Renewable Energy Global Innovations features: Performance of mixed LED light wavelengths on biogas upgrade and biogas fluid removal by microalga Chlorella sp.

Significance Statement

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.

About The Author

Associate professor Dr. Cheng YAN comes from the Department of Environmental Science and Engineering, School of Environmental Studies, China University of Geosciences (Wuhan), No. 388 Lumo Road, Hongshan District, Wuhan 430074, Hubei Province, PR China.

Dr. Cheng YAN focused on Bioenergy with Carbon Capture and Storage (BECCS). He developed several Negative Emission Technologies (NETs), which involve CO₂ capture by biological processes from diffuse and point sources, atmospheric CO₂ capture by microalgae, microalgae as bio-agent for CO₂ mitigation, and CO₂ emission valorization. He also pays close attention to optimizing microalgae photo-bioreactor with artificial lighting system, and upgrading biogas by microalgae system. The research field about purifying anaerobic fermentation slurry by microalgae is also interested.

Dr. Cheng YAN has already published 17 academic papers (10 as the first author, 1 as the second author, and other 6 as cooperator) in refereed international JCR publications in English. He is keen on academic exchanges and participated in several academic international conferences in Barcelona, Prague, Dublin, and Singapore.

Journal Reference

Applied Energy, Volume 178, 15 September 2016, Pages 9–18. 

Cheng Yan^1,2, Liandong Zhu³, Yanxin Wang²

Show Affiliations

1. Laboratory of Basin Hydrology and Wetland Eco-restoration, China University of Geosciences (Wuhan), Wuhan 430074, China
2. Department of Environmental Science and Engineering, School of Environmental Studies, China University of Geosciences (Wuhan), Wuhan 430074, China
3. Faculty of Technology, University of Vaasa, FI65101 Vaasa, Finland

 

Abstract

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.

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