Proceedings World Bioenergy 2010

Chlorine Index: Cl content (%) of the sample dry weight.
According to [1], [7]:
if Cl < 0.2 low slagging trend
if 0.2 < Cl < 0.3 medium trend
if 0.3 < Cl < 0.5 high trend
if Cl > 0.5 very high trend
; whereas [11 ] gives a more conservative threshold:
if Cl > 0.1 corrosion and HCl emissions
2.2.Ash Fusion Temperatures.
Ashes fusion is a continuous process which can be
characterized by the following temperatures, according to
norms ISO 540:2008 and DIN 51730:1998-04:
Deformation temperature (DT). Temperature at
which the first signs of rounding, due to melting, of the
tip or edges of the test piece occur.
Sphere temperature (ST). The temperature at which
the edges of the test piece become completely round,
with its height being equal to the width of the base line.
Hemisphere temperature (HT). The temperature at
which the test piece is approximately hemispherical, with
the height being equal to half the base diameter.
Flow temperature (FT). The temperature at which the
test piece material has spread out so that its height is onethird
of that at the hemisphere temperature.
A large number of authors have proposed predictive
functions of ash fusion temperatures from ash
composition for coals (i.e. see [4] for a comprehensive
review). Information on such predictive functions for
biomass, however, remains scarce (i.e. [12], [13]).
.Mineral composition in biomass differs significantly
from coal, especially in the amount of potassium, calcium
and chlorine, therefore, most appropriate slagging indices
definition and threshold calibration, as well as ash fusion
predictive functions must be different [9].
Additionally, biomass disintegration laboratory tests have
recently been proposed as a complementary means to
characterize slagging tendency of biomass fuels, with
significant potential to reproduce sintering and fouling
behaviour observed in biomass boiler combustion tests
[14].
Results using the above mentioned methodologies for
characterizing slagging and fouling tendency of various
Mediterranean biomass fuels are presented.
The effect of ash composition on ash behaviour as
characterized by laboratory ash fusion and ash
disintegration tests and boiler combustion tests, as well as
the potential of biomass ash slagging indices to predict
ash behaviour in terms of slagging and fouling hazard is
discussed below.
2. SLAGGING AND FOULING RISK OF
MEDITERRANEAN BIOMASSES IN COMBUSTION
Various recent studies (i.e. [01], [02], [03] and [04]) have
recently analyzed the slagging and fouling tendency of
several forest and agricultural residues and energy crops
by means of laboratory ash characterization tests and
combustion tests in Spain.
The results of the ash composition of some of the main
Mediterranean biomasses analyzed by these studies are
synthezised in Table I below.
Table I: Ash composition (wt% dry basis) at 550 ºC of
various Mediterranean Biomasses from studies [01], [02],
[03] and [04] in Spain.
Sample SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O P2O5 Ref.
Poplar I 2.80 -- -- 33 3.70 0.14 18 -- [14]
Poplar II 4.20 0.34 0.36 29 3.00 0.14 16 3.00 [15]
Eucalyptus 41 -- -- 18 4.20 1.90 8.70 -- [14]
Thistle I 13 -- -- 14 2.50 9.20 15 -- [14]
Thistle II 20 -- -- 33 3.30 4.70 6.60 -- [14]
Almond
shell I
Almond
shell II
Olive
stontes
4.60 -- -- 15 1.50 0.30 22 -- [14]
3.50 0.49 0.27 16 2.60 0.49 31 2.40 [17]
24 -- -- 4.40 1.70 0.52 27 -- [14]
Brassica I 7.60 0.61 0.28 23 3.20 0.69 21 9.10 [15]
Brassica II 1.30 0.16 0.60 25 2.70 1.10 27 8.40 [16]
Wheat
straw
44 -- -- 8.10 2.40 0.22 18 -- [14]
Rice straw 51 -- -- 8.9 3.5 2.8 16 -- [14]
Sorghum 40 2.60 1.00 5.50 2.50 0.29 23 2.10 [16]
Forest biomasses (i.e. Poplar, Eucalyptus chips) have
very beneficial properties for combustion, namely a high
Ca content, low silica content, and generally lower K
content when compared to most agricultural feedstocks
and residues, this resulting in high initial deformation
and ash fusion temperatures, generally well above 1200-
1400 ºC, as shown in Table II.
It has to be noted, however, that woody fuels can often be
contaminated during harvest operations with soil
particles, resulting in a higher silica content (i.e.
Eucalyptus sample in Table I), which can lower the ash
fusion temperatures. This situation has been noted by
various authors (i.e. [18]. More careful harvest methods
need to be implemented in order to minimize soil
contamination.
Herbaceous fuels such as brassica, wheat and sorghum
have large contents of both silica and potassium, resulting
in the potential formation of K silicates (i.e. [4], [8],
[19]). These compounds create deposits in the boiler,
potentially causing slagging and fouling problems at
temperatures above 700-800ºC, as illustrated by the
sintering temperatures DT in Table II.
Table II. Ash fusibility temperature of various
Mediterranean feedstocks from Spain.
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