DESULFURIZATION AND DENITRATION METHOD USING CHLORINE DIOXIDE

Abstract

A desulfurization and denitration method includes adding an aqueous solution of a chlorate, an aqueous solution of a peroxide, and an aqueous solution of sulfuric acid to a chlorine dioxide generator to obtain gaseous chlorine dioxide, and mixing the gaseous chlorine dioxide with air to obtain a mixed gas. The gaseous chlorine dioxide is 4-10 vol % of the mixed gas. The method includes letting the mixed gas come into contact with a flue gas to obtain an oxidized flue gas. A molar ratio of the gaseous chlorine dioxide in the mixed gas to nitric oxide in the flue gas is 1-1.8. The final step includes passing the oxidized flue gas to the desulfurization and denitration tower and mixing the oxidized flue gas with a spray of an alkaline absorbent dry powder, and spraying water into the desulfurization and denitration tower to obtain a desulfurized and denitrated flue gas.

Claims

1. A desulfurization and denitration method using chlorine dioxide, which comprises: step (1) of adding an aqueous solution of a chlorate, an aqueous solution of a peroxide, and an aqueous solution of sulfuric acid to a chlorine dioxide generator for a reaction to obtain gaseous chlorine dioxide, and mixing the gaseous chlorine dioxide with air to obtain a mixed gas, wherein the gaseous chlorine dioxide accounts for 4-10 vol % of the mixed gas; step (2) of letting the mixed gas come into contact with a flue gas in a flue gas conduit before the flue gas enters a desulfurization and denitration tower, to obtain an oxidized flue gas, wherein a molar ratio of the gaseous chlorine dioxide in the mixed gas passed to the flue gas conduit per unit time to nitric oxide in the flue gas passed to the flue gas conduit per unit time is 1-1.8; and step (3) of passing the oxidized flue gas to the desulfurization and denitration tower and mixing the oxidized flue gas with a spray of an alkaline absorbent dry powder, and spraying water into the desulfurization and denitration tower to obtain a desulfurized and denitrated flue gas.

2. The method according to claim 1, wherein in step (1), the aqueous solution of a chlorate is an aqueous solution of sodium chlorate having a concentration of 15-40 wt %, and the aqueous solution of sulfuric acid has a concentration of 30-60 wt %.

3. The method according to claim 2, wherein in step (1), the aqueous solution of a peroxide is an aqueous solution of hydrogen peroxide having a concentration of 25-28 wt % or 34-38 wt %.

4. The method according to claim 3, wherein in step (1), sodium chlorate, hydrogen peroxide and sulfuric acid that are contained respectively in the aqueous solution of the chlorate, the aqueous solution of hydrogen peroxide and the aqueous solution of sulfuric acid that are added to the chlorine dioxide generator are at a molar ratio of 1:0.55-1:0.5-1, and the chlorine dioxide generator is at a temperature for the reaction of 40-90° C.

5. The method according to claim 1, wherein in step (2), the flue gas entering the flue gas conduit has a sulfur content of 600-4000 mg/Nm.sup.3, a nitrogen content of 200-600 mg/Nm.sup.3, an oxygen content of 5-23 wt %, and a moisture content of 5-12 wt %.

6. The method according to claim 1, wherein in step (2), the flue gas has a flow rate of 6-15 m/s in the flue gas conduit, and a duration of letting the mixed gas come into contact with the flue gas in the flue gas conduit before the flue gas enters the desulfurization and denitration tower is 1-3 s.

7. The method according to claim 1, wherein in step (3), the alkaline absorbent dry powder is calcium oxide and/or calcium hydroxide, and has a particle size of 150-350 mesh.

8. The method according to claim 1, wherein in step (3), the oxidized flue gas has a flow rate of 1-7 m/s in the desulfurization and denitration tower, and a duration of contact between the oxidized flue gas and the alkaline absorbent dry powder in the desulfurization and denitration tower is 2-15 s.

9. The method according to claim 7, wherein in step (3), a molar ratio Ca/S of elemental calcium contained in the alkaline absorbent dry powder passed to the desulfurization and denitration tower per unit time to elemental sulfur contained in the flue gas passed to the desulfurization and denitration tower per unit time is 1.1-1.5.

10. The method according to claim 9, wherein in step (3), a molar ratio Ca/N of elemental calcium contained in the alkaline absorbent dry powder passed to the desulfurization and denitration tower per unit time to elemental nitrogen contained in the flue gas passed to the desulfurization and denitration tower per unit time is 0.5-1.5.

Description

DETAIL DESCRIPTION OF THE DISCLOSURE

[0024] The present disclosure will be further explained by means of embodiments, but the protection scope of the present disclosure is not limited thereto.

[0025] The present disclosure provides an integrated CFB flue gas desulfurization and denitration process in which chlorine dioxide prepared from sodium chlorate serves as an oxidant, and subsequently, a flue gas is combined with a calcium-based absorbent. The reactions involved in the process are as follows:

[0026] (1) Preparation of gaseous chlorine dioxide [0027] 2NaClO.sub.3+H.sub.2O.sub.2+H.sub.2SO.sub.4.fwdarw.2ClO.sub.2+Na.sub.2SO.sub.4+H.sub.2O+O.sub.2 (main reaction)

[0028] (2) Oxidation of nitric oxide [0029] 2ClO.sub.2+5NO+H.sub.2O.fwdarw.2HCl+5NO.sub.2 (main reaction) [0030] 2ClO.sub.24NO.fwdarw.Cl.sub.2+4NO.sub.2 (side reaction) [0031] 2NO.sub.2+H.sub.2O.fwdarw.HNO.sub.2+HNO.sub.3 (side reaction) [0032] 5HNO.sub.2+2ClO.sub.2+H.sub.2O.fwdarw.5HNO.sub.3+2HCl (side reaction)

[0033] (3) Denitration of flue gas [0034] NO+NO.sub.2+Ca(OH).sub.2.fwdarw.Ca(NO.sub.2).sub.2+H.sub.2O (main reaction) [0035] Ca(NO.sub.2).sub.2+O.sub.2.fwdarw.Ca(NO.sub.3).sub.2 (side reaction) [0036] HNO.sub.2+HNO.sub.3+1/2O.sub.2+Ca(OH).sub.2.fwdarw.Ca(NO.sub.3).sub.2+2H.sub.2O (side reaction) [0037] 4ClO.sub.2+2Ca(OH).sub.2.fwdarw.Ca(OH).sub.2.fwdarw.Ca(ClO.sub.3).sub.2+2H.sub.2O (side reaction) [0038] 2Cl.sub.2+2Ca(OH).sub.2.fwdarw.CaCl.sub.2+Ca(ClO).sub.2+2H.sub.2O (side reaction)

[0039] (4) Desulfurization of flue gas [0040] SO.sub.2+H.sub.2O.fwdarw.H.sub.2SO.sub.3 (main reaction) [0041] 3H.sub.2SO.sub.3+2Ca(OH).sub.2.fwdarw.Ca(HSO.sub.3).sub.2+CaSO.sub.3+4H.sub.2O (main reaction) [0042] Ca(HSO.sub.3).sub.2+2CaSO.sub.3+2O.sub.2+Ca(OH).sub.2.fwdarw.4CaSO.sub.4+2H.sub.2O (main reaction)

[0043] The desulfurization and denitration method of the present disclosure comprises the following steps: (1) preparation of gaseous chlorine dioxide; (2) oxidation of a flue gas; and (3) desulfurization and dentitration. The following is a detailed description of the method of present disclosure.

<Preparation of Gaseous Chlorine Dioxide>

[0044] An aqueous solution of a chlorate, an aqueous solution of a peroxide and an aqueous solution of sulfuric acid are reacted in a chlorine dioxide generator to obtain gaseous chlorine dioxide. The gaseous chlorine dioxide is mixed with air to obtain a mixed gas. In the present disclosure, the gaseous chlorine dioxide may account for 4-10 vol %, preferably 5-8 vol %, and more preferably 7-8 vol % of the mixed gas. Mixing the gaseous chlorine dioxide with air at a ratio falling within the ranges described above increases the rate of conversion of nitric oxide in addition to ensuring a safe production.

[0045] The aqueous solution of a peroxide may be an aqueous solution of hydrogen peroxide. According to one embodiment of the present disclosure, the concentration of hydrogen peroxide in the aqueous solution of hydrogen peroxide may be 25-28 wt %, preferably 26-28 wt %, and more preferably 27-28 wt %. According to another embodiment of the present disclosure, the concentration of hydrogen peroxide in the aqueous solution of hydrogen peroxide may also be 34-38 wt %, preferably 34.5-37 wt %, and more preferably 35-36 wt %. The employment of an aqueous solution of a peroxide having a concentration falling within the ranges described above makes it possible to control the reaction rate and produce gaseous chlorine dioxide with greater safety.

[0046] The chlorate may be one of sodium chlorate, potassium chlorate, and magnesium chlorate, preferably one of sodium chlorate and potassium chlorate, and more preferably sodium chlorate. Sodium chlorate in the aqueous solution of the chlorate, hydrogen peroxide in the aqueous solution of hydrogen peroxide and sulfuric acid in the aqueous solution of sulfuric acid in the chlorine dioxide generator are at a molar ratio of 1:0.55-1:0.5-1, preferably 1:0.6-1:0.6-1, and more preferably 1:0.7-1:0.7-1. The chlorine dioxide generator is at a temperature for the reaction of 50-90° C., preferably 60-80° C., and more preferably 70-80° C. The employment of a molar ratio and reaction temperature that fall within the ranges described above enables chlorine dioxide to be generated at a controlled rate and in greater safety and the generated gaseous chlorine dioxide to be purer.

[0047] The gaseous chlorine dioxide produced by the chlorine dioxide generator and air passed to the chlorine dioxide generator are mixed, resulting in a mixed gas. The mixed gas is output by an induced draft fan.

<Oxidation of Flue Gas>

[0048] The mixed gas is caused to come into contact with a flue gas in a flue gas conduit before the flue gas enters a desulfurization and denitration tower, to obtain an oxidized flue gas. In the present disclosure, a molar ratio of chlorine dioxide in the mixed gas passed to the flue gas conduit per unit time to nitric oxide in the flue gas in the flue gas conduit may be 1-1.8, preferably 1.1-1.6, and more preferably 1.2-1.5. The employment of a molar ratio falling within the ranges described above makes it possible to increase the rate of oxidation of nitric oxide in addition to saving chlorine dioxide.

[0049] According to one embodiment of the present disclosure, the gaseous chlorine dioxide produced by the chlorine dioxide generator and air added thereto are mixed therein, resulting in a mixed gas. The mixed gas is output by an induced draft fan and then input to a flue gas conduit, where the mixed gas comes into contact with the flue gas. The molar ratio of chlorine dioxide in the mixed gas passed to the flue gas conduit per unit time to nitric oxide in the flue gas is 1.2-1.5.

[0050] The flue gas entering the flue gas conduit may have a sulfur content (i.e., a sulfur dioxide content) of 600-4000 mg/Nm.sup.3, preferably 1000-3000 mg/Nm.sup.3, and more preferably 1500-2500 mg/Nm.sup.3. The flue gas entering the flue gas conduit may have a nitrogen content (i.e., a nitric oxide content) of 200-600 mg/Nm.sup.3, preferably 200-400 mg/Nm.sup.3, and more preferably 220-250 mg/Nm.sup.3. The flue gas entering the flue gas conduit may have an oxygen content of 5-23 wt %, preferably 10-20 wt %, and more preferably 15-20 wt %.

[0051] The flue gas entering the flue gas conduit may have a moisture content of 5-12% by weight, preferably 8-12% by weight, and more preferably 10-12% by weight. The employment of a sulfur content, nitrogen content, oxygen content, and moisture content that fall within the ranges described above can help to make the desulfurization and denitration of the flue gas more effective.

[0052] In the flue gas conduit, the flue gas may have a flow rate of 6-15 m/s, preferably 8-15 m/s, and more preferably 10-12 m/s. Also, the duration of letting the mixed gas come into contact with the flue gas in the flue gas conduit before the flue gas enters a desulfurization and denitration tower is 1-3 s. Controlling the flow rate of the flue gas such that it falls within the ranges described above ensures not only an appropriate rate of oxidization of nitric oxide in the flue gas but also an appropriate processing rate.

[0053] The flue gas may have a dust content of 80-200 mg/Nm.sup.3, preferably 100-150 mg/Nm.sup.3, and more preferably 120-150 mg/Nm.sup.3. According to one embodiment of the present disclosure, the flue gas is subjected to a preliminary dust removal before being passed the flue gas conduit. The present disclosure subjects the flue gas to a preliminary dust removal by an electrostatic precipitator.

<Desulfurization and Dentitration>

[0054] In the step of desulfurization and dentitration, the oxidized flue gas is passed to a desulfurization and dentitration tower and mixed therein with a spray of an alkaline absorbent dry powder, and water is sprayed into the desulfurization and denitration tower to obtain a desulfurized and denitrated flue gas.

[0055] In the present disclosure, the alkaline absorbent dry powder is calcium oxide and/or calcium hydroxide, preferably calcium oxide and/or calcium hydroxide, and more preferably hydroxide calcium. In the present disclosure, calcium oxide may have a purity of 80-99 wt %, preferably 80-95 wt %, and more preferably 80-90 wt %. Calcium hydroxide may have a purity of 80-99 wt %, preferably 80-95 wt %, and more preferably 80-90 wt %. In the present disclosure, the alkaline absorbent dry powder may have a particle size of 150-350 mesh, preferably 200-350 mesh, and more preferably 200-300 mesh. The employment of an alkaline absorbent having a particle size falling within the ranges described above in the dry absorption makes the desulfurization and denitration more effective.

[0056] In the desulfurization and denitration tower, the oxidized flue gas may have a flow rate of 1-7 m/s, preferably 2-5 m/s, and more preferably 3-4 m/s. The duration of contact between the oxidized flue gas and the alkaline absorbent dry powder in the desulfurization and denitration tower may be 2-15 s, preferably 3-12 s, and more preferably 5-10 s.

[0057] The molar ratio Ca/S of elemental calcium contained in the alkaline absorbent dry powder passed to the desulfurization and denitration tower per unit time to elemental sulfur contained in the flue gas passed to the desulfurization and denitration tower per unit time may be 1.1-1.5, preferably 1.2-1.5, and more preferably 1.2-1.3. The employment of a molar ratio Ca/S falling within the ranges described above improves the desulfurization efficiency in addition to saving the costs.

[0058] The molar ratio Ca/N of elemental calcium contained in the alkaline absorbent dry powder passed to the desulfurization and denitration tower per unit time to elemental nitrogen contained in the flue gas passed to the desulfurization and denitration tower per unit time may be 0.5-1.5, preferably 0.5-1.0, and more preferably 0.6-0.8. The employment of a molar ratio Ca/N falling within the ranges described above improves the denitration efficiency in addition to saving the costs.

[0059] At the inlet of the desulfurization and denitration tower, the oxidized flue gas may have a temperature of 110-200° C., preferably 110-180° C., and more preferably 120-150° C.

[0060] According to one embodiment of the present disclosure, the desulfurized and denitrated flue gas is dedusted by a bag filter on the top of the desulfurization and denitration tower to become a purified flue gas before being discharged from the chimney. The resultant solid ash is transported to an ash storage tank, or sprayed into the desulfurization and denitration tower to be used once again as the alkaline absorbent dry powder.

EXAMPLE 1

[0061] An aqueous solution of sodium chlorate, an aqueous solution of hydrogen peroxide, and an aqueous solution of sulfuric acid were fed by a metering pump to a chlorine dioxide generator. They reacted therein and generated gaseous chlorine dioxide. In the chlorine dioxide generator, the gaseous chlorine dioxide was mixed with air introduced thereinto, resulting in a mixed gas. The mixed gas was input into a flue gas conduit by an induced draft fan.

[0062] After being dedusted by an electrostatic precipitator, a flue gas was passed to the flue gas conduit and came into contact with the mixed gas input by the induced draft fan. Upon the oxidization of nitric oxide in the flue gas to nitrogen oxides, an oxidized flue gas was obtained.

[0063] The oxidized flue gas was passed to a desulfurization and denitration tower and mixed with a spray of dry calcium hydroxide powder therein. Water was sprayed into the desulfurization and denitration tower to remove nitrogen oxides and sulfur dioxide in the oxidized flue gas, resulting in a desulfurized and denitrated flue gas.

[0064] The method described above was applied to a 90 m.sup.2 sintering machine for a desulfurization and denitration project, and the operating parameters are listed in Table 1. The desulfurized and denitrated flue gas was dedusted by a bag filter at the top of the desulfurization and denitration tower. The purified flue gas obtained in this example has parameters shown in Table 2.

TABLE-US-00001 TABLE 1 Parameters Values Units Flow rate of flue gas at inlet of flue 575824 m.sup.3/h gas conduit (working conditions) Flow rate of flue gas at inlet of flue 400000 Nm.sup.3/h gas conduit (standard conditions) Temperature of oxidized flue gas at inlet 120 ° C. of desulfurization and denitration tower SO.sub.2 content in flue gas 2300 mg/Nm.sup.3 NO content in flue gas 230 mg/Nm.sup.3 Moisture content in flue gas 10 % Oxygen content in flue gas 18 % Dust content in flue gas 120 mg/Nm.sup.3 Flow rate of flue gas in flue gas conduit 12 m/s Flow rate of oxidized flue gas in 3.8 m/s desulfurization and denitration tower Aqueous solution of sodium chlorate 30 wt % Aqueous solution of hydrogen peroxide 27.5 wt % Aqueous solution of sulfuric acid 60 wt % Sodium chlorate: hydrogen peroxide: 1:0.7:0.7 — sulfuric acid (molar ratio) Temperature of chlorine dioxide generator 45 ° C. Percentage by volume of chlorine dioxide 8 vol % in mixed gas Molar ratio ClO.sub.2/NO in flue gas conduit 1.3 — Molar ratio Ca/S 1.3 — Molar ratio Ca/N 0.6 — Purity of dry calcium hydroxide powder 90 wt % Particle size of dry calcium hydroxide powder 200-300 mesh Amount of dry calcium hydroxide powder 1688 kg/h

TABLE-US-00002 TABLE 2 Items Values Units Desulfurization efficiency 99.8 % Denitration efficiency 95.2 %

COMPARATIVE EXAMPLE 1

[0065] In this comparative example, the operating parameters were the same as those in Example 1 except those listed in Table 3. The purified flue gas obtained in this comparative example has parameters shown in Table 4.

TABLE-US-00003 TABLE 3 Parameters Values Units Molar ratio ClO.sub.2/NO in flue gas conduit 0.7 —

TABLE-US-00004 TABLE 4 Items Values Units Desulfurization efficiency 99.2 % Denitration efficiency 89.6 %

COMPARATIVE EXAMPLE 2

[0066] In this comparative example, the operating parameters were the same as those in Example 1 except those listed in Table 5. The purified flue gas obtained in this comparative example has parameters shown in Table 6.

TABLE-US-00005 TABLE 5 Parameters Values Units Molar ratio ClO.sub.2/NO in flue gas conduit 2 —

TABLE-US-00006 TABLE 6 Items Values Units Exhaust temperature 40 ° C. Desulfurization efficiency 99.5 % Denitration efficiency 93.6 %

[0067] The present disclosure is not limited by the above embodiments. Any variation, modification and replacement to the disclosed embodiments which are apparent to those skilled in the art and do not depart from the essence of the present disclosure fall in the scope of the present disclosure.