Effect of SCR denitration on boiler operation

In order to meet the increasingly stringent emission requirements, thermal power units have been denitration transformation, of which, catalytic reduction denitration SCR device, with its mature technology and stable denitrification efficiency, has been widely used in China, while paying attention to denitrification efficiency , The impact of SCR denitrator on the economics and safety of the boiler system should also be highlighted. Unit overview Datang Xinyang Huayu Power Generation Co., Ltd. Phase II 2 × 660MW ultra-supercritical coal-fired power generation units # 3 and # 4 were put into operation in March and October 2009 respectively. Boiler is a boiler (Group) Co., Ltd. produced by DG2000 / 26.15-II2 type once again reheat, ultra-supercritical parameters of transformer operation, with built-in bypass system Bunsen DC boiler. The form of a single hearth, tail double flue structure, hedge combustion before and after the wall. The boiler is equipped with a medium-speed pulverizer and a direct air-blast direct blow system. Each furnace is equipped with 6 medium speed coal pulverizers. B-MCR combustion conditions design coal, 5 transport 1 prepared, fineness of coal powder R90 = 16%, coal uniformity coefficient of 1.05. Each pulverizer supplies a gas-air mixture for a layer of six burners. Three boiler DBC-OPCC-Ⅰ burners are arranged on the front and back walls of the boiler. There are 6 burners on each floor, with a total of 36 burners (the bottom layer of the back wall is a small oil burner). Each furnace installed two three points Junker air preheater, later installed SCR denitration device, denitrification device using urea pyrolysis to produce ammonia gas reductant, selective catalytic reduction (SCR) flue gas out of stock process, The ammonia (NH3) as a reducing agent is sprayed into the flue gas to make the reducing agent react with the NOx in the flue gas to generate non-harmful nitrogen (N2) and water (H2O) so as to achieve the purpose of removing nitrogen oxides , The flue gas treatment capacity of each denitrification device is 100% of the flue gas volume in the corresponding boiler ECR condition, and the denitrification efficiency is guaranteed to be ≥80% when the NOx content in the inlet of the denitrification device is less than 450mg / Nm3; the concentration of NOx emission does not exceed 100mg / Nm3 , Can strictly meet the environmental emission standards, but after the installation of SCR denitration device on the safe and economic operation of the boiler had a greater impact. Effect of SCR denitrification on the operation of ultra supercritical Impact of flue resistance The installation of the SCR device increases the resistance of the boiler flue gas system. The increase in resistance is the biggest difference from the combustion system when there is no SCR device. Increased resistance is generally divided into increased flue resistance, SCR reactor resistance and air preheater increased resistance in three parts. After adding an SCR plant to an ultra-supercritical boiler, the resistance of the SCR reactor is about 500 Pa and the air preheater increases the resistance to about 200 Pa. The entire boiler smoke system resistance is 2000 ~ 3000Pa (BMCR conditions, 330 ~ 660MW). As can be seen, the SCR device can have a significant impact on the flue-resistance characteristics of the boiler, while the fouling of catalysts and air preheaters within the SCR aggravates this effect. The catalyst in the boiler SCR device is a honeycomb, the aperture is 7mm, it is easy to form the fouling blockage. As the running time increases, the pressure loss increases. The NH3 sprayed from the ammonia injection device reacts with NOx and the remaining NH3 will react with SO3 and water vapor to form a strong, corrosive compound of ammonium hydrogen sulfate (NH4HSO4). The reaction equation is as follows: NH3 + SO3 + H2O-NH4HSO4 Ammonium hydrogen sulfate will be attached to the fly ash surface, the surface of the air preheater high temperature and low temperature deposition, so that the air preheater heat element dirty blockage, further increasing the resistance of the flue. Two supercritical boiler factory in the SCR unit put into operation six months after the air preheater resistance differential pressure than just put into an increase of 30%, about 500Pa, resulting in boiler system prone to greater pressure fluctuations. See below. Larger pressure fluctuations in the boiler system will affect the stability of the combustion chamber, the economy of combustion, and even increase the possibility of furnace explosion. At the same time induced draft head pressure will be greatly improved. Not conducive to the safe and stable operation of the boiler. Effect on boiler thermal efficiency The installation of SCR denitrator reduces boiler thermal efficiency. The SCR denitrification device is equipped with a reactor and its connecting flue so as to make the flue longer to increase the heat radiation area of ​​the boiler so that the heat loss of the boiler is increased and the flue gas temperature after the flue gas passes through the ultra-supercritical boiler denitration system is reduced by 5 ℃ About the efficiency of the boiler will have a certain impact. Air preheater heat exchanger clogging the air preheater exhaust temperature. Ammonia and sulfur trioxide react to form hydrogen sulfate. Ammonia bisulfate in the temperature of 180 ~ 200 ℃ environment was "nose" like sticky, therefore, in the air preheater high temperature and low temperature, the dust in the flue gas and ammonia bisulfate easily sticky Attached to the air preheater heat exchanger surface, the air preheater heat element dirty, reducing the air preheater heat exchanger, so that the exhaust gas temperature increases, the boiler efficiency is reduced. Air preheater leakage rate increases. After the supercritical boiler flue gas through the SCR device pressure drop increased by about 500Pa, due to the installation of SCR device air preheater blockage, flue gas through the air preheater to increase the resistance of about 500Pa, the entire flue gas side of the resistance increased by about 1kPa, In order to balance the pressure inside the furnace, the output of the induced draft fan will increase, resulting in the reduction of the pressure of the flue gas inside the air preheater and the increase of the pressure difference between the air preheater and the flue gas side, resulting in the increase of the air leakage rate of the air preheater and the decrease of the boiler efficiency . Impact on the air preheater Relatively speaking, SCR denitration device on the air preheater more prominent. The main reason is the corrosion and adhesion of ammonium bisulfate. Ammonia bisulfate is adhered to the heat exchange element of the air preheater together with the dust, which not only reduces the heat exchange effect, but also low-temperature corrosion occurs in the low-temperature section of the air preheater and fouling of the air preheater. Our SCR denitration plant ammonia escape rate designed for ≤ 2.5mg / Nm3, escape rate exceeds the design value will result in a large number of ammonia sulfate generated, resulting in severe air preheater blockage, which will result in increased suction fan power consumption, a fan The pressure of the mother pipe fluctuates greatly and so on.