Hydrothermal (HT) Processing of Plant Biomass for Petrochemical and Bioenergy Products

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.