Mathematical Modelling of Microalgal Cultivation in Open Ponds and Bioprocess Optimization for Biodiesel Production

Abstract

Algal biofuels are regarded as a greener alternative since microalgae could remediate nutrients from waste sources and sequester carbon dioxide (CO2) from the atmosphere assimilating them into biomass. Though sustained benefits of algal technology are very well known, the economic and technical hindrances exist during scale-up. Also, the ongoing researches have nevertheless provided a direction forward but the specificity associated with the robustness of strain selection and acclimatization with the site-specific and other culture conditions often makes the process unsuitable during extrapolation at a particular location. Thus, the present study comprehensively evaluates the feasibility of National Institute of Technology (NIT) Rourkela for microalgal cultivation and also analyzes alternative strategies to reduce the costs associated with the upstream process of cultivation and downstream process for harvesting and transesterification. The first part of the study aims to estimate the microalgal productivity at NIT Rourkela using a comprehensive mathematical model. Site-specific climatological variables were utilized as the baseline information to predict the biomass, lipid productivity, and CO2 sequestration potential of microalgae using biophysical model formulated in MATLAB ODE 45s solver. Algal productivity was found to be influenced by light intensity (including the effects of photoinhibition), water temperature, and design criteria like pond depth, microalgal concentration. Maximum biomass and lipid productivity of 170.28 kg (dry mass) ha-1 d-1 and 39.42 L ha-1 d-1 respectively were predicted in September with a CO2 capture potential of 224.77 kg ha-1 d-1. A threshold limit of pond depth and algal concentration exists that influences light attenuation, thereby the performance of microalgae in open ponds. Also, it was observed based on the metabolism of microalgae, photoinhibition had a profound effect as it declined the areal productivity by 19%. Later, the study was further extrapolated using the climatologic data sets of a nearby coal-based power plant situated at Jharsuguda, to analyze the technical and economic feasibility at the industrial location. CO2 capture of 147.03 kg ha-1 d-1 with biomass productivity of 111.39 kg ha-1 d-1 was predicted in February, which was thereby used as the baseline information for techno-economic assessment. Process feasibility assessment using SuperPro Designer revealed the technical and economic viability with yearly carbon credits of 52 M$ and, a reasonable rate of returns with an acceptable short payback time of 2.81 years. Sensitivity analysis showed the process to be raw materials and facility dependent. The next part of the study mainly dealt with the use of waste resources during algal cultivation, harvesting, and transesterification process to make the algal technology sustainable. Optimization with response surface methodology resulted in 211.63 mg L-1 d-1 biomass productivity, 26.27% lipids with 6.50% v/v of urine, pH of 7.69 and at a light intensity of 205.40 μ𝑚𝑜𝑙 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑚−2𝑠−1. Subsequently, to wave-off the negative impacts associated with the chemicals during algal flocculation, the harvesting potential of natural plant-based flocculants was explored. It was observed that M. oleifera showed 75.55% biomass removal efficiency at 8 mg mL-1 after 100 min. Further, it was observed that the biomass removal efficiency increased to 95.76% when 4 mg mL-1 M. oleifera extracts were combined with 0.75 mg mL-1 chitosan. The last section dealt with the use of biochar as a solid heterogeneous catalyst for the transesterification of algal oil. Peanut shell pyrolyzed at 400 ℃ with sulfonic acid density of 0.837 mmol g-1 having 6.616 m2 g-1 surface area was selected for efficient catalysis. Biodiesel yield of 94.91% was obtained with 5% wt. catalyst loading, MeOH: oil ratio of 20:1 at 65 ºC after 4 h. GC-MS analysis of algal biodiesel showed the presence of a significant amount of palmitic and oleic acids. Thus, the results obtained in the overall study could act as a benchmark for the policymakers to translate microalgal technology to field scale. [math mode missing closing $]