Key Takeaways & Executive Findings
- •• • Single-pipe 45° oblique purging achieves 57.6% effective clearing area, a 35.7% improvement over conventional horizontal purging, directly reducing the risk of ammonium salt deposition in stagnation zones. • • Increasing pipe diameter from 200 mm to 280 mm yields ~40% improvement in flow uniformity, which is more effective than raising gas velocity alone, indicating that geometric optimization should precede operational adjustments. • • The optimal configuration (single-pipe 45°, 280 mm diameter, 14 m·s−1) attains 88.2% purging efficiency without additional fan power, a 45.6% increase over the baseline, offering a cost-neutral solution to mitigate NH4Cl fouling. • • Low-velocity zones (below minimum fluidization velocity) are identified as the primary cause of acidic combustion product residue and subsequent ammonium salt blockage, validating the need for active flow organization rather than passive countermeasures.
Abstract
Ammonium salt crystallization-induced blockage of the regenerative heat exchanger in regenerative thermal oxidizers (RTOs) remains a critical operational challenge, particularly in pharmaceutical applications where NH4Cl constitutes up to 70% of the fouling deposits. This study employs computational fluid dynamics (CFD) to systematically simulate six purging configurations, varying injection angle and pipe arrangement, and quantifies purging effectiveness via a novel evaluation method based on characteristic observation planes. Using the Realizable k-ε turbulence model coupled with a porous media model, we analyze the velocity distribution and low-velocity failure zones at the gas chamber–regenerator interface. Results demonstrate that a single-pipe 45° oblique injection achieves the highest effective purging area of 57.6%, a 35.7% improvement over conventional horizontal purging. Increasing pipe diameter significantly enhances flow uniformity, yielding an efficiency gain of approximately 40%, outperforming mere increases in gas velocity. A synergistic optimization strategy is proposed, prioritizing high-performance purging structures with coordinated parameter tuning. The recommended configuration—single-pipe 45° injection, 280 mm pipe diameter, and 14 m·s−1 gas velocity—achieves 88.2% purging efficiency without additional fan power, representing a 45.6% improvement over conventional modes. These findings provide a theoretical basis and engineering solution for RTO purging system design and operational optimization.
1. Introduction
Regenerative thermal oxidizers (RTOs) are widely adopted for controlling volatile organic compound (VOC) emissions in petrochemical, pharmaceutical, and electronics industries due to their high heat recovery and stable performance. However, when treating waste gases containing chlorine, nitrogen, or sulfur, acidic combustion products (e.g., HCl, SO2) adsorb onto the heat exchange media and accumulate in the gas chamber and valve sealing areas. During subsequent intake cycles, these acidic residues react with alkaline components (e.g., NH3) in the raw gas, forming solid ammonium salts such as NH4Cl. These salts preferentially crystallize in low-temperature zones and flow stagnation regions, leading to heat exchanger blockage, increased pressure drop, valve failure, and unscheduled shutdowns. Even with advanced six-cycle operating modes that increase purging frequency, ammonium salt fouling persists in complex industrial settings, with NH4Cl comprising up to 70% of the deposits in pharmaceutical applications. This indicates that current purging strategies are insufficient to fully eliminate residual contaminants.
Existing countermeasures, such as periodic water washing or high-temperature reverse burning, are passive and often require system downtime, incurring significant economic losses. Computational fluid dynamics (CFD) has emerged as a powerful tool to simulate complex flow fields inside RTOs, yet systematic studies on purging efficiency and its influencing factors remain scarce. This research addresses this gap by developing a quantitative evaluation method for purging effectiveness using CFD. We systematically investigate the effects of purging pipe configuration (single vs. dual, injection angle) and operational parameters (pipe diameter, gas velocity) on flow field characteristics and low-velocity failure zones. The goal is to identify an optimal purging strategy that actively prevents ammonium salt formation by ensuring complete removal of acidic residues, thereby enhancing RTO reliability and reducing maintenance costs.
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CHEN Hongrui, LU Zhaoyang, QU Xiaolei, LI Ming, XU Zunzhu, CHEN Weijie, WEI Ziqiang (2026). Quantitative Evaluation and Coupling Analysis of Purging Performance in Regenerative Thermal Oxidizers Based on CFD Simulation. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202510004
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Frequently Asked Questions
What is the primary failure mechanism leading to ammonium salt blockage in RTOs, and how does the proposed purging strategy address it?
The primary failure mechanism is the incomplete purging of acidic combustion products (e.g., HCl) from the heat exchange media and gas chamber, which subsequently react with NH3 in the next intake cycle to form NH4Cl crystals. CFD simulations revealed that low-velocity zones (below minimum fluidization velocity) at the gas chamber–regenerator interface are the main sites of residue accumulation. The proposed single-pipe 45° oblique purging configuration enhances flow uniformity and reduces these low-velocity zones, achieving an 88.2% purging efficiency compared to 60.6% for conventional horizontal purging, thereby minimizing the residual acidic species available for salt formation.
How does the purging performance vary with pipe diameter and gas velocity, and what is the optimal combination?
Both increasing pipe diameter and gas velocity improve purging efficiency, but with diminishing returns. Pipe diameter optimization yields a ~40% improvement in flow uniformity, which is significantly greater than the effect of increasing gas velocity alone. The optimal configuration is a single-pipe 45° injection with a 280 mm diameter and 14 m·s−1 gas velocity, achieving 88.2% purging efficiency without additional fan power. This combination balances momentum input and flow distribution, minimizing energy consumption while maximizing contaminant removal.
What are the practical implications of the 45° injection angle compared to 0° and 90°?
The 45° injection angle provides superior turbulent mixing and momentum redistribution across the cross-section, resulting in the smallest and most concentrated low-velocity failure zones. In contrast, 0° (horizontal) and 90° (vertical) injections create larger stagnation regions where ammonium salts can accumulate. The 45° configuration achieves a 57.6% effective clearing area, a 35.7% improvement over horizontal injection, directly reducing the risk of blockage in critical zones.
How does the proposed purging strategy compare to existing passive countermeasures like water washing or high-temperature reverse burning?
Passive countermeasures require system shutdown and incur significant operational costs. The proposed active flow organization strategy optimizes the purging process itself, preventing ammonium salt formation by ensuring complete removal of acidic residues. This approach not only reduces downtime but also improves energy efficiency, as the recommended configuration achieves 88.2% purging efficiency without additional fan power. The strategy aligns with the trend of using pretreatment to mitigate fouling, but offers a more direct and cost-effective solution.
What are the limitations of this CFD study, and what future work is planned?
The study is based on a single-chamber model and does not include experimental validation. The authors acknowledge that independent experimental verification is a key future work. Additionally, the model assumes steady-state conditions and does not account for transient effects during valve switching. Future research should incorporate full-scale multi-chamber simulations and experimental measurements to validate the predicted purging efficiencies and assess long-term performance under real operating conditions.
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