Practices and Explorations on the Resource Utilization of Wastewater and Sludge from Coal Chemical Enterprises

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Practices and Explorations on the Resource Utilization of Wastewater Sludge in Coal Chemical Enterprises / Author/Source: Coal Economics Research / Date: 2019-07-15 / Clicks: 13. Based on the current situation of sludge treatment in China’s coal chemical industry, this article analyzes traditional sludge disposal methods as well as technologies for sludge resource utilization. It focuses on the practices related to sludge resource utilization at China National Coal Group Shaanxi Yulin Energy Chemical Co., Ltd., and provides an economic analysis of such utilization. By exploring new approaches to the resource utilization of sludge, it is proposed that such utilization should be based on the physical and chemical properties of the sludge, adhering to principles such as adaptation to local conditions and safety and environmental protection. Meanwhile, this approach provides valuable experience for coal chemical enterprises in making use of sludge, thereby contributing to the achievement of zero emissions in the coal chemical industry. Keywords: sludge ； Resource utilization ； Energy conservation and environmental protection ； Coal chemical enterprises The 13th Five-Year Plan for National Economic and Social Development, as well as the 13th Five-Year Plan for Energy Development (hereinafter referred to as the \"Plans\") state that coal is China’s primary energy source and an important raw material. Moderate development of the deep processing industry for coal is not only necessary for **strategic technological and capacity reserves in the energy sector**, but it is also an important measure to promote the clean and efficient use of coal and ensure **energy security**. The Plan calls for a comprehensive push to revolutionize energy production and consumption, with the aim of establishing a modern energy system that is clean, low-carbon, safe, and efficient. Developing modern coal chemical industry represents a path for the clean development and utilization of coal that suits China’s national conditions. The coal chemical industry consumes a large amount of water, generates significant amounts of wastewater, and has high pollutant concentrations. In regions rich in coal resources, water is often scarce; therefore, water and environmental issues have become bottlenecks hindering the development of the coal chemical industry. There is a need to find wastewater and sludge treatment technologies that offer better treatment results, greater process stability, and lower operating costs, in order to achieve \"zero discharge\" of wastewater and sludge from coal chemical processes. This has become both an inherent requirement for the development of the coal chemical industry and an external necessity. Sludge is the floc formed when wastewater is treated through flocculation and sedimentation. It is produced in large quantities, has a high water content, and contains large amounts of heavy metal ions as well as small amounts of harmful substances such as pathogenic microorganisms. It has been classified as hazardous waste; if discharged without treatment, it can cause secondary environmental pollution. During the process of urbanization and industrial development, the amount of wastewater generated increases sharply every year. The country’s wastewater treatment capacity exceeds 139 million tons per day, and a large amount of sludge is produced as a result of wastewater treatment in these plants. The main methods for disposing of sludge are landfilling, incineration, and drying. As environmental protection requirements continue to rise, traditional sludge disposal methods can no longer meet the needs of modern society; as a result, technologies for the resource utilization of sludge are receiving increasing attention. This article mainly outlines the main approaches to the resource utilization of sludge, and analyzes them in light of the current status of wastewater and sludge utilization at China Coal Shaanxi Yulin Energy Chemical Co., Ltd., providing a reference for exploring the resource utilization of sludge. 1 Current status of sludge in coal chemical enterprises: Modern coal chemistry is an industry that uses coal as a raw material to produce clean energy and chemical products, including coal-to-gas, coal-to-oil, coal-to-ethylene glycol, and coal-to-olefins. Modern coal chemical industry is one of the important ways for the clean and efficient utilization of coal. The oils and natural gas produced by it help reduce China’s dependence on foreign resources such as oil and natural gas, thereby contributing to the country’s energy security. Therefore, it is of great significance to develop modern coal chemical industry. One of the main issues affecting the development of the coal chemical industry is water. Due to the characteristics of coal chemical wastewater—such as high discharge volume, high toxicity, and significant fluctuations—combined with variations in coal quality, it is difficult to find a specific technological process that can treat such wastewater in a stable, efficient, and reliable manner. With breakthroughs in various key technologies in the field of coal chemical industry in our country, many core technologies now possess independent intellectual property rights. New coal chemical projects are being established everywhere, and the continuous expansion of the scale of this industry has led to a rapid increase in the amount of industrial wastewater. Since this wastewater contains organic substances that are difficult to degrade and remove, it cannot be utilized directly, which in turn hinders the rapid development of the coal chemical industry. Environmental protection requirements in our country are becoming increasingly stringent. In recent years, ** and other enterprises have also increased their investment in the treatment of industrial wastewater; as a result, large coal chemical companies all have corresponding wastewater treatment plants. With the rapid increase in the number of such plants, their capacity to treat wastewater has also improved significantly, which has led to a sharp rise in the amount of sludge generated during wastewater treatment. The rapid development of coal chemical industry, while enabling the clean and efficient conversion of coal, also generates wastewater and sludge. Currently, companies have limited options for disposing of sludge. Due to environmental regulations, they often send the sludge to entities qualified to handle hazardous waste. In Shaanxi Province, the cost of sending sludge with a solid content of 17% for disposal is 3,400 yuan per ton; in some areas, this cost is as high as 6,000 yuan per ton. Disposing of sludge thus increases a company’s production costs significantly. According to incomplete statistics, coal chemical enterprises that primarily use water-coal slurry gasification generate as much as 4.5 million tons of sludge with a solid content of 5% per year, and the cost of disposing of this sludge amounts to around 4.5 billion yuan. The cost of transporting sludge from coal chemical enterprises for disposal is high; by utilizing the sludge as a resource, significant economic benefits can be achieved for these enterprises, and large amounts of raw coal and water resources can also be saved. Therefore, exploring ways to utilize sludge as a resource in conjunction with coal gasification processes has become an important area of research. Since the organic matter contained in sludge has a certain calorific value, this provides a basic condition for the resource utilization of sludge; incorporating it into gasified water-coal slurry represents an excellent approach for such resource utilization. 2 Analysis of Sludge Disposal Technologies and Resource Utilization. The treatment of wastewater sludge is an important aspect in addressing the development of cities and industries. Over the years, as the scale of new wastewater treatment plants in China has continued to increase, wastewater treatment has seen rapid progress, whereas sludge treatment has lagged behind. Globally, traditional sludge disposal technologies have matured over the years through continuous development. The main methods include landfilling, incineration, land use, and reclamation. However, methods such as landfilling and open-air dumping can lead to serious consequences such as secondary pollution of the sludge if not handled properly in practice. As modern society places increasing demands on environmental protection, traditional sludge disposal methods are no longer the best approaches for effectively dealing with sludge. The resource utilization of sludge has become the main approach for its treatment, including composting techniques, construction material production techniques, energy generation techniques, material production techniques, and techniques for utilizing sludge proteins. 2.1 Traditional sludge disposal techniques Currently, there are mainly three types of sludge disposal techniques: sanitary landfilling, sludge incineration, and land utilization. These techniques vary depending on the social development and economic levels of different countries. In developed countries, where industrial development occurred earlier, the sludge disposal techniques are more advanced, with land utilization playing a significant role among them. Although the development of sludge disposal technology in our country started late, with the continuous strengthening of policies aimed at ensuring the harmless treatment of sludge, this technology has now reached a level roughly equivalent to that of developed countries. 1) Landfill disposal. Sludge landfilling is the earliest technique used for treating sludge, and after many years of development, this landfilling method has become quite mature. In recent years, the amount of sludge in our country has been increasing steadily, leading to a rise in the demand for land for landfilling. Landfilling requires large areas of land, and as urbanization accelerates in our country, there is an increasing shortage of available land. Moreover, sludge landfilling techniques cannot prevent environmental pollution; they only delay its occurrence. For example, harmful metals and other substances can seep into groundwater, causing pollution. Therefore, landfilling is not the best method for dealing with sludge. 2) Sludge incineration. Similar problems exist in sludge incineration as well; low incineration temperatures can lead to the generation of harmful gases such as dioxins, thereby polluting the environment. In some areas of our country, landfilling is still used to dispose of sludge, but in most areas, issues such as the location selection for landfills and transportation costs largely limit its widespread use. Therefore, the sludge incineration and landfilling treatment method is not the best way to handle sludge. 3) Land use. Sludge is rich in organic matter, as well as nutrients such as nitrogen, phosphorus, and potassium. It has strong viscosity and water absorption properties, which can improve the physicochemical characteristics of soil. It facilitates the formation of soil aggregates, reduces soil erosion, enhances soil fertility and water retention capacity, and helps to create a sustainable cycle in the agricultural ecological environment. Although sludge has advantages such as improving soil properties and restoring saline-alkali and sandy lands, it contains pathogens and harmful substances. If it is used for land use without treatment, excessive concentrations of these harmful substances can seep into groundwater and cause pollution, while heavy metals may pose a threat to human health through the food chain. Therefore, the use of sludge in land use requires resolution of these issues first. 2.2 Technologies for the resource utilization of sludge 1) Sludge fuel utilization technology. Sludge contains a large amount of organic matter and has a certain calorific value; it can usually be mixed with coal or water-coal slurry to obtain binding properties. When combined in certain proportions with coal powder and other additives, it can be used to create solid fuel or water-coal slurry fuel. Manufacturing briquettes involves using sludge to produce sludge briquettes. Water-coal slurry fuel is a new type of sludge fuel created by mixing sludge, coal, water, and additives in specific proportions and then grinding them together to produce a fluid substance. It has a slurry-like consistency, can be transported through pipes using pumps, and can be burned in an atomized manner similar to liquid fuels. The fuel utilization of sludge can make full use of its inherent calorific value, resulting in significant economic benefits. It can also replace some fresh water, thus saving water resources and enabling the reuse of wastewater, which brings significant social and environmental benefits. 2) Sludge materialization technology. The main ways of utilizing sludge as a resource include its use in construction materials. Some sludge can be processed through special techniques to produce construction material raw materials such as sludge bricks, cement, and powder admixtures, thereby achieving both economic and environmental benefits from sludge utilization. In addition, some scholars have studied the use of sludge to create new materials, such as biochemical fiber boards. These boards are produced by subjecting the protein molecules in sludge to heat pressing, which induces a modifying effect; subsequent treatments such as degreasing and bleaching are then used to press the material into boards. However, this technology is not yet mature in China, and further research is still needed. 3) Sludge composting technology. Sludge composting technology is a sludge treatment method that developed gradually in the 1950s. It typically makes use of microorganisms to degrade the complex organic substances in sludge into more stable forms, while also breaking down the volatile components present in the sludge, thereby improving its physical properties. After final high-temperature treatment, the sludge is free of microorganisms such as pathogens; the resulting compost can be used as a soil conditioner, among other applications. After years of development, sludge composting technology has made significant progress, but there are still many issues that need to be addressed through further research. For example, although composting of sludge solves problems such as the presence of microbial pathogens and high moisture content, heavy metals and other contaminants remain unresolved. Therefore, to turn sludge into a safe and effective composting resource, it is necessary to conduct additional research on these issues. 2.3 Analysis of the development trends in sludge disposal technologies: Traditional sludge disposal methods such as landfilling, incineration, and land use not only incur high operating costs but also fail to completely eliminate the environmental hazards posed by sludge. The harmless disposal of sludge and its effective utilization as a resource are issues of common concern to humanity. The disposal and utilization of sludge should also take into account economic, environmental, and social benefits. The utilization of sludge as a resource in our country is still relatively backward compared to developed countries, with an utilization rate of less than 10%. As the amount of sludge produced continues to increase, this gap will only widen. Therefore, it is of great significance to develop targeted technologies for the resource utilization of sludge in line with the development of China’s industrial sector. 3. Sludge utilization at China Coal Shaanxi Company: China Coal Shaanxi Yulin Energy and Chemical Co., Ltd. is designed to produce 600,000 tons of olefins per year, using water-coal slurry gasification technology. This process generates up to 120,000 tons of sludge with a solid content of 5% per year. Disposing of this sludge as hazardous waste is extremely costly. Therefore, China Coal Shaanxi Company has been continuously exploring ways to recycle this sludge. Since the sludge contains a certain calorific value, it is possible to gasify the harmful organic substances and combustible materials within it under high temperature and pressure conditions. This not only allows for the full utilization of its calorific value but also helps to replace some of the water needed, thereby minimizing environmental impact. At the same time, it helps China Coal Shaanxi Company reduce the costs associated with sludge disposal. China Coal Shaanxi Company analyzed the dewatered sludge on site; the specific analysis results are shown in Table 1. http://img.yf116.cn/image/img/20190715/959533599383.jpg As can be clearly seen from Table 1, the sludge has a high water content and high volatile matter content, as well as a certain calorific value; it can be mixed into coal slurry and then fed into a gasifier for high-temperature gasification. 3.1 Direct co-firing method: After the raw coal is crushed, it is mixed with water and additives through grinding to produce coal water slurry, which is then fed into a gasifier for high-temperature gasification. The main equipment used for preparing coal-water slurry is the rod mill. China Coal Shaanxi Company attempted to directly add sludge water to the rod mill, where it was mixed with the coal slurry prior to gasification. However, during actual operation, it was found that the addition of sludge water caused a sharp increase in the viscosity of the coal slurry, thereby deteriorating its flow properties. This led to the coal slurry clogging the discharge screen of the rod mill, resulting in a higher rate of equipment failures. Consequently, the concentration of the coal slurry had to be reduced during the production process, which significantly diminished the economic efficiency of the gasification process. The main reasons are the large specific surface area of the sludge, its high moisture content, and the presence of polyacrylamide, which acts as a thickening agent. 3.2 Slurry thickening and co-firing method: Direct incorporation of sludge into the coal slurry affects the economics of gasification. China Coal Shaanxi Company sends the sludge to units qualified to handle hazardous waste for treatment; the cost of treating sludge with a solid content of 17% is as high as 3,400 yuan per ton, which significantly increases the company’s production costs. At the same time, the method of reducing sludge volume before sending it for external treatment was also investigated, but this technology requires additional sludge drying equipment, which occupies a large area and is costly; moreover, waste flue gas is generated during the drying process, contaminating the environment, so this sludge reduction technique was not adopted. Through the joint development of new technologies for coal slurry concentration, China Coal Shaanxi Company was able to demonstrate the Sanfeng fractal grading concentration process. Once the coal slurry concentration system was put into use, fine grinders and ultra-fine grinders modified the sludge through high-speed shear and temperature increase; the modified sludge did not affect the concentration of the coal slurry. The system is capable of processing 12 tons of sludge with a solid content of 5% per year, which not only solves the problem of sludge disposal but also replaces some of the water used in slurry production, bringing significant economic benefits to the enterprise. 3.3 Economic analysis of sludge resource utilization: After the coal slurry thickening system is put into operation, it can process 15 tons of sludge with a solid content of 5% per hour, amounting to 120,000 tons per year. China Coal Shaanxi Company pays 12.4 yuan per ton for fresh water, while the cost of transporting and disposing of sludge with a solid content of 17% is 3,400 yuan per ton. The economic benefits generated by the resource utilization of sludge can be calculated. The savings in disposal costs are as follows: Sludge volume (with 17% solid content) = 15 t/h × 5% ÷ 0.17 = 4.41 t/h. The cost of sludge disposal = 4.41 t/h × 3,400 yuan/t = 14,994 yuan/h. The savings in fresh water costs amount to 15 t/h × 95% × 12.4 yuan/t = 176.70 yuan/h. In total, the savings in both sludge disposal costs and fresh water costs amount to 121.3656 million yuan. Assuming that coal chemical plants operate for 8,000 hours per year, the annual savings amount to… yuan. After the resource utilization of sludge, it helped China Coal Shaanxi Company **reduce production costs. China Coal Shaanxi Company utilizes sludge in resource-efficient ways, providing valuable experience for chemical companies. Working together to achieve zero pollution in the coal chemical industry, this approach represents an important attempt at making use of sludge as a resource; it not only reduces the production costs for enterprises but also decreases environmental pollution. 4 Conclusion The disposal and utilization of sludge are important topics in environmental science today. Considering aspects such as economic development, resource exploitation, and environmental protection, and against the backdrop of changes in the current energy structure and environmental policies, the most ideal approach for sludge is its utilization as a resource. Resource utilization technology is an important direction for development, in line with the policy priorities of energy conservation and environmental protection. China’s coal chemical industry is already at the international leading level in terms of equipment and process technology. Achieving zero emissions will be the key factor in enhancing corporate competitiveness; therefore, coal chemical companies should continue to advance the technological upgrading of the energy sector and play an important role in establishing a clean and efficient energy system. The resource utilization of sludge should be based on the characteristics of the sludge, adhering to the basic principles of \"safety and environmental protection, recycling, energy conservation and consumption reduction, adaptation to local conditions, and reliability.\" Taking into account its own characteristics and those of the region, China Coal Shaanxi Company leverages the inherent advantages of coal chemical enterprises by converting sludge into high-concentration sludge coal slurry for gasification. This approach not only addresses the environmental pollution problems caused by sludge but also fully recovers the calorific value and moisture contained in it, thereby achieving the goal of resource utilization of sludge and saving production costs from the company’s perspective. The methods employed by China Coal Shaanxi Company for the resource utilization of sludge can serve as a reference for coal chemical enterprises in the northwestern region, where water resources are scarce and sludge disposal poses significant challenges. Additionally, it can act as a model for the resource utilization of urban and industrial sludge in China. In conclusion, it is of great practical significance to explore appropriate technologies for the disposal and utilization of sludge that suit China’s national conditions. Researching technologies for the resource utilization of sludge can not only address the environmental pollution problems caused by sludge but also save energy, resources, and water, playing an important role in promoting the rapid development of urbanization and industry in China.