(599f) Combustion of an Aqueous Urea/Ammonium Nitrate Alternative Fuel at High Pressure

Bar Mosevitzky, Michael
Epstein, Gennady E. Shter, Gideon S. Grader^*

*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

Implementation of renewable energies is hindered by their intermittent
power generation profiles. To achieve a truly sustainable energy sector,
renewable energy storage is necessary. Chemical fuels are an attractive storage
medium since they possess high energy densities. Aqueous solutions of urea and
ammonium nitrate (UAN) can act as low-carbon alternative fuels suitable for
energy storage applications. Both urea and ammonium nitrate can be synthesized
using renewable hydrogen, atmospheric nitrogen and recovered carbon dioxide. Safe
to store and transport, UAN combusts under pressure to produce mainly water,
nitrogen and some carbon dioxide. However, low concentrations of CO, NO_x
and NH₃ were previously detected in UAN combustion effluent. Previous
UAN combustion experiments suffered from poorly characterized temperature
profiles and complex reactor geometries. Therefore, attempts to simulate the
system resulted in poor agreement, impeding unfolding of the chemical reaction
network involved in UAN combustion.

In this work, a newly constructed 800-centimeter tubular reactor
was used to investigate the combustion of UAN at 1-15 MPa. An inner thermowell tube was used to measure the temperature profile
within the reactor (see Figure 1) using a type-K thermocouple, while effluent
concentrations were measured using an FTIR spectrometer.

Figure 1. The effect of pressure on the inner UAN combustion
temperature profile at 10 ml min^-1 fuel flow rate.

Theoretical gas-phase calculations were applied to characterize the
extent of dispersion of the combustion gasses in the reactor, resulting in
values of 0.012-0.028, indicative of near plug flow behavior. Using a PFR model
and a newly updated reaction mechanism, the effect of pressure on the effluent
composition was simulated and compared to experimental results. Excellent
agreement was achieved for all the monitored effluent species, providing the
basis for rate-of-production and sensitivity analyses of the reaction pathways.
Overall, increasing the system pressure resulted in reduced pollutant emissions
and higher N₂ and CO₂ yields (see Figure 2). These
simulations indicated that the reaction network involved in UAN combustion
becomes less convoluted as the pressure increases, resulting in lower pollutant
emissions. Furthermore, the sensitivity of the pollutant levels to the
mechanism decreased with pressure, affirming the robustness of the mechanism at
high-pressure conditions.

Figure 2. The effect of pressure on the yield of: (a) N₂
and (b) CO₂, as indicated by experimental (♦) and simulation (■) results.

The effect of the combustion pressure of UAN on the temperature profile
(Fig. 1), effluent composition (Fig. 2) and the resulting rate-of-production
and sensitivity analyses will be presented and discussed.