Establishing an efficient staged FGR pattern in the layer combustion zone for upgrading the low-NO performance of an industrial-scale coal-fueled grate furnace

• Still high NO x existing in air-staging coal-fired grate furnaces. • Conventional grate’s even flue gas recirculation showing imperfect low-NO x merits. • Identifying low-NO x effects of flue gas recirculation at different combustion stages. • Determining the primary flue gas recirculation zone and its optimal gas ratio. • Constructing an efficient staged pattern of flue gas recirculation on the grate. Low-NO x coal-fired grate furnaces conventionally apply flue gas recirculation (FGR) uniformly along the grate without distinguishing combustion stages. This FGR pattern discounts the FGR’s low-NO x function. In order to fill the mentioned academic gap and better the FGR’s low-NO x function, this study was devoted to first identifying the low-NO x performances of FGR at different combustion stages on the grate of a 75 t/h coal-fired furnace and then constructing a reasonable staged FGR arrangement, where FGR in the primary and supplementary low-NO x zones performed the major and auxiliary roles, respectively. Accordingly, a 10 % FGR injection (i.e., a FGR ratio of 10 %) was compared among the preceding, primary, and mid-late combustion zones. Subsequently, with FGR supplied in the preceding combustion zone, its FGR ratio was varied. It was found that FGR in the grate’s preceding-combustion period presented the strongest low-NO x effect, followed by FGR in the subsequent primary-combustion period, while FGR in the mid-late combustion period demonstrated no low-NO x function. This FGR performance thus incurred a staged FGR configuration with FGR in the grate’s preceding- and primary-combustion periods respectively acting as the primary and supplementary roles. Increasing the FGR ratio in the preceding combustion zone initially improved but then deteriorated the overall low-NO x combustion performance, where the optimal performance was found at a 10 % FGR ratio. With the 10 % FGR fixed in the preceding combustion zone, additional FGR ratio setups of 0 %, 2.5 %, 5 %, and 7.5 % were regulated in turn in the primary combustion zone to optimize such a staged FGR layout. As this FGR ratio increased, both layer and suspension combustion weakened to raise burnout loss, despite a continuous decrease in NO x emissions. To balance NO x reduction and burnout, the optimal staged FGR setup was composed of 10 % and 2.5 % FGR ratios respectively in the preceding- and primary-combustion periods. NO x data and carbon levels in slag were respectively at 385 mg/m 3 (6 % O 2 ) and 10.5 %, achieving a 23 % NO x reduction rate while raising a little burnout loss when compared with the habitual case without FGR.