Thermal conductivity gas analysis: which constituent is commonly measured in flue gas by thermal-conductivity-based analyzers under plant conditions?

Introduction / Context:Thermal conductivity detectors (TCDs) infer gas composition from changes in thermal conductivity relative to a reference stream. In boiler houses and process heaters, on-line gas benches often include a thermal conductivity channel for certain species, complementing paramagnetic (O2) or NDIR (infrared) sensors.

Given Data / Assumptions:

• Application: flue gas composition measurement.
• Detector: thermal conductivity cell comparing sample vs. reference.
• Species of interest: CO2 among others.

Concept / Approach:CO2 has a significantly different thermal conductivity than air or N2; a TCD can sense concentration changes by monitoring heat loss from heated elements. While O2 is commonly measured by paramagnetic analyzers and CO2 by NDIR for high selectivity, TCDs remain a recognized method for CO2 in mixed streams, especially in older or simpler analyzer trains and in GC-TCD setups.

Step-by-Step Solution:Match technique to property: TCD responds to bulk thermal conductivity of the gas mixture.Identify a species with pronounced conductivity contrast and routine presence in flue gas: CO2.Select CO2 percentage as the typical TCD application among the choices.

Verification / Alternative check:Instrument manuals list CO2 channels based on TCD or NDIR; paramagnetic is reserved for O2, confirming that O2 is not usually measured by TCD in modern stacks.

Why Other Options Are Wrong:O2 percentage: Better served by paramagnetic cells; TCD lacks selectivity.Specific gravity of petrofuels: Determined by hydrometers/densitometers, not TCD gas analyzers.Alloy composition: Requires spectroscopy or XRF, not gas TCD.Moisture by dew point: Uses chilled mirrors or IR absorption, not TCD.

Common Pitfalls:Assuming TCD is species-selective; it senses total thermal conductivity. Cross-sensitivity to other gases must be accounted for or separated upstream (e.g., by columns in GC).

Final Answer:Carbon dioxide (CO2) percentage in flue gas