Solid fuel combustion (2)
Therefore, the overall combustion reaction rate depends on the chemical reaction rate of combustion and the diffusion reaction rate.
(1) V gas through the gas film diffusion rate spread to a solid surface
Where C 0 , C—the oxygen concentration on the surface of air and carbon particles;
δ———film thickness;
D———The expansion factor.
Where DâˆT 1.5-2.0 , the diffusion rate is determined by the film thickness and concentration difference.
(2) chemical reaction rate of V
Of V = KC (6)
Where K—the chemical reaction rate constant, Kâˆe -E/RT ;
E———activation energy;
R———reaction constant;
T——-carbon surface temperature.
When the V is synchronized with the V expansion , the entire reaction proceeds stably, then
The total velocity of the reaction is controlled within the range of diffusion rates.
Improving the particle size of the fuel and accelerating the gas flow rate within this range are advantageous for this reaction to proceed. In the range of chemical reaction speed control, the temperature should be increased to solve. [next]
(III) Calculation of combustion zone in sintering process
(1) R. Schluter calculation method. The combustion of monomeric particulate fuels and the combustion of fuel layers have been studied by many people, but the combustion laws of coke particles in the sintered mixed layer have rarely been studied. The combustion of coke particles in the sinter mix is ​​between the combustion of the monomer particles and the combustion of the layer, so it is very difficult to formulate the combustion process than the coke. In 1962, R. Schluth and G. Bitsinas (BitsiaIleS) proposed the formula for calculating the burning speed in the sintering process at the first international block meeting, and calculated the width of the fuel belt. Although it is quite different from the actual measurement, it is still noticed by the participants. They made the following assumptions before the calculation.
1) Each coke is burned as a separate system and is rarely affected by surrounding particles.
2) The coke grain has a uniform porosity in a circular shape.
3) The combustion is carried out under isothermal conditions, and the operating temperature of the combustion zone is above 1200 ° C, which is a diffusion range.
4) The oxygen content of the gas is fixed.
5) Reactivity and burning rate are constant for different sizes of coke particles.
Using the above assumptions, the oxygen consumption rate equation can be obtained from the binary system diffusion motion differential equation and the chemical reaction velocity equation.
Where P og ——the concentration or partial pressure of O 2 in the gas;
Pi——the logarithmic mean pressure of the carbon film surface layer;
T f ———the average temperature of the film layer, K;
T s ———carbon particle surface temperature;
d———carbon particle diameter, cm;
μ 0 ———the surrounding gas flow rate, cm/sec;
Ï 0 ———the density of surrounding gas, g/cm 3
δ, a, D AB ——— Constant.
The former term of the denominator in the formula represents the diffusion resistance of oxygen through the film layer to the surface of the carbon, and the latter represents the resistance of the chemical reaction.
Carbon consumption is a simple stoichiometric relationship with oxygen consumption in a steady state system:
K=12Ñ„IV. (8)
Where K—the rate of carbon consumption, g/cm 2 · sec;
12—the amount of carbon atoms, gram;
————the chemical calculation factor, related to the form of oxygen becoming carbon oxide, 1≤ф≥2;
N 0 ———Oxygen consumption rate, gram/cm 2 · sec.
Substituting (7) into equation (8), the constant is calculated from the data of Hotter, and the final formula for carbon consumption is obtained. In the case of air and 1 atmosphere, the relationship is:
If the carbon particles are considered to be spherical, the weight loss at the time of combustion is expressed by the following formula:
-dW=-p c dv=KAdt
Where K—the ratio of burning rate, g/cm 2 · sec;
A—the surface area of ​​the particles, cm 2 ;
t———time, second;
Ï c ———density, g/cm 2 ;
V --- volume of the particles, 3 cm.
The volume change is related to the radius, then dV=Adr
-dW=Kadt=-Ï c Adr
∫ t 0 dt=-Ï c ∫ 0 R dr/K
t=Ï c ∫ R 0 dr/K (11)
Where t—the time when the coke particles are completely burned.
Substituting (9) into (11) gives:
t=3730∫ R 0 (rdr)1(r 0.37+0.144 ) (12)
Ï c =0.95
x=μt=t/30 (13)
Where x—the width of the combustion zone;
—————The moving speed of the burning belt is about 2 cm per minute. Graphical integration (13)[next]
Although the calculation and observation of the width of the combustion zone are two to three times different, the combustion of fuel from the combustion kinetics applied to the sintering process is one step ahead.
(2) C.Г. Bratachkov and other calculation methods. Former associate scholars C. Г. Bratachkov and B. ИD.D. (TyMaшB) proposed a new calculation method with a calculated error of no more than 30-40%. The new calculation assumes that the sinter is inert. There is no chemical reaction between the material and the fuel composition, and it is assumed that the combustion reaction of the fuel is dominated by the diffusion process. However, in the calculation process, the effect of the relative surface area of ​​the fuel particles and the factors affecting the gas permeability of the combustion diffusion rate are considered. The time required for carbon pellet combustion is:
Where m, n—the permeability coefficient (porosity and transparency) of the layer;
a lf , a 2f — the rate constant for the formation of CO and CO 2 on the fuel surface;
Ω—the velocity of the airflow at a certain temperature on the horizontal plane of the layer;
d———the diameter of a carbon fuel;
c 0 ,c H ———the final and starting concentration of oxygen;
μ———The moving speed of the burning belt.
According to the above formula, when the air flow rate is 0.5 m / s, the volume of the fuel is 14% (weight 5%), the coke grain diameter is 1 mm, the calculated combustion zone width is 32 mm, and the concentrate is sintered in the laboratory. The actual measured combustion zone width is 28 to 38 mm, which are extremely similar.
From the kinetic analysis of the above fuel combustion, it is known that the burning rate of the carbon particles in the sintered mixture and the width of the combustion zone are closely related to the diameter of the carbon particles. The larger the particle size, the longer the burning time and the wider the burning belt. It is also determined by the flow rate of the gas stream and the permeability coefficient of the layer. When other conditions are fixed, the size of the carbon particles becomes a decisive factor in the yield and quality of the sintering process. The particle size is too coarse, which not only causes segregation during the transportation and cloth distribution of the mixture, but also forms partial over-melting. At the same time, the combustion zone is widened, the resistance of the material layer is increased, and the combustion temperature is lowered. If the particle size is too fine to reduce the gas permeability of the material layer, and because the burning speed is too fast, the combustion band is too narrow, and it is too late to generate sufficient liquid phase, which affects the strength of the sintered ore.
(4) Discussion on the particle size of sintering using fuel
Reasonable fuel particle size is based on the fuel's own conditions and sintering process parameters. The fuel is generally believed that sintering size of 3 mm or less should be 90% and produced a pulverized coal for some species will result, yield and quality of sintered affected. Pangang used different sizes of pulverized coal for sintering test: it was found that it was difficult to sinter from 0 to 0.5 mm, and all indicators were not good. If the carbon content is increased, the strength will increase but the coefficient will decrease. The 0.5-2.0 mm grade is conducive to the improvement of the vertical velocity. The indicators are good in all aspects. The 2.0-3.0 mm grade and the 3.0-4.0 mm grade are poorly breathable for the Panzhihua concentrate. The conditions are still favorable. In order to avoid excessive pulverization of coal powder, it is considered that the upper limit is relaxed, that is, <3 mm is 60%, and coke powder is 70%. If <3 mm is used for 90%, the pulverized coal will produce 35% powder and will account for 35%, which is not conducive to sintering.
The mathematical relationship derived from fuel combustion can be used as a mathematical model to control the quality of sintered ore. If you find the most reasonable width of the combustion zone, you can use mathematical models to control and adjust other factors to meet the requirements, so that you can get the best quality of sintered ore.
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