Research On Regenerative Cycle And Utilization Of MgO Wet Flue Gas Desulfurization Byproducts

Magnesia wet flue gas desulfurization technology has the advantages of highdesulfurization efficiency, not easy to scale and easy to operate. It develops rapidly inrecent years. China has the world’s largest magnesia wet flue gas desulfurization systemand this technology is expected to become one of the leading desulfurizationtechnologies in the future. The key to promote the application of magnesia wet flue gasdesulfurization technology is to recycle the desulfurization byproducts, including theregeneration of magnesium oxide and the recovery of sulfur.This paper studied the material characteristic of the industrial-scale MgOdesulfurization byproducts, the feasibility of the regenerative cycle and utilization, andthe effects of key operating parameters on the regenerative cycle and utilization of theindustrial-scale MgO desulfurization byproducts by the theoretical analysis, a small coldexperimental research, an industrial-scale thermal semi-plant, a method of simulationfor the gas-solid two-phase flow, and so on. The main contents and innovation of thispaper include:①An industrial-scale thermal semi-plant system was designed and built in China.This system is used to study the regenerative cycle and utilization of the industrial-scaleMgO desulfurization byproducts.The semi-plant system includes a furnace (internal diameter of500mm and heightof6780mm), a feed inlet, an air distribution plate, three natural gas jet nozzles, and soon. The auxiliary system includes a feeding system, an air heating system, a flue gasdisposing system, a product recycling system, an air supply and exhaust system, a datameasurement and acquisition system, and so on. The design parameters for the wholesystem are shown as follows: compressed air flow of200Nm3/h, feedrate of80Kg/h,natural gas flow of17Nm3/h, flue gas flow of300Kg/h from the calciner, hot airtemperature of650℃from air preheater, flue gas temperature from a secondary aircooler of250℃, flue gas temperature entering the induced draft fan below250℃, and aslurry circulating pump of50L/min in a flue gas disposing.②The structure characteristic, the thermal decomposition and kinetics of theindustrial-scale MgO desulfurization byproducts were firstly studied by experiment.By Scanning Electron Microscope, it is found that the industrial-scale MgOdesulfurization byproducts which were used to test have a hierarchical structure similar to the sheet. Their appearances are smooth and dense with less porosity. Bythermogravimetric experiment, it is found that the total mass loss of industrial-scaleMgO desulfurization byproducts increased with decreasing particle size. However, theeffects of particle size on the mass loss of each thermal decomposition stage weredifferent. The added charcoal powder in the thermal decomposition process had anobvious effect on the temperature characteristics of the thermal decomposition ofmagnesium sulfate, resulting in decreasing the thermal decomposition temperatures andfacilitating the thermal decomposition process. The final peak of thermal decompositionmoves to the backwards and the reactions move to the high temperature region underthe thermal decomposition of byproducts in an oxygen atmosphere.③The feasibility and influencing factors of sulfur recovery from theindustrial-scale MgO desulfurization byproducts were firstly studied by experimentaland computational analysis in China. The structure characteristic of the regenerativeMgO was analyzed. With a sulfur content of1.65%in coal and a generating capacity of300MW of power plant, the economics of the renewable magnesium oxidedesulfurization technology was compared to that of limestone-gypsum desulfurizationtechnology.The main research results indicate that the chemical reaction of byproducts mainlyoccurred in the lower zone of the furnace. When the excess air coefficient was1.05andthe magnesium sulfite content in the byproducts was60%, the SO2mole fraction in thecalciner gas was10.5%. When the moisture content in the byproducts was15%, the SO2mole fraction reached9.2%. Adopting oxygen-enriched combustion, the oxygen contentin air should be no more than31%. SO2mole content in calciner gas could be increasedobviously by decreasing excess air coefficient, increasing feeding rate, decreasingcrystal content in byproducts, appropriate to reduce the furnace temperature, and so on.The magnesium oxide desulfurization renewable technology was found to be moreeconomic. The generated MgO under the calcination temperature of900-1000°C had ahigh activity. On the contrary, because of the slight sintering, the generated MgO underthe calcination temperature of1100°C had a low activity.④Numerical modeling research on the combustion reactions and the gas-solidflow behavior in furnace of the industrial-scale thermal semi-plant system.Based on Gambit and Fluent soft ware, the appropriate grid structure and quantitywere selected for the calciner by modeling. First, three-dimensional gas-phasecombustion in the furnace was simulated. On this basis, gas-solid flow in the furnace was studied using discrete phase model. The results show that the gas phase combustionfield in the furnace was in line with the flow trend during the experiment on-spot. Theparticle concentration change was big at the bottom of furnace after adding particulatephase, and the particle concentration had larger fluctuation near the feeding port. Theparticle for the upper parts of the furnace mixed well. In the vicinity of the wall of thefurnace, the particle concentration is high, and the particle concentration distribution inthe central region of the furnace is relatively low and relatively uniform.