CN1768904A - Method for removing hydrargyrum from coal burning boiler flue gas by ozone oxidation - Google Patents

Abstract

The invention relates to the technical field of environmental protection and aims to provide a method for controlling mercury emission in boiler flue gas. The method provided by the invention includes spraying ozone into the low-temperature section of the boiler flue with a temperature range of 110-150°C, oxidizing the water-insoluble zero-valent mercury in the boiler flue gas into water-soluble divalent mercury, and then washing Flue gas is washed with water in the tower, divalent mercury is dissolved in water, and H ₂ S fixative is added to the solution to make divalent mercury generate HgS precipitation. Compared with the current situation that the mercury emission removal efficiency of coal-fired power stations is not higher than 50-60%, the flue gas mercury removal method provided by the invention can achieve a total mercury removal efficiency of more than 80%.

Description

Ozone Oxidation Method for Mercury Removal from Coal-fired Boiler Flue Gas

technical field

The invention relates to the technical field of environmental protection, in particular to a method for controlling mercury emission in boiler flue gas, which is suitable for coal-fired and oil-fired boilers and industrial kilns.

Background technique

Mercury is a toxic trace heavy metal element. After various forms of mercury enter the atmosphere and water from pollution sources, they form methylmercury under biological action. Methylmercury can accumulate in water organisms such as algae and fish, and enter the human body through the biological chain. Since mercury cannot be detoxified and discharged in the human body, it will cause long-term harm to human health. The total amount of mercury emitted to the atmosphere globally is 5,000 tons per year, of which 4,000 tons are anthropogenic. The sources of man-made emissions of mercury are mainly factories and mines producing mercury, smelting of non-ferrous metals, chlor-alkali industry, electrical industry and combustion of fossil fuels. Taking the United States as an example, the annual mercury emissions in the United States account for 3% of the global mercury emissions to the atmosphere, about 158 tons, of which 87% come from the combustion industry, 10% from the manufacturing industry, and 1% from other industries. In 1983, 35% of the mercury in the atmospheric environment came from coal combustion. Coal accounted for more than 75% of my country's energy structure, and 80% was used for direct combustion. From 1978 to 1995, my country's accumulated mercury emissions to the atmosphere were 2,493 tons, with an average annual growth rate of 4.8%. , and with the development of the economy, the emission rate will further increase.

Mercury pollution caused by coal combustion has attracted the attention of countries all over the world. The US EPA promulgated preliminary standards for mercury emissions from coal-fired power plants in January 2004. On December 15, 2004, Congress officially promulgated the final standards for mercury emissions from coal-fired power plant boilers. By the end of 2007 Ex-utility boilers will all have to implement mercury emission standards. my country will also control mercury emissions from coal combustion in the near future.

The current mercury emission control methods mainly include:

Activated carbon adsorption method can achieve a mercury removal efficiency of more than 90% by using an appropriate C/Hg ratio in places with high mercury concentrations such as garbage incinerators. For coal-fired boilers with lower concentrations, a higher C/Hg ratio is required Hg ratio, to achieve a mercury removal efficiency of more than 30%, the cost of activated carbon consumption is high, which is often unacceptable.

Calcium-based absorbent method, using CaO, Ca(OH) ₂ , CaCO ₃ , CaSO ₄ 2H ₂ O to remove mercury, the adsorption efficiency of Ca(OH) ₂ for HgCl ₂ can reach 85%, but for elemental mercury ( Hg ⁰ ), 18% of Hg ⁰ can be removed only in the presence of SO ₂ , and 50-60% of the mercury discharged from coal-fired power plant boilers is zero-valent mercury. The removal efficiency is not high, still less than 50%.

Using wet flue gas desulfurization (WFGD) to remove mercury, since Hg ²⁺ such as HgCl ₂ in the flue gas is soluble in water, it can be captured in the wet desulfurization unit. Studies have shown that wet flue gas desulfurization devices can capture 80-95% of Hg ²⁺ and remove them, but the effect on Hg ⁰ capture is not obvious. According to statistics, the capture efficiency of WFGD for total mercury is in the range of 45-55%.

The adsorption of fly ash is used to remove mercury. The fly ash produced by coal combustion has adsorption effect on mercury. The higher carbon content is beneficial to the adsorption of mercury, but the increase of carbon content in fly ash will reduce the boiler efficiency and affect the fly ash. Resistivity reduces the collection efficiency of the electrostatic precipitator for fly ash. In addition, the removal efficiency of this method for total mercury in coal-fired boilers is low, and it is difficult to apply it on a large scale.

Contents of the invention

Aiming at the deficiencies in the prior art, the invention provides a method for removing mercury by ozone oxidation of coal-fired boiler flue gas, comprising the following steps:

(1) Ozone O ₃ is sprayed into the low-temperature section of the boiler flue with a temperature range of 110-150°C to oxidize the water-insoluble zero-valent mercury in the boiler flue gas into water-soluble divalent mercury, and the reaction time is at least 0.5 seconds.

(2) Wash the flue gas with water in the washing tower, dissolve the divalent mercury in water, add H ₂ S fixative to the solution, and make the divalent mercury generate HgS precipitation.

Ozone is injected into the low-temperature section of the boiler flue, and the injection point of the ozone is before or after the flue dust collector.

The molar mass ratio of the injected O ₃ to NO in the boiler flue gas is 1.1˜1.5:1.

The washing tower uses water or lye as the absorbent, and the washing tower is a spray tower or a packed tower.

The lye used as absorbent can be at least one of sodium hydroxide, potassium hydroxide or calcium hydroxide.

The beneficial effects of the present invention are: compared with the present situation that the mercury emission removal efficiency of coal-fired power stations is not higher than 50-60%, the flue gas mercury removal method provided by the present invention can achieve a total mercury removal efficiency of more than 80%.

Description of drawings

Figure 1 is a layout scheme for the control of dust content in flue gas ozone mercury oxide emission of coal-fired boilers

Figure 2 is a dust-free layout scheme for the discharge of ozone and mercury oxides from coal-fired boilers

Detailed ways

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

Studies have found that wet desulfurization equipment can remove 80-95% of divalent mercury Hg ²⁺ , but it can't do anything about zero-valent mercury Hg ⁰ , and 50-60% of mercury in flue gas is Hg ⁰ . This method uses NO in the flue gas of coal-fired boilers, oxidizes it to NO ₃ with ozone, uses NO ₃ to change the form of Hg, and converts gaseous Hg ⁰ into Hg ²⁺ , thus combining wet scrubbing equipment or existing wet scrubbing The flue gas desulfurization device can effectively control the total mercury. Coal-fired boiler flue gas contains a large amount of NO, and its concentration is about 100-500ppm or even higher. The chemical equation of its basic principle is:

The mercury immobilization reaction that takes place in washing liquid is:

The specific process is: spray ozone in the temperature range of 110-150 ℃ in the tail flue of the boiler. The injection position can be before _or after the electrostatic precipitator. 1.5:1 selection, zero-valent mercury Hg ⁰ can be oxidized to divalent mercury Hg ²⁺ , which is easily soluble in water, and removed through a wet scrubber. Water is used as the absorption liquid, and the absorption liquid is recycled to enrich divalent mercury The absorption solution is fed with H ₂ S to form a stable HgS precipitate, which is then landfilled or further treated after precipitation. If a limestone/gypsum wet FGD plant is already in place, it can be integrated with this method.

In the specific embodiment 1, the arrangement method of wet scrubbing and dust-containing method for the ozone oxidation of flue gas from a coal-fired boiler is shown in Fig. 1 . After the air is dried and purified, it is sent to the dry and filtered oxygen generator 2, and the generated oxygen is sent to the ozone generator 3 to prepare high-concentration ozone, and the ozone is sent to the dusty smoke with a temperature of 110°C before the electrostatic precipitator 5 after the air preheater According to the NO concentration of the flue gas, the injection amount is adjusted from time to time according to the O ₃ /NO molar ratio of 1.1:1 to ensure a reaction time of at least 0.5s. The nozzle adopts porous grid injection, and the injection position is 1m away from the flue gas outlet of the air preheater. After passing through the electrostatic precipitator 5, it enters the wet washing tower 7, dissolves and absorbs Hg ²⁺ , and uses H ₂ S to generate stable mercury sulfide HgS. The absorption liquid is recycled, and the mercury sulfide is precipitated for further treatment. After the washing tower 7 and demister 8 are processed, they are sent to the chimney. The washing tower uses water or lye as the absorbent, and the washing tower is a spray tower or a packed tower. The lye used as absorbent is sodium hydroxide, but also water, potassium hydroxide or calcium hydroxide.

In the figure, 1 is the furnace of the boiler; 2 is the dry filter oxygen generator; 3 is the ozone generator; 4 is the tail flue; 5 is the electrostatic precipitator; 6 is the liquid storage tank; 7 is the wet scrubber; device; 9 is a chimney; 10 is a mercury sulfide precipitation treatment device. 11oH ₂ S addition device.

In specific embodiment 2 and specific embodiment 3, the O ₃ injection point temperatures are 130°C and 150°C respectively, ensuring at least 0.5s reaction time; the O ₃ injection amount is based on the O ₃ /NO molar ratio of 1.3 according to the flue gas NO concentration. :1 and 1.5:1 adjustments. Other steps are identical with specific embodiment 1.

In the specific embodiment 4, the dust-free arrangement method of mercury removal by ozonation of coal-fired boiler flue gas is used, and the implementation method is shown in FIG. 2 . Send ozone into the dust-free environment with a temperature of 110°C after the electrostatic precipitator. The amount of ozone to be sent is selected according to the O ₃ /NO molar ratio of 1.2:1 according to the NO concentration. Determine the injection position and ensure a response time of at least 0.5s. Divalent Hg ²⁺ is absorbed and removed after entering the scrubber. The washing tower uses water or lye as the absorbent, and the washing tower is a spray tower or a packed tower. The lye used as absorbent is calcium hydroxide, but also potassium hydroxide or sodium hydroxide. Other steps are identical with specific embodiment 1.

Among Fig. 2, 12 is a dry filter oxygen plant; 13 is an ozone generator; 14 is a tail flue; 15 is an electrostatic precipitator; 16 is a liquid storage tank; 17 is a washing tower; 18 is a mist eliminator; 19 is a chimney; 20 is a mercury sulfide precipitation treatment device; 21 is a boiler furnace; 22 is an H ₂ S adding device.

Finally, it should also be noted that what is listed above are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and many variations are possible. The present invention may be embodied in other specific forms without departing from the spirit and main characteristics of the invention. Therefore, no matter from which point of view, the above-mentioned embodiments of the present invention can only be regarded as descriptions of the present invention and cannot limit the present invention, and the claims have pointed out the scope of the present invention, and the above description does not point out the scope of the present invention. Therefore, any change within the meaning and scope equivalent to the claims of the present invention should be considered to be included in the scope of the claims.

Claims (5)

1. A method for removing mercury by ozone oxidation of coal-fired boiler flue gas, comprising the following steps: (1) Ozone O ₃ is sprayed into the low-temperature section of the boiler flue with a temperature range of 110-150°C to oxidize the water-insoluble zero-valent mercury in the boiler flue gas into water-soluble divalent mercury, and the reaction time is at least 0.5 seconds. (2) Wash the flue gas with water in the washing tower, dissolve the divalent mercury in water, add H ₂ S fixative to the solution, and make the divalent mercury generate HgS precipitation. 2. The flue gas ozone oxidation mercury removal method according to claim 1, characterized in that ozone is sprayed into the low-temperature section of the boiler flue, and the injection point of the ozone is before the flue dust collector or at the flue dust collector after. 3. The flue gas ozone oxidation mercury removal method according to claim 1, characterized in that the molar mass ratio of the injected O ₃ to NO in the boiler flue gas is 1.1-1.5:1. 4. The flue gas ozone oxidation mercury removal method according to claim 1, characterized in that the washing tower uses water or lye as the absorbent, and the washing tower is a spray tower or a packed tower. 5. The flue gas ozone oxidation mercury removal method according to claim 4, characterized in that the lye used as the absorbent can be at least one of sodium hydroxide, potassium hydroxide or calcium hydroxide.

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