Page 98 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

76 Biomass Gasification, Pyrolysis and Torrefaction

content of biomass is generally very small but may play a significant role in
biomass utilization especially if it contains alkali metals such as potassium
or halides such as chlorine. Straw, grasses, and demolition wood are particu-
larly susceptible to this problem. These components can lead to serious
agglomeration, fouling, and corrosion in boilers or gasifiers (Mettanant et al.,
2009).
The ash obtained from biomass conversion does not necessarily come
entirely from the biomass itself. For collection, biomass is often scraped off
the forest floor and then undergoes multiple handlings, during which it can
pick up a considerable amount of dirt, rock, and other impurities. In many
plants, these impurities constitute the major inorganic component of the
biomass feedstock.

3.6.2.3 Moisture
High moisture is a major characteristic of biomass. The root of a plant bio-
mass absorbs moisture from the ground and pushes it into the sapwood. The
moisture travels to the leaves through the capillary passages. Photosynthesis
reactions in the leaves use some of it, and the rest is released to the atmo-
sphere through transpiration. For this reason, there is more moisture in the
leaves than in the tree trunk.
The total moisture content of some biomass can be as high as 90% (db),
as seen in Table 3.12. Moisture drains much of the deliverable energy from a
gasification plant, as the energy used in evaporation is not recovered. This
important input parameter for design must be known for assessment of the
cost of transportation or energy penalty in drying the biomass. The moisture
in biomass can remain in two forms: (1) free or external and (2) inherent or
equilibrium.
Free moisture is that which is above the equilibrium moisture content. It
generally resides outside the cell walls. Inherent moisture, on the other hand,
is absorbed within the cell walls. When the walls are completely saturated,
the biomass is said to have reached the fiber saturation point or equilibrium
moisture. Equilibrium moisture is a strong function of the relative humidity

TABLE 3.12 Typical Moisture Content of Some Biomass
Dairy
Corn Wheat Rice Rice Cattle Wood Food RDF Water
Stalks Straw Straw Husk Manure Bark Sawdust Waste Pellets Hyacinth
Moisture 40 60 8 20 50 80 7 10 88 30 60 25 55 70 25 35 95.3
(wet basis)
Source: Compiled from Kitani and Hall (1989, p. 863).Biomass