Converting a used rotary kiln into activated carbon regeneration: seemingly feasible, but actually difficult.

In recent years, with the increasing demand for resource recycling in the hazardous waste industry, some companies have attempted to convert idle hazardous waste incineration rotary kiln production lines into waste activated carbon regeneration facilities, hoping to reduce investment costs through equipment reuse. However, from multiple dimensions such as technical principles, equipment characteristics, and process adaptability, this conversion path has significant technical limitations and industrialization economic risks. Cases such as a company in Hebei failing to put its converted equipment into production and a company in Ningxia abandoning the conversion under professional guidance have fully demonstrated its feasibility flaws. This article will start from the essential technical differences and systematically analyze the core issues of converting hazardous waste incineration rotary kilns into waste activated carbon regeneration facilities.

1. Limited Equipment Reuse Value and Prominent System Integration Challenges: There are fundamental differences between the core equipment of a hazardous waste incineration rotary kiln production line and the process requirements of a waste activated carbon regeneration system. Apart from the rotary kiln itself potentially retaining some reuse value, the other key supporting equipment is almost impossible to adapt. From a system composition perspective, the waste heat boiler, quench tower, bag filter, wet acid removal system, activated carbon and quicklime injection device of the hazardous waste incineration line are all designed for the incineration process of “high-temperature incineration + flue gas quenching + multi-media purification”. Its core objective is to destroy the toxicity of waste through oxygen combustion and to efficiently remove pollutants such as particulate matter, acidic gases, heavy metals, and dioxins. In contrast, the core requirement of the waste activated carbon regeneration system is “low-temperature pyrolysis + precise activation + tail gas directional purification”. It needs to control the pyrolysis atmosphere (oxygen content control in the rotary kiln), the activation temperature gradient (800-1000℃), and the amount of activation gas (water vapor). Its flue gas composition is mainly pyrolyzed organic matter and a small amount of carbon oxides. The purification focus is on VOCs (volatile organic compounds) and particulate matter, which is completely different from the purification logic of incineration flue gas. This difference necessitates a disruptive reconstruction of the flue gas purification system during the retrofit: the waste heat boiler needs redesign to match the low heat release characteristics of the regeneration process; the quench tower cannot be reused due to the small volume of regenerated flue gas; and the deacidification system needs simplification due to the low acid content in the regenerated flue gas. Therefore, the reuse rate of the main equipment, excluding the rotary kiln, is extremely low, and the retrofit cost far exceeds expectations; the so-called “reuse and cost reduction” is merely a theoretical concept.
2. Differences in equipment structure restrict regeneration efficiency, resulting in inherently insufficient material adaptability. The structural design differences between hazardous waste incineration rotary kilns and activated carbon regeneration rotary kilns fundamentally limit the operational effectiveness after retrofitting. Traditional hazardous waste incineration rotary kilns, to meet the requirements of “rapid heating and complete combustion,” typically employ a small length-to-diameter ratio (generally 5-8), resulting in a short material residence time within the kiln (usually 1 hour), relying mainly on the self-sustaining combustion of waste. Activated carbon regeneration involves three stages: drying, thermal desorption, and activation. Each stage has drastically different requirements for temperature, residence time, and oxygen concentration: the drying stage requires low-temperature drying at 100-200℃, thermal desorption requires a gradient temperature increase of 300-800℃, and the activation stage requires 800-1000℃ with the introduction of activation gas. Therefore, the regeneration rotary kiln must adopt a large aspect ratio (usually around 10) design to achieve economical and efficient regeneration through segmented temperature control and precise control of oxygen content. Converting a small aspect ratio incineration rotary kiln into a regeneration facility will directly lead to two problems: first, the activation stage is difficult to carry out fully, resulting in incomplete cleaning and repair of activated carbon pores and a significant reduction in regeneration efficiency; second, segmented temperature control is difficult, leading to an unreasonable temperature field within the kiln, which can easily cause local overheating leading to activated carbon burnout or local low temperatures leading to incomplete desorption, resulting in large fluctuations in the adsorption performance of the final product, failing to meet the requirements for recycling. Furthermore, the feeding and discharging structures of co-current rotary kilns are designed to accommodate the centralized disposal of multi-source hazardous waste, while spent activated carbon is mostly in powder or granular form, resulting in inherently insufficient material adaptability.
3. The conflicting process mechanisms and incompatible control systems mean that the core differences between activated carbon regeneration and hazardous waste incineration render the existing rotary kiln process control system unsuitable for regeneration requirements. Hazardous waste incineration is essentially a peroxidation reaction, using excess air (excess air coefficient 1.2-1.5) to completely oxidize organic matter into CO₂ and H₂O. The key control points are “combustion efficiency” and “pollutant destruction and removal rate.” The essence of activated carbon pyrolysis activation and regeneration is anoxic pyrolysis and gasification reaction: the pyrolysis stage requires desorption of organic matter in an inert atmosphere (oxygen-free or oxygen-deficient environment) to avoid oxidation and loss of activated carbon; the activation stage requires precise control of the concentration and ratio of activation gas (water vapor) to remove residual carbon deposits in the pores and repair the pore structure through the reaction of carbon and gas (C+H₂O→CO+H₂). This process requires strict atmosphere control (oxygen content <2%) and temperature gradient control (±20℃), which is completely contrary to the oxygen-rich, high-temperature, and strongly oxidizing environment of incineration. The existing control systems of rotary kilns for incineration (such as oxygen meters, burner operating logic, and temperature control strategies) are all designed for oxygen-rich combustion. The retrofit requires a complete reconstruction of the control logic: adding an oxygen content control system to maintain the oxygen-deficient environment of the rotary kiln, replacing the burner with a high-precision temperature control module to achieve temperature gradient control, and adding an activation gas flow control device. This type of modification is not only technically challenging, but also carries risks such as activated carbon burn-out, excessive CO levels, and high energy consumption in the secondary combustion chamber due to outdated existing incineration equipment and insufficient control precision caused by outdated technology. This is the core reason why some enterprises have been unable to put their rotary kilns into production after modification.
4. Poor Site and Auxiliary System Adaptability, Doubtful Industrialization Feasibility Besides the main equipment, the site layout and auxiliary systems of the rotary kiln are also difficult to meet the special needs of activated carbon regeneration. Activated carbon regeneration requires the addition of a dedicated feeding system (such as a dedicated activated carbon silo and a closed screw conveyor to prevent dust leakage), a regenerated activated carbon cooling system (such as an indirect water-cooled screw conveyor to avoid secondary adsorption during cooling), and a regenerated activated carbon screening-sorting and packaging system. The requirements for installation space, load-bearing capacity, and pipeline layout of these devices differ significantly from the existing auxiliary facilities of the incineration line (such as hazardous waste feeding hoppers and slag discharge machines). Some hazardous waste incineration plants have compact layouts, requiring the demolition of existing facilities and replanning of equipment, flue gas, and pipeline layouts during renovation. This not only increases the workload but may also lead to unreasonable equipment placement due to space constraints, affecting operational reliability and safety. More importantly, current activated carbon regeneration equipment manufacturers generally lack experience in incineration line renovation. Their technical reserves are concentrated on building new regeneration systems, with insufficient understanding of the structural characteristics, control logic, and process integration of incinerator kilns, making it difficult to solve systemic problems during renovation. Moreover, most regeneration equipment on the market itself suffers from defects such as unreasonable process selection calculations, incomplete process flows (e.g., missing activation stages), and low control precision (e.g., large temperature fluctuations). Combined with the inherent limitations of rotary kilns, the final operational reliability, product quality, and economic efficiency cannot be guaranteed. Conclusion: Feasible in principle ≠ feasible in practice. The technical renovation path requires careful evaluation. While converting hazardous waste incineration rotary kilns into activated carbon regeneration facilities may seem to conform to the logic of “reusing old equipment and reducing costs and increasing efficiency,” it actually ignores the fundamental differences between the two processes in terms of mechanism, equipment, and system. From an industry practice perspective, such upgrades not only face technical risks such as high equipment reconstruction costs, poor regeneration efficiency, and difficulty in control, but may also result in zero economic viability due to substandard product quality and unstable operation. For hazardous waste disposal companies with investment and construction needs for waste activated carbon regeneration projects, a more reasonable approach is to select specialized regeneration technology and equipment based on the characteristics of the waste activated carbon (type, adsorbent composition, particle size), rather than blindly relying on the reuse of incinerator kilns. Only by respecting the essence of the process and following technical principles can efficient regeneration and safe utilization of waste activated carbon be achieved, avoiding the dilemma of “technical upgrade equals obsolescence.”