In endothermic gas generators, the higher the air ratio, the more H₂O and CO₂ in the outlet gas, the higher the dew point, and the lower the carbon potential; the lower the air ratio, the lower the dew point, and the higher the carbon potential, but the risk of residual methane and carbon black increases. Close-loop adjustment of the air-fuel ratio through an oxygen probe or dew point sensor can stabilize the carbon potential of the RX gas within the range required by the downstream process.
- More air → dew point increases, carbon potential decreases
- Less air → lower dew point, higher carbon potential, but higher CH₄ and carbon black risk
- Control mode: manual proportional valve → automatic closed loop (oxygen probe or dew point)
- Stable air-fuel ratio is the prerequisite for downstream carbon potential accuracy
A word about relationship
The air-fuel ratio is the "carbon potential knob" of the generator. With more air, there are more oxidizing components (H₂O, CO₂), the dew point is high, and the carbon potential is low; with less air, the opposite is true.
Chemical explanation
Under the theoretical ratio (taking natural gas as an example, about 2.4 parts of air to 1 part of methane), the products are mainly CO, H₂ and N₂.
- When the air is slightly more than the theoretical value, the excess oxygen further oxidizes part of H₂ and CO into H₂O and CO₂, and the dew point rises;
- When the air is slightly less than the theoretical value, the methane cracking is incomplete, the residual CH₄ increases, the carbon potential of the gas is higher, and it is easier to precipitate carbon black.
How downstream processes determine setpoints
| Downstream use | Requirements for RX Gas |
|---|---|
| carburizing carrier gas | The carbon potential of the gas itself is usually slightly lower than the target carbon potential, which is increased by the enriched gas in the furnace. |
| Medium carbon steel quenching and heating | RX gas carbon potential is close to the carbon content of the material (for example, about 0.4%) |
| High carbon steel / bearing steel quenching | Need lower dew point, higher carbon potential |
Therefore, there is no unified answer to "How much to set the generator dew point", but it should be derived from the downstream process.
Adjustment method
Manual adjustment: Adjust the air or gas through a proportional valve or needle valve, and check it regularly with a dew point meter. The disadvantage is that changes in gas heating value, pressure and ambient temperature can cause drift.
Automatic closed loop adjustment: Install an oxygen probe or dew point sensor at the generator outlet, and the controller automatically adjusts the air (or gas) branch according to the measured value to stabilize the dew point/carbon potential at the set value. This is the current mainstream practice.
Factors affecting stability
- Fluctuations in gas composition and calorific value: Changes in natural gas composition will change the theoretical air-fuel ratio;
- Air supply pressure fluctuations: Affects the actual ratio of the flow meter and proportional valve;
- Catalyst activity: Carbon deposition or aging causes the residual CH₄ to increase under the same air-fuel ratio;
- reaction temperature: Insufficient temperature leads to incomplete reaction;
- Changes in gas production: The optimal air-fuel ratio is slightly different between low load and high load.
Field experience
- The adjustment range should be small, wait for stability before making a judgment, and avoid back and forth oscillations;
- Look at dew point and CH₄ at the same time. Looking at dew point alone may mask catalyst problems;
- Record the data before and after adjustment to form the generator "health file".
Frequently asked questions
Why does the air-fuel ratio remain unchanged but the dew point gradually increases?
Common causes include carbon deposits on the catalyst (carbon burnout is required), changes in gas composition, wear of the proportional valve or leakage in the sampling pipeline. Check the sampling and sensors first, then the catalyst.
Is it better to control the generator outlet with an oxygen probe or a dew point meter?
The oxygen probe has fast response and is suitable for closed loop; the dew point meter is intuitive and easy to calibrate. Many systems are controlled with oxygen probes and periodically verified with dew point meters or infrared analyzers.
What should we pay attention to when running at low load?
At low load, the residence time of gas in the reaction tank changes, and the air-fuel ratio needs to be fine-tuned accordingly. For long-term low loads, multiple gas production modes or reasonable selection can be considered to avoid energy waste.