Separator is the key component of the main circulation loop, and its function is to complete the gas-solid separation of dusty airflow and send the collected materials back to the furnace to realize ash balance and thermal balance, and ensure the stability and high efficiency of combustion in the furnace. In a sense, the performance of CFB boiler depends on the performance of separator, so the development of circulating bed separator has experienced three generations of development, and the different design of separator marks the development course of CFB combustion technology. Germany Lurgi Company developed CFB boiler earlier, using high-temperature adiabatic cyclone separator with heat preservation, fire resistance and wear resistance materials [1]. The flue gas temperature at the inlet of the separator is about 850℃. The circulating fluidized bed boiler with adiabatic cyclone separator is called the first generation circulating fluidized bed boiler and has been commercialized. Circulating fluidized bed boilers designed and manufactured by Lurgi Company, Ahlstrom Company, Stan, ABB-CE, AEE, EVT, etc. The technology they transfer adopts this form.
This separator has good separation performance, and the circulating fluidized bed boiler using this separator has high performance. However, there are also some problems in this separator, mainly because the cyclone drum is huge, so the steel consumption is high, the boiler cost is high, the floor space is large, the lining of the cyclone drum is thick, the requirements for refractory materials and masonry are high, the consumption is high, the cost is high, the start-up time is long, and faults are easy to occur in operation; The sealing and expansion system is complex; Especially when burning coal with low volatility or poor activity, the temperature of separated materials will rise due to the combustion in the cyclone, which will lead to overheating of the reflux valve of the cyclone or reflux pipe. These problems are more prominent under the actual production conditions in China.
The medium-temperature separation technology of circulating liquid has alleviated the problem of high-temperature cyclone to some extent. A large number of heating surfaces are arranged in the upper part of the furnace, which reduces the temperature and volume of flue gas at the entrance of cyclone, reduces the volume and weight of cyclone, overcomes the defects of adiabatic cyclone technology to some extent, and improves its operation reliability. The superheater and high-temperature economizer are arranged in the upper part of the furnace, which requires tower arrangement, so the furnace is higher, the steel consumption is large, and the boiler cost is increased. At the same time, its CO emission and maintenance problems also limit the development of this technology to some extent. In order to maintain the advantages of adiabatic cyclone circulating fluidized bed boiler and effectively overcome the disadvantages of this type of boiler, Foster Wheeler Company designed an exemplary water (steam) cooled cyclone separator. The circulating fluidized bed boiler with water (steam) cooling separator is called the second generation circulating fluidized bed boiler.
The shell of the separator is formed by bending and welding water-cooled or steam cooling pipes, and the high-temperature insulation layer of the adiabatic cyclone separator is cancelled. Instead, the curved surface made of the heating surface and its inner side are covered with pins and coated with a thin layer of high-temperature wear-resistant castable, and the outer side of the shell is covered with a certain thickness of insulation layer. The water (steam)-cooled cyclone separator can absorb some heat, and the temperature of the materials in the separator will not rise or even drop slightly, and at the same time, the problem of anti-wear inside the cyclone separator is well solved. The circulating fluidized bed boiler put into operation by the company has never experienced the problems of coking in the return system and cyclone wear, which fully shows its advantages. In this way, the advantages of high-temperature adiabatic cyclone separation circulating bed can continue to play, but the disadvantages are basically overcome.
Of course, no design can be perfect. The FW water (steam) cooled cyclone separator has problems in manufacturing technology and production cost, which makes its commercial competitiveness decline and its universality and popularization value are limited. At the same time, there is no essential difference between the structure of the separator and the high-temperature adiabatic cyclone separator, so the boiler structure has not been restored to the perfect form of the traditional boiler. The large expansion joint for thermal expansion of all components has become the weakest link of the furnace type, and damage accidents occur frequently (see 15 FBC international conference operator section). Therefore, adjusting the shape of separator to further improve compactness and reliability has become the key to the development of circulating fluidized bed combustion technology. In order to overcome the structural problems and high manufacturing cost of steam-cooled cyclone boilers, Ahlstrom Company of Finland creatively put forward the design concept of Pyroflow Compact.
Pyroflow compact circulating bed boiler adopts square separator with its unique patented technology. The separation mechanism of the separator is basically the same as that of the circular cyclone separator. The shell still adopts FW water (steam) cooling pipe wall type, but the cylinder is unique because of its plane structure. This is the third generation circulating fluidized bed boiler. The biggest difference between it and the conventional circulating fluidized bed boiler is that it adopts a square gas-solid separation device, and the wall of the separator is a part of the water circulation system of the furnace wall, so it has no thermal expansion connection with the furnace. At the same time, the square separator can be arranged near the furnace, which greatly reduces the volume of the whole circulating bed boiler and makes the arrangement very compact. Drawing lessons from the successful experience of steam-cooled cyclone separator in anti-wear, a thin layer of refractory material was laid on the water-cooled surface of square separator, which made the separator a part of the heating surface and provided conditions for rapid start and stop of boiler.