storage and release of hydrogen from LOHCs, and the electrochemical conversion of LOHCs. For the novel LOHC systems, we briefly focus on reaction enthalpy and storage capacity as main KPIs for the comparison of those systems and discuss the technical availability on a relevant scale. In the field of process- and reactor development our emphasis lies on the power density of the chemical conversion units. The LOHC technology still requires further development to reach the necessary energy efficiency, flexibility and overall research maturity for market competitively and commercial impact.

The utilization of hydrogen as a fuel in free jet burners faces particular challenges due to its special combustion properties. The high laminar and turbulent flame velocities may lead to issues in flame stability and operational safety in premixed and partially premixed burners. Additionally, a high adiabatic combustion temperature favors the formation of thermal nitric oxides (NO). This study presents the development and optimization of a partially premixed hydrogen burner with low emissions of nitric oxides. The single-nozzle burner features a very short premixing duct and a simple geometric design. In a first development step, the design of the burner is optimized by numerical investigation (Star CCM+) of mixture formation, which is improved by geometric changes of the nozzle. The impact of geometric optimization and of humidification of the combustion air on NOx emissions is then investigated experimentally. The hydrogen flame is detected with an infrared camera to evaluate the flame stability for different burner configurations. The improved mixture formation by geometric optimization avoids temperature peaks and leads to a noticeable reduction in NOx emissions for equivalence ratios below 0.85. The experimental investigations also show that NOx emissions decrease with increasing relative humidity of combustion air. This single-nozzle forms the basis for multi-nozzle burners, where the desired output power can flexibly be adjusted by the number of single nozzles.

The benzyltoluene‐based liquid organic hydrogen carrier (LOHC) system enables the safe transport and loss‐free storage of hydrogen. At least 26% of the lower heating value of the released hydrogen, however, has to be invested in form of heat to release the stored hydrogen. The low operation temperatures of catalytic distillation (CD) can facilitate waste heat integration to reduce external heat demand. Herein, the continuous hydrogen release from perhydro benzyltoluene via CD is demonstrated. It is revealed in the experimental results that this mode of operation leads to a high hydrogen release rate and very efficient noble metal catalyst usage at exceptionally mild conditions. The hydrogen‐based productivity of platinum of 0.35 g H2 g Pt −1 min −1 (0.7 kW LHV_H2 g Pt −1 ) at a dehydrogenation temperature of only 267 °C is found to be nearly four times higher than for the conventional continuous liquid‐phase dehydrogenation at the same temperature. Furthermore, simulation results of the CD process are described. The feasibility of a fully heat‐integrated process for electricity generation from the released hydrogen via CD using waste heat from the fuel cell for the CD reboiler is demonstrated. The technical potential of coupling the H12–BT dehydrogenation by CD with high‐temperature fuel cell operation is highlighted by the simulation.

Hydrogen storage in liquid organic hydrogen carriers (LOHC) enables the utilization of renewable energy in different sectors. In this paper, we describe the operational experience with one single LOHC system for bidirectional electrical energy storage at the kW scale. The system includes a reactor for the hydrogenation and dehydrogenation of LOHC, as well as a fuel cell and an electrolyzer based on polymer electrolyte membrane (PEM) technology. The LOHC used is the substance pair dibenzyltoluene/perhydro-dibenzyltoluene. For dehydrogenation, the upflow operation with common discharge of liquid and gaseous product was found to be the preferred mode of operation. For hydrogenation, it was shown that stable operation is possible also with fluctuating hydrogen production from the electrolyzer. After operating the reactor for 725 h in the hot state, i.e., at temperatures above 150 ∘C, samples of the catalyst and LOHC were taken and analyzed. These showed no signs of serious degradation.

Temperature control in heat exchangers in reacting and non-reacting flows is of great importance for process optimization. In this context, phosphor thermometry is a promising technique for remote planar temperature sensing. The thermometry technique is based on exciting a luminescent material by a laser pulse and analyzing the subsequent phosphorescence signal. A particular interesting application is chemical hydrogen storage using liquid organic hydrogen carrier (LOHC) systems. The knowledge of temperature fields is of special interest for the characterization and understanding of hydrogen release from the carrier liquid. We investigated the luminescence properties of the thermographic phosphor (Sr,Ca)SiAIN 3 :Eu 2+ (‘SCASN:Eu 2+ ’) dispersed in different heat transfer fluids, in particular LOHC systems, using a newly developed calibration cell. As heat transfer fluids may be excited to fluorescence by the laser as well, their absorption and florescence behavior is studied to develop an excitation and detection concept for thermometry. We found strong absorption of the heat transfer fluids from the UV range to a wavelength of about 400 nm. In addition, fluorescence signals were found in the visible wavelength range, which can interfere with the phosphor emissions. These fluorescence signals should therefore be circumvented by utilizing the different luminescence decay times in the chosen detection strategy. For thermometry, the SCASN:Eu 2+ particles were excited by a laser sheet of a 532 nm Nd:YAG laser. A spectrometer and photomultiplier tube (PMT) were used to detect the emission spectrum and phosphorescence decay time (PDT). Two temperature evaluation strategies were applied, which are based on either the intensity ratio of two spectral emission regions (two-color laser-induced phosphorescence) or the PDT. The results obtained show an applicable measurement range between 293 K and 598 K for the intensity ratio method with a maximum relative sensitivity of 0.12% K −1 at 293 K. For the PDT method, the phosphor allows measurements between 423 K and 598 K with a maximum relative sensitivity of 0.56% K −1 at 598 K.

In hydrogenation and dehydrogenation processes of liquid organic hydrogen carriers (LOHCs), molecular hydrogen (H2) is present, but its influence on the thermophysical properties of the LOHC compounds is still hardly known. This study provides experimental results from surface light scattering and predictions from molecular dynamics simulations on the influence of dissolved H2 on the liquid viscosity, interfacial tension, and liquid density of the LOHC system based on diphenylmethane at varying degree of hydrogena-tion, process-relevant temperatures up to 573 K, and pressures up to 7 MPa. First-time measurements of the viscosity of bicyclic hydrocarbon compounds in the presence of dissolved H2 at saturation conditions reveal a negligible effect of pressure. The interfacial tension decreases independently of the LOHC composition by about 6% at 7 MPa. The simulations can adequately represent the effect of H2 on the interfacial tension and evi-dence a weak enrichment of H2 at the interface.(c) 2022 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/).

This study presents benzyltoluene/perhydro benzyltoluene as a very favourable liquid organic hydrogen carrier (LOHC) system for potential industrial applications.

2‐Propanol/acetone is a promising liquid organic hydrogen carrier system for fuel cell reactions. Herein, six different concepts for a 2‐propanol/acetone fuel cell system are evaluated in MATLAB simulation with respect to their thermodynamic integration and technical feasibility. Four of the concepts use a direct 2‐propanol fuel cell while the other two first release molecular hydrogen from 2‐propanol and subsequently use a hydrogen fuel cell. The presented liquid phase 2‐propanol fuel cell concept is thermodynamically feasible but cannot be realized technically using commercial Nafion membranes, due to membrane dissolution by the 2‐propanol/acetone/water fuel mixture. Gaseous 2‐propanol fuel cells imply a high heating requirement for the evaporation of the fuel. A direct high‐temperature fuel cell using 2‐propanol is thermodynamically feasible because there is less water in the overall system but is not technically feasible because of the esterification of phosphoric acid. A very interesting option is the conversion of gaseous 2‐propanol to pressurized hydrogen in an electrochemical pumping step followed by a hydrogen fuel cell, because here the waste heat of a sufficiently hot hydrogen fuel cell can drive the 2‐propanol evaporation.

Ionic and covalent cross-linking of an acid–base blend is used to manufacture membranes with high stability against dissolution and reduced isopropanol and acetone crossover for direct-isopropanol fuel cell applications.