SCR flue gas denitrification technology for kilns
1、 Overview
1. Project Overview
Glass kiln. The current NOx emissions cannot meet the new standard of less than 300mg/Nm3. In response to the overall deployment of the Ministry of Environmental Protection on air pollution control, the glass kiln will undergo denitrification transformation.
2. Design principles
This preliminary technical proposal is applicable to the selective catalytic reduction (SCR) flue gas denitrification device for glass kilns. The overall design principles of flue gas denitrification technology include:
(1) Adopting selective catalytic reduction denitrification (SCR) process.
(2) Two sets of furnace reducing agent preparation systems are shared.
(3) Use 20% concentration finished ammonia water purchased externally as a denitrification reducing agent.
(4) The control system of the denitrification device adopts DCS (shared control room and hardware with the DCS of the existing denitrification system).
(5) Under the condition of ensuring the temperature of the reducing agent injection zone is between 285 ℃ and 420 ℃, and the SCR inlet concentration is not higher than 4950mg/Nm3 (dry basis, 10% O2), the denitrification efficiency is not lower than 94%, and the outlet NOx concentration is not higher than 300mg/Nm3.
(6) The NH3 escape amount should be controlled below 10ppm.
(7) The availability rate of the denitrification device is not less than 95%, and the service life is 30 years.
Design parameters
3. Boiler design parameters
3.1 Main equipment and parameters
Rated evaporation capacity (t/h) flue gas volume (Nm3/h) NOx concentration (mg/Nm3) Boiler type: 35000 4950
3.2 Current Power Supply Status
Electric motors of 200kW and above use a voltage of 6kV. Motor power supply voltage: High voltage 6 kV; Low voltage 380 V
4 main design principles determined:
4.1 Denitrification efficiency
When optimizing the conditions, SCR can achieve a denitrification efficiency of about 90%, and during short-term demonstration periods on small and medium-sized boilers, it can achieve a denitrification efficiency of 98%. Typical long-term on-site applications can achieve a NOx removal rate of 90% to 95%.
Therefore, this technical solution is designed with a design efficiency of 94%.
At present, the emission concentration of the boiler is around 4950mg/Nm3, and a design efficiency of 94% can meet the requirements of the atmospheric pollutant emission standards for thermal power plants (GB13223-2011).
4.2 Ammonia escape concentration
This project plans to use ammonia water as a denitrification reducing agent. In practical engineering applications, due to the uneven mixing of NH3 and NOx, as well as the temperature deviation of the reducing agent injection point from the optimal temperature window, NH3 and NOx cannot fully react, resulting in a small amount of ammonia escaping from the reaction device with the flue gas without participating in the reaction. This situation is called ammonia escape.
The escape of ammonia cannot be completely avoided, but it can be controlled. The main reason is that the escaped ammonia is a form of secondary pollution, and another important reason is that the escaped ammonia can cause corrosion and blockage of equipment at the rear of the boiler, including the air preheater. Ammonia escape in denitrification reactions can mainly lead to:
Ammonium bisulfate is generated and deposited in downstream equipment such as air preheaters, causing corrosion and blockage; Causing secondary pollution of aerosols; Increasing NH3 compounds in fly ash has an impact on comprehensive utilization.
During the combustion process of boilers, the majority of elemental sulfur in coal is burned to produce SO2, but a small amount of SO3 is inevitably generated. Although the amount of SO3 generated during combustion is very limited, its impact cannot be underestimated. The reason why SCR has stricter requirements for ammonia escape compared to SNCR denitrification process is because SCR uses a denitrification catalyst, which increases the conversion rate of SO2/SO3. When SO3 in the flue gas reacts with escaped ammonia that does not participate in denitrification reaction, ammonium compounds NH4HSO4 and (NH4) 2SO4 will be generated. NH4HSO4 is liquid at 180-240 ℃ and solid at temperatures below 180 ℃. Ammonium bisulfate has strong corrosiveness and viscosity, which can cause corrosion and damage to boiler tail flue equipment including air preheater, electrostatic precipitator, etc.
In addition, some of this condensate will deposit on the fly ash and some will adhere to the surface of the air preheater, so more than 80% of the escaped ammonia will be adsorbed by the fly ash and enter the dust removal equipment. Research has found that gaseous NH3 is adsorbed onto fly ash in electrostatic precipitators. After passing through the dust collector, a small amount of ammonia will be carried into the downstream FGD.
Therefore, both SCR and SNCR have strict limitations on the escape of ammonia. Usually, when the ammonia escape of SCR is controlled below 10ppm, it will not cause blockage or corrosion on the heating surface of the boiler, and will not have a significant impact on downstream air preheaters, induced draft fans, FGDs, ESP (FF), etc.
Therefore, the ammonia escape in this project plan will be strictly controlled below 10ppm, which will not cause problems such as increased pressure loss and corrosion of the heating surface and dust collector, nor will it affect the comprehensive utilization of fly ash.
5 Technical Requirements
5.1 Scope of this project
Design, equipment supply, installation, system debugging and trial operation, assessment and acceptance, training, etc. of the denitration device (SCR) for glass furnaces.
5.2 Overall requirements for denitrification device
2、 The denitrification device (including all required systems and equipment) must meet at least the following general requirements:
1. Adopting SCR flue gas denitrification technology;
2. When the NOx concentration in the boiler inlet flue gas is 4950mg/Nm3 (10% O2, dry basis), the SCR technology is used to control the NOx concentration in the boiler outlet flue gas below 300mg/Nm3 (10% O2, dry basis).
3. The denitrification device operates effectively within the operating load range of the design temperature of 285 ℃~410 ℃, with a denitrification efficiency of not less than 94%;
4. The denitrification device should be able to start quickly and have good adaptability during load adjustment, and can reliably and stably operate continuously under operating conditions;
5. When the boiler is running, the denitrification device and all auxiliary equipment can be put into operation without any interference to the boiler load and operation mode. The SCR denitrification system increases the flue gas resistance by no more than 600Pa.
6. Under operating conditions, the ammonia escape of the denitrification device is less than 10ppm.
7. Use ammonia water as a denitrification reducing agent, which is directly purchased by the owner. The normal concentration of ammonia water is 20%, but the concentration will fluctuate within the range of 15% -25%. The technical solution can adapt to changes in ammonia water concentration.
8. The electrical loads in the flue gas denitrification project are all low-voltage loads, and only low-voltage distribution devices are installed in the system. The low-voltage system uses a 380V power neutral point ungrounded power supply;
9. The control system of flue gas denitrification project adopts DCS control system, which can operate independently and achieve automatic control of denitrification system. The control objects include: reducing agent flow control system, spray gun mixing control system, cooling water control system, air and air purification control system, temperature monitoring system, etc. The denitrification control system can complete the start stop control of the transportation, metering, water pump, fan, spray gun, etc. of the reducing agent in the denitrification control room without the need for on-site personnel cooperation. It can also monitor, record, print, and handle accidents related to the operating parameters, as well as adjust the operating parameters.
10. The layout of system equipment fully considers the existing site conditions of the project, the transportation of reducing agents, the smooth flow of roads throughout the plant (including fire exits), and the ease of installation and maintenance of all equipment behind the furnace;
11. If wastewater is generated during the flushing and cleaning process of the equipment, it should be collected in the drainage pit of the denitrification device. The wastewater should be discharged into the power plant wastewater treatment facilities for centralized treatment and standard discharge.
12. At a distance of 1 meter from the denitrification device, the noise level should not exceed 85dBA;
13. The manufacturing and design of all equipment fully comply with the requirements of safe, reliable, and continuous effective operation. After passing the performance acceptance test, the device's availability rate is guaranteed to be ≥ 95% during the one-year warranty period;
14. The maintenance interval of the denitrification device is consistent with the requirements of the unit, without increasing the maintenance and repair time of the unit. The maintenance schedule for the unit is: minor repairs once a year, medium repairs every 3 years, and major repairs every 6-7 years;
15. The overall lifespan of the denitrification device is 30 years.
16. In order to ensure the quality of the project and achieve the required denitrification effect throughout its service life, the equipment and components provided by the supplier are guaranteed to be validated, reliable, and of good quality.
