The preparation of reducing agent by liquid ammonia method and ammonia water method has the characteristics of simple process, low energy consumption and convenient maintenance, but both liquid ammonia and ammonia water are toxic substances, and their transportation and storage are major hazards, which have great safety hazards. When liquid ammonia method is used as reducing agent, supporting documents such as safety specifications, transportation line permission, storage safety evaluation and environmental impact assessment certification should be designed, and the use of dangerous chemicals should be registered in relevant management departments;
When urea is used to prepare reductant, it is very safe to transport, store and finally make reductant. Although the process is relatively complicated and the investment and operation costs are relatively high, it can ensure the safety and reliability of the ammonia source. In big cities, densely populated areas and places close to drinking water sources, more and more power plant denitration systems tend to choose safe urea as reducing agent.
In order to develop denitration technology, reduce denitration cost and ensure the safe use of denitration system, our company is committed to developing urea pyrolysis ammonia production technology with independent intellectual property rights, which has been approved by the State Patent Office and has been applied to denitration devices of 100 MW ~ 600 MW units. The successful case shows that the technical indexes of this technology are stable and reliable.
The ammonia production technology of urea pyrolysis in our company uses high-temperature air or flue gas as a heat source to quickly decompose atomized urea aqueous solution into ammonia gas, and low-concentration ammonia gas as a reducing agent enters the flue to mix with flue gas and then enters the SCR reactor, and nitrogen oxides are reduced into harmless nitrogen and water under the action of a catalyst.
Urea pyrolysis ammonia production system generally includes urea storage room, bucket elevator, urea dissolving tank and storage tank, feed pump, urea solution circulating conveying device, electric heater, metering and distribution device, adiabatic decomposition room (including ejector), control device and other equipment. Bagged urea granules are stored in a urea storage room and transported to a dissolving tank by a bucket elevator. The dried urea is dissolved into a urea solution with a mass concentration of 40%~60% by deionized water and transported to a urea solution storage tank by a urea solution delivery pump. The hot primary air provided by the air preheater is heated to about 600℃ by an electric heating device (either directly by air or by various heat sources such as fuel oil, natural gas and high-temperature steam) and enters the adiabatic decomposition chamber. Urea solution enters the adiabatic decomposition chamber in an atomized state through a circulating conveying device, a metering and distributing device, an atomizing nozzle and the like. It decomposes at high temperature to generate NH3, H2O and CO2, and the decomposed products are sprayed into the flue at the front end of the denitration system through the ammonia spraying grid.
The control device ensures that the supply of reducing agent meets the requirements of different loads and denitration efficiency of the boiler. SCR and SNCR are the same, and both react with nitrogen oxides at a certain temperature to generate harmless nitrogen and water. The difference is that the former has the participation of catalyst, which reduces the reaction temperature window (from 800 ~ 1 100℃ without catalyst to 300 ~ 400℃ or lower) and improves the reaction.
The traditional SNCR/SCR mixing process has two reaction zones. Through the injection system arranged on the wall of the boiler, firstly, the reducing agent is injected into the first reaction zone-the boiler hearth, and the urea solution and NO in the flue gas have a non-catalytic reduction reaction at high temperature, thus realizing primary denitrification. The escaping ammonia generated by the high temperature of the boiler is mixed with the boiler flue gas and enters the second reaction zone -SCR reactor. Under the action of catalyst, ammonia and nitrogen oxides undergo chemical reduction reaction to produce harmless nitrogen and water.
The SNCR/SCR mixed denitration technology of our company is based on the traditional mixed process, adopting special urea injection layout design and flow field mixing technology, which can better control the urea injection mode in SNCR section, improve the distribution of ammonia escaping from SNCR, reduce the consumption of reductant, and be more sensitive to the detection and control of terminal emission value of nitrogen oxides. It can effectively eliminate the imbalance of nitrogen oxide emissions from the left and right sides of flue gas in the traditional mixing process, and achieve the purpose of high efficiency and low consumption of denitration process. It is an improved mixed denitration process. SNCR technology, namely selective non-catalytic reduction technology, is one of the main flue gas denitrification technologies. In the narrow temperature range of 850~ 1 100℃ in the furnace, urea, as a reducing agent, can selectively reduce nitrogen oxides in flue gas without catalyst, and basically does not react with O2 in flue gas. Different reductants have different reaction temperature ranges, which are called temperature windows. The optimum temperature range of NH3 reaction is 850 ~ 1 100℃. When the reaction temperature is too high, the reduction rate of nitrogen oxides will decrease due to the decomposition of ammonia. On the other hand, when the reaction temperature is too low, the escape of ammonia will increase, which will also reduce the reduction rate of nitrogen oxides. The key of SNCR process technology is that the reductant injection system must inject reductant into the most effective temperature window in the furnace as much as possible, that is, to ensure that the injected reductant can be well mixed with flue gas at a suitable temperature as much as possible, so that on the one hand, the utilization rate of reductant can be improved, and on the other hand, it can be controlled to obtain less ammonia escape. Compared with SCR technology, SNCR technology has no expensive denitration catalyst used in SCR technology, and its technical advantages are low investment and operation cost and low SO2/SO3 conversion rate. The disadvantage of SNCR is that the denitrification efficiency is relatively low. Generally, the SNCR denitrification efficiency of large boilers is below 40%.
SNCR technology of our company adopts accurate flow field analysis technology, and determines the boundary conditions of key parameters according to different operating conditions of various furnace types. After detailed CFD simulation and CKM calculation, the accurate process design scheme and layout are formulated. Our design experts have rich experience in SNCR engineering design and can provide users with targeted solutions and professional services. At the same time, our company has adopted a new type of injector, which meets the needs of process design to improve denitrification efficiency.