New Climate Warrior. Carbon capture by algae-assisted microbial fuel cell shows promise

It is becoming increasingly evident that carbon capture and sequestration (CCS), which is central to ‘abated coal use’ approaches, is not much of a bang-for-the-buck technology. But recent research work from IIT Jodhpur may have a better solution. In its algae-assisted method, you can not only capture CO₂, but also treat wastewater and generate power.

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Researchers Arti Sharma et al, from the IIT-J tested out their technology and have documented the results in a paper published in Chemical Engineering. What they have developed is to first cool the flue gas (the gas produced from the flue or chimneys of thermal power stations and other industrial plants) in a heat exchanger and then direct it to a sieve-plate absorption column. Here, the sodium carbonate supplemented wastewater absorbs the CO₂, generating flue-gas-derived bicarbonates (FGDBs). The FGDBs are added in plastic bag photobioreacors (PBRs), coupled with algae-assisted microbial fuel cells (MFC). “This study offers a biochemical CO₂ sequestration process that generates power, algae biomass and treats water by utilising algae-assisted MFC for flue gas carbon capture,” the paper says.

The conventional method of carbon capture from flue gases is not only energy intensive but also requires dilution of the gas with nitrogen, which restricts implementation. Further, the absorbent used — monoethanolamine (MEA) — is corrosive, has low oxidative stability and takes energy for regeneration, the authors say, adding that bioenergy with CCS technologies (BECCS) is promising.

Using flue gases to produce useful algae is nothing new, but the paper notes that the conventional method of doing this has been to bubble the flue gases into algal ponds or photo-bioreactors. The problem, however, is the limited solubility of CO₂ in water (0.583 mg per litre) when exposed to the atmosphere at 25^o C.

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So, the authors have suggested a more efficacious “indirect method” for converting CO₂ into carbonates and then use it for algal growth. Solubility of sodium bicarbonate in water is significantly higher (93.2 g/l) at room temperature and atmospheric pressure. “Therefore, the indirect biochemical route of CO₂ fixation is advantageous since more inorganic carbon can stay in the water,” the paper notes.

First of its kind

“The integration of algae MFC with flue gas carbon capture has not been attempted before,” Dr Meenu Chhabra, Professor, Department of Bioscience and Bioengineering, IIT Jodhpur, told quantum. There have been other attempts to make flue gas-generated bicarbonate for algae growth in open ponds. However, the bicarbonate in water strives to achieve equilibrium with the CO₂ in the atmosphere when kept in open ponds. This can cause the bicarbonate to decompose and release CO₂ into the atmosphere. Moreover, this decomposition reaction is endothermic and it causes the pH to become alkaline. “Therefore, closed systems like PBRs are desirable,” the authors note. Further, the process outlined by IIT Jodhpur scientists uses wastewater, where the chosen algal strain —Chlorella vulgaris— is thermo-tolerant and can grow in wastewater.

Once you have the algae, you can put it into a microbial fuel cell to generate electricity. A MFC is a bio-electrochemical device that generates electricity by harnessing the metabolic activity of microorganisms. (When microorganisms break down organic matter — which wastewater is rich in — into simpler molecules, electrons are released in the process. If these electrons are made to flow through an external circuit, you get electric current.)

The researchers say that for a cubic meter of wastewater and FGDB in the microbial fuel cell, they got energy of 0.0066 kWhr. Only a small fraction of algae is used for power generation. The remaining is available for bioenergy. “A major outcome of the present study was an increase in power production through high algae growth,” the authors say.

Theoretically, all the available flue gas can be used to grow algae, but the limitation is with respect to the scale of operation. A tonne per day of CO₂ capture requires 2 sq km algae culture area (aerial) in vertically aligned pipes, says Dr Chhabra. Further, the algae can be filtered out and the rest of it can be used again for CO₂ capture. Typically one tonne of algae captures 180 tonnes of flue gas CO₂.

This, however, is not to conclude that the technology is ready for use—it requires further refinement. The next steps are in this direction and could include developing more robust microbial consortia or genetically engineered strains. And, studies on comprehensive mass balances, feed flow rates and retention times for the efficient CO₂ capture need to be carried out. Also, the device itself could be fine-tuned by adding specialised spargers (gas diffusing devices) to ensure a stable supply of gas for sustained algal growth.

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