Nitric Oxide Emission Reduction in Reheating Furnaces through Burner and Furnace Air-Staged Combustions

In this study, a series of experiments were conducted on a testing facility and a real-scale furnace, for analyzing the nitric oxide (NO) emission reduction. The effects of the temperature, oxygen concentration, and amount of secondary combustion air were investigated in a single-burner combustion system. Additionally, the NO-reduction rate before and after combustion modifications in both the burner and furnace air-staged combustion were evaluated for a real-scale reheating furnace. The air-to-fuel equivalence ratio (lambda) of individual combustion zones for the furnace was optimized for NO reduction without any incomplete combustion. The results indicated that the NO emission for controlling the lambda of a single-zone decreased linearly with a decrease in the lambda values in the individual firing tests (top-heat, bottom-heat, and bottom-soak zones). Moreover, the multi-zone control of the lambda values for individual combustion zones was optimized at 1.13 (top-preheat), 1.0 (bottom-preheat), 1.0 (top-heat), 0.97 (bottom-heat), 1.0 (top-soak), and 0.97 (bottom-soak). In this firing condition, the modifications reduced the NO emissions by approximately 23%, as indicated by a comparison of the data obtained before and after the modifications. Thus, the combined application of burner and furnace air-staged combustions facilitated NO-emission reduction.