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    Product-Pulverized Coal Boiler and Combustion System-Desulfurization and Denitrification Procss
 
The flying ash desulfurizing application show
 
The desulfurization unit
 
       Desulfuration:The elemental sulfur in coal is solidified by calcium base to prevent SO2 generated from combustion.? There are three methods of desulfuration: desulfuration before combustion, desulfuration when combustion, desulfuration after combustion.
1、Desulfuration before combustion
       The elemental sulfur in the coal can be removed from the coal by the physical coal cleaning method, chemical washing coal washing coal, coal gasification and liquefaction, and coal water slurry technology etc or add sulphur-fixing agent in the conveying process of pulverized coal before the pulverized coal combution. In the combustion process, the sulfur-containing compounds in the pulverized coal is reacted with sulfur-fixing agent to generate sulfate etc materials. The sulfate and other materials is stayed in the slag, so the SO2 can be genetated from combustion.
2、Desulfuration when combustion
       
In the combustion process, add sulfur-fixing agent(such as CaO etc) into the furnace to generate sulfate from the elemental sulfur in the coal reacted with sulfur-fixing agent. The sulfur is ddischarged with the slag. The principle is
CaO+SO2=high temperature=CaSO3        2CaSO3+O2===2CaSO4
   3、Desulfuration after combustion
      
          Desulfuration after combustion means Flue gas desulfurization, short for FGD.? It can be 6 methods according to the types           of desulfurizer in the FGD technology: calcium method(on the basis of CaCO3 (limestone), magnesium method (on the basis of           MgO), sodium method(on the basis of Na2SO3), ammonia method(on the basis of NH3), organic alkaline method(on the basis           of organic alkaline) or the fly ash of coal ash method(on the basis of pulverized coal ash).
 

a、Gypsum method
      Gypsum method-limestone method desulfurization technology is the most widely applied desulfurization technology. The working principle: The seriflux generated from pulverized limestone mixed with water is transferred into the absorption tower by pump. The seriflux as absorbent is fully mixed with flue gas. The SO2 in the flue gas, the CaCO3 in the seriflux and the air blasted from the bottom of the toweer are oxidizing reaction to generate calcium sulfate. The dihydrate gypsum is generated from calcium sulfate by crystal formation when the calcium sulfate is reached a certain degree of saturation. The gypsum seriflux discharged from the absorption tower need to be concentrated and dewaterred to make sure the water content is less than 10%.? The gypsum is transferred to the gypsum silos for storage by conveyor. The fogdrop is removed? from the flue gas after desulfuration by defogger.? Then the flue gas is discharged to the atmosphere by chimney after the flue gas is heated by heat exchanger. The absorbent seriflux is repeatedly cycle by circulating pump to contact with the flue gas in the absorption tower. So the utilization rate of absorbent is very high, low calcium sulfur ratio, the desulfurization efficiency can be more than 95%.
       Desulfurization process
       CaCO3+SO2+1/2H
2O→CaSO3·1/2H2O+CO2
       Ca(OH)2+SO
2→CaSO3·1/2H2O+1/2H2O
       CaSO3·1/2H
2O+SO2+1/2H2O→Ca(HSO3)2
       Oxidation process
       2CaSO
3·1/2H2O+O2+3H2O→2CaSO4·2H2O
       Ca(HSO
3)2+O2+2H2O→CaSO4·2H2O+H2SO4

 

b、Spray drying method
      Spray drying method desulfurization technology: The lime as desulfurization absorbent. The lime milk is generated from the lime mixed with water, then they are transferred by pump to the atomization device in the absorption tower. The absorbent is atomized into small droplets, then mixed with flue gas. The CaSO3 is generated from the chemical reaction of SO2 in the flue gas and absorbent. So the SO2 is removal from the flue gas. Meanwhile, the moisture in the absorbent is evaporated quickly to be dry. The temperature of the flue gas is reduced. The reaction products from desulfurization and absorbent which not used are taken out from the absorption tower with flue gas in the form of dry particulate matter. They are collected in the dust-collector.? The flue gas after desulfurization is discharged after fliting the dust in the dust collector. In order to improve the utilization rate of desulfurization? absorbent, some collection materials from the dust collector will be sent to seriflux making system for cyclic utilization.? There two kinds of different spray type: rotating spray atomization, air-liquid two-phase flow.

c、Injecting calcium in furnace and humidification method in tail section
     Injecting calcium in furnace and the flue gas desulfurization by humidification activation technology in tail section: On the basis of injecting calcium in furnace desulfurization technology, equip the humidification section in the tail of the boiler to improve desulfurization efficiency. The limestone? powder as absorbent is injected in to the 850~1150℃ temperature area in the chamber. The limestone is decomposed into calcium oxide and carbon dioxide by heating. The calcium oxide and the SO2 in the flue gas are mixed to generate calcium sulfite. Because the raction is conducted in the two phase of air and solid, the reaction speed is slower and the? utilization rate of absorbent is lower because the influence of mass transfer process. In the humidification activation reactor in tail section, the humidification water is injected by vaporific shape, contacted with the unreacted calcium oxide is to generate calcium hydroxide. Then the? calcium hydroxide is reacted with the SO2 in the flue gas.? When the calcium sulfur ratio is controlled in 2.0~2.5, the desulfurization rate can be reached 65~80%.? The temperature of the flue gas is reduced because of adding the humidification water. Generally, control the flue gas temperature in outlet is higher 10~15℃ than dew point temperature. The humidification water is evaporated quickly because heat transferred from the flue gas. The unreacted absorbent, reaction product are discharged with flue gas by dry state, collected by dust collector.

 

d、?Ammonia water washing method
       In this desulfuration technology, the ammonia water is as absorbent, the by-product is the chemical fertilizer of ammonium sulfate. The temperature of flue gas from the boiler is reduced to 90~100℃ after cooling by heat exchanger. HCI and HF are romoval from the flue gas by washing in the prewasher. The water drop in the flue gas after washing is removal through drop separator. Then the flue gas is into the front washer. In the front washer,? the flue gas is sprayed for washing by ammonia water from the top of tower. The SO2 in the flue gas is removal by washing and absorption. The water drop in the flue gas after washing is removal through drop separator. The flue gas is into the desulfuration washer. In the desulfuration washer, the flue gas is furture washed.? The fogdrop is removal through mist eliminator in the top of the tower. Then the flue gas is into the desulfuration washer.? The flue gas is discharged through chimney after heated by heat exchanger. The 30% concentration of ammonium sulfate solution generated from washing technology is discharged from washing tower. The ammonium sulfate solution can be sent to the chemical fertilizer plant for furture treatment or can be saled as liquid nitrogen fertilizer. The ammonium sulfate solution can be saled by furture concentration, evaporation to process into? granules, crystal, or massive fertilizer.

 

e、.The flying ash method
       The fly ash is composed of many tiny solid particles based on the view of the microscopic structure, a certain pore structure of porous material, which the porosity is 60% ~ 75% and the specific surface area is commonly 25 ~ 50 ㎡ / g. There is a certain amount of alkaline oxide in its chemical composition, such as CaO and MgO etc. The fly ash is applied in flue gas desulfurization, which the flue gas is absorded by the fly ash and the SO2 is absorded by the Hydration products produced by the active substances in the fly ash.
       
The main components of the fly ash is SiO2 and AL2O3. If it is cool quickly in the process of the fly coal, the vitreous (SiO2, AL2O3, Fe2O3) is more. Meanwhile, alkaline mineral is more. The main activity of fly ash is from the vitreous, so that the more content of the vitreous, it is better activity. There is the same activity and adsorption properties with volcanic. It is high calcium ash when CaO content in the fly ash is 7.3%.
(1)、a (OH)
2+SiO2+H2O→(CaO)x(SiO2)y(H2O)z
(2)、Ca(OH)
2+Al2O+H2O→(CaO)x(Al2O3)y(H2O)z
(3)、Ca(OH)
2+SiO2+Al2O3+H2O→(Cao)x(Al2O3)y(SiO2)z(H2O)w
(4)、Ca(OH)
2+Al2O3+SO2+H2O→(CaO)x(Al2O3)y(CaSO3)z(H2O)w
The generated calcium silicate hydrate can promote gas-solid diffusion and increase the specific surface area to absorb the SO2, with the high water performance, keep the surface of desulfurization agent wet in the mortar, reinforcing the reaction between gas and liquid.

 
The denitration unit
 

The Nox process: Denitration in the combustion process, Denitration after combustion.

1、Denitration in the combustion process
       Applying low Nitrogen burner with distrbuting the air for combustion by grade. Digital control the combustion by adjusting the burning angle and returned smoke amount to condcut low temperture(<1000℃)burning, keeping the burning temperature in the low nitrogen state to low nitrogen combustion, reducing the Nox genetation.
 

2、Denitration after combustion
       Denitration after combustion means the flue gas denitration. There are 3 types: dry type, half-dry type and wet type. Ammonia water method: Selective Non-Catalytic Reduction(SNCR),? Selective Catalytic Reduction(SCR), Electron beam combined desulfurization denitration method.? Half-dry method: Activated carbon desulfurization denitration method, Wet method: Ozone oxidation absorption. At present, dry method SCR is the main method.

①SCR(Selective Catalytic Reduction)- Selective Catalytic Reduction denitration technology: Under the function of catalyst(such as TiO2, V2O5,WO3), NH3, CO or carbon-oxygen compound as reducing agent, make the reduction of Nox in the flue gas to non-toxic N2 and H2O at 290-450℃. The NH3-SCR technology is well master and reliable. At present, it is widely apllied in all over the world, especially the develpoed countires.? When the NH3 as reducing agent, SCR reduction chemical equation: 4NH3+4NO+O2→4N2+6H2O.
② SCR(Selective Non-Catalytic Reduction)- Selective Non-Catalytic Reduction denitration technology: Without catalyst, the reducing agent will be injected to make reduction of nitrogen oxide in the flue gas to non-toxic N2 and H2O at the right temperature range which suitable for denitration reaction.? The ammonia, urea or hydrogen glycine are as reducing agent for the Nox reduction.? The reduction agent is just made reduction with Nox in the flue gas, generally not made reduction with oxygen.? Because it do not need catalyst, the reducing agent must be added in the high temperature range. The chamer teperature is 850 ~ 1100℃, the reducing agent is injected to? thermal decomposition into NH3 quickly. Then NH3 make reduction with the Nox in the flue gas to generate N2 and H2O. If the NH3 as reducing agent, in the temperature range of 900℃~1 100℃, Nox reduction chemical equation is:
4NH3 + 4NO+ O2 →4N2 +6H2O
4NH3 + 2NO+ 2O2 →3N2 +6H2O
8NH3 + 6NO2 →7N2 +12H2O

The flow chart of hydrogen peroxide denitration

 

③H2O2 denitration- hydrogen peroxide denitration technology, environmentally friendly treatment process, is to control the flue gas pollutant. The main advantage:
a.H2O2 as oxidizing agent for denitration, Nox can be converted to? nitric acid by H2O2. The by-product can be reuse to improve? economic benefit.;
b.H2O2 washing will not be to generate nitrate, avoiding secondary pollution.;
c.
H2O2 is one kinds of environmental-friendly chemical reagents.? The main method of H2O2 denitration technology: Inject H2O2 for denitration in flue gas pipe, UV(ultraviolet)/H2O2 denitration, absorption tower H2O2 washing for denitration. H2O2 can be decomposed to different product in different temperature.? The chemical equation:
NO + H
2O2→ NO2 + H2O
H
2O2 + 2NO2 → HNO

 

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