Technical Application

Desulfurization process application

Desulfurization Scheme

Main Process

Process Characteristics

Wet Flue Gas Desulfurization (WFGD) Technology Limestone wet method, magnesium method, ammonia method, double alkali method   Fast desulfurization reaction rate Desulfurization absorption is carried out under low temperature and wet saturated conditions; High desulfurization efficiency However, the system is more complex, the construction and operation and maintenance costs are high, and environmental problems such as wastewater and white smoke are generated.
Dry Flue Gas Desulfurization (DFGD) Technology In-furnace calcium injection/post-furnace humidification and activation desulfurization technology/SDS   Solid absorbent reacts with SO2 in a dry state; no secondary pollution from waste liquid. However, the reaction rate is slow, and the desulfurization efficiency and desulfurizer utilization rate are low 。
Semi-dry Flue Gas Desulfurization (SDFGD) Technology Rotary spray drying process, circulating fluidized bed desulfurization process   Desulfurizing agents generally desulfurize in a wet state, with dry by-products; semi-dry processes have some characteristics of both WFGD and DFGD technologies

 

1. Limestone-gypsum wet desulfurization technology
(1) Reaction mechanism and process flow

Reaction mechanism:
Limestone decomposition
CaCO3 (s) ---> CaCO3 (aq)
CaCO3 (aq) + H2O ---> Ca2+ + HCO3- + OH-
SO2 absorption
SO2 (g) ---> SO2 (aq)
SO2 (aq) + H2O ---> H2SO3 ---> HSO- + H+ ---> H+ + SO32-
Forced oxidation in the absorption tower
SO32- + 1/2 O2 ---> SO42-
The basic reactions in the absorption tower are as follows:
SO2 + CaCO3 + 1/2O2 ---> CaSO4 + CO2
Other side reactions:
Ca2+ + SO32- + 1/2 H2O ---> CaSO3.1/2H2O(s)
2 HCl + CaCO3 ---> CaCl2 + H2O + CO2
2 HF + CaCO3 ---> CaF2 + H2O + CO2

 

(2) System composition

 

▷ Limestone slurry preparation system
▷ Flue gas system
▷ SO 2 Absorption system
▷ Gypsum dehydration system
▷ Emergency discharge system
▷ Process water system
▷ Compressed air system
▷ Wastewater treatment system

 

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(3) Unique technology

Gas-liquid Recontaction Device
Because the sprayed slurry in the absorption tower is prone to wall flow phenomenon, the flue gas "escapes" along the tower wall, and the desulfurization efficiency of the flue gas near the tower wall is relatively low. Gas-liquid recontacting sends the slurry flowing down the absorption tower wall back into the flue gas for secondary contact and reaction. Without increasing the gas-liquid ratio, the desulfurization efficiency can be significantly increased by 2-3%, thus achieving energy saving and emission reduction and reducing operating costs. Simulation and schematic diagram as shown:

Gas-liquid Recontaction Device

 

 

Gas distribution device
The flue gas is evenly distributed, and the slurry is more effectively contacted. Under the same flue gas parameter conditions, high-efficiency desulfurization can be achieved with a smaller slurry circulation amount than that of ordinary designs.
Characteristics of gas distribution turbulence technology
1) Good flue gas distribution effect, high desulfurization efficiency
2) More thorough gas-liquid-solid contact, good dust collection effect
3) Low system resistance, low energy consumption
4) Simple structural form, convenient operation and maintenance
5) The gas distribution layer can be used as a support for the repair of the spray layer. During the maintenance of the absorption tower, the gas distribution layer can be used for maintenance

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Tray
1) The slurry sprayed from the high-efficiency nozzle uniformly falls onto the tray, and under the action of the liquid holding layer above the tray from the evenly distributed flue gas entering the absorption tower, the SO2 in the flue gas is fully washed and removed.
2) The tray extends the residence time of flue gas in the absorption tower, and utilizes the condition that the liquid film PH value is lower than that of the slurry pool to promote the dissolution of limestone, thereby improving desulfurization efficiency and reducing the gas-liquid ratio.

High-efficiency nozzle
With the same flow rate, a 120° high-efficiency dual-head nozzle is used, resulting in smaller slurry particle size and higher coverage. The high-efficiency dual-head nozzle can secondary atomize the droplets, dispersing the droplets into smaller particle sizes, increasing the specific surface area of dust removal, and improving dust removal performance and desulfurization efficiency.

High-efficiency demister
Using a three-stage ridged high-efficiency demister and increasing the distance between the topmost spray layer and the demister can control the outlet mist content to below 20 mg/Nm3, thereby reducing the amount of dust carried by the mist droplets and meeting the ultra-low emission requirement of 5 mg/Nm3.

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4) Technical Features
◆The absorption tower is a spray empty tower, with simple operation and high reliability.
◆The absorption tower is designed for high flue gas flow rate (3.4-3.8 m/s), good mass transfer, and small footprint.
◆Low gas-liquid ratio, low power consumption, and low operating cost.
◆The slurry uses multi-layer countercurrent spraying, with a spraying coverage rate of 300%, and a desulfurization efficiency of over 99%.
◆The nozzle is a hollow cone type, convenient for maintenance, and not easily blocked.
◆An eaves is set at the absorption tower inlet to prevent slurry from entering the flue, avoiding scaling.
◆In-tower oxidation is adopted, and the obtained gypsum particles have uniform diameter, which is convenient for gypsum dehydration.
◆A gas-liquid re-contact device is set between the spray layers to prevent flue gas from escaping.
◆Multiple pollution control can be achieved, and the dust removal efficiency can reach 50-70% while desulfurization.

 

2. CFB Circulating Fluidized Bed Semi-dry Desulfurization Technology

1) Reaction Mechanism and Process Flow
CaO + H2O → Ca(OH)2 (1)
SO2 + H2O → H2SO3 (2)
Ca(OH)2 + H2SO3 → CaSO3 + 2H2O (3)
CaSO3 (L) → CaSO3 (S) (4)
Based on the circulating fluidized bed principle, the contact time between the absorbent and the flue gas is extended through multiple recirculation of the absorbent, and the utilization rate of the absorbent is improved. After passing through the Venturi tube at the bottom of the absorption tower, the flue gas is accelerated to obtain the power to form a fluidized bed. After entering the bottom diffusion section, the flue gas speed decreases, and a fluidized bed layer is formed here, which is also the main area for desulfurization chemical reactions. Above the fluidized bed layer is the straight pipe section of the absorption tower, which can be divided into: dense phase zone, transition zone, and dilute phase zone. After passing through the "transition zone", the desulfurization chemical reaction has basically ended. The temperature and pressure are detected in the top turning section of the absorption tower. The water addition amount of the absorption tower is controlled by the temperature, and the desulfurization ash circulation amount is controlled by the inlet and outlet pressure drop of the absorption tower.

2) System Composition
★ Absorbent storage and conveying system
★ Flue gas system
★ SO2 absorption system
★ Process water system
★ Dust removal system
★ Desulfurization ash circulation conveying system
3) CFB Desulfurization System Characteristics
★ Operates at near adiabatic saturation temperature, the flue gas temperature is always above the dew point temperature of 15℃, and no white smoke is produced.
★ The desulfurization efficiency can be controlled by controlling the outlet gas temperature or the desulfurizing agent supply rate.
★ The water content of the ash discharge is less than 1%, and the dust removal system can operate at a temperature close to the adiabatic saturation temperature of the flue gas. After the flue gas containing calcium hydroxide enters the bag dust remover, it undergoes secondary desulfurization through filtration. The desulfurization ash falling into the ash hopper is heated to 85°C to prevent scaling and facilitate the circulation of ash in the tower for desulfurization.
★ No wastewater is produced.

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3. SDA Desulfurization System Characteristics
SDA utilizes the principle of spray drying. After the wet absorbent is sprayed into the absorption tower, it absorbs sulfur dioxide from the flue gas through a chemical reaction. Simultaneously, the flue gas transfers heat to the absorbent, causing it to dry continuously. The desulfurization reaction's waste residue is discharged from the conical outlet of the absorption tower in dry form, thus called semi-dry flue gas desulfurization.

 

 

1) Process Flow
(1) Absorbent Preparation;
(2) Atomization of Absorbent Slurry;
(3) Contact and Mixing of Mist Particles and Flue Gas;
(4) Droplet Evaporation and Sulfur Dioxide Absorption;
(5) Ash Discharge;
(6) Ash Recirculation. (2), (3), and (4) are carried out within the absorption tower.

2) Desulfurization System Characteristics
The absorbed and dried product (mainly CaSO3.1/2H2O) is collected together with fly ash at the bottom of the absorber or in the dust collector. Under ideal operating conditions, the desulfurization efficiency of the SDA process can reach 80%-90%. Its main feature is that the by-product is solid, and no wastewater is produced. However, the absorbent Ca(OH)2 is relatively expensive, resulting in high operating costs. Currently, this technology is mainly used for acid removal from waste incineration flue gas, desulfurization of sintering flue gas in the steel industry, etc.

 

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4. SDS Desulfurization System Characteristics

1) Reaction Mechanism and Process Flow
The SDS dry desulfurization injection technology uniformly injects a highly efficient desulfurizing agent (20-25μm, main component sodium bicarbonate) into the pipeline. The desulfurizing agent is thermally activated in the pipeline, its surface area rapidly increases, and it comes into full contact with the coke oven flue gas. Physical and chemical reactions occur, and the SO2 and other acidic substances in the flue gas are absorbed and purified. The purified flue gas is discharged into the atmosphere through the outlet flue to the original chimney via a booster fan.

The main chemical reactions are:
2NaHCO3 +SO2+1/2O2 → Na2SO4 +2CO2+H2O
2NaHCO3 +SO3 → Na2SO4 +2CO2+H2O
Its main reactions with other acidic substances (such as SO3, etc.) are:
NaHCO3 +HCl → NaCl +CO2+H2O

NaHCO3 +HF → NaF +CO2+H2O

1) SDS Dry Desulfurization Technology Characteristics
★ No desulfurization tower is needed; large-scale solid circulating ash does not need to circulate in the tower, nor does it require slurry injection. The system is simple, has low operating resistance, and is easy to operate and maintain.
★ High desulfurization efficiency due to a reasonable desulfurizing agent distribution device.
★ Very little one-time investment, small footprint, and low operating costs.
★ Completely dry system; no water required.
★ 100% removal rate for acidic substances.
★ High flexibility; it can adapt to the strictest emission standards at any time.
★ Strong adaptability to operating conditions.

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