Did you know that natural gas is naturally odorless? Many will notice that natural gas typically has a penetrating scent. Well, it appears that this penetrating sulfur scent is added to the natural gas on purpose. Odorizing gas is important step in natural gas processing. Let’s discover why this may be.
As natural gas is combustible and odorless by nature, the government enforces certain safety measures. Many countries have similar but different safety regulations regarding how to handle natural gas and biomethane grid-injections. In all cases, gas will need to be odorized. Generally, this the local government’s Health and Safety department (HSE) will regulate the standards for gas safety.
So why does natural gas smell when it is odorless by nature? This is the point where gas odorization comes in. Gas odorization consists of dosing mercaptans, tetrahydrothiophene (THT), or sulfur-free odorants into natural gas with the use of mass flow controllers.
Odorizing natural gas acts as a “warning agent” in case of leakage. The idea is that people can smell the gas prematurely if it is present. Because, if there is too much gas present it can be deadly and/or explosive.
In the picture, you can see the LEL (Lower Explosive Limit) and UEL (Upper Explosive Limit) are critical factors here. If the concentration of the combustible substance present in the air is too low (<LEL), then no combustion will occur. It the mixture is too rich (>UEL), then there is a huge amount of gas in the air and only partial combustion will occur. Gases become dangerous in between the lower and the upper explosive limits. Therefore, it’s important for people in the surrounding areas to be able to smell the gas in time – before concentration is too high and it exceeds the LEL.
As a result, safety regulations dictate that natural gas must be detectable at a concentration level of 20% of the LEL. As a result, gas is odorized to allow detection by human senses. Needless to say, the odorant in the gas is not dangerous to people’s health.
This depends on the type of gas line. There are both distribution lines and transmission lines. Distribution lines are local natural gas utility systems that include gas mains and service lines. For example, commercial gas in domestic environments is a distribution line. All distribution lines need to be odorized. For the transmission lines, various regulations dictate when to odorize the gas.
When odorizing gas, there are many different odorants available such as Tetrahydrothiophene (THT) or a choice of mercaptan. Selecting the odorant depends on the properties of the gas to be odorized, pipeline layout, ambient conditions etc. Tetrahydrothiophene or THT is a well-known odorant. THT, under ambient conditions, is a colorless, volatile liquid with an unpleasant smell.
As a result, Bronkhorst has the pleasure of helping to develop a solution for adding THT to biogas. The biogas generates from anaerobic decomposition of organic matter and must undergo upgrading before injection into the natural gas main. Since commercial natural gas must contain a specific composition of THT per cubic meter of gas, the process of adding this to commercial gas requires high accuracy.
The traditional approach to dose THT into natural gas is by using a dosing pump with a fixed stroke volume. However, low gas flow rates using a pump for batch-wise injection may lead to liquid THT remaining in the gas lines. THT may not mix well with the gas, thus resulting in the wrong concentration. A homogeneous injection of THT therefore offers better results. Besides this, odorants are relatively expensive, which also makes a more economic and accurate injection advantageous.
A better solution is to use a combination of a dosing pump with a mass flow controller. In many cases, a Coriolis based flow instrumentation would be useful, such as a mini CORI-FLOW mass flow controller. Due to an integrated PID controller, the mass flow controller can control the pump directly. This pump and flow meter combination makes it possible to dose both continuously as well as accurately.
Read more about this application in our application note “Controlled supply of odorant to natural gas.”
When odorizing gas, an important consideration is the classification of the area for use. As combustion gases are explosive by nature, it is very common to classify the environment as a hazardous area. Most common classifications (in Europe) are ATEX zone 1 or zone 2. Moreover, it’s critical to select the right material to ensure safety.
For solutions such as THT odorization processes, Bronkhorst can offer both ATEX/IECEx zone 1 and zone 2 solutions.
Liquid Odorization Delivery Sets are available upon request for odorizing gas flows from 10 to more than 8,000 m3 gas flows feeding from 5 up to 50 mg/m3 odorant and are able to cope with virtual any national regulation. Check out our Coriolis mass flow meters for applications in hazardous areas and our thermal instruments for ATEX zone 1.
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