CN103990366A - Method and system for removing mercury through free radicals generated based on ozone/hydrogen peroxide - Google Patents

Abstract

The invention discloses a method and system for removing mercury by free radicals generated by ozone/hydrogen peroxide. The flue gas discharged from a burner first passes through a flue gas heat exchanger to adjust the temperature, and then passes into the liquid phase reaction zone of the reactor; the ozone Inject the flue gas into the flue gas at the inlet flue of the reactor by the ozone injector; the ozone in the flue gas and the hydrogen peroxide in the solution contact in the liquid phase reaction zone to generate hydroxyl radicals to oxidize and remove the elemental mercury in the flue gas, and the reaction product It is divalent mercury that is easily soluble in water; after the reaction solution enters the primary mercury separation tower, it reacts by adding sulfide to form mercury sulfide precipitate, and then continues to pass into the secondary mercury separation tower for precipitation and separation. The method can efficiently remove elemental mercury in the flue gas, and the equipment is simple and reliable, and there is no secondary pollution in the removal process. It is a new flue gas purification method and system with broad application prospects.

Description

A method and system for free radical demercuration based on ozone/hydrogen peroxide

technical field

The invention relates to the field of combustion flue gas pollutant control, in particular to a method and system for removing mercury by free radicals generated by ozone/hydrogen peroxide.

Background technique

Mercury is a highly toxic heavy metal trace element, which is extremely harmful to human health and the ecological environment. my country is the world's largest coal consumer, and the proportion of coal in the energy structure is as high as 75%, and this pattern will not change significantly for a long period of time in the future. With the increasingly stringent air environmental protection standards for coal-fired pollutants, it is expected that the release of mercury pollution control standards for coal-fired flue gas will be an inevitable trend in the near future.

Scholars at home and abroad have done a lot of fruitful work in the field of research on new theories and technologies of mercury removal. At present, among many mercury removal methods, adsorbent adsorption and wet scrubbing are considered to be the two mainstream mercury removal technologies with the most development potential in the field of mercury removal from coal-fired flue gas.

The most studied wet scrubbing mercury removal technology is the application of the existing wet flue gas desulfurization system combined with scrubbing mercury removal. This technology can achieve a high removal rate of Hg ²⁺ (g), but it has no obvious removal effect on insoluble Hg ⁰ (g), and some oxidized mercury may be reduced to elemental mercury. Many scholars try to use some oxidation techniques to oxidize Hg ⁰ (g) in flue gas to Hg ²⁺ (g) before the desulfurization tower, and then use wet flue gas desulfurization system to wash and remove Hg ²⁺ (g) . Selective Catalytic Reduction (SCR), which has been widely studied at present, can convert part of Hg ⁰ (g) into Hg ²⁺ (g), but the mercury removal effect is affected by the coal composition, catalyst type, combustion method and combustion However, the mechanism of catalytic oxidation is still not very clear. Other oxidation technologies, such as plasma oxidation, photocatalytic oxidation and ozone oxidation, are still in the stage of laboratory exploration. The use of traditional oxidants such as potassium permanganate, potassium persulfate and sodium chlorite to oxidize and absorb Hg ⁰ (g) in the absorption tower has also achieved good results, but there are also disadvantages such as expensive absorbents or complex and difficult-to-handle product components. The technology still needs to be further improved.

The adsorption method mainly uses activated carbon or other adsorbents to adsorb Hg ²⁺ (g) and Hg ⁰ (g) in the flue gas, first convert them into particulate mercury, and then use existing dust removal equipment to capture it to achieve mercury removal Purpose. At present, the most researched and most mature activated carbon adsorption method has high mercury removal efficiency, but the application cost is extremely high, which is unaffordable for enterprises.

The disclosed invention patents CN201310684668, CN201310683054, and CN201310648097 use ultraviolet light to directly irradiate mercury in flue gas, while the disclosed invention patent CN201010296592 uses ultraviolet light to catalyze hydrogen peroxide to generate hydroxyl radicals to oxidize and remove mercury in flue gas. Although the above methods can achieve a certain effect of mercury removal, due to the extremely short penetration distance of ultraviolet light, especially the actual coal-fired flue gas contains a large amount of particulate matter, the effective radiation distance of ultraviolet light will be seriously affected. In addition, after long-term use, a large amount of dirt will be deposited on the surface of the UV lamp, which will deteriorate the long-term operating efficiency of the system, and may even cause the system to fail.

In summary, there is currently no coal-fired flue gas mercury removal technology suitable for large-scale commercialization. Therefore, while improving the existing mercury removal technology, it is of great theoretical and practical significance to actively develop a new economical and efficient coal-fired flue gas mercury removal technology.

Contents of the invention

In order to solve the problems in the prior art, the present invention discloses a method and system for removing mercury by generating free radicals generated by ozone/hydrogen peroxide, which uses ozone/hydrogen peroxide to generate free radicals to oxidize and remove elemental mercury in flue gas.

The reaction process principle of the system of the present invention:

The ozone/hydrogen peroxide double oxidant has at first released the hydroxyl radical with strong oxidizing properties, and the specific process can be represented by the following chemical reaction (1):

H ₂ O ₂ +2O ₃ →2·OH+3O ₂ (1)

2. The generated strong oxidizing hydroxyl radicals can oxidize Hg ⁰ in the flue gas to Hg ²⁺ , so as to achieve the purpose of removal:

OH+Hg ⁰ →HgO+H (2)

3. The Hg ²⁺ produced by the reaction can be absorbed by the added S ²⁺ and react to form an insoluble HgS precipitate, which is then recycled after precipitation and separation:

Hg ²⁺ +S ^2- →HgS↓ (3)

4. After the mercury is separated, the residual liquid can be returned to the reactor through the reflux device to continue to participate in the reaction, so as to save water resources and reduce sewage discharge.

In order to achieve the above object, the technical scheme adopted in the present invention and requirements are as follows:

The flue gas discharged from the burner first passes through the flue gas heat exchanger, adjusts the temperature, and then passes into the liquid phase reaction zone of the reactor; the ozone is injected into the flue gas by the ozone injector at the inlet flue of the reactor; the ozone in the flue gas and After the hydrogen peroxide in the solution is contacted in the liquid phase reaction zone, it will generate hydroxyl radicals to oxidize and remove the elemental mercury in the flue gas, and the reaction product is divalent mercury that is easily soluble in water; after the reaction solution enters the primary mercury separation tower, Mercury sulfide precipitation is formed by adding sulfide reaction, and then continue to pass into the secondary mercury separation tower for precipitation and separation to realize the recycling of mercury resources. After passing through the secondary separation tower, the remaining raffinate after separation can be refluxed to the reaction through the reflux device Continue to participate in the reaction in the container to save water resources and reduce sewage discharge.

Because ozone is extremely unstable at high temperature, it is easy to self-decompose into O ₂ , resulting in waste. The high-temperature flue gas should be injected into the flue after cooling through the flue gas heat exchanger. Therefore, the ozone injection system must be located between the flue gas heat exchanger and the flue gas distribution system. If the temperature of the flue gas and solution is too high, the self-decomposition rate of hydrogen peroxide and ozone will increase, and the application cost will be high. The flue gas heat exchanger will adjust the temperature of the flue gas. The temperature of the solution inside is not higher than 90°C.

Increasing the liquid-gas ratio can increase the removal efficiency, but at the same time it will also increase the power consumption of the pump, thereby increasing the cost. The liquid-gas ratio of the reactor is between 2-30L/m ³ . If the concentration of hydrogen peroxide and ozone is too low, the removal efficiency cannot be satisfied; if it is too high, the side reactions will increase and the cost will increase. The molar concentration of hydrogen peroxide is between 0.05-6.0mol/L, and the volume concentration of ozone is between 20-2000ppm between.

If the pH of the solution is too high, hydrogen peroxide and ozone will accelerate self-decomposition, so the pH of the solution should not be greater than 10.0. If the concentration of mercury in the inlet flue gas is too high, the removal efficiency will decrease, which cannot meet the environmental protection requirements. The inlet concentration of mercury in the flue gas from the burner is not more than 800mg/m ³ .

If the flue gas injection distance is too long, ozone will react or decompose prematurely with pollutants in the flue gas, and cannot react with hydrogen peroxide in the reaction zone of the reactor for free radical reaction, thus affecting the entire reaction efficiency. However, if the injection distance is too short, the ozone mixing time and distance will be too short, and the inhomogeneity will increase, which will also affect the overall reaction efficiency. Therefore, the distance H between the inlet of the ozone injector and the flue gas inlet of the reactor is 100-300 cm.

The system is equipped with an induced draft fan, a flue gas heat exchanger, an ozone injector, a reactor, a liquid addition tower, a primary mercury separation tower, a secondary mercury separation tower and a raffinate return device; The heat exchanger and the flue gas heat exchanger are connected to the bottom of the reactor through pipes; the inlet of the ozone injector is connected to the pipe between the flue gas heat exchanger and the reactor; the liquid addition tower is connected to the bottom of the reactor through pipes. The top; the bottom of the reactor and the opposite side of the flue gas heat exchanger pipe are successively connected to a first-level mercury separation tower and a second-level mercury separation tower; a raffinate reflux is connected between the second-level mercury separation tower and the reactor device.

The reactor is a series or parallel connection of one or more of the bubble tower, spray tower, packed tower, stirred tank; the burner is a pulverized coal furnace, a fluidized bed boiler, a grate furnace, Any one of garbage incinerators and cyclone furnaces.

Furthermore, metals have obvious decomposition effects on hydrogen peroxide and ozone, and hydrogen peroxide, ozone, free radicals and acidic solutions produced by the reaction will have a strong corrosion effect on equipment. The lining of the reactor is made of corrosion-resistant or non-metallic materials such as ceramics, polytetrafluoroethylene, quartz or silicate glass to prevent acid corrosion or catalytic decomposition of hydrogen peroxide by metals.

Advantage of the present invention and remarkable effect:

Compared with the disclosed invention patents, CN201310684668, CN201310683054, CN201310648097, which adopt ultraviolet light to directly irradiate mercury in flue gas, and the disclosed invention patent CN201010296592 uses ultraviolet light to catalyze hydrogen peroxide to generate hydroxyl radicals to oxidize and remove mercury in flue gas. HG. The present invention uses ozone as an inducing agent, which can completely overcome various shortcomings of ultraviolet light. In addition, the ozone production system has been applied on a large scale in the field of water treatment. It is a stable and reliable mature system without potential safety hazards, and has incomparable advantages in large-scale industrial applications. Oxidation products are processed by the product post-treatment system to realize resource utilization. The method can efficiently remove heavy metal mercury in the flue gas, the equipment is simple and reliable, and the removal process has no secondary pollution, and the product can be used as a resource. It is a new flue gas purification method and system with broad application prospects.

Description of drawings

Fig. 1 Electron spin resonance (ESR) wave plot of free radicals induced by ozone/hydrogen peroxide dual oxidants.

Fig. 2 is a process flow diagram of a system for free radical removal of mercury based on ozone/hydrogen peroxide in the present invention.

Detailed ways

The specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings.

As shown in FIG. 1 , the electron spin resonance technique was used to detect the generation of hydroxyl radicals in the solution system of the reactor.

Referring to FIG. 2 , the flue gas discharged from the burner 1 is dragged by the induced draft fan 2 , first adjusted to a suitable temperature by the flue gas heat exchanger 3 , and then passed into the liquid phase reaction zone of the reactor 5 . Ozone is injected into the flue gas by the ozone injector 4 at a suitable position of the reactor inlet flue. The ozone in the flue gas and the hydrogen peroxide in the solution contact in the liquid phase reaction zone to generate hydroxyl radicals to oxidize and remove the elemental mercury in the flue gas. The reaction product is divalent mercury that is easily soluble in water. After entering the primary mercury separation tower 10, it can be reacted to generate mercury sulfide precipitation by adding sulfide, and then it can continue to pass into the secondary mercury separation tower 11 for precipitation and separation. Realize the recycling of mercury resources. The remaining raffinate after separation can be returned to the reactor through the reflux device 13 to continue to participate in the reaction, so as to save water resources and reduce sewage discharge. The removed flue gas is discharged into the atmosphere through the chimney 13. Circulation pump 6-9 provides circulation power for solution flow.

Example 1. The concentration of Hg ⁰ in the flue gas is 50 micrograms per cubic meter, the temperature of the flue gas is 50° C., the molar concentration of hydrogen peroxide is 1.5 mol/L, the concentration of ozone injection is 100 ppm, and the liquid-gas ratio is 10 L/m ³ , the distance between the ozone injection port and the flue gas inlet of the reactor is 50cm. The removal efficiency of Hg ⁰ in flue gas was 89.9%.

Example 2. The concentration of ^Hg0 in the flue gas is 50 micrograms per cubic meter, the temperature of the flue gas is 70°C, the molar concentration of hydrogen peroxide is 1.5mol/L, the concentration of ozone injection is 100ppm, and the liquid-gas ratio is 10L/ ^m3 , the distance between the ozone injection port and the flue gas inlet of the reactor is 50cm. The removal efficiency of Hg ⁰ in flue gas was 81.2%.

Example 3. The concentration of Hg ⁰ in the flue gas is 100 μg/m3, the flue gas temperature is 50° C., the molar concentration of hydrogen peroxide is 1.5 mol/L, the ozone injection concentration is 100 ppm, and the liquid-gas ratio is 10 L/m ³ , the distance between the ozone injection port and the flue gas inlet of the reactor is 50cm. The removal efficiency of Hg ⁰ in flue gas was 84.1%.

Example 4. The concentration of Hg ⁰ in the flue gas is 100 μg/m3, the flue gas temperature is 50° C., the molar concentration of hydrogen peroxide is 1.5 mol/L, the ozone injection concentration is 150 ppm, and the liquid-gas ratio is 10 L/m ³ , the distance between the ozone injection port and the flue gas inlet of the reactor is 50cm. The removal efficiency of Hg ⁰ in flue gas was 92.1%.

Example 5. The concentration of Hg ⁰ in the flue gas is 100 μg/m3, the flue gas temperature is 50° C., the molar concentration of hydrogen peroxide is 1.0 mol/L, the ozone injection concentration is 150 ppm, and the liquid-gas ratio is 10 L/m ³ , the distance between the ozone injection port and the flue gas inlet of the reactor is 50cm. The removal efficiency of Hg ⁰ in flue gas was 88.7%.

Example 6. The concentration of Hg ⁰ in the flue gas is 50 μg/m3, the flue gas temperature is 40° C., the molar concentration of hydrogen peroxide is 2.0 mol/L, the ozone injection concentration is 200 ppm, and the liquid-gas ratio is 15 L/m ³ , the distance between the ozone injection port and the flue gas inlet of the reactor is 50cm. The removal efficiency of Hg ⁰ in flue gas is 100%.

Through the comprehensive comparison of the above examples, it can be seen that Example 6 has the best mercury removal effect, and the removal efficiency is as high as 100%, which can be used as the best example for reference.

Claims (6)

1. A method for free radical demercuration based on ozone/hydrogen peroxide is characterized in that: the flue gas discharged from the burner is first passed through the flue gas heat exchanger to regulate temperature, and then passes into the liquid phase reaction zone of the reactor; Ozone is injected into the flue gas by the ozone injector in the flue at the inlet of the reactor; the ozone in the flue gas and the hydrogen peroxide in the solution contact in the liquid phase reaction zone to generate hydroxyl radicals to oxidize and remove elemental mercury in the flue gas, and the reaction The product is divalent mercury that is easily soluble in water; after the reaction solution enters the primary mercury separation tower, it reacts by adding sulfide to form mercury sulfide precipitate, and then continues to pass into the secondary mercury separation tower for precipitation and separation. 2. A method for free radical demercuration based on ozone/hydrogen peroxide according to claim 1, characterized in that: the flue gas heat exchanger regulates the temperature of the flue gas, and the temperature of the flue gas entering the reactor is not higher than 90°C, the temperature of the solution in the liquid addition tower is not higher than 90°C; the liquid-gas ratio of the reactor is between 2-30L/ ^m3 , the molar concentration of hydrogen peroxide is between 0.05-6.0mol/L, the ozone The volume concentration is between 20-2000ppm, the pH of the solution is not greater than 10.0; the inlet concentration of mercury in the flue gas from the burner is not greater than 800mg/m ³ . 3. A method for free radical mercury removal based on ozone/hydrogen peroxide according to claim 1 or 2, characterized in that: after passing through the secondary separation tower, the remaining raffinate after separation can be refluxed through the reflux device Continue to participate in the reaction in the reactor. 4. A system for free radical demercuration based on ozone/hydrogen peroxide, characterized in that: the system is provided with an induced draft fan, a flue gas heat exchanger, an ozone injector, a reactor, a liquid addition tower, and a first-grade mercury Separation tower, secondary mercury separation tower and raffinate reflux device; the burner is connected to the flue gas heat exchanger through the pipe, and the flue gas heat exchanger is connected to the bottom of the reactor through the pipe; the injection port of the ozone injector is connected to the flue gas On the pipeline between the heat exchanger and the reactor; the liquid addition tower is connected to the top of the reactor through the pipeline; the bottom of the reactor and the opposite side of the flue gas heat exchanger pipeline are successively connected to the first-level mercury separation tower, the second-level A mercury separation tower; a raffinate reflux device is connected between the secondary mercury separation tower and the reactor. 5. A system for free radical demercuration based on ozone/hydrogen peroxide according to claim 4, characterized in that: the distance H between the inlet of the ozone injector and the flue gas inlet of the reactor is 100-300cm. 6. A system for free radical demercuration based on ozone/hydrogen peroxide according to claim 4 or 5, characterized in that: said reactor is a bubble tower, a spray tower, a packed tower, a stirred tank One or several of them are connected in series or in parallel; the burner is any one of pulverized coal furnace, fluidized bed boiler, grate furnace, garbage incinerator, and cyclone furnace.