Mill Screen Size Research Articles

Impact of process conditions on the density and durability of wheat, oat, canola, and barley straw briquettes

The present study is to understand the impact of process conditions on the quality attributes of wheat oat, barley, and canola straw briquettes. Analysis of variance indicated that briquette moisture content and initial density immediately after compaction and final density after 2 weeks of storage are strong functions of feedstock moisture content and compression pressure, whereas durability rating is influenced by die temperature and feedstock moisture content. Briquettes produced at a low feedstock moisture content of 9 % (w.b.) yielded maximum densities >700 kg/m3 for wheat, oat, canola, and barley straws. Lower feedstock moisture content of 110 °C and compression pressure >10 MPa minimized the briquette moisture content and maximized densities and durability rating based on surface plots observations. Optimal process conditions indicated that a low feedstock moisture content of about 9 % (w.b.), high die temperature of 120–130 °C, medium-to-large hammer mill screen sizes of about 24 to 31.75 mm, and low to high compression pressures of 7.5 to 12.5 MPa minimized briquette moisture content to 700 kg/m3. Durability rating >90 % is achievable at higher die temperatures of >123 °C, lower to medium feedstock moisture contents of 9 to 12 % (w.b.), low to high compression pressures of 7.5 to 12.5 MPa, and large hammer mill screen size of 31.75 mm, except for canola where a lower compression pressure of 7.5 to 8.5 MPa and a smaller hammer mill screen size of 19 mm for oat maximized the durability rating values.

Ignition and volatilization behavior of dust from loblolly pine wood

Generation of dust particles typically occurs when biomass feedstocks are preprocessed in preparation for conversion to energy, fuels and chemicals. This paper investigated the ignition risk of dust from loblolly pine wood by quantifying (including moisture content and grinding screen size effects on) the amount of dust in ground wood chips, dust physical and chemical properties, hot surface ignition temperature, critical volatilization and exothermic temperatures, volatile release activation energy and exothermic energy. Experimental results showed that up to 22% (on mass basis) of the ground chips can be in dust form and that about 7% dust will be produced from the typical conditions used to grind wood chips (moisture content of about 15% and hammer mill screen size of 3.18mm). The fine dust fraction (dust particles passing through 90μm sieve) had higher ignition risk compared to medium (between 90 and 180μm) and coarse (between 180 and 420μm) dust fractions. Up to 5113kJ/kg of exothermic energy was released during the ignition of the fine dust sample. The results from this study will be useful in the modeling of behavior and design of equipment and systems to minimize risk of dust-causing ignition, fire and explosion in biomass processing plants.

Effect of Fuel Additives on Agricultural Straw Pellet Quality

An investigation was conducted to determine the effect of addition of different levels of AK2, a fuel additive that reduces ash fusion for agricultural biomass, on the physico-chemical properties of biomass pellets. Three different biomass straws, barley, oat, and wheat were ground at two hammer mill screen sizes of 0.8 mm and 1.6 mm. Each ground biomass sample was mixed with three levels of AK2, 0.05%, 0.10%, and 0.15% by mass and also a blank (no AK2) was set aside for comparison. Pellets were made using single-pelleting unit at a pre-set load of 4 400 N corresponding to a pressure of 138.9 MPa. Physical quality of pellets were determined by measuring pellet density, relaxed density, durability, and the specific energy required to make a pellet. Pellets having higher durability values (74%-88%) were obtained from ground straw at hammer screen size of 0.8 mm and AK2 level of 0.15% compared to other treatments. Carbon, hydrogen, nitrogen, and sulfur content of blank pellets and those with 0.15% AK2 at hammer screen size of 0.8 mm were determined. Pellets made with 0.15% AK2 at hammer screen size of 0.8 mm, manufactured by pilot-scale pellet mill, were gasified and the tar content was determined. The tar content of pellets with 0.15% AK2 was significantly lower than blank pellets. Keywords: biomass, biofuels, slag, foul, pelleting, fuel additive DOI: 10.3965/j.ijabe.20140702.011 Citation: Emami S, Tabil L G, Adapa P, George E, Tilay A, Dalai A, et al. Effect of fuel additives on agricultural straw pellet quality. Int J Agric & Biol Eng, 2014; 7(2): 92－100.

Energy and Cost for Pelleting and Transportation of Select Cellulosic Biomass Feedstocks for Ethanol Production

<abstract><title><italic>Abstract. </italic></title> The current forage handling equipment in the cellulosic ethanol industry is severely limited by the low bulk density of baled and ground biomass. Low bulk density contributes to flowability problems and lack of maximizing trailer capacities. Biomass pelleting process can improve the bulk density and flowability characteristics of forages. The objectives of this research were to evaluate: (1) the energy requirements of grinding sorghum stalks, corn stover, wheat straw, and big bluestem through two different screen size openings, (2) the energy requirements of pelleting forages from the two grind sizes, (3) the physical properties of pelleted biomass, and (4) the costs associated with biomass processing, transportation, and storage. The two mill screen size openings (3.2 and 6.5 mm) were found to have significantly different energy consumptions for grinding step from each other. All four forage types, except for big bluestem and corn stover, were also found to have significantly different energy consumptions for grinding. Production rate through the 6.5 mm screen was almost three times higher than that of the 3.2 mm screen (average of 181.4 vs. 68 kg/h). Hammermill screen size opening (i.e., grind size) was found to have significant effects on energy consumption for pelleting process. The four forage types were also found to have significantly different energy consumptions from each other, except for big bluestem versus wheat straw (P=0.1192). Particle length for the 3.2 mm grind ranged from 0.15 to 0.18 cm, while the 6.5-mm grind ranged from 0.20 to 0.31 cm. Pelleting increased bulk density from 99.96 to 160.02 kg/m³ for raw biomass grinds to 499.30 to 701.13 kg/m³ for pelleted biomass. Pellet durability ranged from 93% to 98%. A cost analysis indicated that it would take roughly $22 extra per metric ton for the transportation, pre-processing, and storage of pelleted cellulosic biomass than corn grain. This cost is still almost half that of the cost for baled biomass.

The Effect of Hammer Mill Screen Size on Processing Parameters and Starch Enrichment in Milled Barley

The influence of two levels of hammer mill screen size on processing parameters and the effect of barley particle size, after segregation by sieving, on starch content and bulk density were examined. Barley grains were hammer-milled through a 2mm and 6mm screens and subsequently fractionated on a set of eight sieves size 0.045, 0.125, 0.250, 0.5, 1.0, 1.7 and 2.8mm. The purposes of this study were to characterize the influence of hammer mill screen size (2 and 6mm) on processing parameters: production output, energy consumption, average particle size (dgw), geometric standard deviation (sgw) and to characterize the influence of particle size, after segregation by sieving, on starch enrichment and bulk density. The results of this study show that increasing the hammer milling screen size has resulted in a significant effect on production output, energy consumption, dgw with no effect on sgw. Different particle sizes, when segregated by sieving, have shown significant effect on starch content and bulk density. Particle size retained by sieve size 1mm had the highest starch content, whereas particles retained by sieve size 0.25mm had the lowest, which suggest potential applications in ruminant and monogastric nutrition. Different measurements of bulk density of segregated particle size suggest the possibility of size fractionation of milled barley particles based on their density differences.

Influence of hammer mill screen size on processing parameters and starch enrichment in milled sorghum

The effect of hammer mill screen size on processing parameters and particle size of milled sorghum were examined. Sorghum grains were ground through two levels of hammer mill screen size: 2 mm and 6 mm and then subsequently fractionated by size on a set of eight sieves ranging from 0.045 mm to 2.8 mm. The objective of this experiment were: i) to characterize the influence of hammer mill screen size (2 and 6 mm) on processing parameters: production output, energy consumption, average particle size (dgw), geometric standard deviation (sgw); ii) to characterize the influence of particle size, after segregation by sieving, on starch enrichment. The results of this study showed that using 6 mm hammer milling screen size has resulted in a significant effect on production output, energy consumption, dgw with no effect on sgw. Different particle sizes, when segregated by sieving, have shown significant effect on starch content. Particle size retained by sieve size 1.7 mm had the highest starch content, whereas particles retained by sieve size 0.125 mm had the lowest, which suggest potential applications in ruminant and monogastric nutrition.

Grinding Studies of Mango Ginger: Mathematical Modelling of Particle Size Distribution and Energy Consumption

Mango ginger was ground in hammer mill with three different classifying screens and pin mill to study the particle size distribution and energy consumption. The Rosin-Rammler Bennet (RRB) model fitted well the particle size distribution data over the entire range of the size distribution for grinding in both hammer mill and pin mill with high coefficient of determination (R2) and low values of residual sum square, root mean square error and Chi-square. Relationship between RRB model parameters with hammer mill screen size was obtained with high R2. All the three classical models such as Rittinger’s, Kick’s and Bond’s law were found suitable to explain the energy consumption for grinding. Energy consumption increased exponentially with decrease in classifying screen size of hammer mill. The Work index for grinding increased with increase in size reduction ratios and were in the range of 0.075-0.58 kW/kg.

Determining API domain sizes in pharmaceutical tablets and blends upon varying milling conditions by near-infrared chemical imaging

A blend–mill–blend process is commonly used in tablet manufacturing. This process uses a screening mill to de-lump the agglomerates potentially present in the active pharmaceutical ingredient (API) and/or the excipients to promote better mixing and blend homogeneity. As a part of the study in which various process conditions were explored to better understand the manufacturing of a pharmaceutical formulation in development, two batches were manufactured using different mill screen sizes and were compared to an unmilled batch. Near-infrared (NIR) chemical imaging was used to provide information on API domains in these three samples. Both tablets and blends from the three batches were imaged and the results were compared with the content uniformity data of the tablets. The simplest imaging approach (univariate) was used for producing the API images. Experimentally, the importance and priority was given to the speed of the acquisition and the number of images acquired in order to provide sufficient evidence for the presence or lack of agglomeration. Significant API agglomeration was found in both blends and tablets in the samples from the unmilled batch, clearly differing from the occasional and much smaller lumps found in the blends and tablets in the batches utilizing the milling screens. This study demonstrates an efficient and feasible process analytical technology (PAT) offline application of chemical imaging to provide better process understanding and identify criticality of particular process steps to ensure content uniformity.

Effect of Particle Size and Moisture Content on Cassava Root Pellet Fuel′S Qualities Follow the Acceptance of Pellet Fuel Standard

This research studied about pellet fuel standards around the world and the effect of particle size and moisture content on cassava pellet fuel’s qualities. The sample pellet fuel from cassava root was produced in flat die pellet mill at controlling temperature 80-90 ºC. The studying parameters were particle size and moisture content. The particle size was defined by hammer mill screen sizes : 1, 4, and 10 millimeters. In the part of moisture content was defined by quantities of water addition in cassava root : 10, 20, and 30 % by raw cassava root mass. All different compound samples, cassava root pellet fuel were tested by PFI standard (The United State of America’s pellet fuel standard). The qualities of pellet fuel were diameter, length, moisture content, durability, fine, heating value, and bulk density. The result from testing shown that all cassava root pellet fuel samples passed the criterion of PFI standard and other pellet fuel standards. From this result indicated that the particle size and moisture content didn’t influence with all different compound cassava root pellet fuel samples.

EFFECT OF RAW MATERIAL PROPERTIES AND DIE GEOMETRY ON THE DENSITY OF BIOMASS PELLETS FROM COMPOSTED MUNICIPAL SOLID WASTE

Densification of biomass feedstocks, such as pelletizing, can increase bulk density, improve storability, reduce transportation costs, and ease the handling of biomass using existing handling and storage equipment for grains. In order to study the pelletizing process, compost pellets were produced under controlled conditions. The aim of the work was to investigate the effect of raw material properties and the die geometry on the true density of formed pellets and also find the optimal conditions of the densification process for producing pellets with high density. Compost was extruded into cylindrical pellets utilizing open-end dies under axial stress from a vertical piston applied by a hydraulic press. The effects of independent variables, including the raw material moisture content (35 to 45% (wet basis)), hammer mill screen size (0.3 to 1.5 mm), speed of piston (2 to 10 mm/s), and die length (8 to 12 mm) on pellet density, were determined using response surface methodology. A quadratic model was proposed to predict the pellet density, which had high F and R2 values along with a low p value, indicating the predictability of the model. Moisture content, speed of piston, and particle size significantly affected (P 0.05).

Effects of the pelleting conditions on chemical composition and sugar yield of corn stover, big bluestem, wheat straw, and sorghum stalk pellets

Pelleting of biomass can increase their bulk density and thus improve storability and reduce transportation costs. The objective of this research was to determine the effects of the pelleting conditions on chemical composition and fermentable sugar yield of the biomass. Corn stover, wheat straw, big bluestem, and sorghum stalks were used for this study. Dilute sulfuric acid was used for biomass pretreatment. Accellerase 1500™ was used for cellulose hydrolysis. Effects of mill screen size, die thickness, and L/D ratio of die on chemical compositions and sugar yield were determined. Glucan content of the biomass was positively affected by die thickness and negatively affected by mill screen size. Opposite trend was observed for xylan content. Wheat straw pellets had the highest sugar yield (92.5-94.1%) and big bluestem pellets had the lowest sugar yield (83.6-91.1%). Optimum pelleting condition is 6.5 mm screen size and 44.5 mm die thickness.

Characterization of wheat straw particle size distributions as affected by knife mill operating factors

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.

Physical properties of pellets made from sorghum stalk, corn stover, wheat straw, and big bluestem

Densification of biomass feedstocks, such as pelleting, can increase bulk density, improve storability, reduce transportation costs, and make these materials easier to handle using existing handling and storage equipment for grains. The objectives of this research were to study (1) the physical properties of pellets made from corn stover, sorghum stalk, wheat straw, and big bluestem, (2) the effect of moisture content on bulk density, true density, and durability of the biomass pellets, and (3) the effect of hammer mill screen size and die thickness on bulk density, true density, and durability of the pellets. Biomass pelleting can significantly improve the bulk density from 47 to 60kg/m3 for biomass grinds to 360 to 500kg/m3 for biomass pellets. Of the four types of biomass pellets, wheat straw pellets had the highest bulk density value of 495.8kg/m3, and sorghum stalk pellets had the lowest bulk density value of 365.2kg/m3. An increase in moisture level resulted in a decrease in bulk density and true density of the pellets. The effect of moisture content on durability of the pellets made from corn stover, wheat straw, and big bluestem showed a similar trend; the maximum durability value was 96.8% at the equilibrium moisture content (EMC) range of 9% (d.b.) to 14% (d.b.) for corn stover and wheat straw, and 9% (d.b.) to 11% (d.b.) for big bluestem. A further increase in EMC value resulted in a decrease in pellet durability. For sorghum stalk pellets, the durability value increased initially with increased EMC and reached a maximum of 89.5% at EMC values between 14% (d.b.) and 16% (d.b.). Use of a larger hammer mill screen size (from 3.2mm to 6.5mm screen openings) resulted in increases of bulk density, true density, and durability of biomass pellets, but not in significant levels. Use of a thicker die size (from 31.8mm to 44.5mm in thickness) resulted in significant increase of bulk density, true density, and durability of biomass pellets.

Grinding performance and physical properties of non-treated and steam exploded barley, canola, oat and wheat straw

Grinding experiments were conducted on non-treated and steam exploded barley, canola, oat and wheat straw using a forage chopper and a hammer mill (screen sizes of 30, 6.4, 3.2 and 1.6 mm) to determine specific energy requirements, and geometric mean particle size and distribution of ground material. The bulk density of non-treated biomass was significantly higher than bulk density of steam exploded agricultural biomass. For non-treated agricultural straw, the particle density of canola and oat straw significantly increased with a decrease in hammer mill screen size from 30 to 1.6 mm. The particle density of steam exploded barley and oat straw was significantly higher than non-treated straw, except for barley at 6.4 mm hammer mill screen size. The particle density of steam exploded canola straw was not statistically different from non-treated straw. The chopper consumed highest (3.15 ± 0.09 kWh t−1) and lowest (1.96 ± 0.33 kWh t−1) specific energy to chop barley and canola straw, respectively. The highest and lowest specific energy was consumed by wheat (42.57 ± 2.04 kWh t−1) at 1.6 mm and canola (1.46 ± 0.30 kWh t−1) straws ground using 30 mm hammer mill screen size, respectively. For steam exploded agricultural biomass, the highest and lowest specific energy was consumed by oat (33.18 ± 3.10 kWh t−1) at 1.6 mm and canola (2.69 ± 0.26 kWh t−1) straws ground using 6.4 mm hammer mill screen size, respectively. Specific energy required by hammer mill to grind non-treated and steam exploded barley, canola, oat and wheat straw showed a negative power correlation with hammer mill screen sizes.

Influence of process parameters and biomass characteristics on the durability of pellets from the pruning residues of Olea europaea L.

Abstract The present work aims to investigate the influence of the main process parameters (pressure and temperature) and biomass characteristics (moisture content and particle size) on some mechanical properties (density and durability) of olive tree pruning residues pellets. By means of a lab scale pellet press, able to control process parameters, the biomass, ground with three different hammer mill screen sizes (1, 2 and 4 mm) and conditioned at different moisture contents (5, 10, 15 and 20% w.b.), was pelletized at various process temperatures (60, 90, 120 and 150 °C) and pressures (71, 106, 141 and 176 MPa). Compressed sample dimensions and mass were measured in order to calculate pellet density, while compressive strength tests were carried out to estimate the durability of the final biofuel. The relationships between the factor settings and the responses (density, compression strength and modulus of elasticity) were examined by univariate and multivariate statistical analysis. Temperature resulted the most important variable influencing pellet mechanical properties, followed by the initial moisture content and the particle size of the raw material. In particular, high process temperature, low moisture contents and reduced particle sizes allowed obtaining good quality pellets. The effect of compression force resulted scarcely relevant.

Pelleting characteristics of selected biomass with and without steam explosion pretreatment

Processing and densification of agricultural biomass into high density and durable pellets are critical to facilitate handling, storage and transportation. Biomass pelleting experiments were designed to conduct single and pilot scale pelleting of non-treated and steam exploded barley, canola, oat and wheat straw grinds acquir ed from 6.4, 3.2, 1.6 and 0.8 mm hammer mill screen sizes at 10% moisture content (wb). Single-pelleting was performed by applying compression pressures of 31.6, 63.2, 94.7, and 138.9 MPa using a close-fit plunger die assembly (die length 135.3 mm and diameter of ( 6.30±0.5 ) mm). During pilot scale pelleting, customization of ground straw material was performed by adding steam exploded biomass in increments of 25% to non-treated ground straw for respective biomass at specific grind size. Ground s traw samples were conditioned to 17.5% moisture content and 10% flaxseed oil was added to increase the bulk density and flowability of grinds, which resulted in the production of pellets. The quality of pellets from single pelleting experiments was ascertained by measuring their respective density and durability. In addition, the change in pellet density was measured after a storage period of one month to determine its dimensional stability. It was determined that applied pressure and pre-treatment were significant factors affecting the pellet density. Also, bigger grind sizes and lower applied pressures resulted in higher pellet relaxations (lower pellet densities) during storage of pellets. The pilot scale pellet mill produced pellets from ground non-treated straw at hammer mill screen sizes of 0.8 and 1.6 mm and customized samples having 25% steam exploded straw at 0.8 mm. It was observed that the p ellet bulk density and particle density are positively correlated. The density and durability of agricultural straw pellets significantly increased with decrease in hammer mill screen size from 1.6 mm to 0.8 mm. Customization of agricultural straw by adding 25% of steam exploded straw by weight is possible, but it did not improve pellet quality. In addition, d urability of pellets was negatively correlated to pellet mill throughput and was positively correlated to specific energy consumption. Keywords: b iofuel s , b iomass, d ensity, d urability, p elleting, s pecific e nergy, s team e xplosion, p retreatment DOI: 10.3965/j.issn.1934-6344.20 1 0.0 3 .0 62 -0 79 Citation: Phani Adapa, Lope Tabil, Greg Schoenau , Anthony Opoku . Pelleting c haracteristics of s elected b iomass with and without s team e xplosion p retreatment. Int J Agric & Biol Eng, 20 1 0; 3 ( 3 ): 62 <span style=font-size: 9pt; line-height: 130%; font-family:

Physical and frictional properties of non-treated and steam exploded barley, canola, oat and wheat straw grinds

During storage and handling, accurate knowledge of the physical and frictional behaviors of biomass grinds is essential for the efficient design of equipment. Therefore, experiments were performed on non-treated and steam exploded barley, canola, oat and wheat straw grinds to determine their coefficient of internal friction and cohesion at three hammer mill screen sizes of 6.4, 3.2 and 1.6 mm, three normal stress values of 9.8, 19.6 and 39.2 kPa at 10% moisture content (wb). At any specific hammer mill screen size, the geometric mean particle size and bulk density of non-treated straw was significantly larger than steam exploded straw. The bulk density of ground straw significantly increased with a decrease in hammer mill screen sizes. The steam exploded straw grinds resulted in higher coefficient of internal friction compared to non-treated straw grinds primarily because of lower bulk densities. The coefficient of friction for non-treated barley, canola, oat and wheat straw were in the range of 0.505 to 0.584, 0.661 to 0.665, 0.498 to 0.590, and 0.532 to 0.591, respectively. Similarly, the coefficient of friction for steam exploded barley, canola, oat and wheat straw were in the range of 0.562 to 0.738, 0.708 to 0.841, 0.660 to 0.860, and 0.616 to 1.036, respectively, which were higher than non-treated straw of the kind. Power, logarithmic or exponential equations were developed to predict the coefficient of internal friction and cohesion with respect to average geometric mean particle sizes for non-treated and steam explode barley, canola, oat and wheat straw grinds.

Compression Characteristics of Non-Treated and Steam-exploded Barley, Canola, Oat, and Wheat Straw Grinds

Compaction of low bulk density agricultural biomass is a critical and desirable operation for sustainable and economic availability of feedstock for biofuel industry.A comprehensive study of the compression characteristics (density of pellet and total specific energy required for compression) of ground non-treated and steam-exploded barley, canola, oat, and wheat straw obtained from three hammer mill screen sizes of 6.4, 3.2 and 1.6 mm at 10% moisture content (wb) was conducted. Four preset pressures of 31.6, 63.2, 94.7, and 138.9 MPa were applied using an Instron testing machine to compress samples in a cylindrical die. Ground steam-exploded barley straw at screen sizes of either 3.2 or 1.6 mm produced high density compacts, while ground steam-exploded canola, oat, and wheat straw at screen sizes of 6.4, 3.2 or 1.6 mm produced high density compacts. Steam-exploded barley straw for 3.2 mm at 138.9 MPa produced compacts having 13% higher density and consumed 19% lower total specific energy compared to non-treated straw. Steam-exploded canola straw for 1.6 mm at 138.9 MPa produced compacts having 13% higher density and consumed 22% higher total specific energy compared to non-treated straw. Steam-exploded oat straw for 3.2 mm at 94.7 MPa produced compacts having 19% higher density and consumed 13% higher total specific energy compared to non-treated straw. Steam-exploded wheat straw for 6.4 mm at 138.9 MPa produced compacts having 17% higher density and consumed 17% higher total specific energy compared to non-treated straw. Three compression models, namely: Jones model, Cooper-Eaton model, and Kawakita-Ludde model were considered to determine the pressure-volume and pressure-density relationship of non-treated and steam-exploded straws. Kawakita-Ludde model provided the best fit to the experimental data having R2 values of 0.99 for non-treated straw and 1.00 for steam-exploded biomass samples. The steam-exploded straw had higher porosity than non-treated straw. In addition, the steam-exploded straw was easier to compress since it had lower yield strength or failure stress values compared to non-treated straw.

Knife Mill Comminution Energy Analysis of Switchgrass, Wheat Straw, and Corn Stover and Characterization of Particle Size Distributions

Biomass preprocessing and pretreatment technologies such as size reduction and chemical preconditioning are aimed at reducing the cost of ethanol production from lignocellulosic biomass. Size reduction is an energy-intensive biomass preprocessing unit operation. In this study, switchgrass, wheat straw, and corn stover were chopped in an instrumented knife mill to evaluate size reduction energy and corresponding particle size distribution as determined with a standard forage sieve analyzer. Direct mechanical power inputs were determined using a dedicated data acquisition system for 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. A speed of 250 rpm gave optimum performance of the mill. Optimum feed rates for 25.4 mm screen and 250 rpm were 7.6, 5.8, and 4.5 kg/min for switchgrass, wheat straw, and corn stover, respectively. Total specific energy (MJ/Mg) was defined as the size reduction energy required to operate the knife mill plus that imparted to the biomass. Effective specific energy was defined as the energy imparted to the biomass. For these conditions, total specific energies were 27.3, 37.9, and 31.9 MJ/Mg and effective specific energies were 10.1, 15.5, and 3.2 MJ/Mg for switchgrass, wheat straw, and corn stover, respectively. These results demonstrated that biomass selection affects the size reduction energy, even for biomass with similar features. Second-order polynomial equations for the total specific energy requirement fitted well (R2 > 0.95) as a function of knife mill screen size, mass feed rate, and speed for biomass materials tested. The Rosin-Rammler equation fitted the cumulative undersize mass of switchgrass, wheat straw, and corn stover chop passed through ASABE sieves with high R2 (>0.983). Knife mill chopping of switchgrass, wheat straw, and corn stover resulted in particle size distributions classified as 'well-graded strongly fine-skewed mesokurtic', 'well-graded fine-skewed mesokurtic', and 'well-graded fine-skewed mesokurtic', respectively, for small knife mill screen sizes (12.7 to 25.4 mm) and distributions classified as 'well-graded fine-skewed mesokurtic', 'well-graded strongly fine-skewed mesokurtic', and 'well-graded fine-skewed mesokurtic', respectively, for the large screen size (50.8 mm). Total and effective specific energy values per unit size reduction of wheat straw were greater compared to those for switchgrass. Corn stover resulted in reduced total and effective specific energy per unit size reduction compared to wheat straw for the same operating conditions, but higher total specific energy per unit size reduction and lesser effective specific energy per unit size reduction compared to switchgrass. Data on minimized total specific energy with corresponding particle spectra will be useful for preparing feed material with a knife mill for subsequent grinding with finer size reduction devices.

Specific energy consumption for reducing the size of alfalfa chops using a hammer mill

Measuring the energy requirement for alfalfa size reduction could be important for downstream processes such as densification. Alfalfa chops passed through sieve sizes of 18 mm (SS18mm), 15 mm (SS15mm) and 12 mm (SS12mm) were ground using a hammer mill (1.1 kW) with four screen sizes of 1.68, 2.38, 3.36 and 4.76 mm. Results showed that alfalfa chops with sizes of SS18mm and SS12mm had the highest and lowest specific energy (30.96 and 5.06 kJ kg−1), respectively. Exponential relationships between the specific energy requirement and the hammer mill screen sizes were obtained with coefficients of determination (R2) values ranging from 0.94 to 0.98. The data on specific energy was fitted to Bond, Rittinger and Kick models. The Rittinger model was the best fitted model with R2 > 0.94 for the three sizes of alfalfa chop. A linear model relating the specific energy to the ratio of initial to final screen sizes using the combined data predicted more accurately the specific energy values than all three models.