Low-NO combustion characteristics of a coal-fueled grate furnace with multiple flue gas recirculation

Flue gas recirculation (FGR) in coal-fueled grate furnaces were seldom reported and the related FGR applications were only limited to evenly assigning FGR in the grate zone. These circumstances often led to unsatisfactory FGR’s low-NO x performance and persistently high NO x production in coal-fueled grate furnaces. Accordingly, in this study related to a 75 t/h coal-fueled grate furnace, a staged FGR pattern (with FGR in the preceding and primary layer-combustion periods respectively as the dominant and subordinate FGR) was applied in the grate zone, accompanied by supplementary FGR injections in freeboard sequentially through secondary air and the coal-thrown air. This strategy regulated the multiple FGR involving both the grate zone and freeboard. By performing comprehensive simulations, this work was devoted to exploring the multiple-FGR influence on coal combustion and NO x release in the grate furnace. With a staged FGR in the grate zone, introducing FGR through secondary air in freeboard and raising its FGR ratio suggested that suspension combustion continued to worsen, and the overall low-NO x combustion initially improved but later deteriorated, yielding an optimal low-NO x merit at a 4% FGR ratio in secondary air. Upon the optimal FGR conditions on the grate and in secondary air, additionally delivering FGR in the coal-thrown air to complete the multiple FGR form and then raising its FGR ratio revealed that both layer and suspension combustion weakened again to decrease NO x levels and burnout, and its FGR impact was more apparent than that of secondary air. Under the optimal multiple-FGR conditions, NO x production and burnout loss attained at 297 mg/m 3 (6% O 2 ) and 7.9%, achieving NO x reduction rates of 40% and 22.9% in comparison to the habitual case without FGR and the sole grate-FGR case, respectively. Meanwhile, the incurred burnout-loss increase was controllable. These results confirm the effectiveness of this multiple FGR form in enhancing the grate furnace’s low-NO x performance. The major contributions of this work involve proposing the multiple-FGR conditions featuring a staged FGR on the grate, evaluating the impact of each FGR component, and confirming the multiple-FGR effectiveness. An illustration of the multiple-FGR framework.