GAS RNG Systems’ NMOC and siloxane removal system combines the removal efficiency of activated carbon technologies with the low operating cost of VSA systems.
THERMAL SWING ADSORPTION (TSA) PROCESS
TSA systems are used to treat high loads of siloxanes and VOCs in biogas and landfill gas. Once the media reaches saturation, the thermal desorption process assures that the adsorbent will have an extended usable life, minimizing maintenance and replacement costs.
In a TSA system, gas passes through the adsorbent where the contaminants, siloxanes and VOCs, are adsorbed. During that time, the two other towers are off-line, one being heated, the other one cooled. Once the first tower approaches saturation, it is taken off-line and switched to regeneration mode, while the tower that finished regeneration/cooling mode is brought back on-line.
Regeneration gas (usually vent gas from the CO2 removal unit) is first passed through the tower which finished its regeneration/heating cycle to cool the media bed back to the operation temperature, the gas exiting that tower is further heated in an electric heater then passed in a counter-current mode through the quasi saturated tower, slowly heating the adsorbent bed, and desorbing siloxanes and VOCs. The vent gas is then directed to a thermal oxidizer to be incinerated. Once the complete adsorbent bed has been desorbed the hot bed goes through the cooling stage. At that time, the third tower which has completed its cooling stage is brought back on line and the adsorption, desorption, cooling sequence continues providing a continuous VOCs and siloxanes removal from the biogas.
In cases where regeneration gas is not available, a three tower TSA arrangement is provided where one tower acts as a regeneration gas generator using a split stream of the raw feed gas. In this case, the discharge of VOCs and siloxanes is in the form of a condensed liquid rather than a gas. The adsorption, desorption, cooling sequence is the same as for the TSA with regeneration gas.
In cases of small gas flow and low levels of VOCs and siloxanes, a two tower TSA system can be used. However, as flow and concentration of contaminants becomes higher, the size of a two tower system becomes too large to be practical and a three tower system is implemented.