Hydrothermal Carbonization: An Innovative New Process for the Extraction
of Algal Oil
Kenneth Valentas, BioTechnology Institute, University of Minnesota, St. Paul, MN
S. M. Heilmann, Biotechnology Institute, University of Minnesota, St. Paul, MN
Harvesting the oil produced by algae is a major technical problem that has yet to be economically resolved in an energetically favorable manner. Many research groups are presently engaged in developing techniques to extract lipids from algae and integrating the oil extracts into conventional petroleum processing to produce diesel and jet fuels. The default process for recovering algal oil consists of first drying the algae and then extracting the oil with organic solvents. Most industrial operations that involve drying algae require a significant amount of energy to remove water. This can result in an overall negative energy balance, depending on the algal oil content and often more energy is required for this drying operation than can be obtained from the recovered oil.
Algal slurries as harvested contain 2-3% solids. They can be concentrated through centrifugation or other methods up to as high as 30% solids with expensive equipment and expenditure of energy. To extract the oil effectively the conventional approach is to dry the algal concentrate to about 10% moisture.
Various hydrothermal processing methods for conversion of biomass have been reported. All have an advantage over many conversion processes in that the starting biomass does not need to be dry, and thus the significant energy input required to remove water by evaporation is eliminated. Hydrothermal gasification (HTG) is the most thermally severe process and has been conducted both without catalyst at 400 – 800°C (Matsumura et al. 2005) or with Ni and Ru catalysts at 350 – 400°C (Elliott et al. 2008). Gaseous products from HTG include hydrogen, methane, and carbon dioxide, and this process has also been extended to the use of microalgae (Haiduc et al. 2009). Hydrothermal liquefaction, which is generally conducted at 250 - 450°C (Zhang et al. 2008), provides liquid bio-oils as well as gaseous products and has also been extended to microalgae (Patil et al. 2008).
The mildest reaction conditions, in terms of temperature and pressure, are employed in hydrothermal carbonization (HTC). Lignocellulosic substrates have been extensively examined as reactants at temperatures from 170 – 250°C over a period of a few hours to a day (Titirici et al.2007), and this process has been the subject of a recent review (Titirici et al. 2010). The HTC process takes place effectively only in water, is exothermic, and proceeds spontaneously in the absence of catalysts. In many hydrothermal processing methods, the desired objective of increasing the carbon-to-oxygen ratio (commonly referred to as“carbonization”) has been accomplished by splittingoff carbon dioxide from the reactants (Schumacher et al. 1960). However, this mechanism is undesirable because carbon and oxygen are depleted, as carbon dioxide, and the creation of gaseous products causes even greater reaction pressures. This increases the complexity and cost of reaction equipment. With the HTC process, however, the carbon to oxygen ratio is improved by removing water instead of carbon dioxide, and at the same time, the chemical integrity of lipids is maintained. Thus, the HTC process can be used to separate lipids and oils from the starting biomass.
The HTC process, which is straightforward and environmentally sound, involves heating algal biomass water slurries to temperatures of 190 – 210°C in a confined system at equilibrium pressure for relatively short times on the order of 15 minutes. In the HTC process, oxygen and some hydrogen are removed from the biomass due to the formation of water, even in an aqueous medium. This process occurs through the thermodynamically favored reaction path (Peterson, et al. 2008).
Three valuable product streams have been thus far created by HTC using algae as the starting biomass: (1) a char that is similar compositionally to a very clean coal (algae biochar) but with a highly porous surface and interior morphology, (2) an algal “crude” oil that is “bound” to the biochar, and (3) an aqueous fraction containing soluble products that have utility as a fertilizer to be recycled to support continual algal growth, or for other agricultural applications. The solid and aqueous phases are easily separated by filtration. The algal oil is separated from the char and aqueous fertilizer phase by standard solvent extraction practice.
Consequently, this simple, environmentally friendly, and low energy-requiring process can provide a powerful tool for the economical recovery of algal oil from algal biomass.