JPH11312527A - Molten carbonate type fuel cell power generation-exhaust gas recovery combined system using by-product gas in production of iron - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas with-global-warming-effect fixing type energy saving system for utilizing carbon dioxide transport ability of a molten carbonate type fuel cell, to recover and fix carbon dioxide concentrated in an anode exhaust gas, while supplying two kinds of by-produced gases in iron production as gases for anode and cathode electrodes of the molten carbonate type fuel cell so as to attain highly efficient power generation. SOLUTION: A coke oven gas COG or its steam-reformed gas out of by-produced gases in iron production is used for gas supplied to an anode, combustion exhaust gas of a blast furnace gas BFG after regulated to a prescribed carbon dioxide/oxygen concentration ratio is used for gas supplied to a cathode, and they are supplied separately to a molten carbonate fuel cell MCFC to attain highly efficient power generation, so that iron production by-product gas effectively utilized type energy saving power generation is realized. Carbon dioxide concentrated in the anode is isolated and recovered with low energy by utilizing carbon dioxide transport ability of the molten carbonate type fuel cell MCFC to be utilized for a raw material for existing chemicals so as to be fixed as chemical substances, thereby cross-industrial type process efficiency ranging over iron production-chemistry is improved, and discharge of gases having global warming effect can be reduced at the same time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for separating, recovering and fixing carbon dioxide, which is regarded as a main working gas for global warming, from a by-product gas of an iron making process. Power is generated efficiently from iron by-product gas, which is mainly used as a fuel source for
It is also an energy-saving process that suppresses the generation of carbon dioxide in the production process of the same substance in the chemical substance manufacturing industry by concentrating the generated carbon dioxide, separating and recovering it with high efficiency, and effectively utilizing it as a chemical raw material.

[0002]

2. Description of the Related Art After two oil shocks, the steel industry has been working on drastic improvement of its energy structure. The company has been working to build a stable and energy-saving structure that is mainly made of coal, and has now established a solid foundation with a total energy efficiency of over 60%. However, there is a strong demand for further energy use efficiency to reduce carbon dioxide emissions from a global perspective that crosses borders between industries, as represented by the Third Conference of the Parties to the Climate Change Framework Convention (COP-3), and crosses national borders. Have been

[0003] The steel industry uses coal as a fuel source and a reducing medium. As can be easily inferred from its chemical composition, the ratio of carbon in the gas used and exhausted is higher than that of petroleum and natural gas. The concentration of carbon dioxide in the exhaust gas when burned as fuel is high. Another characteristic of the process is that hydrogen-rich coke oven gas (COG), which is not produced as a by-product of the average coal combustion exhaust gas composition but is recovered as volatile gas during coke production, and a huge iron ore reduction reaction Gases having different compositions, such as carbon monoxide and carbon dioxide-rich blast furnace gas (BFG), which are released from the blast furnace as the exhaust gas of the reduction reaction, are dispersed and generated. Most of these gases are used alone or mixed as fuel gas for thermal power generation and fuel gas for heating furnaces in steelmaking plants, and the energy recovery efficiency does not necessarily exceed the power conversion efficiency of thermal power generation.

As a method for improving the power conversion efficiency of fuel, the development of combined cycle power generation, fuel cells, and the like have been actively developed. In particular, fuel cells have attracted attention because of the high power generation efficiency that can be achieved in principle. , A phosphoric acid type, a molten carbonate type and a solid oxide type are being developed, and some are in the industrialization stage and large-scale demonstration test stage. When utilizing by-product gas for iron production in a fuel cell as fuel, COG is considered to be suitable because it has a high calorific value and is rich in hydrogen.
It is also described in 166179, JP-A-8-206701 and the like. However, BFG has a low calorific value,
The combustible gas component is mainly composed of carbon monoxide, and is applicable only to molten carbonate type and solid oxide type fuel cells. Furthermore, carbon monoxide has a low concentration and is suitable as a fuel gas for fuel cells. It is estimated that it is difficult to stably maintain the power generation efficiency at a high level considering that it is in the verification stage.
Further, even if carbon monoxide contained in BFG is reformed into hydrogen by a shift reaction, purification of hydrogen is indispensable in order to obtain a low concentration fuel gas, which is disadvantageous in terms of energy efficiency. As described above, iron by-product gas has different suitability as a fuel for a fuel cell depending on its type, and in order to use a plurality of steel by-product gases in the same fuel cell power generation system,
Multi-stage processes such as mixing, reaction, and component concentration adjustment of a plurality of gases are required, and a decrease in energy efficiency is inevitable.

On the other hand, techniques for separating and recovering carbon dioxide from a large-scale carbon dioxide source include chemical absorption methods,
There are adsorption method, membrane separation method, cryogenic separation method, crystallization method, etc.
The concentration of carbon dioxide in the gas to be separated is 30% or less,
In the case of an iron making process, it is not easy to separate and recover carbon dioxide into BFG combustion exhaust gas having a high carbon dioxide content to obtain an industrial gas that can be sold outside.

[0006]

The above problems have been overcome,
Taking advantage of the characteristics of multiple steelmaking by-product gases generated by dispersion, they are individually used in the same power generation system to avoid unnecessary mixing of individual steelmaking by-product gases and to generate compact and highly efficient power generation. B is considered to be a major source of carbon dioxide
By constructing a comprehensive system that recovers carbon dioxide in FG combustion exhaust gas with low energy and high purity and uses it as a raw material for chemical products, it aims to achieve not only steelmaking but also cross-industrial overall energy efficiency improvement. is there.

[0007]

The present invention is characterized by utilizing the high-efficiency power generation characteristics of a fuel cell and selectively moving oxygen and carbon dioxide to the fuel side (anode) by forming carbonate. Focusing on molten carbonate fuel cells, COG and / or carbon monoxide and hydrocarbons contained in COG that are hydrogen-rich and suitable as fuels are shifted,
CO reformed to hydrogen and carbon dioxide by steam reforming reaction
G is supplied to the anode while carbon dioxide rich BF
The present invention provides a high-efficiency MCFC power generation system in which G combustion exhaust gas is supplied as a cathode gas after adjusting the carbon dioxide / oxygen concentration ratio to thereby separately use iron by-product gas in a single fuel cell. Further, as a feature of the fuel cell, there is no gas mixture between the anode and the cathode, and in the case of MCFC, the transport of carbon dioxide to the anode side progresses. Since carbon is concentrated, it is easy to recover carbon dioxide at a high recovery rate and high concentration by the adsorption method. Thus, M
Utilizing the high-efficiency power generation function of CFC and the selective transport function of carbon dioxide, two types of by-product steelmaking gas are used in one battery to obtain power with high efficiency, and the carbon dioxide from exhaust gas with low separation energy. By recovering and using it as a raw material for chemical product production, energy saving in the chemical product production process is also achieved.

The gist of the present invention is as follows.

(1) Molten carbonate fuel cell (MCF)
In the power generation system using C), a plurality of by-product gases generated in the iron-making process are supplied as an anode gas and a cathode gas as MCFC power generation, and carbon dioxide is separated and recovered from the anode exhaust gas. Carbonate fuel cell power generation-exhaust gas recovery combined system.

(2) The combined molten carbonate fuel cell power generation-exhaust gas recovery system according to (1), wherein coke oven gas (COG) is supplied as an anode gas among the by-product gases generated in the iron making process.

(3) Shift CO obtained by reforming carbon monoxide contained in COG into hydrogen and carbon dioxide by a shift reaction
G. The combined power generation and exhaust gas recovery system for molten carbonate fuel cells according to (2) above.

(4) Reformed CO obtained by reforming hydrocarbons contained in COG into hydrogen and carbon dioxide by a steam reforming reaction
G. The combined power generation and exhaust gas recovery system for molten carbonate fuel cells according to (2) above.

(5) When blast furnace gas (BFG) is supplied as a cathode gas among the by-product gases generated in the iron making process, carbon monoxide contained in the BFG is oxidized to carbon dioxide by combustion, and then the gas is supplied. The combined use of a molten carbonate fuel cell power generation system and an exhaust gas recovery system according to (1), wherein oxygen is enriched to form a cathode gas.

(6) Air and / or oxygen are mixed with a gas obtained by burning BFG alone or mixed with another fuel, and the concentration ratio of carbon dioxide / oxygen in the gas is reduced to 1/4.
(5) The combined molten carbonate fuel cell power generation-exhaust gas recovery system according to (5), wherein the system is adjusted to 4/1.

(7) The combined power generation and exhaust gas recovery system for a molten carbonate fuel cell according to (1), wherein carbon dioxide is separated and recovered from the anode exhaust gas using a carbon dioxide pressure swing adsorption method.

(8) Carbon dioxide obtained by synthesizing a chemical using carbon dioxide obtained by the combined use of the molten carbonate fuel cell power generation and exhaust gas recovery system according to (1) to (7). Immobilization method.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings.

FIG. 1 is a conceptual diagram of the system of the present invention.
Of the gas supplied to the cathode side of the molten carbonate fuel cell (MCFC), carbon dioxide and oxygen form carbonates and permeate the electrolyte, react with hydrogen in the fuel supplied to the anode side, and form water. This shows the situation where carbon dioxide is produced. At the same time, the carbon dioxide in the BFG is transported from the cathode side to the anode side and is concentrated in the anode exhaust gas, which facilitates recovery. Further, the recovered carbon dioxide is chemically reacted with hydrogen obtained from a hydrogen source. The flow until immobilization as a substance is shown.

FIG. 2 is a simplified flowchart of the present invention.
The combustion exhaust gas generated when BFG is burned as thermal power generation fuel is used as the MCFC cathode supply gas, and the methane-based hydrocarbons contained in the COG are converted into hydrogen and carbon dioxide by the steam reforming reaction as the anode supply gas. It uses reformed COG that has been reformed and has an increased hydrogen concentration. However, the anode supply gas is not limited to FIG. 2 and can be directly used without steam reforming because COG is a hydrogen-rich gas containing 50% or more of hydrogen. A shift COG containing about 5% of carbon oxide and reforming the carbon monoxide into hydrogen and carbon dioxide by a shift reaction shown below to increase the hydrogen concentration.
It is also possible to use.

The CO + H ₂ O = H ₂ + CO ₂ steam reformer is a device for producing hydrogen by reforming a hydrocarbon mainly composed of methane existing in about 30% of COG, and producing hydrogen by a fuel cell electrode reaction. Water vapor and COG, which are self-supplied in the system by a generated water recovery system, are mixed and heated to convert methane-based hydrocarbons into hydrogen and carbon dioxide. This reaction is shown by the following reaction in the case of methane.

CH ₄ + 2H ₂ O = 4H ₂ + CO ₂ This reaction is an endothermic reaction and it is necessary to supply heat to the reactor. As the heat source, the combustion heat of the fuel gas remaining in the anode exhaust gas is used. The reforming reaction is usually operated at 800 ° C., but the operating temperature of the MCFC is 600 to 70 ° C.
Discharge from the reactor at 0 ° C. (stream). The shift reactor is a device for converting carbon monoxide contained in COG or carbon monoxide in the reforming reactor outlet gas into hydrogen. After leaving the reactor, it is introduced into the fuel cell anode. (Stream).

BFG is a gas containing carbon monoxide, carbon dioxide, and nitrogen as main components, but has a low calorific value and a low hydrogen concentration even when converted to hydrogen by a shift reaction of CO. It is inefficient because it requires hydrogen concentration, and it is currently mainly used as a fuel gas for thermal power generation alone, or mixed with a high calorific value fuel such as natural gas, LPG, or coal for combustion. . In order to utilize carbon dioxide in this flue gas as a working medium for MCFC,
Mixing flue gas with air or oxygen-enriched air or pure oxygen, with a carbon dioxide / oxygen volume concentration ratio of 1
/ 4 to 4/1, preferably 2/3 to 3/1. At this time, carbon dioxide and oxygen ideally form carbonate ions at a volume concentration ratio of 2/1 on the surface of the cathode electrode and move in the electrolyte.
Affected by coexisting gas components. If the carbon dioxide / oxygen volume concentration ratio is less than 1/4, the cathode efficiency is reduced due to excessive oxygen, so that the power generation efficiency is reduced. If the carbon dioxide / oxygen volume concentration ratio is more than 4/1, the recovery efficiency of carbon dioxide is undesirably reduced.

The BFG combustion exhaust gas whose concentration has been adjusted is 3%.
The pressure is raised to the atmospheric pressure (gauge) and MC at 600-700 ° C.
Introduced to the FC anode (stream). stream

[0024]

[Outside 1]

Approximately 70% of the components are consumed and are released to the MCFC anode side as water vapor and carbon dioxide. About 50% of the reaction in the MCFC is converted to electric power as a change in free energy, and the remaining 50% is converted to heat as heat in the MCFC.
Although released into the system, it is recovered as enthalpy of steam by a heat recovery mechanism incorporated in the MCFC, and a part is used as steam supplied to the fuel reformer.

The MCFC is operated at a temperature of about 650 ° C. and a pressure of about 3 atm (gauge), and the anode exhaust gas (stream) is recovered in heat, and after removing moisture (stream), is introduced into the carbon dioxide recovery PSA. Exhaust gas (stream) of carbon dioxide recovery PSA is reformed

[0027]

[Outside 2]

Alternatively, it is partially circulated and used as a cathode supply gas. Further, since the cathode exhaust gas is enriched with nitrogen, it is possible to recover high-purity nitrogen by connecting to a nitrogen recovery plant according to the purpose.

The recovered carbon dioxide can be used as a raw material gas in a production process of an existing chemical product synthesized using carbon dioxide as a raw material. For example, methanol,
Chemicals such as dimethyl ether and urea convert carbon dioxide as a raw material into heavy oil, steam reforming of fossil fuels such as natural gas,
Although it is manufactured by partial oxidation, etc., in the system of the present invention, high-purity carbon dioxide can be obtained with low separation energy, so recovered carbon dioxide can be used as a raw material for these chemicals, and only fossil fuel savings Instead, the energy required for manufacturing these chemicals can be reduced.

[0030]

The present invention will be described below with reference to examples.

Table 1 shows the material balance of this example. In this table, the horizontal columns show the stream numbers shown in FIG. 2, and the vertical columns show the gas components constituting each stream and the flow rates of each stream. The concentration of the gas component constituting each stream is shown in mol%, and the flow rate of each stream is
The ratio is shown assuming that the supply flow rate (stream) in the standard state of COG (0 ° C., 1 atm) is 1.

[0032]

[Table 1]

This embodiment is based on the system operation under the conditions shown in Table 2 below.

[0034]

[Table 2]

The COG is mixed with a portion of the steam recovered from the system as shown in FIG. 2 and introduced into the reformer at 550 ° C. (stream). The heat source of the reformer is supplied from the combustion heat of the residual gas and air after dehumidifying the anode exhaust gas and recovering carbon dioxide.

In the carbon monoxide shift reactor, carbon monoxide in COG or unreacted carbon monoxide in the reformer is converted to hydrogen and carbon dioxide, and introduced in a stream to the MCFC anode. On the other hand, on the MCFC cathode, BFG combustion exhaust gas (stream) burned at an air-fuel ratio of about 1.2 is cooled through a heat exchanger, pre-treated, and then has a carbon dioxide / oxygen concentration ratio of 2/1. And pressurized to 3 atm (gauge) and introduced (stream).
The carbon dioxide and oxygen contained in the stream form carbonates and are transported through the MCFC electrolyte to the anode, where they react with the hydrogen in the stream to release water vapor and carbon dioxide and are exhausted. At this time, 70% of the oxygen content in the stream is consumed. The change in free energy of the reaction in the MCFC can be extracted as electric power, but at present, 50% of the reaction heat is converted into electric power, and the remaining 50% is converted into heat in the MCFC. This heat is recovered as enthalpy of steam by a heat recovery mechanism incorporated in the MCFC, and part of the heat is used as steam for steam reforming of COG. In addition, residual steam is steam,
Hot water can be supplied outside the system in any form.

The MCFC has a temperature of about 650 ° C. and a pressure of 3.0.
Operated at atmospheric pressure (gauge). MC

[0038]

[Outside 3]

The gas is circulated and used as a supply gas. Also,
As shown in Table 1, the nitrogen in the cathode exhaust gas was 89%
By connecting to a nitrogen PSA, low-energy, high-purity nitrogen can be produced using process pressure. Nitrogen PSA uses separation sieve carbon as an adsorbent, and adsorbs coexisting gases such as oxygen and carbon dioxide when pressurized, so that high-purity nitrogen can be taken out. The operating conditions of the nitrogen PSA can be selected according to the purpose and the purity of nitrogen can be adjusted. For example, if the nitrogen source is used as a nitrogen source in an ammonia production process, the oxygen concentration in the cathode exhaust gas is low (about 3%).
By taking advantage of these facts, it is possible to introduce directly into the ammonia plant after dehumidification. In the current ammonia plant, air is used as a nitrogen source, but oxidation of hydrogen, which is a main raw material of ammonia synthesis, by oxygen in the air is inevitable and energy loss occurs. If it is used, the oxidation of hydrogen is greatly suppressed because the oxygen concentration is low, and the energy saving of the ammonia plant can be achieved.

The exhaust gas (stream) leaving the anode of the MCFC passes through a COG and steam preheater, is heat-exchanged and cooled, passes through a steam separator, a dehumidifier, and recovers water. Introduced into the recovery system. In the present invention, carbon dioxide pressure swing adsorption (CO ₂ -PSA) is used for the recovery of carbon dioxide. As the carbon dioxide recovery method, in addition to the adsorption method represented by PSA, a chemical absorption method, a membrane separation method, a cryogenic separation method, and the like are known. In the chemical absorption method, an amine-based absorbent is used. There are problems of equipment corrosion, problems of amine toxicity, etc.
In plants having cold sources such as LNG and liquid nitrogen, liquid carbon dioxide can be easily produced using these heats of vaporization, but in plants without these cold sources, the refrigerator is driven. Must be done, which is energetically disadvantageous. On the other hand, the adsorption method is based on the development of activated carbon having a high carbon dioxide adsorption capacity in a normal pressure system.
By repeating the pressure swing (mmHg), it is possible to continuously recover highly efficient and high-purity carbon dioxide, which is advantageous in terms of energy. In the present invention, since the carbon dioxide is highly concentrated at 84% in the anode exhaust gas which has been dehumidified and cooled down to room temperature, it is important that high-purity carbon dioxide can be recovered with low energy by the pressure swing adsorption method. Features.

The anode exhaust gas, which has been dehumidified and lowered to room temperature, is passed through an adsorption column at normal pressure to promote selective adsorption of carbon dioxide on activated carbon, and hydrogen, nitrogen, and oxygen rise in the column as they are and become hydrogen-rich. (Concentration is about 50%) is supplied to the burner of the COG reformer as a gas (stream)
The heat of combustion is the heat source for the COG reformer. On the other hand, after the carbon dioxide adsorption by the carbon dioxide adsorption tower is saturated, part of the carbon dioxide is introduced into the tower, and the remaining gas (hydrogen, nitrogen, oxygen, etc.) inside the tower is washed and off-gas is discharged outside the tower. As exhaust. Thereafter, the pressure is reduced to about 100 mmHg by a vacuum pump, and the carbon dioxide is desorbed and taken out with high purity. Normally, this process is composed of two towers, an adsorption tower and a desorption tower, and carbon dioxide can be continuously extracted by repeating normal pressure and reduced pressure respectively. When a gas having a stream composition is used as a raw material, carbon dioxide is recovered at a recovery of 90% and a purity of 99%. This captured carbon dioxide
It is supplied to a chemical plant that uses carbon dioxide as a raw material, and is immobilized in the form of methanol, dimethyl ether, and urea. At this time, if the chemical plant is not located close to the steelmaking plant and the transportation efficiency needs to be improved, or if even higher purity carbon dioxide is required, CO ₂
-After PSA, the recovered carbon dioxide can be introduced into a cryogenic separation plant to be liquid carbon dioxide.

[0042]

From the viewpoint of power generation efficiency, COG
Power generation efficiency by direct combustion of about 37%,
In a fuel cell, the enthalpy of combustion of hydrogen can be extracted with an efficiency of about 42%, so that an improvement in combustion efficiency of about 5% on a hydrogen basis can be achieved. Also, when steam reforming methane,
Since 4 moles of hydrogen are generated from 1 mole of methane, the heat of combustion of 4 moles of hydrogen is 218.5 kcal, while the heat of combustion of methane is 191.8 kcal / mol. .42 / (191.8 × 0.37) =
1.29, which is an efficiency improvement of less than 30%. Since the ratio of methane in COG is about 30%, the contribution of improving efficiency is about 9%. This efficiency improvement is achievable because the waste heat and fuel of the system can be effectively utilized and a heat source for steam reforming of methane can be provided in the system.
On the other hand, from the viewpoint of carbon dioxide recovery, in the present system, about 63% of the carbon dioxide contained in the BFG combustion exhaust gas is recovered and fixed. Constructed an MCFC fuel cell power generation system that achieved energy-saving power generation using a certain COG or its steam reformed gas as fuel to achieve a reduction in carbon dioxide emissions and, at the same time, combined with the combustion exhaust gas of another steelmaking by-product gas, BFG. In this way, high-purity carbon dioxide can be recovered with low energy, and the recovered carbon dioxide is used as a chemical raw material in the production of chemicals and immobilized in the form of chemicals to produce chemicals. By reducing the amount of carbon dioxide emitted during the process, it is possible to achieve a reduction in carbon dioxide emitted into the atmosphere across industries.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a combined system composed of MCFC power generation using COG and BFG, which are by-product gases of steelmaking, and a process for capturing and fixing carbon dioxide.

FIG. 2 is a diagram showing a simple system flow in a case where a steam reforming gas of COG is used as an MCFC anode supply gas and a mixed gas of BFG thermal power generation exhaust gas and air is used as an MCFC cathode supply gas. It should be noted that the circled numbers on the arrows indicating the flows correspond to the stream numbers, and are shown in Table 1.
Is referenced in

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoshihiro Kaneda 3-35-1 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture Nippon Steel Corporation Technology Development Division

Claims (8)

[Claims] In a power generation system using a molten carbonate fuel cell (MCFC), MCFC power generation for supplying a plurality of by-product gases generated in an iron making process as an anode gas and a cathode gas is performed. A combined power generation and exhaust gas recovery system for molten carbonate fuel cells, which separates and recovers carbon. 2. Among the by-product gases generated in the iron making process,
The combined system of claim 1, wherein coke oven gas (COG) is supplied as an anode gas. 3. The combined power generation and exhaust gas recovery system for a molten carbonate fuel cell according to claim 2, wherein a shift COG obtained by reforming carbon monoxide contained in the COG into hydrogen and carbon dioxide by a shift reaction. 4. The combined power generation and exhaust gas recovery system for a molten carbonate fuel cell according to claim 2, wherein the hydrocarbon contained in the COG is reformed into hydrogen and carbon dioxide by a steam reforming reaction to form a reformed COG. 5. The by-product gas generated in the iron making process,
2. The molten carbon dioxide according to claim 1, wherein, when supplying the blast furnace gas (BFG) as a cathode gas, after carbon monoxide contained in the BFG is oxidized to carbon dioxide by combustion, the gas is enriched with oxygen to form a cathode gas. A combined fuel cell power generation and exhaust gas recovery system. 6. A gas obtained by burning BFG alone or mixed with another fuel, and then mixing air and / or oxygen with the gas.
The volume concentration ratio of carbon dioxide / oxygen in the gas is 1/4 to
The combined system for power generation and exhaust gas recovery of a molten carbonate fuel cell according to claim 5, wherein the system is adjusted to 4/1. 7. The combined molten carbonate fuel cell power generation-exhaust gas recovery system according to claim 1, wherein carbon dioxide is separated and recovered from the anode exhaust gas using a carbon dioxide pressure swing adsorption method. 8. A method for immobilizing a carbon dioxide, comprising synthesizing a chemical using carbon dioxide obtained by the combined power generation and exhaust gas recovery system for molten carbonate fuel cells according to claim 1 as a raw material.