Proceedings World Bioenergy 2010

14 world bioenergy 2010
Fusibility temperatures (ºC)
Sample D S H F Source
Poplar I
>1400 >1400 >1400 >1400 [14]
Eucalyptus 1160 1170 1190 1230 [14]
Thistle I 640 660 1150 1150 [14]
Thistle II 1450 1450 1450 1450 [14]
Almond shell I 750 770 n.d 1450 [14]
Olive stones 1030 n.d 1090 1160 [14]
Brassica I 730 n.d n.d 1450 [15]
Brassica II 770 n.d n.d 1450 [16]
Wheat straw 850 1040 1120 1320 [14]
Rice straw 860 980 1100 1220 [14]
Sorghum 830 1000 1080 1350 [16]
D, deformation; S, sphere; H, hemisphere; F, fluid;
n.d, not detected.
Table III. Laboratory Sintering disintegration test results
for Mediterranean biomasses from Spain.
Disintegration test
Sample 800 °C 900 ºC 1000 °C Source
Poplar I E E E [14]
Eucalyptus VE VE VE [14]
Thistle I VD VD VD [14]
Thistle II D D D [14]
Almond shell
I E D VD
[14]
Olive stones VE E VD [14]
Brassica I VD VD VD [15]
Brassica II - - VD [16]
Wheat straw E VD VD [14]
Rice straw D VD VD [14]
Sorghum - - VD [16]
VE, very easy disintegration; E, easy; D, difficult;
VD, very difficult disintegration.
Disintegration tests seem to offer a complementary
criteria for rapid biomass characterization (Tables II and
III). The development of comprehensive predictive
functions for biomass ash fusion temperature based on
ash composition is still required for a deeper
understanding of ash sintering and melting processes.
Table IV. Slagging indexes and agglomeration level
observed in biomass combustion tests in [14], [15].
Sample
Ash
(%, d.b)
B/A
index
Alkali
index
(kg/GJ)
BedAgg Source
Poplar I 3.40 19.58 0.32 NO [14]
Eucalyptus 4.30 0.8 0.23 NO [14]
Thistle II 13.70 2.38 0.89 Part [14]
Thistle I 14.10 3.13 1.96 Total [14]
Brassica I 5.80 5.87 0.68 Total [15]
Almond shell I 1.00 8.43 0.11 Part [14]
BedAgg, bed agglomeration; NO, not agglomerated;
Part, partially agglomerated; Total, totally agglomerated.
The ash content can be a good indicator of the
problematic nature of a biomass fuel. Herbaceous fuels
typically show a higher value of ash content, this being
correlated with their intrinsic K content [Jenkins et al].
The Base to acid index, which has proven some
predictive potential for prediction of slagging-related
problems in studies such as [6] does not seem capable of
reflecting the bed agglomeration tendency shown in
Table IV. More refined alternative expressions may be
required to account for the role of silica-based
compounds on ash slagging hazard.
Alkali index seems to have potential for discriminating
potentially hazardous fuels in the light of results in Table
IV, with lower values of the index -below the 0.34 kg
alkali/MJ threshold proposed by [9]- corresponding to the
samples where no agglomeration was detected in the
boiler bed during the combustion test. One exception to
this is the almond shell sample, where the low ash
content, considered in the numerator of the alkali index,
amy have masked the intrinsically hazardous nature of
this fuel, with proven agglomeration effects, as proved
both by a DT of 750ºC and a partial agglomeration
observed in the boiler. An alternative alkali index may
allow to account for fuels such as almond with a low
amount of potentially problematic ashes.
The higher alkali (Na+K) content present in the first
sample of cardoon in Table IV (24% vs 11% in Thistle I
and II, respectively), as detected by alkali indices of 1.96
and 0.89, respectively, results in a higher temperature of
sinterizing for the first sample, and a total vs partial
agglomeration observed in the combustion test of these
two biomasses in [01] study.
Alkali metals react with silica contained in the residue's
ash forming silicates with very low melting point
(