Lignocellulose refers to plant dry matter (biomass), so called lignocellulosic biomass. It is the most abundantly available raw material on the Earth for the production ofbiofuels. It is composed of two kinds of carbohydrate polymers,cellulose andhemicellulose, and an aromatic-rich polymer calledlignin.[1] Any biomass rich in cellulose, hemicelluloses, and lignin is commonly referred to as lignocellulosic biomass.[2] Each component has a distinct chemical behavior. Being a composite of three very different components makes the processing of lignocellulose challenging. The evolved resistance to degradation or even separation is referred to as recalcitrance. Overcoming this recalcitrance to produce useful, high value products requires a combination of heat, chemicals, enzymes, and microorganisms.[3][4][5][6] These carbohydrate-containing polymers contain different sugar monomers (six and five carbon sugars) and they are covalently bound to lignin.
Lignocellulosic biomass can be broadly classified as virgin biomass, waste biomass, andenergy crops. Virgin biomass includes plants. Waste biomass is produced as a low value byproduct of various industrial sectors such asagriculture (corn stover, sugarcanebagasse, straw etc.) andforestry (saw mill andpaper mill discards).Energy crops are crops with a high yield of lignocellulosic biomass produced as a raw material for the production of second-generation biofuel; examples include switchgrass (Panicum virgatum) andelephant grass. The biofuels generated from these energy crops are sources of sustainable energy.[7][8]
Lignocellulose consists of three components, each with properties that pose challenges to commercial applications.[10]
Many crops are of interest for their ability to provide high yields of biomass. Some can be harvested multiple times each year. These includepoplar trees andMiscanthus giganteus. The premier energy crop issugarcane, which is a source of the readily fermentablesucrose and the lignocellulosic by-productbagasse.
Lignocellulosic biomass is the feedstock for thepulp and paper industry. In this process lignin and hemicellulose are typically separated from the plant material leaving the fibrous cellulose component to be processed for paper production, or 'chemical cellulose'. Through the pulp process most of the lignin is removed and discharged as waste material in the form of effluent/wastewater before then being used as low-value fuel to generate electricity and heat.[13] In principle, the world's current sugar demand could be fulfilled by repurposing pulp and paper mills for lignocellulosic sugar production, making it a promisingresilient food.[14]
Lignocellulosic biomass, in the form ofwood fuel, has a long history as a source of energy. Since the middle of the 20th century, the interest of biomass as a precursor toliquid fuels has increased. To be specific, thefermentation of lignocellulosic biomass toethanol[15] is an attractive route to fuels that supplements thefossil fuels. Biomass can be acarbon neutral source of energy in the long run. However depending on the source of biomass, it will not be carbon neutral in the short term. For instance if the biomass is derived from trees, the time period to regrow the tree (on the order of decades) will see a net increase in carbon dioxide in theEarth's atmosphere upon the combustion of lignocellulosic ethanol. However, if woody material from annual crop residue is used, the fuel could be considered carbon-neutral. Aside from ethanol, many other lignocellulose-derived fuels are of potential interest, includingbutanol,dimethylfuran, andgamma-valerolactone.[16]
One barrier to the production of ethanol from biomass is that the sugars necessary for fermentation are trapped inside the lignocellulose. Lignocellulose has evolved to resist degradation and to conferhydrolytic stability and structural robustness to the cell walls of the plants. This robustness or "recalcitrance" is attributable to the crosslinking between the polysaccharides (cellulose and hemicellulose) and the lignin viaester andether linkages.[17] Ester linkages arise between oxidized sugars, theuronic acids, and the phenols and phenylpropanols functionalities of the lignin. To extract the fermentable sugars, one must first disconnect the celluloses from the lignin, and then use acid or enzymatic methods tohydrolyze the newly freed celluloses to break them down into simple monosaccharides. Another challenge tobiomass fermentation is the high percentage of pentoses in the hemicellulose, such asxylose, or wood sugar. Unlike hexoses such as glucose, pentoses are difficult to ferment. The problems presented by the lignin and hemicellulose fractions are the foci of much contemporary research.
A large sector of research into the exploitation of lignocellulosic biomass as a feedstock for bio-ethanol focuses particularly on the fungusTrichoderma reesei, known for its cellulolytic abilities. Multiple avenues are being explored including the design of an optimised cocktail of cellulases and hemicellulases isolated fromT. reesei, as well as genetic-engineering-based strain improvement to allow the fungus to simply be placed in the presence of lignocellulosic biomass and break down the matter intoD-glucosemonomers.[18] Strain improvement methods have led to strains capable of producing significantly more cellulases than the original QM6a isolate; certain industrial strains are known to produce up to 100g of cellulase per litre of fungus,[citation needed] thus allowing for maximal extraction of sugars from lignocellulosic biomass. These sugars can then be fermented, leading to bio-ethanol.
Some chemicals could be obtained from lignocellulosic biomass. Almost all are derived from the sugars obtained by hydrolysis of the cellulose component.[19]
Lignocellulosic biomasses has been considered in the production of biocomposites materials such as particle panels, wood-plastic composites, and cement/geopolymer wood composites. Even though the production of biocomposite materials rely mostly on wood resources, in less forest-covered countries or in countries where wood resources are already being overused, it is possible to utilize alternative sources of biomass such as invasive plants, agricultural and sawmills residues for the creation of new "green" composites.[20] Biocomposites produced with lignocellulosic biomass as an alternative to conventional materials, are attracting attention because they are renewable and cheaper but also because they fit perfectly into the policy of the "cascade utilization" of the resources.
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