See also: Biomass – Direct Combustion.
Biomass Composition and Properties
Biomass is a term used to describe any material of recent biological origin, including plant materials such as trees, grasses, agricultural crops, and even animal manure. The bulk composition (in terms of the amounts of cellulose, hemicellulose, and lignin) is, as might be expected variable opening upon the source and types of the biomass (Table B-17).
Table B-17 Bulk composition of different biomass type (% w/w, dry basis).
| Biomass type | Cellulose | Hemicellulose | Lignin |
|---|---|---|---|
| Peat | 10 | 32 | 44 |
| Rice husks | 30 | 25 | 12 |
| Straw (wheat) | 40 | 28 | 17 |
| Wood (bark) | 34 | 16 | 34 |
| Wood (hard) | 39 | 35 | 20 |
| Wood (soft) | 41 | 24 | 28 |
The elemental composition also shows some variation with the source of the biomass (Table B-18).
Table B-18 Elemental composition, ash production, and heat content of biomass types*.
| Biomass, % w/w** | C | H | N | O | Ash |
|---|---|---|---|---|---|
| Miscanthus | 49.5 | 6.2 | 0.6 | 43.7 | 3.3 |
| Peat | 53.1 | 5.5 | 1.3 | 38.1 | 5.6 |
| Reed grass | 49.4 | 6.3 | 1.6 | 42.7 | 8.8 |
| Straw (wheat) | 49.6 | 6.2 | 0.6 | 43.6 | 4.7 |
| Sugar cane | 49.5 | 6.2 | 0.5 | 43.8 | 3.7 |
| Wood (bark) | 47.2 | 5.6 | 0.3 | 46.9 | 3.9 |
| Wood (birch) | 48.8 | 6.0 | 0,5 | 44.2 | 0.5 |
| Wood (pine) | 49.3 | 6.0 | 0.5 | 44.2 | 0.5 |
| *Listed alphabetical rather than by any preference. **Dry basis | |||||
The obvious data are those relating to the amount of oxygen in the biomass which will require (at some stage of the refining) a hydrodeoxygenation step.
In terms of the bulk composition, other biomass components, which are generally present in minor amounts, include (i) triglyceride derivatives, which are ester derivatives of glycerol three fatty acids, (ii) sterol derivatives, which are also known as steroid alcohol derivatives), (iii) alkaloid derivatives, which form a class of naturally occurring organic compounds that mostly contain basic nitrogen atoms, (iv) terpene derivatives, which are the primary constituents of the essential oils of many types of plants and flowers, (v) terpenoid derivatives, which are sometimes referred to as isoprenoids and form a large and diverse class of naturally occurring organic chemicals derived from terpenes – most are multicyclic structures with oxygen-containing functional groups), and (vi) wax derivatives, which are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures and include higher molecular weight alkane derivatives and lipids, typically with melting points above approximately 40°C (104°F) and melt to give low-viscosity liquids. Waxes are insoluble in water but soluble in organic, nonpolar solvents. Natural waxes of different types are produced by plants and animals and occur in crude oil).
This list (above) includes everything from primary sources of crops and residues harvested/collected directly from the land, to secondary sources such as sawmill residuals, to tertiary sources of post-consumer residuals that often end up in landfills. A fourth source, although not usually categorized as such, includes the gases that result from anaerobic digestion of animal manures or organic materials in landfills (Wright et al., 2006).
Thus, knowledge of the composition of a biomass feedstocks is critical to the selection of the varieties with optimized properties for downstream conversion (De Jong, 2014). This can be partially achieved by selecting varieties with biomass composition that are better suited to the conversion process. Lignocellulosic biomass displays considerable recalcitrance to biochemical conversion because of the inaccessibility of its polymer components to enzymatic digestion and the release or production of fermentation inhibitors during pretreatment. If the ratio of hemicellulose, cellulose, and lignin in a woody biomass feedstock was optimized for the specific biochemical conversion method, then the pretreatment methods could be reduced or avoided.
Plants use the light energy from the sun to convert water and carbon dioxide to sugar derivatives (photosynthesis) that can be stored within the plant system. Some plants, such as sugar cane and sugar beets, store energy as