Advances in physical-chemical and biological processes for biogas upgrading boosted by carbon-coated iron based nanoparticles

Abstract

Fossil fuels remain the primary source of energy production, accounting for 80% of the global energy supply in 2023. Their use is the main contributor to global warming due to their greenhouse gas emissions, especially CO₂. In response to this problem, measures such as the Paris Agreement (2015), the European Green Deal, and, in Spain, the national strategy to reduce greenhouse gas emissions by 90% and achieve a 97% share of renewable energy in final energy consumption by 2050, have been promoted in recent years. Biogas and biomethane are key to this energy transition. In 2023, they accounted for 7% of natural gas consumption in the EU, and this figure is expected to rise to 40% by 2050. Biomethane can replace natural gas if it meets strict quality standards, so its production is part of the REPowerEU plan to reduce dependence on Russian natural gas. Biogas upgrading involves removing CO₂ to obtain biomethane. Conventional technologies, while effective, are costly and generate high CO₂ emissions. Therefore, sustainable alternatives need to be explored, such as photosynthetic upgrading, which uses algal-bacterial photobioreactors to remove CO₂ and H₂S under ambient conditions. Furthermore, this process can be integrated with wastewater treatment, utilizing the generated biomass. Another emerging technology is chemical absorption with carbonate-Fe/EDTA-based solvent, which simultaneously removes CO₂ and H₂S, achieving high efficiencies (CO₂ > 95%, H₂S up to 100%). In recent years, the use of nanoparticles in microalgae cultures has been demonstrated to improve biomass and lipid production, thereby increasing the efficiency of biogas upgrading and biomass valorisation. The addition of nanoparticles also enhances CO₂ absorption when added to solvents, forming nanofluids that increase the contact area between the gas and the liquid. However, their application in physicochemical biogas upgrading technologies remains limited. This thesis evaluated the effect of carbon-coated zero-valent iron nanoparticles (70, 140, and 280 mg/L) in a pilot system based on an HRAP photobioreactor interconnected to an absorption column. With the addition of nanoparticles, microalgal productivity doubled, nitrogen and phosphorus assimilation was improved, and a biomethane with CH₄ > 94% and CO₂ < 1% was achieved, complying with European regulations on the use of biomethane in natural gas networks. The improvement in process performance in terms of biogas upgraing was due to the improvement of photosynthesis through the addition of nanoparticles. Strategies to enhance CO₂ mass transfer, such as biogas recirculation, pH adjustment of the centrate/digestate, and the addition of liquid nanoparticles, were also evaluated in a HRAP photobioreactor interconnected to a biogas scrubbing column. Biogas recirculation in the absorption column was not effective, but feeding the centrate/digestate at pH 10 into the column improved CO₂ removal efficiency from 58% to 91%. The addition of liquid nanoparticles increased biomass concentration in the HRAP, although further studies are needed to optimize their use and maximize the biomethane quality. Furthermore, the effect of these nanoparticles was studied for the first time in a closed tubular photobioreactor. Inhibition was observed with high concentrations of ammonium and the punctual addition of 115 mL of liquid nanoparticles, although photosynthetic activity recovered with aeration. Liquid nanoparticles showed higher CO₂ removal efficiency (77%) than solid nanoparticles (49%). Finally, a physical-chemical absorption-desorption process devoted to biogas upgrading using carbonate-Fe/EDTA solutions at ambient pressure and temperature was optimized. Parameters such as pH, inorganic carbon concentration, biogas and air flow, liquid-to-gas ratio, and Fe/EDTA concentration were evaluated. The addition of nanoparticles did not improve the CO₂ absorption of the process. Under optimal conditions, high-quality biomethane (CH₄ ~ 95%, CO₂ ~ 1.7%, O₂ < 1%) was obtained without the need for Fe/EDTA for complete H₂S removal. The process stability was validated during three weeks of continuous operation.