US20130022931A1

US20130022931A1 - Chemical looping combustion method using dual metal compound oxide

Info

Publication number: US20130022931A1
Application number: US13/336,277
Authority: US
Inventors: Yao-Hsuan Tseng; Jia-Long Ma; Young KU; Yu-Lin Kuo; Ping-Chin Chiu; Chung-Sung Tan
Original assignee: National Tsing Hua University NTHU
Current assignee: National Tsing Hua University NTHU
Priority date: 2011-07-21
Filing date: 2011-12-23
Publication date: 2013-01-24
Also published as: TW201304865A

Abstract

A chemical looping combustion method using a dual metal compound oxide includes the following steps: a fuel material combusting with the dual metal compound oxide in a first reactor to obtain a metal product; supplying the metal product obtained in the first reactor into a second reactor, and the metal product reacting with the air in the second reactor to obtain the dual metal compound oxide; and, supplying the dual metal compound oxide obtained in the second reactor into the first reactor. The dual metal compound oxide used in the chemical looping combustion process has high oxidation rate as well as high reduction rate so as to increase the efficiency of the chemical looping combustion process.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to a chemical looping combustion method using dual metal compound oxide, and more particularly, to the chemical looping combustion method capable of increasing reaction rate and decreasing the difficulty in design for the chemical looping combustion.
2. Description of the Prior Art
In recent years, science and technology were developed rapidly, however, the overexploitation of resource and the pollutant in the development of science and technology cause a huge impact to the environment to force people taking the environmental protecting issue seriously. The development of science and technology is highly related to the energy resources, therefore, the impact caused by power plants to the environment and the development of green energy are significant parts in the environmental protecting issue.
The thermal power generating method is the most general power generating means in the world. The power is generated by combusting fossil fuels such as coal, gasoline, or gas to heat water and produce steam to push the power generator. Compared to other power generating methods, the thermal power generating method produces more air pollution. Because of the combustion of the fossil fuels with the oxygen in the air in the reactor, the exhaust gas produced in the thermal power generating method includes carbon dioxide. Carbon dioxide is generally considered as one of greenhouse gases, and the decrement of carbon dioxide emissions is an objective in many countries. Besides the decrement of carbon dioxide emissions, the carbon dioxide storage and reuse also reduce the emissions of carbon dioxide. However, the exhaust gases include pollutants such as nitrogen oxide, and the pollutant must he separated before the storage and reuse of carbon dioxide. The separation of the pollutant from the exhaust gases consumes much energy, and a part of the power generated by the power plant is applied to the separation process, in other words, the power generating efficiency of the thermal power plant is lowered down.
In the prior art, the chemical looping combustion process taking metal oxygen carriers, which is used for replacing air, as comburent has been disclosed to solve the above-mentioned problem. The chemical looping combustion process uses two fluidized bed reactors, the fuel reactor and the air reactor, to process oxidation reaction and reduction reaction respectively to generate heat. In detail, the fuel materials combust with the metal oxygen carriers to release heat, and the combustion reaction of the fuel materials means a reduction reaction of the metal oxygen carriers, so that the metal oxygen carriers are reduced to metal after the combustion. Then, the reduced metal is supplied into the air reactor and oxidized with air or other gas capable of providing oxygen. After the oxidation of the reduced metal in the air reactor, the metal oxygen carriers is generated from the reduced metal and then supplied to the fuel reactor to keep looping.
Compared to the conventional combustion reactions, the chemical looping combustion process provides oxygen by the metal oxygen carriers instead of the air, so that the exhaust gases generated in the chemical looping combustion process includes 99% carbon dioxide after removing the water vapor via condensation. The high-purity carbon dioxide can be stored and reused directly and the separation process of the other pollutants from the exhaust gases which consumes high energy is not necessary. Accordingly, the chemical looping combustion process is beneficial to the decrement of carbon dioxide emissions of the thermal power plant and avoids the energy consumption on separating the gas, that is to say, it is more efficient on power generation.
The ferrous oxygen carrier and the nickel oxygen carrier were often used as the metal oxygen carriers in the chemical looping combustion process. The ferrous oxygen carrier has high oxidation rate but low reduction rate. The nickel oxygen carrier has high reduction rate but low oxidation rate. The mismatch of reaction time results in a difficulty in design for the chemical looping combustion process, and it is harmful to the application of the chemical looping combustion process to the thermal power plant.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a novel chemical looping combustion method to solve the problem in the prior art.
According to an embodiment of the invention, a chemical looping combustion method includes the following steps of: supplying a dual metal compound oxide into a first reactor; a fuel material combusting with the dual metal compound oxide in the first reactor to obtain a metal product and a gas; supplying the metal product obtained in the first reactor into the second reactor; the metal product reacting with the air in the second reactor to obtain the dual metal compound oxide; and, supplying the dual metal compound oxide obtained in the second reactor into the first reactor. The dual metal compound oxide includes an oxide of a first metal, an oxide of a second metal, and an oxide of a compound of the first metal and the second metal.
In this embodiment, the dual metal compound oxide is prepared by the following steps of: mixing a metal salt of the first metal and a metal salt of the second metal uniformly in an aqueous solution to obtain a dual metal solution; adding a water-soluble polymer into the dual metal solution to obtain a precipitate, and then drying the precipitate; and, pulverizing and calcining the dried precipitate to obtain the dual metal compound oxide.
On the advantages and the spirit of the invention, it can be understood further by the following invention descriptions and attached drawings.

BRIEF DESCRIPTION OF THE APPENDED DRAWINGS

FIG. 1 is a flow chart illustrating a chemical looping combustion method according to an embodiment of the present invention.

FIG. 2 is a flow chart illustrating the chemical looping combustion method according to another embodiment of the present invention.

FIG. 3 is a flow chart illustrating the method for preparing the dual metal compound oxide according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Please refer to FIG. 1. FIG 1 is a flow chart illustrating a chemical looping combustion method according to an embodiment of the present invention.
As shown in FIG. 1, the chemical looping combustion method of the embodiment includes the following steps of: in step S10, supplying a dual metal compound oxide into a first reactor; in step S12, a fuel material combusting with the dual metal compound oxide in the first reactor to obtain a metal product, a gas, and heat; in step S14, supplying the metal product obtained by the combustion in the first reactor into a second reactor; in step S16, the metal product reacting with the air in the second reactor to obtain the dual metal compound oxide; and, in step S18, supplying the dual metal compound oxide obtained in the second reactor into the first reactor.
In this embodiment, the dual metal compound oxide in step S10 could be a compound including an oxide of a first metal, an oxide of a second metal, and an oxide of a compound of the first metal chemical bonding with the second metal. For example, if the first metal is iron and the second metal is nickel, the dual metal compound oxide could be a Fe—Ni compound oxide including Fe₂O₃, NiO, and NiFe₂O₄.
In step S12, the fuel material could be fossil fuel, such as methane or propane, generating combustion reaction with the dual metal compound oxide to obtain the metal product, the gas, and heat. For example, a combustion reaction of methane with Fe—Ni compound oxide produces iron, nickel, a compound of iron and nickel, gas, and heat. The reaction in the first reactor is a reduction reaction for the dual metal compound oxide. To ensure the combustion of fuel material with the dual metal compound oxide but not with other oxygen sources, an atmosphere without oxygen can be filled in the first reactor to avoid the reaction of the fuel material with other oxygen sources. In practice, the first reactor can he filled up with an inert gas first, and then the combustion reaction proceeds.
The heat generated in step S12 is used to heating water o produce steam which can drive the power generator. Because the fuel material is fossil fuel such as methane or propane in step S12, the gas obtained includes carbon dioxide and water vapor. According to another embodiment, the gas obtained in step S12 can be processed by the following steps of: drawing the gas from the first reactor; condensing the gas to remove the water vapor from the gas; and, sealing and storing the condensed gas. The water vapor is removed from the gas, so that the condensed gas is high purity carbon dioxide and can be sealed and stored directly. Besides, the condensed gas can be reused directly to reach the purpose of decrement of the carbon dioxide emissions.
Please refer to FIG. 1 again. In the embodiment, the first reactor and the second reactor can he fluidized bed reactors, and the metal product can be supplied into the second reactor (air reactor) directly when it is obtained in the first reactor (combustion reactor), as described in step S14. And then, in step S14, a suitable condition is provided to the second reactor to assist the metal product to be oxidized therein to obtain the dual metal compound oxide. For example, the above-mentioned metal product including iron, nickel and the compound of iron and nickel can be oxidized to obtain a compound oxide including Fe₂O₃, NiO, and NiFe₂O₄, i.e., the dual metal compound oxide reacting with the fuel material in step S12. The reaction of the metal product in the second reactor is an oxidation reaction, and the air or an oxygenic atmosphere can be filled in the second reactor.
As in step S18, the dual metal compound oxide obtained in the second reactor is supplied into the first reactor. In practice, the dual metal compound oxide can react with the fuel material to obtain the metal product, the gas, and heat again if the supply of the fuel material and other conditions in the first reactor is sufficient. Please refer to FIG. 2. FIG. 2 is a flow chart illustrating the chemical looping combustion method according to another embodiment of the present invention. As shown in FIG. 2, after the dual metal compound oxide obtained in the second reactor in step S16 is supplied into the first reactor as described in step S18, step S12 can be executed again to form loops for continuously generating heat. The other steps in this embodiment are substantial the same with the corresponding steps in the above-mentioned embodiments, so that the detail would not be described again.
In this embodiment, step S12 and step S16 are executed stage by stage, that is to say, after reducing the dual metal compound oxide, the product of the reduction is oxidized to obtain the dual metal compound oxide. According to the ratio of the two metals in the dual metal compound oxide, the reacting time of the reduction reaction and that of the oxidation reaction can be adjusted. Summarily, the dual metal compound oxide for the chemical looping combustion method has high reduction rate and high oxidation rate, and then the reaction time of the reduction reaction is close to that of the oxidation reaction. Accordingly, the chemical looping combustion method of the present invention can be provided with high efficiency and easily designed.
Please refer to FIG. 3. FIG. 3 is a flow chart illustrating the method for preparing the dual metal compound oxide according to another embodiment of the present invention. The dual metal compound oxide prepared by the method in this embodiment can be the oxygen carrier provided to the combustion of the fuel material in the above-mentioned embodiments. As shown in FIG. 3, the method for preparing the dual metal compound oxide includes the following steps of: in step S20: mixing a metal salt of the first metal and a metal salt of the second metal uniformly in an aqueous solution to obtain a dual metal solution; in step S22, adding a water-soluble polymer into the dual metal solution to obtain a precipitate, and then drying the precipitate; finally, in step S24, pulverizing and calcining the dried precipitate to obtain the dual metal compound oxide.
Take Fe—Ni compound oxide for example. In step S20, a ferric salt and a nickel salt are mixed in different ratio in the aqueous solution to obtain the Fe—Ni metal solution, wherein the ferric salt can be ferric nitrate and the nickel salt can be nickel nitrate. The water-soluble polymer, which is used to be the dispersing agent, such as polyethylene glycol, is added into the Fe—Ni metal solution, and the precipitate is then obtained and dried, as described in step S22.
In step S24, the dried precipitate is pulverized and then calcined. When the calcining temperature reach a specific temperature range, the iron and nickel is oxidized to form Fe₂O₃and NiO, and iron and nickel is sintered and then oxidized to form NiFe₂O₄. Therefore, the Fe—Ni compound oxide includes the three oxides and the compound of the three oxides. In practice, the temperature range to calcining dried precipitate can be 500° C. to 1600° C., and preferably, can be 700° C. to 1100° C. It should be noted that the calcining temperature range can be adjusted according to the kinds of the two metals.
Please refer to Table. 1. Table. 1 shows different conditions for preparing the Fe—Ni compound oxides according to the above-mentioned embodiments. As shown in Table. 1, the samples 1˜4 are prepared by mixing nickel nitrate and ferric nitrate in different ratio in water, adding the polyethylene glycol selectively, and pulverizing and calcining the precipitate at 700° C. or 1000° C. for 6 hours to obtain the dual metal compound oxides. Please refer to Table. 2. Table. 2 shows the oxidation time and the reduction time required by the chemical looping combustion processes using the Fe—Ni compound oxides of the sample 1˜4, a pure ferric oxide, a pure nickel oxide, and a pure Fe—Ni oxide (the oxide of the compound of iron and nickel).

TABLE 1

		calcining	polyethylene	nickel	ferric
Sample	H2O (mL)	temperature	glycol	nitrate	nitrate

1	15	1000	3	10	30
2	15	700	0	30	10
3	15	700	0	30	10
4	30	700	3	30	10

TABLE 2

	Total reduction	Total oxidation
Sample	time (mins)	time (mins)

1	6.4	4.5
2	7.8	18.9
3	7.6	18.6
4	14.6	6.1
pure ferric oxide	89.5	1.3
pure nickel oxide	3.7	>150.0
pure Fe—Ni oxide	83.0	13.0

As shown in Table. 1 and Table. 2, although the conditions for preparing the Fe—Ni compound oxides in the samples 1˜4 are different and the reduction time and the oxidation time of the chemical looping combustion processes following the conditions are different too, the reduction time and the oxidation time of the chemical looping combustion processes using the Fe—Ni compound oxides of the samples 1˜4 are shorter and closer to each other compared to those using pure ferric oxide, pure nickel oxide, and pure Fe—Ni oxide. In other words, the Fe—Ni compound oxides have high oxidation rate and high reduction rate at the same time for the chemical looping combustion processes. Oppositely, the oxidation rates of the pure fenic oxide and pure Fe—Ni oxide are far larger than the reduction rates of those, but the reduction rate of the pure nickel oxide is far larger than the oxidation rate of that. The close oxidation time and reduction time of the Fe—Ni compound oxides of the samples 1˜4 for the chemical looping combustion processes are advantageous to the designs for the chemical looping combustion processes.
According to another embodiment, the dual metal compound oxide for the chemical looping combustion method can be loaded on a carrier, and then the dual metal compound oxide and the carrier can be supplied together into the first reactor or the second reactor to be reduced or oxidized. For example, after preparing the Fe—Ni compound oxide by the method shown in FIG. 3, the Fe—Ni compound oxide can be loaded on a carrier made of inert material. The Fe—Ni compound oxide and the carrier can be supplied into the first reactor by the method shown in FIG. 1 so that to the fuel material combust with the Fe—Ni compound oxide on the carrier. It should be noted that the combustion reaction of the fuel material is the reduction reaction of the Fe—Ni compound oxide. After the reduction reaction, the metal product is formed on the carrier. The metal product and the carrier is supplied into the second reactor to oxidize the metal product. It further raises the reactivity and decreases the operating temperature of the chemical looping combustion process by supplying the dual compound oxide loaded on the carrier.
To sum up, the chemical looping method of the present invention executes the reduction reaction and the oxidation of the dual metal compound oxide respectively in two reactors so as to keep looping to continuously generate heat for the power generator. Compared to the prior art, the dual metal compound oxide in the present invention has high oxidation rate and high reduction rate at the same time so that the reaction time of the two reactions in the chemical looping combustion process are short and close to each other. Therefore, the problem of difficulty in design for the chemical looping combustion process in the prior art can be solved effectively, so that the chemical looping combustion process can be easily used in the thermal power plant for the decrement of the carbon dioxide emissions.
Although the present invention has been illustrated and described with reference to the preferred embodiment thereof, it should be understood that it is in no way limited to the details of such embodiment but is capable of numerous modifications within the scope of the appended claims.

Claims

1. A chemical looping combustion method using dual metal compound oxide, comprising the following steps of:

supplying a dual metal compound oxide into a first reactor, wherein the dual metal compound oxide comprises an oxide of a first metal, an oxide of a second metal, and an oxide of a compound of the first metal and the second metal;

a fuel material combusting with the dual metal compound oxide in the first reactor to obtain a metal product and a gas;

supplying the metal product obtained in the first reactor into a second reactor;

the metal product reacting with the air in the second reactor to obtain the dual metal compound oxide; and

supplying the dual metal compound oxide obtained in the second reactor into the first reactor.

2. The method of claim 1, wherein the dual metal compound oxide is a Fe—Ni compound oxide, and the metal product comprises iron, nickel, and a compound of iron and nickel.

3. The method of claim 2, wherein the Fe—Ni compound oxide comprises Fe₂O₃, NiO, and NiFe₂O₄.

4. The method of claim 1, further comprising the following steps of:

mixing a metal salt of the first metal and a metal salt of the second metal uniformly in an aqueous solution to obtain a dual metal solution;

adding a water-soluble polymer into the dual metal solution to obtain a precipitate, and then drying the precipitate;

pulverizing and calcining the dried precipitate to obtain the dual metal compound oxide.

5. The method of claim 1, wherein the temperature for calcining dried precipitate is in the range of 500° C. to 1600° C., and preferably 700° C. to 1100° C.

6. The method of claim 4, wherein the dual metal compound oxide is a Fe—Ni compound oxide, the first metal is iron, the second metal is nickel, the metal salt of the first metal is ferric nitrate, the metal salt of the second metal is nickel nitrate, and the water-soluble polymer is polyethylene glycol.

7. The method of claim 11, wherein the fuel material is methane.

8. The method of claim 1, further comprising the following steps of:

loading the dual metal compound oxide on a carrier; and

supplying the dual metal compound oxide and the carrier into the first reactor.

9. The method of claim 1, wherein the first reactor and the second reactor are fluidized bed reactors.

10. The method of claim 1, further comprising the following steps of:

drawing the gas from the first reactor;

condensing the gas to remove the water vapor from the gas; and

sealing and storing the condensed gas.