Tubular Air Preheater for Power Station Boilers and Industrial Boilers

Introduction

Air preheater is a kind of equipment which can improve the heat exchange performance of the boiler and reduce the energy consumption. The function of the air preheater is to heat the flue gas discharged from the tail flue of the boiler and transmit it to the air before entering the boiler through the radiator, so as to preheat the air to a certain temperature.

The air preheater is generally used for coal-fired boilers in power station.It can be divided into tubular type and rotary type. The rotary type can be further divided into wind hood rotary type and heating surface rotary type.Power stations usually use the heating surface rotary air preheater. Generally, the application in boilers is two-compartment, three-compartment and four-compartment, among which the four-compartment is more commonly used in CFB boilers.

Functions

1.Improve and enforce combustion

The air through the heat exchanger enters the furnace, which accelerates the drying, ignition and combustion process of the fuel, ensures the stable combustion in the boiler and improves the combustion efficiency.

2, Enhance heat transfer

Because the combustion in the furnace has been improved and strengthened, and the temperature of hot air entering the furnace has been increased, the average temperature level in the furnace has also been raised, so the radiation heat in the furnace can be strengthened.

3,Reduce the loss of the furnace, lower the exhaust gas temperature and improve the thermal efficiency of the boiler

Because of the stable combustion in the furnace and the enhanced radiation heat exchange, the chemical incomplete combustion loss can be reduced. On the other hand, the air preheater uses the waster heat of flue gas to further reduce the loss of the exhaust gas, thus improving the thermal efficiency of the boiler. According to experience, when the air in the preheater rises by 1.5 degrees, the exhaust gas temperature can be reduced by 1 degree. After installation of the air preheater in the boiler, if the air temperature can be raised by 150-160 degrees by the waste heat, the exhaust gas temperature can be reduced by 110-120 degrees, and the thermal efficiency of the boiler can be increased by 7-7.5E%. Fuel can be saved by 11-12%.

4,Hot air can be used as fuel disiccant.

For layer boilers, if there is hot air, fuels with higher moisture and ash content can be used. For power station boilers, hot air is an important desiccant and pulverized coal conveying medium in grease powder system.

Tubular air preheater

The main heat transfer component of tubular air preheater is thin-walled steel tube. The tubular air preheaters are mostly cubic. The steel tubes are arranged vertically and staggered with each other. Both ends are welded on the upper and lower tube sheets.The tubular air preheater is equipped with an intermediate tube sheet in the tube box. The flue gas passes through the preheater up and down the pipe, while the air passes through the preheater horizontally to complete heat transfer.

Tubular air preheater is widely used in power station boilers and industrial boilers because of its advantages of good sealing, high heat transfer efficiency, easy manufacturing and processing. The shortcomings of tubular air preheater are large volume, easy ash plugging of steel tube content, difficult cleaning and easy wear and tear at flue gas inlet.

Rotary air preheater

The rotary air preheater is the most common form of regenerative air preheater. It uses flue gas and air alternately to heat air through metal heating surface. Rotary air preheater can be divided into heating surface rotation and wind hood rotation according to its movement mode. The two air preheaters of this furnace are three-compartment heating surface rotary air preheater.

 Rotor rotary air preheater consists of a cylindrical rotor, a fixed cylindrical shell and a driving device.

Rotary air preheater can be divided into two types: one is rotor rotary type with heating surface rotation, the other is wind channel rotary type with wind channel rotation.

Rotor rotary air preheater is composed of a rotating circular rotor and a fixed shell. Rotary heating surface is divided into many compartments, which are equipped with regenerative plates. The regenerative plates absorb the heat of gas and accumulate it. When they are transferred to the air side, the attacking heat is released to the air and their temperature is reduced. As the heating surface rotates continuously, the heat will be transferred from the flue gas to the air. The air will be heated and the flue gas will be cooled. This is the working principle of the rotary air preheater.

The main advantages of rotary air preheater are small size, light weight, allowable wear and tear of heat transfer elements, so it is especially suitable for larger boilers. The disadvantages of rotary air preheater is its complex structure, power consumption and large air leakage.

Ancillary system

Rotary air preheater mainly includeds fire alarm(hot spot detection), clearance adjustment, frequency conversion control, lubricating oil system, rotor stop alarm system, soot blowing and cleaning system, etc..

The key problems affecting the performance of rotary air preheater are air leakage, corrosion and ash plugging. In the design of tubular air preheater, air flow rate and tube box size should be reasonably selected, or vibration isolation board should be installed along the direction of air flow to prevent cavity resonance.Vibration isolation board also have the function of eliminating noise. Air leakage of rotary air preheater is an important problem. Measures should be taken in desgin, manufacturing, installation and operation to keep reasonable sealing gap between dynamic and static components in thermal state.When burning high sulfur fuel, both tubular and rotary preheater are prone to corrosion and ash plugging.The preventive measures include:installing heater or hot air recirculation at the air inlet; using low oxygen combustion or mixing combustion additives to reduce the production of SO2 gas in flue gas; blowing ash regularly to keep the heating surface clean; using corrosion-resistant materials on the heating surface, etc.

 Analysis of fire cause

A large amount of unburned combustibles deposited on the heat transfer elements is the necessary condition and key to the ignition of air preheater.The heat transfer elements of air preheater are composed of thin plates, which are arranged densely. The heating surface per unit volume is very high. The metal temperature in the lower part of the air preheater is low, and it is easy to accumulate ash under low load operation. In the intial commissioning stage of the boiler, in the case of ignition(oil ignition) and long-term low load operation(oil injection),  condensate oil mist and unburned combustibles from the furnace accumulate on the middle and lower part of the air preheater and on the heat transfer elements due to incomplete combustion, which is the source of igition.

Parameter