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

Abstract

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 ϕ_CO2) and average CO2 consumption rate (7.48 times by ACR_CO2) 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 ϕ_CO2 and ACR_CO2 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.

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