Hui Pan and Todd F. Shupe

Using biomass as an alternative to petroleum-based products for fuel has attracted interest because of its biodegradable nature and renewable properties. Biomass conversion to liquid and gaseous products has potential to provide a wide range of bio-based energy, fuels and chemicals. Thermochemical conversion, including pyrolysis, liquefaction or gasification, is a main route of biomass utilization that results in biofuels and valuable bio-based chemicals.

Liquefaction refers to two different types of processes – direct liquefaction, which is similar to pyrolysis but conducted under lower temperature and pressure, and solvolysis liquefaction, in which biomass is dissolved in an organic solvent. The term wood liquefaction refers to solvolysis liquefaction.

Wood is a natural polymer consisting of three main components – cellulose, hemicellulose and lignin. Cellulose is a linear polymer of glucose units and can be thought of as one kind of sugar. Hemicellulose is similar to cellulose and bonds to cellulose to form a network of cross-linked fibers. Lignin is generally referred to as the "glue" that holds wood together. For example, lignin must be removed in the pulping process to separate the wood fibers and produce high-quality paper. Lignin is tightly bonded with the other wood components and provides the supporting and strengthening material in wood.

In 1946, a process was patented to convert bio-based materials containing lignin and cellulose, particularly bagasse, into a phenolic-based wood adhesive by reacting the material under elevated temperature and high pressure in the presence of phenol and sulfuric acid. Because phenol formaldehyde resins are derived from petroleum, which at the time was abundant, this approach was not adopted by the industry.

Fast forward to the 1980s when the global marketplace started to undergo demand for "greener" bio-based products in many developed nations, increased mechanization in underdeveloped countries, and an overall increasing human population with a corresponding demand for traditional fossil fuels. The 1980s saw many patents and research papers published about wood liquefaction and its applications for adhesives and moldings and even as a means to develop foam-based products.

Most methods for wood liquefaction include methods that use catalysts and those that do not use catalysts. A catalyst is a chemical added to increase the rate of the reaction. During liquefaction, wood is converted into a viscous liquid. Almost all sources of biomass – trees, bark, corn stalks, bagasse, etc. – can be liquefied successfully.

Indeed, many studies have been conducted to establish the basis of the liquefaction process and its application for various products; however, research on many fundamental aspects is still lacking. Research on wood liquefaction has been ongoing at the LSU AgCenter since 2003. This research involves both improvement and refinement of the liquefaction process and applications of liquefied wood for value-added products.

Because wood is a complex bio-polymer consisting of different components, liquefaction is a complicated process. Figure 1 shows liquefied wood that has undergone reaction with an organic solvent and an acid catalyst at 320 degrees F for 120 minutes. The solid wood powder has been converted into a viscous, dark-brown liquid.

A residue of solid wood always remains after the reaction, and characterization of liquefied wood residues provides a new approach to better understand some fundamental aspects of the wood liquefaction reaction. LSU AgCenter studies of the residues have shown that lignin is the easiest of the three main wood components to liquefy. The liquefaction rate of the three components is, in decreasing order – lignin, hemicellulose and cellulose. In general, a reaction comprising a higher reagent-solvent-to-wood ratio, longer reaction time, higher liquefaction temperature and stronger-acid catalyst will result in less wood residue.

Strong acid catalysts in the reaction, however, may require special equipment to prevent corrosion, and phenol is the most expensive raw material for liquefaction. Therefore, it is desirable to substitute wood for phenol as much as possible in the process of liquefaction. One significant difference of the weak-acid catalyzed system compared with the strong-acid system is the high residue content, indicating incomplete liquefaction.

The commercial application of liquefied wood depends on the reagent solvent used in the liquefaction. Wood liquefied with phenol can be directly developed into molded products or used to produce phenolic resin or foam. Phenolic resin is a leading adhesive for composite panels in moisture-sensitive applications, such as oriented strand board and plywood used for sheathing.

Wood liquefaction is a key technological area being explored in conjunction with the closed-loop recycling program at the LSU AgCenter’s Calhoun Research Station, particularly with regard to chromated copper arsenate-treated wood. When recycling utility poles, some of the poles will be too severely decayed or damaged to be sawn into lumber or otherwise machined. This material and the sawdust generated from sawing sound poles can be used as a feedstock for the liquefaction process.

The process allows for the wood and heavy metals in the preservative to be recovered separately. This has potential for rural economic development because it can be easily integrated with other recycling methods such as re-sawing of spent chromated copper arsenate-treated utility poles. And more than 95 percent of the heavy metals in the preservative can be recovered. Moreover, this is a "green" technology that can reduce the volume of preservative-treated wood in landfills and help maintain the viability of the preservative-treated wood industry. Poles represent a high-value, downstream product for private, nonindustrial forest landowners, and a strong wood preservation industry helps ensure that private nonindustrial forest landowners receive top dollar for their pole-sized timber.

Research on wood liquefaction at the Calhoun Station includes developing value-added products from liquefied wood and scaling up the application of liquefied wood for resin and foam products.

Hui Pan, Assistant Professor, Calhoun Research Station, Calhoun, La., and Todd F. Shupe, Professor, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.

(This article was published in the  fall 2009 issue of Louisiana Agriculture.