Abstract
The aim of this study was to find an inexpensive and environmentally friendly CO2 source for growing the hydrogen-producing microalgae Chlamydomonas reinhardtii. The effect of different flue gas concentrations from a silicomanganese smelter on the growth of these algae at a photon flux density of 200 μmol photons m−2 s−1 applied 24 h day−1 was studied. First, the algae were grown in a laboratory at 1.2, 6.8 and 17.1 % (v/v) pure CO2 gas mixed with fresh air. After 5 days of growth, the dry biomass per litre algal culture was slightly higher (17 %) at 6.8 % CO2 as compared to at 1.2 % CO2. A further increase to 17.1 % CO2 decreased the biomass by about 40 %. Then, the flue gas from a silicomanganese smelter was used as a CO2 source for growing the algae. The flue gas was characterized by a high CO2 concentration (about 17 % v/v), low oxygen concentration (about 4 %), about 100 ppm NOx and 1 ppm SO2. The culture medium bubbled with undiluted flue gas contained about 490 mg L−1 dissolved CO2 and 4.0 mg L−1 dissolved O2, while the lowest flue gas concentration contained about 280 mg L−1 CO2 and 7.1 mg L−1 O2. Undiluted flue gas (17.4 % CO2) decreased the biomass of the algae by about 40 % as compared with 4.8 % pure CO2 gas or flue gas diluted to a concentration of 6.3 % CO2. Flue gas diluted to give 10.0 % CO2 gave less reduction in the growth of the algae (22 %). It was concluded that the high CO2 concentration itself caused the growth reduction and not the air pollutants, and the very low O2 concentrations in the growth medium could not counteract this negative effect.
Highlights
Hydrogen is recognised as a promising future energy alternative because it produces no carbon dioxide that contributes to the greenhouse effect when combusted (IPCC 2013)
C. reinhardtii has been subjected to extensive investigation, the effect of flue gas seems to have been little studied in this species
The results showed a progressive increase in the dissolved CO2 concentration from about 100 to about 500 mg L−1 with increasing CO2 concentration from about 1 % up to about 24 %
Summary
Hydrogen is recognised as a promising future energy alternative because it produces no carbon dioxide that contributes to the greenhouse effect when combusted (IPCC 2013). Today’s atmospheric CO2 concentration of around 400 ppm strongly limits the algae growth, and additional CO2 should be supplied throughout the production phase (Geier et al 2012). In order for the production to be environmentally friendly, waste CO2 from industrial flue gas should be used, which contributes to cleaning the emissions. Several studies have investigated the effect of flue gases on the growth of microalgae (Douskova et al 2009; Kastánek et al 2010; Borkenstein et al 2011; Lara-Gil et al 2014). C. reinhardtii has been subjected to extensive investigation, the effect of flue gas seems to have been little studied in this species. Bark (2012) showed that C. reinhardtii could be grown in simulated flue gas at a concentration of 15 % CO2/100 ppm NO; the growth was better at C. reinhardtii has been subjected to extensive investigation, the effect of flue gas seems to have been little studied in this species. Bark (2012) showed that C. reinhardtii could be grown in simulated flue gas at a concentration of 15 % CO2/100 ppm NO; the growth was better at
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