Source: Ash Desulfurization and Denitrification Technology Alliance
(1) The course of the incident
2016.01.25 14:30 #1脱硝A/B side ammonia spray pneumatic shut-off valve automatic closing, 14:32 #1A侧出口NOX浓度上升至114mg/Nm3, 14:32 #1B侧出口NOX浓度上升至117mg/Nm3. 14:40 After receiving the notice from the supervisor, the denitrification specialist immediately rushed to the spot and manually opened the #1 denitrification A/B side ammonia spray pneumatic shut-off valve, 14:45 #1A侧出口NOX浓度下降至87mg/Nm3, 14:43 #1B侧出口NOX浓度下降至73mg/Nm3; Notify the chief to immediately contact the maintenance person to find the reason for the closure of the ammonia spray pneumatic door, and the maintenance and thermal personnel find the logical trend of the ammonia injection pneumatic door closing protection, and find that there is no protection instruction issued, check the air source pressure of the pneumatic shut-off valve on the spot is 0.12MPA (normal pressure is: 0.3Mpa-0.6 Mpa), and find the reason for the low air source of the pneumatic door on the spot. Check that the pressure of the compressed air pipeline (the same air source as the ammonia injection pneumatic valve) for the pump in the denitrification ash hopper silo is 0.18MPA, and immediately report the value length to restore the compressed air pressure for the instrument. At 16:30, the special engineer of the power generation department confirmed that the compressed air pipeline for the pump instrument of the denitrification chamber had been restored, and switched the ammonia spray pneumatic shut-off valve on the #1 denitrification A/B side to a remote state. At 16:35, the denitrification specialist reported the situation to the project manager and the power plant leader by phone 16:58 The pneumatic shut-off valve for ammonia injection on the #1脱硝A/B side was automatically closed again, and the pneumatic adjustment door for ammonia injection on the A/B side of #1 denitrification was closed, and the ammonia injection flow rate on the #1脱硝A/B side dropped to zero, and at 17:03 #1A侧出口NOX浓度上升至270mg/Nm3, #1B侧出口NOX浓度上升至270mg/Nm3. 17:05 After the denitrification specialist manually opened the #1 denitrification ammonia spraying pneumatic shut-off valve and the ammonia injection bypass door on the #1脱硝A/B side on the spot again, the ammonia injection flow rate on the #1脱硝A/B side rose to 140m3/h (basically reaching the maximum flow).
17:09 #1A侧出口NOX浓度下降至98mg/Nm3, #1B侧出口NOX浓度并无下降趋势 (maintained between 150-180mg/Nm3), ammonia supply pressure 0.23MPA, check the air source pressure of pneumatic shut-off valve in situ is 0.1 MPA, ON-SITE RE-INSPECTION FOUND THAT THE PRESSURE OF THE COMPRESSED AIR PIPELINE (THE SAME GAS SOURCE AS THE AMMONIA INJECTION PNEUMATIC VALVE) FOR THE PUMP INSTRUMENT INCLUDING DENITRIFICATION ASH HOPPER WAS 0.18MPA, AND IMMEDIATELY REPORTED THE VALUE OF THE SITUATION, CONTACTED THE DENITRIFICATION MAINTENANCE PERSONNEL TO THE SCENE, AND FOUND OUT THE CAUSE OF THE COMPRESSED AIR PRESSURE FOR THE INSTRUMENT. 17:41 After the compressed air returns to normal, remotely open the #1 denitrification ammonia spray pneumatic shut-off valve on the A/B side and the ammonia spray pneumatic adjustment door on the #1脱硝A/B side by 42% (maximum opening); After closing the ammonia spraying bypass gate on the A/B side of #1 denitrification, the ammonia injection flow rate on the #1脱硝A/B side is 140m3/h.#1A侧出口NOX浓度50mg/Nm3, #1B侧出口NOX浓度仍无下降趋势 (maintained between 150-180mg/Nm3). At this time, the converted value of NOX concentration at #1 net flue gas outlet: 109mg/Nm3, The denitrification specialist will report the situation to the project manager and the power plant leader, and notify the chief to report the situation to the power plant safety and environmental protection department, and contact the maintenance project manager in time to organize the relevant personnel of the maintenance profession to the site for emergency repair. After the leaders of the power plant, the project engineers (Song Jianwei, Feng Jun, Jiang Hao), maintenance professionals and other relevant personnel arrived at the scene, they immediately carried out accident analysis and organized emergency repairs to find the reason for the insufficient ammonia injection on the A/B side of #1 denitrification. No blockages or abnormalities were found in the air flow meter and other measures.
2016.01.26 02:20 The maintenance personnel checked the ammonia differential pressure flowmeter for the first time, but after taking out the sampling tube, checked the blockage of the sampling tube, and did not disassemble the flowmeter to check the measuring orifice plate, so it was misjudged that the ammonia differential pressure flowmeter was not blocked. 08:50 When the NOX concentration meter of denitrification and desulfurization outlet was calibrated, and the measured NOX concentration was compared with the measured value of NOX concentration according to the distribution of denitrification outlet flue measurement points, the measured NOX concentration at the three points was similar and there was no large deviation, so it was judged that there was no uneven distribution of ammonia spray in the denitrification flue. 09:00 After the denitrification experts arrived at the scene, they re-inspected the ammonia injection system one by one according to the system analysis. 14:20 When inspecting the ammonia injection grid pipeline, it was found that the ammonia injection concentration was low, and the ammonia injection differential pressure flowmeter was checked again (this inspection was a dismantling inspection), and it was found that the measuring orifice plate of the ammonia differential pressure flowmeter on the A/B side of #1 denitrification was seriously blocked. 15:58 After dredging the ammonia differential pressure flowmeter on the A/B side of #1 denitrification, the ammonia supply returned to normal.
(2) Analysis of causes
1. Due to the low temperature, the sewage door gasket at the MCC of the compressed air mother tube boiler for the instrument is damaged and leaked, resulting in the unstable pressure of the compressed air for the instrument, resulting in the frequent automatic closure of the ammonia spray pneumatic shut-off valve and ammonia injection regulating valve on the #1 denitrification A/B side.
2. The measuring orifice plate of the ammonia differential pressure flowmeter on the #1脱硝A/B side is seriously blocked, resulting in insufficient ammonia supply, which is the direct cause of this incident.
3. The time period from the occurrence of the incident to find the cause is long, the main reason is that the maintenance personnel are checking the ammonia flow timer, and do not comprehensively check the flowmeter, resulting in a misjudgment of the cause of the incident, which is the main reason for the long time period of the incident.
(3) Precautionary measures
Through the convening of the accident analysis meeting, the following preventive measures are formulated in view of the deficiencies in safety production management:
The compressed air storage tank for the denitrification platform is installed to make the compressed air supply of the denitrification instrument stable.
Regularly clean and discharge all ammonia pipelines and equipment in the denitrification system.
The blockage is sent for testing, its chemical composition is analyzed, and the liquid ammonia supply is checked to see if the supply of liquid ammonia meets the requirements of the standard or the formation of the blockage is caused by other reasons.
In the event of a blockage or other failure in the ammonia injection system, it is necessary to take correct treatment measures in time to prevent the occurrence of uncontrollable NOx.
Formulate and improve the denitrification system equipment inspection and maintenance operation card, standardize the inspection and processing process, and improve the maintenance skill level and abnormal situation handling ability.