Abstract
The two-step bubble column-photobioreactor photosynthetic biogas upgrading system can enable simultaneous production of biomethane and value-added products from microalgae. However, due to the influence of a large number of variables, including downstream processes and the presence of microalgae, no unanimity has been reached regarding the performance of bubble column reactors in photosynthetic biogas upgrading. To investigate this further, the present work documents in detail, the design and commissioning of a lab-scale bubble column reactor capable of treating up to 16.3 L/h of biogas while being scalable. The performance of the bubble column was assessed at a pH of 9.35 with different algal densities of Spirulina platensis at 20 °C in the presence of light (3–5 klux or 40.5–67.5 μmol m–2 s–1). A liquid/gas flow (L/G) ratio of 0.5 allowed consistent CO2 removal of over 98% irrespective of the algal density or its photosynthetic activity. For lower concentrations of algae, the volumetric O2 concentration in the upgraded biomethane varied between 0.05 and 0.52%, thus providing grid quality biomethane. However, for higher algal concentrations, increased oxygen content in the upgraded biomethane due to both enhanced O2 stripping and the photosynthetic activity of the microalgae as well as clogging and foaming posed severe operational challenges.
Highlights
INTRODUCTION Photosynthetic Biogas UpgradingConventional physicochemical biogas upgrading technologies such as water scrubbing and pressure swing absorption continue to consume a significant amount of energy.[1]
All initial trials were performed with a low algal concentration (0.05 g-dry weight (DW)/L) to establish a working bubble column setup for biogas upgrading with microalgae
Modification to system design, together with the implementation of correction factors to account for residual oxygen and nitrogen, resulted in achieving grid quality biomethane (CO2 below 2% and O2 below 0.55% by volume) above liquid/gas flow (L/G) ratios of 0.4 and low algal concentrations
Summary
Conventional physicochemical biogas upgrading technologies such as water scrubbing and pressure swing absorption continue to consume a significant amount of energy (up to 6% of the energy content in biogas).[1] In addition, it results in a high cost of biomethane, necessitating the need for incentives to achieve financial sustainability.[2] As an alternative, biological biogas upgrading technologies are being investigated to increase the sustainability of biomethane derived from biogas by reducing costs and energy demands.[1] Photosynthetic biogas upgrading is a novel biogas upgrading technology that can be employed to remove CO2 and H2S in biogas by absorption in a carbonate−bicarbonate-based alkaline algal medium.[3,4] The absorbed CO2 would allow the cultivation of microalgae for food, feed, and/or energy.[5] As a consequence, the resulting biomethane could be both economically and environmentally more beneficial than that obtained through traditional biogas upgrading systems.[5,6] The two-step bubble column-photobioreactor (PBR) photosynthetic biogas upgrading configuration is currently under assessment by the scientific community for optimization of both biogas upgrading in a bubble column and growth of microalgae in a PBR.[7,8]
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