Hemicellulose

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.

Lignocellulose Macrostructure

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.

Relevance

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.

Ash

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.

Heating Value

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.