• Feedstocks Analysed at Celignis

Background on Softwood

Trees are classified as softwoods (gymnosperms) and hardwoods (angiosperms). Globally, there are about 30 000 hardwoods and 520 softwood tree species known. However, in Europe there are only 51 and 10, respectively, of these species that exist naturally. The species of most interest for biorefining are those that can be obtained reasonably cheaply whether directly (i.e. specifically grown) or indirectly (as wastes or residues). Such feedstocks include the short-rotation coppice crops willow, poplar, and robina (hardwoods) and residues from the forestry industry (which includes the softwoods Sitka spruce, Norway spruce, and several pine varieties).

Celignis founder Daniel Hayes has considerable experience in the chemical and near-infrared analysis of softwoods and has characterised samples from a number of different species, incuding pines, spruces, willows, poplars, and eucalyptus.

Analysis of Softwood at Celignis

Celignis Analytical can determine the following properties of Softwood samples:

Lignocellulosic Properties of Softwood

Cellulose Content of Softwood

A detailed compilation of the polysaccharide and ligneous composition of wood was carried out by (Fengel and Grosser, 1975). By tabulating the data from more than 350 references in 153 temperate species it was found that, on average, stem wood in softwoods contains 40-45% cellulose.

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Hemicellulose Content of Softwood

There are characteristic differences between hardwoods and softwoods with regard to the composition and structure of the hemicelluloses.

In softwoods, O-acetyl-galactoglucomannan is the principal hemicellulose component. The glucose to mannose ratio is about 1:3, whereas the ratio of galactose to glucose can vary from 1:1 to 1:10. Softwood O-acetyl-galactoglucomannan has been reported to have an approximate degree of polymerization between 100 and 150, equivalent to a molecular weight around 16,00024,000.

Softwoods also contain xylans. Specifically, softwood xylans are an arabino-(4-O-methylglucurono)xylan. The softwood xylan does not contain acetyl groups and is more highly branched and more acidic than the hardwood xylan. These side chains can be removed under mild acid conditions in which the main xylose chain remains intact). The arabinose and uronic acid substituents do stabilise the xylan chain against alkali-catalysed degradation, however.

There are other hemicelluloses that are present in minor quantities in softwoods. These are built up predominantly by units of arabinose, galactose, glucuronic and galacturonic acids. There are also small amounts of starch and pectins.

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Lignin Content of Softwood

The lignin fraction is generally considerably greater in temperate softwoods than in hardwoods. Covalent bonds are thought to exist between the lignins and carbohydrates in woody biomass plants.

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Starch Content of Softwood

Starch content will vary according to the species of softwood and the conditions of its growth and harvest. Starch content is typically lower in the wood than in the foliage and bark.

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Uronic Acid Content of Softwood

Uronic acids can be present as side chains attached to the main backbone of hemicelluloses in softwoods. For example, the arabinoxylans of softwoods have partial substitution of the xylan main chain by glucuronic acid, with the ratio of xylose to uronic acids varying between 4:1 and 9:1. Other softwood hemicelluloses can also contain glucuronic acid as well as galactuornic acid.

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Enzymatic Hydrolysis of Softwood

We can undertake tests involving the enzymatic hydrolysis of Softwood. In these experiments we can either use a commercial enzyme mix or you can supply your own enzymes. We also offer analysis packages that compare the enzymatic hydrolysis of a pre-treated sample with that of the native original material.

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Bioenergy Properties of Softwood

Ash Content of Softwood

The ash of most wood produced in temperate regions is not a particularly significant fraction of the biomass. Generally, softwoods contain somewhat less ash than hardwoods.

Young trees tend to have a higher ash content than mature trees and the ash content tends to be much higher in bark and foliage.

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Heating (Calorific) Value of Softwood

Given that the concentrations of lignin and resins tend to be higher in softwoods than in hardwoods, softwoods tend to have slightly higher heating values. Howard (1973), when examining southern pine, found that extractive content was positively correlated with the heating value and accounted for 54% of the variation, whereas variation in the proportion of the main chemical constituents had only minor effects.

With regard to the extractives, in woods these are substances that tend to be deposited in association with the transition from sapwood to heartwood. They also tend to be present in greater concentrations in barks. Large extractive concentrations are present in woods only in exceptional cases, most of these being tropical species.

Howard (1973) also found that the heating value of mixed stem-bark samples is inversely related to stem height. This is due to a greater proportion of the lower-heating value inner bark at greater heights.

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Ash Melting Behaviour of Softwood

Ash melting, also known as ash fusion and ash softening, can lead to slagging, fouling and corrosion in boilers which may reduce conversion efficiency. We can determine the ash melting behaviour of Softwood using our Carbolite CAF G5 BIO ash melting furnace. It can record the following temperatures:

Ash Shrinkage Starting Temperature (SST) - This occurs when the area of the test piece of Softwood ash falls below 95% of the original test piece area.

Ash Deformation Temperature (DT) - The temperature at which the first signs of rounding of the edges of the test piece occurs due to melting.

Ash Hemisphere Temperature (HT) - When the test piece of Softwood ash forms a hemisphere (i.e. the height becomes equal to half the base diameter).

Ash Flow Temperature (FT) - The temperature at which the Softwood ash is spread out over the supporting tile in a layer, the height of which is half of the test piece at the hemisphere temperature.

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Major and Minor Elements in Softwood

Examples of major elements that may be present in Softwood include potassium and sodium which are present in biomass ash in the forms of oxides. These can lead to fouling, ash deposition in the convective section of the boiler. Alkali chlorides can also lead to slagging, the fusion and sintering of ash particles which can lead to deposits on boiler tubes and walls.

We can also determine the levels of 13 different minor elements (such as arsenic, copper, and zinc) that may be present in Softwood.

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Analysis of Softwood for Anaerobic Digestion

Biomethane potential (BMP) of Softwood

At Celignis we can provide you with crucial data on feedstock suitability for AD as well as on the composition of process residues. For example, we can determine the biomethane potential (BMP) of Softwood. The BMP can be considered to be the experimental theoretical maximum amount of methane produced from a feedstock. We moniotor the volume of biogas produced allowing for a cumulative plot over time, accessed via the Celignis Database. Our BMP packages also involve routine analysis of biogas composition (biomethane, carbon dioxide, hydrogen sulphide, ammonia, oxygen). We also provide detailed analysis of the digestate, the residue that remains after a sample has been digested. Our expertise in lignocellulosic analysis can allow for detailed insight regarding the fate of the different biogenic polymers during digestion.

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Physical Properties of Softwood

Bulk Density of Softwood

At Celignis we can determine the bulk density of biomass samples, including Softwood, according to ISO standard 17828 (2015). This method requires the biomass to be in an appropriate form (chips or powder) for density determination.

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Particle Size of Softwood

Our lab is equipped with a Retsch AS 400 sieve shaker. It can accommodate sieves of up to 40 cm diameter, corresponding to a surface area of 1256 square centimetres. This allows us to determine the particle size distribution of a range of samples, including Softwood, by following European Standard methods EN 15149- 1:2010 and EN 15149-2:2010.

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Examples of Other Feedstocks Analysed at Celignis