Numerical Simulation On The Rotary Air Preheater As An SCR Reactor

In our country, coal is the most important foundation of our energy and the most consumed form of energy; in addition, coal is the main fuel for thermal power, and thermal power generation still dominates the country’s total installed capacity. So pollution from coal combustion in thermal power is one of the main pollution sources of our country and solving the problem of pollution caused by burning coal is also a top priority project of controlling environmental pollution; the nitrogen oxides are one of the major atmospheric pollutants.At present, active coal-fired power plants are building large-scale denitration device and the SCR denitration device between the economizer and the air preheater is the main denitration way. But there are some negative effects of SCR technology, which is expensive and has complex systems, various problems caused by the escape of ammonia; in particular, it will increase congestion and corrosion hazard of air preheater. In addition, the presence of SCR equipment will also increase resistance of the flue gas, so the flue negative pressure of air preheater will increase, causing the air preheater leakage to increase. Currently, SCR catalysts have a variety of catalysts. The most widely used catalysts are high-temperature catalysts, low temperature catalysts are rarely used with very rich research. The range of flue gas temperature in air preheater is 130～370℃, which can meet the high-temperature catalysts’ activity temperature window and also can meet the low-temperature catalysis’ activity temperature window. Based on the above issues, the study of denitration in the air preheater is proposed. The idea will both carry out the denitration process and heat recovery in the air preheater, so that the plant can simplify its structure layout and save a space, but also the negative impacts caused by the denitration system can be solved.In this paper, the study on the conception of denitration in the air preheater mainly includes the following inquiries.(1) Selecting the rotary air preheater as the model for the air preheater denitration. with the heat storage element as the backing material, which realizes the denitration catalyst coated on the surface.(2) For the temperature field distribution of the rotary air preheater, Fluent software is adopted to make numerical simulation. The rotor of the air preheater is simplified as a porous medium model during the numerical simulation. It calculates the temperature curves of the heat storage elements in different locations. The wall temperature of the heat storage element rises linearly and then decreased linearly during endothermic process from the flue gas side and exothermic process to the primary and secondary air. The study of temperature distribution in the rotary air preheater can provide the basis for selecting coated catalyst.(3) The two-dimensional flow models of types of DU3 and NF6 plates of the rotary air preheater are simulated. For different temperature range, high temperature heat storage element selects cerium-manganese-titanium catalyst (molar ratio of Mn: Ce:Ti is 40:13:100) for coating to analyze the heat transfer performances of the heat storage element with coating thicknesses of 0.05mm-0.20mm. Medium temperature heat storage element selects cerium-manganese-titanium catalyst (molar ratio of Mn: Ce:Ti is 40:7:100) for coating to analyze the heat transfer performances of the heat storage element with coating thicknesses of 0.05mm-0.20mm. Low temperature heat storage element also selects cerium-manganese-titanium catalyst (molar ratio of Mn: Ce:Ti is 40:7:100) for coating to analyze the heat transfer performances of the heat storage element with coating thicknesses of 0.05mm-0.30mm. Through the analysis of the heat transfer performance of the heat storage elements coated with catalysts, it can be seen that catalytic weaken the heat transfer performance of heat storage elements, but the overall impact is small. When the thickness of catalyst is 0.20mm and the transient computation time is 5s, the average temperature at the exit of high temperature, medium temperature and low temperature heat storage element increased 0.06%,0.0908% and 0.1217% respectively. So the denitration catalyst can be coated on the surface of the air preheater heat storage element to achieve the aim of denitration and heat transfer in a device.(4) The structural adjustment of the rotary air preheater for denitration. The catalyst coated on the heat storage element makes rotor’s porosity decrease. For DU3 model when the catalyst coating thickness of 0.10mm～0.20mm, the porosity is reduced 3.33%-6.25% correspondingly. To realize the denitration in the rotary air preheater while not affecting the overall heat transfer efficiency, it can increase the overall height of the heat storage element (keeping the original diameter of the air preheater rotor) to adjust the change. For the DU3 model of heat storage element, the height of the high temperature model (the thickness of catalyst is 0.20mm) shall be increased by about 7.5267% and the height of medium temperature section (the thickness of catalyst is 0.20mm) shall be increased by about 8.0808%. For NF6 plate type of low-temperature (the thickness of catalyst is 0.30mm), the height should be increased correspondingly by 13.03%.