One, the role of secondary wind and matching principle?

2022-07-10 0 By

The primary air ensures the bed temperature, adjusts the bed pressure and ensures the normal fluidization of the boiler, and the secondary air adjusts the oxygen well to ensure the balanced temperature field of material mixing. Do you think the boiler operation is so simple as adding coal and reducing coal?Or do you think after a few days of operating a boiler you know all about operation and adjustment?With a little knowledge of the theory learned from the master can be an armchair strategist?Operating a boiler may be easy, but learning it well is really hard!The function and matching principle of primary and secondary air in circulating fluidized bed boiler?What is the primary air of circulating fluidized bed boiler?What does it do?Circulating fluidized bed boiler primary air refers to the air that is sent into the furnace from the bottom of the air distribution plate to fully fluidize the material layer and control the temperature of the bed to ensure the initial fluidized combustion.The primary air needs to overcome the resistance of the combustion chamber and the air distribution plate, and needs to be supplied by a special high pressure head primary fan.It should be pointed out that the outlet air volume of the primary fan does not necessarily become the primary air, and a small part of the air will become the coal seeding air, sealing air or supplementary air source of the secondary air. After deducting these extra bypass air volume, the remaining 90% or more of the primary fan’s air can really be regarded as the primary air.Conventional 300MW circulating fluidized bed boiler, the primary air chamber pressure is generally 9~15kPa.Primary air pressure is mainly related to the static accumulation density of the bed material, the resistance of the air distribution plate, the resistance of the primary air pipeline, the resistance of the primary air preheater, the size of solid particles and the thickness of the material layer and the bed temperature.Primary air has the following functions :1) Form enough air cushion capacity to lift the material layer, ensure the rapid mass transfer of hot particles in the material layer and new fuel particles into the furnace, promote the mixing of cold and hot particles, uniform bed temperature, and strengthen the heat transfer in the lower part.2) Supplement the necessary oxygen in the initial stage of ignition and the early stage of combustion process to produce a good initial stable ignition and combustion state in the reduction zone, and ensure the boiling combustion share and burnout rate of coarse particle layer.3) By adjusting the primary air, the bed temperature is well controlled to meet the combustion requirements of CFB boiler’s material layer temperature in the range of 850~950.4) Under the action of uniform air distribution plate, uniform fluidization of material layer can be realized, and local fluidization anomalies such as eccentric bed, nodal swell and gully flow can be reduced.5) A strong fluidization combustion process is generated to promote the thermal rupture of coarse particles in the material layer and provide basic fine particle groups for the dense phase area and the area above it, forming the transfer of coarse particles to fine particles.What is the secondary air of circulating fluidized bed boiler?What does it do?The secondary air of CFB boiler mainly refers to the auxiliary air which does not participate in the fluidization of the bottom of the bed layer and is used for the combustion of material particles in dense phase zone and suspended section.The secondary air nozzle is generally arranged along the furnace height direction of two or three layers.Coal seeding wind, limestone wind, feed back wind, bed burner cooling wind, sealing wind and other air over the distribution wind, to a certain extent also equal to a small share of secondary air.The secondary air has the following functions :1) To adjust the operating oxygen, form a graded air supply, and further reduce NO, SO and emissions in the low-temperature combustion mode.2) Increase the mutual movement, friction and impact between particles, which is easy to peel the ash shell on the surface of coke particles, promote its contact with oxygen, ensure the rapid fluidization of the suspension section and the combustion efficiency of fuel in the pneumatic conveying area, and improve the burnout rate.3) The good penetration capacity of high-speed secondary air ensures the sufficient supply of oxygen to particles everywhere in the transverse section of the furnace and reduces the oxygen-poor area of high-concentration material particles.4) The strong disturbance generated by the secondary air accelerates the mass transfer and heat transfer process of particles, promotes the temperature balance of materials in the furnace, and strengthens the heat transfer capacity of the heating surface in the furnace.What is the principle of primary and secondary air ratio?First, the general principle of secondary air ratio is: primary risk control bed temperature, secondary air oxygen regulation.When the load is below 35%, the fixed primary air volume at ignition is generally kept unchanged to ensure that the critical air volume exceeds or approaches complete fluidization. At this time, the secondary air is generally 15%~30% in principle.When the load exceeds this value, the primary air volume and the secondary air volume are increased according to the degree of fluidization and bed temperature and oxygen.When the load has increased to 65% or more, the primary air volume basically stops changing.After that, according to the change of oxygen and bed temperature, the secondary air volume is constantly increased until the corresponding air volume under the maximum evaporation volume is reached.On the premise of proper bed temperature, the secondary air rate should be increased as much as possible, and the primary air rate should be reduced, which is beneficial to the efficient staged combustion of CFB boiler, NO, SO and emission reduction.Wind coal ratio, it is more troublesome to adjust, but compared with wind ratio, it is nothing!The ratio of air and coal will not be adjusted under normal circumstances when the bed temperature is stable and the normal fluidized primary air can be guaranteed.Coal volume and air volume is mainly to see the superheater (export oxygen between 3% ~ 5% this is the best, according to the amount of coal increase or decrease the right amount to adjust the secondary air volume).It is quite troublesome to adjust the ratio of wind and air (the ratio of primary and secondary air, the ratio of upper and lower secondary air, and the ratio of front and rear secondary air). The ratio of primary and secondary air is easy to handle. On the premise of ensuring the lowest fluidization air volume, the primary air volume can be increased or decreased appropriately according to the change of bed temperature, and the change range is not very large.Upper and lower secondary air is mainly to provide a layered combustion air volume, you can look at your factory boiler and secondary tuyere location you will find that this arrangement is reasonable, if increase the secondary air volume you can improve the central chamber of a stove or furnace temperature, increase of secondary air can increase the combustion chamber of a stove or furnace bottom, raise the temperature of furnace bottom,The position of the tuyere of the upper secondary air is about 2 meters above the feed inlet. Improving the upper secondary air can increase the combustion in the middle of the furnace. Improving the temperature in the middle of the furnace is a method to increase the boiler load.The ratio of the secondary air before and after should be the same according to the truth, but according to the actual situation and the position of the coal mouth, relative to the layout of the coal mouth on the front wall of the secondary air to be more, specific according to the actual situation and operation of the furnace adjustment!1, circulating fluidized bed boiler, the main way of the primary fan is to send the air into the primary air chamber, through the air distribution device (hood) into the furnace, so that the bed material in the furnace fluidization.Primary fluidized air is the main medium of heat transfer and carrying in furnace.The size of the primary wind speed determines the fluidization of the bedding material and the adjustment of the bed temperature in the furnace.Primary air is the air source of ignition fan and coal seeding fan, so the amount of primary air is the largest in CFB boiler, accounting for more than 60% of the total amount.2, circulating fluidized bed boiler secondary fan is mainly used to supplement the oxygen combustion in the furnace and strengthen the mixing of materials (part is to control the combustion temperature in the furnace, mainly the secondary air classification is different, the role is not the same).Due to the large proportion of primary air volume in CFB boiler, the demand for secondary air volume only accounts for about 40% of the total air volume.The secondary air pressure is also smaller than the primary air, so the capacity of the secondary fan is also smaller than the primary fan.3. The general principle of primary and secondary air is primary risk control bed temperature and secondary risk control oxygen production;At the same time, the negative pressure at the outlet of the furnace is controlled by adjusting the air volume of induced draft fan.4, the boiler design is different, the primary and secondary air ratio is not the same, according to the specific design requirements to adjust the ratio, not according to the experience of others to adjust their own boiler air volume ratio, generally speaking, the optimal scheme is basically the design scheme;As far as possible to reduce NOX generation in the process of boiler operation, reduce an air ventilation panels, a wind effect can ensure the dense-phase zone in the furnace circulating material fluidization, done by secondary air fuel burning, raise the boiler height of the secondary outlet on the dense phase zone, at the same time strengthen dense-phase zone of secondary air classification, adjust the up and down in the process of boiler operation secondary air ratio, increaseAir volume of secondary tuyere and appropriate excess air coefficient are selected to control flue gas oxygen of boiler outlet.Once the wind and the ratio of fluctuation of the secondary air: a secondary air ratio adjustment, can effectively improve the furnace combustion conditions for circulating fluidized bed boiler, the dense-phase zone is in a very special owe oxygen condition, although there are a number of oxygen in the area, but the CO concentration is as high as 2%, in the show dense-phase zone combustion in a state of less oxygen.At the same time from the point of the behavior of gas-solid two-phase flow, the circulating fluidized bed boiler dense-phase zone there is a bubble and emulsion phase, during which the mass transfer resistance to combustion is more outstanding, the influence of oxygen, CO and the incomplete combustion products of coal particles and release of volatile is not fully mixing, reaction, dense-phase zone despite the presence of oxygen, carbon particles combustion is still owe oxygen status.The study shows that there is an obvious hypoxic zone in the center of the combustion chamber on the secondary tuyere. The main function of the secondary air is to provide enough kinetic energy to penetrate the material space to the center of the combustion chamber and provide oxygen for the combustion of carbon particles.The secondary air is divided into upper and lower two layers, and the different distribution ratio determines the oxygen share in different parts of the furnace.Under the condition of constant total air volume, although excessive primary air volume ensures the strength of material extraction and intensifies internal circulation, it cannot provide oxygen needed for combustion on the secondary tuyere, and carbon particles cannot be fully burned out, reducing the boiler efficiency.Excessive secondary air volume can ensure oxygen needed for combustion, but the reduction of primary air volume will weaken the internal circulation, and can not sufficiently bring heat to the dilute phase region to participate in heat transfer, leading to the bed temperature rise.Therefore, adjusting the proportion of primary and secondary air, upper and lower secondary air, adjusting the penetration and diffusion effect of secondary air can effectively improve the mixing degree of air, coal and bed material in the furnace. Under the premise of ensuring full combustion of carbon particles, the total air volume is controlled in the appropriate range, and the exhaust temperature is reduced by reducing the total enthalpy of flue gas!In circulating fluidized bed boiler, the combustion air is generally divided into primary and secondary air into the furnace, and the determination of the proportion of primary and secondary air mainly depends on the following factors:1) The primary air is sent from the air distribution plate in the dense phase area, and the primary air volume should meet the needs of fuel combustion in the dense phase area. From the perspective of good fluidization, the proportion of primary air is relatively large, and the primary air volume should be matched according to the combustion share in the dense phase area.2) From the perspective of reducing NOX emissions, the excess air coefficient in the dense phase area should be close to 1, so that the dense phase area presents a reducing atmosphere, and the secondary air is sent by the side wall to ensure complete combustion of fuel. The primary air must be fed with sufficient secondary air segmented air supply combustion.To sum up: primary air proportion: 50—-60% secondary air proportion: 40– 50% When burning low-quality volatile fuel, the use of a higher primary air rate.Lower primary air rates are used when high volatile fuels are used.When the furnace flame center is too high, the top secondary air baffle should be opened.When the furnace flame center is too low, the lower secondary air baffle should be opened.When the center of the furnace flame skew, the rigidity of the primary air should be improved to prevent the formation of reducing gas in the burner area, and to prevent coking and scouring the water wall.When the unit is in normal operation, the outlet pressure of the furnace should be kept between 0 and -100Pa.Keep flue gas oxygen in the range of 3 ~ 5%;In order to prevent coking and ash accumulation in the furnace, the air volume can be appropriately increased and the primary and secondary air can be rationally proportioning.In operation, control the smoke exhaust temperature < 130℃, maintain oxygen 5 ~ 6% when 150MW load, maintain oxygen 4 ~ 5% when 200MW load, maintain oxygen 3 ~ 5% when 250MW and above load, adjust the upper secondary air door appropriately, so that the NOx content in the flue gas meets the standard.Unit load changes, timely adjustment of air volume, coal volume, to maintain steam temperature, steam pressure stability.When the load is added, the boiler air volume should be increased first, and then the coal quantity should be increased;Load reduction, should first reduce coal feed, then reduce boiler air volume.