CO2 (carbon dioxide) is one of the building blocks of algae biomass. Algae use the energy collected from light plus CO2 to create organic building blocks (via photosynthesis) which get incorporated into the biomass. Although the concentration of CO2 in the atmosphere is increasing, the rate at which it dissolves into the culture medium is too slow to support vigorous biomass production. Therefore, cultivation schemes need to accommodate strategies to add CO2 to the culture medium.
Our industrialized society produces CO2 at a higher rate than can be used by plants, algae or absorbed into the oceans (see curve, above, showing the atmospheric CO2 concentration between the late 1950s and 2016). CO2 produced by industrial processes using (directly or indirectly) fossil fuels represents prehistoric CO2 that had been originally captured by algae and plants. It has been proposed that algae could play a role in reducing the rate of accumulation of CO2 in the atmosphere one more time.
CO2 from smoke stacks is contaminated with various concentrations of SOX and NOX which acidify the culture medium. Earlier work seemed to indicate that this would preclude algae from using flue gases directly in cultivation. Our research at Mera, funded by the US Department of Energy in the early 2000s, showed, however, that algal cultures can use flue gasses directly if one pays close attention to pH management of the cultures. Under the right conditions, algal photosynthesis is not negatively affected.
During our research, we determined that algae cultures can handle raw flue gas produced from a household, gas-fired, water heater and a coal combustor burning sub-bituminous coal (see photos above these lines). The water heater flue gas was bubbled directly into outdoor photobioreactors. The coal combustor flue gas was only filtered through glass wool before being injected into the PBRs. Gas analysis showed that the cultures lowered the concentration of NOX (gas-fired) and NOX and SOX (coal-fired). We reported our results at the Third National Conference on Carbon Sequestration in 2004.
We continued working on the utilization of flue gases while at Greenfuel Technologies. In 2006 we successfully produced algae biomass using flue gases from a NRG coal burning power plant in western New York (photos above). The flue gas was taken from the bottom of the stack and bubbled directly into hanging bag photobioreactors. The productivity of biomass was as good as that of cultures fed purified CO2.
A second field trial consisted in feeding a thin film reactor (about 1/4 acre) with combustion gases taken directly from a smoke stack at a gas-fired APS power plant (Redhawk) in Arizona. The Google Earth photograph above shows the location of the greenhouse-covered photobioreactor at the power plant in 2007. The results were summarized by Dr. Otto Pulz, who designed the photobioreactor.
We have repeatedly shown that algae can thrive on smoke stack gases. This is highly relevant when one considers that CO2 is a significant expense in algae cultivation and that, we expect one day, CO2 emitters will have to find uses for their "waste" product to avoid costs.