What chemical properties of a sample are most important will depend on whether the biomass sample is to be processed in hydrolysis or thermochemical processes. For hydrolysis technoloies (e.g. enzymatic hydrolysis, acid hydrolysis) the lignocellulosic (cellulose, hemicellulose, lignin) constituents of a sample are critical whilst for thermochemical technologies the elemental and ash composition of the sample are of greater importance. Relevant lignocellulose chemistry is discussed below, for background on elemental and ash composition click here.
Lignocellulose ChemistryLignocellulosic biomass is that which predominately contains the polysaccharides cellulose and hemicellulose and the biopolymer lignin. There will also be lesser amounts of ash and extractive components.
Cellulose is the major mass constituent of most naturally occurring biomass. It is a polysaccharide (a polymer containing sugar units) made up of interlinked glucose units. These bonds are beta-linked and result in significant hydrogen bonding, both within and between cellulose molecules, meaning that cellulose is relatively hard to hyrolyse (compared with the alpha-linked polymers in starch).
Cellulose tends to consist of amorphous regions and crystalline regions. The amorphous regions tends to be easily hydrolysable by acids while crystalline cellulose is more resistant. The result is that hydrolysis with a dilute acid tends to remove the amorphous regions from cellulose leaving a polysaccharide of increased crystallinity that is resistant to further hydrolysis.
Paper and cardboard samples can have large cellulose contents.
Hemicelluloses constitute the cell wall polysaccharides of land plants that are not cellulose or pectins. Hemicelluloses tend to be heteropolysaccharides (i.e. containing more than one type of sugar unit) with branches attached to the main backbone. Hemicelluloses are mostly built up of: the pentoses D-xylose and L-arabinose; the hexoses D-glucose, D-mannose and D-galactose; with smaller amounts of L-rhamnose, in addition to uronic acids such as glucuronic acid, 4-O-methyl-D-glucuronic acid, and galacturonic acid. While the proportions of these substituents vary between hemicellulose and feedstock types, the majority tend to be pentoses. Click here to read more about uronic acids.
There are key differences between cellulose and hemicelluloses. These include the absence of a highly ordered state in hemicelluloses, and that their molecular weight is much lower ??? their degree of poymerisation is only 100 to 200 units (compared to 1000's in cellulose). Such factors help explain their comparatively easy hydrolysis.
However, many cellulases designed for enzymatic hydrolysis of cellulose would not be active in hemicellulose hydrolysis meaning that specific enzymes would be needed if the hemicelluloses are not hydrolysed in the pretreatment steps. Also, hemicelluloses contains sugars other than glucose, including C5 sugars, and these tend to require different biota to those used for glucose fermentation. Each of these sugars may also have different fermentation efficiencies.
Lignin is a structurally important polymer in biomass. It is said to function primarily as a supporting agent in cell structure and it also assists in the resistance of biomass against microbial attack and decay.
Structurally, lignin can be described as a complex three-dimensional polymer of phenylpropane units (approx. 25). The phenylpropane units are derelatives of carbohydrates, coming from the dehydration and cyclisation of sugars. They are mostly either 4-hydroxycinnamyl alcohol (para-coumaryl alcohol, H) or its 3- and/or 3,5-methoxylated derivatives - coniferyl (guaiacyl, G) alcohol, and sinapyl alcohol (or syringal, S), respectively. The ratio of these units varies between plants; for example in hardwoods S and G forms dominate, whereas softwood lignins contain only G units. Ether linkages between the phenylpropane units are very resistant to cleavage, a factor in explaining the low lignin degradation rates by most biota. It has been shown that a high lignin content is correlated with the recalcitrance of polysaccharides to enzymatic hydrolysis and that increasing the relative proportion of the S monomer may aid in biofuel conversion mechanisms.
A small fraction of lignin is soluble under acid conditions. The amount of acid soluble lignin (ASL) varies between lignin and feedstock types and its presence can interfere with enzymatic hydrolysis and the acid-catalysed hydrolysis of cellulose.
Cellulose is said to form a skeleton which is surrounded by other substances functioning as matrix (hemicelluloses) and encrusting (lignin) materials.
More specifically, disordered (amorphous) cellulose molecules, as well as hemicelluloses and lignin, are located in the spaces between crystalline cellulose microfibrils. The hemicelluloses are considered to be amorphous even though they are apparently orientated in the same direction as the cellulose microfibrils.
The complex associations of lignin with the polysaccharides is of particular importance for many hydrolysis technologies. Researchers have postulated the effects of cross-linking on the properties of plant cell walls, such as accessibility and degradability. The resistance of polysaccharide-lignin bonds to hydrolysis not only depends on the type of linkage involved but also on the types of sugars associated with the linkage, and the chemical structure of the lignin unit attached to the sugar.
For the cost-effective production of second generation biofuels in high yields it is important that a pre-treatment is employed to break apart the lignocellulose structure, enabling easier hydrolysis of cellulose. At Celignis we have extensive experience in the analysis of pre-treated biomass samples and have characterised biomass pre-treated by several different processes under varying degrees of severity.
Click here to see the Celignis Analysis Packages related to the characterisation of lignocellulosic samples.
Extractives are defined as extraneous components that may be separated from the insoluble cell wall material by their solubility in water or neutral organic solvents. Solvents of different polarities are required to remove different types of extractives. Hence the extractives are often classified according to which solvent can extract them (e.g. ethanol-soluble-extractives).
There are a large number of different extractives, many of which are species-specific. These are often classified into categories according to the similarity of their chemical structures. Major categories include monosaccharides, polysaccharides, volatile oils, terpenes, fatty acids and their esters, waxes, polyhydric alcohols, alkaloids, and aromatic compounds.
For lignocellulosic compositional analysis it is very important to remove the extractives prior to hydrolysing the sample with acid. If the extractives are not removed they will cross-react with the acid and condense to acid insoluble components that will be associated-with and classified-as Klason lignin (KL). The presence of these extractives during the acid hydrolysis process may also impact on the hydrolysis dynamics and ultimate fate of the monosaccharides that are liberated.
Researchers have found that hydrolysis with no extraction significantly increases the reported value for KL and acid soluble lignin (ASL). Water and/or 95% ethanol can be effective in removing a large proportion of the extractives, allowing more accurate lignocellulosic analysis.
Click here to see the Celignis Analysis Packages related to the removal and quantification of extractives.
The moisture content of biomass is a crucial efficiency parameter when biomass combustion is the main consideration, but less important in some lignocellulose hydrolysis technologies. Its determination is also necessary in most carbohydrate analytical procedures and it has a huge influence on the near infrared spectra of lignocellulosic materials.
Water is generally held in biomass in two ways - either as a free liquid and vapour that is contained in the cell cavities, or as a molecule that is bound within the cell walls.
Moisture content tends to vary widely with biomass species, age, geographic locations and genetic differences. It also varies between different anatomical fractions of the same plant and throughout the year.
The moisture content of biomass can either be measured on a wet or dry basis. The wet-basis expresses the ratio of moisture mass to the total mass of the substance whilst the dry basis expresses the ratio of the moisture mass to the mass of dry matter. All Celignis data are reported using the wet-basis.
Ash is generally considered to be the residue remaining after the material has been incinerated. It therefore has no energy value and, being made up of the inorganic elements in the biomass, is of no direct value in hydrolysis technologies. High ash-contents can cause problems in many thermochemical processes (e.g. pyrolysis, gasification, and combustion).
Ash content can vary greatly between plant species, and is generally higher in agricultural residues. The ash present in plants will depend on their stage of growth, the time of year, and their location. The leaching of stored biomass may reduce the level of inorganics in some instances.
The major cations present in ashes from lignocellulosic materials are Calcium, Potassium and Magnesium. Other elements such as Manganese, Sulphur and Phosphorus are present in minor amounts. Trace constituents (such as Al, Fe, Zn, Cu, Ti, Pb, Ni, V, Co, Ag and Mo) are also found in many substrates. The anions that are usually present are Chloride, Carbonate, Sulphate and Silicate. With waste feedstocks (municipal solid wastes in particular), ashes are often more abundant and more diverse.
The elemental analysis of biomass will provide the mass concentrations of the major elements (carbon, oxygen, hydrogen, nitrogen and sulphur) in the sample. These concentrations will depend on the chemical composition of the biomass sample.
Lignocellulosic biomass is mainly composed of carbohydrates (in the form of the polysaccharides cellulose and hemicellulose) and lignin. Carbohydrates, having an elemental composition of (CH2O)n, will have a generally uniform carbon content of 40%, which is lower than many other biomass mass constituents such as lignin which has an average carbon content of approximately 60-65%.
Typically carbon, hydrogen, nitrogen, and sulphur can be determined in the same analytical method and the oxygen content can then be calculated as the remaining mass after the ash content has been considered. Alternatively, oxygen can be measured directly using a dedicated elemental analysis method.
We use a Vario MACRO Cube Elemental Analyser to determine the carbon, hydrogen, nitrogen, and sulphur contents of biomass.
The heating value is currently one of the most important properties of biomass given the predominance of combustion facilities over second-generation biorefining facilities.
There are several units of measurement. The caloric, or higher heating value (HHV), is independent of moisture content and reliant on the chemical composition of the material. A linear relationship exists between the heat of combustion and the carbon content of the substrate while oxygen, nitrogen and inorganic elements tend to reduce the value.
The Lower Heating Value (LHV), or effective heating value, is perhaps more relevant than the HHV in practical operations. It considers not only the energy required to vaporise the moisture of the biomass but also that necessary to vaporise the water generated when the hydrogen and oxygen elements of the biomass combine. Hydrogen content then becomes a reducing factor in the heating value.
The heating value of samples can be determined directly, by combusting the sample in a bomb calorimeter, or it can be calculated based on elemental composition. The heating value can be reported on a dry or wet basis. The wet-basis value considers the moisture content of the biomass and reduces the heating value to incorporate the latent heat of condensation.
Click here to see the Celignis Analysis Packages related to the characterisation of biomass samples for utilisation in thermal processes.