CN101187475A - Method for reducing elemental mercury and nitrogen oxide emissions in flue gas during staged reburning of coal-fired boilers - Google Patents

Abstract

The invention discloses a method for reducing the emission of elemental mercury and nitrogen oxides in the flue gas during the staged reburning of a coal-fired boiler, comprising the following steps: a. Select _CH4 gas as the staged reburning gas, and simultaneously select a mixture of _CH4 and _NH3 The gas is two-stage reburning gas; b. The reburning fuel is sent into the coal-fired boiler in two stages, among which the reburning nozzle (1) sends CH ₄ into the reburning nozzle (2), and the reburning nozzle (2) sends CH ₄ and NH ₃ mixed gas. An overhead air nozzle is arranged between the nozzles (1) and (2),

; c. Combined application with wet flue gas desulfurization method. The invention converts zero-valent mercury into divalent mercury that is easily adsorbed by fly ash and slurry, and then achieves the purpose of simultaneous removal of multiple pollutants through combined application with the existing wet flue gas desulfurization method.

Description

Method for reducing elemental mercury and nitrogen oxide emissions in flue gas during staged reburning of coal-fired boilers

technical field

The invention relates to a method for reducing the emission of gas pollutants produced during combustion of a coal-fired boiler, and more specifically relates to a method for reducing the emission of elemental mercury and nitrogen oxides in flue gas during staged reburning of a coal-fired boiler.

Background technique

Nowadays, the problem of environmental pollution has become the focus of attention all over the world. Coal combustion is the largest source of NOx and Hg pollution. At present, electricity production mainly relies on coal-fired power plants, especially in China, whose proportion accounts for about 75% of the total power generation. It can be said that coal-fired power plants are concentrated areas of coal consumption and NOx and Hg pollution. Especially due to the massive burning of coal, the total amount of mercury released from coal burning in the world reaches more than 3000t every year. Mercury in coal is gasified due to high temperature during the coal combustion process. As the combustion gas cools, it interacts with other combustion products to produce oxidized mercury and particulate mercury. The three forms are called total mercury and are represented by Mostly gaseous mercury. Elemental mercury is volatile, has low solubility, high toxicity, stable chemical properties, is not easy to react with other chemical substances, spreads widely, stays in the environment for a long time, and is difficult to remove. A large amount of mercury may pollute the water body through dry and wet deposition, and the highly toxic methylmercury is formed after the biomethylation reaction. This kind of organic mercury is the most toxic and can damage the brain. It is enriched in organisms, and the half-life of staying in the human body is as long as 70 days. Even if the dose is small, it can accumulate and cause toxicity, causing great harm to the human body.

Aiming at the status quo of mercury pollution in the coal combustion process, researchers have proposed various control methods, including sorbent injection method, fixed bed filtration method, wet flue gas desulfurization combined with mercury removal method, etc., the first two require high-efficiency Inexpensive adsorbents are commonly known as adsorbent adsorption methods, and the third method requires an oxidant to oxidize elemental mercury in the flue gas to divalent mercury. Due to the stable form of elemental mercury, it is difficult to directly remove elemental mercury from flue gas, and it will cause a wide range of air pollution when discharged into the atmosphere, while divalent mercury in flue gas is easily adsorbed on ash and collected by various dust collectors. The removal process is relatively simple, so if the elemental mercury after combustion can be converted into divalent mercury as much as possible, it can effectively reduce the mercury pollution to the environment caused by coal combustion. At the same time, reducing the pollution of NOx compounds produced by coal combustion has always been one of the research focuses of environmental protection workers.

Contents of the invention

The technical problem to be solved by the present invention is to provide a method for converting zero-valent mercury into divalent mercury that is easily adsorbed by fly ash and slurry during the staged reburning of coal-fired boilers, so as to achieve the simultaneous removal of various pollutants Purpose.

The technical scheme adopted in the present invention: the method for reducing the emission of elemental mercury and nitrogen oxides in the flue gas during the staged reburning of the coal-fired boiler comprises the following steps:

a. CH ₄ gas is selected as the graded reburning gas, and the mixed gas of CH ₄ and NH ₃ is selected as the secondary reburning gas;

b. The reburning fuel is sent into the coal-fired boiler in two stages, wherein the reburning nozzle (1) is fed into CH ₄ , and the reburning nozzle (2) is fed into the mixed gas of CH ₄ and NH ₃ , and the reburning nozzle (1) and (2) ) are arranged with top-mounted air nozzles, the ratio of the distance between each nozzle is the reburning nozzle (1) height h ₁ : total height H=0.01～0.5, top-mounted air nozzle height h ₂ : total height H=0.01～0.2, reburning Height of nozzle (2) h ₃ : total height H=0.01～0.2, amount of reburning CH ₄ injected into reburning nozzle (1) is Q _(CH4)1 , amount of CH ₄ injected into reburning nozzle (2) is Q _{(CH4 )2} , the _amount of NH injected into the reburning nozzle (2) is Q _NH3 , Q _{(CH4) 1} : Q _{(CH4) 2} = 0.01～0.20, Q _{(CH4) 2} : Q _NH3 = 1: 1;

c. The combined application of the above staged reburning and wet flue gas desulfurization methods can achieve the joint removal effect of pollutants and reduce the emission of elemental mercury and nitrogen oxides in flue gas.

Beneficial effects of the present invention: the present invention converts zero-valent mercury into divalent mercury that is easily adsorbed by fly ash and slurry, and then achieves the simultaneous desulfurization of various pollutants through the joint application with the existing wet flue gas desulfurization method purpose of removal. In principle, the denitrification mechanism of natural gas reburning is that when the generated NO meets the hydrocarbon radical CHi and the incomplete combustion products CO, H ₂ , C, CnHm, the reduction reaction of NO will occur. The overall reaction equation is:

4NO+CH ₄ →2N ₂ +CO ₂ +H ₂ O

2NO+2C _n H _m +(2n+m/2-1)O ₂ →N ₂ +2nCO ₂ +mH ₂ O

2NO+2CO→N ₂ +2CO ₂

2NO+2C→N ₂ +2CO

2NO+2H ₂ →N ₂ +2H ₂ O

The reburning and denitrification mechanism of NH ₃ is: NH ₃ can react with NOx in the flue gas for SNCR reaction at high temperature to generate N ₂ , which is beneficial to nitrogen removal. The reaction is: 4NH3+4NO+O2=4N2+6H2O

The mechanism of the natural gas reburning process on Hg can be analyzed from the following aspects:

During the reburning process of natural gas, it reacts with CH ₄ and NH ₃ in the presence of oxygen to produce HCN and other substances: 2CH ₄ +3O ₂ +2NH ₃ =2HCN+6H ₂ O

HCN is not very stable, and will be converted into other substances (such as HCNO, etc.) in the presence of oxygen, but there will still be a small amount of HCN or CN ^- in the flue at the back of the furnace outlet, which will react with mercury and oxidize elemental mercury Or immobilize divalent mercury so that it can be transformed in a direction that is conducive to capture and fixation:

2Hg+8CN ^- +2H ₂ O+O ₂ → 2Hg(CN) ₄ ^2- +4OH ^-

Hg ²⁺ +2CN ^- ＝Hg(CN) ₂

Hg(CN) ₂ +2CN ^- ＝Hg(CN) ₄ ^2-

HgO+4CN ^- +H ₂ O＝Hg(CN) ₄ ^2- +2OH ^-

HCNO also has an oxidation effect on Hg. At the same time, the H ₂ O generated during the combustion of CH ₄ is also conducive to the oxidation and capture of Hg by Cl ₂ , HCl and other substances in the flue gas, thereby facilitating the conversion of the form of mercury to the direction that is convenient for adsorption and removal.

Description of drawings

Figure 1 is a schematic diagram of staged reburning;

Figure 2 is a schematic diagram of the test furnace system.

Figure 3 Schematic diagram of the single-section furnace structure

Figure 4 Simplified diagram of furnace body and sampling location

Detailed ways

Below by accompanying drawing and embodiment the present invention is described in further detail: the method for reducing elemental mercury, nitrogen oxide discharge in flue gas when coal-fired boiler staged reburning, comprises the following steps, as shown in Figure 1:

a. CH ₄ gas is selected as the graded reburning gas, and the mixed gas of CH ₄ and NH ₃ is selected as the secondary reburning gas;

b. The reburning fuel is sent into the coal-fired boiler in two stages, wherein the reburning nozzle (1) is fed into CH ₄ , and the reburning nozzle (2) is fed into the mixed gas of CH ₄ and NH ₃ , and the reburning nozzle (1) and (2) ) are arranged with top-mounted air nozzles, the ratio of the distance between each nozzle is the reburning nozzle (1) height h ₁ : total height H=0.01～0.5, top-mounted air nozzle height h ₂ : total height H=0.01～0.2, reburning Height of nozzle (2) h ₃ : total height H=0.01～0.2, amount of reburning CH ₄ injected into reburning nozzle (1) is Q _(CH4)1 , amount of CH ₄ injected into reburning nozzle (2) is Q _{(CH4 )2} , the _amount of NH injected into the reburning nozzle (2) is Q _NH3 , Q _{(CH4) 1} : _{Q (CH4) 2} = 0.01～0.20, Q _{(CH4) 2} : Q _NH3 = 1: 1;

c. The combined application of the above staged reburning and wet flue gas desulfurization methods can achieve the joint removal effect of pollutants and reduce the emission of elemental mercury and nitrogen oxides in flue gas.

Example 1

The experimental research is carried out on the one-dimensional pulverized coal combustion test system. The whole test system is composed of one-dimensional test furnace furnace body, hot air system, feeding system, combustion system, heating system, cooling system, dust removal system and monitoring system. , Sampling and analysis of trace heavy metal element mercury in coal-fired flue gas, using Ontario Hydro Method, measurement of NO _x and SO ₂ in flue gas using electrochemical method for flue gas analysis. The hearth body is a building block structure with a total height of 5m and an inner diameter of 300mm, which can be divided into a base and five segmented furnace bodies with the same structure. The furnace body is supported by channel steel brackets, and the sections are fixed with bolts and sealed. The overall structure of the furnace body is shown in Figure 2. 1. Blower 2. Preheater 3. Feeding device 4. Burner 5. Sampling hole 6. Flue gas cooler 7. Bag filter 8. Induced fan 9. Chimney 10. Thermometer 11. Pressure gauge 12. Valve 13 .Flow meter 14. Furnace body, the entire furnace body from top to bottom is the first stage, second stage, third stage, fourth stage, fifth stage and base. Each section of the furnace body is divided into four layers from the inside to the outside: the innermost layer is a silicon carbide sleeve, which can withstand high temperature; the second layer is a heating layer, with a resistance wire in the middle, which is used to heat the air in the furnace to the set temperature ; The third layer is a thermal insulation cover, which can reduce heat loss; the outermost layer is a shell made of refractory fiber. Fig. 3 is a schematic diagram of the structure of a single furnace body. Sampling holes are arranged on the second section to the fifth section. A hole with a diameter of 60mm is opened above the furnace body as a fuel inlet. There is a flue gas outlet on the side of the base. The locations of the furnace body and flue gas sampling holes are shown in Figure 4. CH ₄ is sent into one of the air supply ports in one stage; CH ₄ +NH ₃ is sent into the air supply port in the third stage; the air supply port in the second stage is a top-mounted air nozzle, and an appropriate amount of air is sent in. The pulverized coal used in the experiment comes from a coal-fired power plant, which is a kind of mixed coal, which has been mechanically milled and vibrated before the test. The test conditions are: pulverized coal feed rate 8kg/h, excess air coefficient 1.2, primary air volume 15.18Nm ³ /h, secondary air volume 35.42Nm ³ /h, exhaust gas temperature 70°C. The form distribution of mercury in coal combustion products in this example is: gaseous divalent mercury accounts for about 60%, gaseous elemental mercury accounts for about 20%, mercury content in fly ash is 15%, and mercury content in slag is 5%. Converted to 6% oxygen, the concentration of NO in the flue gas is 300mg/m ³ , and the content of SO ₂ is 560mg/m ³

Example 2

The test equipment and the basic conditions of the test conditions are the same as in Example 1, except that certain modifications have been made in the combustion process: the calcium-sulfur ratio is 2 mixed with limestone powder for desulfurization, and natural gas is injected into the second section of the furnace, and the fifth The combustion air is sprayed into the section to investigate the effect of desulfurization of limestone in the furnace while denitrification of natural gas, and its comprehensive influence. In this example, the form distribution of mercury in coal combustion products is as follows: gaseous divalent mercury still accounts for about 60%; but the content of gaseous elemental mercury is almost zero and can be ignored; the content of mercury in fly ash is correspondingly increased to 30%, and the content of mercury in slag is also increased to 10%. Converted to 6% oxygen, the NO concentration in the flue gas is 310mg/m ³ , and the SO ₂ content is 450mg/m ³ . From the above two examples, it can be seen that by reburning natural gas, limiting the amount of natural gas injection inlets and the height ratio between nozzles, and adding limestone for desulfurization, the zero-valent mercury in coal combustion products is significantly reduced, and the mercury in ash and fly ash is large The mercury emission reduction effect is remarkable; the content of NO and SO ₂ in the flue gas is also reduced by about 1/3, and the removal effect is remarkable.

The above content is only a basic description of the concept of the present invention, and any equivalent transformation made according to the technical solution of the present invention shall fall within the scope of protection of the present invention.

Claims (1)

1. The method for reducing the emission of elemental mercury and nitrogen oxides in flue gas during staged reburning of coal-fired boilers includes the following steps: a. CH ₄ gas is selected as the graded reburning gas, and the mixed gas of CH ₄ and NH ₃ is selected as the secondary reburning gas; b. The reburning fuel is sent into the coal-fired boiler in two stages, wherein the reburning nozzle (1) is fed into CH ₄ , and the reburning nozzle (2) is fed into the mixed gas of CH ₄ and NH ₃ , and the reburning nozzle (1) and (2) ) are arranged with top-mounted air nozzles, the ratio of the distance between each nozzle is the reburning nozzle (1) height h ₁ : total height H=0.01～0.5, top-mounted air nozzle height h ₂ : total height H=0.01～0.2, reburning Height of nozzle (2) h ₃ : total height H=0.01～0.2, amount of reburning CH ₄ injected into reburning nozzle (1) is Q _(CH4)1 , amount of CH ₄ injected into reburning nozzle (2) is Q _{(CH4 )2} , the _amount of NH injected into the reburning nozzle (2) is Q _NH3 , Q _{(CH4) 1} : _{Q (CH4) 2} = 0.01～0.20, Q _{(CH4) 2} : Q _NH3 = 1: 1; c. The combined application of the above staged reburning and wet flue gas desulfurization methods can achieve the joint removal effect of pollutants and reduce the emission of elemental mercury and nitrogen oxides in flue gas.

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