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          Dalian Bihai Environmental Protection Equipment Co., Ltd. is specialized in research, development, design, manufacture, installation, commissioning, after-sales service and technical consultation for environmental protection and gas/fume scrubbing.


          Flue Gas Desulfurization 

          1. Brief introduction

          Flue Gas Desulfurization (FGD) is a chemical process for removing sulfur dioxide (SO2) in gas and is also the most effective technology means to restrain the acid rain formation and prevent the SO2 pollution. Our company has successfully completed many SO2 scrubbing projects for many companies by using wet scrubbing, spray-dry scrubbing and dry scrubbing methods.

          Wet FGD process has been developed for dozens of years and has increasingly become a mature and perfect technical process. Some processes such as lime (limestone) and/or plaster process have come to their second even third generations of process system which provide a great improvement on the problems of blockage and fouling in traditional wet FGD process. As per the introduction of some data, about 85% of the FGD process used in foreign countries is wet scrubbing process. In particular, almost all of the FGD process adopted in Japan is the wet scrubbing process. According to the difference of sorbent, wet FGD process has many varieties and the most commonly used one at present is the lime stone(lime) and/or plaster process . Our company has brought in the FGD technology of America and made further improvement in it so that a self contained wet FGD technology is formed.

          2. System composition

          As shown in below flow chart, a typical FGD process mainly includes the flue gas system (flue gas damper, flue gas heat exchanger, booster fan, etc.), absorption system (absorption tower, circulating pump, oxidation fan, defogger, etc.), absorbent preparation system (limestone silo, limestone ball mill, limestone slurry tank and slurry pump, etc.), plaster dewatering and storage system (plaster slurry pump, hydraulic cyclone, vacuum belt dewaterer, etc.), wastewater treatment system and utility system (process water, power, compressed air, etc.)

          (Typical process flow)


          3. Reaction principle



          SO2 and SO3 in flue gas are absorbed by the water of slurries.

                SO2+H2O H2SO3

                SO3+H2O H2SO4



          The neutralization reaction takes place among H2SO3, H2SO4 and the fine limestone particle of suspension slurries.

                CaCO3+H2SO3 CaSO3+CO2+H2O

                CaCO3+H2SO4 CaSO4+CO2+H2O

                CaCO3+2HCI CaCI2+CO2+H2O


          The above reactions are ionic reactions taking place in the slurry pond. The slurry dwell time should be long enough so as to ensure the sufficient reaction time.


          The absorption liquid is sprayed into the absorption tower after nozzle atomization and then dispersed into fine droplets to cover the whole cross section of the tower. SO2, SO3 and HCI, HF in flue gas are absorbed when these droplets are in contact with flue gas counterflow inside the tower and have mass transfer reaction and absorption reaction. The products of SO2 after absorption reaction finally form plaster after the completion of oxidization and neutralization reaction in the oxidation zone on the bottom of the absorption tower. In order to keep the constant PH value of the absorption liquid and reduce the consumption of limestone, the limestone is continuously added into the tower and the distribution and dissolution of the limestone in slurry is accelerated though continual stirring the absorbent slurry by the blender, oxidation air and the circulating pump of the absorption tower.

          Absorption tower system


          SO2, SO3, HCI and HF are absorbed after the reaction between the flue gas and the slurry of
          limestone/plaster and finally form the byproduct plaster (CaSO4 · 2H2O). The limestone slurry enters the slurry pond of absorption tower and mixes with plaster slurry. The mixture slurry in the slurry pond of absorption tower is transmitted to the nozzle by circulating pump and sprayed after atomization, producing pretty fine suspension slurry. Generally one set of circulating pump matches with one spraying layer. SO2 and SO3 react with the limestone in slurry and form calcium sulfite which is oxidized as calcium sulfate by the oxidation air in the slurry pond of absorption tower and finally the supersaturated solution is crystallized into plaster.



          The absorption tower can be divided into the following sections.

          Oxidization and crystallization take place mainly in the slurry pond of the absorption tower in which the PH value, controlled by the dosage of limestone added, is about 5.6~5.8. The dwell time in the tower should be long enough so as to ensure the better formation of the plaster crystal (CaSO4 · 2H2O). When the flue gas in reaction section is cooled to the saturation temperature, it is saturated by the steam of the circulating suspension slurry. The circulating process water partially makes up the water loss through evaporation and the rest part of circulating water is used to cleaning the defogger on the top of tower. Then the flue gas leaves the tower after the defogger removes the droplets in it. The tower is usually equipped with two-stage defogger and each defogger with spraying tube that set in the bottom carrying nozzle. The water is sprayed to the bottoms of components with great power so as to reach the cleaning effect, and then the scrubbed wet flue gas leaves the system from the top of absorption tower. The absorption tower is designed as automatic control mode so as to keep continuous operation.

           Sketch of vacuum belt dewaterer


          Absorbent preparation system


          Absorbent adopts limestone of which 90~95% particle sizes are 200~300 mesh. There is limestone producing system on FGD device which usually adopts wet type ball mill to make the limestone block (20mm purchased in power plant) into limestone slurry, and then the slurry with 20% of concentration produced by the limestone producing system is sent to the absorption tower. The FGD device can only set the limestone slurry distribution system without the limestone producing system. In this case, the power plant is in charge of purchasing the proper limestone powder and the powder will be made into slurry though the limestone slurry distribution system and sent to the absorption tower.


          Plaster handling system


          The plaster in the slurry pond of absorption tower is produced constantly and transmitted to the plaster hydraulic cyclone station by plaster slurry pump. The plaster slurry extracted from the absorption tower is concentrated into the slurry with 40% to 50% of concentration by plaster hydraulic cyclone. The concentrated underflow of the cyclone containing coarse plaster particle is sent to vacuum belt dewaterer and after the dehydration, the water content of each share of the plaster is less than 10%.

          Partial overflow of the hydraulic cyclone station in plaster returns to the absorpt ion tower under the force of gravity and the plaster inside the absorption tower is further dissolved and produces minor plaster crystals, while the other part of overflow enters into the wastewater clarifier.

          (Sketch of GGH structure)

          (Sketch of double-ply flue gas damper)

          Performance parameter