| SO2 is a most common air pollutant and the flue gas emission from coal-fired boiler is a main source of SO2 in the air. The flue gas desulphurization is an effective way to reduce SO2 emission.There exist many ways for flue gas desulphurization and the Wet MgO Process is widely used owing to its low investment, short process flow, less area occupied, high efficiency and reliable running. In recent years, the MgO Process of recovery type has more and more been concerned by the industry, of which the process of desulfurized by-product pyrolysis to recover SO2 and MgO is much acclaimed, the recovered SO2 is oxidized to produce acid and MgO used as a sorbent for cycling. This article, based upon the study on pyrolysis conditions for magnesium sulfite and magnesium sulfate---main chemicals of desulfurized by-product, has carried out a study on pyrolysis of actual desulfurized by-product from industrial plant to obtain a higher recovery rate and activity of MgO.In the study, the pyrolysis conditions being investigated cover pyrolysis temperature, temperature-rise rate, holding time of constant temperature and moisture content of sample. The evaluation on pyrolysis effect includes sample decomposition rate, SO2 yield, MgO content and MgO activity which is respectively by means of simple hydration method and citric acid method, further, the analysis and investigation of change of lattice structure of MgO by X-ray diffraction (XRD).This study has verified that the pyrolysis of magnesium sulfite starts at about 400℃and as the temperature reaches 550℃the yield of SO2 to 92.0%. XRD analysis of MgO product shows that the choice of 550℃as pyrolysis temperature for magnesium sulfite is appropriate, and when the temperature raises higher the size of mosaic block of MgO grains may gradually increase and the crystallization level of crystal increase whereas the activity is affected and lowered. At 550℃pyrolysis temperature the analysis of activity of MgO---pyrolysis product of magnesium sulfite---has verified that its activity is better than that of MgO of industrial grade. In the course of analysis, as the pyrolysis temperature reaches 550℃and the holding time range of constant temperature in between 0.5---2.0 hours the study so carried out also verified that the yield of both SO2 and MgO is maximized when the holding time of constant temperature is 2.0 hours. In addition, the study also discovers that the increase of moisture content of magnesium sulfite is not conducive to its pyrolysis reaction, but has an effect on the activity of pyrolysis product---MgO. The addition of activated carbon as a reducer basically does not work in the pyrolysis of magnesium sulfite; hence, during the magnesium sulfite pyrolysis there is no need to add any reducer. The lower pyrolysis temperature-rise rate is beneficial to obtain a higher activity of MgO.The study on pyrolysis of magnesium sulfate has verified that under the condition that activated carbon as reducer is added, the pyrolysis of magnesium sulfate starts at about 650℃and when the temperature reaches 850℃the yield of SO2 is 98.7%, this represents a basic completion of pyrolysis process. At the pyrolysis temperature 850℃the measured activity of MgO---pyrolysis product---is close to that of MgO of reagent grade, this manifests that this 850℃taken as a pyrolysis temperature for magnesium sulfate is appropriate. The XRD analysis of MgO product also shows that the 850℃taken as a pyrolysis temperature for magnesium sulfate is appropriate; a continuous rise of pyrolysis temperature for magnesium sulfate may make the lattice of MgO perfect, but reduce the MgO activity. Meanwhile, it is confirmed that a two-hour holding time of constant temperature is used after the 850℃pyrolysis temperature for magnesium sulfate reached is necessary for obtaining a higher MgO activity. In addition, it is also discovered that a lower pyrolysis temperature-rise rate is beneficial to obtain a higher activity of MgO.The dry residues of desulfurized by-product from industrial plant are relatively stable at 35℃. However, its heat weightlessness is 31% at 550℃and 62% at 650℃. Under 600--700℃the crystallization level of MgO---pyrolysis product---is lower but activity higher, to choose two hours as the pyrolysis constant temperature holding time is appropriate. When the pyrolysis temperature is up to over 800℃, even to 900℃, then a part of MgO is caked and a reduction of activity resulted in.The technological process for thermal decomposition and recovery of desulfurized by-product by a pyrolysis process is feasible and this basic theoretical study may provide theoretical and practical basis to the implementation of commercialized plant. |
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