CN111603916A - Low-temperature waste gas denitration method and byproduct recycling application - Google Patents
Low-temperature waste gas denitration method and byproduct recycling application Download PDFInfo
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Abstract
The invention discloses a low-temperature waste gas denitration method and byproduct recycling application, wherein the method comprises the following steps: s1, ozone is fully mixed with the normal-temperature or low-temperature waste gas in the flue, NO which is insoluble in water in the waste gas is oxidized into high-valence nitrogen oxide which is soluble in water, and then the high-valence nitrogen oxide waste gas enters an absorption tower; s2, absorbing liquid is contained in the absorbing tower, and after high-valence nitrogen oxides in the waste gas are absorbed and denitrated by the absorbing liquid in the absorbing tower, the waste gas reaching the standard is discharged from the absorbing tower; the absorption liquid comprises calcium hydroxide slurry and an additive; and S3, absorbing the high-valence nitrogen oxides by the absorption liquid to obtain a solution rich in calcium nitrate. The solution rich in calcium nitrate can be used as fertilizer for plant growth. The method has the advantages of low operation cost, resource of denitration byproducts, high low-temperature denitration efficiency and the like.
Description
Technical Field
The invention relates to the technical field of energy conservation and environmental protection, in particular to a low-temperature waste gas denitration method and byproduct recycling application.
Background
At present, the main technologies applied to the denitration of exhaust gas are Selective Catalytic Reduction (SCR) and selective non-catalytic reduction (SNCR), but the conventional SCR and SNCR methods can only treat medium-high temperature flue gas above 200 ℃, and are not suitable for the treatment of low-temperature exhaust gas. The low-temperature waste gas can be treated only by an oxidation method, wherein ozone denitration is the most widely used oxidation denitration technology at present. The existing ozone oxidation denitration utilizes the strong oxidizing property of ozone to oxidize nitrogen monoxide which is insoluble in water into high-valence nitrogen oxides such as soluble nitrogen dioxide, dinitrogen pentoxide and the like, and then sodium hydroxide is used as an absorbent to absorb the nitrogen dioxide and the dinitrogen pentoxide to achieve the purpose of removing.
At low temperature, O3The main reactions with NO are as follows:
NO+O3→NO2+O2(1)
2NO2+H2O＝HNO3+HNO2(2)
HNO3+HNO2+2NaOH＝NaNO3+NaNO2+2H2O (3)
the overall reaction equation is:
4NO+4O3+4NaOH＝2NaNO3+2NaNO2+2H2O+4O2(4)
it can be seen from the reaction equations (1) - (4) that, in the process of ozone denitration, nitrogen oxides in the exhaust gas are finally absorbed by sodium hydroxide and converted into sodium nitrate and sodium nitrite, so that the denitration wastewater contains a large amount of nitrates, the self-treatment difficulty is high, and the outsourcing treatment cost is high. Therefore, there is a need to develop a low-temperature denitration method for waste gas, which has the advantages of low cost, low price of absorbent, high denitration efficiency, and easy recycling of byproducts.
Disclosure of Invention
Aiming at the defects of the prior art, the inventionThe invention provides a method for absorbing NO generated in the ozone denitration process by adopting calcium oxide or calcium hydroxide as an absorbent based on an ozone denitration system2、N2O5The method for completely converting the nitrogen oxides into calcium nitrate by adding the additives to control reaction conditions for the nitrogen oxides with high valence states achieves the purposes of low cost, high denitration rate and easy recycling of denitration byproducts.
The technical scheme of the invention is realized as follows:
a low-temperature exhaust gas denitration method comprises the following steps:
s1, firstly, ozone is fully mixed with the normal-temperature or low-temperature waste gas in the flue, NO which is insoluble in water in the waste gas is oxidized into high-valence nitrogen oxide which is soluble in water, and then the waste gas containing the high-valence nitrogen oxide is introduced into an absorption tower;
s2, absorbing liquid is contained in the absorption tower, and after high-valence nitrogen oxides in the waste gas are absorbed by the absorbing liquid in the absorption tower, the purified waste gas is discharged from a waste gas outlet of the absorption tower; the absorption liquid comprises calcium hydroxide slurry and an additive;
s3 absorbing the high-valence nitrogen oxides by the absorption liquid to obtain a calcium nitrate-rich solution which can be used as a fertilizer for plant growth.
According to the method, the absorption liquid is recycled in the absorption tower to absorb the high-valence nitrogen oxide waste gas, when the pH value of the absorption liquid is less than 8, the circulation of the absorption liquid is stopped, and the absorption liquid is discharged out of the absorption tower. The main component in the absorption liquid is calcium nitrate, and the absorption liquid can be directly used as a foliar fertilizer for promoting plant growth, so that the pollutant recycling is realized.
According to the method, the absorption liquid is recycled in the absorption tower to absorb and denitrate high-valence nitrogen oxide waste gas, when the pH value of the absorption liquid is less than 8, the absorption liquid stops recycling, the absorption liquid is conveyed to evaporation concentration equipment, calcium nitrate in the extraction solution is separated, and the plant foliar fertilizer is prepared.
According to the method, the calcium hydroxide slurry is prepared by dissolving calcium oxide or calcium hydroxide in water, and the initial concentration of the calcium hydroxide slurry is 30-50 g/L.
In the method, the additive is one or more than one phosphate and/or amino carboxylate.
The method, the phosphate and/or aminocarboxylate is sodium tripolyphosphate, sodium pyrophosphate, sodium hexametaphosphate, sodium aminotriacetate, ethylenediaminetetraacetate (disodium EDTA or tetrasodium EDTA), or Diethylenetriaminepentacarboxylate (DTPA).
According to the method, the concentration of the additive in the absorption liquid is 0.5 g/L-2.0 g/L.
According to the method, the molar ratio of NO to ozone in the waste gas is 1 (1.0-2.0); preferably, 1 is (1.5-2.0).
According to the method, the absorption tower is a spray-type absorption tower, and the absorption tower is sequentially divided into the following parts from bottom to top: the circulating tank is positioned at the bottom and used for containing absorption liquid, the waste gas spraying absorption area is positioned in the middle, and the waste gas outlet is positioned at the upper part; the top of the waste gas spraying area is provided with a plurality of spraying heads, the plurality of spraying heads are connected with the circulating pool through an absorption liquid pipeline, and the absorption liquid pipeline is provided with a circulating pump; the bottom of the waste gas spraying area is provided with a high-price nitrogen oxide waste gas inlet; absorption liquid in the circulating tank is sprayed out from a spray header at the top of the waste gas spray zone under the action of the circulating pump, and falls into the circulating tank for continuous recycling after being in countercurrent contact with high-valence nitrogen oxide waste gas, and waste gas after denitration is discharged from a waste gas outlet.
In the method, the effective volume of the waste gas in the absorption tower is 18-25 m3。
By adopting the treatment method, the absorption liquid absorbs high-valence nitrogen oxides to form a calcium nitrate-rich solution, and the calcium nitrate-rich solution can be applied to preparation or directly used as a plant growth fertilizer.
The invention has the following beneficial effects:
(1) the invention adopts calcium oxide or calcium hydroxide as an absorbent to absorb NO generated in the process of ozone denitration2、N2O5The nitrogen oxides with high valence are completely converted into nitre by controlling the reaction conditionsCalcium carbonate realizes the recycling of denitration byproducts. Adding additive A into calcium hydroxide absorbentnBmAnd dissociation of calcium hydroxide is promoted, so that more hydroxide ions are generated, and the denitration efficiency is improved. The added small amount of phosphate or amino carboxylate is just phosphate fertilizer and nitrogen fertilizer needed by plant growth and can also play a role in promoting plant growth.
(2) The operation cost is low. The conventional ozone denitration technology usually adopts sodium hydroxide as an absorbent, and the sodium hydroxide is expensive. The invention adopts low-price calcium oxide or calcium hydroxide as an absorbent, and the operation cost is only one tenth of that of sodium hydroxide.
(3) The denitration by-product can be recycled. The nitrogen oxides are finally converted into calcium nitrate, the calcium nitrate is a foliar fertilizer with the effects of quickly supplementing calcium and nitrogen, and can be widely applied to basal application, topdressing, flushing application and foliar spraying in agriculture and also can be used as a nutrient solution for soilless culture. The method not only solves the problem of difficult treatment of the denitration wastewater, but also realizes the purpose of recycling the denitration byproducts.
(4) And (4) low-temperature denitration. The method is not influenced by the temperature of the waste gas, and can realize high-efficiency denitration on the low-temperature and normal-temperature waste gas which can not be treated by the traditional SCR and SNCR methods, and the denitration by-products are recycled.
(5) The denitration efficiency is up to more than 90 percent, the denitration device is not influenced by high-concentration dust or solid particles in the waste gas, can be used for carrying out combined denitration with SNCR or PNCR and the like, meanwhile, the ultralow emission standard of NOx and ammonia escape is realized, and the SCR or other denitration devices with high investment and operation cost are avoided.
Drawings
FIG. 1 is a flow chart of a method for producing a low-temperature waste gas denitration byproduct resource according to the present invention.
Wherein: 1-oxygen gas storage tank, 2-ozone generator, 3-mixing reactor, 4-absorption tower, 4.1-circulation pool, 4.2-spraying absorption zone, 4.3-flue gas outlet and 5-waste water treatment and crystallization system.
Detailed Description
For a more clear understanding of the technical features, objects and advantages of the present invention, reference is now made to the following detailed description of the embodiments of the present invention taken in conjunction with the accompanying drawings, which are included to illustrate and not to limit the scope of the present invention.
As shown in figure 1, an oxygen gas storage tank 1 is connected with an ozone generator 2 through a pipeline, ozone is prepared by the ozone generator 2, the generated ozone enters a flue mixing reactor 3 and is fully mixed with nitrogen oxide-containing waste gas in a flue, and NO which is difficult to dissolve in water in the waste gas is selectively and forcibly oxidized into NO2、N2O5High-valence nitrogen oxides which are easy to dissolve in water enter the absorption tower 4. This absorption tower 1 is a fountain absorption tower, the absorption tower has by lower supreme in proper order: a circulating pool 4.1 positioned at the bottom for containing absorption liquid, a spraying absorption area 4.2 positioned at the middle part and a flue gas outlet 4.3 positioned at the upper part; the top of the spray absorption area 4.2 is provided with a plurality of spray heads, the spray heads are connected with the circulating pool through absorption liquid pipelines, and the absorption liquid pipelines are provided with circulating pumps; the bottom of the spraying absorption area 4.2 is provided with a high-price nitrogen oxide waste gas inlet; absorption liquid in the circulating tank is sprayed out from a spray header at the top of the spray absorption area 4.2 under the action of a circulating pump, and falls into the circulating tank to be recycled after being in countercurrent contact with high-valence nitrogen oxide waste gas, the high-valence nitrogen oxide is absorbed by the absorption liquid sprayed in the tower, so that the high-valence nitrogen oxide in the waste gas is irreversibly removed, and the waste gas after denitration is discharged from a waste gas outlet 4.3.
The main reaction equation is as follows:
at low temperature, O3The main reactions with NO are as follows:
NO+O3→NO2+O2(5)
2NO2+H2O＝HNO3+HNO2(6)
CaO+H2O＝Ca(OH)2(7)
2HNO3+2HNO2+2Ca(OH)2＝Ca(NO3)2+Ca(NO2)2+2H2O (8)
the overall reaction equation is:
4NO+4O3+2Ca(OH)2＝Ca(NO3)2+Ca(NO2)2+2H2O+4O2(9)
from the reaction formula (8), it is understood that the reaction molar ratio of NO to ozone is theoretically 1:1, and NO can be completely converted into calcium nitrate and calcium nitrite. But in practice, NO2And Ca (OH)2The reaction speed is low, the absorption and denitration effect is not ideal, the dosage of ozone is often required to be increased, and NO is directly oxidized into N which is easier to absorb2O5Not only can improve the denitration effect, but also can remove NOxAll converted into calcium nitrate with recycling value. The reaction process is as follows:
NO+O3→NO2+O2(10)
NO2+O3→NO3+O2(11)
NO3+NO2→N2O5(12)
N2O5+H2O＝2HNO3(13)
CaO+H2O＝Ca(OH)2(14)
2HNO3+Ca(OH)2＝Ca(NO3)2+2H2O (15)
the overall reaction equation:
2NO+3O3+Ca(OH)2＝Ca(NO3)2+H2O+3O2(16)
as can be seen from the reaction formula (16), the reaction molar ratio of NO to ozone is theoretically 1: 1.5, the nitrogen oxides can be completely converted into calcium nitrate. However, in the actual operation process, if a better denitration effect is to be achieved, such as ultra-low emission and the like, the amount of ozone is often excessive, and the reaction molar ratio of NO to ozone in the actual engineering case needs to be controlled to be 1: 1.5 to 2.0. In this case, the absorbing liquid (i.e., the denitration wastewater) is an aqueous solution containing only calcium nitrate. Calcium nitrate is a foliar fertilizer with the function of rapidly supplementing calcium and nitrogen, is widely applied to basal application, topdressing, irrigation and foliar spraying in agriculture, and can also be used as a nutrient solution for soilless culture. After the absorption liquid is circulated for many times, calcium nitrate is continuously enriched. After reaching a certain concentration, the fertilizer is separated by a conveying device and can be directly used as foliar fertilizer. Or conveying the calcium nitrate into a wastewater treatment and crystallization system 5 for evaporation concentration, separating and extracting calcium nitrate, and using the calcium nitrate to prepare the plant foliar fertilizer.
When the waste gas contains SO2When ozone is first reacted with NOx, SO may also be added if ozone is in excess2Oxidized to higher valent SO3。SO2And SO3Finally absorbed by calcium hydroxide and all converted into calcium sulfite and calcium sulfate. After the absorption liquid is circulated for many times, the denitration product calcium nitrate and the desulfurization product calcium sulfite/calcium sulfate are continuously enriched. After reaching a certain concentration, the calcium sulfite and the calcium sulfate are separated by a conveying device, and because the calcium sulfite and the calcium sulfate are insoluble in water and are settled at the bottom of an absorption liquid after standing, the supernatant is a calcium nitrate aqueous solution and can be directly used as a foliar fertilizer.
As the solubility of calcium hydroxide in water is very small, the solubility of calcium hydroxide at normal temperature (20 ℃) is only 0.165g/100 g of water, and sodium hydroxide is very soluble in water, and the solubility of sodium hydroxide at normal temperature (20 ℃) can reach 109g/100g of water and is far higher than that of calcium hydroxide. This is also the main reason why calcium oxide or calcium hydroxide is rarely used as an acid gas absorbent in practical engineering. Therefore, the invention adds the additive A into the calcium hydroxide absorption liquidnBmAdditive AnBmIs one or more of phosphate and aminocarboxylate, such as sodium tripolyphosphate, sodium pyrophosphate, sodium hexametaphosphate, sodium aminotriacetate, ethylene diamine tetraacetic acid salt (disodium or tetrasodium EDTA), and diethylenetriamine pentacarboxylate (DTPA). The reaction equation is as follows:
as shown in reaction formula (19), additive AnBmAnd the complex is combined with calcium ions in the absorption liquid to generate a complex, so that the concentration of the calcium ions in the solution is reduced, the reaction formula (17) is promoted to be performed rightward, the dissociation of calcium hydroxide is promoted, more hydroxide ions are generated, and the denitration efficiency is improved. The added small amount of phosphate or amino carboxylate is just phosphate fertilizer and nitrogen fertilizer needed by plant growth and can also play a role in promoting plant growth. As the reaction time is prolonged, the concentration of calcium hydroxide in the absorption solution gradually decreases, and the concentration of free calcium ions decreases, so that reaction formula (19) proceeds to the left, Ca2+From the complex CanB2And dissociated. Therefore, only a small amount of additive is added to the absorption liquid, and only a small amount of additive or complex is contained in the finally discharged denitration wastewater.
Example 1
A low-temperature exhaust gas denitration method comprises the following steps:
s1 will contain NOxThe process waste gas is input into a mixing reactor, and the waste gas amount is 10000m3H, NO in exhaust gasxHas a concentration of about 700mg/m3And the temperature of the waste gas is 40-50 ℃. The ozone generation amount of the ozone generator is 15 kg/h. Ozone is fully mixed with the waste gas in the flue to oxidize NO which is difficult to dissolve in water in the waste gas into NO2、N2O5After high-valence nitrogen oxides which are easily dissolved in water are obtained, the high-valence nitrogen oxide waste gas enters an absorption tower;
s2 absorption tower is filled with absorption liquid which is calcium hydroxide slurry generated by reaction of calcium oxide and water, and is added with additive sodium pyrophosphate with concentration of 1g/L, and the effective volume of the absorption tower is 20m3The initial concentration of the calcium hydroxide slurry in the column was 30 g/L.
The absorption liquid is sprayed from the top of the absorption tower, the waste gas is input from the bottom of the absorption tower, and NO in the waste gas is in countercurrent contact2、N2O5High-valence nitrogen oxides which are soluble in water equally are absorbed by the absorption liquid, the waste gas after denitration is discharged from the top of the absorption tower, and the concentration of the treated NOx is 68mg/m3The denitration rate reaches 90.3 percent. And (3) recycling the absorption liquid in the absorption tower, stopping the circulation of the absorption liquid when the pH value of the absorption liquid is less than 8, and discharging the solution subjected to denitration by the absorption liquid out of the absorption tower.
The absorption liquid discharged from the absorption tower at S3 contains calcium nitrate as a main component and very small amounts of sodium pyrophosphate, calcium pyrophosphate, and sodium nitrate. Therefore, the denitration wastewater can be directly used as a foliar fertilizer for promoting plant growth, so that the pollutant recycling is realized.
Example 2
A low-temperature exhaust gas denitration method comprises the following steps:
s1 will contain NOxThe process waste gas is input into a mixing reactor, and the waste gas amount is 10000m3H, NO in exhaust gasxHas a concentration of about 600mg/m3The temperature of waste gas is 90-100 ℃. The ozone generation amount of the ozone generator is 5 kg/h. Ozone is fully mixed with the waste gas in the flue to oxidize NO which is difficult to dissolve in water in the waste gas into NO2、N2O5After high-valence nitrogen oxides which are easily dissolved in water are obtained, the high-valence nitrogen oxide waste gas enters an absorption tower;
s2 the absorption tower is filled with absorption liquid which is calcium hydroxide serous fluid generated by the reaction of calcium oxide and water, and is added with 0.5g/L additive disodium ethylene diamine tetraacetate, the effective volume of the absorption tower is 20m3The initial concentration of the calcium hydroxide slurry in the column was 50 g/L.
The absorption liquid is sprayed from the top of the absorption tower, the waste gas is input from the bottom of the absorption tower, and NO in the waste gas is in countercurrent contact2、N2O5High-valence nitrogen oxides which are soluble in water equally are absorbed by the absorption liquid, the denitrified waste gas is discharged from the top of the absorption tower, and the concentration of the treated NOx is 70mg/m3And the denitration rate reaches 88.3 percent. And (3) recycling the absorption liquid in the absorption tower, stopping the circulation of the absorption liquid when the pH value of the solution is less than 8, and discharging the solution subjected to denitration by the absorption liquid out of the absorption tower.
S3 the main component of the absorption liquid is calcium nitrate, and very small amount of disodium ethylene diamine tetraacetate, calcium ethylene diamine tetraacetate and sodium nitrate.
Therefore, the denitration wastewater can be directly used as a foliar fertilizer for promoting plant growth, so that the pollutant recycling is realized.
Comparative example 1
The process for low-temperature denitration of exhaust gas is the same as that of example 1, except that sodium pyrophosphate as an additive in the absorbent is removed and only calcium hydroxide slurry is used as the absorbent.
The absorption liquid is sprayed from the top of the absorption tower, the waste gas is input from the bottom of the absorption tower, and NO in the waste gas is in countercurrent contact2、N2O5High-valence nitrogen oxides which are soluble in water equally are absorbed by the absorption liquid, the denitrified waste gas is discharged from the top of the absorption tower, and the concentration of the treated NOx is 230mg/m3The denitration rate reaches 67.1 percent.
The above embodiments are merely provided to help understand the method and core principle of the present invention, and the main steps and embodiments of the present invention are described in detail by using specific examples. To those skilled in the art, the various conditions and parameters may be varied as desired in a particular implementation in accordance with the principles of the invention, and in view of the foregoing, the description is not to be taken as limiting the invention.
Claims (10)
1. A low-temperature waste gas denitration method is characterized by comprising the following steps:
s1, fully mixing ozone with the normal-temperature or low-temperature waste gas in the flue, oxidizing the nitrogen oxide which is difficult to dissolve in water in the waste gas into high-valence nitrogen oxide which is easy to dissolve in water, and introducing the waste gas containing the high-valence nitrogen oxide into the absorption tower;
s2, absorbing liquid is contained in the absorption tower, and after high-valence nitrogen oxides in the waste gas are absorbed by the absorbing liquid in the absorption tower, the purified waste gas is discharged from a waste gas outlet of the absorption tower; the absorption liquid comprises calcium hydroxide slurry and an additive;
and S3, absorbing the high-valence nitrogen oxides by the absorption liquid to obtain a calcium nitrate-rich solution.
2. The method of claim 1, wherein the absorption liquid is recycled in the absorption tower to perform absorption denitration on the waste gas containing high-valence nitrogen oxides, when the pH value of the absorption liquid is less than 8, the circulation of the absorption liquid is stopped, and the solution obtained after absorption denitration by the absorption liquid is discharged out of the absorption tower.
3. The method according to claim 1, wherein the absorption liquid is recycled in the absorption tower to carry out absorption denitration on the high-valence nitrogen oxide waste gas, when the pH value of the absorption liquid is less than 8, the circulation of the absorption liquid is stopped, the absorption liquid absorbs the denitrated solution and is conveyed to evaporation concentration equipment, and calcium nitrate in the extraction solution is separated.
4. The method according to claim 1, wherein the calcium hydroxide slurry is prepared by dissolving calcium oxide or calcium hydroxide in water, and the initial concentration of the calcium hydroxide slurry is 30g/L to 50 g/L.
5. The method of claim 1, wherein the additive is one or more phosphates and/or aminocarboxylates.
6. The method of claim 5, wherein the phosphate and/or aminocarboxylate is sodium tripolyphosphate, sodium pyrophosphate, sodium hexametaphosphate, sodium aminotriacetate, ethylenediamine tetraacetate, or diethylenetriamine pentacarboxylate.
7. The method of claim 1, wherein the concentration of the additive in the absorption solution is between 0.5g/L and 2.0 g/L.
8. The method according to claim 1, wherein the molar ratio of NO to ozone in the exhaust gas is 1 (1.0-2.0).
9. The method according to claim 1, wherein the absorption tower is a spray-type absorption tower, and the absorption tower is divided into the following components in sequence from bottom to top: the circulating tank is positioned at the bottom and used for containing absorption liquid, the waste gas spraying absorption area is positioned in the middle, and the waste gas outlet is positioned at the upper part; the top of the waste gas spraying area is provided with a plurality of spraying heads, the plurality of spraying heads are connected with the circulating pool through an absorption liquid pipeline, and the absorption liquid pipeline is provided with a circulating pump; the bottom of the waste gas spraying area is provided with a waste gas inlet containing high-valence nitrogen oxides; absorption liquid in the circulating tank is sprayed out from a spray header at the top of the waste gas spray zone under the action of the circulating pump, and falls into the circulating tank for continuous recycling after being in countercurrent contact with high-valence nitrogen oxide waste gas, and waste gas after denitration is discharged from a waste gas outlet.
10. The application of recycling the low-temperature waste gas denitration by-products is characterized in that by adopting the method as claimed in any one of claims 1 to 9, the absorption liquid absorbs high-valence nitrogen oxides to form a calcium nitrate-rich solution, and the calcium nitrate-rich solution can be applied to preparation or direct application as a plant growth fertilizer.
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