Hydrothermal carbonization is a process in which biomass is heated in water under pressure to create a char product. With higher plants, the chemistry of the process derives primarily from lignin, cellulose and hemicellulose components. In contrast, green and blue-green microalgae are not lignocellulosic in composition, and the chemistry is entirely different, involving proteins, lipids and carbohydrates (generally not cellulose). Employing relatively moderate conditions of temperature (ca. 200 °C), time (<1 h) and pressure (<2 MPa), microalgae can be converted in an energy efficient manner into an algal char product that is of bituminous coal quality. Potential uses for the product include creation of synthesis gas and conversion into industrial chemicals and gasoline; application as a soil nutrient amendment; and as a carbon neutral supplement to natural coal for generation of electrical power.
Bibliographical noteFunding Information:
Chlamydomonas reinhardtii (CC-125 wild type mt+137c) was obtained from The Chlamydomonas Resource Center (supported by the National Science Foundation) at the University of Minnesota. The alga was inoculated into 20 L glass carboys containing 18 L of TAP medium  . Synechocystis sp. strain PCC 6803 [N-1] was obtained from the American Type Culture Collection (Manassas, VA) and used to inoculate a 20 L glass carboy containing 18 L of BG-11 medium  . The inoculated carboys were placed within fluorescent light rings, producing 5960 cd and sparged with air containing 5% carbon dioxide for several days until the cell count reached a plateau as determined using a hemocytometer. Algae were harvested by centrifugation (8000 × g at 22 °C for 15 min). When crossflow filtration was employed, the alga was resuspended in 1 L of water and diafiltered against 8 L of water by passage through an Amersham Biosciences CFP-2-E-5A Hollow Fiber Cartridge using a peristaltic pump and a flow rate of 10 L/min. Centrifugate pastes were freeze-dried in order to employ accurate and reproducible masses in experiments. Aphanizomenon flos-aquae was purchased from Klamath Lake, Inc. (Klamath Falls, OR). Spirulina spp. and Chlorella spp. were both purchased as food-grade materials from a local health food store; the spray-dried materials were utilized as received. Dunaliella salina containing 2% β-carotene was a food-grade product obtained from Alibaba Inc. and was used as received.
Financial assistance was provided by the BioTechnology Institute of the University of Minnesota and the Initiative for Renewable Energy and the Environment (IREE) and is gratefully acknowledged. Dr. Kannan Seshadri of 3M is also thanked for plotting the data of the designed experiment.
Copyright 2010 Elsevier B.V., All rights reserved.
- Algal coal
- Hydrothermal carbonization
- Synthetic coal