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