| Since NOx released from thermal power plants is one of the main sources of NOx emissions, it is urgent to suppress the production of NOx in coal combustion process for relieving the stress on the environment.Until now, several urea-based post-combustion NOx control technologies have been proposed such as selective catalytic reduction (SCR), selective Non-catalytic reduction (SNCR), and Hybrid SNCR/SCR. Especially for the hybrid SNCR/SCR is an updated technology combining the SCR, the SNCR and the pyrolysis of urea technology. Therefore, it has several advantages including reduction of NOx concentration in the entrance of SCR reactor, and elimination of unnecessary equipment. Besides, this hybrid technology is specially suitable for the coal-fired boiler that has low NOx concentration and can not use liquid ammonia as reductant. In this study, both experimental and numerical studies were performed to solve key problems of the application of the hybrid SCR/SNCR in 300MW coal-fired power plants, focusing on the characteristics of the multiphase flow and optimized design for uniform mixing in both SCR reactor and the tail flue gas duct of the SNCR reactor. Several important experimental data and design parameters were obtained, which could facilitate future industrial application of the hybrid SCR/SNCR.Considering that the urea solution should spray into the flue gas with high temperature in both the furnace and the subsequent tail flue gas duct, the pyrolysis process of urea solution spray droplet in the hot flue was studied by experiments and simulationswith the purpose of investigating the influences of temperature, residence time and oxygen concentration on the NH3 generation. The results revealed that there exists an effective temperature range for the pyrolysis of urea from 450K to 730K. The pyrolysis ration of the urea at the upper limit of the temperature range could reach 98% with 85% conversion efficiency of NH3. and the characters of denitration catalyst on the catalytic hydrolysis of HNCO. All the findings provide the basis and reference for the design of this mixed SNCR/SCR denitration system. the thermal decomposition of urea is basically completed, while the temperature exceeds 800℃, due to the REDOX reaction, the concentration of NH3 and HNCO begin to reduce, accompanied by a small amount of NO and N2O formation, and its concentration increases with the rising temperature. The resulting SNCR/SCR denitration system for ammonia injection location (to urea pyrolysis chamber flue area) of the reasonable basis. In flue gas, the thermal decomposition product HNCO was not completely converted to NH3, A catalytic reaction module test device was established at the outlet of the urea pyrolysis furnace in order to observe the catalytic hydrolysis reaction of the catalyst for HNCO, The generated NH3 from the reaction of water vapor and HNCO are accelerated with the denitration catalyst in low temperature. What’s more, the generation rate of HNCO increases with the rising temperature, and when the temperature reaches 300℃, the conversion rate of HNCO close to 95%. The flue gas temperature at the exit of coal-fired boiler economizer is between 300 and 420℃, which is full compliance with HNCO hydrolysis reaction conditions.In order to study the influencing factors of gas solid two phase flow and uniform mixing in SNCR/SCR combined denitrification system, the cold test device of the hybrid SNCR/SCR denitration system was built. The reducing agent is replaced by CO gas. When the SNCR/SCR denitration system channel are in a state of the flow self-modeling, the tests and analysis of gas-solid flow characteristics were conducted at different operating conditions, which including the reductant concentration distribution, the pressure drop, the gas velocity and the particle concentration distribution. The results show that the reductant concentration distribution uniformity is poor in the tail flue duct and the reactor, because the SNCR injection nozzles are mainly gathered in front wall of the furnace. Hence, the position of the ammonia spray gun is placed more closely to the side wall in the steering chamber, The L type non uniform distribution of the large concentration gradient is presented in the boiler tail flue and the reactor, the reductant concentration distribution is more uniform, but the effect is not obvious. In this paper, a new compound X type mixer is designed to improve the implements the flue gas composition in the long and narrow space inside the long distance migration and partition of strong mixing effect. The well concentration distribution of ammonia nitrogen was obtained in the first layer of the SCR reactor. Besides, the flue gas from the furnace is of high dust content, and experience a number of 90 degree elbow, so the standard deviation coefficient of the fly ash concentration and the flue gas flow rate in the inlet section of the first layer of the reactor are all large. Therefore, the reactor with ceiling circular turbulence pole structure is designed, to improve uniformity of the first layer of catalyst inlet velocity field distribution, also, it is beneficial to the uniform distribution of fly ash particles.Combined with thermal decomposition characteristics of urea solution and the cold experimental results, a 3-D full-size numerical simulation of a 300 MW hybrid SNCR/SCR denitration system is established to gain the velocity field, pressure field, and the concentration field. By utilizing the injection of urea solution to replace the regular urea solution reactor, the optimization study on injection method of NH3 and coupled control scheme with urea solution in SNCR system are conducted. Thus, the instability of sole SNCR denitration system and the excessive NH3 concentration in the outlet of SCR reactor do not occur. The results show that the denitration effect for the the SNCR system is the best, when the coal boiler is in full capacity, the average size of atomized droplets is 0.4 mm, and the initial injection velocity of droplets is 30 m/s. The mole ratio of ammonia to nitrogen has a nonlinear relationship with the SNCR denitration rate. Besides, when the mole ratio of ammonia to nitrogen is greater than 1.2, the influence on the SNCR denitration rate are not obvious. And the distribution of escaping NH3 from the SNCR reactor is in agreement with the cold experimental result. Further, the design parameters of the new composite X type mixer used at the entrance of SCR and the ceiling circular turbulence pole structure of the reactor are achieved, to acquire a more uniform distribution of the catalyst inlet velocity, NH3 concentration and the fly ash particle concentration.Finally, the integrated design and development of the hybrid SNCR/SCR denitration system is finished, including the flue gas system, urea station system, measuring module, SNCR injection module, ammonia spray gun module as well as the guide plate, mixer, turbulence bar. The thermal experiments are undertaken in the hybrid SNCR/SCR denitration system. It is found that, for the single SNCR system, when the mole ratio of ammonia to nitrogen reaches 1.12, the SNCR denitration efficiency could surpass 33% and the concentration of escaping ammonia is 25 ppm. The SNCR denitration efficiency is affected by the injection temperature, droplet diameter and injection speed. With the installation of the new compound X type mixer at the entrance of SCR reactor, the supplement of ammonia injections on the side wall in the boiler, and the addition of multiple sets of circular turbulence bar component on the reactor ceiling, the NH3 concentration distribution can distribute tend to rather uniform in the hybrid SNCR/SCR denitration system. When the total mole ratio of ammonia to nitrogen of the hybrid SNCR/SCR denitration system is 1.418, the whole SNCR/SCR denitration efficiency reaches 85.96, including the SNCR denitration efficiency of 35% and the SCR denitration efficiency of 78.4%. Besides, NOx concentration of outlet of the economizer of A and B side is different after SNCR reaction, and the A side is greater than the B side, which the calculation is in accordance with the experimental values. The urea solution spraying into the boiler has a great impact on the boiler efficiency. When the urea solution concentration is 5%, the thermal efficiency of this boiler is reduced by 0.8114%; when the urea solution concentration is 10%, the thermal efficiency of this boiler is reduced by 0.4148%. The escaping NH3 leads to the increase of ammonia content in the fly ash, and also can react with the SO3 in the flue gas, which cause great influence on the downstream equipment of the boiler. Last but not least, the urea solution droplet should be avoided leaking and contacting with the water wall and heater screens, which can reduce the corrosion of heat exchange components and guarantee the security and stable operation of the boiler. |
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