Endothermic gas (commonly known as RX gas, Endo gas) is a gas generated by mixing air and natural gas (or propane) in a certain proportion and cracking them in a nickel catalyst reaction tank at a temperature above about 1000 °C. When natural gas is used as raw material, the typical composition is about 40% N₂, 40% H₂, 20% CO, with a small amount of CO₂, H₂O and CH₄. Because the reaction is endothermic, continuous external heating is required; the composition of the generated gas is stable, and it is the mainstream choice for carburizing carrier gas and neutral quenching protective gas.
- Raw materials: air + natural gas (or propane), mixed in a fixed ratio
- Conditions: high temperature (above about 1000 °C) + nickel-based catalyst + external heating
- Product: Approximately 20% CO, 40% H₂, 40% N₂ (natural gas feedstock)
- The outlet needs to be cooled quickly to avoid carbon black precipitation in the mid-temperature zone.
- Quality indicators: dew point/CO₂, residual CH₄, ingredient stability
Reaction principle
The generation of endothermic gas can be approximately expressed as (taking methane as an example, O₂:N₂ ≈ 1:3.76 in air):
CH₄ + 2.38 × (0.21 O₂ + 0.79 N₂) → CO + 2H₂ + 1.88 N₂
That is, each part of methane reacts with about 2.4 parts of air to produce CO, H₂, and N₂. This reaction as a whole is endothermic reaction, must be maintained by continuous heating (electric heating or gas heating) outside the generator reaction tank. The catalyst is generally a nickel-loaded refractory material that is used to complete the reaction at an acceptable temperature and suppress methane residue.
In actual production, the ratio of air to natural gas is often fine-tuned around 2.5:1 to control the dew point or CO₂ of the outlet gas, thereby determining the "carbon potential" of the gas.
Typical ingredients
| Components | Natural gas feedstock (approx.) | Propane feedstock (approx.) |
|---|---|---|
| CO | 20% | 23% |
| H₂ | 40% | 31% |
| N₂ | 40% | margin |
| CO₂ / H₂O / CH₄ | A small amount, changes with air-fuel ratio | A small amount |
The composition data of natural gas raw materials are consistent with the industry's public information; the data of propane raw materials refer to the comparison in the Gos Control CQI-9 carbon potential management article.
Why cool quickly
After the cracked gas leaves the high-temperature reaction zone, if it stays in the range of about 450–700 °C for too long, a reverse reaction of 2CO → C + CO₂ will occur, precipitating carbon black and changing its composition. Therefore, a cooler (water cooling or air cooling) is provided at the generator outlet to quickly cool down the gas and "freeze" the equilibrium components at high temperatures.
Main purpose
- Carburizing / Carbonitriding Carrier Gas: Provide stable CO and H₂, and adjust the carbon potential with enriched gas;
- Quenching and heating protection for medium and high carbon steel: Adjust the dew point to match the carbon potential with the carbon content of the material to prevent decarburization;
- Recarburization, brazing, sintering and other occasions that require a restorative atmosphere.
Due to the stable composition and relatively constant CO content, after the endothermic atmosphere enters the furnace, CO and CO₂ are in a stable state and CH₄ is at a low level, which is conducive to precise and stable carbon potential control. This is its main advantage over nitrogen-methanol atmospheres.
What to look at for gas quality
- Dew point or CO₂: Reflects the air-fuel ratio and the carbon potential level of the gas;
- Residual CH₄: High indicates a decrease in catalyst activity, insufficient reaction temperature, or deviation of the air-fuel ratio;
- Composition fluctuations: Directly affects the carbon potential control accuracy in the downstream furnace;
- carbon black: The appearance of carbon black indicates the need to burn charcoal or check the cooling.
Frequently asked questions
Are endothermic gases toxic?
Containing about 20% CO, it is a toxic and flammable gas. Workshops require ventilation, CO alarms, reliable safety interlocks and regulated ignition/purging procedures.
Can the gas be stored after the generator is shut down?
Generally, there is no storage and continuous production on demand. When continuous gas supply is required, double furnace generators can be used, one in use and one in standby, or multiple generators can be connected in parallel.
What does RX mean?
RX was originally the trademark name of the American Surface Combustion Company for endothermic gas, and was later widely used in the industry to refer to endothermic gas.