METHOD FOR CAPTURING CARBON DIOXIDE AND NITROGEN OXIDES IN FLUE GAS AND CONVERSION THEREOF TO CARBON SOURCE AND NITROGEN SOURCE NEEDED FOR ALGAE GROWTH

Abstract

A method for treating a flue gas, comprising: step (A) desulfurizing a flue gas to obtain a desulfurized flue gas, comprising NO.sub.x, SO.sub.x, and CO.sub.2; step (B) providing oxygen for an oxidization to obtain an oxidized flue gas; step (C) rinsing the oxidized flue gas with water to obtain a rinsed flue gas and a nitric acid solution; and step (D) contacting the rinsed flue gas with a basic solution to absorb the CO.sub.2 in the rinsed flue gas. The method for treating a flue gas can largely reduce the content of CO.sub.2 and NO.sub.x in the flue gas. Besides, the obtained product can be used as a carbon source and nitrogen source for algae cultivation.

Claims

1. A method for treating a flue gas, comprising: step (A): desulfurizing a flue gas to obtain a desulfurized flue gas, wherein the flue gas comprises nitrogen oxides, sulfur oxides, and carbon dioxide, wherein the nitrogen oxides comprise nitric oxide; step (B): providing oxygen to react with the nitric oxide in the desulfurized flue gas for an oxidation reaction to obtain an oxidized flue gas, wherein the oxidized flue gas comprises nitrogen dioxide and carbon dioxide; step (C): rinsing the oxidized flue gas with water to dissolve nitrogen dioxide in the oxidized flue gas in water to obtain a rinsed flue gas and a nitric acid solution; and step (D): contacting the rinsed flue gas with a basic solution to absorb the carbon dioxide in the rinsed flue gas by the basic solution to obtain a basic-washed flue gas and a weak basic solution, wherein the pH value of the basic solution is from 9.5 to 14, and the pH value of the weak basic solution is from 8 to 9.

2. The method as claimed in claim 1, wherein the method further comprises step (E): adjusting the pH value of the nitric acid solution to 6.5 to 8 to obtain a pH-adjusted nitric acid solution.

3. The method as claimed in claim 2, wherein in the step (E), sodium hydroxide is added to the nitric acid solution to obtain the pH-adjusted nitric acid solution, wherein the pH-adjusted nitric acid solution is a pH-adjusted sodium nitrate solution.

4. The method as claimed in claim 1, wherein in the step (D), the basic solution is a sodium hydroxide solution.

5. The method as claimed in claim 2, wherein the method further comprises step (F): adding the weak basic solution and the pH-adjusted nitric acid solution to a microalgae cultivation tank containing microalgae.

6. The method as claimed in claim 5, wherein the microalgae is Botryococcus braunii, Chlorella sp., Crypthecodinium cohnii, Cylindrotheca sp., Dunaliella primolecta, Isochrysis sp., Monalanthus Salina, Nannochloris sp., Nannochloropsis sp., Neochloris oleoabundans, Nitzschia sp., Phaeodactylum tricornutum, Schizochytrium sp., Tetraselmis suecica, Arthrospira maxima, Arthrospira platensis, Cyanobacteria, or of any combination thereof.

7. The method as claimed in claim 1, wherein a total contacting time of the basic solution with the rinsed flue gas is more than 5 seconds.

8. The method as claimed in claim 3, wherein a total contacting time of the basic solution with the rinsed flue gas is more than 5 seconds.

9. The method as claimed in claim 6, wherein a total contacting time of the basic solution with the rinsed flue gas is more than 5 seconds.

10. The method as claimed in claim 1, wherein the step (C) is conducted at a temperature below 90? C.

11. The method as claimed in claim 3, wherein the step (C) is conducted at a temperature below 90? C.

12. The method as claimed in claim 6, wherein the step (C) is conducted at a temperature below 90? C.

13. The method as claimed in claim 1, wherein in the step (C), a concentration of the basic solution is from 1 wt % to 50 wt %.

14. The method as claimed in claim 3, wherein in the step (C), a concentration of the basic solution is from 1 wt % to 50 wt %.

15. The method as claimed in claim 6, wherein in the step (C), a concentration of the basic solution is from 1 wt % to 50 wt %.

16. The method as claimed in claim 1, wherein in the step (A), the desulfurizing is dry desulfurizing.

17. The method as claimed in claim 3, wherein in the step (A), the desulfurizing is dry desulfurizing.

18. The method as claimed in claim 6, wherein in the step (A), the desulfurizing is dry desulfurizing.

19. The method as claimed in claim 1, wherein the step (B) is step (B), dedusting the desulfurized flue gas and providing oxygen to react with the nitric oxide in the desulfurized flue gas for the oxidation reaction to obtain the oxidized flue gas, wherein the oxidized flue gas comprises the nitrogen dioxide and the carbon dioxide.

20. The method as claimed in claim 3, wherein the step (B) is step (B), dedusting the desulfurized flue gas and providing oxygen to react with the nitric oxide in the desulfurized flue gas for the oxidation reaction to obtain the oxidized flue gas, wherein the oxidized flue gas comprises the nitrogen dioxide and the carbon dioxide.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0057] FIG. 1 is a schematic diagram of the system for treating flue gas of Example 2 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] The present invention is further explained through the following embodiments. The present invention should not be limited to the contents of the embodiments. A person having ordinary skill in the art can do some improvement or modifications which are not departing from the scope of the present invention.

Example 1 Method for Treating a Flue Gas

[0059] First, in step (A): a flue gas was desulfurized to obtain a desulfurized flue gas. Specifically, an emitted flue gas collected from the high-temperature boilers of steelmaking comprising nitric oxide, nitrogen dioxide, sulfur oxide, carbon dioxide, and dust particles were dry-desulfurized by sodium bicarbonate to obtain a desulfurized flue gas, wherein the desulfurized flue gas comprises dust particles, nitric oxide, nitrogen dioxide and carbon dioxide.

[0060] Next, in step (B): the desulfurized flue gas was dedusted, and oxygen was provided to react with the nitric oxide in the desulfurized flue gas for an oxidation reaction to obtain an oxidized flue gas. Specifically, the desulfurized flue gas was dedusted by passing through a ceramic fiber filter coated with a platinum metal catalyst. Then the oxygen was provided to react with the nitric oxide in the desulfurized flue gas for oxidation reaction to obtain an oxidized flue gas, wherein the oxidized flue gas comprises nitrogen dioxide and carbon dioxide. Besides, the volume ratio of oxygen provided to nitric oxide in the flue gas is 1:1.5 and the fully contacting time of gas and catalyst for reaction was 0.04 second (sec) to 1 sec.

[0061] Then, in step (C), the oxidized flue gas was rinsed with water to dissolve nitrogen dioxide in the oxidized flue gas in water to obtain a rinsed flue gas and a nitric acid solution. Specifically, the oxidized flue gas was rinsed with water spraying to dissolve nitrogen dioxide in the oxidized flue gas in water to obtain a 70 wt % nitric acid solution and a rinsed flue gas.

[0062] After that, in step (D1) the rinsed flue gas, having a flow rate of 330 m.sup.3/hr, was preliminarily contacted with a 2 wt % sodium hydroxide solution, having a flow rate of 500 L/hr, in the way of co-current flow. The preliminary contact was a circulation spray lasting for 5 seconds. In the meantime, the flow rate ratio of the basic solution to the gas was 1:660. Said circulation spray refers to the rinsed flue gas sprayed by a circulated sodium hydroxide solution in a chamber, so that the carbon dioxide in the rinsed flue gas was absorbed by the sodium hydroxide solution, to obtain a preliminary basic-washed flue gas and a first sodium hydrogen carbonate solution, wherein the preliminary basic-washed flue gas comprises the remaining carbon dioxide after preliminary contact; and step (D2): the preliminary basic-washed gas, having a flow rate of 330 m.sup.3/hr, was again contacted with the 2 wt % sodium hydroxide solution, having a flow rate of 500 L/hr, in the way of co-current flow. Besides, the term again contacting refers to a circulation spray lasting for 5 seconds, resulting in a second sodium hydrogen carbonate solution and a second basic-washed gas. After that, step (D2) was repeated once, in order to obtain the third sodium hydrogen carbonate solution and a basic-washed gas. The first sodium hydrogen carbonate solution, second sodium hydrogen carbonate solution and third sodium hydrogen carbonate solution, obtained from the three stages of contacting, were collected together into a storage pool, and the pH value of sodium hydrogen carbonate solution in the storage pool was measured to be 8 to 9, while the concentration of the sodium hydrogen carbonate solution in the storage pool was about 0.5 to 2 wt %. Therefore, in step (D), the rinsed flue gas was separately contacted with 2 wt % sodium hydroxide solution for three times sequentially, so that the carbon dioxide in the flue gas was mostly absorbed by the sodium hydroxide solution. Finally, a basic-washed flue gas was obtained. The content of carbon dioxide of the basic-washed flue gas was detected by a carbon dioxide detector and the result thereof showed that the content of carbon dioxide largely reduced compared to the original flue gas.

[0063] In step (D), when the sodium hydroxide solution contacted the carbon dioxide, the acid-base neutralization reacted immediately. Besides, by contacting the rinsed flue gas with the basic solution in the way of co-current flow, the contacting time of the sodium hydroxide solution with the carbon dioxide was extended. Therefore, the acid-base neutralization can fully react.

[0064] Next, in step (E): the sodium hydroxide was added to the nitric acid to obtain a pH-adjusted nitric acid solution. Specifically, the addition amount of sodium hydroxide depended on the amount of the microalgae solution in the subsequent microalgae cultivation step. In the present embodiment, 3.3 kg sodium hydroxide was added to 5.29 kg 70 wt % nitric acid solution to obtain a pH-adjusted sodium nitrate solution with a pH value from 7 to 7.5.

[0065] Finally, in step (F): the weak basic solution and the pH-adjusted sodium nitrate solution were added to a microalgae cultivation tank containing microalgae. Specifically, every 7 days, 25 kg pH-adjusted sodium nitrate solution obtained from step (E) was added to a microalgae cultivation tank containing 10 metric ton microalgae solution as nitrogen source. Meanwhile, 0.5 wt % to 2 wt % sodium bicarbonate obtained from step (D) was dried to obtain solid sodium bicarbonate and then 95 kg solid sodium bicarbonate was added to said microalgae cultivation tank containing 10 metric ton microalgae solution as carbon source every 7 days. In the present embodiment, a sodium nitrate solution with specific pH value obtained by adding specific amount of sodium hydroxide in step (E), and the specific addition amount of pH-adjusted sodium nitrate solution and sodium bicarbonate solution in step (F) can make cyanobacteria grow optimally, so that harvested microalgae powder can be maximum: 34.5 kg harvested microalgae powder every 7 days. Compared with the better addition amount of pH-adjusted sodium nitrate solution and sodium bicarbonate solution, if the addition amount of the nitrogen source and carbon source is not controlled in the optimal range, the produced microalgae powder would be less, which is only about 1/10 of the present invention, that is, 10 metric tons of algae water can only produce about 3 to 3.78 kg of microalgae powder. Generally, the weight of dried microalgae powder is about 1/3500-1/2500 of the wet weight of microalgae solution after 7 days of cultivation.

Example 2 System for Treating a Flue Gas

[0066] FIG. 1 is a schematic diagram of the system for treating flue gas of the present example, wherein the double solid line indicates the pipeline where liquid or gas flows.

[0067] As shown in FIG. 1, the system for treating flue gas 1 of the present invention comprises: a desulfurization part 11, an oxidization part 12, a rinsing part 13 and a basic washing part 14, wherein the desulfurization part 11 contains a flue gas inlet 111, a desulfurization reaction unit 112 and a desulfurized flue gas outlet 113, wherein the desulfurization reaction unit 112 is respectively in fluid communication with the flue gas inlet 111 and the desulfurized flue gas outlet 113. The flue gas inlet 111 is for introducing a flue gas into the desulfurization reaction unit 112, wherein the flue gas comprises nitrogen oxides, such as nitric oxide and nitrogen dioxide, sulfur oxides, and carbon dioxide. The desulfurization reaction unit 112 provides a desulfurization agent for reacting with the sulfur oxides in the flue gas to obtain a desulfurized flue gas. The desulfurized flue gas outlet 113 is for discharging the desulfurized flue gas.

[0068] The oxidization part 12 contains an oxygen providing unit 121, an oxidation reaction unit 122, and an oxidized flue gas outlet 123, wherein the oxidation reaction unit 122 is respectively in fluid communication with the desulfurized flue gas outlet 113, the oxygen providing unit 121, and the oxidized flue gas outlet 123. The oxygen providing unit 121 provides oxygen to the oxidation reaction unit 122 for oxidizing the nitric oxide in the desulfurized flue gas to obtain an oxidized flue gas, which is discharged from the oxidized flue gas outlet 123, wherein the oxidized flue gas comprises nitrogen dioxide and carbon dioxide

[0069] The rinsing part 13 contains a rinsing tower 131, a rinsed flue gas outlet 132, and a nitric acid solution outlet 133, wherein the rinsing tower 131 is respectively in fluid communication with the oxidized flue gas outlet 123, the rinsed flue gas outlet 132 and the nitric acid solution outlet 133. The rinsing tower 131 provides water for rinsing the oxidized flue gas and dissolving the nitrogen dioxide in the oxidized flue gas in water to obtain a rinsed flue gas and a nitric acid solution. The rinsed flue gas outlet 132 is for discharging the rinsed flue gas. The nitric acid solution outlet 133 is for discharging the nitric acid solution. In other embodiments, the nitric acid solution outlet 133 is in fluid communication with a pH adjustment unit 134, which can adjust the pH value of the nitric acid solution.

[0070] The basic washing part 14 contains a basic solution spraying unit 141, a basic-washed flue gas outlet 142, and a weak basic solution outlet 143, wherein the basic solution spraying unit 141 is respectively in fluid communication with the rinsed flue gas outlet 132, the basic-washed flue gas outlet 142, and a weak basic solution outlet 143. The basic solution spraying unit 141 provides a basic solution to absorb carbon dioxide in the rinsed flue gas to obtain a basic-washed flue gas and a weak basic solution. The basic-washed flue gas outlet 142 is for discharging the basic-washed flue gas, and the weak basic solution outlet 143 is for discharging the weak basic solution.

[0071] The method of utilizing the system for treating flue gas of the present invention is stated as follows:

[0072] First, an emitted flue gas collected from the high-temperature boilers of steelmaking was introduced to the desulfurization part 11 through the flue gas inlet 111; wherein the flue gas comprised nitrogen oxides such as nitric oxide and nitrogen dioxide, sulfur oxides, carbon dioxide. The sulfur oxides in the flue gas were absorbed by the desulfurization agent: sodium bicarbonate powders in the desulfurization reaction unit 112, so that a desulfurized flue gas was obtained.

[0073] Next, the desulfurized flue gas was introduced to the oxidation reaction unit 122 in the oxidization part 12 through the desulfurized flue gas outlet 113. The oxygen was provided from the oxygen providing unit 121 to the oxygen reaction unit 122, so that the nitric oxide in the desulfurized flue gas was oxidized and an oxidized flue gas was obtained.

[0074] Then, the oxidized flue gas was introduced to the rinsing tower 131 in the rinsing part 13 through the oxidized flue gas outlet 123, wherein the rinsing tower 131 provides water for rinsing the oxidized flue gas, so that the nitrogen dioxide in the oxidized flue gas was dissolved in water and a rinsed flue gas and a nitric acid solution were obtained. The nitric acid solution can be introduced to a cultivation tank 15 containing microalgae via a nitric acid solution outlet 133 for algae cultivation. In another embodiment, sodium hydroxide can be provided to the nitric acid solution by pH adjustment unit 134, so that the pH value of the nitric acid solution can be adjusted to pH 6.5 to 8, which can be introduced to the cultivation tank 15 containing microalgae through the nitric acid solution outlet for algae cultivation.

[0075] Finally, the rinsed flue gas was introduced to a basic solution spraying unit 141 in the basic washing part 14 via the rinsed flue gas outlet 132 for absorbing carbon dioxide. Specifically, the carbon dioxide in the rinsed flue gas was absorbed by the basic solution provided by the basic solution spraying unit 141, and a weak basic solution and a basic-washed flue gas were obtained. In the present embodiment, the basic solution adopted in the basic solution spraying unit 141 was sodium hydroxide solution, so the weak basic solution obtained was sodium bicarbonate solution, which can be used as a carbon source for microalgae cultivation. Said sodium bicarbonate solution was introduced to the cultivation tank 15 containing microalgae via the weak basic solution outlet 143 for microalgae cultivation. The basic basic-washed flue gas, the carbon dioxide in which was largely decreased after detected by a detector, can be discharged via the basic-washed flue gas outlet 142. After the step (D) was conducted, the carbon dioxide removal rate can be almost 90%.

[0076] To sum up, the method for treating flue gas and using the system for treating flue gas can effectively absorb carbon dioxide and nitrogen oxides. Besides, the weak basic solution and the nitric acid solution obtained from the system for treating flue gas can be used as carbon source and nitrogen source respectively for microalgae cultivation.

[0077] The above embodiments are only preferred embodiments of the present invention, not intended to limit the present invention in any aspect. It is apparent to those skilled in the art that various modifications and variations can be made without departing from the scope and spirit of the invention. Although the present invention has been described in terms of specific preferred embodiments, it should be understood that the invention should not be unduly limited to those specific embodiments. In fact, the various modifications that are obvious to those of ordinary skill in the art are also encompassed within the scope of the following claims.