Coke oven construction, oven baking and start-up (4)

2. Preparation of baking oven

The amount of work during the baking oven is large and the time is short. Full preparation before baking oven is the main link to ensure the smooth progress of baking oven and improve the quality of baking oven.

1 Projects that must be completed before baking the furnace

(1) Chimney and flue engineering All chimneys have been inspected and accepted, the flue joints have been grouted, the expansion joints have been cleaned, the temperature and pressure measurement management has been set up, and the small stove and fuel pipeline for baking the furnace have been built.

(2) Brickwork and cleaning of the furnace body Make a cold inspection record of the carbonization chamber, clean the 30mm expansion joints of the furnace body and furnace wall, fill the expansion joints on the front of the furnace body with asbestos rope, and build a temporary small furnace head.

(3) Sealing work of various parts The riser hole can be covered with a spare coal loading hole cover, the surrounding area of the coal loading hole cover is sealed with mortar, the small flue opening is sealed, the exhaust gas switch base and the heat storage chamber are sealed, and the upper part of the protective plate furnace door frame is covered with a rainproof covering layer (a layer of horse manure paper is placed on it, and cement mortar is applied on it).

(4) Installation project includes: furnace iron parts (furnace door frame, protection plate, furnace column, pull adjustment and spring), exhaust gas switch, machine and coke side operation joint, measuring wire rack and measuring platform, and burying of central nails of resistance wall.

(5) Preparation work before furnace ignition includes: 1) Marking of furnace chamber number and measuring points; numbering of heat storage chamber and combustion chamber. Furnace length, furnace height and spring measuring points, 30mm expansion joint measuring points of furnace end wall, gap measuring points between furnace column and protection plate, etc. 2) Measurement of various original records: including the temperature of combustion chamber, heat storage chamber, grate brick, small flue, temperature and suction of main flue and sub-flue, resistance wall temperature, atmospheric temperature, furnace column curvature and load of large and small springs, lifting height of longitudinal pull bar and spring load, upward movement of furnace door frame, gap between furnace column and protection plate, machine coke side operation platform and resistance wall inclination, furnace height and furnace length, etc.

(6) Furnace baking personnel staffing Due to different furnace baking methods, different furnace baking management personnel are required. Comparing the three baking methods, the solid fuel baking method requires the most personnel, while the gas fuel baking method requires the least personnel.

(7) Preparation of baking fuel According to the different baking methods, the required amount of solid, liquid and gas fuels is related to many factors, such as the speed of baking, the length of insulation time, etc. Under normal baking conditions, the required amount of fuel is as follows:

Solid fuel can be calculated as 32 tons per carbonization chamber: 42-hole coke ovens require a total of 1,400 tons of lump coal. The required fuel before 300°C accounts for 27% of the total fuel, and 73% from 300 to 950°C.

Liquid fuel is calculated at an average of 20 kg/h·hole.

(8) Other preparations

1) Various consumable materials and tools required for baking, such as thermocouples, various thermometers, various pressure (suction) gauges, optical pyrometers, etc.

2) The coal preparation, coke screening system, and primary cooling and final cooling system projects must be completed and meet the test operation conditions or be mostly completed. (Especially the blower installation should meet the test run conditions)

2. Formulation of heating plan for oven drying

The whole oven drying process can be divided into two stages: drying and heating. The basis for formulating heating plans at different stages is also different. The drying period is mainly to ensure that the moisture inside the masonry diffuses outward and the evaporation rate of moisture on the surface of the masonry is coordinated; the heating stage is mainly to consider the slow and uniform expansion of various parts of the masonry, while the relevant equipment is still in a cold state, but they are all related to the temperature ratio of each interval of the masonry and the expansion of the refractory material.

Therefore, the whole oven heating plan should be formulated based on the above factors.

(1) Determination of drying period A newly built 58-type coke oven (42 holes) has a total moisture content of about 300t in the masonry. This moisture must be discharged before the coke oven is heated normally. The drying period is the time required to raise the furnace temperature to 100℃. In fact, when the furnace temperature reaches 100℃, the moisture in the masonry is not completely discharged, it is only a customary division.

Before the masonry is dried, the internal moisture and surface moisture are basically uniform. After the drying begins, the surface moisture is first taken away by the hot air flow, and the balance between the internal and surface moisture of the masonry is destroyed. The moisture diffuses from the inside to the surface, and then is taken away by the hot air flow until the masonry is completely dry. The diffusion rate of moisture from the inside of the masonry to the outside is related to the temperature. The higher the temperature, the faster the diffusion rate. However, when the temperature is too high, the internal moisture will be directly vaporized and generate considerable pressure to rush out from the mortar joints of the masonry, making the mortar joints loose, thereby destroying the tightness of the masonry. In addition, too fast a speed will lead to another disadvantage. When the hot air flows through the carbonization chamber, combustion chamber, ramp, heat storage chamber, small flue and other parts, the temperature will gradually drop. If the drying speed is increased, the temperature of each part will increase. When the hot air reaches saturation in the combustion chamber and flows to the lower part of the masonry, the water vapor in it may cool down in the small flue due to the lower temperature. This will not only prolong the drying period of the lower masonry, but also wash away the mortar joints, destroy the integrity of the masonry, and affect the firmness and tightness of the masonry. This situation must be prevented in the oven. Therefore, the drying speed of the masonry cannot be too fast. The drying period depends on the moisture content of the masonry, the solar term when laying bricks, the fuel used for the oven and the excess and sparse air in the early stage of the oven. Generally, 6-10 days is appropriate.

(2) Selection of daily expansion rate Most of the coke oven masonry is made of silica bricks. It is known that silica bricks expand unevenly when heated. At temperatures of 117, 163, 180~270℃, due to the crystal transformation, the volume changes sharply, and the silica bricks themselves and the masonry will produce great internal stress, resulting in cracks or pulling the masonry apart and destroying its tightness.

The faster the heating rate, the greater the temperature difference between the parts, and the easier it is to produce destructive tension. In order to prevent the occurrence of such destructive expansion, the daily expansion rate is used to control the heating rate. According to many years of practical experience, it is believed that the heating rate should be planned at a maximum daily expansion rate of 0.035% before 400℃, and 0.05% after 400℃. The selection of the daily expansion rate, that is, the speed of the furnace, is not only determined by the properties of the refractory materials, but also related to many factors such as the furnace baking method, the level of operation and management, the progress of the hot engineering and the construction force. Therefore, an advanced and reliable furnace heating plan should be formulated according to different situations.

(3) Determination of brick sample expansion curve The expansion characteristics of brick samples are the main basis for formulating the oven heating plan. Usually, brick numbers with representative lateral and vertical expansion are selected from the three areas of the combustion chamber, ramp area, and regenerator, that is, the brick numbers with the largest usage, larger brick size, and more difficult to make, to measure their expansion curves.

According to the quality of silica bricks and the route of the coke oven, 2 to 4 brick numbers are generally selected for measurement in each part. Two sets of brick samples should be prepared, one for measurement and one for reference.

When measuring thermal expansion data, considering that the temperature before 250℃ is the stage where the crystal transformation points of silica are relatively concentrated and the volume change is the largest, a data point is taken every 25℃ in the range of 20~250℃, a data point is taken every 50℃ between 250~300℃, and a data point is taken every 100℃ between 300~500℃. Since there is a crystal change between 500~600℃, a data point is taken every 50℃, and a data point is taken every 100℃ above 600℃. Finally, 850℃ is measured, and the measurement results are plotted in curves and charts.

(4) The ratio of the upper and lower temperatures in each temperature range Due to the different temperature distribution in the coke oven and the nonlinear expansion properties of silica bricks, the expansion amount and expansion speed of each part of the coke oven are different. In order to keep the coke oven parts as much as possible to expand accordingly and minimize the relative displacement, the masonry will not be pulled apart and unreasonable relative displacement will not be generated. The moisture in the masonry can be effectively discharged during the drying period. Therefore, the temperature of each part of the coke oven should be controlled at a certain ratio during the oven baking process.

The temperature ratio of the upper and lower parts of the furnace body during the furnace baking process is determined by the thermal expansion properties of silica bricks.

Take the silica bricks produced by a certain factory as an example: the total expansion when heated to 850℃ is 1.268%, and the expansion of 0~300℃ is 0.762%, (0~100℃ is 0.08%), accounting for 60% of the total expansion. The corresponding expansion for every 1℃ temperature difference in the range of 100~300℃ is

For this reason, it is required that the temperature of the heat storage chamber before 300℃ is 95% of the combustion chamber temperature. As the temperature rises, this ratio can be gradually reduced, but it should not be lower than 85% at the end of the furnace baking.

The temperature of the small flue should be about 85% of the combustion chamber temperature in the early stage, and should be close to the normal production temperature in the late stage. It should not be too low to avoid the sharp drop in the temperature of the small flue, resulting in shrinkage and cracking. Destroying the tightness of the masonry, the excessive temperature of the small flue will cause the foundation platform to overheat, deform and crack.

According to the above four conditions, a furnace heating plan can be formulated. First, according to the expansion curve of the brick sample, the temperature ratio of the upper and lower parts of the coke oven and the specified daily expansion rate, the number of days for each temperature interval is calculated, and then the daily temperature rise and the maximum daily expansion of each temperature interval are calculated according to the number of days adopted, and the temperature rise and expansion curves are tabulated.

By analogy, the number of days for each part in each temperature interval, the number of day and night temperature rise and the maximum day and night expansion rate can be calculated.

The temperature of the combustion chamber in the table is measured by the vertical fire channel, the temperature of the regenerator is measured by the temperature measuring hole at the top of the regenerator, and the temperature of the grate brick is measured by the temperature measuring hole at the bottom of the regenerator. The expansion values listed in the table also represent the expansion of the corresponding parts.

Because the clay brick coke oven brick itself has no obvious crystal transformation, its expansion is close to a straight line, so the heating plan of each section can be evenly controlled, generally determined by a daily expansion rate of about 0.05%.

3. Baking furnace management

1 Ignition

During the baking process, the hot air flow relies on the suction of the chimney to overcome the resistance and flow through various parts of the furnace body. To ensure that the chimney has sufficient suction, the flue and chimney must be baked before the small stove in the carbonization chamber is ignited. Generally, the chimney is baked 3 to 8 days in advance (depending on the fuel used in the baking furnace), and the flue is baked 1 to 2 days in advance. In order to meet the requirements of the coke oven at low temperatures, only half of the small stoves in the carbonization chamber are ignited at the beginning (the odd and even numbers on the machine and coke sides are staggered). When the temperature of the combustion chamber reaches 70 to 80°C, the remaining half of the small stoves are ignited, and the small stoves against the wall are ignited two days later. According to the furnace temperature, when the chimney has sufficient suction, the baking of the chimney and flue can be stopped successively.

2 Temperature management

(1) Temperature measurement Temperature measurement is the basic method to understand the temperature rise of various parts of the coke oven during the baking process, and it is also a major basis for thermal regulation. Depending on the temperature range, three tools are used for temperature measurement: mercury glass thermometer, thermocouple and optical pyrometer. Each temperature measuring tool must be calibrated one by one before use. When switching from one temperature measuring tool to another, the error between the two temperature measuring tools must be calibrated. The temperature measurement items during the baking process are shown in Table 12.

(2) Pressure measurement The pressure measurement items include: the pressure of the fire hole of the whole furnace, the suction of the middle combustion chamber, and the suction of the total and divided flue.

The fire hole pressure turns into positive pressure at about 150~180℃. When using solid fuel to bake the furnace, try to create conditions for the fire hole pressure to turn into positive pressure in advance to prevent cold air from being sucked into the furnace, reducing the furnace head temperature and damaging the furnace wall. However, when heating with blast furnace gas, for safety reasons, try to keep the fire hole pressure slightly negative. During the furnace baking period, the flue suction should be kept stable and reach the specified value so as to correctly control the temperature distribution ratio of the upper and lower parts of the furnace body. The flue suction does not change much during the whole furnace baking period.

(3) Temperature regulation In order to ensure uniform expansion of the furnace body, the hot air flow should be evenly distributed to each combustion chamber, and the temperature of the combustion chamber and the heat storage chamber should be close. In order to make the temperature of the upper and lower parts of the gas evenly distributed, it is mainly achieved by adjusting the amount of fuel and air.

In the early stage of furnace baking, in order to prevent water vapor from condensing in the small smoke, a large air excess coefficient should be maintained to increase the exhaust gas volume and reduce the exhaust gas inlet and outlet temperature. Therefore, the secondary air door is fully opened at this time. As the temperature rises and the combustion volume increases, the air excess coefficient will gradually decrease. Therefore, it is necessary to pay attention to controlling a certain flue suction and the opening of the small stove air inlet door. The size of the flue suction directly affects the amount of air entering the furnace, thereby affecting the temperature change.

During the whole process of furnace drying, the value of flue suction is roughly stable within a certain range. In the early stage of furnace drying, the air volume is large to facilitate the drying of the furnace body, but because the combustion volume is small and the secondary air door is fully open, the flue suction is slightly large. After the drying period, the secondary air door is fully closed, and the air volume gradually decreases, but because the fuel volume gradually increases, the flue suction does not decrease much. When the temperature of the fire channel reaches about 150~180℃, in order to make the pressure of the fire hole turn into positive pressure, the flue suction is reduced to the minimum. Later, with the increase of fuel volume, the flue suction gradually increases.

In addition, the flue suction also affects the temperature distribution above and below the furnace body. Generally, increasing the suction is conducive to the increase of the lower temperature, and reducing the suction is conducive to the increase of the upper temperature. Since the change of flue suction has a more obvious effect on the temperature, and the flue suction is also related to the amount of fuel, the type of fuel, the opening of the air door, etc., during the whole furnace drying period, attention should be paid to stabilizing the flue suction, and the furnace temperature and its distribution should be adjusted slightly to control the furnace temperature and the upper and lower temperature distribution ratio.

When the temperature of the combustion chamber reaches 150~180℃, the pressure of the fire hole in the vertical fire channel is mainly turned into positive pressure by the action of thermal buoyancy in the furnace, which is conducive to the drying and heating of the top of the furnace and to preventing cold air from entering the furnace. Appropriately reducing the suction of the flue is conducive to the positive pressure of the fire hole, but this is not the main means, because as mentioned above, the suction of the flue should ensure that the furnace temperature rises steadily and in proportion to the plan.

Changes in atmospheric temperature (especially in the early stage of furnace drying, when the air volume is the largest) have a great impact on the furnace temperature. Changes in wind direction will also affect the furnace temperature. Therefore, attention should be paid to timely adjustment. The tightness of the furnace body is also an important measure to ensure the uniform rise of the furnace temperature. Especially in the negative pressure stage, if the various parts of the furnace body are not tightened in time, cold air will be inhaled and the temperature of various parts of the furnace body will be increased, and cracks will easily occur on the surface of the furnace body. Therefore, the sealing work of the sealing wall, small stove and various parts of the furnace body should be strengthened. In the furnace temperature management, attention should also be paid to the adjustment of the horizontal temperature. Due to the heat dissipation of the burner head, as well as the influence of factors such as the flame length and the position of the furnace hole, the horizontal temperature of the combustion chamber is generally uneven, with the burner head slightly lower and the middle of each side slightly higher. During the furnace baking process, the horizontal temperature is generally not easy to adjust, but the temperature of the burner head fire channel should be prevented from being too low by strictly sealing the wall and controlling the primary air door. This is especially important when the furnace temperature is high.

The temperature is not allowed to drop during the furnace baking, but it cannot be heated beyond the plan. If the temperature has reached or even exceeded the heating plan of this shift, it should be kept warm and should not continue to heat up.

The temperature adjustment method varies depending on the furnace fuel. When baking with solid fuel, the main means of uniform heating is to rely on the reasonable use of fuel and the management of the combustion of small stoves. In the low temperature stage, the combustion of small stoves has a more significant impact on the temperature of the burner head.

The burning method should pay attention to adding thin layers at regular intervals and quantitatively, removing slag at regular intervals, and adjusting the opening of the secondary air door to control the combustion intensity; when keeping warm, compact the coal layer, close the secondary air door, and control the slag removal; in the later stage of the furnace, add coal and remove slag frequently to speed up the temperature rise.

When using gas fuel to bake the furnace, the furnace temperature is controlled by adjusting the pressure of the gas branch pipe and replacing orifice plates of different diameters. Several sets of orifice plates of different diameters are required: For example, the 58-type coke oven has 5 types of orifice plates, made of 0.5mm thick steel plates, with inner diameters of 4, 6, 9, 13 and 18mm, respectively, and 80 of each type (42 holes). The diameter of the small orifice plate in the side carbonization chamber is 1.3~1.5 times that of the middle one, and 8 of each type are prepared. The small orifice plate should be processed smoothly (accuracy is ±0.1mm). Practice shows that when using an orifice plate of any diameter, when the gas pressure reaches 1961Pa, the gas flow rate will not increase significantly if the gas pressure is increased, and the furnace temperature is difficult to continue to rise. At this time, the orifice plate should be replaced. The operation should be carried out quickly to prevent the furnace temperature from dropping or fluctuating, and special attention should be paid to fire prevention, explosion prevention and poison prevention. After replacing the orifice plate, the gas pressure should be adjusted to stabilize the gas flow.

When the furnace is dried with liquid combustion, the furnace temperature is controlled by adjusting the oil volume and air volume. The branch pipe opener and the needle valve on the nozzle are usually used to increase or decrease the oil volume. The air volume and pressure of the atomized fuel oil should ensure that the fuel oil does not drip and burns continuously.

During the drying process, the excess air coefficient of fuel combustion should be measured and calculated regularly.

3 Iron parts and expansion management

In order to prevent the tightness of the furnace body from being destroyed during the expansion process of the drying furnace, protective pressure is applied to the furnace body through the furnace protection iron parts. The longitudinal direction of the coke oven is tightened by the concrete resistance wall and the longitudinal brace, so the longitudinal expansion of the furnace body is absorbed by the expansion joint.

The coke oven is tightened horizontally by the upper and lower horizontal braces of the furnace column. If the furnace body expands unevenly during the baking process, the furnace guard iron parts can protect the integrity and tightness of the furnace body. During production, they can also resist the mechanical force that damages the furnace body when pushing coke and loading coal.

During the baking process, the spring load and furnace column curvature should be adjusted accordingly with the increase of furnace body temperature and the expansion of the furnace body. If the adjustment is not timely, the furnace column will be deformed and the pull rod will be broken. The pressure of the furnace body should be kept within the specified range. Insufficient or excessive pressure will cause damage to the furnace body.

The life of the coke oven depends to a large extent on the quality of the furnace guard iron parts during infrastructure construction. Therefore, during installation, baking and production, special attention must be paid to the installation quality and hot adjustment of the furnace guard iron parts. (1) In order to maintain a certain spring load, the nut of the large spring must be tightened in time. When the pull rod diameter is 50mm, loosening the nut one turn can relax the spring by 5mm. The measurement and calculation method of the furnace column curvature can be found in Chapter 8.

(2) Management of the furnace door frame Due to the influence of the furnace body temperature expansion, the furnace door frame (or protective plate) also moves up. Under such conditions, the grinding plate surface is constantly approaching the bottom of the carbonization chamber. If it is not properly controlled, the grinding plate surface will be higher than the bottom of the carbonization chamber. Therefore, the upward movement of the furnace door frame should be checked frequently during the entire furnace baking period to keep the bottom of the carbonization chamber higher than the grinding plate surface. When the furnace door frame moves up and the gap between its bottom and the furnace body boss increases by 8~10mm, the top screw on the upper part of the furnace column presses the furnace door frame. When the furnace body continues to expand, the furnace door frame is pressed to its original position. During the furnace baking period, check once every 25℃ until the furnace door frame is poured.

This inspection method is more laborious, so during the furnace baking period, adjust the top screw to a range of 10~15mm from the upper edge of the furnace door frame. When this range is reduced, it means that the furnace body is expanding in the vertical direction, and the gap between the furnace body boss and the furnace door frame is increasing. At this time, the upward movement of the bottom gap should be checked for timely control. If the distance at the top does not change, it means that the gap at the bottom does not change either, and there is no need to check the gap at the bottom. It should be noted that the value of the upper reduction does not represent the value of the lower increase. It is only used as a reference for inspection. Therefore, the value of the upper gap reduction cannot be used to determine the time to press the furnace door frame. The time to press the furnace door frame is determined by the value of the bottom gap.

(3) Management of longitudinal tie rods The large spring load of the longitudinal tie rods is maintained at around 176×103N during the entire furnace baking period. During the furnace baking period, the tightness of the longitudinal tie rods is adjusted by the longitudinal tie rod adjuster. After the coke oven is changed to normal heating, the adjuster is removed and the nut is tightened.

(4) Determination of the expansion of the resistance wall During the furnace baking process, the resistance wall also expands due to heat absorption. Therefore, the measured furnace length expansion value should be added with the correction value of the resistance wall lateral expansion to obtain the actual expansion value of the furnace length. This expansion correction should be performed at furnace temperatures of 300℃, 600℃ and after coal loading.

(5) Expansion management of furnace length and furnace height In order to ensure the quality of furnace baking, the temperature rise amplitude, the upper and lower temperature ratio and the daily average expansion value of the coke oven should be mastered. Therefore, the actual expansion value of the furnace body should be measured during the furnace baking. Due to the introduction conditions of the heat carrier, the upper and lower heating of the furnace body is uneven, and the expansion is also inconsistent. For example, when the expansion of the combustion chamber is basically completed, the regenerator area is expanding. Therefore, the furnace body expansion should be checked regularly during the furnace baking process to timely find the maximum expansion position and calibrate the furnace baking chart according to this maximum value.

The expansion measurement of the furnace length is selected at the upper cross iron, lower cross iron, ramp and grate brick, and steel wires are pulled along both sides of the machine coke. These steel wires are fixed on the wire rack of the resistance wall.

The four steel wires on each side should be kept in the same vertical horizontal plane and the distance from the center point of the coke oven should always remain unchanged.

As the furnace body expands, the distance between the steel wires and the furnace body is also shortened. The shortened value is the furnace body expansion value. Furnace height expansion measurement: The height expansion of the coke oven is checked with a spirit level, and the measuring point is selected at the standard vertical fire channel of the standard combustion chamber.