Limestone FGD

Limestone Flue Gas Desulfurization

SO2 absorption efficiency	99.2% and above
Ammonium sulfite oxidation effect	99.0% and above
Ammonia recovery rate	99%
S02 content after desulfurization	<35mg/m3
NOx content after desulfurization	<50mg/Nm3
NH3 content after desulfurization	<3mg/Nm3
aerosol content after desulfurization	<3mg/Nm3
particles content after desulfurization	<5mg/Nm3
PH value	5.5-6

The Technology of Limestone Flue GAS Desulfurization

2CaCO3+H2O+2SO2＝2CaSO3·1/2H2O+2CO2↑
2CaSO3·1/2H2O+O2+3H2O＝2CaSO4·2H2O

Limestone or lime is used as desulfurization absorbent, limestone is broken and mixed with water, then ground into powder to form absorbent slurry (when lime is used as absorbent, lime powder is digested and mixed with water to make absorbent slurry).
In the absorber, SO2 in flue gas reacts with CaCO3 in slurry and oxidized air to form gypsum dihydrate, and SO2 is removed. The gypsum slurry discharged from the absorption tower is recovered after dehydration by dehydration device.
After desulphurization, the flue gas is removed from water by the mist remover, and the heat exchanger heats up and enters the chimney into the atmosphere.

The Engineering System of Limestone flue gas desulfurization

It is composed of slurry preparation system, flue gas system, absorption and oxidation system, gypsum dehydration system and discharge system.

1slurry preparation system

The tank car is used for conveying the lime powder to the lime silo, and the lime powder is quantitatively sent to the lime slurry tank through weighing belt conveyor to form slurry. Then the slurry is pumped to the inlet of absorption tower circulating pump. Lime silo is carbon steel structure, its effective volume storage shall be designed according to the rated working condition of the unit for 3 days(24hrs/day) of lime consumption(the content of CaO is designed according to **%).
The effective volume of slurry tank shall be designed according to the the rated working condition of the unit for 6hrs slurry consumption of the desulfurization unit . The bottom of the lime silo is “conical”, “conical” bottom has a slope of at least 55°, and there is a sealed inspection/manhole door at the top of the silo. The door can be opened quickly with hinges and handles. A bag filter is arranged at the top of the discharge hopper and lime silo, and the pipeline is arranged to the lime slurry tank to prevent dust from the tank car hen unloading. Lime silo is equipped with material level meter and can also be used for remote instructions. In order to reach the top of the inspection level meter, install stairs, and provide a certain number of staircase platforms at the appropriate height.

2flue gas system

1) Under the condition of 40% to 100% working conditions, the flue gas system of the desulfurization unit can operate normally, and the flue gas system can still operate safely and continuously under the condition of 100% working condition, the flue gas temperature of the desulfurization system can still operate safely and continuously under the condition of 10℃ margin.
2) Install instruments for operation control and observation on the flue ducts, such as pressure gauges, thermometer and SO2 analyzers, etc., and send signals to the DCS system.

3SO2 absorption system

Lime slurry is pumped to the spray system in the tower from the slurry tank to absorb SO2 in the flue gas by contact with the flue gas countercurrent, and the calcium sulfite is oxidized to calcium sulfate in the circulating slurry pool of tower. The gypsum discharge pump sends calcium sulfate slurry from the absorption tower to the gypsum dehydration system.
3.1 absorption tower The absorption tower is an air tight structure, in order to ensure the integrity of the shell structure, welding connections are used as much as possible, flanged and bolted connections are used only if necessary. Manhole, passage, connecting pipe and so on need to be sealed in the place where the shell is pierced to prevent leakage. The shell of the absorption tower is designed to withstand pressure load, pipe force and torque, wind load, snow load and seismic load, as well as all other loads added to the absorption tower. The absorption tower is provided with supports and reinforcements to fully prevent the tilt and sloshing of the tower. 3.2 slurry spray system The slurry spray system in the absorption tower is composed of distribution pipe network and nozzles. The spray system can reasonably distribute the required spray quantity. The computer simulation will be used to make the flue gas flow direction uniform, and to ensure the full contact and reaction between limestone slurry and flue gas. For the selection of slurry spray pipe material, it is technically feasible and ensure the service life of 20 years. 3.3 accident flue gas cooling system The accident flue gas cooling system includes the distribution pipe network and the nozzles, and the cooling medium adopts fire-fighting process water. The system shall meet the temperature of the flue gas at the inlet of the desulfurization unit and the range of the flue gas flow, so as to ensure the lining of the absorption tower when the circulation pump is in fault.

4gypsum dehydration system

The calcium sulfate slurry is driven into the gypsum cyclone by gypsum discharge pump, and the concentrated slurry is dehydrated by vacuum belt filter, and the gypsum is washed at the same time to meet the quality requirements of comprehensive utilization of gypsum. The water content of the dehydrated gypsum is less than 10%(wt), and sent to the gypsum reservoir for storage. One part of the filter is returned to the absorption tower as supplementary water to maintain the water balance in the absorption tower, one of which goes into the lime pulping system for reuse, the other part is sent to the wastewater cyclone, and the upper liquid is pumped to the wastewater discharge system for centralized treatment in order to reduce the cumulative Cl-in the desulfurization tower. The gypsum dehydration system is divided into two stages, the first stage is gypsum cyclone concentrator, the solid content of slurry leaving cyclone is 40% to 50%, and the second stage is filter, through which gypsum cake with moisture content less than 10%(wt) is produced, and gypsum filtrate is returned to absorption tower or wastewater treatment.

The Features of Limestone flue gas desulfurization(FGD)

Mature technology, wide application
High operation reliability
Rich absorbent resources
Low straight investment
90%-95% desulphurization efficiency
Adaptable to the variety of coal and load

Limestone/Limestone-Gypsum Desulfurization Process

1.Working Principle

Limestone/limestone-gypsum flue gas desulfurization uses limestone or lime as the desulfurization absorbent. Limestone is crushed and ground into powder and mixed with water to form absorption slurry. When limestone is used as absorbent, lime powder is digested and added with water to form absorbent slurry. In the absorption tower, the absorbent slurry is contacted and mixed with the flue gas, and the sulfur dioxide in the flue gas is removed by chemical reaction with the calcium carbonate in the slurry and the blown-in oxidized air. The final reaction product is gypsum.
2.Reaction Process

1.Absorption

SO₂+ H₂O—> H₂SO₃

SO₃+ H₂O—> H₂SO₄

2.Neutralization

CaCO₃+ H₂SO₃ —> CaSO₃+CO₂+ H₂O

CaCO₃+ H₂SO₄ —> CaSO₄+CO₂+ H₂O

CaCO₃+2HCl—> CaCl₂+CO₂+ H₂O

CaCO₃+2HF —>CaF₂+CO₂+ H₂O

3.Oxidization

2CaSO₃+O₂—>2 CaSO₄

4.Crystallization

CaSO₄+ 2H₂O —>CaSO₄·2H₂O
3.System Composition

The desulfurization system mainly consists of flue gas system, absorption and oxidation system, limestone/ limestone slurry preparation system, by-product treatment system, wastewater treatment system, emergency slurry tank system, utility system (process water, compressed air, etc.) and electrical control system, etc.
4.Technological Process

Boiler/ Kiln-> Dust Collector-> Induced Draft Fan-> Absorption Tower-> Chimney<br />

The flue gas from the boiler or kiln enters the absorption tower after dust removal under the action of the induced draft fan. The absorption tower is a countercurrent spraying empty tower structure, which integrates the absorption and oxidation functions. The upper part is the absorption zone, the lower part is the oxidation zone, and the flue gas after dust removal is in reverse contact with the circulating slurry in the absorption tower. The system is generally equipped with 3-5 sets of slurry circulating pumps, and each circulating pump corresponds to an atomized spray layer. When only one unit is running or the load is low, 1-2 spray layers can be stopped. At this time, the system still maintains high liquid-gas ratio, thus achieving the required desulphurization effect. The secondary demister is installed in the upper part of the absorption zone, and the free water in the flue gas of the demister outlet is not more than 75mg/Nm³. The slurry after absorbing SO₂ enters the circulating oxidation zone, in which calcium sulfite is oxidized into gypsum crystals by the blown-in air. At the same time, fresh limestone slurry is supplied from the absorbent preparation system to the absorption oxidation system to supplement the depleted limestone so as to make the absorbent slurry maintain at a certain pH value. When the reaction product slurry reaches a certain density, it is discharged into the desulphurization by-product system, and then dehydrated to form gypsum.
5.Application Field

Flue gas desulfurization for coal-fired power generation boilers, cogeneration boilers, central heating boilers, sintering machines, pellet kilns, coking furnaces, glass kilns, etc.

Friendly reminder: The process has the most extensive application, mature technology, good adaptability to flue gas load and variety of coal types, as well as the high desulphurization efficiency. It is especially suitable for high sulfur coal and strict environmental protection emission requirements, but the system is relatively complex, the cost of investment is high, and the chimney needs anti-corrosion treatment

Special Topics on Desulfurization Process Selection

Wet desulphurization is the most widely used desulphurization method, accounting for 90% of the total amount of desulfurization. According to the different raw materials of desulphurization, the common wet desulphurization process can be divided into limestone method and calcium-sodium dual-alkali method, among which limestone-gypsum method is widely used because of its strong system buffering capacity, long-term stable operation of desulphurization facilities and wide sources of desulfurizer. At present, large domestic power plants all adopt this process.
The following table mainly focuses on the comparison of several desulfurization processes that are widely used.

S.l No.	Item	DSI FGD	Semi-dry FGD	Wet FGD
1	electrical consumption	no	low	high
2	water consumption	60%-85%	60%-85%	80%-99%
3	desulfurization efficiency	The maintenance of the equipment is simple and the cost is very low.	Low, equipment maintenance is simple, but compared with dry desulphurization, semi-dry tower interior needs maintenance and maintenance.	High, wet desulphurization equipment corrosion is serious, maintenance rate is high.
4	maintenance and repair cost	High, the equipment in the high temperature operation, will not produce the knot, the operation is stable.	Low, equipment maintenance is simple, but compared with dry desulphurization, semi-dry tower interior needs maintenance and maintenance.	High, wet desulphurization equipment corrosion is serious, maintenance rate is high.
5	availability of equipment	High, the equipment in the high temperature operation, will not produce the knot, the operation is stable.	A little high, the equipment runs at medium and low temperature, due to the characteristics of water content, internal consolidation is easy to appear, resulting in parking maintenance and so on.	High, Because of its high desulphurization efficiency, some equipment problems will not affect the flue gas treatment with low sulfur content under normal maintenance.

As can be seen from the table above, all the above methods can achieve more than 95% desulfurization efficiency. The lime/ gypsum method is the leading desulphurization technology in the world, accounting for more than 90% of all flue gas desulphurization devices. It is characterized by mature technology, high system reliability, high absorption efficiency, wide sources of absorbent and wide application scope.

Analysis of Operating Cost

1. The amount of SO2 removed annually
2. Power consumption of desulfurization system
3. Consumption calculation
3.1 Consumption and costs of desulfurizer
3.2 Electricity charges
3.3 Labor costs
3.4 Water consumption costs
The design data, specific parameters related to the list, operation cost and implementation plan need to be calculated according to the actual situation.