Key Technology Study Of SCR Flue Gas DeNOx System In Large Power Plant Boiler

Nitrogen oxides (NOx) are recognized as a major source of air pollution in the world. The NOx emission is becoming greater and greater with the rapid development of power industry in recent years. The selective catalytic reduction (SCR) denitrification technology has been widely used because of the highest efficiency and is regarded as the first choice among the technologies for removing NOx in flue gas at present. It will be used more and more widely in our country as the NOx emission standard becomes more strict. However, the SCR technology in our country has just started, the technology imported from overseas leads to some problems such as high cost, low availability and unstable performance. Therefore, it is of significance to develop the SCR denitrification technology that possesses independent intellectual property. In this paper, study on key technology of SCR denitrification based on a practical SCR project in a660MW unit of coal fire plant was carried out, and the main results are shown as following:1. The correlation between preparation parameters and catalyst forming as well as DeNOx efficiency was studied, and effect of binder, pore former, drying condition and calcination temperature on catalyst forming and microstructure is determined. The results show that increase of micropore between1-5nm can offer more inner surface area and active sites which can improve reaction rate; uniform evaporation of catalyst can avoid catalyst cracking in drying process; calcination temperature has a great influence on crystal morphology and surface area. The best preparation condition of monolith catalyst is adding0.85%A,0.75%B and0.5%D, drying at30℃for48h,40℃for24h and60℃for24h. and calcining at550℃. At the conventional SCR reaction temperature (300～400℃),90%of NO conversion was obtained over the monolith catalysts.2. The effect of flue gas parameters on the DeNOx performance of monolith catalysts was studied. In the actual operating condition, the SCR reaction is first-order for NO. zero-order for O2when the O2concentration is more than2%and zero-order for NH3when the molar ratio of NH3to NO is no less than1. H2O has an inhibiting effect on SCR reaction, however, when H2O concentration is more than6%, change of H2O concentration has no effect on catalyst activity and SCR reaction is zero-order for H2O. SO2can be adsorbed on the catalyst surface to form new sulfate, then the surface acidity is enhanced and the catalyst activity is improved. The promotion effect of SO2on catalyst activity is higher than the inhibiting effect of H2O, and the NO conversion can be kept stable at99%in the condition of10%H2O,1000ppm SO2and molar ratio of NH3/NO equal to1. At last the kinetic parameters such as reaction rate constant and activation energy in the SCR reaction is determined.3. The velocity, concentration and pressure field of SCR reactor was studied using combination method of cold test and numerical simulation, and the guiding plates of variable cross-section at AIG upstream was optimization designed. The results show that the guiding plates with different installation angles plays an important role in guiding the flue gas through the variable cross-section, and after optimization design, the velocity distribution Cvl at AIG upstream decreases from17.4%to12.7%. Then based on computational fluid dynamics (CFD) technology and structuralized-non-structuralized hybrid grid technology, the standard turbulent model which describe the flue gas turbulent motion in SCR reactor was simulated with the SIMPLE calculation method. The results show that when flow guide device remains unchangeable in the flue from AIG to catalyst, increase of the velocity distribution uniformity at AIG inlet can enhance the velocity and concentration distribution uniformity at catalyst inlet effectively.4. The10000Nm3/h pilot test-bed of SCR system was established, and the flue gas system, ammonia supply and preparation system and reactor system are designed. The flow field of SCR system was studied adopting numerical simulation method, and the activity of monolith catalyst was tested. In pilot system, the maximum NO conversion can reach97%as increase of molar ratio of NH3/NO; the SCR reaction is still first-order for NO in the practical condition; the DeNOx efficiency of the catalyst decreases when increasing gas flow, the activity of the catalyst can reach86.7%of initial activity after5500h steady operation.5. The guiding device is designed and simulated using numerical simulation method for SCR system in the practical project, and the results have been used in the project. SCR system operation test shows that the system is stable, all of equipments are in normal operation and the catalyst activity is high. The influence of adjustment of ammonia flow in ammonia injection pipe on the uniformity of ammonia concentration and DeNOx activity was studied, and the optimization condition can make the flue gas and ammonia well-mixed and increase the DeNOx efficiency.