Abstract
Statement of Problem
The growth of algal biomass can vary substantially depending on environmental characteristics, photobioreactor (PBR) design, and feeding strategies. This can range from the amount of light exposure, broth temperature, nutrition, turbulence of the mixture, and more. If the algae are being used as a mechanism for carbon capture and sequestration (CCS) purposes, then it is important to understand these growth parameters so that total carbon dioxide (CO2) sequestration can be forecasted when implementing these CCS systems. First, these parameters are defined in detail, and a literature review of experimentation that has been conducted to see the relative impact of each parameter is discussed. Then the literature review investigates many of the various photobioreactor designs and how the parameters play an influence. Following the literature review is a brief introduction to a design for connecting a photobioreactor to a pilot-scale biomass gasifier and engine. Afterwards, a simple model is produced that can assess the energetic “success” of various algal biomass strains should they be processed into a fuel. In doing so, the optimal algal species for a CCS application may be selected. In preparation for experimentation, a series of preliminary designs and experiments are presented and attempted to investigate critical aspects in PBR design ranging from temperature control to turbulence and light exposure. Finally, an experiment is conducted that evaluates how changes in CO2 feeding schedule and light exposure may impact implementation of an algal CCS.
Sources of Data
Data utilized in this paper comes from a mixture of peer reviewed sources, theoretical calculations, and experimentation carried out by the researcher on multiple closed PBRs. Qualitative and minor quantitative data was taken for preliminary experiments in the Spring of 2021. The preliminary turbulence experiments were performed in an outdoor closed PBR consisting of four 9-liter aquariums with baffles of varying size. Samples were taken and concentrations were measured using haemocytometer cell counts. Primary experiment feed-schedule data was collected throughout Spring and Summer of 2022. The primary experiment consisted of four 56-liter subgroups comprised of three closed-circuit aquariums being sparged with 2.5 sL/min of air or CO2-enriched air. Concentrations and growth rates were determined by measuring the mass of the dehydrated algal broth.
Conclusions Reached
When growing algae for biofuel production, the simple model indicates that the most energetically potent strain is Chlorella vulgaris algae. However, this Among the most energetically costly aspects to the post-production process for all strains are the steps that require heat transfer. This heat can be supplied from the waste heat via a combined heat and power process. Capturing this energy would not only create a chemical battery in the form of biofuel, it would lead to a greater energetic return on investment, giving algae an additional edge as a mechanism for CCS. In the preliminary design for an interconnector between a gasifier engine and PBR, the heat energy pulled from the flue exhaust could be used to provide make the biofuel production process more efficient. Since the health of algae is critical to the potency of biofuels produced, it is essential to optimize growth parameters to produce as much biomass as possible, and in as healthy a way as possible. The growth parameters discussed in the literature review all influence the success of an algal CCS to varying capacities- turbulence is one especially complicated factor; low turbulence limits a colony’s size but could prevent a colony collapse. Temperature, on the other hand, has a very clear relationship with algal growth. It is still unclear the extent to which time-of-day feedings have on algae. Results indicate that there is slight variation in maximum concentration by time of day and feed concentration. However, there were issues with a few assumptions, such as flow rate, that may have an unseen influence on the data. The algae that were fed at approximately .3% concentrations for 7 hours per day experienced the highest maximum average concentration at 1.97±.17g/L on day 17. However, this overlapped considerably with the batch-fed averages. The 9am, and 1pm groups saw their maximum concentrations reach 1.94±.14g/L and 1.89±.15g/L on days 17 as well. Meanwhile, the 5pm group reached a maximum concentration of 1.89±.22g/L on day 13. Despite the higher concentration, the subgroup that was fed all day received considerably more CO2, around 40% more; thus, while the maximum concentration was highest (without factoring in uncertainties), the efficiency of sequestration is lower, and may therefore be less desirable in practice. The results and methods presented in this paper are useable as a means for a simple evaluation on the potential for an algal CCS system.