Slagging Characteristics during Combustion of Woody Biomass

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Energy Fuels 2010, 24, 3456–3461 Published on Web 05/20/2010

: DOI:10.1021/ef901571c

Slagging Characteristics during Combustion of Woody Biomass Pellets Made from a Range of Different Forestry Assortments § € Erica Lindstr€ om,*,† Sylvia H. Larsson,‡ Dan Bostr€ om,† and Marcus Ohman †

Energy Technology and Thermal Process Chemistry, Umea˚ University, S-901 87 Umea˚, Sweden, ‡Unit of Biomass Technology and Chemistry, Swedish University of Agricultural Sciences, S-901 83 Umea˚, Sweden, and §Division of Energy Engineering, Lulea˚ University of Technology, S-971 87 Lulea˚, Sweden Received December 21, 2009. Revised Manuscript Received April 28, 2010

In this study, multivariate methods were used to select representative raw materials of the pellet assortments prior to combustion. The fuels were selected to form a range of expected slagging tendencies. During combustion, temperatures and O2, CO, NO, and SO2 were measured continuously. The deposits (i.e., slag and bottom ash) were quantified after every experiment and collected for analysis to identify the crystalline phases and to study the morphology and elemental composition respectively. As expected, the slagging was most severe for the whole-tree assortments because of their content of branches, foliage, and twigs. In the most severe case over three-quarters of the total amount of ash melted to form slag. This study indicates that certain concentrations of silicon, inherent in the fuel but also as silicates from contamination, together with alkali metals, mainly potassium, are prerequisites for the initiation of and progress of slag formation. Generally the concentrations of silicon and potassium are low in stemwood but higher in bark, foliage, and living tissues of the tree. Also, the contamination from sand and/or soil is present in the bark and foliage.

harvest, difficult to handle and process, and are variable in quality.6 The average ash content of bark and foliage from Scots pine, Norway spruce, and birch is 6-11 times higher than the ash content of stemwood.7 The effective heating values (MJ kg DM-1) of all above-ground components vary as follows: Scots pine, 19.3 -21.0; Norway spruce, 19.0 -19.8; birch, 18.6 -22.7. 8 Bark, foliage, and wood in branches have higher heating values than wood in stems. Consequently, small dimension assortments will have higher ash content and slightly higher heating value (due to a higher ratio between bark and stemwood) than large dimension assortments and the amount of branches and foliage will have a substantial impact on the ash content. Also, compared to pure stemwood, small dimension assortments have a broader variation in total ash content, as well as in composition of ash forming elements. 9 Results from previous work have shown that the operation of pellet burning equipment is relatively sensitive to variations of the ash-forming elements in the fuel due to slag formation on the burner grates and that high concentration of silica (i.e., contamination of sand/soil) in wood raw material leads to severe slagging. 5,10

Introduction From 1997 to 2009, the delivered amount of fuel pellets to Swedish consumers increased from approximately 0.5 to 1.9 million tons per year.1 The raw materials used in Swedish fuel pellet production are residues from the forest industry, mainly sawdust and cutter shavings. The availability of these raw materials, being side products, is limited by the demand of traditional forest industry products. The increased demand of raw material for fuel pellet production has increased the sawdust prices and created a competitive situation where the pellet production industry has been experiencing feedstock shortages.2 To be able to grow further, the pellet production industry will have to introduce new raw materials. For pellet feedstock, certain properties are desirable, e.g., low ash content, high heating value, and low slagging tendency.3-5 Stemwood from large dimension lumber has high fuel quality and low extraction costs, compared to residues from conventional forestry that are expensive to *To whom correspondence should be addressed. Telephone: þ46 (0)10 480 22 91. Fax: þ46 (0)10 480 23 63. E-mail: erica.lindstrom@ chem.umu.se, [email protected]. (1) PiR (Swedish Association of Pellet Producers), 2010. Delivery Statistics Swedish Market 1997-2009. http://www.pelletsindustrin.org/ web/Statistik.aspx. (2) Helby, P.; B€ orjesson, P.; Hansen, A. C.; Roos, A.; Rosenqvist, H.; Takeuchi, L. Market Development Problems for Sustainable Bio-Energy Systems in Sweden. (The BIOMARK Project). Section for Environmental and Energy Systems Studies; Lund University: Lund, Sweden, 2004; pp 151-191. (3) Lehtikangas, P. Quality properties of pelletised sawdust, logging residues and bark. Biomass and bioenergy 2001, 20, 351–360. (4) Obernberger, I.; Thek, G. Physical characterisation and chemical composition of densified biomass fuels with regard to their combustion behaviour. Biomass Bioenergy 2004, 27, 653–669. € (5) Ohman, M.; Nordin, A.; Hedman, H.; Jirjis, R. Reasons for slagging during stemwood pellet combustion and some measures for prevention. Biomass Bioenergy 2004, 27, 597–605. r 2010 American Chemical Society

(6) Andersson, G.; Asikainen, A.; Bj€ orheden, R.; Hall, P. W.; Hudson, J. B.; Jirjis, R.; Mead, D. J.; Nurmi, J.; Weetman, G. F. Production of Forest Energy. In Bioenergy from Sustainable Forestry: Guiding Principles and Practice; Richardson, J., Bj€orheden, R., Hakkila, P., Lowe, A. T., Smith, C. T., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2002. (7) Hakkila, P.; Kalaja, H. The technique of recycling wood and bark ash. Folia For. 1983, 552. (8) Nurmi, J. Heating values of the above ground biomass of smallsized trees. Acta For. Fenn. 1993, 236. (9) Nordin, A. Chemical elemental characteristics of biomass fuels. Biomass Bioenergy 1994, 6, 339–347. € (10) Ohman, M.; Boman, C.; Hedman, H.; Nordin, A.; Bostr€ om, D. Slagging tendencies of wood pellet ash during combustion in residential pellet burners. Biomass Bioenergy 2004, 27, 585–596.

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Table 1. Descriptive Data on the Five Tested Pellets Assortments code

assortment

species

mean stem diameter at breast height (cm)

stems (ha-1)

stemwood REFERENCE pulpwood delimbed Norway spruce thinning delimbed Scots pine thinning whole-tree Scots pine thinning whole-tree MIX

stemwood thinning/final felling early thinning early thinning early thinning

Norway spruce/Scots pine Norway spruce Scots pine Scots pine mixed (>50% birch)

14 14 11

2900 2900 7200

Table 2. Fuel Characteristics parameter (units) moisture content (%) ash content (wt % d.b.) bulk density (kg/m3) C (% d.b.) H (% d.b.) O (% d.b.) N (g/kg) S (g/kg) Cl (g/kg) P (g/kg) Si (g/kg) Ca (g/kg) K (g/kg) Mg (g/kg) Al (g/kg)b a

stemwood REFERENCE

pulpwood delimbed Norway spruce

thinning delimbed Scots pine

thinning whole-tree Scots pine

thinning whole-tree MIX

8 0.2 650 51.9a 6a 41.8a 0.12a

7.2 0.7 680 50.4 6.1 42.52 0.8 0.1 0.1 0.08 0.06 1.14 0.72 0.22

5.7 0.5 666 50.4 6.2 43.2 2 0.1 0.1 0.09 0.85 0.6 0.46 0.13

7.6 1.1 629 51.2 6.4 42.1 3 0.2 0.1 0.24 1.98 1.21 1.21 0.25

8.6 1.3 656 50.6 6.1 42.52 3 0.3 0.1 0.3 2.34 1.69 1.2 0.29

0.002 0.005 0.06 0.022 0.006

Nordin, A. Chemical elemental characteristics of biomass fuels. Biomass Bioenergy 1994, 6 (5), 339-347. b Values not available.

The major ash forming elements in silvicultural raw materials are Si, Ca, Mg, K, P, and depending on the degree of contamination from sand and soil, Al. However, variations in these elements between different tree components are considerable. Werkelin et al.11 has studied the concentration of different elements in Scots pine, Norway spruce, and birch. The relation between the Si content in stemwood compared to needles, twigs, and shoots for Scots pine and Norway spruce was 1:4 to 77, whereas the relation between the Si content in stemwood and bark was 1:1 to 2. For birch, the relations between Si content in different above-ground components were more even (1:1 to 3). The branch-bark of birch contains much higher amounts of silica than pine bark (1:14). The K content and P content in fuels are of significant importance, since products of these two have a major impact on the ash melting temperature. The study by Werkelin et al.11 showed that the potassium content and phosphorus content were strongly correlated. In young foliage, the K content and P content were higher than in older foliage and twigs. Generally, the K content and P content were high in branches, though the P content was lower than the K content. The objectives in this study were to quantify the slagging of pellets made from different forestry assortments. Also, the impact of delimbing as a fuel quality improving action was evaluated. The assortments to be studied are considered to be interesting silvicultural raw materials for future fuel pellets production.

harvested in August in Harrsele (64.00 N, 19.34 E), NE Sweden, and delimbing was preformed on site prior to storage. All delimbing was performed on site, before transport to storage. Thinning assortments were stored for 1 year in uncovered piles on a paved surface in Umea˚ (63.49 N, 20.18 E), NE Sweden. Raw materials were shredded with a mobile shredder (Willibald SR 5000, Willibald GmbH, Wald-Sentenhart, Germany) and dried down from approximately 50% to 15% moisture content through forced ventilation of the material bed with hot air (approximately 85 °C). After drying, materials were reshredded using a 15 mm screen size (Lindner Micromat 2000, Lindner-Recyclingtech GmbH, Spittal, Austria), hammer-milled (screen sizes: 3 mm for pulpwood; 6 mm for thinning assortments) and pelletized with a SPC pelletizer (Sweden Power Chippers, Bora˚s, Sweden). The characteristics of the different pellet assortments are presented in Table 2. Multivariate methods were used to evaluate how representative the raw materials of the pellet assortments were. From the literature,12-21 data on the nutrient content of trees matching (12) Alriksson, A.; Eriksson, H. M. Variations in mineral nutrient and C distribution in the soil and vegetation compartments of five temperate tree species in NE Sweden. For. Ecol. Manage. 1998, 108, 261–273. (13) Helmisaari, H.-S. Nutrient cycling in Pinus sylvestris stands in eastern Finland. Plant Soil 1995, 168-169, 327–336. (14) Holmen, H. Forest Ecological Studies on Drained Peat Land in the Province of Uppland, Sweden. Parts I-III, Stockholm; Epsilon-SLU: Uppsala, Sweden, 1964. (15) M€alk€ onen, E. Effect of Complete Utilization on the Nutrient Reserves of Forest Soils; University of Maine: Orono, ME, 1973. (16) M€alk€ onen, E. Annual Primary Production and Nutrient Cycle in Some Scots Pine Stands; University of Helsinki: Helsinki, Finland, 1974. (17) Ovington, J. D.; Madgwick, H. A. I. Distribution of organic matter and plant nutrients in a plantation of Scots pine. For. Sci. 1959a, 5, 344–355. (18) Ovington, J. D.; Madgwick, H. A. I. The growth and composition of natural stands of birch. 1. Dry-matter production. Plant Soil 1959b, 10, 271–282. (19) Ovington, J. D.; Madgwick, H. A. I. The growth and composition of natural stands of birch. 2. The uptake of mineral nutrients. Plant Soil 1959c, 10, 389–400. (20) Tamm, C. O. Site Damages by Thinning Due to Removal of Organic Matter and Plant Nutrients; Royal College of Forestry: Stockholm, Sweden, 1969. (21) Wright, T. W.; Will, G. M. The nutrient content of Scots and Corsican pines growing on sand dunes. Forestry 1958, 31, 13–25.

Materials and Methods Fuels. Five different (L 8 mm) pellet assortments were used in the study (see Table 1). The stemwood pellets were produced from sawdust at a large pellet mill in northern Sweden. The raw materials for the other assortments were gathered as follows: The pulpwood (i.e., Norway spruce stemwood with bark) was harvested and delimbed 10 km east of Bora˚s (57.43 N, 12.55 E), SW Sweden. The thinning assortments (pine and birch) were (11) Werkelin, J.; Skrifvars, B.-J.; Hupa, M. Ash-forming elements in four Scandinavian wood species. Part 1: Summer harvest. Biomass Bioenergy 2005, 29, 451–466.

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Figure 2. p1/p2 loading plot for PCA using nutrient content (N, P, K, Ca) in different pellet assortments.

expected high fuel quality, to the whole-tree thinning assortments with the lowest expected fuel quality from a slagging point of view. Experimental Procedure. The combustion study was performed in a commercial underfed pellet burner (20 kW) installed in a reference boiler used for the national certification test of residential pellet burners in Sweden. Temperatures at three positions located on and in the vicinity of the burner grate were measured continuously with shielded type N thermocouples. Continuous measurements of O2, CO, NO, and SO2 were also performed with conventional instruments in the exhaust gas directly after the burner. The residual matter after a full conversion of fuel pellets in an underfed pellet burner is distributed between (i) melted ash (i.e., slag) deposited in the burner or pushed over the burner grate edge down to the bottom of the boiler, (ii) nonmelted ash at the bottom of the boiler (bottom ash), and (iii) fly ash. All melted particles greater than 3 mm was removed from the bottom ash by sieving and classified as slag, while the melted ash fraction smaller than 3 mm was included in the bottom ash fraction. The amount of deposited matter in the burner as well as in the boiler (i.e., bottom ash and slag) was quantified after every experiment, and the products were collected for analysis. Minimal variation in the slag formation has been seen in a previous study between replicate combustion procedures.22 Visual Examination and Chemical and Phase Analysis of the Collected Deposits. The collected deposits were characterized with respect to phase composition using a Bruker D8 Advance X-ray diffractometer (XRD) equipped with a primary G€ obel mirror and a superspeed VA˚NTEC-1 detector. Semiquantitative analyses were performed with Rietveld technique using TOPAS R, version 2.1,23 where structures from ICSD database24 were used as starting models. The morphology and elemental composition of the slags were investigated by means of environmental scanning electron microscopy (ESEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The samples were mounted in epoxy resin and polished prior to ESEM/EDS analysis. The resulting cross sections were carefully examined by spot- and area-mapping methods. Inductively coupled plasma mass spectrometry (ICP-AES/ICP-MS) was also used to analyze the composition of the ashes.

Figure 1. PCA score plot (t1/t2) of the nutrient content (N, P, K, Ca) of literature samples (unfilled symbols) and the pellets samples used in this study (filled symbols): (blue circles) pulpwood delimbed Norway spruce; (orange triangles) thinning delimbed Scots pine; (green boxes) thinning whole-tree Scots pine; (magenta triangles) thinning whole-tree birch; (brown diamond) thinning whole-tree mix; (orange asterisk) stemwood REFERENCE.

the age and tree species (Birch whole-tree (24-27 years), Pine delimbed (18-47 years), Pine whole-tree (15-47 years), Spruce delimbed (50-85 years)) of the studied pellet feedstock assortments were gathered. Principal component analysis (PCA) was carried out on a data matrix of 39 samples (used 5 pellet samples and 34 literature samples) and 4 variables (N-, P-, K-, and Ca-content). It would have been desirable to include more ash forming elements in the selection of variables, but the four variables were decided by the availability of literature data. PCA is an orthogonal linear transformation that transforms the data to a new coordinate system such that the greatest variance by any projection of the data comes to lie on the first coordinate (called the first principal component), the second greatest variance on the second coordinate, and so on. For the modeled data, using two principal components, the goodness of fit (R2X ) and goodness of prediction (Q2X ) was 0.94 and 0.63 respectively. The score plot of components 1 and 2 (Figure 1) concentrates the most relevant information on the samples where the variation of the variables is described by t1 = 0.77 and t2 = 0.17. The score plot shows that there is a clustering of the literature samples that is meaningful. The pellet samples from pulpwood and thinnings, used in this study, fall within their corresponding clusters of literature samples, and the clustering is divided as delimbed species to the left and whole-tree species to the right, which makes sense considering the PCA loading plot in Figure 2. This (Figure 2) shows that all nutrients are located to the far right, which indicates that the stemwood pellets sample, located to the far left, has a low content of all nutrients. Conclusively, the PCA analysis shows that the pellet samples used in this study have N, P, K, and Ca, which are representative for their species, treatment, and age group, and this is well illustrated in the score plot (Figure 1). The chosen raw materials form a range of expected slagging tendencies, from stemwood and rejected pulpwood, with an

€ (22) Gilbe, C.; Ohman, M.; Lindstr€ om, E.; Bostr€ om, D.; Backman, R.; Samuelsson, R.; Burvall, J. Slagging characteristics during residential combustion of biomass pellets. Energy Fuels 2008, 22, 3536–3543. (23) DIFFRACplus TOPAS R, version 2.1; Bruker AXS GmnH: Karlsruhe, Germany, 2003. (24) ICSD, Inorganic Crystal Structure Database; Fachinformationszentrum: Karlsruhe, Germany, 2005.

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The deposits were classified into four categories by visual inspection using the following criteria:25 (category 1) nonsintered ash residue, i.e., nonmelted ash; (category 2) partly sintered ash, i.e., brittle particles containing clearly melted ash, easy to break apart with bare hands; (category 3) totally sintered ash, i.e., the deposited ash that was melted into smaller blocks, still breakable with bare hands; (category 4,) totally sintered ash, i.e., the deposited ash that was completely melted into larger solid blocks, with no possibility of being broken apart with bare hands.

Results Amount of Deposited Slag Sintering-Degree and Melting Behavior. The in-going fuel load was just above 3 kg/h, which corresponded to a total burned pellet amount of 48 kg during 15-16 h of combustion. Because of an insufficient amount of available pulpwood pellets, the combustion time was only 4 h for this experiment (corresponding to 12 kg of combusted pellets). The excess oxygen levels varied between 5% and 9% during the respective combustion experiments, and the CO emissions were between 250 and 550 mg/Nm3 at 10% O2. From continuous measurements the maximum combustion temperature in the vicinity of the burner grate, where the slag was formed, was determined to be 1090 ( 80 °C. None of the experiments was interrupted due to severe slagging. All the collected slag was found on the burner grate as large fused lumps (see Figure 3). As can be seen in Figure 4, the different fuels produced different amounts of slag with different degrees of sintering. The pellets from stemwood and pulpwood showed a low slagging tendency, while the slagging tendency from the whole-tree thinning assortments was high. The most severe slagging occurred during combustion of the whole-tree thinning MIX assortment (>50% birch). By comparision of the two Scots pine assortments (i.e., thinning delimbed Scots pine and thinning whole-tree Scots pine), it is clear that the whole-tree assortment showed higher tendency for slagging. Chemical Composition of the Formed Bottom Ash and Slag. The results from the elemental analysis showed that the ashes of the three slag forming fuels (the thinning assortments) were dominated by silicon, calcium, and potassium in order of occurrence (Figure 5). In the ash of the non-slag-producing fuels (stemwood REFERENCE and pulpwood delimbed Norway spruce) calcium was the major element with potassium as the second most abundant. Generally, the composition of the melted ash part of the slag (with sand minerals excluded) was remarkably homogeneous. It was dominated by almost equivalent amounts of silicon and calcium and lesser amounts of potassium and aluminum, respectively (Figure 6). The thinning whole-tree Scots pine assortment contained more potassium compared to the delimbed Scots pine assortment, while the thinning whole-tree MIX had the highest amount of Si of all the slag forming fuels. The results from the XRD analysis on the bottom ash of the two non-slag-forming fuels showed that these were dominated by magnesium and calcium oxides (see Table 3). Carbonates of calcium and potassium and calcium hydroxide were also found. These ashes contained no sand minerals (quartz (SiO 2), microcline (KAlSi3 O8 ), and albite

Figure 3. Lump of fused ash from thinning whole-tree MIX that covered the whole burner grate as a lid in comparison to one L 8 mm pellet.

Figure 4. Fraction of fuel ash that formed slag (sintering categories 3 and 4) for the studied fuels.

(NaAlSi3O8)), but another silicate, merwinite (Ca3MgSi2O8), was identified in both cases. The bottom ash of the three slag forming fuels, on the other hand, was dominated by sand minerals, mostly quartz. The same non-silicate phases as in the previous ashes, except for K2Ca(CO3)2, were also found. In addition, two other silicates were identified, a˚kermanite and Ca2SiO4. The slags (formed from the three thinning assortments) displayed a more homogeneous phase composition (see Table 4). Besides sand minerals, mainly quartz, the slags consisted of the silicates a˚kermanite, leucite (KAlSi2O6), and in the case of thinning whole-tree Scots pine, also kalsilite (KAlSiO4). Enhanced baselines were observed in the XRD patterns of most of the ash and slag samples. This may be an indication of the presence of significant amounts of glassy materials.

€ (25) Ohman, M.; Bostr€ om, D.; Nordin, A.; Hedman, H. Effect of kaolin and limestone addition on slag formation during combustion of wood fuels. Energy Fuels 2004c, 18, 1370–1376.

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Figure 5. Elemental distribution (normalized on O and C free basis) of the formed ash obtained with ICP-AES/ICP-MS. Only one ash sample was sent for analysis.

Figure 6. Average elemental distribution (normalized on O and C free basis) of the melted ash obtained from five separate area analyses using SEM/EDS. Uncertainty bars correspond to the standard deviation of these five areas. Table 3. Identified Crystalline Phases from the Analyzed XRD Data Collection of the Bottom Asha stemwood REFERENCE SiO2 (quartz) KAlSi3O8 (microcline) NaAlSi3O8 (albite) Ca2MgSi2O7 (a˚kermanite) Ca3MgSi2O8 (merwinite) Ca2SiO4 CaO (lime) MgO (periclase) CaCO3 (calcite) K2Ca(CO3)2 (fairchildite) Ca(OH)2 (portlandite) total a

pulpwood delimbed Norway spruce

12

9

49 25 6 3 5 100

51 23 2 6 9 100

thinning delimbed Scots pine

thinning whole-tree Scots pine

thinning whole-tree MIX

59 9

27 8 11 7

51 13 25 3

13 9 12 6

8

7 100

100

6 6 6 14

100

Values are given in weight percent.

to pine needles according to Werkelin et al.11 This can be explained by the much higher amounts of silica in bark of birch compared with the bark of pine. However, the higher silicon content in thinning delimbed Scots pine and thinning whole-tree Scots pine compared to the stem- and pulpwood assortment could not be explained by the inherent Si content in the fuel.11 This must be an effect of sand contamination. The latter was also confirmed by the XRD analysis of the corresponding produced deposits which contained significant amounts of sand based minerals, i.e., quartz, microcline, and

Discussion The used wood assortments in this study showed clear differences in their fuel ash composition. Some major trends were observed that correspond with the results from the studies made by Werkelin et al.11 More potassium was found in the thinning whole-tree Scots pine and in the thinning whole-tree MIX assortment because of the high content of needles, shoots, twigs, and leaves, compared to the delimbed species. More silica was found in the thinning whole-tree MIX assortment, though leaves have lower silica content compared 3460

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Conclusion

Table 4. Identified Crystalline Phases from the Analyzed XRD Data Collection of the Slaga thinning delimbed Scots pine SiO2 (quartz) KAlSi3O8 (microcline) NaAlSi3O8 (albite) Ca2MgSi2O7 (a˚kermanite) KAlSi2O6 (leucite) KAlSiO4 (kalsilite) total a

thinning whole-tree Scots pine

11

7

47 42

58 22 13 100

100

The chosen raw materials formed a range of expected slagging tendencies, from no melted ash at all up to as high as almost 77% of the total ash melted. The slagging tendency was low for pellets from stemwood and pulpwood. The slagging tendency increased for the thinning assortments in the following order: pine delimbed, pine whole-tree, and mixed whole-tree. The concentrations of silicon and potassium show considerable variations in woody biomass. Generally the concentrations are low in stemwood (dead tissue) but higher in bark, foliage, and living tissues of the tree. Also, the contamination from sand and/or soil is, of natural reasons, preferentially present in the bark and foliage. This study indicates that certain concentrations of silicon, inherent in the fuel but also as silicates from contamination, together with alkali metals, mainly potassium, are prerequisites for the initiation of and progress of slag formation. In order to abate the risk of slagging, measures such as delimbing or defoliation of the forestry rejects could be recommended. Care must also be maintained to avoid unnecessary contamination of sand and/soil during harvesting, transport, and storage of the fuel or to debark the logs prior upgrading. Thus, the uneven distribution of silicon and potassium in different parts of woody biomass has to be carefully considered in order to find efficient ways, from an operational point of view, of utilizing assortments as forestry residues for energy conversion purposes. Measures such as delimbing or defoliation may be considered to reduce the content of potassium and silicon to decrease the potential risk of slag formation. Furthermore, the correlation of slagging with the presence of sand minerals in the fuel motivates efforts in reducing sand/ soil contamination during harvesting, transport, and storage of the fuel. One measure of prevention could be to debark the logs prior to upgrading.

thinning whole-tree MIX 25 13 11 7 44 100

Values are given in weight percent.

albite. This circumstance clearly indicates that these “new” types of wood-based assortments are susceptible to contamination during production and handling. The experimental combustion results showed that the slagging tendency of the silicon rich forestry biomass assortments was relatively high compared to the silicon poor assortments, i.e., the stem- and the pulpwood (stem þ bark). These results are qualitatively in agreement with previous experiences that showed that silicon rich fuels generally have relatively high slagging tendencies and that sand contamination to woody biomass fuels enhances the slag formation.10,22 Previous work has also suggested that gaseous potassiumcontaining species reacts with silicate surfaces and/or inherent Si in the fuel, forming sticky potassium rich silicate melts during the char burnout and devolatilization phase.10 This is considered to be the initiation of the slagging process.10,22 The results from the chemical analysis of the slags from the silicon rich assortments in this study also confirmed the presence of alkali rich silicates. For the nonslagging, silicon poor fuels, no slag are formed, and in the bottom ash potassium is found as harmless (nonsticky) potassium containing carbonates. The present results clearly show that a high slagging tendency in the combustion appliance can be expected when pellets made from wood based material containing significant amounts of branches, needles, and living tissues are used. Also, it has been seen that enhanced silicon rich biomass due to sand and/or soil contamination results in enhanced silicon content in residual ash that promotes slagging, as previous work also point out.5

Acknowledgment. This work was partly financed by the EU INTERREG IIIA via the project Bioenergy from forest thinning and partly by the Swedish Energy Agency. Pellets were produced at Biofuel Technology Center, SLU, Umea˚. For the linguistic corrections Dr. Dipanjan Banerjee is gratefully acknowledged.

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