Toward the Development of Low-NO Ammonia Cofiring Furnaces: Numerical Investigation into the Secondary Injection System Using the Conjugate Heat Transfer Method

In the context of utilizing ammonia for cofiring power generation, the air-staged strategy is recognized as an effective method for controlling NO emissions due to its ability to stage air input, thereby creating a fuel-rich region within the furnace. To explore the NO formation and reduction characteristics employing the secondary injection system under various experimental conditions, the present study utilized three-dimensional numerical simulations to investigate the impact of the primary air ratio (λ1) and the secondary nozzle diameter (D2). The conjugate heat transfer method is used to accurately simulate thermal conditions at the furnace walls. Results indicate that with a constant total air ratio of 1.2, simply reducing the primary air ratio, which enhances fuel-rich combustion in the primary air zone, does not lead to a linear decrease in NO emissions. Instead, NO emissions exhibit a V-shaped trend, reaching a minimum when the primary air ratio equals 0.6. An analysis of combustion characteristics reveals two distinct combustion modes appear as the primary air ratio varies, which is the primary reason for this phenomenon. When λ1 ≥ 0.6, as fuel and most of the oxidizers are injected through the primary nozzle, reactions are predominantly concentrated in the primary air region of the furnace where the reaction temperatures are higher. The formation of a fuel-rich region in this central zone leads to a notable decline in NO emissions. Conversely, when λ1 < 0.6, the flow from the secondary nozzle increases, shifting the main combustion reactions toward the secondary air zone, where the flame temperature significantly decreases. In these conditions, the main reactions do not occur in the fuel-rich region of the primary air zone, leading to a trend where NO emissions increase again as the primary air ratio decreases. Regarding the impact of the secondary nozzle diameter at lower ammonia cofiring ratios, results demonstrate that increasing the nozzle size enhances reduction reactions and controls NO emissions. These findings provide insights for the parameter design and selection of low-NO ammonia cofiring furnaces in future studies.