A "New" Resource for Bioprocessing April 2013
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Until recently, lignin was mainly a low-value by-product of pulp and papermaking. Lignin is an abundant natural biopolymer, taking second place in plant-structure volume to cellulose but with a complex chemical structure that makes it less accessible to processing—with the result that researchers have given little time and effort to developing the material for commercial applications. However, more recently, the forestry industry has been seeking ways to become not only more efficient but also more than simply a producer of high-volume, low-value products. Countries in the northern hemisphere, with well-established forestry interests, lead the way in lignin research and development.
Lignin Availability for Bioprocessing
Vascular plant material contains lignin, and it is associated with cellulose; lignin acts as glue, binding the cellulose fibrils into a structure that will support the plant. Being hydrophobic, lignin also aids the transport of water through the plant by impeding the passage of water through hydrophilic and permeable cell walls. Some 30% of plant biomass is lignin, with softwoods having a lignin component of 27% to 33% by mass; hardwoods, 18% to 25%; and grasses, 17% to 24%. The pulp and paper industry must separate lignin from cellulose in order to produce its end product of high-quality paper (newsprint production retains more lignin than does high-quality paper production), and worldwide, this industry generates extracted lignin at a rate of some 50 million tonnes a year. Of this output, about 2% sells on for use in low-value products such as concrete additives, animal feed, and dispersing and binding agents; typically the pulp and paper facility uses the remainder as a fuel to power its production process.
A driver for ramping up research into lignin is increasing pressure from governments and pressure groups for the biofuel industry to move away from first-generation biofuel feedstocks (producing sugars for bioethanol from crops such as sugarcane and corn) that derive from food material or from crops that farmers grow in place of food crops. Lignin-containing plant material is one of many second-generation feedstocks that are less suitable than first-generation biofuel feedstocks for the production of biofuels because the removal of lignin and the breakdown of cellulose are an additional and not yet cost-effective processing stage necessary before the sugar that derives from the process is available for conversion to bioethanol. To widen the choices of material suitable as feedstocks for biofuel production, researchers must deliver cost-effective processes that remove or break down lignin and cellulose. A processing route that extracts lignin and provides a means to utilize it in bioprocessing would mean, for example, that a wide variety of waste plant material—stalks, leaves, husks, and wood chippings—could provide an alternative to corn or sugarcane for biofuels and bioplastics.
The development of lignocellulosic biofuels production alongside paper and pulp production could generate a further and significant supply of lignin as a by-product. Because the raw material is already available, the challenge for researchers moves to capitalizing on the potential of lignin as a feedstock for high-value chemical processing. The chemical industry could access phenols, hydrocarbons (benzene, toluene, xylene), and oxidized products such as vanillin by exploiting the aromatic structure of lignin. Lignin is an amorphous polymer with a complex aromatic substructure that is based on a crosslinked network of C9 phenylpropenyl units. In addition, lignin could feed into the production of macromolecular materials such as carbon-fiber fillers, polymer extenders, and thermoset resins.
To maintain its structural-gluing function in plants, lignin is resistant to degradation, and it is this feature that makes it a difficult material to extract and process. In the pulp and paper industry (the largest source of lignin today), paper manufacturers use the acid-sulfite pulping process to remove lignin by using salts of sulfurous acid to produce water-soluble sulfonated lignin. An alternative process—the kraft process—is less in use and produces unsulfonated lignin. These lignin products have limited potential for further chemical processing because they are typically contaminated with salts, sugars, particulates, and volatiles. Researchers are looking for more efficient processes to provide lignin of higher purity in order to supply a full range of chemical-processing opportunities.
Lignin is available from both hardwood and softwood trees and monocots (grasses), but all types of plants have a characteristic profile of lignin chemical subunits. For example, all contain common aromatic compounds, but whereas hardwoods have a mix of coniferyl alcohol and sinapyl alcohol residues, softwoods have mainly coniferyl alcohol residues and only trace amounts of sinapyl alcohol residues.
An acid-catalyzed steam pretreatment makes the cellulose content of softwood available for further processing to bioethanol. This stage in the production of bioethanol is energy intensive and expensive. Researchers are looking for alternative routes, such as a biological pretreatment step (see below) that would release cellulose at lower cost and would provide lignin as a by-product.
Wealth Out of Waste is an apt phrase that introduces a research program at the University of Warwick (Warwick, England) that includes a focus on lignin and its potential as a chemical feedstock for a range of chemical syntheses. In the natural environment, lignin breakdown occurs by the action of fungi. For example, white-rot fungi produce an extracellular lignin peroxidase enzyme that breaks down lignin. According to the Warwick team, researchers' efforts to harness these enzymes have not yet delivered a commercial process for lignin breakdown. The following examples highlight some research that could contribute to more effective lignin extraction.
- The Warwick researchers are screening an alternative group of microorganisms—bacteria—to identify novel lignin-degrading systems; in local soil samples, they have found ten new lignin-degrading bacterial strains. Their next research phase is to isolate the enzymes in the degradation; with that information, they will have the potential to engineer bacteria that might selectively become bioprocessors for specific, and commercially useful, lignin-breakdown products such as vanillin. The Warwick team reports on its identification of a lignin peroxidase and the characterization of a recombinant form of the enzyme in a 2011 paper in the journal Biochemistry.
- Researchers at Central South University (Changsha, China) reported in the journal Biotechnology for Biofuels in January 2013 on the lignin-degradation capability of beta-proteobacterium Cupriavidus basilensis B-8, which they isolate from bamboo. Their studies focus on the degradation of kraft lignin, a product of the alkaline-sulfide treatment of lignocellulose, and extend to an analysis of the bacterial genome and identification of degradation steps and intermediates.
- Recognizing that a challenge for the utilization of cellulose is the difficulty of extracting it from lignocellulose plant material, researchers at the Beijing Academy of Agriculture and Forestry Sciences (Beijing, China) are working on the transgenic modification of lignin in poplar to improve the availability of cellulose for bioprocessing. The transgenic trees have a reduced lignin content of up to 10%, and down-regulation of CoA 3-O-methyltransferase enzymes increases the potential for breaking down cellulose into monosaccharide components (the process of saccharification).
- Focusing more on the extraction of cellulose, an aqueous-solvent-extraction system under development by researchers at the University of Wisconsin (Stevens Point, Wisconsin) separates plant material into cellulose and lignin. The cellulose fraction can serve for any of the regular applications for cellulose, including its use as a feedstock in biofuel production. The separated lignin fraction has value as a fuel, either alone or in combination with biodiesel, but may also be a valuable input into bioprocessing because the material is reportedly a purified product without the additional chemicals associated with lignin extraction by other methods. The researchers have patented the process and are looking to move their research to a demonstration-scale plant.
Gasification of Lignin
Extracted lignin in a less pure form could provide a feedstock for a power-generation technology. For example, in biofuel (bioethanol) production, the process that releases cellulose from plant material leaves some 15% to 30% of the initial biomass as unconverted lignin. This material, essentially a by-product of biofuel production, is a potential input for the production of synthetic fuels (syngas) by gasification and recombination of the gas constituents into alcohol. According to Biomass Magazine, biomass gasifiers are still at an early stage of development, and the challenge for researchers in this area is to produce clean synthetic gas from lignin. The University of North Dakota Energy & Environmental Research Center (Grand Forks, North Dakota) is one organization pursuing improved gasification technology; its focus is on thermocatalytic conversion of lignin residue to synthetic fuels.
Industry Interest in Lignin
Until recently, other sources of feedstock than lignin were more attractive to chemicals producers as alternative starting materials to oil and natural gas for chemical processes. However, the current wider agenda of governments and industries that look for greater value in waste products is driving research in the structure, degradation, and utilization of lignin. The availability of lignin and its potential in bioprocessing, and in product applications, have potential to change the profile of this renewable material. Market-research-company Frost & Sullivan published a market report in 2012 that highlights lignin's position saying, "It is the only renewable source for industrial aromatics production and is de-correlated from the fluctuating price of oil."
The pulp and paper industry has an interest in improving its production processes and is looking at ways to utilize the lignin it produces. Industry associations in North America and Europe are active in promoting the potential for lignin and in supporting its development. Governments also see the potential rewards of utilizing lignin and support its place as a component in a bio-based economy. For example, in the European Union, the Biosynergy project supports research into the use of biomass for synthesis and energy and the development of biorefining.
The following examples highlight some recent industry-related developments.
- Alberta-Pacific (Alpac; Edmonton, Ontario) is a large forestry-products company that recently joined the Lignoworks Network (Ilderton, Ontario), an organization that aims to "create technology platforms for novel materials and chemicals based on lignin to replace fossil-fuel based chemicals and products." Alpac is looking for opportunities to develop value-added lignin applications and thereby use the lignin resulting from its core business, bleached kraft pulp, to greater purpose than its use as a fuel.
- Innventia (Stockholm, Sweden) is a research company developing technology that supports greater use of forestry products. The company has a small team of researchers developing technology for carbon-fiber production (in its LigniCarb project) using lignin as the raw material. The researchers report success in spinning carbon fibers from softwood and note that different wood types produce material with distinct characteristics. A high-value application for lignin would be an attractive development, and a bio-based route to carbon fibers would support aims to reduce dependence on fossil fuels. Innventia has also developed LignoBoost technology in partnership with Chalmers University of Technology (Gothenburg, Sweden). The technology enables economic expansion of the capacity of pulp mills to extract high-quality lignin from the pulp-production process.
- In early 2013, Domtar Corp (Montreal, Canada) completed its installation of a commercial-scale lignin-separation plant (using Innventia's LignoBoost technology). The facility produces BioChoice lignin, and at target capacity, the output will be 75 tonnes per day. Domtar sees its product going to a range of industrial applications as a bio-based alternative to fossil fuels and for petroleum-based thermoset and thermoplastic resins.
- Tecnaro GmbH (Ilsfeld-Auenstein, Germany) mixes lignin with natural fibers from hemp, flax or similar plants, and natural additives and processes the mixture at raised temperatures to produce high-quality thermoplastics that are disposable by incineration. The material, Arboform, finds use in products such as automotive interior panels and casings for electronics and in the construction industry. An early product application was a wood-effect casing for a watch. Fujitsu has used Arboform to make a palm rest for its computer keyboards. In the construction industry, Arboform finds use instead of wood and has the advantage of being uniform (no grain) and able to withstand loads in any direction.
- The Maersk Group (Copenhagen, Denmark) recently announced its intention to develop a lignin-based fuel for marine applications. The March 2013 announcement does not say how its researchers will achieve this goal, but Maersk will be working with Progression Industry (Eindhoven, Netherlands), an organization that conducts R&D to develop a range of green solutions for the automotive industry. Maersk is also part of the Biomass for the 21st Century project that has funding from the Danish National Advanced Technology Foundation, with an academic lead from the University of Copenhagen. Maersk sees the potential for the company to buy some 50 000 tonnes of a lignin-based fuel per annum.