Lignocellulosic Energy Crops, Production, and Provision

Abstract

Glossary C3 Grasses Grasses possessing the C3 photosynthetic mechanism, so called because the first product of carbon fixation contains three carbon atoms. C4 Grasses Grasses possessing the C4 photosynthetic mechanism, so called because the first product of carbon fixation contains four carbon atoms. Cellulose Component of plant cell walls made up of chains of D-glucose (C6 sugar) molecules linked via hydrogen bridges consisting of 42 % carbon (C), 6 % hydrogen (H), and 52 % oxygen (O). Energy crops Crops grown for the purpose of producing biomass for energetic use. Energy grasses Perennial grasses with high biomass potential which can preferentially be harvested once a year at low moisture content. Fischer-Tropsch diesel Biofuel produced from lignocellulosic biomass via a multistep process. Gasification produces synthesis gas, which is cleaned and then transformed to liquids at high temperature and pressure by Fischer-Tropsch catalysis. Hemicellulose A heteropolysaccharide that contains various different sugar monomers such as glucose, xylose, mannose, galactose, rhamnose, and arabinose. Hemicellulose contains mostly D-pentose or so-called C5 sugars and is found in plant cell walls. Lignin Complex three-dimensional polymers built from phenylpropane derivatives consisting of 64 % carbon, 6 % hydrogen, and 6 % oxygen. Rhizomes Underground shoot systems in perennial grasses serving as overwintering organs and for storage of nutrients. Short rotation coppice (SRC) Fast-growing trees, such as poplar or willow, harvested in cycles of 3–5 years and which regrow from the stools after harvesting. Definition of the Subject More than 90 % of the dry matter of lignocellulosic biomass consists of the three biopolymers: cellulose, hemicellulose, and lignin. Cellulose is made from chains of D-glucose molecules linked via hydrogen bridges. It is composed of 42 % carbon (C), 6 % hydrogen (H), and 52 % oxygen (O). Cellulose is the most common organic substance found in nature and forms the structural component of cell walls. Hemi-celluloses are polysaccharides, which have shorter chains of sugar units than cellulose. They have various functions in the plant including support and holding together the cell membranes. Lignin is a complex three-dimensional polymer built from phenylpropane derivatives, which is stored between the cellulose fibrils to strengthen the cell wall. It consists of 64 % C, 6 % H, and 6 % O. Lignocellulosic energy crops are characterized by the high lignin and cellulose content of their biomass. Lignin in particular is very suitable for direct combustion due to its high carbon content. Direct combustion as a solid fuel is therefore the preferred use of lignocellulosic biomass so far. But applications based mainly on the use of the cellulose fraction are also promising future energetic applications. These include the production of biofuels such as ethanol produced via enzymatic hydrolysis and diesel produced via gasification and Fischer-Tropsch synthesis.