(497a) Integrating Direct Air Carbon Capture and Algal-Based Bioenergy Production

Introduction: Microalgae are sun-powered factories that efficiently convert carbon dioxideand water into biomass, with minimal nutrient or land requirements, especially when compared with traditional energy crops. As such, they are widely recognized for their potential to provide both a sustainable source of fuels and in abating carbon dioxide emissions.

A rapidly growing microalgal culture quickly depletes the dissolved inorganic carbon resulting in growth limitation. Carbon limitation can be ameliorated by using a gas stream with a high CO₂ content (i.e. flue gas containing up to 15% CO₂), thus improving mass transfer rates [1]. Reliance on a flue gas stream, or other highly concentrated CO₂ sources, however, implies that algal cultivation facilities must be collocated immediately adjacent or in close proximity with large point source emitters, for example near large industrial complexes or fossil fuel driven power plants. These locations, however, are likely to lack sufficient land available for setting a large-scale algal cultivation facility; or if available, the land is likely to be quite expensive.

Recently, a natural photosynthetic microbial community capable of growing at high pH and high alkalinity has been reported as suitable for the production of biofuels and for carbon capture applications. This microbial community was derived from soda lakes located in the Caribou Plateau, in central British Columbia, Canada, where the lakes frequently reach sodium carbonate concentrations near saturation level, and pH can be as high as 10.5 [2][3].

As indicated above, microalgal culture productivity is tightly linked to the availability of CO₂ in the growth medium, which is limited by physicochemical constraints such as mass transfer rates, complex thermodynamic equilibrium, and reaction kinetics. Alkalinity and pH are known to have a strong effect on the mass transfer rate and thermodynamic equilibrium of inorganic carbon species in the medium. Consequently, the availability of a microbial community adapted to grow at high pH and high alkalinity offers a unique opportunity to greatly improve CO₂ mass transfer rates and the overall availability of dissolved inorganic carbon species to support growth. Furthermore, the increase in CO₂ mass transfer rates opens the possibility for direct air capture applications, where the algal cultivation facilities do not need to be collocated with large point source emitters.

Here we report on the design of an integrated direct air capture and algal cultivation facility for the conversion of carbon dioxide harvested from the atmosphere into biomass at high pH and high alkalinity.

Algal cultivation and processing: Algae were cultivated in a 0.5 M carbonate/bicarbonate solution for 6 days at pH=10.5 and at 21±0.5°C and harvested by gravitational settling. During the growth process, pH was not regulated reaching 11.0 after about 4 days. After harvesting, the algal biomass was processed in an anaerobic digester where it was converted into methane and carbon dioxide. Carbon dioxide produced at the anaerobic digestion stage can be recovered and further used in the algal cultivation stage.

Direct air capture: In order for a solvent to be suitable for direct air capture of carbon dioxide, it must have a negligible vapor pressure, high thermal and chemical stability, and provide a high mass transfer rate [4]. A highly alkaline aqueous solution, like the growth medium used for microalgae growth in this proposed process [3], satisfies most of these criteria. The high pH and alkalinity of the growth medium promote the carbon dioxide absorption through the following series of reactions [5].

(I) CO₂ + OH^– ⇄ HCO₃^–

(II) HCO₃^– + OH^– ⇄ CO₃^2– + H₂O

(III) OH^– + H₃O⁺ ⇄ 2H₂O

(IV) CO₂ + 2H₂O ⇄ HCO₃^– + H₃O⁺

This system of reactions was modeled in Aspen Plus^® using the electrolyte template which selects Electrolytic NRTL (ELECNRTL) as the thermodynamic package to account for the electrochemical reactions, that allow the absorption of the CO₂ into the highly alkaline media. It includes the mass transfer, kinetics, and thermodynamic limitations of the chemical absorption of CO₂ in a carbonate/bicarbonate solution, which is the main constituent of spent algal growth medium. Additionally, it considers the CO₂ as a Henry component, therefore, including the diffusion constraint to the mass transfer model.

A direct air capture unit consisting of a packed bed absorption column was modeled with the RadFrac model, which rigorously models two phase columns and includes a rate-based calculation method for the stage calculations instead of an equilibrium-based method, accounting for the non-equilibrium behavior observed in real applications. This model considers the actual rates of multicomponent mass, heat and chemical parameters which are taken into account simultaneously. Furthermore, the modified Pitzer model is used for the gas-liquid equilibrium calculations of aqueous strong electrolytes and it was selected because it distinguishes between anions and cations.

A laboratory scale unit capable of processing up to 200 mL/min of spent medium was built and packed with stainless steel Pall rings. CO₂ sensors both in the gas inlet and the gas outlet of the absorption column were installed to evaluate the resulting change in concentration. The solution’s pH was measured at the inlet and the outlet to identify the amount of CO₂ that was being absorbed chemically into the system, maintaining the same alkalinity. This setup was able to capture ~60% of the CO₂ in the inlet stream which contained about 400 ppm using the spent algal cultivation medium, which has an initial pH at about 11.0.

Experimental results were used to adjust model parameters and to further validate model predictions. After the experimental validation, this model was successfully used to design a unit that can meet the desired CO₂ requirements for the cultivation of microalgae. The column specifications include all the internal design parameters such as packing, which provides surface area for media/air contact and the overall column dimensions to fulfill the specified demand. The model is able to predict, with a 99% reliability, the column’s performance for variations in process conditions which include pH level, air temperature, and humidity and carbon dioxide concentration in the inlet gas. Pressure drop along the absorption column was minimal at just 12% of the inlet pressure, which translates in reduced pumping and compression costs. Water losses in the air capture unit were estimated to just under 1% of the water circulating in the system, which is significantly less than the water losses estimated for open pond cultivation systems.

References:

[1] J. Doucha, F. Straka, and K. Lívanský, “Utilization of flue gas for the cultivation of microalgae (Chlorella sp.) in an outdoor open thin-layer photobioreactor,” J. Appl. Phycol., vol. 17, no. 5, pp. 403–412, 2005.

[2] C. E. Sharp, S. Urschel, X. Dong, A. L. Brady, G. F. Slater, and M. Strous, “Biotechnology for Biofuels Robust, high ‑ productivity phototrophic carbon capture at high pH and alkalinity using natural microbial communities,” Biotechnol. Biofuels, pp. 1–13, 2017.

[3] K. A. Canon-Rubio, C. E. Sharp, J. Bergerson, M. Strous, and H. De la Hoz Siegler, “Use of highly alkaline conditions to improve the cost-effectiveness of algal biotechnology,” Appl. Microbiol. Biotechnol., vol. 100, no. 4, pp. 1611–1622, 2016.

[4] A. Kothandaraman, L. Nord, O. Bolland, H. J. Herzog, and G. J. McRae, “Comparison of solvents for post-combustion capture of CO2 by chemical absorption,” Energy Procedia, vol. 1, no. 1, pp. 1373–1380, 2009.

[5] H. Hikita, S. Asai, and T. Takatsuka, “Absorption of carbon dioxide into aqueous sodium hydroxide and sodium carbonate-bicarbonate solutions,” Chem. Eng. J., vol. 11, no. 2, pp. 131–141, 1976.