RU2105257C1 - Method for calcination of bricks in tunnel kiln - Google Patents

Abstract

FIELD: construction materials industry. SUBSTANCE: according to method, bricks are loaded into kiln, heated, calcined, and cooled, with simultaneous loading and unloading of equal amounts of bricks. Duration of keeping bricks in zones of heating and cooling is determined from relations given in formula brought forth in description of invention. Aforesaid method reduces power and labour cost, cuts calcination period due to reduced duration of heating-cooling processes proper. EFFECT: high efficiency. 2 dwg

Description

 The proposed method relates to the field of the construction industry, in particular the production of building materials, for example bricks, where one of the important operations that determine the efficiency of the process and the quality of the products obtained is firing products.

 Firing products, in particular bricks, is carried out in special tunnel kilns, which are a straight section of a tunnel with two ends. In existing methods, firing products are usually loaded into one of the ends, and finished fired and cooled products are unloaded from the other.

The firing process in modern tunnel kilns is continuous and includes moving operations with products on the ends of the furnace / loading, unloading and reloading /, the heating-cooling process, consisting of drying and the actual heating-cooling, and firing at t ≈ 900-1000 ^o C.

 The main directions of improving the firing of products, including bricks, are reducing labor and energy costs and reducing the time of the process, and as a consequence of the latter, reducing the dimensions of the furnaces.

 So, in order to maximize the use of heat of products during cooling according to [1], the zones of drying and heating of products are located in a common channel under or above the firing zone, and the movement between the zones of cooling, drying and heating is organized in a counterflow.

 However, the counterflow in this case is organized in such a way that it is necessary to change the direction of movement of the products several times inside the furnace, which determines a significant complication of the design and technology and additional heat loss, since the drying-heating and cooling zones are located one above the other and it is impossible to effectively implement heat transfer due to limited heat transfer area.

 According to [2], to reduce fuel consumption during transportation of products in a heating furnace, the value of step movement is determined by a special dependence. However, in this case, the heat of cooling products is not used at all to heat the new ones.

 For the same purpose, according to [3], the loading of long cylindrical products into a thermal furnace is carried out so that the product axes in plan form a certain angle with the direction of movement of the latter. This method also does not allow the heat of cooling products to be regenerated.

 Closest to the claimed is a method of firing ceramic products in a continuous furnace according to [4], including moving operations with products at each end of the furnace, a heating-cooling process / including drying and heating-cooling / by heat exchange between products and firing.

 Relocation operations in the known method are loading and unloading products from each end face in equal quantities, as well as reloading the dried products from the feed conveyor of the raw brick to the main one.

 According to this method, from each end of the furnace, bricks are loaded onto conveyors of raw feed channels, and then at the opposite ends partially dried products are loaded onto conveyors of working channels moving in the opposite direction and after the heating, firing and cooling zone are unloaded from the same end of the furnace, where downloaded.

 The process of heating-cooling bricks, consisting of drying and the actual heating and cooling, is carried out, according to the description, by convective heat transfer, i.e. with the help of an intermediate medium - air / fired bricks give heat to the air, and the latter heats the products supplied for firing /.

 Thus, labor costs are greatly increased, because from each end of the furnace, not only loading and unloading products in equal amounts is carried out, but also reloading from one vehicle to another / the latter is a necessary operation for organizing convective heat transfer during heating / cooling of products /.

 The objective of the proposed method is to reduce energy and labor costs and reduce firing time by reducing the duration of the actual processes of heating and cooling bricks.

где k - коэффициент, час•K³;
T₁ - температура обжига, К;
T₂ - температура охлажденных кирпичей, К;
ΔT - перепад температур процесса собственно охлаждения-нагрева кирпичей, К.According to the claimed method of firing, as relocation operations at each end of the furnace, only simultaneous loading and unloading of bricks in equal quantities is used, and the residence time of bricks in the heating-cooling zones is determined by the condition:

where k is the coefficient, hour • K ³ ;
T ₁ - firing temperature, K;
T ₂ - temperature of chilled bricks, K;
ΔT - temperature difference of the actual process of cooling-heating bricks, K.

In FIG. 1 shows a diagram of a tunnel furnace with two ends for implementing the proposed method (a - in plan, b - transverse section of the furnace in the firing zone). The furnace is a single-channel heat-insulated tunnel [1] with two ends at the ends of the tunnel. There are several zones along the tunnel length, with a brick burning zone in the middle, and heating-cooling, loading-unloading zones symmetrically located to the left and right of it, and the heating-cooling zones in general consist of drying zones and heating-cooling proper / in the manufacture of bricks by dry or semi-dry molding, the drying zone may be absent. Electric heaters [2] are located inside the firing zone to create a temperature of about 900-1000 ^o C in this zone and to burn bricks laid in several continuous rows [3] several bricks high. In this case, there are no partitions between the rows, and any two adjacent rows [3] move on the conveyors [4] in opposite directions, i.e. half of the bricks loaded in the furnace move in one direction, and the other in the opposite direction, and in a straight line. In practice, this nature of the movement of bricks can be accomplished by using two independent drives.

The inventive method is as follows. At each end of the tunnel kiln, bricks are loaded only simultaneously, laying bricks in rows that move in the direction of the burning zone, and loading from those rows that move from the burning zone. Since the number of those and other rows and the speed of their movement are equal, then the number of loaded and unloaded bricks are equal. Bricks loaded from one of the ends with an ambient temperature T _о from the loading zone go to the beginning of the heating-cooling zone, where they are found with already burnt bricks moving towards and radiating heat. If drying is necessary in the drying zone, the drying temperature T _c is maintained, and the drying time is determined in a known manner depending on the method of forming bricks. Next, the bricks enter the heating zone itself, where the rows of bricks moving towards towards them after firing by means of radiant heat exchange heat the first ones, while at the beginning of this zone the heated bricks have a temperature T _c , and those that transfer heat - T ₂ . At the end of the heating zone, the bricks are heated to a temperature of T ₃ and the heat-releasing ones have a calcination temperature, i.e. T ₁ .

The average temperature difference in the heating-cooling zone will be:
ΔT = 0.5 (T ₁ -T ₃ + T ₂ -T _c ). .

где k - коэффициент, определяющий скорость процесса передачи тепла между соседними рядами кирпичей и зависящий от высоты ряда (т.е. от количества n_k уложенных по высоте кирпичей - см. фиг.2).The residence time of bricks τ in the heating-cooling zone is determined by the condition:

where k is a coefficient that determines the speed of the heat transfer process between adjacent rows of bricks and depends on the row height (i.e., on the number n _{k of} bricks stacked along the height - see figure 2).

After the bricks with a temperature of T ₃ exit the heating-cooling zone, they enter the firing zone, where, using the heaters [2], they are finally heated to the firing temperature T ₁ and maintained at this T _{1 for a} certain time according to the technology. After leaving the firing zone, the bricks are cooled to the second heating-cooling zone, where they are cooled to the temperature T ₂ at the same average temperature difference, giving off heat by means of radiant heat exchange towards the rows of bricks and heating the latter to the temperature T ₃ . Bricks with a temperature T ₂ enter the drying zone, where they continue to cool and maintain the drying temperature T _c in this zone and then are unloaded at the other end of the furnace. If the bricks are molded semi-dry or dry method, then the drying zone is absent and the bricks are unloaded with a temperature T ₂ .

 Thus, if only simultaneous loading and unloading of bricks in equal amounts is used as relocation operations at each end of the furnace, and the processes of self-heating and cooling are carried out by radiant exchange between rows of bricks moving in opposite directions, then the disadvantages of existing firing methods are eliminated.

Example 1. Using the proposed method, it is required to calculate the parameters of the baking process of bricks and the dimensions of the furnaces for the following initial conditions: bricks are molded by dry or semi-dry method, productivity Q = 350 bricks per hour, firing temperature T ₁ = 900 ^o C / 117ЗК /, temperature of bricks when loading T ₀ = 20 ^o C / 293K /, the speed of the conveyor belt of the furnace V = 2.5 cm / min = 0.025 m / min. According to the claimed method, we accept / and then specify / the number of rows of bricks along the width of the furnace tunnel n _p = 10, while 5 rows / for example, odd rows / move in the same direction along the axis of the tunnel, and the remaining 5 rows / even / move in the opposite direction . Thus, on each of the 2 ends of the furnace, 5 rows of bricks are used for loading, and the remaining 5 for unloading. There is no drying zone.

 We take the temperature difference ΔT = 100K / from the condition for ensuring a sufficiently effective heat transfer between adjacent rows /.

.We accept, and then check / the value of the coefficient k = 0.9 • 10 ⁹ according to the graph in FIG. 2, assuming that the height of each row will be at least 4-5 bricks. The temperature of the chilled bricks will be T ₂ = T ₀ + ΔT = 293 + 100 = 393K. According to the stated ratio, we determine the residence time of the bricks in the heating-cooling zone:

.

We take τ = 2 hours. Then the length of the heating-cooling zones will be
l _but = v • 60 • τ = 0.025 • 60 • 2 = 3.0 m.

Because from a technological point of view, the firing time τ _o should not be less than 3, then the length of the firing zone will be:
l _o = v • 60 • τ _o = 0.025 • 60 • 3 = 4.5 m.

The total length of the furnace will be:
L _n = 2l _but + l _o = 2 • 3.0 + 4.5 = 10.5m.

When using the method according to [4], the length of the heating-cooling zones would be l $\genfrac{}{}{0ex}{}{\text{etc}}{\text{but}}$ ≈ 19 m, and the total length of the furnace L $\genfrac{}{}{0ex}{}{\text{etc}}{\text{P}}$ ≈ 42.5 m, which would lead to the impossibility of the process without the use of additional handling operations at the ends of the furnace.

где l_k = 0,3м - длина кирпича /кирпичи в ряду укладывают без зазора/, принимаем n_k = 7.The row laying height for a given capacity will be:

where l _k = 0.3 m - the length of the bricks / bricks in a row are laid without a gap /, we take n _k = 7.

Check the value of the adopted coefficient k: for n _k = 7 / the graph of FIG. 2 /. k = 0.8, i.e. virtually the same as accepted and no recount required.

Для увеличения величины ζ необходимо уменьшить перепад температур, однако это связано с увеличением практически в тех же пропорциях времени τ , а следовательно, размеров /длины/ печи. Поэтому в каждом случае необходимо исходить из конкретных условий, в частности из наличия производственных помещений для размещения печи.Thus, the degree of heat recovery in the above example is:

To increase the ζ value, it is necessary to reduce the temperature difference, however, this is due to an increase in the time τ in almost the same proportions, and hence the size / length / furnace. Therefore, in each case, it is necessary to proceed from specific conditions, in particular from the availability of production facilities for placing the furnace.

 According to example 1, in the INTERM software, a brick kiln was developed and put into operation / humidity after molding 2-4% /.

 Example 2. Using the proposed method to calculate the parameters of the process of firing bricks and the dimensions of the furnace for bricks molded plastic / t. E. wet / method, humidity after molding 20-25%.

The parameters Q, T ₁ , T ₀ , V are similar to example 1.

Let, according to technology and experimental data, the molded bricks should dry at T ₀ = 150 ^o C / 423K / 4 hours (τ _c = 4). . We accept Δ = 100K.

Принимаем τ = 1,5 час. Тогда длина зон нагрева-охлаждения составит
l_но= v•60•τ = 0,025•60•1,5 = 2,25 м.
Длина зон сушки составит:
l_c= v•60•τ = 0,025•60•4 = 6,0м. .The temperature of the cooled bricks at the exit from the zone of the actual heating-cooling / i.e. at the entrance to the drying zone / will be:
T ₂ = T _c + ΔT = 423 + 100 = 523K.
The residence time of bricks in the actual heating-cooling zone will be:

We take τ = 1.5 hours. Then the length of the heating-cooling zones will be
l _but = v • 60 • τ = 0.025 • 60 • 1.5 = 2.25 m.
The length of the drying zones will be:
l _c = v • 60 • τ = 0.025 • 60 • 4 = 6.0 m. .

The total length of the furnace will be: L _n = 2 (l _no + l _c ) + l _o = 2 (2.25 + 6) + 4.5 = 21 m. When using the method according to [4], the length of the heating-cooling zones would be l $\genfrac{}{}{0ex}{}{\text{etc}}{\text{but}}$ ≈ 15.0 m, and the total length of the furnace l $\genfrac{}{}{0ex}{}{\text{etc}}{\text{P}}$ ≈ 46.5 m.

Claims (1)

The method of firing bricks in a tunnel kiln, including loading, heating, firing and cooling, while loading and unloading equal amounts of bricks, characterized in that the residence time of the bricks in the heating and cooling zones is determined from the condition

where k is the coefficient, h • K ³ ;
T ₁ firing temperature, K;
T ₂ temperature of bricks at the exit of the heating-cooling zone, K;
ΔT is the temperature difference of the heating-cooling zones, K.

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