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Predicting Slagging Tendencies for Biomass Pellets Fired in Residential Appliances: A Comparison of Different Prediction Methods Carl Gilbe,*,† Erica Lindstro¨m,‡ Rainer Backman,‡ Robert Samuelsson,§ Jan Burvall,| and ¨ hman† Marcus O DiVision of Energy Engineering, Luleå UniVersity of Technology, S-971 87 Luleå, Sweden, Energy Technology and Thermal Process Chemistry, Umeå UniVersity, S-901 87 Umeå, Sweden, Unit for Biomass Technology and Chemistry, Swedish UniVersity of Agricultural Sciences, Box 4097, S-904 03, Umeå, Sweden, and Skellefteå Kraft AB, S-93180, Skellefteå, Sweden ReceiVed May 8, 2008. ReVised Manuscript ReceiVed September 8, 2008
In this paper, a comparison between four different types (both empirical and theoretical) of techniques to predict the slagging tendencies in residential pellet combustion appliances was performed. The four techniques used were the standard ash fusion test (SS ISO-540) used in the Swedish pellet standard (SS 18 71 20), thermal analysis (TGA/DTA), thermochemical model calculations, and a laboratory-scale sintering test. The tests were performed with 12 pelletized biomass raw materials, and the results were compared with measured slagging tendencies in controlled combustion experiments in a commercial under-fed pellet burner (20 kW) installed in a reference boiler. The results showed significant differences in the prediction of slagging tendencies between different predicting techniques and fuels. The method based on thermal analysis (TGA/DTA) of produced slags must be further developed before useful information could be provided of the slagging behavior of different fuels. The used sintering method must also be further improved before the slagging tendency of fuels forming slags extremely rich in silicon (e.g., some grasses) can be predicted. Relatively good agreement was obtained between results from chemical equilibrium calculations and the actual slagging tendencies from the combustion tests. However, the model calculations must be further improved before quantitative results can be used. The results from the standard ash fusion test (SS ISO 540) showed, in general, relatively high deformation temperatures, therefore predicting a less problematic behavior of the fuels in comparison to the actual slagging tendencies obtained from controlled combustion experiments in commercial pellet burner equipment. Nevertheless, the method predicted, in most cases, the same fuel-specific slagging (qualitatively) trends as the corresponding combustion behavior.
1. Introduction Upgraded biomass fuels, that is, pellets and briquettes, have become more common during recent years, especially fuel pellets, which are well suited for heating applications in the residential sector. Limited availability of sawdust and cutter shavings together with an increasing demand for wood pellets in Scandinavia are pushing the market toward new and potentially more problematic raw materials with higher ash content than normal stem wood assortments, that is, ash content >0.5 wt % on dry basis. Examples of such raw materials are some bark, logging residues, whole tree assortments, and reed canary grass. Compared to ordinary stem wood, these raw materials have a broader variation in the total fuel ash content, as well as in the composition of the ash-forming elements.1 The amount of total ash and critical inorganic elements in some of these raw materials could, potentially, result in ash-related problems such * Corresponding author. Phone: +46 911 211027; fax: +46 911 232399; e-mail:
[email protected]. † Luleå University of Technology. ‡ Umeå University. § Swedish University of Agricultural Sciences. | Skellefteå Kraft AB. (1) Nordin, A. Chemical elemental characteristics of biomass fuels. Biomass Bioenergy 1994, 6, 339–347.
as slagging in pellet appliances.2,3 These problems can lead to a reduced accessibility of the combustion system as well as bad publicity for the pellet market. It would therefore be of considerable interest to predict the slagging tendencies for different biomass pellet assortments. Many ash behavior prediction techniques are available.4-8 Most of these have, however, been developed for addressing fouling and slagging during coal combustion. The standard ash ¨ hman, M.; Boman, C.; Hedman, H.; Nordin, A.; Bostro¨m, D. (2) O Slagging tendencies of wood pellet ash during combustion in residential pellet burners. Biomass Bioenergy 2004, 27, 585–596. (3) Lindstro¨m E.,Bostro¨m D.,Ohman M. Effect of kaolin and limestone addition on slag formation during combustion of woody biomass pellets In 14h European Biomass for Energy Industry and Climate Protection, Paris, France, 17-21 October, 2005; ETA-Florence Renewables Energies: Italy, 2005; ISBN: 88-89407-07-7. (4) Raask, E. Sintering characteristics of coal ashes by simultaneous dilatometry-elictrical conductance measurements. J. Therm. Anal. 1979, 16, 91. (5) Skrifvars, B. J.; Hupa, M.; Hiltunen, M. Sintering of ash during fluidized bed combustion. Ind. Eng. Chem. Res. 1992, 31, 1026–1030. ¨ hman, M.; Nordin, A.; Hupa, M. Predicting bed (6) Skrifvars, B.-J.; O agglomeration tendencies for biomass fuels fired in FBC boilers a comparison of three different methods. Energy Fuels 1999, 13, 359–363. ¨ hman, M.; Nordin, A. A new method for quantification of fluidized (7) O bed agglomeration tendencies: a sensitivity analysis. Energy Fuels 1998, 12, 90–94. (8) Stallmann, J. J.; Neavel, R. C. Technique to measure the temperature of agglomeration of coal ash. Fuel 1980, 59, 584.
10.1021/ef800321h CCC: $40.75 2008 American Chemical Society Published on Web 10/23/2008
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Table 1. Main and Ash-forming Elements of the Used Fuels wheat reed canary reed canary hemp straw grass (ha) grass (la) (ha) 6.6 asha cal. HVb 18.5 Ca 46.2 Ha 5.6 Oa 40.7 Na 0.9 Sa 0.14 Cla 0.22 Sia 1.45 Ala 0.006 Fea 0.010 Caa 0.39 Mga 0.086 Pa 0.10 Naa 0.013 Ka 1.19 a
10.7 17.9 44.2 5.5 38.8 0.8 0.09 0.05 3.8 0.31 0.11 0.29 0.073 0.090 0.052 0.36
3.1 19.5 48.2 6.0 41.6 1.1 0.12 0.02 0.68 0.081 0.031 0.34 0.078 0.13 0.009 0.23
hemp (la)
5.9 1.6 18.91 19.6 47.2 48.8 5.7 5.8 40.4 43.5 0.8 0.3 0.08 0.06 0.03 0.02 1.53 0.21 0.22 0.051 0.18 0.034 0.57 0.37 0.055 0.042 0.077 0.029 0.057 0.014 0.26 0.082
salix 2.0 19.7 48.8 6.0 42.9 0.3 0.03 1550
1070 1520 1290 1210 1270 >1550 >1550 1380 1230 1240 >1550 >1550
1140 1530 1310 1270 1270 >1550 >1550 1450 1230 1310 >1550 >1550
From the calculations, the melting behavior of the produced ash (condensed phases) as a function of temperature could be extracted. 3. Results 3.1. Combustion Results. By continuous and extensive measurements during the experiments, the maximum combustion temperature in the region where the slag was formed, that is, on the burner grate, was estimated to be about 1200-1250 °C. These temperatures did not vary between runs with different fuels. Clear differences in the slagging tendency were determined for the fuels. Four fuels had a high slagging tendency (wheat straw, reed canary grass-low ash, hemp-high ash, and logging residue-stored), that is, a large fraction (>50 wt-%) of the ingoing fuel ash formed a slag with a high sintering degree (according to the classification in Section 2.1). All of the bark fuels and hemp (la) showed a moderate tendency to form slag (10-40 wt % slag of fuel ash) during combustion, whereas the reed canary grass with high ash content, salix, and the fresh logging residue had low slagging tendencies (50 wt%), whereas reed canary grass (ha), logging residues (fresh), salix, and bark from spruce and pine had moderate fraction of melts (10-30 wt%) at 1200 °C. No slag was predicted to form at 1200 °C for the sawdust fuel. In general, the calculations gave
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Figure 1. Calculated fraction of oxide/silicate melt of condensed species at 1200 °C based on fuel ash (fuel) and produced slag composition (slag). Table 4. Results Determined by the Different Methods wheat reed canary hemp logging residues bark bark hemp bark logging residues reed canary straw grass (la) (ha) (stored) spruce birch (la) pine (fresh) grass (ha) actual combustion results, fraction of fuel ash that form slag (wt %) Thermochemical model calculations based on fuel composition, calculated fraction of melt of condensed species at 1200 °C (wt-%) Ash fusion test, deformation temperature (°C) Sintering test, initial sintering temperature (°C) a
salix
sawdust
75
71
70
59
34
22
20
11
5
3
1
0
78
55
51
65
10
59
53
17
29
30
28
0
820
1120
1170
1170
1400
1200 1250 1350
>1550
1390
n/aa
1020
830
1010
1030
1010
950
970
n/a
960
>1550 >1530 n/a
n/a
n/a; data not available.
relatively the same amounts of melt for the slag and the fuel ash for the all-straw fuels except for reed canary grass (ha). However, for the forest-derived fuels and for salix, the discrepancies between the calculations based on slag and fuel ash composition were, in general, higher (see Figure 1). The sintering test of the used slag samples resulted in initial sintering (defluidization) temperatures of 830-1030 °C. The lowest sintering temperature, (830 °C) was obtained for the slag produced when using wheat straw. The bark fuels followed, with sintering temperatures of 950-970 °C. The reed canary grasses, the hemp (ha), and the stored logging residues resulted in agglomeration temperatures between 1010 and 1030 °C. Because of too low amount of sample, the slags from hemp (la), salix, and the fresh logging residue were not subjected to the test. The result from the thermal analysis (DTA) of the collected slag’s showed only minor differences between different fuels. 4. Discussion A comparison between the results achieved from all of the different methods except from the thermal analyses is presented in Table 4. The results from the thermal analysis did not provide useful information of the slagging behavior of the different fuels. In general, the ash fusion test method predicted the fuelspecific slagging (qualitatively) trends relatively well. Fore many fuels, the thermochemical model calculations also managed to qualitatively predict the fuel-specific slagging trends relatively well. However, both the DTA analysis and the sintering method failed to predict these trends. The detailed results from the different methods are further discussed in this section.
The ash fusion test gave relatively high deformation temperatures in comparison to the actual slagging tendencies results obtained in the combustion experiments. For three fuels (hemp (la) and spruce and pine bark) which show a moderate slagging tendency, the deformation temperatures were well above the actual measured temperatures in the burner. Comparing the actual slagging tendency of the different fuels with the deformation temperature (Figure 2) it seems that if the deformation temperatures obtained are decreased by approximately 200 °C then the ash fusion test would, in all cases except for three fuels, manage to relatively well classify the fuels according to the actual combustion performance that is, slagging tendency. The standard ash fusion test has been extensively criticized in the coal literature,. One general criticism is the relevance of the ash sample, which is subjected to the test. The ashing temperature is often much lower (in this work 550 °C compared to approximately 1200 °C) and the history and atmosphere are often quite different to what the ashes have experienced in a real combustion situation. However, for about 50% of the used fuels there are only minor differences between the elemental composition of the slag and fuel ash for all elements except Cl and S, which are depleted from the slag (see Figure 3). For reed canary grass (ha), bark spruce, bark pine, logging residues (fresh), and salix there are major differences in the elemental composition between the fuel ash and the slag. The result from the equilibrium calculations also indicates (see Figure 1) that these discrepancies in composition affect the melting behavior of the ash. Gerald et al.15 have revealed that significant melting of the ashes occurs far below (200-400 °C) the TDT. In addition,
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Figure 2. Resulting deformation temperature TDT detected by SS-ISO 540 and TDT-200 °C versus actual slagging tendency obtained from the combustion experiment.
Huggins et al.16 and Huffman and Huggins17 showed that most ashes are completely molten at temperatures far below the TDT
Gilbe et al.
and that the progression from the TDT to the fluid temperature can be accomplished by holding the TDT constant for a period of 45 min. These previous results are in good agreement with the results from the equilibrium calculations in this work, which in general showed a relatively high fraction of melt (62 ( 22 wt %) at the deformation temperature. In addition, a better agreement with the actual combustion results was obtained if the TDT values were lowered by 200 °C. The results in this work therefore suggest that the ash fusion test (SS-ISO 540) could possibly be further improved by lowering the heating rate and by using a significantly higher ashing temperature. Further, strict control and observance of the test conditions is necessary to obtain reproducible results. Both repeatability and reproducibility have recently been shown to be poor in other ash fusion standards.18,19 In one investigation, the reproducibility has been demonstrated to be as poor as 140 °C. However, in the used method (ISO-540) the height and the width are continuously measured online and a reproducibility of (5 °C has been reported in the standard.10 Figure 4 shows the amount of ash at 1200 °C calculated with the chemical equilibrium model for both fuel ash (Table 1) and slag (Figure 3) compositions given as a function of the estimated fraction of the fuel ash that forms slag. For some of the fuels, a correlation can be seen, that is, higher slag forming tendency
Figure 3. Comparison between elemental distribution in fuel ash and slag.9 The elemental composition of the formed slag is given as average values and the standard deviations are shown as error bars.
Predicting Slagging Tendencies for Biomass Pellets
Figure 4. Calculated fraction of oxide/silicate melt of condensed species at 1200 °C based on fuel ash and produced slag composition versus actual slagging tendency obtained from the combustion experiment.
corresponds to higher amount of melt. It is important to notice that the equilibrium model used to estimate the amount of melt is not yet fully developed. The effect of formation of calcium and potassium phosphates on the melting behavior is not known and cannot be predicted by the model. In addition, some of the ternary compounds in the system SiO2-CaO-K2O are not implemented in the model. Thus, the estimated amounts of melt formed contain a rather large uncertainty. Nevertheless, the results can be used for semiquantitative comparisons between the fuel, that is, predicted trends are indicative. Some of the fuels used in this study were contaminated by soil and sand.9 The quartz from the sand probably reacts differently compared to the fuel ash related silica. In the model for estimation of the amount of molten phase, no distinction is made between these two forms of silicon. Because of this, the model may estimate higher amount of melt than is formed in the real case due to kinetical effects. This may explain why the two fuel ashes with lowest slag forming tendency (Figure 4) show relatively high amounts of melt at 1200 °C (45-53%). For one of the other slags, in parenthesis in Figure 4, the composition is far outside the validity for the chemical equilibrium model. Probably the amount of melt at 1200 °C for this slag is around 40% and not 10% as estimated. The results of the thermal analysis of the slag samples cannot be fully interpreted in terms of slag formation. Some of the (14) Bale, C. W.; Pelton, A. D. FACT-database of FACT-Win, v. 3.05; CRCT E´cole Polytechnique de Montre´al: Quebec, Canada, 1999. (15) Gerald, P.; Huggins, F. E.; Dunmyre, G. R. Investigation of the high-temperature behavior of coal ash in reducing and oxidising atmospheres. Fuel 1981, 60, 585. (16) Huggins, F. E.; Deborah, A. K.; Gerald, P. Correlation between ash fusion temperatures and ternary equilibrium phase diagrams. Fuel 1981, 60, 577. (17) Huffman, G. P.; Huggins, F. E. Fouling and Slagging Resulting from Impurities in Combustion Gases. Eng. Found. 1983, 25, 9–279. (18) Wall, T. F.; Creelman, R. A.; Gupta, R. P.; Gupta, S.; Coin, C.; Lowe, A. Applications of AdVanced Technology to Ash-Related Problems in Boilers, Baxter, L., Desollar, R. Eds.; 1995; Plenum Press: NewYork, 1996, p 541. (19) Coin, C.; Kahraman, H.; Peifenstein, A. P. Applications of AdVanced Technology to Ash-Related Problems in Boilers, Baxter, L., Desollar, R., Eds.; 1995; Plenum Press: NewYork, 1996, p 187. (20) Slegeir, W. A.; Singletary, J. H. How reliable are correlations between coal ash chemistry and ash fusibility, Proc. Int. Conf. Santa Barbara; 1988. California.
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slag samples contained unburned carbon, which give upward peaks (exothermic) in the temperature range 350-650 °C due to ignition of the char. The resulting curves reflect the course of gradual melting, that is, no sharp peaks occur, but the decreasing trend at higher temperatures indicates continuous melting and/or other endothermic reactions. The downward slope between different samples varies considerably, but based on these single runs it is not possible to fully identify the reason for the differences. Differences in heat capacity between the samples may have an influence, as well as possible sintering reactions that cause physical changes of sample. Most of the samples probably had a high content of glass, which means that the endothermal behavior to a large extent reflects physical glass properties and not the melting properties of the slag in burner conditions. An additional factor may be that the analyzer was not fully calibrated for calorimetric determinations, that is, the baseline may vary between runs. Thus, this method must be further developed in order to provide useful information about the slag-forming tendencies of fuels. The results from the sintering test of the collected samples showed a lower initial defluidization temperature for the used bark fuels than for the more problematic reed canary grass (la), hemp (ha), and the stored logging residue. The determined defluidization temperature is most probably describing the stickiness factor of the collected slag. This stickiness factor is probably influences both the viscosity and the amount of produced melt, both of which are also a function of the slag composition. The slags produced from the bark fuels had significantly lower silicon content than the slag produced using the reed canary grass (la), hemp (ha), and the stored logging residue. It can be speculated that the discrepancies in the predicted slagging characteristics could therefore be an effect of glass content in the slags. Previous studies where slags produced from combustion of different woody biomass fuels have generally showed good agreements with the actual slagging tendencies from corresponding combustion tests.2 5. Conclusions Twelve pelletized raw materials were predicted for their tendency to form slag during residential pellet combustion. Four different ways of predicting the slagging tendency were used: (i) the standard ash fusion test (SS ISO-540), (ii) thermal analysis (TGA/DTA), (iii) thermochemical model calculations, and (iv) a laboratory-scale sintering test. The results showed significant differences in the predicting slagging tendencies between different predicting techniques and fuels. The method based on thermal analysis (TGA/DTA) of produced slags must be further developed before useful information could be provided of the slagging behavior of different fuels. The used sintering method must also be further improved before the slagging tendency of fuels forming slags extremely rich in silicon (e.g., some grasses) can be predicted. Relatively good agreement was obtained between results from chemical equilibrium calculations and the actual slagging tendencies from the combustion tests. However, the model calculations must be further improved before quantitative results can be used. The results from the standard ash fusion test (SS ISO 540) showed, in general, relatively high deformation temperatures, therefore predicting a less problematic behavior of the fuels in comparison to the actual slagging tendencies obtained from controlled combustion experiments in commercial pellet burner equipments. Nevertheless, the method predicted, in most cases,
3686 Energy & Fuels, Vol. 22, No. 6, 2008
the same fuel-specific slagging (qualitatively) trends as the corresponding combustion behavior. The results from this work therefore suggest that the standard ash fusion test must be further improved before the method could also give useful quantitative information of the slagging tendencies of different biomass fuels. Acknowledgment. The financial support from the Swedish Energy Agency (STEM), EU Objective 1 So¨dra Skogsla¨n region, the counties of Va¨sternorrland, Va¨sterbotten, and Norrbotten, the Kempe Foundations, the Swedish University of Agricultural
Gilbe et al. Science (SLU), the Swedish pellet production industry (PIR), Skellefteå Kraft AB, Bioenergi i Luleå AB, SCA Bionorr, and the commune of Sollefteå is gratefully acknowledged. Additional funding from the National Technology Agency of Finland through the research project ChemCom at Åbo Akademi University (RB) is acknowledged. The authors also want to thank Henrik Sjo¨berg, Michael Finell, Mikael Thyrel, and Magnus Rudolfsson at Swedish University of Agricultural Sciences, Biomass Technology and Chemistry in Umeå for help with drying and pelletizing of the fuels. EF800321H
