1.Wet flue gas desulfurization
(1) Limestone (lime)-gypsum flue gas desulfurization
The limestone or lime slurry reacts with the SO2 in the flue gas, and the desulfurization product is 15-20% aqueous gypsum. Magnesium oxide flue gas desulphurization is the reaction of magnesium oxide slurry with SO2 in flue gas, and the desulfurization product is a solid absorption product of magnesium sulfite and magnesium sulfate containing crystal water. Ammonia flue gas desulfurization uses ammonium sulfite (NH4)2SO3 to absorb SO2 to produce ammonium bisulfite NH4HSO3. Recycle ammonia in the recycle tank regenerates NH4HSO3 ammonium bisulfite to (NH4)2SO3 ammonium sulfite for recycling. Double-alkali flue gas desulfurization uses sodium hydroxide solution as a start-up desulfurizer. The prepared sodium hydroxide solution is directly injected into the desulfurization tower to remove SO2 in the flue gas to achieve flue gas desulfurization, and then the desulfurization product is subjected to desulfurization. The regeneration tank is reduced to sodium hydroxide and then recycled back to the desulfurization tower. The seawater is used for flue gas desulfurization. The seawater is normally weakly alkaline. It has natural sulfur dioxide absorption capacity, generates sulfite ions and hydrogen ions, and the washed seawater is acidic. After being processed, it will be discharged into the sea.
(2) Dry or semi-dry flue gas desulfurization
The so-called dry flue gas desulphurization means that the final product of desulfurization is a dry spray method: by using a high-speed rotary atomizer, the lime slurry is atomized into fine droplets and flue gas for heat transfer and reaction to absorb SO2 in the flue gas. . In-furnace Ca-steam tailing humidification activation method: Spray calcium-based absorbents such as limestone, dolomite, etc. into the upper part of the combustion chamber of the furnace and the temperature is lower than 1200
In the region of °C, limestone is calcined into calcium oxide, and newly formed calcium oxide CaO reacts with SO2 to form CaSO4 calcium sulfate, which is collected with the fly ash in the dust collector and sprayed with water in the activation reactor to promote desulfurization. Circulating fluidized bed method: the dry powder absorbent powder is sprayed into the tower to react with the SO2 in the flue gas, and a certain amount of atomized water is sprayed at the same time to humidify the surface of the particles to increase the reaction and control the temperature of the tower outlet flue gas. The absorbent and the produced product are collected together by a dust collector, and are further cycled to prolong the contact time between the absorbent and the flue gas, thereby greatly improving the utilization of the absorbent and the desulfurization efficiency.
(3) Charge dry spray desulfurization method:
The absorbent dry powder passes through the high-voltage electrostatic corona charging zone at a high speed so that the same negative charge on the dry powder charge is injected into the smoke. The charged dry powder repels the same charge, forming a uniform suspension state in the flue gas, and the ion surface is fully exposed. , increased the chance of reaction with SO2. At the same time, the charged particles enhance the activity, shorten the residence time required for the reaction, and increase the desulfurization efficiency.
2. Sintering machine lime-gypsum wet desulfurization process overview
(1) Flue gas characteristics of sintering machine
a. Sintering machine has a high annual operating rate of more than 90%, with a large amount of smoke emission;
b. The composition of smoke is complicated, and there are many changes in gender depending on the ingredients;
c. Fluctuation of the temperature of flue gas is relatively large and the fluctuation range is from 90 to 170°C;
d. Flue gas humidity is generally about 10%;
e. Due to the sulfur content of the sintering raw materials, the SO2 concentration of the emitted flue gas will change greatly with the change of the batch ratio;
f. Sintered flue gas has high oxygen content, accounting for about 10% to 15%;
g. Contains corrosive gases. Sintering machine ignition and the sintering process of the mixture produce a certain amount of hydrogen chloride (HCl), sulfur oxides (SOx), nitrogen oxides (NOx), and hydrogen fluoride (HF).
(2) Lime-gypsum wet desulfurization process principle
The desulfurizer uses lime powder (above 150 mesh, calcium content ≥80%, sieve residue ≤ 5%). After the desulfurization slurry absorbs S02 in the flue gas, it is oxidized to form gypsum. The reaction equation is as follows:
a. SO2 and SO3 dissolution in flue gas; SO2 contained in the flue gas and the absorbent slurry undergo full gas/liquid contact, mass transfer occurs at the gas-liquid interface, and gaseous SO2 and SO3 are dissolved in the flue gas Converted to the corresponding acidic compounds: SO2 + H2O ← → H2SO3 sulfite SO3 + H2O ← → HSO4 Some other acidic compounds in the hydrogen sulphide gas (such as: HF (hydrogen fluoride), HCl (hydrogen chloride), etc.), in the smoke and spray down When the slurry is contacted, it is also dissolved in the slurry to form hydrofluoric acid, hydrochloric acid, and the like.
b. Dissociation of acid SO2 formed by dissolution of SO2 is quickly dissociated as follows: H2SO3 sulfurous acid ← → H + hydrogen ion + hydrogensulfite HSO3- (lower pH) HSO3-sulfite → H + hydrogen Ion + SO32 Sulfite – (higher pH)
c. Dissolving and Neutralization of Absorbing Agent First of all, lime must be digested, that is, the quick lime reacts with water to produce mature lime Ca(OH)2 (calcium hydroxide) slurry: CaO calcium oxide (solid) + H2O ← → Ca(OH)2 ( Calcium Hydroxide) (Solid) Ca(OH)2 (Solid) + H2O ← → Ca(OH)2 (Slurry) + H2OCa(OH)2 Partially Ionized to Produce Ca(OH)2 (Slurry) ←→ Ca2+ Positive-divalent calcium ion + 2OH-(2 hydroxide) absorbent slurry neutralizes the H+ strong acid generated in the dissociation reaction in the spray zone of the absorber: OH-hydrogen ion + strong acid H++ hydrogensulfite Root HSO3-→ Sulfite SO32-+H2OCa(OH)2+H++ Bisulfite HSO3-→Ca2++SO32-+H2O Under acidic conditions, the sulfite SO32 generated in the reaction can also be reacted as follows :SO32-+H+←→Hydrosulfite HSO3-
d. Oxidation reaction and crystallization of SO2 absorbed slurry, containing a large amount of sulfite SO32-, hydrogen sulfite HSO3-, these ions in the bottom of the absorber in the pool, the oxygen is oxidized by the Roots blower: 2SO32 -+O2→2SO42-2HSO3-+O2→2H++2SO4-sulfite SO32-, hydrosulfite HSO3- are continuously oxidized to sulfate SO42-, and calcium sulfate CaSO4 is continuously combined with divalent calcium Ca2+. This eventually leads to supersaturation of the solution, which in turn produces gypsum crystals. The pH of Ca2++SO42-+2H2O→CaSO42H2O (gypsum) absorber slurry pool is controlled by adding lime slurry. The reaction in the absorber slurry pond needs to be long enough for the gypsum to produce good gypsum crystals (CaSO42H2O) calcined gypsum. . Oxidation fans are used to provide enough oxygen to the absorber slurry pool to facilitate the formation of gypsum (ie, further oxidation from calcium sulfite to calcium sulfate) because the oxygen contained in the flue gas does not satisfy the need for oxidation.
(3) Characteristics of lime-gypsum method:
a. High desulfurization efficiency. The desulfurization rate of limestone (lime)-gypsum wet desulfurization process is as high as 95%. After the desulfurization, the concentration of sulfur dioxide is very low, and the dust content of flue gas is also greatly reduced. Large units adopt wet desulfurization technology and large amounts of sulfur dioxide are removed, which is conducive to the implementation of total control in regions and power plants.
b. The technology is mature and the operational reliability is good. The operating rate of limestone (lime)-gypsum wet desulfurization equipment in thermal power plants in foreign countries is generally over 98%. Due to its long history of development, mature technology and many operating experiences, it will not affect the normal operation of boilers due to desulphurization equipment. In particular, the newly-built large units adopt the wet desulfurization process, which has a long service life and can achieve good investment returns.
c. Adaptable to changes in coal types. The process is suitable for flue gas desulfurization of any sulphur-bearing coal species, whether it is high-sulphur coal with sulphur content greater than 3% or low sulphur coal with sulphur content below 1%, limestone (lime)-gypsum wet method. Desulfurization process can adapt.
d. The land area is large, and the one-off construction investment is relatively large. The limestone (lime)-gypsum wet desulfurization process has a larger footprint than other processes, so the existing power plant adopts this process without a desulfurization site, which has certain difficulties, and its one-time construction investment is also higher than other processes. To be higher.
e. Absorbent resources are abundant and the price is cheap. As a limestone (lime)-gypsum wet desulfurization process absorbent limestone, widely distributed in our country, rich in resources, limestone grade in many areas is also very good, calcium carbonate content in more than 90%, excellent up to 95%. Among the various absorbents in the desulfurization process, the price of limestone is the cheapest, the crushing and grinding is simpler, and the calcium utilization rate is higher.
f. Desulfurization byproducts are easy to use comprehensively. The desulfurization by-product of limestone (lime)-gypsum wet desulfurization process is dihydrate gypsum. In Japan and Germany, the annual output of desulfurized gypsum is about 2.5 million tons and about 3.5 million tons respectively, which can basically be utilized comprehensively. The main purpose is to produce building material products and cement retarders. The comprehensive utilization of desulfurization by-products can not only increase the efficiency of the power plant, reduce operating costs, but also reduce the disposal costs of desulfurization by-products and extend the service life of the ash yard.
g. Technological progress is fast. In recent years, the limestone (lime)-gypsum wet process has undergone in-depth research and continuous improvement in foreign countries. For example, the original cooling, absorption, and oxidation towers of an absorption device are combined into a tower, and the flow rate in the tower is greatly improved. Further improvement etc. Through technological advancement and innovation, it is hoped that the problem of large area and high cost of the process will be gradually and properly solved.
(3) Process description of lime-gypsum method
A flue is added to the original horizontal flue duct between the draft fan of the sintering machine and the original chimney, and the flue duct is led to the booster fan. In addition, an electric double blind bypass damper door is installed on the horizontal flue between the horizontal flue interface and the chimney. The flue gas is pressurized by the booster fan and sent to the reaction tower. The gas flows upward in the tower and reacts with the sprayed spray slurry. The SO2, dust, some heavy metals, and oxides in the flue gas are removed or removed. After washing and purifying the wet flue gas, after the defogging and defogging by the secondary demister, the chimney is discharged through the top of the tower, and the slurry that absorbs SO2 generates calcium sulfite CaSO3 in the slurry tank of the absorption tower and is oxidized in the tower. The compressed air is forced to oxidize, and the mature gypsum CaSO4•2H2O is continuously aggregated to reach the sedimentation density. After the gypsum discharge valve is discharged into the gypsum accidental slurry pool and reaches a certain volume, the gypsum pump is then sent to the automatic membrane filter press. Dehydration treatment. Gypsum with moisture content ≤ 20% is taken out of the plant and the generated waste water flows into the waste water storage tank, flows into the waste water treatment system, and is reused after treatment.
3. Lime-gypsum desulphurization process system
(1) Flue gas system
a. Function of flue gas system The flue gas system adopts a scheme in which a booster fan is arranged on the flue gas side upstream of the absorption tower to ensure that the entire FGD system is operated at positive pressure and at the same time avoids the low temperature smoke that the booster fan may suffer from. Corrosion, thus ensuring the safe long-life operation of the booster fan and the entire FGD system. The raw flue gas from the sintering machine passes through the flue gas damper door through the flue and is led by the booster fan to the desulfurization tower system. The original flue gas is desulfurized in the absorber tower. In the absorption tower, the original flue gas is in full contact with the lime slurry, and the SO2, SO3, and other gases are removed by the reaction. After passing through the desulfurization tower, the flue gas temperature is reduced to about 50°C. After the desulfurization, the net flue gas passes through the demister, passes through the net flue gas chimney and chimney, and is discharged into the atmosphere. In order to separate the FGD system from the sintering machine, a flue gas damper with zero leakage is provided in the entire flue gas system, including the bypass damper door and the original flue inlet baffle. door. When the desulfurization system operates normally, the bypass baffle is closed, and the original flue gas passes through the original flue gas baffle and enters the FGD device for desulfurization reaction. In an emergency state that requires the FGD system to be turned off, the bypass damper automatically opens quickly and the original smoke damper automatically closes. In order to prevent smoke from leaking in the flapper door, the flapper door is provided with a sealed air system. The flue adopts ordinary steel flue. The main body of the tower is carbon steel plus glass flake resin coating.
b. Flue gas system equipment The main mechanical equipment of the flue gas system includes: booster fan and its auxiliary equipment (blade adjustment mechanism, cooling fan, lubricant station, hydraulic oil station), bypass flue gas damper door, inlet flue gas Baffle door, baffle door seal blower, expansion joint and so on. Fan system thermo-mechanical instruments: mainly include fan vibration measuring device, stall probe alarm device, motor bearing and stator coil temperature measuring device, fan bearing temperature measuring device, motor current transmitter signal, blade electric actuator regulating opening device, Lubricating station measuring points; electric drive devices, including the drive motor and the corresponding electrical control equipment, etc.; corresponding DCS control system.
(2) Absorption tower system
a. The function of the absorption tower system The absorption tower is the main site of the SO2 absorption reaction and is the core of the flue gas desulfurization system. In the absorption tower, the SO2 in the flue gas is washed by the absorption slurry and reacts with Ca(OH)2 in the slurry. The calcium sulfite generated in the reaction is forced to oxidize by the air blown by the oxidation fan in the circulating slurry pool at the bottom of the absorption tower. The final production of gypsum, gypsum discharged from the gypsum discharge valve, sent to the gypsum processing system pressure filter dehydration. The flue gas passes through the demister at the top of the tower to remove the fine droplets entrained by the flue gas after desulfurization, so that the amount of fog in the flue gas is within the required range. Desulfurization unit The absorption tower is a counter-flow spray absorption tower, the absorption tower is a cylinder, the bottom is a circulating pool, and the upper part is the spray washing area. Generally, a three-layer nozzle is arranged. The flue gas flows through the spray zone from bottom to top, and after desulfurization, it is discharged through the absorption tower at the top of the absorption tower.
b. The equipment composition of the absorption tower system mainly includes absorption towers, demisters, slurry circulation pumps, oxidation fans, absorption tower mixers, gypsum slurry discharge valves and other equipment. Thermal instrumentation includes: Density meter, PH meter, absorption tower liquid level meter, defogger differential pressure meter, export chimney smoke analyzer, etc. The absorber tower is a steel structure with a glass flake resin liner. Three centrifugal slurry circulation pumps were used to drive the slurry from the bottom of the absorption tower to the spray layer in the tower. The slurry was atomized through nozzles and contacted with flue gas to achieve the purpose of absorbing SO2; usually one Roots type forced oxidation was installed per tower. The fan provides enough oxygenated air for the slurry in the tower. Oxidation of calcium sulfite can be accomplished with in-situ enhanced slurry oxidation without the addition of sulfuric acid or other compounds in the absorber’s circulating slurry tank.