ARTICLE pubs.acs.org/EF
Effect of Process Variables on the Quality Characteristics of Pelleted WoodXylite Mixtures Bastian Lehmann,*,†,‡ Hans-Werner Schr€oder,† Ralf Wollenberg,§ Raimund Lange,‡ and Jens-Uwe Repke† †
Institute of Thermal, Environmental and Natural Products Process Engineering, TU Bergakademie Freiberg, Leipziger Strasse 28, D-09596 Freiberg, Germany ‡ Refinement Division, RWE Power AG, Ludwigstrasse, D-50226 Frechen, Germany § Institute of Mineral Processing Machines, TU Bergakademie Freiberg, Lampadiusstrasse 4, D-09596 Freiberg, Germany ABSTRACT: Xylite is a byproduct of lignite coal production in Germany and, up to now, has normally been used as an additive for potting-soil production. Another possibility to use the residual xylite, however, is to produce fuel pellets out of it. These pellets can be used like wood pellets in domestic and industrial firing systems. Using a novel process based on wet grinding, agglomeration of the wet xylite, and subsequent drying of the raw xylite pellets, it is possible to convert xylite into high-quality fuel pellets without being dependent upon the mechanics of lignin softening. An important aspect for fuel pellets is their CO2 emissions. This is especially important for xylite, because it is ranked among the fossil fuels. In this work, xylite is mixed with CO2-neutral wood to produce composite pellets out of xylite and wood with reduced CO2 emissions. The topic of this study is the production of composite pellets, which have been produced using the mentioned novel pelleting process under variation of four parameters: wood content, moisture content, material temperature, and binder content. The interaction of these four parameters has been investigated using the design of experiments (DOE) methods. Models could be derived for different pellet quality parameters (i.e., pellet abrasion resistance), which describe the effects and interactions of the four parameters. Evaluation of the significant parameters for pellet qualities was carried out with statistical tests and graphical analysis. It became evident that, in the wet-pelleting process, the moisture content of the raw material and the mass fraction of wood are of great relevance for pellet quality. In contrast, the temperature of the material has no significant effect. The addition of potato starch as a binder results in a steady increase in pellet quality across the entire study.
’ INTRODUCTION Lignite coal is an intermediate product in the coalification series, and because of the different deposit conditions and the large bandwidth of original plant material, it is neither chemically nor morphologically homogeneous. Residues of coalified wood are normal in a lignite coal seam. These wood residues, whose morphological fiber structure is often clearly visible, are called xylite or lignite coal woody fiber.1 Individual pieces of xylite in the seam can reach considerable sizes, up to complete stumps. The fibrous structure of xylite is currently the main barrier for its usage. To use lignite coal in power generation, a coarse grinding of the raw coal is necessary, which is performed by swivel-arm crushers. These machines are designed for crushing brittle material, so that the fibrous and elastic xylite is often insufficiently crushed. Because of that, big pieces of xylite are ejected in the crushing process. This xylite has, until now, remained an under-used byproduct of lignite coal production. The lignite coal deposits in Germany contain varying amounts of xylite. However, while the lignite coal in the lower Rhine and Lusation (or “Lausitz”) regions contain xylite, the lignite coal in central Germany contains almost no xylite.24 Because of its high organic content, xylite is currently used as an additive in the production of potting soil. Focus of this work is the usage of xylite from the lower Rhine region for the production of fuel pellets. Other applications for different types of xylite that are known from the literature could be (1) soil improvement or ground cover,5 (2) production of fiberboards,6 (3) coke replacement in the electrothermal industry,7 (4) algae reduction in water,8 and (5) adsorbent usage for gas or water cleaning.911 r 2011 American Chemical Society
A new method for the use of xylite would be to process it into fuel pellets. Because of the fossil origin of xylite, CO2 emissions of pelleted xylite have to be recognized. One possibility to mitigate CO2 emissions of such xylite pellets is to mix it with CO2-neutral biomass, such as wood. Therefore, the addition of wood to produce composite pellets out of xylite and wood has been investigated as one parameter in this study. These pellets could be used as a fuel for small or domestic heating systems with high user comfort, such as wood pellets. Wood pellet production is based on high-pressure compaction of pure wood shavings in flat or ring die presses. The principle behind the wood pelleting process is the thermoplastic behavior of wood lignin. Because of friction between wood shavings, die and roller heating takes place in the pellet press. The high temperature causes the lignin in the wood to soften, with binding of the wood particles taking place in the press channel of the pellet press.12 The degree of heating in the pellet press depends upon the wood type, wood composition, particle size, and moisture content.1315 Lignin softening in the pellet press, however, only takes place if sufficiently dry material is used. Because of the high moisture content (up to 60%) in its raw condition, xylite drying results in a highly energy-intensive operation. Furthermore, the wear between die and rollers in the pellet press will rise significantly with drier material, because of the higher ash content of xylite in comparison to wood. Received: April 14, 2011 Revised: June 9, 2011 Published: June 20, 2011 3776
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Energy & Fuels It therefore becomes necessary to use an alternative conversion process, which works without pre-drying. This requirement is satisfied by the novel wet pelletizing procedure.16 In this process, the raw material is comminuted without pre-drying using a double twin screw extruder (DSE), followed by a normal flat die pellet press for producing pellets from the wet material. After pelleting, the wet pellets are dried in a convection dryer at 105 °C to moisture contents below 10% to gain final strength. The main factors affecting the strength and durability of densified biomass products are12 (1) feed constituents, (2) moisture content, (3) particle size, (4) steam conditioning or preheating, (5) adding binders or additives, (6) mixing feeds or biomasses, (7) densification equipment, and (8) post-production treatment conditions, such as heating or cooling and storage conditions. The influence of these factors is known for specific biomass materials and for the traditional pelleting process with wood. These factors will also influence the quality of pellets produced with the novel process, but there is limited knowledge about the main effects and interactions between the mentioned parameters. The aim of this study was to use xylite with wood chips, therefore, raw material constituents were given. There is probably a high influence of particle size on the pellet quality also, but in the used novel process, a highly fibrous and wet material is being produced by the DSE in the wet comminution step. Thus, this fibrous and partly agglomerated intermediate product could not be examined by sieve analysis. Therefore, an influence of particle size to pellet quality in the novel process is difficult to investigate. Previous investigations showed that the material supply for a ring die press is difficult for such fibrous material because of its low flowability and the axial feed in such a machine.17 Therefore, a flat die press has been used. With regard to these aspects, the testing parameters were reduced to four: (1) mass fraction of wood mbio as an option for CO2 reduction of the pellets, (2) mass fraction of the binding agent mb, (3) temperature of the raw material T (preheating), and (4) moisture content of raw material mc. During the investigations, both pure xylite and mixtures of xylite and biomass (wood chips) were treated and pelleted. The implementation of the pelleting tests was performed using design of experiments (DOE) methods. Using the results, a statistical model was developed to describe the effects of the four parameters (mbio, mb, T, and mc) on the physical properties of the pellets and to identify possible parameter interactions. The densities of the wet crude pellets and the dried pellets (gross and bulk) and strength (abrasion and compression resistance) were then analyzed. A statistical analysis and a regression analysis were subsequently used to determine the model parameters that describe the effects of the factors influencing pellet quality. Through a graphical representation of the models as surface plots, the interpretation of such reactions can be achieved. The objectives of the study are (1) to identify what process conditions lead to high agglomerate qualities, (2) to identify the most important of the four chosen parameters (amount of wood, binder content, temperature of the material to be pelleted, and moisture content), and (3) to examine the applicability of the chosen statistical method for the agglomeration process of xylite and xylitewood mixtures in a flat die pellet press under wet-pelleting conditions.
’ MATERIALS AND METHODS Materials. Samples of xylite from the Rhenish area in Germany (provided by RWE Power AG) were used for the study. The used xylite
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Figure 1. Pictures of the used xylite.
Table 1. Proximate Composition of the Raw Materials Used
moisture content (wt %) ash contenta (dry) (wt %)
a
xylite
wood chips
59.4
20.0
2.27
1.87
volatiles (dry) (wt %)
69.10
80.27
fixed carbon (dry) (wt %)
28.63
17.86
Determined at 550 °C according to ref 18.
is a byproduct of lignite production in the open-cast mines Inden and Garzweiler and has been separated from the lignite in the coarse grinding step performed by swivel-arm crushers in the power plants of Neurath and Weisweiler. The exact origin of the xylit in the seam cannot be given because it occurs in all parts or distributed in lenses. The separated xylite reaches dimensions up to several meters; therefore, it was crushed by RWE with a mobile shredder to particle sizes below 40 (Figure 1). The so treated xylite represented the starting material. The biomass used consisted of pre-dried wood chips (mainly pine). For the materials used, the proximate analysis data are shown in Table 1, the elemental analysis data are shown in Table 2, and the ash 3777
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Table 2. Elemental Composition (daf) and Upper (Hs) and Net (Hi) Calorific Values of the Raw Material xylite
wood chips
C (wt %)
60.45
52.88
H (wt %) N (wt %)
5.66 0.15
6.10 0.21
S (wt %)
0.44
0.10
O (wt %)
33.51
40.73
Hs (MJ/kg)
23.542
20.543
Hi (MJ/kg)
22.336
19.237
Table 3. Ash Composition of the Used Raw Materials Determined by X-ray Fluorescence Spectroscopy element
xylite
wood chips
C (wt %)
0.82
3.21
O (wt %)
43.84
42.46
Na (wt %)
2.15
0.95
Mg (wt %) Al (wt %)
4.97 1.45
2.62 2.25
Si (wt %)
10.59
13.58
P (wt %)
0.05
1.03
S (wt %)
9.01
0.84
Cl (wt %)
0.13
0.14
K (wt %)
0.78
6.49
Ca (wt %)
17.01
20.15
Ti (wt %) Fe (wt %)
0.18 8.47
0.24 4.63
Ba (wt %)
0.20
0.11
Mn (wt %)
0.10
0.88
Sr (wt %)
0.19
Zn (wt %)
0.16
composition data are shown in Table 3. The ash composition was measured using X-ray fluorescence spectroscopy. Similar to the lignite coal of the Rhenish area, xylite exhibits high moisture contents. The relatively high ash content of the wood chips is due to included bark. According to DIN 14775,18 the ash content for both materials was determined at 550 °C. Commercial potato starch was used as a binder, because it is very common in wood pellet production. Production Process for the Composite Pellets. The major steps of the wet-pelleting process are wet comminution in a DSE, followed by pelleting with a flat die pellet press with subsequent drying of the wet pellets to attain final strength. In contrast to normal wood pelleting, the materials in the wet-pelleting process have relatively high moisture contents of up to 60% when pelleted. The basic process is shown in Figure 4. To activate the binding potential of the materials, hydromechanical activation of the raw material is essential. This is especially possible for soft lignite coal but is also possible for xylite and biomass.19 The raw materials (xylite and wood chips) were premixed using an intensive mixer and then wet-ground using the DSE (type MSZT15/2, Lehmann Maschinenbau, Germany). The comminution step in the DSE leads to a very fibrous structure of both xylite and xylitewood mixtures (Figure 2). For those fibrous and wet materials, the particle size could not be determined by sieve analysis. The pellets were then produced out of the treated material using a flat die pellet press (type 14-175, Amandus Kahl, Germany). The press
Figure 2. Stereomicroscopic images of the raw materials after wet grinding in the DSE. channels of the die were 6 mm in diameter and 20 mm in length. The roller speed was 95 min1. The flat die presses was operated with a fixed gap between rollers and die of 0.75 mm. Because of the rotation of rollers on the surface of the die, the cutting blade below the die, and the small space above and below the die, there is no possibility for measuring the compression pressure in press channels. The wet crude pellets produced in the pelleting process only reach their ultimate strength during the drying process, where stable bonds triggered by the formation of solid bridges are formed. Furthermore, the fibrous structure of xylite and biomass after the wet-grinding process in the DSE promotes the formation of interlocking bonds between the particles.16,17 In this study, drying was performed using a convection dryer with an air temperature of 105 °C. The pellets were dried to moisture contents below 10%. To vary the material temperature, it was preheated in pressure-tight containers, after which the temperature was measured for each sample. Each pelleting test was performed with 46 kg of raw material, resulting in an operating time of the flat die press of 1015 min. For the test with preheated material, the die of the pellet press was also preheated in a drying oven. A part of the so produced wet pellets was then used for determining the pellet quality parameters, and the other part was dried below moisture contents of 10%. Quality parameters were 3778
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Figure 3. Device for measuring the gross density of irregular-shaped agglomerates by displacement of mercury (own construction).
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Pellet Bulk Density Fbulk. Determination of bulk densities for the pellets was carried out using a 1 L vessel. For preparation, the pellets were screened on a 3.15 mm sieve. The empty container was weighed and then filled with pellets for measurement. Excess material was scraped off the top of the container. After the vessel was weighed to the nearest 0.1 g, the bulk density could be calculated from the pellet mass and container volume. In fulfilling the requirements of DIN EN 14961-2, minimum bulk densities of 0.6 kg/dm3 are required. Abrasion Resistance A. According to DIN EN 15210-1,22 a tumbling box is suitable for measuring abrasion resistance. To test the abrasion resistance, 500 g of the pellets were screened on a 3.15 mm sieve and placed in the tumbling box. At a speed of 50 rpm, the box was rotated 500 times. Afterward, the pellets were screened again on a 3.15 mm sieve. The abrasion resistance is then calculated from the residue on the sieve in relation to the initial weight of the pellets. According to DIN EN 14961-2, benchmark values for abrasion resistance of wood pellets are above 96.5%. Although the abrasion resistance of the pellets does not correlate with the density,23 it represents the only current standard stability criterion and was therefore determined for comparison within the context of this study. Compression Resistance C. Another parameter for pellet strength is their resistance to force under compression (see ref 17). In this procedure, a pressure cylinder is filled with pellets. The pellet bed is then compacted in a press at a pressure of 1 MPa. The ratio of the immersion depth of the press stamp to the original height of the pellet bed is equal to the compression of the pellet bed. The higher the compression, the lower the compression resistance of the pellets, because the pellet bed may be compacted more at constant pressure. The compression resistance is especially important for wet agglomerates, because the fine particles in the tumbling box can reagglomerate if sufficiently wet material is used. This could lead to inappropriately high values for the abrasion resistance of wet agglomerates. Both resistance values should therefore be used to compare wet agglomerates. Furthermore, it is possible that very stable agglomerates may exhibit very high abrasion resistances that make comparison difficult. In this case, the compression resistance can be used to make a more meaningful comparison between different pellet qualities.
Experimental Plan and Parameter Identification Using DOE. Experimental Plan. For investigation of the four parameters Figure 4. Process scheme for production of composite pellets out of xylite and biomass according to ref 17. tested for both wet (index I) and dried (index II) pellets to compare the increase in quality resulting from the drying process. Determination of the Pellet Quality. The gross density Fgross, bulk density Fbulk, abrasion resistance A, and compression resistance C of the pellets were determined as common parameters to evaluate pellet quality. Comparable values for the quality parameters were taken from DIN EN 14961-220 (class B wood pellets) and DIN 51731.21 Moisture Content mc. The moisture content of raw material and pellets was measured using a thermogravimetric moisture analyzer (type Mytron FAB-1/2, accuracy of 10 mg). The samples were heated at about 110 °C up to a constant mass. The moisture content could then be determined directly from the difference in weight. A total of 10 samples were used to measure the moisture content. According to DIN EN 14961-2, pellets should have a moisture content of less than 10%. Pellet Gross Density Fgross. The gross density of the pellets was measured using a mercury displacement device (own construction; see Figure 3). Five representative samples of each charge of pellets were tested to determine the pellet gross density. High gross densities result in high bulk densities and, therefore, in high energy densities. The gross density is thus an important factor for logistics and storage space. In DIN 51731, a minimum of 1.0 kg/dm3 is specified.
(mass of wood chips mbio, mass of the binder mb, pelleting temperature of the raw material T, and moisture content of the pelleted material mc), a statistical plan of experiments with 25 test runs was formulated and applied. Using this type of statistical plan, all interactions and quadratic effects could be estimated and nonlinear relations could be determined.24 Five levels were used for every parameter, whereby the basis for the plan of experiments was a complete 2k plan (cubic point tests) together with star point experiments (R values) and a central point experiment. The transformed levels are indicated with R, 1, 0, 1, and +R (Figure 5). The R levels are selected such that the plan of experiments resulted in a composite orthogonal experimental design, offering the following advantages:24 (1) the estimates of the coefficients in the model were independent of each other (i.e., they did not influence each other), and (ii) at fixed values of 1 and 1 in the transformed experiment plan, narrow confidence intervals for the coefficients could be obtained with a predetermined number of individual attempts. The distance between the test points from the center (R) and the number of experiments (N) were functions of the number of variables (k) and could be obtained by the following equation:25 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi R ¼ 0:5ð Ntotal 2k 2k Þ ð1Þ Interval from star point to central point Ntotal ¼ N(1 þ N(R þ N0 3779
ð2Þ
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Table 4. Plan of Experiments with Parameter Levels and Results of Pellet Quality Tests parameter levels
wet pellet qualities
dry pellet qualities
experiment number mc (%) mbio (%) T (°C) mb (%) Fgross,I (kg/dm3) Fbulk,I (kg/dm3) AI (%) CI (%) Fgross,II (kg/dm3) Fbulk,II (kg/dm3) AII (%) CII (%) 1
27.2
7.32
45.86
0.29
1.124
0.536
96.6
75.3
1.101
0.506
97.4
79.9
2
37.8
7.32
45.86
0.29
1.012
0.505
88.3
62.7
0.866
0.440
94.3
79.4
3
27.2
42.68
45.86
0.29
1.044
0.510
97.7
71.3
1.037
0.500
98.8
81.5
4
27.2
7.32
74.14
0.29
1.142
0.533
96.3
80.5
1.041
0.530
97.6
85.8
5
27.2
7.32
45.86
1.71
1.182
0.573
98.4
81.1
1.140
0.568
98.9
88.4
6
37.8
42.68
45.86
0.29
0.918
0.388
86.8
52.4
0.734
0.331
96.7
70.8
7
37.8
7.32
74.14
0.29
0.959
0.501
86.7
64.1
0.901
0.445
94.2
78.4
8 9
37.8 27.2
7.32 42.68
45.86 74.14
1.71 0.29
1.053 1.131
0.499 0.502
94.5 97.5
64.8 77.7
0.928 1.061
0.450 0.506
97.2 98.3
76.1 82.5
10
27.2
42.68
45.86
1.71
1.174
0.556
98.6
79.5
1.084
0.550
99.1
87.8
11
27.2
7.32
74.14
1.71
1.205
0.561
98.0
85.8
1.156
0.556
98.6
83.5
12
37.8
42.68
74.14
0.29
0.878
0.370
86.4
52.6
0.744
0.344
96.0
65.0
13
37.8
42.68
45.86
1.71
0.879
0.396
92.6
53.8
0.773
0.327
97.9
69.0
14
37.8
7.32
74.14
1.71
1.057
0.504
94.8
66.2
0.956
0.451
97.6
79.0
15
27.2
42.68
74.14
1.71
1.218
0.574
98.9
81.0
1.155
0.553
99.0
87.7
16 17
37.8 25.0
42.68 25.00
74.14 60
1.71 1
0.832 1.162
0.408 0.531
93.4 92.4
53.8 83.1
0.818 1.139
0.336 0.549
97.9 93.3
70.0 82.9
18
40.0
25.00
60
1
0.890
0.398
88.3
51.8
0.714
0.324
96.6
69.2
19
32.5
0.00
60
1
1.179
0.553
96.6
74.2
1.144
0.529
98.0
84.0
20
32.5
50.00
60
1
0.981
0.447
96.7
70.5
0.947
0.429
98.6
80.1
21
32.5
25.00
40
1
0.991
0.463
95.8
63.4
1.018
0.448
98.2
81.3
22
32.5
25.00
80
1
0.974
0.479
96.3
70.7
0.989
0.481
98.2
80.1
23
32.5
25.00
60
0
0.981
0.469
94.0
67.5
0.911
0.456
97.0
81.0
24 25
32.5 32.5
25.00 25.00
60 60
2 1
1.094 1.005
0.512 0.463
97.8 95.8
73.7 68.2
1.052 0.945
0.481 0.456
98.8 98.1
85.7 83.5
The total number of experiments were N(1 = 2k cubic point experiments, N(R = 2k star point experiments, and N0 = 1 central point experiment. The formulated plan of experiments used for this study is shown in Table 4. Parameter Identification. To describe the dependence of the pellet quality characteristics upon the process parameters, a second-grade polynomial model was used (eq 3). A similar model has been successfully used elsewhere to study the pelleting behavior of wheat distiller’s dried grains with solubles, a byproduct of bioethanol production.26 The model was not based on the (unknown) physical contexts but was adapted from a purely statistical point of view and was fitted to the experimental points by regression. y ¼ b0 þ
k
k
k
k
∑ bi xi þ i∑¼ 1 bii xi 2 þ i∑¼ 1 j∑¼ 1 bij xi xj i¼1
ð3Þ
The model contains the dependent variable y, the independent variables (mass of wood chips mbio, mass of the binder mb, pelleting temperature of the raw material T, and moisture content of the pelleting material mc) in coded form (xi and xj), the model coefficients b0, bi, bii, and bij, and the total number of independent variables (i.e., the number of parameters) k. In this study, we work with k = 4 (mbio, mb, T, and mc). The model parameters were estimated by means of regression analysis and, subsequently, the dependence of the pellet quality characteristics upon the parameters. The statistical analysis, model parameter identification, and graphical representation were carried out using the software package Statgraphics Centurion 16.27 For each identified characteristic of the raw and dry pellets (gross density, bulk density, abrasion resistance, and compression resistance), a specific model was calculated.
Figure 5. Graphical representation of a centrally composed plan of experiments consisting of cubic point experiments (1 and +1 values), central point experiment (ZP), and star point experiments (R values) (k = 2). Evaluation of the Identified Model Parameters. The evaluation of the identified model parameters was performed by comparing the p values for the model coefficients. The p value of an estimated model parameter is the probability to obtain the present or a greater parameter value, if the variable is set to zero. That means, for comparing the p values, important effects or interactions had p values of p e 0.05 (low probability of obtaining the parameter value by setting the variable to zero), less important effects or interactions had p values of p e 0.15, and unimportant effects or interactions had p values of p g 0.15. Because the model parameters present themselves in differing dimensions, such a comparison was only possible and appropriate in dimensionless form. Another parameter to evaluate the quality of the identified model parameters was the coefficient of determination R2, which describes how well the calculated model data matches the experimental data. 3780
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Figure 6. Effect of the moisture content and biomass addition on gross density Fgross,II (in g/cm3) of dried pellets (T = constant = 60 °C). A descriptive assessment of the effects and interactions of the parameters was achieved by presentation of the models as surface plots.
’ RESULTS AND DISCUSSION Surface Plots of the Calculated Models. The effect of the process parameters on pellet quality could be clearly represented by surface plots. In these plots, points of equal height of an objective are represented by a contour line. The following surface plots show the interaction between the mass of wood chips mbio and moisture content mc on the different pellet quality properties (gross density Fgross, abrasion resistance A, and compression resistance C). Because they represent the final product, the pellet quality parameters are shown as surface plots for the dried pellets.
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Figure 7. Effect of the moisture content and biomass addition on bulk density Fbulk,II (in g/cm3) of dried pellets (T = constant = 60 °C).
Because of the low significance of the pelleting temperature of the raw material T (see Table 6), surface plots with this parameter are not shown. In Figure 6, the model for gross density of dried pellets Fgross,II is shown, dependent upon the moisture content of the pelleted material mc and the mass of wood chips mbio. The amount of binder was considered as a constant parameter (mb = 0, 1, and 2%). The pelleting temperature of the raw material T was set to 60 °C for all of the figures. The interactions between the biomass and moisture content were visible as follows: high pellet densities can be achieved at low moisture contents (mc = 25%) with no biomass (mbio = 0%), or with a maximum addition of biomass (mbio = 50%). The pellet bulk density Fbulk,II (Figure 7) showed similar trends. However, because of the high pre-drying 3781
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Figure 8. Effect of the moisture content and biomass addition on abrasion resistance AII (in %) of dried pellets (T = constant = 60 °C).
effort for xylite (high moisture content combined with its fibrous structure), a process design with raw xylite drying is not very economical. The low density of the pellets at high moisture contents was a result of the incompressibility of the water as the material is pressed into pellets in the pellet press. On the other hand, the water caused a reduction of wear because of its effect as a lubricant. Furthermore, the water could act as a binder because of its ability to form liquid bridges between particles.28 The addition of a binder (potato starch) resulted in an increase in pellet density for all conditions tested. This means that the binder effect is not reduced given high water or high biomass contents. This evidence brings us to the conclusion that, in further investigations of the pelleting process, binder addition can be neglected as a process parameter, because binder addition in the
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Figure 9. Effect of the moisture content and biomass addition on compression resistance CII (in %) of dried pellets (T = constant = 60 °C).
Table 5. Coefficients of Determination of the Calculated Models for Pellet Quality Properties
3782
pellet quality factor
coefficient of determination R2
Fgross,I
0.9604
Fbulk,I
0.9935
AI
0.9531
CI
0.9877
Fgross,II
0.9763
Fbulk,II AII
0.9941 0.7209
CII
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Table 6. Results from the Variance Analysis of the Model Coefficients (p Values) p values linear terms mc
parameter
quadratic terms
bilinear terms
mbio
T
mb
mc2
mbio2
T2
mb2
mcmbio
mcT
mcmb
mbioT
mbiomb
Tmb
Fgross,I
0
0.0001
0.9412
0.0058
0.5643
0.0202
0.2794
0.3142
0.0062
0.0544
0.0724
0.7207
0.3866
0.9340
Fbulk,I
0
0
0.7146
0
0.6121
0.0001
0.4995
0.0014
0
1
0.0009
0.5622
0.0068
0.0917
AI
0
0.8015
0.8981
0.0001
0.0003
0.2443
0.5564
0.6599
0.1466
0.9568
0.0028
0.6533
0.6279
0.5315
CI
0
0
0.0031
0.0008
0.1380
0.0480
0.0827
0.4034
0.0012
0.0660
0.0522
0.5357
0.8705
0.4530
Fgross,II
0
0.0001
0.4110
0.0007
0.0136
0.0670
0.7896
0.5213
0.0048
0.6264
0.6366
0.3493
0.9011
0.2841
Fbulk,II
0
0
0.0428
0.0003
0.0038
0.0201
0.6021
0.2753
0
0.8533
0.0006
0.7319
0.6178
0.2316
AII
0.0762
0.1464
0.8564
0.0217
0.0218
0.3426
0.3968
0.5904
0.6296
0.9482
0.2702
0.7954
0.3751
0.8696
CII
0
0.0025
0.8013
0.0386
0.0035
0.8702
0.3475
0.5470
0.0016
0.6183
0.0952
0.4830
0.2578
0.9003
Table 7. Model Coefficients for the Objectives Gross Density Ggross, Bulk Density Gbulk, Abrasion Resistance A, and Compression Resistance C of the Composite Pellets model coefficients linear terms objectives constant
mbio
mb
mc2
mbio2
a
T
T2
Fgross,I
0.8606 0.0080
0.0023
ns
0.1508
ns
0.0001
ns
Fbulk,I
0.5778
0.0029
0.0031
ns
0.0158
ns
5.2 105
ns
AI
31.8535
5.1119
CI Fgross,II
82.7373 1.9558 0.3603 1.3084 11.2543 0.0363 0.0046 0.0061 0.4740 0.0622 0.0020 ns 0.0631 0.0013 7.8 105 ns
Fbulk,II AII CII a
mc
quadratic terms
ns
10.6338 0.0910
ns
2.9 105
ns
mcmbio
mcT
mcmb
0.0003 0.0003 0.0047
0.0232 0.0003 ns
0.0057
ns ns
0.0213 0.0003
mbioT mbiomb ns
Tmb
ns
ns
ns
0.0023
ns
ns
0.3538
ns
ns
ns
0.2624 ns
ns ns
ns ns
ns ns
0.0123 ns
0.0005 0.0003
ns
ns
0.0003
ns
0.0030
ns
ns
ns
ns
2.856
0.0436
ns
ns
ns
ns
ns
ns
ns
ns
ns
154.4530 7.4202 0.8175
ns
8.039
0.1116
ns
ns
ns
ns
ns
ns
66.4316
0.0237
0.1184 0.0004
ns
mb2
2.5675 0.0576
0.1985
0.0052 0.0007
ns
bilinear terms
0.0267
ns
0.2857
ns = not significant (p values > 0.15).
Table 8. Calculated Parameter Values for Creating High-Quality Composite Pellets parameters mc (%) 26.8 25.0
mbio (%) 0 0
T (°C) 75 75
mb (%)
calculated quality
comparable value
2
Fgross,II = 1.2 g/cm
DIN 51731, Fgross = 1.0 g/cm3
2
Fbulk,II = 0.6 g/cm
DIN EN 14961-2, Fbulk > 0.6 g/cm3
3
3
32.6
50.0
40
2
AII = 100%
DIN EN 14961-2, A > 96.5%
30.5
46.7
46
1.9
CII = 100%
wood pellets DINplus, C = 84.6%
studied process conditions always led to an improvement of pellet quality. In looking at the abrasion resistance (A), it can be seen that the addition of wood had a positive effect (Figure 8). This could be a result of the fibrous structure of wood, especially evident after the wet-grinding process in the DSE (see Figure 2). These fibers created interlocking bonds in the pellet and, therefore, increase pellet abrasion resistance. Therefore, the composite pellets showed higher abrasion resistance at wood contents of 30% or above. A decrease in abrasion resistance was evident at higher moisture contents, which is a result of the incompressibility of the water and the resulting inadequate compaction of the raw material in the press channel of the pellet press.
The clear increase in abrasion resistance because of binder addition is also remarkable; the addition of a binder resulted in a uniform increase of abrasion resistance over the studied parameter range. In examining the compression resistance C of pellets produced without a binder (Figure 9), it could be seen that the addition of wood chips has a negative effect. High compressive strengths could be achieved at a low moisture content and with no biomass addition. This changed when adding a binder; then, composite pellets with high compressive strength could be produced if maximum biomass is used. One reason for this could be the high fiber content of the wood. The binder was able to strengthen the interlocking bonds between the fibers or particles, especially when the pellets are 3783
dx.doi.org/10.1021/ef200573c |Energy Fuels 2011, 25, 3776–3785
Energy & Fuels dried. Such an effect had also been observed for pelleting of corn stover and switchgrass.29 Substance-specific binders were activated through pelleting with lower moisture content and higher temperatures in this case. Moreover, is it known that wet grinding in a DSE (especially the high shearing stress) led to mechanical activation of the raw materials, combined with dissolution and colloidal dispersion of soluble components (e.g., humid acid and/or proteins) out of the material.30,31 This type of mechanochemical activation of solid materials can also be performed using mills.32 When using lignite coal in this process, the amount of extractives can be increased through mechanochemical treatment33 and humic acids can be activated.34 The compression of the loose, wet material in the press channel of the pellet press led to the formation of liquid bridges and interlocking bonds between fibers and particles.28 Through drying of the wet pellets, the dissolved colloidally dispersed components crystallized, thereby strengthening the contact positions of interlocking fibers.31 This effect had also been observed in the production of compressed biomass products.12 The increase in pellet quality through drying was therefore caused by a successful mechanochemical activation of substancespecific binding agents from the raw materials xylite and wood chips, using a DSE. Coefficient of Determination, p Values, and Model Coefficients. The calculated coefficients of determination R2 were used to compare the rate of fit between the model and experimental data (Table 5). With the exception of abrasion resistance, all models showed good accuracy (R2 > 0.90). This showed that the chosen model approach is feasible as a model for assessing pellet quality on a laboratory scale and that the model is able to describe the effect of the parameters (mass of wood chips mbio, mass of the binder mb, pelleting temperature of the raw material T, and moisture content of the pelleted material mc) on pellet quality. A variance analysis was carried out for the calculated models, which produced the p values for the model coefficients (Table 6). The calculated p values and model parameters allow for the following observations on the effects of the four process parameters: (1) The linear terms (mc, mbio, and mb), except the term for the temperature T, had the greatest influence on pellet quality in all model parameters. The quadratic (mc2, mbio2, T2, and mb2) and interaction (mcmbio, mcT, mcmb, mbioT, mbiomb, and Tmb) terms had therefore little to no influence on the respective objective. (ii) The moisture content mc can have both positive and negative impacts on pellet quality. It had a negative influence on compression strength C and a positive effect on abrasion resistance A. (iii) The effect of the potato starch binder mb was particularly evident in wet pellets (index I). (iv) The pelleting temperature of the raw material T was found to influence only two of eight dependent variables (CI and Fbulk,II). Evaluation of the Agglomerate Qualities. The models calculated allow for the determination of operation conditions with which optimal pellet qualities can be achieved. In this study, high pellet quality means high gross and bulk densities, as well as high abrasion and compression resistance. Table 7 showed the values for optimal quality of each considered parameter, together with comparable values from wood pellet quality standards. Furthermore, an optimization calculation for four objectives (pellet gross density Fgross, pellet bulk density Fbulk, abrasion resistance A, and compression resistance C) had been performed to identify the parameter values for composite pellets with high values in all four quality criteria (Table 8). With values of mc = 28.3%, mbio = 8.0%, T = 68.2, and mb = 2.0%, it is possible to
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produce pellets with high quality, exhibiting qualities of Fgross,II = 1.16 g/cm3, Fbulk,II = 0.57 g/cm3, AII = 99.1%, and CII = 87.9%. With no pre-drying of the material, the optimization led to values of mc = 40.0%, mbio = 0%, T = 48.6, and mb = 2.0%, producing composite pellets with qualities of Fgross,II = 0.92 g/cm3, Fbulk,II = 0.45 g/cm3, AII = 97.3%, and CII = 75.0%. The results showed that composite pellet qualities that fulfill the current wood pellet standards could only be produced with pre-dried material. As mentioned, however, pre-drying of the raw material is uneconomical. More investigations are therefore necessary to improve the pellet quality, so that xylite and wood can be processed into fuel pellets without pre-drying. This includes, in particular, further investigations of the drying kinetics and the influence of drying conditions on pellet quality. Higher pellet qualities may be expected using dies with longer press channels, so that the material is compacted more in the agglomeration process. The use of other press channel geometries could also make the addition of binders unnecessary and could, therefore, reduce the costs for producing composite pellets from xylite and wood.
’ CONCLUSION This study showed that the production process for composite pellets from xylite and wood, comprised of wet grinding, agglomeration in a flat die pellet press, and drying of the raw pellets, can be investigated using statistical DOE methods. The calculated models described the influence of the process parameters (mass of wood chips mbio, mass of the binder mb, pelleting temperature of the raw material T, and moisture content of the pelleted material mc) on different pellet quality properties (gross density Fgross, bulk density Fbulk, abrasion resistance A, and compression resistance C). With the aid of the calculated models, it was possible to investigate the effects and interactions of the parameters on pellet quality. Using a variance analysis for the calculated models, it was possible to identify important and less important parameters. The calculated p values and model parameters led to the following conclusions for the process: (1) The mass of wood chips mbio and moisture content of the pelleted material mc were identified as the most important parameters for pellet quality in the used process. (2) The high influence of the linear terms on the pellet quality in the calculated models showed that the process parameters have minimal effects on each other. (3) In the studied process, the temperature T was not significant for any of the determined pellet quality parameters. (4) Because of the lack of significance of the interaction terms, it could be concluded that the potato starch effectiveness was not influenced by moisture content mc, mass of wood chips mbio, or pelleting temperature of the raw material T. Therefore, the addition of potato starch as a binder led to a steady increase of agglomerate quality in the studied parameter range. (5) Only two of eight dependent variables in the calculated models were influenced by the temperature. That means that, in the tested temperature range of 4080 °C, almost no effect on pellet quality is caused. It also shows that, within the temperature range above, no lignin softening takes place, although it can occur in wood at a temperature of 80 °C.15,35 (6) Overall, the small effect of tempering and binder addition means that they would not have to be varied in further investigations. The pellet qualities produced show that it is possible to produce moderate quality fuel pellets out of xylitewood 3784
dx.doi.org/10.1021/ef200573c |Energy Fuels 2011, 25, 3776–3785
Energy & Fuels mixtures using the wet-pelleting process. This process allows for the use of raw materials with high moisture contents to produce agglomerates with moderate quality. Therefore, pre-drying of the materials becomes unnecessary, and furthermore, the positive effects of water (as a binder and as a lubricant) can be used. To produce fuel pellets that can be used as a substitute for wood pellets, more investigations of the production process are necessary. Thus far, pellet quality has been negatively affected by strong re-expansion of biomass in the pellet structure and inadequate compression of the material, if it had high moisture content. Using dies with longer press channels, it should be possible to compensate these disadvantages, so that high-quality fuel pellets can be produced. With our study, the novel process of wet pelleting had not been conclusively investigated, so that further studies especially for investigating the drying process and the binding mechanism in the pellets are planned for the future.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank the reviewers who read an earlier draft of the manuscript carefully and gave many valuable suggestions to improve its quality. ’ REFERENCES (1) Preu, E.; Lissner, A. Xylit—Vorkommen, Eigenschaften, Verarbeitung, Verwendung, Freiberger Forschungshefte A148; Akademie-Verlag: Berlin, Germany, 1959; pp 5057. (2) Lehmann, H. Xylit—Vorkommen, Eigenschaften, Verarbeitung, Verwendung, Freiberger Forschungshefte A148; Akademie-Verlag: Berlin, Germany, 1959; pp 79. (3) Schneider, W. Int. J. Coal Geol. 1995, 28, 229–248. (4) Naeth, J. Int. J. Coal Geol. 2004, 60, 17–41. (5) Naundorf, W.; Schr€oder, H.; Strassburger, U. Stoffliche Nutzung von nachwachsenden Rohstoffen, Freiberger Forschungshefte A866; TU Bergakademie Freiberg: Freiberg, Germany, 2002; pp 100111. (6) Kunze, E. Xylit—Vorkommen, Eigenschaften, Verarbeitung, Freiberger Forschungshefte A148; Akademie-Verlag: Berlin, Germany, 1959; pp 106124. (7) Peter, E. Xylit—Vorkommen, Eigenschaften, Verarbeitung, Freiberger Forschungshefte A148; Akademie-Verlag: Berlin, Germany, 1959; pp 143150. (8) Vattenfall Europe Mining AG. Mittel zur Vermeidung und Reduzierung des Wachstums von Gr€unalgen, insbesondere fadenbildenen Algenarten. DE Patent 102006016715A1, 2007. (9) Predeanu, G.; Panaitescu, C. Int. J. Coal Geol. 2007, 71, 542–553. (10) Papanicolaou, C.; Pasadakis, N.; Dimou, D.; Kalaitzidis, S.; Papazisimou, S.; Foscolos, A. Int. J. Coal Geol. 2009, 77, 401–408. (11) B€ohmer, U.; Kirsten, C.; Bley, T.; Noack, M. Eng. Life Sci. 2010, 10, 26–34. (12) Kaliyan, N.; Vance Morey, R. Biomass Bioenergy 2009, 33, 337–359. (13) Nielsen, N.; Gardner, D.; Poulsen, T.; Felby, C. Wood Fiber Sci. 2009, 41, 1–12. (14) Sadoh, T. Wood Sci. Technol. 1981, 15, 57–66. (15) Becker, H.; Noack, D. Wood Sci. Technol. 1968, 2, 213–230. (16) Naundorf, W.; H€age, K.; Biegel, A.; Trommer, D. Verfahren zur Herstellung eines kleinst€uckigen Brennstoffes mit sehr guten Dosier- und Verbrennungseigenschaften. DE Patent DE10150074C1, 2003. (17) Lehmann, B.; Schr€oder, H.; Wollenberg, R.; H€artel, G. BergHuettenmaenn. Monatsh. 2010, 155, 257–263.
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