Dual Alkali FGD is introduced
1The status quo, desulfurization technology

In order to control sulfur dioxide in the atmosphere, as early as nineteenth Century, humans began to conduct relevant research, but to carry out large-scale research and application of desulfurization technology is from the beginning of twentieth Century 50's.. More than 200 kinds of SO2 control technology has been developed after years of research. These techniques according to the desulfurization process and combustion combination can be divided into: pre combustion desulfurization (such as coal, microbial desulfurization); the combustion desulfurization (industrial briquette, calcium spray furnace); the combustion desulfurization, flue gas desulfurization (Flue Gas Desulfurization, referred to as FGD). FGD is currently the world's only large-scale commercial application of desulfurization method, is the most important technology measure to control acid rain and sulfur dioxide pollution.
Flue gas desulfurization technology is mainly used absorbent or adsorbent to various alkaline trapping of sulfur dioxide in the flue gas, will be transformed into a more stable and easy to mechanical separation of sulfur compounds or sulfur, so as to achieve the purpose of desulfurization. According to the method of FGD desulfurization and desulfurization products content can be divided into two types: the wet, which uses liquid absorbent such as water or alkaline solution (or slurry) washing to remove sulfur dioxide. The use of dry, powdered or granular absorbent, absorbent and catalyst to remove sulfur dioxide. According to whether the desulfurization product reuse can be divided into recovery method and discard method. According to the way of dealing with absorption of sulfur dioxide can be divided into regeneration method and non regeneration method (abandon).
2Abroad, flue gas desulfurization
Study on flue gas desulfurization abroad began in 1850, after years of development, so far, the world has more than 2500 sets of FGD devices, the total capacity has reached 200000MW (with a generating capacity of power plant flue gas volume meter), 700Mm3/h, a year of sulfur dioxide removal of nearly 10Mt, these devices in Japan and Germany, 90% USA.
Although many FGD methods developed by other methods, but the real industrial application is only limited dozen. The wet wash method (including abandon method and gypsum) accounted for 73.4% of the total number of the spray drying method, accounting for 17.7% of the total number of the other methods, accounting for 9.3%. The FGD system America, abandon the majority. In wet lime / limestone method, accounted for more than 90%. Visible, wet lime / limestone method in today's dominant FGD system.
Although countries have made great progress in terms of FGD, but the operating cost is quite amazing, but each method has its limitations, therefore, many researchers are still in the continuous research and development of more advanced, more economical FGD technology.
At present the main technology of industrialization has:
The wet lime / limestone gypsum method the lime or limestone slurry absorbing SO2 in flue gas, generating half water calcium sulfite or re oxidation of gypsum. The high degree of technological maturity, stable desulfurization efficiency, more than 90%, is currently the main method at home and abroad.
The spray drying method this method is the use of lime as absorbent is sprayed into the desulfurization tower, the desulfurization and desulfurization slag discharged into powder after drying, semi dry desulfurization, desulfurization efficiency of about 85%, the investment ratio of wet limestone gypsum method low. It is mainly used in USA.
The main method of ammonia absorption and regeneration method, Magnesium Oxide method, double alkali method, W-L method. The desulfurization efficiency can reach about 95%, more mature technology.
The furnace calcium injection - humidification desulfurization method this method is a kind of powdered calcium desulfurizing agent (Shi Huishi) is directly injected into the boiler furnace combustion desulfurization technology, suitable for low sulfur coal, the boiler, the desulfurization efficiency of about 85%.
3The domestic status quo, flue gas desulfurization
Exhaust gas desulfurization technology in China in the early1950 in the sulfuric acid industry and non-ferrous metallurgy industry, the technology for removing sulfur dioxide in flue gas of boiler combustion has started in twentieth Century 70's and the national "six five" to "Nine Five" period has made great progress. There have been more than 60 colleges and universities, scientific research units and production experiments were carried out on a variety of desulfurization process.
Although the research of FGD system in China started very early, the surface is very wide, but most of the technology only in laboratory or pilot plant scale, far from the large area industrial application level. In the demonstration project heavily in the introduction of advanced equipment, although stable, but the huge investment, operation cost is quite high. Therefore, to accelerate the absorption of advanced foreign technology, make its localization, low cost, is an important and arduous task. Here is a partial list of FGD devices imported by china. In recent years, China has increased investment in FGD technology research, "eight five", "Nine Five" during the period there have been large project support in this area of research, has made gratifying achievements, among them, cyclone tower desulfurization technology is studied, in this period of time, the development of the development.
Sodium calcium dual alkali desulfurization method is one of the more commonly used, the law in foreign countries (such as Japan, American) has been successfully applied to large-scale, there are at least 50 sets of double alkali desulfurization device in Japan and American, successfully applied to large scale power plants and industrial boilers, the Central Illinois Public Service USA, Newtow 1#, 575MW.
Process selection

At present, the most widely used at home and abroad is the desulfurization technology of wet limestone gypsum method, but the technology of large investment, high operation cost, equipment and piping system easy to wear and clogging.
Double alkali method is to use alkaline liquid soluble as absorbentSO2, then the milk of lime or lime for regeneration of absorption liquid, due to the absorption and absorption liquid treatment, using different types of alkali, called double alkali method. Sodium calcium dual alkali is the first alkali absorption of SO2 in flue gas with sodium carbonate or sodium hydroxide solution, and then use lime or slaked lime as a secondary treatment, the absorption liquid, the absorption liquid after regeneration to use absorber. Due to the use of sodium lye as absorption solution, there is no fouling and clogging slurry, and the sodium absorption rate than calcium salt rate, liquid gas ratio required is much lower, can save power consumption.
Therefore, sodium calcium dual Alkali FGD process used in this project.

1Sodium calcium dual alkali process, reaction principle
The method of using Na2CO3 or NaOH liquid absorption in flue gas SO2, HSO32-, SO32- and SO42- formation, the reaction equation as follows:
A, desulfurization process
(1)
(2)
(3)
Which type: (1) for the start-up phase Na2CO3 solution absorption reaction of SO2;
Type (2) for regeneration liquid at higher pH value (higher than 9), the main reaction solution absorption SO2;
Type (3) for the solution of low pH value (5~9) of the main reaction.
Two, the oxidation process (reaction)
(4)
(5)
Three, the regeneration process
(6)
(7)
Type (6) as the first step of the reaction regeneration reaction type, (7) the main reaction regeneration to pH > 9 after going on.
The project choice of sodium calcium dual alkali method desulfurization process, using lime as the main desulfurization agent, sodium alkali desulfurization agent for help. Because of the absorption liquid in the absorption process in Sodium Alkali FGD system, there is no problem such as scale, safe and reliable operation. And because the rate of sodium alkali absorption liquid and reaction of sulfur dioxide than calcium alkali is much faster, smaller than in liquid gas conditions, achieve high rate of sulfur dioxide removal.

2Spray, low resistance and high efficiency desulfurization process
The spray tower has become a spray tower, is in the upper layers of the absorption tower arrangement of nozzle, desulfurizing agent is ejected through the nozzle to form liquid mist, through the full contact of droplet and gas, to complete the transfer process. Empty tower spray absorption tower main body is a rectangular tower, the tower body has a plurality of configuration and high efficient nozzle demister, slurry in absorption tower through efficient atomization, spray coverage area up to200%, the formation of gas-liquid contact reaction interface is good, after the flue gas entering the tower, rising in the tower of uniform, comprehensive and efficient mixed contact spray liquid, removal of acid gases such as SO2. According to the sulfur content of coal, desulfurization efficiency, desulfurization tower in the general layout of the nozzle layer. Nozzle and spray pressure has obvious influence on the droplet diameter. Reduce the droplet diameter, can increase the mass transfer area, prolong the residence time in the drop tower, both on the desulfurization efficiency plays a positive role. The droplet residence time in the tower and the droplet diameter, jet velocity and flow direction of flue gas. With the rise to efficient flue gas spray demisting device, the demisting device, gas-liquid contact two absorption and get the effective separation, so as to avoid gas entrainment, minimizing the flue gas with water phenomenon.
Empty tower spraying flue gas scrubbing technology is now international and domestic mature technology, the most empty tower spraying popular use of the technology frontier.
1, hollow spraying tower is a spray tower after a large limestone gypsum method evolved, with the desulfurization efficiency is high, the maximum of 95%;
2, flexible operation, strong adaptability to changes in the coal, sulfur content below 4% can ensure that the concentration of sulfur dioxide emissions, the boiler can be below 110% and so on;
3, small system resistance, low operation cost, the right for large wet 1/10;
4, using imported defogging techniques, the moisture content of the flue gas to ensure compliance with the requirements;
5, there is no blocking problem;
6, high utilization rate of equipment, and ensure the synchronous operation of boiler is more than 100%;
7, other investment and not much difference between empty tower tower;
8, simple operation, convenient maintenance, stability is three to five times the other tower.
The mist eliminator can be installed in the upper part of the absorption tower, to separate the flue gas entrained droplets. Demister outlet flue gas humidity is not more than 75mg/Nm3, divided into two levels of layout in the upper part of the desulfurization tower, set up two level four channel flat plate demister, a layer of thick mist, a layer of fine mist.
The mist eliminator type can guarantee the high availability and good removal effect of droplet, and ensure that the flue gas desulfurization with uniform velocity through the demister, prevent the occurrence of two carry, blocking demister.
Mist eliminator systems are designed to consider the FGD device of the entrance concentration of fly ash effect. The system also includes the removal of demister sediment flushing and drainage system, operation according to the given or change procedures, which can be used for automatic washing, can also be artificial irrigation. The design of a reasonable flushing time and flushing water, can clean the demister, and prevent the formation of two times to carry.
Located below the first class defogger is a large droplet separator, blade gap is slightly larger, larger droplets to rise with flue gas separation. Above the second level mist eliminator is a fine droplet separator, blade distance is small, used to separate the rise of flue gas in the slurry droplets and small flushing water. The flue gas flows through the demister, droplet due to inertia, in the baffle. As the droplet detention also contains solid material, so there is a risk of baffle on the scale, at the same time to ensure that the flue gas passes through the mist removing pressure drop does not exceed the set value, should be regularly cleaned. Therefore, set up regular operation of cleaning equipment, including nozzle system. Washing medium for industrial water.
A demister on the bottom and two level mist eliminator is arranged under the flushing nozzle, the normal operation of the lower demister bottom surface and the top surface, the bottom surface of the upper defogger automatically according to the procedure in cleaning area. The mist eliminator flushing each layer according to the gas load, the demister at both ends of the pressure automatic regulating irrigation frequency.
Water is the flushing pump, wash water is used to supplement the moisture absorption of evaporation loss in the tower.

3The desulfurization system,Form
The desulfurization system of the process flow diagram and layout diagram shown in Figure 1 and figure 2.
The whole process consists of five parts:
(1) desulfurizer preparation system
By product of lime (particle size less than 10mm (100%) of the powdered lime) transported to the factory after manual lime digester digestion, digestion after the lime slurry flow into the regeneration pool for desulfurizing solution regeneration reaction.
Sodium alkali feeding by the transport vehicle to the concentration of sodium alkali pool, mixing until the desired process with water in the pool, self regeneration tank.
(2) flue gas system
The hot flue gas from the boiler after entering the absorber, through the spray layer to flow, the flue gas is cooled to saturation temperature, SO2 in flue gas and other pollutants by desulfurization liquid absorption. After saturated flue gas spray after washing, the mist eliminator to remove water, through a flue after fan emptying into the chimney.
From the outlet of the boiler to connecting flue desulfurization tower inlet section by A3 steel, and according to the need to set expansion joint. Connecting flue baffle system for flue gas desulfurization system, accident bypass operation. The manual pumping plate baffle valve, including 1 entrance baffle, 1 bypass baffle and 1 outlet baffle desulfurization device. During normal operation, the entrance baffle and the outlet baffle opening, closed bypass baffle. In the case of faults, open the gas bypass baffle, baffle closed the entrance and outlet baffle, flue gas is directly discharged to the chimney through the flue gas bypass bypass flue gas desulfurization system.
(3) SO2 absorption system
In the absorption tower, desulfurizing liquid, sodium hydroxide and capture from flue gas of SO2, SO3, HF, HCl and other chemical reactions, the formation of sodium sulfite and Sodium Bisulfite material etc.. Clean the flue gas desulfurization by mist eliminator to remove air entrained droplets discharged after absorption tower.
The spray tower as absorption tower, spray tower is currently widely used in the small boiler desulfurization device in the desulfurization tower, the liquid has the advantages of big circulation, pressure drop, operating flexibility, efficiency, wide is not easy to block stability.
The main absorption tower desulfurization reaction principle is as follows:
A) absorption
In the absorption tower, flue gas SO2 and SO3 according to the following reaction by solution of water absorption:
SO2 + H2O<==> H2SO3
SO3 + H2O<==> H2SO4
B) neutralization reaction
H2SO3 and H2SO4 must be quickly neutralized to ensure effective SO2 and SO3. absorption.
H2SO3, H2SO4, HCl and HF and the suspension base according to the following reaction reaction:
Na2CO3 + H2SO3 <==>Na2SO3+CO2 +H2O.
Na2CO3 + H2SO4 + CO2 + H2O Na2SO4 <==>.
Na2CO3 + HCl NaCl +CO2 +H2O <==>.
Na2CO3 + HF <==>NaF +CO2 +H2O.
C) side reaction
The oxygen contained in the flue gas will generate desulfurization reaction of sodium sulfite (Na2SO3) oxidation of sodium sulfate (Na2SO4):
2 Na2SO3+O2 <==>2 Na2SO4
(4) the desulfurization liquid circulation system and desulfurization slag disposal system
The desulfurization liquid pump forebay to flue gas desulfurization tower and the contact reaction through a circulating pump after the pump, discharged from the desulfurization device at the bottom, CaSO4, CaSO3 and a small amount of dust containing slag discharged (most of dust in the original dust removal) mixed slurry liquid into the regeneration tank, sedimentation tank, regeneration reaction with lime slurry. From the lime slurry tank, and the by-product of the precipitation, flows through the supernatant before the pump pool, the pool bottom ash slurry after precipitation by manual cleaning, return back to the filtrate before the pump tank, desulfurization recycling by the circulating water pump pumping to desulfurization device.
(5) electrical control system
The power supply mode
The power equipment in the system for decentralized arrangement, for three-phase power supply, the civil power and civil lighting into a single three-phase power supply distribution, power supply design processing system uses the emanant power supply mode, has the advantages of safe and reliable.
The grounding system
Treatment of ground zero protection using TN--C--S system for low voltage power distribution system, all metal electrical equipment shall be grounded reliably and zero connection, civil power, lighting ground zero protection system with TT.
To determine the position of the low voltage distribution
The design requirements of low-voltage distribution location as close as possible to the load center, due to the large power electrical equipment in the main circulation pump, slurry pump, power and other lighting load is small, it is a control room in the pump room installation, supply cabinet, power cabinet and instrument cabinet etc..
The power equipment control and start method
All the power devices are equipped with undervoltage, short-circuit and overload protection, power supply cabinet set overcurrent protection.
The civil power and civil lighting with short circuit, overload and leakage protection.
The power cables with armoured cables along the cable trench laying dark, no local cable channel soft cable and signal cable are used in steel concealed underground laying, cable trench support reliable grounding, grounding form.
The supplier is responsible for the design, supply of desulfurization system of all devices in the cable. Delivery and the island part (cut-off point for the desulphurization island 1 meters) laying by the owner is responsible for.
The FGD by manual control.
This relates to the engineering system of all specifications, standards or specifications (including all valid supplement or appendix) is the latest version, the effective date of the contract as the deadline by the latest version.
Alarm monitor and adjust the start / stop control, normal operation and abnormal and accident conditions of desulfurization system and auxiliary system. Process control system and instrument, equipment design, supply can meet the above requirements.
The power supply system adopts 380V, 50HZ AC power supply, power distribution cabinet and control cabinet according to the power load by the design institute responsible for the design.

4The main advantage of this technology,

• Advanced technology, technical indicators can meet the environmental requirements and the requirements of manufacturers;
• Using a special anti-corrosion, wear-resistant, high inlet nozzle, spray drops from 800 to 1200 m, with a huge surface area, and is not easy to cause two entrainment;
• Good desulfurization, desulfurization efficiency of 65% ~ 95%, the desulfurization tower smoke outlet concentration is not higher than 130mg/m3;
• Investment, low operation cost, has a good economy;
• Anti fouling, anti blocking performance, stable operation, high safety performance;
• Good corrosion resistance, long service life (the main equipment in more than 20 years);
• Small resistance, low pressure (wet FGD system is less than 1000Pa);
• Wide operation flexibility, convenient operation and management, the system is simple, investment province;
• The fan can ensure the long-term safe and reliable operation.

5, material consumption
(1)Lime consumption
Lime consumption: 90% purity lime dosage of 126.5kg. for an hour
The lime dosage varies with lime purity, sulfur content and gas desulfurization rate changes.
(2)Sodium alkali consumption
In theory, the first dual alkali alkali (sodium carbonate or sodium hydroxide) without the need to increase, but the absorption liquid in the absorption process of evaporation cycle with flue gas, desulfurization slag and take away with partial discharge. According to the actual situation in the operation of the project, the project of the consumption of sodium alkali for 13.1kg/ days (with 100% sodium hydroxide.).
(3)Use of electricity
Preliminary estimate of power consumption

The power supply adopts 380V, 50HZ AC.
(4) water
Water consumption is mainly composed of three parts, namely, with evaporation with flue gas desulfurization slag and take away part of each furnace emissions, water consumption is about 7~8m3/h.
(5) the amount of desulfurization by-products and slag amount
The desulfurization product is mainly calcium sulfite and calcium sulfate, except in ash water absorption down in flue gas dust, desulfurization agent is mainly calcium sulfate, calcium sulfate and a small amount of unreacted.
After calculation, the removal of sulfur dioxide is about 130.1kg/h, the final product moisture content of about 30%, while emissions of approximately 500kg/h, annual emissions of 4000 tons. Due to the actual gas sulfur content, factors influencing the desulfurization rate, the actual production of slag will have great changes.
(6) wastewater
Desulfurizing solution for water recycling, basically nothing more than row, but in order to maintain the balance of chloride ion, to discharge 3~5 m3 / h water, this part of the treated wastewater into the wastewater neutralization and reached the standard of wastewater discharge after the corresponding emission.

Desulfurization project content

1Desulfurizing agent, preparation system
Desulfurizing agent preparation system includes: Lime digester, soda tank, agitator and corresponding valves, pipes and pipe fittings etc..

(2)Lime digester
Slurry preparation system configuration of a lime slurry tank, the effective volume of 1 Taiwan (slurry pool utilities by all two boilers considering) boiler run lime slurry for 3 hours in the condition of maximum consumption, lime digester volume is 3m3. The basic dimensions of 1.4 x 2:, using the reinforced concrete structure, semi underground style layout, tank is provided with a stirrer (the power of 4.5Kw, the blade is made of SUS316L), ash water as process water.
(3)Sodium alkali pool
The effective volume of sodium alkali pool is 5M3, with sodium lye into the regeneration tank.
(4)Valves, pipes and pipe fittings
Valves, pipes and pipe fittings are made of SUS304 material.

2, the flue gas system
The flue gas system comprises a flue, bentoniteExpansion joints and electric baffle.
From the outlet of the boiler to connecting flue desulfurization tower inlet section by A3 steel, and according to the need to set expansion joint. Connecting flue baffle system for flue gas desulfurization system, accident bypass operation. The baffle adopts manual gate valve, comprising 1 entrance baffle, 1 bypass baffle and 1 outlet baffle desulfurization device. This system can realize 100% flue gas bypass. In order to meet the needs of the desulfurization tower repair and maintenance to ensure the normal operation of the boiler stable.

3SO2, absorption system
SO2 absorption system is mainly composed of desulfurization tower, connecting flue (auxiliary tower), spray layer, combination type demister, embedded parts and external steel structure, washing system.

1. The desulfurization tower

The desulfurization tower is the core of the system, the desulfurization tower material is the key, the desulfurization sulfur project can long-term stable operation in accordance with the requirements, making use of the desulfurization tower body made of granite. According to the standard of national execution.
The absorption tower adopts hollow spraying tower structure (according to the needs of the desulfurization rate setting 2 layer atomizing spray layer).
The absorption tower with granite, can withstand the wear of fly ash and flue gas desulfurization process of suspended solids. All the parts including the tower body and internal structure design are considered corrosion requirements.
The absorption tower design can prevent liquid leakage. The tower on the manhole, channel, connecting pipes are sealed in the casing perforation, prevent leakage.
The absorption tower shell design can withstand all kinds of loads, including the absorption tower and its role in weight, absorption equipment and pipelines on the tower of medium weight, heat heavy, and wind load, snow load, seismic load etc..
The absorption tower bottom can be completely emptying the slurry.
The overall design of the tower, convenient maintenance and maintenance of tower components, guide plate, internal absorption tower and spray system support is not easy to accumulate dirt and fouling.
The absorption tower design considerations prevent entrance flue flue gas reflux and solids accumulation.
The absorption tower is equipped with a sufficient number of suitable size and manhole, set in the vicinity of platform and walkway.
The spray volume of sprinkler system can design, reasonable distribution requirements, the flue gas flow uniformity, and ensure full contact with the gas and liquid absorption reaction.
All the nozzles can avoid rapid wear, fouling and clogging. The wear life of the nozzle for at least 2 years.
The design of the nozzle and the pipeline for maintenance, washing and replacement, can be realized without stopping the furnace maintenance.

(2)Connecting flue
The connecting flue (side tower) is the main tower desulfurization tower outlet and the fan inlet section. The use of carbon steel connecting flue. According to the requirements of the national standard execution.
(3)Spray layer
In the desulfurization system, in order to achieve good absorption effect, absorption tower design of countercurrent spray tower, set the spray layer 2 layers, each layer of spray layer is composed of a plurality of atomizing nozzle and nozzle of solid, each self-contained system, can be opened and closed and can adjust the spray volume. The absorption liquid sprayed from the nozzle, the nozzle are evenly arranged on the cross section of the tower, ejected into solid cone type slurry can cover the entire cross section, to meet the required SO2 absorption surface area at the same time, the technology to reduce the pressure loss caused by the spray to the minimum. The mass transfer and absorption time for 2-3 seconds.
The nozzle is the key part of the purifying device, it has the following characteristics:

1. The atomizing nozzle due to processing, domestic material limit, and not imported to be compared, in order to improve the uniformity degree of atomization and atomization desulfurization liquid, we introduce the original 316L atomizing nozzle.
   The original high degree of atomization, atomization particle size, desulfurization agent is bigger than the surface area, scientific and reasonable arrangement plus nozzle, makes the pretreatment area formed without loopholes, little overlap in the absorption liquid atomization section, compared with the domestic technology, hundreds of thousands of times to improve the contact with flue gas desulfurization at the same time the opportunity to spray liquid, can be greatly reduced, resulting in the flue gas temperature is decreased, the high temperature flue gas, large contact area, between the SO2 and the desulfurizing agent reacts violently, fast reaction speed, which is one of the main factors of high desulfurization efficiency, but also to a ball of smoke has provided good conditions.

The liquid in the nozzle channel large and smooth, with good anti blocking performance; using special stainless steel, with good corrosion and wear resistance. The nozzle has the advantages of small size, convenient installation and cleaning.
The spray layer is mainly composed of annular distribution pipe, nozzle, casing, valve, spray pipe.
(4)Combined type demister
Flat baffle demister, two channel four, to ensure that the defogging effect. Here is no longer described.
(5)Flushing system
Due to the use of dual alkali desulfurization process technology, theory of demister, nozzle and pipe scaling problem does not exist, but the actual operation of the mist eliminator, the bottom of the desulfurization tower and part of the pipeline are sediment and scaling phenomenon exists in the process, so this project in these places are equipped with washing device, washing water for process water. To regularly wash the demister, the bottom of the desulfurization tower and other parts due to possible long term operation of the dead ash, solved without stopping the furnace cleaning problems of demister.
Water flushing system for the owner of the water supply pressure is 0.3MPa by the process of water mist, except separate sets of washing water pump.

After-sale service
The company to undertake the "three wastes" treatment engineering, with advanced technical design and excellent quality as the principle. To undertake the project, make the following commitments:

1. Ensure the advanced technology, process design, feasibility and rationality.
2. To ensure system stability and durability.
3. To ensure the quality of construction projects and technical indicators.
4. For users to formulate feasible operation rules.
5. Responsible for technical training for operators.
6. In the long-term, preferential prices provide a variety of spare parts, and give technical guidance.
7. The received information quality problems reflect, make a reply within 24 hours, and from time to time to send technical personnel arrived at the scene.
8. The main equipment warranty for one year, dedicated door-to-door service, technical problems of life return.
9. The implementation of regular visits to the engineering equipment of this project, and is responsible for tracking service life.