Application and Maintenance of Heavy Oil Burners for Asphalt Mixing Equipment

Preface

The burner of asphalt mixing equipment mainly provides the required heat for the drying of aggregates. Its performance directly affects the fuel consumption and environmental protection indicators of the asphalt mixing plant. With the continuous rise in oil prices, the use of heavy oil as fuel has become an inevitable trend. However, the combustion performance of heavy oil is poor. How to select and use heavy oil burners well naturally becomes the key. In 2004, Yunnan Road and Bridge Construction Group purchased a set of LQC240 asphalt mixing plant equipped with heavy oil combustion from Zoomlion in Changsha for the construction of the Anchu Expressway. This equipment features advanced technology and a high degree of automation. Its heavy oil burner is imported from Germany in its original form, featuring high combustion efficiency, reliable operation, and environmental friendliness and energy conservation. Since the equipment was debugged and put into use at several construction sites, it has been operating smoothly all along. The average actual heavy oil consumption per ton of material is below 5.6 kilograms, achieving excellent economic benefits. The author would like to share some personal insights on the application and maintenance of heavy oil burners.

I. Composition and Working Principle of Heavy Oil Combustion System

Due to the high viscosity and poor atomization performance of heavy oil, the combustion system must be equipped with a heavy oil heating device. The system composition is shown in Figure 1.

Figure 1 shows the structural principle diagram of the heavy oil combustion system

Figure 1 shows the structural principle diagram of the heavy oil combustion system

The system is mainly composed of heavy oil tanks, diesel tanks, oil filters, heavy oil supply pumps, heat exchangers, light and heavy oil switching valves, fuel pumps, pressure regulating valves, burners and their conveying pipelines, etc. The heavy oil part is heated and insulated with heat transfer oil. The heat exchanger finally rapidly heats the heavy oil to the required temperature. Its automatic temperature control system ensures the stability of the heavy oil temperature at the outlet.

The burner that is matched with Zoomlion's LQC240 asphalt mixing plant uses high-pressure air atomized fuel. Compared with the widely used mechanical atomized burners on the market, it has a better atomization effect and more complete combustion. The requirements for oil quality are low, and the service life of the oil pump is relatively long.

Zoomlion LQC240 < a burner of asphalt mixing station supporting SRC = "http://tec.lmjx.net/UploadFiles_7130/200812/20081208132859765.bmp" width = 284 border = 0 >

The burner for the Zoomlion LQC240 asphalt mixing plant

Its main performance parameters are as follows:

The rated heat load is 23.72MW

The maximum fuel capacity is 2000kg/h

The adjustment ratio is 1:8

Fuel types: light fuel and heavy fuel

The calorific value of the fuel is ≥ 42,000 kJ/kg

The viscosity of the inlet fuel is ≤5°E

The fuel supply pressure is 1.2MPa

The atomization method is high-pressure air atomization

The atomization gas pressure is 0.5 to 0.6MPa

The atomization gas flow rate is 2m ³ /min

Ii. Selection of Fuel

At present, there are many types of oil that can be used in asphalt mixing equipment in our country. Fuel oil includes 4#, 5# light, 5# and 6#, while heavy oil includes domestic grades 20, 40, 60, 100, 180 and 200, as well as imported grades 180 and 380CST, among others. The quality varies greatly. How to choose the right oil is of vital importance.

Heavy oil is generally made by blending cracked heavy oil, vacuum heavy oil, atmospheric heavy oil or wax oil in different proportions. When making a selection, the overall requirements should be high calorific value, good combustion performance, moderate viscosity, low corrosiveness and solid impurities, and easy to store and transport.

1. Viscosity

Viscosity indicates the quality of fuel flow. The higher the viscosity, the worse the atomization effect. It is the most important quality and performance indicator of heavy oil, directly affecting the working efficiency of the fuel pump and injector as well as the fuel consumption. As the temperature rises, the viscosity of heavy oil gradually decreases. Therefore, high-viscosity oil must be heated and raised in temperature for smooth transportation and atomization.

Therefore, when selecting oil products, in addition to understanding the conventional indicators of the oil products, it is also necessary to understand the viscosity-temperature curve of the oil products to ensure that heating can make the heavy oil reach the viscosity required by the burner before atomization. For mechanical atomizing burners, the optimal fuel viscosity before the nozzle is 2.5 to 3.5°Et, and for high-pressure air atomizing burners, it is 4.5 to 5.8°Et.

There are two units for expressing viscosity: Engler viscosity (°Et) and kinematic viscosity νt (mm ² /s, also known as centistos cSt). The conversion relationship between them is: νt=7.310°Et-6.31/°Et.

2. Sulfur content

In heavy oil, sulfur can exist in various forms such as elements and compounds. The SO2, SO3 and other substances generated by sulfur after combustion, when discharged into the atmosphere, will seriously pollute the environment. When combined with water vapor, they will form highly corrosive sulfurous acid, sulfuric acid and other substances, which will affect the service life of flue ducts, dust collectors and fans. In addition, sulfides in the oil have varying degrees of influence on pipelines, valves, pumps, sealing rings, spray guns, etc. Therefore, when choosing heavy oil, the lower its sulfur content, the better, generally should be less than 3%.

3. Mechanical impurities

Mechanical impurities can clog the filter screen, spray gun and fuel valve, accelerate the wear of the fuel pump and nozzle, and seriously affect the service life of the components. Therefore, it is generally required that the content of mechanical impurities in the fuel be less than 0.5 to 1%.

4. Flash point and ignition point

Flash point and ignition point are important indicators of the fire and explosion safety of fuel oil, and the higher the temperature, the better. When choosing oil products, it is essential to know their flash point and ignition point to determine the heating temperature. In pressure-free open heavy oil storage tanks, the heating temperature should be 10 to 20 degrees Celsius lower than the flash point, which can effectively prevent fires.

5. Calorific value (calorific value

Calorific value is an important economic indicator of fuel and is directly related to the fuel consumption of asphalt plants. The higher the calorific value, the better. The lower calorific value of oil is usually 38.5 to 44MJ/kg.

6. Moisture content

Moisture is one of the main impurities in fuel. It not only reduces the content of combustible components in the fuel but also makes it difficult for the fuel to ignite. Excessive moisture can increase the corrosion of pipelines and equipment, raise heat loss from flue gas exhaust and energy consumption for transportation. Meanwhile, uneven moisture content can cause flame pulsation or even flameout. Fuel oil should be dehydrated before use, and the moisture content should generally be controlled between 1% and 3%.

Iii. Commissioning of the Burner

Burner commissioning is a key link to ensure efficient combustion, which requires very professional basic knowledge and practical experience. The key point of the debugging is the ratio of air to oil in the burner.

When adjusting the air-oil ratio, if conditions permit, a flue gas monitoring instrument should be used in conjunction. The adjustment should be made based on the CO content and the remaining O2 content detected by the instrument. Generally speaking, it is ideal for the remaining oxygen content to be between 3% and 5%, and for the CO content to be less than 100ppm. However, most users do not have such conditions. In actual use, they all rely on experience, such as adjusting by visually observing the color of the flame. Under normal circumstances, the center of the flame is light orange, bright and uniform, with a clear outline. There is no black smoke at the tail end, no snowflake sparks on the outer circle, and the oil mist at the base of the flame is uniform without black strips. The smoke is transparent. Otherwise, it can be changed by adjusting the air supply volume or oil volume. The adjustment methods are shown in Figures 2 to 5.

In Figure 2, the closer the oil valve connection rod is adjusted to the root of the crank, the faster the oil valve rotates and the faster the oil volume increases.

As shown in Figure 3, the closer the oil valve connection rod is adjusted to the head of the crank or to the root of the crank at the motor end, the slower the oil valve rotates and the slower the oil volume increases.

As shown in Figure 4, if the damper connection rod is adjusted towards the damper head, the slower the damper rotates, the slower the increase in air volume will be.

As shown in Figure 5, if the damper connection rod is adjusted towards the root of the damper, the faster the damper rotates, the faster the air volume increases.

If the combustion effect is not good at a certain point or within a certain section, at this time, the position of the spring plate of the CAM air distribution plate should be appropriately adjusted to change the air supply volume, so that the air-oil ratio is in a normal state and the fuel is completely burned.

Iv. Use of the Burner

After the burner is debugged, its normal use is relatively simple. The operation method and precautions are as follows:

Heat the heavy oil in the heavy oil tank to the required temperature (generally 80 to 100 degrees Celsius);

2. Preheat each insulation pipe to the required temperature (above 80℃ on the pipe surface).

3. Set and adjust all operating parameters properly: Oil pressure 1.0 to 1.2Mpa; The pressure of high-pressure gas is 0.5 to 0.6MPa.

4. Based on the oil type and the viscosity-temperature curve, determine the heavy oil heating temperature at the outlet of the heat exchanger.

5. Start the burner. The system will operate according to the preset procedure. First, ignite and burn with diesel oil. After it stabilizes, switch to heavy oil combustion. 10 to 15 minutes before the shutdown, switch to diesel for combustion and use diesel to clean the pipelines thoroughly.

Closing remarks

The burner is an important component of the asphalt plant, but there are many factors that affect the normal combustion of the burner. In practical application, apart from properly using and adjusting the burner itself, attention should also be paid to many issues such as the regulation of the induced draft air volume of the dust removal system and the negative pressure changes of the drum, to ensure complete and thorough combustion and truly achieve environmental protection and energy conservation. The component content should be controlled between 1% and 3%.