ingeniería y arquitectura

Optimization of photosynthetic biogas upgrading combined with algal biomass valorization: Assessing alternative operating strategies and photobioreactor configurations.

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Optimization of photosynthetic biogas upgrading combined with algal biomass valorization: Assessing alternative operating strategies and photobioreactor configurations.

PhD in Chemical and Environmental Engineering
Doctor: Roxana Ángeles Torres
Date of reading: 16-07-2021
Directors: Raquel Lebrero Fernández, Raúl Muñoz Torre
Court: Silvia Bolado Rodríguez (Chairwoman), Fernando González Fermoso (Secretary), César Sáez Navarrete (Member)

Introduction

Fossil fuels account for 80 % of the total energy consumption worldwide, significantly contributing to greenhouse gas emissions. Therefore, many recently developed international policies have focused on mitigating climate change and promoting renewable energy production, with the objective to reduce GHG emissions and fossil fuel consumption. Among renewable energy sources, biogas from the anaerobic degradation of organic waste constitutes a sustainable energy vector due to its high CH4 content (~40 to 75 % v/v). However, a preliminary purification stage (known as upgrading) is recommended prior biogas use owing to the presence of other gas pollutants such as carbon dioxide (CO2, 15 – 60 % v/v), hydrogen sulphide (H2S, 0.005 – 3 % v/v), nitrogen (N2, 0 – 2 % v/v), oxygen (O2, 0-1 % v/v), siloxanes (0 – 0.02 % v/v), volatile organic compounds (VOC, < 0.6 % v/v), ammonia (NH3, < 1 % v/v) or carbon monoxide (CO, < 0.6 % v/v).

One of the most promising biological technology alternatives for the simultaneous removal of CO2 and H2S is photosynthetic biogas upgrading in algal-bacterial photobioreactors (PBRs). This biological technology is based on the fixation of CO2 by microalgae via photosynthesis using solar light and the concomitant oxidation of H2S by sulphur-oxidizing bacteria utilizing the oxygen photosynthetically produced.

Objectives

The main objective of this PhD thesis was the engineering of innovative operational strategies to obtain a biomethane composition complying with most international standards for utilization as a vehicle fuel or injection into natural gas grids under two different photobioreactor configurations. This approach combined a reduction in the costs of biogas upgrading using photosynthetic processes with the creation of a circular economy model through biomass valorization.

Development

The performance of photosynthetic biogas upgrading combined with algal biomass valorization was evaluated under different operational strategies and photobioreactor configurations.

Thus, two new operational strategies aiming at minimizing the content of N2 and O2 in the upgraded biogas in a conventional open PBR were explored. The first approach was based on the degassing of the culture broth using PDMS liquid-gas membranes operated at varying vacuum pressures before being fed to the scrubbing column. The second approach focused on minimizing the desorption of N2 and O2 from the liquid to the biomethane using an external pressurized scrubbing column. An enclosed tubular-PBR was used to evaluate the influence of the alkalinity of the cultivation broth on biomethane quality. In addition, after optimization of the culture conditions for integral biogas upgrading, the tubular-PBR was operated under nitrogen deprivation cycles with the purpose of assessing the influence on the chemical composition of the algal biomass. Furthermore, the influence of the biogas supply regime (feeding only during the illuminated period or continuous feeding during the dark/light periods) on the photosynthetic biogas upgrading performance and algal biomass production was investigated.

Finally, the thesis focused on the optimization of the growth of the cyanobacterium N. muscorum and the accumulation of value-added products such as glycogen and poly–β–hydroxybutyrate using CO2 from either biogas or flue-gas as the carbon source under different nutrient deprivation strategies.

Conclusions

Photosynthetic biogas upgrading has been successfully confirmed as a sustainable and environmentally friendly alternative that guarantees high performance in the elimination of the main biogas contaminants (i.e., CO2 and H2S) while generating valuable algal biomass from the nutrients contained in the wastewater.