Development of a control strategy to cope with biogas flowrate variations during photosynthetic biogas upgrading

Abstract

The design and evaluation of a control system for a photosynthetic biogas upgrading unit was successfully carried out in this study. This control system ensured a specific biomethane quality under any disturbance in the biogas flowrate. The recycling liquid flowrate, and indirectly the liquid to biogas (L/G) ratio, was selected as the manipulated variable in order to maintain the CO2 and O2 content of biomethane, and therefore comply with the requirements for its use as natural gas substitute (<2.5% and <1.0%, respectively). The control system was able to maintain the biomethane CO2 content below the set point value under a stepwise increase in the biogas flowrate from 60 to 150 ml/min, together with negligible H2S concentrations and an O2 stripping from the recycling liquid to the biomethane lower than 1%, thus obtaining a consistent biomethane quality over time. On the contrary, the biomethane CO2 content increased up to 13.2% under this stepwise increase in the biogas flowrate without control system. Successful results were also obtained when the control system was challenged with stepwise surges in the biogas flowrate between 60 and 120 ml/min under different temperatures (15 and 35 ºC) and inorganic carbon concentrations (1500, 500 and 100 mg/L) when the recycling liquid entering the absorption column presented a pH 10. However, the high liquid flowrates required at a cultivation broth pH of 8.5 as a result of the low CO2 mass transfer led to an excessive O2 desorption to the biomethane, resulting in biomethane O2 contents >1%.