Slagging Characteristics during Combustion of Corn Stovers with and

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Energy & Fuels 2008, 22, 3465–3470

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Slagging Characteristics during Combustion of Corn Stovers with and without Kaolin and Calcite ¨ rberg,† Gunnar Kalen,† Mikael Thyrel,† Shaojun Xiong,*,† Jan Burvall,†,‡ Håkan O § ¨ Marcus Ohman, and Dan Bostro¨m| Unit of Biomass Technology and Chemistry, Swedish UniVersity of Agricultural Sciences, SE-904 03 Umeå, Sweden, DiVision of Energy Engineering, Luleå UniVersity of Technology, SE-971 87 Luleå, Sweden, and Energy Technology and Thermal Process Chemistry, Umeå UniVersity, SE-901 87 Umeå, Sweden ReceiVed NoVember 29, 2007. ReVised Manuscript ReceiVed June 11, 2008

Ash-related problems have more than occasionally been observed in biomass-fired boilers and also recently in biopellet burners. These problems can lead to reduced reliability of the combustion systems as well as bad publicity for the market. When agricultural residues are used as biofuel feedstock, slagging problems will be worse. The objectives of the present work were, therefore, to examine the effects of kaolin and calcite addition on the slagging tendency of corn stover fuel when corn stover pellets are burned in a small-scale appliance and determine the slag characteristics during combustion. Pellets with an additive/fuel ratio of 3% (dry mass) were combusted in an underfed burner (50 kW) that is installed in a boiler with 90 kW output. The choice of 3% additive/fuel ratio was based on analyses of the ash melting behavior of seven fuel mixtures that combine either 0-3% kaolin or 0-3% calcite and corn stovers. The 3% kaolin and calcite addition increased the ash melting temperature (IT) by about 100-200 °C. When the 3% kaolin or calcite was added to the corn stover raw material, the severe slagging tendency of the fuel was considerably reduced. The slag quantities from burning kaolin- and calcite-added fuels were about half and one-third, respectively, of that from nonadditive pellets. The slag deposits from the burner were characterized with scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDS) and X-ray diffraction (XRD). The XRD was also used to examine the chemical composition of corresponding bottom ash in the boiler. The results indicated that the reduction of slagging when using additives can be attributed to a change from relatively low melting temperature silicates to higher melting temperature silicates. For the corn stover without additives, the low melting fractions of the slag were assumed to consist mainly of potassium calcium silicate, indirectly observed as a glass by the XRD. When kaolin was added, a depletion of potassium was observed because of the extensive formation of leucite (KAlSi2O6) and the glass became dominated by calcium, aluminum, and silicon. This process was accompanied by a considerable reduction of glass amount. In the case of CaCO3 addition, however, calcium magnesium silicates formed to an extent that the glass (low melting material) finally became dominated by potassium silicate. This process was also accompanied by a substantial reduction of the amount of glass.

1. Introduction During recent years, an increasing growth of sustainable energy production has been seen globally. A major contribution to the growth of sustainable energy is expected to come from biomass, because it is a renewable and CO2-neutral energy source. New and upgraded biomass fuels (i.e., pellets, briquettes, and powder) have become more common, and especially fuel pellets have proven to be well-suited to the residential market. Unlike virgin biomass, pellets are a densified fuel with higher energy content per unit volume that can easily be transported and traded. The raw materials presently used for pellets are in general stem-wood-based residues from sawmills and the woodworking industry, for example, in Sweden, Finland, Austria, and Canada. These wood pellets for residential heating are often used in more automated and optimized operating systems, approaching the user friendliness of oil firing and also * To whom correspondence should be addressed. E-mail: shaojun.xiong@ btk.slu.se. † Swedish University of Agricultural Sciences. ‡ Current address: Skellefteå Kraft, SE-931 80 Skellefteå, Sweden. § Luleå University of Technology. | Umeå University.

resulting in higher combustion efficiencies and less emission compared to traditional wood log firing. However, in the near future and in the regions/countries where stem-wood-based raw materials are limited, agriculture-derived feedstock will be increasingly used. This is especially true for some agricultural countries, such as China, where crop residues are surplus and energy is urgently needed. In comparison to stem-wood-based biofuels, agricultural residues normally have a higher ash content and lower ash melting temperature and, therefore, may be problematic in combustion.1,15 Studies to understand these problems and solutions to the problems are crucial. China has put bioenergy as a priority for the development and use of renewable energy sources, to achieve social and environmental sustainability and to meet the increasing energy demand.2 Developing bioenergy will help to make up the energy shortage and reduce dependence on coal that is about 67.1% of the total energy consumption in China. Coal combustion has (1) Stro¨mberg, B. Fuel Handbook; Va¨rmeforsk: Stockholm, Sweden, 2006; ISSN . (2) Li, J. F. Status of energy in China 2005. Presented at 1st Workshop of CAE-IVA Project “Cooperation on Renewable Energy and Environment”, Beijing, China, May 30-31, 2005.

10.1021/ef700718j CCC: $40.75  2008 American Chemical Society Published on Web 07/29/2008

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largely contributed to China being the world’s second largest emitter of the greenhouse gas carbon dioxide (CO2) and to the adverse effects of acid rain and particle emissions that are evident almost everywhere in China. Agricultural residues will be one of the major bioenergy resources in China.3 It has been estimated that about 551 million tons of agricultural residues are available for energy purposes, of which 30-40% consists of corn stovers (i.e., residues, mostly consisting of leaves and stalks, of corn plants left after a harvest of cereal grains).3,4 Today, most of this biomass is either abandoned or used traditionally for domestic cooking and space heating with low efficiency and high emission. In some regions, farmers have to burn the residues in fields to clear the area for the coming season cultivation, which has caused a lot of emissions and affected traffic because of heavy smoke. Upgrading the biomass into pellets may provide people with not only solid fuels for cooking, heating, and even power generation but also better environments, life convenience, and working opportunities. The densification of biomass into pellets makes it economically possible to be an energy carrier for long-distance delivery, and therefore, it can even be used as an alternative fuel for large-scale units. A goal has been set by the Chinese government that 50 million tons of biomass pellets will be produced and used annually by 2020.2 Feasibility studies are, however, missing. A potential establishment of a sustainable residential pellet market will be dependent upon preferences such as economic consideration and the attitudes of households. Ash-related problems, such as slagging on the grates, are often observed in biomass-fueled plants and in different pellet-fired burners and furnaces, especially when agricultural residues and biomass contaminated with impurities are used. These problems can lead to a reduced accessibility and durability of the combustion systems as well as higher emissions, which may in turn result in bad publicity for the residential pellet market. Previous work5–7 has indicated that the operation of pellet burners is relatively sensitive to variations in the total ash content and variations of the ash-forming elements in the fuel and that the sintering tendency may even occur among stem-wood-based pellets classified as “first-class” pellets according to the Swedish standard SS 18 71 20. Studies have also demonstrated that the ash-sintering tendency of biomass pellets is closely correlated to Si and alkali element contents in the fuels that vary with plant species, growing conditions, such as soil type and fertilization, harvest season, etc.8,9 Although the engineering and economical aspects of collection, densification, and some chemistry analyses of corn stovers as biofuel feedstock have been studied, for example, in North America,10–13 a gap exists in understanding the ash melting behavior and characteristics (3) Liao, C. P.; Yan, Y. J.; Wu, C. Z.; Huang, H. B. Biomass Bioenergy 2004, 27, 111–117. (4) Li, J. F.; Hu, R. Q.; Song, Y. Q.; Shi, J. L.; Bhattacharya, S. C.; Salam, P. A. Biomass Bioenergy 2005, 29, 167–177. ¨ hman, M.; Boman, C.; Hedman, H.; Nordin, A.; Bostro¨m, D. (5) O Proceedings of the First World Conference on Pellets, Stockholm, Sweden, Sept 2-4, 2002; pp 213-219. ¨ hman, M.; Nordin, A.; Hedman, H.; Jirjis, J. Proceedings of the (6) O First World Conference on Pellets, Stockholm, Sweden, Sept 2-4, 2002; pp 93-97. ¨ hman, M.; Bostro¨m, D.; Nordin, A.; Hedman, H. Energy Fuels (7) O 2004, 18, 1370–1376. (8) Landstro¨m, S.; Lomakka, L.; Andersson, S. Biomass Bioenergy 1996, 11, 333–341. (9) Burvall, J. Biomass Bioenergy 1997, 12, 149–154. (10) Sokhansanja, S.; Turhollow, A.; Cushmana, J.; Cundi, J. Biomass Bioenergy 2002, 23, 347–355. (11) Sokhansanj, S.; Turhollow, A. Appl. Eng. Agric. 2004, 20, 495– 499.

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of slag during the combustion of corn stovers. The increased market of pellets and other biofuels in the world suggests a big potential for using corn stovers if the slagging-related problems are understood and solved, because corn stovers are very common and available in large quantity not only in China but also in the rest of the world.10 Several authors have previously proposed the use of various kinds of mineral additives, for example, clay minerals5–7,14–16 and lime- and/or dolomite-based additives,5,14,16 to combat ashrelated operational problems during the combustion of biomass fuels. In a comparative study between kaolinite, bauxite, and emalthite, kaolinite (the major substance in kaolin) proved to be the most efficient mineral/additive.17 Combustion experiments have also been previously conducted, in which kaolin has been mixed into straw with positive results.18 The initial melting temperature was increased by 250 °C by adding kaolin in a proportion corresponding to 20 wt % of the fuel ash content of the straw. In another study, the ash fusion temperatures of the ash formed during combustion were effectively increased when kaolin and calcite, respectively, were mixed with peat before pelletizing.5 However, few, if any, studies have been published in which the effects of kaolin and/or calcite addition on corn stover fuels have been qualitatively and quantitatively determined. The objectives of the present work were therefore to (i) examine the effects of kaolin and calcite addition on the slagging tendencies of corn stover fuels when corn stover pellets are burned in small-scale appliances and (ii) determine the slag characteristics during combustion and therefore contribute to the understanding of the process of slagging tendency. 2. Experimental Section The methodologies employed in the present study were (1) fuel characterization analyses of corn stover fuel blended with kaolin or calcite [0-3 wt % dry mass (DM)], (2) subsequent combustion tests for corn stover pellets with and without the additives in a small-scale under-fed burner (EcoTec 50 kW) based on the results from 1 [an additional test with stem-wood pellets (see section 2.2) was also conducted for comparison], and (3) chemical and visual analysis as well as quantification of the sintered material from the collected deposits (see section 2.3). 2.1. Fuels, Additives, and Appliance. Corn stover fuel materials (Table 1) originating from Jilin, northeast China, were milled in a hammer mill with a sieve size of 4 mm prior to pelletizing. The pelletizing was conducted on a SPC PP300 Compact pelletizer (Sweden Power Chippers, Borås, Sweden), with a maximum capacity of 300 kg h-1. Three types of corn stover pellets were produced and used in the study: (1) pure corn residue pellets (i.e., no additives were blended); (2) kaolin-blended corn residue pellets (with 3% kaolin dry mass); and (3) calcite-blended corn stover pellets (with 3% calcite dry mass). The 3% was considered as the should be optimal additive/fuel ratio, which was based on analyses of the ash melting behavior of seven fuel mixtures (two replicates each) that combine either 0-3% kaolin or 0-3% calcite and corn (12) Pordesimoa, L. O.; Hamesb, B. R.; Sokhansanjc, S.; Edensd, W. C. Biomass Bioenergy 2005, 28, 366–374. (13) Mani, S.; Sokhansanj, S.; Bi, X.; Turhollow, A. Appl. Eng. Agric. 2006, 22, 421–426. (14) Ivarsson, E.; Nilsson, C. Special Report 153, Department of Farm Buildings, Swedish University of Agricultural Sciences, 1998; ISBN 91576-3500-5. (15) Steenari, B. M.; Lindqvist, O. Biomass Bioenergy 1998, 14, 67– 76. (16) Oravainen, H. Res. Thermochem. Biomass ConVers., Ed. Rev. Pap. Int. Conf. 1988; 667-679. (17) Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M.; Zhou, J.; Hiraki, T. T.; Masutani, S. M. J. Inst. Energy 1998, 71, 163–177. (18) Wile´n, C.; Staahlberg, P.; Sipila, K.; Ahokas, J. Energy Biomass Wastes 1987, 10, 469–484.

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Table 1. Fuel Characteristics of Corn Stovers from Qian, Jilin item

method

unit

result

ash at 550 °C chlorine (Cl) gross calorific value net calorific value carbon at 1050 °C (C) hydrogen at 1050 °C (H) nitrogen at 1050 °C (N) oxygen (O2) sulfur at 1350 °C (S) phosphor (P) potassium (K) calcium (Ca) silicon (Si) sodium (Na) magnesium (Mg) aluminum (Al)

SS 18 71 71:1 SS 18 71 54:1 SS-ISO 1928:1 SS-ISO 1928:1 LECO-method 1 LECO-method 1 LECO-method 1 calculated SS 18 71 77:1 NMKL161 mod.; ICP-AES NMKL161 mod.; ICP-AES NMKL161 mod.; ICP-AES ALC208:201; AAS (flame) NMKL161 mod.; ICP-AES NMKL161 mod.; ICP-AES NMKL161 mod.; ICP-AES

% of DM % of DM MJ/kg DM MJ/kg DM % of DM % of DM % of DM % of DM % of DM mg/kg DM mg/kg DM mg/kg DM mg/kg DM mg/kg DM mg/kg DM mg/kg DM

5.3 0.36 18.12 16.82 45.7 6.0 0.6 42.0 0.09 690 8100 3500 8300 120 3700 270

stovers (Table 2). The 3% was considered feasible with a consideration that the ash content in the fuel should not be too high after blending the additives. The mixing and pelletizing processes are summarized in the next paragraph. The slagging tendency of the ashes from the three types of pellets were further evaluated by combustion in an under-fed pellet burner. To make blended material/pellets, diluted additive suspensions (slurries) were supplied to the corn residues by a manually pumped knapsack sprayer. To facilitate a good performance of the sprayer, to increase the total reactive surface of the added additives, and to increase the adhesiveness to the raw material during the pelletizing process, particles of 1-2 µm were used in the preparation of the slurries of both additives. The mixing was conducted in a diagonal mixer (Mafa Diagonal Mixer type D2) for 30 min at a speed of 50 rpm. Water was added until the desired moisture level was reached. After preparation, the material was sacked and stored at approximately 15 °C for a minimum of 48 h. The additives used were a crystalline calcite slurry (76 wt % DM) and a kaolin slurry (66 wt % DM). The main constituent of the kaolin used was pure (99.9%) mineral kaolinite, Al2Si2O5(OH)4, with a small amount of halloysite Al2Si2O5(OH)4(H2O)2. The major constituent of the calcite was pure CaCO3 (99.9%). The different additive/fuel ratios were achieved by varying the dilution ratio of the original additive suspension. Pellets with nonadditive and optimal additive/fuel ratios were produced under similar conditions (e.g., total water content added to the process, production capacity, and raw material particle fraction) to minimize the variation in the physical characteristics between the different produced pellets. As shown in Table 3, the physical characteristics of the produced pellets corresponded well to normal full-scale produced wood pellets. A commercial pellet burner (EcoTec 50 kW) installed in a boiler of 90 kW (Teem, Eryl, Falun, Sweden) was used for the research tests. The burner is commonly used on the European market and also represents one class of burner design, with under-feeding of the fuel. 2.2. Combustion Procedure. Pellets of the corn residues with and without an additive were initially combusted in the under-fed burner (Figure 1) to evaluate the effect of different additives on the slagging tendency. The under-fed burner was chosen because of its better characteristics when burning ash-rich fuels in our earlier experiences. For a comparison, commercial stem-wood pellets were also burned. Every combustion test lasted for about 30-40 min at a nominal load of about 40 kW, corresponding to a total pellet amount of 4-6.2 kg for every experiment. A lot of slagging/ash deposition was piling up and blocking the primary and secondary air outlets upon combusting the corn stover pellets, making a longer test impossible. Temperatures were measured with a type N thermocouple sensor above the center of the burner cup aligning the top. Better positions for measuring the combustion temperature in the region where the slag was formed, i.e., on the burner grate, were proven not possible because of the burning behavior of the examined fuel and the design of the burner cup in which a ring grate was clockwise-rotating when operating. Continuous measure-

ments of O2, CO2, CO, SO2, NO2, NO, and NOx were also performed with conventional instruments in the exhaust gas directly after the boiler. All of the deposits from the bottom of the boiler were sieved to separate the ash from the slag and collected for analyses. All of the melted particles greater than 3 mm were removed from the ash as slag. The amount of deposited ash and slag were quantified after every experiment. The major parameters concerning the combustion process can be found in Table 4. 2.3. Chemical Analysis of the Collected Deposits. The collected deposits from the combustion experiments with and without an “optimal” (3% of DM) additive/fuel ratio were characterized with scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDS) and X-ray diffraction (XRD). The samples for SEM/EDS analysis were embedded in epoxy resin and polished, and the resulting cross-sections were carefully examined by SEM/EDS spot- and area-mapping methods. The samples for XRD analyses were carefully ground and mounted for data collection with a Bruker d8Advance X-ray diffractometer. Semiquantitative analyses were performed with the Rietveld technique using TOPAS R 2.1,19 where structures from the Inorganic Crystal Structure Database (ICSD)20 were used as starting models for the ash and slag phases.

3. Results and Discussion 3.1. Slagging Tendencies of a Corn Stover with and without Additives. The fuel analyses in this study (Table 1) indicated that the examined corn residues have generally similar characteristics to autumn-harvested energy grasses, such as reed canary grass and rather higher contents of ash, silicon, and alkali elements in comparison to wood fuels in Sweden.9 This suggests a high risk of slagging when burning the corn stovers from Jilin. Indeed, the examined corn stover pellets without additives caused severe slagging when they were burned in an under-fed pellet combustor in this study. Because of severe slagging in the burner cup, the combustion experiment of corn stover pellets without any additives was stopped after about 30 min from starting the fire, because of a piling-up of slag/ash and completely blocked air inlets for both primary and secondary air. A large fraction, 40 wt % of the ingoing fuel ash, formed a slag with a high sintering degree; i.e., the slag pieces reached as large as 1-2 tennis balls (cf. Table 5) and were as hard as a stone. In contrast, the stem-wood pellets without any additives did not produce any slagging at all in the same conditions of combustion in this study. The results of fuel analysis in this study suggested that the corn stover ash started to melt at about 1170 °C when no additive was blended (Table 2) according to the ash fusion test. Previous work has also shown that maximum temperatures in the region where the slag is formed, i.e., on the burner grate, were about 1200-1250 °C in similar under-fed pellet burners.21 In the combustion test of this study, however, the temperature was measured at a position above the center of the burner cup (Figure 1). The recorded maximum temperature was 860-900 °C, i.e., several hundred degrees lower than the ash melting temperature according to the ash fusion test (Table 2). The measure temperature did not correspond to the point where slagging occurred. The technique to measure the temperature in such a difficult situation (rotating grate with piling-up ash and slag) must be improved/developed in future studies. During (19) Bruker AXS GmbH DIFFRACplus TOPAS R 2.1, Karlsruhe, Germany, 2003. (20) Fachinformationszentrum, ICSD, Inorganic Crystal Structure Database, Karlsruhe, Germany, 2005. ¨ hman, M.; Lindstro¨m, E.; Gilbe, R.; Backman, R.; Samuelsson, (21) O R.; Burvall, J. Proceedings of the Second World Conference on Pellets, Jo¨nko¨ping, Sweden, June 1-2, 2006.

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Table 2. Ash Melting Behaviors of Corn Stovers with and without Additives, According to the Ash Fusion Testa +kaolin

initial deformation temperature, IT (°C) softening temperature, ST (°C) hemispherical temperature, HT (°C) fluid temperature, FT (°C)

+calcite

nonadditive

1%

2%

3%

1%

2%

3%

1170 1180 1200 1230

1270 1280 1310 1330

1350 1360 1370 1390

1370 1380 1390 1390

1170 1190 1210 1220

1240 1260 1280 1290

1290 1300 1320 1330

a Method: ASTM D1857-68. The percentage of additives was based on dry weight. Slurries of additives were prepared and blended in the raw materials before the test.

Table 4. Combustion Parametersa

Table 3. Characteristics of Three Types of Produced Corn Residue Pellets size (length × diameter, mm) bulk density (kg of DM/m3) durability (%, Ligno test) moisture (%)

nonadditive

+kaolin

+calcite

15-40 × 8 629 98.0 13.5

15-40 × 8 665 98.3 13.4

15-40 × 8 649 98.6 13.8

a test run, the temperature varied also a lot, because of the changing air flow when slag deposition blocked the air inlets. The additives used in this study increased the ash melting temperature by 100-200 °C, according to the ash fusion test (Table 2), and had a distinct positive effect on the slagging tendency (cf. Figure 2). The 3% calcite addition to the pellets had the best results and reduced slag formation, thereby improving the operation of the burner considerably. As shown in Table 5, about 13% of the ingoing fuel ash formed slag and only less than 1/3 of the slag quantity formed when burning the nonadditive pellets. The slag particles had a size of less than

corn stover pellets test time (min) pellets used (kg) thermal input (kW) O2 (mg N-1 m-3) CO (mg N-1 m-3) SO2 (mg N-1 m-3) NO (mg N-1 m-3) NO2 (mg N-1 m-3) a

steam wood pellets

nonadditive

+kaolin

+calcite

nonadditive

39 6.2 39 8.1 54 100 240 12

35 5.6 41 7.8 30 120 270 19

34 5.4 42 7.6 57 98 270 6.7

30 4.0 38 7.2 380 2.0 56 3.0

Emission values are measured at 11% O2 of dry gas.

10 mm and were so fragile that a press between fingers could turn them into powder. The 3% kaolin addition also reduced slag formation but not as effectively as the calcite. About 20% of the ingoing fuel ash formed slag, and the slag particle size was between that obtained from burning calcite-blended and

Figure 1. Schematic illustration of the boiler and pellet burner cup used in this study. The illustrations are modified pictures from EcoTec (http:// www.ecotec.se).

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Figure 2. Optical microscopy (6.3 × 10) pictures showing the morphology of the slag formed when combusting corn stover pellets with and without additives. (A) Nonadditive, (B) 3% kaolin, and (C) 3% calcite. The scale is the same for all three pictures. Table 5. Slag Characters combustion of pellets blended with nonadditive fraction of fuel ash that forms slag (sinters >3 mm in ash, % of DM) average size of top 10 sinters, mm (length × width)

kaolin

calcite

40.6

20.0

13.3

79 × 51

21 × 16

9×7

nonadditive pellets; however, the slag was very hard, comparable to that from nonadditive pellets. Even though an under-fed burner with a rotated ring grate was chosen to conduct the combustion test for the ash-rich fuel in this study, slag formation was substantial when nonadditive fuels of corn stover were used. However, in the cases of fuels with additives where slag formation was low, the combustion process was still difficult to maintain longer than 40 min after ignition because of ash accumulation in the burner. 3.2. Chemical Compositions of Slag and Bottom Ash. The collected samples of bottom ash and slag were analyzed with SEM/EDS to determine the concentration of the main elements. The result shows a typical image of a polished cross-section of a slag sample in epoxy resin and indicates that the melted ash (i.e., slag) was relatively homogeneous. The average elemental composition of the slag for all assortments is given in Figure 3. The slag was dominated by Si, K, Mg, Ca, and Al, thus presumably consisting of different silicates. The slag produced when burning calcite-blended pellets had significantly higher Ca- concentrations, whereas the slag produced when using kaolin as the additive was richer in Al. The corn stover fuel showed moderate to high slagging tendencies (Table 5). It contained rather high melting temperature phases, such as, for instance, forsterite (Mg2SiO4), monticellite (CaMgSiO4), åkermanite (Ca2MgSi2O7), and leucite (KAlSi2O6) (Table 6). Considering the high potassium and silicon content of both the corn stover fuel and the corresponding slag (Figure 3), it is plausible that a certain amount of low melting temperature potassium silicates formed in the ash and remained melted at combustion conditions. When cooled, these melted materials to a certain extent formed glasses, which are not directly observable by XRD. Upon adding an aluminosilicate, such as kaolinite (Al2Si2O5(OH)4), the provision was made for the precipitation of potassium in the form of kalsilite (KAlSiO4) or leucite. It should be noted that, during the formation of the latter phase, equivalent amounts of potassium and silica are needed in addition to kaolinite. From Figure 3, it can be seen that the potassium content of the slag decreased only by about 10 atom % at the same time as the total amount of ash-forming matter increased by about 60% (weight) upon the kaolinite addition. Thus, the kaolinite additive is serving dual purposes: to capture potassium in the bottom ash and to neutralize reactive silica into a more

Figure 3. Average elemental composition in the formed slag (presented on a carbon- and oxygen-free basis) during combustion of corn stover pellets with nonadditive, kaolin, and calcite. Areas of 100 × 100 µm of the slag were analyzed. Standard deviations are shown as error bars. Table 6. Composition (wt %) of Components in Bottom Ash and Slag Resulting from XRD Analysis nonadditive ash SiO2 (quartz) Mg2SiO4 (forsterite) CaMgSiO4 (monticellite) CaSiO3 (wollastonite) Ca2MgSi2O7 (a˚kermanite) Ca3Mg(SiO4)2 (merwinite) Ca14Mg2(SiO4)8 (bredigite) CaO (lime) MgO (periclase) CaCO3 (calcite) KAlSi2O6 (leucite) KAlSiO4 (kalsilite) sum

1 31 13 7 6

slag 57 4

+kaolin slag

ash

slag

30

1 22 1

9

9

16 44 17 5 3 3

15 41 17 6 3 4

3 100

5 100

11

1 29 12 99

17 10 100

+CaCO3

ash

65 5 100

76 100

high melting temperature and favorable phase from a slagging point of view (Tables 2 and 5). The addition of calcite changed the composition of the ash from being dominated by leucite and forsterite to more calcium containing orthosilicates, such as merwinite [Ca3Mg(SiO4)2] and bredigite [Ca14Mg2(SiO4)8]. These are also high melting temperature phases and contribute to an increase in ash melting temperature compared to the control material with no additive. Notable is the total disappearance of leucite. As mentioned above, the analyzed ash and slag is composed of a mixture of crystalline and glassy materials. Their ratio is a function of the original ash composition and the thermal history. The morphological information from SEM analysis revealed that

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present in crystalline phases after cooling. This estimate resulted in 55, 23, and 25 mol % amorphous material for the control, the kaolin-added, and the CaCO3-added fuel, respectively. Given these ratios, the elemental distribution between amorphous and crystalline material could be calculated (see Figure 4). In the first case (control), the glass mainly consisted of a potassium calcium silicate. When kaolin was added, a depletion of potassium was observed because of the extensive formation of leucite (KAlSi2O6) and the glass became dominated by calcium, aluminum, and silicon. This process was accompanied by a considerable reduction of glass amount. In the case of calcite addition, calcium magnesium silicates formed to the extent that the content of the glass finally became dominated by potassium. This process was also accompanied by a substantial reduction of the amount of glass. The simplifications involved in the calculations and estimations above deprive the results of too strict inferences but can be used to interpret the major chemical trends in the slag upon kaolinite and calcite addition. 4. Conclusions

Figure 4. Elemental distribution between glass (amorphous material) and crystalline material of the analyzed slag. The composition is normalized to the content of K, Mg, Ca, Al, and Si. The glassy fraction amounted to 55, 23, and 25% for the pure corn stover and kaolin- and calcite-added fuel, respectively.

under combustion conditions certain fractions of the residue were probably melt and other fractions solid, preferentially crystalline. Upon cooling, the latter part increased as several phases successively crystallized from the melt. However, in all cases, it is likely that at some temperatures a certain fraction of the melted material ceased to crystallize, became supercooled, and finally formed a glass. The sinters estimated and quantified in Table 5 thus consist of solidified melt (crystalline or glass) and agglomerated ash with melt acting as “glue” (cf. Figure 2). The amounts of sinters given in Table 5 must therefore not be regarded as the amount of “pure” melt, originally present in situ under combustion conditions. An attempt to estimate the relation between glass (amorphous material) and crystalline material of the analyzed slag was performed by accounting for the amount of crystalline phases from the XRD measurements (Table 6), the elemental analysis from SEM/EDS results (Figure 3) and simplifying the system to only comprise Si, Ca, Mg, K, and Al and under the assumption that all of the magnesium content of the slag was

The examined corn stover pellets without an additive caused severe slagging when they were burned in an under-fed pellet combustor in this study. About 40 wt % of the ingoing fuel ash formed slag, and the slag pieces could reach as large as 1-2 tennis balls and were as hard as stones. By adding a kaolin or calcite suspension (1-2 µm particles in water) with an additive/fuel ratio of 3 wt % DM to the corn stover raw material, the severe slagging tendency of the fuel could be considerably reduced. The slag quantities (dry mass) formed with kaolin- and calcite-added fuels were about a half and one-third, respectively, of that of the pure corn stover fuel. The reason for the reduction of slagging can be attributed to a change from relatively low melting temperature silicates to more high melting temperature silicates. For the corn stover without additives, the low melting fractions of the slag were estimated to consist mainly of potassium calcium silicate, indirectly observed as a glass. When kaolin was added, a depletion of potassium was observed because of the extensive formation of leucite (KAlSi2O6) and the glass became dominated by calcium, aluminum, and silicon. This process was accompanied by a considerable reduction of glass amount. In the case of CaCO3 addition, calcium magnesium silicates formed to an extent that the glass (low melting material) finally became dominated by potassium silicate. This process was also accompanied by a significant reduction of the amount of glass. Acknowledgment. The financial support from China Kerchin Cattle Industries Ltd. and the Swedish Energy Agency (STEM 30040-1) are gratefully acknowledged. The help given by Mr. Li He, Li Yi, Li Feng, and Song Yifei at China Kerchin Cattle Industries Ltd. is very much appreciated. We are grateful to the editor and four reviewers who provided valuable comments on the manuscript and to Ann-Sofi Hahlin who helped us with optical pictures of slag morphology. EF700718J