CO₂ capture from air by Chlorella vulgaris microalgae in an airlift photobioreactor

Highlights

• The effect of gas superficial velocity on gas holdup and liquid circulation was investigated.

• The effect of high gas superficial velocity on the Chlorella vulgaris microalgae growth and CO₂ removal efficiency in an airlift photobioreactor was investigated.

• In high gas superficial velocity, the amount of algae cell concentration and CO₂ removal efficiency is inversely proportional to input gas superficial velocity.

• CO₂ removal efficiency by Chlorella vulgaris was achieved equal to 80% in the gas superficial velocity of 7.458 × 10⁻³ m/s.

Abstract

In this work, hydrodynamics and CO₂ biofixation study was conducted in an airlift bioreactor at the temperature of 30 ± 2 °C. The main objective of this work was to investigate the effect of high gas superficial velocity on CO₂ biofixation using Chlorella vulgaris microalgae and its growth. The study showed that Chlorella vulgaris in high input gas superficial velocity also had the ability to grow and remove the CO₂ by less than 80% efficiency.

Introduction

Human population growth and industrial expansion lead to increased energy consumption and the use of fossil fuels. Fossil fuel combustion releases carbon dioxide (CO₂) and water vapour into the atmosphere. The CO₂ is a pollutant that causes the greenhouse effect (Basu et al., 2014, Cheah et al., 2015). More than 50 percent of global warming is due to CO₂ emissions into the atmosphere. Hence, it is necessary to reduce the uncontrolled emissions of CO₂ and to prevent its release into the atmosphere (Wilbanks and Fernandez, 2014). Great efforts have been made by the international community to reduce CO₂ emissions in the atmosphere. For instance, the Kyoto Protocol in 1997 and Paris COP 21 in 2015, advocate international restrictions on CO₂ release (Nations, 2015).

There are two general approaches to reduce emissions of CO₂ into the atmosphere. One way is to reduce consumption of fossil fuels, and the other, moving toward renewable energy, is to capture and utilize CO₂. Among the CO₂ capture methods, can be cited chemical absorption and the biological fixation process (Chiang et al., 2011, Kaithwas et al., 2012). The biological sequestration process can be performed using plants or microorganisms. Microalgae and cyanobacteria are among the microorganisms that consume CO₂ to grow and lead to CO₂ fixation. Microalgae and cyanobacteria also have the most conversion efficiency of CO₂ into oxygen and biomass products (Chisti, 2007). The CO₂ removal by microalgae depends on different parameters such as microalgae species, type of cultivation system, nutrients ratio, light intensity, temperature, pH, CO₂ concentration, and gas flow rate (Cheng et al., 2013).

Many studies have been done on CO₂ fixation using different algae species. Among them, Chlorella vulgaris, which can tolerate high concentrations of CO₂, has high photosynthetic capacity, and can maintain high growth rate and CO₂ fixation rate in a wide range of CO₂ concentrations from 0.04 to 18% (v/v), can be considered as a good species to fix CO₂ (Singh and Singh, 2014, Yang et al., 2015).

Algae cultivation systems play a crucial role in the process of CO₂ fixation. There are two types of algae cultivation system for fixing CO₂ that include open raceway ponds and closed photobioreactors. Closed photobioreactors compared to open ponds provide better control of operating conditions and increase the efficiency of biofixation (Kasiri et al., 2015, Razzak et al., 2013). Other advantages of the bioreactor in the field of biochemicals are the ease of sterile operations and suitable hydrodynamics for biocatalysts sensitive to the tension and turbulence. Moreover, bioreactors have industrial applications such as in wastewater treatment, and in chemical and biochemical processes (Chisti, 1998).

Different bioreactors could be employed in biochemical processes. Among them, simple bubble column and airlift reactor (ALR) with internal loop and external loop can be cited. ALR has better mixing, more suitable heat, and mass transfer than bubble column due to the existence of the draft tube, and some of its advantages include simple construction without moving parts such as agitator and low consumption of energy (Chen et al., 2016, Chisti, 1998). As CO₂ and nutrients are essential for algal growth, ALR hydrodynamic parameters such as gas holdup and liquid circulation velocity play a key role. The gas holdup and liquid circulation velocity are a function of the input gas flow rate (Bitog et al., 2014, Nayak et al., 2014). In addition, the gas flow rate plays a great role in the algae growth, control of pH, creating optimal internal mixing, and preventing the accumulation of oxygen in the system (Kumar et al., 2010). Most of the studies in the CO₂ biofixation field have focused on the effect of different levels of CO₂, the impact of microalgae species, the effect of temperature, and the effect of light intensity on the rate of CO₂ fixation (Basu et al., 2013, Cheng et al., 2013).

The aim of this work was to investigate the effect of high gas superficial velocity on CO₂ capture from air using Chlorella vulgaris microalgae and its growth in an airlift photobioreactor with internal loop and internal sparger in constant light intensity and gas CO₂ level. This technology reduces CO₂ emissions into the atmosphere and helps in reducing global warming. ALR hydrodynamics was also evaluated under different input gas superficial velocities.