Biomass Cofiring and Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 365

From a combustion standpoint, ash types in biomass may be divided into
three classes, with their distinct fouling characteristics (Hiltunen et al., 2008):

1. Rich in calcium and potassium but lean in silica (e.g., woody biomass).
2. Rich in silica but lean in calcium and potassium (e.g., rice husk, straw,
and bagasse).
3. Rich in phosphorous, calcium, and potassium.

Biomass ash is generally more reactive than that from coal (Hiltunen et al.,
2003). Biomass with type 1 ash forms deposits of CaO, CaSO 4 ,andK 2 SO 4 on
high-temperature tubes of superheater and reheaters of a PC-fired boiler. These
deposits harden with time. In fluidized bed boilers the Ca, K could react with
quartz in bed material coating them with low softening temperature Ca or K sili-
cate layers that causes agglomerates. Contrary to type 1 ash, type 2 ash that is
rich in silica, separate sticky and molten ash particles are formed in a fluidized
bed and cause agglomeration. Here, the quality of bed material does not affect
the behavior of bed agglomeration in this case as was the case with type 1 ash.
Type 3 ash being rich in phosphorous could form low-temperature melting
potassium phosphate and make it high-fouling fuel. To avoid this, one can add
limestone to fluidized-bed boilers to facilitate formation of high-temperature
melting calcium phosphate, instead of low temperature potassium phosphate
(Hiltunen et al., 2003).
On the basis of 10,000 fuel samples from 150 operating circulating
fluidized-bed boilers, Foster Wheeler Corporation developed a probability index
to give a quantitative assessment of how different biomass fuels ranks for foul-
ing, agglomeration, and corrosion potential. They are shown in Table 10.3 .The
index is ranked as follows: 0 2 low probability, 2 4 medium probability, 4 5

high probability, and 5 10 very high probability.

10.6.5 Biomass Variability
Variability is a major issue in biomass supply. Unlike coal, it is difficult to
get a consistent supply of large quantities of biomass of a given composition.

TABLE 10.3

Probability for Agglomeration, Fouling, and Corrosion for
Several Biomass Fuels as Developed by the Foster Wheeler Corporation

Probability Index
for

Rice
Husk

Sunflower
Husk Sawdust Wood Chips Straw

Agglomeration 0 1 1 4 5

Fouling 0 2 2 6 6

Corrosion 2 1 2 5 7

Source: Data from Janati et al. (2003).