NO_X formation from the radiant tube burner in a continuous annealing line

Due to its simplicity and high productivity, continuous annealing lines (CAL) has been widely adopted as a method to anneal cold rolled steel strip. The China Steel Corporation (CSC) introduced CAL to its cold rolling mill in the 1980s to treat its cold rolled steel strip in order to take advantage of its high throughput benefits. In a CAL, steel strip is heated by a series of radiant tube (RT) burners which use fossil fuel gas (coke oven gas in the case of CSC). After some years of operation, the concentration of NO_X emitted from a CAL is sometimes up to the limit of the Taiwanese Environmental Protection Administration emission standards. To solve this problem, a series of tests have been conducted in CSC to investigate the influence of burner operating parameters on NO_X emissions. The investigated operating parameters include off-stoichiometric combustion, air/fuel ratio (Φ_T), primary air distribution (Φ₁), and secondary air swirl number (Sg).

Based on the results of a number of experiments, it has become apparent that NO_X concentration increases with burner load, that the initial left eccentric combustion has lower NO_X emissions than the bottom eccentric flame, that NO_X formation is quite low in the fuel-rich combustion region until the Φ reaches nearly 0.8, and that Φ₁ and Sg have lower and greater effects on the NO_X concentration for the left eccentric combustion, respectively. The suggested low NO_X operating parameters for the left and bottom eccentric combustion are Φ₁ = 0.54 & Sg = 0.0 and Φ₁ = 0.35 & Sg = 0.56, respectively. Regarding the temperature of the RT outer surface, the bottom eccentric flame has better temperature uniformity, even if it has a higher NO_X emission concentration, and the left eccentric flame has worse temperature uniformity due to its proximity to the RT, as well as to the production of a higher localized temperature.