Application of slag discharge equipment for circulating fluidized bed boiler.
2. Differences in application of 2.CFB boiler slag discharge equipment at home and abroad.
CFB boilers abroad are equipped with ash cooling devices and ash conveying devices to cool high-temperature ash and convey low-temperature ash. The application of these devices and equipment is basically successful, which is not only related to their design, manufacture and operation level, but also related to their fuel characteristics (low ash content and small particle size). Due to the small amount of ash and small particle size, the cooling and transportation of ash and slag are easy to realize. Most CFB boilers in China burn coal with high ash and low calorific value, which contains a lot of ash and coarse particles. The problems of cold slag and slag transportation have not been fundamentally solved. In the 75t/h and 130t/hCFB boilers currently in operation in China, there are few molded and perfect dry slag cooling and conveying equipment. Most of them only have slag cooling equipment and no slag conveying equipment. A few CFB boilers are completely cooled by manual discharge of hot slag, natural stacking or direct water cooling. It not only increases the heat loss of ash, but also pollutes the environment, which is not conducive to the comprehensive utilization of ash.
3. Present situation of slag discharge equipment for CFB boilers in China
Slagging methods of circulating fluidized bed boilers can be divided into wet slagging and dry slagging.
3. 1 wet slag discharge system
It is a hydraulic slag removal system composed of industrial water circulation system, slag ditch, slag flushing pump, sedimentation tank and other equipment. It is a typical slag discharge method in large power plants. Although the application technology has passed the standard, it is not suitable for comprehensive utilization of slag after water quenching, and it occupies a large area and has a large physical heat loss of slag. At the same time, it also brings water pollution and waste of water resources, so wet slag removal of CFB boiler is not a reasonable slag removal method and will not be popularized.
3.2 Dry slag discharge system
Dry slag discharge is the main slag discharge method widely used in CFB boilers. Dry slag discharge system generally consists of slag cooling equipment, slag conveying equipment, slag storage bin, feed valve of slag cooler and discharge valve of slag storage bin. At present, there are mainly the following types of dry slag cooling and conveying equipment used in domestic 75t/h and 130t/hCFB boilers.
3.2. 1 spiral water-cooled slag cooler
Commonly known as water-cooled wringer, hot slag advances along the spiral channel, and cooling water with a certain pressure flows in the water jacket of the wringer shell, the shaft center and the water jacket of the blade, and the two media flow in the opposite direction for heat exchange, so that the hot slag is cooled from 850℃ to about 200℃, and the speed is adjusted by the speed regulating motor to realize automatic control. Its advantages are large heat exchange, small reburnibility, stable operation, convenient adjustment and small overall size. Disadvantages are that the main shaft and blades are worn and easy to leak, and the wear-resistant protective shoes of blades need to be replaced every year, and the quality of cooling water is high (except salt water or softened water). To set up a reasonable water circulation system, many users are still using it.
3.2.2 Drum-type water-cooled slag cooler
After the hot slag enters the drum, it advances along the spiral groove of the inner drum wall. The inner and outer cylinder liners exchange heat reversely with hot slag through cooling water, and at the same time, they can be connected to an air cooling system to cool the hot slag at 850℃ to 200 ~ 300℃. The advantages of drum slag cooler are less wear, less maintenance, long service life, simple structure and convenient operation. The requirement for cooling water quality is not high (softened water or industrial water is enough). The disadvantage is that the heat exchange capacity is slightly lower, so the overall size is slightly larger, and the current application range is very wide.
3.2.3 steel strip air-cooled slag cooler
This equipment is a new type of dry slag discharge equipment applied to 220t/h pulverized coal boiler, and can also be applied to large CFB boiler. Its structure is mainly composed of a large number of strip-shaped heat-resistant steel plates, which are driven by chains on both sides to move at a low speed. Hot slag falls on the steel plates and is cooled to 200℃ in a large area by negative pressure ventilation, and the cold air absorbs heat and raises the temperature to 300 ~ 400℃, which can be used as air supply. The advantages of this equipment are clean and sanitary, stable operation, high utilization rate of heat energy and easy self-control. However, the equipment cost is too high, the investment is large, and the equipment volume is huge, which is only suitable for large CFB boilers.
Moving bed slag cooler
In the moving bed slag cooler, the slag * gravity moves from top to bottom, contacts with the heating surface or air to exchange heat, and the cooled slag is discharged from the slag outlet. Moving bed slag coolers include water cooling, air cooling and wind-water cooling. The equipment is simple in structure and easy to operate. The disadvantages are huge volume, poor heat exchange effect and few applications, and it is only used as a small-capacity or low-ash fluidized bed.
3.2.5 Fluidized bed of Feng Shui slag cooler
The slag cooler uses the gas-solid two-phase flow characteristics of fluidized bed to transfer heat, with air cooling as the main method and water cooling as the auxiliary method. The slag cooling temperature decreases with the increase of air volume and the decrease of slag quantity, and the slag cooling effect is the best. Reasonable air-cooled fluidized bed slag cooler has no mechanical equipment, simple structure, low maintenance cost, no need to set up a separate fan to save electricity, the outlet air temperature is higher than 200℃ and can be used as secondary air to enter the furnace, and the slag cooling water can be low-temperature feed water or other condensed water. The slag discharge temperature is about 120℃, which has good heat recovery and the best energy-saving effect, making the supporting slag conveying equipment work safely. This is a promising slag cooling equipment, which is worth popularizing.
3.2.6CFB slag conveying equipment for CFB boiler
At present, the slag conveying equipment used in China mainly includes scraper slag conveyor, temperate slag conveyor, bucket elevator, chain bucket conveyor, positive and negative pressure pneumatic slag conveying equipment, etc. Scraper and bucket conveyor have high failure rate, and the belt has poor high temperature resistance, which can only bear 150℃, and the lifting angle is small, while chain bucket and pneumatic conveyor have better use effect.
3.3 Several Typical Slag Removal Systems in China
3.3. 1 Mechanical slag discharge system consisting of roller or water-cooled cage slag cooler, ash scraper, bucket elevator and slag storage bin.
The system was put into use in Jinzhou No.2 Thermal Power Plant 1993. After many transformations, the scraper and bucket elevator can run for a long time with large output. However, when the slag discharge temperature is between 200℃ and 300℃, the scraper expands when heated, which leads to long conveying distance of the total scraper, many mechanical failures, large wear of bucket shell and bucket, long conveying distance, large tensile load, easy fracture, large maintenance, poor continuity of operation and great environmental pollution, and is not recommended to be popularized.
3.3.2 Mechanical slag discharge system consisting of roller or water-cooled cage, chain bucket conveyor and slag storage bin.
The running state of chain bucket conveyor has been improved, the running state of chain bucket is stable and the output is large. However, at higher slag temperature, the rotating parts are also easy to be damaged and need further improvement. However, the lifting angle of chain bucket can reach 60 degrees, which is suitable for high-position transmission and can be long or short, and it is widely used at present.
3.3.3 A relatively perfect dry slag discharge system developed by National Electric Power Construction Research Institute for 220t/h pulverized coal boiler.
It can be applied to large CFB boilers. The original temperature-resistant belt slag conveyor was changed into a chain bucket due to burning loss, which has stable operating conditions and clean equipment and meets the requirements of environmental protection. But the equipment investment is too large, which is not suitable for small and medium-sized CFB boilers.
3.3.4 Slag cooler with roller or water-cooled cage
The matching pneumatic slag conveying system has been running in Shenyang Xinbei Thermal Power Company for four years. After multi-directional transformation, a reasonable and perfect dry slag discharge system has been gradually formed, which has certain successful experience.
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