Nitrogen-Based Fuels: Renewable Hydrogen Carriers

Nitrogen-based Fuels: Renewable Hydrogen Carriers

Gideon S. Grader^*, Michael Epstein, Ayillath Kutteri Deepa, Oren Elishav, Gennady E. Shter, Bar Mosevitzky

*Corresponding author: grader@technion.ac.il

 Wolfson Department of Chemical Engineering, Nancy and Stephen Grand Technion Energy Program, Technion-Israel Institute of Technology, Haifa 3200003, Israel

Renewable energy sources such as solar and wind suffer from an intermittent power output, making energy storage a key element in future energy infrastructure. Fuels offer both high energy densities and efficient transport compared to other energy storage alternatives. One energy storage solution is water electrolysis. However, the generated hydrogen is incompatible with the global fuel infrastructure, inhibiting its implementation as an energy vector. Storing hydrogen on carrier atoms provides a safe and convenient way to utilize and transport renewable energies. While carbon–based fuels are commonly suggested, using nitrogen as a hydrogen carrier can potentially offer a superior option. In this talk we will present the culmination of several studies on nitrogen-based fuels. Firstly, an analysis comparing the energy-based efficiency of several carbon-based synthetic fuels to their nitrogen-based counterparts will be presented. Secondly, the effect of the equivalence ratio on the thermal autoignition of a non-carbon nitrogen-based fuel will be shown. Thirdly, the effect of Ru/Pt/Al₂O₃ catalysts used for effluent gas treatment of a low-carbon nitrogen-based fuel will be discussed.

To evaluate the feasibility of nitrogen-based fuels, seven carbon- and nitrogen-based synthetic fuels were analyzed on a power-to-fuel-to-power (PFP) basis. Methane, methanol and dimethyl ether were compared to ammonia, ammonium hydroxide with urea (AHU), ammonium hydroxide with ammonium nitrate (AAN) and urea with ammonium nitrate (UAN). This analysis took into account the energy costs of air separation, water splitting, fuel synthesis and distribution (Fig. 1). Both possible sources of carbon dioxide (atmospheric and flue gas) were considered in this investigation. The results of this analysis indicate that ammonia is the most efficient candidate. This study demonstrates the feasibility of nitrogen-based synthetic fuel economy.

Figure 1. Principal material and energy balance for fuel analysis by PFP

An aqueous solution of ammonium hydroxide and ammonium nitrate (AAN) fuel includes a reducer and a net oxidizer. Therefore, the fuel equivalence ratio (ϕ) can be defined as the ratio of ammonia to nitric acid divided by the stoichiometric ratio of the two. The effect of ϕ on the thermal autoignition of AAN was studied using a combined DTA/DBA (differential thermal/barometric analysis) system. Results indicated an increase in the autoignition temperature (AIT) with ϕ. Gas-phase kinetics simulations, used to investigate the cause for this behavior, indicated that nitric acid decomposition promotes the thermal autoignition. On the other hand, reduction of NO₂ and NO₃ to nitric oxide inhibited the ignition. This was due to the role of NO₂ and NO₃ radicals in reactions producing amidogen from ammonia. The potential for selecting efficient catalysts for future use in AAN's catalytic ignition using this data will be discussed.

Continuous combustion of UAN yields primarily steam, nitrogen and carbon dioxide. However, small amounts of pollutants such as carbon monoxide, ammonia, nitrous oxide and NO_x are present in UAN's effluent. Increasing the system pressure was previously found to reduce pollutant levels and increase the combustion product yield (Fig. 2). However, high pressures exceeding 15 MPa were required to achieve this effect.

Figure 2. The nitrogen yield as a function of system pressure

Catalysts can be used to reduce the concentrations of these pollutants at lower pressure values. Platinum, ruthenium and bimetallic Pt/Ru catalysts on an Al₂O₃ support were prepared and tested at pressures of 1 – 7.5 MPa and temperatures of 250 – 500ºC. Outlet concentrations were measured using online FTIR analysis. While pure platinum increased the ammonia effluent by a factor of three at 5 MPa and temperatures up to 300ºC, it eliminated all other pollutants. At higher temperatures the ammonia generation ceased, but nitrous oxide was generated instead. The N₂ yield was 95.9% – 98.5% in the 250 – 400^oC range as shown in Fig. 3.

Figure 3. The nitrogen yield as a function of temperature for the Pt₂₅Ru₇₅/Al₂O₃ at 5 MPa and non-catalytic systems

With pure ruthenium and bimetallic Pt/Ru at temperatures above 350ºC and 300ºC, respectively, all pollutants but N₂O were eliminated at 5 MPa. However, nitrous oxide was generated in low amounts at these high temperatures. Results indicated that at 5 MPa Pt/Ru at an atomic ratio of 25:75 was capable of increasing the N₂ yield from 96% to 99.7% (Fig. 3) and the CO₂ yield from 99% to 100%. The improved N₂ yield of the Ru/Pt catalysts in Figure 3 demonstrates its advantage over the pure Pt alternative. Comparison of these results to current regulatory standards will be discussed.