Energy Fuels 2010, 24, 4866–4871 Published on Web 08/11/2010
: DOI:10.1021/ef1005995
Corn Stalk Ash Composition and Its Melting (Slagging) Behavior during Combustion § € Shaojun Xiong,*,†,‡ Marcus Ohman, Yufen Zhang,‡ and Torbj€ orn Lestander† †
Unit of Biomass Technology and Chemistry, Swedish University of Agricultural Sciences, SE-901 83 Umea, Sweden, ‡ College of Life Science, Beijing Normal University, Beijing 100875, China, and §Division of Energy Engineering, Lulea University of Technology, SE-971 87 Lulea, Sweden Received May 12, 2010. Revised Manuscript Received July 26, 2010
The objectives of this study were to determine the fuel ash composition of corn stalk samples from 23 locations in Jilin province, China, and to estimate, via laboratory and theoretical methods, their ash-melting (slagging) behavior during combustion. The ash-forming matter is in general dominated by Si, K, Ca, and Mg, although there is large variation between the samples in fuel characteristics and contents of ashforming main elements. The results from the alkali index, the ash fusion test, and the thermochemical model calculations all indicated that the corn stalk fuel showed moderate to high slagging tendencies during combustion. However, there are large differences in predicted slagging behavior for the different samples. Samples with a low K/(Ca þ Mg) ratio showed moderate ash-melting temperatures in the American Society for Testing and Materials (ASTM) test and a moderate amount of melt in the temperature range of 1000-1200 °C in the thermochemical model calculations. In the samples with a high K/(Ca þ Mg) ratio, however, low ash-melting temperatures and a high amount of melt in the range of 1000-1200 °C was found in the ASTM test and in the thermochemical model calculations, respectively.
combustion behavior are rather well-understood,6-11 very few studies are available on corn stalk used as a biofuel. Biofuel characteristics often vary with crop species and can also even be affected by soil and cultivation conditions.32 After complete combustion of the fuel particles, a major part of the ash-forming elements will form a solid residue, known as residual ash. Previous work has shown that residual ash formation during biomass combustion depends upon the amount and composition of ash-forming elements as well as actual temperatures on the grate, the mixing between the fuel and oxidizer, and residence time on the grate.12 One of the major problems when burning corn stalk fuels is melting of the residual ash-forming slag in the furnace and on the grates. As indicated by previous research, corn stalks originating from the same area as in this study had a severe ash-melting (slagging) problem when used for direct combustion.5 Up to 40% of ingoing ash formed slag with a high degree of sintering, and the combustion in an under-fed pellet combustor had to be stopped about 30 min after starting the fire, because of severe slag blocks in the burner cup. 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 biofuel market. It is therefore important to determine the ash-melting behavior of corn stalk.
1. Introduction Biomass is expected to make a large contribution to an increase in the growth of sustainable energy production globally, because it is a renewable and CO2-neutral energy source. To use local indigenous biomass for energy purposes will reduce the dependency upon fossil fuels, mitigate carbon emissions, improve environments, and encourage social development. In the context of sustainable development, it is important to study the possibilities of using crop residue biomass for energy purposes, because food production must be secured worldwide and crop residue resources are rather abundant. Corn stalks (mostly consisting of leaves and stems of corn plants left after the harvest of corn grains) are a major crop residue throughout the world. For example, corn stalk comprises roughly 75% of total agricultural residues in the U.S.A.1 and nearly 30% of agro-biomass available for energy in China.2,3 The gross energy value of corn stalk is reported as being of around 16.7-20.9 MJ/kg4,5 compared to stem wood fuels in Sweden (18-21 MJ/kg). This suggests that the corn stalks may be a considerable biofuel resource. However, corn stalk may be similar to other agricultural residues, and this can be problematic for energy conversion, as suggested by our previous study.5 In comparison to other agricultural residues, such as wheat and rice straws, whose fuel characteristics and
(7) Bakker, R. R.; Jenkins, B. M.; Williams, R. B. Energy Fuels 2002, 16, 356. (8) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998, 54, 47. (9) Skrifvars, B. J.; Yrjas, P.; Kinni, J.; Siefen, P.; Hupa, M. Energy Fuels 2005, 19, 1503. (10) Steenari, B. M.; Lindqvist, O. Biomass Bioenergy 1998, 14, 67. (11) Str€ omberg, B. Fuel Handbook; V€armeforsk: Stockholm, Sweden, 2006. (12) Frandsen, J. F.; Moiraghi, L.; van Lith, S.; Jensen, P. A.; Glarborg, P. Aerosols in Biomass Combustion; International Energy Agency (IEA) Bioenergy, Task 32 Workshop, Graz University of Technology, Graz, Austria, March 18, 2005.
*To whom correspondence should be addressed. E-mail: shaojun.xiong@ btk.slu.se. (1) Sokhansanj, S.; Turhollow, A.; Cushman, J.; Cundiff, J. Biomass Bioenergy 2002, 23, 347. (2) Li, J. F.; Hu, R. Q.; Song, Y. Q.; Shi, J. L.; Bhattacharya, S. C.; Salam, P. A. Biomass Bioenergy 2005, 29, 167. (3) Liao, C. P.; Yan, Y. J.; Wu, C. Z.; Huang, H. T. Biomass Bioenergy 2004, 27, 111. (4) Pordesimo, L. O.; Hames, B. R.; Sokhansanj, S.; Edens, W. C. Biomass Bioenergy 2005, 28, 366. € (5) Xiong, S. J.; Burvall, J.; Orberg, H.; Kalen, G.; Thyrel, M.; € Ohman, M.; Bostr€ om, D. Energy Fuels 2008, 22, 3465. (6) Arvelakis, S.; Koukios, E. G. Biomass Bioenergy 2002, 22, 331. r 2010 American Chemical Society
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: DOI:10.1021/ef1005995
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The composition of the fuel ash, i.e., variation in the ashforming elements in fuel, is believed to play a key role in determining ash-melting behavior/slagging tendency.6,8,13-15 Among others, K, Ca, Si, and Mg have been proposed as important elements affecting the slagging process13,16,17 at least for nonphosphorus-rich fuels. Ashes originating from different fuel species or origins (sites) may differ in melting temperature, which can largely depend upon a relative dominance of K, Ca, Mg, and Si in the fuels. It is, therefore, of considerable interest to determine both the fuel ash composition and the ashmelting behavior during combustion of corn stalk samples of different origins. The objectives of this study were both to determine the fuel ash composition of a significant amount of different corn stalk samples from 23 agricultural fields in different soils in Jilin province in northeast China and to estimate, via laboratory and theoretical methods, the ash-melting (slagging) behavior during combustion of representative samples.
Table 1. Methods of Fuel Characterization of Corn Residues items
methods
gross calorific value ash at 550 °C ash fusion chlorine (Cl) sulfur (S) silicon (Si) phosphorus (P) aluminum (Al) calcium (Ca) magnesium (Mg) potassium (K) sodium (Na) iron (Fe)
SS-ISO 1928:1 SS 18 71 71:1 ASTM D1857-68 silver nitrate titration nitric acid; ICP-AES gravimetric method (ISO 5997: 1984) nitric acid; ICP-AES nitric acid; ICP-AES nitric acid; ICP-AES nitric acid; ICP-AES nitric acid; ICP-AES nitric acid; ICP-AES nitric acid; ICP-AES
2.3. Prediction of the Ash-Melting Behavior during Combustion. Five samples were selected from the original set of 23 samples for further prediction of their ash-melting behavior during combustion. The techniques used to predict the ash-melting (slagging) behavior of the fuels were the standard ash fusion test (ASTM D1857-68),18 thermochemical model calculations, and a previously used alkali index for predicting the fouling and slagging behavior of a fuel sample.19 The samples were selected to represent the main variation in the composition of the 23 samples with respect to the K/Si (wt % DM) and the K/(Ca þ Mg) (wt % DM) ratios. These ratios contain the four main dominating inorganic elements in the fuel samples (Table 3). These ratios have also been mentioned in previous studies as interesting ratios when discussing different ash-forming processes during combustion (e.g., slag formation) of biomass fuels.13,20 2.3.1. Ash Fusion Tests. The fuel ashes from the five selected fuel samples, achieved through a standardized ashing procedure at 550 °C (SS 187171),21 were used for the ash fusion test (ASTM D1857-68). In the ash fusion test, the external shapes, i.e., the initial deformation (TIT), spherical (TST), hemispherical (THT), and fluid (TFT) temperatures, are identified from an ash cone. The heat treatment of the ash cone was performed in an oxidizing atmosphere (air) with a heating rate of 8 °C/min. 2.3.2. Alkali Index. The alkali index is expressed as the mass of alkali oxides per unit fuel energy (on the basis of the high calorific value of the fuel). Fuels with an alkali index below 0.17 kg/GJ are of low fouling and slagging severity, whereas a fuel with an index above 0.34 kg/GJ is of high severity.19 2.3.3. Thermochemical Model Calculations. To predict the ash-melting behavior during combustion for the different selected corn stalk samples, chemical equilibrium model calculations were performed using the software program FactSage 5.4.1. This program uses the method of minimization of the total Gibb’s free energy of the system. Thermodynamic data were taken from the FACT database,22 including stoichiometric data as well as non-ideal solid and liquid solution models. The melting behavior of the fuel ashes were estimated. The melting behavior calculations of the fuel ashes were based on the fuel characteristics taken from Table 3. The calculations were performed using a global approach for atmospheric pressure (1 bar) and an air/fuel ratio (λ) of 1.4, corresponding to the typical global
2. Experimental Section 2.1. Fuel Sampling. The corn stalk samples were taken from one of the golden belts of corn production in the world. The studied area covered Qianan, Changling, and Nongan counties in Jilin province (latitude, 43.8-45.0° N; longitude, 122.7125.1° E). In the first week of November 2007, about 1 month after the corn grain harvest, 23 corn stalk samples were taken in the region. All samples were from different farmers who were randomly chosen. A total of 19 of 23 farmers used the same variety “Jidan 1”, but the remaining 4 farmers could not confirm their variety names. Stalks were piled in the gardens of the farmers or standing in their fields, and they were all harvested by hand. Before sampling, an examination on biomass quality was conducted. A gentle shaking was carried out to remove impurities (e.g., soil). A total of 20-30 plants without decay and contamination were randomly and primarily selected at each site. Leaves (including some outer husk blades) and stems of middle sections only between 50 and 80 cm from the stem base were cut and selected for samples. The stalk base sections (0-50 cm) were excluded because they were more frequently contaminated with impurities than other parts. To keep “average” fuel characteristics, we did not include the top (>80 cm) sections either. Therefore, each sample was composed of 38% leaves and husk blades and 62% stem. Care was taken to avoid impurities and contaminates during the operation. All samplings were made within 1 week. The samples were cut respectively into pieces using scissors, taken back in plastic bags, dried at 60 °C for 48 h, milled to pass a 0.2 mm sieve, and then analyzed in accredited laboratories of the Beijing Normal University. The weather conditions are rather the same among the sites, and the soil conditions vary greatly and influence the corn stalk fuel characteristics, as indicated in a previous study.32 2.2. Fuel Analysis. The methods used for fuel analyses of the corn residues are summarized in Table 1. The fuel analyses were mostly conducted at the Analytical and Testing Centre, Beijing Normal University, and at the Institute of Plant Nutrition and Resources, Beijing, China, except for the ash fusion that was performed in the BTK laboratory of the Swedish University of Agricultural Sciences, Umea, Sweden.
(18) American Society for Testing and Materials (ASTM). ASTM D1857-68, Standard Test Method for Fusibility of Coal and Coke Ash; ASTM: West Conshohocken, PA, 2009 (19) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Biomass Bioenergy 1996, 10, 125. € (20) Lindstr€ om, E.; Ohman, M.; Backman, R.; Bostrom, D. Energy Fuels 2008, 22, 2216. (21) Swedish Standards Institute (SIS). Biofuel;Determination of Ash Content; SIS: Stockholm, Sweden, 1984; SS 187171. (22) Bale, C. W.; Pelton, A. D. FACT Database of FACT-Win, Version 3.05; Center for Research in Computational Thermochemistry (CRCT), Ecole Polytechnique de Montreal: Montreal, Quebec, Canada, 1999.
(13) Gilbe, C.; Lindstrom, E.; Backman, R.; Samuelsson, R.; Burvall, € J.; Ohman, M. Energy Fuels 2008, 22, 3680. € (14) Gilbe, C.; Ohman, M.; Lindstrom, E.; Bostr€ om, D.; Backman, R.; Samuelsson, R.; Burvall, J. Energy Fuels 2008, 22, 3536. € (15) Ohman, M.; Boman, C.; Hedman, H.; Nordin, A.; Bostr€ om, D. Biomass Bioenergy 2004, 27, 585. (16) Nilsson, D.; Bernesson, S. Processing biofuels from farm raw materials;A systems study. Report 001, Department of Energy and Technology, Swedish University of Agricultural Sciences: Uppsala, Sweden, 2008; ISSN . € (17) Paulrud, S.; Nilsson, C.; Ohman, M. Fuel 2001, 80, 1391.
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Table 2. Elements and Solution Models Used in the Chemical Equilibrium Model Calculationsa elements phases solution phases solution models
a
C, H, O, N, S, Cl, P, K, Na, Ca, Mg, Fe, Mn, Si, and Al 220 9 slag SLAGA (MgO, SiO2, CaO, K2O, and Na2O) (liquid) salt SALTA (NaCl, KCl, NaOH, KOH, Na2SO4, K2SO4, Na2CO3, K2CO3, NaNO3, and KNO3) (liquid) chloride ACL (Na, K, Ca, and Mg/Cl) (solid solution) CSOB (solid solution) CO3SO4 (Ca, Mg) liquid K, Ca/CO3, and SO4 (LCSO) solid K, Ca/CO3, and SO4 (SCSO) solid Ca(SO4) and Mg(SO4) (SCMO) liquid Ca, Mg, and Na/(SO4) (LSUL) solid Ca, Mg, and Na/(SO4) (SSUL)
The designations of the solution models are the ones used in FactSage 5.4.1.
Table 3. Fuel Characteristics: Ash-Forming Main Elements (wt %), Ash Content (wt %), Heat Value (MJ/kg), K/Si and K/(Ca þ Mg) Weight Ratios, and Alkali Index (kg of Alkali Oxides/GJ)a sample/site code KEC-Q-01 KEC-Q-03 KEC-Q-05 KEC-Q-07 KEC-Q-09 KEC-Q-11 KEC-Q-13 KEC-Q-15 KEC-Q-17 KEC-C-19 KEC-C-21 KEC-C-23 KEC-C-25 KEC-C-27 KEC-C-29 KEC-C-31 KEC-C-33 KEC-C-35 KEC-C-37 KEC-N-39 KEC-N-41 KEC-N-43 KEC-1-51 mean standard deviation a
Al
Fe
Ca
Mg
K
Na
S
Cl
P
Si
ash
HV
K/Si
K/(Ca þ Mg)
alkali index
0.038 0.083 0.060 0.061 0.114 0.057 0.081 0.061 0.083 0.074 0.104 0.065 0.161 0.066 0.061 0.052 0.040 0.055 0.041 0.170 0.172 0.389 0.028 0.092 0.076
0.016 0.013 0.016 0.021 0.015 0.019 0.015 0.014 0.032 0.023 0.018 0.020 0.016 0.026 0.036 0.015 0.019 0.017 0.020 0.032 0.024 0.018 0.020 0.020 0.006
0.611 0.400 0.555 0.755 0.683 0.318 0.441 0.479 0.783 0.790 0.625 0.453 0.775 0.620 0.428 0.568 0.411 0.756 0.452 0.781 0.661 1.230 0.369 0.606 0.204
0.433 0.413 0.478 0.342 0.424 0.401 0.224 0.379 0.480 0.351 0.517 0.607 0.349 0.259 0.591 0.266 0.288 0.300 0.493 0.264 0.392 0.671 0.378 0.404 0.118
0.434 0.992 0.532 0.994 0.504 1.002 1.069 1.164 1.017 0.511 0.443 0.293 0.850 0.951 0.725 0.896 1.754 0.838 0.931 1.373 0.742 0.613 0.777 0.844 0.332
0.031 0.035 0.043 0.034 0.038 0.030 0.042 0.072 0.043 0.031 0.043 0.034 0.043 0.031 0.029 0.038 0.030 0.036 0.022 0.026 0.016 0.054 0.034 0.036 0.011
0.085 0.089 0.086 0.097 0.094 0.079 0.083 0.117 0.113 0.096 0.099 0.135 0.102 0.077 0.094 0.064 0.093 0.115 0.069 0.102 0.078 0.163 0.064 0.095 0.023
0.343 0.240 0.187 0.425 0.113 0.369 0.082 0.275 0.399 0.089 0.082 0.103 0.331 0.266 0.253 0.252 0.363 0.144 0.263 0.196 0.338 0.128 0.218 0.237 0.108
0.047 0.054 0.052 0.041 0.047 0.141 0.048 0.148 0.084 0.081 0.041 0.056 0.073 0.059 0.050 0.041 0.150 0.091 0.036 0.094 0.069 0.104 0.035 0.072 0.035
1.611 1.995 1.224 2.518 1.112 2.307 1.800 2.427 1.346 1.852 1.391 2.306 1.139 1.340 2.346 1.130 1.570 1.093 1.788 1.790 1.145 2.569 1.688 1.717 0.500
6.091 5.802 6.016 6.153 5.836 6.094 6.366 6.468 6.048 5.460 5.786 5.102 5.356 5.862 5.571 5.562 6.172 5.810 5.924 6.324 5.326 6.111 5.602 5.863 0.356
18.66 18.35 18.61 18.71 18.39 18.45 18.52 18.70 18.62 18.68 18.58 18.62 18.92 18.83 18.52 18.35 18.26 19.00 18.53 18.35 18.06 18.50 18.72 18.562 0.215
0.269 0.497 0.435 0.395 0.453 0.434 0.594 0.480 0.755 0.276 0.319 0.127 0.746 0.709 0.309 0.792 1.117 0.767 0.521 0.767 0.649 0.239 0.460 0.526 0.233
0.416 1.221 0.515 0.906 0.455 1.394 1.609 1.357 0.804 0.448 0.388 0.276 0.757 1.082 0.712 1.074 2.512 0.794 0.984 1.314 0.705 0.322 1.040 0.917 0.513
0.302 0.677 0.375 0.664 0.358 0.677 0.726 0.802 0.689 0.352 0.318 0.214 0.572 0.630 0.493 0.616 1.179 0.557 0.621 0.921 0.507 0.439 0.524 0.574 0.218
All data are based on dry mass.
predicted amounts of melt (melting behavior) and solid phases as functions of the temperature were extracted.
oxidizing conditions present during combustion. The calculations were carried out covering a temperature range of 700-1300 °C. Two separate liquid phases were assumed, comprising both an oxide/silicate (slag) melt and an alkali salt melt. The slag was assumed to contain SiO2, CaO, MgO, K2O, and Na2O. Aluminum was excluded from the slag melt because melting temperatures for the fuel mixtures when including aluminum were found to be unexpectedly low and because data for the K2O-Al2O3SiO2 system is considered to be very approximate.22 The results from our previous combustion study of corn stalk also showed that aluminum was mainly found in the bottom ash in the high melting temperature phase of leucite (KAlSi2O6) and kalsilite (KAlSiO4).5 Iron was also excluded from the slag melt because the fuel mixtures when including iron were found to be very low and because Fe is distributed as FeO (not as Fe2O3) in the most proper slag solutions that are found in FactSage 5.4.1, i.e., containing the main slag-forming compounds (Si, Ca, Mg, K, and Na). The results from previous combustion research of both corn stalk5 and other straws14 have also shown that iron is only found in very low concentrations in the slag/melt. In addition, all relevant binary solid and liquid solutions with K, Na, Ca and Mg were included. The elements and solution models used in the calculations are shown in Table 2. From the calculations, the
3. Results 3.1. Fuel Analysis: Major Ash-Forming Elements. The dominant ash-forming elements in the fuel samples are silicon (Si), potassium (K), calcium (Ca), and magnesium (Mg) (Table 3). These four elements constitute about 87% of the sum of the main ash-forming elements (or about 61% of the total ash content) on average. Among these four elements, Si has the largest contribution to the sum of the content of the main ashforming elements, nearly 42%, followed by K (20%), Ca (15%), and Mg (10%). Considerable variations can be found among contents of Si, K, Ca, and Mg in corn stalk samples originating from different sites, which are indicated by standard variations in Table 3. There are therefore also large variations in the K/Si and K/(Ca þ Mg) ratios between different samples. The fuel samples are also relatively rich in chlorine, with an average content of 0.24% and standard deviation of 0.108. The Al, P, Na, and Fe contents in each of the fuel samples are normally 4868
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: DOI:10.1021/ef1005995
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silicates (e.g., SiO2, Mg, Mg-Ca, and K-Al silicates) are the dominating phases in the lower temperature range (1000 °C. However, relatively high variations in the results could be found between fuel samples with different K/(Ca þ Mg) ratios, indicating that fuel samples with low K/(Ca þ Mg) ratios, i.e., C-23 and C-25, gave moderate amounts of melt in the temperature range of 1000-1200 °C. The main phases predicted to form in the calculated temperature range, i.e., 700-1300 °C, according to the equilibrium calculations are shown in Figure 2. Different solid
(23) Burvall, J. Standortens Inverkan pa Br€ anslekvalitet hos Stabr€ anslen; Vattenfall Utveckling AB: Stockholm, Sweden, 1998; ISSN .
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Energy Fuels 2010, 24, 4866–4871
: DOI:10.1021/ef1005995
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Figure 2. Predicted amounts of melt (melting behavior) and solid phases as functions of the temperature for the five selected samples with different K/Si and K/(Ca þ Mg) weight ratios. Table 5. Literature Review and Comparison of Major Ash-Forming Elements reference
feedstock
origin
ash % of DM
Si (wt % in ash)
K (wt % in ash)
Ca (wt % in ash)
Mg (wt % in ash)
sum (wt % in ash)
this study 13 25 27 30 8 6 7 9 30 19 19 13 29 31 26 28 26 13 30 13
corn stalk wheat straw wheat straw wheat straw wheat straw wheat straw wheat straw rice straw rice straw rice straw rice straw switchgrass reed canary grass mixed straw mixed straw mixed straw cotton stalk wood wood wood salix
China Sweden Turkey Australia U.S.A. U.S.A. Greece U.S.A. Finland U.S.A. U.S.A. U.S.A. Sweden Finland Denmark Denmark China Denmark Sweden U.S.A. Sweden
5.9 6.6 7.9 7.8 9.8 8.1 7.6 18.3 15 22.1 18.7 9.0 10.7 5.9 7.4 3.9 1.8 0.6 0.3 1.2 2.0
29.28 21.96 16.64 28.03 26.86 31.56 20.78 33.75 32.66 35.22 34.89 30.45 35.51 14.67 17.64 17.75 4.91 10.00 3.55 4.37 2.60
14.40 18.03 20.5 17.53 13.79 11.98 13.22 13.80 12.75 9.96 10.25 9.67 3.36 18.67 25.78 22.33 56.57 10.08 14.52 8.93 13.00
10.34 5.9 13.64 2.37 2.00 2.20 10.29 1.18 2.50 1.14 2.15 4.00 2.70 7.14 5.70 8.86 14.28 20.00 30.00 22.9 23.00
6.89 1.36 3.13 1.40 1.10 1.73 1.61 0.90 0.96 0.99 1.05 1.81 0.68 2.23 1.36 1.57 6.86 3.98 6.77 2.97 2.05
60.91 47.25 53.91 49.33 43.75 47.47 45.90 49.63 48.87 47.31 48.34 45.93 42.25 42.71 50.48 50.51 82.62 44.06 54.84 39.17 40.65
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.5,14 Nevertheless, in most cases, the method predicted (qualitatively) the same
Previous work has shown good qualitative agreement between results from chemical equilibrium calculations and the actual slagging tendencies obtained in well-designed combustion experiments with biomass.14 The results from the standard ash fusion test showed, in general, relatively high deformation 4870
Energy Fuels 2010, 24, 4866–4871
: DOI:10.1021/ef1005995
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Table 6. Results Determined by the Different Methods samples methods
KEC-C-23
KEC-C-25
KEC-Q-11
KEC-C-33
KEC-1-51
fuel characteristics: K/(Ca þ Mg) weight ratio fuel characteristics: K/Si weight ratio thermochemical model calculations: calculated fraction of silicate melt of condensed species at 1000 °C (mol %) ash fusion test: initial deformation temperature (°C) alkali index (kg of alkali oxides/GJ)
0.276 0.127 22
0.757 0.746 16
1.394 0.434 67
2.512 1.117 84
1.040 0.460 75
1250 0.214
1200 0.572
1090 0.677
990 1.179
1240 0.524
fuel-specific slagging trends as the corresponding combustion behavior. The examined corn stalk was harvested in autumn of 2007. The slagging tendencies would be weaker, however, if a delayed harvest had been applied to the corn stalks, as indicated by a previous study, where alkali elements in corn stalks dropped considerably while the heat value increased a bit when they were harvested in late winter or the following spring.24 On the other hand, the examined biomass was harvested and picked up by hand and, therefore, would be much less contaminated than those harvested and collected by machines. Any contamination by soil will definitely change the ash composition and slagging tendencies of the biofuel, although it varies with soil composition.
other graminoid crops, such as wheat straw, rice straw, reed canary grass, and switchgrass. The corn stalk fuel characteristics and ash-forming main elements show large variations between the samples taken from different sites, which may be attributed to the soil and growth conditions because the samples were from the same variety and harvest time. The results from the alkali index, the ash fusion test, and the thermochemical model calculations all indicated that the corn stalk fuel will have a moderate to high slagging tendency during combustion in typical grate firing appliances (i.e., process temperatures above 1000 °C). However, there are large differences in predicted ash-melting behavior (slagging) during combustion for the different studied samples. Samples with a low K/(Ca þ Mg) ratio showed moderate ash-melting temperatures in the ASTM test and a moderate amount of melt in the temperature range of 1000-1200 °C in the thermochemical model calculations. Furthermore, samples with a high K/(Ca þ Mg) ratio showed low ash-melting temperatures in the ASTM test and a high amount of melt in the temperature range of 1000-1200 °C in the thermochemical model calculations.
5. Conclusions In summary, the examined corn stalk fuel samples from Jilin province have an ash content of about 6 (5.9 ( 0.4) wt % DM and the ash-forming matter is dominated by Si, K, Ca, and Mg. The ash composition of corn stalk is comparable to (24) Liu, J. L.; Cheng, X.; Xie, G. H.; Xiong, S. J.; Zhu, W. B. Sci. Agric. Sin. 2009, 42, 2229. (25) Bakisgan, C.; Dumanli, A. G.; Yurum, Y. Fuel 2009, 88, 1842. (26) Capablo, J.; Jensen, P. A.; Pedersen, K. H.; Hjuler, K.; Nikolaisen, L.; Backman, R.; Frandsen, F. Energy Fuels 2009, 23, 1965. (27) Nutalapati, D.; Gupta, R.; Moghtaderi, B.; Wall, T. F. Fuel Process. Technol. 2007, 88, 1044. (28) Sun, Z.; Jin, B. S.; Zhang, M. Y.; Liu, R. P. Chem. Eng. Technol. 2008, 31, 1605. (29) Theis, M.; Skrifvars, B. J.; Zevenhoven, M.; Hupa, M.; Tran, H. Fuel 2006, 85, 1992. (30) Thy, P.; Jenkins, B. M.; Grundvig, S.; Shiraki, R.; Lesher, C. E. Fuel 2006, 85, 783.
Acknowledgment. The authors thank Mr. Zhuo Yue for the sampling and fuel analyses. Financial aid was from the Swedish Energy Agency (Project 30040-1), Beijing Normal University 985-II Project, and Kerchin Cattle Industry. Shaojun Xiong is also grateful to Dr. Robert Samuelsson for cooperative work in the BIONORM II project. (31) Zheng, Y. J.; Jensen, P. A.; Jensen, A. D.; Sander, B.; Junker, H. Fuel 2007, 86, 1008. (32) Xiong, S. J.; Zhang, Y. F.; Zhuo, Y.; Lestander, T.; Geladi, P. Renewable Energy 2010, 35, 1185.
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