High-value regeneration of powdered waste activated carbon: Pathways for technological innovation and industrial upgrading

I. Current Status of Powdered Activated Carbon Applications In recent years, with the rapid development of industries such as water treatment, food decolorization, chemicals, new materials, and new energy, the demand for activated carbon has continued to grow. Among them, powdered activated carbon is widely used in industrial fields due to its large specific surface area, fast adsorption rate, and flexible application. After powdered activated carbon becomes saturated, it needs to be regenerated online or replaced with new activated carbon. Currently, the technology and equipment for online regeneration coupled with activated carbon adsorption are relatively few, resulting in a large amount of waste powdered activated carbon. Currently, the main methods for treating waste powdered activated carbon include incineration, landfill, and recycling. Landfill not only occupies valuable land resources but also poses a significant environmental pollution risk. Incineration, although it can recover some heat, damages the pore structure of activated carbon, making resource recycling impossible. In contrast, recycling can restore the adsorption performance of waste activated carbon and allow for reuse, which is a key way to achieve green, low-carbon, and circular economy. In terms of technology, commonly used regeneration methods at home and abroad include thermal regeneration, chemical regeneration, biological regeneration, and emerging methods such as microwave regeneration and ultrasonic regeneration. Among these, thermal regeneration is the most widely used regeneration technology in industry due to its strong adaptability and high processing efficiency.

II. Major Problems in Powdered Activated Carbon Regeneration While thermal regeneration technology is mature and widely used in centralized regeneration enterprises and industrial enterprises for granular activated carbon, powdered activated carbon regeneration still faces numerous industrialization bottlenecks, resulting in insufficient centralized regeneration capacity and a large amount of powdered activated carbon not being recycled for high-value utilization.

1. Internally Heated Rotary Kiln Regeneration Technology Internally heated rotary kilns transfer heat directly to powdered activated carbon through a heat medium. Their advantages include fast heat transfer rate and high thermal efficiency. However, they have significant shortcomings in engineering applications: Temperature gradient is difficult to control precisely: the temperature requirements for drying, pyrolysis, and activation differ significantly, and the direct heat exchange method within an internally heated rotary kiln makes it difficult to create a dynamically optimized temperature gradient along the axial direction; if the heating power is too high and the regeneration temperature is too high, it can easily cause ablation of the activated carbon matrix, reducing the specific surface area; if the heating temperature is too low and the heating rate is too low, the drying section will be too long within the kiln, occupying too much effective space, thus compressing the space for subsequent pyrolysis and activation stages. This not only leads to incomplete desorption of organic matter but also restricts the recovery of pore structure due to insufficient activation space, ultimately reducing the adsorption performance of the regenerated carbon. Dust entrainment and equipment blockage: High-speed hot flue gas easily carries fine particulate carbon into the subsequent tail gas purification system, not only reducing the regenerated carbon yield but also causing carbon particles to deposit in critical equipment such as equipment, flues, and dust collectors, leading to blockage and increased system pressure drop, increasing the frequency of equipment shutdowns for maintenance and cleaning, and raising operating and maintenance costs. Tar formation and blockage: When processing complex multi-source carbon particles with high organic matter content, improper regeneration temperature control can generate a large amount of tar in the flue gas. This tar condenses and adheres with the airflow, combining with dust to form deposits, further exacerbating equipment and flue blockage and reducing the system’s continuous and stable operating capability.
2. Externally Heated Rotary Kiln Regeneration Technology: Externally heated rotary kilns rely on heat conduction through the kiln wall, with the heat medium not directly contacting the material. Its main technical bottlenecks are: Low regeneration temperature: Due to limitations in the long-term temperature resistance of the equipment’s main material, the kiln shell needs to operate stably at high temperatures for extended periods. However, existing heat-resistant steels have limitations in high-temperature strength and creep resistance, making long-term operation under high-temperature conditions impossible. Currently, the operating temperature of internally heated rotary kilns in the industry is typically around ~600℃, far below the ideal high-temperature conditions required for activated carbon activation. This results in incomplete desorption of organic pollutants, limited recovery of activated carbon pore structure, and low iodine and methylene blue adsorption values ​​in the regenerated carbon, making high-value reuse difficult. Limited heat transfer area hinders linear scaling of regeneration capacity. The heat transfer area of ​​an externally heated rotary kiln is determined by the kiln wall surface area. However, in actual operation, the actual increase in activated carbon regeneration capacity is very limited due to the limited activated carbon filling rate (which is usually low to ensure sufficient tumbling and uniform heating of the activated carbon). Even with a linear increase in the cylinder diameter and length, the actual increase in activated carbon regeneration capacity is very limited. 3. The mainstream fluidized bed regeneration technology in China currently follows this process: drying of powdered waste activated carbon → rapid fluidized bed regeneration and activation → flue gas waste heat recovery → powdered carbon collection → tail gas purification. While this process achieves continuous production, it suffers from the following main problems in actual operation: low efficiency of flue gas waste heat recovery and high system energy consumption. The rapid fluidized bed generates a large amount of high-temperature flue gas during the activated carbon activation and regeneration process. Although a flue gas waste heat recovery stage is included, the recovered heat is difficult to efficiently reuse in the drying section due to limitations in heat exchange efficiency and system design. The powdered carbon drying process still requires a certain amount of external heat source, resulting in high overall energy consumption. The powdered carbon collection and tail gas purification system is complex. To reduce the loss of fine powdered carbon with the flue gas and meet environmental emission standards, the system needs to be equipped with multi-stage cyclone separators, bag filters, and other regenerated carbon collection equipment. The tail gas then enters the tail gas purification unit. High system integration and reliability are required. The system integration and process control requirements between units such as activated carbon drying, regeneration, waste heat recovery, powdered carbon collection, and tail gas purification are high. Technical defects or design omissions in any link can affect the reliability of the entire system. In summary, the industrial application of centralized powdered carbon regeneration still faces several technological challenges: high energy consumption and severe activated carbon matrix erosion. Powdered carbon typically has a moisture content as high as 40%–70%, requiring significant heat energy for drying before direct entry into the thermal regeneration system. Simultaneously, localized overheating is prone to occur during high-temperature activation, leading to activated carbon matrix erosion and a decrease in regenerated carbon yield. Regeneration efficiency and capacity are limited. Powdered carbon easily clogs equipment and pipelines, restricting the continuous and stable operation of the unit and its annual actual processing capacity. Product performance and quality are low. Some powdered carbon regeneration projects suffer from numerous technical defects, poor process reliability, and unreasonable equipment selection, resulting in low recovery of adsorption performance after activated carbon regeneration. During thermal regeneration, if the activation temperature is too low or the activation time is insufficient, the microporous structure cannot be fully opened; if the temperature is too high or the residence time is too long, pore wall erosion and a decrease in specific surface area will occur. Furthermore, improper atmosphere control during regeneration can also affect surface chemical properties and reduce the adsorption selectivity for specific pollutants. These factors collectively lead to the difficulty in fully restoring key indicators such as the iodine value of regenerated carbon to the level of new carbon, with some even only reaching 60%–70% of the performance of new carbon. This not only affects its reuse in high-requirement fields but also reduces the market competitiveness of regenerated carbon, hindering the high-quality development of the industry. Safety and environmental risks also exist. The volatile organic compounds and dust released during the thermal regeneration of powdered carbon pose an explosion risk; insufficient system integration technology in the regeneration tail gas treatment facilities restricts the normal operation of the regeneration equipment.

III. Technical and economic limitations of powdered carbon granulation regeneration Currently, domestic hazardous waste enterprises adopt a technical route of granulating powdered carbon before thermal regeneration to simplify equipment and operation and reduce treatment difficulty. While this approach solves the problem of powdered carbon transportation and collection to some extent, it has significant drawbacks: a substantial decrease in product quality and value; the addition of binders and mechanical extrusion during granulation compresses or even blocks the original microporous structure of activated carbon. While the regenerated product retains some adsorption capacity, it cannot restore the high specific surface area and rapid adsorption kinetics of the original powdered carbon. It can only be used as a low-grade product in applications with less stringent requirements, severely impacting the efficient utilization of resources and economic value. Mechanical damage to pore structure: The mechanical damage to activated carbon caused by the granulation process is irreversible. High-pressure molding leads to the formation of dense contact interfaces between powdered carbon particles, significantly reducing the proportion of mesopores and micropores. This structural change is fundamentally different from the regeneration of undisturbed activated carbon. Original regeneration primarily addresses the issue of adsorbates occupying pore channels, while granulation regeneration alters the essential characteristics of activated carbon at the structural level. Fundamental differences in process: Original powdered carbon regeneration is a performance restoration process based on maintaining the original particle morphology and pore structure; granulation regeneration, on the other hand, first changes the morphology and then regenerates, essentially performing secondary processing on the material. This difference results in significant differences in the regeneration mechanism, optimal process parameters, and final product performance of the two processes. Granulation regeneration struggles to achieve the product quality level of undisturbed regeneration. Furthermore, charcoal powder has a wide range of sources and types, involving multiple fields such as municipal water supply, industrial wastewater treatment, food processing, pharmaceuticals, dyes and chemicals, and metallurgy. The charcoal powder generated by different industries varies significantly in terms of pollutant types, adsorbate content, moisture content, ash content, and physicochemical properties, resulting in complex and variable properties. This diversity places extremely high demands on the material universality and technical reliability of the regeneration process, and also poses a severe challenge to the operational flexibility of the regeneration equipment (such as temperature adaptability, atmosphere control capability, and material handling capacity). If the regeneration process and equipment lack sufficient adaptability and controllability, it will be difficult to ensure both regeneration effectiveness and operational stability and economy. Therefore, in the process of charcoal powder regeneration technology innovation, it is essential to strengthen process optimization design and system integration to ensure the reliability and sustainability of industrial applications.

IV. Xuye Machinery has developed a charcoal powder thermal regeneration technology, dividing the charcoal powder regeneration process into three independent but continuous stages: drying, pyrolysis, and activation. Through segmented temperature control, atmosphere regulation, and heat recycling, high-quality virgin charcoal powder regeneration and economical and reliable equipment operation are achieved. Drying Section Control: For powdered activated carbon with high moisture content, low-temperature flue gas and indirect heating are used for drying under inert gas protection, optimizing the drying load and volatile organic compound (VOC) control of the drying section equipment. The drying heat source utilizes the waste heat from the activated carbon activation section, which is introduced into the drying section after cascade utilization, achieving internal heat circulation in the core activation and regeneration system. Pyrolysis Section Control: The dried activated carbon enters the pyrolysis section, where adsorbed organic matter and some impurities undergo thermal decomposition and volatilization at 400–600℃ in an oxygen-free atmosphere. The pyrolysis waste gas is then recycled back to the pyrolysis process after high-temperature dust removal, secondary combustion (1100℃), and waste heat recovery in a waste heat boiler. Activation Section Control: At approximately 900℃, steam generated by the waste heat boiler is used as an activator to activate and restore the microporous structure of the activated carbon. The activation temperature and time are dynamically adjusted according to the target iodine value of the regenerated carbon. The activation waste gas is returned to the pyrolysis and drying sections, achieving cascade energy utilization in the activation and regeneration process.