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Carbon Membranes for Biogas Upgrading

In general, the composition and quality of the biogas varies a lot, and is obviously highly dependent on the source or substrate on which the bacteria are fed.

This means that each individual plant will require customizations/adaptations. On top of this, the proposed use of the upgraded biomethane may also put constraints on the upgrading process. Similarly, local or global legislations on methane emissions in the slip (rejected stream or permeate) may add to the complexity of the upgrading plant. To utilize the biogas as a fuel, several issues must be clarified in the initial stages of planning:

  • • A biogas upgrading pre-study should be performed, as described in the next section.
  • • The intended end use of the final biomethane must be identified.

Biogas Upgrading Pre-Study

A pre-study of the applicability of (carbon) membrane technology for the upgrading of biogas to biomethane ought to start with a detailed high- resolution, time-averaged analysis of the composition of the particular biogas generated. Several components in the "normal" biogas are of interest: nitrogen gas, oxygen gas, sulfur-containing components (including SOx and H2S), alcohols, acids, nitrogen oxides, water vapor (expected to be at saturation at fermentation temperature), and, of course, CO, and methane.

Detailed knowledge of these parameters will assist in choosing the appropriate amount and order of any necessary pretreatment steps needed to comply with all environmental legislation and methane utilization standards, and to ensure the carbon membrane has a sufficient lifetime.

Biomethane Utilization

Three typical uses are common:

  • • Dual fuel (typical methane purity 80-85 vol%).
  • • Vehicle fuel (typical methane purity > 95 vol%).

Gas-grid injection (typically, inert less than 7 vol%, with specific demand for 02 level).

The actual specifications for these three qualities are highly dependent on region and need to be obtained and complied with for each case.

 
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