| In China,the total consumption of coal based primary energy is large,and the intensity of coal consumption per unit area in key regions is high.The fine particles(PM2.5),SO2,NOx and other pollutants emitted from coal combustion are the main reasons for the frequent occurrence of regional haze pollution,which seriously endangers people's lives and health.At present,ultra-low emission technologiesfor controlling coal-fired flue gas pollutants have been widely applied in China,which have greatly reduced the emission of pollutants.However,the research and application of ultra-low emission technologiesfor controlling the flue gas pollutants from inferior coals such as high sulfur and high alkali coals are still facing many challenges.In this thesis,laboratory cold-state experiments,pilot tests,numerical simulations and project demonstrationswere used to study the key technologies of ultra-low emission for high sulfur/high alkali coal flue gas,focusing on break-through in key technologies for low NOx combustion,high efficiency denitrification and desulfurization.In this way,technical routes and schemes of ultra-low emission technology for high sulfur/high alkali coals are achieved,and industrial verification and project demonstration are realized.For the opposed wall-fired combustion system burning high sulfur/high alkali coals,cold-state physical tests using a 18 MW single swirl pulverized coal burner were firstly carried out to study the influence of swirl intensity and air/fuel ratio on the recirculation zone.It is found that the larger the swirl strength is,the larger the circumfluence area is,and the larger the opening of the secondary air valve is,the worse the circumfluence area is.Then pilot tests were carried out on a full-scale swirl burner combustion test rig to study the influence of the swirl angles of internal and external tertiary blades and the opening of the secondary air damper on NOx emission.It was found that annular recirculationcreated an oxygen-rich atmosphere near the water walls,reducing slagging and high-temperature corrosion which leads to the construction of the new low NOx swirl burner.Finally,the industrial verification of the new type of low NOx swirl pulverized coal burner was carried out on a 600 MW domestic supercritical coal-fired power station unit.After the installation of the new type of low NOx swirl pulverized coal burners,the performance test results showed that the NOx emission concentration was 185mg/Nm3(@O2=6%).The burners can effectively control the generation of NOx in the combustion process together with the use of over-fire air to ensure a certain residence time in the reduction zone,and prevent the high temperature corrosion and slagging of the water walls.For the tangential-fired combustion system burning high sulfur/high alkali coals,the influence from the size,installation angle and inlet air velocity of the multi-louver damper on the characteristics of the fuel/air separation of the burner was studied firstlyusing the cold-state test rig for the single direct-flow pulverized coal burner.It was found that the influence of the inlet cross-section velocity on the fuel/air separation is not obvious,which is mainly related to the size and installation angle of the blade.Better fuel biasing performance can ensure low NOx combustion,strengthen the oxygen-rich atmosphere near the water walls,and prevent high temperature corrosion and slagging,so as to give the burner construction.After that,pilot tests were carried out to study the effect of reduction zone and air distribution on the low NOx effect of the burner.Finally,the industrial verification on a 660 MW coal-fired unit showed that the average NOxemission concentration at the outlet of the boiler was 186 mg/Nm3,and there was no slagging and high temperature corrosion in the furnace.By analyzing the fly ash characteristics of high alkali coals,the influence of high alkali on the denitrification system was learnt.It is found that the high alkaline oxide in the high alkali coal ash is easy to cause the fly ash particles to have strong viscosity,and it is easy to form large particles of fly ash.Firstly,the research ontechnologies for the prevention of fly-ash blockageshowed that the components including the ash baffle installed at the economizer outlet can effectively capture the large ash particles in flue gas,and the collection rate of the ash hopper at the lower end of economizer is in direct proportion to the particle size,and reduce the ash deposit area by 63%,and reduce NOxemission from the original 100 mg/Nm3(@O2=6%)to 50 mg/Nm3(@O2=6%),and has little effect on the system pressure resistance.Secondly,by optimizing the flow field in the denitration zone,it was found that although the flue gas flow velocity fluctuated at the turning of flow direction and the change of cross-sectional area after the installation of the guide vanes,the fluctuation range was less than 15%at BMCR,and the angle of the flue gas entering the catalyst was less thaną10°from the vertical direction,which basically solved the problems caused by the uneven distribution of the flow field,and the maximum pressure drop of the denitrification system was not higher than 1000 Pa.Finallythrough the SCR'scatalyst activity test,it was found that the catalyst V-B/Ti has good alkali-resistance performance and good reaction selective performance.The technology was verified using a 350 MW coal-fired unit.The results showed that the NOxemission of the boiler was lower than 50 mg/Nm3(@O2=6%),and the deviation of NOxconcentration on both sides of the denitration zonecan be controlled within 5%.By combining theoretical analysis with pilot tests,the influence of important parameters such as slurry pH value,liquid/gas ratio and inlet SO2 concentration on desulfurization efficiency was studied.It was found that with the continuous increase of slurry pH value,the rate of desulfurization efficiency growing rate clearly drops;with the increase of liquid/gas ratio,the desulfurization efficiency increases rapidly;with the increase of inlet SO2 concentration,the desulfurization efficiency measured in pilot testsreduces.In order to meet the ultra-low emission requirements of medium and high sulfur coals,the wet limestone gypsum self-adaptive sieve plate technology and zonal controlled pH value high-efficiency desulfurization technology were developed.Industrial verification tests of these technologies were carried out in the coal-fired unit ultra-low emission project.The results showed that the desulfurization efficiency can reach 98.94%.High-efficiency desulfurization is realized.The results of the high sulfur/high alkali coal-fired unit demonstration projectshowed that by adopting technologies for low NOx combustion,high-efficiency denitrification and high-efficiency desulfurization,the problems of water wall corrosion and slagging,catalyst blockage,and low desulfurization efficiency,existing in burning high sulfur/high alkali coals in China can be resolved.The emission concentration of dust,SO2 and NOx of boiler unit shall not exceed 5 mg/Nm3(@O2=6%),35 mg/Nm3(@O2=6%)and 50 mg/Nm3(@O2=6%),so as to meet the requirements of ultra-low emission of boilers. |
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