Cascade upgrading

Cascade upgrading is the simple, low-cost route. Anaerobic supernatant, the liquid from the digester, is pumped over a series of waterfalls. Carbon dioxide leaves the liquid as it falls and is allowed to escape. The gas that remains is richer in methane. The methane share rises to 65 percent.

There is no gas compressor and no solvent circuit. The energy in the step is the pump that lifts the supernatant. Sixty-five percent methane is not a pipeline specification and it is not a feed for liquefaction. It is the right product where that share is enough, or a first enrichment ahead of a later process.

The other routes are membrane separation, an amine scrubber, and hydrogen biomethanisation. All four are listed on biomethane upgrading.