A Brief Discussion on the Selection Criteria for Heavy Oil in Marini MAP Series Asphalt Mixing Plants

Abstract: This paper introduces the requirements and application effects of heavy oil in the combustion system of asphalt mixing plants, and particularly proposes selection criteria for heavy oil applicable to the combustion system of Marini MAP series asphalt mixing plants in Italy.

The asphalt mixing plant is suitable for three types of fuel: heavy oil, residual oil and diesel, collectively referred to as fuel oil. When conditions permit, straight-run or cracked heavy oil or diesel should be used as much as possible, as their quality remains relatively stable within a certain period. Fuel oil refined from small refineries' mixed oil should be avoided as much as possible, as the quality of such oil products varies greatly among different batches, which is not conducive to the stable operation of the system.

The most important requirement for fuel oil in the combustion system of Marlene MAP series asphalt mixing plants is that the Engler viscosity when entering the atomization plate should be between 1.8 and 2.0 OE, and it is absolutely not allowed to exceed 3.0 to 4.0OE. This is a prerequisite for whether the oil can be used in the system. Fuel with a viscosity between 1.8 and 2.0OE is most suitable for use in this system.

The overall requirements for fuel selection should be moderate viscosity, good combustion performance, high calorific value, good atomization performance, complete combustion, less carbon deposits and ash produced, low corrosiveness to burner components, and easy to store, transport and pressurize. Heavy oil is generally made by blending cracked heavy oil, vacuum heavy oil, atmospheric heavy oil or wax oil in different proportions. Compared with straight-run (vacuum and atmospheric pressure) heavy oil, cracked heavy oil has a higher viscosity and relative density, and contains more solid impurities. These impurities are prone to sedimentation, clogging pipelines and fuel preheaters, and are not easy to burn. Therefore, the proportion of cracked heavy oil in the selected heavy oil should be as small as possible.

At present, there are no national or departmental/industry standards to restrict heavy oil. This system has the following several requirements for the quality of heavy oil:

1 Engler viscosity

Viscosity indicates the quality of fuel flow. The greater the viscosity, the worse the fluidity. Viscosity has a significant impact on both combustion and transportation. High viscosity increases the resistance of fuel transportation in pipelines, making loading, unloading and atomization more difficult. Temperature has the most significant impact on viscosity. As the oil temperature rises, its viscosity decreases. Therefore, high-viscosity oil must be heated and raised in temperature for smooth transportation and atomization.

The more colloidal asphaltenes there are in the oil or the larger the oil molecules are, the worse it is. If the oil contains a relatively high amount of gelatinous asphalt, carbon precipitation and sediment are likely to occur during the heating stage of the transportation process, resulting in sludge. This can clog the filter screen and even thinner pipes, and coking is likely to form at the nozzle during combustion.

More importantly, the higher the viscosity of the oil, the more difficult it is to burn, especially to burn completely. Basically, over 90% of the burners in mixing plants adopt mechanical pressure atomization. Since viscosity has little to do with pressure, the higher the viscosity of the oil, the more difficult it is to atomize, resulting in poor combustion effect, difficulty in ignition and instability after ignition, low flame intensity, and a tendency to break out due to incomplete combustion.

In engineering, the viscosity of fuel is generally expressed by Engler viscosity. Nowadays, kinematic viscosity is also widely used. The fuel oil used in the mixing plant generally requires that its Engler viscosity at 50℃ should not exceed 25 to 300E.

Engler viscosity is a conditional viscosity, which is the ratio of the time it takes for 200mm ³ of oil at t℃ to flow out of the small hole of the Engler viscometer to the time it takes for the same volume of distilled water at 20℃ to flow out. This ratio is called the Engler viscosity 0Et of this oil at t℃. Clearly, it is dimensionless.

The relationship between Engler viscosity and kinematic viscosity is: Vt=7.310Et to 6.31/0 ET

Viscosity is the most important quality and performance index of heavy oil. It determines the possibility and conditions of its application and directly affects the working efficiency of oil pumps and fuel injectors as well as fuel consumption. The relationship between viscosity and temperature is as follows: As the temperature rises, the viscosity initially decreases sharply, but when the temperature reaches a certain level, the viscosity change of heavy oil tends to stabilize.

2. Sulfur content

In petroleum refining, about 80% of sulfur remains in the refined heavy oil. In heavy oil or residual oil, sulfur can exist in various forms such as elements and compounds. The SO2, SO3 and other substances produced by sulfur after combustion are toxic. When discharged into the atmosphere, they will seriously pollute the environment. Moreover, when passing through flue ducts, dust collectors, fans and other devices, they will combine with water vapor to form highly corrosive sulfurous acid, sulfuric acid and other substances, which seriously affect the service life of the dust removal system. Moreover, during storage and transportation, sulfides in the oil have serious adverse effects on both human health and equipment, and have a significant impact on the service life of pipelines, valves, pumps, sealing rings, spray guns, etc.

The sulfur content in heavy oil should be as low as possible. Generally, the sulfur content should be less than 3%.

3 Mechanical impurities

The damage caused by mechanical impurities to equipment is obvious. They can clog the filter screen, fuel injectors and valves, accelerate the wear of the fuel pump, and seriously affect the service life of the components. Therefore, the smaller the mechanical impurities in the fuel, the better. Generally, the content of mechanical impurities in the fuel is required to be less than 0.5 to 1%.

4. Freezing point and boiling point

The freezing point is a performance indicator of the low-temperature flowability of fuel oil. It refers to the temperature point at which the oil surface of the sample oil in a 45° inclined test tube remains unchanged after one minute. The freezing point of fuel is related to the paraffin content. Oil with a higher paraffin content has a higher freezing point. The freezing point is also related to the water content in the oil. As the water content in the oil increases, the freezing point gradually rises. The freezing point is an important reference for the low-temperature fluidity of heavy oil, but it is not equivalent to the highest temperature at which heavy oil indicates fluidity in actual use.

Fuel oil is composed of a complex mixture of various hydrocarbons and has no fixed boiling point but only a temperature range. The boiling of oil begins at a certain temperature and continues continuously as the temperature rises.

5. Flash point and ignition point

Flash point is an important indicator of the fire and explosion safety of fuel. At a certain temperature, when the mixture of oil vapor and air on the oil surface comes into contact with an open flame and flashes briefly (then goes out), the temperature of the oil at this time is called the flash point. The more small molecular components there are in the oil, the lower the flash point. Oil with a low boiling point also has a low flash point. When the oil pressure rises, the flash point also increases accordingly. The temperature of heavy oil in pressure-free open storage tanks should be 10 to 20 degrees Celsius lower than the flash point, which can effectively prevent fires. The flash point of heavy oil is generally between 135 and 245 degrees Celsius.

The temperature at which a fuel gas can continue to burn when ignited by an open flame (for no less than 5 seconds) is called the ignition point of the fuel. The general flash point is 10 to 50 degrees Celsius higher than the flash point.

6. Calorific value

Calorific value is an important economic indicator of fuel. Generally, the lower calorific value of fuel is 38.5 to 44MJ/kg. Generally speaking, the heavier the oil, the relatively lower its hydrogen content and the lower its calorific value. The calorific value of heavy oil is slightly lower than that of gasoline, generally ranging from 40 to 42MJ/kg.

7 Residual carbon

Residual carbon refers to the mass percentage of fixed carbon remaining in the fuel after the oil vapor evaporates when the fuel is heated in an air-free environment. Oil with a high carbon residue content has a high flame blackness when burning, which can enhance flame radiation. However, it is prone to release a large amount of fixed particles during combustion. This not only makes it difficult to achieve complete combustion but also easily leads to coking and blockage of the atomizer, affecting the atomization effect, damaging system performance, and increasing the maintenance workload of the combustion device. As this system employs a mechanical pressure atomizer, the residual carbon in heavy oil should not exceed 0.25%.

Aromatic hydrocarbon compounds with a benzene ring

Benzene and its homologues have a strong aromatic smell. Their vapors have a significant effect on the human body, a strong corrosive effect on natural rubber, are difficult to ignite spontaneously, and have extremely strong destructive power on the sealing performance of pumps, valves, etc. Therefore, the content should be as small as possible.

9 Moisture content

Water is one of the main impurities in fuel. Generally speaking, water in oil is harmful. 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%.

On the other hand, emulsifying heavy oil with water can easily achieve low-oxygen combustion, improve the combustion effect of heavy oil, save fuel and reduce pollution. The water evenly mixed in the oil in an emulsion not only does not undermine the stability of the flame but also promotes better and more complete combustion of the fuel, reduces the difficulty of ignition, and plays a role in secondary atomization.

Generally speaking, uniformly adding water for emulsification can reduce the amount of carbon black, resulting in less soot near the ignition point, raising the temperature of the flame itself by 10 to 20 degrees Celsius, and also lowering the temperature during the ignition stage. Water vapor can achieve a secondary atomization effect, making it flammable and smoothly transitioning to low-oxygen combustion. The optimal amount of water to be added should be determined based on the viscosity of the oil, generally between 2% and 5%, and should not exceed 6.5%.

Good emulsification and mixing is a prerequisite for the effect of adding water to heavy oil. When the quality of heavy oil (especially its viscosity) is close to that of residual oil or residual oil is used as fuel oil, adding water is a good choice.

In conclusion, the quality of fuel selection has a significant impact on the stable operation of equipment and its economic benefits. Appropriate fuel should be chosen based on the characteristics of the equipment and the fuel source to better bring out the performance of the equipment and achieve the best overall benefits.

References

1. Oil application and management. Xiong Yun, Li Xiaodong and Xu Shihai. Sinopec Press. May 2004

2. Comprehensive List of China's Petrochemical Products. Compiled by the Business Division of Sinopec Corporation. China Petrochemical Press. June 2002