Gypsum wet limestone FGD process

Process wet limestone / gypsum FGD desulfurization apparatus may Island Department

The system is divided into three subsystems

Flue gas treatment and SO2

Absorption subsystem

Gypsum dewatering subsystem

Preparation subsystem reactants

It can be further subdivided into seven subsystems

Limestone - gypsum wet flue gas desulfurization system on the principles of the system configuration by the following structure:

1. Limestone preparation system consists of limestone powder silos and limestone grinding and measuring station consisting of

2. By a wash cycle, the defogger and oxidation step consisting of absorber

3. by the Rotary smoke - smoke flue gas heat exchanger, flue gas cleaning a cooling tower or wet stack exhaust emissions constitute reheat systems

4. Desulfurization fan

5. By a hydrocyclone and filter belt consisting of gypsum dewatering apparatus

6. gypsum storage means

7. Wastewater treatment systems.

We are to be introduced below.

Limestone slurry preparation system

Selective absorbent preparation system should be determined after a comprehensive technical and economic comparison absorber according to sources, investment, operating costs and transportation conditions. When the resources to implement reasonable price should be given priority when direct purchase program limestone by wet milling plant self absorbent preparation system when conditions permit and reasonable solution. When must a new limestone processing flour mill should give priority to regional cooperation that is focused on the construction and should be based on investment and management, processing methods,

Site location factors, transport conditions such as a comprehensive technical and economic feasibility.

Limestone slurry preparation system is mainly composed of limestone powder silo, limestone powder metering and delivery means, stirred slurry tank, slurry pumps and other components shown. The limestone from the tankers to the storage silo then feeder, metering and lose powder machine powder into limestone slurry preparation tank in. And circulating the water in the tank from the process formulated with a limestone content of 15% -20% slurry. The slurry was pumped into the absorber pipe along the bottom tank via the circulation flow rate measuring device.

Absorber

Absorber device is the core of FGD System Requirements absorbing reaction gas-liquid contact area is large gas pressure loss good and suitable for large-capacity flue gas treatment. Absorber shall be determined by the number of boiler capacity, the capacity and reliability of the absorber. 300MW or more units should be equipped with a furnace a tower. 200MW units and below, preferably two furnaces with a tower. generally two furnace desulfurization tower according to the company's experience of foreign investment is lower than the desulfurization unit means a furnace a tower of 5% -10% configure multiple furnace using a column on the following grades of 200MW units in favor of saving investment.

Absorber in the wet FGD system design is crucial. Absorber tower is the most important spray absorber a prominent position in the world of wet FGD systems mostly countercurrent spray tower.

Flue gas from the lower section into the spray absorber with a uniform flow of discharged absorbent slurry into contact flue gas flow rate of 3 ~ 4m / s or so liquid-gas ratio and the desulfurization rate of coal sulfur content and the relationship between large generally 8 ~ 25L / m3 between. Advantages spray tower is the tower parts so less fouling potential small pressure loss. Reverse airflow running in favor of the flue gas and absorption liquid contact but larger than the downstream pressure loss. Absorption zone height 5 ~ 15m such as by column flow rate of 3m / s computing contact reaction time of 2 ~ 5s. Parking area for 3 to 6 spray level spray level are each equipped with a plurality of spray nozzles arranged cross coverage of 200% to 300%. Nozzle inlet pressure

force can not be too high at between 0.5 × 10 ^ 5 ~ 2 × 10 ^ 5Pa. Nozzle outlet flow rate of about 10m / s droplet diameter of about 1320 ~ 2950μ large droplet residence time in the tower 1 ~ 10s small droplets in suspension under certain conditions. The picture shows the countercurrent absorber structure.

In addition to slurry washing absorber system there defogge ME and oxidation systems. Clean the flat flue flue gas outlet is provided for the two demister defogger usually mounted on the tower top of the cylinder vertically arranged  or tower outlet after the bend level

Arrangement. The latter allows the flue gas flow rate is higher than the former. And set the wash water rinse  intermittent mist. Cold flue gas residual moisture generally can not exceed 100mg / m3 now mostly requires no more than 75mg / m3 otherwise it will tarnish the heat exchanger, flue and fan and so on. Wet flue gas desulfurization tower used mainly for demister demister baffle, cyclone demister.

Typically baffle plate defogger distance between the plates is 20-30m for vertical placement baffles average gas flow rate of 2-3m /s for horizontal baffle gas flow rate may be higher generally 6-10m / s. Gas velocity is too high will cause re-entrainment. Baffle structure and demister demisting principle shown in Figure.

Swirl board structure as shown airflow into the sheet as it passes through the gap where the whirling airflow droplets in the inertia to a certain elevation exit for spiral motion and thrown to the outside collection left to overflow tank achieve the purpose of the demister demisting efficiency can reach 90-99%.

Within the absorber demister

• usually two defogger mounted on top of the tower.

• flue gas residual moisture after processing can not exceed 75mg / m3

Preferably not more than 50mg / m3

• desulfurization mainly baffles followed swirl plate.

Oxidative desulfurization system mode

There are limestone wet flue gas desulfurization forced oxidation and natural oxidation of the points The difference is that the bottom of the holding tank desulfurization tower filled if forced oxidation air. For the natural oxidation process absorbent slurry HSO3

Desulfurization by-products are mainly calcium sulfite and calcium bisulfite. Natural oxidation due to boiler and flue gas desulfurization system parameters vary different degree of oxidation when the oxidation rate of 15 ~ 95% calcium utilization is less than 80% calcium sulfate range Guinean easy scaling because the oxidation rate is high 15% generated calcium sulfate precipitation can not be together with calcium sulfite precipitation oxidation rate of less than a certain extent 95% can not produce enough seed leaving the gypsum plaster gypsum crystals led to rapid growth in the desulfurization tower within fouling.

Oxidative desulfurization system mode

There are limestone wet flue gas desulfurization forced oxidation and natural oxidation of the points The difference is that the bottom of the holding tank desulfurization tower filled if forced oxidation air. For the natural oxidation process absorbent slurry HSO3 flue gas in the absorption tower is remaining oxygen power plant flue gas oxygen content is generally about 6%, the desulfurization by-products are mainly calcium sulfite and calcium bisulfite. Natural oxidation due to boiler and flue gas desulfurization system parameters vary different degree of oxidation when the oxidation rate of 15 ~ 95% calcium utilization is less than 80% calcium sulfate range Guinean easy scaling because the oxidation rate is high 15% generated calcium sulfate precipitation can not be together with calcium sulfite precipitation oxidation rate of less than a certain extent on 95% can not produce enough of a seed crystals of gypsum plaster rapid growth cause gypsum in the desulfurization tower fouling. Oxidative desulfurization system mode

It has forced oxidation and natural oxidation of the points limestone wet flue gas desulfurization. The difference is that the holding tank at the bottom of the desulfurization tower filled if forced oxidation air.

For the natural oxidation process absorbent slurry HSO3 desulfurization by-products are mainly calcium sulfite and calcium bisulfite. Natural oxidation due to boiler and flue gas desulfurization system parameters vary different degree of oxidation when the oxidation rate of 15 ~ 95% calcium utilization is less than the range of 80% calcium sulfate Guinea Yi Fouling because higher oxidation rate 15% generated calcium sulfate precipitation can not be together with calcium sulfite precipitation oxidation rate of less than a certain extent 95% can not produce enough seed and plaster the rapid growth of gypsum crystals cause gypsum in the desulfurization tower fouling. Gypsum dewatering system. Gypsum is a limestone forced oxidation byproducts of wet flue gas desulfurization gypsum crystals particle size 250μm, mainly concentrated in the 30-60μm, in normal operation of desulfurization unit generates nearly white color of gypsum, limestone - gypsum take off. Sulfur gypsum purity is generally between 90% 95% gypsum crystals (CaSO4 • 2H2O). Physical and chemical properties of gypsum and natural gypsum have common characteristics, but as an industrial by-product, which has some of the characteristics of renewable gypsum and natural gypsum have some differences compared to the content in which the gypsum than natural gypsum also many high. The gypsum wallboard or cement manufacturing as general and sell. Because of its stability harmful to the environment so that the land can be used for backfill. WFGD in gypsum dewatering system as shown in FIG. Major equipment gypsum dewatering system is the hydrocyclone and vacuum belt filter. Hydrocyclone gypsum dewatering gypsum, also known as the master device It mainly consists of the cyclone inlet distributor, swirl promoter, an upper dilute liquid storage tank and the bottom of the gypsum slurry dispenser components. Swirl child is using the principle of centrifugal separation separated. The effect can be controlled by the inlet pressure. Figure 1-18 Schematic diagram of the hydrocyclone. Overflow gypsum solids cyclone is generally about 1% ~ 3% fine solid particles mainly incomplete reaction absorbent, gypsum small crystallizatio former continue to participate in the latter as the desulfurization reaction slurry pond crystallization nuclei grow influence the formation of the next phase of large gypsum crystals. Bottom cyclone flow solid content is generally about 45% ~ 50% solid phase crystallization predominantly coarse plaster vacuum dewatering belt aim is to remove the free water of large crystalline particles.Gypsum plaster processing system hydrocyclone

• heavy, coarse particles flow in the secondary dehydration

• lighter fine particles including fly ash limestone overflow out

• No transmission parts

Dehydration vacuum belt dehydrating machine principle is to liquid or gas mixture to be separated is placed on one side having a fine pore filtration media another fluid under pressure driven by the action of the filter media pore channels flowing medium side fluid solid particles were trapped order to achieve separation of liquid and solid particles. Study on the operation of different manufacturers of vacuum belt machine showed product gypsum is mainly affected by the moisture content and physical properties of gypsum belt running operation. Gypsum crystallized the better, the larger the particle size, the lower the chloride ion content, the lower the content of impurities such as fly ash more conducive to dehydration of gypsum by-product that is beneficial to improve the quality of gypsum. Currently gypsum comprehensive utilization is mainly used as construction gypsum and cement additives in two ways. Gypsum do you need before calcination by drying therefore need to plaster water content determined according to the number of major energy drying equipment generally calcined gypsum by necessary moisture content should be less than 10% in order to reduce drying energy consumption. There are two cases do when used in high-grade cement cement additives still needs by calcination, molding requirements and with the same time of building plaster Another case is added directly to the gypsum cement this case the moisture content should be controlled 15% or less.

Gypsum and gypsum storage system utilization

Gypsum wet storage method depends on the power plant FGD gypsum production, user demand, means of transport as well as the size of the plaster in the middle of the silo. For a capacity of 300 ~ 700m3 intermediate silo plaster in which time is not stored. It should be more than one month. Therefore recommended with a bottom discharge system of a type silo shown. Gypsum warehouse should take measures and attempts to prevent corrosion measures. plaster warehouse freezing measures should be taken in cold areas. If the desulfurization by-product utilization conditions No one may be delivered to the cyclone concentrated storage sites can also be transported to the storage site by dehydration but the possibility of the future utilization of the ash should be stacked separately left and to prevent secondary pollution caused by byproducts of measures to be taken. Desulfurization wastewater treatment

Produce waste water is wet limestone desulfurization disadvantage. Some national and regional governments for wastewater treatment is strictly regulated. Germany, for example for wastewater discharge is strictly regulated limit emission concentrations of trace metals and other harmful ingredients. In Japan, waste water treatment also reduced COD mainly in the form of sulfate dithionite. Its processing of precipitate formed by the addition of alkali and desulfurization wastewater and wastewater most heavy metal flocculant is added to precipitate the sludge become concentrated sludge is sent to the ash field stacking. Wastewater pH and suspended solids compliance after the direct discharge

FIG typical wastewater treatment system.

Desulfurization wastewater treatment include the following four steps

A, and the wastewater. Reaction cell consists of three compartments trough each compartment rear pocket full of gravity into the next compartment slot. Desulfurization wastewater into the first compartment of the tank while adding a certain amount of lime slurry of about 10% constantly stirring the pH value can be increased to 9.0 from 5.5 and above.

B, heavy metal precipitate Ca (OH) 2 is added to not only increase the pH value of the waste water but also Fe3 +, Zn2 +, Cu2 +, Ni2 +, Cr3 + and other heavy metal ions generated hydroxide precipitation. Under normal circumstances trivalent heavy metal ions more easily precipitate when the pH reaches 9.09.5 when most of the heavy metal ions are formed insoluble hydroxides than divalent. Meanwhile lime slurry Ca2 +

Also part of the wastewater react with F- CaF2 form insoluble complexes with As3 + generate Ca3 (AsO3) 2 and other insoluble substances at this time Pb2 +, Hg2 + ions form still remain in the wastewater so in the second compartment tank by adding organic sulfides agent TMT it Pb2 +, Hg2 + to form insoluble sulfides deposited. TMT-15 is a triazine component (C3N3S3Na3, three polysulfide triazine acid trisodium salt) energy at room temperature, with a variety of heavy metal ions in wastewater mercury, lead, copper, cadmium, nickel, manganese, zinc , chromium and other rapid response water-insoluble and has good chemical stability so as to capture the removal of chelate

Objective heavy metals can also be removed has been transformed into a complex of heavy metals. TMT-15 is 15% wt% C3N3S3Na3 solution even very small quantities of heavy metals also exhibit a very high removal efficiency.

C, flocculation

After the first two steps by chemical precipitation reaction wastewater also contains many fine particles and dispersed colloidal material so in the third compartment tank by adding a certain percentage of flocculant FeClSO4 so that they condense into large. Particles deposited. In the waste water outlet of the reaction cell was added as a cationic polyelectrolyte polymer coagulant to reduce the surface tension of the particles of the particle growth process strengthen further promote hydroxide and sulfide precipitation make small floes slowly getting into larger, more easily deposited floc simultaneous desulfurization wastewater suspended solids can settle.

D, clarified and concentrated

Flocculated wastewater enters from the reaction vessel equipped with a stirrer clarified overflow pool concentrated floc deposited on the bottom by gravity and the upper portion was concentrated to a sludge water. Most of the sludge discharged by the sludge pump Mortar pool small part of the sludge is returned as the contact sludge wastewater precipitate the desired reaction tank provide nuclei. The upper portion of water by clarifying thickener overflow gravity to the periphery of the tank net net water tank set up to monitor pH and the suspension of online monitoring instrumentation If the pH and suspended solids reach the drainage design standards through the net pump efflux otherwise be returned to the wastewater treatment reaction tank continues until the compliance date.

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