1) has excellent fuel adaptability. Circulating fluidized bed boiler is a fly ash recovery system through separator and reflux valve group. The change of fly ash circulation produced by coal combustion can adjust the heat absorption and bed material temperature in the combustion chamber. As long as the calorific value generated by fuel combustion is greater than the heat required to heat the fuel itself and the air needed for combustion to a stable temperature (850 ~ 950℃), this kind of coal can be stably burned in a fluidized bed boiler. Therefore, the fluidized bed boiler can burn almost all kinds of coal, and it is best to burn all kinds of inferior fuels. It is a good choice for enterprises with unstable coal quality supply.
However, pulverized coal boilers have higher requirements for coal quality. When there is a big difference between coal burning and design coal, coking of furnace burner and superheater and coal breakage of coal feeder are easy to occur. As a result, the boiler can not operate normally, and the adaptability of pulverized coal boiler to coal is poor.
2) The fuel system is relatively simple. Fluidized bed boilers are suitable for burning wide-screened fuels (the granularity of coal is required to be 0- 10mm, and the 50% cutting granularity d50=2mm), and the fuel preparation and crushing system is greatly simplified. Therefore, the cost of circulating fluidized bed boiler is higher than that of pulverized coal boiler with the same capacity, eliminating the complicated pulverizing system, and the overall investment including civil engineering is still lower than that of pulverized coal boiler.
3) high combustion efficiency. For conventional pulverized coal boilers, if the coal type can't reach the design value, the efficiency can generally reach 85-95%, while circulating fluidized bed boilers adopt fly ash recirculation system, and the combustion efficiency can reach 95-99%.
4) Wide load adjustment range and good adjustment performance. The load regulation range of pulverized coal boiler is usually 70 ~ 1 10%. When the load is low, the pulverized coal boiler needs to be ignited and burned with an oil gun. However, the circulating fluidized bed boiler has a large amount of bed material and strong heat storage capacity. The fly ash recirculation system is adopted, and the regulation range is much wider than that of pulverized coal boiler, generally 30 ~ 1 10%, and the load regulation rate can reach (5 ~ 10) b-MCR/min. Therefore, the circulating fluidized bed is especially suitable for cogeneration, heating boilers with large thermal load changes or boilers with peak shaving units.
5) Low emission of combustion pollutants. Sulfur dioxide (SO2) produced in the combustion process can be removed by adding desulfurizer (limestone or dolomite powder) into the circulating fluidized bed boiler. The amount of limestone is determined according to the sulfur content in the fuel. When the molar ratio of calcium to sulfur is 2 ~ 2.5, the desulfurization efficiency can reach 90%. Compared with pulverized coal boiler (wet desulfurization cost of pulverized coal boiler: average cost of foreign technology 1300 ~ 1500 yuan /KW, average cost of domestic technology 1000 yuan /KW), fluidized bed boiler has greater cost advantage when burning high-sulfur coal. The optimum combustion temperature of fluidized bed boiler is 850 ~ 950℃, which is suitable for desulfurization reaction. The output of nitrogen oxides is obviously reduced, and the emission concentration is 100 ~ 200 ppm, which is lower than that of pulverized coal boiler, and other pollutants such as CO, HCl and HF in circulating fluidized bed boiler are also lower than that of pulverized coal boiler. For pulverized coal boilers, the cost of removing nitrogen oxides from flue gas is much higher than that of desulfurization of pulverized coal boilers. The emission of SO2 and nitrogen oxides from circulating fluidized bed boilers can completely meet the national environmental emission standards, and it is absolutely superior to pulverized coal boilers in environmental emission competition.
6) High combustion heat intensity and strong heat transfer capacity in the furnace. Because the circulating fluidized bed boiler adopts the fly ash recirculation system, the combustion heat intensity is high, the cross-section heat load can reach 3 ~ 8mw/m2, which is close to or higher than that of pulverized coal boiler, and the furnace volume heat load is 1.5 ~ 2mw/m3, which is 8 ~ 10 times that of pulverized coal boiler. The heat transfer in fluidized bed furnace is mainly convection heat transfer and radiation heat transfer between rising flue gas and material and heating surface. The heat transfer coefficient of gas-solid two-phase mixture in the furnace to the water wall is much larger than that in the pulverized coal boiler furnace. Compared with pulverized coal boiler, it can greatly save the metal consumption of heating surface.
7) The number of coal feeding points is small and the arrangement is simple. Circulating fluidized bed boiler has better lateral mixing characteristics, and there are fewer coal feeding points than pulverized coal boiler. For example, there are only four coal feeding points at 220 t/h. The reduction of coal feeding points simplifies the arrangement of coal feeding devices, and the coal feeding points are not easy to coke and operate reliably.
8) It is easy to realize the comprehensive utilization of ash. Generally, the carbon content in the bottom slag of fluidized bed is 1 ~ 3%, and the carbon content in fly ash is 4 ~ 15%. The optimum combustion temperature of fluidized bed boiler is 850 ~ 950℃. Compared with pulverized coal boiler, it belongs to medium-low temperature combustion, and the generated ash will not soften and bind, so it has good activity. It can be used as admixture for manufacturing cement or building materials, and has broad comprehensive utilization prospects.
(2) disadvantages of circulating fluidized bed boiler compared with pulverized coal boiler:
1) Circulating fluidized bed boilers have high power consumption and high flue resistance. Compared with pulverized coal boiler, the pressure heads of primary fan, secondary fan and fluidized fan of fluidized bed boiler are higher; The unique air distribution plate device and fly ash recirculation combustion system in fluidized bed make the resistance of air supply system much greater than that of pulverized coal boiler. The blower pressure of pulverized coal boiler is generally below 2KPa, while that of fluidized bed boiler is generally above 10 kPa, which is characterized by high power consumption, high noise and large vibration. Generally speaking, the power consumption ratio of circulating fluidized bed boiler is at least 4 ~ 5% higher than that of pulverized coal boiler.
2) The boiler parts are badly worn. Due to the characteristics of high material concentration and high wind speed of fluidized bed boiler, the parts of boiler are seriously worn. Although anti-wear measures have been taken, such as refractory and wear-resistant castable treatment, spraying treatment, giving way to pipes in dense phase area and dilute phase area, the wear rate of heating surface of circulating fluidized bed boiler is still much faster than that of pulverized coal boiler in actual operation. It is much more difficult to treat and repair the wall wear at the junction of dense phase zone and dilute phase zone than pulverized coal boiler.
3) Wear, cracking and shedding of refractory materials and wear-resistant layers are difficult problems for fluidized bed boilers. The types and quantities of refractories used in fluidized bed boilers are much more than those used in pulverized coal boilers. However, due to improper selection of refractory and wear-resistant materials, or unreasonable construction technology, or improper temperature control when baking and ignition are started, the evaporated water vapor in refractory materials can not be discharged in time, or the thermal stress is too large, resulting in cracking and falling off of refractory lining. The loss of refractory in dense phase zone will destroy the normal fluidization state of bed material and cause slagging of bed material. The loss of refractories in the separator, dipleg and return valve system will block the return system from slagging and destroy the material circulation. Circulating fluidized bed boiler will become bubbling fluidized bed boiler, evaporation can not be maintained, and boiler will be forced to stop. However, this problem does not exist in pulverized coal boilers, and there are few accidents that affect boiler shutdown because the refractory of cold ash hopper of pulverized coal boilers falls off and slags.
4) Long ignition start-up time. The ignition start-up time of circulating fluidized bed boiler is not only affected by the temperature rise rate of drum, but also limited by the temperature rise of lining material of refractory and wear-resistant layer and the thermal stress it can bear. If the temperature rises too fast, the thermal stress of the lining material of the refractory and wear-resistant layer will exceed the allowable thermal stress and crack. Therefore, the circulating fluidized bed boiler has strict requirements on ignition start-up time and heating rate. The time from cold start to full load of circulating fluidized bed boiler with steam-cooled cyclone separator is generally controlled in 6 ~ 8 hours. However, the pulverized coal boiler does not have a large area of refractory and wear-resistant lining material, and the ignition start-up only considers the temperature rise rate of the drum, and the ignition time is relatively short, and the design load can be reached in 5 ~ 6 hours in cold state.
5) Circulating fluidized bed boilers have wide adaptability to fuels, but strict requirements on coal particle size. The coal particle size of circulating fluidized bed boiler is generally between 0 ~ 10 mm, and the average particle size is between 2.5 ~ 3.5 mm If this requirement is not met, it will bring adverse consequences to the operation, such as the boiler can not meet the design evaporation, the main steam temperature is difficult to guarantee, the carbon content of ash is high, and the heating surface is seriously worn.
6)N2O output is higher than that of pulverized coal boiler. Compared with the combustion process of high-temperature pulverized coal boiler, the combustion temperature of circulating fluidized bed boiler is lower, and the amount of NOx (nitrogen oxides such as NO and NO2) is less, but the amount of N2O is larger. Commonly known as "nitrous oxide", it is a strong greenhouse gas, which has a destructive effect on the atmospheric ozone layer, causing ultraviolet rays to directly irradiate the earth and cause skin cancer. At present, the international concern is the emission of "laughing gas".
7) The wear of the heating surface at the tail of circulating fluidized bed boiler is greater than that of pulverized coal boiler. The fly ash share of circulating fluidized bed boiler is smaller than that of pulverized coal boiler, but the particle size of fly ash is much larger than that of pulverized coal boiler. If the separator effect is poor or the flue gas velocity is high during operation, the heating surfaces such as superheater and economizer are seriously worn.
8) The smoke hood, the core component of circulating fluidized bed boiler, is easy to wear. The transverse scour between the ventilation holes of the hood and the abrasion of the hood by high-speed bed material are likely to cause slag leakage in the air chamber, poor fluidization effect, coking and channeling, which will affect the boiler load. However, the maintenance of the range hood is extremely difficult, so it is necessary to remove dozens of tons of inert bed material from the air distribution plate before reinstalling it. The maintenance cycle is long and the labor demand is large. Pulverized coal boilers do not have this problem.
9) High operation and maintenance cost and short operation cycle. The main body of circulating fluidized bed boiler, including refractory and wear-resistant layer, metal heating surface and hood, is seriously worn, which leads to the daily maintenance cost of fluidized bed higher than that of pulverized coal boiler. Because the accidents of main engine and auxiliary engine are more than those of pulverized coal boiler, the continuous cumulative operation time of circulating fluidized bed boiler is shorter than that of pulverized coal boiler, and the annual operation time of pulverized coal boiler can reach more than 8000h/y, while fluidized bed is almost impossible, so it is good that the operation period can reach 100 day. There is a certain gap with the requirements of safe, stable and long-term operation of chemical system.
10) It is difficult to realize automatic control of circulating fluidized bed boilers. The combustion system of circulating fluidized bed boiler is much more complicated than that of pulverized coal boiler. Pulverized coal boiler does not have the control of bed pressure, bed temperature and air volume of return air system. In addition, the furnace is seriously worn, and the reliability of continuous operation of pressure and temperature measuring points cannot be guaranteed, so automatic control is much more difficult than pulverized coal boiler. The automatic control of the flue gas system can realize the automatic control of single impulse, and the automatic control of the combustion system can be realized by debugging the pulverized coal boiler, which is very good. This is not available in circulating fluidized bed boilers. To sum up, there are more problems in circulating fluidized bed boiler operation than pulverized coal boiler, and the continuous operation hours are shorter than pulverized coal boiler. In chemical industry selection, if the fuel quality is reliable and the fuel sulfur content is low, pulverized coal boiler can be considered. It has the advantages of stable combustion, mature auxiliary technology, high degree of automation, simple operation, long operation cycle and relatively little maintenance, and is suitable for long-term safe and stable operation of chemical systems. On the other hand, if it is based on burning inferior coal, the quality of coal supply is unstable, the sulfur content of coal quality is high, and the environmental protection emission requirements are harsh. It belongs to the heating form of heating, peak shaving and cogeneration, with low desulfurization cost and good adaptability to various coal qualities, so circulating fluidized bed boilers can be considered.