Scaling of Pulverized-Fuel Jet Flames That Apply Large Amounts of Excess Air—Implications for NOx Formation

Abstract

Measures to reduce nitrogen oxides (NOx) formation in industrial combustion processes often require up-scaling through pilot-scale facilities prior to being implemented in commercial scale, and scaling is therefore an important aspect of achieving lower NOx emissions. The current paper is a combined experimental and modelling study that aims to expand the understanding of constant velocity scaling for industrial jet flames applying high amounts of excess air. These types of flames are found in e.g., rotary kilns for production of iron ore pellets. The results show that, even if the combustion settings, velocity, and temperature profiles are correctly scaled, the concentration of oxygen experienced by the fuel during char combustion will scale differently. As the NO formation from the char combustion is important in these flames, the differences induced by the scaling has important impacts on the efficiencies of the applied primary measures. Increasing the rate of char combustion (to increase the Damköhler number), by using, for example, smaller-sized particles, in the pilot-scale is recommended to improve scaling.

Evaluation of NOx-Reduction Measures for Iron-Ore Rotary Kilns

“…As has been previously observed, 20 NO x emissions are significantly higher in pilot-scale than in full-scale. Replacing the reference burner with the precessing jet burner in pilotscale reduced NO x by 14% in Hood A and ∼17% in Hood B.…”

“…The discussion on NO x control in iron-ore rotary kilns starts at the conclusion from our previous work, − based mostly on the FlexiFuel 7 campaign, that the net conversion of char-N to NO is responsible for the high NO x emissions. Our simulations (using the same model as in this Article) showed that the large amounts of air expose the char particles to high partial pressures of oxygen and reduce the concentrations of NO and char by dilution.…”

“…The net conversion of char-N to NO is also increased, primarily because it is heavily dependent on how much of the formed NO is reduced by char, both in the pores and by other char particles. , A high excess air ratio will reduce the amount of char available for NO reduction by fast oxidation of char and by dilution of NO and char (thus reducing the reaction rate). Our recent work has shown that when coal is combusted in iron-ore rotary kilns, the NO x formation is dominated by the conversion of char-N to NO, , which has an impact on the scaling procedure for such jet flames . A recent study by Wiinikka et al showed that when coal was fully replaced by woody biomass (with a low nitrogen content) in their pilot-scale furnace, a NO x reduction of 15–20% was observed.…”

“…Our recent work has shown that when coal is combusted in iron-ore rotary kilns, the NO x formation is dominated by the conversion of char-N to NO, 18,19 which has an impact on the scaling procedure for such jet flames. 20 A recent study by Wiinikka et al 21 showed that when coal was fully replaced by woody biomass (with a low nitrogen content) in their pilot-scale furnace, a NO x reduction of 15−20% was observed. The reduction was not proportional to the reduction in fuel-bound nitrogen which indicates that the thermal NO formation may increase when biomass combustion replaces coal combustion.…”

Alternative Fuel Substitution Improvements in Low NOx In-Line Calciners

“…Standard ILC calciners lack a combustion chamber; the body's average oxygen content is between 8 and 10%. Any energy from combustion that is present in the calciner body is promptly consumed by meal, thus limiting access to the required oxygen [58].…”