Experimental Research And Application Of Optimization And Adjustment Of SCR Device Under Ultra-low Emission Condition In Coal-fired Power Station

The conventional retrofit technology of NO_x ultra-low emission for coal-fired units is to control NO_x production by low nitrogen combustion technology as far as possible under the premise of ensuring the operation economy and safety of coal-fired units,the NO_x ultra-low emission can be realized by increasing the capacity of denitrification unit.With the passage of operation time,the relative deviation of N O_x concentration online monitoring data between reactor outlet and total discharge port is large gradually in SCR denitrification unit.The ammonia escape exceeds the standard,which leads to the blockage of the downstream air preheater and the increase of resistance.Some units NO_x emission concentration is difficult to control,can not meet the discharge and other typical problems.The fundamental reason is that the inlet velocity distribution of SCR denitrification unit is not uniform and the molar ratio of NH₃/NO_x is deviated,which leads to the uneven catalytic reduction of NO _x in the reactor.In order to make the NO_x emission concentration and ammonia escape amount stable and controllable after the ultra-low emission transformation,this paper takes the SCR denitrification device which completes the ultra-low emission transformation as the research object,and studies the above typical problems of the SCR denitrification unit.Firstly,the comprehensive performance of ammonia injection grille in selected SCR denitrification unit is studied by using CFD numerical simulation technology,and the theoretical basis of uniform mixing realizability of NH₃ and NO_x after adjustment of ammonia injection grille is demonstrated.Secondly,on the basis of field measurement,the existing problems are summarized and analyzed by measuring the NO_x,NH₃ concentration field and velocity field at the entrance and exit of SCR denitrification unit,combined with the structural characteristics of the reactor.Thirdly,according to the above research,the optimization adjustment strategy of ammonia injection is worked out and its effectiveness is verified by field practice.It has been proved by practice that the outlet NO_x can be evenly distributed by adjusting the opening of the ammonia injection grille regulating valve.Taking the research object as an example,under the load 550MW load condition,the average relative standard deviation of NO_x distribution at the outlet of both sides of reactor A was 47.05%and 13.22%respectively before and after optimization adjustment,and decreased 33.83%after optimization.After optimization,the average outlet value of SCR denitrification unit is close to the CEMS indication value of the same position and the CEMS indication value of the total discharge port,and the relative deviation is 5.9%,5.7%and 7.3%,respectively.Therefore,the CEMS indication value at the outlet of SCR denitrification unit can be used as the true value for ammonia injection regulation.While the problem of CEMS indication distortion is solved,the NO_x emission concentration at the total discharge port of coal-fired unit can be adjusted and controlled,and the amount of ammonia escape from the outlet of SCR denitrification unit can reach the standard.Finally,according to the test data,the relationship between NH₃ and NO_x at the outlet of SCR denitrification unit is drawn,and it is evaluated that when the ammonia escape volume concentration is not greater than 3ppm,the minimum control value of NO_x concentration is 20mg/m³,but in order to reduce the blockage risk of air preheater,It is recommended that the optimal control value of NO_x concentration is 40 mg·m³,which not only meets the emission requirements,but also ensures that the concentration of ammonia escape volume is less than 1 ppm.The results show that the uneven flow rate at the entrance of SCR denitrification unit,the uneven distribution of NO_x concentration and the relative deviation of NH₃/NO_x molar ratio at different positions are the main reasons for the poor NO_x uniformity and high ammonia escape at the outlet of SCR denitrification unit.According to the optimization and adjustment of ammonia injection,the ammonia air mixed gas flow rate of each ammonia injection branch can be arranged according to the inlet flue gas parameters of SCR denitrification unit,the uniformity of inlet NH₃/NO_x molar ratio can be improved,the NO_x at outlet can be evenly distributed,and the amount of ammonia escape can be reduced.The CEMS monitoring values of NO_x concentration at the outlet of the reactor are basically consistent with those of the total discharge port of the chimney.