Exploration on deep pulverized coal activation and ultra-low NOx emission strategies with novel purifying-combustion technology

Under the strategic objectives of carbon peaking and carbon neutrality, increasingly stringent NOx emission standard was hard to meet in coal-fired boilers, and it was imperative to develop clean coal combustion technologies. As a novel combustion technology, purifying-combustion technology had good prospects in NOx emission reduction, and pulverized coal modification and modified fuel combustion occurred in purifying burner and down-fired combustor (DFC) respectively with this technology. The novelty of this study was associated with a first-time systematical analysis of the advantage of purifying burner in deep pulverized coal activation and the matching relationship of multistage combustion air in reduction region of DFC. Experiments were performed in 30 kW purifying-combustion test rig to investigate the two-stage modification characteristics of pulverized coal in purifying burner and the difference in influence of reducing intensity on combustion and NOx emission characteristics at different reduction region lengths and the influence of staged air distribution on them in DFC. Two-stage purifying burner demonstrated greater advantages in improving particle properties of pulverized coal compared to single-stage self-preheating burner: specific surface area, pore volume, pore diameter, density of carbon defect structure and fuel-N conversion rate increased from 19.01 m3/g, 29.15 mm3/g, 4.34 nm, 3.93 and 52.96 % to 34.39 m3/g, 42.49 mm3/g, 4.57 nm, 4.41 and 66.43 %, respectively. In DFC, increasing reducing intensity in reduction region or extending its length reduced NOx emission, albeit at the expense of combustion efficiency (η). Decreasing reducing intensity resulted in decrease of η difference and increase of NOx emission difference between different lengths. Staged air distribution in reduction region promoted clean and efficient combustion, and increasing staged air ratio to ∞ realized minimal NOx emission of 39.50 mg/m3 with η of 99.23 %.