Proceedings World Bioenergy 2010

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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 (
The DOMOHEAT European project is focused on the demonstration of two innovative and sustainable medium size Centre European heating systems, for domestic and tertiary buildings, using as fuel mediumlow quality wastes from South European Regions agro/forest/wood production. Biomasses selected for this project are olive stone, pine wood chips, pine cone chips, pine cone seed shells, almond shells, hazelnut shells, eucalyptus wood chips, straw pellets, oak and pine sawdust pellets, vine shoot chips, olive pruning chips, oak chips, poplar chips, and paulownia chips. Fuel and ash characterization is being carried out for each one of the different biomasses at laboratory to determine the proximate analysis (ash, volatiles, fixed carbon), moisture content, ultimate analysis (C, H, N, S, O), Cl content, gross and net calorific value, contents of major and minor ash forming elements, particle size distribution and mass density. Laboratory evaluations of ash slagging tendency are being performed. Combustion tests in 25, 100, 150 kW experimental boilers are currently being carried out both in Vigo (Spain) and KWB Austria, in order to achieve knowledge about biomass behaviour during combustion, ash melting behaviour and relevant emissions (CO, CO , H O, CxHy, 2 2 NOx, O2, SO2 and HCl) are being determined. Slag deposits composition will be analyzed and ash-limiting thresholds will be explored as a means to limit the amount of silica, alkali and chlorine present in the fuels as validated by boiler combustion tests and to predict the slagging tendency of the biomasses. The tendency of slagging and ash deposition will be evaluated based on these combustion tests to select the most promising fuels for further combustion test in two demonstrative boilers at Spain (Vigo and Leon). Based on biomass characterization results and combustion test results, criteria for rejecting/accepting biomasses will be developed. On a second stage, based on established ash slagging risk thresholds calibrated with combustion test results, biomass mixtures will be performed as a strategy to diminish the sintering and slagging tendency of initially rejected biomasses in 10 new combustion tests of biomass mixtures. More information of the project can be found at http://www.escansa.com/domoheat/ REFERENCES. [1] Fernandez J. Los cultivos energéticos en España y las tendencias de su desarrollo. [Energy Crops in Spain and the trends for their development]. In: I International Congress Bioenergia. Valladolid, Spain, 18-20 October 2006. [2] Vega-Niena, D. J., Dopazo, R., Ortiz. L. Reviewing the potential of Forest Bioenergy Plantations: Woody Energy crop plantations management and breeding for increasing biomass productivity. In: World Bioenergy 2008. Jönköping, Suecia 27-29 Mayo 2008 [3] Dopazo, R. D. J. Vega, Ortiz, L. A Review of Herbaceous Energy Crops for Bioenergy Production in Europe. In: 17 th European Biomass Conference & Exhibition, Hamburgo (29 Junio-3 Julio 2009). [4] Bryers, R. W. Fireside slagging, fouling, and hightemperature corrosion of heat-transfer surface due to impurities in steam-raising fuels. Prog. Energy Combust. Sci. 22 (1996) 29-120 [5] Tortosa-Masiá, A.A., Ahnert F., Spliethoff H., Loux J.C., Hein K.R.G. Slagging and fouling in biomass cocombustion. Thermal science 9 (2005) 3, 85-98. [6] Salour, D. Jenkins, B. M. Vafaei, T M., Kayhanian M. Control of in-bed agglomeration by fuel blending in a pilot scale straw and wood fueled AFBC. Biomass and Bioenergy Vol. 4, No. 2, pp. 117-133, 1993 [7] Pronobis M. Evaluation of the influence of biomass cocombustion on boiler furnace slagging by means of fusibility correlations. Biomass and Bioenergy 28 (2005) 375-383 [8] Jenkins B.M., Baxter L.L., Miles T.R. Jr., Miles T.R. Combustion properties of biomass. Fuel Processing Technology 54 (1998) 17-46. [9] Miles T.R., Miles T.R. Jr., Baxter L.L., Bryers R.W., Jenkins B.M., Oden L.L. Boiler deposits from firing biomass fuels. Biomass and Bioenergy 10 (1996) 125- 138. [10] Vamvuka D., Zografos D. Predicting the behaviour of ash from agricultural wastes during combustion. Fuel 83 (2004) 2051-2057. [11] Obernberger, I., Brunner, T., Bärnthaler G. Chemical properties of solid biofuels-significance and impact. Biomass and Bioenergy 30 (2006) 973–982 [12] Friedl A., Padouvas E., Rotter H., Varmuza K. Prediction of heating value of biomass fuel and ash melting behaviour using elemental compositions of fuel and ash. In: 9th International Conference on Chemometrics in Analytical Chemistry, September 2004. Lisbon, Portugal. [13] Seggiani, M. Empirical correlations of the ash fusion temperatures and temperature of critical viscosity for coal and biomass ashes. Fuel 78 (1999) 1121–1125 [14] Fernandez Llorente, M.J. Carrasco Garcia. J.E. Comparing methods for predicting the sintering of biomass ash in combustión. Fuel 84 (2005) 1893–1900 world bioenergy 2010 15
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