Hydrothermal (HT) Processing of Plant Biomass for Petrochemical and Bioenergy Products
Introduction The Chinese tallow tree has simple, deciduous leaves which are alternate, broad rhombic to ovate in shape and have smooth edges, heart shaped and sometimes with an extended tail. (Photo courtesy of North Carolina State University). Hydrilla is a naturalised and invasive plant that is commonly found in the southeastern United States. (Photo courtesy of Purdue University)
Plant biomass represent a vast, renewable source of biobased feedstocks for chemical and thermal processing into biobased energy and petrochemical products. This ongoing research uses hydrothermal (HT) treatment, which simply refers to chemical reactions conducted in water that has been heated (200-600 ºC) and pressurized (50-500 bar) in the absence of dissolved molecular oxygen. Water under these conditions has very unusual properties, and a mixture of water and biomass can rapidly be transformed to products that contain many of the long-chain hydrocarbons present in modern combustible fuels.
HT conditions provide for reactions that are not achievable in other common treatment settings (e.g, pyrolysis, superheated steam, organic reaction systems, liquefaction and gasification). HT systems allow for biomass conversions to industrially useful or energy-yielding chemical mixtures in configurations that allow for (1) recovery of carbon, (2) detoxification of many pollutant organic chemicals, (3) recovery of elements including toxic metals and (4) recycling of heat and water in a closed-loop configuration.
Previous HT transformation studies were generated under simple reaction conditions (i.e., the substrates were mixed with tap water, sealed in a metal reactor vessel, treated in heated water under pressure with no added reactants or catalysts) with the intention of (1) estimating gas-phase and semi-volatile hydrocarbon yields, and (2) identifying the most abundant hydrocarbons generated in this process.
Research to date has demonstrated the technical applicability of HT processing for a wide range of plant biomass types, including invasive plant species and preservative-treated wood waste, both of which have little or no economic value and represent an environmental and ecological problem. This paper will summarize our results using various plant feedstocks for HT treatment with respect to hydrocarbon yields, chemical composition of semi-volatile mixtures and volume reduction.
Currently most decommissioned preservative-treated wood is disposed in landfills. This option is neither environmentally friendly nor economically advantageous for those disposing large volumes of treated material (e.g., utility and telecommunication industries). Since nearly half of all southern pine is preservative-treated, it is important for Louisiana forest landowners that the wood-preservation industry remain solvent so forest landowners can obtain maximum value for their timber.
Past research has extensively looked at the feasibility of HT to detoxify preservative-treated wood. The preservatives of interest have included chromate copper arsenate (CCA), creosote and pentachlorophenol (penta). We found that during treatment, the creosote-derived hydrocarbon residues in the wood were nearly completely recovered, and the wood itself was transformed into a mixture of hydrocarbons. These wood-derived transformation products served to reconstitute the "light end" of the creosote, which largely had been lost via leaching while in service. Thus, the hazardous waste (creosote-hydrocarbon mixture) was recovered, and the solid waste (wood) was transformed into a complimentary product mixture in a single pass. For the CCA- and penta-treated wood studies, wood particles were transformed into liquid and gaseous hydrocarbon mixtures irrespective of pH conditions and preservative.
It is cumbersome to segregate decommissioned preservative-treated wood by preservative type. Therefore, a successful recycling method must be able to accommodate mixtures of wood treated with different preservative types. Accordingly, current research is looking at the technical feasibility of HT treatment of equal parts of CCA-treated wood, creosote-treated wood and penta-treated wood. The initial data have shown that during HT treatment, the creosote-derived hydrocarbon residues were recovered in the decommissioned, treated wood and the wood mass itself was transformed into a mixture of industrially useful hydrocarbons. The metals from the CCA-treated wood were partially recovered. It is speculated that some arsenic was transformed into a gas, which could be trapped and recovered under industrial conditions. The penta was detoxified. The HT process also resulted in the generation of industrially useful mixed hydrocarbons with substantial reductions in substrate mass. Thus, the preservative-treated wood was transformed into a liquid mixture that contained many industrially useful products, some of which can be used for bioenergy and others for biobased chemicals.
It is important to note that this work was performed in large reactors, which were heated with hot air in a muffle furnace. Hence the incubation time was long. Our current work with small reactors heated rapidly (under two minutes) in tin baths to 400º C has confirmed that HT reactions occur on the order of seconds to minutes.
Invasive Aquatic Plants
This work examined HT treatment of three invasive aquatic plants (i.e., Lemna sp., Hydrilla sp. and Eichhornia sp.) Identical HT treatments yielded similar semi-volatile product mixtures for Hydrilla. sp. and Eichhornia sp. versus a significantly different mixture for Lemna sp. No semi-volatile hydrocarbons were found in any of the species prior to HT treatment. Post-HT-treatment product mixtures were composed of complex mixtures of compounds. All three plant HT product mixtures were dominated by compounds found in the top 100 industrially useful chemicals.
Results of wet chemical analyses showed that a major difference between Lemna sp. and the other two plants was significantly higher extractives levels in the former. It was found that this fraction accounted for much of the complexity in the post-HT treatment liquid of the Lemna sp. biomass. For all HT treatments, the substrate mass was reduced by 95% or more.
Chinese Tallow Tree
The principal tree species investigated to date is the Chinese tallow tree (Triadica sebifera [syn. Sapium sebiferum]. The species is extremely well adapted to numerous environments, and there are no known diseases that debilitate it. Also, it produces aboveground biomass at a significantly faster rate than most other tree species and is able to establish a dense stand quickly.
Work to date has explored the potential of Chinese tallow tree (wood/bark, leaves and seeds) as a raw material for biobased chemical and energy production using hydrothermal (HT) conversion. Seeds were HT treated as both whole and ground. Ground wood/bark, leaves and seeds yielded similar aromatic compound assemblages after HT treatment. Ground seeds yielded unique minor byproducts and did not contain naphthalene, which was present in the other tissue types. Whole HT-treated seeds yielded a material that resembled asphalt in appearance, odor and chemical properties but did not produce any phenol. In contrast, ground seeds did not yield any particulate matter and had substantial amounts of phenol.
With regards to the energy input/output of this work, the HT treatment had a fairly neutral effect on energy content of the tallow seeds. The energy values of the tallow seeds are much higher than those typically reported for hardwood stemwood.
In recent years, the authors have documented the transformation of underused biomass (WB) into gas phase and semi-volatile hydrocarbon mixtures in hydrothermal (HT) reaction systems. HT systems allow for biomass conversions to industrially useful or energy-yielding chemical mixtures in configurations that allow for (1) recovery of carbon as volatile and semi-volatile petrochemicals, (2) transformation of many pollutant organic chemicals, (3) recovery of elements including toxic metals and (4) recycling of heat and water in a closed-loop configuration. Studies to date have documented treatability efficiencies on the order of 90-99% on a mass basis and the generation of saleable and/or value-added chemical product mixtures including volatile and semi-volatile compounds including over one dozen chemicals in the top 100 commercial petrochemicals.