Biogas upgrading – Review of commercial technologies - SGC

SGC Rapport 2013:270
Figure 37 The Lidköping Biogas plant. In front the filling station is shown, and behind
that the liquefaction unit is situated.
The technology for methane liquefaction is based on a reverse nitrogen Brayton
cycle. This process extracts heat from methane, liquefying it in a plate-fin heat exchanger.
The reverse nitrogen Brayton cycle is shown in Figure 38. The process
can be divided into the following steps, which are also shown in Figure 38 .
1. Nitrogen is compressed inside a centrifugal compressor.
2. Compressed nitrogen is then over-pressurised in a turbo-booster before
entering the heat exchanger to be cooled down
3. The nitrogen stream is expanded, generating the cold power for the system
at a temperature of 110 K (-163 °C),
4. The low pressure stream flows back in the plate heat exchanger and cools
down both methane and the high pressure nitrogen stream,
5. Biomethane is liquefied at 110 K and is released at a pressure of
3.5 bar(a). The outlet pressure can be adjusted depending on the customer
requirements.
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