The growth of Chlorella sorokiniana as influenced by CO2, light, and flue gases

In order to achieve recognition as environmentally friendly production, flue gases should be used as a CO2 source for growing the microalgae Chlorella sorokiniana when used for hydrogen production. Flue gases from a waste incinerator and from a silicomanganese smelter were used. Before testing the flue gases, the algae were grown in a laboratory at 0.04, 1.3, 5.9, and 11.0 % (v/v) pure CO2 gas mixed with fresh air. After 5 days of growth, the dry biomass per liter algal culture reached its maximum at 6.1 % CO2. A second experiment was conducted in the laboratory at 6.2 % CO2 at photon flux densities (PFD) of 100, 230, and 320 μmol photons m−2 s−1. After 4 days of growth, increasing the PFD increased the biomass production by 67 and 108 % at the two highest PFD levels, as compared with the lowest PFD. A bioreactor system containing nine daylight-exposed tubes and nine artificial light-exposed tubes was installed on the roof of the waste incinerator. The effect of undiluted flue gas (10.7 % CO2, 35.8 ppm NOx, and 38.6 ppm SO2), flue gas diluted with fresh air to give 4.2 % CO2 concentration, and 5.0 % pure CO2 gas was studied in daylight (21.4 ± 9.6 mol photons m−2 day−1 PAR, day length 12.0 h) and at 135 μmol photons m−2 s−1 artificial light given 24 h day−1 (11.7 ± 0.0 mol photons m−2 day−1 PAR). After 4 days’ growth, the biomass production was the same in the two flue gas concentrations and the 5 % pure CO2 gas control. The biomass production was also the same in daylight and artificial light, which meant that, in artificial light, the light use efficiency was about twice that of daylight. The starch concentration of the algae was unaffected by the light level and CO2 concentration in the laboratory experiments (2.5–4.0 % of the dry weight). The flue gas concentration had no effect on starch concentration, while the starch concentration increased from about 1.5 % to about 6.0 % when the light source changed from artificial light to daylight. The flue gas from the silicomanganese smelter 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 biomass production using flue gas significantly increased as compared with about 5 % pure CO2 gas, which was similar to the biomass produced at a CO2 concentration of 10–20 % mixed with N2. Thus, the enhanced biomass production seemed to be related to the low oxygen concentration rather than to the very high CO2 concentration.