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 (NOx) 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 NOx 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 NOx and nitrous oxide (N2O) levels if not designed correctly, could cancel the global warming 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. NOx, N2O, and NH3 exhaust emissions were recorded while burning rich premixed NH3-air with primary equivalence ratios of ϕprimary = 1.13, 1.15, and 1.18. Two quartz lengths of 76 mm and 178 mm were first implemented for the primary combustion zone while injecting secondary air through a second stage geometry of 5 holes with 2.03 mm diameter. Strong interaction was found between the primary flame and the secondary combustion zone only for the 76 mm liner, increasing NOx emissions prior to flame blowout. In addition, a longer combustion chamber is more desirable for an RRQL system since it could allow for NOx 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 178 mm long quartz. It was observed that ϕprimary = 1.13 yields simultaneously low NOx-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 ϕglobal, yields lower NOx-N2O emissions. However, these differences between geometries are less noticeable by decreasing ϕprimary from 1.18 down to the optimum 1.13. Also, diffusion-like combustion was observed at low secondary air additions (0.90 ≤ ϕglobal ≤ 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 ϕprimary = 1.13 and 0.70 ≤ ϕglobal ≤ 0.90 with a theoretical outlet burner gas temperature of 1720–1970 K.