Controlled supply of odorant to natural gas

You might think that natural gas has a typical pungent smell by itself. But nothing could be further from the truth, as natural gas is virtually odourless. The typical smell originates from THT, which is a compound that has been added artificially to commercial natural gas. Its aim is to act as a ‘warning agent’ in case of a leakage of this highly flammable gas during transport or use.

THT, short for tetrahydrothiophene, is under ambient conditions a colourless volatile liquid with an unpleasant smell. Commercial natural gas in the Netherlands has to contain at least 18 mg of THT per cubic meter gas. Natuurgas Overijssel B.V. generates biogas from anaerobic decomposition of organic matter, which is upgraded to natural gas quality for injection into the Dutch natural gas mains. Natuurgas Overijssel requested Bronkhorst for a solution to supply THT to their biogas in a controlled way.

Process solution

The heart of the new process solution is a Bronkhorst Coriolis mass flow controller (M12 mini CORI-FLOW series) that injects a small but continuous liquid THT flow into the biogas flow. A master mass flow meter measures the biogas flow and sends a signal to the mass flow controller via a PLC. The flow controller - acting as a slave - has been set to a dosing value of typically 18 mg of THT per cubic meter gas. Hence, this mass flow controller continuously injects the correct amount of THT, based on the actual biogas flow.

In this configuration, a 40 liter THT storage tank is pressurised by means of nitrogen gas. A pulsation damper is incorporated to remove pressure surges and to guarantee a stable control. This results in a precharge pressure upstream of the flow controller of 10 to 13 bars. Downstream of the device, in the biogas flow, the pressure is typically 8 bars. The full scale of this flow controller is occasionally set to maximum dosing value of 40 mg/m³ .

Using this solution, THT can be dosed very accurately, within a bandwith of 1-2%, which is a major improvement compared with the traditional batch-wise method. Furthermore, the continuous dosing results in a homogeneous THT/biogasmixture, which is a more efficient way of dealing with the amount of odorant as overdosing is avoided.

Another important aspect is the operational reliability of this solution - the ability to supply to the natural gas mains. Prior to entering the mains, the biogas composition is analysed by GC. If the biogas contains not enough THT, the grid operator will shut off the supplier from the natural gas mains. By default, the monitoring of the dosing occurs by PLC, but it can also be done by means of the mini CORI-FLOW itself. In this solution, it is exactly known how much THT has been dosed - and an alarm will be given when no odorant is detected. Moreover, a prediction can be made when an empty THT storage tank has to be replaced.