Biofiltration of Carbon Dioxide Using Algae

As this paper was being finalized, world leaders were meeting in Copenhagen to lay the ground work for a global agreement on climate change. Carbon dioxide is one of the main gases that has been implicated in global climate change or global climate disruption. Methods to control the atmospheric accumulation of carbon dioxide include scrubbing with alkaline solutions, adsorption, and bioconversion using cyanobacteria and phototrophic cultures. The first two methods are energy intensive processes and produce several environmentally unfriendly waste streams. In this paper, a novel technique for biofiltration of carbon dioxide by Tetraselmis suecica is presented here. The major advantages of using Tetraselmis suecica include: (1) the inherent buffering action of the marine algae that maintains the pH at 8.5; and (2) the requirement of a limited spectrum of light by the marine algae. Gallium-Aluminum-Arsenic (GaAlAs) light emitting diodes (LED's) emit light in the range required by Tetraselmis suecica. These GaAlAs LED's were put in series on both sides of the photobioreactor in a cascading manner for maximum photosynthesis activity. Laboratory scale experiments were carried out in a bubbling photobioreactor with different liquid recycle rate and photobioreactor conditions to determine the reaction kinetics and develop a process model that can be applied for a full-scale design. For liquid residence time of about 1 second, the carbon dioxide removal efficiency was about 4% in the photobioreactor. Further, the experimental data matched closely with the model calculations. Area requirement and pressure drop calculation carried out between a sparged bioreactor and spray chamber bioreactor for the same operating condition indicated spray chamber bioreactor to be a better alternative than a sparged bioreactor. In spray chamber bioreactor, the area requirements are about a 100 fold less than that for a sparged bioreactor. Also the pressure drop is negligible across the spray chamber bioreactor in comparison to a sparged bioreactor. Based on the process model, it has been determined that for separation of 7000 million tons CO2/year and 95 % removal efficiency, 15 units of 25 m diameter and 52 m height will be required.