Method and device for using CO2 mineralization to produce sodium bicarbonate or sodium carbonate and output electric energy

Abstract

Disclosed are a method and device for using CO.sub.2 mineralization to produce sodium bicarbonate or sodium carbonate and output electric energy. The device comprises an anode area, an intermediate area, and a cathode area. The anode area and the intermediate area are spaced by a negative ion exchange membrane (2). The intermediate area and the cathode area are spaced by a positive ion exchange membrane (3). The anode area, the intermediate area, and the cathode area can accommodate corresponding electrolytes. An anode electrode (1) is disposed in the anode area, a cathode electrode (4) is disposed in the cathode area, and the cathode electrode and the anode electrode are connected through a circuit. A raw material hydrogen gas inlet is disposed in the anode area, and a CO.sub.2 inlet and a product hydrogen gas outlet are disposed in the cathode area. The method is based on the principle of CO.sub.2 mineralization and utilization, combines the membrane electrolysis technology, facilitates spontaneous reaction by using the acidity of CO.sub.2 and the alkalinity of the reaction solution and realizes separation of the products, and converts through a membrane electrolysis apparatus the energy released by the reaction into electric energy at the same time when producing the sodium bicarbonate or sodium carbonate and outputs the electric energy. The method and device have low energy consumption, high utilization rate of raw materials and little environmental pollution, and can output electric energy while producing sodium carbonate at the same time.

Claims

1. A method for coproducing chemicals and electric energy, comprising: providing a fuel cell having an anode region containing an anode electrode and anolyte, a cathode region containing a cathode electrode and a catholyte, and an intermediate region containing electrolyte; adding an alkaline material into the anolyte; adding a sodium salt into the electrolyte; feeding CO.sub.2 into the catholyte whereby forming hydrogen at the cathode electrode and bicarbonate ions in the catholyte; contacting hydrogen with the anode electrode to produce proton in the anolyte; and producing an electric current between the cathode electrode and the anode electrode when the cathode electrode and the anode electrode are connected.

2. The method according to claim 1, wherein an anion exchange membrane is disposed between and separates the anode region and the intermediate region, a cation exchange membrane is disposed between and separates the intermediate region and the cathode region, wherein sodium ions migrate from the intermediate region into the cathode region through the cation exchange membrane.

3. The method according to claim 1, further comprising forming sodium bicarbonate in the cathode region; and separating solid sodium bicarbonate from the catholyte.

4. The method according to claim 1, wherein the sodium salt is sodium chloride, sodium sulfate, or sodium nitrate.

5. The method according to claim 1, wherein the anolyte and/or catholyte contain sodium ions.

6. The method according to claim 1, wherein the alkaline material added to the anode region comprises at least one compound selected from the group consisting of calcium hydroxide, sodium hydroxide, ammonia, and potassium hydroxide.

7. The method according to claim 1, wherein the anode electrode is a gas diffusion electrode.

8. The method according to claim 1, characterized in that the cathode electrode is made of a nickel foam supported Pt/C catalyst.

9. The method according to claim 1, wherein hydrogen generated at the cathode electrode is fed to the anode region.

10. The method according to claim 1, further comprising storing the anolyte in a first vessel; and circulating the anolyte between the first vessel and the anode region.

11. The method according to claim 1, further comprising storing the intermediate electrolyte in a second vessel; and circulating the intermediate electrolyte between the second vessel and the intermediate region.

12. The method according to claim 1, further comprising storing the catholyte in a third vessel; and circulating the catholyte between the third vessel and the cathode region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a method for using CO.sub.2 mineralization to produce sodium carbonate and output electric energy; and

(2) FIG. 2 is a schematic diagram of a CO.sub.2 mineralized fuel cell as described in example 4 of the invention.

MARKS IN FIGURES

(3) 1: gas diffusion electrode (anode electrode); 2: anion exchange membrane; 3: cation exchange membrane; 4: nickel foam supported. Pt/C electrode (cathode electrode); 5: hydrogen buffer tank; a: anode current collecting layer; b: anode gas chamber plate and frame; c: hydrogen diffusion electrode; d: anolyte chamber plate and frame; e: anion exchange membrane (AEM); f: intermediate chamber plate and frame; g: cation exchange membrane (CEM); h: cathode chamber plate and frame; i: cathode electrode; j: cathode current collecting layer; I: anode buffer tank; II: intermediate buffer tank; and III: cathode buffer tank.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) The above-mentioned contents of the invention will be further described in detail in combination with preferred embodiments of examples. However, the following examples should not be construed as limitation to the scope of the invention. Any modification made within the spirit and principle of the invention, and equivalent replacement or improvement according to the common technical knowledge and common means of the art are included in the protection scope of the invention.

Example 1

(5) The process of using CO.sub.2 mineralization to produce sodium carbonate and output electric energy in this example is shown in FIG. 1. As the housing of a CO.sub.2 generating device, a container was divided into an anode region, an intermediate region and a cathode region by a cation exchange membrane 3 which only allowed cation to permeate and prevented anion from permeating in the container and an anion exchange membrane 2 which only allowed anion to permeate and prevented cation from penetrating. A solid calcium hydroxide was added to 1 mol/L NaCl solution to form a cloudy solution containing saturated Ca(OH).sub.2, and the solution was added to the anode region as an anolyte. To the cathode, 0.3 mol/L NaHCO.sub.3 solution was added as a catholyte, and 6 mol/L NaCl solution was added to the intermediate region as an intermediate electrolyte. A gas diffusion electrode 1 was used as the anode electrode, and an electrode 4 using nickel foam supported Pt/C catalyst was used as the cathode electrode. CO.sub.2 gas was bubbled into the cathode region from the bottom of the container as the housing of the CO.sub.2 generating device, and the hydrogen generated from the cathode electrode was collected and introduced into a buffer tank 5. The hydrogen from the buffer tank was introduced into the gas diffusion electrode for 1 h electrolytic reaction at 25 C., the current density was controlled to 7 mA/cm.sup.2, and the tank voltage was maintained at 0.15V. After the reaction, the concentration of the Ca.sup.2+ dissolved out after acid-base reaction in the anode region was determined with an atomic absorption spectrometer, and the concentration of the bicarbonate ion produced in the cathode region was tested by chemical titration. Compared with the theoretical formula, the current efficiency of sodium bicarbonate produced in the electrolytic process was 89%, and that of Ca.sup.2+ produced in the anode region was 91%.

Example 2

(6) The mineralization process of the example is shown in FIG. 1. As the housing of a CO.sub.2 generating device, a container was divided into an anode region, an intermediate region and a cathode region by a cation exchange membrane 3 which only allowed cation to permeate and prevented anion from permeating in the container and an anion exchange membrane 2 which only allowed anion to permeate and prevented cation from penetrating. To the anode region, 1 mol/L aqueous ammonia was added as an anolyte, 1 mol/L NaHCO.sub.3 solution was added to the cathode region as a catholyte, and saturated NaCl solution was added to the intermediate region as an intermediate electrolyte. A gas diffusion electrode 1 was used as the anode electrode, and an electrode 4 using nickel foam supported Pt/C catalyst was used as the cathode electrode. CO.sub.2 gas was bubbled into the cathode region from the bottom of the container as the housing of the CO.sub.2 generating device, and the hydrogen generated from the cathode electrode was collected and introduced into a buffer tank 5. The hydrogen from the buffer tank was introduced into the gas diffusion electrode for reaction. At 25 C., the resistance of the applied load was controlled to be 0, and the current density of tank reached 10 mA/cm.sup.2. When the resistance of the applied load was controlled to +, the tank voltage reached 0.31V, and the measured maximum output power of the CO.sub.2 generating device was 3.2 W/m.sup.2.

Example 3

(7) The mineralization process of the example is shown in FIG. 1. As the housing of a CO.sub.2 generating device, a container was divided into an anode region, an intermediate region and a cathode region by a cation exchange membrane 3 which only allowed cation to permeate and prevented anion from permeating in the container and an anion exchange membrane 2 which only allowed anion to permeate and prevented cation from penetrating. To 1 mol/L NaCl solution, 1 mol/L Na(OH).sub.2 was added to form a mixed solution. The mixed solution was added to the anode region as an anolyte, saturated NaHCO.sub.3 solution was added to the cathode region as a catholyte, and saturated NaCl solution was added to the intermediate region as an intermediate electrolyte. A gas diffusion electrode 1 was used as the anode electrode, and an electrode 4 using nickel foam supported Pt/C catalyst was used as the cathode electrode. CO.sub.2 gas was bubbled into the cathode region from the bottom of the container as the housing of the CO.sub.2 generating device, and the hydrogen generated from the cathode electrode was collected and introduced into a buffer tank 5. The hydrogen from the buffer tank was introduced into the gas diffusion electrode for reaction. At 25 C., the resistance of the applied load was controlled to be 0, and the current density was maintained at 12 mA/cm.sup.2. Under the conditions above, electrolytic reaction was performed for 1 h, and the sodium bicarbonate produced after the 1 h electrolytic reaction was cooled, filtered, and dried at 110 C. to obtain 0.210 g sodium carbonate. A theoretical value of 0.237 g of sodium carbonate was produced by comparing with the faradaic current efficiency theory, the conversion rate of the produced sodium carbonate was up to 88.6%, and the measured maximum output power was 3.3 W/m.sup.2. After the reaction, the solution in the anode region was subject to acid-base titration, and NaOH consumption was measured. At the same time, carbonate ions produced in the cathode region were subject to chemical titration. By comparing with the theoretical formula, the current efficiency of the produced sodium bicarbonate was up to 93%, and the current efficiency of acid-base reaction in the anode region reached 96%.

Example 4

(8) As shown in FIG. 2, a CO.sub.2 mineralized fuel cell comprises an anode current collecting layer, an anode gas chamber plate and frame, a hydrogen diffusion electrode, an anolyte chamber plate and frame, an anion exchange membrane, an intermediate chamber plate and frame, a cation exchange membrane, a cathode chamber plate and frame, a cathode electrode and a cathode current collecting layer arranged successively. The anode current collecting layer and the cathode current collecting layer were connected by a circuit. The anode gas chamber plate and frame was provided with a raw hydrogen inlet and a raw hydrogen outlet. The cathode gas chamber plate and frame was provided with a carrier gas inlet (the inlet can be also enclosed) and a product hydrogen outlet. The anolyte chamber plate and frame, the intermediate chamber plate and frame, and the cathode chamber plate and frame were respectively provided with an inlet and an outlet for the corresponding electrolyte. The electrolyte inlet on the cathode chamber plate and frame was connected to the cathode buffer tank, and the electrolyte outlet on the cathode chamber plate and frame was also connected to the cathode buffer tank to form a circulation loop, thus returning the reacted electrolyte back to cathode buffer tank for recycle. Meanwhile, a CO.sub.2 gas inlet was arranged on the cathode buffer tank and a solid-liquid separator was arranged in the cathode buffer tank or the outlet thereof. The electrolyte inlet on the intermediate chamber plate and frame was connected to the intermediate buffer tank and supplies electrolyte to the intermediate region through the buffer tank. The electrolyte outlet on the intermediate chamber plate and frame was connected to the intermediate buffer tank, and returns the reacted electrolyte back to the intermediate buffer tank for recycle. A sodium salt feeding inlet was arranged on the intermediate buffer tank. The electrolyte inlet on the anolyte chamber plate and frame was connected to the anode buffer tank and supplies electrolyte to the anode region through the buffer tank. The electrolyte outlet on the anolyte chamber plate and frame may be connected to a concentration and separator. The electrolytes in the three liquid chambers (anolyte chamber, intermediate chamber and cathode chamber) flow or circulate under the drive of a peristaltic pump. The inner volume of three liquid chambers was 15 ml and the anion exchange membrane (AEM) was placed between the intermediate chamber and the anode chamber (area S=4 cm.sup.2). The anion exchange membrane CEM (S=4 cm.sup.2) was placed between the intermediate chamber and the cathode chamber. The anode electrode was gas diffusion electrode (S=3.24 cm.sup.2) and the cathode was electrode with Pt/C (S=3.24 cm.sup.2) as the catalyst. An H.sub.2 cylinder was connected to the anode side of the device to provide a hydrogen source for the gas diffusion electrode, and the hydrogen enters from the inlet of the anode and goes out of the outlet thereof. A N.sub.2 cylinder was connected to the cathode inlet of the device, and N.sub.2 was introduced into the carrier gas inlet at a flow rate of 10 ml/min so as to purge the H.sub.2 generated at the cathode out of the hydrogen outlet (the purge step aims at detecting the hydrogen production more accurately, and nitrogen purge and the corresponding device could be omitted in the actual application). A CO.sub.2 cylinder was connected to the external buffer tank at the cathode side. Both the cathode and anode were of current collecting layers made of stainless steel mesh.

(9) Preparation of Electrode

(10) The following three layers were provided on the gas diffusion electrode actually: current collecting layer, carbon cloth and catalyst layer. A 1 mg/cm.sup.2 Pt catalyst (Shanghai Hesen Electric Co., Ltd.) was loaded on the carbon cloth with a filling and leveling layer. Afterwards, the carbon cloth and the titanium mesh of the current collecting layer were stacked together to press for 10 min at pressure of 5 MP and temperature of 25 C., thus obtaining a gas diffusion electrode. A Pt/C catalyst with the same load (1 mg/cm.sup.2) as the anode was loaded on a nickel foam with the same surface area, dried for 2 h at 60 C., and lastly pressed for 10 min at 3 MP to obtain a cathode electrode. Optionally, Pt/C catalysts were loaded on the anode electrode and cathode electrode by spraying suspension. The preparation method of the Pt/C catalyst suspension is as follows: first, placing 0.1 Pgt/C (40% Pt) catalyst in a beaker; then, adding 1 ml of distilled water, 4 ml of absolute ethanol and 4.5 g of perfluorosulfonic acid solution, wherein ethanol and perfluorosulfonic acid play a role in dispersion and adhesion respectively; and spraying the black suspension obtained by sonicating the mixture.

(11) Optionally, the preparation of the cathode electrode is divided into the following steps: first, placing 0.1 Pgt/C (40% Pt) catalyst in a beaker; then, adding 1 ml of distilled water, 4 ml of absolute ethanol and 4.5 g of Nafion solution, wherein ethanol and Nafion play a role in dispersion and adhesion respectively; spraying the black suspension obtained by sonicating the mixture to the carbon paper at 80 C. and allowing the support amount amount of Pt/C on the carbon paper to be 1 mg/cm.sup.2; drying at 60 C. for 2 h, and finally pressing at 3 MPa for 10 min.

(12) Firstly, 50 ml of 1 mol/L NaCl solution was added to each buffer tank during use, and the membrane potential caused by the difference in ion concentration was counteracted by a peristaltic pump at a pump speed of 15 ml/min. Before the formal start of the reaction, a voltage of 3V was applied to two electrodes of the system for 5 min so as to completely dissipate the O.sub.2 adhering to the electrode surface. After the operation, 100 mg of Ca (OH).sup.2 was added to the anode buffer tank to prepare solution (Part I in FIG. 2). The H.sub.2 flow rate was controlled to 10 ml/min by a mass flow meter, and the gas was introduced into the gas inlet of the gas diffusion electrode (part b in FIG. 2). The rate of introducing CO.sub.2 into the cathode buffer tank was controlled to 10-20 ml/min. The flow of liquid between the extrinsic cycle and the reaction chamber was controlled to 15 ml/min throughout the experiment so as to ensure stability of the whole system. An electronic load (ItechIT 8511) was connected between the cathode and the anode of the system. After the start of the reaction, the generation of current was capable of being detected immediately, and the output voltage and the output power density was capable of being controlled by adjusting the load.

(13) After the start of the experiment, 100 mg of analytically pure calcium hydroxide was added to the anode side of the CMFC system, saturated NaCl solution was added to the intermediate chamber, and 1 mol/L NaCl solution was added to the cathode side. CO.sub.2 (20 ml/min) with purity of 99.99% was then introduced into the catholyte, and a current was generated immediately at this time. The voltage produced by the system with the continuous injection of CO.sub.2 and power density became stable. The representative reaction of anode in the system was Ca(OH).sub.2+H.sub.2+2Cl.sup..fwdarw.CaCl.sub.2+2H.sub.2O+2e.sup., and the cathode produced NaHCO.sub.3 through the reaction 2CO.sub.2+2H.sub.2O+2e.sup.+2Na.sup.+.fwdarw.2NaHCO.sub.3+H.sub.2.

(14) Samples were quantitatively taken every 30 minutes at a current density of 2.5 A/m.sup.2 to determine the concentration change of HCO.sub.3.sup. at the cathode side. A linear increase in HCO.sub.3.sup. concentration indicated that sodium bicarbonate was continuously produced in the catholyte. The average current efficiency (percentage of electrons entering the NaHCO.sub.3 product) of the produced NaHCO.sub.3 was 91.4% by calculation. The amount of CaCl.sub.2 produced at the anode side was determined by determining the concentration change of chloride ions at the anode side by ion chromatography. The linear increase of chloride ion content was as expected, and the calculated average current efficiency of CaCl.sub.2 generated within 120 min was 93.4%.

(15) In the electrogenesis process, the compositions of inlet gas and outlet gas in the anode gas chamber plate and frame of the CMFC system were determined by gas chromatography. In the experiment, it was detected that the anode consumed hydrogen and the cathode produces hydrogen. As the role of H.sub.2 was accelerating the electron transfer rate in the whole process, net stoichiometric H.sub.2 generation and consumption should not exist in the reaction formula 1. The ratio of generated and consumed H.sub.2 was calculated by gas chromatography. We found that the ratio of H.sub.2 consumed by the anode was very close to that of H.sub.2 generated by the cathode, which also experimentally proved that the amounts of H.sub.2 generated and consumed by the anode and cathode were equal.

(16) In the experiment, the solubility of NaHCO.sub.3 in the solution was determined by acid-base neutralization titration: 0.5 ml of sample was taken from the cathode every half hour and transferred into a conical flask; 3 drops of methyl red-bromocresol green indicators were added and a proper amount of distilled water was added; the solution became bright green; and then 1 mol/L HCl with concentration of 0.001 was titrated into the solution till that the solution becomes dark red. In order to avoid the effect of Na.sub.2CO.sub.3 in the experiment, phenolphthalein indicator was added to the control group. The amount of generated NaHCO.sub.3 was calculated according to the concentration difference (c) of the HCO.sub.3.sup.+ in solution before and after reaction and the volume (V) of the electrolyte, m=cV. At the anode side, the concentration of CaCl.sub.2 in the solution was calculated by measuring the concentration of Cl.sup. in the solution (C1) and the concentration of Na.sup.+ (C2): C(C1C2)/2. The content of Na.sup.+ was determined by atomic absorption spectrometry, and the content of Cl.sup. was determined by ion chromatography. According to the experimental results, the anode generated CaCl.sub.2, cathode generated NaHCO.sub.3. At the cathode side, CO.sub.2 was introduced into the solution to form H.sub.2CO.sub.3, the produced H.sub.2CO.sub.3 was quickly decomposed into H.sup.+ and HCO.sub.3.sup.. The H.sup.+ gains electrons to generate H.sub.2, and the HCO.sub.3.sup. remains in the solution. At the anode side, H.sub.2 lost two electrons to become H.sup.+ and the H.sup.+ dissolved Ca(OH).sub.2 to produce H.sub.2O and Ca.sup.2+. Under the action of the internal electric field, the intermediate salt solution tank provided Na.sup.+ and Cl.sup. for two electrodes. AEM and CEM in the system avoided the mix of the produced CaCl.sub.2 and NaHCO.sub.3 by selectively making the Na.sup.+ enter the anode and the anion enter the cathode. It was found from the XRD and TGA analysis on the sample that the purity of NaHCO.sub.3 was 99.4%.

Example 5

(17) Effect of CO.sub.2 on System

(18) Test was carried out using the same device and operating procedure as that in Example 4. The only difference was that when the electrogenesis process became stable, 10 ml/min N.sub.2 instead of CO.sub.2 was introduced into the cathode reaction buffer tank. Afterwards, the output voltage and output power density gradually decreased to 0 eventually, at which time, CO.sub.2 instead of N.sub.2 was reintroduced into the cathode buffer tank, and the system immediately generated voltage and current again. In the whole process, the output voltage and power were recorded every 60 s, and the change in pH of the cathode side was determined and recorded at the same time. The results showed that the produced H.sub.2CO.sub.3 reduced the pH of the cathode when CO.sub.2 was introduced into the system, accompanied by the generation of electric energy. Once CO.sub.2 was replaced with N.sub.2, the H.sup.+ in the solution would not be sufficient to capture the electrons produced by the anode. In such case, H.sub.2O would play the receptor of electron and produce OH.sup.+ while producing H.sub.2, which would increase pH and gradually stop the electrogenesis process.

(19) Effect of Ca(OH).sub.2 on System

(20) Test was carried out using the same device and operating procedure as that in Example 4. The only difference was that 1 ml of saturated Ca(OH).sub.2 solution was added to the anode at the initial stage of the reaction. As electrogenesis process progresses, the Ca(OH).sub.2 of the anode was gradually consumed, and the output voltage and power density gradually decreased to zero eventually. Afterwards, 1 ml of saturated Ca(OH).sub.2 solution was again added to the anode, and the system externally outputted electric energy again.

(21) One milliliter of saturated Ca(OH).sub.2 solution was added to the anode, and then the pH of the anode immediately rose and electric energy was generated. As the reaction progressed, the Ca(OH).sub.2 in the solution was gradually consumed, the pH decreased, and the output energy reduced to zero. After 1 ml of new saturated Ca(OH).sub.2 solution was added, the pH of the solution increased and electric energy was output again.

(22) Based on the experimental results, we can conclude that the roles of CO.sub.2 and CaCl.sub.2 in the system are building a pH difference between the cathode and anode. When H.sub.2 is introduced into the anode, the pH difference will be converted into the potential difference of the oxidation pair (H.sup.+/H.sub.2) at the anode and cathode. During external conduction, the system will output current. In the process, CO.sub.2 provides H.sup.+ for the anode, and Ca(OH).sub.2 provides OH.sup. for the cathode. According to the Nernst equation, the theoretical cell voltage can be calculated according to the equation (2):
Ece=011.0591(pHanodepH(cathode)(formula 2)

(23) In order to further confirm this theory, we plotted a relation curve between the cell voltage and the pH difference between anode and cathode. The results show that increasing the pH difference between the anode and cathode will increase the open-circuit voltage, which is consistent with result of the equation (2).

(24) Hydrolysis of the NaHCO.sub.3 produced in the cathode tank in the electrogenesis process will affect the pH of the solution. To test such effect, different electrolytes (NaCl instead of NaHCO.sub.3) were added to the cathode. The results showed that the electrogenesis effect of 1 mol/L NaCl was better than that of saturated NaHCO.sub.3 solution in the CMC system. This is because that the pH difference generated by the former in the system was greater than that of the latter, so that the former had higher output voltage and energy. The maximum output energy was 5.5 W/m.sup.2 and the maximum open-circuit voltage (OCV) was 0.452V in the experiment.

Example 6

(25) With the device as described in Example 4, Ca(OH).sub.2 was added to the anode in the experiment, saturated NaCl solution was added to the anode, and saturated NaHCO.sub.3 solution was added to the cathode. CO.sub.2 was not introduced at the beginning of the reaction. At the energy density of 30.86 A/m.sup.2, the residual HCO.sub.3.sup. in the solution would provide H.sup.+ for the reaction and become CO.sub.3.sup.2 because of no introduction of CO.sub.2 into the cathode. However, as the reaction progresses, the output energy density will gradually decrease from 3.5 W5/m.sup.2 to 2.9 W6/m.sup.2. At this time, excessive CO.sub.2 was introduced into the cathode buffer tank. In this process, CO.sub.3.sup.2 will be converted to HCO.sub.3.sup.. When the solubility of sodium bicarbonate in the solution reaches the maximum value, crystal will be separated out. After solid-liquid separation, the solid phase can be used to prepare sodium bicarbonate or sodium carbonate. The liquid phase returns back to the CMFC cell system. The output energy immediately rises to 3.55 W/m.sup.2 again. With recycle above, electric energy and sodium bicarbonate or sodium carbonate were produced continuously.

Example 7 Mineralization and Electrogenesis Performance of Other Basic Raw Materials

(26) At the proof-of-concept phase, we used analytically pure Ca(OH).sub.2 in our test. In order to test whether this system can effectively utilize the industrial solid wastes containing Ca(OH).sub.2, carbide slag and cement kiln dust obtained from the chemical plant were added to the anode region as the alkali source. In the experiment, 50 ml of 1 mol/L NaCl solution was added to the anode and cathode respectively, and 50 ml of saturated NaCl solution was added to the intermediate chamber. In the electrogenesis operation, the current value was adjusted from 0 to 25 mA at a gradient of 1 mA and every current value was held for 120 min. Throughout the process, the temperature of the system was maintained at 25 C. The results show that the two kinds of slag can be used to produce electricity, and the reactivity of carbide slag is very close to that of the analytically pure Ca(OH).sub.2.

(27) In the experiment using different amines as the alkali sources, relevant experiments were carried out according to the same procedure as that described above. The results show that three different kinds of amines (10% ammonia, MEA and TEA) can produce the power density of 3.71, 2.81 and 1.02 W/m.sup.2 respectively under the same reaction conditions.

Example 8 Stability of CMFC System

(28) The stability of the CMFC system was investigated. In specific experiment, 300 ml of 1 mol/L NaCl solution was added to the cathode and anode of the system respectively, and 300 ml of saturated NaCl solution was added to the intermediate chamber. After the start of the experiment, an appropriate amount of carbide slag was added to the anode, and CO.sub.2 was introduced into the catholyte at a certain flow. The fixed current density was 30.86 A/m.sup.2, and the system continuously operated for 17 h or more. During this period, output power was determined every 2 min. The results show that the system has a good stability.

Example 9 Effect of CO2 Concentration on Electrogenesis Effect

(29) The concentration of CO.sub.2 in the flue gas emitted from thermal power plants is low generally (usually <20%). In order to study the possibility of treating industrial flue gas directly with the CMFC system, mixed N.sub.2/CO.sub.2 was used to investigate the effect of CO.sub.2 concentration on the electrogenesis process. Fifty milliliter of 1 mol/L NaCl solution was added to the cathode anode and cathode region respectively, and 50 ml of saturated NaCl solution was added to the intermediate chamber. In experiments, mixed CO.sub.2 gases with different concentrations (10%, 20%, 50% and 100%) were introduced into the catholyte at a speed of 100 ml/min. The concentration of CO.sub.2 was controlled by adjusting the flow rates of N.sub.2 and CO.sub.2. The device and operation method used in associated electrogenesis process are described in Example 4. The experimental results show that electric energy can be produced even when the CO.sub.2 content is as low as 10%. As different concentrations of CO.sub.2 result in different pH values at the cathode side, the higher the CO.sub.2 concentration, the higher the output power density.

(30) The examples are only preferred examples of the invention, and they are illustrative for the invention instead of limitation thereto. Those skilled in the art should understand that many changes, modifications and even equivalent alternations can be made to the invention without departing from the spirit and scope as defined by the claims of the invention, but will fall into the protection scope of the invention.