Principle of system
　　Process of semi-dry FGD plus BF system is flue gas with sulfur in the outlet of the boiler enter into lower tower. Metering pump will pump lime slurry into inside lower tower, and atomize lime slurry into droplets by fixed spray nozzle under blowing of compressed air, that contact and mix with flue gas contains sulfur at the same time. Heating of flue gas makes droplets dry, meanwhile, SO2、HCL、HF such acidic gas are absorbed by taking chemical reaction with slurry droplets. The droplets are fully dried before rising to outlet at the top of the tower and form into solid reactant particle entered into outlet duct with flue gas.
　　Cooled flue gas from outlet of the tower enter into bag filter, unreacted Ca(OH)2 powder stick on the surface of the bag after entered into the bag filter, those Ca(OH)2 continually react with acidic gas like SO2、HCL、HF, that improve sulfur removal efficiency.
　　By-products are calcium sulfate, sulfate, combination of fly ash and unreacted lime.
　　Spray drying is widely applied on low sulfur coal and middle sulfur coal fired boiler with small and medium capacity. Spray drying process can remove 70-95% SO2, even as high as 98% is available.
　　Semi-dry FGD process is mainly applicable to small and medium scale power plants (<200MW) and sulfur content in the coal is less than 2%, as well as flue gas treatment of city waste incinerator and medical waste incinerator

　　technical process
　　(1)quicklime for lime slurry
　　CaO + H2O → Ca(OH)2
　　(2)SO2 is absorbed by droplets of lime slurry
　　SO2 + H2O →  H2SO3
　　(3)reaction between sorbent and SO2
　　Ca(OH)2 + H2SO3 → CaSO3 + 2H2O
　　(4)CaSO3 in the slurry droplets are supersatuated and pricipitated out
　　CaSO3（aq）→ CaSO3（s）
　　(5) Part of CaSO3（aq）are oxidized by oxygen dissolve in the droplets and form calcium sulfite
　　CaSO3（aq）+ 1/2O2 → CaSO4（aq）
　　(6)CaSO4 is hard to dissolved in the water and it quickly pricipitate out solid CaSO4
　　CaSO4（aq） → CaSO4（s）
　　meanwhile, vapor phase HCL and HF in the flue gas are captured by the sorbent, reaction is as follows:
　　2HCl + CaO → CaCl2 + H2O
　　2HF + CaO → CaF2 + H2O