Abstract
Contents
- Introduction
- Topicality of the theme
- Purpose and problems of the work
- The practical value of the results
- Conclusion
- List of literature
Introduction
A blast furnace is a shaft furnace designed for smelting of cast iron from iron ore. It is the only metallurgical object of smelting of primary product of the ferrous metallurgy. Under the terms of heat exchange blast furnaces are furnaces with layerwise mode working with a dense layer.
Topicality of the theme
In the blast-furnace production the problem of reducing the consumption of expensive coke remains relevant to the present. One of the solutions is using of substitutes for coke (fuel oil, natural gas, pulverized-coal fuel). This research aims to studying of the heat exchange processes occurring at inflation of pulverized-coal fuel (PCF).
Purpose and problems of the work
The aim of the work is to work out a methodology for calculating of the heat exchange processes in the tuyere area of the blast furnace with the condition of PCF supply and analysis of these processes.
The practical value of the results
Research of the heat exchange in the tuyere area of the blast furnace allowed to work out the methodology for calculating heat exchange processes by convection and radiation. The numerical values for the consumption of pulverized-coal fuel were established. The work allows to make a conclusion about importance of inflation of pulverized-coal fuel and its influence on economic indexes of coke consumption in the furnace.In the work the tuyere area is represented as spherical cavity in which there is an intense combustion of coke and PCF with formation of reducing forehearth gas consisting of a mixture of CO, Н2 gases. Between forehearth gases and materials on the periphery of the tuyere zone heat exchange occurs by convection and radiation.
The following method of calculation was developed to estimate the convective heat exchange. Density of convective heat flow is determined by the law of Newton-Richman [1]:
where – αк - is a coefficient of heat irradiation by convection, W/(m2·К); Тф – is the temperature of gas in the tuyere zone, K; Тпр – is the temperature of products of blast furnace fusion and coke surrounding the tuyere zone, K.
A model of the motion of gases in the tuyere zone is proposed to determine the coefficient of heat irradiation by convection. It is schematically shown in Figure 1. Jet of blow air flowing into the tuyere zone expands, reaches the opposite "wall" and spreads on the spherical surface. At that zones with different intensity of heat exchange can be identified. Intensive heat exchange zone is located in the jet blow place with square fинт and heat irradiation coefficient αинт, as well as the zone of jet upstream where the heat irradiation coefficient αобр is significantly lower.
In these areas heat irradiation coefficients by convection are found by Nusselt criterion [2]:
which in turn is determined from the criterion dependence [3]:
where Reynolds criterion [1]:
Prandtl criterion [2]:
(in formulas 1-5: w - the calculated gas velocities in areas m/s; d - estimated size, m;; ν – the kinematic viscosity coefficient, m2/s; a - thermal diffusivity coefficient, m2/s; λ – coefficient of thermal conductivity of tuyere gases, W/(m·К)).
Coefficient of influence of the angle of jet meeting β and between the jet and backflow with the surface of the tuyere zone φ is determined by Table 1 [3].
| β | 0 | 10 | 40 | 60 | 70 | 90 |
| φ | 1,00 | 1,05 | 1,85 | 2,32 | 2,42 | 2,50 |
For an area of intense heat exchange φ = 2,5; Coefficient for dust-laden flow is: φ = (1+2,5)/2=1,7.
Estimated gas velocities in the zone of intense convection are calculated using formulas of Abramovich H.N. [4]:
where w0 – is velocity of outflow from the tuyere, m/s; dф.з. – is diameter of the tuyere zone, m; d0 – is diameter of the tuyere, m.
The estimated thickness of the backflow lрасч is determined as half the difference of tuyere zone radius and the radius of the jet in the middle of the tuyere zone rстр. The latter is determined by the following formula of Abramovich H.N. [4]:
where r0 – is tuyere radius, m; rф.з. – is tuyere zone radius, m.
Example of calculation of density of convective heat flow is given for the tuyere zone with the consumption of blast Vд.(1)=1,881 m3/s, temperature of blast tдутья=1000 0С, dф.з.=1 m, d0=0,14 m. It is determined in the research that the area of intense heat exchange is 12% of the total surface area of the tuyere zone.
In the case of the maximum possible consumption of PCF equal to 250 kg/t of cast iron the following values are obtained: αинт=111 W/(m2·К), αобр=29 W/(m2·К); the average coefficient of heat irradiation is equal to expression αср=0,12·111+0,88·29=35,2 W/(m2·К). The heat flow density (1) for tф=2000 0C and tпр=1500 0C, is qк=17600 W/m2.
The heat flow density by radiation is determined by the Stefan-Boltzmann formula [1]:
where the reduced coefficient of radiation [1]
the reduced degree of emissivity of the tuyere zone volume is determined by the formula [5,6]:
where εг – is degree of emissivity of the tuyere zone volume; εповерх – is degree of emissivity of the internal surface of the tuyere zone.
Degree of emissivity of the tuyere zone volume is determined taking into account the known point of the Kirchhoff law that the degree of emissivity which characterizes the intensity of the radiation «ε» is equal to the absorptance of the body «а». Due to the fact that the combustion of coke and PCF into the tuyere zone occurs with formation of diatomic gas (CO, Н2), which are known not to participate in the radiation heat exchange heat only dense particles can radiate heat. Almost instantaneous combustion of PCF particles occurs in the volume of tuyere zone, as a result a dusty stream forms consisting of molten ash particles. Absorptive power of dusty stream may be determined by the methodology of Kutateladze-Borishansky [5]:
where К is effective value of coefficient of attenuation of the rays in this medium; l is ray path length, m.
The calculation of absorptive power is performed in the following order:
1. Choosing of the fuel type involved in combustion: PCF and coke.
2. Determination of the diameter of particles of PCF and coke ashes. Calculation of the diameter of the ash particles of the selected fuel is carried out taking into account their ash level and density.
3. Calculation of the effective value of the ray attenuation coefficient in the environment [5]:
where Кn is the effective attenuation coefficient, the proportion of units [5]; μ is density of dusty gas stream [5], F is average specific surface of dust m2/g,
where d is average diameter of the ash, microns; ρ – is density of combustibles kg/m3;
where Аc is ash content in the fuel used, the proportion of units; G is fuel consumption, kg/t of cast iron; Vгг is outlet of hearth gases, m3/t of cast iron.
4. Calculation of absorptive power is performed on formula (7).
The dependence of the degree of emissivity of the gas flow from the particle size and PCF consumption is shown in Figure 2. As shown in Figure 2 degree of emissivity of volume reaches the unit at sufficiently low supply of PCF and particle of any size.
Degree of emissivity of the surface of the tuyere zone εповерх is determined as average weighted value for cast iron, coke and slag, and is 0,47-0,51. The reduced emissivity degree with the use of PCF amounts the value substantially equal εповерх, and the heat flow density q (6) reaches 450 000 W/m2, для tф=2000 0C and tпр=1500 0C. The heat flow density without PCF i.e. when burning only coke is only 1100 W/m2. Therefore the heat flows transferred by convection and radiation are: when burning only coke q∑=17600+1100=18700 W/m2, and when using PCF q∑=17600+450000=467600 W/m2.
Conclusion
On the basis of research carried out in the work a method of determining the heat flows which are transferred by convection and radiation in the tuyere zone has been developed. The proposed method allows to determine the numerical values of heat flows and makes it possible to compare them. Flow values are significantly different numerically. When using PCF heat flow is 25 times greater than it is when burning only coke.
List of literature
1. Исаченко В.П., Осипова В.А., Сукомел А.С. Теплопередача - М.: Энергия,1975. – 488с.
2. Казанцев Е.И. Промышленные печи - М.:Металлургия: Справочное руководство для расчетов и проектирования, 1975.-368 с.
3. Курбатов Ю.Л., Василенко Ю.Е. Металлургические печи: учебное пособие – Донецк: ГВУЗ «ДонНТУ», 2013. – 388 с.
4. Курбатов Ю.Л., Масс М.С., Кравцов В.В. и др. Гидрогазодинамика в теплотехнике – Донецк: Норд Пресс, 2009. – 234 с.
5. Кутателадзе С.С., Боришанский В.М. Справочник по теплопередаче – М.: Гос. энергетическое издательство,1959. - 414 с.
6.Мишин И.В., Курбатов Ю.Л., Ярошевский С.Л. Методика расчета температуры продуктов плавки на выпуске при вдувании в горн дополнительных топлив: труды междунар. науч.-практ. конф.,18-21 сент. 2012 г. - Ек.: Уральский федеральный университет.