In an endothermic or nitrogen-methanol atmosphere, H₂O (expressed as dew point), CO₂, and oxygen partial pressure are related to each other through water gas reactions and boundary reactions. As long as the atmosphere is close to equilibrium and the CO and H₂ contents are stable, any of the three plus temperature can be converted to the carbon potential. Dew point is often used for generator outlet control. CO₂ infrared analysis is suitable for calibration and diagnosis. The oxygen probe has the fastest response and is most suitable for real-time closed-loop control in the furnace.
- The lower the dew point, the lower the CO₂, the lower the oxygen partial pressure → the higher the carbon potential
- The three are related to each other through the equilibrium CO + H₂O ⇌ CO₂ + H₂
- The dew point is normally controlled at the generator outlet and controlled by an oxygen probe in the furnace.
- When the atmosphere deviates from equilibrium (e.g. high residual CH₄), the single index conversion will be distorted
The same set of reactions behind the three indicators
Endothermic atmospheres are composed primarily of CO, H₂, N₂, with small amounts of CO₂, H₂O, and CH₄. The main equilibria related to carbon potential are:
- Boundary reaction: 2CO ⇌ [C] + CO₂
- Water gas reaction: CO + H₂ ⇌ [C] + H₂O
- Water gas conversion: CO + H₂O ⇌ CO₂ + H₂
When the atmosphere is close to equilibrium, CO₂, H₂O and oxygen partial pressure are three expressions of "the same information": knowing one of them, and then knowing the temperature and CO (and H₂) content, you can deduce the other two and the carbon potential.
Comparison of three measurement methods
| Index | Measurement means | Advantage | Limitations |
|---|---|---|---|
| Dew point (H₂O) | Dew point meter, chilled mirror, capacitive sensor | Intuitive, suitable for generator outlet gas quality monitoring | The sampling tube is prone to condensation and slow response; the carbon potential cannot be given separately |
| CO₂ | Infrared (NDIR) Analyzer | It can measure CO and CH₄ at the same time, with strong diagnostic ability and easy calibration. | Requires sampling and regular calibration; instrument cost is high |
| partial pressure of oxygen | Zirconia oxygen probe (plug-in type) | In-situ measurement, fast response, most suitable for closed-loop control in the furnace | The probe will age and accumulate carbon, and requires maintenance and calibration. |
Division of labor in practical applications
- Generator outlet: Control the air/gas ratio with dew point or CO₂ to ensure stable RX gas quality;
- Carbon potential control in the furnace: Use the oxygen probe as the main signal, calculate the carbon potential based on the temperature and set CO%, and adjust the enriched gas or diluted air in real time;
- Routine calibration and diagnosis: Use a three-gas analyzer to measure CO, CO₂, and CH₄, compare the controller to display the carbon potential, and judge the status of the probe and atmosphere.
Common misunderstandings
- Determine the carbon potential by looking only at the dew point: Under the same dew point, the temperature and CO content are different, and the carbon potential is also different;
- Ignore CH₄: High methane content indicates that the atmosphere deviates from equilibrium, and the conversion based on the equilibrium relationship will deviate. This is more common in nitrogen-methanol + propane enriched atmospheres;
- Condensation in sampling line: Dew point and CO₂ If the temperature of the sampling tube is too low or leaks, the readings will be seriously distorted.
Frequently asked questions
At what level is the dew point at the generator outlet generally controlled?
Depends on the carbon potential required for downstream processes. When used as a carburizing carrier gas, the dew point is usually kept low; when used for neutral quenching and heating, it is selected according to the carbon potential corresponding to the carbon content of the material. It should be set according to process requirements rather than fixing a value.
Do I need an infrared analyzer when I have an oxygen probe?
Recommended equipment. The oxygen probe is responsible for real-time control, and the infrared analyzer is responsible for calibration and diagnosis (such as measuring whether CO% deviates from the set value and whether CH₄ is too high). The two are complementary and help meet the backup verification requirements of CQI-9.
Why do dew point readings fluctuate so much?
Common causes include air leakage or condensation in the sampling line, fluctuations in the generator air/gas ratio, carbon deposits or aging of the catalyst, and sensor contamination. The sampling system should be checked first, then the generator status.