Studies on practical removal system of NOx from boiler flue gas using microalgae

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Nitric oxide (NO) is a main component of NOx in flue gas. We found that it was possible to remove NO from flue gas using the microalgal culture system. For practical application of this system, improvement of NO removal ability, stable and continuous NO removal, and NO removal under outdoor conditions are required. In this study, therefore, the following investigations were done to evaluate practicability of this system.1. Rate limiting factors in the NO removal system was investigated. Based on that information, the ability of NO removal in this system could be increased by improving cultivation conditions.2. NO could be continuously removed under light condition. For outdoor system, it is important to drive this system under light-dark cycle. It was found that stable and continuous NO removal was successfully achieved by illuminating weak light during dark period in the light-dark cycle.3. By investigation of NO removal mechanism in the cells, it was shown that dissolved NO was directly taken in the cells, oxidized to nitrite and nitrate, and then consumed by the nitrogen assimilation pathway. The oxidation ability of NO to nitrite and nitrate might be supplied by the photosynthesis under light condition and the respiration under dark condition.4. The effect of SOx on NO removal from flue gas was investigated. It was shown that this system was available to remove NO from the desulfurized flue gas containing low concentration of SOx.