A method for treating exhaust gas by recovering carbon dioxide and nitrogen oxides from the exhaust gas and converting them into carbon and nitrogen sources necessary for algae growth.

Abstract

To capture carbon dioxide and nitrogen oxide in an exhaust gas, and convert the same to other product in order to use the captured object thereafter.SOLUTION: An exhaust gas treatment method comprises: a step (A) at which an exhaust gas containing NOX, SOX and CO2 is desulfurized to obtain the desulfurized exhaust gas; a step (B) at which oxygen is supplied for oxidation to obtain the oxidized exhaust gas; a step (C) at which the oxidized exhaust gas is washed with water to obtain the water-washed exhaust gas and a nitric acid solution; and a step (D) at which the water-washed exhaust gas is brought into contact with a basic solution to absorb CO2 in the water-washed exhaust gas. The exhaust gas treatment method can extremely reduce contents of CO2 and NOX in the exhaust gas. Further, the obtained product can be used as a carbon source and a nitrogen source for culture of algae.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The technology disclosed herein relates to a gas treatment method, and more particularly to a treatment method for reusing pollutants in exhaust gas. [Background technology]

[0002] With the development of industry, the amount of exhaust gas emitted from boilers in factories and thermal power plants has increased significantly, causing serious air pollution. In particular, polluted air containing greenhouse gases such as sulfur oxides, nitrogen oxides, and carbon dioxide is generated mainly by the combustion of fossil fuels. When sulfur oxides and nitrogen oxides are released into the atmosphere, acid rain occurs immediately, causing soil acidification and affecting the growth of agricultural crops. Carbon dioxide is one of the greenhouse gases that cause global warming. Currently, environmental protection regulations stipulate that exhaust gases emitted from chimneys must be subjected to treatment processes such as desulfurization and denitrification before being released, and sulfur oxides and nitrogen oxides must be reduced to a certain concentration. In existing technologies, the method of reducing nitrogen oxides to harmless stable nitrogen and releasing it into the atmosphere is common. However, it is unfortunate that this method does not allow for effective use of the discharged waste.

[0003] Furthermore, emitted waste gases increase the amount of greenhouse gases, strengthening the greenhouse effect and causing global warming. As a result, extreme weather events become more frequent, floods and droughts become more frequent, and extreme cold and heat become the norm. These events affect crop yields, cause ice sheets to melt and sea levels to rise, and have serious effects on ecosystems and the environment. For this reason, the goal of global efforts is to reduce carbon dioxide, the main component of greenhouse gases.

[0004] In order to solve the problem of carbon dioxide and the greenhouse effect, many technologies for carbon dioxide capture, storage and utilization have been developed. These technologies involve converting carbon dioxide into various chemicals, for example, chemical plastic products such as acetic acid and polycarbonate (PC). These methods seem rational at first glance, and economic benefits can be expected through industrial recycling. However, in reality, since human beings do not consume that many chemical products, it is not possible to remove much carbon dioxide from the atmosphere. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] CN205360917U Summary of the Invention

[0006] The fossil fuels currently burned by human beings are organic matter accumulated since ancient times by plant photosynthesis. These fossil fuels contain large amounts of carbon and nitrogen elements, which are essential for plant growth. If we were to use only fossil fuel energy and arbitrarily emit large amounts of these carbon and nitrogen elements, it would not only destroy the natural carbon cycle, but also cause adverse effects such as pollution and global warming. Therefore, introducing large amounts of these carbon and nitrogen elements into the natural carbon cycle is the fundamental solution to the pollution and global warming problems currently facing human beings.

[0007] In view of the fact that existing technologies are unable to reduce the large amount of carbon dioxide in the exhaust gas emitted after burning fossil fuels, the objective of the present invention is to capture the large amount of carbon dioxide and nitrogen oxides in the exhaust gas emitted after burning fossil fuels, and further convert the captured objects into other products for further use, for example, by applying these products to algae farming, a large amount of carbon dioxide can be returned to the natural carbon cycle and quickly transferred to other layers of the carbon cycle, thereby significantly reducing the carbon dioxide in the exhaust gas and significantly reducing the acid rain caused by nitrogen oxides and its environmental impact.

[0008] In order to achieve the above object, the exhaust gas treatment method of the present invention comprises: The exhaust gas is desulfurized to obtain a desulfurized exhaust gas, the exhaust gas being nitrogen oxides (NO X ), sulfur oxides (SO X ) and carbon dioxide (CO 2 (A) comprising a step of: Oxygen is supplied for an oxidation reaction to react with nitrogen monoxide in the desulfurized exhaust gas to obtain an oxidized exhaust gas, and the oxidized exhaust gas is nitrogen dioxide (NO 2 ) and carbon dioxide; and (C) washing the oxidized exhaust gas with water to dissolve nitrogen dioxide in the oxidized exhaust gas in water to obtain a washed exhaust gas and a nitric acid solution; and (D) contacting the water-washed flue gas with a basic solution to cause the basic solution to absorb carbon dioxide in the water-washed flue gas, thereby obtaining a base-washed flue gas and a weakly basic solution, wherein the basic solution has a pH value of 9.5 to 14 and the weakly basic solution has a pH value of 8 to 9.

[0009] In the existing gas treatment method, desulfurization and denitrification technology is used in which nitrogen oxides are decomposed into nitrogen by a catalyst, and then desulfurization treatment such as wet desulfurization and dry desulfurization is performed. Nitrogen is a stable gas and cannot be absorbed or utilized by plants, and nitrate ions are a form of nitrogen source that can be directly absorbed and utilized by plants. According to the present invention, the flue gas is preliminarily purified by a desulfurization process, and the sulfur element that hinders the growth of algae thereafter can be significantly reduced. Then, an oxygen supply process and a water washing process are performed, and the nitrogen dioxide obtained by oxidizing the nitrogen oxides in the flue gas is dissolved in water to obtain a nitric acid solution, and this basic solution is brought into contact with the washed flue gas to perform an acid-base neutralization reaction. Therefore, the present invention can achieve the effects of significantly reducing carbon dioxide and nitrogen oxides in the flue gas, reducing the emission of greenhouse gases and acidic gases, purifying air quality, and improving ecology and the environment. Furthermore, the products obtained by the treatment method of the present invention can be used as carbon and nitrogen sources for subsequent algae cultivation. As a result, the present invention not only significantly reduces greenhouse gases in the flue gas, but also fully utilizes waste materials in the flue gas. That is to say, the present invention is not only environmentally friendly, but also has commercial value.

[0010] Preferably, the nitrogen oxide is nitrogen dioxide (NO 2 ).

[0011] In accordance with the present invention, sulfur oxides include, but are not limited to, sulfur dioxide and sulfur trioxide.

[0012] According to the invention, the desulfurized exhaust gas contains nitric oxide, nitrogen dioxide and carbon dioxide.

[0013] According to the present invention, the source of exhaust gas is exhaust gas generated by burning coal, natural gas, heavy oil, etc. in thermal power plants, steel mills, concrete plants, petrochemical plants, oil refineries, paper plants, heating plants, etc., but is not limited to these.

[0014] Preferably, the concentration of said sulfur oxides in said exhaust gas is less than 500 ppm.

[0015] Preferably, in step (B) of the method, the volume ratio of oxygen to nitric oxide (i.e., the volume ratio of oxygen to nitric oxide) is 1:1.5 to 1:2.0. More preferably, in step (B) of the method, the volume ratio of oxygen to nitric oxide is 1:1.7 to 1:1.9, for example, 1:1.8.

[0016] Preferably, in step (B) of the method, a catalyst may be used to promote the oxidation reaction of oxygen reacting with nitrogen monoxide in the desulfurized exhaust gas to obtain the oxidized exhaust gas, and the catalyst may be a metal or activated carbon. More preferably, the metal is gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), etc.

[0017] Preferably, the concentration of the nitric acid solution obtained in step (C) of the method is between 50 weight percent (wt%) and 70 wt%, more preferably between 55 wt% and 65 wt%.

[0018] Preferably, the water-washed tail gas obtained from step (C) of the above process is continuously oxidized so that the nitric oxide therein can be used up after several oxidation cycles.

[0019] Preferably, the method further comprises a step (E) of adjusting the pH value of the nitric acid solution to 6.5-8 to obtain a pH-adjusted nitric acid solution. Due to this pH range, the pH-adjusted nitric acid solution is suitable for the subsequent step of adjusting the pH range to an optimal range for the growth of microalgae, and can provide the nitrogen element required for the rapid growth of microalgae.

[0020] Preferably, the method further comprises a step (F) of adding the weakly basic solution and the pH-adjusted nitric acid solution to a microalgae culture tank containing microalgae. This can provide a carbon source for photosynthesis to the microalgae in the water, and the microalgae can be further processed to produce functional foods, animal feed, and biodiesel. In addition, the microalgae culture liquid can be reused for carbon dioxide adsorption. Therefore, the product of the present invention can be effectively used. Preferably, the microalgae culture tank containing microalgae contains a microalgae solution, and the microalgae solution contains 0.01 wt% to 0.5 wt% of microalgae. In another embodiment, the weakly basic solution (i.e., carbon source) or the pH-adjusted nitric acid solution (i.e., nitrogen source) is dried before being added to the microalgae culture tank containing microalgae. Since the nitrogen source or carbon source can be added to the microalgae culture tank containing microalgae in solid form, the drying process makes it easier to transport the carbon source or nitrogen source to another location for microalgae culture.

[0021] Preferably, the weight ratio of the amount of weakly basic solution (i.e., carbon source) added to the pH-adjusted nitric acid solution (i.e., nitrogen source) in the microalgae solution in the microalgae culture tank containing the microalgae (i.e., the weight ratio of the weakly basic solution to the pH-adjusted nitric acid solution to the microalgae solution) is 100-2000:10-100:8000-20000.

[0022] Preferably, in step (E) of the method, sodium hydroxide is added to the nitric acid solution to obtain the nitric acid solution with the adjusted pH value, and the nitric acid solution with the adjusted pH value is a sodium nitrate solution with the adjusted pH value. In order to avoid the microalgae growing environment becoming acidic as a result of directly adding a large amount of nitric acid solution to the microalgae culture tank, an alkaline substance can be added to the nitric acid solution to adjust the pH range so that the microalgae can grow in a suitable environment. Adjusting the pH value with sodium hydroxide can promote the growth of microalgae more than other alkaline substances such as potassium hydroxide, magnesium oxide, calcium carbonate, etc. Taking the cultivation of cyanobacteria as an example, adding 2kg to 4kg of sodium hydroxide to 3kg to 5kg of 70wt% nitric acid solution and then adding it to 1 ton of microalgae solution can provide the best growing conditions for cyanobacteria, i.e., the biomass of cyanobacteria can grow rapidly. More preferably, 10kg of NaHCO 3 as a carbon source is added to the nitric acid solution. 3 When added simultaneously, the total biomass of microalgae can be grown within 48 hours under proper water flow and sufficient lighting.

[0023] Preferably, step (D) comprises: (D1) preliminarily contacting the water-washed flue gas with the basic solution, and allowing the basic solution to absorb carbon dioxide in the water-washed flue gas, thereby obtaining a preliminarily base-washed flue gas and a first weakly basic solution, wherein the preliminarily base-washed flue gas contains carbon dioxide remaining after preliminarily contacting; and (D2) contacting the preliminarily base-washed flue gas with the basic solution again, causing the basic solution to absorb carbon dioxide in the preliminarily base-washed flue gas, and obtaining the base-washed flue gas and a second weakly basic solution, the weakly basic solution including the first weakly basic solution and the second weakly basic solution.

[0024] More preferably, the step (D2) can be repeated one or more times. For example, when the step (D2) is repeated once, it means that in the step (D), the water-washed tail gas is contacted with fresh basic solution three times in sequence.

[0025] Preferably, the desulfurization method is dry desulfurization. By adopting dry desulfurization, the problem of the wall of semi-dry desulfurization being sticky can be avoided. Specifically, sodium bicarbonate powder can be used as a desulfurization agent for dry desulfurization. The activation of sodium bicarbonate by high-temperature exhaust gas forms a microporous structure on the surface of sodium bicarbonate, which can react with sulfur oxides quickly and sufficiently.

[0026] Preferably, the basic solution in step (D) of the method is a sodium hydroxide solution, one reason being that the product obtained by the reaction of sodium hydroxide with carbon dioxide, sodium bicarbonate, does not cause environmental pollution, and can be recovered and used, particularly as a good carbon source, and can be added to the microalgae culture tank in step (E).

[0027] Preferably, the microalgae is selected from the group consisting of 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., and the like. sp., Tetraselmis suecica, Arthrospira maxima, Arthrospira platensis, cyanobacteria, or combinations thereof.

[0028] Preferably, the step (B) is a step (B') of de-dusting the desulfurized flue gas and supplying oxygen for an oxidation reaction with nitric oxide in the desulfurized flue gas to obtain the oxidized flue gas, the oxidized flue gas including nitrogen dioxide and carbon dioxide. This step can further purify the flue gas. Specifically, dust particles in the flue gas and / or dust particles generated by the aforementioned desulfurization process can be removed. In some embodiments, the particle size of the aforementioned dust particles is greater than 0.1 μm. Preferably, the de-dusting step is filtering the dust particles by a nanofiber tube.

[0029] Preferably, in step (D) of the method, the total contact time between the basic solution and the washed tail gas is at least 5 seconds. More preferably, the total contact time is at least 15 seconds. More preferably, the total contact time is at least 1 minute.

[0030] In step (D) of the method, the higher the concentration of the basic solution, the easier it is for the basic compounds in the basic solution to precipitate, resulting in clogging of the flow path and increasing the operational risk. Meanwhile, the lower the concentration of the basic solution, the higher the flow rate of the basic solution required. That is, it will result in a relative waste of energy. Therefore, preferably, the concentration of the basic solution is 1 wt% to 50 wt%, but is not limited thereto. In one embodiment, the concentration of the basic solution is 3 wt% to 5 wt%, which is safe for operation and relatively energy-saving. In another embodiment, a high concentration basic solution is adopted to produce salt crystals. For example, sodium hydroxide having a concentration of 10 wt% or more can be adopted for spraying to produce sodium bicarbonate crystals, and the sodium bicarbonate can be transported to other places in solid form for algae cultivation.

[0031] Preferably, step (D) of the method is carried out at a temperature of 90°C or less. This temperature range can prevent the water contained in the basic solution from evaporating due to an excessively high temperature, and can partially precipitate the dissolved basic compound, thereby preventing the flow path for transporting the basic solution from being easily clogged. More preferably, step (D) is carried out at a temperature of 4°C or more and 90°C or less, since the reaction rate of acid-base neutralization decreases at temperatures below 4°C. Furthermore, taking into consideration the solubility of the salt obtained by the reaction of the basic solution with carbon dioxide, step (D) is more preferably carried out at a temperature of 50°C or more and 60°C or less.

[0032] The present invention further provides a system for treating an exhaust gas, the exhaust gas treatment system comprising: A desulfurization section including an exhaust gas inlet, a desulfurization reaction unit, and a desulfurized exhaust gas outlet, the desulfurization reaction unit being in fluid communication with the exhaust gas inlet and the desulfurized exhaust gas outlet; an oxidation section including an oxygen supply unit, an oxidation reaction unit, and an outlet of an oxidized exhaust gas, the oxidation reaction unit being in fluid communication with the outlet of the desulfurized exhaust gas, the oxygen supply unit, and the outlet of the oxidized exhaust gas, respectively; a water washing tower, a water washing section in fluid communication with the oxidized exhaust gas outlet, the water washed exhaust gas outlet, and the nitric acid solution outlet, respectively; a basic solution spray unit, a base-washed tail gas outlet, and a weakly basic solution outlet, the basic solution spray unit comprising a base wash section in fluid communication with the water-washed tail gas outlet, the base-washed tail gas outlet, and the weakly basic solution outlet, respectively; The exhaust gas inlet is for introducing the exhaust gas into the desulfurization reaction unit, the exhaust gas including nitrogen oxides such as nitric oxide and nitrogen dioxide, sulfur oxides, and carbon dioxide; The desulfurization reaction unit supplies a desulfurization agent to react with sulfur oxides in the exhaust gas to obtain a desulfurized exhaust gas; the desulfurized exhaust gas outlet is for discharging the desulfurized exhaust gas, The oxygen supply unit supplies oxygen to the oxidation reaction unit to oxidize nitrogen monoxide in the desulfurized exhaust gas to obtain an oxidized exhaust gas; the oxidized flue gas outlet is for discharging the oxidized flue gas, the oxidized flue gas comprising nitrogen dioxide and carbon dioxide; the water washing tower supplies water for washing the oxidized flue gas with water and dissolving nitrogen dioxide in the oxidized flue gas in water to obtain a washed flue gas and a nitric acid solution; the water-washed exhaust gas outlet is for discharging the water-washed exhaust gas; The nitric acid solution outlet is for discharging the nitric acid solution, The basic solution spray unit supplies a basic solution to absorb carbon dioxide in the water-washed flue gas, and obtains a basic-washed flue gas and a weakly basic solution; the base-washed exhaust gas outlet is for discharging the base-washed exhaust gas; The weakly basic solution outlet is for discharging the weakly basic solution.

[0033] Preferably, the oxidation reaction unit is a ceramic fiber filter containing a catalyst. Preferably, the catalyst is a metal or activated carbon. More preferably, the metal may be gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), or ruthenium (Ru).

[0034] Preferably, a nitric oxide detection valve is installed at the outlet of the water-washed exhaust gas, and the nitric oxide detection valve is connected to the oxidation unit via a circulation pipe, and can detect the concentration of nitric oxide in the outlet of the water-washed exhaust gas; when the concentration of nitric oxide in the outlet of the water-washed exhaust gas is higher than 10 ppm, the water-washed exhaust gas is reintroduced into the oxidation reaction unit.

[0035] Preferably, the nitric acid solution outlet is further in fluid communication with a pH value adjustment unit, which provides sodium hydroxide to adjust the pH value of the nitric acid to provide optimal conditions for algae growth.

[0036] The advantage of the present invention is that the nitrogen oxides in the flue gas are retained by desulfurization without denitrification. Then, the nitrogen dioxide is dissolved in water to form a nitric acid solution through the steps of oxygen treatment and water washing, and this basic solution is brought into contact with the water-washed flue gas to carry out an acid-base neutralization reaction. Therefore, the present invention can achieve the effects of greatly reducing carbon dioxide and nitrogen oxides in the flue gas, reducing greenhouse gas emissions, purifying air quality, and improving ecology and the environment. In addition, the products obtained by the treatment method of the present invention can be used as carbon and nitrogen sources for subsequent algae cultivation. As a result, the present invention not only greatly reduces greenhouse gases in the flue gas, but also fully utilizes the waste materials in the flue gas. That is, the present invention not only protects the environment, but also has commercial value. [Brief description of the drawings]

[0037] [Figure 1] Second Embodiment FIG. 1 is a schematic diagram of an exhaust gas treatment system according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] The present invention will be further described through the following embodiments. The present invention should not be limited to the contents of the embodiments. Those skilled in the art can make various improvements and modifications without departing from the spirit of the present invention.

[0039] (Example 1 Exhaust gas treatment method) First, in step (A), the exhaust gas was desulfurized to obtain a desulfurized exhaust gas (desulfurized exhaust gas). Specifically, the exhaust gas, which contains nitrogen monoxide, nitrogen dioxide, sulfur oxides, carbon dioxide, and dust particles and is collected from a high-temperature boiler of a steelmaking plant, was dry-desulfurized with sodium bicarbonate to obtain a desulfurized exhaust gas. The desulfurized exhaust gas contains dust particles, nitrogen monoxide, nitrogen dioxide, and carbon dioxide.

[0040] Next, in step (B'), the desulfurized exhaust gas was dedusted, and oxygen was supplied to carry out an oxidation reaction with the nitric oxide in the desulfurized exhaust gas to obtain an oxidized exhaust gas (oxidized exhaust gas). Specifically, the desulfurized exhaust gas was passed through a ceramic fiber filter covered with a platinum metal catalyst to be desulfurized. Then, oxygen was supplied to carry out an oxidation reaction with the nitric oxide in the desulfurized exhaust gas to obtain an oxidized exhaust gas. The oxidized exhaust gas contains nitrogen dioxide and carbon dioxide. In addition, the volume ratio of the supplied oxygen to the nitric oxide in the exhaust gas (i.e., the volume ratio of oxygen to nitric oxide) was 1:1.5, and the complete contact time between the gas and the catalyst for the reaction was 0.04 to 1 second.

[0041] Next, in step (C), the oxidized exhaust gas was washed with water to dissolve the nitrogen dioxide in the oxidized exhaust gas in water, and a washed exhaust gas (water-washed exhaust gas) and a nitric acid solution were obtained. Specifically, the oxidized exhaust gas was washed with water spray, and the nitrogen dioxide in the oxidized exhaust gas was dissolved in water, and a 70 wt% nitric acid solution and a water-washed exhaust gas were obtained.

[0042] Then, at step (D1), 330m 3 The water-washed exhaust gas having a flow rate of 330 m3 / hr was pre-contacted with a 2 wt% sodium hydroxide solution having a flow rate of 500 L / hr in parallel flow. The pre-contact was a continuous circulation spray for 5 seconds. During that time, the flow rate ratio of the basic solution to the gas was 1:660. The circulation spray refers to the water-washed exhaust gas sprayed with the sodium hydroxide solution circulating in the chamber, whereby the carbon dioxide in the water-washed exhaust gas was absorbed by the sodium hydroxide solution, and a pre-base-washed exhaust gas (pre-base-washed exhaust gas) and a first sodium bicarbonate solution were obtained, and the pre-base-washed exhaust gas contains carbon dioxide remaining after the pre-contact. In step (D2), 330 m3 of water was sprayed with 2 wt% sodium hydroxide solution having a flow rate of 500 L / hr. 3The preliminary base-scrubbed flue gas having a flow rate of 1000 L / hr was "contacted again" in parallel with a 2 wt% sodium hydroxide solution having a flow rate of 500 L / hr. The term "contact again" means continuous circulating spray for 5 seconds, which resulted in a second sodium bicarbonate solution and a second base-scrubbed flue gas. Step (D2) was then repeated once to obtain a third sodium bicarbonate solution and a base-scrubbed flue gas. The first sodium bicarbonate solution, the second sodium bicarbonate solution, and the third sodium bicarbonate solution obtained by this three-stage contact were collected together and collected in a storage pool, and the pH value of the sodium bicarbonate solution in the storage pool was measured to be 8 to 9, and the concentration of the sodium bicarbonate solution in the storage pool was about 0.5 to 2 wt%. Therefore, in step (D), the water-scrubbed flue gas was contacted with a 2 wt% sodium hydroxide solution three times in sequence, and most of the carbon dioxide in the flue gas was absorbed by the sodium hydroxide solution. Finally, a base-scrubbed flue gas (base-scrubbed flue gas) was obtained. When the carbon dioxide content of the base-cleaned exhaust gas was detected using a carbon dioxide detector, it was found that the carbon dioxide content was significantly reduced compared to the original exhaust gas.

[0043] In step (D), when the sodium hydroxide solution contacted the carbon dioxide, the acid-base neutralization reaction occurred immediately. In addition, by contacting the water-washed exhaust gas with the basic solution in parallel, the contact time of the sodium hydroxide solution with the carbon dioxide was extended. Therefore, the acid-base neutralization reaction can be fully carried out.

[0044] Next, in step (E), sodium hydroxide was added to the nitric acid to obtain a nitric acid solution with an adjusted pH value (a pH-adjusted nitric acid solution). Specifically, the amount of sodium hydroxide added depended on the amount of the microalgae solution in the subsequent microalgae culture process. In this embodiment, 3.3 kg of sodium hydroxide was added to 5.29 kg of a 70 wt% nitric acid solution to obtain a sodium nitrate solution with an adjusted pH value of 7 to 7.5 (a pH-adjusted sodium nitrate solution).

[0045] Finally, in step (F), the weakly basic solution and the pH-adjusted sodium nitrate solution were added to the microalgae culture tank containing the microalgae. Specifically, 25 kg of the pH-adjusted sodium nitrate solution obtained in step (E) was added as a nitrogen source to the microalgae culture tank containing 10 tons of microalgae solution every 7 days. Meanwhile, the 0.5 wt% to 2 wt% sodium bicarbonate obtained in step (D) was dried to obtain solid sodium bicarbonate, and then 95 kg of solid sodium bicarbonate was added as a carbon source to the microalgae culture tank containing 10 tons of microalgae solution every 7 days. In this embodiment, a specific amount of sodium hydroxide was added in step (E) to obtain a sodium nitrate solution with a specific pH value, and a specific amount of the pH-adjusted sodium nitrate solution and sodium bicarbonate solution were added in step (F), thereby optimally growing the cyanobacteria and maximizing the harvested microalgae powder. That is, 34.5 kg of microalgae powder can be harvested every 7 days. When the amount of nitrogen source and carbon source added is not controlled within the optimal range, the amount of microalgae powder produced is less than that when the amount of pH-adjusted sodium nitrate solution and sodium bicarbonate solution added is better, and is only about 1 / 10 of that of the present invention. In other words, only about 3 to 3.78 kg of microalgae powder can be produced from 10 tons of algae aqueous solution. Generally, after 7 days of cultivation, the weight of the dry microalgae powder is about 1 / 3500 to 1 / 2500 of the wet weight of the microalgae solution.

[0046] (Example 2 Exhaust Gas Treatment System) FIG. 1 is a schematic diagram of an exhaust gas treatment system according to this embodiment, in which double solid lines indicate pipes through which liquid or gas flows.

[0047] As shown in FIG. 1, the flue gas treatment system 1 of the present invention includes a desulfurization section 11, an oxidation section 12, a water washing section 13, and a base washing section 14. The desulfurization section 11 includes an flue gas inlet 111, a desulfurization reaction unit 112, and an outlet (desulfurized flue gas outlet) 113 for desulfurized flue gas. The desulfurization reaction unit 112 is fluidly connected to the flue gas inlet 111 and the desulfurized flue gas outlet 113. The flue gas inlet 111 is for introducing the flue gas into the desulfurization reaction unit 112. The flue gas contains nitrogen oxides such as nitric oxide and nitrogen dioxide, sulfur oxides, and carbon dioxide. The desulfurization reaction unit 112 supplies a desulfurizing agent to react with the sulfur oxides in the flue gas to obtain a desulfurized flue gas (desulfurized flue gas). The desulfurized flue gas outlet 113 is for discharging the desulfurized flue gas.

[0048] The oxidation section 12 includes an oxygen supply unit 121, an oxidation reaction unit 122, and an outlet for oxidized exhaust gas (oxidized exhaust gas outlet) 123, and the oxidation reaction unit 122 is fluidly connected to the desulfurized exhaust gas outlet 113, the oxygen supply unit 121, and the oxidized exhaust gas outlet 123, respectively. The oxygen supply unit 121 supplies oxygen to the oxidation reaction unit 122 to oxidize nitrogen monoxide in the desulfurized exhaust gas to obtain oxidized exhaust gas (oxidized exhaust gas), and the oxidized exhaust gas is discharged from the oxidized exhaust gas outlet 123, and the oxidized exhaust gas contains nitrogen dioxide and carbon dioxide.

[0049] The water washing section 13 includes a water washing tower 131, an outlet (washed flue gas outlet) 132 for the washed flue gas, and a nitric acid solution outlet 133, and the water washing tower 131 is fluidly connected to the oxidized flue gas outlet 123, the washed flue gas outlet 132, and the nitric acid solution outlet 133, respectively. The water washing tower 131 supplies water for washing the oxidized flue gas, and dissolves nitrogen dioxide in the oxidized flue gas in the water to obtain a washed flue gas (washed flue gas) and a nitric acid solution. The washed flue gas outlet 132 is for discharging the washed flue gas. The nitric acid solution outlet 133 is for discharging the nitric acid solution. In another embodiment, the nitric acid solution outlet 133 is fluidly connected to a pH value adjustment unit 134 that can adjust the pH value of the nitric acid solution.

[0050] The base washing section 14 includes a basic solution spray unit 141, an outlet for base-washed exhaust gas (base-washed exhaust gas outlet) 142, and a weakly basic solution outlet 143, and the basic solution spray unit 141 is fluidly connected to the water-washed exhaust gas outlet 132, the base-washed exhaust gas outlet 142, and the weakly basic solution outlet 143, respectively. The basic solution spray unit 141 supplies a basic solution to absorb carbon dioxide in the water-washed exhaust gas, and obtains a base-washed exhaust gas (base-washed exhaust gas) and a weakly basic solution. The base-washed exhaust gas outlet 142 is for discharging the base-washed exhaust gas, and the weakly basic solution outlet 143 is for discharging the weakly basic solution.

[0051] The method of using the exhaust gas treatment system of the present invention is as follows.

[0052] First, exhaust gas collected from a high-temperature boiler of a steel plant is introduced into the desulfurization section 11 through an exhaust gas inlet 111. The exhaust gas contains nitrogen oxides such as nitric oxide and nitrogen dioxide, sulfur oxides, and carbon dioxide. The sulfur oxides in the exhaust gas are adsorbed by a desulfurizing agent (i.e., sodium bicarbonate powder) in the desulfurization reaction unit 112, and desulfurized exhaust gas (desulfurized exhaust gas) is obtained.

[0053] Next, the desulfurized exhaust gas was introduced into the oxidation reaction unit 122 of the oxidation section 12 from the desulfurized exhaust gas outlet 113. Oxygen was supplied from the oxygen supply unit 121 to the oxidation reaction unit 122, whereby the nitrogen monoxide in the desulfurized exhaust gas was oxidized, and an oxidized exhaust gas (oxidized exhaust gas) was obtained.

[0054] The oxidized exhaust gas is then introduced from the oxidized exhaust gas outlet 123 into the water washing tower 131 of the water washing section 13, and water for washing the oxidized exhaust gas is supplied from the water washing tower 131 to dissolve nitrogen dioxide in the oxidized exhaust gas in the water, thereby obtaining a washed exhaust gas and a nitric acid solution. The nitric acid solution can be introduced into the culture tank 15 containing microalgae through the nitric acid solution outlet 133 for algae cultivation. In another embodiment, the pH value of the nitric acid solution can be adjusted to pH 6.5 to 8 by supplying sodium hydroxide to the nitric acid solution by the pH value adjustment unit 134, and this nitric acid solution can be introduced into the culture tank 15 containing microalgae through the nitric acid solution outlet 133 for algae cultivation.

[0055] Finally, the water-washed exhaust gas was introduced into the basic solution spray unit 141 of the base washing section 14 through the water-washed exhaust gas outlet 132 to absorb carbon dioxide. Specifically, the carbon dioxide in the water-washed exhaust gas was absorbed by the basic solution supplied from the basic solution spray unit 141, and a weakly basic solution and a base-washed exhaust gas were obtained. In this embodiment, the basic solution used in the basic solution spray unit 141 was a sodium hydroxide solution, and the resulting weakly basic solution was a sodium bicarbonate solution. The sodium bicarbonate solution can be used as a carbon source for microalgae culture. The sodium bicarbonate solution was introduced into the culture tank 15 containing microalgae through the weakly basic solution outlet 143 for microalgae culture. The base-washed exhaust gas, in which the carbon dioxide was greatly reduced as a result of detection by the detector, can be discharged through the base-washed exhaust gas outlet 142. After step (D) is performed, the carbon dioxide removal rate is approximately 90%.

[0056] In summary, the exhaust gas treatment method and the exhaust gas treatment system can effectively absorb carbon dioxide and nitrogen oxides, and the weakly basic solution and the nitric acid solution obtained from the exhaust gas treatment system of the present invention can be used as a carbon source and a nitrogen source, respectively, for microalgae cultivation.

[0057] The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the present invention in any manner. It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope and spirit of the present invention. Although the present invention has been described in terms of certain preferred embodiments, it should be understood that the present invention should not be unduly limited to those specific embodiments. Indeed, various modifications that are obvious to those skilled in the art are also encompassed within the scope of the following claims.

Claims

1. (A) desulfurizing an exhaust gas to obtain a desulfurized exhaust gas, the exhaust gas comprising nitrogen oxides, sulfur oxides and carbon dioxide, the nitrogen oxides comprising nitric oxide; (B) supplying oxygen for an oxidation reaction to react with nitrogen monoxide in the desulfurized exhaust gas to obtain an oxidized exhaust gas, the oxidized exhaust gas comprising nitrogen dioxide and carbon dioxide; (C) washing the oxidized exhaust gas with water to dissolve nitrogen dioxide in the oxidized exhaust gas in water to obtain a washed exhaust gas and a nitric acid solution; and (D) a step of contacting the water-washed flue gas with a basic solution to absorb carbon dioxide in the water-washed flue gas with the basic solution, thereby obtaining a base-washed flue gas and a weakly basic solution, wherein the basic solution has a pH value of 9.5 to 14, and the weakly basic solution has a pH value of 8 to 9.

2. The exhaust gas treatment method according to claim 1, The method for treating an exhaust gas further comprises a step (E) of adjusting a pH value of the nitric acid solution to 6.5 to 8 to obtain a nitric acid solution having an adjusted pH value.

3. The exhaust gas treatment method according to claim 2, In the step (E), sodium hydroxide is added to the nitric acid solution to obtain the pH-adjusted nitric acid solution, and the pH-adjusted nitric acid solution is a pH-adjusted sodium nitrate solution.

4. The exhaust gas treatment method according to claim 1, In the step (D), the basic solution is a sodium hydroxide solution.

5. The exhaust gas treatment method according to claim 2, The exhaust gas treatment method further comprises a step (F) of adding the weakly basic solution and the nitric acid solution with the adjusted pH value to a microalgae culture tank containing microalgae.

6. The exhaust gas treatment method according to claim 5, The microalgae include 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 suesica, and the like. suecica), Arthrospira maxima, Arthrospira platensis, cyanobacteria, or a combination thereof.

7. The exhaust gas treatment method according to any one of claims 1 to 6, The step (B) and (B') removing dust from the desulfurized exhaust gas and supplying oxygen for an oxidation reaction that reacts with nitric oxide in the desulfurized exhaust gas to obtain the oxidized exhaust gas, the oxidized exhaust gas containing nitrogen dioxide and carbon dioxide.

8. The exhaust gas treatment method according to any one of claims 1 to 6, The exhaust gas treatment method according to claim 1, wherein the total contact time between the basic solution and the washed exhaust gas is 5 seconds or more.

9. The exhaust gas treatment method according to any one of claims 1 to 6, The exhaust gas treatment method, wherein the step (D) is carried out at a temperature of 90° C. or less.

10. The exhaust gas treatment method according to any one of claims 1 to 6, In the step (D), the concentration of the basic solution is 1 wt % to 50 wt %.

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