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 CHEMICAL ENGINEERING   TRANSACTIONS
 

VOL. 45, 2015 

A publication of 

 
The Italian Association 

of Chemical Engineering 

www.aidic.it/cet 
Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Sharifah Rafidah Wan Alwi, Jun Yow Yong, Xia Liu  

Copyright © 2015, AIDIC Servizi S.r.l., 

ISBN 978-88-95608-36-5; ISSN 2283-9216 DOI: 10.3303/CET1545167 

 

Please cite this article as: Skryja P., Bělohradský P., Hudák I., Juřena T., 2015, Experimental study on nox formation in gas-

staged burner, Chemical Engineering Transactions, 45, 997-1002  DOI:10.3303/CET1545167 

997 

Experimental Study on NOx Formation 

in Gas-staged Burner 

Pavel Skryja, Petr Bělohradský, Igor Hudák, Tomáš Juřena 

Brno University of Technology, Faculty of Mechamical Engineering, Technická 2, Brno, 616 69, Czech Republic. 

skryja@fme.vutbr.cz 

The paper deals with the experimental investigation of combustion operating parameters and burner’s 

constructional parameters on the formation of nitrogen oxides. The tests were performed using the burner 

prototype with the maximal capacity of 1.5 MW. The burner is both fuel-staged and air-staged. During the 

testing campaign, various geometries of secondary nozzles and their position towards the burner quarl, 

and various geometries of the so-called swirl generator were investigated. The observed parameters 

included NOx and CO emissions, turndown ratio, flame stability, flame length and flame shape, heat 

transfer from hot flue gas to the combustion chamber’s shell and the distribution of in-flame temperatures 

measured in the horizontal symmetry plane of the chamber. 

The results showed that in cases when the minimum of NOx emissions was reached, the flame instability 

was observed. When the length of flame was about 4 m, the NOx emissions were low too, however, the 

heat transfer in the radiation section significantly decreased. Better results were obtained when the flame 

length was up to 2 m at the maximal burner capacity and the primary stage was highly fuel-lean while the 

balance of fuel was combusted in the secondary stage. The minimum NOx was about 30 ppm. 

1. Introduction 

Due to rapidly growing energy consumption and increasing volume of pollutant emissions, the main 

objective in the field of industrial combustion for upcoming years is to improve the efficiency of combustion 

process and to reduce the emissions, especially the emissions of nitrogen oxides (NOx). The main factors, 

which contribute to the NOx formation, are the temperature of combustion air, combustion air excess, 

temperature peaks in the flame zone and long residence time of reactants in high temperature zones. 

Many review works on NOx abatement have been performed recently, e.g., Normann et al. (2009), Hill and 

Smoot (2000). The common targets of all primary NOx-control techniques are the reduction of high 

temperature peaks, creation of oxygen deficient stoichiometric conditions and short residence time of 

reactants in high temperatures zones (Baukal, 2004). The primary techniques have been investigated by 

many research groups and include (a) fuel-staging (Bělohradský and Kermes, 2012), air-staging (Ballester 

et al., 2008), flue gas recirculation (Liuzzo et al., 2007), and fuel-reburning (Smoot et al., 1998). 

Swirl motion of combustion air is one of key burner design parameters that significantly influence the 

mixing intensity of fuel with combustion air and subsequently the flow pattern in the combustion chamber, 

in-flame temperatures distribution and NOx formation. The swirling flow is particularly important for the 

flame stabilization. For example, Vondál and Hájek (2014) have been investigated the influence of swirl 

generator’s geometry on the flow pattern downstream the swirl generator. 

The aim of the present work was to optimize the current geometry of the mono-block automatic burner 

equipped with the standard burner head in terms of NOx reduction, thermal efficiency increase and stable 

flame. Newly designed burner head utilizes both fuel-staging and air-staging, and the swirl generator for 

the flame stabilization. In the present work, the effects of fuel distribution between primary and secondary 

stage, position of secondary nozzle heads, combustion air distribution between primary and secondary 

stage and geometry of swirl generator on NOx and CO emissions, flue gas temperature, heat flux 

distribution from hot flue gas to the wall of combustion chamber, in-flame temperatures distribution in the 

horizontal symmetry plane of the combustion chamber, and flame stability were investigated in detail. 



 

 

998 

 
2. Experimental setup 

2.1 Testing facility 
The combustion tests were carried out at the burners testing facility located at Brno University of 

Technology. The key apparatus of the facility, shown in Figure 1, is the two-shell horizontal water-cooled 

combustion chamber with the inner diameter of 1 m and the length of 4 m. The cooling shell of chamber is 

divided into seven individual sections with independent supply of cooling water. Each section is equipped 

with sensors for measurement of cooling water flow rate, inlet and outlet temperature. This unique 

construction thus enables to partially simulate conditions similar to the ones in fired process heaters and to 

assess the heat extracted from the hot flue gas to the combustion chamber wall lengthwise the flame. The 

chamber is equipped with eight inspection windows along the cylindrical part and two inspection windows 

on the rear side of the chamber opposite the burner. The windows can be used for the installation of 

additional measurement instrumentation (e.g., thermocouples) and also allow to observe the flame on the 

burner. Flue gas is exhausted from the combustion chamber through the flue gas stack where the 

measurements of pressure in the combustion chamber, flue gas temperature and flue gas composition 

(O2, CO, NO, NO2) are located. The detailed description of the testing facility can be found in the work of 

Bělohradský et al. (2014). 

 

Figure 1: Water-cooled combustion chamber in burners testing facility. 

2.2 Tested burner 

The tested gas burner is shown in Figure 2. The burner is of a diffusion-mixed type fired with natural gas 

and its maximal capacity is 1.5 MW. Non-preheated atmospheric air is used as the oxidizer. The burner is 

designed as a mono-block and thus the air fan is a part of the burner’s main body. This type of burners is a 

suitable source of heat for overpressure and under pressure applications, especially for steam and hot 

water boilers. Originally, the burner was designed with one-staged fuel supply and one-staged air supply. 

The standard achieved values of NOx emissions were 50 - 70 ppm. 

Within the scope of the research work a new burner head was designed to suppress NOx formation. This 

new burner design enables two-staged supply of both fuel and combustion air (Figure 3). The gas inlet 

consists of twelve primary nozzles and eight secondary nozzles. The primary nozzles are drilled in the 

primary nozzle head and are aligned in two circular sets. There are six nozzles with the diameter of 

3.0 mm in the first set and eight nozzles with the diameter of 2.6 mm in the second set. The maximum 

thermal input of the primary stage can be regulated by the exchangeable primary gas throttle of different 

diameters placed before the inlet to the primary stage of the burner. 

The secondary gas inlet is provided by four secondary nozzle heads with the varying pitch angle of head’s 

tip from 0° to 30°. Each head has two nozzles. The first nozzle with the diameter of 6 mm is located on the 

pitched tip of the head. The second nozzle with the diameter of 2.3 mm is located on the cylindrical part of 

the head. It is possible to change the position of secondary nozzle heads towards the burner tile in 

tangential direction. In the reference tangential position the secondary heads are oriented directly towards 

the burner axis, i.e. angle 0°. The head’s orientation can be changed both clockwise (i.e. in the direction of 

flame’s swirl motion – positive angle) and counter clockwise (i.e. negative angle). 

The burner is equipped with the flame holder that has the form of swirl generator. The swirl generator 

consists of six pitched blades and is mounted on the burner head. 

The burner channel is split into two channels with the cylinder that is inserted between the pipes for 

secondary fuel and the swirl generator. The primary combustion air flows through the inner channel where 



 

 

999 

the combustion of primary fuel also takes place. The combustion of primary and secondary fuel is then 

completed by the secondary combustion air that flows through the outer channel. 

 

Figure 2: Tested mono-block burner. 

 

Figure 3: Detail of new burner head design. 

3. Plan of combustion tests 

The aim of combustion tests was to investigate the combustion quality and flame properties while 

changing various constructional burner’s parameters and operating parameters. Measured and observed 

parameters included concentrations of NOx and CO in dry flue gas, flue gas temperature, heat transfer 

from hot flue gas to the combustion chamber’s shell, the distribution of in-flame temperatures measured in 

the horizontal symmetry plane of the combustion chamber, and the observation of flame stability, flame 

shape and its colour. During all tests, the target oxygen concentration in dry flue gas was kept at 3 % by 

volume (corresponding to air excess of 1.15) and the pressure in the combustion chamber was kept 

around -100 Pa. 

The combustion tests were carried out both with the standard burner head and with the new burner head 

design. During the tests with the new burner head design, following parameters were varied: 

 Pitch angle of swirl generator’s blades: 30° and 45°. 

 Pitch angle of secondary nozzle head’s tip: 0° and 30°. 

 Tangential orientation of secondary nozzle heads: 0° and 30°. 

 Ratio of primary/total fuel: 0.17 and 0.2 (the ratio changed by the diameter of primary gas 

throttle). 

 Ratio of primary/total combustion air: 0.5, 0.7, 0.75, 0.8 and 0.9 (the ratio changed by 

decreasing/increasing flow area of the secondary air channel). 

4. Results and discussion 

In Table 1 the results obtained during the burner testing with the standard burner head are summarized. 

The results show that NOx emissions are within the range between 59 and 70 ppm. The efficiency of heat 

transfer in the radiation part of the chamber ranges from 48 % to 54 % and decreases with increasing 

burner capacity. 



 

 

1000 

 
Table 1: Results from testing with the standard burner head. 

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Standard 500 70 1.8 0.3 748 54.2 

Standard 750 68 2.3 0.3 800 54.1 

Standard 1,000 61 2.8 0.4 880 53.1 

Standard 1,200 60 3.0 0.5 931 50.2 

Standard 1,400 59 3.5 0.6 970 47.8 

Table 2 summarises the results of tests with the new burner head design. First, the tests were performed 

using the secondary nozzle heads with the pitch angle of the tip equal to 30°. When the secondary heads 

were directed at the burner axis (i.e. the angle of rotation is 0°) and the secondary air channel was fully 

opened, NOx emissions were ranging from 50 to 55 ppm (at 3 % O2 in dry flue gas). When the tangential 

orientation of the secondary heads was set up to 30° and the secondary air channel was gradually closed, 

NOx decreased to 44 ppm. During all tests, CO emissions did not exceed the value of 5 ppm, the flame 

was observed stable and was characterized with distinctly separated primary and secondary flames. 

Moderate improvement was achieved when the angle of the tip of secondary heads was 0°. Then NOx 

emissions decreased to 40 - 42 ppm. However, when the ratio of primary fuel was less than 0.2 or the ratio 

of primary air was higher than 0.8, the burning began to be unstable. 

The burning stability was enhanced when the swirl generator with the pitch angle of blades of 45° was 

used. This enabled to operate the burner with the ratio of primary fuel less than 0.2 while keeping the 

burning stability. The redistribution of fuel between primary and secondary stage resulted in the reduction 

of NOx to the level of 37 ppm. Further NOx reduction (to 30 ppm) was achieved when the ratio of primary 

air was increased to 0.9, however, then the primary flame began to be unstable with the danger of blow-

off. 

The dependence of NOx concentration on the burner capacity is displayed in Figure 4(a). When the burner 

capacity increases, NOx has the descending behaviour for the standard burner head while NOx slightly 

increases for the new burner head. 

Table 2: Results from testing with the new burner head design. 

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30° 0.20 30° 0° 0.50 55 2.8 830 54.2 

30° 0.20 30° 30° 0.50 55 2.5 820 55.0 

30° 0.20 30° 30° 0.70 45 2.3 815 55.3 

30° 0.20 30° 30° 0.80 44 2.3 817 55.9 

30° 0.20 0° 0° 0.50 43 3.2 850 51.5 

30° 0.20 0° 0° 0.75 40 3.0 830 54.1 

45° 0.17 0° 0° 0.50 44 3.2 835 54.2 

45° 0.17 0° 0° 0.75 36 3.2 835 54.1 

45° 0.17 0° 0° 0.90 30 3.0 825 58.1 

45° 0.20 0° 0° 0.50 48 3.2 833 56.2 

45° 0.20 0° 0° 0.75 46 3.2 835 56.6 

45° 0.20 0° 0° 0.90 44 3.0 830 56.6 

45° 0.20 30° 0° 0.50 49 3.1 830 56.8 

45° 0.20 30° 0° 0.75 45 2.9 828 57.2 

45° 0.20 30° 0° 0.90 43 2.7 825 57.5 



 

 

1001 

 

Figure 4: (a) Comparison of NOx emissions for the standard and newly designed burner head, (b) 

distribution of heat fluxes for the standard and newly designed burner head. 

Figure 4(b) shows the distribution of heat flux along the radiation part of the combustion chamber. The 

heat flux to the wall of each chamber’s section was evaluated based on the measurement of cooling water 

flow rate, inlet and outlet temperature of cooling water. It is evident that the heat transfer rate was higher 

when the new burner head was used. This also corresponded to higher thermal efficiency. 

Figure 5 shows the thermal field in the horizontal symmetry plane of the combustion chamber measured 

during the tests with the standard burner head. The-in-flame temperatures were measured using the 

water-cooled thermocouples of type R that were installed through the inspection windows. It is evident 

from the figure that the temperature peaks occur near the burner region in the core of the primary flame. It 

is generally known (Baukal, 2004) that the temperatures above 1,100 °C are favourable for the formation 

of thermal NOx. For this burner configuration, the highest temperatures were about 1,350 °C and NOx 

emissions were about 70 ppm. 

 

Figure 5: Temperature field for the standard burner head. 

Figure 6 shows the thermal field measured during the tests with the new burner head design. The 

configuration of the burner was following: pitch angle of swirl generator’s blades 45°, ratio of primary fuel 

0.17, and pitch angle of secondary nozzle head’s tip 0°. It can be seen that temperatures in the zone of 

primary flame are lower and distributed more widely compared with the temperature field of the standard 

burner head. Lower temperatures then contributed to lower NOx formation. 

  

Figure 6: Temperature field for the new burner head. 



 

 

1002 

 
5. Future work  

The future work will be focused on the optimisation of burner geometry to achieve higher flame stability 

when the burner is operated with small ratio of primary fuel or high ratio of primary combustion air. The aim 

will also be to investigate the dependence of heat flux along the chamber’s shell on the flame length and 

on the type of fuel. 

6. Conclusions 

The results showed that the fuel-staging and air-staging are efficient methods to suppress the NOx 

formation. The NOx reduction is achieved by decreasing of temperature peaks in the flame core and more 

uniform temperature distribution in the combustion chamber. This then resulted in higher heat transfer rate 

in the radiation part of the combustion chamber compared to the burner design without both low-NOx 

methods. The lowest NOx concentration was measured when the ratio of primary fuel was about 0.17 and 

the ratio of primary air was about 0.9. For this burner configuration NOx was about 30 ppm. However, the 

flame was observed to be slightly unstable. 

Acknowledgement 

The research leading to these results has received funding from the MEYS under the National 

Sustainability Programme I (Project LO1202) and from the TA CR under the ALFA Programme (Project 

TA03021017). 

References 

Normann F., Anderson K, Leckner B., Johnsson F., 2009, Emission control of nitrogen oxides in the oxy-

fuel process, Prog. Energy Combust Sci. 35, 385-397. 

Hill S.C., Smoot L.D., 2000, Modelling of nitrogen oxides formation and destruction in combustion systems, 

Prog. Energy Combust Sci. 26, 417-458. 

Baukal C.E., 2004, Industrial combustion pollution and control, Marcel Dekker, Inc., New York, USA. 

Bělohradský P., Kermes V., 2012, Experimental study on NOx formation in gas-staged burner based on 

the design of experiments, Chemical Engineering Transactions, 29, 79-84. 

Ballester J., Sanz A., Gonzalez M.A., 2008, Investigation of the characteristics and stability of air-staged 

flames, Exp. Therm Fluid Sci. 32, 776-790. 

Liuzzo G., Verdone N., Bravi M., 2007, The benefits of flue gas recirculation in waste incineration, Waste 

Manage. 27, 106-116. 

Smoot L.D., Hill S.C., Xu H., 1998, NOx control through reburning, Prog. Energy Combust. Sci. 24, 385-

408. 

Vondál J, Hájek J., 2014, Numerical Flow Field Prediction Downstream of Axial Guide, Vane Swirl 

Generator, Chemical Engineering Transactions, 39, 907-912. 

Bělohradský P., Skryja P., Hudák I., 2014, Experimental study on the influence of oxygen content in the 

combustion air on the combustion characteristics, Energy 75, 116-126.