1362 Biomass Fuel Characteristics

Biomass is produced by complex photochemical and biological reactions that involve the conversion of carbon dioxide and water to stored chemical energy. The simplified reaction scheme is:

Because biomass resources are very diverse, their characteristics as energy feedstocks also vary significantly. As shown in Table 13.3, the energy content of woody biomass is typically about 19 to 20 MJ/kg (HHV, dry basis, or about 8300 BTU/lb), while that of other biomass or wastes is usually less. In practice, the energy content of the biomass is also highly dependent on both the moisture content of the biomass and the bulk density. Woody biomass typically contains about 50% moisture (total weight basis), while other feedstocks and wastes range anywhere from 5 to 95% moisture as harvested. The lower-moisture feedstocks can be dried for thermal conversion systems such as gas-ifiers by a combination of storage practices and the use of waste heat without significant loss of overall thermal efficiency. Higher-moisture feedstocks can be used without drying in biological conversion systems such as anaerobic digesters. The bulk density of the feedstock will also vary depending on whether it is stored as chips, powder, hogged fuel, etc.

Biomass contains inorganic material (ash) that varies by species, as shown in Table 13.4. Woody biomass typically has about 1 to 2% ash, while herbaceous species or wastes such as rice hulls may contain 15 to 20%. This ash is composed primarily of salts and oxides of calcium, potassium, and silicon. Sulfur in harvested biomass is typically less than 0.5%, although the sulfur content of selected wastes such as animal manures can be high.

It is important to note that, despite the variability of the characteristics, biomass is used extensively worldwide as an energy resource for power generation. Effective feedstock preparation, handling, and conversion systems have been designed and deployed by considering the types of biomass a specific facility is likely to use. Hundreds of facilities worldwide routinely use a wide range of biomass feedstocks and wastes. Each system is typically capable of handling a range of feedstocks (such as forest residues, orchard prun-ings, nut shells, cocoa bean pressings, and similar materials at the same site).

TABLE 13.3

Energy Content and Inorganic Content of Biomass Feedstocks

Inorganic Material, HHV,

Material dry wt% MJ/dry kg

Woody

Cottonwood 1.1 19.5

Hybrid poplar 1.0 19.5

Loblolly pine 0.5 20.3

Eucalyptus 2.4 18.7

Herbaceous

Switchgrass 10.1 18.0

Sweet sorghum 9.0 17.6

Miscellaneous

Rice straw 19.2 15.2

Paper 6.0 17.6

Cattle manure 23.5 13.4

Brown kelp 45.8 10.3

Pine bark 2.9 20.4

Source: Klass, D.L., Biomass for Renewable Energy, Fuels, and Chemicals, Academic Press, San Diego, 1998 (with permission).

TABLE 13.4

Composition of Ash from Selected Biomass

Ash Composition by Constituent, Dry wt%

TABLE 13.4

Ash Composition by Constituent, Dry wt%

CaO

K2O

P2O5

MgO

Na2O

SiO2

SO3

Other

Hybrid Poplar

47.2

20.0

5.0

4.4

0.2

2.6

2.7

17.9

Pine

49.2

2.6

0.3

0.4

0.4

32.5

2.5

12.1

Source: Klass, D.L., Biomass for Renewable Energy, Fuels, and Chemicals, Academic Press, San Diego, 1998 (with permisison).

Source: Klass, D.L., Biomass for Renewable Energy, Fuels, and Chemicals, Academic Press, San Diego, 1998 (with permisison).

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