Natural gas odorization

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I. Why is natural gas odorized? Natural gas is colorless and odorless, and its main component, methane, is also colorless and odorless. However, methane is a flammable gas; when its concentration in air reaches a certain range (4.9%–16%), it can explode upon contact with an open flame. What’s more dangerous is that when the methane concentration in the air exceeds 16%, it can burn quietly, although it will not explode. Therefore, in order to ensure that users can detect a gas leak in a timely manner and take action to use the gas safely, natural gas is generally odorized. II. A set of relevant data on common odorants used in natural gas
| Parameter | Methyl mercaptan | Ethyl mercaptan | Tetrahydrothiophene |
|-----------|------------------|------------------|---------------------|
| Boiling point | 6.2°C | 36.2°C | 119°C |
| Odor threshold | 0.000067 ppm | 0.0000087 ppm | 0.00062 ppm |
| Explosive limit | 3.9%–21.8% | 2.8%–18.0% | 1.1%–12.1% |
| Difference | Not very large | | |
| Density | 0.87 g/cm³ | 0.84 g/cm³ | 1.00 g/cm³ |
| Solubility | No specific data | No specific data | No specific data |
| State at room temperature | Gas phase | | |
| Actual dosage | 5–8 mg/m³ | 4–6 mg/m³ | 18–25 mg/m³ |

For ethyl mercaptan, the values are based on CJJT-148-2010.

III. Analysis of the above data
1. According to the “Technical Specifications for Odorization of Urban Gas” CJJT-148-2010, the dosage should be higher in summer. In other words, the lower the boiling point, the better the vaporization effect. The vaporization temperature of liquefied natural gas approaches room temperature quite quickly, while methyl mercaptan (with a boiling point of -6.2°C) is in gas form at room temperature. Gases are easier to disperse evenly, resulting in a homogeneous gas phase and thus an optimal odorization effect; therefore, methyl mercaptan has the best performance. 2. Among substances with the same olfactory threshold, ethanethiol is the one for which the smell can be detected at the lowest dose. Roughly speaking, the olfactory threshold for ethanethiol is 1/10 that of methanethiol, while the olfactory threshold for methanethiol is 1/10 that of thiophene ； Based on actual olfactory thresholds, ethanethiol is theoretically the most effective. 3. In terms of the explosion limit, the difference is not very large. 4. Solubility: Based on the principle of like dissolves like, they are mutually soluble; solubility is better within the same phase. In terms of the actual amount to be added, the less that is added, the higher the cost-effectiveness ； Methanethiol is a gas at room temperature, offering good dispersion properties and a low olfactory threshold; as a result, natural gas supplemented with methanethiol can achieve a warning effect in very small quantities. IV. Relevant Data 1. Other **addition details: Serial Number Country Gas Type Amount of odorant added Remarks 1 China Natural gas **Thiophene 18–25 mg/m³ Thiol 5–8 mg/m³ 2 France Natural gas **Thiophene 25 mg/m³ Thiol 5 mg/m³ 3 United Kingdom Natural gas **Thiophene 16 mg/m³ Thiol 6 mg/m³ 4 Germany Natural gas **Thiophene 15–18 mg/m³ Thiol 5–8 mg/m³ 5 Italy Natural gas **Thiophene 20–40 mg/m³ Thiol 4–8 mg/m³ 2. **Standards for sulfur content in natural gas:** The Natural Gas Standards (GB17820 2018) specify that the total sulfur content in natural gas should be ≤20 mg/m³ for Category 1 gas and ≤100 mg/m³ for Category 2 gas. 3. The strictest European standard, EN16726-2016: total sulfur content ≤ 20 mg/m3 (without odorizers), and total sulfur content ≤ 30 mg/m3 (with odorizers). V. In summary, methanethiol-based natural gas odorants exhibit relatively good performance: they have a low boiling point which facilitates vaporization, they provide good dispersion, require less dosage, offer effective warning capabilities, and are relatively inexpensive. According to national standards (GB17820 2018) and European standards (EN16726-2016), the dosage used has an impact on the total sulfur content but does not exceed the allowed limits. Ethanethiol is recognized as the most pungent gas, with the lowest odor threshold and a relatively low boiling point; it is also a viable option ； Personally, I think methanethiol is the most suitable in terms of usage purpose, effectiveness, and dosage.