Protective atmospheres for heat-treatment shops (captive or commercial) can fall into five categories: endothermic, blended gases, exothermic, elemental gases, vacuum and air.
Endothermic-based atmospheres are manufactured in an endothermic gas generator. The gas generator is a very simple furnace construction, and its principle of operation is extremely simple. The generated gas is simply a mixture of a hydrocarbon, gas and air in very specific ratios. As one is aware, air will always carry moisture. The two gases of air and hydrocarbon are mixed together followed by compression. The compressed gas is then passed through a nickel-based catalyst, which will decompose and clean the gas of heavy carbon (soot). The compressed gas is passed through a heated chamber that holds the nickel catalyst at a temperature of approximately 1900°F. The gas output composition will be (approximately) in the following percentage by volume:
- Nitrogen (N2) = 45.1%
- Carbon monoxide (CO) = 19.6%
- Methane (CH4) = 0.3%
- Carbon Dioxide (CO2) = 0.4%
- Hydrogen (H2) = 34.6%
This is based on an atmospheric temperature of 72°F to produce a dew point at approximately 50°F. The carbon potential of the gas will be approximately 0.3%. The gas ratio of gas to air should be approximately 2.8 volumes air to 1 volume of gas. This will vary according to the “natural gas” source.
On exhausting from the furnace, the process gas passes through a cooler, which is there to condense out any heavy carbon and to prevent it from being carried over to the process furnace. One can decide how to blend the gas with the enriching gas at the furnace.
Troubleshooting the Endothermic Generator
If atmosphere problems arise at the process furnace, it is prudent to check the performance of the endothermic gas generator. The following sequence is a suggestion to commence troubleshooting.
The function of the endothermic generator is based on the performance of the nickel-catalyst cubes, and it is necessary to keep the catalyst tubes clean and clear of residual carbon. A simple operation is necessary to ensure the cleanliness and the ability of the generator to produce good, clean gas. The procedure is called burnout. The sequence of the burnout procedure is simply to reduce the cracking temperature of the generator to approximately 1500°F, with the process gas turned off. The generator is then run with only the air compressor operating. The presence of air inside the process retort at 1500°F will cause the carbon to ignite. The time taken to complete the burnout procedure will be determined by the amount of carbon present in the nickel catalyst. Generally, the time can be between one to four hours.