As we all know, fluidized bed boilers are divided into two categories: bubbling fluidized bed boilers (CF-BB) and circulating fluidized bed boilers (CF-BB). There is no clear and authoritative classification between the two. Some people advocate the classification by fluidization speed, but from the point of view of gas-solid two-phase dynamics, wind speed is meaningful relative to particle size and density. Others argue that the dense zone is distinguished by bubbling bed or turbulent bed or fast bed. However, the boiler uses wide screen fuel, and the boiler with coal ash as bed material is a drum bed, so the classification method is still not comprehensive. Some people take whether there is a gray cycle as the standard, and so on, all of which pay attention to one thing and lose sight of another. In my opinion, it may be divided from the mechanism of combustion. The combustion of fluidized bed boiler mainly occurs in the dense phase area at the lower part of the furnace. For example, as recommended in the Technical Manual for Industrial Boilers (Volume II) compiled by China, the proportion of dense phase zone of general coal gangue bituminous coal, lean coal and anthracite is as high as 75%-95%, and the air required for combustion is mainly sent to the bed with primary air. The primary air share of circulating fluidized bed boiler is generally 50%-60%. The combustion share of dense bed is affected by fluidization velocity, fuel particle size and properties, bed height and bed temperature. , fluctuating up and down in the above values. The rest of the fuel is suspended in the dilute phase region at the upper part of the furnace, so the combustion mechanism is that BFBB is close to the layer furnace, while CFBB is closer to the chamber furnace. There are great differences between them in this respect, so it seems more reasonable to separate them.
The combustion share of dense bed in bubbling fluidized bed boiler is large, so it is necessary to arrange buried heating surface to absorb combustion release. The heat transfer coefficient of buried tube is as high as 220-270 kW/mc, which is much higher than 100-500kw/m2℃ of the heating surface of CFBB furnace. Although the ratio of heat transfer coefficient in dilute phase zone of fluidized bed boiler is very low, the share of heat absorption in dilute phase zone is very small. Generally speaking, the heating surface of small-capacity boilers uses less steel, and BFBB combustion mainly feeds coal in the phase bed. Although the buried pipe has been worn, the horizontal buried pipe can be used for five years and the vertical buried pipe can be used if proper anti-wear treatment is carried out.
The cross-sectional heat load of CFBB is 2-3 times that of BFBB (the heat load increased from top to bottom is not one layer), which is beneficial to scale, uniform temperature in the furnace, low emission of atmospheric pollutants and high combustion efficiency (up to more than 99%). It is an improvement of BFBB technology and has superior performance. However, because the separator can't catch fine coal, it needs a higher furnace, which requires higher crushing granularity and operation control of coal. Therefore, CFBB furnace type has no obvious advantages for small and medium-sized boilers, so some foreign researchers believe that BFBB is suitable for the capacity below 50t/h and CFBB is suitable for the capacity above 220t/h, both of which exist in the range of 50-220t/h. ..
In the past many years, nearly 3,000 fluidized bed boilers have been built in China. Although it has played a great role in burning inferior coal, it has been running at a low level, with a large amount of fly ash, high carbon content and low boiler efficiency. In addition, the investment in dust removal is insufficient, and the dust control has not been well solved, which leads to the poor reputation of the fluidized bed. After the appearance of CFBB, people branded the circulating fluidized bed boiler, and introduced many types of boilers, such as the circulating bed boiler with low portability introduced by Tsinghua, and the circulating bed boiler with buried tube trough separator developed by Harbin Institute of Technology and Northland. In fact, they are all BFBB. However, they are improved boiling furnaces, which raise the technology of boiling furnaces to a higher level. These furnace types have strong vitality in both industrial boilers and cogeneration boilers. We should cheer for the new achievements of BFBB, instead, restore its reputation and develop this BFBB within a certain boiler capacity.
China has the largest number of fluidized bed boilers in the world and has long-term operation experience, so the improved fluidized bed boiler technology has a high maturity. However, CFBB technology needs to be improved and improved. In the selection of many furnace types, we must first distinguish whether it belongs to BFBB or CFBB, and then consider other technical indicators and reliability. The following chapters of this paper are mainly aimed at CFBB, and are applicable to some common technologies.
fluidizing velocity
The most direct and main influence of fluidization speed on CFBB is its lifting and entrainment of circulating materials. With the increase of V, the entrainment increases rapidly. In the early stage, foreign CFBB, such as Luqi Technology, had a V as high as 8- 12m/s, but with the problems of wear and energy consumption caused by high flow rate, it has gradually dropped to about 6m/s at present. In China, CFBB technology developed late. In the early stage, due to the above problems, some furnaces were designed with low V (4-5m/s), and recycled materials were found.
Particle size and quality analysis of coal
The fluidization speed of CFBB is very high, which can fluidize bed materials with large particle size. It can be seen from the literature that the range of coal particles into the furnace can reach 0- 12, 0-20, 0-25mm and so on. The allowable range varies with different manufacturers and coal types, wider than BFBB, and the maximum allowable particle size is also large. However, according to our research and some foreign literature reports, the average particle size of the fuel used by CFBB is actually much smaller than that of BFBB. The average particle size of fuel in BFBB is as high as 1-2mm, while that in CFBB is only 300-400μ m. Strictly speaking, CFBB requires a large proportion of fine particles in the fuel whose terminal velocity is less than the fluidization velocity, so that these fine particles can be blown into the suspended section space for combustion as soon as they enter the furnace, and at the same time, the amount of circulating materials can be increased. The influence of fuel particle size is mainly manifested in the influence on the combustion share and material balance of dense bed. There are many fine particles in fuel, small combustion share in dense bed and many circulating materials.
The determination and selection of particle size distribution of CFBB fuel is related to the choice of fluidization velocity. It can be seen that the particle size has a great influence on them. The selected particle size distribution should ensure that there are enough fine coal particles blown into the suspension section under the condition of determining the fluidization speed, so as to ensure the upper combustion share, form enough bed material and maintain the material balance.
The main factors affecting the particle size of fuel are the thermal explosion properties and volatile content of coal. The coal with strong thermal explosion can choose a larger particle size, and the larger coal particles can increase their share after thermal explosion. At this time, the particle size distribution of coal can be relaxed.
The ratio of primary air to secondary air
The air required for combustion is divided into primary air and secondary air, which are sent to the fluidized bed combustion chamber from different positions to form a reducing atmosphere in the dense bed and realize staged combustion, which can greatly reduce the generation of thermal NOX, which is one of the main advantages of CFBB. However, the purpose of dividing primary air and secondary air is not limited to this. The primary air rate (the share of primary air volume in the total air volume) directly determines the combustion share of dense bed. Under the same conditions, the primary air ratio is large. This will inevitably lead to a high combustion share of dense bed. At this time, more low-temperature circulating materials need to return to the dense bed to take away the heat released by combustion to maintain the temperature of the dense bed. If the circulating material is not enough, the temperature of the fluidized bed will be too high, and no more coal can be added and the load will not go up. This material for cooling the bed may come from the cooled circulating ash collected by the separator or from the circulating ash falling along the membrane wall around the furnace. During the falling process, the ash will be cooled in contact with the membrane wall.
From the combustion and heat balance of dense bed, the smaller the primary air rate, the lower the material balance requirement for circulating ash. But in fact, the choice of primary air rate is also restricted by factors such as fuel particle size and properties. The small primary air rate requires that the proportion of large particles in the fuel that cannot be blown into the suspension section should be small, otherwise the large particles will burn incompletely due to insufficient oxygen, and the discharged bed ash has a very high carbon content, and the primary air rate is generally about 50%.
Generally, secondary air is injected into the furnace on the dense bed, which can supplement the air needed for combustion and also play a disturbing role in strengthening gas-solid mixing. The lower part of CFBB furnace is designed as a cone, and the secondary air can be divided into several streams and sent from different heights to keep the flue gas velocity in the furnace relatively uniform. The position of the secondary air outlet also has a great influence. For example, if the secondary tuyere is set at a place with high ash concentration in the transition zone above the dense bed, more carbon particles and materials can be blown into the space, and the fuel share and material concentration in the upper part can be increased.
segregator
No one will doubt the important role of separator in CFBB. Without separator, there would be no CFBB. Because of this, considerable attention has been paid to the research of separator at home and abroad. The type and structure of separator is one of the distinguishing signs of CFBB schools.
The main performance index of CFBB separator is still separation efficiency, which must be high enough, one is to provide enough circulating materials, the other is to collect fine carbon particles and send them back to the furnace for re-combustion, so as to improve combustion efficiency. The main part of CFBB circulating material is 200-300 WM particles. The designed separator not only has high separation efficiency (> 99%) for this particle size, but also should be as small as possible to improve the carbon burnout rate. The carbon content analysis of CFBB fly ash shows that the carbon content reaches the peak at a certain material particle size, and then decreases. The particle size corresponding to this peak is closely related to the efficiency of the separator.
At present, the separators used in CFBB are mainly divided into two types, cyclone separators and inertial separators. Generally speaking, the cyclone separator has higher efficiency and larger volume, and the inertia separator is less efficient, but its volume is small, which makes the boiler structure more compact.
According to the operating conditions, separators can be divided into two categories: high temperature separation and medium temperature separation. As far as the influence on boiler performance is concerned, high temperature separation is superior, because the high concentration of solid materials in CFBB furnace leads to poor mixing and high CO concentration in the furnace. The secondary combustion in the high temperature separator can reduce the CO concentration, and the temperature increase caused by the secondary combustion is beneficial to the reduction of N2O and N2O emission concentration.
The selection of separator should also consider the capacity range of boiler and make technical and economic comparison. For example, if a cyclone separator is used in a small industrial furnace, the furnace must be high enough considering that both the cyclone and the dipleg need to have a certain height, otherwise reducing the height of the cyclone and the dipleg will inevitably affect its performance. At this time, it is necessary to conduct a comprehensive analysis of technology and economy.
Ash returning device
Except for a few mechanical valves (such as Luirgl's conical valve), the ash return control device of CFBB ash circulation system generally adopts mechanical valves, such as J valve, L valve and V valve. Non-mechanical valves have no moving parts, and their opening and closing are controlled by gas supply, which is self-evident.
Non-mechanical valves are divided into two categories: self-balancing and adjustable. J-valve, V-valve and ring seal are all self-balanced, that is, the outflow is automatically adjusted according to the inflow, and the valve itself has a weak function of adjusting the flow. L-valve is adjustable, so the flow can be adjusted as needed. The author knows from his own practice that the biggest problem in the operation of L valve is the material level measurement in the vertical section of the valve. Because the material level in the vertical section is too low, the loose wind may not take the ash out of the horizontal section, but blow it up from the vertical section, which will not seal the valve and may also lead to coking. Pay attention to this problem.
In the design of non-mechanical valves, firstly, we should pay attention to the selection of proper ash flow cross section; secondly, if the ash is high temperature ash, we should calculate the heat balance in the valve, that is, oxygen in loose wind burns in contact with carbon in the ash, and part of the released heat is converted into the enthalpy of hot flue gas, and the rest of the heat heats the circulating ash and becomes the sensible heat of the ash. The temperature rise of ash should be controlled to prevent coking due to too high ash temperature, which is also part of the reason for the development of water-cooled material feet abroad in recent years.
Wear of heating surface
There is a buried heating surface in BFBB dense bed, and the metal surface has been worn to some extent due to the scouring of fluidized bed materials. The wear of BFBB mainly occurs on the buried pipe, and the heating surface of the buried pipe is not arranged in the dense layer of CFBB, so the wear problem has not been solved. Due to a little carelessness in design, serious wear may occur in any part of the furnace and ash removal system.
In terms of mechanism, metal wear can be divided into two types: one is that the metal surface gradually loses weight due to friction under the scouring of solid particles, and the other is that an oxide film is formed on the metal surface, which is hard but brittle. Under the scouring of material particles, the oxide film quickly peels off, forming a new oxide film on the peeled metal surface, and the wear is carried out in this process. The hardness comparison between oxide layer and other substances is given in the following table (3): Table 1 Material Hardness Table (at 20℃).
Material Limestone Silicate Steel Coating Oxide Film
Hardness (HV)140-160 800130-250 500-1800 600-1800.
It can be seen that the hardness of the oxide film is extremely high. If an oxide film can be formed on the surface of the pipeline, it is extremely beneficial to reduce wear. The formation rate of oxygen film is very important. If it is less than the wear rate, an oxide film cannot be formed on the metal surface. It is found that when the wall temperature is higher than 300 degrees Celsius, the oxide film is easy to form.
The dense layer of CFBB is generally in a reducing atmosphere, which is not conducive to the formation of oxide film on the metal surface. The pipe can be covered with wear-resistant materials to avoid serious wear. At the junction of reducing and oxidizing atmosphere, because this interface will fluctuate up and down, it will also lead to increased wear, so it should be treated as a reducing zone.
At the junction of the vertical section and the conical section of the lower wall of the furnace, the top of the furnace and the outlet of the furnace. All parts are prone to serious wear, so the structure should be considered or anti-wear measures should be added in the design. The wear of the tail convection heating surface is also a problem that must be paid attention to, and some CFBB units put into operation in China have worn out. Some people think that CFBB is equipped with a separator, and the concentration of fly ash in the tail flue is lower than that in BFBB. This understanding is not comprehensive. The installation of separator will send the collected ash back to the furnace, which will lead to the increase of ash concentration in the furnace. Separators have been designed for this high ash concentration. In order to maintain the ash circulation required for normal operation, the separation efficiency is as high as 99%. Although it is so high, the absolute value of the discharged ash may still be high because the high concentration separator in the furnace failed to collect it. In the tail flue, the flue gas flows downward, and the particles flow with the flue gas while being subjected to gravity. The absolute velocity of particles is flue gas velocity, and the particle size is large, which leads to serious wear of the heating surface at the tail of economizer. If there is a big gap between the elbow of the tube bundle and the tube wall of the heating surface at the end of the economizer, a flue gas corridor will be formed and the wear will be accelerated. The relationship between metal wall wear rate and velocity is 3-3.5 power, and the relationship between wear rate and ash particle diameter is square. In the design of the tail flue, the above factors should be fully considered, the appropriate wind speed should be selected, and the reasonable structure should be designed to avoid serious wear of the heating surface.