Fired Brick and Tile Production Technology (XIV)
Xiao Hui
Section 2 Internal Combustion Blending

Internal combustion bricks are produced by mixing fuels or combustible industrial waste (coal gangue, pulverized coal mixed with ash, slag, etc.) of a certain fineness into brick-making raw materials in a specific proportion, uniformly mixing them to form combustible brick bricks, and firing the bricks by burning fuel within the bricks. Implementing internal combustion bricks fully utilizes industrial waste and saves fuel, making it one of the main energy-saving measures in my country's brick and tile industry. Under the same draft conditions, internal combustion bricks have a faster firing speed, which is beneficial for increasing kiln output and reducing the labor intensity of workers.
4. Preparation of internal fuel
Before use, the calorific value of internal fuel must be measured to determine its appropriate level, which serves as the basis for blending. Inaccurate or inconsistent calorific value not only makes the firing process difficult to control but also affects the quality of the bricks. The calorific value can be measured using an oxygen bomb tester or determined through industrial analysis.
Secondly, the internal fuel must be crushed to a particle diameter of less than 3mm. Coarse internal fuel particles not only accelerate wear on the extruder and cause the cutting wire to break easily, but also reduce the product's appearance quality. Furthermore, incomplete combustion during firing increases fuel consumption. Commonly used coarse crushing equipment includes jaw crushers, while fine crushing equipment includes high-speed pulverizers or fan-type crushers. In addition, when selecting internal fuel, attention should be paid to whether it contains substances harmful to the brick body or prone to efflorescence. Slag containing high levels of limestone, acids, alkalis, and soluble salts, as well as coal gangue containing a large amount of limestone, should not be used.

5 Factors affecting internal combustion blending
The factors affecting the amount of internal combustion fuel used are multifaceted and should be considered comprehensively from the perspective of the entire brick and tile production process.
(1) First, it is necessary to ensure that the brick has sufficient mechanical strength. To this end, the amount of admixture must be determined based on the properties of the clay, the type of internal fuel, and the amount of moisture. For example, when the clay has high plasticity, more can be added, and vice versa.
(2) Determine the internal heat according to the baking requirements. In order to make the baking operation easier to control, the temperature easier to adjust, and the degree of embossing and blackening less, the internal heat is generally 70%-80% of the total baking heat.
(3) For industrial waste with good drying and bursting properties, adding it can reduce shrinkage and decrease drying sensitivity, which can both speed up the drying process and improve the drying quality. It is beneficial to both drying and baking, so it can be added in larger quantities. For industrial waste with poor drying properties, caution should be exercised. When adding it, both the calcination heat value and the drying requirements should be considered.
(4) Determine the type of internal fuel based on the principle of using local materials. When there are multiple types of internal fuel, the above-mentioned factors can be considered comprehensively.

6. Determination of Internal Combustion Blending Quantity
The internal combustion engine blending ratio can usually be determined by calculation and trial blending methods.
6.1 Calculation Method
The calculation method involves determining the appropriate total calorific value for firing based on the kiln's heat balance, and then determining the mixing ratio of internal fuel and clay based on the internal combustion calorific value. The following formulas can be used to determine the weight of internal fuel per 10,000 bricks and the volume ratio of internal fuel to clay.
(1) Calculation of weight ratio

G - Weight of internal combustion fuel added per 10,000 bricks (kg/10,000 bricks); B - Heat required to fire one brick (kJ/brick); Q - Dry calorific value of internal fuel (the calorific value of fuel obtained after drying internal fuel at 105~110℃, kJ/kg); Wm - Relative moisture content of internal fuel; n - Degree of internal combustion, which indicates the ratio of internal combustion heat to total firing heat.
Example: A factory uses coal gangue as fuel. Its dry basis calorific value is 2000 x 4.18 kJ/kg, and its relative moisture content is 2%. Each brick fired produces 1100 x 4.18 kJ, with an internal combustion degree of 80%. Calculate how much coal gangue needs to be added per 10,000 bricks?
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(2) Calculation of volume ratio

 In the formula, Z is the volume ratio of internal fuel to clay; G is the amount of internal fuel added per 10,000 bricks (kg/10,000 bricks); r is the bulk density of internal fuel (kg/m2); V is the volume of 10,000 brick bricks (m2/10,000 bricks). For example, if the volume of 10,000 brick bricks is 18m2 and the bulk density of internal fuel coal gangue is 1600kg/m2, what should the volume ratio of internal fuel to clay be in the above example?

 6.2 Trial Doping Method
Based on the experience of other factories and the specific conditions of our factory, a preliminary internal calorific value is determined. This value is then converted to its weight based on the internal fuel calorific value and added to the brick blank. In production practice, if it can be achieved that no coal is added in the middle, and only some coal is added at the edges to adjust the kiln temperature, the firing temperature is easy to control, the operation is relatively stable, and the brick quality is good, then the internal calorific value can be considered to basically meet the requirements. If, during production, it is found that the amount of externally added coal is still too large, or the firing temperature increases rapidly; or if the bricks are severely underfired or overfired, it indicates that the internal calorific value is too low or too high. Corresponding measures should be taken to adjust the internal fuel blending ratio to make the internal calorific value more reasonable.

Section 3. Structure and Firing Principle of the Rotary Kiln

7. Kiln Structure and Firing Principle
7.1 Kiln Structure
The main structure of the rotary kiln consists of a firing channel, kiln door, coal feeding hole, main flue, branch flue, smoke damper, and air supply, as shown in Figure 1.

Figure 1. Cross-sectional view of the rotary kiln
The firing channel is the firing space of a rotary kiln, consisting of two parallel straight kiln sections and curved kilns at both ends. The firing channel is 3m to 4m wide, and the arch is an approximate semi-circular arch composed of two 90-degree arches with double centers. Semi-circular arches and other types of arches with varying curvatures are also used. The internal height is generally 2.5m to 3m. Kiln doors are equidistantly spaced along the firing length. The kiln passage between two adjacent kiln doors is called a kiln chamber, which is generally about 5m long. The scale of a rotary kiln is expressed by the number of doors.
The coal feeding hole, also called the fire eye, has a diameter of 150mm~200mm. It has a cast iron fire eye ring on top, which reinforces the seal with the fire eye cover to prevent air leakage. The coal feeding holes are arranged in 5 rows per kiln chamber along the length of the kiln passage, with a row spacing of about 1m ; and 4 rows horizontally along the width of the kiln passage, with the coal feeding holes on both sides about 0.3m~0.4m from the kiln wall, and the row spacing in the middle about 0.9m~1m. For curved kilns, it is advisable to add another row of fire eyes between the two rows, on the outer edge and in the outer middle.
The kiln door of a rotary kiln should not be too large, ideally just large enough for a cart to pass through, generally 1m wide and 1.6m to 1.8m high. To prevent the cart from colliding with the side wall of the kiln door, a groove (two layers of bricks thick, recessed 20mm) can be built at the point where the kiln door wall collides with the axle, allowing the axle to pass through the groove.
The flue gas inside the rotary kiln is connected to the main flue between the two kiln passages through the branch flues and the flue damper. The main flue is also connected to the chimney or exhaust fan, forming the rotary kiln ventilation system.
The cross-section of a branch flue is typically 500mm x 600mm. A cross-section that is too small will not only affect the draft of the chimney, but also increase the difficulty of removing accumulated ash.
The cross-section of the branch flue is generally 1.2m x 1.5m for a single-fired rotary kiln and 1.5m x 1.95m for a two-fired rotary kiln.
The flue gas inlet, also known as the exhaust duct, is typically 0.4m-0.6m wide and 0.5m-0.7m high to ensure smooth exhaust of flue gas. In a rotary kiln, one exhaust duct is provided per kiln chamber in the straight section; in a curved kiln, due to the longer outer bend and greater resistance, an additional exhaust duct is required to increase draft. An external exhaust duct is always provided in the curved section, while in the straight kiln, an internal exhaust duct should be provided at the bend exit to reverse the gas flow direction. External exhaust ducts can be provided elsewhere. In kilns with multiple chambers, a small number of internal exhaust ducts may also be provided to regulate the inner and outer fires.
Smoke dampers, also known as cone dampers, should ideally have a diameter of 700mm to 800mm to reduce airflow resistance when open.
7.2 Calcination Principle
(1) Changes in brick and tile bricks during the firing process
During the heating and firing process, brick and tile bricks undergo a series of physicochemical changes, including the removal of residual moisture and chemically bound water, the burning off of organic matter, the transformation of quartz crystals, solid-phase reactions, and the formation of new chemical compounds. Eutectic compounds begin to form at approximately 800°C, and some clay minerals melt to produce a liquid phase. As the firing temperature and time increase, the amount of liquid phase increases. This molten liquid phase flows between the unmelted clay particles, firmly binding them together. Upon cooling, it recrystallizes into hard, stone-like brick and tile products.
Therefore, sufficient liquid phase is required to obtain qualified brick and tile products. For this purpose, a reasonable firing system must be established to ensure the final firing temperature and firing time are achieved in order to obtain high-quality products.
(2) Establishment of baking system
The baking regime includes temperature regime and pressure regime.
The temperature regime consists of the heating rate, the maximum temperature and holding time, and the cooling rate. Since the rotary kiln operates continuously, the firing temperature progresses at a uniform rate during normal production. Therefore, controlling the length of each zone means controlling the time that the brick and tile bricks undergo preheating, firing, and cooling.
The maximum firing temperature depends on the properties and composition of the raw materials. Different raw material compositions result in different sintering properties, and the time and amount of liquid phase appearance also vary. Generally, higher Al2O3 and SiO2 content, coarser clay particles, and lower content of fluxes such as CaO, KO, and Na2O lead to higher firing temperatures, and vice versa. The typical firing temperature range for clay is between 930°C and 1050°C. The holding time at the maximum temperature depends on the kiln size, the shape of the green body, and its thickness. Generally, larger kiln capacities and more complex, thicker green body shapes allow for longer holding times, typically 1.5 to 2 hours.
The same firing effect can be achieved through two methods: one is rapid heating, resulting in a slightly higher firing temperature and a shorter holding time; the other is slow heating, resulting in a slightly lower firing temperature and a slightly longer holding time. For internally fired bricks, the latter method is preferable as it helps eliminate embossed patterns and black cores.
The pressure regime is expressed as the difference between gas pressure and atmospheric pressure at various points along the length of the firing channel. To ensure a certain firing speed without the top bricks being constantly under negative pressure, which would affect product quality, a positive and negative pressure regime is generally used inside the kiln. The firing operator achieves the predetermined pressure regime by adjusting the air damper and opening the kiln door. In daily production management, the number of re-firing rows is an indicator of a particular pressure regime.
In normal factory production, as long as the length of each strip remains constant, the maximum roasting temperature is stable, and the number of re-fired rows is controlled and determined, high-quality products can be continuously produced.
8 kilns
Brick loading, also known as brick stacking, involves placing green bricks and tiles in a specific pattern inside the kiln, and is a crucial step in the firing process. Brick loading and firing are interdependent and inseparable.
For each rotary kiln, the importance of kiln stacking lies in the fact that, under certain flue gas equipment conditions, once the brick stacks are stacked, the uniformity of ventilation within the kiln and the distribution and dispersion of internal fuel are largely determined. This directly affects the kiln's output, product quality, and coal consumption. The kiln workers' actions, such as monitoring the fire, adding coal, and adjusting the valves, can only regulate and adapt to these established firing conditions within a certain range; they cannot fundamentally change the firing conditions already formed by the brick stacks. Therefore, kiln stacking must be coordinated with the thermal characteristics of the bricks. The traditional experience of "seven parts stacking, three parts firing" fully illustrates the relationship between kiln stacking and firing. Internally fired bricks incorporate fuel into the brick bricks, reducing external coal input and making kiln stacking play a more decisive role in production. Therefore, to ensure successful rotary kiln production, it is essential to master the kiln stacking process.
The performance of a stack of bricks can be evaluated using the following four parameters.
(1) Stack resistance and resistance coefficient. The lower the stack resistance, the stronger the ventilation capacity. The resistance coefficient represents the resistance generated when gas flows through a 1m long stack of billets at a velocity of 1m/s. Its value depends on the form of the channels in the stack and the size of the equivalent diameter. In actual stacking, the larger the spacing between straight billets and the smaller the spacing between horizontal billets, the smaller the stack resistance, which is beneficial to ventilation. If the stacking pattern of straight billets pressing two horizontal billets is changed to straight billets pressing three horizontal billets, not only can the resistance be reduced, but the increased capacity will also help increase production. When stacking billets at an angle, the larger the angle, the greater the resistance, which is detrimental to ventilation.
(2) Effective cross-sectional porosity. This refers to the ratio of the area of the pores through which gas can pass on the cross-section of the billet to the area of the billet. The larger the effective cross-sectional porosity, the smaller the gas flow resistance, and the more beneficial it is for ventilation.
(3) Heat transfer area. The heat transfer area of the brick stack refers to the area of the brick exposed to the outside and in contact with the gas. The larger the heat transfer area, the better for the preheating, firing and cooling of the brick. Therefore, it is desirable for the heat transfer area of the brick stack to be as large as possible, and the longitudinal heat transfer area should account for as much as possible.
(4) Stability of the stack of bricks. This is a prerequisite for stacking bricks. Any method of stacking bricks must ensure that the stack remains stable and does not collapse throughout the firing process.
The four parameters above measure the performance of the billet stack from different perspectives, and sometimes they contradict each other. Therefore, it is necessary to introduce a comprehensive index for comparing the performance of the billet stack, namely the resistance number. The smaller the resistance coefficient of the billet stack and the higher the stacking density in the kiln, the smaller the resistance number. In production practice, it has been found that the smaller this index is, the less energy is consumed for firing, while the kiln productivity is higher, all other things being equal. In other words, a billet stack with a lower resistance number has better performance.
8.1 Palletizing method
Any stack of bricks can be broadly divided into the brick support, the stack body, and the fire hole.
Also known as the legs, they are the bottommost foundation part of the entire stack of bricks. The function of the brick legs is to support the entire stack and provide a bottom fire channel to ensure unobstructed fire transmission.
(1) Kang Leg
Common types of kang legs include lantern-hanging kang legs, two-section kang legs, and upright kang legs. A kang leg that stacks two bricks in a grid pattern is called a lantern-hanging kang leg.
Two brick bricks are stacked sequentially, and the third layer connected by a horizontal band is called the second sequential brick kang leg.
The brick bricks are stacked upright at the bottom of the kiln, with the edges of the upright bricks left open. A horizontal bridge is placed on top of the upright bricks, and the fourth layer of bricks is connected by horizontal bands to form the kiln legs, which are called the upright blank kiln legs.
Comparison of several types of kang legs:
The advantages of hanging lanterns on kang legs are that they are sturdy and easy to stack, but the disadvantages are that they are numerous and create a lot of resistance. To accelerate the advance of the bottom fire, the kang legs often need to be raised. When the kang legs are too high, the ventilation under the kang is too great, the front fire burns poorly and the back fire cools down quickly, which increases coal consumption.
Advantages of the two-stage kiln leg: 1. The airflow is evenly distributed across the cross-section, reducing the risk of a bluish bottom and flashback, resulting in a stable and easily controlled firing temperature; 2. The increased height of the kiln stack with this type of leg increases the coal-receiving area while reducing the amount of coal falling to the bottom, thus lowering coal consumption; 3. This type of kiln leg is relatively sturdy. Disadvantages of this type of kiln leg: The kiln legs are connected end-to-end, and when the ash content is high, they can easily block the bottom ventilation channels, affecting the progress of the bottom fire.
Advantages of vertical brick kiln legs: 1. Low resistance, sufficient air supply under the kiln, and smooth bottom fire; 2. There is a transverse air passage between the front and rear vertical bricks, which reduces the resistance of gas flow to the air inlet and facilitates the discharge of water vapor from the preheating zone, which plays a good role in improving the quality of the lower part of the kiln; 3. There is a cross passage at the bottom of the kiln, which allows the fuel put into the kiln to spread out to the left and right, and the air to circulate up, down, left and right, so that the fuel can be completely burned and the amount of coke and black bricks is reduced.
In the kiln, bricks are stacked on a slant instead of a horizontal strip above the legs. This reduces resistance, improves ventilation, and avoids brick breakage and blackening caused by the horizontal strip, thus improving the quality of the bricks in this area. When using slant strips instead of horizontal strips, the distance between the bottom fire channels should be slightly reduced, and attention should be paid to the stacking quality to ensure stability.
Among the three types of brick-laying legs mentioned above, the upright brick-laying legs have been widely adopted in recent years because they improve ventilation and combustion conditions under the brick stack, resulting in faster fire spread, better fire retention, and more complete combustion. When stacking bricks in a two-row pattern, sometimes one less batch of bricks is stacked under the brick stack than on the brick stack between two rows of fire holes; this is called "sinking the brick stack." The purpose of sinking the brick stack is to ensure even firing. Sinking the brick stack sometimes occurs in every row of fire holes, and sometimes every few rows, depending on the fire temperature under the brick stack. During sinking, three batches of bricks are stacked on the brick stack at the front and back of the brick stack, while only two batches are stacked under the brick stack with gaps. This longitudinal sparseness and gaps reduce the lateral resistance of airflow entering the vent, while the lateral density of the legs increases the longitudinal resistance of airflow. This reduces the gas flow under the brick stack, increases the firing temperature under the brick stack, and reduces the production of underfired bricks.
(2) Stack body
The stack body is the stack of bricks above the kang leg. The stack body can be stacked in two ways: straight horizontal barcode method, straight diagonal barcode method and large hole barcode method.
The straight and horizontal barcode method is a kiln stacking method in which billets are stacked alternately in the direction of the direction and in the direction of the direction. This type of method can be further divided into one straight barcode and one horizontal barcode, and two straight barcodes and one horizontal barcode.
The straight-slant barcode method is a kiln stacking method that alternates between stacking billets along the direction of travel and stacking them at an angle. This type of method can be further divided into one straight and one angled barcode, two straight and one angled barcode, and three straight and one angled barcode.
The large-hole method involves overlapping horizontal and vertical billets from side to side.
The vertical barcode method has high resistance, poor ventilation, and slow firing speed, making it a relatively poor kiln stacking method, and it is generally not used except for mechanical stacking of bricks.
The large-hole stacking method offers the least resistance and the strongest ventilation capacity, providing excellent ventilation conditions for rapid firing. However, it places strict requirements on the firing operation. If the fuel supply and calorific value cannot be matched with the ventilation volume, it will inevitably increase the amount of excess air in the firing zone, causing the firing temperature to drop sharply below the brick's firing temperature, resulting in a "green bottom" phenomenon where bricks are formed at the bottom. To maintain the necessary temperature, a large amount of coal must be added, increasing coal consumption. For high-internal-combustion bricks, where very little external coal is added and the ventilation volume is excessive, the large-hole stacking method is more prone to causing the "green bottom" phenomenon and is therefore not suitable.
In summary, straight and inclined bars and large holes are two good kiln stacking methods, each with its own advantages and disadvantages. In actual production, the choice should be made based on specific equipment conditions, technical conditions, fuel quality, internal combustion intensity, and climatic conditions. Generally, in order to adjust the balance of resistance at the top, middle, and bottom of the billet stack, two or more kiln stacking methods are often used on the same kiln cross-section.
(3) Fire-eye blank stack
The stack of coal briquettes arranged in a specific pattern below the coal feeding hole is called the fire-eye stack. Its function is to receive the fuel thrown in through the fire-eye and ensure that each layer of the stack receives a certain proportion of coal, with 10% to 15% of the coal falling directly to the bottom of the kiln. In addition, the fire-eye stack should have low resistance to ensure sufficient air supply near the fire-eye so that the added coal can be completely burned.
There are many types of fire eyes, but practice has shown that the fire eye with a large hole in the upright leg and the fire eye with a bridging structure without a horizontal band are the best.
The large hole of the vertical leg is a two-section large hole composed of straight and horizontal billets. The hole has a large cross section and low resistance, which can improve ventilation conditions. The coal billets in the large hole can be adjusted and increased or decreased as needed, so that the coal is evenly distributed in the kiln. The two or four sides are degassed to ensure sufficient air supply near the fire hole. The airflow is readjusted and distributed at the degassed point to even out the temperature difference between the upper and lower parts of the roasting zone and the preheating zone.
The fire-eye without transverse bridging, except for the connecting transverse strip on the kiln, is entirely aligned with the billet, minimizing fire-eye resistance and creating numerous transverse fire channels at the fire-eye location. This ensures uniform mixing of coal and air, accelerating the combustion speed. The disadvantages are that this fire-eye requires strict kiln stacking, and the billet placement is not ideal. The billet positions are often vertically aligned, causing coal to tend to concentrate at the bottom of the kiln, resulting in a large bottom fire during firing and insufficient burning in the upper and middle fires.
8.2 Kiln Stacking Principles
(1) General principles for determining kiln density
The number of brick bricks stacked per cubic meter of volume within the firing tunnel is called the kiln stacking density. Years of production practice have proven that regardless of the stacking pattern, reducing the kiln stacking density always improves ventilation. Improved ventilation accelerates fuel combustion, increasing the heat generated per unit time, thus meeting the firing needs of more bricks and tiles. Increased ventilation also significantly improves the preheating conditions of the brick bricks, accelerating preheating and thus effectively speeding up the firing process in the kiln. Therefore, we consider sparse stacking as the general principle for kiln stacking density.
It must be pointed out that the concepts of "sparse" and "dense" are only relative and have no clear boundaries. Furthermore, sparse stacking is not always better. While sparse stacking of externally fired bricks increases ventilation, excessive sparse stacking will result in an excessively high excess air coefficient, causing the firing temperature to drop below the brick's firing temperature. Additionally, sparse stacking reduces the volume of bricks. Generally speaking, the limit for sparse stacking of externally fired bricks is to stack them sparse enough, under existing conditions, to maintain the minimum firing temperature of the bricks. For internally fired bricks, the principle is to stack them sparse enough that no scorched bricks are produced in the center.
The above describes the overall kiln packing density, which is the average packing density of each kiln chamber and each part. To ensure that bricks and tiles are uniformly fired throughout the cross-section of the kiln firing channel, the packing density of each part of the cross-section must also be considered.
(2) Determination of local kiln density
There are no clear boundaries between the upper and lower or the inner and outer parts of the firing chamber space. However, when stacking kilns, there must be some distinction. Usually, the width of the kiln is divided into three equal parts, and the height is divided into three equal parts, namely upper, middle, lower, inner, middle, and outer.
1. Dense at the top and sparse at the bottom
This principle applies to both internally fired bricks and externally fired bricks, as well as straight and curved kilns.
As previously mentioned, when gas moves within the kiln, it is subjected to both the draft from the chimney (or fan) and the upward force of the hot gas. The combined effect of these two forces causes the gas to move upwards at an angle, resulting in a faster gas flow velocity at the top and a slower velocity at the bottom. Consequently, the temperature of the billet stack in the preheating zone is higher at the top and lower at the bottom. Adopting a denser stacking density at the top and a sparser stack at the bottom increases the resistance to gas movement in the upper part of the stack, forcing the airflow downwards and accelerating the gas velocity in the lower part of the stack, thus balancing the airflow and temperature between the upper and lower parts. Simultaneously, with a denser upper stack, the outer combustion bricks increase the coal-receiving area at the top, and the inner combustion bricks increase the fuel quantity. Both effectively increase the firing temperature of the upper part of the stack during firing, preventing and reducing the underfiring of bricks commonly found at the top of the kiln.
The degree of upper reinforcement should be determined based on the kiln's suction force. Higher suction force requires less or no reinforcement, while lower suction force requires more. In actual kiln stacking, a stacking method with higher resistance can be used instead of reinforcement.
2. Medium-dense edge-sparse and medium-sparse edge-dense
Because the combustion and heat transfer mechanisms of fuel are different during the firing of internal combustion bricks and external combustion bricks, different kiln stacking densities are used in the cross section.
The principle of denser stacking at the center and sparser stacking at the edges applies to the case of bricks being burned outside the straight section of a rotary kiln.
When gas moves within the kiln, the airflow in the center only overcomes the resistance of the stack of billets, while the airflow on the sides must overcome not only the resistance of the stack but also the frictional resistance of the kiln walls. When the kiln door is not level with the kiln walls, there is also local resistance from the kiln door itself. This results in a phenomenon where the airflow velocity is faster in the center, less so at the inside, and slower at the outside. To change this, the resistance in the center of the kiln must be increased, and the resistance at the inside and outside must be decreased sequentially. This involves stacking the billets in a denser middle, less so at the inside, and less so at the outside, thus balancing the resistance in the center and the edges, and achieving airflow equilibrium. Adopting this local principle of denser middle and sparser edges also allows for full utilization of the central area with better firing conditions and creates conditions for adding more coal to the edges, compensating for heat loss on both sides of the kiln walls.
The "sparse in the middle, dense at the edges" stacking method is suitable for bricks burned within the straight section of a rotary kiln. It addresses heat loss on both sides. Since most of the required fuel is incorporated into the brick bricks for internal combustion, the stacking density in different parts of the kiln determines the amount of fuel allocated to each area and the firing temperature. Bricks in the middle of the kiln dissipate less heat and are exposed to radiant heat from the surrounding bricks, making them prone to over-firing. Using a sparse in the middle and dense at the edges effectively reduces the amount of fuel in the middle, preventing scorched bricks, while increasing the amount of fuel at the edges to compensate for heat loss, thus improving the edge temperature and preventing scorched bricks. Furthermore, since the outer walls have greater heat loss than the inner walls, a stacking pattern of sparse in the middle, denser at the inner, and denser at the outer can be used.
3 miles dense outside sparse
The method of stacking materials with denser inner layers and sparser outer layers is suitable for the curved kiln section.
When firing in the straight section of a rotary kiln, the firing is long and even, the temperature is easy to control, and the fire travels quickly. However, when entering the curved section, uneven firing often occurs inside and outside, and the fire travels slowly. This is mainly because the curved kiln stacking method is not compatible with the gas movement.
Gas exhibits the following characteristics when passing through a curved kiln: First, the gas must both move forward and bend when passing through a curved kiln, resulting in a change in its direction of movement and generating localized resistance. Experiments show that when the stacking of billets in both straight and curved kilns is identical, the resistance to gas flow through a curved kiln is 35% greater than that in a straight kiln. Second, the gas travels a longer distance on the outer bend and a shorter distance on the inner bend when passing through a curved kiln. Third, except for the gate near the outer fire extraction point, the extraction force of all other gates in a curved kiln is directed towards the outer fire.
To maintain consistent firing progress between curved and straight kilns, the overall stacking density in curved kilns tends to be more concentrated. The kiln density should be lower than that in straight kilns to maintain resistance balance between the curved and straight sections, ensuring consistent firing progress. Generally, curved kilns have two fewer stacks than straight kilns.
Furthermore, based on the principle that the gas flow is longer on the outside and shorter on the inside, and the suction is stronger on the inside and weaker on the outside, in order to ensure that the outer and inner fires of the curved kiln progress in unison when they reach the lower bend, it is necessary to adhere to the principle of stacking the kiln with a denser inner fire and a sparser outer fire. Increasing the resistance of the inner fire stack and increasing the gas flow through the outer bend will make the gas flow through the inner and outer bends compatible with the distance between the inner and outer bends. Only in this way can the temperature of the inner and outer fires of the curved kiln be consistent, allowing it to advance forward along the fan shape.
To ensure a denser interior and sparser exterior, the following should be observed when stacking bricks in a curved kiln: 1. The legs should be lower on the inside and higher on the outside. Use three, four, or five layers of legs for the inside, middle, and outside sections respectively. 2. The spacing between bricks should be denser on the inside and sparser on the outside. Use the kiln center line as the boundary to distinguish the inside and outside. The inside half of the legs should have one more leg than the outside half, and the inside half of the basic kiln head should have 2-3 more legs than the outside half. 3. The stack spacing should be open on the inside and outside. There should be no gaps between the stacks of bricks from one batch to the next, ensuring solid stacking at the front and back, with gaps or openings on the outside to reduce resistance. 4. All diagonal strips in the curved kiln should face outwards.
4. Hafeng Seam
The gap of a certain width (usually 200mm~240mm) left from the inside to the outside at the air inlet of the rotary kiln for the billet stack is called the air inlet gap.
The benefits of using tuyere (air-blown duct) for gas extraction are numerous. First, the tuyere creates discontinuous gaps in the kiln's brick stacks, minimizing the channels blocked by slanted bricks. Second, as gas passes through these gaps, expansion and contraction occur, redistributing the airflow at the gaps, regulating the airflow within the brick stacks, and evenly distributing the temperature difference between the firing and preheating zones, resulting in better preheating and more uniform firing of the product. Third, the tuyere provides a transverse channel for gas to flow from the kiln into the tuyere, reducing lateral resistance in the brick stacks near the tuyere. This allows the chimney (or fan) to fully exert its draft, facilitating the timely removal of moisture from the kiln, preventing condensation, dampness, and the formation of white-headed bricks, thus improving brick quality. In short, tuyere is an effective measure to increase output in kilns with poor draft.
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