Nitrogen oxide (NO
x) is a pollutant involved in the formation of several hazardous phenomena, most notably acid rain and photochemical smog. Legislation on emissions has motivated significant research into pollutant mitigation technologies, and this has resulted in reductions of NO
x emissions during the last decades [
1,
2,
3,
4]. Regarding NO
x emissions from stationary sources, the focus of legislative measures has been on power generation, which has driven the development of technological measures that are suited to these types of facilities, whereas industrial combustion processes have received less attention. However, this situation is changing, as emissions from industrial combustion plants now typically lie significantly above heat and power plants applying state-of-the-art technologies. Some of the industrial combustion processes differ significantly from conventional combustion systems and state-of-the-art technologies are not always applicable. There is, therefore, a need to adapt existing technologies to the conditions of industrial combustion or to develop new technologies for controlling the emissions from these processes. Since measures that affect the combustion process are usually tested in pilot-scale facilities prior to being applied in full-scale, it is critical that the effects of scaling are well understood.
Two commonly used principles for scaling combustion processes are: (1) constant velocity scaling; and (2) constant residence-time scaling. To relate these scaling criteria to the heat input, it is helpful to write the fuel input as:
where
ρ0 and
u0 are the inlet density and velocity of the combustion air, respectively [
5].
D0 is the diameter of the combustion air inlet, and
K is a proportionality constant that relates the air flow to the fuel input. Constant velocity scaling implies that u
0 is kept constant, and constant residence-time scaling implies that
D0/u0 is kept constant. The
D0/u0 ratio, which will be referred to hereinafter as the mixing time, is representative of the flame residence time. Both scaling criteria aim at maintaining the fractional degree of mixing over the normalized length of the combustor [
6]. If the reaction rates are faster than the mixing rate, combustion is controlled by mixing and should proceed in a similar manner independently of the scale and scaling method. For slower processes, constant residence-time scaling achieves a greater degree of similarity and is, in theory, the superior scaling method [
6,
7,
8]. However, constant residence-time scaling is rarely applied owing to practical problems associated with severe pressure drops and low velocities, and constant velocity scaling is, thus, often preferred [
7,
9,
10]. Weber and Breussin [
8] have stated that for swirling pulverized fuel (PF) flames, both scaling methods achieve NO
x emissions that are representative of the commercial scale when the thermal input of the pilot-scale is above 4 MW. At lower thermal inputs, NO
x formation is underestimated, especially for constant velocity scaling. Weber and Breussin have attributed the decreased formation of NO
x to a deeper penetration depth into the reverse flow zone in the vicinity of the burner. For thermal inputs <1 MW, they recommend constant residence-time scaling as well as a more finely milled fuel. The differences in particle trajectories between large scale and small scale flames, using constant velocity scaling, are demonstrated by CFD simulations in the recent work by Weber and Mancini [
11], who also provide a good overview of scaling issues. The scaling studies in literature are mainly based on flames with a swirl-induced reverse flow zone (Type 1 and Type 2 flames, see [
12,
13] for more information about the flame classification system used by the International Flame Research Foundation (IFRF)). The novelty in this paper is the focus on jet flames (Type 0) and the application to rotary kilns for which little, if any, pilot-scale data that relates to scaling work, exists.
One industrial process that applies a PF-jet flame is the Grate–Kiln process for iron ore induration. In this process, combustion at high temperatures and with large volumes of excess air (4–6-times the stoichiometric requirement) powers the heat treatment of iron ore pellets in a rotary kiln. High temperatures and excess air are known to promote NO
x formation, and the levels of NO
x emissions from these units are indeed high. Several of the commonly used mitigation strategies are not easily applicable to the Grate–Kiln process due to practical limitations associated with the rotary kiln and the high content of iron dust in the off-gases. The Swedish iron ore company Luossavaara-Kiirunavaara Aktiebolag (LKAB) has worked with measures to reduce NO
x emissions and are investigating how to overcome these limitations. Our recent work [
14,
15] on the LKAB pilot-scale kiln led to the conclusion that most of the NO in the Grate–Kiln process originates from the char-bound nitrogen in the fuel. The main premise for this conclusion was that the measured flame temperatures were not sufficiently high for significant thermal NO formation, and that the NO emissions decreased almost linearly with the amount of fuel-N introduced (30% of the coal, containing 1.4% nitrogen, was replaced with biomass that contained 0.1% nitrogen) [
14]. LKAB has conducted several investigations of primary NO
x mitigation and combustion efficiency in a similar pilot-scale setup [
14,
15,
16,
17,
18,
19,
20,
21,
22]. However, the PF-jet flame suffers from scaling issues and the ways in which the pilot-scale results should be interpreted and transferred to the commercial scale are not clear, and low-NO
x combustion remains to be implemented in industrial-scale iron ore kilns.
The current paper examines constant velocity scaling of PF-jet flames that apply a high degree of excess air and its implications on NOx formation. The overall aim is to derive a methodology for implementation of efficient primary NOx mitigation measures in such processes. As a case study, this paper assesses the LKAB pilot-plant kiln and compares it to a LKAB full-scale rotary kiln using detailed reaction modeling.