Abstract This paper presents some experimental results on the integral reburning process including all reburning stages in a down-fired furnace. A kind of high volatile bituminous coal (DT coal) and a kind of low volatile anthracite (SX coal) were focused. Natural gas as reburning fuel was also chosen when DT coal used as the main fuel. Reburning fuel fractions and residence times of reburning zone were considered. The deep and middle degree air-staged combustion cases were also conducted. Results show that the NOx reduction efficiency at the cases with natural gas as reburning fuel is significantly higher than that at the cases with two kinds of coal as reburning fuel. At the residence time of 0.82 s and 1.0 s, the conversion ratio of fuel nitrogen to NOx is reduced by 30–70%. At the coal reburning cases, the reduction of NOx and the oxidation of volatile and char to form NOx occur simultaneously in the reburning zone. The NOx concentration values at the reburning stage of DT coal rise faster and reach higher than that of SX coal. The NOx emission values at air-staged combustion cases are lower than that at all reburning cases. Air staging is more likely to inhibit or dissolve the conversion of fuel nitrogen to NOx during the combustion process. Further analysis shows that the release rate of fuel nitrogen from DT coal is significantly higher than that from SX coal, which leads to a faster and higher increase of NO formation at the latter stage of reburning zone.