The heat loss of exhaust gas is one of the largest power plant boiler the heat loss, generally from 5% to 8%, the total heat loss of the boiler 80% or higher. The main factor affecting the heat loss of flue gas is the flue gas temperature of the boiler. Generally speaking, the heat loss of flue gas increases by 0.6%-1.0% with the increase of the flue gas temperature by 10 C. 1. The exhaust gas temperature of the boiler in active thermal power units in China is generally about 125-150 C. It is a common phenomenon that the actual exhaust gas temperature is higher than the design value.
Our company has cooperated with well-known domestic scientific research institutions such as Xi'an Jiaotong University to develop an efficient and energy-saving low-temperature waste heat recovery system, which is in the leading position in the technology of waste heat recovery of boiler tail flue.
"Low-temperature economizer" can greatly reduce the flue gas temperature, so that the flue gas temperature can be further reduced by 40-50 degrees or more. The energy absorbed by the cryogenic economizer can be used to heat condensate water or to raise the temperature of combustion-supporting air by heating air with a heater. The utility boiler uses the "low temperature economizer", which can save coal 1~4g/kWh, and the efficiency of the unit can be increased by 0.2 to 1.2%. The low temperature economizer not only fully recovers and utilizes the waste heat of flue gas, but also makes the flue gas temperature reach the optimum desulfurization efficiency state, which greatly reduces the water cooling water consumption of the desulfurization tower in order to reduce the flue gas temperature. Normally, the power plant can recover and increase the investment of "low temperature economizer" and its system equipment within 1 to 3 years, and the economic benefit is very huge.
The economy of waste heat recovery from flue gas is a sensitive topic. How to recover investment in the shortest time and achieve the greatest economic benefits is the most concerned issue for users. This problem needs to be analyzed in detail according to the specific temperature difference of flue gas and the use of heat recovery.
1. flue gas waste heat recovery system applied to tail flue of utility boiler
The flue gas waste heat recovery system has low heat energy and low quality, usually only about 100 degrees Celsius. This part of the recovered heat energy can not be calculated by equivalent conversion method, but by equivalent conversion method or heat balance calculation to determine the effective utilization value of the recovered heat energy (the second law of thermodynamics).
Take the condensate water of thermal power plant as an example: a 200 MW unit in China has exhaust gas temperature of 169 C. After installing the flue gas heat exchanger, the exhaust gas temperature is reduced to 120 C, and the recovery power is 13557 KW (the same in three working conditions).
From the above table, it can be seen that the higher the temperature of condensate is heated, the lower the heat consumption of the turbine, and the better the economy of the whole unit. The economy of waste heat recovery from flue gas used for heating will be better, because the quality of steam extraction used for heating in power plants is higher (usually around 250 C, 0.4 MPa). Generally speaking, when the waste heat recovery system of flue gas is installed in the power plant, the coal consumption can be reduced by 1 g/kwh to 3 g/kwh after considering the energy consumption of the system, and the recovery period of equipment investment is from 1 to 3 years.
2. flue gas waste heat recovery device applied to other industrial boilers
Under normal circumstances, the economy is superior to the application in power plants. This is because the steam resources available in other industrial boiler systems are relatively limited. For users with insufficient steam, the heat recovered can be regarded as 100% of the available resources, and its economy is the most objective.
(1) advantages of flue gas heat exchanger
Improving cycle efficiency and reducing coal consumption;
Improving dust removal efficiency (before installation of dust collector);
Reduce evaporation of desulfurizing tower and save water
(2) problems and solutions in installing flue gas heat exchangers at the rear flue of boilers
Solution to low temperature corrosion
When designing, choose reasonable metal wall temperature and use metal surface temperature control technology to control in real time.
Selection of corrosion resistant materials
ND steel (09CrCuSb) - the ideal steel for low sulphuric acid dew point corrosion at home and abroad.
Low oxygen combustion
Acceptable limited corrosion rate: corrosion rate is less than 0.2 mm / year.
Wear solution
Main factors affecting wear:
A. fly ash particles characteristics and concentration: SiO2 content is high, wear is aggravated.
B. tube bundle arrangement and scouring mode: Rank: first row; wrong row: second row.
C. maximum wear position: the first row 45 degrees - 60 degrees (on both sides of the windward side center angle) second rows: 30 degrees -45 degrees.
D. air flow to the direction: gravity effect
E. wall material and wall temperature: the temperature of the tube wall increases and the hardness of the oxide film increases.
Choose a reasonable flue gas velocity: no less than 6m/s
Adopt anti abasion device.
1. anti abrasion tile 2. anti abrasion curtain
Expand the comprehensive utilization technology of heating surface
When choosing reasonable fin height and fin pitch for H-type finned tube, the boundary layer will be formed on the fin surface when the smoke flows through the finned tube under the action of viscous force, and a smaller eddy zone will appear. Large particles of fly ash can not touch the surface of the base tube. In addition, under the action of fins, the rule of flue gas lateral scour is not like that of light tube concentrated on scour. Strike 45 degree range, but relatively uniform distribution along the surface of the pipe, reducing the local wear of the outer surface of the pipe.
Solution to ash deposition:
Select a reasonable flue gas flow rate (not less than 6m/s).
Adopt soot blower
The heat exchanger surface should be properly arranged: staggered layout and proper pitch to enhance airflow disturbance.
A heat exchange element with multiple gains - H finned tube
Solution of flue gas resistance:
The allowance of induced draft fan
Reasonable arrangement of pipe rows and reduction of resistance to acceptable limits.
It is arranged before the draught fan.