Discussion of the current state of application of circulating fluidized bed boilers

       From 1988, the first domestic 10t/h evaporation circulating fluidized bed boiler was put into operation after more than ten years, the application of circulating fluidized bed boiler in the domestic centralized heating field has developed very rapidly. In small and medium-sized thermal power plants, the first domestic 35t/h circulating fluidized bed boiler was put into operation in 1989, the first domestic 75t/h circulating fluidized bed boiler was put into operation in 1992 in Zhejiang Sheng County Thermal Power Plant, the first domestic 220t/h circulating fluidized bed boiler was put into operation in 1995 in Dalian Chemical Industry Company's own thermal power plant, the first domestic 130t/h circulating fluidized bed boiler was put into operation in 2000 in Shijiazhuang In 2000, the first domestic 130t/h circulating fluidized bed boiler was put into operation in Shijiazhuang Cogeneration Plant No.3, and the first domestic 420t/h circulating fluidized bed boiler was put into operation in Baoding Cogeneration Plant in 2002.

       In terms of district heating boilers, the first domestic 29MW circulating fluidized bed high temperature water boiler was put into operation in Puyang Zhongyuan Oilfield in Henan Province in 1993, the first domestic 14MW circulating fluidized bed low temperature water boiler was put into operation in Shijiazhuang Time Heating Station in 1999, the first domestic 58MW circulating fluidized bed high temperature water boiler was put into operation in Jinan in 2000, the first domestic 116MW circulating fluidized bed high temperature water boiler was put into operation in Shijiazhuang Time Heating Station in 2001. The first domestic 116MW circulating fluidized bed high temperature water boiler was put into operation at Shijiazhuang Time Heating Station in 2001. Shanxi Lantian Lang Feng'e said when talking about the current situation of circulating fluidized bed boiler applications.

　　1 Circulating fluidized bed boiler application characteristics

　　From the circulating fluidized bed boiler has been put into operation, its advantages in the practical application are mainly the following:

　　1.1 Boiler thermal efficiency is high

　　Due to the strong circulation disturbance between gas and solid in the circulating bed, the heat and mass transfer process in the furnace is strengthened, so that the fresh fuel particles just entering the bed are instantly heated to the furnace temperature (≈850℃), and the combustion and heat transfer process can be carried out at constant temperature along the height of the furnace, thus extending the combustion reaction time. The fuel is recycled back to the furnace several times through the separator, which prolongs the residence and reaction time of the particles and reduces the incomplete combustion loss of solids, so that the circulating bed boiler can achieve 98-99% combustion efficiency. According to the operational tests of different capacities and models of boilers in the relevant cogeneration enterprises, the circulating fluidized bed boiler of the same capacity is about 10% higher than the operating thermal efficiency of the chain furnace. The carbon content of circulating bed boiler slag is between one and five percent, and the carbon content of fly ash is between 8 and 35%, while the carbon content of chain furnace slag is between 15 and 25%, and the carbon content of fly ash is between 40 and 50%. The combustion efficiency of circulating fluidized bed boiler is less than that of pulverized coal furnace, but due to the complexity of pulverized coal furnace system, poor fuel adaptability, high concentration of nitrogen oxides and sulfur dioxide emissions and other shortcomings, in small and medium-sized thermal power plants and regional boiler room has been less and less used.

　　1.2 Stable operation, simple operation

　　The circulating fluidized bed boiler fuel system has less rotating equipment, mainly coal feeder, slag cooler and fan, which eliminates the complex pulverizing and powder feeding system equipment compared with pulverized coal furnace, and eliminates the frequent failure of the grate part compared with chain furnace, which creates conditions for stable operation of the combustion system.

　　Circulating fluidized bed in the 90s in the early application in China, due to the lack of experience in research, design, manufacturing, installation, operation and other aspects, its application does have a short continuous operation time, insufficient output, ignition difficulties, serious wear and tear, easy coking, high failure rate of auxiliary machinery, many problems, but after more than ten years of continuous improvement in all aspects of the work, not only to ensure continuous operation time higher than 4000h, to have the experience of design, manufacturing, installation, and operation of the system. Other problems have also been overcome for experienced design, manufacturing, installation and operation units. The two 116MW circulating fluidized bed boilers of Shijiazhuang Time Heating Station operated continuously and steadily throughout the heating season of 2003-2004 without a single breakdown. Many users are proficient in circulating fluidized bed boilers from ignition start, load lifting and lowering, continuous slag discharge, pressure fire standby to troubleshooting. As long as to ensure uninterrupted coal feeding, keep the chamber material difference stable, control the furnace temperature, in 50 ~ 110% of the load under continuous stable operation is not a problem.

　　1.3 Wide adaptability of fuel, strong adaptability to fluctuations in coal supply market

　　Circulating fluidized bed boiler has a very high combustion heat intensity, its cross-sectional heat load of 4-6MW/m2, is 2-6 times the chain furnace, its chamber volume heat load of 1.5-2MW/m3, is 8-11 times the pulverized coal furnace, so it can burn almost all kinds of fuels such as poor coal, anthracite, gangue, etc., which are difficult to ignite and burn out in pulverized coal furnace or chain furnace, and achieve a high thermal efficiency, This is important for burning local poor quality fuels, to deal with the coal supply situation has important significance. Shijiazhuang in recent years several coal market fluctuations, heating enterprises with coal is not guaranteed. The heat content of coal fluctuates greatly between 3000~6000kcal/kg, but the circulating fluidized bed boiler always operates stably, and its superiority is very obvious. And chain furnace, pulverized coal furnace due to large changes in coal, either not reach the output, or frequent fire, coking and other failures.

　　1.4 Low pollutant emissions

　　The combustion temperature in circulating fluidized bed can be controlled in the range of 850-950℃ for stable and efficient combustion, this combustion temperature inhibits the formation of thermal reaction type NOx, while the use of graded combustion method to feed about 30-40% of the secondary air into the furnace, and can control the generation of fuel type NOx. In addition, according to the size of the sulfur content in the coal, directly sprayed into the furnace or mixed with a certain amount of 0-1mm limestone powder in the coal feed, can remove the SO2 generated in the combustion process, and the desulfurization efficiency can reach 90%. Table 2 is a plant 220t / h circulating fluidized fluidized bed boiler desulfurization effect monitoring results. The coal received a base low calorific value of 24200 KJ/kg, and the sulfur content is 1.34%. Burning coal volume 25t/h, adding limestone powder (containing impurities 4%) is 2.6 t/h, Ca/S molar ratio is 2.64, its desulfurization efficiency reached 93.39%.

　　2 Application selection of circulating fluidized bed boiler

　　Circulating fluidized bed boiler differs from chain furnace and pulverized coal furnace in that the main combustion devices are: air distribution device, separator and return device, of which the separator is the key technology to distinguish and select circulating fluidized bed boiler, which directly affects the operating characteristics of the boiler. According to the different working temperature of the separator, it can be divided into three kinds:

　　2.1 High temperature separation

　　That is, the separation is carried out at the exit of the furnace chamber, and the operating temperature is above 850℃. The current application of the typical structure of the separator type adiabatic cyclone separator, steam (water) cooled cyclone separator and square water-cooled cyclone separator, the first two boiler separation efficiency is high, the latter boiler compact structure, start and stop flexible. Adiabatic separator volume, start and stop time is long, but the operation is stable, the application history is long, the accumulated experience is rich, steam (water) cold separator volume is small, start and stop flexible, but the structure is complex, manufacturing and installation process requirements are high, large boilers are used more. (Lang Feng'e, a native of Xinzhou City, Shanxi Province, gave up her job as a teacher in a small county and spent four years developing new high-efficiency pulverized coal industrial boiler system technology, which was marketed to fill the technical gap of the difficulty of miniaturizing pulverized coal boilers in China. With her persistence and perseverance, her intuition and unique perspective, she has single-handedly created the country's leading integrated energy solution provider of new energy-efficient and environment-friendly pulverized coal boilers).

　　2.2 Medium temperature separation

　　That is, the separation after the superheater, the working temperature is generally between 400 ~ 600 ℃, with the Crcofulid furnace type of the German Bagauk company as the representative, domestic boiler plants such as Tang pot, Beipot production. Its circulation rate is low, the separator volume is small, less wear, but the combustion efficiency and desulfurization rate is low.

　　2.3 Combined separation

　　That is, the combination of high-temperature separation and low-temperature separation, such as high-temperature separation using louvers or impact plane separation, low-temperature separation using a combination of cyclone separation, but due to the high failure rate of inertial separation, separation efficiency is low, and is rarely used.

　　3 circulating fluidized bed boiler application difficulties

　　3.1 Wear problem

　　A large number of theoretical and experimental studies have shown that the wear of circulating fluidized bed boiler is proportional to the third power of flue gas flow rate and proportional to the diameter and concentration of fuel particles. Design and operation of the boiler to control the reasonable flue gas flow rate of each part is the fundamental factor to reduce wear. Domestic circulating fluidized bed boilers mostly use low cycle times, the design flue gas flow rate in the furnace chamber for 4.5 ~ 5m / s, it should be said that under normal circumstances wear is relatively light, but in the local flue gas flow changes, the degree of wear is very serious, such as the furnace wall, water-cooled wall tube, separator, return device, filter, coal conservator, preheater and other places. Some furnace models with buried pipes under the bed, the wear problem is more prominent. In the easy to scour and local vortex at the use of high temperature, wear-resistant silicon carbide brick or casting material, corundum brick, high alumina brick and other anti-wear materials, strict control of the quality of furnace construction can achieve the desired anti-wear effect.

　　3.2 Coking problem

　　Control of stable bed temperature is the key to prevent coking, the main factors affecting coking in operation are as follows:

　　1) When the ignition temperature or operation, the fuel input quantity is suddenly increased, and the air volume is not increased in time to make the bed temperature rise rapidly to reach above the ash melting point, the bed as a whole will coke at high temperature.

　　2) When operating slag discharge, the material layer is too thick or uneven, resulting in too large or too small fluidization air volume, easy to form high temperature in the local, resulting in local coking. The use of continuous mechanical slagging can largely avoid this problem.

　　3) If the high-temperature circulating return ash into the furnace too much, causing bed temperature can not be controlled is also very easy to cause high-temperature coking. In operation, attention should be paid to the uniform control of the amount of return material.

　　4) When normal pressure fire, cold air should be strictly avoided in the furnace, otherwise it is easy to make combustible materials burn and cause local over-temperature coking.

　　3.3 Ignition problems

　　At present, circulating fluidized bed boiler ignition is generally light diesel ignition under the bed, but there are many users to save costs still use bed charcoal or firewood ignition. As some users do not have sufficient preparation for ignition or operating experience, ignition is prone to fire or coking failure, and some even serious furnace explosion accidents. From ignition to stable operation mainly after several or even a dozen times to success, so many users feel headache.

　　Practice has proved that each type of circulating fluidized bed boiler ignition process, both common and characteristic, the commonality is the main, the need to operate the management staff in practice to continuously explore and summarize, learn from the successful experience of others. A successful ignition process, first of all, the cold state simulation test data as a guide, followed by attention to the bed material thickness, bed material sieving characteristics and ratios, strict control of the ignition air volume and the amount of coal to the operation, pay attention to the close observation of bed temperature changes. Many users have mastered the operating procedures of oil ignition under the bed, Shijiazhuang West suburban heating station will bed wood charcoal ignition method successfully applied to a large 116MW boiler, its cost is only one-fifth of the oil ignition.

　　4 Circulating fluidized bed boiler application problems

　　4.1 Comprehensive utilization of fly ash

　　Theoretically, circulating fluidized bed boiler combustion temperature is low, the resulting ash has a good activity, can be used to make cement admixture or other raw materials for construction materials, but the actual operating results are, when the raw coal in more fine particles or crushing process of crushing serious, the amount of unburned fine particles fly out, especially in the combustion of low-volatile poor coal or anthracite coal, fly ash carbon content is often as high as 20 ~ 40%, up to Not up to the cement mixer requirements for fly ash carbon content must be less than 8% of the standard, fly ash can not be used comprehensively, but also can not find a dumping site. The carbon content of slag can be controlled at less than 5%, which is easier to be used comprehensively. The carbon content of fly ash of chain furnace is as high as 40-50%, although the thermal efficiency of the boiler is low, the fly ash can be used as fuel in brick making, so it can be used comprehensively. Pulverized coal furnace due to high combustion efficiency, its fly ash carbon content can be controlled at less than 8%, and therefore easier to comprehensive utilization.

　　In order to reduce the fly ash carbon content of circulating fluidized bed boiler, some scientific research institutions and manufacturers have taken measures to increase the height of the furnace, set the bottom feeding back to combustion system, etc., although a certain effect has been received, but the low volatile content of poor coal, the fly ash carbon content is still as high as 15 ~ 20%. The main solution to this problem should be to improve the efficiency of the separator and to control the proportion of fine particles in the furnace coal. At present, the domestic 410t/h circulating fluidized bed boiler with Oslon steam-cooled cyclone separation technology has reduced the carbon content of fly ash to 8% when burning lean coal and adding desulfurizer. This problem is expected to be solved by carefully summarizing the experience.

　　4.2 The effect of environmental protection has not been given full play

　　Circulating fluidized bed is a clean combustion technology, with low NOx emissions and furnace desulfurization and other very good environmental performance, this performance is not only theoretically well-founded and from the domestic and international engineering practice has been verified. However, due to the domestic environmental protection policy is not implemented, resulting in most users did not install desulfurization facilities, some although the installation of advanced desulfurization facilities, and even investment to build a supporting limestone powder plant, but due to limestone desulfurization to increase the cost of heating, but also did not really put into operation. To make the user motivated to take the initiative to implement desulfurization and have the ability to afford, but also need to develop comprehensive supporting policy measures.

　　5 circulating fluidized bed boiler application prospects

　　Compared to pulverized coal and chain furnace, circulating fluidized bed boiler development history is still relatively short, especially circulating fluidized bed boiler in the early 1990s in some domestic projects left a deep lesson, so that some users are very cautious about the application of circulating fluidized bed. But carefully study and absorb the experience of successful manufacturing plants, design institutes, installation and operation units, it should be said that there is no insurmountable technical obstacles.

　　Since entering the twenty-first century, China's rapid economic development, coal and electricity to the economic development of the bottleneck constraints reappear, a considerable period of time in the future, coal supply will be more tight, many small and medium-sized thermal power plants and regional boiler room to burn local poor quality coal or multi-channel solution to the problem of coal supply, and thus the characteristics of circulating fluidized bed fuel to adapt to a wide range will be more play out. At the same time, with the introduction of more scientific and strict policies and measures for the emission of pollutants in the atmosphere, will also make circulating fluidized bed boilers more competitive due to the characteristics of clean combustion, therefore, the application of circulating fluidized bed boilers should be expanded according to local conditions.