Comparative analysis of kinetic and diffusion modes of natural gas combustion in EAF burners

Introduction

Diffusion combustion of natural gas with process oxygen examined under conditions where the gas mixture forms in the free volume of the furnace, outside the burner [1 – 3]. In the diffusion combustion mode, gas and oxidizer are supplied separately until they meet at the interface between the gas and oxidizer streams. Due to molecular diffusion, a mixture is formed that is characteristic of a laminar diffusion flame at low flow velocities, or a molar diffusion process promoting large-scale turbulence typical of a turbulent diffusion flame. The gas–oxidizer mixture is partially formed both before ignition and during combustion, which complicates analysis of this process. The temperature, composition of combustion products, and their physical and thermophysical properties in the combustion zone – separated by the flame front from the initial mixture – affect both the primary stage of mixing and the rate of chemical reactions.

Temperature in the diffusion flame zone corresponds to the combustion temperature of a stoichiometric premixed mixture, provided that the thermal diffusivity coefficient equals the mutual diffusion coefficient of the gas and oxidizer and that heat losses to the surroundings are negligible. The fundamental principles of diffusion combustion theory were developed and published in [4 – 6]. In operating fuel-fired furnaces, taking into account their design features, technological processes, and thermal conditions, it is advisable to implement the kinetic combustion mode of natural gas with oxygen, which involves preparing a well-mixed premixed gas–oxygen mixture [7 – 10]. In this case, the mixture-formation stage is effectively excluded from the sequence of physicochemical processes. Heating and melting of the initial materials occur primarily through heat and mass transfer from the combustion products, with no unburned hydrocarbons remaining in the gas phase [11 – 14].

Thus, the aim of this study is to carry out a comparative analysis of kinetic and diffusion combustion modes of natural gas with an oxygen–air mixture (OAM) in EAF burners, based on computer modeling of temperature fields and CO/H₂ concentrations.

Objectives:

To compare the torch characteristics of the VG burner (kinetic mode) and its analogues (VAI FUCHS, SMS DEMAG, and NTPF Etalon) during combustion of natural gas with oxygen and to evaluate the influence of combustion modes on.

– distribution of the temperature fields;

– the maximum concentrations of CO and H₂.

Materials and methods

A comparative study was carried out to analyze the torch characteristics during natural gas combustion with an oxygen–air mixture (OAM) in burners of various designs. The investigation included:

– diffusion burners used in modern electric arc furnaces (EAFs) manufactured by NTPF Etalon, SMS DEMAG, and VAI FUCHS;

– the VG burner, a promising design operating in the kinetic combustion mode, which ensures preliminary mixing of the components in the diffuser (Fig. 1).

Experimental studies of combustion require significant time and financial resources for developing test procedures and using specialized measuring equipment. In contrast, computer modeling makes it possible to optimize burner design without manufacturing intermediate prototypes and to quickly analyze how design parameters and operating modes affect torch formation and its characteristics. Computer modeling greatly shortens development time and provides a reliable basis for evaluating the efficiency of various design modifications and technical solutions [15 – 17].

Combustion modeling was performed in the ANSYS software package (CFX module) using the following models and settings:

– extended Coherent Flamelet Model (ECFM);

– Total Energy heat transfer model;

– k–ε turbulence model.

Boundary conditions for the burners were as follows:

– oxygen flow rate: 0.553 kg/s (95 wt. %);

– natural gas flow rate: 0.092 kg/s (the natural gas composition was normalized to an equivalent methane concentration of 100 wt. %).

The variation of temperature (Т, K) and the maximum CO and H₂ concentrations in the cross-section of combustion products along the torch length were determined (Figs. 2 – 4).

As a result, comparative performance characteristics were obtained for burners with kinetic and diffusion combustion modes: the VG burner (kinetic mode) and the diffusion burners of NTPF Etalon, VAI FUCHS, and SMS DEMAG, operating at equal natural gas and oxygen flow rates.

Based on the analysis of the results, zones of intensive combustion were identified, corresponding to regions of minimal CH₄ concentration and maximum temperature:

– for the NTPF Etalon burner – at a torch length of about 500 mm;

– for the VG burner – at a torch length of 100 – 200 mm;

– for the SMS DEMAG burner – at a torch length of about 500 mm;

– for the VAI FUCHS burner – at a torch length of over 1000 mm.

Afterburning regions can be diagnosed by elevated CO and H₂. The VG burner showed the lowest maxima – 32 vol. % CO and 35 vol. % H₂ – whereas diffusion burners exhibited 36 – 42 vol. % CO and 41 – 56 vol. % H₂. These reductions (by 4 – 12 vol. % for CO and 6 – 14 vol. % for H₂) indicate more complete and uniform natural-gas combustion in the kinetic combustion mode than in diffusion systems.

The criterion for the absence of flame-slip risk was taken to be the absence of regions with temperatures above 800 °C inside the burner’s internal cavity. No such high-temperature zones were detected for any of the burners examined. The maximum temperature of the combustion products in the torch ranged from 2757 to 2792 °C, while, at the same time, the gas-dynamic velocity in local flow regions varied widely – from 250 to 750 m/s (see Table). It should be noted that the temperature of the combustion products in the VG burner torch (2776 °C) corresponds to a gas-dynamic flow velocity 1.15 – 3.75 times higher than that of burners operating in the diffusion combustion mode [18; 19].

The optimal efficiency of the VG burner under EAF operating conditions can also be achieved by maintaining the proper vertical distance from the molten bath surface (bath level), the angle of inclination in the vertical plane, and the tangential direction of combustion product flow in the horizontal plane [15; 20]. This configuration enables the most efficient utilization of the thermal energy of the combustion products for rapid and uniform heating of the charge, which enhances the steelmaking intensity, reduces power consumption, and increases overall productivity.

Conclusions

This study shows that the VG burner, operating in the kinetic combustion mode, outperforms diffusion burners. It provides complete, rapid combustion of hydrocarbons in the premixed stream and prevents flame slip into the burner internal volume.

The results support the adoption of kinetic-type burners in modern steelmaking operations.