Method and system for fixing carbon dioxide in cement hydrate-containing materials

Abstract

To provide an immobilization method and an immobilization system of carbon dioxide to cement hydrate inclusion where carbon dioxide contained in a combustion exhaust gas is efficiently immobilized by using the cement hydrate inclusion, and where elution of hexavalent chromium from the cement hydrate inclusion is reduced by reducing a content of hexavalent chromium in the obtained cement hydrate inclusion.SOLUTION: The present invention relates to an immobilization method and an immobilization system of carbon dioxide, which immobilizes carbon dioxide on the cement hydrate inclusion by supplying an overheat steam contact heat of a temperature of 120°C or more to the cement hydrate inclusion to heat the cement hydrate inclusion and reduces elution of the hexavalent chromium from a treated cement hydrate inclusion by making at least any of a reductive gas, a reductive material or an organic material exist in the overheat steam treatment.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method and a system for immobilizing carbon dioxide in a cement hydrate-containing material, and particularly to a method and a system for immobilizing carbon dioxide in a cement hydrate-containing material using superheated steam.

Background Art

[0002] In order to immobilize carbon dioxide in carbon dioxide-containing gases such as factory exhaust gases and reduce the amount of carbon dioxide discharged into the atmosphere, various technical developments have been carried out. For example, Patent Document 1 discloses a method for reducing exhaust carbon dioxide gas, which comprises contacting an exhaust gas containing CO2 with an aggregate of solid particles containing CaO and / or Ca(OH)2 as a composition, and immobilizing CO2 in the exhaust gas as CaCO3 on the solid particles, thereby reducing the CO2 concentration in the exhaust gas. According to this method, CO2 in the exhaust gas generated in an industrial process or the like can be efficiently absorbed and removed, and the amount of CO2 discharged into the atmosphere can be reduced.

[0003] Patent Document 2 describes a method for immobilizing carbon dioxide, which comprises collecting materials obtained by crushing waste concrete, supplying water and stirring to make it in a wet state, supplying exhaust gas with waste heat to the wet material to dry the material, and repeating an alternating process consisting of supplying water again, stirring the materials, and supplying exhaust gas, thereby immobilizing carbon dioxide in the exhaust gas in the materials. According to this method, immobilization of carbon dioxide in exhaust gas with waste heat can be realized using recycled sand of waste concrete.

[0004] Patent Document 3 describes a carbon dioxide-immobilized concrete structure comprising a carbon dioxide-immobilized molded body obtained by curing a concrete composition containing water, cement, admixture, and aggregate on the surface of a concrete structure, having voids in the surface layer portion, and immobilizing carbon dioxide in the atmosphere in the surface layer portion. According to this structure, carbon dioxide in the atmosphere can be effectively immobilized.

[0005] Patent Document 4 describes a method for fixing carbon dioxide, which includes a contact step in which a carbon dioxide-containing gas having a temperature of 350°C or higher is brought into contact with a cementitious hardened body, thereby fixing the carbon dioxide contained in the carbon dioxide-containing gas to the cementitious hardened body.

[0006] However, the carbon dioxide fixation methods described in Patent Documents 1 to 4 had the problem that hexavalent chromium contained in concrete easily leached out of the treated material after carbon dioxide fixation, causing environmental pollution. This made the disposal of the treated material itself difficult.

[0007] On the other hand, as a method to suppress the leaching of hexavalent chromium from soil containing hexavalent chromium, Patent Document 5 proposes a method of heat-treating soil containing hexavalent chromium at 200 to 600°C under a reducing atmosphere. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2000-197810 [Patent Document 2] Japanese Patent Publication No. 2009-90198 [Patent Document 3] Japanese Patent Publication No. 2008-75391 [Patent Document 4] Japanese Patent Publication No. 2020-131074 [Patent Document 5] Japanese Patent Publication No. 2003-334532 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The problem that the present invention aims to solve is to provide a method and system for fixing carbon dioxide into a cement hydrate-containing material that solves the above-mentioned problems, efficiently fixes carbon dioxide contained in combustion exhaust gas and the like using a cement hydrate-containing material, and reduces the elution of hexavalent chromium from the cement hydrate-containing material by reducing the hexavalent chromium content in the obtained cement hydrate-containing material. [Means for solving the problem]

[0010] To solve the above problems, the carbon dioxide fixation method and fixation system for cement hydrate-containing materials of the present invention have the following technical features. (1) A method for fixing carbon dioxide, characterized by a superheated steam treatment step in which a cement hydrate-containing material is brought into contact with superheated steam at a temperature of 120°C or higher and heated, and carbon dioxide being added during the superheated steam treatment step to fix the carbon dioxide in the cement hydrate-containing material.

[0011] (2) The carbon dioxide fixation method described in (1) above is characterized by the presence of at least one of a reducing gas, a reducing substance, or an organic substance during the superheated steam treatment step, thereby reducing the elution of hexavalent chromium from the cement hydrate-containing material after treatment.

[0012] (3) The carbon dioxide fixation method described in (1) or (2) above, characterized in that the cement hydrate-containing material is any of the following: recycled aggregate, waste building materials consisting of concrete or mortar, waste cement paste, or sludge generated in ready-mixed concrete.

[0013] (4) A carbon dioxide fixation system characterized by a superheated steam treatment means that contact-heats a cement hydrate-containing material with superheated steam at a temperature of 120°C or higher, and by supplying carbon dioxide to the superheated steam treatment means and fixing the carbon dioxide in the cement hydrate-containing material.

[0014] (5) The carbon dioxide immobilization system according to (4) above supplies at least one of a reducing gas, a reducing substance, or an organic substance to the superheated steam treatment means, and reduces the elution of hexavalent chromium from the cement hydrate-containing material after treatment.

[0015] (6) In the carbon dioxide immobilization system according to (4) or (5) above, the cement hydrate-containing material is any one of waste materials of building materials composed of recycled aggregates, concrete or mortar, waste materials of cement paste, or sludge generated from ready-mixed concrete. [[Effect of the Invention]]

[0016] In the carbon dioxide immobilization method and system of the present invention, in the cement hydrate-containing material, contact heating is performed with superheated steam at a temperature of 120°C or higher, and carbon dioxide is added during heating with the superheated steam, and the carbon dioxide is immobilized in the cement hydrate-containing material. Therefore, the cement hydrate-containing material can be efficiently heated, and carbonation can be promoted. Moreover, the oxygen concentration in the treatment furnace can be kept very low with superheated steam, and it can be easily maintained in a reduced state by using a reducing substance or the like. As a result, the treated product treated by the carbon dioxide immobilization method and system of the present invention has a reduced elution amount of hexavalent chromium, and the utilization fields such as recycling of the treated product can be expanded. [[Brief Description of the Drawings]]

[0017] [Figure 1] It is a diagram for explaining the outline of the carbon dioxide immobilization method of the present invention. [Figure 2] It is a diagram for explaining a method of mixing carbon dioxide or a reducing gas into superheated steam. [Figure 3] It is a flowchart showing an example of the carbon dioxide immobilization method of the present invention. [[Modes for Carrying Out the Invention]]

[0018] Hereinafter, a method and a system for immobilizing carbon dioxide in a cement hydrate-containing material according to the present invention will be described in detail using preferred examples while referring to the drawings. As shown in FIG. 1, the present invention provides a method and a system for immobilizing carbon dioxide, which are characterized in that superheated steam at a temperature of 120° C. or higher is supplied to a cement hydrate-containing material to heat the cement hydrate-containing material, and carbon dioxide is added to the cement hydrate-containing material to immobilize carbon dioxide in the cement hydrate-containing material.

[0019] In the present invention, superheated steam is used instead of hot gas in order to perform efficient heating. Since superheated steam has more energy than hot gas, heating can be performed efficiently. The temperature of the superheated steam supplied to the treatment furnace needs to be 120° C. or higher, preferably 200° C. or higher, and more preferably 300° C. or higher. Further, as will be described later, when the temperature is 300° C. or higher and 500° C. or lower, a high carbon dioxide solidification rate (solidification amount) can be expected. Furthermore, when the temperature of the superheated steam is too high, a reaction of re-converting to hexavalent chromium also occurs, the suppression effect of the elution amount of hexavalent chromium decreases, and the consumption amount of the energy used also increases. Therefore, considering the treatment effect (the solidification amount of carbon dioxide and the elution amount to hexavalent chromium) with respect to the energy used, it is set to 700° C. or lower, preferably 600° C. or lower, and more preferably 500° C. or lower.

[0020] In the present invention, since superheated steam is used, it is possible to keep the oxygen concentration very low in the treatment furnace for performing the heat treatment. Further, by using a reducing substance in combination, it is possible to easily keep the object to be treated in a reduced state, and it is also possible to effectively suppress the elution amount of hexavalent chromium from the treated product.

[0021] By the way, when superheated steam is used, in addition to the low oxygen concentration in the treatment furnace, organic substances such as high-performance AE water-reducing agents contained in waste concrete are thermally decomposed by superheated steam (oxygen-free state), and it is also possible to generate reducing substances. In such a case, it is not necessary to separately supply a reducing gas.

[0022] When carbon monoxide is used as a reducing agent in the processing furnace, the concentration of carbon monoxide should be set to 50 ppm or higher, preferably 100 ppm or higher.

[0023] The cement hydrate-containing materials used in this invention include demolished concrete blocks, finely crushed concrete blocks, recycled aggregate and its fine powder portion produced from concrete blocks, ready-mix concrete sludge discharged from the water treatment facilities of ready-mix concrete plants, wood cement boards, ALC waste, and other materials that utilize cement and have cement hydrate attached to or contained within them. Among these, recycled aggregate, waste building materials made of concrete or mortar, waste cement paste, or sludge generated in ready-mix concrete are more preferable from the viewpoint of cost and effective utilization of waste materials.

[0024] In this invention, as shown in Figure 1, it is possible to directly introduce the cement hydrate-containing material into the processing furnace, but as shown in Figure 2, it is also possible to preheat the material using hot gas (hot air) before heating it with superheated steam. High-temperature exhaust gas can be used as the hot gas. Furthermore, preheating the cement hydrate-containing material makes it possible to perform superheating with superheated steam more efficiently.

[0025] Furthermore, as shown in Figure 2, after heating and drying the cement hydrate-containing material in a heating furnace, the particle size can be reduced by grinding or crushing, making it easier to incorporate carbon dioxide. This also has the potential to remove mortar and other materials from the aggregate. Moreover, treatment with superheated steam makes it even easier to remove mortar from the aggregate. The treated material discharged from the processing furnace can be separated by particle size, for example, into fine powder with a particle size of 1 mm or less and recycled aggregate with a particle size of 1 mm or more, and each can be used for different purposes. Alternatively, the treated material can be ground or crushed before separation.

[0026] As shown in Figure 1, it is possible to supply carbon dioxide, reducing gas, etc., individually to the processing furnace that supplies superheated steam, but as shown in Figure 3, it is also possible to supply carbon dioxide and reducing gas (CO) mixed with superheated steam.

[0027] As an example, water is converted into steam in a steam boiler to generate saturated steam at approximately 120°C and 0.5 MPa. Carbon dioxide is mixed with this and supplied to a superheated steam generator. There are various methods for converting saturated steam into superheated steam, but for example, a method of inductively heating a heating element using high frequency generated from a high-frequency power supply is preferred because the temperature can be precisely set and the structure can be simplified. The superheated steam can be set in the range of 120°C to 1000°C, and is set according to the capacity of the processing furnace and the amount of cement hydrate-containing material being processed. Furthermore, in order to sufficiently reduce the amount of hexavalent chromium leached out, it is more preferable to set the temperature of the superheated steam in the range of 300°C to 500°C.

[0028] In this invention, when performing treatment with superheated steam, at least one of a reducing gas, a reducing substance, or an organic substance is present, and the treatment is performed in a reducing atmosphere. This makes it possible to reduce the elution of hexavalent chromium from the cement hydrate-containing material after treatment. As shown in Figure 3, it is also possible to supply carbon monoxide as a reducing substance to the superheated steam between the superheated steam generator and the treatment furnace, and then supply it to the treatment furnace.

[0029] As shown in Figure 1, if the temperature cannot be sufficiently maintained in the processing furnace by heating with superheated steam alone, it is possible to use auxiliary heating means such as electric heaters in combination. Furthermore, in order to maintain the appropriate pressure inside the processing furnace, unwanted gases are released into the atmosphere after dust and harmful substances have been removed. Furthermore, in order to operate the processing furnace continuously, it is also possible to use a tilting rotary furnace, such as the one shown in Patent Document 5.

[0030] To confirm the effectiveness of the carbon dioxide fixation method of the present invention, the following experiment was conducted. (Experimental procedure) (1) A steam boiler generated 100 kg / h of saturated steam (0.2 MPa, 120°C) and supplied it to a superheated steam generator. (2) Using a superheated steam generator (Nomura Giko Co., Ltd., GE-100 model, high-frequency power output 20kW), superheated steam at a maximum of 550°C was generated and supplied to the processing furnace (internal dimensions W400mm × H400mm × D400mm). (3) The temperature inside the furnace was controlled by temperature control of the supplied superheated steam. (4) To restore the initial temperature drop to the set temperature as quickly as possible, an electric heater was used as an auxiliary device. (5) The sample (using RC10 roadbed material manufactured from waste concrete) was laid to a thickness of approximately 10 mm and placed inside the processing furnace. (6) For carbon dioxide, the flow meter was used to adjust the partial pressure so that it could be controlled between 0-20%. (7) The carbon monoxide concentration was also adjusted using a flow meter to be between 0 and 200 ppm.

[0031] The treated materials processed according to the experimental procedure described above were analyzed using the following method. (Analysis method) (1) The amount of hexavalent chromium leached is in accordance with the Ministry of the Environment Notification No. 46. (2) Increase in calcium carbonate content Based on TG thermogravimetric analysis, the weight change between 500 and 850°C is considered to be calcium carbonate. Analysis was performed before and after superheated steam treatment to determine the amount of increase. The calculation was based on the dry sample state. Specifically, it was calculated using the following formula. (Formula) Increase in calcium carbonate (%) = (Qb - Qa) / Qa × 100 Here, the meanings of Qa and Qb are as follows: Qa: Amount of calcium carbonate contained in untreated sample per unit sample volume (on a 105°C dry basis) Qb: Amount of calcium carbonate contained in the processed sample per unit sample volume (based on 105°C dryness)

[0032] The experiment was conducted using two methods: fixing carbon dioxide using only superheated steam (Table 1) and fixing carbon dioxide using a combination of superheated steam and reducing gas (CO) (Table 2). The results of each experiment are as follows. The carbon dioxide supply time in each experiment was 60 minutes. As a comparative example, heating was performed using an electric heater instead of superheated steam.

[0033] [Table 1]

[0034] [Table 2]

[0035] Based on the results in Tables 1 and 2, and referring to Comparative Example 1 and Example 3, or Comparative Example 2 and Example 7, it can be seen that when superheated steam is used, the increase in calcium carbonate is approximately 20 times greater at the same 300°C compared to heating with an electric heater, confirming that carbon dioxide fixation is more efficient when using superheated steam.

[0036] Furthermore, by utilizing superheated steam, particularly as seen in Examples 3 and 4, it is understood that when superheated steam at 300°C or higher is used, the amount of hexavalent chromium leaching from the treated material is effectively suppressed. This is presumed to be due to the reduction in oxygen supply caused by the use of superheated steam, as well as the decomposition of organic matter contained in waste concrete by heating with superheated steam, which generates reducing substances.

[0037] Naturally, as shown in Examples 5 to 9, the elution of hexavalent chromium can be effectively suppressed by supplying and treating with a reducing substance (CO). Moreover, it can be confirmed that the amount of hexavalent chromium eluted is suppressed even at temperatures lower than 300°C. [Industrial applicability]

[0038] As described above, the present invention makes it possible to provide a method and system for fixing carbon dioxide into a cement hydrate-containing material, which efficiently fixes carbon dioxide contained in combustion exhaust gas and the like using a cement hydrate-containing material, and reduces the elution of hexavalent chromium from the cement hydrate-containing material by reducing the hexavalent chromium content in the obtained cement hydrate-containing material.

Claims

1. The cement hydrate-containing material is subjected to superheated steam treatment, which involves contact heating with superheated steam at a temperature of 120°C or higher. A method for fixing carbon dioxide, characterized by adding carbon dioxide during the superheated steam treatment to fix the carbon dioxide in the cement hydrate-containing material, and by presenting at least one of a reducing gas, a reducing substance, or an organic substance during the superheated steam treatment to reduce the elution of hexavalent chromium from the cement hydrate-containing material after treatment.

2. The cement hydrate-containing material is equipped with a superheated steam treatment means that performs contact heating with superheated steam at a temperature of 120°C or higher. A carbon dioxide fixation system characterized by supplying carbon dioxide to the superheated steam treatment means to fix the carbon dioxide in the cement hydrate-containing material, and supplying at least one of a reducing gas, a reducing substance, or an organic substance to the superheated steam treatment means to reduce the elution of hexavalent chromium from the cement hydrate-containing material after treatment.

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