With increasingly stringent environmental regulations imposed on the coking industry, Volatile Organic Compounds (VOCs) and odorous pollutants generated from coke oven operations, by-product recovery systems, and wastewater treatment facilities have become major targets of emission control. Due to the complex composition of coking exhaust gas—which contains not only organic pollutants such as benzene homologs, tar, and naphthalene, but also corrosive and irritating gases including ammonia (NH₃) and hydrogen sulfide (H₂S)—traditional single-stage treatment technologies can no longer satisfy current ultra-low emission requirements or ensure long-term stable operation.

Regenerative Thermal Oxidizer (RTO) has become one of the most effective technologies for deep VOCs treatment in the coking industry because of its high destruction efficiency, excellent thermal energy recovery, and reliable long-term performance. In particular, the Rotary Regenerative Thermal Oxidizer （RTO can achieve a VOC removal efficiency exceeding 99% while recovering more than 95% of the thermal energy generated during oxidation, significantly reducing fuel consumption. It has therefore been widely applied in coking, coal chemical, petrochemical, and other industrial VOC treatment projects.

VOC emissions from coking plants mainly originate from the gas cooling section, ammonia recovery, crude benzene recovery, fine desulfurization units, wastewater treatment stations, and storage & transportation systems. Exhaust gas from the gas cooling process typically contains tar mist, naphthalene, benzene compounds, and hydrocarbons, while ammonia recovery and desulfurization units emit ammonia, hydrogen sulfide, and other inorganic pollutants. Wastewater treatment facilities continuously release low-concentration VOCs together with odorous gases. These exhaust streams are generally characterized by large air volume, complex composition, fluctuating VOC concentrations, condensation and crystallization tendencies, and a high risk of equipment fouling. Therefore, multi-stage pretreatment processes such as oil scrubbing, acid scrubbing, and alkaline scrubbing are normally installed upstream of the RTO system to remove tar, naphthalene, ammonia, and acidic pollutants, ensuring reliable long-term operation of the RTO.

1. Process Selection

1.1 Technology Selection

Among various VOC treatment technologies, thermal oxidation remains the most effective and reliable solution. Common thermal treatment technologies include Thermal Oxidizer (TO), Regenerative Thermal Oxidizer (RTO), Catalytic Oxidizer (CO), and Regenerative Catalytic Oxidizer (RCO).

For this project, RTO was selected as the core treatment equipment.

The operating principle of an RTO is to heat VOC-containing exhaust gas to temperatures above 760°C, where VOCs are oxidized into carbon dioxide and water. The high-temperature clean gas then passes through specially designed ceramic regenerative media, transferring heat to the ceramic bed. The stored thermal energy is subsequently used to preheat incoming untreated exhaust gas, thereby significantly reducing auxiliary fuel consumption.

Each regenerative chamber cyclically undergoes the processes of heat recovery, heat release, and purge. After completing the heat release stage, a portion of the purified exhaust gas is introduced to purge the chamber before it re-enters the heat recovery cycle, preventing untreated VOCs from escaping.

RTO systems are generally classified into valve-switching (fixed-bed) and rotary types.

Valve-switching RTOs include first-generation two-bed systems and second-generation three-bed or multi-bed systems. These systems utilize multiple ceramic regenerative chambers and switching valves to periodically reverse the airflow direction for heat recovery.

Generally, increasing the number of regenerative chambers improves destruction efficiency. A Rotary RTO features 12 regenerative chambers arranged in a cylindrical structure. Instead of multiple switching valves, a continuously rotating distribution valve directs the airflow between chambers. As the third generation of RTO technology, Rotary RTO is widely recognized as the most advanced RTO design available today.

1.2 Operating Principle of Rotary RTO

A Rotary Regenerative Thermal Oxidizer oxidizes combustible VOCs into carbon dioxide and water at high temperatures while recovering the thermal energy released during oxidation.

The recovered heat is stored in ceramic regenerative media and reused to preheat incoming exhaust gas, enabling a thermal recovery efficiency exceeding 95% while maintaining high VOC destruction efficiency.

1.3 Structure of Rotary RTO

A Rotary RTO mainly consists of:

• Combustion chamber
• Ceramic regenerative beds
• Rotary distribution valve

The reactor body is divided into 12 chambers, including:

• 5 inlet chambers
• 5 outlet chambers
• 1 purge chamber
• 1 isolation chamber

Driven by an electric motor, the rotary distribution valve rotates continuously at a constant speed, allowing the exhaust gas to switch smoothly among the twelve chambers.

The rotary sealing system adopts a contact-type sealing structure, providing excellent resistance to wear, high temperature, and corrosion.

2. Process Flow

VOC Treatment Process for the Coking Industry Using RTO

Exhaust gas from the gas cooling section contains tar, naphthalene, hydrocarbons, and other organic compounds.

After collection, the gas is pressurized by a booster fan before entering the oil scrubbing tower. The gas flows upward through the packed section while the scrubbing oil flows downward in a counter-current arrangement, enabling efficient heat and mass transfer.

Tar, naphthalene, benzene, and other organic compounds are absorbed into the circulating wash oil. Fresh lean oil is periodically added according to the liquid level, while the VOC-rich oil is returned to the original production process for recovery and reuse. Following oil scrubbing, the concentration of hydrocarbons, benzene, and naphthalene in the exhaust gas is significantly reduced.

The treated gas is then combined with exhaust streams from the ammonia recovery, ammonium phosphate, and fine desulfurization units before entering the acid scrubbing tower.

Ammonium sulfate mother liquor is used as the scrubbing solution. The gas flows upward through the packed bed. It contacts the descending acidic solution counter-currently, allowing ammonia and other alkaline contaminants to react with the scrubbing liquid and transfer into the liquid phase.

The circulating solution is continuously recycled. Rich liquor is discharged back to the ammonium sulfate process, while fresh scrubbing solution is periodically replenished to maintain stable removal efficiency.

The outlet gas from the acid scrubber is then mixed with exhaust gas from the wastewater treatment station before entering the alkaline scrubbing tower.

A diluted sodium hydroxide solution (prepared from 40% NaOH) is used as the absorbent. The gas flows upward through the packed bed and reacts with the alkaline solution, effectively removing acidic pollutants such as hydrogen sulfide.

The absorbent is continuously circulated, and spent alkaline liquor is discharged to the biological wastewater treatment system. Fresh alkali is added periodically to maintain stable scrubbing performance.

Following the combined oil scrubbing + acid scrubbing + alkaline scrubbing pretreatment process, VOCs (including tar, hydrocarbons, benzene, and naphthalene) together with odorous pollutants such as ammonia and sulfur compounds are significantly reduced before entering the RTO for final thermal oxidation.

The pretreated exhaust gas first enters a mixing chamber equipped with an LEL detector. After passing through a flame arrestor, the gas is delivered by the main fan into the Rotary RTO.

The exhaust gas is preheated through the five inlet regenerative chambers to temperatures above 760°C before entering the oxidation chamber, where complete turbulent mixing occurs.

The combustion chamber temperature is maintained at approximately 850°C, with a gas residence time of no less than 1.2 seconds, ensuring complete oxidation of VOCs.

The purified gas then passes through the five outlet regenerative chambers, where thermal energy is stored within the ceramic media before the treated exhaust is discharged.

The outlet NMHC concentration is below 50 mg/Nm³.

3. Key Technical Features

Although the pretreatment process significantly reduces tar mist and naphthalene, long-term operation of the RTO still requires measures to prevent asphalt-like deposits or crystallized substances from blocking flame arrestors and ceramic regenerative media.

The following design measures are adopted to minimize the risk of fouling.

(1) Advanced Pretreatment System

A wire-mesh filter is installed inside the mixing chamber and consists of:

• One vane-type mist eliminator
• One layer of high-efficiency wire-mesh filter

The vessel is equipped with an automatic steam backflushing system.

Differential pressure sensors monitor filter resistance and automatically initiate steam cleaning when the preset pressure drop is reached.

The high-efficiency mesh effectively captures dust, tar mist, naphthalene, condensed water vapor, and other particulates before the gas enters the RTO.

Steam Backflushing

The exhaust gas enters the filter vessel from the side and exits through the top after passing through the wire mesh.

As contaminants accumulate, the differential pressure gradually increases.

When the preset value is reached, the pressure transmitter activates the steam backflushing system.

To ensure thorough cleaning under high airflow conditions, multiple steam nozzles are arranged in different zones to perform sequential backflushing from the top downward.

Condensate is discharged through the drain outlet at the bottom.

Superheated steam is used until the differential pressure returns to its normal operating level.

(2) Steam Heat Tracing

Since the methanol plant generates abundant steam, steam tracing can be installed on the main duct upstream of the RTO to prevent condensation and crystallization inside the piping system.

 

                                    Stainless Steel Wire Mesh Mist Eliminator                                       Stainless Steel Vane Mist Eliminator

(3) Large-Cell Ceramic Regenerative Media

Large-cell honeycomb ceramic media are employed to provide larger flow passages, thereby reducing the risk of blockage.

In addition, 12 maintenance doors are installed between the bottom ceramic layer and the second ceramic layer, allowing convenient inspection and replacement of the ceramic media.

If small amounts of tar or naphthalene bypass the pretreatment system, these substances are most likely to crystallize on the lowest ceramic layer.

During maintenance, operators can open the inspection doors and use steam to clean or replace the bottom ceramic media.

The accumulated ammonium salts and deposits can be dissolved and flushed out through the drainage system located at the bottom of the RTO.

3. Conclusion

VOCs treatment in the coking industry requires not only high destruction efficiency but also reliable long-term operation and economical energy consumption.

By integrating oil scrubbing, acid scrubbing, alkaline scrubbing, wire-mesh filtration, steam backflushing, steam tracing, and large-cell ceramic regenerative media, the system effectively minimizes the risk of tar and naphthalene deposition while ensuring stable, continuous operation of the RTO.

As the final treatment unit, the Rotary RTO provides highly efficient destruction of VOCs and odorous pollutants while recovering the majority of the thermal energy released during oxidation. This substantially reduces natural gas consumption and improves overall energy efficiency.

With increasingly stringent environmental regulations, the combination of comprehensive pretreatment and Rotary RTO technology has become one of the most reliable solutions for VOC emission control in the coking industry. Continued optimization of exhaust gas collection, pretreatment, and RTO system design will further improve emission compliance, extend equipment service life, and reduce operating costs, supporting the sustainable and low-carbon development of coking enterprises.