Abstract: Based on the analysis of existing cleaning methods for air preheaters and the actual usage of power plants, a new type of fully scalable online air preheater has been developed, and the design concept is introduced. Specific operational steps and precautions have been proposed for cleaning the air preheater in power plants.
Background: With the increasingly serious environmental pollution problem, the country has increased its environmental protection requirements for various industries. Specifically in the thermal power generation industry, reducing emissions of sulfur dioxide and nitrogen oxides has become a major issue. In November 2011, the website of the Ministry of Environmental Protection released the newly revised "Air Pollution Emission Standards for Thermal Power Plants" jointly issued by the Ministry of Environmental Protection and the General Administration of Quality Supervision, Inspection and Quarantine of China, which came into effect on January 1, 2012. In order to reduce the pollutants contained in flue gas emissions, thermal power plants have carried out flue gas denitrification transformation. After installing flue gas denitrification facilities, the blockage of the air preheater is more severe than before, especially after two or three years of denitrification operation. With the deactivation of the catalyst, the blockage becomes more frequent and severe. This is because in the denitrification process, there is inevitably ammonia escape, which reacts chemically with sulfur oxides present in the flue gas to form ABS (ammonium bisulfate). Ammonium bisulfate exists in three forms in the air preheater: gaseous, liquid, and solid. Liquid ammonium hydrogen sulfate adsorbs ash in flue gas to form ash scale. Due to the strong adhesion of the liquid, it adheres to the cold end heat exchange element of the air preheater and forms blockage.
Blockage of the air preheater not only affects heat transfer, but may also limit boiler output due to a sharp increase in flue resistance. In addition, the presence of ash accumulation increases flue gas resistance and exhaust gas temperature, becoming a hidden danger to the safe operation of the boiler. Therefore, air preheater cleaning has become a necessary problem to be solved in power plant production.
1. Existing cleaning methods for air preheater
The commonly used cleaning methods for air preheaters include steam blowing, shock wave blowing, and sonic blowing. The specific introduction is as follows:
Steam soot blowing method is currently the main soot blowing method, which can achieve good blowing effect in most cases under the premise of ensuring steam pressure and superheat. But in some cases, such as coal quality deviating significantly from the design (such as high sulfur and ash content), high ammonia escape rate, etc., the steam blowing method cannot completely remove the blockage. Refer to Figure 1 for steam soot blowing equipment.
1.2 Shock wave soot blowing method
Shock wave soot blowing is achieved by the shock wave generated by the explosion of acetylene and oxygen mixture to achieve the purpose of cleaning. Due to the fact that the cold end heat storage components of the air preheater after denitrification are generally covered with enamel layers, this strong impact can cause damage to the enamel and result in peeling, so it is generally not recommended to use it. The shock wave equipment is shown in Figure 2.
1.3 Acoustic soot blowing method
Acoustic soot blowing utilizes the principle of resonance to remove accumulated dust. Ammonium bisulfate that forms blockages is in liquid or paste form, and its natural frequency is generally not fixed. It is difficult for sound waves to capture the resonance frequency of constantly changing liquid substances. Currently, the effectiveness of solving stubborn ammonium bisulfate blockages needs to be verified. Refer to Figure 3 for the sound wave equipment.
2. A new generation air preheater cleaning system that can be installed and maintained online without shutting down the furnace
Traditional soot blowing equipment needs to be installed in advance during power plant construction or during shutdown maintenance. Once the nozzle or other accessories embedded in the air preheater malfunction, they cannot be maintained in a timely manner and can only be repaired and replaced when the furnace is shut down. Forcefully shutting down the furnace will result in significant losses during shutdown and start-up, and the unit cannot be shut down arbitrarily during operation.
After extensive visits to power plants and understanding the practical needs of soot blowing and the inconveniences of various existing methods, we have developed a new generation of air preheater soot blowing system that can be installed without shutting down the boiler and maintained online.
2.1 Development Approach
The existing steam soot blowing equipment has an integrated structure for the gun barrel, and mostly uses steel pipes with an overall length equivalent to that of an air preheater. Generally, it is not possible to provide a site for its complete withdrawal on site. Due to its length limitation, it needs to be pre embedded in the air preheater in advance. The sealing between the wall of the air preheater and the soot blowing gun also limits the online maintenance and repair of the steam soot blowing gun.
Based on actual research, we use low-pressure, high flow rate, and high-pressure water as the blowing medium, spraying vertically from the bottom of the air preheater to clear the accumulated dust between the radiator fins and achieve cleaning of the air preheater. The high-pressure water pipeline adopts high-pressure hoses and is supported by expandable steel sleeves on the periphery, enabling the nozzle to move and fully withdraw along the diameter direction of the air preheater. Installing an openable automatic door on the wall of the air preheater, combined with the cleaning of the telescopic gun, can achieve online cleaning. When not cleaned, the telescopic gun can fully retract, the automatic door closes, and the air preheater is sealed.
2.2.2 System Principle and Configuration
The fully retractable online cleaning system consists of a high-pressure water cleaning unit, a fully retractable cleaning device, and a control system. Refer to Figure 4 for the system schematic diagram. The overall parameters of the system are as follows:
Type: Fully retractable cleaning medium: Clear water
Work pressure: maximum 50Mpa Flow rate: maximum 240L/Min
Cleaning time: Online cleaning: about 240 minutes Offline cleaning: about 600 minutes
2.2.4 High pressure water cleaning unit
The high-pressure water cleaning unit adopts the 200TJ3 type unit manufactured by our company, and the maximum pressure can reach 280 MPA. The online cleaning unit is designed with a maximum pressure of 50MPA and a maximum flow rate of 240 L/min. The cleaning medium is clean water. The high-pressure water cleaning unit is equipped with a JETECH high-pressure pump from the United States, powered by an electric motor. The fully enclosed stainless steel water tank has an automatic water replenishment function to prevent safety hazards caused by upper overflow. A stainless steel filter screen type filter is added to the inlet pipeline in front of the water tank to ensure the quality of the incoming water. The unit has the following characteristics:
Short downtime and easy maintenance
Less moving parts than other plunger pumps
No special tools required
Generally, there is no need to move the inlet and outlet chambers - there are no heavy components
All bolts can be seen from the front end of the pump
The "removable" valve body makes inspection and maintenance simple and easy. Remove the valve body without releasing the plunger.
No need to disconnect the hose from the inlet and outlet chambers.
2.2.5 Fully retractable cleaning device
The fully retractable cleaning device adopts a three-level retractable structure, and the body is made of stainless steel square tube processing. The power source adopts Delta servo motor as the main power source, matched with high-precision L-shaped reducer, providing powerful drive for the telescopic mechanism, driving the nozzle at the gun head to advance along the diameter direction of the air preheater, thereby cleaning the air preheater.
The front end of the telescopic arm carries two high-pressure water gun tubes, with six nozzles arranged in a zigzag pattern to achieve the best cleaning effect.
Main technical parameters:
Retract overall dimensions: 3550mm × 830mm × 1700mm
Extended overall size: 10750mm × 830mm × 1700mm
Maximum external dimensions: 330mm × 290mm
Equipment power: 1KW
Adjustable speed range: 0-43m/min
Spray gun (standard): dual guns with 6 nozzles
Scope of work: 0-7200mm
The device effect is shown in Figure 6, with the left side in a fully retracted state. The right side is in a fully extended state.
2.2.6 Control System
The control system adopts Siemens 200 series PLC, combined with Siemens SMART series touch screen as the human-machine interaction interface. The servo motor control adopts Delta servo drive for precise control of the gun head, and the minimum walking control accuracy can reach millimeter level. Through the human-machine interface, various control parameters such as walking speed, step distance, and dwell time can be set.
The control system of the fully retractable cleaning device can achieve the following three modes of action for the mechanical telescopic arm: 1. reciprocating cleaning mode (simulating manual cleaning). Step by step cleaning mode. 3. Direct feed mode. Regardless of the operating mode of the cleaning device, the implementation of the action can be achieved through two methods: first, the touch screen configuration button. (Software). Secondly, DCS remote control.
When the device is in operation, the touch panel will display the real-time operation status and various parameters collected by the controller.
Comparison of advantages and disadvantages between 3 fully retractable online cleaning devices and traditional soot blowing guns
4、 Precautions for online cleaning operation
The load of the unit during online water flushing should be greater than 90%, and the higher the load, the smaller the impact on the unit.
When cleaning the air preheater, it is necessary to make corresponding preparations to avoid adverse effects on the operation of the unit.
Firstly, it is necessary to clear the exhaust outlet of the flue, switch the pipeline valves to the appropriate position, etc., to prevent the flue from being blocked, and the cleaned ash cannot be removed in a timely manner, resulting in excessive load on the wall of the air preheater. Before formal cleaning, experimental flushing can be conducted for a short period of time to check the normal operation of the unit before normal flushing.
During flushing, the bottom surface of the flue and the supports and hangers should be monitored to prevent the deposition of ash and scale that may cause damage to the flue and supports and hangers. During the flushing process, maintenance personnel should always clear the bottom drainage and ash discharge pipe of the air preheater to prevent blockage. If the exhaust temperature is too low, adjust the flow rate of the flushing side exhaust gas in a timely manner to increase the exhaust temperature. If it continues to decrease, the flushing should be stopped.
During and after flushing, observe the current of the air preheater drive motor, the resistance of the air preheater, the exhaust temperature, the situation of flue gas discharge, and whether the dust collector is operating normally. Observe and record parameters such as high-pressure water flushing time, pressure, unit load before and after air preheater flushing, pressure difference between flue gas and air sides, current of supply and induced draft fans, furnace pressure, hot air and exhaust gas temperature. During flushing, the operation of the high-pressure cleaning unit should be monitored. The unit itself has various protections to achieve fault alarm and shutdown, but it should be decided in a timely manner whether to stop flushing and troubleshoot when the fault alarm occurs
Two key parameters need to be focused on during cleaning:
（1） Flow rate: Pay attention to checking the overflow rate of the overflow port. When the overflow rate significantly increases, it indicates that the nozzle may be blocked and should be replaced or unblocked.
（2） Pressure: When the flushing pressure does not reach the rated pressure, nozzle wear may cause the aperture to increase or the nozzle to fall off, and a new nozzle should be replaced.
In general, multiple flushes should be performed, even if the resistance of the air preheater no longer continues to decrease, it is best to rinse once or twice. The cleaner the heat exchange element is flushed, the longer the interval between two flushes. This cleaning unit can perform online accompanying flushing or cleaning during shutdown. Different power plants can flush their air preheaters according to their operating conditions, and a well functioning air preheater is an indispensable link in the stable operation of the unit.
5、 Summary
This article analyzes the advantages and disadvantages of several existing methods for cleaning air preheaters, proposes new product design ideas based on the actual use of power plants, and designs a new set of air preheater cleaning equipment. Specific precautions have been proposed for cleaning the air preheater in power plants.