This paper presents experimental study of fluidized absorption process for flue gas purification of co-combustion of municipal solid waste (MSW) and coal in a circulating fluidized bed Combustor (CFBC) test rig. The test rig is composed of a CFBC, coal/MSW feeding subsystem, ash cycle subsystem and flue gas purification subsystem. In the circulating fluidized bed, section area of fluidized bed is 230mm × 230mm and the freeboard is 460mm × 395mm. The total height of the test facility is 8m; height of bed and freeboard are 1.5m and 6m respectively. The preheated air enters the bed as primary air passing through distributor and provides oxygen for combustion. Six movable tubes immerged within the bed are used in adjusting the bed temperature. The cyclone separator is fixed up at the exit of chamber. The separated ashes return to chamber through the recycling feeder for decreasing the carbon content in fly ash and promoting the combustion efficiency. The flue gas from the exit of cyclone separator enters the air preheater to preheat the cold air at first, then enters the flue gas purification facility, finally be discharged into air by induced drafted fan passing through the stack. Coal is carried to a positive pressure feeding entrance by screw feeder and enters the bed. Secondary air is injected into a sealed end feeding pipe under MSW feeder, for enhancing the mixture in furnace, providing the oxygen for combustion and preventing from MSW remaining in the feeding pipe. The material of bed is silicon sand. Fluidized absorption facility for flue gas purification in MSW incineration is mainly composed of humidification system, absorption tower, flue gas reheater, fabric filter, slurry making pool, sediment pool and measurement subsystem. The temperature of flue gas from boiler by induced draft fan reduces to 120°C when flue gas enters the humidification region, which can increase the ability of acid gas absorption and prevent the slurry evaporation. When flue gas and limestone slurry enter the absorption tower, the three-phase material of gas, liquid and solid generates intense mixing and forms bubbling layer. The acid gases in flue gas are absorbed by limestone slurry, and a large amount of dusts are collected in reaction tank. Feeding oxidation air into slurry and agitating slurry simultaneously so as to promote the inner circulation of slurry and oxygenization of calcium sulphite. Flue gas passes through undulate demister which has high efficiency and low resistance, then enters fabric filter after reheating, finally be discharged into the stack by induced draft fan. The mixture of slurry and gypsum is emitted into the sediment pool through bottom and clear liquid in sediment pool returns to slurry making pool or absorption tower. The test results are as follows: the combustion efficiency is greater than 95%, the carbon content of fly ash is lower than 8%, and the loss of slag combustion is lower than 5%. When sorbent is limestone slurry, the concentration of slurry is 1%, the circulating ratio is 3, the jet rate is 5∼15m/s. The immerged depth of bubbling pipe under the slurry is 140mm. In the fluidized absorption facility for flue gas purification of MSW incineration, the desulfurization efficiency is >90%, the de-nitrification efficiency is 20∼30%, the de-chlorination efficiency is >80%, the removal efficiency of dust, heavy metal and dioxins are >99%, >98.6% and 99.35% respectively. After passing through fluidized absorption facility for flue gas purification of MSW incineration, when the concentration of O2 is 11%, the emission concentration of every components in flue gas are: SO2 is 20∼50mg/Nm3, NOx is 130∼270 mg/Nm3, HCl is 7∼12 mg/Nm3, HF is ∼8 mg/Nm3, CO2 is7∼8%, dust is 23∼67 mg/Nm3, Cr is 0.2172 mg/Nm3, Cu is 0.0454 mg/Nm3, Pb is 0.2963 mg/Nm3, Zn is 0.2074 mg/Nm3, Fe is 2.834 mg/Nm3, As is 1.112 × 10−3 mg/Nm3, Hg is 2.38 × 10−4 mg/Nm3 and dioxins is 0.1573 ng/Nm3. These emission concentrations are all lower than the Chinese emission standards. Some of them come close to the emission standards of developed country.
Mercury removal from MSW incineration flue gas by mineral-based sorbents
