Session: 04-38 Ammonia Combustion II

Paper Number: 151784

Towards the Development of an NH3-RRQL System Part 2: Effects of the Primary Combustion Zone Length and Secondary Stage Number of Holes on Stability and Emissions 

Staged combustion systems, such as the Rich-Relaxation-Quench-Lean (RRQL), have the potential for low nitrogen oxides (NO_x) levels while burning ammonia (NH₃). This system relies on burning fuel rich premixed NH₃-air in a primary stage, then allowing enough residence time to relax NO_x down to equilibrium values, followed by a lean secondary combustion zone due to secondary air injection. However, the secondary combustion zone in RRQL systems can also lead to significant NO_x and nitrous oxide (N₂O) levels if not designed correctly, could cancel the climate impact benefits of its carbon-free nature. This study reports on the effects of the primary combustion zone length and second stage number of holes on stability and emissions of a laboratory scale RRQL system. Experiments were conducted at atmospheric pressure in a modular axial swirl burner with a geometrical swirl number of 1.1 and 16 straight vanes. NO_x, N₂O, and NH₃ exhaust emissions were recorded while burning rich premixed NH₃-air with primary equivalence ratios of ϕ_primary = 1.13, 1.15, and 1.18. Two quartz lengths of 3” and 7” were first implemented for the primary combustion zone while injecting secondary air through a second stage geometry of 5 holes with 0.08” diameter. Strong interaction was found between the primary flame and the secondary combustion zone only for the 3” long quartz, increasing NO_x emissions prior to flame blowout. In addition, a longer combustion chamber is more desirable for an RRQL system since it could allow for NO_x relaxation and NH₃ cracking before the lean second stage. Then, three different second stage geometries with 5, 10, and 16 holes were tested with the 7” long quartz. It was observed that ϕ_primary = 1.13 yields simultaneously low NO_x-N₂O-NH₃ emissions. Also, the second stage geometry plays a role, as a lower number of holes, meaning higher momentum flux ratio for the same ϕ_global, yields lower NO-N₂O emissions. However, these differences between geometries are less noticeable by decreasing ϕ_primary from 1.18 down to the optimum 1.13. Also, diffusion-like combustion was observed at low secondary air additions (0.90 ≤ ϕ_global ≤ 1.10), producing higher O₂ concentration in the flue gases compared to equilibrium O₂, demonstrating that such a combustion regime is not desirable for an RRQL system due to low combustion efficiency. Finally, an optimum operability range was found with ϕ_primary = 1.13 and 0.50 ≤ ϕ_global ≤ 0.70 with a theoretical outlet burner gas temperature of 1450 – 1720 K.

Presenting Author: Cristian D. Avila Jimenez Georgia Institute of Technology

Presenting Author Biography: Dr. Avila earned his B.Sc and M.Sc in Mechanical Engineering from Universidad de Antioquia, Colombia. In 2023 he earned his Ph.D. in Mechanical Engineering from King University of Science and Technology (KAUST) under the guidance of Prof. William L Roberts and Prof. Thibault F. Guiberti. In March 2024 he joined the Ben T. Zinn combustion laboratory at Georgia Tech as a Postdoctoral Fellow under the supervision of Prof. Tim Lieuwen. Dr. Avila's research is towards integrating zero-carbon fuels, such as ammonia and hydrogen, into real-scale gas turbines for electrical power generation to reduce CO2 emissions.

Authors:

Cristian D. Avila Jimenez Georgia Institute of Technology
Renee Cole Georgia Institute of Technology
David R. Noble Electric Power Research Institute (EPRI)
Robert Steele Electric Power Research Institute (EPRI)
David Wu Georgia Institute of Technology
Benjamin L. Emerson Georgia Institute of Technology
Timothy C. Lieuwen Georgia Institute of Technology