Gas Dynamic Air Distribution for Post-Reaction Gas Afterburning in a Metallurgical Furnace

. In the paper, the course of post-reaction gas afterburning generated during the metallurgical process where the reduction of metal and semi-metal oxides is performed with the use of carbon is presented. Process waste gases can be an alternative source of energy to be used in the same process or converted to another. The participation of chemical enthalpy in the total energy stream of the post-reaction gas is 93 %, the rest is physical enthalpy. In the study, particular attention was paid to the proper mixing of fuel and oxidizer and to maintaining an appropriate ratio of excess combustion air λ a . The dynamics of the post-reaction gas combustion processes was calculated according to several popular models but the best results were obtained using the two-step mechanism with reaction constants according to Westbrook-Dryer.


Introduction
Due to increasing costs of energy carriers, it is economically necessary to make every effort to reduce their consumption. During chemical processes in industrial furnaces, post-reaction gases are frequently generated which demonstrate high levels of chemical and physical enthalpies [1][2][3][4][5]. Therefore, there is a need to utilise these properties not only for economical reasons, but also for protection of traditional fossil fuel resources and natural environment.
Management of the resulting waste energy requires its conversion to another convenient form. The first stage of this process is the usual gases afterburning, i.e. the conversion of their chemical enthalpy into a physical one. For solving similar problems of gas combustion, but in objects with smaller geometry, good compliance with the experimental data is provided by the method of stochastic fields [6], while an overview of the various modelling techniques can be found in the work [7].
A success in this field requires performing a deep analysis of the technological process, the potential to change it and availability of economical resources necessary to achieve intended goals. Another issue is the human factor, often neglected in technical considerations, which is associated with new technological solutions that require developing new skills and acquiring new knowledge.
The aim of the paper is to present the results of estimated energetic potential of high-temperature postreaction gases during their afterburning in metallurgical furnaces. is 6.98 % to 7.23 %. This shows that a major part of usable energy of post-reaction gases is associated with their chemical enthalpy. Therefore, a very important factor is proper organisation of the post-reaction gas afterburning process in the restricted furnace space with controlled air distribution.

Organisation of the combustion process
While designing the combustion process, several assumptions were made necessary to be met in order to effectively recover a part of the energy of post-reaction gases (defined, to distinguish, as post-process or exhaust gases). The major assumptions were as follows:  The combustion air must be sucked through the charging windows and blown in through the nozzles.  A proper gas dynamics for mixing post-reaction gases with the air must be ensured to achieve their complete combustion for various phases of cyclic delivery of new charge portions.  A controlled delivery of the air necessary for post-reaction gas combustion must be ensured so that the exhaust gas temperature at the hood outlet is 900 °C to 950 °C. Meeting the presented assumptions required solving a series of technical problems associated not only with gas dynamics but also with metrology and automatics.
Another challenge for the organisation of the combustion process was the presence of gaseous SiO in the post-reaction gas (Table 1). Its afterburning generates SiO2 dust in the exhaust gases. Although the fraction of molar SiO in the post-reaction gas is small, it carries a lot of chemical enthalpy. The comparison of the reaction enthalpy increase, taking into account phase transitions for CO and SiO with an average gas temperature of 750 °C and the pressure of 1014 hPa, is presented in equations (1)

and (2) [8]:
The data from equations (1) and (2) indicate that with a nearly three-fold lower concentration of gaseous SiO, the achieved thermal effects are comparable to those for CO oxidation. In addition, the rate of SiO oxidation to SiO2 is approximately four-fold higher at 800 °C than the rate of CO oxidation. Therefore, an important factor is the impact of SiO combustion on the energy balance in the discussed combustion process.
During the earlier furnace operation, it was found that the post-reaction gas combustion might be longer and partly occur even in the exhaust gas channel depending on the degree of the charging window shutters   opening. This phenomenon occurred due to insufficient mixing of the post-reaction gases and the oxidizer despite the relevant ratio of the excess air. The problem was solved by delivering the combustion air through additional ceiling nozzles. The ceiling nozzles have been designed to ensure that part of the air flow is axial and part of the stream is twirled. This division was set in half and cannot be changed without intervention in the nozzle geometry. The nozzle design is presented in Fig. 1 and its assembly in the furnace ceiling is shown in Fig.2.
The mass flow rate of the pumped air was selected based on the stoichiometric calculations of the combustion of post-reaction gases with the composition presented in Table 1. For the calculations, modified equations of substance balances for the gas components were used as follows [9]: The theoretical amount of oxygen required for the complete combustion of the post-reaction gases was calculated based on the following equation: As the combustion air is the air with the oxygen molar fraction of 0.2099, the theoretical demand for dry oxygen was calculated as follows: Due to imperfection of the process of mixing the oxidizer with the fuel, its excess is introduced defined as the ratio of excess air λa. Assuming λa, the real amount of the air used in the combustion process is calculated with the use of the following equation due to the organisation of the combustion process or additional technological requirements: For the stream of post-reaction gases generated in the furnace during the proper course of the technological process, the flow rate regarding the theoretical demand for dry air, calculated using the equation (9), is 2.66 to 2.70 kg/s. The influence of λa values from equation (10) on the exhaust gas temperature is illustrated in Fig. 3.
 C balance (kmol C/kmol of the gas) , =  H2 balance (kmol H2/kmol of the gas)  Si balance (kmol SiO/kmol of the gas)  S balance (kmol H2S /kmol of the gas)  O2 balance (kmol O2/kmol of the gas) The dynamics of the post-reaction gas combustion was determined according to several popular models [10][11][12][13]. The best results were achieved using the Westbrook and Dryer two-step mechanism with chemical reaction constants (see Table 2) [14,15] and the reaction rate was determined with the use of the following equation: In the Westbrook and Dryer model [10-13], = 0; therefore, the exponent n = 0. The proper selection of the equation (11), proposed by Westbrook and Dryer as the most beneficial, is confirmed not only by the computational results that are comparable to the observations, but also by the literature reports of the use of this mechanism for fuel-scarce gaseous flames. The computational effects of the model for two-step mechanism were comparable to the values obtained in the measurements. These values are presented in Table 3 and the way the measurement probe was introduced is illustrated in Fig. 4.
The composition of the exhaust gases was determined using an IMR 3000 P exhaust gas analyser equipped with the following electrochemical sensors:  an O2 sensor with the range of 0 -20.9 %,  a CO sensor with the range of 0 -60 000 ppm,  an SO2 sensor with the range of 0 -4 000 ppm,  an NO sensor with the range of 0 -2 000 ppm,  an NO2 sensor with the range of 0 -4 000 ppm.
Due to a large range of the CO sensor in the IMR 3000 P analyser, a WAG-1 analyser was also used for the measurements, equipped with the following sensors:  an electrochemical CO sensor with the range of 0 -2 000 ppm,  an electrochemical O2 sensor with the range of 0 -100 %,  a FTIR CO2 sensor with the range of 0 -100 %,  Temperature measurements were performed with the use of a thermocouple type K located in an aspiration probe. Comparable measurement results were obtained from a temperature probe located in the IMR 3000 P exhaust gas analyser.
The gas concentrations presented in Table 3 were converted to conventional 6 % O2 in the exhaust gases. The measured low CO concentrations measured in unbeneficial conditions of post-reaction gas combustion (opened charging windows to perform measurements, excess amounts of the sucked combustion air, a low temperature of the gases) confirm a good design of the new furnace gas dynamics. High-temperature post-reaction gases generated in the combustion process in controlled conditions demonstrate a high usable energetic potential. Controlled, multi-step combustion air delivery ensures achievement of their high and stable temperature within the range of 700 °C to 900 °C. The measured temperatures of the exhaust gases at the gas outlet window and in the exhaust gas channel which delivers them to the recuperation system are presented in Fig. 5. Table 3. Components of the post-reaction gases at the level of 2 metres above the edge of the furnace bath [16]. The thermocouple located in the furnace outlet window demonstrates higher than real temperatures of the flowing exhaust gases mainly due to the effects of the radiant flux generated during the process of post-reaction gas combustion in the furnace space.

Conclusions
1. Waste gases from technological processes can constitute an alternative energy source to be used in the same process or after. 2. Conversion in another process. 3. The gas generated in the reduction process demonstrates a chemical enthalpy comparable to that of the coke oven gas which was a valuable fuel in metallurgy ensuring its development. The participation of chemical enthalpy in the total energy stream of the post-reaction gas is 93 %, the rest is physical enthalpy. 4. The important elements of the gas combustion process are proper mixing of the fuel and the oxidizer and maintaining the proper ratio of excess air λa.
5. The dynamics of the post-reaction gas processes was determined according to several popular models.
The best results were achieved using the Westbrook and Dryer two-step mechanism with chemical reaction constants. 6. The correctness of calculations according to the Westbrook and Dryer model was verified by the measurements. Their results are comparable to the computational values.