What technologies are used for the utilization of low-grade waste heat?

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In recent years, China has seen rapid development in energy-saving renovations such as waste heat utilization and waste heat power generation. At the same time, significant progress has been made in waste heat recovery within China’s industrial sector, particularly in the utilization of high-temperature and medium-temperature waste heat resources. However, compared with developed countries, there is still a certain gap in the utilization of low-temperature waste heat resources (low-grade waste heat resources) in the industrial sector, and there is also significant disparity among different companies within this industry. As energy-saving efforts continue to advance, the utilization of low-temperature waste heat resources has increasingly become a key focus and challenge in this field. This paper analyzes the characteristics of such low-quality waste heat resources and summarizes the current methods of utilization as well as technological advancements in this area. 1 Classification of waste heat resources: Industrial waste heat refers primarily to low-grade forms of energy such as waste heat, wastewater, and waste gas generated by the thermal conversion equipment and energy-consuming devices in industrial enterprises during the production process. Waste heat resources are commonly found in industries such as steel, chemicals, petroleum, building materials, light industry, and food processing. These industries possess abundant waste heat resources of varying grades. Utilizing waste heat recovery technologies to recycle such low-grade energy sources is one of the important means of saving energy. Based on the temperature of the carriers of waste heat resources, these can be classified by their quality; a higher temperature indicates a greater capacity for utilizing the waste heat, and it can be easily utilized even through direct heat transfer – this is what is referred to as \"high-quality waste heat resources\". A low temperature indicates that the quality of this waste heat resource is relatively low. At present, most of the high-grade high-temperature and medium-temperature waste heat resources in industrial enterprises have been put to good use; considerable efforts have also been made in exploring the utilization of low-grade waste heat resources. Overall, however, the utilization of low-grade waste heat resources is still in its infancy. 2 Sources and utilization challenges of low-grade waste heat resources The main sources of waste heat resources are: ① Waste heat from flue gases; ② Waste heat from high-temperature products and slag; ③ Waste heat from cooling media; ④ Waste heat from combustible waste gases, waste liquids, and waste materials; ⑤ Waste heat from waste steam and wastewater; ⑥ Waste heat generated by chemical reactions. Typical low-grade waste heat sources include: ① flue gas emitted from boilers (heating furnaces), with a temperature of 140–220°C; ② water used for flushing blast furnace slag and steelmaking slag, with a temperature of 60–90°C; ③ steam condensate, with a temperature of 90–120°C; ④ cooling water from diesel and gasoline production processes in the oil refining industry, with a temperature of around 150°C. The difficulties in utilizing low-grade waste heat resources lie in the following: ① Most low-grade waste heat resources contain corrosive substances, which pose a significant threat to the long-term safe operation of equipment; ② Some of these low-grade waste heat sources are intermittent in nature, making it difficult to operate them continuously; ③ Due to their low grade, it is hard to find suitable heating/cooling loads in the vicinity of the source; ④ When used for power generation, their efficiency is low, the relevant technology is not yet fully developed, and the economic benefits are modest. 3 Exploration of utilization methods for low-grade waste heat resources. The utilization of low-grade waste heat resources can be divided into three methods: direct thermal utilization, cooling and heating, and thermodynamic conversion. 3.1 Heat exchange technologies for direct heat utilization utilize low-temperature waste heat without changing the form of its energy; rather, they transfer this waste heat energy directly to the energy-consuming processes in the respective processes through heat exchange equipment. This represents one of the direct and efficient methods for waste heat recovery. Due to the low temperature of low-temperature waste heat resources, it is necessary to find suitable applications for them, while also taking into account the losses that occur during transmission. Common examples include: circulating water (such as oil field production water and blast furnace slag flushing water) being used for heating, boiler flue gas being utilized to provide waste heat for air (or feedwater), and the waste heat from heating furnace flue gas being used to heat materials. Enterprises such as Xuansteel, Shougang, Jigang, and Taigang have carried out projects that utilize blast furnace slag water for residential heating, achieving excellent energy-saving results. Shougang Jingtang Iron and Steel Company uses blast furnace slag washing water for seawater desalination, successfully addressing the issue of long-term operation of low-temperature waste heat recovery systems throughout the year; this is a highly reasonable way of utilizing such resources. In the power industry, Germany and Japan were the first abroad to adopt waste heat recovery systems for boiler flue gas. In China, facilities such as the third phase of the Shanghai Waigaoqiao Power Plant, the Datang Ningde Power Plant, and the Shanghai Caojing Power Plant have successfully implemented such systems, thereby improving the operational efficiency of the boilers. In some boiler rooms in Beijing, low-temperature economizers have been installed in the flue gas ducts at the rear of natural gas-fired boilers, thereby reducing natural gas consumption. Some oil fields have also attempted to use produced water from oil fields for heating. 3.2 Cooling and heating: In cases where direct heat exchange is not suitable for use, the low-temperature waste heat can also be utilized for absorption cooling or heat pump heating, thereby changing the grade of the waste heat energy. Tsinghua University and Jiangsu Shuangliang have developed heat sources for heat pumps that utilize circulating water from power plants; a small amount of extracted steam is used as the driving energy. The heat pumps raise the temperature of this circulating water to 95°C before sending it into the urban heating network, resulting in significant energy savings. Chemical enterprises have many process units that require cooling. Utilizing lithium bromide absorption chillers, with waste heat serving as the heat source to generate cooling capacity for production purposes, is also a very effective energy-saving measure for making use of low-temperature waste heat. However, generally speaking, aside from chemical companies that can utilize low-temperature waste heat internally, it is somewhat difficult for other companies to make use of such waste heat within their own facilities; they usually need to establish connections with areas outside their own company in order to make full use of this low-grade waste heat. 3.3 Thermal-to-mechanical conversion Thermal-to-mechanical conversion refers to the transformation of low-grade waste heat into mechanical energy or electrical energy for utilization. Generally speaking, if it is very difficult to utilize low-grade waste heat resources either for direct thermal application or for cooling and heating purposes, thermodynamic conversion can serve as an alternative option. If only mechanical energy is to be converted, there must be a reasonable mechanical load at the site where waste heat is utilized. By further converting it into electrical energy, it can be transmitted over longer distances, allowing for more flexible usage. The main technical methods for heat-to-work conversion include ORC organic Rankine cycle expansion engine power generation technology, steam turbine power generation technology (using organic working fluids), and saturated steam power generation/drive technology. Due to the low quality of low-grade waste heat resources, the energy utilization efficiency for thermal-to-mechanical conversion is very low; in many cases the economic benefits are not significant, requiring a detailed technical and economic comparison. 4 Key Technologies for the Utilization of Low-Grade Waste Heat Resources 4.1 Efficient heat pipe heat exchange technologies, represented by phase-change heat transfer. The utilization of low-grade waste heat resources requires a large heat transfer area due to the small temperature difference, which in turn leads to increased use of steel and more space occupancy; as a result, traditional heat exchangers are not suitable for such applications. The efficient heat pipe heat exchange technology based on phase-change heat transfer can **increase the heat transfer coefficient, reduce the amount of steel required, and also facilitate the control of the wall temperature of the heated surface. 4.2 Corrosion and wear resistance technologies for heat exchangers: Industrial waste heat sources are often subject to corrosive factors; flue gases contain acidic gases and dust, while wastewater contains chloride ions, among other substances. These corrosive factors have a serious impact on the long-term stable operation of heat exchangers. To make use of low-grade waste heat, it is necessary to address the corrosion issue; using specific materials for heat exchangers and designing their structure appropriately can help reduce corrosion. Attention should also be paid to the economic efficiency of utilizing these heat exchangers. 4.3 Heat pumps and lithium bromide refrigeration using low-temperature waste heat: The use of heat pumps and lithium bromide refrigeration systems to make use of low-temperature waste heat enables more efficient utilization of such heat, but it also presents challenges related to improving energy efficiency and preventing corrosion of the heating surfaces; these issues need to be taken into account in the design of heat pumps and lithium bromide refrigerators. 4.4 Expander power generation technology: As a technology for reducing emissions in low-grade power generation, expansion engine technology can utilize superheated steam, saturated steam, hot water, hot fluids, and other heat sources. It can use hot water at temperatures above 70°C as well as sulfur-free flue gas at temperatures above 130°C (some sources indicate temperatures ranging from 180 to 200°C), and it can also make use of low-pressure steam. 4.5 Organic Rankine cycle power generation and other low-temperature power generation technologies are useful for utilizing the low-grade waste heat prevalent in industry at temperatures of 200 °C or even below 300 °C, where it is difficult to use conventional steam/saturated steam/hot water as working fluids for power generation. At this point, an organic working fluid circulation scheme can be considered. The Organic Rankine Cycle (ORC) does not use water as the working fluid; instead, it employs low-boiling-point organic substances to absorb the waste heat from exhaust gases, causing them to vaporize. The resulting vapor then enters an expander where it expands to generate power, which is used to drive a generator. This system is simple and compact. The organic working fluids currently in use include low-boiling-point organic substances such as n-butane and R245fa, and some systems employ mixtures of organic working fluids. There is extensive research on this technology abroad; countries such as the United States, Germany, and Japan have companies that specialize in the research, development, and application of ORC technology. This technology in our country is still in the stage of development, with various domestic institutions currently conducting research on it. Other low-temperature power generation technologies, including those using the Kalina cycle, employ ammonia-water mixtures as working fluids. 5 Principles for the recovery and utilization of low-temperature waste heat: From the perspective of making the most of low-grade waste heat resources, it is recommended that the utilization of such resources follow the following principles: (1) First and foremost, the production process should be optimized as much as possible, heat exchange processes should be organized reasonably to reduce energy consumption in the process, and the generation of low-temperature waste heat should be minimized within the production process. (2) The utilization of low-temperature waste heat resources requires a comprehensive consideration of the industrial and domestic heating (cooling, power) demands within the plant area and its surrounding regions; it cannot be considered in isolation. When utilizing such resources, a proper planning approach should be adopted, following the principles of giving priority to users who are closer first, as well as giving preference to longer utilization periods. (3) The utilization of low-temperature waste heat resources should be carried out in a hierarchical manner, based on the quality and quantity of the waste heat as well as user demands, through energy-level matching, in order to minimize the temperature difference for heat transfer. (4) Regarding the utilization methods of low-temperature waste heat resources, they should be arranged in order of \"direct heat utilization – refrigeration and heating – heat-to-work conversion\". (5) When designing the specific utilization plan, it is also necessary to consider the corrosion resistance of the heat transfer elements and the maturity of the utilization technology, as well as to conduct a thorough analysis and comparison of economic benefits.