Hydrothermal carbonization of microalgae
Introduction
With the apex of the world's petroleum production from known reserves having already been achieved or will be attained in the very near future, increased utilization of coal as an energy source seems a certainty. Aside from formidable health problems associated with increased atmospheric particulate and heavy metal contents, coal, like petroleum, is a fossil fuel and burning massively increased quantities of coal will greatly exacerbate the very serious problem of global warming. In contrast, combustion of biomass that has not been stored for eons in subterranean reservoirs releases carbon dioxide that is not “new” to the earth's atmosphere and constitutes a “carbon neutral” event.
Green and blue-green (cyanobacteria) microalgae have been on the earth for millions of years and differ substantially from higher plants. They are single-celled microorganisms that live in aquatic environments, and all components necessary for life and procreation are located within a single cell. In higher terrestrial plants, specialized cells with specific functions are required that make up roots, stems, flowers and other functional parts. Cellulose, hemicellulose and lignin often provide structural support for these specialized cells and are present in significant quantities. In contrast, microalgae and cyanobacteria are not lignocellulosic in composition but are comprised of proteins, lipids, non-cellulosic carbohydrates, and nucleic acids.
Various hydrothermal processing methods have been reported. All enjoy the significant advantage that starting biomass does not need to be dry, and the significant energy input required to remove water by evaporation is eliminated. Hydrothermal gasification is the most thermally severe and has been conducted both without catalyst at 400–800 °C [1] and in the presence of Ni and Ru catalysts at 350–400 °C [2]. Gaseous products include hydrogen, methane, and carbon dioxide, and this process has also been extended to microalgae [3]. Hydrothermal liquefaction, generally conducted at 250–450 °C [4], provides liquid bio-oils as well as gaseous products and has also been extended to microalgae [5].
The mildest reaction conditions in terms of temperature and pressure are employed in hydrothermal carbonization (HTC). Lignocellulosic substrates have been extensively examined [6] as reactants at temperatures from 170 to 250 °C over a period of a few hours to a day, and this process has been the subject of a recent tutorial review [7]. The process takes place effectively only in water, is exothermic, and proceeds spontaneously. Two product streams are created that are isolated by filtration: 1) an insoluble, char product and 2) water soluble products. In general, the desired objective of increasing the carbon-to-oxygen ratio (commonly referred to as “carbonization”) has been accomplished by endeavoring to split off carbon dioxide [8], [9], [10]. This mechanism is undesirable because, with loss of carbon dioxide, carbon is depleted as well as oxygen, and creation of gaseous products causes even greater reaction pressures that increase complexity/cost of reaction equipment. No published technical reports of algal species being subjected to HTC have been found.
The principal objective of the present work was to focus on the char product and to obtain a high level of carbonization and yield, while simultaneously minimizing processing time. Hopefully, relatively brief reaction times can be employed in a batch mode that would suggest the potential for continuous processing. This is regarded as being imperative if the technology is to have longer term practical impact. Microalgae should be excellent biomass substrates for this purpose because their small size will facilitate rapid thermal transfer to processing temperatures. A secondary objective was to accomplish carbonization by a mechanism other than by loss of carbon dioxide, and the non-cellulosic carbohydrate composition of microalgae may allow dehydration to occur at relatively moderate temperatures in the manner of soluble biomass substrates, e.g., glucose [11]. Reaction parameters examined included time, temperature, and algal concentration. Potential catalysts for the HTC process with microalgae were also evaluated. Compositions and energy contents of resulting algal char products were determined and compared with a natural coal and a char obtained by HTC of a lignocellulosic biomass substrate.
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Material and methods
Elemental analyses, heats of combustion, ash, and carbonate determinations for the various products were performed by Galbraith Laboratories, Inc. (Knoxville, TN). SEM analyses were performed at the University of Minnesota Imaging Center, College of Biological Sciences, St. Paul, MN.
Calculations
Computations for the following sections were conducted using experimental details of the HTC of C. reinhardtii of Section 2.2.1.
Effect of metal salt additives and acids
An early report [16] examining the hydrothermal carbonization of sucrose focused on the development of turbidity in the presence of various metal salt additives at 100–120 °C. Of the metal salts reported to have a high degree of influence on turbidity development, only CaCl2 and MgCl2 were environmentally acceptable as additives; oxalic acid was also reported to be highly effective. Another report [17] indicated that ferrous ion and iron oxide nanoparticles were effective catalysts in HTC of
Conclusions
Hydrothermal carbonization of microalgae provided char products of unique composition and with energy contents in the bituminous coal range. Process conditions were remarkably mild, e.g., ca. 200 °C and times as brief as 0.50 h, for developing acceptable levels of carbonization and yields of algal char materials. The relatively brief reaction time demonstrated in batch processing suggested that a continuous process might be developed for the HTC processing of algae. Some strains of
Acknowledgement
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.
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