The odorization of biomethane is the process by which odorizing compounds are added to biomethane to ensure safety during its use and transportation. Since biomethane, like natural methane, is odorless, odorization allows for the easy detection of potential leaks through the sense of smell. However, the presence of chemical compounds that may interact with the odorant poses a challenge, requiring advanced technical solutions and strict process management.
The chemistry of interfering compounds in the odorization of biomethane was the central theme of a presentation by Marco Pontello, a technical specialist from LOD, during Ecomondo 2024. The presentation, titled "Odorization of biomethane: the chemistry of interfering compounds", addressed the new UNI TS 11537 standards, the impact of interfering compounds on biomethane’s odorization, and the importance of olfactory tests to ensure safety and compliance within the distribution network.
We participated in this initiative, organized by CIB - Consorzio Italiano Biogas (as an associated laboratory), as part of a conference where various industry stakeholders explored multiple aspects of biogas quality, from the pre-upgrading phase to compression.
The role of LOD in odorization verification
LOD, the Dynamic Olfactometry Laboratory, utilizes a rhino-analytical chamber compliant with the UNI 7133-3:2023 standard to verify the proper level of odorization/odorability of combustible gases, including biomethane. In addition, LOD conducts experimental tests on next-generation fuels, which are essential for the energy transition. During the event, LOD shared this expertise with other industry stakeholders.
Marco Pontello at Ecomondo with his presentation “Odorization of biomethane: the chemistry of interfering compounds”
Below is the transcript of Marco Pontello's presentation at this event:
As in the vast majority of countries worldwide, Italy requires the odorization of combustible gas intended for domestic use. The primary compounds currently used for the odorization of combustible gas are Tetrahydrothiophene (THT) and a mixture of mercaptans (TBM, IPM, NPM).
Regulatory updates on the obligation to odorize combustible gas for domestic use: UNI TS 11537
Let’s talk about regulatory references: as many of you may know, in January 2024, the new version of UNI TS 11537 was published. This standard provides the reference guidelines for injecting biomethane into networks.
This new revision incorporates the content previously included in Technical Report 11722, which described the analyses to be performed and, most importantly, how to conduct them to ensure the quality and safety of biomethane being injected into the network.
Verification requirements and chemical composition
What does this revision tell us in detail? What verifications need to be performed? A key element concerns the extended chemical-physical characterization of the gas, which is essential to determine the composition of the matrix, both at a macro-compositional level and in terms of microcomponents. These microcomponents, as we will see, play a crucial role in odorization.
Additionally, the standard requires the drafting of gas safety data sheets and includes our specific contribution: the verification of odorability.
The standard builds upon the content previously outlined in Technical Report 11722, noting that various interfering compounds may be present in biomethane. Among these, terpenes represent the most significant family of compounds due to their impact, but other substances, such as methyl ethyl ketone and cumene, can also have a substantial influence.
Interfering compounds: a challenge to manage
There are no strict limits for these two compounds; however, there is a general reference to a maximum concentration of 52 mg/Sm³, calculated in limonene equivalents, which should ensure proper odorization. Since this reference alone is not sufficient to guarantee that biomethane is effectively odorizable, the standard emphasizes the need to conduct olfactory tests to verify compliance.
LOD's expertise on interfering compounds
Let’s talk about masking (or interfering) compounds. We know that, on a macro-compositional level, biomethane is essentially comparable to natural gas. However, at the microcomponent level, a range of compounds is present due to the matrices from which the gas is produced. These compounds interact with traditional sulfur-based odorizing agents, reducing their effectiveness or altering their olfactory properties.
To proceed with testing, the relevant technical standards recommend performing and evaluating chemical-physical screenings on the produced gas beforehand. Accordingly, our laboratory maintains a historical archive of chemical analyses conducted on biomethane injected into the grid in recent years.
Recently, we have started leveraging this extensive data to assess gas quality. In general, the test evaluates biomethane compliance for authorization in two aspects.
First, let’s briefly describe the test process: biomethane is mixed with odorants at concentrations specified by the standards, across different sets of trials at varying concentrations. The purpose is twofold: to verify that the odor intensity of the gas-odorant mixture in air meets the requirements of the technical standards, and to ensure that the odor type remains unchanged. Naturally, if the odorant loses its characteristic smell, it can no longer fulfill its intended function.
Issues identified: terpenes and other interfering compounds
In more than half of the samples analyzed by LOD, the presence of high terpene concentrations resulted in non-compliant outcomes, both in terms of odor intensity and odor type. Other sulfur compounds or microcomponents may also contribute to these non-conformities.
What is our experience?
What we have observed is that biomethane can occasionally be non-compliant in two specific areas, which we will discuss shortly. While we cannot clearly determine the exact cause that renders a given biomethane non-compliant, we can make some observations, especially in light of the chemical data provided by the client.
Odor Intensity
A non-compliant result in odor intensity is observed when the odorant is perceived with much lower intensity. In more than half of the non-compliant biomethane samples we have evaluated so far, we found significantly higher concentrations of terpenes among the chemical components.
We have observed that biomethane with substantial traces of terpenes presents a series of challenges during grid injection. It is important to note that odorability analyses are mandatory and the responsibility of the biomethane producer, both to proceed with grid injection and to maintain authorization for grid use.
Odor Type
In cases where biomethane is non-compliant with the odor type parameter—meaning it alters the characteristic odor of the odorant injected into the grid—the situation is even more extreme. Based on our observations, this occurs in biomethane with high concentrations of various components that are typically not present in “purer” biomethane. We hypothesize that this may be due, among other causes, to the presence of high concentrations of sulfur compounds.
Finally, it is worth noting that while the initial list of interfering compounds we mentioned earlier is included in the standards, the standards themselves acknowledge that this list is likely not exhaustive.
In conclusion, the UNI TS 11537 standard lists some interfering compounds but acknowledges that the list is not exhaustive. It is essential to expand research into other potential interferents to improve the assessment of biomethane quality and ensure optimal odorization.
LOD will continue to leverage historical data and advanced analyses to support biomethane producers, contributing to the safety and efficiency of the odorization process in the transition toward more sustainable energy.
