Controlled supply of odorant to natural gas

For a customer in the renewable natural gas industry, Bronkhorst supplied flow instruments for adding odorant to Biomethane in a continuous and controlled way.

Renewable natural gas

Biomass waste, such as agricultural, municipal or food waste, is fermented by using Micro-organisms. In this process biomass waste is converted into biogas, a blend of gases containing mainly methane gas and carbon dioxide. After a purification process removing carbon dioxide and impurities you get ‘biomethane’. Biomethane can be used as a substitution for natural gas, a renewable natural gas. Biomethane is suitable for domestic heating and cooking, but only when it is odorized.

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Odorization of biogas with flow instruments

Odorization of biomethane

Odorization of biomethane is mandatory when it comes to grid injection because it is odourless by nature. The goal is to make a gas leak detectable for human smell. After odorization the gas smells like rotten eggs. Overdosing means that the smell will be much too strong which leads to higher costs and even false alarms. Underdosing can lead to undetected danger and may even lead to human casualties.

Odorization with THT

In most European countries a highly concentrated odorant like mercaptan or THT (tetrahydrothiophene) is used for odorization. In this article we focus on THT. The required concentration of THT is typically 10-30 milligram per cubic meter gas. (This can vary per country). This is a very small amount of odorant which needs to be added into a fluctuating gas flow speed.

Traditionally, THT is dosed using shot dosing. Meaning a controller will switch a pump or valve on-and off. Shot dosing always results in a pulsating type of dosing. Each shot of odorant is a relatively large amount of liquid, especially in low flow speed conditions.

The actual injection rate over time can be influenced by wear and tear of the pump and system pressure variations. In combination with the lack of real feedback or adaptive control, this may lead to under-and overdosing over periods of time and inaccurate dosing of odorant.

Flow instruments for odorization

The key component of the dosing solution used to add the odorant is a Coriolis mass flow controller (mini CORI-FLOW series). This device takes care of the liquid delivery within the injection system.

A mass flow meter measures the main biogas flow and reports the actual value to the mass flow controller via a PLC. The Coriolis flow controller is configured “to ratio”: following the master mass flow rate. It delivers a dosing value of 18 mg of THT per (normal) cubic meter gas. The flow rate will automatically adapt to systematic fluctuations based on mass-based control.

By customer configuration, a THT storage tank is pressurised by means of nitrogen gas. In other cases, pumps may be used also directly controlled by the Coriolis flow controller. The pressure upstream of the flow controller is 10 to 13 bar. The downstream (injection) pressure of the system is typically up to 8 bar.

Flow scheme THT dosing in odorization process

Mass flow control

Using the liquid mass flow controller, THT can be dosed and measured very accurately, within a range of 1-2% on actual dosing rate.

The system uses an inherent feedback loop because the dosing is based on true mass measurement. The amount of actual dosed odorant and corresponding gas flow is automatically monitored and reported to the customers “gate keeper” (allowing grid injection based on correct properties). Even considerable systematic wear and tear (e.g., on pumps or filter clogging) are automatically corrected without influence on the accuracy of the actual dosing rate.

The fluent continuous dosing results in a homogeneous THT/biogas mixture. The effective evaporation rate, in low- and high flow condition, can as such be guaranteed without the need for additional evaporators.

Important topics

Actual measurement of liquid
Accurate and controlled dosing of odorant
Adding odorant continuously

Customer review

"We are much more confident to operate closer to the minimal required odorization rate now, without risking underdosing using this accurate flow controller. The odorant costs haven been cut down, and the lifetime of the odorant tank is much longer. As it is possible now to predict, and actual measure, the total used amount of odorant, we now can replace the tank in time."

Odorization of Biogas with flow instruments