Questions&Answers

The combustion reaction belongs to the chain reaction, and each activation of the intermediate active product or activation center involved in the reaction can cause the basic reaction to produce more chain carriers, so that the reaction selectivity increases rapidly and continues until the reactants are exhausted, which is the ignition and combustion of fuel.

In combustion equipment, the ignition of combustible gas must rely on forced ignition. The ignition conditions are related to the temperature and concentration of the gas, and the concentration is determined by the pressure of the combustion system and the composition of the gas.

At a certain pressure or temperature, not any concentration of gas can ignite, but there is a certain concentration range, known as the "ignition concentration limit". When the pressure or temperature drops beyond a certain point, the mixture of any concentration cannot ignite. The local atmospheric pressure in Xining is only 76.3% of the standard atmospheric pressure, and the ignition range of gas is greatly reduced accordingly. Improper control of the proportion of gas and air may cause the ignition difficulty of the burner, thus directly affecting the ignition and normal combustion operation.

Electric spark ignition is commonly used in engineering. When the two electrodes placed in the combustible mixture are energized to generate a spark, the local combustible molecules ionize, causing a rapid chain reaction and raising the local temperature of the spark zone. The spark zone becomes the ignition source, forming the center of the initial flame, and propagating steadily toward the unignited mixture for combustion.

Therefore, according to the principle of spark ignition, the problem of gas ignition under the condition of low pressure on the plateau is solved. High energy ignition devices must be used; Expand the spark zone, form a thicker ignition flame, improve the local ignition temperature, to ensure the normal ignition operation of the burner.

After the transformation of the low nitrogen burner, the nozzle of the low nitrogen burner is about 1m lower than the original. During the analysis, with the bottom burner as the benchmark, the height of the remaining burner moves downward, causing the flame center to drop simultaneously, reducing the heat absorbed by all heated surfaces, thereby lowering the outlet smoke temperature. The modified low nitrogen burner has a great influence on the temperature of reheating steam. Due to the slow coordination of boiler unit and the incoordination of pressure tracking and adjustment, it is easy to cause the problem of excessive adjustment, which will also lead to the instability of steam temperature.

In addition, due to the low oxygen combustion should be paid attention to in the primary and secondary air ratio process, so the steam temperature adjustment will be limited, the whole adjustment time can not be shortened, the unit operation efficiency can not be improved. In order to ensure that the outlet temperature is within the standard range, the upper powder manufacturing system can be used in the low load stage, and the height of the nozzle can be appropriately raised, but this adjustment can make the low nitrogen steam into the standard range, which has become an important issue in the transformation of the low nitrogen burner.

In the process of transformation can be appropriate to adjust burnt out the nozzle of the wind, if adjust the nozzle of the burning wind be upward swing, reheat steam temperature would rise in the state, if adjust the nozzle of the burning wind become downward swing, reheat steam temperature will reduce until in standard range, therefore, Through this principle, the low nitrogen burner can be adjusted appropriately in the process of boiler operation.

Oxygen and fuel mix into the combustion chamber for combustion, the generated flue gas into the recirculation flue gas channel, again into the combustion chamber for combustion, cycle many times, so that the fuel is fully burned, improve the combustion efficiency, reduce the emission of NOx and soot in the combustion products, to achieve clean combustion and efficient combustion.

Because thermal NOx and fast NOx are derived from nitrogen in the air, oxygen is used as oxidant, so that the generated NOx is mainly from the N in the fuel, that is, fuel NOx, so it will greatly reduce the total emissions of NOx.

Oxygen-coal flameless combustion technology is generally divided into three zones: diffusion zone, diffusion dynamic zone and chemical reaction dynamic zone. The combustion process of diffusion zone and diffusion dynamic zone is determined by material diffusion or partial material diffusion, which is manifested as the occurrence of flame, such as oxygen and coal flame combustion technology. Oxygen-coal flameless combustion technology works in chemical reaction power zone.

Oxygen coal combustion technology of pure oxygen by liquid oxygen gasification, high pressure liquid oxygen can easily get close to the speed of sound of oxygen jet, flameless combustion technology of flue gas oxygen coal outside loop is used to dry pulverized coal and conveying pulverized coal, the flow rate is needed for combustion oxygen flow rate of 1 times or so, around 200 m/s in the hearth of the secondary air oxygen jet pulverized coal jet, the entrainment CO2 a wind At the same time, due to the large slip speed between pulverized coal and flue gas, the full premix of pulverized coal and oxygen can be achieved instantly.

Many provinces in the western region are located in the plateau, low atmospheric pressure, relatively thin air, so the fuel combustion process and burner selection methods compared to the plains have their own special characteristics. Compare the technical requirements of boiler gas burner, except for the structure, the rest are directly related to the low air pressure, as follows

1.low air pressure affects the density of the gas mixture, so that the range of the gas jet changes, which in turn affects the flame shape and size and the matching problem of the furnace chamber structure.

2.the air density in the Xining area is 1/4-1/3 lower than the standard state, which generally exceeds the adjustment range of the burner air-fuel ratio, and the combustion conditions are unstable.

3.due to the instability of the combustion road conditions, directly affecting the rated output of the burner.

4.incomplete combustion under the conditions of oxygen deficiency, so that the harmful components in the flue gas increased.

There is no very clear international specification for the definition of the flameless combustion state, and the one that is more recognized is the study of Wunning and Sons, who obtained the relationship between the internal circulation rate Kv and the temperature of the flue gas under the diffusion combustion mode of methane, where the mixing and combustion are carried out simultaneously, through a large number of experiments.

The axial evolution and radial diffusion of the fuel jet MF and the oxidizer jet MA induce a coiling effect of the surrounding air ME, so that the central zone of the jet is gradually diluted by combustion products (CO2 and H20) and non-reacting gases (N2) to achieve "low oxygen" combustion conditions.

Kv is the ratio of the circulating flue gases (CO2, H2O and N2) sucked by the combustion jet to the mass flow rate of the incoming jet (fuel and air).

KV=ME/ (MF+MA)= (MJ-MF-MA) / (MF+MA)

In addition, there are some other methods of determining this. For example, the temperature rise is used to determine whether the combustion is in a flameless combustion state, and Kumar et al. concluded that the combustion can be defined as flameless combustion if the temperature rise in the reactor is normalized to a spatial temperature change of about 15%. The inlet temperature is higher than the self-ignition temperature and the temperature rise is higher than the inlet temperature, which is called high temperature combustion.

1.The burner can reach the rated output under the rated gas pressure.

2.The shape and size of the flame should match the size of the furnace chamber structure, and there should be good flame fullness.

3.Has good regulation characteristics, in the boiler from the lowest load to the highest load operation, the combustion conditions should be stable, that is, there is a large regulation ratio, regulation ratio = 1:5.

4.High combustion efficiency, combustion pollutant emissions standards.

5.Compact structure, easy to install and operate, flexible adjustment, low noise.

1.Burner operation mode.

Low nitrogen burner transformation, the focus needs to be placed on how to enhance the rated temperature of the furnace and the stability of the pulverized coal combustion, as far as possible to avoid a sudden stoppage of the burner or a power shortage situation.

2.The boiler secondary air distribution method.

Boiler secondary air distribution method should meet certain basic needs, to ensure that the surplus air coefficient in the main combustion area is within a reasonable range; burner needs to stop, the corresponding dampers should be closed appropriately; the degree of opening and closing the dampers is determined by the corresponding flow curve to determine; boiler operation needs to ensure that the air pressure in the wind box is normal, to meet the rigid needs of the wind jet.

3.Boiler operation oxygen level.

The oxygen content of the boiler operation is controlled within a reasonable range, and the best oxygen content of the boiler combustion needs to be determined according to the test results of the actual combustion.

4.The control of the fine degree of coal powder.

If the combustion classification technology of SOFA is used, the fineness of coal powder needs to be adjusted.

Disadvantages of traditional flame-fired combustion mode

With the wide application of modern heat storage technology in industry, some heating equipment burning high calorific value fuel usually operates in high temperature environment. In the traditional flame-fired combustion mode, the chemical reactions are concentrated on a relatively small flame surface, resulting in uneven temperature distribution and high flame front temperatures, which inevitably leads to a large amount of thermodynamic NO generation and CO generation and emission.

Technical defects of new combustion modes

1.Scholars have proposed a variety of new combustion modes, but all of them have certain technical defects. Such as catalysts used in catalytic combustion are expensive to prepare and have poor stability and short lifetime at high temperature operating conditions.

2.Although oil-rich-quenched-oil-poor combustion can achieve ultra-low emissions (less than 20.5mg/m3), research on it is still mainly focused on gas turbines.

3.Although oxyfuel combustion can achieve near-zero NOx emissions, the investment in oxyfuel enrichment is very large in the early stage.

Advantages of flameless combustion

Compared with other new low-pollution combustion methods, flameless combustion technology has the advantages of simple equipment, uniform combustion, stable combustion, high efficiency, low emissions, energy saving, low NOx generation inhibition, and reduced CO content, which has a broad application prospect in industry. Compared with traditional combustion technology, flameless combustion technology extends the range of fuel combustion, stable combustion process, uniform temperature distribution in the combustion zone, low pollution emission and low noise.

Single-stage, two-stage burner air regulating device

1.single-stage burner air regulating device

For the single-stage burner, the air door is fixed after the commissioning.

2.Draft control device of two-stage burner

The two-stage burner has a servo motor-driven damper regulating device. When the damper rotates, the opening of the air inlet changes, thus changing the inlet air volume.

Sliding burner air regulator

1.The principle of wind oil adjustment

The cam mechanism is driven by the servo motor to turn clockwise to the large fire position, adjustable spring belt, drive the damper baffle connector and open the damper for the front blowing.At the end of the front purge, the servo motor drives the cam disk to open the oil volume adjustment valve, which turns the damper baffle to the ignition load position, in which the oil volume regulator bases open, which means that only a small part of the oil is atomized in the nozzle, and most of it flows back to the oil tank through the return pipe. At the same time, the damper is closed to make the air supply meet the need of atomized oil volume. Servo motor drive linkage mechanism from small fire to large fire for continuous movement, the air door open, oil regulator close small, only a small part of the oil back to the tank.

2.Air volume adjustment

By adjusting the sliding screw on the linkage mechanism, drive the air baffle opening, and measure the smoke composition at each load point, so that the wind volume is suitable for the oil volume, after the above adjustment is completed, the outer locking screw should be tightened.

3.Oil volume adjustment

Use the hexagonal wrench to turn the hexagonal screw to make the eccentric steel piece convex or concave, through the eccentric steel piece to move the oil volume regulator (oil pressure rise or fall), so as to achieve the purpose of oil volume adjustment, wind and oil adjustment should be coordinated.

1.Mechanical atomization

Mechanical atomization nozzle is to use high oil pressure to spray oil from the spray hole at high speed to achieve atomization. The fuel with certain pressure and temperature enters the tangential groove through the inlet hole and even oil groove to obtain a high speed, and then enters the cyclone chamber. The oil produces strong centrifugal rotation in the cyclone chamber, causing pulsation in the groove, and is sprayed out from the spray hole to form a hollow conical film. As the oil in the turbulent flow situation, through the spray hole diffusion, coupled with the centrifugal effect, the formation of surface tension and was crushed into many fine oil particles, to achieve the purpose of atomization.

2.Rotating cup atomization

The working principle of rotating cup atomization is that the fuel oil is sent to the cup-shaped device rotating at a high speed of 3000~5000r/min through the hollow shaft, and under the action of centrifugal force, the oil forms a very thin oil film in the inner wall of the rotating cup, and flies out along the mouth of the cup, and is atomized into very fine oil particles. At the same time, the fan blade mounted on the same shaft will blow the primary wind along the mouth of the rotating cup through the guide piece, the direction of rotation of the primary wind is opposite to the direction of the oil droplets, so that the oil droplets are further atomized.

3.Steam atomization

The principle of steam atomization is to use high-speed steam through the jet to bring out and break the oil into oil particles, and then by the expansion of steam and oil in the furnace heated by the flue gas, so that the oil particles are further crushed into a finer oil mist. Steam atomization nozzle can be divided into two types of nozzles: internal mixing nozzle and external mixing nozzle according to the mixing position of oil and steam, and internal mixing nozzle for mixing steam and oil inside the nozzle, and external mixing nozzle for mixing steam and oil outside the nozzle. Steam atomization quality is better, fine and uniform oil particles, available in a variety of fuel oil.

4.Air atomization

The oil is sprayed out from the nozzle at low pressure, and the air with high speed (about 80 m/s) is shot in from all around the oil, thus atomizing the oil. Air atomization nozzle according to the different air pressure used, divided into three categories: high pressure, medium pressure and low pressure. High and medium pressure air atomizing nozzle to obtain high pressure air, you must use the air compressor, so the application is less in the boiler, the boiler is mostly used in low pressure air atomizing nozzle.

Oil and gas boiler and kiln combustion conditions are mainly dependent on the operating performance of the burner. Poor quality of oil burner atomization or unreasonable air distribution will cause the following adverse effects.

(1) incomplete combustion, pollution of the boiler tail heating surface, the exhaust temperature rises, and even cause secondary combustion.

(2) Increasing heat loss from incomplete combustion of combustible gases.

(3) Increasing heat loss from incomplete combustion of combustible solids.

(4) Coking in the oil burner outlet or furnace chamber.

(5) Accidents such as flameout, gunning and even furnace explosion.

(1) High combustion efficiency

In order to ensure the operation of high combustion efficiency, for oil burners require a certain operating regulation range, with good atomization performance, that is, the fuel oil by atomization of the oil droplets particle size fine and uniform, atomization angle appropriate, oil mist along the circumference of the flow density distribution and air distribution consistent, oil mist and air mixture is good, etc..

For gas burner at rated gas pressure, it should be able to pass the rated amount of gas and burn it fully to meet the production of rated heat load.

(2) Reasonable air distribution to ensure stable and complete fuel combustion.

In the root of the atomizing torch should supply the right amount of air in time to prevent the oil and gas from pyrolysis to carbon black due to lack of oxygen at high temperature; at the exit of the combustion airflow should form a moderate size, properly located reflux area, so that the fuel and air are in a higher temperature field to ensure rapid and stable ignition; in the middle and late stages of combustion to make the air and oil mist mix quickly and evenly to ensure complete combustion, and make the combustion flue gas generated by harmful substances (CO, NO, etc.) the less the better.

(3) Combustion flame shape and length should be compatible with the furnace, flame fullness, flame temperature and blackness should be in line with the requirements of the furnace. Should not make the flame scour the furnace wall, furnace bottom and export window at the convection heating surface.

(4) good adjustment performance. The burner should be able to adapt to the needs of the kiln load regulation, that is, between the lowest and highest load of the kiln, the burner can work steadily, without tempering and de-fire.

(5) the nozzle in the atomization of the less energy consumed the better.

(6) The resistance of the air conditioning device is small, and the operating noise is small.

(7) Ignition and operation adjustment is convenient, safe and reliable.

(8) Simple and compact structure, reliable operation, high degree of automation, easy maintenance.