INVESTIGATION OF THE CARBON MONOXIDE POST-COMBUSTION FLAME IN THE WORKING SPACE OF A STEELMAKING UNIT

In order to optimize thermal mode of the steelmaking process and to bring down energy consumption, we examined effect of thermophysical parameters of the carbon monoxide post-combustion flame considering aerodynamic processes on the thermal-technological parameters during melting. Based on modern approaches and methods, we obtained data on the character of macro-physical processes that occur in the working space of the unit and in the reaction zone, taking into consideration the influence of aerodynamic processes in the bath of a steelmaking unit. We conducted a comparative analysis of the shape and temperature fields of the flame taking into consideration the influence of aerodynamic processes at different intensities of blowing the bath of a steelmaking unit with oxygen for different types of blowing devices. It was established that the shape and magnitude of temperature fields of the drigted flame varies depending on the content of carbon (from 4 to 0.1 %) in the melt, the intensity of blowing the bath with oxygen (from 1,800 to 2,400 m 3 /h), as well as design features of the blowing device (nozzle diameter and inclination angle). In this case, lances with an increase in the inclination angle of the nozzles to 50° and varying nozzle diameters from 10 to 20 mm, compared to lances of basic design (at the same inclination angles), make it possible to improve flame organization, to increase the length and temperature of the flame, to improve uniformity of the structure of flame, to increase heat exchanging surface between the flame and the bath, and to improve heating capability of the bath in a steelmaking unit. The studies reported in the present paper are applicable to industrial steelmaking units with the intensity of blowing the bath with oxygen in a range of 1,800 ‒ 2,400 m 3 /h. The results obtained bring us closer to the development of a rational design of the blowing device to optimize the thermal mode of steelmaking process that will make it possible to reduce energy consumption in steel production.


Introduction
Development of ferrous metallurgy under contemporary conditions is characterized by a significant consumption of natural gas in the process of steelmaking. Relevant tasks [1,2] in this regard are the development of theoretical and practical aspects of the new energy-and resource-saving techniques for steel smelting in steelmaking units with oxygen blowing (O 2 ) and post-combustion of carbon monoxide (CO).
In order to meet these challenges, promising is the application of modes of steel smelting with an increased degree of CO post-combustion in the flue gases by jets of O 2 with the subsequent transfer of heat from the CO post-combustion flames to the melt and a steelmaking bath.
The rational mode of CO post-combustion by jets of oxygen in the flow of high temperature gases discharged from the zone of blowing [3] should ensure more efficient use of the heat released from the CO post-combustion in the system of gas flows in order to heat the bath, reduce natural gas consumption and to improve other technical-economic indicators without compromising resistance of the unit's lining.

Literature review and problem statement
In paper [4], authors studied effect of oxygen jet flow on the process of CO post-combustion when changing the inclination angles of a blowing device from 8° to 12°. Other ranges of change in the inclination angles and their influence on temperature fields of the CO post-combustion flame were not, however, examined.
Article [5] investigated influence of the blowing device's nozzle inclination angle, equal to 16°, on the process of CO post-combustion and agitation of the melt in the bath of a steelmaking unit. It, however, did not address other possible variations of inclination angles and their effect on the temperature field of a CO post-combustion flame.
The blowing process of the bath of a steelmaking unit using a cold model simulation was studied in paper [6]. The work, however, was limited to modeling the intensity of blowing up to 450 m 3 /h. Authors of article [7] examined conditions of overheating reaction zones relative to the peripheral part of the bath and estimated temperature gradients. In this case, the research is limited only to surface measurements of the bath's temperature fields, without detailed examination of macrophysical processes in the reaction zone and of the effect of change in the intensity of blowing on the temperature fields of a CO post-combustion flame.
In paper [8], authors theoretically studied physical-chemical processes in the reaction zone when blowing the melt with oxygen. Results of the work, however, were not tested under industrial conditions. Authors of article [9] performed theoretical modeling of the effect of changing the intensity of blowing in the bath of 75 tons of arc furnace. Results of the study, however, were not tested at the industrial unit; formation of the temperature fields of a CO post-combustion flame was not studied. Parameters and shape of reaction zones in steelmaking units when designing and applying experimental multi-nozzle lance and their influence on the process of CO post-combustion were investigated in paper [10]. In this case, the authors did not take into consideration the influence of aerodynamic processes in the bath and changes in the temperature fields of a CO post-combustion flame.

INVESTIGATION
The process of blowing the bath of a steelmaking unit using a cold model simulation was studied in [11]. The authors, however, did not specify the effect of changing blowing intensity on the temperature fields in the unit.
Intensive splashing and the existence of a post-combustion flame of carbon monoxide in the region of the lance in a general form was recorded by a photographing method [12] excluding the impact of change in the intensity of blowing.
It is worth noting that all the above studies consider behavior of a post-combustion flame of carbon monoxide and the effect on it resulting from the carbon monoxide flow discharged from the reaction zone, including the bubbles of CO formed when oxygen interacts with the melt. They do not take into consideration the influence of aerodynamic processes in the bath of an industrial steelmaking unit, formed under the influence of thrust produced by the fume collection vanes. Their influence on the temperature fields of a CO post-combustion flame was not examined either.

The aim and objectives of the study
The aim of present study is to examine a CO post-combustion flame in the working space of a two-bath steelmaking unit. This will make it possible to proceed to the optimization of thermal mode of steelmaking process, which would reduce energy consumption per unit.
To achieve the set aim, the following tasks have been solved: -based on existing approaches and methods, to obtain data on the nature of macro-physical processes that occur in the workspace of the unit and in the reaction zone, taking into consideration the effect of aerodynamic processes in the bath of a steelmaking unit; -to establish dependences of thermophysical parameters of a post-combustion flame of carbon monoxide, taking into consideration the effect of aerodynamic processes on the thermotechnical parameters of steel melting; -to perform comparative analysis of the shape and temperature fields of the flame considering the influence of aerodynamic processes under different intensities of blowing the bath of a steelmaking unit with oxygen for various types of blowing devices.

Methods applied for studying working space of the unit and the reaction zone
The methods of theoretical research are the formalization and synthesis.
The methods of empirical studies are the laboratory and field experiment (industrial tests).
The most common methods for examining a reaction zone of the interaction between oxygen jets and the melt in a steelmaking unit are the methods of photo-and video recording and filming, which were widely used in paper [12].
We propose to employ these methods to study the reaction zone and a post-combustion flame of carbon monoxide. Additionally, we used the thermal imaging camera NEC H2640 (NEC Avio Infrared Technologies Co. Ltd., Japan) and the infrared pyrometer Raynger (Raytek, USA) with additional blocks.

Results of examining the working space of a steelmaking unit during interaction between oxygen jets and the melt
While conducting balance melting during blowing the bath of a two-bath steelmaking unit (TSU) (Fig. 1) at the PAO ZMK (Ukraine), we obtained information on the character of macro-physical processes that occur in the reaction zone. The experiments were carried out using oxygen lance with an oxygen flow rate of 1,800 m 3 /h and above. The furnace operates in the following way: one bath (hot) is used for melting and finishing with intense blowing of the metal with oxygen while the second bath (cold) is used at the same time for filling and warming the hard charge. Gases under the influence of thrust created by the fume collection vanes, are directed from the "hot" part of the furnace to the "cold". In the cold part of the furnace, CO burns to CO 2 with warming the solid charge by the released heat. The heat lacking for the heating process is replenished by supplying natural gas through the burners installed in the roof of the furnace.
Some fragments of imaging managed to record that the discharge of CO proceeded in several separate regions corresponding to the jets of oxygen that enter the bath from each nozzle of the lance. A diameter of the area of the released carbon monoxide from one such region is 0.25-0.31 m.
There are metal splashes observed within the range of a near-lance flame on the circle with a radius of up to 0. At the same time, we registered, when oxygen is fed through the lance placed over the surface of the bath, the interaction modes between the blowing and the melt with an increase in the pressure from jet to the bath, which has a slag cover of insignificant thickness (0.2-0.3 m). The formation of a near-lance flame is observed during operation mode of blowing when the lance head is located at the slag-metal boundary and above, within a circle of radius up to 0.5-0.8 m (Fig. 2). The flame is formed as a result of after-burning of the carbon monoxide flow released from the reaction zone in the flow of air streams. The oxidizer flow moves inside TSU from one bath to another under the influence of thrust produced by the fume collection vanes. A shape of the near-lance flame is typical for jets, blown into entraining flow (Fig. 3).
When captured by a photo camera, it is possible to see that the flame is displaced to the right from the lance body (in the direction of the discharged gas motion). At the same time, the CO that is released from the melt is sucked by the flame, followed by its pulsating ignition at a distance of 0.4-0.5 m from the lance. The formation of flame along the length of the bath, carried away by the flow of flue gases at a different content of carbon in the melt is shown in Fig. 4.
Based on the photo-recording (Fig. 4) and the measurement of temperatures in the visible part of the carried-away flame on TSU throughout the entire melting, we constructed a dependence of change in the length and temperature of the carrying flame at a different content of carbon in the melt (Fig. 5).
In this case, correlation coefficient R reached 0.95, the relative error when testing the regression equation against actual data amounted to 1.5-2 %.
-Temperature of the drifted flame on the content of carbon in the melt, о С: where [C] is the carbon content in the melt, %. In this case, correlation coefficient R amounted to 0.87, the relative error when testing the regression equation against actual data reached 1.7-2.5 %.
In order to perform a detailed analysis of temperature fields of the visible part of the flame, we split the flame lengthwise into 3 conditional sections (zones). Each zone was divided into 2 vertical parts (top and bottom) and 2 horizontal parts (beginning and end of the zone). According to a special test program, during balance melting, we conducted 8 experimental measurements of the visible part of temperature fields of the drifted flame: 4 experimental measurements on basic lances (a) with the same inclination angle of nozzles relative to the melt (30°) and 4 experimental measurements on the tested lances (b) with a combined inclination angle of the nozzles relative to the melt (20/50°). During tests, the intensity of blowing the bath with oxygen varied from 1,800 to 2,400 m 3 /h at a content of carbon in the melt of 3 %. The measurements were carried out using the infrared pyrometer Raynger 3i and thermal imager NEC H2640.
Results of experiments are shown in Fig. 6-10 and are summarized in Table 1 The blowing was conducted through basic six-nozzle lances (inclination angle of the nozzles to the vertical is 30, diameter of nozzles -15 mm) and experimental (at inclination angles of the nozzles up to 50° to the vertical and with varied nozzle diameters from 10 to 20 mm).  ---polynomial (experimental lance) Table 1 Results of measurements of temperature fields of the drifted flame (basic lances) at a carbon content of 3 %  Table 2 Results of measurements of temperature fields of the drifted flame (experimental lances) at a carbon content of 3 %