Did you know that natural gas is odorless? I didn’t… I always find it has a penetrating sulfur scent. Well, it appears that this penetrating scent is added to the natural gas on purpose. Let’s see why this is.
As natural gas is combustible and odorless by nature, the government requires some safety measures here. Many countries have established safety regulations how to handle natural gas and which gas needs odorisation. This is mostly done by the Health and Safety department (HSE) of the local government.
What about natural gas odorisation?
Today’s question is about this subject. Why does gas smell when it is odorless by nature? This is the point where gas odorisation comes in.
Odorisation of natural gas is done to act 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 explosive.
As shown in the picture, the LEL (lower explosive limit) and UEL (upper explosive limit) are crucial here. If the concentration of the combustible substance present in the air is too low (< LEL), then no combustion will occur. If the mixture is too rich (> UEL), there is a huge amount of gas in the air and only partial combustion will occur. Gases become dangerous in between the LEL and UEL. Therefore, it is most important that nearby people to smell the gas in time, before the concentration is too high and it exceeds the LEL.
As a result, it is stated in the safety regulations that natural gas must be detectable at a concentration level of 20% of the LEL and this is done by odorisation. It’s important to note that the gas’s odor itself is not dangerous to people’s health.
When is an odor added to gas?
The answer depends on the type—distribution or transmission— of the gas line. Distribution lines are local natural gas utility systems that include gas mains and service lines, such as the commercial gas used at domestic environments. All these distribution lines need to be odorised. For the transmission lines it is stated in the regulations when to odorise it.
There are many different odorants available, such as tetrahydrothiophene (THT) and Mercaptan. Selecting the odorant depends on the properties of the gas to be odorised, pipeline layout, ambient conditions, etc. Tetrahydrothiophene or THT is a well-known odor. THT is under ambient conditions a colourless volatile liquid with an unpleasant smell.
Controlled supply of THT using mass flow controllers
Bronkhorst had the pleasure of working with a Dutch customer to develop a solution to add THT to their biogas. Biogas was generated from anaerobic decomposition of organic matter and upgraded to natural gas quality to inject into the Dutch natural gas main. As commercial natural gas in the Netherlands has to contain at least 18 mg of THT per cubic meter, the process of adding this to the commercial gas had to be done really accurately.
The traditional approach to add THT is using a 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 be mixed well with the gas and might have the wrong concentration. A homogeneous injection of THT is therefore much better. Besides this, THT is relatively expensive, so an accurate injection eliminates wasted capital.
A better solution in this case would be using a combination of a pump with a Coriolis mass flow controller, in our case the mini CORI-FLOW™ series mass flow controllers. The Coriolis instruments make it possible to dose both continuously as well as accurately.
An area’s classification must also be taken into account. As gases in principle are explosive, it is very common for the environment around gases to be classified as a hazardous area. Most common classifications (in Europe) are marked as ATEX zone 1 or zone 2, so engineers should select gases accordingly.
For solutions such as THT odorisation processes, Bronkhorst can offer both ATEX/IECEx zone 1 and zone 2 solutions. Our mini CORI-FLOW Ex d mass flow meter, for zone 1 applications, is a collaboration with one of world’s leading manufacturers in explosion protection, Electromach member of the R.STAHL Technology Group.