Microalgae Biomass as a New Potential Source of Sustainable Green Lubricants.

Lubricants are materials able to reduce friction and/or wear of any type of moving surfaces facilitating smooth operations, maintaining reliable machine functions, and reducing risks of failures while contributing to energy savings. At present, most worldwide used lubricants are derived from crude oil. However, production, usage and disposal of these lubricants have significant impact on environment and health. Hence, there is a growing pressure to reduce demand of this sort of lubricants, which has fostered development and use of green lubricants, as vegetable oil-based lubricants (biolubricants). Despite the ecological benefits of producing/using biolubricants, availability of the required raw materials and agricultural land to create a reliable chain supply is still far from being established. Recently, biomass from some microalgae species has attracted attention due to their capacity to produce high-value lipids/oils for potential lubricants production. Thus, this multidisciplinary work reviews the main chemical-physical characteristics of lubricants and the main attempts and progress on microalgae biomass production for developing oils with pertinent lubricating properties. In addition, potential microalgae strains and chemical modifications to their oils to produce lubricants for different industrial applications are identified. Finally, a guide for microalgae oil selection based on its chemical composition for specific lubricant applications is provided.

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.

Photorespiration in an outdoor alkaline open-photobioreactor used for biogas upgrading.

The rates of oxygenic and carbon fixation photosynthetic processes of a microalgae consortium were simultaneously evaluated under steady-state performance in an bench scale alkaline open-system exposed to outdoor conditions in Mexico City. A synthetic methane-free gaseous stream (SMGS) similar to biogas was used as inorganic carbon source and model of biogas upgrading. The microalgae CO2 fixation rates were calculated through a novel methodology based on an inorganic carbon mass balance under continuous scrubbing of a SMGS similar to biogas, where the influence of pH and temperature time-depended oscillations were successfully incorporated into the mass balances. The oxygenic activity and carbon fixation occurred at different non-stoichiometric rates during the diurnal phase, in average carbon fixation predominated over oxygen production (photosynthesis quotient PQ≈ 0.5 mol O2 mol-1 CO2) indicating photorespiration occurrence mainly under dissolved oxygen concentrations higher than 10 mg L-1. The oxygen and inorganic carbon mass balances demonstrated that photorespiration and endogenous respiration were responsible for losing up to 66% and 7% respectively of the biomass grew at diurnal periods under optimal conditions. In favoring photorespiration conditions, the microalgae biomass productivity (CO2 effectively captured) can be severely decreased. A kinetic mathematical model as a function of temperature and irradiance of the oxygenic photosynthetic activity indicated the optimal operation zone for this outdoor alkaline open-photobioreactor, where irradiance was found being the most influential parameter.

A study of photosynthetic biogas upgrading based on a high rate algal pond under alkaline conditions: Influence of the illumination regime.

Microalgal-bacterial processes have emerged as environmental friendly systems for the cost-effective treatment of anaerobic effluents such as biogas and nutrients-laden digestates. Environmental parameters such as temperature, irradiation, nutrient concentration and pH effect the performance of the systems. In this paper, the potential of a microalgal-bacterial photobioreactor operated under high pH (≈9.5) and high alkalinity to convert biogas into biomethane was evaluated. The influence of the illumination regime (continuous light supply vs 12h/12h light/dark cycles) on the synthetic biogas upgrading efficiency, biomass productivity and nutrient removal efficiency was assessed in a High-Rate Algal Pond interconnected to a biogas absorption bubble column. No significant differences in the removal efficiency of CO2 and H2S (91.5±2% and 99.5%±0.5, respectively) were recorded regardless of the illumination regime. The high fluctuations of the dissolved oxygen concentration during operation under light/dark cycles allowed to evaluate the specific growth rate and the specific partial degradation rate of the microalgae biomass by photosynthesis and respiration, respectively. The respiration reduced the net microalgae biomass productivity under light/dark cycles compared with process operation under the continuous light supply.