• Feedstocks Analysed at Celignis

Background on Hardwood

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 hardwoods and has characterised samples from a number of different species, incuding ash, alder, birch, and paulownia.

Analysis of Hardwood at Celignis

Celignis Analytical can determine the following properties of Hardwood samples:

Lignocellulosic Properties of Hardwood

Cellulose Content of Hardwood

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, whilst stem wood in temperate-zone hardwoods contains 40-50% cellulose.

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Hemicellulose Content of Hardwood

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

The major group of hemicelluloses found in hardwoods are the glucuronoxylans. Specifically, this xylan is a O-acetyl-(4-O-methylglucurono)-b-D-xylan. The content of glucuronoxylan content of hardwood is typically between 15 and 30% by weight. In a few species, for example in some birches, the xylan content can reach as high as 35%. The xylan backbone has glucuronic acid substituents.

Also, unlike softwoods, the 2,3 positions of the xylose backbone may be partially acetylated with about seven acetyl residues per ten xylose units. The xylosidic bonds between the xylose units are easily hydrolysed by acids, whereas the linkages between the uronic acid groups and xylose are very resistant. The acetyl groups are easily cleaved by alkali.

Hardwoods also usually contain small amounts (2-5%) of glucomannan. It is composed of b-D-glucopyranose and b-D-mannopyranose linked by (1->4) bonds. The glucose to mannose residues are generally in the ratio of 1:2. The mannosidic bonds between the mannose units are more rapidly hydrolysed than the corresponding glucosidic bonds and glucomannan is easily depolymerised under acidic conditions.

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Lignin Content of Hardwood

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

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Starch Content of Hardwood

Starch content will vary according to the species of hardwood 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 Hardwood

Uronic acids can be present as side chains attached to the main backbone of hemicelluloses in hardwoods. For example, the xylan backbone of the hardwood glucuronoxylans can be substituted with glucuronic acid with the xylose-to-uronic acid ratio varying from 3:1 to 20:1, with the most common ratio being from 7:1 to 12:1.

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Enzymatic Hydrolysis of Hardwood

We can undertake tests involving the enzymatic hydrolysis of Hardwood. 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 Hardwood

Ash Content of Hardwood

The ash of most wood produced in temperate regions is not a particularly significant fraction of the biomass. Generally, hardwoods contain somewhat more ash than softwoods, and tropical hardwoods contain more than temperate hardwoods, but this is not always the case.

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 Hardwood

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 Hardwood

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 Hardwood 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 Hardwood 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 Hardwood ash forms a hemisphere (i.e. the height becomes equal to half the base diameter).

Ash Flow Temperature (FT) - The temperature at which the Hardwood 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 Hardwood

Examples of major elements that may be present in Hardwood 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 Hardwood.

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Analysis of Hardwood for Anaerobic Digestion

Biomethane potential (BMP) of Hardwood

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 Hardwood. 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 Hardwood

Bulk Density of Hardwood

At Celignis we can determine the bulk density of biomass samples, including Hardwood, 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 Hardwood

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 Hardwood, 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