JP4474533B2 - Method for firing powdered calcium carbonate - Google Patents

Description

The present invention uses a system for producing calcium oxide (CaO) by calcining powdered calcium (CaCO ₃ ), and in particular, calcining fine particles, ie, powdered calcium carbonate, in order to obtain fine calcium oxide. Regarding the method.

In general, calcium oxide (CaO), that is, quicklime is produced by firing limestone, shells, and the like, which are raw materials for calcium carbonate (CaCO ₃ ), at 1100 to 1200 ° C. When calcium hydroxide (Ca (OH) ₂ ), that is, slaked lime is produced by the digestion reaction of calcium oxide, calcium carbonate is first fired to produce calcium oxide. Conventionally, as disclosed in Patent Documents 1 and 2, a method of firing massive limestone and shells in a vertical shaft furnace is common. The vertical shaft furnace is a relatively large system in which raw materials and fuels are charged from the upper part of a solid furnace, and combustion air is blown from the lower part to take out products accumulated at the bottom of the furnace. Therefore, when limestone is used as a raw material, a lump having a particle size of about 40 to 80 mm is used.

  Here, as a known incinerator, there is a fluidized bed furnace often used for incineration of waste (Patent Document 3, etc.), but calcining calcium carbonate using a fluidized bed furnace to produce calcium oxide, Not very common. When using a fluidized bed furnace, the particle size of calcium carbonate as a raw material can be made smaller than in the case of the vertical shaft furnace described above, but at present, the minimum is about 1 to 3 mm.

JP-A-5-170494 Japanese Patent Laid-Open No. 2002-60254 Japanese Patent No. 3999995 JP 2000-256047 A JP-A-11-76808

Zement-Kalk-Gips, Vol.42, No.12, p621,1989

  Currently, in various fields, in order to obtain highly reactive high-activity calcium oxide, high-activity calcium hydroxide (slaked lime) and calcium, there is a demand for a technique for efficiently firing powdered calcium carbonate.

  One of them is in the field of dry flue gas purification technology. In recent years, a flue gas purification system for removing harmful substances (sulfur oxides, nitrogen oxides) contained in flue gas from various incinerators has been proposed. For example, it is disclosed in Patent Document 3. In the dry type flue gas purification system, calcium-based flue gas purifying agents such as calcium oxide and calcium hydroxide are used to absorb and remove harmful substances such as sulfur oxide, nitrogen oxide or hydrogen chloride in the flue gas. is doing. Since the desulfurization reaction, desulfurization reaction or desalting reaction in this dry flue gas purification system is a solid-gas reaction, high reaction activity is required for calcium oxide or calcium hydroxide used as a calcium-based flue gas purification agent. In order to obtain highly active calcium oxide or calcium hydroxide, it is necessary that the firing temperature of calcium carbonate as a raw material be set in the vicinity of its thermal decomposition temperature range without being unnecessarily high and as short as possible. In order to satisfy such firing conditions, it is desirable to fire fine particles, that is, powdered calcium carbonate.

  As another field, in the field of sugar production technology, in a sugar solution refining process, a large amount of calcium oxide is added to a high concentration sugar solution to form calcium hydroxide, and carbon dioxide is blown into this to produce calcium carbonate. The purified sugar solution is obtained by adsorbing and removing the suspension, which is an impurity in the sugar solution, in the calcium carbonate production process. As a result, a large amount of waste (lime cake) containing calcium carbonate as a main component is generated. The particle size of calcium carbonate in the lime cake is 10 μm or less. If this is calcined and recalculated, it can be recycled. Also in this case, a powdered calcium carbonate baking technique is required.

  However, as described above, the current calcium carbonate firing technology has a minimum particle size of about 1 to 3 mm, and powder calcium carbonate firing technology with a particle size of 0.3 mm or less has been established. Absent.

When firing powdered calcium carbonate, there are the following problems (i) and (ii).
(I) FIG.1 and FIG.2 is a graph which shows the result of having performed the baking experiment of calcium carbonate with a particle size of 1-3 mm using the conventional fluidized bed furnace. FIG. 1 shows the measurement results of the calcining temperature of calcium carbonate (the temperature of the free board part in the fluidized bed furnace) and the activity. In this measurement, the activity of calcium oxide is evaluated by the consumption of hydrochloric acid (by a 4N solution) required to neutralize the calcined calcium oxide in 10 minutes. The maximum value of activity is shown in a relatively low temperature range of calcination temperature 850 to 900 ° C. On the other hand, FIG. 2 shows the measurement results of the firing temperature (temperature of the free board portion in the fluidized bed furnace) and the firing rate. The lower the firing temperature, the greater the loss on ignition of the calcined calcium oxide (corresponding to the uncalcined portion of calcium carbonate).

  From FIG. 1 and FIG. 2, in order to obtain highly active calcium oxide using a conventional fluidized bed furnace, it is desirable to calcine calcium carbonate at a firing temperature at which a maximum activity of 850 to 900 ° C. is obtained. In the case of calcium carbonate having a diameter of 1 to 3 mm, it is understood that about 10 to 15% is not fired. A certain amount of residence time is required for complete firing in a relatively low temperature range. However, the smaller the particles, the more likely it is to scatter, and there is a problem that it is difficult to ensure the residence time for firing. In particular, in the case of a powder with a particle size of 0.3 mm or less, even if it is put into a conventional fluidized bed furnace, it flows out together with the combustion gas in several seconds, so that a considerable part remains unfired. Therefore, there is a need for a technique that ensures a sufficient residence time of the powder in a fluidized bed furnace having an optimum firing temperature.

(Ii) In addition, when the calcium carbonate fine particles are calcined into calcium oxide fine particles and coexist with the combustion gas, carbon dioxide (CO ₂ ) generated by combustion of fuel and carbon dioxide generated by thermal decomposition of calcium carbonate As a result, the combustion gas contains a high concentration of carbon dioxide. Under such an atmosphere, recalcification of calcium oxide occurs due to the reversible reaction of Equation 1, and calcium carbonate adheres and solidifies on the inner walls of equipment and piping ducts that serve as combustion gas passages, resulting in a so-called coating blockage phenomenon. . In order to remove the adhered and solidified calcium carbonate, the operation had to be stopped, and the burden of the removal work was large.

Here, patent document 4 is disclosing the method of preventing recarbonation of calcium oxide. Paragraph 0014 of Patent Document 4 includes carbon dioxide which causes recalcification of quicklime in an atmosphere of limestone or quicklime when operating an apparatus that handles limestone (calcium carbonate CaCO ₃ ) or quicklime (calcium oxide CaO) at T ° C. (CO ₂ ) partial pressure P

P [MPa] = 452exp (−21441 / (T + 179) +17.01)

It is calculated that the re-carbonation can be prevented by lowering the carbon dioxide in the limestone or quicklime atmosphere to less than P. FIG. 3 is a graph showing the relationship between the thermal decomposition equilibrium temperature of calcium carbonate and the carbon dioxide partial pressure. The solid line is based on the formula in Patent Literature 4, and the broken line is based on the formula in Non-Patent Literature 1. Although there are some differences depending on the production area and properties of limestone, it shows almost the same tendency. The higher the carbon dioxide partial pressure, the higher the thermal decomposition equilibrium temperature of calcium carbonate. With these curves as a boundary, the high temperature side becomes a thermal decomposition region (CaO + CO ₂ ), and the low temperature side becomes a bonding region (CaCO ₃ ). Therefore, in the case of preventing the recalcification of calcium oxide in the combustion gas containing calcium oxide at a predetermined temperature, it is necessary to adjust the carbon dioxide partial pressure so that it becomes a thermal decomposition region at that temperature. That is, when the partial pressure of carbon dioxide is too high, it is necessary to reduce this.

  In view of the above problems, an object of the present invention is to provide a firing method capable of efficiently firing powdery calcium carbonate (particularly, a particle size of 0.3 mm or less) at a high firing rate.

In order to solve the above problems, the present invention provides the following configurations. The numerals in parentheses are the reference numerals of the drawings showing examples to be described later, and are attached for reference.
(1) The method for firing powdered calcium carbonate according to the present invention comprises a first stage fluidized bed furnace (20) for forming a bubble fluidized bed and a second stage fluidized bed furnace (30) for forming a spouted bed. A method for producing calcium oxide (CaO) by calcining powdered calcium carbonate (CaCO ₃ ) using a two-stage calcium carbonate firing furnace, wherein the first-stage fluidized bed is formed with a bubble fluidized bed. A first step in which powdered calcium carbonate and fuel are supplied to the furnace and calcined; and a combustion gas accompanied by calcium oxide and uncalcined calcium carbonate produced by the calcining in the first step is converted into the second stage fluidized bed. A second step of firing the uncalcined calcium carbonate by forming a spouted bed by flowing into the furnace and swirling along the inner wall of the second stage fluidized bed furnace.

(2) In the method of (1), the second stage fluidized bed furnace has an upright cylindrical shape, and the combustion gas is allowed to flow in a tangential direction of a circumferential section of the horizontal section of the second stage fluidized bed furnace. Is preferred.

(3) In the method of (2) above, an inner cylinder that is arranged coaxially with the second stage fluidized bed furnace and opens through the upper end surface (33a) of the second stage fluidized bed furnace. It is preferable that a pipe (32) is provided and the combustion gas flows out through the inner cylinder pipe.

(4) In any one of the above methods (1) to (3), cold air is supplied to the combustion gas flowing out of the second-stage fluidized bed furnace (30) accompanied by calcium oxide produced by firing. By mixing, it is preferable to rapidly cool to 600 ° C. or lower and simultaneously reduce the partial pressure of carbon dioxide contained in the combustion gas to separate the calcium oxide and the combustion gas (40, 41).

(5) In any of the above methods (1) to (3), water is sprayed on the combustion gas flowing out of the second-stage fluidized bed furnace (30) accompanied by calcium oxide generated by firing. Thus, it is preferable to rapidly cool to 600 ° C. or lower, separate calcium oxide and combustion gas, and simultaneously digest calcium oxide to produce calcium hydroxide (40, 41).

(6) In any one of the above methods (1) to (5), in addition to the powdered calcium carbonate, coal ash or clay serving as a silicon dioxide (SiO ₂ ) supply source is mixed with 3 to 3 of calcium carbonate. It is preferable to mix 30% by weight and put into the first stage fluidized bed furnace.

(7) In the method of (1) above, an indirect external heating kiln is provided that includes an inner cylinder (94) and an outer cylinder (95), and heats the inside of the inner cylinder by flowing a heating thermal fluid through the outer cylinder. In addition, the powdered calcium carbonate is charged into the inner cylinder of the indirect external heat kiln prior to charging the first fluidized bed furnace, and accompanied by calcium oxide from the second stage fluidized bed furnace. The preheated powder further includes a preheating step of preheating the powdered calcium carbonate and combustion air by flowing the discharged combustion gas as a heating fluid into the outer cylinder of the indirect external heating kiln. It is preferable to put body-like calcium carbonate into the first fluidized bed furnace,

(8) In the method of (7) above, by supplying cold air to the combustion gas while the combustion gas as the heating heat fluid flows through the outer cylinder of the indirect external heat kiln, It is preferable to rapidly cool the combustion gas to 600 ° C. or lower and simultaneously reduce the partial pressure of carbon dioxide contained in the combustion gas to separate calcium oxide from the combustion gas.

(9) In the method of (7) above, by supplying water to the combustion gas while the combustion gas as the heating heat fluid flows through the outer cylinder of the indirect external heat kiln, It is preferable to rapidly cool the combustion gas to 600 ° C. or less to separate calcium hydroxide generated from calcium oxide from the combustion gas.

(10) In the method for calcining powdered calcium carbonate according to the present invention, the inner cylinder is heated using a combustion gas not accompanied by calcium oxide as a heating-side heat fluid of an indirect external heating kiln to form powdered calcium carbonate (CaCO in ₃₎ firing the method of calcining calcium oxide (CaO), feedstock and cogeneration in air to burn off organic matter-containing lime cake when lime cake burning organic matter, the indirect outside thermal kiln Supplying cold air or water to the more outflowing calcium oxide and associated gas to rapidly cool to below 600 ° C, while air cooling has means to reduce carbon dioxide partial pressure and prevent re-carbonation It is.

  The present invention is a method for calcining powdered calcium carbonate using a two-stage fluidized bed furnace, in which a first stage of firing is performed in a first stage fluidized bed furnace for forming a bubble fluidized bed, and then firing is performed. Combustion gas accompanied by calcium and uncalcined calcium carbonate is caused to flow into the second-stage fluidized bed furnace to form a spouted bed by swirling flow to perform second-stage firing. By using a two-stage fluidized bed furnace, in particular, a sufficient residence time in the second-stage fluidized bed furnace is ensured, so that uncalcined calcium carbonate is almost completely fired.

  The second stage fluidized bed furnace is an upright cylinder, and the bottom of the second stage fluidized bed furnace (for example, when the second stage fluidized bed furnace is provided directly above the first stage fluidized bed furnace) or near the lower end (for example, In the case where the upper part of the first stage fluidized bed furnace and the lower part of the second stage fluidized bed furnace are connected by a horizontal connecting pipe), the inner wall It is possible to reliably form a spout layer that rises while swirling along.

  In addition, the combustion gas is disposed in the inner cylinder pipe by being provided coaxially with the second stage fluidized bed furnace and opening in the furnace space through the upper end surface of the second stage fluidized bed furnace. The outer surface swirls and descends, and rises and flows out from the lower end of the inner tube, contributing to securing the residence time of the powder.

  Cold air is mixed with the combustion gas flowing out of the second stage fluidized bed furnace accompanied by calcium oxide produced by firing, and rapidly cooled to below 600 ° C. At the same time, the partial pressure of carbon dioxide contained in this combustion gas Is reduced to separate calcium oxide and combustion gas. Thereby, re-carbonation does not occur and the coating blockage phenomenon can be prevented.

  At the same time as separating calcium oxide and combustion gas by spraying water on the combustion gas flowing out of the second stage fluidized bed furnace accompanied by calcium oxide produced by firing and rapidly cooling to 600 ° C. or lower, Digests calcium oxide to produce calcium hydroxide. Thereby, re-carbonation does not occur and the coating blockage phenomenon can be prevented. When it is better not to reduce the partial pressure of carbon dioxide, such as when producing light calcium carbonate (calcium carbonate synthesized again from calcium oxide generated by baking) or recycling lime cake (requires high concentration of carbon dioxide) Is suitable).

In addition to powdered calcium carbonate, coal ash or clay as a silicon dioxide (SiO ₂ ) supply source is mixed with 3 to 30% by weight of calcium carbonate and charged into the first-stage fluidized bed furnace. As a known technique (Patent Document 6), it is a flue gas treatment agent using quick lime (calcium oxide) as a raw material, and a highly active silicon compound is produced by coexisting non-crystalline silicon dioxide to cause quick lime digestion reaction In addition, a flue gas treating agent is known in which this highly active silicon compound is kneaded as a component. This flue gas treating agent has high desulfurization / desorption performance. In the present invention, by adding silicon dioxide to calcium carbonate and baking, highly active calcium oxide in which thermal bonding with silicon dioxide is promoted can be obtained. By digesting this highly active calcium oxide, highly active calcium hydroxide can be obtained.

According to the present invention, it becomes possible to calcinate fine calcium carbonate having a particle size of 0.3 mm or less, and by producing highly active calcium oxide, it is possible to produce a high-performance dry smoke purification agent at low cost. became.
Waste (lime cake) containing a large amount of fine calcium carbonate produced in the sugar solution refining process can be baked, recycled as calcium oxide, and water required for the refining process from the obtained calcium oxide. Calcium oxide can be produced in a sugar factory.

  Furthermore, in the method using the indirect external heat kiln, the combustion gas flowing through the kiln outer cylinder and the calorific value of calcium oxide (CaO) accompanying the combustion gas are used for indirect heating to the kiln inner cylinder. The heat can be recovered and used for drying and preheating the powdered calcium carbonate (for example, lime cake) in the kiln before being supplied to the fluidized bed furnace. It can also be used for preheating combustion air supplied to a fluidized bed furnace. Thereby, the combustion efficiency in a fluidized bed furnace improves.

  Furthermore, in the method using the indirect external heat kiln, the combustion gas is generated in the combustion gas generation furnace and flows into the outer cylinder of the kiln, and is used for indirect heating to the inner cylinder of the kiln. Inside, it can utilize for baking of powdery calcium carbonate (for example, lime cake).

It is a graph which shows the relationship between the calcination temperature of calcium carbonate, and activity. It is a graph which shows the relationship between the calcination temperature of calcium carbonate, and a calcination rate. It is a graph which shows the relationship between the thermal decomposition equilibrium temperature of calcium carbonate, and a carbon dioxide partial pressure. It is a firing system diagram of powdered calcium carbonate (using gas gas heater). It is a calcination system figure (indirect external heat type kiln utilization) of powdery calcium carbonate. It is a calcination system figure (calcination by an indirect external heat type kiln) of powdery calcium carbonate. (A) is a schematic sectional side view showing a first stage fluidized bed furnace and a separately installed second stage fluidized bed furnace in one embodiment of the system of FIG. 4a. (B) is AA sectional drawing of (a). (C) is BB sectional drawing of (a). FIG. 4b schematically shows the flow of combustion gas in the second stage fluidized bed furnace of the system of FIG. 4a. 4A is a schematic cross-sectional side view showing a first stage fluidized bed furnace and a second stage fluidized bed furnace installed immediately above in another embodiment of the system of FIG. 4A. FIG. (B) is CC sectional drawing of (a).

The present invention relates to a firing method for producing calcium oxide (CaO) by firing powdered calcium carbonate (CaCO ₃ ), and further, a process for producing calcium hydroxide (Ca (OH) ₂ ) from calcium oxide. The present invention also relates to a method for firing powdered calcium carbonate that also contains water. Embodiments of the present invention will be described below with reference to examples of systems to which the method of the present invention is applied.

    4a, 4b, and 4c are flow configuration diagrams of embodiments of the powdered calcium carbonate firing system to which the method of the present invention is applied. The particle size of the powdered calcium carbonate to which the present invention is applied is 0.3 mm or less. The particle size of calcium carbonate contained in the lime cake discharged in the sugar solution refining process for sugar production is 10 μm or less, which is a suitable object for the present invention.

  The system shown in FIGS. 4a and 4b includes two fluidized bed furnaces, a first stage fluidized bed furnace 20 and a second stage fluidized bed furnace 30, and performs two-stage firing. The system shown in FIG. 4 c performs firing using a combustion gas generation furnace 20 a and an indirect external heat kiln 90.

First, the system of FIG. 4a will be described.
The first stage fluidized bed furnace 20 is a so-called bubble fluidized bed furnace, and performs the first stage firing process. It has an upright cylindrical first furnace body 23, and a bubble fluidized bed part 22 filled with a particle medium having a particle size of 0.5 mm or more is formed in the lower part of the furnace space, and above that is a free board part. 24, a gas outlet is provided at or near the upper end, and one end of a connecting pipe 25 to the second fluidized bed furnace 30 is attached.

  Below the bubbling fluidized bed portion 22, a supply port for preheated air for combustion and an activation burner 21 as a first heating means are provided. It is preferable to maintain the furnace temperature at the time of firing in the first stage fluidized bed furnace 20 at 870 to 1000 ° C. by this first heating means. In order to obtain highly active calcium oxide, it is desirable to fire at a relatively low temperature range of 850 to 900 ° C. However, the lower the firing temperature, the lower the ignition loss of the calcined calcium oxide (the uncalcined calcium carbonate). ) And the firing rate tends to vary. Therefore, to obtain a stable firing rate of approximately 95% or more, the temperature is set to 870 to 1000 ° C. Incidentally, even if it is 850 degreeC, the favorable calcination rate 92-95% is obtained in the comparison with the conventional baking furnace (refer the test result mentioned later).

  The second stage fluidized bed furnace 30 is a so-called spouted fluidized bed furnace, and performs a second stage firing process. It has an upright cylindrical second furnace body 33, a gas inlet is provided at the lower end or in the vicinity of the lower end, and the other end of the connecting pipe 25 for transferring the combustion gas generated from the first stage fluidized bed furnace 30 is attached. ing. Therefore, a swirling flow from the lower side to the upper side is formed in the second stage fluidized bed furnace 30 of this embodiment. Near the lower end of the furnace body 33, an auxiliary burner 31 as a second heating means is provided. The combustion gas transferred from the first stage fluidized bed furnace 20 maintains its temperature substantially. The second heating means assists to maintain the furnace temperature at the time of firing in the second-stage fluidized bed furnace 30 as appropriate at 870 to 1000 ° C. as necessary.

  The powdered calcium carbonate 10 is charged into the bubble fluidized bed portion 22 of the first stage fluidized bed furnace 20. As a fuel, coal 12 is basically suitable, but oil 13 may be used as appropriate, and supplied to heating means 21 and 31 of each of the first stage fluidized bed furnace 20 and the second stage fluidized bed furnace 30. Is done.

The coal ash 11 is ash after combustion of fuel coal, and is optionally added. Coal ash 11 is added as a silicon dioxide (SiO ₂ ) supply source. Another example is clay. Preferably, 3 to 30% by weight of calcium carbonate is mixed. By adding coal ash, calcium oxide produced by firing and silicon dioxide are combined to obtain calcium oxide with higher activity. By using calcium oxide obtained by adding coal ash, it is possible to produce a flue gas purifying agent having high absorbability of SO _x , NO _x , and HCl.

By the two-stage firing in the first-stage fluidized bed furnace 20 and the second-stage fluidized bed furnace 30, the powdered calcium carbonate is fired into calcium oxide fine particles. At the same time, combustion gas is generated by combustion of fuel and decomposition of calcium carbonate. The combustion gas contains carbon dioxide (CO ₂ ) and water vapor (H ₂ O), and as described above, carbon dioxide is contained at a high concentration. The combustion gas flows out from a gas outlet provided at the upper end of the second stage fluidized bed furnace 30 accompanied by calcium oxide fine particles.

  The combustion gas containing calcium oxide fine particles flows out of the second-stage fluidized bed furnace 30 and then is guided to the two-phase mixer 40. In the two-phase mixer 40, the combustion gas is rapidly cooled. There are the following two examples of the rapid cooling means.

  In the first embodiment of the rapid cooling means, cooling air (which may be room temperature) is supplied to the two-phase mixer 40 by the pump 82 and mixed with the combustion gas, so that the temperature of the combustion gas is about 600 ° C. Cooling. As described above, when coexisting with a combustion gas having a high carbon dioxide partial pressure, calcium carbonate may be re-carbonated, and the calcium carbonate may adhere to the surroundings. Here, since the partial pressure of carbon dioxide in the total gas after mixing is reduced by mixing air, re-carbonation of calcium oxide does not occur. Subsequently, in the classifier 41, the combustion gas and the powdered calcium oxide are separated. The powdered calcium oxide is cooled by the powder cooler 50, collected by the bag filter 60, and stored in the silo 61. The combustion gas is cooled by being passed through a preheater 70 that performs exhaust heat recovery. Thereafter, the combustion gas is exhausted by the draft fan 81 through the bag filter 60. The preheater 70 preheats the combustion air sent from the supply fan 80 by the heat recovered from the combustion gas, and supplies it to the first stage fluidized bed furnace 20.

In the second embodiment of the rapid cooling means, the temperature of the combustion gas is cooled to about 600 ° C. by mixing the spray water supplied from the pump 82 to the two-phase mixer 40 with the combustion gas. Combustion gas and separated calcium oxide undergo a digestion reaction due to the presence of water and become calcium hydroxide. In the case of this example, calcium hydroxide is immediately generated, and therefore, recarboxylation of calcium oxide does not occur. Subsequently, in the classifier 41, the combustion gas and powdered calcium hydroxide are separated. The powdered calcium hydroxide is cooled by the powder cooler 50, collected by the bag filter 60, and stored in the silo 61. The treatment of the combustion gas is the same as that in the first embodiment.
This example is suitable for applications that require high concentrations of carbon dioxide, such as when producing light calcium carbonate or recycling lime cake.

Next, the system of FIG. 4b will be described.
The first stage fluidized bed furnace 20 and the second stage fluidized bed furnace 30 are basically the same as the system of FIG. 4a described above. However, the second-stage fluidized bed furnace 30 is configured such that a gas inlet is provided at the upper end or near the upper end, and combustion gas and accompanying calcium oxide flow out from the lower end or near the lower end. Therefore, a swirling flow from the upper side to the lower side is formed in the second-stage fluidized bed furnace 30 of this embodiment.

  The indirect external heat kiln 90 is a component at the front stage of the first stage fluidized bed furnace 20 and a component at the rear stage of the second stage fluidized bed furnace 30. The indirect external heat kiln 90 has a double cylindrical shape with the shaft installed in the horizontal direction, and the target is placed in the inner cylinder 94, which is the kiln body, and the heating fluid flows through the outer cylinder 95. It is the structure which heats an object indirectly.

  The powdered calcium carbonate 10 is charged from an inlet provided at one end of the inner cylinder 94 of the indirect external heat kiln 90. At the same time, combustion air is also supplied from the inlet of the inner cylinder 94 via the combustion air supply fan 80. On the other hand, the combustion gas accompanying calcium oxide that has flowed out of the second-stage fluidized bed furnace 30 flows into the outer cylinder 95 from the thermal fluid inlet 91 as a heating fluid for heating the indirect external heating kiln 90 and exits from the thermal fluid outlet. It flows toward 92 and flows out. The thermal fluid inlet 91 of the outer cylinder 95 is provided at the end opposite to the inlet of the inner cylinder 94. The combustion gas as a heating heat fluid heats the inner cylinder 94 while passing through the kiln, whereby the internal calcium carbonate 10 and the combustion air are heated. Calcium carbonate 10 and combustion air flow out from an outlet provided at the other end of the inner cylinder 94 and are then introduced into the first stage fluidized bed furnace 20. Therefore, heating the calcium carbonate 10 and the combustion air in the inner cylinder 94 is a preheating process prior to firing. In addition, when the calcium carbonate 10 is a lime cake containing moisture, a drying process is also included. Thereby, the firing efficiency in the fluidized bed furnace is improved. The temperature suitable for the preheating process is about 400 ° C.

  Further, the combustion gas as the heating thermal fluid flowing through the outer cylinder 95 and the accompanying powdered calcium oxide are lowered in temperature by heating the inner cylinder 94. At this time, in order to prevent the recalcification of calcium oxide by carbon dioxide contained in the combustion gas, as a means for rapidly cooling the combustion gas, the thermal fluid cooling is performed between the thermal fluid inlet 91 and the thermal fluid outlet 92 of the outer cylinder 95. A port 93 is provided to supply cold air. As a result, the heating fluid is rapidly cooled to 600 ° C. or lower, and re-carbonation is prevented by reducing the partial pressure of carbon dioxide in the combustion gas by supplying air. The combustion gas and the powdered calcium oxide associated therewith are separated from the combustion gas and calcium oxide in the bag filter 60 after flowing out from the indirect external heat kiln 90. Thereafter, the processing is the same as that of the system of FIG.

  In the system of FIG. 4b, as another example of the rapid cooling means for rapidly cooling the combustion gas as the heating thermal fluid to 600 ° C. or less, water may be supplied from the thermal fluid cooling port 93, although not shown. Calcium oxide undergoes a digestion reaction due to the presence of water, and immediately becomes calcium hydroxide so that re-carbonation does not occur. The combustion gas and the powdered calcium hydroxide associated therewith are separated from the combustion gas and calcium hydroxide in the bag filter 60 after flowing out from the indirect external heat kiln 90. After that, it becomes the same processing as the system of FIG. 4a.

Next, the system of FIG. 4c will be described.
In this embodiment, powdered calcium carbonate is fired using an indirect external heat kiln 90. The indirect external heat kiln 90 has a double cylindrical shape with the shaft installed in the horizontal direction, and the target is placed in the inner cylinder 94, which is the kiln body, and the heating fluid flows through the outer cylinder 95. It is configured to indirectly heat the object.

  Furthermore, in the system of FIG. 4c, a combustion gas generating furnace 20a is provided to generate combustion gas as a heating fluid. The combustion gas generation furnace 20a basically uses coal 12 as a fuel, and appropriately uses petroleum 13 together, and generates combustion gas by burning these fuels. Since the calcium-based raw material is not charged into the combustion gas generating furnace 20a, the generated combustion gas does not accompany calcium oxide.

  The powdered calcium carbonate 10 is charged into an inlet provided at one end of the inner cylinder 94 of the indirect external heat kiln 90. On the other hand, the combustion gas flowing out of the combustion gas generation furnace 20a flows into the outer cylinder 95 from the thermal fluid inlet 91 as a heating fluid for heating the indirect external heating kiln 90, flows toward the thermal fluid outlet 92, and is heated. It flows out from the outlet 92 and is exhausted through the induction ventilation fan 81. The thermal fluid inlet 91 of the outer cylinder 95 is provided at the end opposite to the inlet of the inner cylinder 94. The combustion gas as the heating heat fluid heats the inner cylinder 94 while passing through the kiln, and in this embodiment, the calcium carbonate 10 is thereby fired. Therefore, in this case, the temperature inside the inner cylinder 94 is a temperature range suitable for the above-described calcium carbonate firing, considering the heat resistance strength of the inner cylinder 94, and when an indirect external heat kiln is used as a firing furnace, The firing residence time is set to 800 to 950 ° C. because it can be easily extended. Thus, the calcium carbonate in the inner cylinder 94 is decomposed into calcium oxide and carbon dioxide and flows out.

  When the feedstock to the inner cylinder 94 is lime cake, air for incinerating the organic matter containing lime cake is supplied and the combustion gas is also accompanied. In order to prevent the re-carbonation of calcium oxide by carbon dioxide contained in the outflowing gas, cold air is supplied through the two-phase mixer 40 as a rapid cooling means for the outflowing gas. As a result, the combustion gas is rapidly cooled to 600 ° C. or lower, and re-carbonation is prevented by reducing the carbon dioxide partial pressure in the combustion gas by supplying air. Thereafter, the combustion gas and the accompanying calcium oxide are separated into the combustion gas and calcium oxide in the bag filter 60. Thereafter, the processing is the same as that of the system of FIG.

  In the system of FIG. 4 c, as another example of the rapid cooling means for rapidly cooling the combustion gas flowing out from the inner cylinder 94 to 600 ° C. or lower, although not shown, water may be supplied via the two-phase mixer 40. Good. Calcium oxide undergoes a digestion reaction due to the presence of water, and immediately becomes calcium hydroxide so that re-carbonation does not occur. Thereafter, the combustion gas and the powdered calcium hydroxide associated therewith are separated into combustion gas and calcium hydroxide in the bag filter 60. Thereafter, the processing is the same as that of the system of FIG.

  The preheater 70 recovers heat from the combustion gas and preheats the combustion air sent from the supply fan 80 with the recovered heat. The preheated combustion air is supplied to the combustion gas generation furnace 20a.

  Here, the combustion air 96 supplied to the inlet of the inner cylinder 94 of the indirect external heat kiln 90 will be described. When the raw material calcium carbonate 10 is pure calcium carbonate, this combustion air 96 is unnecessary. However, since a suitable raw material is a lime cake that is a waste from the sugar production process, it usually contains about 10% of organic matter. Combustion air 96 is supplied to completely incinerate the organic matter.

FIG. 5 (a) is a side sectional view schematically showing portions of the first stage fluidized bed furnace 20 and the second stage fluidized bed furnace 30 in one embodiment of the system shown in FIG. 4a. (B) is AA sectional drawing of (a). (C) is BB sectional drawing of (a).
In the present embodiment, the first-stage fluidized bed furnace 20 and the second-stage fluidized bed furnace 30 are separately provided and connected by a connecting pipe 25. After the start-up burner 21 of the first stage fluidized bed furnace 20 is started, coal whose particle medium in the bubbling fluidized bed portion 22 is fuel is burned, thereby calcining calcium carbonate into calcium oxide. However, as described above, since the powdered calcium carbonate cannot ensure a sufficient residence time with only one fluidized bed furnace, a part of the powdered calcium carbonate remains from the first-stage fluidized bed furnace 20 as unfired calcium carbonate. Leaked. Accordingly, the combustion gas ascends the free board portion 24 accompanied by the calcium oxide fine particles and the uncalcined calcium carbonate fine particles generated by firing, and from the gas outlet 26 near the upper end of the first furnace body 23. It flows out and flows into the gas inlet 36 near the lower end of the second furnace body 33 in the second stage fluidized bed furnace 30.

5A is a horizontal cross section at the position of the connecting pipe 25. FIG. The connecting pipe 25 is connected on the tangent line of the horizontal section circumferential portion of the second furnace body 33. Accordingly, the combustion gas flowing into the second stage fluidized bed furnace 30 flows in the tangential direction of the circumferential section of the second furnace body 33, and then along the circumferential inner wall of the second furnace body 33. To form a swirling flow that swirls (see solid arrows). In addition, an upward flow is also formed by flowing in from the vicinity of the lower end of the second furnace body 33. As a result, the combustion gas rises while swirling along the inner wall of the second furnace body 33. This forms a spouted bed. 5B is a horizontal cross section in the vicinity of the upper end of the second stage fluidized bed furnace 30. FIG. The gas outlet of the second stage fluidized bed furnace 30 is constituted by a cylindrical inner tube 32 provided at the upper end of the second furnace body 33.
In another embodiment (not shown), the combustion gas may flow into the second-stage fluidized bed furnace 30 from the upper part of the furnace. Also in this case, the second furnace body 33 is caused to flow in the tangential direction of the horizontal section circumferential portion.

  FIG. 6 is a diagram schematically showing the flow of combustion gas in the second-stage fluidized bed furnace 30 of the system shown in FIG. 4a. The inner tube 32 is disposed coaxially with the second furnace body 33 and penetrates the upper end surface 33a of the second furnace body 33 and opens in the furnace space. Preferably, the diameter D2 of the inner tube 32 is less than or equal to one half of the diameter D1 of the second furnace body 33, and the length L of the inner tube 32 in the furnace space is equal to that of the inner tube 32. It should be at least twice the diameter D2.

  The combustion gas rises while swirling along the inner wall of the second furnace body 33, and reaches the upper end surface 33a (solid line with arrow in FIG. 6). The uncalcined calcium carbonate fine particles flow with a swirling speed slower than the swirling speed of the combustion gas due to friction with the inner wall of the furnace body 33 while accompanying the combustion gas. This extends the residence time of the uncalcined calcium carbonate.

  The combustion gas that has reached the upper end surface 33a then descends while swirling along the outer wall of the inner cylindrical tube 32.When the combustion gas reaches the opening 32a at the lower end of the inner cylindrical tube 32, the combustion gas reaches the inside of the inner cylindrical tube 32 from the opening 32a. Enters and rises and exits the second stage fluidized bed furnace 30 (dashed line in FIG. 6). On the other hand, among the uncalcined calcium carbonate fine particles contained in the combustion gas, relatively large particles that require a long time for firing are less likely to flow out because they are delayed from the flow of the combustion gas, and circulate in the upper part of the furnace space. Become. For example, even if relatively large fine particles reach the opening of the inner tube 32 in association with the downward swirling flow, they cannot accompany the flow of combustion gas that goes straight upward from the opening, and again ascend swirling along the inner wall. It circulates in the flow. By providing the inner tube 32, a classification action based on the particle diameter of the fine particles can be obtained. Thus, the uncalcined calcium carbonate can secure a sufficient residence time in the second-stage fluidized bed furnace 30 and is completely calcined to become calcium oxide.

  Note that particles circulating in the upper part of the second furnace body 33 are partially discharged due to airflow turbulence or the like, so that the discharge amount naturally increases as the circulating particle concentration increases. Therefore, it has been experimentally confirmed that the amount of fine particles reaches equilibrium in a state where the operation of the apparatus is not hindered.

FIG. 7A is a side sectional view schematically showing portions of a first stage fluidized bed furnace 20 and a second stage fluidized bed furnace 30 in another embodiment of the present system. A second stage fluidized bed furnace 30 is installed immediately above the first stage fluidized bed furnace 20. (B) is CC sectional drawing of (a). In this embodiment, the connecting pipe 25 that communicates the gas outlet 26 at the upper end of the first stage fluidized bed furnace 20 and the gas inlet 36 at the bottom of the second stage fluidized bed furnace 30 includes the furnace bodies 23 and 33. It is provided coaxially. The connecting tube 25 includes a plurality of (four in the illustrated example) plate-like blades 25b around the central shaft portion 25a, and the surfaces of the plate-like blades 25b are inclined at the same angle with respect to the horizontal plane and are adjacent to each other. A gap is formed between the two plate-like blades 25b.
Combustion gas that has risen in the freeboard section 24 of the first stage fluidized bed furnace 20 passes through the gap between two adjacent plate blades 25b and flows into the bottom of the second stage fluidized bed furnace 30. This direction is inclined with respect to the vertical direction by the plate-like blade 25b. As a result, when it flows into the second stage fluidized bed furnace 30, it flows in the tangential direction of the circumferential section of the horizontal section, and then rises while swirling along the inner wall of the second furnace body to form a spouted bed. To do. Also in the present embodiment, the flow state in the vicinity of the upper end where the inner tube 32 is provided with 33 is the same as in FIG.

The lime cake discharged in the sugar solution purification process in sugar making was tested using the baking system according to the method of the present invention. As a comparative example, a similar test using a conventional one-stage furnace was also performed. The test conditions and results are as follows.
<Sample>
Lime cake (LC): 100 kg / h (feed rate), particle size of 10 μm or less (component in LC) Calcium carbonate CaCO ₃ 55.5%
Organic 10.5%
Water 33.0%
Other 1.0%
<Baking conditions>
Firing furnace: Fluidized bed firing furnace Two-stage fluidized bed (diameter: 250mm, 150mm)
Fuel: Kerosene Firing temperature: It was set to maintain several temperatures between 850 and 1000 ° C., and a plurality of tests were conducted at each temperature.

<Result>
The theoretical yield of calcium hydroxide Ca (OH) ₂ from the lime cake is 41 kg / h. The firing rate was calculated based on the actual yield relative to the theoretical yield. For example, the yield at a firing temperature of 950 ° C. was 39.5 kg / h, and the firing rate was 96%.
Table 1 summarizes the results of the firing temperature and the firing rate. In the examples, there was slight variation in the firing rate in the range of 850 to 900 ° C., but a stable firing rate was obtained at 900 ° C. or higher, particularly 950 ° C. or higher. In addition, in the entire range of 850 to 1000 ° C., a higher firing rate was obtained in the example than in the comparative example.

  In the above test, a lime cake having a particle size of 10 μm or less was used, but for a particle size range larger than this, the residence time in the furnace becomes longer, and naturally, better baking than lime cake. Rate is obtained.

Conventionally, it has been common to calcine massive limestone to produce calcium oxide, so coke or heavy oil is used as the fuel. According to the present invention, high-activity calcium oxide or calcium hydroxide having high reactivity is produced at a higher firing rate than before by firing powdered calcium carbonate using coal as fuel. Is possible.
By the powdered calcium carbonate baking technology according to the present invention, a lime cake mainly composed of calcium carbonate, which is a waste product in a sugar production process, can be calcined to form calcium oxide or calcium hydroxide, and recycled in the sugar production process. Is possible.
Also. From using coal fuel, by coal ash caused by coal burning binds to calcium oxide is silicon dioxide source, producing a SO _x, NO _x, highly absorbent flue cleaning agents HCl be able to.

10: Powdered calcium carbonate
11: Coal ash
12: Coal
13: Oil
20: First stage fluidized bed furnace
20a: Combustion gas generation furnace
21: Startup burner
22: Bubble fluidized bed
23: First furnace body
24: Freeboard
25: Connecting pipe
25a: Center axis
25b: Plate-shaped feather
26: Gas outlet
30: Second stage fluidized bed furnace
31: Auxiliary burner
32: Inner tube
32a: Opening
33: Second furnace body
33a: Top surface
36: Gas inlet
40: Two-phase mixer
41: Classifier
50: Powder cooler
60: Bug filter
61: Silo
70; Combustion air preheater
80: Combustion air supply fan
81: Combustion gas induction fan
82: Pump
90: Indirect external heat kiln
91: Thermal fluid inlet
92: Thermal fluid outlet
93: Thermal fluid cooling port
94: Kiln inner cylinder
95: Kiln outer cylinder
96: Combustion air

Claims (3)

Powdered carbon dioxide using a two-stage calcium carbonate calcining furnace having a first-stage fluidized bed furnace (20) for forming a bubbling fluidized bed and a second-stage fluidized bed furnace (30) for forming a spouted bed In the method for producing calcium oxide (CaO) by calcining calcium (CaCO ₃ ), first calcium powder and fuel are charged into the first stage fluidized bed furnace in which a bubbling fluidized bed is formed and calcined. A combustion gas accompanied by calcium oxide and uncalcined calcium carbonate produced by firing in the step, the first step flows into the second stage fluidized bed furnace, and swirls along the inner wall of the second stage fluidized bed furnace A second step of firing the uncalcined calcium carbonate by forming a spouted bed, the second stage fluidized bed furnace is upright cylindrical, and the combustion gas is passed through the second stage fluidized bed furnace. of allowed to flow into the horizontal cross-sectional circumference tangential direction, the second-stage fluidized bed furnace And an inner cylinder pipe (32) that passes through the upper end surface (33a) of the second-stage fluidized bed furnace and opens in the furnace space, and allows the combustion gas to flow out through the inner cylinder pipe. A method for firing powdered calcium carbonate.   Water is sprayed on the combustion gas flowing out of the second-stage fluidized bed furnace (30) accompanied by calcium oxide generated by firing, thereby rapidly cooling to 600 ° C. or less to separate calcium oxide and combustion gas. The method for calcining powdered calcium carbonate according to claim 1, wherein calcium hydroxide is produced by simultaneously digesting calcium oxide. Using an indirect external heating kiln that includes an inner cylinder (94) and an outer cylinder (95) and heats the inside of the inner cylinder by flowing a heating thermal fluid through the outer cylinder, the powdered calcium carbonate is Prior to charging into the first fluidized bed furnace, it is charged into the inner cylinder of the indirect external heating kiln, and the combustion gas flowing out of the second stage fluidized bed furnace accompanied by calcium oxide is used as a heating hot fluid. In the preheating step of preheating the powdered calcium carbonate and combustion air by flowing it through the outer cylinder of the indirect external heating kiln, the combustion gas is flown through the outer cylinder of the indirect external heating kiln as the heating fluid. Then, by supplying water to the combustion gas, the combustion gas is rapidly cooled to 600 ° C. or less to separate calcium hydroxide generated from calcium oxide from the combustion gas. How to fire powdered calcium carbonate as described in .

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