Low-NOx approach for ammonia cracked gas micro-mixing type combustion using rich–lean staging

In this paper, a low-NOx approach is proposed in which micro-mixing type array combustion of ammonia (NH3) cracked gas is coupled with rich–lean staged combustion. A numerical investigation was performed to determine the optimal primary rich conditions and the most effective secondary-air configuration. The results suggest that achieving efficient combustion of NH3 cracked gas under rich-lean staging requires satisfying two criteria: (i) the cracking ratio (Cr) should not exceed 0.4, and (ii) the equivalence ratio (Φ) suitable for the primary rich combustion conditions shows a positive correlation with Cr. The optimal primary rich condition, identified through screening of operating conditions, was Cr = 0.4 and Φ = 1.3. This condition was chosen for its ability to leverage the inherent advantages of the fuel-rich zone, namely reduced unburned NH3 slip, while also maintaining lower NO formation under the rich-lean staging configuration. Furthermore, by coordinating the global equivalence ratio (Фglob) and combustor length, NO was reduced to below 250 ppm (dry basis, 15% O2), H2 to under 40 ppm (dry basis, 15% O2), and NH3 slip was minimized to nearly zero. On this basis, the secondary-air configuration was optimized to further reduce NOx. The results indicate that inclined-jets were found to axially extend the rich zone and generate high near-wall axial velocities. With this configuration, NO was reduced to below 200 ppm (dry basis, 15% O2), while H2 remained below 25 ppm (dry basis, 15% O2). These findings are expected to inform the design and optimization of NH3-fueled gas turbine combustors, thereby aiding in the reduction of fossil fuel consumption and furthering the goal of carbon neutrality.