Lignocellulosic biomass
Encyclopedia
Lignocellulosic biomass refers to plant biomass
that is composed of cellulose
, hemicellulose
, and lignin
. The carbohydrate polymers (cellulose and hemicelluloses) are tightly bound to the lignin. Lignocellulosic biomass can be grouped into four main categories: agricultural
residues (including corn stover
and sugarcane bagasse
), dedicated energy crop
s, wood residues (including sawmill
and paper mill
discards), and municipal paper waste.
trees and Miscanthus giganteus
. The premier energy crop is sugarcane
, which is a source of the readily fermentable sucrose
and the lignocellulosic by-product bagasse
.
. This energy-intensive industry focuses on the separation of the lignin and cellulosic fractions of the biomass.
, has a long history as a source of energy. Since the middle of the 20th century, the interest of biomass as a precursor to liquid fuels has increased. To be specific, the fermentation
of lignocellulosic biomass to ethanol
is an attractive route to fuels that supplements the fossil fuels. Biomass is a carbon-neutral source of energy: Since it comes from plants, the combustion of lignocellulosic ethanol produces no net carbon dioxide into the earth’s atmosphere. Aside from ethanol, many other lignocellulose-derived fuels are of potential interest, including butanol
, dimethylfuran, and gamma-Valerolactone
.
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 confer hydrolytic
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 via ester
and ether
linkages. Ester linkages arise between oxidized sugars, the uronic acid
s, 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 acid-hydrolyze the newly freed celluloses to break them down into simple monosaccharides. Another challenge to biomass fermentation is the high percentage of pentoses in the hemicellulose, such as xylose
, or wood sugar. Unlike hexoses, like glucose, pentoses are difficult to ferment. The problems presented by the lignin and hemicellulose fractions are the foci of much contemporary research.
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
that is composed of cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
, hemicellulose
Hemicellulose
A hemicellulose is any of several heteropolymers , such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...
, and lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
. The carbohydrate polymers (cellulose and hemicelluloses) are tightly bound to the lignin. Lignocellulosic biomass can be grouped into four main categories: agricultural
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
residues (including corn stover
Corn stover
Corn stover consists of the leaves and stalks of maize plants left in a field after harvest and consists of the residue: stalk; the leaf, husk, and cob remaining in the field following the harvest of cereal grain.” Stover makes up about half of the yield of a crop and is similar to straw...
and sugarcane bagasse
Bagasse
Bagasse is the fibrous matter that remains after sugarcane or sorghum stalks are crushed to extract their juice. It is currently used as a biofuel and as a renewable resource in the manufacture of pulp and paper products and building materials....
), dedicated energy crop
Energy crop
An energy crop is a plant grown as a low cost and low maintenance harvest used to make biofuels, or combusted for its energy content to generate electricity or heat. Energy crops are generally categorized as woody or herbaceous ....
s, wood residues (including sawmill
Sawmill
A sawmill is a facility where logs are cut into boards.-Sawmill process:A sawmill's basic operation is much like those of hundreds of years ago; a log enters on one end and dimensional lumber exits on the other end....
and paper mill
Paper mill
A paper mill is a factory devoted to making paper from vegetable fibres such as wood pulp, old rags and other ingredients using a Fourdrinier machine or other type of paper machine.- History :...
discards), and municipal paper waste.
Dedicated energy crops
Many crops are of interest for their ability to provide high yields of biomass and can be harvested multiple times each year. These include poplarPoplar
Populus is a genus of 25–35 species of deciduous flowering plants in the family Salicaceae, native to most of the Northern Hemisphere. English names variously applied to different species include poplar , aspen, and cottonwood....
trees and Miscanthus giganteus
Miscanthus giganteus
Miscanthus giganteus is a large perennial spoon hybrid of Miscanthus sinensis and Miscanthus sacchariflorus native to Japan. It is currently used in the European Union as a commercial energy crop. It is used as a source of heat and electricity, or converted into biofuel products such as ethanol.-...
. The premier energy crop is sugarcane
Sugarcane
Sugarcane refers to any of six to 37 species of tall perennial grasses of the genus Saccharum . Native to the warm temperate to tropical regions of South Asia, they have stout, jointed, fibrous stalks that are rich in sugar, and measure two to six metres tall...
, which is a source of the readily fermentable sucrose
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...
and the lignocellulosic by-product bagasse
Bagasse
Bagasse is the fibrous matter that remains after sugarcane or sorghum stalks are crushed to extract their juice. It is currently used as a biofuel and as a renewable resource in the manufacture of pulp and paper products and building materials....
.
Pulp and paper industry
Lignocellulosic biomass is the feedstock for the pulp and paper industryPulp and paper industry
The global pulp and paper industry is dominated by North American , northern European and East Asian countries...
. This energy-intensive industry focuses on the separation of the lignin and cellulosic fractions of the biomass.
Biofuels
Lignocellulosic biomass, in the form of wood fuelWood fuel
Wood fuel is wood used as fuel. The burning of wood is currently the largest use of energy derived from a solid fuel biomass. Wood fuel can be used for cooking and heating, and occasionally for fueling steam engines and steam turbines that generate electricity. Wood fuel may be available as...
, has a long history as a source of energy. Since the middle of the 20th century, the interest of biomass as a precursor to liquid fuels has increased. To be specific, the fermentation
Ethanol fermentation
Ethanol fermentation, also referred to as alcoholic fermentation, is a biological process in which sugars such as glucose, fructose, and sucrose are converted into cellular energy and thereby produce ethanol and carbon dioxide as metabolic waste products...
of lignocellulosic biomass to ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
is an attractive route to fuels that supplements the fossil fuels. Biomass is a carbon-neutral source of energy: Since it comes from plants, the combustion of lignocellulosic ethanol produces no net carbon dioxide into the earth’s atmosphere. Aside from ethanol, many other lignocellulose-derived fuels are of potential interest, including butanol
Butanol
Butanol or butyl alcohol can refer to any of the four isomeric alcohols of formula C4H9OH:*n-Butanol, butan-1-ol, 1-butanol, n-butyl alcohol;*Isobutanol, 2-methylpropan-1-ol, isobutyl alcohol;...
, dimethylfuran, and gamma-Valerolactone
Gamma-Valerolactone
gamma-Valerolactone is an organic compound with the formula C5H8O2. This clear liquid is one of the more common lactones. It is a structural isomer of delta-valerolactone. Because of its herbal odor, it is used in the perfume and flavor industries....
.
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 confer hydrolytic
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
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 via ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
and ether
Ether
Ethers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
linkages. Ester linkages arise between oxidized sugars, the uronic acid
Uronic acid
thumb|300px|The [[Fischer projection]]s of [[glucose]] and [[glucuronic acid]]. Glucose's terminal carbon's hydroxyl group has been oxidized to a [[carboxylic acid]]....
s, 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 acid-hydrolyze the newly freed celluloses to break them down into simple monosaccharides. Another challenge to biomass fermentation is the high percentage of pentoses in the hemicellulose, such as xylose
Xylose
Xylose is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional group. It is the precursor to hemicellulose, one of the main constituents of biomass...
, or wood sugar. Unlike hexoses, like glucose, pentoses are difficult to ferment. The problems presented by the lignin and hemicellulose fractions are the foci of much contemporary research.