• Energy Research

Abstract Temperature profiles and effluent concentrations were measured during combustion of aqueous urea and ammonium nitrate at pressures of 1–15 MPa. Pollutant levels decreased, while the combustion temperature showed a non-monotonic change with increasing pressure. Experimental temperature profiles were applied in kinetic gas-phase simulations, and resulting species concentrations were in good agreement with experimental values. Sensitivity analyses indicated the kinetic parameters of isocyanic acid hydrolysis are the main source of uncertainty, possibly leading to the lower agreement observed for experimental and simulation carbon species. Rate of production analyses indicated that isocyanic acid is mainly consumed by hydrolysis to carbon dioxide, while nitric acid reacts with nitrous acid to produce water. Ammonia exhibited two channels of decomposition, reacting with either hydroxyl or nitrogen dioxide to form amidogen and either water or nitrous acid, respectively. Nitrogen was mainly formed by the three-body reaction of diazenyl. As pressure increased, the aforementioned pathways became increasingly dominant.

AbstractReview: 90 refs.

Abstract Production of transportable and environmentally friendly synthetic chemical fuels using hydrogen produced by water splitting, using renewable energy will facilitate energy storage and incorporation of renewable energy into the grid. Both carbon and nitrogen can serve as hydrogen carriers leading to carbon- or nitrogen-based fuels carriers. Although the carbon route is vastly reported, the nitrogen-based analog is only scarcely described in the literature, and its economic potential is completely overlooked. Using levelized cost of storage analysis, this work evaluates for the first time the economic feasibility of a nitrogen economy, where liquid nitrogen-based fuels serve as alternative hydrogen carriers. The results indicate that an aqueous solution of ammonium hydroxide and urea is competitive with other future large-scale energy storage solutions such as methanol and batteries. At a hydrogen price below 2.5 $/kg, this fuel can be competitive with currently-used mature technologies.

AbstractAqueous urea ammonium nitrate has been previously suggested as a hydrogen‐carrying monofuel. The addition of helium and water to the fuel is known to increase its autoignition temperature. Nevertheless, both the reason for this behavior and the reaction pathways that lead to the ignition of the fuel are not completely clear. In this work, the effect of fuel‐rich conditions (1.1≤φ≤2.5) on the thermal autoignition of this fuel was investigated. The pre‐ignition reaction network was explored using a kinetic gas‐phase model. Furthermore, the effects of helium, water, and fuel‐rich conditions on these reactions were studied using simulations. The results indicated the autoignition temperature rises with the increase of the urea content of the fuel mixture. Moreover, this behavior was suggested to stem partially from the inhibiting effect of low nitric acid levels on amidogen generation. Additionally, helium and water were identified to act as diluents that increase the pressure and the flux through three‐body reactions. This work provides a valuable in‐depth look into the reactions that lead to the ignition of aqueous urea ammonium nitrate and their dependence on the process parameters.

Solar water splitting provides a promising path for sustainable hydrogen production and solar energy storage. One of the greatest challenges towards large-scale utilization of this technology is reducing the hydrogen production cost. The conventional electrolyzer architecture, where hydrogen and oxygen are co-produced in the same cell, gives rise to critical challenges in photoelectrochemical (PEC) water splitting cells that directly convert solar energy and water to hydrogen. Here we overcome these challenges by separating the hydrogen and oxygen cells. The ion exchange in our cells is mediated by auxiliary electrodes, and the cells are connected to each other only by metal wires, enabling centralized hydrogen production. We demonstrate hydrogen generation in separate cells with solar-to-hydrogen conversion efficiency of 7.5%, which can readily surpass 10% using standard commercial components. A basic cost comparison shows that our approach is competitive with conventional PEC systems, enabling safe and potentially affordable solar hydrogen production.

Abstract Conversion of hydrogen into a transportable and environmentally friendly chemical fuels can facilitate the implementation of renewable energy sources in our energy portfolio. In this paper we analyze a liquid mono-fuel composed of an aqueous solution of ammonium nitrate and urea (UAN), a commercial fertilizer commodity and a hydrogen carrier. In order to implement this fuel, additional information regarding its safety and combustion behavior is needed. We report on an important safety parameter of UAN, namely the auto ignition temperature (AIT). The AIT was measured in an original experimental system based on an ASTM standard under applied pressure at different water contents. The water content impacts the crystallization temperature which allows utilization of the fuel at different weather conditions. Specifically, the role of water in stabilizing and inhibiting the ignition is demonstrated, where ignition occurs in a gaseous phase after nearly complete water evaporation. Additionally, the influence of initial pressure on the ignition process was investigated. The AIT increases with pressure due to the increased water vaporization temperature. This study outlines the boundaries that support auto ignition and useful combustion of this fuel.

Electrolytic hydrogen production faces technological challenges to improve its efficiency, economic value and potential for global integration. In conventional water electrolysis, the water oxidation and reduction reactions are coupled in both time and space, as they occur simultaneously at an anode and a cathode in the same cell. This introduces challenges, such as product separation, and sets strict constraints on material selection and process conditions. Here, we decouple these reactions by dividing the process into two steps: an electrochemical step that reduces water at the cathode and oxidizes the anode, followed by a spontaneous chemical step that is driven faster at higher temperature, which reduces the anode back to its initial state by oxidizing water. This enables overall water splitting at average cell voltages of 1.44–1.60 V with nominal current densities of 10–200 mA cm−2 in a membrane-free, two-electrode cell. This allows us to produce hydrogen at low voltages in a simple, cyclic process with high efficiency, robustness, safety and scale-up potential. Conventionally, the two half reactions involved in water electrolysis occur simultaneously, presenting materials and process challenges. Here, the authors decouple these to split water efficiently in two steps: electrochemical hydrogen evolution, followed by spontaneous oxygen evolution at elevated temperature.

Alternative fuels are essential to enable the transition to a sustainable and environmentally friendly energy supply. Synthetic fuels derived from renewable energies can act as energy storage media, thus mitigating the effects of fossil fuels on environment and health. Their economic viability, environmental impact, and compatibility with current infrastructure and technologies are fuel and power source specific. Nitrogen-based fuels pose one possible synthetic fuel pathway. In this review, we discuss the progress and current research on utilization of nitrogen-based fuels in power applications, covering the complete fuel cycle. We cover the production, distribution, and storage of nitrogen-based fuels. We assess much of the existing literature on the reactions involved in the ammonia to nitrogen atom pathway in nitrogen-based fuel combustion. Furthermore, we discuss nitrogen-based fuel applications ranging from combustion engines to gas turbines, as well as their exploitation by suggested end-uses. Thereby, we evaluate the potential opportunities and challenges of expanding the role of nitrogen-based molecules in the energy sector, outlining their use as energy carriers in relevant fields.