Strategies for decreasing the O2 content in the upgraded biogas purified via microalgae-based technology.

Microalgae-bacteria consortium based technology using a High Rate Algal Pond (HRAP) interconnected to an Absorption Bubble Column (ABC) has emerged as an environmentally friendly promising option to upgrade biogas. However, the oxygenic photosynthesis of microalgae induces oxygen contamination in upgraded biogas, which could limit its further applications. Several strategies were proposed to favor the oxygen desorption and oxygen uptake in parts and accessories of the upgrading system. The effect of the volumetric ratio liquid recirculation rate/biogas rate (L/G = 5.0, 1.0 y 0.5) was evaluated in conjunction with the application of a novel accessory called Open Trickling Column (OTC). The O2 content in upgraded biogas was around 2.1%v, attaining CO2 removal efficiencies around 90%, at L/G ratio of 1.0 during diurnal and nocturnal periods. The inclusion of an OTC at the previous L/G, enhanced 54% the removal of O2 by stripping and uptake compared with the basal condition. Mass balances of H2S and methane showed that L/G > 1.0 favored the complete oxidation of H2S but promoted the loss of methane in dissolved form. Additionally the effect of increasing linear velocity of liquid broth in the lab-scale HRAP (from 15 cm s-1 to 20 cm s-1) showed to improve the O2 stripping with a consequential increase of biomass concentration under steady-state (from 0.7 to 1.4 g L-1) besides achieving O2 content in the upgraded biogas around 1.5%v.

Long-term photosynthetic CO2 removal from biogas and flue-gas: Exploring the potential of closed photobioreactors for high-value biomass production.

The long-term performance of a tubular photobioreactor interconnected to a gas absorption column for the abatement of CO2 from biogas and flue-gas was investigated. Additionally, a novel nitrogen feast-famine regime was implemented during the flue-gas feeding stage in order to promote the continuous storage of highly-energetic compounds. Results showed effective CO2 (~98%) and H2S (~99%) removals from synthetic biogas, supported by the high photosynthetic activity of microalgae which resulted in an alkaline pH (~10). In addition, CO2 removals of 99 and 91% were observed during the flue-gas operation depending on the nutrients source: mineral salt medium and digestate, respectively. A biomass productivity of ~8 g m-2 d-1 was obtained during both stages, with a complete nitrogen and carbon recovery from the cultivation broth. Moreover, the strategy of feeding nutrients during the dark period promoted the continuous accumulation of carbohydrates, their concentration increasing from 22% under normal nutrition up to 37% during the feast-famine cycle. This represents a productivity of ~3 g-carbohydrates m-2 d-1, which can be further valorized to contribute to the economic sustainability of the photosynthetic CO2 removal process.

Seasonal variation of biogas upgrading coupled with digestate treatment in an outdoors pilot scale algal-bacterial photobioreactor.

The yearly variations of the quality of the upgraded biogas and the efficiency of digestate treatment were evaluated in an outdoors pilot scale high rate algal pond (HRAP) interconnected to an external absorption column (AC) via a conical settler. CO2 concentrations in the upgraded biogas ranged from 0.7% in August to 11.9% in December, while a complete H2S removal was achieved regardless of the operational month. CH4 concentrations ranged from 85.2% in December to 97.9% in June, with a limited O2 and N2 stripping in the upgraded biogas mediated by the low recycling liquid/biogas ratio in the AC. Biomass productivity ranged from 0.0 g m-2 d-1 in winter to 22.5 g m-2 d-1 in summer. Finally, microalgae diversity was severely reduced throughout the year likely due to the increasing salinity in the cultivation broth of the HRAP induced by process operation in the absence of effluent.

Influence of the seasonal variation of environmental conditions on biogas upgrading in an outdoors pilot scale high rate algal pond.

The influence of the daily and seasonal variations of environmental conditions on the quality of the upgraded biogas was evaluated in an outdoors pilot scale high rate algal pond (HRAP) interconnected to an external absorption column (AC) via a conical settler. The high alkalinity in the cultivation broth resulted in a constant biomethane composition during the day regardless of the monitored month, while the high algal-bacterial activity during spring and summer boosted a superior biomethane quality. CO2 concentrations in the upgraded biogas ranged from 0.1% in May to 11.6% in December, while a complete H2S removal was always achieved regardless of the month. A limited N2 and O2 stripping from the scrubbing cultivation broth was recorded in the upgraded biogas at a recycling liquid/biogas ratio in the AC of 1. Finally, CH4 concentration in the upgraded biogas ranged from 85.6% in December to 99.6% in August.

In-situ biogas upgrading process: Modeling and simulations aspects.

Biogas upgrading processes by in-situ hydrogen (H2) injection are still challenging and could benefit from a mathematical model to predict system performance. Therefore, a previous model on anaerobic digestion was updated and expanded to include the effect of H2 injection into the liquid phase of a fermenter with the aim of modeling and simulating these processes. This was done by including hydrogenotrophic methanogen kinetics for H2 consumption and inhibition effect on the acetogenic steps. Special attention was paid to gas to liquid transfer of H2. The final model was successfully validated considering a set of Case Studies. Biogas composition and H2 utilization were correctly predicted, with overall deviation below 10% compared to experimental measurements. Parameter sensitivity analysis revealed that the model is highly sensitive to the H2 injection rate and mass transfer coefficient. The model developed is an effective tool for predicting process performance in scenarios with biogas upgrading.

CO2 Removal from Biogas by Cyanobacterium Leptolyngbya sp. CChF1 Isolated from the Lake Chapala, Mexico: Optimization of the Temperature and Light Intensity.

In the present study, the capacity of the cyanobacterium Leptolyngbya sp. CChF1 to remove CO2 from real and synthetic biogas was evaluated. The identification of the cyanobacterium, isolated from the lake Chapala, was carried out by means of morphological and molecular analyses, while its potential for CO2 removal from biogas streams was evaluated by kinetic experiments and optimized by a central composite design coupled to a response surface methodology. Results demonstrated that Leptolyngbya sp. CChF1 is able to remove CO2 and grow indistinctly in real or synthetic biogas streams, showing tolerance to high concentrations of CO2 and CH4, 25 and 75%, respectively. The characterization of the biomass composition at the end of the kinetic assays revealed that the main accumulated by-products under both biogas streams were lipids, followed by proteins and carbohydrates. Regarding the optimization experiments, light intensity and temperature were the studied variables, while synthetic biogas was the carbon source. Results showed that light intensity was significant for CO2 capture efficiency (p = 0.0290), while temperature was significant for biomass production (p = 0.0024). The predicted CO2 capture efficiency under optimal conditions (27.1 °C and 920 lx) was 93.48%. Overall, the results of the present study suggest that Leptolyngbya sp. CChF1 is a suitable candidate for biogas upgrading.