Transforming Sludge into Treasure: The Synergistic Miracle of Disc Drying and Coal-fired Power Plants

The Dilemma of Sludge Disposal: Urgent Need for Breakthrough

With the acceleration of urbanization and the continuous improvement of sewage treatment capacity, the global sludge production has been increasing year by year. Sludge is rich in a large amount of organic matter, pathogens, heavy metals, and other harmful substances. If not disposed of properly, it is very likely to cause serious pollution to the soil, water bodies, and the atmospheric environment, threatening the ecological balance and human health.

Traditional sludge disposal methods, such as sanitary landfilling, land application, and incineration, each have obvious limitations. Sanitary landfilling not only occupies a large amount of valuable land resources but also may lead to the pollution of groundwater by leachate. Moreover, with the increasing shortage of available land, the landfill space is becoming more and more limited. Land application faces the risk of excessive heavy metals and organic pollutants in the sludge. Long-term use may lead to problems such as a decline in soil quality and contamination of crops. Traditional direct incineration not only has high energy consumption but also generates a large amount of harmful gases during the combustion process, such as dioxins and nitrogen oxides, which seriously damage the atmospheric environment. At the same time, the treatment of the ash after incineration is also a difficult problem. The disadvantages of these traditional disposal methods make it extremely urgent to find a more efficient, environmentally friendly, and sustainable sludge disposal solution.

Disc Drying: The Prelude to the Rebirth of Sludge

In the innovative exploration of sludge disposal, the disc drying technology has stood out and become the key prelude to achieving efficient sludge treatment. Through its unique technical principle, it transforms the originally difficult-to-treat sludge into a form that is easier to dispose of, laying a solid foundation for subsequent collaborative disposal.

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(I) In-depth Analysis of the Technical Principle

The core equipment of the disc drying technology is the disc dryer, which has a sophisticated structure and is mainly composed of a jacket, a single-shaft rotor, and a transmission device. The key component of the equipment, the disc, is the core element for realizing sludge drying. The disc is a hollow structure and is connected to the hollow shaft. The cavity of the shaft and the disc forms a complete heat conduction channel. During actual operation, high-temperature steam is used as the heat medium and flows into each hollow disc through the hollow shaft. For example, in the disc drying system of a certain sewage treatment plant, the steam temperature can reach about 159°C, and the pressure is maintained at 0.5 - 0.6MPa, providing sufficient heat energy for sludge drying.

After the sludge enters the dryer from the feed port, it is distributed in the space between the disc and the outer shell. As the single-shaft rotor rotates, the disc also rotates. The propelling agitators on the edge of the disc play a crucial role. They are like diligent "mixers" that continuously stir the sludge. During the stirring process, the drying surface of the sludge is continuously updated, allowing the sludge to fully contact the heat transferred by the disc. At the same time, the longer scraper fixed on the inner wall of the outer shell is also working silently. It extends into the gap between the discs, not only stirring the sludge but also cleaning the disc surface in a timely manner to prevent the sludge from sticking to the rotating disc and ensuring the smooth progress of the drying process.

Under the action of heat transfer, the moisture in the sludge gradually evaporates. The water vapor formed by evaporation accumulates in the dome space above the disc and is then carried out of the dryer by a small amount of the ventilation system, thus achieving efficient dehydration and drying of the sludge. Throughout the process, the steam does not directly contact the sludge, avoiding the pollution of the sludge by the steam, and also making the amount of insoluble gases in the tail gas small, reducing the burden on the subsequent tail gas treatment.

(II) Highlights of Technical Advantages

The disc drying technology has emerged prominently in the field of sludge treatment due to its remarkable advantages.

Energy conservation is a major outstanding feature of the disc drying technology. It adopts an indirect heating method. The heat medium flows in the hollow shaft and the disc, and the heat is transferred to the sludge through the disc surface, resulting in high thermal efficiency. Taking the sludge treatment project of TDG TIC Technology Co., Ltd. using the disc drying technology of Mitsubishi as an example, compared with the traditional direct heating drying method, its energy consumption is reduced by about 30%. Moreover, this technology can use a low-temperature heat source (≤170°C) for heating, further reducing energy consumption while ensuring the drying effect.

Excellent environmental performance is also a major advantage of the disc drying technology. Since the steam does not come into contact with the sludge, the amount of insoluble gases in the tail gas is small, and the generated tail gas can be directly incinerated in the furnace, reducing the risk of secondary pollution. The equipment is equipped with an air supply port, making the tail gas emission smoother, reducing the load of the tail gas treatment system, and reducing the impact on the surrounding environment.

In terms of the floor area, the disc drying technology also performs excellently. The dryer conducts heat transfer through the outer shell wall, the hollow shaft, and the left and right sides of each vertical disc. With a large heat transfer area, the equipment has a compact structure and a small external dimension. In the current situation where land resources are becoming increasingly scarce, this advantage makes the disc drying technology more attractive in urban sludge treatment projects, which can effectively save land costs and improve land utilization efficiency.

High treatment efficiency is another remarkable advantage of the disc drying technology. It has a large solid waste treatment load, can ensure stable operation even if the feeding is uneven, has a small internal operation resistance, and low equipment energy consumption. For example, after a large-scale sewage treatment plant adopted the disc drying equipment, it can treat hundreds of tons of wet sludge every day, and the moisture content of the dried sludge can be stably reduced to below 40%, greatly improving the efficiency and production capacity of sludge treatment.

The disc drying technology can also ensure the good quality of the dried sludge. After being crushed and stirred, the sludge is in a uniform granular shape, which is conducive to subsequent disposal. These granular dried sludges can be used as raw materials for incineration media, fertilizers, building materials, activated carbon, etc., realizing the resource utilization of sludge and contributing to sustainable development.

Collaborative Disposal with Coal-fired Power Plants: Opening a New Chapter Hand in Hand

When the sludge goes through the key prelude of disc drying, it ushers in a new chapter of collaborative disposal with coal-fired power plants. This innovative cooperation model has opened up an efficient, environmentally friendly, and sustainable path for the final disposal of sludge.

(I) Comprehensive Analysis of the Collaborative Disposal Process

In the process of collaborative disposal of sludge with coal-fired power plants, a series of rigorous and orderly processes ensure that the sludge can be properly treated and its energy can be utilized.

The dried sludge is first transported to the coal-fired power plant through specialized transportation equipment. These transportation equipment usually adopt a closed design to prevent the diffusion of odors and leakage of the sludge during transportation, avoiding pollution to the surrounding environment. For example, in some urban sludge disposal projects, special sludge transportation vehicles with sealed cabins are used to ensure the safety and environmental protection of the sludge during transportation.

After arriving at the power plant, the sludge is stored in a special sludge storage bin. The design of the storage bin fully considers the characteristics of the sludge and has good sealing and ventilation systems to prevent the accumulation of biogas generated by the piled-up sludge and avoid the moisture absorption of the sludge, which may affect the subsequent mixing and combustion effects. Some power plants also install a stirring device in the storage bin to maintain the uniformity of the sludge and ensure a stable supply during the retrieval process.

In the mixing link, the dried sludge is mixed with coal according to an accurate proportion. The determination of this proportion needs to comprehensively consider various factors, including the calorific value of the sludge, the quality of the coal, and the combustion characteristics of the boiler. Generally speaking, in order to ensure the stable operation of the boiler and the combustion efficiency, the co-firing proportion is usually controlled within a certain range, such as 5% - 8%. For example, through long-term operation practice and experimental research, a certain power plant has determined that the dried sludge and coal are mixed at a ratio of 6%, which can not only make full use of the calorific value of the sludge but also ensure the normal operation of the boiler. The mixing process is realized with the help of specialized mixing equipment, which can fully stir the sludge and coal to make them evenly mixed, laying a foundation for subsequent efficient combustion.

The mixed fuel is transported to the boiler for combustion. In the boiler, the high-temperature environment promotes the rapid combustion of the fuel, releasing a large amount of heat energy. The organic matter in the sludge cooperates with the coal during the combustion process to jointly release energy. The high-temperature flue gas generated by the combustion passes through the heating surface of the boiler, transferring the heat energy to the working medium, which is then converted into steam to drive the steam turbine to generate electricity. Throughout the combustion process, the automated control system of the power plant monitors various operation parameters in real time, such as temperature, pressure, and oxygen content, and adjusts the combustion conditions in a timely manner according to the monitoring data to ensure the stability and high efficiency of the combustion.

(II) Multiple Benefits Brought by Collaboration

The innovative model of collaborative disposal of sludge with coal-fired power plants has brought significant benefits in many aspects and has shown unique advantages in the fields of environmental protection, economy, and energy utilization.

From an environmental protection perspective, this model has greatly reduced the potential harm of sludge to the environment. Traditional sludge disposal methods often have many environmental risks. Through collaborative disposal with coal-fired power plants, the harmful substances in the sludge are effectively decomposed and solidified during the high-temperature combustion process, reducing the pollution risks to the soil, water bodies, and the atmosphere. For example, the pathogens contained in the sludge are completely killed at high temperatures, and the heavy metals and other harmful substances are fixed in the slag, avoiding their diffusion in the natural environment. At the same time, the exhaust gas generated during the combustion process is treated through the existing high-efficiency desulfurization, denitrification, and dust removal equipment of the power plant to achieve the standard discharge of pollutants, further reducing the impact on the atmospheric environment.

At the economic level, collaborative disposal has brought a win-win situation for coal-fired power plants and sewage treatment plants. For coal-fired power plants, sludge, as a low-cost alternative fuel, reduces the dependence on coal to a certain extent, thereby reducing the fuel procurement cost. Taking a certain power plant as an example, by co-firing sludge, it can save millions of yuan in coal procurement costs every year. In addition, in some regions, the government will give certain subsidies to power plants to encourage the resource utilization of sludge, which has also become an additional source of income for power plants. For sewage treatment plants, cooperating with coal-fired power plants to outsource the sludge disposal business reduces their own investment in the construction and operation of sludge treatment facilities and lowers the operation cost. At the same time, it also avoids the high fines and legal risks that may be faced due to improper sludge disposal.

In terms of energy utilization, the collaborative disposal of sludge with coal-fired power plants has realized the recycling of resources and transformed the originally regarded as waste sludge into usable energy. The organic matter contained in the sludge has a certain calorific value. By co-firing with coal, its energy is released and converted into electrical energy, improving the energy utilization efficiency. This not only conforms to the concept of sustainable development but also helps to alleviate the pressure of energy shortage and contributes to the optimization of the energy structure. For example, some power plants can generate millions of kilowatt-hours of additional electricity every year by co-firing sludge, realizing the energy utilization of waste and providing more clean energy for society.

(II) Experience Summary and Enlightenment

In project planning and site selection, full consideration should be given to the collaborative utilization of surrounding resources. Selecting a site close to relevant enterprises such as coal-fired power plants and cement plants can shorten the transportation distance of sludge, reduce transportation costs, and facilitate the resource sharing and recycling between sludge and other industries. Cooperating with power plants can use their waste heat steam as the drying heat source to reduce energy consumption; cooperating with cement plants can use the fly ash after incineration as a production raw material to improve the comprehensive utilization rate of resources.

The selection and optimization of technology are of vital importance. Adopting advanced disc drying technology and collaborative incineration process can improve the efficiency and quality of sludge treatment. In the selection of drying equipment, attention should be paid to the energy conservation, stability, and treatment capacity of the equipment to ensure the drying effect while reducing energy consumption. In the collaborative incineration link, the optimal co-firing ratio of sludge and coal should be accurately determined through experiments and actual operation data, and the combustion conditions should be optimized to ensure the stable operation and high-efficiency combustion of the boiler. Strengthen the research and application of pollutant control technologies during the drying and incineration processes to ensure that the project meets environmental protection requirements.

During the project implementation process, the close cooperation and coordination of all participants are indispensable. Sewage treatment plants, power generation enterprises, equipment suppliers, and relevant government departments should establish a good communication and coordination mechanism, clarify the responsibilities and obligations of all parties, and jointly solve the problems that arise during the project construction and operation. In terms of sludge quality control, production plan arrangement, equipment maintenance management, etc., all parties should cooperate closely to ensure the smooth progress of the project.

Future Prospects: An Infinite Future

(I) Prospects for Technological Development Trends

With the rapid development of science and technology, the technologies of sludge disc drying and collaborative disposal with coal-fired power plants are expected to achieve major breakthroughs in aspects such as intelligence, high efficiency, and greenness. In the field of intelligence, with the help of advanced technologies such as artificial intelligence, big data, and the Internet of Things, an intelligent monitoring and management system will be constructed to achieve real-time monitoring and precise control of the entire sludge treatment process. Real-time data such as the composition, moisture content, and calorific value of the sludge, as well as the operation parameters of the equipment, such as temperature, pressure, and flow rate, are collected through sensors. Big data analysis technology is used to deeply mine and analyze these data, providing a scientific basis for optimizing the production process and adjusting the co-firing ratio. Artificial intelligence algorithms are adopted to achieve automatic control of the equipment, automatically adjusting the combustion conditions according to the characteristics of the sludge and coal to ensure the stable and efficient operation of the boiler, improving energy utilization efficiency, and reducing operation costs.

In terms of high efficiency, new disc drying equipment and combustion technologies will be developed to further improve the drying efficiency and combustion efficiency. For example, by improving the structure and heat transfer method of the disc dryer, the heat transfer efficiency will be improved, the drying time will be shortened, and the energy consumption will be reduced. Develop efficient combustion technologies, such as new burners and optimized combustion processes, to make the sludge and coal fully mixed and rapidly burned in the boiler, releasing more energy and improving the power generation efficiency. At the same time, strengthen the research on sludge pretreatment technologies, such as using physical, chemical, or biological methods to deeply treat the sludge, removing impurities and harmful substances in it, and improving the quality and calorific value of the sludge, providing better raw materials for subsequent drying and combustion.

(II) Far-reaching Significance for Industry Development

The promotion and application of the technologies of sludge disc drying and collaborative disposal with coal-fired power plants have far-reaching significance for the environmental protection industry and sustainable development. In the environmental protection industry, it provides a reliable solution for the harmless treatment of sludge, effectively reducing the pollution of sludge to the environment and reducing the risks to the soil, water bodies, and the atmospheric environment. This helps to improve the quality of the ecological environment, protect the health of the people, and promote the healthy development of the environmental protection industry. This technology also promotes the technological innovation and upgrading of the environmental protection industry, driving the development of related fields such as equipment manufacturing, engineering design, and operation management, creating more employment opportunities and economic benefits.

From the perspective of sustainable development, the resource utilization of sludge is an important measure to achieve resource recycling and sustainable development. By transforming sludge into energy and usable resources, it reduces the dependence on natural resources and improves the resource utilization efficiency. The organic matter and combustible components in the sludge release energy during the combustion process, realizing the recycling of energy; the slag after incineration can be used for the production of building materials, realizing the resource utilization of waste. This not only conforms to the concept of sustainable development but also helps to alleviate the problems of energy shortage and resource shortage, providing strong support for the sustainable development of the economy and society. All sectors of society should strengthen their attention and understanding of the sludge treatment problem, improve environmental protection awareness, and jointly promote the development of the sludge treatment industry.