1 Introduction
The high-speed gas burner is the embodiment of modern combustion technology in the industrial combustion equipment industry. The injection speed of high-temperature combustion products at its outlet can reach 100 m/s ~ 300 m/s. It has technical advantages such as energy saving, high efficiency, and controllable flame momentum. In developed countries, it has been widely used in various heating furnaces in aviation, steel, chemical industry, light industry and other industries. my country introduced this technology in the 1980s, and some imitation products were put into the market. However, due to its high cost, easy-to-break refractory lining and short service life, it affected its promotion.
The high-speed gas burner is a device that converts the chemical energy of the fuel into the potential energy and kinetic energy of the combustion products. The liquid rocket engine is a device that converts the chemical energy of the propellant into the thermal energy and kinetic energy of the combustion products. Both are working. There are similarities in principle. Utilizing its rich experience in the development of liquid rocket engines, a high-performance high-speed gas burner with an all-metal structure and regenerative cooling was developed.
1.1 Technical characteristics of high-speed gas burner
a) Accurately organizes combustion, with a combustion efficiency of 99.9%;
b) Wide operating conditions: heat load adjustment ratio 1:20, air coefficient 0.5~10;
c) Using staged combustion, harmful gas (NOx) emissions comply with national environmental standards;
d) It has a flue gas injection and reflux function, which can lead the waste flue gas back from the back of the furnace and put it back into the furnace;
e) Full metal structure, continuous service life of 3 years.
1.2 The mechanism of high-speed burner to improve heat transfer efficiency. In traditional industrial furnace design, the flame speed of the burner is about a few meters per second. When the temperature of the combustion product is between 600 °C and 800°C, convection heat transfer and radiation in the furnace Heat exchange each accounts for 50%; when the combustion product temperature is above 800°C, radiation heat transfer is dominant; when the combustion product temperature reaches 1,400°C, radiation heat transfer is 10 times that of convection heat transfer, so most furnaces The design is based on radiation heat transfer. However, after using a high-speed gas burner, even in the high-temperature zone, the proportion of enhanced convection heat transfer in the furnace in the comprehensive heat transfer is greatly increased, as detailed below.
When using an ordinary burner, the flame speed is low, the flow of combustion products on the surface of the heated object is laminar flow, and the convective heat transfer coefficient of laminar flow is h1-Nu*λ/d where the Nuschelt number Nu=0.332Pr1/ 3*Re1/2; Pr is Prandtl number;
Re is Reynolds number;
λ is the thermal conductivity of the gas;
d is the equivalent diameter of the flow channel.
Using a high-speed gas burner, the flame injection speed is high (100 m/s ~ 300 m/s), the flow on the surface of the heated body is mainly turbulent, and the local heat release coefficient of the turbulent boundary layer is h2=Nu*λ/d In the formula, Nu=Pr1/3 (0.036Re0.8-836) assumes that the inner cavity size of the heating furnace is 6.45m×2.3m×2.9m, the temperature of the combustion product is 1790°C, and the temperature of the heated body is 900°C.
Using an ordinary burner, when the combustion product flow rate is 5 m/s, the convective heat transfer specific heat flow between the combustion product and the surface of the heated body is q1=h1(tg-tw)=2671X4.18kJ/h*m2
Using a high-speed gas burner, when the combustion product flow rate is 150 m/s, the convection heat transfer specific heat flow between the combustion product and the surface of the heated body is q2=h2(tg-tw)=10685X4.18kJ/h*m2
q2 is 4 times of q1.
Comparative tests have been conducted abroad on radiant heating furnaces and high-speed convection heating furnaces. In the heating process of 0 ℃ ~ 1 200 ℃, the heating time required by the radiant heating furnace is 6 times that of the high-speed convection heating furnace. In the heating process of 750 ℃ ~ 1 200 During the heating process at ℃, the heating time required by the radiant heating furnace is 10 times that of the high-speed convection heating furnace.
In 1980, high-speed gas burners were introduced into the country for technical transformation of well-type heating furnaces. The original radiant heating furnace took 24 hours to heat up from 0 ℃ to 650 ℃, while the heating furnace using high-speed gas burners heated from 0 ℃ to 650 ℃. It only takes 4 hours to heat up to 650°C, and fuel consumption can be saved by 25% to 30% due to the use of high-speed gas burners.
In the oil field, a high-speed gas burner was transformed into a three-in-one heating furnace, which doubled the heating efficiency and saved 20% in fuel consumption.
The combustion products of the high-speed gas burner rush into the heating furnace at high speed, and the gas in the furnace is stirred multiple times and mixed accordingly, which can greatly improve the temperature uniformity in the furnace. After a foreign factory retrofitted its heating furnace with a high-speed gas burner, the temperature uniformity in the furnace could be increased from ±15°C to ±2°C. After domestic well-type heating furnaces adopt high-speed gas burners, the furnace temperature uniformity reaches ±7°C.
2 Key points in the design of high-speed gas burner
2.1 Precise tissue burning
In order to achieve complete combustion, the newly developed high-speed gas burner allows gas and air to enter the combustion chamber through numerous nozzle holes. The gas jet and air jet collide one by one and mix evenly. The direction of the synthetic jet vector should be parallel to the main axis of the combustion chamber. , to prevent one jet from penetrating another jet and causing uneven mixing. In order to reduce the generation of NOx, zoned combustion is adopted. The combustion temperature in the front zone is lower than the combustion temperature in the rear zone, and the combustion temperature is the highest at the exit of the combustion chamber. The front zone is oxygen-depleted combustion, the rear zone increases oxygen supply, and reaches the excess air coefficient α = 1 at the combustion chamber outlet.
Such a design can not only reasonably distribute air, ensure complete combustion and prevent the generation of CO, but also maintain a lower combustion temperature and reduce the generation of harmful gas NOx.
2.2 Combustion chamber wall cooling The combustion chamber is filled with high-temperature combustion products, and its metal chamber walls must be sufficiently cooled to work safely. Following the regenerative cooling of liquid rocket engines, in the design of high-speed gas burners, air is used to flow in the outer chamber of the combustion chamber to absorb the heat conducted by the chamber wall to ensure that the chamber wall operates within a safe temperature range. At the same time, preheating the air can promote complete combustion and increase the theoretical combustion temperature. In the local high-temperature area of the combustion chamber nozzle, an air film is specially designed to protect the metal wall. Due to the above design, the service life of the high-speed gas burner can reach 3 years, while the combustion chamber of the high-speed gas burner designed and produced by other units is lined with refractory materials, and the service life is several months, and even some products , its refractory lining cracked after less than 3 months of work.
2.3 Flow resistance design
In high-speed gas burner design, in order for the combustion products to have sufficient momentum, the combustion chamber should have a certain pressure. According to the gas physical and chemical parameters and speed requirements provided by the user, after performing combustion calculations to determine the various thermal parameters of the combustion products, the pressure of the combustion products in the combustion chamber can be calculated, namely
pc=pa/[1-(k-1)w2/2kRT]k/(k-1)
In the formula, pc is the combustion chamber pressure; pa is the ambient pressure; k is the adiabatic index of the combustion products; R is the gas constant of the combustion products; T is the combustion temperature; w is the flow rate of the combustion products at the nozzle.
Air and gas enter the burner, flow through their respective channels, and enter the combustion chamber through the nozzle hole. There is a certain flow resistance along the way. Figure 1 shows the relationship between the burner air flow Qk and the air pressure drop Δpk; Figure 2 shows the relationship between the burner gas flow Qr and the gas pressure drop Δpr.
Figure 1 The relationship between burner air flow rate and pressure drop (20 ℃)
2 Relationship between burner natural gas flow rate and pressure drop (20°C)
Burner air inlet pressure
pk is the combustion chamber pressure
pc and air pressure drop
The sum of Δpk, the gas inlet pressure of the burner
pr is the combustion chamber pressure
pc and gas pressure drop
The sum of Δpr. Right now:
pk=pc+Δpk
pr=pc+Δpr
pk and pr cannot be too high and should meet the usage conditions provided by the user.
When a high-speed gas burner uses natural gas as fuel, the combustion product speed is 100 m/s, and the natural gas inlet pressure is
2
500 Pa, air inlet pressure is 2 100 Pa.
3 Design calculations
3.1 Combustion calculation
Determination of air requirements for gas combustion through combustion calculations
L0, the amount of combustion products
Vα, combustion product density
ρ, gas constant
R, adiabatic index
K and theoretical combustion temperature T.
a) Based on the selected gas, determine the low heating value Qyd and the volume percentage of the components.
b) Confirm combustion
Theoretical air volume required for 1m3 gas
L0 is L0=4.672/100*[1/2*CO+1/2*H2+(n+m/4)CnHm+3/2*H2S-O2]
In the formula, CO, H2, and CnHm are the volume percentages of gas components.
c) Confirm combustion
The amount of combustion products produced by 1m3 of gas
Vy is Vy=αL0+0.38+0.075Qyd/1000
e) Determine the theoretical combustion temperature
T is T=(Qyd+CrTr+CkTkLα)/(VyCy) (α>1)
T=(Qy+CrTr+CkTkLα)/(VyCy) (α<1)
In the formula, Cr, Ck, and Cy are the average specific heats of gas, air, and combustion products respectively;
Tr and Tk are the starting temperatures of gas and air respectively; Qy is the effective calorific value of gas.
Qy=Qyd-QVy where Q is the heat contained in the combustion products.
Q=3022Xco + 2581Xh2
In the formula, Xco and Xh2 are the volume percentage content of combustion CO and H2).
f) Determine the density of combustion products when α=1
ρy=(44Xco2 + 18XH2O +28XN2)/22.4
When α<1
ρy=(44Xco2 + 28Xco + 18XH2O + 2XH20 + 28XN2)/22.4
in the formula
Xco2, Xco, Xh2O, Xh2, and Xn2 are respectively
The volume of CO2, CO, H2O, H2, and N2 in combustion products
Percent content.
g) Determine combustion product gas constants
R is
R=8.314/Meq
In the formula, Meq is the reduced molecular weight of combustion products.
In the formula, Xi is the volume percentage content of the i-th component in the combustion products;
Mi is the molecular weight of the i-th component.
h) Determine the adiabatic index of combustion products
In the formula, Cpi is the specific heat of the i-th component in the combustion product.
3.2 Combustion chamber structure design calculation
a) Select the heat load Q0 of the burner.
b) Determine gas consumption
Vr (volume) and Gr (mass) are where ρr is the gas density.
c) Determine air consumption
Vk (volume) and Gk (mass) are Vk=L0Vr
Gk=Vkρk where ρk is the air density.
d) Determine the amount of combustion products generated Gy as Gy=Gr+Gk
e) Select the velocity w of the combustion products at the nozzle according to the usage conditions.
f) Determine the combustion chamber outlet cross-sectional area
Fe, diameter
de is Fe=GyWe/ρy(1+T/273)
g) Determine the cross-sectional area of the combustion chamber cylinder section
F1, diameter
d1 is
h) Determine the length L1 of the combustion chamber cylinder section as L1=(1~1.4)d1
3.3 Design and calculation of combustion chamber nozzle holes
3.3.1 Gas nozzle design calculation
a) Selection of gas injection speed wrj.
The selected principle is: after the gas jet collides with the corresponding air jet, the direction of the resultant jet is parallel to the axis of the combustion chamber.
b) Determine the total area Frj of the gas nozzle hole as Frj=Vr/wrj
c) Determine the percentage distribution of gas injection flow along the length of the gas nozzle.
3. The percentage distribution of gas flow in the mixed combustion zone of the combustion chamber determines the percentage distribution of gas injection flow along the length of the gas nozzle.
3 Distribution of air and gas flow in the combustion chamber
d) Determine the distribution of gas injection holes along the length of the gas nozzle.
There are n groups of gas nozzles along the length of the gas nozzle, with a spacing of about 20 mm; along the circumferential direction of the gas nozzle, there are m nozzles in each group, m = 12 to 24.
e) Determine the diameter of each group of gas nozzles.
According to the percentage distribution of gas flow along the length of the nozzle, we can know the area percentage distribution of the nozzle holes and the total area of each group of nozzle holes. Assuming that the diameter of the nozzle holes in the same group is the same, the diameter of the nozzle hole can be found.
3.3.2 Air nozzle design calculation
a) Selection of air injection speed wkj.
3.3.1. Under rated design conditions, it is recommended that wkj≈15m/s.
b) Determine the total area of air nozzles Fkj as Fkj=Vk/wkj
c) Determine the distribution of air injection flow on the inner wall of the combustion chamber.
3. Determine the air flow rate. Under rated design conditions, it is recommended that the air excess coefficient α≈0.6 in the ignition zone; α≈0.8 at the end of the mixed combustion zone; α≈1 at the end of the tail combustion chamber.
4 is the gas flow chart in the combustion chamber.
4 Gas flow chart in combustion chamber
d) Grouping of air nozzles in the mixed combustion zone.
8 Determine the air nozzle grouping according to the fuel nozzle grouping, and the number of air nozzle holes in each group is m.
e) Determination of air nozzle holes in the tail combustion area.
The air flow in the tail combustion zone accounts for about 20% to 30% of the total air flow. The air nozzles should be small in diameter and large in number to form a uniform air cooling film on the inner wall of the tail combustion zone tube.
4 Combustion test
The high-speed gas burner has been tested by natural gas combustion and proved to be reliable and easy to operate.
4.1 Ignition method
Two electric ignition methods were used in the combustion test. One is a high-energy igniter DHZ-103 with an ignition frequency of 1.5 times/s and an energy storage of 12J/time. The other is a flame monitoring igniter HJ-1 with an ignition voltage of 15 000 V. , the ignition nozzle is a car spark plug.
4.2 Ignition procedure
a) Supply air according to 1/4 of the total air volume required by the rated load;
b) The electric igniter is powered on;
c) Turn on the natural gas switch, and when α≈0.6, ignition can be successful.
4.3 Heat load adjustment
A burner with a rated heat load of 30 m3/h can still maintain combustion when the amount of natural gas is reduced to 1.6 m3/h during the test.
(α≈14).
4.4 Air coefficient adjustment
In the test, the air coefficient adjustment range of the burner was 0.5 to 20, which could maintain combustion.
4.5 Gas temperature adjustment
9 In the test, the actual measured temperature change range of the burner outlet was 90°C ~ 1300°C (α≥2).
10Conclusion
The high-speed gas burner is a high-efficiency, energy-saving, low-pollution burner developed for industrial heating furnaces using the combustion technology of liquid rocket engines. The natural gas combustion test has proven that it works reliably, is simple to operate, has a large heat load adjustment range, air excess coefficient and gas temperature adjustment range. The burner has been successfully used in heating furnaces in oil fields, ceramics, steel and other industries.