Endothermic Gas Generators
The father of the endothermic gas generator was a gentleman by the name of Norbert K. Koebel, who was fond of saying to young engineers such as The Doctor, “Treat ‘em right and they’ll treat you right.” He knew that the endothermic gas generator was the heart of any atmosphere heat-treat operation. True then, true now. Let’s learn more.
Endothermic gas (aka RX® or endo gas) is primarily used for neutral hardening and as a carrier gas for gas carburizing and carbonitriding. Today, endo gas is typically supplied to the furnace so that the furnace atmosphere is essentially neutral to the surface of many steels and can be made chemically active by the addition of enrichment (hydrocarbon) gas, ammonia or air at the furnace proper.
Endothermic gas is produced when a mixture of air and fuel is introduced into an externally heated retort at such a low air-to-gas ratio that it will normally not burn. The retort contains an active catalyst, which is needed for cracking the mixture. Leaving the retort, the gas must be cooled rapidly enough to avoid the so-called carbon reversal or carbon reformation reaction, where carbon monoxide breaks down into carbon dioxide and carbon (in the form of soot) before it reaches the furnace. The gas needs to be rapidly cooled in the temperature range of approximately 705°C (1300°F) to 480°C (900°F) or below to avoid this reaction.
The endothermic gas reaction (Equations 1-2) occurs in two steps and produces an atmosphere of nitrogen, hydrogen and carbon monoxide with varying percentages of carbon dioxide, water vapor and residual hydrocarbon as methane if natural gas is the feedstock.
CH4 + air (2O2 + 8N2) → CO2 + 2H2O + 8N2 (1)
2C3H8 + air (3O2 + 11.4N2) → 6CO + 8H2 + 11.4N2 (2)
The endothermic gas composition (Table 1), by volume, varies depending on the type of hydrocarbon gas feedstock. The use of a nickel-based catalyst (Fig. 1) accelerates the reaction. The nickel (Ni) attracts the hydrogen atoms of the methane, which attaches to the catalyst. The oxygen molecules approach and are attracted to the carbon atoms. The carbon atoms combine with the oxygen atoms to form carbon monoxide (CO). The hydrogen atoms combine to form H2 and are released from the nickel attraction. The now-available nickel attracts new methane to continue the reaction (cracking) process. After the passage of the air-gas mixture over the catalyst, the reaction is “frozen” by chilling the gas rapidly to around 315°C (600°F) in either an air-cooled or water-cooled heat exchanger.
Endothermic gas generators consist of several basic components: a gas mixer, burner, combustion chamber and heat exchanger. They are available in single-retort (Fig. 2) and multiple-retort (Fig. 3) designs. The products of combustion of a fuel (e.g., natural gas) and air are combined at air/gas ratios typically between 2.5:1 and 3.5:1 to create the atmosphere. The reaction requires heat to proceed (hence the name endothermic), and, as such, these generators typically have heated combustion chambers.
Endothermic gas generators are common equipment in the heat-treat shop. The main components of an endothermic generator
(Fig. 4) are relatively simple and consist of:
- Heated reaction retort with catalyst
- Air-gas proportioning control components
- Pump to pass the air-gas mixture through the retort
- Cooler to “freeze” the reaction and prevent soot formation
- Fire check valve to prevent backfire in the fuel supply line
- Burnoff vent to combust excess gas produced
- Thermocouples (control, over-temperature, recording) and control instrumentation
The retort for an endothermic gas generator is typically a cast alloy – HU (38% Ni, 18% Cr) and HK (20% Ni, 25% Cr) are common. In some instances, retorts are fabricated from Inconel 600® (preferred alloy choice) or made of silicon carbide.
Retorts in most industrial generators are either thin and tall or thick and short. They vary in diameter from about 150 mm
(6 inches) to 300 mm (12 inches). In larger-diameter designs, either the inlet pipe runs down through the center of the retort (to preheat the gas) or the space is occupied by a closed-ended pipe, typically 50-75 mm (2-3 inches) in diameter to avoid issues with a cold center in the catalyst bed.
For economic reasons only, manufacturers have gone away from supplying pure nickel shot as a catalyst and today utilize insulating firebrick catalyst cubes typically 25 mm (1 inch) in size coated with 3-7% nickel sulfate (NiSO4). Smaller-sized cubes, 17.5 mm (11/16 inch) and spheres of 19 mm (3/4 inch) diameter, have also been used, but the pressure drop through the catalyst bed must be monitored due to increased packing density. The use of a refractory catalyst often suggests a smaller-diameter retort to assure both proper heat distribution throughout the catalyst bed and adequate dwell time at temperature for complete dissociation.
Endothermic gas generators have a long and proven track record of success. The gas produced is relatively stable and adequate for a broad spectrum of process applications. Maintenance is relatively simple, and problems with the equipment and technology are well understood and solvable on the shop floor.
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A mixing pump and (optional) carburetor control the air/gas ratio of an endothermic gas generator in the range of 2.5:1 to 3.5:1 for natural gas (Fig. 5a) and 7.25:1 and 9.25:1 for propane (Fig. 5b). In some generator designs air/gas ratios have been known to run as low as 2.0:1 for natural gas.
Today, most generators run a dew point (Table 2) in the range of +4.5 ± 0.2˚C (+40 ± 2˚F) to minimize maintenance concerns (e.g., sooting). For carburizing applications, some heat treaters prefer to run generators in the range of -1.1 ± 0.2˚C (+30 ± 1°F). Final adjustments to the atmosphere are typically made at the furnace.
Similar to their exothermic gas generator counterparts, endothermic generators can be equipped with advanced software packages combined with regenerative blowers capable of delivering “flow on demand” throughout the working range of the generator (Fig. 6).
The heating chamber of an endothermic gas generator can either be refractory-lined or lined with ceramic-fiber insulation. Modular designs and clam-shell-style chambers (to facilitate retort removal) housing single retorts are more popular today than the past practice of using large heating chambers housing multiple retorts. Cracking of the refractory and shrinkage of the ceramic fiber result in hot spots on the shell that can be avoided by proper design and heat-up practices of the unit from ambient temperature.
Endothermic gas generators are either gas-fired or electrically heated. If gas-fired, ring burners or combustion burners are commonplace. Electrically heated units either use nickel-chromium or silicon-carbide heating elements. If metallic, they often include the addition of rare-earth elements (e.g., Hf, Y) to extend operating life since the elements run in air. Power is regulated by on/off or proportional control (zero-fired or phase-angle-fired SCRs). The typical operation temperature of an endothermic gas generator is 1010-1095°C (1850-2000°F), depending on design. Most units run in the 1065°C (1950°F) range.
Most generators use either type K or type S thermocouples. Type N thermocouples are often used as a check (SAT) thermocouple.
The firecheck (Fig. 7) is a safety device designed to prevent backfire into the incoming gas supply line. The functionality of the firecheck should be checked every six months or more frequently if recommended by the manufacturer. The sad reality is that most heat treaters and plant maintenance personnel don’t understand its function and never check that it is operating properly!
The uses of endothermic gas (Table 3) include the following heat-treatment processes:
- Bright hardening
- Carbon restoration
- Neutral hardening
One of the tasks The Doctor was assigned that first week on the job was to change the catalyst in an endothermic generator – a task that left an indelible impression on a heat-treat rookie. It was fun, it was informative and it was hard work! Maintenance on endothermic gas generators today is even more critical than it was back then and includes the following general tasks (depending on the generator involved):
Test to determine if the catalyst bed is sooted. Indicators are:
- Small ratio adjustments do not result in a change of dew point.
- Very high dew-point readings
- Methane (CH4) higher than 0.5% after a proper air burnout operation
- When operating between 30-40°F, the CO2 should be approximately the equivalent to the dew point divided by 100 (0.30-0.40% CO2).
Over the years, checklists have been developed (and customized) for most endothermic gas generators. Here’s a general list of maintenance activities by suggested frequency:
- Check the temperature control instrumentation for proper operating temperatures.
- Check for proper flow and pressure of the generated atmosphere.
- Check for proper inlet air-gas ratios.
- Check either the gas analysis or the dew point of the unit. Make sure that manual and automatic readings coincide. Recalibrate automatic gas analyzers.
- Check that the floats in the gas flow tubes are free and operating.
- Check that the compressor is operating and functional.
- Check that the gas cooler is operating. If installed, check the temperature of the exiting gas to confirm that the carbon reversal reaction is not occurring (and that soot is not being formed) on gas discharge from the generator to the furnace.
- If the system is water-cooled, check sight drains or temperature gauges (or both) to confirm proper water flow, pressure and temperature.
- Check that there are no leaks from any of the joints on the process retort, particularly at the point of entry of the process gas from the compressor.
- Check the heating chamber and visually confirm it is incandescent.
- If gas-fired, check the combustion equipment including pilots, spark igniters and flame rods for proper operation. Check burners for proper ignition and combustion characteristics.
- If electrically heated, check the current draw on the heating elements.
- Make sure atmospheric burners or pilots (or both) are protected from drafts.
- Check the burnoff stack to confirm ignition of flammable atmosphere gases.
- Monitor the carbon monoxide (CO) level in the immediate area of the generator (confirm it is <0.01%).
- Check for proper operation of the exhaust hoods and stacks.
- Check for excessive temperature in all areas of the generator.
- Check hand valves, manual dampers, secondary air openings or adjustable bypasses, valve motors, and control valves for smooth action, proper position and adjustment.
- Check all pressure switches for proper pressure settings.
- Check blowers, compressors and pumps for unusual noise or vibration.
- Check belt tension.
- Check for evidence of any damage, from any cause.
- Burnout/regenerate the catalyst as per the recommended manufacturer’s instructions and at the frequency recommended by the manufacturer.
- Remove the air filter from the compressor, clean and/or replace.
- Once the burnout/regeneration is complete, start the gas-making procedure. Check either the gas analysis or the gas dew point.
- Make sure the flame-sensing equipment is in good condition, properly located and free of foreign debris.
- Clean the burner flame rod.
- Check ignition spark electrodes for proper operation and gap.
- Test thermocouples and leadwire for shorts and loose connections. Check protection tubes for sagging, cracks and proper insertion depth.
- Test visible and audible alarm systems for proper functionality.
- Remove the air filter from the compressor, clean and or replace.
- Check ignition spark electrodes for proper operation and gap.
- Remove the floats from the flow-meter glass tube and clean the internal and external surfaces of the flow meter and re-assemble.
- Check the thermocouples for calibration.
- Check the gas pressure of the gas at the compressor.
- Check the instrumentation for calibration. This means temperature as well as gas analysis or dew point.
- Test interlock sequences of all safety equipment. Manually make each interlock fail, noting that related equipment closes or stops as specified by the manufacturer.
- Test pressure-switch settings by checking switch movements against pressure settings and comparing with actual impulse pressure.
- Inspect all electrical devices for proper current and voltage and be sure that all electrical contacts and switches are functioning properly.
- Clean or replace the air blower filter.
- Clean any filters or strainers.
- Inspect burners and pilots.
- Check ignition cables and transformers.
- Test automatic and manual turndown equipment.
- Test pressure-relief valves; clean as necessary.
- Check backpressure regulators; inspect and clean/replace diaphragms.
- Inspect the catalyst and fill (if necessary) to the recommended mark, or replace if necessary.
- Inspect and clean the burners. Check the gas train for functionality.
- Remove the gas delivery line from the generator to the furnace and clean. There may be soot present if there have been any problems with the gas cooler.
- Check all safety solenoids and safety controls.
Semiannual or Annual Checks
- Inspect the retort, refractory, heat exchangers, refrigerators, dryers and other accessories; repair or replace as necessary.
- Lubricate the instrumentation, valve motors, valves, blowers, compressors, pumps and other components.
- Test instrumentation; clean slidewires and electrical components.
- Test flame safeguard units.
- Burn out carbon in the retort(s).
- Check for plugging of hot pipes, tube bundles and jacketed pipes.
The most common problems experienced with endothermic gas generators involve:
- Temperature – Efficiency of the endothermic reaction is thermally dependent. The entire atmosphere must reach a minimum temperature in order for the gas to be completely reacted. If the temperature is not reached no reaction will occur.
- Gas coolers – If the gas coolers are not operating efficiently, sooting will occur due to the carbon-reversal reaction. This typically takes place outside the retort and causes a restriction in the outlet piping and the piping to the furnace, which causes backpressure to occur and a loss of flow. Soot can accumulate in a gas cooler in a matter of minutes, which is why ratio control is so important.
- Catalyst – The catalyst is often a nickel-impregnated refractory chosen for its capability to support itself at the required high operating temperature to withstand catalyst regeneration cycles and to maintain physical stability in the presence of the reaction products (mostly carbon monoxide). If the temperature is not high enough and the gas is not completely reacted, then sooting in the catalyst bed will result. Once the catalyst starts to soot, it becomes ineffective, and the gas composition will drift and produce higher percentages of methane, carbon dioxide and water vapor.
Like any other piece of atmosphere equipment that uses a combustible gas, great care must be taken when starting up, producing gas and operating endothermic gas generators. Returning the unit to service after a shutdown, the generator temperature should be raised slowly to reduce the risk of thermal shock to the refractory and creating stress on the process retort. Under no circumstances should you consider putting gas or any other combustible gas mixture into the retort (or into the furnace) when the temperature is below 760°C (1400°F). Otherwise, a serious explosion will most likely occur and can result in serious injury or death and significant damage to the equipment. All NFPA 86 standards should be followed.
- Herring, Daniel H., Atmosphere Heat Treatment, Volume I, BNP Media, 2014
- Herring, Daniel H., Atmosphere Heat Treatment, Volume II, BNP Media, 2015
- Metals Handbook,, Volume 2: Heat Treating, Cleaning and Finishing, 8th Edition, ASM International, 1964
- Berry, Theodore P., “An Overview of Endothermic Generators,” Super Systems, Technical Data
- “Endogas: Endothermic Atmosphere for Hardening, Brazing, Sintering, and Gas Carburizing,” SECO/WARWICK Corporation, Bulletin AG-301.2
- Herring, Daniel H., and David Pye, “Understanding the Endothermic Gas Generator: Maintenance and Safety Checklists,” HOT TOPICS in Heat Treatment and Metallurgy, January 2004
- Mr. Donald Bowe, Lead Engineer, Air Products & Chemicals Inc. (www.airproducts.com), technical and editorial review, private correspondence
- Mr. James Oakes, Vice President Business Development, Super Systems, Inc. (www.supersystems.com), technical and editorial review, private correspondence
- Mr. Jason Jossart, Atmosphere Engineering Company (www.atmoseng.com), private correspondence
- Mr. Michael Schmidt, SECO/WARWICK Corporation (www.secowarwick.com), private correspondence
- RX® is a registered trademark of Surface Combustion, Inc.