Ceramic kiln flue gas

1、 Overview of Smoke Pollution from Ceramic Manufacturing Kilns

The production process of building ceramics is roughly as follows: raw material batching, ball milling, pulping, slurry screening and iron removal, spray pulverizing, powder aging, brick forming, drying, glazing, firing, deep processing, etc. During spray pulverizing and firing stages, fuel gas combustion is used to achieve the effect of pulverizing and brick firing. The process of fuel gas combustion will produce flue gas emissions. The flue gas contains gaseous and solid substances generated by fuel combustion, pulverizing and physical and chemical reactions during brick firing, mainly including SO2 and NOX; Fluoride ions, chloride ions, dust (particulate matter); Lead, cadmium, mercury and other heavy metal ions. If these substances are discharged directly without treatment and control, they will cause pollution and harm to the atmospheric environment.

(1) SO2: The first source of SO2 is fuel, such as coal, gas, heavy oil, etc; The second is pyrite (FeS2), sulfate, etc. in the billet. If the fuel is natural gas, only SO2 produced during the burning process of pyrite, sulfate, etc. in the raw material is emitted in the flue gas. If only natural gas is used as the fuel for the firing kiln, but coal slurry or gas or heavy oil or coal is used for spray pulverizing, the flue gas emission of the firing kiln still contains SO2 released from the hydrogen sulfide carried by the powder during the firing process, so the pure natural gas is used for the kiln while natural gas is not used for the spray tower, and SO2 in the flue gas will still not meet the emission standards. At present, most of the flue gas desulfurization technologies used are wet processes, where different absorbents generate different sulfuric acid substances. Add calcium based to generate calcium sulfate, and add sodium based to generate sodium sulfate. The principle is that in a wet absorption tower, the flue gas and absorbent slurry flow in opposite directions, and a reaction is achieved during the contact process.

(2) Fluorine and chloride ions: The first source of F and CI is the decomposition of fluorine-containing and chloride minerals in the billet into gaseous fluoride and chloride ions at high temperatures; The second is that the chemical raw materials added to the glaze decompose at high temperatures and are emitted in the form of gases. The current treatment method is mostly wet desulfurization combined with removal. The principle is that fluoride and chloride ions in the flue gas react with the absorbent to produce fluoride and chloride, which are then removed.

(3) Dust (particulate matter): The first source of dust is fuel such as coal, gas, etc; The second is the surface of the billet and the carrying conditions of the kiln; The third is the secondary dust generated during the flue gas desulfurization process. The current treatment method uses filtration or water washing to remove it. The filtering method usually uses bag filtration, while the water washing method uses water mist to spray and settle the smoke and dust.

(4) Heavy metals (lead, cadmium, mercury, etc.): The main source of heavy metals is the decomposition of minerals in the billet at high temperatures, which precipitate in an ionic state. The current treatment methods use filtration and water washing to remove heavy metal ions. Bag filtration is usually used for filtration, while water washing is usually carried out using water mist spraying to settle heavy metal ions.

(5) NOX: The first source of NOX is fuel, such as coal and gas; The second is the NOx generated by nitrogen and oxygen in the air at high temperatures during the combustion process. The current treatment methods mostly use non catalytic reduction methods, which involve adding reducing agents in the temperature range where NOX can be reduced during combustion to reduce it to nitrogen and water. The low-temperature catalytic reduction method currently being researched and explored uses a catalyst to reduce NOX to nitrogen and water at temperatures below 200 ℃.

2、 Kiln flue gas treatment process plan

After the flue gas is collected, it enters the desulfurization tower tangentially and collides with the desulfurization liquid to form a foam layer. The gas and liquid in the foam layer are fully contacted, the two-phase interface is expanded, and mass transfer is intensified to achieve purification; Due to the tangential entry of exhaust gas into the desulfurization tower, the exhaust gas rotates around the tower and comes into contact with the desulfurization liquid flowing along the tower wall. Three layers of cyclones, three layers of spiral nozzles, and one layer of high-efficiency defogging device are arranged inside the tower. The cyclone causes a series of complex reactions such as collision, gas-liquid mass transfer, acid-base neutralization, flocculation, etc. between the exhaust gas cyclone and the desulfurization solution; The purified gas continues to swirl upwards and passes through an efficient swirl dehydration device. The water droplets contained in the gas undergo inertial collision and centrifugal separation, and larger droplets are removed, ultimately forming dry and clean gas. The flue gas is concentrated at the top of the tower and discharged in compliance with standards.

Description and characteristics of each major system

Desulfurizer preparation system

The desulfurizer preparation system mainly includes a pulp tank, a pulp tank agitator, and a slurry pump. This process uses externally purchased CaO powder and caustic soda as desulfurizers. The basic physical and chemical indicators of CaO and caustic soda are as follows:

Purity: CaO content ≥ 80%, NaOH content ≥ 90%. Particle size: Lime, 90% pass through 180 mesh sieve; Caustic soda, 300 mesh flakes.

Desulfurization absorption system

Structure of desulfurization tower

Moisturizing, cooling, and initial purification of flue gas: Dust and sulfur-containing flue gas first enters the pretreatment chamber and comes into good contact with the atomized purification liquid in turbulent flow, allowing the dust to undergo initial infiltration. Sulfides and alkali mist undergo mass transfer, and large smoke particles are collected, while fine smoke particles increase the bonding cohesion and coalesce into the absorption liquid. Generally, during this process, the purification efficiency of sulfides can reach 20-30%, and the purification efficiency of smoke and dust can reach 30-40%. At the same time, this process also has a cooling effect, which helps with subsequent acid-base neutralization reactions.

Self excitation enhancement of flue gas: After wetting and initial purification, the flue gas flow breaks through the surface gas film of fine dust particles under the action of the airflow, allowing the fine dust particles to be further wetted. At the same time, it also enhances the gas-liquid mass transfer process, allowing sulfides in the airflow to be well absorbed, and collecting larger surface fine dust particles and sulfides that have not yet participated in the reaction. At the same time, the absorption liquid accumulated in the tower will automatically intensify the liquid mist under the action of strong atmospheric flow force to form a foam liquid mist layer with efficient capture and absorption. During this process, the dust removal efficiency can generally reach over 40%, and the desulfurization efficiency can reach around 40%.

Absorption and dedusting of foam layer: under the strong airflow force in the pretreatment room, the gas stirs up the absorption liquid accumulated in the tower to stimulate the droplets of foam, which optimizes the mass transfer of gas, solid and liquid, creating good conditions for the further purification of the flue gas flow after twice purification, and has good effects on the removal of sulfides and the purification of smoke and dust.

Stage cyclone effect: after the waste gas hits the desulfurization liquid to form a foam reaction, it rotates and rises tangential around the inner diameter of the tower, and contacts the desulfurization liquid flowing along the tower wall. At the same time, a cyclone and an atomizing nozzle are arranged in the tower, which make the waste gas swirl up, and at the same time make the atomized desulfurization liquid evenly spread in the tower. A series of complex effects, such as collision, gas-liquid mass transfer, acid-base neutralization, flocculation, occur between the desulfurization liquid flowing down the cyclone and the waste gas flowing up the cyclone, and dust and sulfide are effectively purified to achieve three-stage purification; The general dust removal efficiency can reach 60%, and the desulfurization efficiency can reach over 60%.

The total efficiency of the above four levels of action can be calculated as U=1- (1-u1) (1-u2) (1-u3) (1-u4), and the total desulfurization efficiency is U>90%; Total dust removal efficiency: U>90%

Absorption liquid circulation spray system

After sufficient gas-liquid contact and mass transfer reaction inside the tower, the desulfurization slurry contains substances such as CaSO3, Ca (OH) 2, and unreacted CaO. These unreacted desulfurization slurries are circulated and sprayed again by the circulation pump, reacting with the flue gas multiple times to make the equivalence ratio of the entire reaction close to 1. After multiple cycles of absorption, CaSO3 gradually increases and the pH value of the desulfurization slurry decreases. When the pH value is lower than the design value, the electric valve of the desulfurization circulation pump will automatically open to replenish the desulfurizer.

recirculating pump

The desulfurization circulating pump adopts a desulfurization dedicated pump, which has excellent corrosion resistance, wear resistance, and strong impact resistance.

Dehydration and dehazing system

The dust and sulfur dioxide in the flue gas are well purified, but there are also liquid droplets and aerosols brought into the flue gas, becoming liquid containing flue gas, also known as flue gas with water, which can have a serious impact on the exhaust system and fans. Therefore, it is necessary to separate the liquid in the flue gas as much as possible. To this end, based on the actual situation of the desulfurization tower, the centrifugal separation mechanism with the strongest separation ability is selected, and an efficient demister is arranged to achieve good separation of gas and liquid. After removing most of the aerosol, it is discharged through the chimney; The separated dusty and sulfur-containing liquids flow into the bottom of the tower along the separator cylinder wall and reflux together with the accumulated liquid.

circulating water system

Circulating water volume

This project adopts wet desulfurization technology, and the desulfurization liquid is recycled. Two sets of SCX-V5.0 wet desulfurization and dust removal devices have a circulating water supply of 600m3/h. During actual operation, the circulating water volume can be adjusted appropriately according to the characteristics of the flue gas.

Circulating sedimentation tank

The slurry in the wet desulfurization process is mainly desulfurization slag, and the solid-liquid separation in the circulating water system can adopt a circulating sedimentation tank structure. The desulfurization wastewater overflows into the sedimentation zone through the diversion channel. Calcium sulfite is oxidized to calcium sulfate, which is easy to settle. The settled slurry is pressurized by the slurry pump and transported to the high-level sedimentation tank. The supernatant after sedimentation overflows into the clear liquid tank, adjusts the pH value, and is pressurized by the circulation pump and transported to the desulfurization and dust removal device. After sedimentation and concentration, the slurry is transported by a slurry pump to a filter press for dewatering and then handed over to a qualified unit for recycling.