The combustion process applied in the grate-kiln process for iron ore pellet production employs air-to-fuel equivalence ratios in the range of 4–6, typically with coal as fuel and high-temperature air (>1000 °C) as oxidant. The NOx emissions from these units are in general significantly higher than those in other combustion systems, and the large flows of flue gases make the implementation of secondary measures for NOx control costly. Therefore, it is of importance to investigate NOx formation under combustion conditions relevant for iron ore production, in order to control the emissions from these units. The present work examines NOx formation during the combustion of four pulverized coals, as well as during cofiring with biomass in a pilot-scale kiln (580 kWfuel) based on a two-week experimental campaign. The influence of burner settings was also included in the investigation. Based on the presented experimental results and the results of previous modeling and experimental studies, we suggest that the NOx emissions are mainly the result of a high conversion of fuel-bound nitrogen (fuel-N) to NO. In particular, char-bound nitrogen (char-N) conversion appears to be higher than in conventional pulverized fuel flames, presumably due to the high levels of oxygen present in the char-burnout region. The temperatures in the kiln varied between the test cases, but thermal NO formation is estimated to be of low importance.

A new type of Mn-based oxygen carrier was prepared by impregnating manganese ore with copper nitrate solution. Cyclic reduction and oxidation reactivity of the materials was investigated in a fluidized-bed reactor. The potential use of these oxygen carriers for chemical looping combustion (CLC) was examined. The reactivity of the manganese ore can be highly improved by impregnation of copper. The reactivity of the oxygen carrier reduction is higher with a larger amount of copper impregnated. However, the degree of the reactivity enhancement is not proportional to the amount of copper doped on the oxygen carriers. An important finding is that, even with low Cu loading, such as 0.5 wt % copper impregnated on manganese, the period with full CO conversion can be enhanced 6 times. A very interesting phenomenon is that the Cu-modified manganese ore can completely convert CO, even at a low temperature, such as 500°C. This study proves that the reactivity of the manganese ore could be significantly improved by impregnating copper, even with a small amount. The copper impregnation method could be very promising to improve the reactivity of the manganese ore as oxygen carriers for CLC.

Given that more stringent NO x emission limits are expected in the near future, several industrial processes are in need of NO x mitigation measures. The Grate-Kiln process, applied in the iron ore industry, is one such process. NO x formation is inherently high in the process, and due to the combustion conditions, several standard mitigation strategies are impractical. Alternative solutions are thus needed. The current paper aims at developing a model capable of describing the NO formation under conditions relevant in iron ore rotary kilns and to identify governing parameters that may be modified for mitigation purposes. The developed model uses detailed reaction modeling for the homogeneous combustion chemistry combined with simpler modeling with apparent kinetics for the heterogeneous chemistry. The main findings are that thermal NO is of low significance and that the NO formation during char combustion is the main contributor to the high NO x emissions. Attempting to control the partitioning between the volatile nitrogen and the char-bound nitrogen is suggested as a mitigation strategy, since the combustion of char is challenging to control compared to the combustion of volatiles.

The grate-kiln process is employed for sintering and oxidation of iron-ore pellets. In this process, a fuel (typically coal) is combusted with a large amount of excess air in a rotary kiln, and the high air-to-fuel ratio leads to significant NO x formation. The current Article is an assessment of NO x reduction measures that have been tested in pilot-scale and in full-scale by the Swedish iron-ore company Luossavaara-Kiirunavaara Aktiebolag (LKAB). The results show that the scaling between the full-scale kiln and the pilot-scale kiln is crucial, and several primary measures that reduce NO x significantly in pilot-scale achieve negligible reduction in full-scale. In the investigated full-scale kiln, thermal NO x formation is efficiently suppressed and low compared with the NO formation from the fuel-bound nitrogen (especially char-bound nitrogen). Suppressing the NO formation from the char-bound nitrogen is difficult due to the high amounts of excess air, and all measures tested to alter mixing patterns have shown limited effect. Switching to a fuel with a lower nitrogen content is efficient and probably necessary to achieve low NO x emissions without secondary measures. Simulations show that replacing the reference coal with a biomass that contains 0.1% nitrogen can reduce NO x emissions by 90%.

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.