JP4111458B2 - Exhaust gas recirculation combustion system - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to an exhaust gas recirculation combustion apparatus that recirculates exhaust gas taken out from a combustion chamber to a flame in the combustion chamber.
[0002]
[Prior art]
For the purpose of reducing the amount of NOx generated by combustion, exhaust gas recirculation combustion is performed in which a part of exhaust gas is circulated into a flame and combustion is performed. When exhaust gas having a reduced oxygen concentration is sent to the flame, the oxygen concentration in the combustion air is lowered, the flame temperature is lowered, and the flame combustibility is deteriorated, but the amount of NOx generated can be reduced. In addition, a fuel amount adjusting device for adjusting the fuel supply amount and a combustion air amount adjusting device for adjusting the combustion air supply amount are provided, and the combustion amount is controlled by adjusting the fuel and fuel air supply amounts. It is also widely done. In the case of a combustion device that performs exhaust gas recirculation combustion, even if the combustibility decreases due to the inclusion of exhaust gas, as long as combustion is continued at a constant combustion amount, combustion does not become unstable. When the change was made, the combustion became unstable, and vibration combustion and misfire could occur.
[0003]
[Problems to be solved by the invention]
The problem to be solved by the present invention is to prevent instability of combustion during combustion in a combustion apparatus performing exhaust gas recirculation combustion.
[0004]
[Means for Solving the Problems]
The invention according to claim 1 is a burner that receives supply of fuel and combustion air and burns a flame in the combustion chamber, a fuel amount adjusting device that adjusts the fuel supply amount to the burner, and a combustion air supply to the burner A combustion air amount adjusting device for adjusting the amount is provided, and the combustion amount is controlled by adjusting the supply amount of fuel and combustion air, and the exhaust gas taken out from the combustion chamber is circulated to the flame in the combustion chamber. In an exhaust gas recirculation combustion device that has an exhaust gas recirculation path and performs combustion while circulating the exhaust gas to the flame, a temperature detection device that detects the temperature of the exhaust gas is provided, and the temperature detected by the temperature detection device is lower than the set temperature Therefore, it is prohibited to change the amount of combustion by changing the amount of fuel supplied to the burner and the amount of combustion air.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a flow diagram of a boiler implementing the present invention. The boiler is provided with a combustion chamber 11 in the center, the periphery of the combustion chamber 11 is surrounded by a large number of water pipes, and a burner 3 for burning a flame toward the combustion chamber is provided above the combustion chamber 11. The boiler has a water supply pipe in the lower part and a steam pipe in the upper part. The water pipe is heated by the heat of the burner 3 to heat the water in the water pipe and take out the steam. The pressure of the steam generated in the boiler Is provided.
[0007]
A blower 9 for sending combustion air to the burner 3 is provided, and a blower path 12 connects between the blower 9 and the burner 3. A combustion air amount adjusting device 6 that adjusts the amount of combustion air sent to the burner 3 by changing the flow path area is provided in the air passage 12. In the middle of the fuel line for supplying fuel to the burner 3, a fuel amount adjusting device 4 comprising a fuel solenoid valve 8 provided in each of the parallel pipe branched from the fuel line and the parallel pipe is provided. An exhaust gas passage 1 for taking out exhaust gas from the boiler and discharging it to the outside is connected to the side surface of the boiler, and a temperature detection device 2 for detecting the exhaust gas temperature is provided in the exhaust gas passage 1. The exhaust gas passage 1 is provided with an exhaust gas circulation path 5 that is branched in the middle and connected to the blower 9 at the other end, and a part of the exhaust gas is branched in the middle by the exhaust gas circulation path 5.
[0008]
In the operation of the boiler, the combustion amount of the boiler is controlled so that the pressure of the steam generated in the boiler is kept within a predetermined range. In this embodiment, the combustion amount is controlled at three positions of high combustion, low combustion, and stop. If the steam pressure is lower than the lower limit pressure, the combustion is high, and if it is between the lower limit pressure and the upper limit pressure, the low combustion is set. A combustion amount control device 7 is provided to stop if the pressure is higher. The combustion amount control device 7 is connected to each of the pressure detection device 10, the combustion air amount adjustment device 6, the fuel amount adjustment device 4, and the blower 9, and controls the combustion amount of the burner 3. When performing high combustion, the combustion amount control device 7 opens both the high-combustion and low-combustion fuel electromagnetic valves 8 and increases the opening of the combustion air amount adjusting device 6 to thereby increase the fuel and combustion. When the amount of supply air is increased and low combustion is performed, only the fuel solenoid valve 8 for low combustion is opened, and the opening of the combustion air amount adjusting device 6 is reduced to reduce the amount of fuel and combustion air. When the supply amount is reduced and the combustion is stopped, both the fuel electromagnetic valves 8 are closed, the operation of the blower 9 is stopped, and the supply of fuel and fuel air is stopped.
[0009]
In addition, a circulation amount determination device 13 is provided in the combustion amount control device 7. The circulation amount determination device 13 is connected to the temperature detection device 2 and determines the amount of the exhaust gas circulation amount based on the temperature detected by the temperature detection device 2. The combustion amount control device 7 controls the combustion amount of the burner 3 based on the information from the circulation amount determination device 13 in addition to the information from the pressure detection device 10.
[0010]
When starting the operation of the boiler, even if the steam pressure at the time when the operation switch of the boiler is turned on is lower than the lower limit pressure, the combustion is started from low combustion at first. At the start of combustion, the blower 9 is first operated, the combustion air amount adjusting device 6 is set to a low combustion opening with a small opening, and the low combustion fuel electromagnetic valve 8 is opened to start low combustion.
[0011]
When the flame is burned in the burner 3, a high-temperature combustion gas is generated in the combustion chamber, and the combustion gas heats the water pipe installed around the combustion chamber 11. The temperature of the combustion gas decreases by exchanging heat with the water pipe, and the exhaust gas whose temperature has decreased is exhausted to the outside through the exhaust gas passage 1. Part of the exhaust gas passing through the exhaust gas passage 1 branches in the middle of the exhaust gas passage 1, passes through the exhaust gas circulation path 5, and travels toward the blower 9. In the blower 9, a mixture of the exhaust gas from the exhaust gas circulation path 5 and normal air taken from the outside is used as combustion air, and the combustion air is sent to the burner 3. When exhaust gas recirculation combustion is performed, the oxygen concentration of the combustion air decreases, so that an increase in flame temperature is suppressed and the amount of NOx generated decreases. If the exhaust gas circulation amount is maintained appropriately, the NOx generation amount can be kept small while performing stable combustion.
[0012]
Combustion amount control of high combustion, low combustion, and stop is determined based on the steam pressure value detected by the pressure detection device 10, but the exhaust gas temperature is low because it is immediately after the start of boiler operation, and is detected by the temperature detection device 2. When the exhaust gas temperature is lower than the set temperature, the combustion amount is not changed from low combustion to high combustion. When the boiler is continuously operated, the temperature of the boiler is stabilized at a high temperature, so that the exhaust gas exhausted through the exhaust gas passage 1 is also stabilized at a constant temperature. However, when combustion is started from the cold can state, the water pipe in the boiler and the refractory material covering the combustion chamber and the wall surface of the exhaust gas passage 1 are also low at the start of combustion, and the combustion gas is not only water in the water pipe. In order to exchange heat with the entire boiler, the exhaust gas temperature is greatly reduced.
[0013]
In the case of a combustion apparatus that performs exhaust gas recirculation combustion, when the exhaust gas temperature changes, the ratio of the exhaust gas contained in the combustion air changes. If the pressure is constant, the volume of the gas is proportional to its absolute temperature (Charles' law), so that the density of the exhaust gas changes when the temperature of the exhaust gas changes. Exhaust gas and normal air are mixed at a constant flow ratio, and the mixture ratio is determined based on the exhaust gas density when the exhaust gas temperature is stable at high temperatures, which occupies most of the operating time. The amount is greater than the proper amount of exhaust gas mixture.
[0014]
For example, if the exhaust gas temperature during continuous combustion is 300 ° C., and the exhaust gas temperature immediately after the start of combustion is 100 ° C., and the pressure is constant, the exhaust gas temperature changes from 100 ° C. to 300 ° C. Will expand about 1.5 times. If there is a temperature difference between the exhaust gas and the normal air, the partial pressure of the exhaust gas decreases due to the volume reduction due to the temperature decrease of the exhaust gas when the combustion air has a uniform temperature by mixing. When the exhaust gas temperature is high and the air is greatly expanded, the partial pressure of the exhaust gas is greatly reduced due to a decrease in the exhaust gas temperature due to mixing with normal air, but when the exhaust gas temperature is low and the degree of expansion is low, it is usually mixed with air. However, the partial pressure drop of the exhaust gas is reduced.
[0015]
In other words, in the exhaust gas low temperature period immediately after the start of combustion, even if the exhaust gas at the same flow rate as the exhaust gas temperature high temperature stable period is recirculated, the substantial amount of exhaust gas recirculation increases, and the ratio of exhaust gas to the combustion air growing. When the ratio of the exhaust gas in the combustion air exceeds the appropriate range, the NOx reduction effect changes in an increasing direction, but combustion defects such as vibration combustion and misfire tend to occur. Even if the exhaust gas circulation amount exceeds the appropriate range, as long as constant combustion is continued, vibration combustion or the like will hardly occur, but if the combustion amount is changed in this state, the combustion amount will be changed. Sometimes the balance of combustion may be lost, causing vibrational combustion and the like.
[0016]
Therefore, the combustion amount control device 7, when the exhaust gas temperature detected by the temperature detection device 2 is lower than the set temperature, determines that the substantial circulation amount of the exhaust gas is larger than the set value. When the exhaust gas circulation amount is substantial, the combustion amount is not changed.
[0017]
The set temperature for determining the amount of exhaust gas circulation is set to a temperature slightly lower than the temperature stabilized by continuous combustion. Immediately after the start of combustion, the exhaust gas temperature suddenly rises, and then the rising curve gradually becomes gentle and eventually stabilizes at a constant value. The temperature at is the set temperature. Assuming that the set temperature is 280 ° C. and the time required for the exhaust gas temperature to rise to the set temperature of 280 ° C. after starting combustion in the cold can state is 8 minutes, the circulation amount discriminating device 13 is in the cold can state. It is determined that the exhaust gas circulation amount is larger than the set value until 8 minutes have passed after the combustion is started and the exhaust gas temperature rises to 280 ° C. or higher, and the combustion amount control device 7 does not change the combustion amount. In order to prevent the occurrence of vibration combustion.
[0018]
When the exhaust gas temperature detected by the temperature detection device 2 is higher than the set temperature, the circulation amount determination device 13 determines that the circulation amount of the exhaust gas is within the set value, and enables the change of the combustion amount. Thereafter, the combustion amount control device 7 controls the operation of the fuel amount adjustment device 4, the combustion air amount adjustment device 6, and the blower 9 so that the combustion amount corresponds to the steam pressure detected by the pressure detection device 10. To do. After the substantial exhaust gas circulation amount is reduced to an appropriate range, the partial pressure of the exhaust gas is appropriate, so that the possibility of vibration combustion or the like is reduced even if the combustion amount is changed.
[0019]
In the present embodiment, the exhaust gas circulation amount is determined based on the exhaust gas temperature detected by the temperature detection device 2 provided in the exhaust gas passage 1, but the temperature detection device 2 includes the exhaust gas circulation path 5, Any part that can detect the temperature of the exhaust gas, such as the blower 9, the air passage 12, and the burner 3, can be freely arranged. The temperature of the combustion air, which is a mixture of exhaust gas and normal air, decreases when the exhaust gas temperature is low and rises as the exhaust gas temperature rises. Therefore, the temperature of the exhaust gas can also be detected by detecting the combustion air temperature. It is possible to determine the amount of exhaust gas circulation.
[0020]
It is also possible to measure the time required for the exhaust gas temperature to rise in advance and determine the amount of exhaust gas circulation based on the elapsed time from the start of combustion. However, the temperature at the start of combustion of the boiler varies depending on the elapsed time since the previous combustion was stopped, etc., and the time required for the exhaust gas temperature to rise to the set temperature varies depending on the boiler temperature at the start of combustion. When determining the amount of exhaust gas circulation based on the elapsed time, it is necessary to correct the set time.
[0021]
【The invention's effect】
Since the change of the combustion amount is performed only when the exhaust gas circulation amount is within an appropriate range, it is possible to prevent the combustion from becoming unstable during the combustion.
[Brief description of the drawings]
FIG. 1 is a flowchart of an embodiment of the present invention.
DESCRIPTION OF SYMBOLS 1 Exhaust gas path 2 Temperature detection apparatus 3 Burner 4 Fuel quantity adjustment apparatus 5 Exhaust gas circulation path 6 Combustion air quantity adjustment apparatus 7 Combustion quantity control apparatus 8 Fuel electromagnetic valve 9 Blower 10 Pressure detection apparatus 11 Combustion chamber 12 Blower path 13 Circulation amount discrimination apparatus

Claims (1)

A burner that receives the supply of fuel and combustion air and burns the flame in the combustion chamber, a fuel amount adjustment device that adjusts the fuel supply amount to the burner, and a combustion air amount adjustment that adjusts the combustion air supply amount to the burner A combustion device is controlled by adjusting the supply amount of fuel and combustion air, and has an exhaust gas recirculation path for circulating the exhaust gas extracted from the combustion chamber to the flame in the combustion chamber. In an exhaust gas recirculation combustion device that performs combustion while circulating to a flame, a temperature detection device that detects the temperature of the exhaust gas is provided, and if the temperature detected by the temperature detection device is lower than the set temperature, the amount of fuel supplied to the burner And an exhaust gas recirculation combustion apparatus that prohibits a change in the amount of combustion by changing the amount of combustion air.

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