Application Research on the Fuel-to-Coal Conversion Technology of ASTEC-50 Asphalt Mixing Plant

Abstract: The application of fuel-to-coal technology in asphalt mixing plants has a significant effect on reducing production costs. This article introduces the complete set of fuel-to-coal conversion technologies suitable for Aastec-50 asphalt mixing plants and the matters needing attention during use, and analyzes its value from the economic aspect, which can be referred to by relevant enterprises.

Key words: Aastec-50 asphalt mixing plant, fuel-to-coal conversion technology, precautions, Application research

1. The raising of the question

The Aastec-50 asphalt mixing plant, introduced by Guangdong Changda Highway Engineering Company in 2002, is a dual-purpose intermittent and continuous asphalt concrete mixing equipment made in the United States. With an intermittent production capacity of 360 tons per hour, it is currently the asphalt mixing equipment with the highest output and the most advanced technology in Asia. After the efforts of the company's technical personnel, it has been maturely applied in pavement projects such as the Guangzhou-Huizhou Expressway and the Guangzhou-Zhuhai West Expressway, and has achieved good benefits.

In recent years, due to the increasingly fierce competition in the expressway construction market and the shrinking market profit margin, in the construction of high-grade highways, not only are the requirements for modern construction and building techniques constantly rising, but also corresponding conditions and strict requirements have been put forward for the investment, cost and expenses of highway construction. For a construction enterprise, it is not only necessary to strictly control the quality, ensure the progress of the project, and meet all reasonable requirements of the owner, but also to do everything possible to reduce production costs and ensure its own reasonable profits to maintain the normal operation of the company. It is not uncommon for many enterprises to exit the market due to the inability to reduce production costs. Among the mechanical costs of several road construction projects, the fuel consumption of asphalt mixing plants accounts for approximately 30 to 40% of the mechanical costs. In addition, the continuous rise of diesel in petroleum products in recent years has a direct impact on the production cost increase of black pavement construction enterprises.

How to reduce the construction cost of engineering projects, including mechanical costs, is the primary concern for construction units. Among the various supporting machinery involved in the construction of asphalt pavement, the investment proportion of asphalt concrete mixing equipment is the largest. Fuel consumption cost is the most significant component in the operating expenses of asphalt concrete mixing equipment. Especially in recent years, with the continuous rise in fuel prices, the proportion of fuel costs in asphalt concrete mixing equipment will further increase. This poses a very important and very realistic issue for engineering construction units. Whether technological improvements can be made to find fuel substitutes to achieve the same heating effect, thereby significantly reducing the cost of mechanical use, lowering the production cost of construction enterprises, increasing profit margins, and ensuring the competitiveness of enterprises is a very important issue that concerns the survival of enterprises.

Requirements for fuel in the heating of mixed aggregates

The fuel is required to be able to raise the temperature to an appropriate level after combustion and not cause pollution to the aggregates.

3 Feasibility Analysis

To solve this problem, through the analysis of the current practices of domestic and foreign peers in this regard, the following several situations exist:

3.1 Use 0# diesel as fuel

0# diesel is used as fuel. It can produce a stable flame when burned and has been widely adopted by construction units. And it has been well applied.

3.2 Use natural gas as fuel

The use of natural gas as fuel is greatly influenced by the gas source and the state has relatively strict regulations on the storage of gaseous fuels. It is not widely used.

3.3 Use gas as fuel

The use of gas as fuel is not widely applied (for the same reason as "3.2").

3.4 Use heavy oil as fuel

Many construction units also adopt the scheme of using heavy oil as fuel. The more controversial points are that the impact of SO2 produced by the sulfur-containing combustion of heavy oil on dust removal bags and the influence of high-concentration residues produced by incomplete combustion adhering to crushed stones on asphalt mixtures are both controversial and require further research.

At present, Changda Company has purchased this system. Judging from the usage at several construction sites, it is not very satisfactory. Heavy oil has poor fluidity, incomplete combustion and unstable quality at low temperatures. To achieve the ideal low-temperature fluidity, the common practice is to blend one-third diesel into heavy oil, which undoubtedly increases the usage cost. After using heavy oil, the machine has cumulatively discharged 150,000 tons of materials. However, a set of cloth bags has been replaced, and the service life of the cloth bags is far shorter than the designed discharge capacity of 500,000 tons.

3.5 Use bituminous coal as fuel

The disadvantages of bituminous coal as fuel are unstable quality. Some coals have a low calorific value, unstable flame and difficult temperature control. Secondly, the ash produced during the combustion process enters the drying cylinder. However, its advantage is its low price. But after investigating the mixing plants that currently use bituminous coal as fuel at home and abroad, it is found that there are quite a few successful cases of its use. The disadvantages of bituminous coal as fuel can be improved. The key lies in the improvement of combustion furnaces and the adoption of advanced processing technologies to control the quality of bituminous coal materials.

4 Grinding and combustion equipment and their working principles

In light of the characteristics of the Aastec-50 asphalt mixing plant, this oil-to-coal conversion technology is prepared to adopt the RMF series pulverized coal combustion furnace and coal grinding powder spraying machine. The organic combination of the features of the pulverized coal combustion furnace and the coal grinding powder sprayer and Aastec-50 is conducive to improving the thermal efficiency of bituminous coal, enabling it to burn completely in a closed combustion chamber and form a stable spiral flame.

4.1 Combustion Furnace

The structure of the combustion furnace is shown in Figures 1 and 2.

The relevant technical parameters of the combustion furnace are shown in Table 1.

4.2 Coal grinding powder Spraying machine

The connection between the coal grinding pulverizer and the combustion furnace is shown in Figure 3.

The relevant technical parameters of the coal grinding and powder spraying machine are shown in Table 2.

4.3 Working Principle

(1) Combustion furnace

The combustion process is as shown in Figure 2: Open the diesel control valve, and the diesel burns first to increase the temperature of the combustion furnace. Then, open the coal quantity control valve, and the pulverized coal enters the furnace through the pulverized coal pipeline for initial combustion → increase the dosage for sudden combustion → (strong wind and large coal) sudden combustion → generate a large amount of heat → volatilize the moisture and ash → part of it deposits in the furnace slag (part enters the drum).

Before the pulverized coal enters the furnace, the combustion furnace is heated by diesel combustion, providing an appropriate temperature for the subsequent combustion of the pulverized coal. The primary air duct's incoming air (internal air) and pulverized coal are thoroughly mixed and initially ignited, generating CO. The secondary air passes through a small air outlet with high air pressure, creating a high-speed circulation in the secondary combustion zone of the furnace hall to ensure complete combustion, thereby enhancing thermal efficiency. The central temperature inside the furnace can reach 1000℃ to 1200℃.

(2) Coal grinding powder spraying machine

The coal grinding and powder spraying machine is a multi-functional device integrating coal grinding and centrifugal air supply. It is a stepless speed-regulating high-pressure coal grinding and powder spraying machine, used for crushing coal blocks with a particle size of less than 15mm, and mixing coal powder and supply air in the conveying pipeline to provide conditions for the complete combustion of coal powder in the furnace.

Issues to be noted in the application of fuel-to-coal technology

5.1 Requirements for matching the combustion furnace with the coal grinding and powder spraying machine

The matching of the combustion furnace and the coal grinding powder injector is determined by the output of the mixing plant. A mixing plant with a large output requires a large heating capacity, and the output of the grinding powder injector also needs to be large.

The Aastec asphalt mixing plant's complete set of technology for converting fuel oil to coal was originally designed to use one combustion furnace and three coal grinding and powder spraying machines. Judging from the actual effect, it is better to use four coal grinding and powder spraying machines.

5.2 Requirements for Coal quality

To ensure the quality of the combustion flame, corresponding requirements must be made for the coal quality. The specific indicators are as follows:

(1) The particle size of the feed should be less than 15mm, and the particle size after grinding must be controlled at around 200 mesh (with an error of ±10%).

(2) Calorific value ≥5500kcal/kg. After investigation, Datong coal is the standard.

(3) Ash content ≤12%. Ash content affects the combustion value, resulting in a decrease in calorific value. The sulfur content in the slag is high, making slag removal difficult.

(4) Volatile matter ≥25%. When the volatile matter reaches this level, the ignition point rises and combustion becomes difficult.

(5) The moisture content should be controlled at 3% to 5%. If the moisture content is too high, the coal powder and air will mix and form lumps, which will affect combustion.

(6) The flash point of coal ash is ≥1450℃.

5.2 Venue Requirements

From the application of the fuel-to-coal technology in several expressway construction sites, the following issues must be noted in terms of the site:

(1) Most of the construction work on expressways is carried out outdoors, and the mixing sites are mostly temporary land. When considering the site layout, sufficient space should be reserved for the storage of coal. Before starting work, prepare sufficient quantities to avoid quality fluctuations caused by untimely purchase or temporary purchase that cannot be monitored during the usage process.

(2) Coal storage should be covered with a shed to prevent it from being soaked by rain. If the humidity is too high, it should be dried. The water content in coal should be less than 5%.

5.3 Other measures that need adjustment

(1) The increase in ash content produced by pulverized coal combustion has intensified the workload of dust removal. It is recommended to adjust the air damper to increase the negative pressure inside the dust removal box and enhance the dust removal effect. Strengthen the monitoring to prevent untimely powder discharge.

(2) The combustion of pulverized coal is prone to generating SO2, which increases the acidity of the aggregates. It is recommended that when dust is removed during the production of aggregates in stone quarries, lime water with a water-cement ratio of 9:1 be used for flushing to enhance the alkalinity of the aggregates.

(3) The head brick of the pulverized coal combustion furnace is formed with high-temperature resistant casting material, and the casting body is very dense. It is the first time to use pulverized coal combustion

When roasting, wood or coal must be used to remove the moisture from the casting. The baking method is as follows:

Heat at 25℃ per hour to 110℃ and maintain for more than 6 hours. Heat at 40℃ per hour to 400℃ and maintain for more than 6 hours.

When baking, it is essential to strictly control the temperature. Do not increase the temperature too quickly; otherwise, the lifespan of the furnace head bricks will be greatly shortened, and they may even burst, leading to the scrapping of the combustion furnace and making it unusable.

6. Economic benefit analysis

The cost of converting the Astec-50 combustion device from oil to coal is approximately 400,000 yuan. Taking the eighth section of the Helong Expressway pavement project of the Third Branch of Guangdong Changda Highway Engineering Company as an example for economic analysis, the total amount of asphalt mixture in the mixing plant of the eighth section of the Helong pavement project is 160,000 tons. The cost of heating diesel, heavy oil and coal per ton of mixture is shown in Table 3

The unit prices of diesel, heavy oil and coal are based on the current market listed prices.

The unit consumption of diesel, heavy oil and coal is based on the empirical values of Changda Highway Company (considering the high moisture content of materials due to the rainy weather in Guangdong).

By the end of the project, coal combustion can save 3.504 million yuan compared with diesel combustion, and coal combustion can save 1.712 million yuan compared with heavy oil combustion. It can be seen from this that only by investing a certain amount of modification costs, the technology of converting oil to coal for asphalt mixing plants can achieve very good economic benefits.

7 Conclusion

The technology of heating asphalt mixture to convert fuel oil into coal is a highly technical technique. Beneficial trials have been conducted in many provinces and cities in China, but the application effects vary greatly. From the perspective of the application of the Aastec-50 asphalt mixing plant's complete set of technology for converting fuel oil to coal, as long as there is high-quality coal, a combustion furnace suitable for this mixing plant is adopted, and some technical details are mastered in the early stage of use, and the working reliability of the grinding and combustion equipment components is enhanced, the application effect is quite satisfactory.

During the application of the Aastec-50 asphalt mixing plant's complete set of technology for converting fuel oil to coal, rich experience has been accumulated, mature complete sets of technology have been mastered, and remarkable economic benefits have been achieved.

This article discusses the technology of replacing fuel oil with coal mainly because of its good economic benefits and wide application prospects.

References

1. Comprehensive Information on Asphalt Mixing Equipment 2001.5 ASTEC Machinery Co., LTD., USA

2. Modern Construction Machinery, May 1997, Communications Press, Li Wanli et al

Performance Analysis of German teltomat-200sm Semi-Mobile Asphalt Mixing Plant 2002.10 Road and Road Transport Xu Chunhui, Zeng Liwen, et al