Strategies for N2 and O2 removal during biogas upgrading in a pilot algal-bacterial photobioreactor

Abstract

Photosynthetic biogas upgrading in algal-bacterial photobioreactors interconnected to external absorption columns has been proven as a feasible, low cost and environmentally friendly technology, ensuring a high removal performance of the main biogas contaminants (i.e., CO2 and H2S). However, despite the promising results, the upgraded biogas is commonly contaminated with N2 and O2 desorbed from the cultivation broth, which limits biomethane applications. In this work, two innovative strategies to improve the quality of biomethane were evaluated: degasification of the cultivation broth by means of a polydimethylsiloxane (PDMS) gas-liquid membrane, and biogas scrubbing at high pressure. The hollow fiber PDMS membrane demonstrated a superior degassing performance at 100–300 mbar of vacuum pressure, exhibiting dissolved oxygen removals of 61.7 ± 7.8% in the cultivation broth and reducing N2 and O2 concentrations in the upgraded biomethane to ~0.8 and ~0.1%, respectively. On the other hand, biogas scrubbing at 2 atm (over atmospheric pressure) promoted CO2 and H2S mass transfer from the biogas to the recirculating broth, achieving removals of ~98% and 100%, respectively, and N2 and O2 concentrations in the biomethane of ~1.7 and ~0.1%, respectively. Both strategies provided a biomethane composition complying with most international standards for utilization as vehicle fuel or injection into natural gas grids.