Transfer Process And Chemical Reactions In Simultaneous Removal Of SO₂ And NO_x From Coal-Fired Power Generation Units

Coal is still the main energy of China in the foreseeable future,but the emissions of SO2,NOx and particulate matter result in a series of environmental problems during coal combustion.The coal-fired power generation unit is one of the main sources of anthropogenic pollutant emissions.To meet with the requirement of the ultra-low emission standard for air pollutants,both equipment investment and energy consumption for the coal-fired power unit are inevitably increased with stricter emission limits.Therefore,it is urgent to optimize the existing equipment for reducing the operational cost for flue gas purification system.More importantly,it is necessary to develop the novel simultaneous removal of multi-pollutants,such as SO2 and NOx,which should decrease the overall energy penalty together with utilization of wastes.The industrial-scale wet flue gas desulfurization(WFGD)unit is investigated in this dissertation.A computational fluid dynamics(CFD)model is established to describe the gas-slurry transfer behaviors and reactions in WFGD scrubber of coal-fired power unit.A series of intensification methods are suggested to improve SO2 absorption performance,which have been used in commercial-scale scrubber.Furthermore,the simultaneous removal of SO2 and NOx in the existed WFGD system is explored by combined with O3 pre-oxidation technology.The main contents and conclusions are as follows:Firstly,a comprehensive CFD model is established by involving the hydrodynamics of gas-slurry flow,interphase heat transfer combined with H2O evaporation and SO2 chemical absorption process in WFGD scrubber,which is validated by the measured data from a desulfurization system of 330 MW coal-fired power unit.Based upon the gas-slurry flow patterns and SO2 transfer behaviors,three regions are identified in the spray zone of WFGD scrubber,including Gas Inlet Region(GIR),Dominant Absorption Region(DAR)and Slurry Dispersed Region(SDR).Furthermore,a better understanding of desulfurization process is obtained from the fundamental relationship between SO2 absorption performance and complicated transfer process and chemical reactions in three regions.Numerical results show that the overall desulfurization performance of spray scrubber is largely determined by both gas-slurry momentum transfer and SO2 chemical absorption process,which is hardly influenced by the heat transfer together with H2O evaporation.The gas-slurry flow in DAR is dominated by counter-current mode due to the droplets approximately in forcebalanced state,which is more favorable for both interphase momentum exchange and SO2 absorption compared with GIR and SDR.The overall desulfurization performance of WFGD scrubber is directly affected by SO2 absorption process in DAR,while GIR and SDR provide the initial conditions of transfer process and chemical reactions in DAR.Moreover,the process intensification schemes for SO2 absorption are proposed by regulating gas-slurry flow pattern,including optimizing spray conditions in SDR and installing internals in GIR.Secondly,the influences of spray conditions and internals on desulfurization performance are explored to intensify SO2 absorption process in commercial-scale WFGD scrubber.The considered spray conditions are consisted of droplet diameter,injection direction and spray morphology,and the internal includes the baffle and sieve tray.For the spray conditions in SDR.the optimal diameter of droplet in non-ideal flow is obtained by considering the synergistic effect of the uniformity of gas-slurry distribution and the dynamics of SO2 mass transfer.The dual-direction spray reaches a great balance between the competitive effects of the uniformity of gas-slurry flow and the interfacial area for mass transfer.Compared with the single-direction spray,the desulfurization efficiencies could be increased from 91.6%-97.2%to 95.6%-99.0%by dual-direction mode.The appropriate combination of hollow-cone and full-cone nozzles optimizes the gas-slurry contacting condition in DAR,which leads to increases of desulfurization efficiencies from 94.8%-98.8%to 95.8%-99.6%compared with the arrangement scheme with identical spray morphology.For the gas distribution in GIR,the baffle configurated at the gas inlet decreases the velocity gas flow around the wall.The desulfurization efficiency of scrubber could be increased from 98.4%to 99.1%by the baffle with appropriate structural parameter.Moreover,the sieve tray could effectively intensify the modification performance for the gas distribution at the whole cross section of WFGD scrubber.The dual-sieve tray further improves SO2 absorption performance at different initial concentrations of SO2 in flue gas,which could increase the desulfurization efficiency from 98.0%to 99.7%corresponding to SO2 initial concentration of 6000 mg·Nm-3.Some of key schemes have been successfully employed in the 330 MW ultra-low emission coal-fired power unit,which verifies that the above intensification methods are available and reliable for improving the SO2 absorption performance of commercial-scale WFGD scrubber.Thirdly,the NO oxidation by O3 is one of the key issues for the overall denitration efficiency during simultaneous removal of SO2 and NOx in WFGD scrubber of coalfired power unit.The simplified 11-step reaction model is determined to describe the conversion of NO into NOx with higher chemical valances.The hydrodynamics and O3 oxidation of NO are numerically simulated in an industrial-scale oxidation reactor.To meet with the normal stoichiometric ratio of O3 to NO.an optimized structure of pipelines for O3 distribution is obtained for the obvious difference in flow rates between flue gas and injections.The uniformity of O3 distribution is improved in the oxidation reactor by regulated the disturbance to both flue gas and injection gas.NO2 and N2O5 are the main products during O3 oxidation of NO in flue gas.The production of N2O5 is significantly decreased when the temperature increases from 363K to 423 K,whilst the production of NO2 is slightly increased.Therefore,the temperature of 373 K is suggested for actual commercial process to avoid the acid corrosion of wet flue gas.A new type of combinational layout of distributors is proposed,which could oxidize almost all NO into NO2 or N2O5,and the proportion of N2O5 is approximately 51%in flue gas.The above findings have been applied in the oxidation reactor of 256 m2 sintering machine,which confirms that the intensification scheme is applicable for the deep oxidation of NO in full-scale O3 oxidation reactor.Finally,the simultaneous removal of SO2 and NOx in WFGD scrubber is further explored based on the above SO2 absorption together with the NO conversion into NOx by O3 oxidation process.A CFD model is established for describing mass transfer of SO2 and NOx coupled with chemical under complicated hydrodynamics.The removal performance of NOx is identified based on transfer and reaction conditions.Furthermore,the simultaneous removal efficiency of SO2 and NOx is predicted in the WFGD scrubber of 330 MW coal-fired power unit.For NOx absorption by S(Ⅳ)solution,the overall denitration efficiency in scrubber could be improved by the increase of conversion rate for NO into N2O5 in pre-oxidation process.NO2 absorption is much slower than N2O5 and more sensitive to chemical compositions in droplets or hydrodynamics in scrubber.Based on the sensitivity analysis,a correlation is proposed to predict the volumetric mass transfer coefficient(KGa)for NO2 absorption in WFGD scrubber.The overall mass transfer coefficient(KG)for NO2 is mainly determined by S(Ⅳ)concentration and pH in droplets,while droplet diameter is the most sensitive parameter for the specific interfacial area(a)between two phases.For the simultaneous absorption of SO2 and NOx by limestone slurry in commercial-scale WFGD spray scrubber,the absorption rate of NO2 is slower than that of SO2,while N2O5 can be easily removed in scrubber.Under the condition of pH≤6.0 for slurry droplets,the absorption rate of NO2 could be improved by the increase of SO2 concentration in flue gas due to higher concentration of dissociated HSO3-in droplets.The removal efficiency of NO2 is increased by 2.4%when the molar ratio of SO2 to NO2 increases from 0 to 30.Moreover,the absorption performance of NO2 is effectively improved by the increase of S(Ⅳ)initial concentration or pH value in droplets.The predicted removal efficiency of NO2 is approximately 50%corresponding to S(Ⅳ)initial concentration of 0.2 kmol·m-3 and pH of 6.0.The findings in this dissertation could provide useful information for designing the simultaneous removal of SO2 and NOx in flue gas from coal-fired power units.