What are the different requirements for process operation when using lignite as fuel in circulating fluidized bed boilers?

This post was last edited by zgj2405 on 2011-8-26 at 12:53. Since lignite has a moisture content on a basis of its ash content of over 30%, with internal moisture as high as 11%, the coal particles after being crushed come into contact with hot materials at around 860°C when they enter the furnace; these particles are rapidly heated, causing the internal moisture and volatile substances to expand and be released, which results in the particles bursting. Therefore, when using this type of coal, the particle size of the raw coal can be allowed to range from 12 to 15 mm； Furthermore, due to the high volatile content of this coal, its ignition temperature is reduced, allowing it to ignite at around 500°C in a fluidized state ； Furthermore, due to the low ash content in this coal and the small amount of slag, the reliability of the boiler is greatly improved ； Furthermore, due to the low SiO2 content in the ash of this coal, it causes less wear on the boiler’s heating surfaces. Taking all the above points into account, boilers that burn lignite have significant advantages. The furnace chamber features a pant-leg and dual air distribution plate design, while the evaporation heating surfaces inside the furnace utilize a membrane-type water wall along with water wall extension walls. Water-cooled air distribution plates and large-diameter bell-type air caps are used. 　　Two high-temperature insulated cyclone separators are arranged on each of the left and right sides at the upper part of the furnace chamber; the upper part of these separators is cylindrical, while the lower part is conical. A return valve and an external heat exchanger are arranged under each return leg of the high-temperature adiabatic separator. The return material valve is of pneumatic self-balancing type, and the fluidization air is supplied by a high-pressure blower. One side of each return valve is connected to the furnace, while the other side is connected to an external heat exchanger. Part of the high-temperature material separated by the separator is sent back directly to the furnace, while the other part passes through a cone valve into an external heat exchanger. The distribution of the material between the external heat exchanger and the return valve is controlled by adjusting the opening degree of the cone valve. Four external heat exchangers are symmetrically arranged at the lower parts on both sides of the furnace chamber. High-temperature reheaters and low-temperature superheaters are installed within the two external heat exchangers located near the furnace; the main function of these external heat exchangers is to regulate the temperature of the reheated steam ； Superheater I and Superheater II are located within the two external heat exchangers situated behind the furnace; the primary function of these external heat exchangers is to regulate the bed temperature. 　　The furnace, separator, return valve, and external heat exchanger constitute the material circulation loop of a circulating fluidized bed boiler; coal and limestone come into contact with each other repeatedly within the combustion chamber, thereby enabling the combustion of coal and the desulfurization reaction to take place. Flue gas and fine particulate ash enter the tail convective flue, while the ash is collected by the electrostatic precipitator. To reduce heat loss of fuel to the outside and prevent wear on the heated surfaces and furnace walls, a large amount of wear-resistant and fire-resistant material is used as lining inside circulating fluidized bed boilers. This material is mainly installed in the water-cooled air chambers, the dense-phase zone of the combustion chamber, the cyclone separators, the return valve, the external bed, and the slag cooler. Key control points during operation 　　1. Coking prevention control 　　Circulating fluidized bed boilers rely on primary air at the bottom of the boiler to fluidize the material and enable cyclic combustion; their circulation circuits are divided into internal circulation and external circulation. The internal circulation takes place within the furnace, with the fluidization force being the primary air ； The external circulation takes place within the separator, return valve, and external bed, with high-pressure fluidization air serving as the fluidization force. Coking on the bed surface refers mainly to the coking of the material on the air distribution plate due to poor fluidization and overheating. 　　(1) Methods for preventing poor fluidization: ① It is necessary to ensure that the small holes in the air distribution plate nozzles remain unobstructed; this requires cleaning those holes as well as the surface of the bed before adding material to it ； 　　②After operation, the primary air volume must be greater than the critical fluidization air volume ； 　　③During the process of increasing temperature and pressure, control the rate of temperature rise to prevent the wear-resistant and fire-resistant materials inside the furnace from falling off and blocking the air caps ； ④The particle size of the raw coal should be kept between 6 and 10 mm, to prevent poor fluidization due to excessively large particle sizes ； 　　⑤Control the content of gangue and iron pieces in the coal, and regularly remove large particles to ensure good fluidization. 　⑥During load increase and adjustment, coal feeding and air adjustment should not be changed abruptly. 　　(2) Methods for preventing and controlling overheating-induced coking: ① Since the deformation temperature of the Xiaolongtan lignite ash is only 1060°C, the bed temperature must not be allowed to rise too high; it should be maintained between 860°C and 900°C during operation ； The bed temperature control in the furnace’s air distribution plate system is achieved by adjusting the amount of material returned to the external bed and the height of the material in the dense phase zone ； The bed temperature of the return material valve air distribution plate, the external bed air distribution plate, and the cold slagger air distribution plate is controlled by adjusting the volume of high-pressure fluidizing air and the amount of bed material. 　　②Maintain an appropriate bed pressure to prevent the coal from coming into direct contact with the air cap, which could lead to coal accumulation, deflagration, overheating, and coking. 　　③During the ignition start-up phase, the air supply to the fuel injectors is controlled properly to ensure complete combustion of the fuel, thereby preventing unburned fuel mist from adhering to the coal particles and causing coking. 　　2. Prevention and control of bed instability: Since the furnace of this boiler is trapezoidal in shape, dual air distribution plates and dual combustion chambers are employed. The burners located beneath the bed are placed under each of the two air distribution plates, while the oil injectors positioned above the bed are situated below the dense-phase regions of the two combustion chambers. The coal feeding points are located above the two combustion chambers, with two sets of coal feeding systems on each side. Therefore, if there is a significant difference in the combustion loads on both sides, it may lead to instability in both beds, causing fluctuations in bed pressure; in severe cases, this can affect the safe operation of the unit. During operation, the following aspects should be given attention to: (1) The coal feeding amount, return material amount, and slag discharge should be controlled properly to ensure consistent bed pressure on both sides. 　　(2) When adjusting the coal feeding rate, it is necessary to ensure uniform coal supply at all points so that the coal is distributed evenly across the entire bed surface. If the coal feeding rate on one side is reduced, the feeding rate on the other side should be decreased immediately, in order to keep the pressure difference between the two sides of the furnace at less than 2.5 kPa. 　　(3) The return material amounts for the external beds on both sides of the furnace should be adjusted to be roughly the same, in order to prevent deviations in bed temperature and pressure caused by differences in the return material amounts. 　　(4) Adjust the air volume and coal feeding rate on both sides of the furnace to ensure balanced bed temperatures and primary air flow on both sides. 　　3. Prevention and control of superheated steam temperature 　　Due to the low bulk density of the fly ash from Xiaolongtan lignite (0.85 t/m3), the proportion of fly ash in the dilute phase increases, resulting in deviations in the heat distribution within the furnace compared to the design values. Therefore, it is necessary to strengthen the control of various air flows, cooling water amounts, and the external beds, in order to prevent incidents of excessive superheated steam temperature and reheat steam temperature. During debugging and operation, the following aspects should be given attention to: (1) When increasing or decreasing the load during operation, it should be done slowly; when increasing the load by adding coal, the extent of the adjustment should not be too large, and air should be added first before coal is added. Similarly, when decreasing the load by reducing coal usage, the extent of the adjustment should also not be too large – coal should be reduced first before air flow is decreased, and this process should be carried out in small amounts multiple times. When the coal feeder runs out of coal, the amount of primary air supply should be reduced immediately to prevent coal particles from burning inside the separator, which could cause an increase in the smoke temperature at the separator’s outlet and lead to overheating of the superheater and reheater. After restoration, the rate of adding coal and air should not be too fast, to ensure rapid ignition of the coal. 　　(2) Due to the high water content in the Xiaolongtan lignite, although its thermal explosion properties are good, the particle size of the coal fed into the furnace should still be kept between 8 and 10 mm. 　　(3) Adjust the fluidization air volume of the external bed appropriately to prevent poor fluidization and coking in the external bed, which could lead to excessive temperatures in the superheater and reheater. Adjust the feed rate to the external bed appropriately to keep the steam temperatures at the inlets and outlets of each stage of superheaters and reheaters within the designed range. 　　(4) Adjust the amount of primary air flow to ensure a proper distribution of heat between the dense-phase and dilute-phase regions in the furnace, control the flue gas temperature at the furnace outlet, and maintain reasonable heat transfer coefficients in the exhaust gas superheater and reheater tubes, thereby preventing overheating due to excessively high heat transfer coefficients. 　　(5) Strengthen soot blowing on the rear heating surfaces to reduce ash accumulation and the overheating of steam temperature caused by deviations in flue gas temperature. 　　(6) When the superheater is shut down or operating at low load, pay attention to adjusting the amount of water used for temperature reduction as well as the amounts of primary and secondary air fed into the furnace. 　　(7) During operation, the bed pressure can be appropriately increased to raise the ash concentration in the furnace, ensuring an appropriate amount of heat absorption in the furnace chamber. 　　4. Coal clogging prevention and control: Circulating fluidized bed boilers do not have a coal powder preparation system; coarse and fine coal crushers break the raw coal into particles of 6–8 mm in size, which then enter the coal hopper and from there, via the coal feeder, directly into the furnace. Due to the increased surface area of the crushed coal particles, and considering that the total moisture content of Xiaolongtan lignite is as high as 30% while its internal moisture content is as high as 11%, coal clogging easily occurs in components such as coal crushers, raw coal hoppers, and the coal feed ports of feeders. Coking of the coal will directly threaten the stable operation of the boiler. The main faults include: (1) Blockage of the coal crushing equipment: This occurs when coal sticks to the outlets and inlet pipes of the crusher, resulting in poor coal flow and interruption in coal delivery, or when coal sticks inside the crusher itself, causing it to become blocked ； 　　(2) Coal blockage in the raw coal hopper: Coal blockage in the raw coal hopper occurs because the crushed coal particles are compressed within the hopper, resulting in a buildup that prevents smooth flow of coal ； Furthermore, the raw coal hopper is designed to be square, which increases the contact area between the raw coal and the hopper; this increased resistance to the flow of coal leads to clogging of the hopper ； 　　(3) Coal blockage at the coal discharge opening: The coal particles that enter this opening are heated by the returning ash, which causes a large amount of moisture within the coal particles to evaporate. The rising water vapor accumulates at the coal discharge opening and condenses into water droplets, ultimately leading to the coal particles forming bridges that block the opening. 　　(4) During operation, it is necessary not only to strengthen the monitoring and maintenance of the coal feeding equipment but also to check whether there is coal blockage in the aforementioned areas; if such blockage occurs, it should be cleared promptly. Once coal feeding resumes, attention should be paid to controlling combustion and steam temperature. 　　5 Prevention and control of wear-resistant and refractory material shedding Since circulating fluidized bed boilers have a large amount of wear-resistant and refractory materials lining their air ducts, burners, water-cooled air chambers, furnace, cyclone separators, return valves, external beds, and slag coolers, improper control of temperature rise can lead to the shedding of these materials, thereby affecting material fluidization and the safe operation of the boiler. To prevent the wear-resistant and fire-resistant materials from falling off, it is necessary to ensure high installation quality at the time of installation. During drying, the process must be carried out in accordance with the temperature rise curve specific to these materials, and the moisture content must be reduced to below 2% ； During ignition and startup, ensure that the overall temperature rise of the furnace chamber as well as the local temperature rise in various sections is less than 100°C/h. Also, pay attention to maintaining a proper ratio of air flow rates to avoid localized overheating.

What should be the negative pressure levels at the top and bottom of a circulating fluidized bed boiler? Is it permissible to install observation doors that can be opened at any time to check for loose material or coking issues?