(I) Proposal of the problem EW Voyes (UK ) from 1951 to 1952 found that the exhaust gas rate ( m3 /ton mix) was similar regardless of the type of raw materials and the amount of carbon. Except that the air outside the fuel for combustion, but also to meet the needs of heat transfer, so he designed a program, with an inert material such as quartz, silicon, aluminum, brick, aluminum trioxide instead of slag, the molten slag to avoid exothermic or endothermic. Considering the source of heat, one set of tests was supplemented with fuel, and the other set was heated externally, that is, the heat carrier was heated to 1300 ° C, heated on the surface, and then subjected to two sets of tests under the same operating conditions. The former is called the sintering test, the latter is called the heat transfer test, and the results are shown in Figure 1. 1) Regardless of the sintering test and the heat transfer test, the shape of each horizontal temperature change curve of the material layer is somewhat different, but the rate at which the high temperature zone passes through the material layer is very similar for each material; Where h g ——— the heat capacity per unit volume of gas; We analyzed the heat storage of the Shougang No. 2 high-sinter sintering layer: υ———Trolley speed, at 1.5 m/min; In the formula a———sintering yield. 6) Total heat income Q=q 1 +q 2 +q 3 +q 4 +q 5 =830.06 million joules [next] Matt Black Oxidized Parts Processing,Oxidation Parts Processing,Aluminum Oxide Parts Processing,Double Oxidation Parts Processing Dongguan Kingswin Wing Tat Metal Products Co., Ltd. , https://www.milling-turning.com
2) Regardless of the sintering test or heat transfer test, the heat transfer time of the heat wave reaching the highest temperature of the exhaust gas through the material layer is also similar;
3) The exhaust gas rate is very close, that is, the sintering test and the heat transfer test are close to the exhaust gas rate.
This test shows that the heat transfer process is the determining factor of the exhaust gas rate, that is, the amount of air required for sintering determines not only the combustion of the fuel but also the heat transfer.
To further confirm this conclusion, heat transfer tests were carried out with gases of different heat capacities. If Ar, He with a small heat capacity and CO 2 with a large heat capacity are used, if heat transfer is used, the exhaust gas rate when Ar and He are used should be large, and the exhaust gas rate of CO 2 should be small, which indicates the sintering process. Heat transfer determines the exhaust rate. The exhaust gas rate is inversely proportional to the heat capacity and its product is constant. In order to further confirm the above results, the fuel type (using titanium and carbon) was tested. If the combustion process is used as a factor in determining the exhaust gas rate, the amount of gas required for carbon combustion is large, and there should be a high exhaust gas rate. The test results show that the burning of angular titanium and carbon has similar exhaust gas rates, which proves that the exhaust gas rate is determined by the heat transfer process rather than the dry fuel combustion process. [next]
(2) Heat wave forward and flame forward
To further understand the heat transfer characteristics of the sintering process, EW Voith and his colleagues proposed two concepts: “hot front†and “flame frontâ€:
1) When there is no internal heat source, the moving speed of the thermal wave front determines the moving speed of the thermal wave curve. In order to calculate the thermal front, it is stipulated that when the temperature of the material layer begins to rise uniformly, it indicates that the thermal wave front has arrived, generally with an isotherm of 1000 °C. Prevail. When equipped with fuel, the concept of a flame front is used, which stipulates that when the temperature of the material layer rises rapidly, it indicates that the flame front arrives, generally 600 ° C or 1000 ° C isotherm.
2) The characteristic of the heat wave curve is a curve with two sides symmetrically centered on the highest temperature. Because the entire layer is only air and quartz, the same air flow rate, the air flow rate is the same; and the flame wave curve is asymmetrical, and the two sides of the curve are asymmetrical, which is a non-isothermal curve.
3) The heat wave curve advances with the heat wave, the maximum temperature gradually decreases, and the heat wave curve is continuously widened. The flame wave curve moves downward with the flame wave (or combustion zone), and the temperature at the highest point rises.
The heat wave moving speed, BB Blaskett proposed the following formula:
h s ———The heat capacity of solid material per unit volume;
Ω—the flow of gas per unit area per minute;
f———the porosity of the unit volume layer.
It can be seen from the above formula that the heat wave velocity is proportional to the gas heat capacity and the gas flow rate, and inversely proportional to the heat capacity of the solid material, the porosity is large, and the heat wave velocity is also large. In addition, it can be inferred:
1) When the material diameter is large, the heat transfer rate is slow, the heat transfer efficiency is low, the heat is maintained in the gas, and the corresponding heat wave velocity is increased;
2) Increasing the content of CO 2 and H 2 O in the gas, and increasing the heat wave velocity because the heat capacity of CO 2 and H 2 O is large;
3) If the air flow rate increases a lot, the gas-solid phase heat transfer efficiency decreases, and the exhaust gas rate increases.
The influence of heat wave and flame front moving speed on the sintering process, E.Ф. Wegman (B ecMaH ) pointed out that the moving speed of "hot front" and "flame front" must be distinguished during the sintering process. In general, they are mutually It is different in quantity. In the case of normal or slightly higher carbon, the burning rate of carbon determines the total speed of the sintering process. The moving speed of the "flame front" tends to lag behind the moving speed of the "hot front". This sintering system is insufficient due to insufficient oxygen supply. The coke particles have been heated to the ignition point and will not burn. Oxygen enrichment should be used to accelerate the combustion, or pressure sintering can be used to accelerate the movement of the flame front, thereby accelerating the entire sintering process. If the carbon is low, the residual oxygen is very large. In this case, all the coke powder heated to the ignition point is burned without exception, so the total speed of the sintering process is determined by the moving speed of the "hot front". For example, when sintering sulfur-containing ore, the hot front is mainly slower than the burning speed, so the gas heat capacity can be improved, the gas permeability can be improved, and the gas flow rate can be increased, thereby accelerating the sintering process. [next]
(III) Heat storage in the sintering process The maximum temperature distribution of each layer in the sintering layer is gradually increased, which is mainly caused by the automatic heat storage in the sintering process.
Soviet AA West Goff (C uco8 ) quantitatively studied the heat storage of the sintering process. The normal carbon-mixed mixture was divided into small units along the height of the layer by 1 cm, and the heat balance per unit was calculated in an area of ​​1 m 2 . The heat income of the first unit is the ignition heating and the combustion of coke in the mixture. The heat expenditure is:
1) heating required for heating the small unit layer mixture (ignition temperature calculated at 1200 ° C);
2) heat required to heat the exhaust gas (to calculate the amount of exhaust gas);
3) heat required to heat the aqueous mixture to 100 ° C and water vapor;
4) the heat required to convert Fe 2 O 3 to Fe 2 SiO 4 in the layer;
5) Heat loss.
The heat recovery project of the second unit below should increase the heat of the upper layer to heat the material (heat from the hot ore cooling process) and the heat that enters the exhaust gas through the upper layer (the heat brought in by the exhaust gas). The heat can be calculated by:
Q=V g C tm 50 (t m -50°C)
Where t m is the highest temperature in a layer;
V g ——— volume of exhaust gas, meter 5 ;
C tm 50 ———The heat capacity of the exhaust gas (from 50 ° C to the highest temperature of the layer).
The thermocouple multilayer test shows that when the temperature of the layer increases from 50 °C to t m (about 1400 ° C ~ 1600 ° C), the edge of the layer is 1.0 to 1.5 minutes, and the center of the layer is 1.5 to 3.0 minutes. Take 1.5 minutes. If the vertical speed is 2 cm/min, the heat of the exhaust gas should be distributed to the three units. According to the rising temperature curve, the distribution ratio is 45%, 35% and 20%.
The heat of the heated air through the upper layer of sinter: Since the sinter is cooled slowly, the temperature curve drops gently, often extending to 4 to 6 minutes (average 5 minutes). According to the sintering process of each small unit is 0.5 points, It is believed that 95% of the sinter is supplied to the following 10 layers of small units. Determine the heat distribution amount of each unit according to the temperature drop curve (2 units 16%, 3 to 15%, 4 to 13%, 5 to 12%, 6 to 10%, 7 to 9%, 8 to 7%, 9 to 6) %, 10 to 4%, 11 to 3%).
Figure 2 depicts the change in heat of each layer. It can be seen from the figure that the eighth layer of the heat storage reaches 62.8% of the total heat. [next]
Take a production trolley of Shougang No.2 Burning, the height of the material layer is 425 milliseconds (minus the bottom material 25 mm, the actual material layer height is 400 mm). The trolley area is 2.5 m × 1.0 m, according to the height of the material layer 400. In millimeters, divide it into 5 layers and then calculate the heat balance by layer.
The first layer of heat income:
1) Ignition heating
Q --- coal calorific value gas, MJ / m 3;
Q———The gas consumption per unit time, m3 /min.
2) Fuel combustion heat q 2 = W × C × (0.9q co + 0.1q co2 ) 400 × 3.34% × (0.9 × 34.18 + 0.1 × 10.4) = 424.87 million joules in the W - - the unit is mixed Feed quantity, kilograms;
C———The mixture contains carbon, %.
q co , q co2 — the calorific value of CO, CO 2 per kg of carbon burned, millions of joules (33.5 and 9.83 million joules per kilogram C);
3) Fe 3 O 4 oxidation exothermic reaction
4) Physical heat brought into the mixture q 4 =400×60×0.2×0.0042+400×8.85%×60×0.0042=29.08 million joules 0.2×0.0042—the specific heat capacity of the mixture, kJ/kg · °C;
400———The amount of mixed material in this unit, in kilograms.
5) Mineral generation heat
The first layer of heat expenditure:
1) Limestone decomposition q 6 = W cao · qcao + W MgO · qMgO = 2415 × 3.19 + 771 × 2.78 = 98.52 million joules W CaO , W MgO - the unit CaO, MgO content, kilograms;
q CaO , q MgO — the heat of decomposition per kilogram of CaO, MgO, millions of joules per kilogram.
2) Water evaporation q 7 = W H2O · qH2O = 35.394 × 2.499 = 88.45 million joules W H2O · qH2O — The water content of the unit mixture, kilograms, and the heat of evaporation of water, millions of joules per kilogram.
3) External heat loss q 8 =0.15Q=0.15×830.06=124.51 million joules 4) Heat taken away by exhaust gas and sinter q 9 =Qq 6-8 =518.578 million joules This q 9 is all the following units Absorption, 70% of the change in exhaust gas temperature is estimated as the second layer absorption, and 30% is the third layer absorption, whereby the heat balance of each layer and the heat storage rate of each layer can be calculated.
From the above calculations, we can know:
1) The heat storage in the material layer is gradually accumulated as the height of the material layer increases. When the material layer height reaches 400 mm, the heat storage rate reaches 65%.
2) The source of heat storage is brought about by the preheating of cold air from the previous unit of exhaust gas and hot sinter. When the material layer is increased from 300 mm to 400 mm, the heat storage capacity is increased to 175.1 megajoules, equivalent to 5.95 kilograms of standard coal, equivalent to 6.58 kilograms of ordinary fuel (containing 80% of fixed carbon, and the heat of combustion per kilogram of carbon in sintering is 31.204). Millions of joules, fuel savings per ton of sinter will be 4.1 kg. This is why thick layer sintering reduces fuel consumption.
3) Due to the automatic heat storage effect of the sintering process, the temperature of the sinter layer is increased as the layer of the sinter increases, so that the strength of the sinter is better. This is why sintering high-layer operations can increase the strength of the sintered ore.