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Development Trends and Countermeasures for the Comprehensive Utilization of Water Resources in Coal Chemical Industry Author/Source: Date: 2017-09-13 Clicks: 32 This article explores the methods for utilizing water supply and wastewater in the coal chemical industry, taking into account relevant policies in this field. It introduces measures for the comprehensive use of water resources in coal chemical production, focuses on analyzing and discussing the combined processes needed to achieve nearly zero wastewater discharge, and explains the water treatment technologies involved in achieving such zero-discharge levels as well as their future development directions. In the coal chemical industry, the amount of water used and discharged during the production process is enormous. Taking coal-to-olefins as an example, producing one ton of product requires 3–5 tons of coal and 9–14 tons of water. Coal resources and water resources generally exhibit an inverse distribution pattern. The layout of the coal chemical industry is dictated by coal resources, which makes the issue of water resource allocation particularly prominent in the industry’s development. In areas where water resources are scarce, the water environment capacity is often insufficient, and there may even be a lack of bodies of water capable of receiving wastewater; as a result, large amounts of wastewater end up with no place to be discharged. How to save water resources and improve water utilization is of great significance for chemical enterprises to enhance economic efficiency and achieve sustainable development. This paper discusses measures for the comprehensive utilization of water resources in chemical enterprises, in conjunction with the production processes of modern coal chemical industry. Chemical enterprises are both major consumers of water and significant sources of water pollution. The Provisional Environmental Access Requirements for Modern Coal Chemical Industry Projects state that water-saving measures should be strengthened to reduce the use of fresh water; in areas where conditions permit, mine drainage water and recycled water should be given priority over other water sources, and the use of groundwater for production purposes is prohibited. Secondly, a wastewater treatment and disposal plan is designed based on the principles of separating clean water from polluted water, treating different types of pollutants separately, carrying out advanced treatment, and reusing water according to its quality. Each drainage system in coal chemical enterprises carries out classified treatment and hierarchical control of the wastewater discharged. Dikes are installed in the areas where pollution occurs to collect polluted rainwater, and the wastewater discharged from various units is treated before being reused as process water at different levels whenever possible. The conditions also stipulate that in water-scarce areas, water-saving technologies such as air cooling and closed-loop circulation should be given priority. The closed-loop cooling water system for the entire plant is equipped with a soft-water-based closed-loop cooling water system; the make-up water for the spraying system comes from fresh water obtained through wastewater desalination or condensed water produced by evaporation and crystallization, which helps to **reduce the demand for fresh water**. The comprehensive utilization of wastewater can reduce water consumption costs and improve economic efficiency; it also **reduces the environmental problems caused by excessive extraction of groundwater, thereby helping to protect water resources. Based on this approach, and in accordance with the quality requirements for water used in production, the water used throughout the plant is utilized in a differentiated and tiered manner. For production units with high water requirements, fresh process water, as well as fresh water produced through reverse osmosis and evaporation, is used. The waste water generated across the plant is collected and treated in a differentiated way before being reused in the corresponding process units, thereby enabling a hierarchical use of water within the entire plant’s water system. The highly concentrated brine generated by the wastewater reuse treatment system is processed in an evaporation and crystallization unit; the resulting industrial salt is separated and sold as a by-product. The residual salts are sent to a hazardous waste treatment center for disposal, enabling the project to achieve near-zero emissions. 1. Main water-saving measures for water utilization in the coal chemical industry 1.1 Comprehensive utilization of mine dewatering water. As an industry with high water consumption, the coal chemical industry brings about environmental pressures as it develops. Social problems such as the decline in water levels due to over-extraction of groundwater and difficulties for residents in accessing drinking water have also emerged as a result. A large amount of groundwater is discharged to the surface, further exacerbating the decline in the water table, accelerating the reduction of surface vegetation, and expanding the area of desertification. Moreover, the discharge of vast quantities of mine dewatering water onto the surface also causes environmental pollution. As a by-product of coal mining, mine dewatering water is discharged without proper management in terms of its utilization and disposal; as a result, large amounts of this water are wasted, which not only leads to the loss of valuable water resources but also causes environmental pollution. Mine water in our country mainly contains suspended solids and a small amount of oil; its quality is good, and it generally does not contain toxic substances. The treatment process generally involves \"air flotation for oil removal—coagulation and sedimentation—filtration,\" followed by further advanced treatment steps, after which the water can be used as process water throughout the plant. If used as potable water, a disinfection step should also be added at the end of the process. The treatment methods for mine water vary depending on its water quality characteristics. Mine water containing suspended solids is treated using coagulation-sedimentation or air flotation; mine water with high mineral content is treated via ion exchange or membrane separation techniques; and acidic mine water is treated by neutralization. In situations where coal mining enterprises must drain large amounts of water, while chemical enterprises lack water sources, coal mines can supply this drained water to other enterprises under agreement or free of charge, or use it to irrigate ecological vegetation. This not only meets the water needs of those enterprises but also facilitates nearby discharge by coal mining firms, helping to reduce costs and environmental impact. It also ensures that the drained water from mines is made full use of, bringing about positive economic, social, and environmental benefits. 1.2 Water conservation in the circulating cooling water system: In the production process of coal chemical projects, the main points of water consumption are the makeup water for the circulating cooling water and the makeup water for demineralized water, with these amounts accounting for over 70% of the total water used in production across the plant. Therefore, recycling various types of condensate and reducing the makeup water volume for circulating cooling can significantly decrease a company’s water consumption. 1.2.1 Water-saving measures for open-loop circulating water: Water savings in open-loop circulating water are primarily achieved through the control of the concentration ratio. During the cooling process, direct contact with air causes dust, microorganisms, and dissolved oxygen from the air to enter the circulating water system, leading to a deterioration in water quality. By controlling the concentration ratio, it is possible to reduce the levels of harmful substances in the circulating water. Additionally, after some wastewater is discharged, the concentrations of chemicals such as scale inhibitors and biocides in the water are diluted; therefore, it is necessary to replenish these chemicals to maintain the normal operation of the system. (1) Increasing the concentration ratio: The concentration ratio of circulating water is an important indicator for assessing the operational condition of a circulating water system. By increasing this ratio, it is possible to reduce the amount of water that needs to be added and the volume of wastewater discharged, as well as minimize the amount of chemicals required. This is beneficial both for water conservation and environmental protection. According to statistics, when the temperature difference between the inlet and outlet water is 10°C, the concentration factor increases from 3 to 5, which allows for a 16% reduction in the amount of make-up water required. Enterprises should adopt a higher concentration factor within the allowable range for water quality control, taking into account their specific production conditions. Currently, in the coal chemical industry, the circulating water concentration factor, which uses recycled water as make-up water, is generally greater than 3. (2) Controlling corrosion and scaling. Methods for controlling corrosion and scaling in circulating water systems include chemical and physical methods. Currently, physical treatment methods are generally used in small-scale water treatment systems, and there are relatively few related reports. In China, engineering projects have already adopted magnetic treatment and electrostatic treatment. For coal chemical projects, especially medium and large-scale ones, chemical treatment methods are widely used, and such technologies are more mature, reliable, and effective. In circulating water systems, composite chemicals such as polyphosphates are typically added to control corrosion and scaling in the circulating water. However, the effect of a single corrosion inhibitor is often not ideal; currently, combinations of two or more corrosion inhibitors are generally used, and through their synergistic action, better water quality conditions for circulating water can be achieved. (3) Controlling microbial growth: Microbial sludge can act as a bridge-forming and flocculating agent for colloidal substances, suspended particles, and corrosion products in water, causing pollutants to adhere together and form scale. The presence of sludge can also cause electrochemical corrosion on the inner surfaces of equipment and pipes; it simultaneously reduces the heat exchange efficiency of the equipment, leads to overheating of the process medium, and increases production energy consumption. The commonly used method at present is the addition of chlorine-based oxidizing disinfectants. Wang Jianna et al. described the application of the QS-14 fungicide, which is a combination of polyquaternary ammonium cationic polymers with other fungicides and additives, at Qilu Petrochemical Company. It achieves a fungicidal efficacy of ≥98% against heterotrophic bacteria within 4–24 hours, and it also exhibits good performance in controlling sludge formation. (4) Employ appropriate side-stream filtration technology. Currently, side-stream filtration equipment is widely used in open recirculating water systems. It helps improve the quality of the recirculating water by removing certain amounts of turbidity, suspended solids, etc., and can also increase the concentration ratio of the recirculating water. In actual design and operation, the amount of water used for side filtration typically accounts for 3% to 5% of the total volume of circulating cooling water. For side filtration equipment, modern types that offer good water quality, low backwash water consumption, easy installation and maintenance, and stable system operation are generally chosen. Examples of such devices include shallow sand filters, medium-to-high speed filters, disc filters, and high-efficiency fiber filters, which are currently widely used. 1.2.2 Water-saving measures for closed-loop cooling water systems: The core of a closed-loop cooling water system is the air cooler. The hot water returned from the process units is cooled within the tubes of the air cooler; thereafter, it is pressurized by the circulation cooling water pump and sent back to the process units to absorb heat there, before returning to the air cooler for further cooling, in a continuous cycle. Throughout the process, the cooling water inside the pipes does not come into direct contact with the atmosphere, ensuring excellent water quality. Compared to open-loop water cooling systems, it features lower operating costs and significant water-saving effects. Closed-loop cooling water systems can be classified by cooling method into: dry air cooling, wet air cooling, and combined air cooling. (1) Dry air cooling: Dry air cooling uses ambient air as the cooling medium. It enhances heat transfer outside the tubes by increasing the heat transfer area through finned tubes, and the heat is removed by the rise in temperature of the air flowing over these finned tubes, thereby achieving cooling and condensing the process fluid inside the tubes. Considering technical and economic factors, dry air cooling is suitable for applications with large heat transfer temperature differences, and it is rarely used in coal chemical circulating water systems. Wet air cooling is suitable for applications with a small temperature difference for heat transfer, and it is widely used. (2) Wet air cooling: Wet air coolers are further divided into surface evaporation air coolers, humidified wet air coolers, and spray evaporation air coolers. I. Surface Evaporation Air Coolers: In surface evaporation air coolers, cooling water is sprayed onto the tube bundle, creating a thin layer of water on its outer surface. At the same time, air flows across the tube bundle; the rapid evaporation of this surface water layer enhances heat transfer outside the tubes, thereby cooling the fluid inside them, with the temperature reaching levels close to the ambient wet-bulb temperature. Surface evaporation air coolers integrate water cooling with air cooling, as well as heat transfer and mass transfer processes, combining the advantages of both. They feature a compact structure, high heat transfer efficiency, low investment costs, low operating expenses, and ease of installation and maintenance. They have broad application prospects in industries such as petroleum refining, chemicals, metallurgy, and light manufacturing, representing a new direction in the development of air cooling technology. However, its heat exchange element has a small area and high water consumption, making it unsuitable for applications with high water-saving requirements. II. Humidifying type air cooler: The humidifying wet air cooler sprays water mist at the air inlet; the evaporation of this misty water humidifies the dry air at the inlet, thereby reducing the temperature there. As the dry-bulb temperature of the air drops, the temperature difference between the air inlet temperature and the exit temperature of the fluid inside the tubes increases, which enhances heat transfer. Compared to surface air coolers, humidified wet air coolers incorporate water baffles; the wet air passes over these baffles to have any entrained water droplets removed, after which it flows across the finned tube bundle, using the rise in air temperature to remove heat from the fluid. The lower the relative humidity of the air at the inlet, the greater the temperature drop after humidification, and the more significant the cooling effect. When the ambient temperature is high, the cooling effect is enhanced by adjusting the water flow rate of the spray pump to cool the inlet air through spraying. When the ambient temperature is low, turning off the spray water pump achieves a cooling effect while also reducing water consumption. III. Spray-evaporation type air cooler. The working principle of the spray-evaporation type air cooler is based on the water film formed on the surface of the finned tubes in the tube bundle; as air passes over this tube bundle, the water film on the fin surfaces is affected both by the airflow and by the hot fluid inside the tubes, resulting in forced evaporation and thus a high heat removal capacity. The heat transfer coefficient of the outer tube membrane is 3 to 5 times greater than that of ordinary dry air coolers. Furthermore, due to the evaporation caused by water spraying, the wind temperature at the inlet of the air cooler drops from the dry-bulb temperature to a level close to the wet-bulb temperature. Moreover, because the latent heat of vaporization of water is high, the increase in wind temperature at the outlet of the air cooler is minimal; as a result, the temperature difference for heat transfer is greater compared to that in ordinary humidified air coolers. Spray-evaporation type air coolers are suitable for applications where the cooling temperature of the fluid medium in the finned tubes is required to be low. Due to the combined effect of air flow and the hot fluid inside the tubes on the water film on the fins of spray-evaporation type air coolers, forced evaporation occurs, which results in significantly higher efficiency compared to other conventional humidifying wet air coolers. (3) Combined air cooling: A combined air cooler integrates a dry air cooler and a wet air cooler; the first section is a wet air cooler, while the second section is a dry air cooler. The evaporation of the spray water on the finned tube surfaces of the first section of the wet air cooler enhances the heat transfer in the wet air cooler. After passing through the first wet air cooler, the air on its surface changes from dry air to wet air, and its temperature drops to near the wet-bulb temperature of the air. The cooler air discharged from the wet air cooler enters the second stage of dry air coolers, thereby increasing the temperature difference for heat transfer and enhancing the heat transfer capacity of the dry air coolers. The series connection of dry and wet air coolers increases the resistance for air to pass through the air coolers, and the number of tube banks in the wet and dry air coolers should be restricted. Dry-wet combined air coolers have high design requirements; it is necessary to allocate the heat transfer area and pressure drop of such coolers in a reasonable manner in order to take advantage of the strengths of both the wet and dry sections and achieve optimal performance. Generally, the number of rows in a wet air cooler bundle is 2–3; for dry air coolers, the number of rows should not exceed 4. For instance, in the design of a large coal chemical project in the northwest, after comparing aspects such as equipment investment, land occupation, operating costs, system reliability, and water consumption indicators of cooling towers, a hybrid dry-wet closed-circuit cooling tower with a water savings rate of 60% was selected. 1.3 Recovery of closed-loop condensate: Closed-loop condensate recovery refers to a process in which the condensate does not come into direct contact with air during recovery, thereby reducing corrosion of pipes and equipment as well as minimizing flash loss. Condensate recovery mainly involves the collection of turbine condensate and process condensate. Condensate is gathered by installing additional recovery devices such as surface heat exchangers, and after simple preprocessing and ion exchange, it can be reused as demineralized water. 1.4 Classification, collection, and cascade utilization of wastewater. The classified treatment and quality-based reuse of sewage mainly fall into two categories: direct reuse of untreated wastewater and reuse after treatment. Since different processing stages have varying requirements for water quality, the degree of pollution generated in each stage also differs. By taking into account the water quality requirements of different processes, wastewater with varying degrees of pollution is treated and reused in a targeted manner. This improves the rate of wastewater reuse, while simultaneously reducing the scale and complexity of wastewater treatment, thereby achieving both water conservation and economic benefits. For example, the ammonia-laden wash water generated by the acid gas removal unit in a coal-to-olefins plant is sent directly to the CO conversion unit. The wastewater from the gasification drum containing low levels of salts, as well as wastewater from methanol-to-olefins processes and domestic sewage, contain pollutants primarily in the form of COD and ammonia nitrogen; such wastewater is reused after undergoing biochemical treatment followed by advanced treatment processes. Wastewater with a high salt content, such as that from circulating water systems, desalination plants, and chemical cleaning processes, has salt as its main contaminant; the treated water is reused after undergoing pretreatment and membrane concentration. 1.5 Near-zero wastewater discharge technology: Constrained by both water resource and water environment issues, domestic coal chemical projects have successively proposed near-zero wastewater discharge solutions. \"Zero\" or near‑\"zero\" wastewater discharge refers to a situation in which the wastewater generated by coal chemical projects is concentrated and then treated or reused in the form of this concentrated liquid, with no liquid being discharged into surface waters. That is, a design approach that, on the basis of a reasonable division of the water systems and taking into account the characteristics of wastewater, achieves maximum degree of treatment and reuse, so that the wastewater is no longer discharged into natural water bodies. Upon analysis, in coal chemical projects, the amount of clean wastewater discharged, combined with the production wastewater, can account for 40%~60% of the total water used in production. The near-zero wastewater discharge technology involves collecting and treating almost all of the production wastewater as well as the clean wastewater before reusing it in the process units; this also serves as an effective supplement to fresh water, allowing for a reduction in fresh water consumption by approximately 30%~50%, thus achieving significant water-saving effects. The near-zero discharge technology for wastewater from coal chemical projects mainly consists of four stages: biological treatment, reuse treatment, membrane concentration treatment, and evaporation crystallization. A typical near-zero discharge scheme is shown in Figure 1. http://img.yf116.cn/image/img/20170913/103903834083.jpg The achievement of near-zero discharge of coal chemical industry wastewater is primarily constrained by four stages and four factors: the first is source control, which involves collecting wastewater of different quality categories and treating them accordingly ； The second aspect is the selection of wastewater pretreatment technologies; that is, for the high-concentration, hard-to-degrade wastewater in wastewater treated on a differentiated basis, an appropriate oxidation technology (such as WAO) is chosen through comparative analysis to break down these hard-to-degrade organic substances into wastewater with higher biodegradability or wastewater whose main pollutants are salts, which is then treated using biochemical or membrane separation technologies ； Thirdly, regarding the selection of treatment technologies for high-salinity wastewater, since the wastewater resulting from membrane desalination contains high levels of calcium, magnesium, silicon, and other substances, this can easily lead to fouling or scaling of the subsequent concentration membranes. Therefore, choosing appropriate technologies for removing calcium, magnesium, and silicon (such as the HERO process) should be a key aspect of the membrane concentration treatment process ； Fourth, the selection of evaporation crystallization technology. The operation of evaporation crystallization units is unstable, and there are no effective ways to comprehensively utilize the crystallized salts. Therefore, ensuring the stable operation of these units and the resource utilization of the by-product crystallized salts should be the focus of future research efforts. 2 Prospects for the Development of Water-saving Technologies in Coal Chemical Industry. Water-saving technologies in the coal chemical industry should be implemented not only at the source and throughout the entire production process, through the establishment of appropriate water-saving rules and improved management practices, but also by focusing on the treatment and utilization of high-concentration wastewater at the end of the wastewater treatment process, in order to minimize the impact of wastewater discharge or leakage on the environment. With increasingly stringent environmental regulations and water scarcity, research on near-zero emission technologies is advancing steadily. The complexity of the components in coal chemical industry wastewater makes it difficult for single treatment methods to be effective; therefore, the integration and optimization of various technologies will undoubtedly be a key trend in the development of wastewater treatment techniques for this industry. At present, the research and application of near-zero emission technologies in China’s coal chemical industry are still in their initial stages. There are few successful examples of the use of such technologies, and some of them are derived from other industries and fields; it remains to be determined whether the adoption of these technologies can improve the treatment of wastewater generated by the coal chemical industry. Furthermore, the investment costs for implementing near-zero emission technologies are extremely high, and achieving near-zero emissions cannot be imposed as a mandatory requirement on coal chemical enterprises. The utilization pathways for the crystalline salts produced at the end of the evaporation and crystallization process, as well as the standards for these crystalline salts, should be established as soon as possible. The treatment of crystalline salts is also set to become a focus of research. Solidification/stabilization techniques are mature processes for dealing with hazardous waste containing heavy metals; for the crystalline salts generated in the coal chemical industry, further research is needed regarding the suitability of such techniques and the auxiliary chemicals used. There is relatively little research on this topic in China, and this is an area that needs to be strengthened urgently. Through continuous investment in technology, it will **promote the continuous improvement of wastewater treatment technologies for the coal chemical industry, thereby enabling harmonious development of this industry with society, the economy, and the environment. With the introduction of increasingly stringent environmental protection policies, near-zero emissions will surely become the trend in the development of water-saving technologies for coal chemical industries. 3 Conclusion The comprehensive utilization of water resources in coal chemical enterprises is not only related to the direct economic benefits of these enterprises but also to their sustainable development. Only by continuously innovating technologically, comprehensively analyzing the production capacities of various production units and the parameter requirements of cooling units, improving the system, and ensuring energy balance between upstream and downstream processes to formulate the optimal process route, can enterprises gradually establish a scientific water usage system. In addition to adopting various effective technical measures, it is also very important for the smooth progress of water conservation efforts to strengthen daily water-saving management, enforce strict control standards for various processes, raise employees’ awareness of water conservation, and organize competitions related to water-saving performance. In addition, management measures should be strengthened, with comprehensive planning and rational utilization to ensure balanced use of water resources.