Biogas and Digester Gas Upgrading

Biogas from digesters (waste water treatment plants and other digesters) typically have a methane (CH₄) content of about 50-60% with the remaining gas being mostly carbon dioxide (CO₂) and saturated moisture.  Hydrogen sulfide (H₂S), Volatile Organic Chemicals (VOC) and siloxanes generally amount to less than 0.5% of the biogas.

The common industry approach to upgrading the gas, pictured below, consists of using the following process flow:

• Flow into H₂S removal skid using a non-regenerable fixed bed media which is replaced on a regular basis (usually 3-4 months)
• From the H₂S removal skid, flow into a Refrigerated Dryer skid using a cooling process to remove the bulk of the water
• Compress the resulting gas to about 200 PSIG 
• Feed the compressed gas into a Pressure Swing Adsorption (PSA) dryer system to remove the water down to pipeline standards
• Using a VOC Trap, condense the Volatile Organic Compounds out of the flow to protect the downstream membranes
• If siloxanes are present in the feed gas, the gas flows to a non-regenerable carbon-based or alumina-based adsorbent bed vessel to remove siloxanes, which is also regularly replaced 
• Multiple stages of membranes and associated compression to remove the CO₂

This results in a complicated system, forcing the owner/operator to understand and maintain multiple technologies.  The Guild Associates approach is to use the Molecular Gate^TM advantage in a single step process.  The process flow is much simpler:

• Compress the gas to 100 psig
• Remove the bulk of the moisture using a cooling process, reheating the resulting gas with waste heat from the compressor
• Remove moisture, CO₂, siloxanes and VOCs in a Molecular Gate Pressure Swing Adsorption system
• Deliver the resulting product gas to the local grid at 90 psig.  
  
  

This simple-to-operate technology has been installed at waste water treatment plants and commercial digesters such as hog farms and rendering plants.  The technology offers the benefits of:

• Single step removal of impurities
• All media is regenerated – no need for replacement
• H₂S completely removed from any level up to 7000 PPM
• Siloxanes removed to non-detect levels
• Dehydration to CNG requirements without a separate dryer required
• No chillers required
• Dry system operation, with no waste product other than condensed water from compression

• Automatic start-up and shutdown at a push of a button
• Automatic product purity control without operator attention