Bio-decarbonization by microalgae: a comprehensive analysis of CO2 transport in photo-bioreactor

Microalgae biofilm is a typical porous structure where CO₂ is converted by microalgae into organic matter. Therefore, CO₂ transport and its distribution in porous biofilm are crucial for microalgae decarbonization and its energy utilization. In order to get detailed process information of CO₂ transport and its bioconversion, a mathematical model considering the microalgae growth and its material consumptions was established for substances' flow and transport processes in an immersed microalgae biofilm reactor. Modeling results showed that CO₂ concentration on the surface of biofilm reduced about 26.57% along the flow direction. Increased inlet CO₂ concentration (0.2–5.2 ​mM) significantly promoted the average specific growth rate of biofilm, which was more dramatical at a low flow rate with an enhancement about 7.38 times. Essential reason for it is a synchronous increase on total transfer flux of CO₂ (8.25 times by $\varphi {\_}_{C{O}_2}$) and average CO₂ consumption rate (7.48 times by $ACR{\_}_{C{O}_2}$) in biofilm. However, such promotion gradually waned with a growing initial carbon supply concentration. Enhanced CO₂ transport in biofilm caused by increasing culture medium's flow rate (1–6 ​mL ​min⁻¹) doesn't always result in synchronous improvements on biofilm growth. At sufficient carbon supply, increased flow rate doesn't further effectively improve biofilm growth but greatly reduced CO₂ removal. Whether increasing carbon supply concentration or flow rate, biofilm growth can't be significantly promoted unless the $\varphi {\_}_{C{O}_2}$ and $ACR{\_}_{C{O}_2}$ in biofilm were increased by almost the same level. This work provides a new and deeper mechanistic insight into macroscopic growth characteristics of biofilms from the perspective of CO₂ transport in it, as well as providing some theoretical guidance towards the cultivation of immersed microalgae biofilm for bio-decarbonization.