Development of bicarbonate-based microalgae cultivation system combined with electrochemical CO2 absorption process

Abstract

Recent concerns about ever-increasing CO2 emissions and global warming have attracted interest in microalgae-based CO2 capture and utilization (CCU) technology. Indeed, there have been many studies using microalgae to fix the CO2 in industrial flue gas. Despite the theoretical feasibility of this technology, there exist some critical issues in directly utilizing flue gas: low carbon utilization efficiency, high cost for CO2 transportation, and inhibition of algal growth by toxic compounds. All these difficulties arise because the CO2 being managed is gaseous. In this doctoral study, therefore, I focused on utilization of an ionic form of CO2, specifically bicarbonate (HCO3−), as a carbon source for microalgae cultivation. To make it possible to implement this novel approach, I proposed a novel process combining microalgae cultivation and electrochemical CO2 absorption, which is able to provide sodium bicarbonate (NaHCO3) from flue gas in an eco-friendly manner. With this process, CO2 in the flue gas is captured and subsequently turned into NaHCO3, which is then utilized as a carbon source for the microalgae. This approach, which is innovative in itself, allows avoidance of the high energy demands for gas compression and the low carbon utilization efficiency of conventional CO2-based cultivation. For this approach to bicarbonate-based cultivation to work, it is important to utilize microalgae species that are capable of growing on bicarbonate. To this end, a size-based microalgae separation technique was developed to isolate the microalgal strains that are favorable for bicarbonate-based cultivation. A contraction–expansion array microchannel utilizing inertial microfluidics was employed as a simple and effective method for the separation of two model microalgae, Chlorella vulgaris and Haematococcus pluvialis. Fluorescent microbeads with diameters of 6 and 20 μm were first used as surrogate particles, and they were clearly separated at a total flow rate of 7.4 mL h−1. In this predetermined optimal condition, the two algal species were also successfully separated without affecting cell viability: the separation yielded 97.9% of purity for C. vulgaris and 94.9% for H. pluvialis. This inertia-based separation technology is expected to be able to overcome the labor-intensive and time-consuming disadvantages of conventional microalgae separation technologies. In addition, the ability of ten algal species to utilize bicarbonate as their carbon source was tested, and their optimal culture conditions, such as bicarbonate concentration, culture temperature, and light intensity, were also determined. The specific growth rates of all the algal cells were found to increase with NaHCO3, suggesting that bicarbonate can be utilized as a carbon source. It is worth noting that this study was the first to systematically demonstrate that many eukaryotic microalgae are able to utilize NaHCO3, which has not been widely accepted to date, in contrast to prokaryotic cyanobacteria. Moreover, the optimal culture conditions were found to be different depending on the carbon source used. It is my anticipation and belief that the basic physiological database obtained during this study will be useful when the bicarbonate-based technology becomes widespread. In addition, the potential of the bicarbonate-based cultivation was compared to that of conventional CO2-based cultivation in terms of biomass productivity and carbon utilization efficiency. This bicarbonate-fed cultivation process could yield biomass productivity similar to that of a CO2-based system as long as pH is controlled. Moreover, the theoretical carbon utilization efficiency when using NaHCO3 was calculated to be 86.7–100.5%, much higher than that using CO2 (10–25%). These results were also in agreement with experimentally obtained carbon utilization efficiencies. While CO2 supplied in gaseous form was mostly lost (only 3.6% of it was usable for biomass synthesis), bicarbonate was effectively incorporated into the biomass with carbon utilization efficiency of 91.4%. The environmental benefits of using NaHCO3 were assessed via carbon footprint analysis from the perspective of the entire process, ranging from carbon source preparation to microalgae cultivation. This analysis revealed that conventional CO2-based microalgae cultivation is not as effective as expected because most of the CO2 supplied is outgassed. Additionally, this also showed that the use of bicarbonate can provide other advantages over the other two CO2-based carbon sources (direct flue gas and MEA-extracted pure CO2), with respect to net CO2 emissions and biomass productivity. These benefits were even more pronounced when the transport of carbon sources was considered. The low carbon outgassing rate (maximum 9.6 ppm day−1) and substantially higher carbon utilization efficiency (>86.7%) with bicarbonate were found to be the determining factors. This high carbon utilization efficiency could even outweigh the disadvantage of providing the substantial electrical energy required to produce NaHCO3 from CO2. Finally, a novel water electrolysis-based microalgae cultivation system, which is capable of both pH control and continuous supply of the carbon source, was designed and its performance was confirmed. Daily operation of the water electrolytic cell could effectively lower the pH of an algal culture medium within 6–90 minutes depending on the voltage applied, and at the same time, approximately 100–150 mg L−1 of carbon could be successfully supplemented into the anodic algal culture solution. Interestingly, the final biomass concentrations were the same (1.7 g L−1) regardless of the voltage applied and the concentration of catholyte, and the applied voltage (3–6 V) did not affect the viability and lipid productivity of algal cells. I believe that all these results, which I produced during my doctoral program, strongly support my assertion that bicarbonate-based microalgae cultivation is indeed feasible and practical, in particular when integrated with the electrochemical CO2 mineralization technology. Given that long-distance transport of gaseous CO2 is not economical and that a plant-adjacent co-located algae facility is also hardly available, this bicarbonate-based technology appears even more desirable. It is also more practical because the use of an aqueous or powder form of NaHCO3 product would allow the microalgae cultivation sites to be located well away from the CO2 sources, such as power plants or other CO2 emitters, extending the scope of application of this technology.