CO Measurement in Potentially Explosive Atmospheres: Challenges in RTO Systems
Regenerative Thermal Oxidation (RTO) systems are indispensable in the chemical industry for treating VOC-containing exhaust gases. However, when different emission streams—for example, from cryogenic VOC processes and CO-containing exhaust gases—are combined in a common mixing chamber, special measurement requirements arise. Continuous CO measurement up to the lower explosion limit (LEL) represents a safety-critical measurement task that requires specialized analytical technology.
At Fresenius Umwelttechnik, we have developed solutions for such applications that ensure explosion protection, fast response times, and reliable measurement values even under demanding process conditions. In this article, we demonstrate the technical challenges involved in high-concentration CO measurement in RTO systems and how modern LEL analyzers with NDIR technology overcome them.
Process Requirements: When Two Gas Streams Become a Challenge
In the application under consideration, two different emission streams are combined in a mixing chamber before being fed together to the RTO system. The emission monitoring takes place in the common exhaust duct after mixing, with the following characteristic conditions:
- Volume flow: 3,500 Nm³/h
- Process temperature: normally 35-40°C, maximum up to 60°C
- Pressure range: 0.7-1.1 bar (pressure compensation required)
- CO concentration: up to 12.5 vol.% (equivalent to 145 g/m³ or 100% LEL)
The central challenge lies in the measurement of CO in explosive concentrations. At 12.5 vol.%, the gas mixture is exactly at the lower explosion limit of carbon monoxide. At the same time, fast response times must be achieved to detect process-relevant changes promptly and avoid exceeding safety-relevant limit values.
Technical Challenges of High-Concentration CO Measurement
Condensate Formation and Sample Conditioning
When combining different gas streams—particularly when cryogenic VOC processes are involved—temperature gradients can occur, leading to condensate formation. Condensates not only distort measurement results but can also cause corrosion and blockages in the analytical system. Effective gas cooling and condensate separation before the actual measurement is therefore essential.
Fast Response Times Despite Sample Conditioning
In safety-critical applications, fast response times are crucial. The T90 time—the time until 90% of the final value is reached during a concentration change—must be as short as possible, even considering the necessary sample conditioning. This requires optimized flow paths and sufficient sample volume flows for workplace atmosphere measurement methods.
The Solution: SC300 LEL Analyzer with NDIR Dual-Beam Technology
For this demanding measurement task, we employ the SC300 LEL analyzer, which was specifically developed for measuring combustible gases in high concentrations. The system is based on proven NDIR dual-beam technology (Non-Dispersive Infrared) and offers decisive advantages for this application:
NDIR Dual-Beam Technology for Precise CO Measurement
The non-dispersive infrared measurement utilizes the specific absorption property of carbon monoxide in the infrared range. The dual-beam principle automatically compensates for contamination and aging effects of the infrared source, resulting in high long-term stability and low maintenance requirements. The temperature-stabilized measuring cell is constantly maintained at 50°C to minimize cross-sensitivities due to temperature fluctuations—a decisive advantage over catalytic pellistor sensors.
Integrated Explosion Protection
The SC300 LEL features flame arresters at both the inlet and outlet of the analyzer. These reliably prevent the propagation of flames or explosions between the analyzer and the process. The design meets the requirements for use in potentially explosive atmospheres and ensures the highest safety standards.
High-Performance Sample Gas Delivery
An integrated sample gas pump continuously delivers 4 l/min of sample gas through the system. This comparatively high volume flow is crucial for fast response time and ensures that representative samples are extracted from the main gas stream. The pump is designed to maintain a constant sample gas flow even with slight pressure fluctuations in the process (0.7-1.1 bar).
System Integration and Sample Conditioning
External Gas Cooler for Condensate Removal
Before entering the analyzer, the sample gas passes through an external gas cooler. This controllably reduces the gas temperature and reliably separates condensates formed. The cooled and dried sample then reaches the temperature-stabilized measuring cell, where precise and reproducible measurements can be performed. This sample conditioning is essential to avoid measurement errors due to moisture and condensates.
Pressure Compensation for Stable Measurement Values
Since the process pressure can fluctuate between 0.7 and 1.1 bar, the SC300 features integrated pressure compensation. This automatically corrects the measurement values and ensures precise concentration measurements even during pressure fluctuations. Particularly in high-concentration measurements, where even small deviations can be safety-relevant, this function is indispensable.
Daily Zero Point Adjustment
To ensure the highest measurement accuracy, an automatic zero point adjustment is performed daily using uncontaminated ambient air. This calibration routine compensates for drift effects and ensures that the measurement remains reliable even over long operating periods. The zero point adjustment is automated and requires no manual intervention by operating personnel.
Performance Characteristics: T90 Time and Measuring Range
The overall performance of the system is characterized by the following parameters:
| Parameter | Value / Specification |
| CO measuring range | 0 – 12.5 vol.% (0 – 145 g/m³, 0 – 100% LEL) |
| T90 time (incl. sample conditioning) | 5 seconds |
| Sample gas volume flow | 4 l/min |
| Measuring cell temperature | 50°C (stabilized) |
| Pressure compensation | 0.7 – 1.1 bar |
| Measurement accuracy | ±2% LEL |
Particularly noteworthy is the T90 time of only 5 seconds including the entire sample conditioning. This enables near real-time monitoring of CO concentration and allows rapid responses to process changes. Especially in safety-critical applications, where approaching the explosion limit must be detected in time, this fast response time is of crucial importance.
Conclusion: Safety Through Specialized Measurement Technology
The high-concentration CO measurement in RTO systems with mixed gas streams represents one of the most demanding tasks in process gas analysis. The combination of explosive concentrations, different emission sources, and the requirement for fast response times demands specialized analytical technology that goes far beyond standard gas analyzers.
With the SC300 LEL analyzer based on NDIR dual-beam technology, we offer a solution that meets all safety-relevant and measurement technology requirements. The integration of flame arresters, high-performance sample gas delivery, external gas cooling, and automatic pressure compensation ensures reliable measurement values even under the most demanding process conditions.
The T90 time of 5 seconds including sample conditioning enables quasi-continuous monitoring and rapid response to critical concentration values. This contributes significantly to process safety and protects both equipment and personnel from the hazards of explosive gas mixtures.
For plant operators and process engineers facing similar measurement tasks, we offer comprehensive consultation on the design and integration of LEL analyzers in existing or new RTO systems. Contact us to develop the optimal solution for your specific application together.
Frequently Asked Questions (FAQ) on CO Measurement in RTO Systems
Why Must CO Be Measured up to the Explosion Limit?
Measurement up to the lower explosion limit (LEL) is required to detect and prevent explosive gas mixtures early. For CO, the LEL is 12.5 vol.%. Below this limit, there is no ignition risk; above it, explosions can occur in the presence of an ignition source. Continuous monitoring enables timely countermeasures in accordance with TRGS 722 and protects personnel and equipment.
What Certifications Are Required for LEL Analyzers in RTO Systems?
LEL analyzers in potentially explosive atmospheres require ATEX certification according to Directive 2014/34/EU. Additionally, the operator must perform zone classification according to ATEX 1999/92/EC and prepare an explosion protection document according to GefStoffV § 6 Para. 9. For emission monitoring, conformity with EN 14181 is required.
Why Is NDIR Technology Better Than Catalytic Sensors?
NDIR sensors offer several advantages over catalytic pellistor sensors: They are insensitive to catalyst poisons (e.g., sulfur compounds), have a longer service life (no sensor wear), require no regular calibration, and deliver more precise measurement values at high CO concentrations. The dual-beam technology also automatically compensates for aging effects of the infrared source.
How Often Must LEL Analyzers Be Tested?
According to TRGS 500 and ATEX 1999/92/EC, gas warning devices must be tested by qualified persons before commissioning, after significant modifications, and at regular intervals. The test intervals must be defined in the risk assessment, typically annually. The SC300 LEL analyzer performs automatic zero point adjustments daily, which extends maintenance intervals.
What CO Limit Values Apply to RTO Systems?
According to TA Luft, CO emission limits for RTO systems in the chemical industry are typically < 100 mg/Nm³ in the cleaned exhaust gas. However, for safety monitoring upstream of the RTO, LEL monitoring is critical: At 12.5 vol.% CO (equivalent to 145 g/m³ or 100% LEL), the system must be shut down immediately to prevent explosions.
