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Technical Characteristics
Dual-Alkali desulfuration method is developed to overcome the shortcomings that lime stone/lime-gypsum method is easy to scale. Traditional lime stone/lime-gypsum desulfurating art of fule gas adopts calcium based desulfurizing agent to absorb sulfur dioxide to produce calcium sulfite and calcium sulfate. Because their solubility is low, conditions of scaling and jamming are easy to form. The problems of scaling and jamming seriously influence the normal running of the desulfuration system. What is more serious, the normal running of the boiler can be influenced. To avoid the adverse factors of using calcium based desulfurizing agent, calcium resulfuration art should be supported with corresponding compulsory oxidative system (aerating system), and the investment and running cost would be increased. Using cheap desulfuration agent is easy to cause problems of scaling and jamming. It costs too high and the desulfurationg products are hard to deal with to only use calcium based desulfurizing agent. There is contradictions between them. Dual-alkali flue gas desulfuration art of gas emerges and it solves the above contradictions.
Basic principle of the art
Dual-alkali method adopts natrium desulfurizing agent to desulfurize in the tower. Because of the strong alkalinity of the natrium desulfurizing agent, the solubility of the substance produced after absorbing the sulfur dioxide is high and no supersaturated crystal can be formed to cause the problems of scaling and jamming. On the other hand, the desulfuration products are discharged into the regenerating pool to be regenerated by calcium hydroxide. The natrium desulfurizing agent produced after regeneration can be used again in the desulfurating tower. Dual-alkali method reduces the invest and running cost and is suitable for the desulfuration deduster for middle or small boilers.
Dual-alkali technology uses sodium hydroxide solution as starting desulfurizing agent. The sodium hydroxide solution are put directly into the desulfurating tower to remove SO2 in the flue gas to achieve the purpose of desulfuration. Then products of desulfuration can be deoxidized to sodium hydroxide in the regenerating pool to be used in the desulfurating tower circularly.
The desulfurating arts of the deduster includes 5 parts:
(1) Production and supplement of absorbent;
(2) Spray of absorbent serous fluid;
(3) Fog drop contacts and mixes with the flue gas;
(4) Serous fluid in the regenerating pool deoxidizes natrium-based alkali.
(5) Dewatering treatment of gypsum
The principle of dual-alkali desulfurating art is similar to other wet desulfurating reactions like limestone/lime. The main reaction is that SO2 in the flue gas dissolves in the absorption liquid and then dissociates to H+ and HSO3 -SO2(g)= = = SO2(aq) ( 1 )
SO2(aq)+H2O(l) = = =H++HSO3- = = = 2H++SO32 - ( 2 )
Formula (1) is slow reaction. It is one of the controlling process of the speed.
Then H+ counteracts with OH- in the solution. Salt and water are poduced to accelerate the absorption and dissolution of SO2. The concrete equation is as follows:
2NaOH + SO2 → Na2SO3 + H2O
Na2SO3 + SO2 + H2O → 2NaHSO3
Products after desulfuration comes into regenerating pool to be deoxidized by another alkali(generally Ca(OH)2). The reaction process is as follows:
Ca(OH)2 + Na2SO3 → 2 NaOH + CaSO3
Ca(OH)2 + 2NaHSO3 → Na2SO3 + CaSO3 · 1/2H2O +1/2H2O
Oxygen exiting, the following reaction can happen:
Ca(OH)2 + Na2SO3 + 1/2O2 + 2 H2O → 2 NaOH + CaSO4 · H2O
The sulfur removed come out in form of calcium sulfite and calcium sulfate. They can be pumped into gypsum to be dewatered. They can also be piled or discharged. The regenerated NaOH can be used circularly.
Introduction to the techniques process
The flue gas from the boiler will firstly be dedusted by the deduster. Then the flue gas will come into the desulfuration tower through the smoke duct and will be divided into several layers of swirling board (according to the practical case). Tower of swirling board has good conditions for the contact of gas and liquid. The alkaline liquid sprayed from the top of tower will be atomized on the swirling board to make SO2 mix and react with alkaline liquid. The clean gas will enter the heat exchanger after dehydrating in the demister. The warm-up gas will be discharged by chimney to the atmosphere through induced draft fan.
There is only one circulatory pool for the original dual-alkali method. NAOH, lime and flying ash collected in the desulfurating process are all mixed in one circulatory pool. When ash residues are cleaned in the circulatory pool, smoke ash, calcium sulfite, calcium sulfate, lime residues and limes which have not been reacted are cleaned at the same time. The mixtures, which are cleaned out, are not easy to be utilized and become wastes. To overcome the disadvantages of traditional dual-alkali method, we have improved it. The main process is: Put sodium hydroxide into clean water pool at one time to make desulfurating liquid. The liquid would be pumped into the absorption tower to desulfurate. The three resultants all dissolve in water. In the process of desulfuration, the flying ash in the flue gas would be humidified and collected by the circulatory liquid and flow into the sedimentation tank with the circulatory liquid discharged by the absorption tower. Ash residues can be cleaned periodically after sedimentation and be recycled, for example making brick. Supernatant fluid flows into the reaction pool to react with limes. Sodium hydrate produced by the reaction dissolves in the circulatory water. Calcium sulfite, calcium sulfate and calcium carbonate, which are water, are produced at the same time. They can be cleaned after precipitation.
Explanations to the techniques process
The dual-alkali desulfurating deduster includes five parts of absorbent making and adding system, flue gas system, SO2 absorbing system, gypsum desulfurating and dehydrating system and electric controlling system.
A. Absorbent making and adding system
Sodium hydroxide is used as absorbent when starting the desulfurating equipment. Dry powders of sodium hydroxide are added into alkali pot. Water is added in to produce sodium hydroxide alkali solution. The alkali solution is put into reverting pool and pumped into the desulfurating tower to desulfurate. For regenerating the desulfurating products by using natrium based desulfurizing agent, a pulping pot is needed. Lime powders are added into the pulping pot with water to produce lime serum. Lime serum is then put into reverting pool to react with sodium sulfite and sodium sulfate. In the whole process, many solid residues produced by desulfuration are pumped into gypsum dehydrating system by residue serum pump. Because some sodium hydroxide will be lost in the discharging process of residues, sodium hydroxide solution should be added periodically in the alkali pot to ensure the normal running of the whole desulfurating system and eligible discharging of flue gas. To avoid that calcium sulfite and calcium sulfate can also be pumped in to desulfurating tower to cause jamming and scaling, aeration system can be equipped to proceed compulsive oxidation or the water pool can be made larger. The regenerated desulfurizing agent solution will be sufficiently precipitated through three-grade sedimentation tank to make sure that big particulate matters won't be pumped back into the tower. On the other hand, filter can be equipped in front of the circulatory pump to settle big particulate matters and liquid impurities.
B. Flue gas system
Flue gas of boiler enters deduster to be dedusted and then enters desulfurating tower. The cold flue gas after desulfuration will be demisted by the two-grade demister and enter the main smoke duct. The flue gas will be discharged into the atmosphere after it is warmed up. When the desulfuration system malfunctions or is overhauled, the system will close the entrance and exit dam-board. The flue gas will enter the chimney to be discharged through the side of original smoke duct of the boiler.
C. SO2 absorption system
The flue gas enters the absorption tower and flows upward and then will be washed in the way of adverse current with spraying lime stone serum. Then he gas contacts sufficiently with the liquid. Several layers of built-in swirling board are equipped in the desulfuration tower. On the top desulfuration swirling board in the tower, there is a sprayer nozzle. The sodium hydroxide is sprayed on the distributor of the swirling board columella through the spraying serum layer. Then the alkali liquid is distributed uniformly. Under the diversion of swirling board, the flue gas rises reelingly and contacts with the even alkali liquid on the swirling board. It atomizes the alkali further and absorbs acid gases like SO2, SO3, HCL and HF to produce NaSO3 and NaHSO3. Sodium hydroxide as absorbent is consumed at the same time. The added sodium hydroxide alkali liquid enters the reverting pool to be pumped into the absorption tower with the sodium hydroxide solution regenerated by the lime through circulatory pump.
Two-grade swirling board (or baffle plate) demister is equipped on the exit of the absorption tower to demist the water mist, which is produced in the washing process, in the flue gas. In this process, the demister collects the dust and other solid matters carried by flue gas. Two grades of demisters are both equipped with water-washing nozzles. They can wash the demisters periodically to avoid jamming of the demisters.
D. The desulfurating products treating system
The final products by the desulfurating system will still be gypsum serum (solid content 20%). The specific components are CaSO3, CaSO4 and part of oxidized NaSO4. They are discharged through serum discharging pipe on the bottom of the sedimentation tank into the waterpower swirler by serum discharging pump. Because there are several kinds of ash and NASO4 in the solid products, the quality of gypsum is seriously influenced. Thus they should be thrown away. In the waterpower swirler, gypsum serum is condensed (solid content 40%)and then pumped to the residue treating area. The overflowing liquid flows away into the regeneration pool.
E. Electric controlling system
The electric power supply of desulfurating equipment is led from the switchboard of electric factory and then connected to the switchboard in the control room of the desulfurating equipment. In the electric control room of the desulfurating equipment, electric power is divided into two ways. One directly connects to the high-pressure electrical engineering (serum circulatory pump) through switchboard and control switch cupboard. The other connects to desulfuration transformer. The output end of it connects to the low-pressure electrical appliance through switchboard and control switch cupboard. The power of low-pressure power distribution is supplied by power centric electromotor control center.
The system is equipped with low-pressure DC electrical source to supply parts of power for the electric concentratly control part.
The desulfurating agent feeding device and swirling separator are controlled on the spot. The others are controlled by desulfuratiung switchboard in the control room. They can also be controlled by hand on the spot.
When running normally, every control valve is controlled automatically by vertical switchboard to control the supplying of lime and the supplement of sodium hydroxide. When the load of boiler is changing, it should be automatically adjusted. The controlling of flue gas is according to the exhaust smoke level of the boiler. It is controlled by feeding signals of flue gas quantity and practical flue gas quantity led to the desulfuration equipment through boiler loading signals by the entrance board of the induced draft fan. The flow rate of absorbent serum is controlled by the quantity of SO2 and PH value of the serum in the circulatory pool. The byproduct serum allowance is controlled by the flow rate of absorbent serum. The flow rate of washing water of the demister, the pressure of washing water of the absorption room entrance and flow rate of the discharging liquid by spin-dryer are controlled separately. The liquid on the bottom of the desulfuration tower is also controlled separately by the supplement quantity of water. The quantity of lime added is controlled by the rotate speed of feeder, which is adjusted by the quantity of water supplement. Then the thickness of absorbent serum can be controlled. The supplement water sprayed by the spray valve periodically washes the demister of the absorption room entrance.
The problem of solving the secondary pollution
Sodium hydroxide is used as desulfurating agent. The action of sulfur dioxide absorption is fast and the desulfuration rate is high. However, the desulfurating product, Na2SO4, is hard to deal with and it is easy to cause serious secondary pollution. Using dual-alkali art, the products, which are produced after sodium hydroxide absorbs sulfur dioxide, are regenerated by the lime. There is only a little Na2SO4 taken into the gypsum serum. The gypsum mixed with a little Na2SO4 is pumped into the swirling separator to be separated into solid and liquid. The large amount of solid residues of low moisture content is pumped to the residue area to be piled. The solution flows back to the regenerating pool to be used again. So there is no secondary pollution.
Features of Art
Compared with limestone and wet lime desulfurating art, dual-alkali method has advantages in principle as following.
(1) Using NAOH, the circulatory water is basically solution of NAOH, so in the circulatory process, water pump, pipe and equipment won’t be corrupted and jammed. It is easy for the running and maintenance of the equipment.
(2) The regeneration of absorbent and precipitation of desulfuration residues happen out of the tower. Then the jamming and abrasion can be avoided, the reliability can be raised and the operation cost can be reduced. High efficient board tower and filling tower can be used to take place of empty tower to make the system more compact and the desulfuration efficiency higher.
(3) The absorption speed of natrium-based absorbent is fast, so it can use smaller vapour-liquid ratio to achieve higher desulfurating efficiency, generally above 90%.
(4) Compared with desulfurating and dedusting integration, the utilization ratio lime can be raised.
The disadvantages are: The oxidation side reaction product of NaSO3, Na2SO4 is hard to regenerate, so NAOH or Na2CO3 should be added constantly. So the consumption of alkali will be raised. On the other hand, the existence of NA2SO4 will reduce the quality of the gypsum.
Dual-alkali method is mature and used widely at home and abroad. It is a desulfurating technique that is suitable of middle and small boilers. It has wide markets. |
