Biogas is the most important renewable energy resource that attracts attention all around the world. In Europe, the biogas production increases from 1.18 million MMBtu d-1 in 2010 to 1.46 million MMBtu d-1 in 2013, while the total biogas potential is estimated as 16 million MMBtu d-1. In China, the biogas production of small scale projects raises from 180 million m3 in 1996 to 1000 m3 in 2007, and the medium and large scale biogas projects raises from 12 billion m3 in 1996 to 600 billion m3 in 2007.
Raw biogas usually consists of methane (CH4, approximately 60 vol. %), carbon dioxide (CO2, approximately 40 vol. %), and other trace compositions including hydrogen sulfide (H2S), water vapor, etc. However, the relatively high concentration of CO2 in raw biogas will lower its heat content as well as increase its energy demand of compression and transportation usage. The biogas upgrading is removing CO2 from raw biogas. It is the precondition for biogas high efficient usage. When the CO2 content in the biogas is decreased, the CH4 concentration in the biogas is increased. The biogas CH4 concentration should be upgraded to at least higher than 90% (vol. %) to meet the criterion for using as fuel for vehicles or even substitute for natural gas.
There are several biogas upgrading techniques that have been widely applied nowadays, including absorption of liquids with physics/chemical adsorbent, membranes separation, pressure swing adsorption, and cryogenic separation. However, they usually need high capital cost when build construction and consume a large amount of energy during treating process. This makes above mentioned techniques achieve high economic benefit only when they are used in large-scale industrial biogas projects. Most of these techniques also require complicated operating systems, and produce unwanted end products that need further treatment or result in secondary pollution. Further, the CO2, which removed from the raw biogas, is always discharged into the atmosphere as greenhouse gas in these techniques. In addition, most physical/chemical technologies for CO2 removal require a prior removal of H2S.
An alternative technique to upgrade biogas is to use photosynthetic CO2 uptake by microalgae. Microalgae have high carbon fixation ability and rapid growth rate, and can be adapted to various environmental conditions. When microalgae are utilized for biogas upgrading, the photosynthesis can efficiently convert CO2 in raw biogas into its biomass. This allows the valorization of biogas CO2 in the form of a valuable microalgae biomass, which can be used as feedstock to produce biofuels or even high value-added by-product. Anaerobic digestion not only produce raw biogas, but also nutrient-rich waste stream, called biogas slurry, which can be uptake freely during microalgae growth process and made the major contribution to the nitrogen and phosphorus removal from biogas slurry wastewater. Therefore, removing CO2 from raw biogas by culturing microalgae with biogas slurry is a highly potential technique for simultaneous biogas upgrading and biogas slurry decontamination.
However, as far as we know, there is a little literature available about the simultaneously biogas upgrading and biogas slurry decontamination by using of the photosynthetic CO2 uptake of microalgae, particularly about its effects under various light intensities and wavelengths. Therefore, this research focused on the effects of various LED artificial light source’s light wavelengths, light intensities, and photoperiods on biogas upgrading and simultaneously biogas slurry decontamination by using of microalgae photo bioreactor. Furthermore, the most appropriate light wavelength was discussed. The lighting control strategy was also optimized by analyzing the microalgae growth, as well as the efficiencies of biogas CO2 removal and simultaneously biogas slurry decontamination under various light intensities and photoperiod’s treatments.
Cheng Yan1,2, Liandong Zhu3, Yanxin Wang2Show Affiliations
- Laboratory of Basin Hydrology and Wetland Eco-restoration, China University of Geosciences (Wuhan), Wuhan 430074, China
- Department of Environmental Science and Engineering, School of Environmental Studies, China University of Geosciences (Wuhan), Wuhan 430074, China
- Faculty of Technology, University of Vaasa, FI65101 Vaasa, Finland
Anaerobic digestion not only produces raw biogas which needs to be upgraded, but also nutrient-rich waste stream biogas slurry which needs decontamination. Therefore, this research focused on the effects of various light wavelengths, light intensities, and photoperiods on biogas upgrading and simultaneously biogas slurry decontamination by using of microalgae photobioreactor. The microalgae photobioreactor was a transparent polyethylene bag (80 cm × 60 cm × 11 cm). The results demonstrated that biogas upgrading and simultaneously biogas slurry decontamination was successfully achieved by the use of the photosynthetic CO2 uptake by microalgae photobioreactor. The optimal light wavelength was the mixed LED red:blue = 5:5; whereas the optimized lighting control strategy was: low light intensity (300 μmol m−2 s−1) with long photoperiod (16 h light:8 h dark) for the time course of 0–48 h, moderate light intensity (600 μmol m−2 s−1) with middle photoperiod (14 h light:10 h dark) for the time course of 48–96 h, and high light intensity (900 μmol m−2 s−1) with short photoperiod (12 h light:12 h dark) for the time course of 96–144 h. Its biogas CO2 removal efficiency was 85.46 ± 6.25%. Its removal efficiency of chemical oxygen demand, total nitrogen, and total phosphorus were 85.23 ± 8.32%, 87.10 ± 7.55%, and 92.40 ± 3.05%, respectively.Go To Applied Energy