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Air preheater for CFB boiler

key word:Heat exchange element

Category:


Product description

Air preheater for CFB boiler

 

1. Introduction

1.1. Purpose

The capacity of circulating fluidized bed (CFB) boilers is increasing continuously. The installed capacity of units has reached 350 MW. At present, several boiler manufacturers have designed 600 MW units. At the same time, with the continuous increase of unit capacity, it is increasingly urgent to improve boiler efficiency (reduce exhaust gas temperature) and reduce investment. Considering the above two requirements, compared with tubular air preheater which is widely used in small units, rotary regenerative air preheater is more widely used for units with capacity over 100MW. The obvious advantages and attractiveness of rotary regenerative air preheater in dimension and cost determine that it will be the first choice for future application.

However, compared with the traditional balanced draft pulverized coal boiler, the inlet air pressure of CFB boiler is much higher. This not only increases the pressure load of the system, but also potentially increases the air leakage rate of the air preheater. Therefore, in order to better adapt to this harsh condition, the air preheater must be specially designed. In addition, in the selection and design of air preheaters with desulfurization (SO2) and denitrification (NO2) systems upstream, the washable characteristics of heat exchange elements should be considered to further resist the side effects of limestone or escaping ammonia carried in flue gas on air preheaters.

1.2. Model meaning

30.5 (G) V(H) N(A,S,F) (T,Q,C) (1500)

30.5 ------ rotor flow area serial number

G -----G refers to flue gas heat exchanger; without this item, it indicates air preheater

V ------- V represents vertical arrangement of rotor shaft; H represents horizontal arrangement of rotor shaft

N (A, S, F) ---- N indicates that the sector plate and the axial sealing plate are not adjustable; A indicates that the hot end sector plate is dual position adjustable; S indicates that the hot end sector plate is adjustable by automatic tracking; F indicates that the hot end sensor is adjustable

T. Q, C - the form of air preheater (without this item, it is divided into two parts; T is three parts; Q is four parts; C is ring sleeve type)

1500 -- total height of heat exchange element (mm)

1.3. Arrangement of air preheater for CFB boiler

Figure 5-1 shows the general layout of the alholmen CFB boiler with flue gas flow, including fans and rotary air preheaters.

For ventilation equipment, although the unit is only equipped with one air preheater, all fans of air duct and flue gas flow are dual configuration, as follows:

1.3.1. 2 sets of centrifugal primary air fans (PA) to provide fluidizing air. In order to reduce the fan power and improve the efficiency, the two sets of fans are arranged and operated together with the forced draft fan.

1.3.2. 2 sets of single-stage axial flow fans with adjustable moving blades, providing secondary air and over burning air.

1.3.3.2 sets of two-stage axial flow fans with adjustable moving blades are used for flue gas emission.

Figure 5-1 general layout of typical CFB boiler (including rotary air preheater)

 

2. Performance index and design condition of CFB boiler air preheater

2.1. Performance index of air preheater (specified by the user)

2.1.1. Temperature.

2.1.2. Resistance.

2.1.3. Air leakage rate.

2.2. Design conditions (considering the following factors)

2.2.1. The pressure of air and flue gas and the pressure difference between smoke air flow.

2.2.2. Fuel type, ash Plugging Tendency and washability.

2.2.3. Low maintenance design.

2.2.4. The initial investment and operating costs are minimized.

2.3. Design of VN rotary air preheater with four compartments

2.3.1. Temperature, resistance and air leakage rate of air preheater:

2.3.1.1. Temperature. Since the reduction of exhaust gas temperature is mostly achieved by further improving the heat transfer performance of air preheater, rather than by reducing the inlet flue gas temperature of air preheater, the rotary regenerative air preheater is more competitive. At the same time, the volume and mass of rotary regenerative air preheater are much smaller, so the cost is greatly reduced.

2.3.1.2. Resistance. The resistance of the air preheater directly affects the power consumption of the fan. Therefore, the performance of the air preheater will have a great impact on the economic operation of the whole unit.

The resistance of the air preheater can be reduced by increasing the flow area and reducing the flow velocity, so as to reduce the power consumption of the fan. Due to the compactness of heat transfer surface and low resistance level of rotary regenerative air preheater, with the continuous increase of unit capacity and the decrease of exhaust gas temperature, rotary regenerative air preheater has gradually become the preferred scheme. Taking alholmen project as an example, the resistance level and exhaust gas temperature of rotary air preheater are lower.

When calculating the type selection of rotary air preheater and determining its final size (i.e. rotor diameter), it is noted that 1/2 is added to each wall of silo air preheater (the rotor diameter is increased by about 5%), and the resistance is reduced by about 20%. Therefore, the power consumption of the fan can be greatly reduced. However, it must be considered that after increasing the type of air preheater, the air leakage rate and fan power consumption of the air preheater will be reduced And equipment investment also increased slightly.

2.3.1.3. Air leakage rate. The air leakage control of rotary air preheater is very important. In order to compensate the air leakage of air preheater, the inlet air flow of air preheater and the power consumption of forced draft fan must be increased. In the rotary air preheater, most of the air leakage is the direct leakage of air into the flue gas through the seal gap. Figure 5-2 shows the typical air leakage path of rotary air preheater with two compartments.

2.3.2. Flue gas pressure and air to flue gas side pressure difference of CFB boiler air preheater. Compared with the traditional pulverized coal boiler, the air pressure sent to CFB boiler is much higher. After the air flow pressure increases, the structural strength and stiffness of the air preheater are required to be much greater than those of the same kind of equipment in the ordinary pulverized coal boiler. The top load-bearing components of the air preheater must be appropriately thickened to ensure sufficient strength and stiffness.

Another important point in the design of multi compartment air preheater is that the pressure acting on the top and bottom of the air preheater and flowing through the air preheater are different, so the bending moment produced by the above pressure difference on the rotor of the air preheater must be comprehensively considered. A part of the force in the air duct of the air preheater can be well balanced, so the design of the air preheater mentioned above can be simplified by using the structural stress characteristics of the four compartment air preheater.

The pressure difference between air side and flue gas of air preheater increases with the increase of air side pressure. Table 5-1 shows the comparison results between the hot end flue gas pressure difference and the design air leakage rate level of the air preheater for typical pulverized coal fired boilers and alholmen CFB boilers burning peat.

Table 5-1 comparison of design level of pressure difference and air leakage rate between pulverized coal fired boiler and CFB boiler

Unit type Pulverized coal boiler CFB
Application site Typical Alhomens
working condition 100%MCR 100%MCR
Fuel Design coal Peat
Air preheater type Tri-section Quarter-section
Pressure balance of hot layer (kPa) Secondary air/flue gas 2.9 8.1
Primary air/Secondary air 6.2 7.7
Primary air/flue 9.1 15.8
Leakage rate%) 5.72 5

It should be noted that up to now, the pressure difference between primary air and flue gas side in alholmen CFB boiler is the largest (15.8kPa), which is about 17 times that of typical pulverized coal boiler (9.1kPa).

In addition, the pressure difference between secondary air and flue gas in alholmen CFB boiler is 8.1kPa, which is 2.8 times of that of typical pulverized coal boiler, and is only 11% smaller than that of primary air to flue gas (9.1kPa) of air preheater of typical pulverized coal boiler.

It can be seen from the data in Table 5-1 that the design air leakage rate of alholmen CFB boiler air preheater is lower than that of traditional pulverized coal boiler air preheater.

Obviously, in order to meet the above stringent design requirements, it is necessary to improve the sealing system of the air preheater of the traditional pulverized coal boiler in a large range. For example, the air preheater commonly used in the traditional pulverized coal boiler is changed into the four compartment design, as shown in Figure 5-3. The biggest advantage of the four compartment air preheater is that it arranges the high-pressure primary air compartments between two secondary air compartments to overcome the high-pressure difference between the primary air and the flue gas side.

2.3.3. Number of sealing strips in sealing pair. Before the 1970s, the number of rotor compartments of all air preheaters almost did not exceed 24, that is, the angle of each rotor compartment was not less than 15. As shown in Fig. 5-4, the above-mentioned rotor compartment forms a sealing pair with a fan-shaped plate with fixed width. In this way, only one radial diaphragm and sealing strip of the rotor pass through the sector plate at any time, which is known as the single seal.

At the end of 1960s, Howden was the first professional air preheater manufacturer in the world to propose the use of adjustable seal baffle technology to compensate for the hot end seal clearance and leakage area increased by the hot "mushroom" deformation of the rotor. However, the long-term operation experience shows that the air leakage rate of the air preheater with adjustable seal plate will rise sharply during the operation of the unit. As shown in the previous curve in Figure 5-5, the air leakage rate of a two compartment air preheater with adjustable sector plate will generally double after 2-3 years of operation.

With CEGB (Central Electricity Generation - ting board, After a lot of extensive research and development work, Howden has established the theoretical basis for the increase of air leakage rate of air preheater, that is, when the sector plate is adjustable, a set of flexible sealing device should be added to ensure the sealing between the fan-shaped plate and the static parts when it is adjusted and moved; Under the condition of long-term high temperature and fly ash scouring, the sealing effect of the above flexible sealing device will continue to deteriorate, and the secondary air leakage will increase accordingly.

According to the above research results, Howden has adopted the fixed double seal  structure shown in Fig. 5-6, namely VN sealing system, and obtained a patent. In order to achieve double sealing effect in the sector plate structure with the same width, the number of radial diaphragms of the rotor is increased by 1 times. For the seal pair with the same seal clearance, the double seal structure can reduce the direct air leakage by 1/,  which can make up for the adverse effect of the increase of the seal gap at the hot end caused by the "mushroom" deformation of the rotor, which provides conditions for abandoning the adjustable seal design. Since the VN seal design cancels all moving parts and does not rely on the sealing device with long-term continuous friction movement or expansion movement, the variation trend of common leakage rate is shown in the next curve in Figure 5-5. The improvement of the sealing system completely eliminates the defect that the air leakage rate of the adjustable sealing system rises sharply in the long-term operation time.

For the air preheater of alholmen CFB boiler, due to the user's extremely low requirement on the air leakage rate, in addition to the double sealing structure adopted between the primary air side and the secondary air side, the three seal design is also adopted on the fan-shaped plates at the secondary air side and the flue gas side. In this way, the calculated air leakage rate can be reduced by about 0.75%.

Like other air preheaters provided by Howden, the design of alholmen air preheater follows the guiding principles of minimum initial investment, low maintenance cost and high comprehensive economic benefits.

Figure 5-7 shows Howden's axle sleeve type central drive device, which has been used for many years by Howden company. It is compact and easy to disassemble and assemble. The gearbox is directly set on the rotor drive shaft to avoid the wear problem of shroud pin and gear in other peripheral belt transmission modes.

2.4. Design and application features of air preheater for howdencfb boiler.

2.4.1. Four compartment design is adopted, that is, the secondary air duct is arranged between two secondary air ducts to reduce the air leakage rate from air to flue gas.

2.4.2. The comprehensive application of double seal and triple seal technology can further reduce the air leakage rate of air preheater.

2.4.3. The fixed sealing system can reduce the initial investment and greatly improve the comprehensive economic benefits in the whole operation period.

2.4.4. Increasing the diameter of air preheater rotor can reduce resistance and reduce fan power consumption.

2.4.5. When SNC unit is put into operation, ammonium sulfate generated by the reaction of sulfur-containing escape from ammonia in flue gas will aggravate ash blockage of air preheater. Therefore, semi telescopic soot blower can be used and mobile high-pressure water washing device can be used to clean the cold end of air preheater.

2.4.6. Compared with the traditional unit air preheater, considering the high-pressure characteristics of CFB boiler, it is required that the bearing members of the top structure of the air preheater should be appropriately thickened, and the number of struts in the flue gas duct should also be appropriately increased.

2.4.7. Due to the self balance of the air pressure acting on the flue gas duct of the air preheater, the air preheater has good structural bearing characteristics.

 

3. Selection method

Howden adopts computer model selection, which can meet the requirements of users to the maximum extent. Users who wish to use Howden's air preheater should provide the following data:

3.1. Performance parameters

The following data, if available, shall be included in the user's inquiry:

3.1.1. Flue gas flow direction, such as whether the flow direction is vertical or horizontal.

3.1.2. The number of air preheaters matched with one boiler and the number of boilers.

3.1.3. Location of air preheater, such as indoor or outdoor.

3.1.4. Altitude or atmospheric pressure.

3.1.5. Properties of the fuel being burned.

3.1.6. Mass flow at flue gas inlet of air preheater.

3.1.7. Ash content. If the ash content is very high, the ash analysis and specific heat capacity should be given.

3.1.8. Outlet mass flow of secondary air and / or primary air of air preheater.

3.1.9. Ambient humidity and temperature range and average value.

3.1.10. Flue gas inlet temperature.

3.1.11. Flue gas outlet temperature after air leakage correction and uncorrected and any requirements for minimum temperature.

3.1.12. Temperature of primary and secondary air at inlet.

3.1.13. Inlet air temperature regulation methods, such as secondary air hot air recirculation, adding heater before primary air or secondary air, etc.

3.1.14. Hot end pressure difference.

3.1.15. Maximum pressure loss on flue gas side and air side or pressure loss on comprehensive flue gas and air of pressure boiler.

3.1.16. Any possible unstable factors, such as high flue gas inlet temperature or the need to reserve the position of heat exchange elements, etc.

3.1.17. Design mode of boiler, such as natural balance or forced circulation boiler.

3.1.18. The maximum allowable air leakage rate shall be expressed by the mass percentage of flue gas inlet or by the change of O2 content on the flue gas side. In this case, the oxygen content of flue gas shall be informed.

3.1.19. Performance guarantee requirements.

3.1.20. Hot end sector plate control.

3.2. Site conditions

If the following site conditions are available, it will be helpful to select the type:

3.2.1. The voltage, phase and frequency of the power supply provided.

3.2.2. The pressure and temperature of soot blowing medium provided, such as steam or air.

3.2.3. Pressure, temperature and composition of the water source provided.

3.2.4. Compressed air provided.

3.3. Supply requirements

3.3. Any special material requirements of the rotor, such as the air duct.

3.3.2. Any special material, thickness and life requirements of heat exchange elements.

3.3.3. The type of sootblower and whether local control is required, and whether the supply requirements beyond the end point of water pipe (flange plate) are exceeded. In this case, the material requirements of pipes and valves shall be specified.

3.3.4. Any additional requirements related to measuring instruments, such as pressure drop measuring instruments, bearing temperature measurement, etc. At the same time, the end points of the scope of supply should be specified, such as the junction box and wire of electrical appliances. The general scope of supply includes rotor stop alarm device, bearing oil temperature thermocouple, etc.

3.3.5. Number and type of driving motor, such as AC, DC or compressed air.

3.3.6. Any other requirements, such as locking bolts, etc.

3.3.7. Special requirements for surface protection and paint packaging.

3.3.8. Special delivery place.

3.3.9. Any requirements on document language.

3.3.10. Whether there are site test requirements for Howden company.

 

4. Selection examples

The following is a typical example of air preheater of 300MW CFB Unit. The type selection parameters of air preheater are shown in table 5-2.

Selection parameters BMCR Working condition ECR  Working condition
Flue gas flow at inlet(kg/s) 408.05 382.7
Air flow at secondary air outlet(kg/s) 154.3 134.4
Air flow at primary air outlet(kg/s) 142.5 140.3
Flue gas temperature at inlet () 308 299
Flue gas temperature at outlet () 137.2 134.1
Inlet temperature of primary air () 45 45
Outlet temperature of primary air () 275.7 269.5
Inlet temperature of secondary air () 35 35
Outlet temperature of secondary air () 273.9 268.7
Pressure balance from primary air to secondary air (kPa) 11.2 11.2
Pressure balance from secondary air to flue gas hot side (kPa) 14.56 14.56
Flue gas side pressure decrease(kPa) 0.976 0.864
Primary air pressure decrease (kPa) 0.599 0.579
Secondary air pressure decrease(kPa) 0.62 0.486

According to the selection parameters in table 5-2, the following results can be obtained:

4.1. Each boiler is equipped with one set of 32vnq1600 air preheater.

4.2. Heat exchange surface area of each air preheater (single side): 42892m2.

4.3. Net weight of each air preheater: 431t.

4.4. Speed of air preheater: 0.75r/min

 

5. Scope of supply and order requirements

The scope of supply includes product design, procurement, manufacturing, factory testing, installation and commissioning drawings, operation and maintenance manuals.

5.1. Plant and equipment:

5.1.1. Rotor and heat exchange elements.

5.1.2. Rotor bearing.

5.1.3. Rotor seal.

5.1.4. Rotor housing.

5.1.5. Transition flue and air duct of air preheater body and its necessary fixing parts.

5.1.6. Main bearing bracket.

5.1.7. Top and bottom platforms.

5.1.8. Driving device, including main and auxiliary motors and their frequency converters.

5.1.9. Test windows and doors.

5.1.10. Access door.

5.1.11. Steam soot blowing and water washing device, including local control cabinet.

5.1.12. Fire nozzle header with nozzle cover flange.

5.1.13. Thermocouple fire detection system.

5.1.14. Rotor speed and fault alarm.

5.1.15. Rotor bearing oil temperature thermocouple.

5.1.16. Inject lubricating oil for the first time.

5.1.17. Name plate.

5.1.18. Standard surface protective paint.

5.1.19. Special tools.

5.2. Termination point:

2.5. The inlet and outlet flange is not included.

5.2.2. The connection point between bottom beam and side column of air preheater and steel frame of power plant boiler, excluding connecting bolt.

5.2.3. Upper and lower platforms of air preheater body structure.

5.2.4. Fire fighting water at smoke and air pipes.

5.2.5. Upper and lower soot blower steam and water pipe flange.

5.2.6. Motor terminals.

5.2.7. Flange of cooling water for rotor top bearing shell.

5.3 those not within the scope of supply include:

5.3.1. Connect the pipe and insulation baffle.

5.3.2. Expansion connection of pipeline at connection flange of air preheater.

5.3.3. Pipes, valves and instruments outside the air preheater.

5.3.4. Connecting wires and cables of electrical equipment.

5.3.5. Motor starter.

5.3.6. Lubricating oil (except for primary injection).

5.3.7. Slide rail beam and crane.

5.3.8. Insulation, cladding and anchoring.

5.3.9. Maintenance platform and ladder (except the bottom and upper platform on the body).

5.3.10. Support steel frame.

5.3.11. Foundation and civil engineering.

5.3.12. Installation, commissioning and field test.