Basic principle and method of flue gas desulfurization

The basic principle of

Basic desulfurizer +SO2= sulfite (absorption process)

Sulfite +SO2= sulfate (oxidation process)

Alkaline desulfurization sorbent absorbs SO2, reacts to form sulfite, then oxidizes oxygen to form stable sulfate, and then processes the sulfate into the desired product. Therefore, any desulfurization method is a chemical process.

Main flue gas desulfurization methods

There are many technical methods for flue gas desulfurization. The types of absorbent can be divided into:

1) Calcium method (mainly limestone/lime-gypsum)

2) Ammonia process (ammonia or ammonium bicarbonate)

3) Magnesium method (magnesium oxide)

4) Sodium process (sodium carbonate, sodium hydroxide)

5) Organic alkali method

6) Activated carbon method

7) Seawater method, etc

At present, the most used method is calcium method, followed by ammonia method. The calcium method includes limestone/lime-gypsum method, spray drying method, in-furnace spraying calcium method, circulating fluidized bed method, in-furnace spraying calcium tail humidification method, GSA suspension absorption method, etc., among which the most used is limestone/lime-gypsum method. Ammonia method is also a variety of, such as thiamine method, thiamine production method and sulfuric acid method, combined production of ammonium phosphate method, etc., mainly to ammonium phosphate method.

Introduction of flue gas desulfurization technology

(1) Limestone/lime-gypsum wet flue gas desulfurization technology

Limestone/lime-Gypsum wet flue gas desulfurization process USES cheap and easy limestone as desulfurizer, lime is crushed and ground into powder and mixed with water to make absorbent slurry. When lime is used as an absorbent, lime powder is digested and mixed with water to make an absorbent slurry. In the absorber, the slurry is absorbed and mixed with the flue gas. The sulfur dioxide in the flue gas reacts with the calcium hydroxide in the slurry and the air pumped in. The final reaction product is gypsum. At the same time can remove other impurities in the flue gas. The flue gas after desulphurization is removed by a defogging device to remove the tiny droplets that are brought out, and then heated by a heat exchanger to form a chimney. Desulphurizing gypsum is recovered after dehydration in a dehydration unit.

Composition of the system

The limestone/lime-gypsum flue gas desulfurization unit consists of absorber preparation system, flue gas absorption and oxidation system, desulfurization by-product treatment system, desulfurization wastewater treatment system, flue gas system, automatic control and online monitoring system, etc.

Dry circulating fluidized bed flue gas desulfurization technology

Dry circulating fluidized bed flue gas desulfurization technology is a new dry flue gas desulfurization technology developed in the late 1980s. This technology has the advantages of less investment, small land area, simple structure, easy operation, efficient dust removal and flue gas purification, low operating cost and so on. Therefore, the research center of Power station Combustion Engineering technology and the State Key Laboratory of Clean Coal Combustion Technology of Tsinghua University have carried out theoretical and experimental studies on the reaction mechanism and mathematical model of the reaction process respectively. The process flow is shown in Figure 3. The actual flue gas generated from the pulverized coal combustion unit enters the reactor through an induced draft fan, then goes through a cyclone dust collector, and finally is discharged from the chimney through an induced draft fan. The desulfurizer is the high quality lime powder produced from rotary kiln, which is continuously added with spiral pulverizer according to the given ratio of calcium and sulfur. Part of the desulphurization ash removed from the cyclone dust catcher is recycled into the reactor by circulating ash hopper and spiral feed to the aircraft. There is a spray atomizer in the Venturi tube, which controls the temperature in the reactor by regulating the amount of water.

Dry circulating fluidized bed flue gas desulfurization (FGD) technology is especially suitable for flue gas with low SO2 concentration. It has the following characteristics:

(1) As a circulating material, the boiler fly ash has uniform solid particle concentration in the reactor, strong solid internal circulation, good gas-solid mixing and contact, and ideal heat and mass transfer between gas and solid. 

(2) In the reaction tower, due to the water evaporation and water adsorption of particles and the strong contact friction between solid particles, the reaction activity and reaction surface area between gas, solid and liquid phases are extremely great, which is very ideal for the removal of SO2 from flue gas.

(3) Solid materials are collected by efficient cyclone separator and dust collector outside the reactor and sent back to the reaction tower, which makes the removal agent circulate repeatedly and prolong the residence time in the reactor, thus improving the utilization rate of the removal agent and reducing the operating cost.

(4) By spraying water into the reactor, the flue gas temperature was reduced to the saturation temperature under the partial pressure of water vapor, so as to improve the desulfurization efficiency.

(5) The reactor is not prone to corrosion and wear.

(6) Fly ash in the system has a catalytic effect on desulfurization reaction

System composition

The flue gas fluidized bed mainly consists of absorber preparation system, sulfur dioxide absorption system, dust removal system, absorber recirculation system, automatic control and on-line monitoring system.