Numerical Calculation Of Bypath Flue Reform For SCR Denitrification System On 600MW Units

Nitrogen oxides is one of the main sources of atmospheric pollutants,which can cause a series of environmental problems.With the introduction of various national environmental protection policies,most thermal power units,especially coal-fired power units,have equipped low-nitrogen combustions and flue gas nitrogen oxides reduction devices.In several flue gas nitrogen oxide reduction technologies,SCR(selective catalytic reduction)technology has become an extensive technology for thermal power units because of its high denitrification efficiency,better technological maturity and applicability.In addition,due to the structural reform of coal-fired power on the supply side,some backward coal-fired power units have been eliminated and the high-quality production capacity is released.Under the background of national energy structure adjustment and thermal power generation making way for renewable energy generation,it has become a trend for coal-fired power units to participate in deep peak-regulating operation.While 600 MW units,as the main coal-fired power units in China,will gradually operate under low load conditions to participate in the Ancillary Services Market.However,the temperature of flue gas in SCR Denitrification System can't maintain under low load.Especially when the load is reduced to 40%,the SCR flue gas temperature drops below 280 ? and the denitrification system has to be out of operation.At this time,the concentration of NOx produced by the boilers is higher than that under rated load.With more stringent national requirements issued on atmospheric pollutant emission,it is necessary to improve the load adaptability of SCR system by increasing the flue gas temperature at the entrance of SCR under low load conditions in order to meet the requirements of NOx emission limits within full load operation range,including deep peak load regulation.By comparing various wide-load denitrification schemes,this paper chooses the flue bypass scheme as the research object because of its short transformation cycle,small investment,strong applicability and high value of flue gas temperature increasing.The results of numerical calculation can forecast the actual effect in engineering design and renovation,and the numerical calculation is economical and feasible.However.there are few related numerical calculation cases reported in the reference materials about this kind of engineering renovation scheme.Therefore,combined with the actual operation data of a 600 MW unit under 40%load,the numerical calculation method is used to calculate several parameters in the flue bypass scheme,including the high temperature flue gas extraction position,the flue gas extraction ratio,bypass flue size and mixing angle of high and low temperature flue gas.And the optimal scheme selected is implemented in the unit.By comparing the numerical calculation results with the reconstruction effect,the following main conclusions are drawn:(1)The location of high-temperature flue gas extraction is selected at the entrance of low-temperature superheater with higher flue temperature,which can achieve the desired effect with the premise of less influence on the flue gas flow in the main flue.When the proportion of high-temperature flue gas is 40%,the flue gas temperature on the surface of the three layers of denitrification catalyst are 324.12 C,324.12 C and 324.10? respectively.The average temperature is higher than 320 ?,which meets the requirements of the denitrification catalyst for flue gas temperature and ensures the denitrification's safety under low load.(2)The sectional dimension of bypass flue is determined to be 7000mm x 1200mm.In order to reduce the influence of the uniformity of flue gas velocity distribution on the catalyst surface,the flue gas mixing position is arranged in front of the diversion plates of the gradual expansion section of the horizontal flue,and the cross-section length of the bypass flue is 1200 mm in accordance with the design criteria and the bypass flue gas flow rate.The cross-section width is calculated to be 7000 mm.(3)When the incident angle of high temperature flue gas is 60 degrees,the mixing degree of high and low temperature flue gas is better.At this time,the relative standard deviation coefficients of surface velocity on three layers of catalyst are 9.7%,4.2%and 2.1%,respectively.The uniformity of flue gas velocity distribution on catalyst surface meets the standard requirements.At the same time,the pressure field on the surface of the three layers of catalyst is uniform,and the total pressure loss is 150 Pa,which meets the standard requirements.(4)After retrofitting,when the bypass flue is put into operation,the distribution of temperature field and velocity field in SCR denitrification system is in good agreement with the numerical simulation results,and the operation effect is expected.After the modification of bypass flue,the average temperature of flue gas in SCR denitrification system was 325.9 C under 40%BMCR load,and the relative standard deviation coefficient of flue gas velocity on the surface of the first catalyst was 10.8%.The results were basically in agreement with the simulation results,which ensured the normal operation of denitrification system at low load.In a word,using the scheme and data provided by the simulation calculation to guide the actual engineering transformation,the unit runs well and the denitrification system works normally.