Particle size distributions of Corn stover (
Zea mays L.) created by a knife mill were determined using integral classifying screens with sizes from 12.7 to 50.8
mm, operating at speeds from 250 to ...500
rpm, and mass input rates ranging from 1 to 9
kg
min
−1. Particle distributions were classified using American Society of Agricultural and Biological Engineers (ASABE) standardised sieves for forage analysis that incorporated a horizontal sieving motion. The sieves were made from machined-aluminium with their thickness proportional to the sieve opening dimensions. A wide range of analytical descriptors that could be used to mathematically represent the range of particle sizes in the distributions were examined. The correlation coefficients between geometric mean length and screen size, feed rate, and speed were 0.980, 0.612, and −0.027, respectively. Screen size and feed rate directly influenced particle size, whereas operating speed had a weak indirect relation with particle size. The Rosin–Rammler equation fitted the chopped corn stover size distribution data with coefficient of determination (
R
2)
>
0.978. This indicated that particle size distribution of corn stover was well-fit by the Rosin–Rammler function. This can be attributed to the fact that Rosin–Rammler expression was well suited to the skewed distribution of particle sizes. Skewed distributions occurred when significant quantities of particles, either finer or coarser, existed or were removed from region of the predominant size. The mass relative span was slightly greater than 1, which indicated that it was a ‘borderline narrow to wide’ distribution of particle sizes. The uniformity coefficient was <4.0 for 19.0–50.8
mm screens, which indicated particles of relatively uniform size. Knife mill chopping of corn stover produced ‘fine-skewed mesokurtic’ particles with 12.7–50.8
mm screens. Size-related parameters, namely, geometric mean length, Rosin–Rammler size parameter, median length, effective length, and size guide number, were well predicted at
R
2 values of 0.981, 0.982, 0.979, 0.950 and 0.978, respectively as a function of knife mill screen size, feed rate, and speed. Results of this analysis of particle sizes could be applied to the selection of knife mill operating parameters to produce a particular size of corn stover chop, and could serve as a guide for the relationships among various analytic descriptors of biomass particle distributions.
Biomass particle size impacts handling, storage, conversion, and dust control systems. Size reduction mechanical energy was directly measured for switchgrass (
Panicum virgatum L.), wheat straw (
...Triticum aestivum L.), and corn stover (
Zea mays L.) in an instrumented hammer mill. Direct energy inputs were determined for hammer mill operating speeds from 2000 to 3600 rpm for 3.2 mm integral classifying screen and mass input rate of 2.5 kg/min with 90°- and 30°-hammers. Overall accuracy of specific energy measurement was calculated as ±
0.072 MJ/Mg. Particle size distributions created by hammer mill were determined for mill operating factors using ISO sieve sizes from 4.75 to 0.02 mm in conjunction with Ro-Tap® sieve analyzer. A wide range of analytical descriptors were examined to mathematically represent the range of particle sizes in the distributions. Total specific energy (MJ/Mg) was defined as size reduction energy to operate the hammer mill plus that imparted to biomass. Effective specific energy was defined as energy imparted to biomass. Total specific energy for switchgrass, wheat straw, and corn stover grinding increased by 37, 30, and 45% from 114.4, 125.1, and 103.7 MJ/Mg, respectively, with an increase in hammer mill speed from 2000 to 3600 rpm for 90°-hammers. Corresponding total specific energy per unit size reduction was 14.9, 19.7, and 13.5 MJ/Mg mm, respectively. Effective specific energy of 90°-hammers decreased marginally for switchgrass and considerably for wheat straw and it increased for corn stover with an increase in speed from 2000 to 3600 rpm. However, effective specific energy increased with speed to a certain extent and then decreased for 30°-hammers. Rosin–Rammler equation fitted the size distribution data with
R
2
>
0.995. Mass relative span was greater than 1, which indicated a wide distribution of particle sizes. Hammer milling of switchgrass, wheat straw, and corn stover with 3.2 mm screen resulted in ‘well-graded fine-skewed mesokurtic’ particles. Uniformity coefficient was <
4.0 for wheat straw, which indicated uniform mix of particles, and it was about 4.0 for switchgrass and corn stover, which indicated a moderate assortment of particles. Size-related parameters, namely, geometric mean diameter, Rosin–Rammler size parameter, median diameter, and effective size had strong correlation among themselves and good negative correlation with speed. Distribution-related parameters, namely, Rosin–Rammler distribution parameter, mass relative span, inclusive graphic skewness, graphic kurtosis, uniformity index, uniformity coefficient, coefficient of gradation and distribution geometric standard deviation had strong correlation among themselves and a weak correlation with mill speed. Results of this extensive analysis of specific energy and particle sizes can be applied to selection of hammer mill operating factors to produce a particular size of switchgrass, wheat straw, and corn stover grind, and will serve as a guide for relations among the energy and various analytic descriptors of biomass particle distributions.
Size reduction energy inputs for switchgrass, wheat straw, and corn stover were directly measured in an instrumented hammer mill for 90°- and 30°-hammers. Particle size distributions were determined using ISO sieve sizes from 4.75 to 0.02 mm. A wide range of analytical particle size descriptors were examined. The 90°-hammers consumed less specific energy than 30°-hammers. Rosin–Rammler equation fitted the size distribution data with
R
2
>
0.995.
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Bulk density of comminuted biomass significantly increased by vibration during handling and transportation, and by normal pressure during storage. Compaction characteristics affecting the bulk ...density of switchgrass, wheat straw, and corn stover chopped in a knife mill at different operating conditions and using four different classifying screens were studied. Mean loose-filled bulk densities were 67.5
±
18.4
kg/m
3 for switchgrass, 36.1
±
8.6
kg/m
3 for wheat straw, and 52.1
±
10.8
kg/m
3 for corn stover. Mean tapped bulk densities were 81.8
±
26.2
kg/m
3 for switchgrass, 42.8
±
11.7
kg/m
3 for wheat straw, and 58.9
±
13.4
kg/m
3 for corn stover. Percentage changes in compressibility due to variation in particle size obtained from a knife mill ranged from 64.3 to 173.6 for chopped switchgrass, 22.2–51.5 for chopped wheat straw and 42.1–117.7 for chopped corn stover within the tested consolidation pressure range of 5–120
kPa. Pressure and volume relationship of chopped biomass during compression with application of normal pressure can be characterized by the Walker model and Kawakita and Ludde model. Parameter of Walker model was correlated to the compressibility with Pearson correlation coefficient greater than 0.9. Relationship between volume reduction in chopped biomass with respect to number of tappings studied using Sone’s model indicated that infinite compressibility was highest for chopped switchgrass followed by chopped wheat straw and corn stover. Degree of difficulty in packing measured using the parameters of Sone’s model indicated that the chopped wheat straw particles compacted very rapidly by tapping compared to chopped switchgrass and corn stover. These results are very useful for solving obstacles in handling bulk biomass supply logistics issues for a biorefinery.
This paper presents a comprehensive and unitary stochastic mathematical model of grain threshing and separation in both axial and tangential threshing units. The derived equations describe and ...quantify the percentages of unthreshed grain, free grain, and separated grain (cumulative and distribution) over the length of the threshing space, be that the rotor or the concave length. Grain threshing and separation losses are also quantified.
Two coefficients of the mathematical model can be expressed as functions of functional and design parameters of the threshing unit. The model was validated with data from a large database obtained from extensive testing of various axial and tangential threshing units. Exceptional agreement of predicted and experimental data shows that all equations are highly significant.
The threshing process - by which the grains are removed from the ears - begins effectively at the feeding zone of threshing unit. At the same time, grain separation occurs in the feeding zone of the ...threshing unit if the rotor cage has openings. This paper presents the application of mathematical model for grain threshing and separation (described in Part I) in an axial threshing unit with separation beginning in the tangential feeding zone. The model describes and quantifies the cumulative percentages of unthreshed grain, free grain, and separated grain over the entire length of the rotor. Grain threshing and separation losses are also quantified. The mathematical model was validated using reliable and comprehensive experimental data. Very good agreement was found between theoretical (predicted) and experimental data for separated grain as well as for threshing, and separation losses of grain over the entire range of testing conditions. Assuming that a set of minimum experimental data is provided for initialization of model parameters, the model can be further used for: - process simulation that means reduction of testing time and associated costs, - optimization of design and functional parameters of threshing unit, - development of control software and process controllers for combine harvesters. This is the only model ever published for this type of axial threshing units.
This paper presents a study of the movement of vegetal material through the threshing space of an axial threshing unit. The developed kinematical equations are based on a nonlinear law governing the ...non-uniform movement of the material on an uneven helical path between the rotor and concave-cage. These equations describe:
•
the characteristics of material trajectory (length, angle, pitch length, and number of material revolutions) as functions of longitudinal position along the rotor;
•
the components of material velocity.
The model was validated with experimental data from a large database obtained from laboratory testing.
The model can be used for practical analysis of material movement influence on threshing unit performance in terms of grain separation and damage, as well as for threshing unit design and process optimization. The kinematics model proved to be highly significant and useful for predictions.
Lengthy straw/stalk of biomass may not be directly fed into grinders such as hammer mills and disc refiners. Hence, biomass needs to be preprocessed using coarse grinders like a knife mill to allow ...for efficient feeding in refiner mills without bridging and choking. Size reduction mechanical energy was directly measured for switchgrass (
Panicum virgatum L.), wheat straw (
Triticum aestivum L.), and corn stover (
Zea mays L.) in an instrumented knife mill. Direct power inputs were determined for different knife mill screen openings from 12.7 to 50.8
mm, rotor speeds between 250 and 500
rpm, and mass feed rates from 1 to 11
kg/min. Overall accuracy of power measurement was calculated to be ±0.003
kW. Total specific energy (kWh/Mg) was defined as size reduction energy to operate mill with biomass. Effective specific energy was defined as the energy that can be assumed to reach the biomass. The difference is parasitic or no-load energy of mill. Total specific energy for switchgrass, wheat straw, and corn stover chopping increased with knife mill speed, whereas, effective specific energy decreased marginally for switchgrass and increased for wheat straw and corn stover. Total and effective specific energy decreased with an increase in screen size for all the crops studied. Total specific energy decreased with increase in mass feed rate, but effective specific energy increased for switchgrass and wheat straw, and decreased for corn stover at increased feed rate. For knife mill screen size of 25.4
mm and optimum speed of 250
rpm, optimum feed rates were 7.6, 5.8, and 4.5
kg/min for switchgrass, wheat straw, and corn stover, respectively, and the corresponding total specific energies were 7.57, 10.53, and 8.87
kWh/Mg and effective specific energies were 1.27, 1.50, and 0.24
kWh/Mg for switchgrass, wheat straw, and corn stover, respectively. Energy utilization ratios were calculated as 16.8%, 14.3%, and 2.8% for switchgrass, wheat straw, and corn stover, respectively. These data will be useful for preparing the feed material for subsequent fine grinding operations and designing new mills.
Biomass particle size impacts handling, storage, conversion, and dust control systems. Switchgrass (
Panicum virgatum L.) particle size distributions created by a knife mill were determined for ...integral classifying screen sizes from 12.7 to 50.8
mm, operating speeds from 250 to 500
rpm, and mass input rates from 2 to 11
kg/min. Particle distributions were classified with standardized sieves for forage analysis that included horizontal sieving motion with machined-aluminum sieves of thickness proportional to sieve opening dimensions. Then, a wide range of analytical descriptors were examined to mathematically represent the range of particle sizes in the distributions. Correlation coefficient of geometric mean length with knife mill screen size, feed rate, and speed were 0.872, 0.349, and 0.037, respectively. Hence, knife mill screen size largely determined particle size of switchgrass chop. Feed rate had an unexpected influence on particle size, though to a lesser degree than screen size. The Rosin–Rammler function fit the chopped switchgrass size distribution data with an
R
2
>
0.982. Mass relative span was greater than 1, which indicated a wide distribution of particle sizes. Uniformity coefficient was more than 4.0, which indicated a large assortment of particles and also represented a well-graded particle size distribution. Knife mill chopping of switchgrass produced ‘strongly fine skewed mesokurtic’ particles with 12.7–25.4
mm screens and ‘fine skewed mesokurtic’ particles with 50.8
mm screen. Results of this extensive analysis of particle sizes can be applied to selection of knife mill operating parameters to produce a particular size of switchgrass chop, and will serve as a guide for relations among the various analytic descriptors of biomass particle distributions.
Knowing the effect of mill operating factors on biomass size reduction would be useful for predicting or adjusting particle size distributions that affect supply chain efficiency and biomass-to-fuel ...conversion processes. Wheat straw (
Triticum aestivum L.) particle size distributions generated by a knife mill were determined for integral classifying screen sizes from 12.7 to 50.8 mm, operating frequencies from 4.17 to 8.33 Hz, and mass input rates from 2 to 9 kg min
−1. Particle distributions were obtained with standardized sieves for forage analysis that included horizontal sieving motion with machined-aluminum sieves of thickness proportional to sieve opening dimensions to reduce inadvertent particle spearing through sieve openings. Several analytical descriptors were examined to mathematically represent the range of particle sizes in the distributions. Screen size and feed rate directly influenced particle size, whereas operating frequency had a weak indirect relation with particle size. The Rosin-Rammler equation adequately fit the knife-milled wheat straw particle size distribution data (
R
2 > 0.982). Mass relative span was >1, which indicated a wide distribution of particle sizes. Uniformity coefficient was <4.0 for 12.7 and 19.0 mm screens, which indicated particles of relatively uniform size. Knife mill chopping of wheat straw produced fine-skewed mesokurtic particles with 12.7–25.4 mm screens and strongly fine-skewed mesokurtic particles with 50.8 mm screen. Other size related parameters, such as, geometric mean length, Rosin-Rammler size parameter, median length, effective length, and size guide number, were well predicted as a function of knife mill screen size, feed rate, and mill frequency. Results showed the relative effects of knife mill operating factors to produce particular particle sizes of wheat straw.
► Wheat straw chop obtained from various operating factors of knife mill was characterized. ► Wheat straw chop particle size distributions were modeled in terms of knife mill operating factors. ► Ready reckoner is established in order to produce a specific wheat straw chop size.
