Thermal-conductivity-type continuous flue gas analyzer: among common flue gas constituents CO2, N2, O2, and CO, which has the highest thermal conductivity and therefore produces the greatest conductive signal change?

Introduction / Context:
Thermal-conductivity gas analyzers detect composition changes via heat transfer from a heated element to the surrounding gas. The signal depends on the thermal conductivity k of the gas mixture. Recognizing relative k values of typical flue gas components guides interpretation and instrument selection.

Given Data / Assumptions:

• Operating near ambient pressure and moderate temperature.
• Constituents considered: CO2, N2, O2, CO (not including H2 or He, which would be much higher).
• Analyzer element is balanced against a reference cell.

Concept / Approach:
At room temperature, approximate thermal conductivities are: CO2 ≈ very low (about 0.016 W/m·K), CO ≈ lower mid (≈ 0.024), N2 and O2 ≈ around air (≈ 0.026). Between N2 and O2, oxygen is slightly higher than nitrogen under common conditions. Therefore, among the listed flue gas species, O2 has the highest thermal conductivity and produces the largest conductive signal relative to the others.

Step-by-Step Solution:

Verification / Alternative check:
Property tables show air ≈ 0.026 W/m·K, O2 slightly above N2, validating the ranking for these species.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming air’s k equals nitrogen exactly; O2’s slight increase matters when discriminating among listed gases.

Final Answer:
O2