| At present,China’s energy consumption structure is still dominated by coal.A large part of coal consumption is used for thermal power generation,which causes sulfur dioxide emission pollution and has serious harm to human body and environment.In recent years,the emission limit of sulfur dioxide in flue gas of coal-fired power plant is more and more strict.The traditional limestone wet flue gas desulfurization(WFGD)has some problems,such as large investment,pipeline corrosion and blockage.Magnesium oxide WFGD has many advantages,such as high desulfurization efficiency,and its application is gradually increasing in China.However,the technology route of desulfurization product resource utilization is not mature and needs to be studied.In this paper,the models of limestone-gypsum WFGD and magnesium oxide-magnesium sulfate heptahydrate WFGD were established by Aspen Plus,and the technical and economic comparative analysis is made.Technical analysis showed that the suitable liquid gas ratio of magnesium oxide WFGD is 7-9 L/Nm3,while that of limestone process is 17-19 L/Nm3.When the desulfurization rate is about 97.3%,the desulfurization mole ratio of limestone WFGD is about 1.014 mol/mol,and that of magnesium oxide WFGD is about 1.006 mol/mol.When the SO2 content of raw flue gas varied from 773 mg/Nm3 to 1288 mg/Nm3,the desulfurization performance of magnesium oxide WFGD is always slightly better than that of limestone WFGD.Economic analysis showed that in 300 MW coal-fired power plant,the investment cost of limestone WFGD tower is about 4.49 million Chinese Yuan(CNY),while that of magnesium oxide WFGD tower is about 3.04 million CNY.The operation cost of limestone WFGD tower is about 5.39 million CNY per year,and that of magnesium oxide WFGD tower is about 3.20 million CNY per year.In view of the immature technical route of desulfurization product resource utilization in magnesium oxide WFGD,the whole production process utilizing waste heat of flue gas is proposed.The whole production process inclues concentratiton and crysallization and its desulfurization product is magnesium sulfate heptahydrate.The models of multi-effect evaporation concentration system utilizing waste heat of flue gas and thermal vapor compression crystallization system utilizing waste heat of flue gas were established by Aspen Plus.The analysis of the multi-effect evaporation concentration system utilizing waste heat of flue gas showed that compared with the forward-feed benchmark system and the parallel-feed optimization system with different optimization process,the parallel-feed optimization system with 7-process makes more efficient use of the heat of secondary steam,condensed water and concentrated liquid,so it has better thermal and economic performance,and the increase of flue gas temperature helps to increase the upper limit of system effect,so as to improve the system efficiency.Among the four main factors that affect the total annual cost of parallel-feed optimization system with 7-process,annual utilization hours and steam price have a greater impact,ND steel price takes the second place,and 304 stainless steel price has almost no impact.When the capacity of power plant changes,the thermal performance of parallel-feed optimization system with 7-process is almost unaffected,while the economic performance changes greatly.The analysis of thermal vapor compression crystallization system utilizing waste heat of flue gas showed that compared with steam turbine extraction as heat source,the heat source of flue gas waste heat has worse thermal performance and higher equipment investment cost,but lower annual total cost.For the thermal vapor compression crystallization system utilizing waste heat of flue gas,crystallization temperature and mixed steam pressure have more obvious influence,while flue gas temperature has less influence,and the system has better thermal and economic performance in high crystallization temperature,low mixed steam pressure and high flue gas temperature.When the capacity of power plant changes,the thermal performance of the system is almost unaffected,while the economic performance changes greatly.In general,compared with limestone WFGD,magnesium oxide WFGD can achieve better desulfurization effect with lower investment and operation cost of desulfurization tower.When the resource utilization technical route of magnesium sulfate heptahydrate utilizes waste heat of flue gas,magnesium oxide WFGD can bring additional profit about 7.09 million,14.71 million and 21.66 million CNY per year for 300,600 and 1000 MW coal-fired power plants. |
