Exhaust gas circulation type limestone firing system

Abstract

To provide highly efficient limestone firing system which can radically suppress the generation of CO2 from a fuel gas, and enables the reuse of CO2 from raw stone, which is unavoidable in principle.SOLUTION: A fuel gas containing CO and H2 is combusted in a firing chamber 12A of an exhaust gas circulation type limestone firing system 100 in the presence of a combustion supporting gas (oxygen), thus limestone is thermally decomposed and fired into quicklime. An exhaust gas containing CO2 and H2O is added with an H2 gas in a reformer 90 to obtain a reformed exhaust gas containing CO and H2O generated by reverse water gas shift reaction and unreacted H2, CO2 and H2O.The reformed exhaust gas is dehumidified in a dehumidifier 92 to obtain a dehumidified reformed exhaust gas containing CO, H2 and CO2. At least a part of this dehumidified reformed exhaust gas is again fed to the firing chamber 12A as a fuel gas. In this way, the cyclic use of the dehumidified modified exhaust gas is executed.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas circulation type limestone calcination system that can drastically reduce CO2 emissions, which accelerate global warming. [Background technology]

[0002] As calls for a reduction in CO2 emissions, which contributes to global warming, grow on a global scale, the development of CO2 emission reduction technologies, which contradicts current economic rationality, is now becoming the global standard for credit transactions between companies. Because neglecting CO2 emission reduction measures could lead to damage to corporate brands, various efforts are being made across industries, but the current situation is that no progress has been made in the development of fundamental technologies aimed at reducing non-energy-related CO2 emissions from the generation stage.

[0003] The lime industry is one of the industries struggling to reduce non-energy-related CO2 emissions. Raw limestone (CaCO3) and dolomite (CaMg(CO3)2) (hereinafter referred to as "limestones") are "fossil raw materials" that contain CO2. Therefore, reducing CO2 emissions directly leads to a decrease in the production of the finished products, quicklime (CaO) and lightly burned dolomite (CaO·MgO) (hereinafter referred to as "quicklime"). In reducing furnaces such as blast furnaces, carbon sources such as coke are merely auxiliary raw materials, but in limestone calciners, carbon sources are the main raw material. Therefore, limestone calciners are more troublesome than reducing furnaces such as blast furnaces when it comes to reducing CO2 emissions.

[0004] Known methods for reducing CO2 emissions include separating and capturing the generated CO2, followed by storage, sequestration, or effective utilization. For example, Patent Document 1 describes a "method for separating and capturing carbon dioxide from by-product gases generated in steelworks by chemical absorption, characterized in that low-grade exhaust heat of 500°C or less generated in the steelworks is utilized in the process of absorbing carbon dioxide from the gas with a chemical absorption solution and then heating the chemical absorption solution to separate the carbon dioxide." Patent Document 2 also describes a regenerative co-flow vertical shaft furnace (limestone calciner) that uses an oxidizing mixed gas, in which a portion of the recovered exhaust gas is mixed with high-concentration oxygen, instead of combustion air, to increase the CO2 concentration in the exhaust gas and improve the efficiency of CO2 utilization and capture.

[0005] One method for effectively utilizing CO2 is methanation, which uses CO2 as a raw material. For example, Patent Document 3 describes a method in which CO2 contained in combustion exhaust gas is brought into contact with a CO2 absorbent, which is then heated to extract a gas mainly composed of CO2, to which hydrogen is added, which is then passed through a desulfurizer to remove sulfur compounds, and further hydrogen is added before passing through a methanation catalyst to convert it into methane. Patent Document 4 describes a quicklime production system in which CO2-containing exhaust gas and hydrogen gas produced in a limestone calciner are introduced into a methane generator, and the methane produced is recycled and reused as a thermal energy source for the limestone calciner. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-292298 [Patent Document 2] Special Publication No. 2023-533226 [Patent Document 3] Japanese Patent Application Publication No. 2019-172595 [Patent Document 4] Japanese Patent Publication No. 2022-96876 Summary of the Invention [Problem to be solved by the invention]

[0007] Most current CO2 emission reduction technologies are based on the separation and capture of CO2 from exhaust gases, and there are almost no technologies that attempt to prevent CO2 from being generated. Furthermore, the costly method of synthesizing methane from separated and captured CO2 and recycling it as fuel for calcining limestone does not seem to be sufficient in terms of efficiency and economy.

[0008] In view of the above problems, the present invention aims to provide a highly efficient limestone calcination system that can drastically suppress the generation of CO2 derived from fuel gas and enables the reuse of CO2 derived from raw stones, which is inevitably generated in principle. [Means for solving the problem]

[0009] To solve the above problems, the present inventors conducted extensive research and discovered the following. The present inventors conducted extensive research into how to realize an efficient limestone calciner that can recycle exhaust gas as a starting fuel. Considering that 1 mole of methane has a higher lower heating value than the 4 moles of hydrogen required to synthesize 1 mole of methane, it seems more advantageous to reduce CO with hydrogen to convert it into CO, which has a higher lower heating value than hydrogen, and then recycle the CO as fuel, rather than spending money to synthesize methane from separated and captured CO and recycle it as fuel for limestone calcination. Therefore, the present inventors discovered that exhaust gas can be efficiently recycled by using the so-called reverse water-gas shift reaction (CO + H → CO + H O).

[0010] Exhaust gas generated in a limestone calciner generally contains CO2 derived from the combustion of fuel gas and the calcination of limestone, H2O (water vapor) derived from the combustion of fuel gas, and nitrogen and oxygen derived from air, etc. For example, the exhaust gas from a limestone calciner that uses a mixed gas containing H2, CO, CO2, and N2, and optionally CH4 and C2H4, with the balance being unavoidable impurities (hereinafter also referred to as "M gas") as fuel gas, and air as the combustion-supporting gas and cooling gas, contains CO2 derived from the thermal decomposition of limestone and the combustion of M gas, N2 derived from air and M gas, H2O (water vapor) derived from the combustion of M gas, and O2 derived from the air. The inventors have conceived the idea of ​​adding H gas to such exhaust gas to cause the reverse water gas shift reaction, thereby converting the CO in the exhaust gas into CO to produce reformed exhaust gas, and then removing the water vapor contained in the reformed exhaust gas to produce dehumidified reformed exhaust gas, which can be recycled and reused as fuel gas.

[0011] The present invention, which was completed based on the above findings, has the following gist and configuration. [1] A furnace body having a calcination chamber in which a fuel gas containing CO and H2 is burned in the presence of a combustion-supporting gas to thermally decompose and calcinate limestone contained therein to produce quicklime; a limestone supply passage for supplying the limestone to the calcination chamber; a quicklime discharge passage for discharging the quicklime from the burning chamber; a fuel gas flow path for supplying the fuel gas to the firing chamber; a combustion-supporting gas flow path for supplying the combustion-supporting gas to the firing chamber; a reformer that adds H gas to exhaust gas containing CO and H O generated by the reverse water-gas shift reaction and unreacted H, CO, and H O, and that is generated by the combustion of the fuel gas and the thermal decomposition of the limestone, thereby obtaining a reformed exhaust gas containing CO and H O generated by the reverse water-gas shift reaction and unreacted H, CO, and H O; a dehumidifier that dehumidifies the reformed exhaust gas generated by the reformer to obtain a dehumidified reformed exhaust gas containing CO, H, and CO; a dehumidified reformed exhaust gas flow path that guides at least a portion of the dehumidified reformed exhaust gas discharged from the dehumidifier to the fuel gas flow path; and supplying at least a portion of the dehumidified reformed exhaust gas as the fuel gas to the calcination chamber via the fuel gas flow path.

[0012] [2] An exhaust gas circulation type limestone calcination system as described in [1] above, which has an O2 gas supply device connected to the combustion-supporting gas flow path, and the combustion-supporting gas supplied to the calcination chamber via the combustion-supporting gas flow path consists of O2 gas.

[0013] [3] An initial fuel gas supply device connected to the fuel gas flow path, In the initial stage of operation, an initial fuel gas having a low heating value capable of calcining all of the limestone contained in the calcination chamber is supplied as the fuel gas from the initial fuel gas supply device to the calcination chamber through the fuel gas flow path; Thereafter, in the reformer, the amount of H2 gas added is adjusted so that the dehumidified reformed exhaust gas has a lower heating value equal to the lower heating value of the initial fuel gas, and only the dehumidified reformed exhaust gas is recycled and used as the fuel gas. [1] or [2] The exhaust gas circulation type limestone calcination system described in the above [1] or [2].

[0014] [4] An exhaust gas circulation type limestone calcination system according to any one of [1] to [3] above, in which when the amount of CO2 in the exhaust gas is divided into A produced by the combustion of the fuel gas and B produced by the thermal decomposition of the limestone, the amount of the dehumidified reformed exhaust gas A / (A+B) is supplied to the calcination chamber as the fuel gas.

[0015] [5] The furnace body is a Mertz furnace in which a combustion side shaft and a heat storage side shaft are connected, and the preheating zone, the firing zone, and the cooling zone of the combustion side shaft constitute the firing chamber; (I) supplying the H gas from the bottom of the heat-storage-side shaft to the inside, thereby causing the inside of the heat-storage-side shaft to function as the reformer; and (II) supplying the H gas from the bottom of the combustion-side shaft to the inside, thereby causing the cooling zone of the combustion-side shaft and the inside of the heat-storage-side shaft to function as the reformer, The exhaust gas circulation type limestone calcination system according to any one of [1] to [4] above, wherein the reformed exhaust gas generated in the reformer is discharged from the top of the heat storage side shaft and supplied to the dehumidifier.

[0016] [6] An exhaust gas circulation type limestone burning system according to [5] above, in which cooling air is not supplied from the bottom of the combustion side shaft to the inside and from the bottom of the heat storage side shaft to the inside. [Effects of the Invention]

[0017] The exhaust gas circulation type limestone calcination system of the present invention can drastically suppress the generation of CO2 derived from fuel gas, and also makes it possible to reuse CO2 derived from raw stones, which is unavoidable in principle to be generated. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram showing an example of a conventional limestone calcination system 200, the amount of limestone charged during operation, and the flow rates of fuel gas (M gas), combustion-supporting gas (air), cooling gas (air), and exhaust gas. [Figure 2] 1 is a graph showing the ratio of CO and CH4 in a C-containing gas relative to the amount of hydrogen input in Example 1 of the present invention. [Figure 3] 1 is a graph showing the total calorific value of the dehumidified reformed exhaust gas versus the amount of hydrogen input in Example 1 of the present invention. [Figure 4] FIG. 1 is a diagram showing an example of an exhaust gas circulation type limestone calcination system 100 according to Example 1 of the present invention, the amount of limestone input during operation, and the flow rates of fuel gas (dehumidified reformed exhaust gas), combustion-supporting gas (oxygen), cooling gas (hydrogen), and dehumidified reformed exhaust gas. [Figure 5] 1 is a graph showing the transition of fuel gas components accompanying the circulating use of dehumidified reformed exhaust gas in Example 1 of the present invention. [Figure 6] 1 is a graph showing the transition of the amount of fuel gas and the amount of oxygen required with the circulating use of dehumidified reformed exhaust gas in Example 1 of the present invention. [Figure 7] FIG. 10 is a diagram showing an example of circulating dehumidified reformed exhaust gas and utilizing surplus dehumidified reformed exhaust gas in a conventional furnace according to Example 2 of the present invention. [Figure 8] FIG. 10 is a diagram showing an example of circulating dehumidified reformed exhaust gas and utilizing surplus reformed exhaust gas within the premises according to Example 3 of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Before describing the exhaust gas circulation type limestone calcination system according to one embodiment of the present invention, a conventional limestone calcination system 200, which is the premise of the system, will be described with reference to FIG.

[0020] 1 shows a limestone calcination system 200 using a conventional, general Mertz kiln. The limestone calcination system 200 has a furnace body 10, which is a Mertz kiln. The furnace body 10 is composed of a combustion-side shaft 12 and a heat-storage-side shaft 14, which are connected by a joint 16 located at the center of their respective heights. The interior of the combustion-side shaft 12 forms a calcination chamber 12A, and in this calcination chamber 12A, a fuel gas is combusted in the presence of a combustion-supporting gas to thermally decompose and calcinate the limestone contained therein into quicklime.

[0021] The reaction formula for converting limestone into quicklime by thermal decomposition and calcination is as follows: CaCO3+heat→CaO+CO2

[0022] The reaction formula for pyrolyzing and calcining dolomite to produce lightly calcined dolomite is as follows: CaMg(CO3)2+heat→CaO·MgO+2CO2

[0023] Limestone is supplied as needed from the hopper 20 through a limestone supply passage 22 to the inside of each shaft from the top 12C of the combustion-side shaft 12 and the top 14C of the thermal storage-side shaft 14. The quicklime obtained by burning the limestone is discharged to the outside of the furnace body 10 through a quicklime discharge passage 30 located at the bottom of the furnace body 10.

[0024] The fuel gas flow path 40 extending from the initial fuel gas supply device 44 branches into a first fuel gas flow path 40A and a second fuel gas flow path 40B, with the first fuel gas flow path 40A terminating inside the combustion-side shaft 12 and the second fuel gas flow path 40B terminating inside the thermal-storage-side shaft 14. In Fig. 1, the first fuel gas shutoff valve 42A provided in the first fuel gas flow path 40A is open, and the second fuel gas shutoff valve 42B provided in the second fuel gas flow path 40B is closed. Thus, fuel gas is supplied from the initial fuel gas supply device 44 through the fuel gas flow path 40 (from its midpoint to the first fuel gas flow path 40A) to the inside of the combustion-side shaft 12, i.e., to the firing chamber 12A.

[0025] Similarly, the combustion supporting gas flow path 50 extending from the combustion air blower 54 branches into a first combustion supporting gas flow path 50A and a second combustion supporting gas flow path 50B. The first combustion supporting gas flow path 50A is connected to the top 12C of the combustion-side shaft 12, and the second combustion supporting gas flow path 50B is connected to the top 14C of the regenerator-side shaft 14. In FIG. 1, the first combustion supporting gas shutoff valve 52A provided in the first combustion supporting gas flow path 50A is open, and the second combustion supporting gas shutoff valve 52B provided in the second combustion supporting gas flow path 50B is closed. Therefore, air as the combustion supporting gas is supplied from the combustion air blower 54 through the combustion supporting gas flow path 50 (from the first combustion supporting gas flow path 50A to the top 12C of the combustion-side shaft 12) to the inside, i.e., the firing chamber 12A.

[0026] In this way, in the calcination chamber 12A, the tip of the first fuel gas flow path 40A functions as a burner, and the fuel gas injected from the tip burns in the presence of a combustion-supporting gas (air) supplied from above. The combustion heat generated here causes the limestone to thermally decompose and calcinate, turning into quicklime.

[0027] A cooling gas flow path 60 extending from the cooling air blower 62 is connected to the bottom 12B of the combustion-side shaft 12 and the bottom 14B of the thermal storage-side shaft 14. Therefore, air as a cooling gas is supplied from the bottom 12B of the combustion-side shaft 12 and the bottom 14B of the thermal storage-side shaft 14 via the cooling gas flow path 60 extending from the cooling air blower 62 to the inside of each shaft.

[0028] With these fuel gas, combustion-supporting gas, and cooling gas supply systems, the firing chamber 12A is composed of, from top to bottom, a preheating zone 12A1, a firing zone 12A2, and a cooling zone 12A3. The preheating zone 12A1 is the region from the top 12C of the combustion-side shaft 12 to the tip (burner) of the first fuel gas flow path 40A. The firing zone 12A2 is the region from the burner to the lower end of the joint 16. The cooling zone 12A3 is the region from the lower end of the joint 16 to the bottom 12B of the combustion-side shaft 12.

[0029] Limestone supplied into the calcination chamber 12A from the top 12C of the combustion-side shaft 12 is preheated while passing through the preheating zone 12A1, and is thermally decomposed and calcined while passing through the calcination zone 12A2 to produce quicklime. The resulting quicklime is cooled while passing through the cooling zone 12A3 and discharged from the quicklime discharge path 30 located at the bottom of the furnace body 10. The above-mentioned pyrolysis reaction proceeds at temperatures above 900°C, but it is preferable to accelerate the reaction in the calcination zone 12A2 by setting the temperature at 1000 to 1100°C.

[0030] Exhaust gas generated by the combustion of fuel gas and the thermal decomposition of limestone is cooled as it merges with the cooling gas flowing from the cooling zone 12A3, and moves to the thermal-storage-side shaft 14 via the joint 16. The exhaust gas that has moved to the thermal-storage-side shaft 14 merges with the cooling gas flowing from the bottom 14B of the thermal-storage-side shaft 14, and is further cooled in the process of preheating (storing heat) the limestone contained in the thermal-storage-side shaft 14, and is discharged to the outside from the top 14C of the thermal-storage-side shaft 14 via the exhaust gas flow path 70. The exhaust gas that passes through the exhaust gas flow path 70 is introduced into the exhaust gas dust collector 72, where dust is removed, and then emitted into the atmosphere.

[0031] In FIG. 1, the left side of the pair of furnace bodies is shown as the combustion-side shaft 12, and the right side as the regenerator-side shaft 14. However, in a Mertz furnace, this is reversed at regular intervals (e.g., approximately 10 to 20 minutes). Specifically, by closing the first fuel gas shutoff valve 42A and opening the second fuel gas shutoff valve 42B, fuel gas is supplied to the interior of the right furnace body via the fuel gas flow path 40 (from its middle to the second fuel gas flow path 40B). Furthermore, by closing the first combustion-supporting gas shutoff valve 52A and opening the second combustion-supporting gas shutoff valve 52B, air as the combustion-supporting gas is supplied to the interior of the right furnace body via the combustion-supporting gas flow path 50 (from its middle to the second combustion-supporting gas flow path 50B). In this way, the tip of the second fuel gas flow path 40B serves as a burner, allowing the right furnace body to serve as the combustion-side shaft, and the left furnace body to serve as the regenerator-side shaft. In this way, by alternately burning fuel in two furnace bodies, waste heat is recovered and preheating and soaking are carried out effectively, resulting in high thermal efficiency and the production of high-quality quicklime.

[0032] The amount of limestone fed from the hopper 20 may be set appropriately depending on the operating capacity, such as the size of the furnace body 10. For example, the amount of limestone fed can be set within a range of 5 to 16 t / h. The amount of quicklime discharged from the quicklime discharge conduit 30 differs depending on whether the raw material is limestone or dolomite. When the raw material is limestone, the amount of quicklime discharged is approximately 0.56 times the amount of limestone fed.

[0033] The fuel gas supplied from the initial fuel gas supply device 44 is a mixed gas (M gas) containing H, CO, CO, and N, and optionally CH and CH, with the remainder being unavoidable impurities. The combustion components in M ​​gas are H, CO, CH, and CH, and the following reactions occur when each is completely combusted: 2H2+O2→2H2O 2CO+O2→2CO2 CH4+2O2→CO2+2H2O C2H4+3O2→2CO2+2H2O

[0034] The component composition of the M gas is not particularly limited, but may include, by volume, 2-56% H2, 7-62% CO, 2-22% CO2, 5-51% N2, 0-26% CH4, and 0-3% C2H4, with the remainder consisting of unavoidable impurities of 0.5% or less. The temperature of the fuel gas supplied from the initial fuel gas supply device 44 is not particularly limited, but may be 3-60°C. The flow rate of the fuel gas supplied from the initial fuel gas supply device 44 is set so that the pyrolysis and calcination of limestones are completed by combustion of the fuel gas. Once the composition of the fuel gas is determined, its lower heating value is also determined. Therefore, an appropriate flow rate of the fuel gas is determined depending on the amount of limestones charged and the composition of the fuel gas. As an example, for the amount of limestone charged described above and the composition of the M gas shown in Table 1, the flow rate of the M gas supplied from the initial fuel gas supply device 44 is 950-2600 Nm 3 / h.

[0035] In the conventional example shown in FIG. 1, the combustion-supporting gas is air. The temperature of the air is not particularly limited, but may be 3 to 40°C. The flow rate of the air as the combustion-supporting gas is appropriately set by determining the theoretical amount of air required for complete combustion of the fuel gas according to the flow rate of the fuel gas. As an example, in the case of the flow rate of the aforementioned M gas, the flow rate of the air as the combustion-supporting gas is 2500 to 7000 Nm 3 / h.

[0036] In the conventional example shown in FIG. 1, the cooling gas is air. The temperature of the air is not particularly limited, but may be 3 to 40°C. From the viewpoint of being able to cool the quicklime, not damaging the transport system, and maintaining the furnace body temperature, the flow rate of the air used as the cooling gas is preferably set to about 5 / 7 of the flow rate of the air used as the combustion-supporting gas, which correlates with the calorific value of the exhaust gas. As an example, in the case of the flow rate of the air used as the combustion-supporting gas described above, the flow rate of the air used as the cooling gas is 1800 to 5000 Nm 3 / h.

[0037] In the conventional example shown in Fig. 1, the exhaust gas discharged to the outside through the exhaust gas flow path 70 has a composition consisting of CO2 produced by the combustion of fuel gas and the thermal decomposition of limestone, O2 derived from air used as a cooling gas, H2O (moisture) produced by the combustion of the fuel gas, and N2 derived from air used as a combustion-supporting gas and a cooling gas, and the fuel gas. The temperature of the exhaust gas discharged to the outside through the exhaust gas flow path 70 is preferably within a range of 70 to 120°C. In the conventional example, the exhaust gas containing CO2 was released into the atmosphere.

[0038] An exhaust gas circulation type limestone calcination system 100 according to one embodiment of the present invention will now be described with reference to Fig. 4. The exhaust gas circulation type limestone calcination system 100 according to this embodiment is intended to reform and dehumidify exhaust gas containing CO2, which would have been released into the atmosphere in conventional systems, to produce "dehumidified reformed exhaust gas," which is then recycled and reused as fuel gas.

[0039] The following describes the device configuration and operating conditions that differ from those of the conventional example shown in Figure 1. The other device configurations and operating conditions are the same as those of the conventional example shown in Figure 1, so the previous explanations will be used and repeated explanations will be omitted.

[0040] The flue gas circulation type limestone calcination system 100 has an O2 gas supply device 56 instead of the combustion air blower 54 shown in FIG. 1. During operation, O2 gas as a combustion-supporting gas is supplied from the O2 gas supply device 56 through the combustion-supporting gas flow path 50 (which then becomes the first combustion-supporting gas flow path 50A) extending from the O2 gas supply device 56 to the top 12C of the combustion-side shaft 12 and into the interior, i.e., the calcination chamber 12A. As described above, in this embodiment, the combustion-supporting gas is preferably O2 gas. If the combustion-supporting gas is air, as in the conventional example, the flue gas will contain a large amount of N2 derived from the air as the combustion-supporting gas. As a result, the dehumidified reformed flue gas also contains N2, which is not a combustion component. During the process of circulating the dehumidified reformed flue gas inside the furnace body 10, N2 derived from the air accumulates in the furnace, significantly reducing the calorific value of the dehumidified reformed flue gas per unit volume. By using O2 gas (pure oxygen) as the combustion-supporting gas, the N2 gas derived from the air used as the combustion-supporting gas is not contained in the dehumidified reformed exhaust gas. This ensures the calorific value of the dehumidified reformed exhaust gas and enables the continuous burning of limestone.

[0041] The temperature of the O2 gas as the combustion supporting gas is not particularly limited, but may be 3 to 40°C. The flow rate of the O2 gas as the combustion supporting gas is appropriately set by determining the theoretical amount of oxygen (approximately 21% of the theoretical amount of air) required for complete combustion of the fuel gas according to the flow rate of the fuel gas. As an example, the flow rate of the O2 gas as the combustion supporting gas is 530 to 1500 Nm 3 / h.

[0042] The exhaust gas circulation type limestone calcination system 100 also has an H2 gas flow path 80 and an H2 gas supply device 82 instead of the cooling gas flow path 60 and the cooling air blower 62 shown in Fig. 1. As an example, the H2 gas flow path 80 extending from the H2 gas supply device 82 is connected to the bottom 12B of the combustion side shaft 12 and the bottom 14B of the thermal storage side shaft 14. During operation, H2 gas is supplied from the H2 gas supply device 82 via the H2 gas flow path 80 to the inside of each shaft from the bottom 12B of the combustion side shaft 12 and the bottom 14B of the thermal storage side shaft 14. The H2 gas has the following two roles.

[0043] The primary role of H2 gas is to reform the exhaust gas produced by the combustion of fuel gas and the thermal decomposition of limestone, producing reformed exhaust gas. Specifically, CO2 and H2 produced by the combustion of fuel gas and the thermal decomposition of limestone are converted into CO and H2O by the reverse water-gas shift reaction. This converts the CO2 in the exhaust gas into CO, a combustion component. In this embodiment, the cooling zone 12A3 of the combustion-side shaft 12 and the interior of the thermal storage-side shaft 14 function as a reformer 90 that converts the exhaust gas into reformed exhaust gas. Reverse water gas shift reaction: CO2 + H2 → CO + H2O (endothermic reaction)

[0044] Because the upstream stage of the reformer 90 (specifically, the cooling zone 12A3 inside the combustion-side shaft 12) is at a high temperature of 1000 to 1100°C, thermal fluctuations may be greater than the activation energy of the reverse water-gas shift reaction. Therefore, the reverse water-gas shift reaction proceeds to some extent even without a catalyst. However, from the perspective of promoting the reaction, it is preferable to use a catalyst to lower the activation energy. Known catalysts for the reverse water-gas shift reaction, such as nickel-based or precious metal-based catalysts, may be used. The location of the catalyst is not particularly limited, but, as an example, the catalyst can be installed at the joint 16 connecting the combustion-side shaft 12 and the regenerator-side shaft 14.

[0045] The second role of H gas is to replace the cooling gas, which was air in the conventional system. Exhaust gas generated by the combustion of fuel gas and the thermal decomposition of limestone is cooled as it merges with H gas flowing from the cooling zone 12A3 as a cooling gas, and moves to the thermal-storage-side shaft 14 via the joint 16. The exhaust gas that has moved to the thermal-storage-side shaft 14 merges with H gas flowing from the bottom 14B of the thermal-storage-side shaft 14, and is further cooled in the process of preheating (storing heat) the limestone contained in the thermal-storage-side shaft 14. The exhaust gas is then discharged to the outside from the top 14C of the thermal-storage-side shaft 14 via the exhaust gas flow path 70. In addition to cooling due to heat exchange between the high-temperature exhaust gas and the low-temperature H gas, cooling also occurs due to the endothermic reverse water-gas shift reaction.

[0046] In this embodiment, because H gas functions as a cooling gas, there is no need to supply cooling air from the bottom 12B of the combustion-side shaft 12 to the interior and from the bottom 14C of the heat-storage-side shaft 14 to the interior, as in the conventional example. As a result, N derived from the air used as a cooling gas is not contained in the dehumidified reformed exhaust gas. This ensures the calorific value of the dehumidified reformed exhaust gas and enables continuous calcination of limestone. Furthermore, O derived from the air used as a cooling gas is not contained in the exhaust gas. If O is contained in the exhaust gas, it will react with a portion of the added H gas to produce water (H O), which is undesirable because it reduces the efficiency of the reverse water-gas shift reaction. However, this does not occur in this embodiment.

[0047] The temperature of the exhaust gas drops in the later stages of the reformer 90 (specifically, in the upper part inside the heat-storage-side shaft 14). If the temperature of the exhaust gas falls below 700°C, a methanation reaction occurs as a side reaction, and some of the CO2 in the exhaust gas may be converted into CH4, a combustion component. However, since the methanation reaction is an exothermic reaction and low temperatures are advantageous, it is unlikely that the thermal fluctuations will be greater than the activation energy. For this reason, there is a possibility that the reaction will hardly proceed without a catalyst. Methanation reaction: CO2 + 4H2 → CH4 + 2H2O (exothermic reaction)

[0048] As described above, the reformed exhaust gas discharged from the reformer 90 to the exhaust gas flow path 70 via the top portion 14C of the thermal-storage-side shaft 14 contains the following components. (A) CO and HO produced by the reverse water-gas shift reaction (B) CH4 and HO produced by the methanation reaction (if side reactions occur) (C) Unreacted H2, CO2, and H2O The unreacted H2 is the unreacted portion of the added H2 gas. The unreacted CO2 is the unreacted portion of CO2 produced by the combustion of the fuel gas and the thermal decomposition of limestone. The unreacted H2O is the H2O produced by the combustion of the fuel gas. The component composition of the reformed exhaust gas is not particularly limited, but may be, in volume %, 15-60% H2, 5-20% CO, 0-5% CH4, 5-40% CO2, and 25-35% H2O.

[0049] The flue gas circulation type limestone calcination system 100 has a reformed flue gas flow path 92 extending from the flue gas dust collector 72, a dehumidifier 94 to which the reformed flue gas flow path 92 is connected, and a dehumidified reformed flue gas flow path 96 extending from the dehumidifier 94. The reformed flue gas that has passed through the flue gas dust collector 72 flows through the reformed flue gas flow path 92 and is introduced into the dehumidifier 94, where the reformed flue gas is dehumidified to obtain dehumidified reformed flue gas. The dehumidified reformed flue gas is reformed flue gas from which moisture (H2O) has been removed, and contains the following components: (I) CO produced by the reverse water gas shift reaction (II) CH4 produced by the methanation reaction (if a side reaction occurs) (III) Unreacted H2 and CO2 Of these, H2 and CO (and CH4 if a side reaction occurs) can be combustion components. The component composition of the dehumidified reformed exhaust gas is not particularly limited, but can be, by volume, 20-70% H2, 10-30% CO, 0-10% CH4, and 10-50% CO2.

[0050] The dehumidified reformed exhaust gas flow path 96 is connected to the fuel gas flow path 40. Therefore, at least a portion of the dehumidified reformed exhaust gas discharged from the dehumidifier 94 is guided from the dehumidified reformed exhaust gas flow path 96 to the fuel gas flow path 40 and supplied as fuel gas to the firing chamber 12A via the fuel gas flow path 40. In this embodiment, by reforming and dehumidifying the exhaust gas containing CO2 in this way to produce "dehumidified reformed exhaust gas," and circulating and reusing this as fuel gas, CO2 emissions can be effectively reduced.

[0051] The temperature of the H2 gas to be added is not particularly limited, but can be within the range of 3 to 40°C.

[0052] The flow rate of the H2 gas to be added is preferably determined in consideration of the operating conditions as follows.

[0053] First, in the initial stage of operation, as in the conventional example shown in Figure 1, an initial fuel gas having a low heating value capable of calcining all of the limestone contained in the calcination chamber 12A is supplied from the initial fuel gas supply device 44 to the calcination chamber 12A via the fuel gas flow path 50.

[0054] Thereafter, in the reformer 90, the amount of H2 gas added is adjusted so that the dehumidified reformed exhaust gas has a lower heating value equal to that of the initial fuel gas. Details will be explained in the Examples section with reference to FIG. 3, but the lower heating value of the dehumidified reformed exhaust gas varies depending on the amount of H2 gas added. Therefore, by setting the flow rate of H2 gas added so that the dehumidified reformed exhaust gas has a lower heating value equal to that of the initial fuel gas, it is possible to continue the thermal decomposition and calcination of limestone using only the dehumidified reformed exhaust gas. Therefore, it is possible to recycle and use only the dehumidified reformed exhaust gas as fuel gas without additionally supplying fuel gas (M gas) from the initial fuel gas supply device 44. As an example, the flow rate of H2 gas added is 7000 to 8000 Nm 3 / h.

[0055] As an example, as shown in Figure 1, the flow rate of the initial fuel gas (M gas) is set to 2120 Nm 3 / h, the first hour after the start of operation is 2120Nm 3 The initial fuel gas is supplied into the firing chamber 12A at a constant rate. After that, the dehumidified reformed exhaust gas is added midway and supplied as fuel gas into the firing chamber 12A. After one hour has passed since the start of operation, the initial fuel gas (M gas) is shut off and only the dehumidified reformed exhaust gas is circulated as fuel gas.

[0056] Here, the CO2 in the exhaust gas is classified as CO2 produced by the combustion of fuel gas and CO2 produced by the thermal decomposition of limestone. If the entire amount of dehumidified reformed exhaust gas is recycled, CO2 produced from the raw limestone (ore) accumulates in the exhaust gas every time new limestone (ore) is fired, and eventually the exhaust gas reaches its limit of accumulation in the furnace (gas balance is disrupted). Therefore, when the amount of CO2 in the exhaust gas is divided into A produced by the combustion of fuel gas and B produced by the thermal decomposition of limestone, it is preferable to supply A / (A+B), the amount of dehumidified reformed exhaust gas, to the firing chamber as fuel gas. This maintains gas balance, allowing continuous firing of limestone while circulating only the dehumidified reformed exhaust gas as fuel gas. Specifically, as shown in Figure 4, a surplus gas flow path 98 branches off from the middle of the dehumidified reformed exhaust gas flow path 96, and the amount of dehumidified reformed exhaust gas A / (A+B) is supplied from the dehumidified reformed exhaust gas flow path 96 to the firing chamber 12A via the fuel gas flow path 40, and the remaining B / (A+B) can be effectively utilized by being used as fuel gas for another furnace or sold externally via the surplus gas flow path 98 from the dehumidified reformed exhaust gas flow path 96.

[0057] According to the flue gas circulation type limestone calcination system 100 of this embodiment described above, the dehumidified reformed flue gas obtained by reforming and dehumidifying flue gas can be recycled and reused as fuel gas, eliminating the need to purchase fuel gas (M gas) and drastically reducing CO2 emissions from fuel gas. Furthermore, surplus dehumidified reformed flue gas can be effectively utilized by using it as fuel gas for other furnaces or by selling it externally, making it possible to reuse CO2 derived from raw ore, which is unavoidable in principle. In this way, all CO2 emitted from the furnace body 10 can be reused.

[0058] 4 shows an example in which the H gas flow path 80 is connected to both the bottom 12B of the combustion-side shaft 12 and the bottom 14B of the thermal-storage-side shaft 14, but it may be connected to either one. That is, it may be possible to either (I) supply H gas from the bottom 14B of the thermal-storage-side shaft 14 to the interior, or (II) supply H gas from the bottom 12B of the combustion-side shaft 12 to the interior. In the case of (I), the interior of the thermal-storage-side shaft 14 functions as a reformer 90, and in the case of (II), the cooling zone 12A3 of the combustion-side shaft 12 and the interior of the thermal-storage-side shaft 14 function as the reformer 90.

[0059] 4, an embodiment in which the furnace body 10 is a Maertz furnace has been described, but in the present invention, the type of furnace body 10 is not limited to a Maertz furnace. The furnace body 10 may be, for example, a rotary kiln, which is a horizontal furnace, or may be any known or arbitrary furnace for calcining limestone, such as a Maertz furnace (regenerative type), a shaft furnace, a top furnace (counter-flow type), or a Beckenbach furnace (double-cylinder type) as a vertical furnace.

[0060] To utilize the reverse water-gas shift reaction, it is necessary to secure a reaction region (reformer 90 shown in FIG. 4) in the exhaust route. For example, in the case of a rotary kiln, a reformer (reaction vessel) for the reverse water-gas shift reaction is installed in the exhaust route, and hydrogen is introduced from upstream within the reformer. When the furnace body 10 is a Mertz furnace as in this embodiment, this is preferable because the reaction region (reformer 90) can be secured within the furnace, as described above. [Example]

[0061] [Comparative Example (Conventional Example)] Figure 1 shows an example of the structure of a Maerz furnace, the amount of limestone charged during operation, and the flow rates of fuel gas (M gas), combustion-supporting gas (air), cooling gas (air), and exhaust gas. A mixed gas (M gas) with the component composition shown in Table 1 is used as the fuel gas. From this component composition, the lower heating value of M gas is 2754 kcal / Nm 3 Air is used as the combustion supporting gas and cooling gas.

[0062] The fuel gas (M gas), combustion support gas (combustion air), and cooling gas (cooling air) input rates, as well as the exhaust gas generation rate, when raw limestone is input at a rate of 11.1 t / h, are shown in Table 2. The fuel gas (M gas) input rate is set based on the lower heating value calculated from the amount of limestone input and the M gas's component composition. The combustion support gas (combustion air) input rate corresponds to the theoretical air volume required for complete combustion of the M gas. The cooling gas (cooling air) input rate is set to approximately 5 / 7 of the combustion support gas (combustion air) input rate. The calcination temperature of limestone is typically around 1000°C. The exhaust gas generation rate and component composition when calcined at 1000°C were calculated using thermodynamic equilibrium calculation software and thermodynamic database Fact Sage 8.2 from Computational Mechanics Research Center, Inc., and are shown in Tables 2 and 3, respectively. The exhaust gas, including CO2, generated during the calcination process is released into the atmosphere.

[0063] [Table 1]

[0064] [Table 2]

[0065] [Table 3]

[0066] [Example 1] As can be seen from the exhaust gas composition (Table 3) described in the comparative example, when 11.1 tons of limestone per hour is converted into quicklime, the amount of CO2 generated in the exhaust gas is (8.0 + 18 2) / 8.0 = 3.28 times the amount of CO2 generated by burning M gas. If this CO2 is converted into dehumidified reformed exhaust gas and the entire amount is recycled, it is clear that C-containing gas derived from limestone will continue to accumulate in the furnace with each repeated firing, and the accumulation limit will soon be reached. Therefore, it is rational to recycle the reformed exhaust gas equivalent to one furnace of M gas, and use the reformed exhaust gas equivalent to 2.28 furnaces of limestone-derived M gas in another furnace. The reformed exhaust gas, which generates a total heat equivalent to 3.28 furnaces of M gas, has a calorific value of 2754 kcal / Nm 3 ×2120Nm 3 This shows that a lower calorific value per hour of 19.15 Gcal / h is required.

[0067] First, the M gas listed in Tables 1 and 2 is completely combusted with oxygen to convert limestone into quicklime at around 1000°C. The theoretical amount of oxygen required for complete combustion is 1184 Nm3, which is 21% of the theoretical amount of air. 3 / h. Using Fact Sage 8.2, the composition and generation rate of exhaust gases when limestone is burned at 1000°C were calculated. The combustion of M gas and the thermal decomposition of limestone resulted in the emission of 5228 Nm3 of the composition shown in Table 4. 3 / h of exhaust gas was generated.

[0068] [Table 4]

[0069] H2 gas is sequentially injected into this exhaust gas from the bottom of the combustion-side shaft and the bottom of the heat storage-side shaft, where cooling air is usually injected. The injection of H2 gas generates CO in the exhaust gas due to the reverse water-gas shift reaction. Because the reverse water-gas shift reaction is an endothermic reaction, when the gas temperature drops below 700°C due to its cooling effect, CH4 may also be generated as a side reaction due to the methanation reaction. Figure 2 is a graph showing the equilibrium ratios of CO and CH4 in the carbon-containing gas versus the amount of hydrogen injected, calculated using Fact Sage 8.2.

[0070] Figure 3 shows the calorific value of these three components (H2, CO, CH4) in the dehumidified reformed exhaust gas plotted against the amount of H2 gas added. 3 It can be seen that the calorific value of the dehumidified reformed exhaust gas reaches the required amount (19.15 Gcal / h) mentioned above at around / h.

[0071] For the reasons already mentioned, the amount of dehumidified reformed exhaust gas recycled as fuel gas is limited to the amount equivalent to the exhaust gas resulting from the combustion of M gas (CO2 composition ratio in Table 3: 8.0 / (8.0+18.2) = 1 / 3.28 of the total amount of dehumidified reformed exhaust gas). An example of the gas balance when this reformed exhaust gas is recycled is shown in Figure 4.

[0072] By distributing the dehumidified reformed exhaust gas in this way, the components and amount of fuel gas and the amount of combustion-supporting gas (oxygen for combustion) rapidly stabilize after one hour, as shown in Figures 5 and 6. The small changes after one hour can be explained by the effect of nitrogen in the starting fuel M gas being distributed outside the furnace during the circulation process.

[0073] [Example 2] If there are 10 Mertz furnaces of the same type, by recycling and using the dehumidified reformed exhaust gas equivalent to the amount of exhaust gas resulting from M-gas combustion in three furnaces and using the remaining dehumidified reformed exhaust gas (equivalent to the amount of exhaust gas resulting from the thermal decomposition of limestone) as an alternative fuel to M-gas in the seventh furnace using the conventional method, it will be possible to reduce the M-gas and CO2 emissions of the 10 furnaces by 30% (Figure 7).

[0074] [Example 3] As shown in Figure 8, if the dehumidified reformed exhaust gas can be recycled in all 10 identical Maerz furnaces, it will be possible to reduce the M gas and CO2 emissions of the 10 furnaces by 30%, and 100% of the CO2 emissions from the 10 furnaces can be reused. Furthermore, the reformed exhaust gas (59,100 Nm3), which has a calorific value 81% of that of M gas, can be recycled. 3 / h can be sold or used in other furnaces on the premises. [Industrial Applicability]

[0075] The exhaust gas circulation type limestone calcination system of the present invention can be used in the lime industry, which produces quicklime by thermally decomposing and calcining limestone. [Explanation of symbols]

[0076] 100 Exhaust gas circulation type limestone burning system 200 Limestone Calcination System 10 Furnace body (Mertz furnace) 12 Combustion side shaft 12A Firing Room 12A1 Pre-hot zone 12A2 firing zone 12A3 Cooling Zone 12B Bottom of combustion side shaft 12C Top of combustion side shaft 14 Heat storage side shaft 14B Bottom of the heat storage side shaft 14C Top of the heat storage side shaft 16 Joint 20 Hopper 22 Limestone supply route 30 Quicklime discharge channel 40 fuel gas flow path 40A First fuel gas flow path 40B Second fuel gas flow path 42A First fuel gas shutoff valve 42B Second fuel gas shutoff valve 44 Initial fuel gas supply device 50 Combustion-supporting gas flow path 50A First combustion-supporting gas flow path 50B Second combustion-supporting gas flow path 52A First combustion-supporting gas shutoff valve 52B Second combustion-supporting gas shutoff valve 54 Combustion air blower 56 O2 gas supply device 60 Cooling gas flow path 62 Cooling air blower 70 Exhaust gas flow path 72 Exhaust gas dust collector 80 H2 gas flow path 82 H2 gas supply device 90 Reformer 92 Reformed exhaust gas flow path 94 Dehumidifier 96 Dehumidified reformed exhaust gas flow path 98 Excess gas flow path

Claims

1. In the presence of a combustion-supporting gas, CO and H ２ a furnace body having a calcination chamber in which a fuel gas containing the above is burned to thermally decompose and calcinate limestone contained therein to produce quicklime; a limestone supply passage for supplying the limestone to the calcination chamber; a quicklime discharge passage for discharging the quicklime from the burning chamber; a fuel gas flow path for supplying the fuel gas to the firing chamber; a combustion-supporting gas flow path for supplying the combustion-supporting gas to the firing chamber; CO produced by the combustion of the fuel gas and the thermal decomposition of the limestones ２ and H ２ In exhaust gas containing O, H ２ By adding gas, CO and H produced by the reverse water gas shift reaction can be ２ O and unreacted H ２ , CO ２ , and H ２ a reformer for obtaining a reformed exhaust gas containing O; The reformed exhaust gas generated in the reformer is dehumidified to remove CO, H ２ , and CO ２ a dehumidifier for obtaining a dehumidified reformed exhaust gas containing the a dehumidified reformed exhaust gas flow path that guides at least a portion of the dehumidified reformed exhaust gas discharged from the dehumidifier to the fuel gas flow path; and supplying at least a portion of the dehumidified reformed exhaust gas as the fuel gas to the calcination chamber via the fuel gas flow path.

2. The O gas connected to the combustion-supporting gas flow path ２ The combustion-supporting gas supplied to the firing chamber through the combustion-supporting gas flow path is O ２ 2. The exhaust gas circulation type limestone burning system according to claim 1, wherein the exhaust gas circulation type limestone burning system comprises a gas.

3. an initial fuel gas supply device connected to the fuel gas flow path; In the initial stage of operation, an initial fuel gas having a low heating value capable of calcining all of the limestone contained in the calcination chamber is supplied as the fuel gas from the initial fuel gas supply device to the calcination chamber through the fuel gas flow path; Then, in the reformer, the H is added to the dehumidified reformed exhaust gas so that the dehumidified reformed exhaust gas has a lower heating value equal to the lower heating value of the initial fuel gas. ２ 3. The exhaust gas circulation type limestone burning system according to claim 1, wherein the amount of gas added is adjusted, and only the dehumidified reformed exhaust gas is circulated and utilized as the fuel gas.

4. CO in the exhaust gas ２ 4. The exhaust gas circulation type limestone calcination system according to claim 3, wherein when the amount of the dehumidified reformed exhaust gas is divided into A produced by combustion of the fuel gas and B produced by thermal decomposition of the limestone, A / (A+B) of the amount of the dehumidified reformed exhaust gas is supplied to the calcination chamber as the fuel gas.

5. The furnace body is a Mertz furnace formed by connecting a combustion-side shaft and a heat-storage-side shaft, and the preheating zone, the firing zone, and the cooling zone of the combustion-side shaft constitute the firing chamber; (I) The H ２ (II) supplying gas from the bottom of the heat-storage-side shaft to the inside of the heat-storage-side shaft to cause the inside of the heat-storage-side shaft to function as the reformer; and ２ supplying gas from a bottom portion of the combustion-side shaft to the inside thereof, thereby causing the cooling zone of the combustion-side shaft and the interior of the heat-storage-side shaft to function as the reformer; 5. The exhaust gas circulation type limestone burning system according to claim 4, wherein the reformed exhaust gas generated in the reformer is discharged from a top of the heat-storage-side shaft and supplied to the dehumidifier.

6. 6. The exhaust gas circulation type limestone burning system according to claim 5, wherein cooling air is not supplied from the bottom of the combustion side shaft to the inside and from the bottom of the heat storage side shaft to the inside.

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