In Part 1, the author underscored the importance of understanding the changes in gas composition through three steps of its production: first, the production in the combustion chamber; second, the cool down of gas to bring the Exothermic gas (Exo gas) to below the ambient temperature; and third, the introduction of the gas to the heat treat furnace. Read Part 1, published in Heat Treat Today’s August 2023 Automotive Heat Treat print edition, to understand what Exo gas is and to learn about the composition of gas in the first step.
Founder and President TAT Technologies LLC Source: TAT
As the author demonstrated in Part 1, Exo gas composition changes in its chemistry for heat treatment; this first step is how the gas composition changes when it is produced in the combustion chamber. The composition of reaction products, temperature, Exothermic energy released, various ratios, and final dew point are all factors that need to be considered to protect metal parts that will be heat treated in the resulting atmosphere.
Now, we’ll turn to Steps 2 and 3.
Step 2: Composition of Exo Gas after Exiting the Reaction Chamber Being Cooled Down
The two examples that follow demonstrate how lean and rich Exo under equilibrium conditions change as they are cooled from peak equilibrium temperature in the combustion chamber down to different lower temperatures (Table B). This cool down brings the Exo down to below ambient temperatures to avoid water condensation.
Example 1: Lean Exo Gas with a 9:1 Air to CH₄ Ratio
The first column highlighted in blue shows the composition of the lean Exo gas as generated in the reaction chamber with an air to natural gas ratio of 9:1. The peak temperature as generated in the combustion chamber is 3721°F. The next four columns show how the composition changes when the lean Exo gas is slowly cooled from 3721°F to 2000°F, 1500°F, 1000°F, and 500°F under equilibrium condition. The following key changes take place as the temperature of the lean Exo is lowered from the peak temperature to 500°F:
- Hydrogen volume almost triples from 0.67% to 1.97%.
- H₂O volume decreases slightly from 19.1% to 17.5%, but still is very high at all temperatures.
- Oxidation-reduction potential (ORP) changes as the H₂ to H₂O ratio increases from 0.035 to 0.111. At all temperatures, it is very low.
- CO and the CO to CO₂ ratio drop in a big way, making lean Exo from being decarburizing at higher temperatures to being highly decarburizing at lower temperatures.
- The percentage of N₂ remains at 70.34 at all temperatures.
- There is no C (carbon, i.e., soot) or residual CH₄ at all temperatures.
- For all practical purposes, at an air to natural gas ratio of 9:1, the Exo gas as generated is predominantly an N₂ and H₂ (steam) atmosphere with some CO₂ and small amounts of H₂ and CO.
Example 2: Rich Exo Gas with a 7:1 Air to CH₄
The column under ratio of seven is highlighted as red to show the composition of the rich Exo gas as generated in the reaction chamber with an air to CH₄ ratio of seven. The peak temperature is 3182°F — significantly lower than that for lean Exo. The next four columns show how the composition changes when the rich Exo gas is slowly cooled from 3182°F to 2000°F, 1500°F, 1000°F, and 500°F. The following key changes take place as temperature of the rich Exo is lowered from the peak temperature to 500°F:
- Hydrogen volume almost doubles from 5.58% at peak temperature to 9.91% at 1000°F, and then it drops to 5.70% at 500°F. The overall volume of H₂ in rich Exo is significantly higher than in lean Exo.
- H₂O volume decreases slightly from 17.9% to 15.1%, but it is still very high at all temperatures.
- Oxidation-reduction potential (ORP) changes as the H₂ to H₂O ratio increases from 0.312 at peak temperature to 0.737 at 1000°F before decreasing to 0.377 at 500°F. Overall, ORP in rich Exo is significantly higher than that in lean Exo.
- CO and the CO to CO₂ ratio drop in a big way, making it mildly decarburizing to more decarburizing
- The percentage of N₂ remains at 65– 67%, which is lower than lean Exo.
- There is no C (carbon, i.e., soot) at any temperature. However, there is residual CH₄ at 1000°F and lower. This increases rapidly when cooled slowly below 1000°F.
- For all practical purposes, the rich Exo gas (at air to natural gas ratio of 7:1) generated is still predominantly a H₂
and H₂O (steam) atmosphere, but with more H₂; hence, it has somewhat higher oxidation-reduction potential (ORP) than lean Exo and a bit higher CO to CO₂ ratio (less decarburizing than lean Exo).
In summary, rich Exo as generated in the combustion chamber differs from lean Exo as follows:
- It has a little less N₂ % as compared to lean Exo.
- It has significantly more H₂ , but a little less H₂O than lean Exo. As such, it has a significantly higher H₂ to H₂O ratio (ORP).
- It is decarburizing, but less than lean Exo.
- It has residual CH₄ at temperatures below 1000°F. Therefore, it must be cooled very quickly to suppress the reaction of developing too much residual CH₄.
Discussion
Let us take the example of rich Exo (an air to natural gas of 7:1) exiting from the reaction chamber in Table B (see column highlighted in red). The total volume is 853.3 SCFH and has H₂O at 152.4 SCFH (17.9% by volume). This is equivalent to dew point of 137°F. Its H₂ content is 47.6 SCFH (5.58% by volume). And the H₂ to H₂O ratio is 0.312.
If this were quenched to close to ambient temperature “instantly,” this composition would be “frozen,” except most of the H₂O vapor will become water. Let us assume the Exo gas was instantly quenched to 80°F (3.6% by volume after condensed water is removed). Rough calculation shows that the final total volume of H₂O vapor has to be reduced from 152.4 SCFH to about 26.0 SCFH in order to meet the 80°F dew point goal. This means 152.4 – 26.0 = 126.4 SCFH of H₂O vapor got condensed to water.
Now the total volume of Exo gas after cooling down to 80°F= 853.35 – 126.4 = 726.95 SCFH, or almost 15% reduction in volume of Exo gas as compared to what was generated in the reaction chamber.
Of course, the composition of Exo gas will not be the same as calculated above. The exact composition after being cooled down depends upon the following:
a. Cooling rate of the reaction products from the peak temperature in the reaction chamber to some intermediate temperature, typically around 1500°F.
b. Cooling rate of the gas from the intermediate temperature to the final (lowest) temperature via water heat exchangers — typically 10–20°F below ambient temperature unless a chiller or dryer is installed on the system.
Depending upon the overall design of the generator, especially how Exo gas coming out of the combustion chamber is cooled and maintained during the period of its use, the expected Exo gas composition should be in the range of the light red columns in Table B — where temperatures are between 1500°F to 1000°F — however:
- Total volume closer to 727 SCFH (since a major portion of H₂O was condensed out)
- N₂ between 74–77%
- Dew point between 80–90°F
- CH₄. between 0.1–0.5%
- H₂ percentage between 7–9%
Step 3: Composition of Exo Gas after Being Introduced into the Heat Treat Furnace
The cooled down Exo gas will once again change its composition depending upon the temperature inside the furnace where parts are being thermally processed.
As an illustration, let us assume the following composition of the rich Exo gas (with a 7:1 air to natural gas ratio) at ambient temperature just before it enters the furnace:
- Total volume: 727 SCFH
- H₂: 8% (58.16 SCFH)
- Dew Point 86°F or 4.37% (31.77 SCFH)
- CO: 6% (43.62 SCFH)
- CO₂: 6% (43.62 SCFH)
- CH₄ : 0.4% (2.91 SFH)
- Balance N₂ (%)
- 75.23% (546.92 SCFH)
Table C shows how the composition changes once it reaches the high heat section of the furnace where parts are being thermally treated. The column highlighted blue shows the composition of Exo gas as it is about to enter while it is still at the ambient temperature. The next three columns show the composition of the Exo gas in the high heat section of furnaces operating at three different temperatures depending upon the heat treat application — 1100°F like annealing of copper, 1500°F like annealing of steel tubes, and 2000°F like copper brazing of steel products. The H₂ to H₂O ratio decreases as temperature increases.
Other general comments on Exo generators:
- Generally, they are horizontal.
- Size ranges from 1,000 to 60,000 SCFH.
- Rich Exo generators use Ni as a catalyst in the reaction chamber. Lean Exo does not.
- Lean Exo generators typically operate at a 9:1 air to natural gas ratio. There is no carbon/soot buildup.
- Rich Exo generators typically operate at a 7:1 air to natural gas ratio. Below about 6.8 and lower ratios, soot/carbon deposits start appearing that require carbon burnout as part of the maintenance procedure.
Conclusions
A walkthrough of the entire cycle of gas production to cool down to use in the high heat section of the furnace clearly shows that as temperature changes, so does the Exo gas composition for any air to natural gas ratio.
Having a well-controlled composition of Exo gas requires the following:
- Well-controlled composition of the natural gas used
- Air supply with controlled dew point
- Highly accurate air and natural gas mixing system
- Highly controlled and maintained cooling system
- A reliable ORP analyzer or the H₂ to H₂O ratio analyzer as part of the Exo gas delivery system.
Protecting metallic workpieces is paramount in heat treating, and in order to do this, the atmosphere created by Exothermic gas must be understood, both in the cool down phase and within the heat treat furnace. For further understanding of the good progress made in the improvement of Exo generators, see Dan Herring’s work in the reference section below.
References
Herring, Dan. “Exothermic Gas Generators: Forgotten Technology?” Industrial Heating, 2018, https:// digital.bnpmedia.com/publication/?m=11623&i=53 4828&p=121&ver=html5.
Morris, Art. “Exothermic Atmospheres.” Industrial Heating (June 10, 2023), https:// www.industrialheating.com/articles/91142-Exothermic-atmospherees.
About the Author
Harb Nayar is the founder and president of TAT Technologies LLC. Harb is both an inquisitive learner and dynamic entrepreneur who will share his current interests in the powder metal industry and what he anticipates for the future of the industry, especially where it bisects with heat treating.
For more information: Contact Harb at harb.nayar@tat-tech.com or visit www.tat-tech.com
