Flue gas recirculation can realize the control of combustion temperature and oxygen concentration, improve the temperature field and flow field of the combustion chamber, so as to achieve the purpose of reducing emissions and improving combustion efficiency. This paper summarizes the application and research progress of flue gas (exhaust gas, gas) recirculation technology in different fields, and summarizes them as follows: Combustion such as high-temperature air combustion, lean combustion, oxygen-enriched combustion and soft combustion can be realized through flue gas recirculation technology Way. In order to achieve the purpose of reducing emissions and improving combustion efficiency. The difficulty in realizing flue gas recirculation lies in the control of flue gas return flow. Which way to return the flue gas and how to strengthen the backflow is an important research content of the flue gas recirculation combustion technology.
Whether it is an ordinary gas burner, an oil burner, or a pure oxygen burner in industrial burners, the flue gas recirculation technology has broad application prospects. High-temperature air combustion is an effective method to achieve industrial combustion, and its core is to achieve air preheating through flue gas recirculation. This combustion method has significant advantages such as doubled flame volume, uniform flame temperature field distribution, and low NOx emissions.
Xia Dehong et al proposed to use the contraction-expansion structure for the air channel of the burner. A flue gas self-circulation burner for high-temperature air combustion has been developed. This burner uses the negative pressure zone formed by the air passing through the scaling channel to entrain a large amount of flue gas in the combustion chamber. The combustion-supporting air is heated and diluted to a relatively high temperature before combustion. low oxygen content. The effect of throat area on flue gas self-circulating combustion is analyzed by numerical simulation. In actual operation, compared with conventional burners, the exhaust gas temperature of this burner is reduced by 30K, the efficiency is increased by 9%, and the fuel saving rate is 13%, which has considerable economic benefits.
In addition, flue gas recirculation also has a certain improvement effect on fuel atomization. Liu Liansheng et al. studied the effect of flue gas recirculation on the macroscopic characteristics of swirling liquid mist flame downstream of bubble atomizing nozzle and the composition of combustion products. The research shows that: on the one hand, the flue gas recirculation increases the evaporation speed of the oil mist particles, changes the macroscopic characteristics of the fuel mist swirling flame, shortens the flame length, increases the rigidity, and weakens the existence of the high temperature zone, significantly reducing the The incomplete combustion products in the flue gas improve the combustion efficiency; on the other hand, the flue gas recirculation greatly reduces the oxygen volume fraction in the high temperature zone of the flame and inhibits the formation of NOx, thereby greatly reducing NOx emissions.
The application of pure oxygen combustion technology in glass furnaces and metallurgical industrial furnaces has achieved good energy-saving and emission-reduction effects. In the process of transforming the existing combustion system from ordinary air combustion to pure oxygen combustion, due to the extremely high temperature of pure oxygen combustion, it is necessary to use the burner to return a large amount of flue gas to achieve temperature control, thereby avoiding burners and The problem of local high temperature caused by overheating of burner bricks and reduction of flue gas. Mark D and others studied a typical flue gas external circulation pure oxygen burner. The return flue gas of the burner enters the pre-combustion chamber from the pipe through the control valve, and after being fully mixed with fuel and oxygen in the pre-combustion chamber, it enters the main combustion chamber for combustion reaction. The burner is mainly used in the transformation of pure oxygen combustion equipment for ordinary air-assisted combustion, and can achieve flame characteristics similar to ordinary combustion.
The flue gas recirculation technology in the combustion of coal-fired boilers is mainly combined with technologies such as air separation and oxygen-enriched pressurized combustion to form a combustion technology with its own characteristics. Horne and Steinburg proposed air separation/recirculation technology, also known as O2/CO2 combustion technology.
Air separation separates oxygen from the air, and the separated pure oxygen is mixed with part of the flue gas burned by the boiler to form a new mixture, which replaces the original air as the oxidant of the fuel. Since nitrogen is separated during air separation, the The content of CO2 in the process combustion products reaches more than 95%. Most of the flue gas is directly liquefied. The remaining recirculated flue gas is mixed with pure oxygen according to a certain ratio and then enters the furnace for a combustion process similar to the conventional combustion method. The air separation/flue gas recirculation combustion technology not only makes it easy to separate and collect CO2, but also has the functions of relatively low NOx emissions and high desulfurization efficiency, which can effectively control coal combustion pollution.
Due to the conventional air separation/flue gas recirculation combustion technology, it takes a lot of energy to compress carbon dioxide into a liquid state. Therefore, the American Thermo Energy Company proposed a pressurized oxygen-enriched combustion technology based on the conventional air separation/flue gas recirculation combustion technology. The whole process of combustion and capture of CO2 in the integrated power generation system of pressurized oxygen-enriched combustion is completed at a high pressure of 6-8 MPa, which greatly reduces compression power consumption and pressure drop loss. Compared with atmospheric pressure oxygen-enriched combustion, multi-stage compression and Compared with refrigeration to capture CO2, the power consumption of compression is greatly reduced. There will still be a certain pressure drop before the flue gas returns to the combustion chamber, so it is necessary to use the flue gas recirculation compressor to compress and boost the recirculated flue gas and then circulate it back to the combustion chamber. The gas is compressed and boosted under high pressure, and the power consumption of the recirculation compressor is large, which reduces the overall system economy. In view of the above problems, since the liquefaction temperature of CO2 under 6MPa high pressure is about 25°C, a new flue gas recirculation system is proposed that firstly liquefies CO2 and boosts the pressure, then vaporizes and evaporates it into gaseous CO2, and then circulates it back to the combustion chamber. The power consumption of the booster pump for compressing and boosting liquid CO2 is far less than that of the compressor for compressing and boosting gaseous CO2, so this new flue gas recirculation system that liquefies and then evaporates CO2 can make the whole The economy of the power generation system is improved.
Flue gas recirculation can realize the control of combustion temperature and oxygen concentration, improve the temperature field and flow field of the combustion chamber, so as to achieve the purpose of reducing emissions and improving combustion efficiency. This paper summarizes the application and research progress of flue gas (exhaust gas, gas) recirculation technology in different fields, and summarizes them as follows: Combustion such as high-temperature air combustion, lean combustion, oxygen-enriched combustion and soft combustion can be realized through flue gas recirculation technology Way. In order to achieve the purpose of reducing emissions and improving combustion efficiency. The difficulty in realizing flue gas recirculation lies in the control of flue gas return flow. Which way to return the flue gas and how to strengthen the backflow is an important research content of the flue gas recirculation combustion technology.
Whether it is an ordinary gas burner, an oil burner, or a pure oxygen burner in industrial burners, the flue gas recirculation technology has broad application prospects. High-temperature air combustion is an effective method to achieve industrial combustion, and its core is to achieve air preheating through flue gas recirculation. This combustion method has significant advantages such as doubled flame volume, uniform flame temperature field distribution, and low NOx emissions.
Xia Dehong et al proposed to use the contraction-expansion structure for the air channel of the burner. A flue gas self-circulation burner for high-temperature air combustion has been developed. This burner uses the negative pressure zone formed by the air passing through the scaling channel to entrain a large amount of flue gas in the combustion chamber. The combustion-supporting air is heated and diluted to a relatively high temperature before combustion. low oxygen content. The effect of throat area on flue gas self-circulating combustion is analyzed by numerical simulation. In actual operation, compared with conventional burners, the exhaust gas temperature of this burner is reduced by 30K, the efficiency is increased by 9%, and the fuel saving rate is 13%, which has considerable economic benefits.
In addition, flue gas recirculation also has a certain improvement effect on fuel atomization. Liu Liansheng et al. studied the effect of flue gas recirculation on the macroscopic characteristics of swirling liquid mist flame downstream of bubble atomizing nozzle and the composition of combustion products. The research shows that: on the one hand, the flue gas recirculation increases the evaporation speed of the oil mist particles, changes the macroscopic characteristics of the fuel mist swirling flame, shortens the flame length, increases the rigidity, and weakens the existence of the high temperature zone, significantly reducing the The incomplete combustion products in the flue gas improve the combustion efficiency; on the other hand, the flue gas recirculation greatly reduces the oxygen volume fraction in the high temperature zone of the flame and inhibits the formation of NOx, thereby greatly reducing NOx emissions.
The application of pure oxygen combustion technology in glass furnaces and metallurgical industrial furnaces has achieved good energy-saving and emission-reduction effects. In the process of transforming the existing combustion system from ordinary air combustion to pure oxygen combustion, due to the extremely high temperature of pure oxygen combustion, it is necessary to use the burner to return a large amount of flue gas to achieve temperature control, thereby avoiding burners and The problem of local high temperature caused by overheating of burner bricks and reduction of flue gas. Mark D and others studied a typical flue gas external circulation pure oxygen burner. The return flue gas of the burner enters the pre-combustion chamber from the pipe through the control valve, and after being fully mixed with fuel and oxygen in the pre-combustion chamber, it enters the main combustion chamber for combustion reaction. The burner is mainly used in the transformation of pure oxygen combustion equipment for ordinary air-assisted combustion, and can achieve flame characteristics similar to ordinary combustion.
The flue gas recirculation technology in the combustion of coal-fired boilers is mainly combined with technologies such as air separation and oxygen-enriched pressurized combustion to form a combustion technology with its own characteristics. Horne and Steinburg proposed air separation/recirculation technology, also known as O2/CO2 combustion technology.
Air separation separates oxygen from the air, and the separated pure oxygen is mixed with part of the flue gas burned by the boiler to form a new mixture, which replaces the original air as the oxidant of the fuel. Since nitrogen is separated during air separation, the The content of CO2 in the process combustion products reaches more than 95%. Most of the flue gas is directly liquefied. The remaining recirculated flue gas is mixed with pure oxygen according to a certain ratio and then enters the furnace for a combustion process similar to the conventional combustion method. The air separation/flue gas recirculation combustion technology not only makes it easy to separate and collect CO2, but also has the functions of relatively low NOx emissions and high desulfurization efficiency, which can effectively control coal combustion pollution.
Due to the conventional air separation/flue gas recirculation combustion technology, it takes a lot of energy to compress carbon dioxide into a liquid state. Therefore, the American Thermo Energy Company proposed a pressurized oxygen-enriched combustion technology based on the conventional air separation/flue gas recirculation combustion technology. The whole process of combustion and capture of CO2 in the integrated power generation system of pressurized oxygen-enriched combustion is completed at a high pressure of 6-8 MPa, which greatly reduces compression power consumption and pressure drop loss. Compared with atmospheric pressure oxygen-enriched combustion, multi-stage compression and Compared with refrigeration to capture CO2, the power consumption of compression is greatly reduced. There will still be a certain pressure drop before the flue gas returns to the combustion chamber, so it is necessary to use the flue gas recirculation compressor to compress and boost the recirculated flue gas and then circulate it back to the combustion chamber. The gas is compressed and boosted under high pressure, and the power consumption of the recirculation compressor is large, which reduces the overall system economy. In view of the above problems, since the liquefaction temperature of CO2 under 6MPa high pressure is about 25°C, a new flue gas recirculation system is proposed that firstly liquefies CO2 and boosts the pressure, then vaporizes and evaporates it into gaseous CO2, and then circulates it back to the combustion chamber. The power consumption of the booster pump for compressing and boosting liquid CO2 is far less than that of the compressor for compressing and boosting gaseous CO2, so this new flue gas recirculation system that liquefies and then evaporates CO2 can make the whole The economy of the power generation system is improved.