Flame adjustment of asphalt mixer combustion system

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Author of flame adjustment of asphalt mixer combustion system: Summary of Zhang Jianfeng, Anhui Luqiao Pavement Engineering Co., Ltd.: This article introduces the judgment basis and causes of the flame quality of the asphalt mixer combustion system, and mainly analyzes the adjustment methods in five aspects: ignition point position, air blast volume, induced air volume, air-to-oil ratio, and atomization quality. keywords: The flame adjusts the ignition point position, air-to-oil ratio, atomization quality. In asphalt mixers, mature products are generally used as combustion equipment for drying aggregates. Whether the heavy oil is burned completely in the drying cylinder has a lot to do with the adjustment of the combustion process. The debugging and working conditions of the combustion system are ultimately reflected and determined by a series of intuitive characteristics such as the color, brightness, and shape of the flame. For industrial, mining, and engineering construction site equipment, the most concise and effective way to judge whether the combustion system is working properly and whether it needs to be adjusted is by observing the flame. When fuel burns in the drying cylinder, the flame seen through the observation hole at the front of the cylinder should be: the center of the flame is centered in the drying cylinder, and the flame is evenly distributed around the cylinder without touching the cylinder wall. The degree of filling is high, the distance between the ignition point and the air outlet (nozzle) is appropriate (the nozzle is not burned); the flame length is about 1/3 of the length of the drying barrel, basically filling the entire combustion zone, not too short to affect the material copying efficiency, and not too long to directly hit the material curtain and cause burnt material; the center of the flame is light orange, white and even, the entire flame has a clear outline, and there are no snowflake-like sparks in the outer ring. Outside the flame, the smoke is translucent, and the cylinder wall material plate is clearly visible; the flame in the cylinder chamber is stable, white-orange, and generally has a "rumbling" sound, but the flame has no beating or abnormal noise. Flame adjustment mainly adjusts the position of the ignition point, flame shape, color (complete combustion), etc. 1. Adjustment of the ignition point. The ignition point of the oil mist should be close to the nozzle and there should be no backfire. A close ignition point is conducive to complete combustion and stability of the oil mist, but if it is too close, the nozzle will easily be burned. The barrel temperature, oil type, atomization quality, air volume, wind speed and heavy oil temperature will all affect the distance of the ignition point. When adjusting, the cause of the improper ignition point should be analyzed and determined, and targeted measures should be taken to adjust the parameters. When the oil, pressure, and temperature are stable, and there is no obvious change in the atomization quality, the ignition point position is mainly determined by the wind speed and air distribution conditions, that is, the position of the deflector; pushing in will reduce the air speed of the nozzle and move the ignition point forward; conversely, it will delay the ignition point. When the oil pressure and oil temperature are too low or the aperture of the atomization piece is too large (worn or improperly selected), poor oil atomization will also delay the ignition point. Generally speaking, when the system is working normally, the ideal ignition point can be obtained by adjusting the position of the deflector in a small range, that is, the position of the return flow area moves with the deflector. 2. Air blast volume adjustment. By increasing the air blast volume within a certain range, the mixing degree of oil mist and air can be improved, which is beneficial to combustion. However, too much air volume will reduce the barrel temperature and increase incomplete combustion loss and exhaust heat loss; insufficient air volume will cause incomplete combustion, resulting in dark red flames and increased carbon deposits. Generally, highway construction sites judge whether the air volume is suitable and whether it needs to be adjusted based on the flame conditions. If the combustion condition is poor or the atomizer sheet with a different hole diameter is replaced, the minimum opening of the air duct damper should be adjusted to achieve normal combustion. The corresponding relationship between the air and oil ratios can be appropriately corrected. The usual approach is to first adjust the low-fire damper to match it properly, and then adjust the height of each screw accordingly to form a smooth curve (equivalent to the cam principle); or first adjust the minimum-fire and maximum-fire dampers, and then adjust the height of all the screws in the middle accordingly to form a smooth curve. This generally ensures a more appropriate air volume for different flame sizes. The amount, rotation direction and rotation intensity of the wind at the base of the flame are all controlled by the deflector. The main function of the deflector is to enhance the mixing of oil mist and air before ignition to form a suitable high-temperature reflow area to ensure smooth ignition of the oil mist. Each blade of the deflector is tilted at a certain angle, so when the airflow passes through, a rotating airflow is formed. The rotation of the airflow increases the expansion angle of the injected oil and airflow, and at the same time enhances the mixing of oil and air. The size and position of the high-temperature reflow zone should be appropriate. If it is too large, it will extend all the way to the nozzle, which will not only easily burn the nozzle, but also be detrimental to the early mixing of oil and gas; if it is too small or the position is too far, the ignition will be delayed and the flame will be elongated. The wind passing through the deflector plays the role of supplying air (primary wind) to the root of the flame. Adjust the position of the deflector to control the appropriate airflow rotation intensity. Too strong or too weak corresponds to too large or too small the return flow area, which is inappropriate. The size and location of the reflow zone play a decisive role in the location of the ignition point. When the number of baffle blades is reduced, the recirculation area will be reduced due to poor flow guidance of the blades and weakened airflow rotation, and the combustion situation will worsen. Therefore, it is very important to promptly clean up the coking and restore the deformation of the deflector. The baffle can enhance the air flow disturbance at the burner outlet, and plays a big role in the difficulty of ignition and the appropriate position. However, this disturbance area is small and disappears quickly. The fundamental measure to enhance the overall flame mixing is to increase the airflow velocity. On the one hand, the function of the deflector is to rotate the wind passing through the blades, so that a small part of the airflow at the root and center of the flame creates a confluence area, and promotes the mixing of the root wind (primary wind) and the oil mist; on the other hand, the wind passing through the circular channel between the deflector wall and the pipe (most of the airflow of the blower is DC wind) uses the slow attenuation of the DC wind speed to strengthen the later oil and gas mixing, so that the oil and gas can be strongly mixed throughout the combustion process, achieving low oxygen and complete combustion. There is an enlargement in the air duct after passing through the deflector, which guides the airflow to a certain extent. The flaring angle also affects the size of the confluence area. When the flaring angle increases, the maximum diameter of the confluence area becomes correspondingly larger. Generally, the flare angle should not exceed 30°, which depends on the flame shape. The air volume of the blower is self-distributed by the deflector, that is, it is divided into swirling wind (primary wind or root wind) flowing through the deflector and DC wind (secondary air) that does not pass through the deflector. The flow ratio of the two air flows depends entirely on the structural size of the baffle. The equipment has been designed before leaving the factory and generally cannot be changed at will. When the deflector is located at the air duct expansion, what affects the air volume distribution is the ratio of the deflector diameter of the section where the deflector inlet is to the air duct diameter. The deflector shrinks inward (close to the nozzle), the diameter of the air duct decreases, the swirling wind increases, and the confluence area becomes larger. The function of the center hole of the deflector is to send a small amount of central axial air volume so that the confluence area is not close to the deflector and prevent the flame from burning the deflector and nozzles. The outlet of the deflector blade is concave inward, which can prevent end contamination, and the nozzle position has more room for adjustment, which is conducive to the mixing of air and oil. When the outlet plane of the deflector is level with the air outlet, the flame will roll back and the smoke in the chamber will be turbid, causing the flame detection element to malfunction. As the deflector retracts, the flame becomes shorter and the flame curling phenomenon will gradually reduce. Generally, the distance between the deflector and the air outlet is about 0.1-0.2 times the radius. The flame will also roll back when the nozzle is flush with the deflector. When the nozzle is gradually retracted, the flame volume decreases, the flame gradually becomes brighter, and the length becomes shorter. But when it is retracted too much, the root of the flame will become dark and the smoke in the chamber will be turbid. Under normal combustion conditions, the air volume should be reduced as much as possible to achieve low-oxygen combustion to promote complete combustion of heavy oil and reduce corrosion. 3. Induced draft adjustment: The induced draft volume is adjusted according to changes in fuel injection volume and blast volume. When the load changes, the amount of fuel, blast volume, and stone dust that enters the drying cylinder for combustion will change, and the amount of smoke generated by combustion will also change. At this time, the induced draft must be adjusted so that the drying cylinder chamber still maintains a good negative pressure combustion environment. For example, the combustion system of the Marini MAP series asphalt mixer uses a slightly negative pressure combustion environment, and the negative pressure in the barrel is generally required to be maintained between 8-15Psi. When the negative pressure is too large, on the one hand, it increases the heat loss of the exhaust smoke, which is very detrimental to preventing low-temperature corrosion and the formation of carbon deposits, and the flame will be broken. If the negative pressure is too small or positive pressure appears, the flame and smoke in the barrel will eject in the opposite direction, and even equipment burnout and personal accidents may occur. Generally speaking, when the load changes and air volume adjustment is required, in order to avoid positive pressure or lack of oxygen and air in the barrel, when the load increases, the induced air volume is usually increased first, and then the blast volume and fuel volume are increased; conversely, when the load decreases, the fuel volume and blast volume are first reduced, and then the induced air volume is reduced. The adjustment of the induced air volume is achieved by controlling the opening of the induced air damper baffle using a servo motor. 4. Air-to-oil ratio adjustment. In the combustion system of Marini MAP series asphalt mixer, the amount of fuel input is linked to the air blast volume. When the fuel is input for a long time, the amount of air input also increases proportionally. It increases or decreases synchronously according to the preset air excess coefficient (air door opening curve), thereby controlling the size of the flame. It is realized by synchronously controlling the damper damper and fuel saving valve through the servo motor, drive cam and connecting rod installed on the burner. The opening of the air valve is controlled by a set of adjustable jack screws, and the cam position controls the opening of the fuel throttle valve. During operation, the servo motor drives the cam to rotate (change the opening of the fuel throttle valve) through the reducer transmission mechanism, and the cam drives the air valve connecting rod (the opening of the air valve). As the cam rotation angle changes, the throttle valve and damper change their openings at the same time, thereby synchronously controlling the air blast and fuel supply to achieve control of the flame size. The temperature change signal feeds back to the blast servo to control the size of the flame, and the wind pressure change signal feeds back to the air induction servo to control the air induction damper to ensure negative pressure in the barrel. By weakening or strengthening combustion, the control of negative pressure and temperature is achieved accordingly. In order to ensure complete combustion, the damper rod curve can be adjusted or the initial opening of the throttle valve can be changed, or both can be adjusted simultaneously to achieve the optimal air-oil ratio. The proper coordination of the air-to-oil ratio is an important condition for achieving low-oxygen combustion. If the air damper is too large, the air volume and wind speed are too large, the flame will be extinguished, making it difficult to burn, and may even lead to tar deposition and a foul smell in the cylinder; if the air damper is too small, the combustion will be incomplete and even lead to waste, equipment and personal accidents. 5 Atomization with good atomization quality is an important condition for complete combustion of heavy oil. Appropriate fuel types should be selected according to the requirements of the atomizer, and the working conditions of the atomizer should be checked regularly. The oil pump pressure is also the key to atomization. Always pay attention to whether the indication of the oil pressure gauge is within the appropriate pressure range. In short, there are many factors that affect the combustion conditions of asphalt mixers, but for the combustion system of Marini MAP series asphalt mixers, there are mainly three key links: atomization quality, air volume and deflector position. Inspection and analysis based on specific phenomena in actual work and targeted measures can generally ensure good combustion. References 1. Advanced Combustion Science. Zhejiang University Press. December 2002. 2. Combustion Theory and Pollution Control. Machinery Industry Press. June 2004 3. Practical Technology of Oil and Gas Boilers. China Electric Power Press. March 2001