Is it feasible to compress cleaned Biogas?

Question

Biogas generated from anaerobic digestion mainly contains around 50-60% $CH_4$, 35-40% $CO_2$, small amount of $N_2$, saturated $H_2O$ and trace of $H_2S$. It's upgraded to automotive fuel grade by removing $H_2S$ up to, $H_2O$ and $CO_2$. The upgraded Biogas (contains >90% of $CH_4$ and <4% $CO_2$) is compressed up to 250 bar to bottle, transport and to use as an alternative of Natural Gas.

Now, my question is, is it feasible to compress biogas after cleaning out $H_2S$ and $H_2O$ but without removing $CO_2$ so that it can be bottled and transported? Or what problem can arise while compressing methane with $CO_2$? Up to which pressure can it be compressed?

Answer 1

Possible, yes. But there are reasons not to do it:

• the vapor pressure of CO₂ at 20°C is about 57 bar, in other words at the high pressures you are talking about you get liquid CO₂. Makes more sense, then, to remove the CO₂
• Compression takes energy. Removing the CO₂ before compression saves some of that energy
• Compression generates heat, liquefying CO₂ generates additional heat
• Biogas has a different Wobbe-Index than pure methane, meaning the air-fuel mixture will need to be changed in a consumer that is designed for methane. Often a very minor change, like the setting of a carburetor in an engine or adjusting a nozzle in a burner, but nonetheless
• You would haul the additional weight of the CO₂ around

As far as I know, the most common biogas upgrading technologies these days are membrane filtration and pressure water scrubbing. However biogas upgrading by CO₂ liquification (by cooling and compression) is or was available on the market (some Swedish company, forgot the name) and used in gas-to-grid plants. These plants would sell the liquid CO₂.