1 Introduction
With the gradual completion of the West-East Gas Pipeline Project, natural gas has been widely used as a clean fuel in the west of my country. Since many provinces in the western region are located on plateaus, the atmospheric pressure is low, and the air is relatively thin, so the combustion process of fuel and the selection method of burners have their particularities compared with plains. This article will mainly discuss the calculation process and selection method of plateau correction when gas burners are used in high altitude areas.
2 The particularity of the burner when used in high altitude areas
The burner is a device that converts the combustible components in the fuel into heat energy after fully mixing and burning the oxygen in the air. The oxygen required for fuel combustion comes from air, and the volume percentage of oxygen in air is about 21%. The theoretical air volume required for the complete combustion of 1m3 natural gas under standard conditions is about 9.64m3[1]. However, in high-altitude areas, as the altitude changes, the atmospheric pressure, air density, and oxygen content will be significantly different from the standard state, as shown in Table 1 [2].
It can be seen from Table 1 that as the altitude increases, the atmospheric pressure decreases, the air density decreases, and the oxygen content decreases accordingly. Due to the change of oxygen content in the air, the theoretical air volume value calculated by us in the standard state is not suitable for calculation in high altitude areas.
For example: Many boiler drums and induced draft fans used in high-altitude areas often use the same model as those in plain areas, and the boilers are far below the rated output power at this time. Some users will choose a larger size for the drum and induced draft fan, but even so, the use effect is still unsatisfactory. The main reason for this result is that the amount of air provided by the blower is not suitable for the amount of oxygen required for complete combustion of the fuel. It is not difficult to see from this that only by taking the oxygen content in the air corresponding to the altitude as the main parameter and correcting the actual amount of air required for combustion can we achieve the goal of complete combustion of fuel with the minimum excess air coefficient. This correction process is what makes the burner special when used at high altitudes.
Table 1 Atmospheric pressure, air density,
Oxygen content and boiling point of water
Altitude (m) []0[]3000[]4000[]5000[]6000[]7000 Atmospheric pressure (kPa)[]101.32[]70.7[]62.4[]54.9[]48.1[]42 Air density (g /m3)[]1292[]892[]802[]719[]644[]573Oxygen content (g/m3)[]260[]206[]186[]166[]149[]133Boiling point of water ( ℃) []100[]90[]87[]84[]80[]773 Correction calculation when the burner is used in high altitude areas
Since the air pressure, temperature and density conform to the state equation of an ideal gas, at a certain temperature, the atmospheric pressure is proportional to its density, and the relationship between the three is expressed by the ideal gas state equation [3]:
p=RρT(1)
or ρ=p/RT(2)
In the formula: p—atmospheric pressure, Pa;
ρ—air density, kg/m3;
T—thermodynamic temperature, K;
R—gas constant, J/(kg·K).
For the gas constant of air R=287J/(kg·K), when we know two basic parameters of a certain state, we can calculate the air density in this state according to formula (2). Atmospheric pressure in each region can be found from relevant data, and can also be calculated by the following formula [4];
pH=p0(1-H/44340)5.256 (3)
In the formula: pH—atmospheric pressure at altitude H, Pa;
p0—atmospheric pressure at sea level, Pa;
H—altitude above sea level, m.
The air temperature, pressure and density values at commonly used altitudes at 15°C are shown in Table 2 [4].
Table 2 Air temperature, pressure and density at commonly used altitudes (at 15°C)
Altitude H (m) [] Temperature T (K) [] Pressure p (kPa) [] Density ρ (kg/m3) 0 [] 288 [] 101.325 [] 1.2251000 [] 281.651 [] 89.872 [] 1.11172000 [] 275.154[]79.501[]1.00663000[]268.659[]70.121[]9.0925×10-14000[]262.166[]61.66[]8.1935×10-1 If we approximate the volume percentage of atmospheric oxygen content as a constant, through the formula ( 4) The relative oxygen density can be calculated under the same conditions at different altitudes and on the ground.
ρ0′=CρH(4)
In the formula: ρ0'—relative oxygen density at different altitudes, kg/m3;
C—the percentage of oxygen content near the ground;
ρH—air density at different altitudes, kg/m3.
Because density is the mass a substance has per unit volume, the mass ratio between two substances with the same volume but different densities is equal to the ratio of their densities. Therefore, the ratio of the relative oxygen density at different altitudes to the oxygen density in the standard state is equal to the mass ratio of the oxygen content per unit volume of the two. This ratio is actually the correction value of the actual air volume required for combustion at different altitudes, namely:
K=ρ0′/ρ0 (5)
In the formula: K—plateau correction coefficient;
ρ0—Oxygen density in standard state, kg/m3.
The values listed in Table 3 are the relative oxygen density at common altitudes at 15°C calculated according to formula (4) and the values in Table 2.
Table 3 Relative oxygen density at commonly used altitudes (15°C)
Altitude H
(m) []1000[]2000[]3000[]4000 relative oxygen content ρ0′
(kg/m3)[]0.233[]0.211[]0.191[]0.17215°C (T=288K) The volume percentage of oxygen content in the air near the ground where the altitude is zero is 20.95%[5], according to the formula (4) Oxygen density near the ground for this condition:
ρ0=Cρ=20.95%×1.225=0.2566kg/m3
Table 4 shows the plateau correction values of commonly used altitudes at 15°C (T=288K).
Table 4 Plateau Correction Values for Commonly Used Altitudes (15°C)
Altitude H (m) [] 1000 [] 2000 [] 3000 [] 4000 Plateau correction coefficient K [] 0.908 [] 0.823 [] 0.744 [] 0.67 According to reference materials [1] Average atmospheric pressure in winter and summer in various regions of my country The difference is small, which proves that the temperature has little influence on the pressure and density of the same gas. Therefore, the author basically calculates according to the values in Table 4 in actual use. As long as a margin is properly reserved in the selection of the burner, the influence of air temperature on the oxygen density can be completely eliminated.
4 Selection of burners in high altitude areas
At present, oil and gas boilers are usually equipped with imported burners, which have their own blowers. Its output characteristics can be obtained from the characteristic curve diagram of each model. The abscissa in the figure is the burner output power, and the ordinate is the blower pressure. For a certain point on the characteristic curve, its abscissa value not only indicates the output power of the burner, but also represents the amount of air that can be supplied by the blower attached to the burner to meet the complete combustion of fuel corresponding to this output power, and its ordinate value It is the ability of the burner to overcome the total resistance of the flue gas side of the boiler under the output power condition. Therefore, when the burner is used in a plain or near-standard state, we generally follow the steps below to determine the burner model:
(1) Calculate the output power Qb of the burner
Qb=Q/η
In the formula: Q—the rated output power of the boiler, MW;
η—boiler thermal efficiency.
(2) Determine the total resistance ΔP on the flue gas side of the matching boiler
(3) Check the burner blower pressure
According to the burner characteristic curve, draw a line upward from the output power Qb and intersect the characteristic curve at a point Pf, and select the burner model corresponding to the Pf>ΔP characteristic curve to complete the selection of the burner.
When the burner is used on a plateau, since the plateau correction coefficient K value is less than 1, that is, the oxygen content in the air is only K times that of the standard state, the air volume corresponding to the Qb/k point on the characteristic curve can meet the output of the burner. The complete combustion of fuel when the power is Qb, and the ordinate value of this point must also be greater than the total resistance ΔP on the flue gas side of the boiler. The burner selection steps under plateau conditions are as follows:
(1) Calculate plateau correction coefficient
K=ρ0′/ρ0
(2) Calculate the actual output power of the burner
Qb=Q/η
(3) Calculate the output power of the burner when it meets the plateau air volume
Qbh=Q/η·K
(4) Check the burner blower pressure
According to the burner characteristic curve diagram, from Qbh up to a point where the characteristic curve intersects Pfh. Select the burner model corresponding to the Pfh > ΔP characteristic curve, that is, complete the selection of the burner plateau operating conditions.
For example, the correction process of a gas burner used in a 2.8MW hot water boiler at an altitude of 3000m:
(1) When the known altitude is 3000m, K=0.744
(2) Take η=91% for boiler efficiency
The output power Qb=2.8MW/0.744×91%=4.136MW when the burner meets the air volume at an altitude of 3000m
(3) Boiler total resistance ΔP=12mbar=1.2kPa
(4) The characteristic curve of the burner is shown in Figure 1 [6]
Figure 14. At 136MW, the ordinate Pf corresponding to several burners is the pressure head of the burner blower
a. GI350DSPGNPf=11mbar<ΔP
b. GI420DSPGNPf=20mbar>ΔP
Therefore, at an altitude of 3000m, the choice of GI420DSPGN can ensure that the boiler can reach the rated load output of 2.8MW. After the plateau correction calculation is completed, finally select a solenoid valve group with an appropriate caliber according to the actual gas volume of the burner.
5 Several common problems in the use of burners in high altitude areas
(1) No type selection calculation
Since the burners currently used for oil and gas boilers are basically integrated with a blower, many manufacturers only check whether the output power of the burner meets the rated output power of the boiler when selecting the burner, and do not check the combustion. The ability of the burner to overcome the resistance at the rated output of the boiler has resulted in high noise and severe vibration when the burner is running, and poor combustion effect. In this case the operator usually resorts to reducing the output of the burner to barely operate. Therefore, it is very important to carefully check the ability of the burner to overcome the resistance of the boiler when calculating the type of the burner in the high altitude area.
(2) Incorrect load regulation
Due to the relatively high price of natural gas as a fuel, many users adopt a low-temperature constant supply operation mode similar to coal-fired boilers in order to save fuel when the burner is running, and the burner runs on low fire for a long time. Due to the characteristics of high-altitude areas, the amount of combustion air is 1/K times that of the plain, and its theoretical combustion temperature is relatively lower than that of the plain under the same load conditions. At this time, long-term small-load operation will undoubtedly further reduce the exhaust gas temperature. When it is lower than the dew point temperature A large amount of condensed water appears, and serious cases gather in the rear smoke box, causing rapid corrosion of the boiler and reducing the service life of the boiler. Therefore, considering several aspects of saving fuel, ensuring the heating effect and prolonging the service life of the equipment, it is a better choice to use the high-temperature intermittent heating operation mode when the burner operates in high-altitude areas.
(3) There is no regular maintenance and maintenance plan
As a mechatronic product, any hidden danger in operation of the burner will involve the life and property safety of people and equipment. Due to the relatively short time for the burner to enter our country and the relative lack of professional burner maintenance personnel, many users no longer carry out the inspection and maintenance required by the product standards from the beginning of the burner installation, resulting in frequent burner operation accidents. , and shows an upward trend with the increase of social holdings. Therefore, it is very necessary for each user unit to strengthen the shift inspection, weekly inspection, monthly maintenance, and quarterly maintenance of the burner before the national competent authority has issued the mandatory inspection and maintenance regulations for the burner.
1 Introduction
With the gradual completion of the West-East Gas Pipeline Project, natural gas has been widely used as a clean fuel in the west of my country. Since many provinces in the western region are located on plateaus, the atmospheric pressure is low, and the air is relatively thin, so the combustion process of fuel and the selection method of burners have their particularities compared with plains. This article will mainly discuss the calculation process and selection method of plateau correction when gas burners are used in high altitude areas.
2 The particularity of the burner when used in high altitude areas
The burner is a device that converts the combustible components in the fuel into heat energy after fully mixing and burning the oxygen in the air. The oxygen required for fuel combustion comes from air, and the volume percentage of oxygen in air is about 21%. The theoretical air volume required for the complete combustion of 1m3 natural gas under standard conditions is about 9.64m3[1]. However, in high-altitude areas, as the altitude changes, the atmospheric pressure, air density, and oxygen content will be significantly different from the standard state, as shown in Table 1 [2].
It can be seen from Table 1 that as the altitude increases, the atmospheric pressure decreases, the air density decreases, and the oxygen content decreases accordingly. Due to the change of oxygen content in the air, the theoretical air volume value calculated by us in the standard state is not suitable for calculation in high altitude areas.
For example: Many boiler drums and induced draft fans used in high-altitude areas often use the same model as those in plain areas, and the boilers are far below the rated output power at this time. Some users will choose a larger size for the drum and induced draft fan, but even so, the use effect is still unsatisfactory. The main reason for this result is that the amount of air provided by the blower is not suitable for the amount of oxygen required for complete combustion of the fuel. It is not difficult to see from this that only by taking the oxygen content in the air corresponding to the altitude as the main parameter and correcting the actual amount of air required for combustion can we achieve the goal of complete combustion of fuel with the minimum excess air coefficient. This correction process is what makes the burner special when used at high altitudes.
Table 1 Atmospheric pressure, air density,
Oxygen content and boiling point of water
Altitude (m) []0[]3000[]4000[]5000[]6000[]7000 Atmospheric pressure (kPa)[]101.32[]70.7[]62.4[]54.9[]48.1[]42 Air density (g /m3)[]1292[]892[]802[]719[]644[]573Oxygen content (g/m3)[]260[]206[]186[]166[]149[]133Boiling point of water ( ℃) []100[]90[]87[]84[]80[]773 Correction calculation when the burner is used in high altitude areas
Since the air pressure, temperature and density conform to the state equation of an ideal gas, at a certain temperature, the atmospheric pressure is proportional to its density, and the relationship between the three is expressed by the ideal gas state equation [3]:
p=RρT(1)
or ρ=p/RT(2)
In the formula: p—atmospheric pressure, Pa;
ρ—air density, kg/m3;
T—thermodynamic temperature, K;
R—gas constant, J/(kg·K).
For the gas constant of air R=287J/(kg·K), when we know two basic parameters of a certain state, we can calculate the air density in this state according to formula (2). Atmospheric pressure in each region can be found from relevant data, and can also be calculated by the following formula [4];
pH=p0(1-H/44340)5.256 (3)
In the formula: pH—atmospheric pressure at altitude H, Pa;
p0—atmospheric pressure at sea level, Pa;
H—altitude above sea level, m.
The air temperature, pressure and density values at commonly used altitudes at 15°C are shown in Table 2 [4].
Table 2 Air temperature, pressure and density at commonly used altitudes (at 15°C)
Altitude H (m) [] Temperature T (K) [] Pressure p (kPa) [] Density ρ (kg/m3) 0 [] 288 [] 101.325 [] 1.2251000 [] 281.651 [] 89.872 [] 1.11172000 [] 275.154[]79.501[]1.00663000[]268.659[]70.121[]9.0925×10-14000[]262.166[]61.66[]8.1935×10-1 If we approximate the volume percentage of atmospheric oxygen content as a constant, through the formula ( 4) The relative oxygen density can be calculated under the same conditions at different altitudes and on the ground.
ρ0′=CρH(4)
In the formula: ρ0'—relative oxygen density at different altitudes, kg/m3;
C—the percentage of oxygen content near the ground;
ρH—air density at different altitudes, kg/m3.
Because density is the mass a substance has per unit volume, the mass ratio between two substances with the same volume but different densities is equal to the ratio of their densities. Therefore, the ratio of the relative oxygen density at different altitudes to the oxygen density in the standard state is equal to the mass ratio of the oxygen content per unit volume of the two. This ratio is actually the correction value of the actual air volume required for combustion at different altitudes, namely:
K=ρ0′/ρ0 (5)
In the formula: K—plateau correction coefficient;
ρ0—Oxygen density in standard state, kg/m3.
The values listed in Table 3 are the relative oxygen density at common altitudes at 15°C calculated according to formula (4) and the values in Table 2.
Table 3 Relative oxygen density at commonly used altitudes (15°C)
Altitude H
(m) []1000[]2000[]3000[]4000 relative oxygen content ρ0′
(kg/m3)[]0.233[]0.211[]0.191[]0.17215°C (T=288K) The volume percentage of oxygen content in the air near the ground where the altitude is zero is 20.95%[5], according to the formula (4) Oxygen density near the ground for this condition:
ρ0=Cρ=20.95%×1.225=0.2566kg/m3
Table 4 shows the plateau correction values of commonly used altitudes at 15°C (T=288K).
Table 4 Plateau Correction Values for Commonly Used Altitudes (15°C)
Altitude H (m) [] 1000 [] 2000 [] 3000 [] 4000 Plateau correction coefficient K [] 0.908 [] 0.823 [] 0.744 [] 0.67 According to reference materials [1] Average atmospheric pressure in winter and summer in various regions of my country The difference is small, which proves that the temperature has little influence on the pressure and density of the same gas. Therefore, the author basically calculates according to the values in Table 4 in actual use. As long as a margin is properly reserved in the selection of the burner, the influence of air temperature on the oxygen density can be completely eliminated.
4 Selection of burners in high altitude areas
At present, oil and gas boilers are usually equipped with imported burners, which have their own blowers. Its output characteristics can be obtained from the characteristic curve diagram of each model. The abscissa in the figure is the burner output power, and the ordinate is the blower pressure. For a certain point on the characteristic curve, its abscissa value not only indicates the output power of the burner, but also represents the amount of air that can be supplied by the blower attached to the burner to meet the complete combustion of fuel corresponding to this output power, and its ordinate value It is the ability of the burner to overcome the total resistance of the flue gas side of the boiler under the output power condition. Therefore, when the burner is used in a plain or near-standard state, we generally follow the steps below to determine the burner model:
(1) Calculate the output power Qb of the burner
Qb=Q/η
In the formula: Q—the rated output power of the boiler, MW;
η—boiler thermal efficiency.
(2) Determine the total resistance ΔP on the flue gas side of the matching boiler
(3) Check the burner blower pressure
According to the burner characteristic curve, draw a line upward from the output power Qb and intersect the characteristic curve at a point Pf, and select the burner model corresponding to the Pf>ΔP characteristic curve to complete the selection of the burner.
When the burner is used on a plateau, since the plateau correction coefficient K value is less than 1, that is, the oxygen content in the air is only K times that of the standard state, the air volume corresponding to the Qb/k point on the characteristic curve can meet the output of the burner. The complete combustion of fuel when the power is Qb, and the ordinate value of this point must also be greater than the total resistance ΔP on the flue gas side of the boiler. The burner selection steps under plateau conditions are as follows:
(1) Calculate plateau correction coefficient
K=ρ0′/ρ0
(2) Calculate the actual output power of the burner
Qb=Q/η
(3) Calculate the output power of the burner when it meets the plateau air volume
Qbh=Q/η·K
(4) Check the burner blower pressure
According to the burner characteristic curve diagram, from Qbh up to a point where the characteristic curve intersects Pfh. Select the burner model corresponding to the Pfh > ΔP characteristic curve, that is, complete the selection of the burner plateau operating conditions.
For example, the correction process of a gas burner used in a 2.8MW hot water boiler at an altitude of 3000m:
(1) When the known altitude is 3000m, K=0.744
(2) Take η=91% for boiler efficiency
The output power Qb=2.8MW/0.744×91%=4.136MW when the burner meets the air volume at an altitude of 3000m
(3) Boiler total resistance ΔP=12mbar=1.2kPa
(4) The characteristic curve of the burner is shown in Figure 1 [6]
Figure 14. At 136MW, the ordinate Pf corresponding to several burners is the pressure head of the burner blower
a. GI350DSPGNPf=11mbar<ΔP
b. GI420DSPGNPf=20mbar>ΔP
Therefore, at an altitude of 3000m, the choice of GI420DSPGN can ensure that the boiler can reach the rated load output of 2.8MW. After the plateau correction calculation is completed, finally select a solenoid valve group with an appropriate caliber according to the actual gas volume of the burner.
5 Several common problems in the use of burners in high altitude areas
(1) No type selection calculation
Since the burners currently used for oil and gas boilers are basically integrated with a blower, many manufacturers only check whether the output power of the burner meets the rated output power of the boiler when selecting the burner, and do not check the combustion. The ability of the burner to overcome the resistance at the rated output of the boiler has resulted in high noise and severe vibration when the burner is running, and poor combustion effect. In this case the operator usually resorts to reducing the output of the burner to barely operate. Therefore, it is very important to carefully check the ability of the burner to overcome the resistance of the boiler when calculating the type of the burner in the high altitude area.
(2) Incorrect load regulation
Due to the relatively high price of natural gas as a fuel, many users adopt a low-temperature constant supply operation mode similar to coal-fired boilers in order to save fuel when the burner is running, and the burner runs on low fire for a long time. Due to the characteristics of high-altitude areas, the amount of combustion air is 1/K times that of the plain, and its theoretical combustion temperature is relatively lower than that of the plain under the same load conditions. At this time, long-term small-load operation will undoubtedly further reduce the exhaust gas temperature. When it is lower than the dew point temperature A large amount of condensed water appears, and serious cases gather in the rear smoke box, causing rapid corrosion of the boiler and reducing the service life of the boiler. Therefore, considering several aspects of saving fuel, ensuring the heating effect and prolonging the service life of the equipment, it is a better choice to use the high-temperature intermittent heating operation mode when the burner operates in high-altitude areas.
(3) There is no regular maintenance and maintenance plan
As a mechatronic product, any hidden danger in operation of the burner will involve the life and property safety of people and equipment. Due to the relatively short time for the burner to enter our country and the relative lack of professional burner maintenance personnel, many users no longer carry out the inspection and maintenance required by the product standards from the beginning of the burner installation, resulting in frequent burner operation accidents. , and shows an upward trend with the increase of social holdings. Therefore, it is very necessary for each user unit to strengthen the shift inspection, weekly inspection, monthly maintenance, and quarterly maintenance of the burner before the national competent authority has issued the mandatory inspection and maintenance regulations for the burner.