Raw material properties
We have divided the properties of wood to physical and chemical properties for discussion. Physical properties are measurable, for example temperature, moisture and particle size. Chemical composition on the other hand, cannot be determined just by viewing or touching the substance. A tree consists of mainly four chemical components: cellulose, hemicellulose, lignin in addition to relatively small amount of extractable small molecules i.e. extractives.
Potential applications of these components are presented in figures on the right.
Below you can find information about raw material properties and methods for their characterization.
The moisture content of a living tree can be 50 % or more depending on species and time of year. During the growing season, for example, the living wood is saturated as water is in transit from the root system to the leaves. A tree felled in winter, however, contains less water. Once a tree is harvested, the amount of moisture plays an important role in material quality.
Characterization
Moisture content of solid biofuels is determined by a standard method (EN 14774-1:2009). A sample with the minimum mass of 300 g is dried at a temperature of (105 ± 2°C) and in which the air atmosphere changes between 3 and 5 times per hour, until constant mass is achieved. The moisture content as per cent is calculated from the loss in sample mass through drying.
Frictional forces between wood particles place more limitations on conveying and feeding systems compared to coal and aggregate materials. For example, the combination of low bulk density and non-uniform particle size can cause bridging of material in conveying lines.
Characterization
The particle size distribution (PSD) of a powdered or granular material is determined by a standard method for solid biofuels (EN 15149-1:2010). The method uses an oscillating screen and sieves to separate a particulate material into groups based on particle diameter. PSD gives information about the relative particle size differences that exist in a sample or pile of a material.
The heating value (also called calorific value) of a fuel is the amount of thermal energy released from its combustion mainly due to the oxidation of the fuel components which include carbon (C) and hydrogen (H). The main combustion reactions and their released thermal energy (heats of reaction) can be described as
C (s) + O2 (g) -> CO2 (g) ΔH = -393 kJ
CO (g) + ½O2 (g) -> CO2 (g) ΔH = -283 kJ
2H (g) + ½O2 (g) -> H2O (g) ΔH = -242 kJ
The symbols s and g here mean solid and gas, respectively. The products of these reactions are heat, gaseous carbon dioxide and water vapour.
The determination of calorimetric heating value is done with a bomb calorimeter. Calorimetric heating value includes the heat of condensation of water created during combustion when hydrogen combines with oxygen. To calculate the net calorific, sometimes called effective heating value, the heat of condensation of the water vapor created during combustion need to be subtracted from the calorimetric heating value. To do this it is necessary to know the hydrogen content of the fuel.
In practice biofuels always contain moisture which has to be evaporated in the first stage of combustion. The heat energy for evaporation comes from the burning fuel which lowers the amount of usable energy. This is called the gross or effective heating value of biomass with moisture and is proportionate to the fuel moisture content.
In measuring gross calorific value using this method, the gaseous H2O formed from combustion products is condensed to a liquid. In this case, the heating value is call the gross heating value of the fuel.
C (s) + O2 (g) -> CO2 (g) ΔH = -393 kJ
CO (g) + ½O2 (g) -> CO2 (g) ΔH = -283 kJ
2H (g) + ½O2 (g) -> H2O (g) ΔH = -242 kJ
The symbols s and g here mean solid and gas, respectively. The products of these reactions are heat, gaseous carbon dioxide and water vapour.
The determination of calorimetric heating value is done with a bomb calorimeter. Calorimetric heating value includes the heat of condensation of water created during combustion when hydrogen combines with oxygen. To calculate the net calorific, sometimes called effective heating value, the heat of condensation of the water vapor created during combustion need to be subtracted from the calorimetric heating value. To do this it is necessary to know the hydrogen content of the fuel.
In practice biofuels always contain moisture which has to be evaporated in the first stage of combustion. The heat energy for evaporation comes from the burning fuel which lowers the amount of usable energy. This is called the gross or effective heating value of biomass with moisture and is proportionate to the fuel moisture content.
Characterization
Determination of a fuel’s heating value is carried out at constant volume and at a reference temperature of 25 °C using a bomb-type calorimeter according to a standard procedure (EN 14918).In measuring gross calorific value using this method, the gaseous H2O formed from combustion products is condensed to a liquid. In this case, the heating value is call the gross heating value of the fuel.
Ash consists of the inorganic portion of the tree. Wood-based materials (0.5-2.8 %) generally have a low ash content in comparison with peat (5.7 %) or mineral coal (3-20 %). of not more than a few per cent, depending on species and part of the tree.
Characterization
The ash content of organic materials is determined by a standard method for solid biofuels (EN 14775:2009). The ash content (expressed as per cent) is calculated from the mass of the residue remaining after a sample of the material is heated in air controlled conditions (i.e. combusted) for a specific time 550 ± 10 °C.
Carbohydrates are biomolecules consisting of carbon, hydrogen and oxygen with a hydrogen-oxygen atom ratio of 2:1. In biochemistry carbohydrates are seen as synonymous with saccharides, which include sugars, starch and cellulose, however in the context of lignocellulosic biomass the term carbohydrate refers primarily to cellulose and hemicellulose.Cellulose is the main component in a tree and constitutes of long straight chains of the sugar glucose.
Hemicellulose is also a carbohydrate but consists of six different sugars and the chains are much shorter and branched as compared to cellulose.
Read more about carbohydrates here.
Characterization
The carbohydrate content of the extractives free raw material can be determined by using a acid hydrolysis (TAPPI T 222 om-02) and acidic methanolysis (Sundberg et al. 1996) methods.In acid hydrolysis carbohydrates of the raw material are hydrolyzed via mixing cold sulphuric acid with the analyte in 30 °C water bath for 1 hour and in autoclave at 1 bar for 1 hour. The hydrolyzate is then analyzed for its monosaccharide content with either gas or liquid chromatographic methods.
In acidic methanolysis the hemicellulose fraction of the analyte is hydrolyzed with HCl/MeOH solution by holding the mixture in 100 °C oven for 3 hours in capped bottle and consequently analyze the hydrolyzed monosaccharides with chromatographic methods. The quantification/qualification of the monosaccharides in both methods is done with sugar standards.
Lignin gives stiffness to the cell wall and, hence, contributes to the mechanical strength of the wood.
It is a complex, polyphenolic, amorphous polymer with irregular chemical structure. Lignin is primarily composed of up to three different aromatic, phenyl propane monomers, namely p-coumaryl, coniferyl, and sinapyl alcohols.
Here you can read more about lignin.
The amount of acid soluble lignin may be analyzed from the filtrate after acid hydrolysis via UV-Vis spectrophotometer at 205 nm according to standard method (TAPPI T 222).
It is a complex, polyphenolic, amorphous polymer with irregular chemical structure. Lignin is primarily composed of up to three different aromatic, phenyl propane monomers, namely p-coumaryl, coniferyl, and sinapyl alcohols.
Here you can read more about lignin.
Characterization
The amount of acid insoluble lignin (Klason lignin) in extractives free raw material can be determined by filtering, drying and weighing the solid residue from the mixture formed after acid hydrolysis (see analysis of carbohydrates).The amount of acid soluble lignin may be analyzed from the filtrate after acid hydrolysis via UV-Vis spectrophotometer at 205 nm according to standard method (TAPPI T 222).
Wood extractives provide a potential source for several types of high value platform and specialty chemicals, such as pharmaceutical or nutritional products, cosmetics, beverages, wood adhesives, paints, wood protection agents, plant-protective products and detergents.
Extractives can be defined as the nonstructural constituents of wood which can be removed via extraction with neutral organic solvents or water. The extractives are important factors influencing such properties of the wood as odor, color, light stability, decay and insect resistance, density, flammability, hygroscopicity, permeability, ease of pulping, density, paintability, etc. Extractives comprise an extraordinarily large number of diverse substances, i.e., several thousands of individual components, mainly with low molecular masses.
The extractives content and composition varies between tree parts and tree species. This in turn has effects on the supply of raw materials, sampling and sorting if the aim is to procure raw materials containing specific extractives or to avoid certain extractives in the raw materials.
Even in a given species, the amount and composition of wood extractives may vary significantly between trees. Further variation can be observed, within a single stem section from the base to the top and from the pith to the bark. In addition, the age of a tree has effects on the extractives of the tree; for example, older trees contain more extractives in their heartwood than young trees.
If you like to know more about extractives, please read here.
Extractions are done using a series of different solvents in order to collect extracts of differing polarity, for example, using hexane to collect lipophilic and water or acetone to collect hydrophilic extractives.
The total amount of extractives is first determined gravimetrically. The extractive groups, and individual free and esterified extractives are then analyzed via gas chromatography both quanitatively (GC-FID) and qualitatively (GC-MS) (Örså and Holmblom 1994) using internal standards.
Extractives may also be further fractionated, e.g. via different gels and columns, and analyzed separately via various different methods like HPLC, NMR etc.
Extractives can be defined as the nonstructural constituents of wood which can be removed via extraction with neutral organic solvents or water. The extractives are important factors influencing such properties of the wood as odor, color, light stability, decay and insect resistance, density, flammability, hygroscopicity, permeability, ease of pulping, density, paintability, etc. Extractives comprise an extraordinarily large number of diverse substances, i.e., several thousands of individual components, mainly with low molecular masses.
The extractives content and composition varies between tree parts and tree species. This in turn has effects on the supply of raw materials, sampling and sorting if the aim is to procure raw materials containing specific extractives or to avoid certain extractives in the raw materials.
Even in a given species, the amount and composition of wood extractives may vary significantly between trees. Further variation can be observed, within a single stem section from the base to the top and from the pith to the bark. In addition, the age of a tree has effects on the extractives of the tree; for example, older trees contain more extractives in their heartwood than young trees.
If you like to know more about extractives, please read here.
Characterization
The extractives may be obtained from the raw material via various different methods, e.g. using more traditional extractions methods such as soxhlet or soxtec apparatus, or more novel methods like accelerated solvent extractor (ASE, Dionex) or various kind of pressurized hot water extraction (HWE) equipments.Fractionation of spruce bark hot-water extract with Sephadex gel.
The total amount of extractives is first determined gravimetrically. The extractive groups, and individual free and esterified extractives are then analyzed via gas chromatography both quanitatively (GC-FID) and qualitatively (GC-MS) (Örså and Holmblom 1994) using internal standards.
Extractives may also be further fractionated, e.g. via different gels and columns, and analyzed separately via various different methods like HPLC, NMR etc.
