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Impact of process conditions on the density and durabilityof wheat, oat, canola, and barley straw briquettes
J. S. Tumuluru & L. G. Tabil & Y. Song & K. L. Iroba &V. Meda
Published online: 10 September 2014# The Author(s) 2014. This article is published with open access at Springerlink.com
Abstract The present study is to understand the impact ofprocess conditions on the quality attributes of wheat oat,barley, and canola straw briquettes. Analysis of variance indi-cated that briquette moisture content and initial density imme-diately after compaction and final density after 2 weeks ofstorage are strong functions of feedstock moisture content andcompression pressure, whereas durability rating is influencedby die temperature and feedstock moisture content. Briquettesproduced at a low feedstock moisture content of 9 % (w.b.)yielded maximum densities >700 kg/m3 for wheat, oat, cano-la, and barley straws. Lower feedstock moisture content of110 °C and com-pression pressure >10 MPa minimized the briquette moisturecontent and maximized densities and durability rating basedon surface plots observations. Optimal process conditionsindicated that a low feedstock moisture content of about 9 %(w.b.), high die temperature of 120–130 °C, medium-to-largehammer mill screen sizes of about 24 to 31.75 mm, and low tohigh compression pressures of 7.5 to 12.5 MPa minimizedbriquette moisture content to 700 kg/m3. Durability rating >90 % is achievableat higher die temperatures of >123 °C, lower to mediumfeedstock moisture contents of 9 to 12 % (w.b.), low to highcompression pressures of 7.5 to 12.5 MPa, and large hammer
mill screen size of 31.75 mm, except for canola where a lowercompression pressure of 7.5 to 8.5MPa and a smaller hammermill screen size of 19 mm for oat maximized the durabilityrating values.
Keywords Agricultural straws . Briquettes . Unit density .
Durability rating . Response surface models . Optimization .
Lignocellulosic biomass includes agricultural residues, hard-wood, and softwood, as well as dedicated biomass crops.According to Kim and Dale , the estimated residue fromcrops such as corn, wheat, oats, rice, sorghum, and sugarcanethroughout the world is approximately 1.5 billion metrictonnes. These residues also constitute one of the most impor-tant biomass feedstocks in Canada due to its vast agriculturalbase. Major cereal crops (such as wheat, barley, and oats) aregrown on the Canadian prairies. The total straw available forindustrial use after soil and livestock requirements is approx-imately 15 Mt . The Saskatchewan Ministry of Agriculturereported that the province of Saskatchewan alone produced12.44 Mt of wheat, 5.63 Mt of canola, 4.59 Mt of barley, and2.3 Mt of oats straws. This abundant, inexpensive, and readilyavailable biomass source indicates that Canada can play asignificant role in the development of an environmentallybenign bioenergy sector.
Agricultural biomass, due to its low density limitations,must be processed and handled in an efficient manner toenhance its flowability, reduce material waste, and increaseits bulk density to reduce transportations costs. The densifica-tion of biomass involves the compression or compaction ofbiomass to eliminate inter- or intraparticle voids.Different densification technologies are available for
J. S. Tumuluru (*)Biofuels and Renewable Energy Technologies Department, EnergySystems and Technologies Division, Idaho National Laboratory,P.O. Box 1625, Idaho Falls, ID 83415-2025, USAe-mail: [email protected]
L. G. Tabil :K. L. Iroba :V. MedaDepartment of Chemical and Biological Engineering, University ofSaskatchewan, Saskatoon, SK, Canada S7N 5A9
Y. SongCollege of Engineering, Shenyang Agricultural University,Shenyang, China
Bioenerg. Res. (2015) 8:388–401DOI 10.1007/s12155-014-9527-4
producing a uniform feedstock commodity for bioenergy ap-plications . Among these technologies, pelleting andbriquetting are the two most widely used. Briquetting hasadvantages over pelleting in terms of feedstock process vari-ables such as particle size, where bigger particle sizes can beused. In general, briquettes produced using a hydraulic presshave uniform shape and size, typically 40×40-mm cylinders,and unit densities of about 700–800 kg/m3. The quality ofbriquettes or pellets can be managed by proper control ofprocess and feedstock conditions, formulation, and use ofadditives. A number of researchers have discussed in detailhow process variables (i.e., die temperature, compressionpressure, die geometry, and retention time), feedstock vari-ables (i.e., moisture content and particle size and shape), andbiomass composition (i.e., protein, fat, cellulose, hemicellu-loses lignin, etc.) play a major role in densified biomassquality [3–13]. Pelleting at high die temperatures can resultin plasticization of components like protein and starch, andsoftening of lignin . Starch, if present in biomass, gelati-nizes and lignin undergoes softening due to temperature andpressure and helps in biomass binding . Also, the pressureapplied can result in crushing of biomass particles, thus open-ing up the cell structure and exposing the protein and pectin,which can act as a natural binder. Feedstock moisture contentis considered an important process variable as it impacts thephysical properties of briquettes produced . Low-moisture,chopped corn stover (5 to 10 %, w.b.) resulted in more dense,stable, and durable briquettes than high-moisture stover (15 %w.b.) . Grover and Mishra  indicated that agriculturalmaterials can be briquetted even at 22 % (w.b.) feedstockmoisture content. During briquetting, the moisture in thematerial forms steam under high compression pressure, hy-drolyzing the hemicellulose and lignin into lower molecularcarbohydrates, lignin products, sugar polymers, and otherderivatives which act as adhesives and bind the particlestogether . The same authors indicated that a briquette pressis more flexible in terms of particle size where bigger particlesare suitable as bonding is more due to interlocking of biomassparticles. In general, briquetting or cubing may require coarsegrinding or one-stage particle size reduction, whereas pellet-izing requires two or more stage particle size reduction, mak-ing for a lower energy requirement of the process. Kaliyan andMorey [6, 7] and Adapa et al.  investigated the effects ofparticle size on the densification characteristics of corn stover,switchgrass, and alfalfa and concluded that decreasing thegeometric mean particle size helps to increase the density ofdensified biomass.
In most of the earlier studies, researchers have used particlesizes of 19 mm in combination with other process
variables influence the quality of the briquettes produced.Also, making briquettes using bigger particle sizes can elim-inate more than two-stage grinding (which is typically used togrind biomass to smaller particle sizes), and can reduce theoverall briquette production cost. The specific objective of thispresent study is to investigate the effect of larger hammer millscreen size in the range of 19.1 to 31.75 mm, compressionpressure in the range of 7.5 to 12.5 MPa, feedstock moisturecontent in the range of 9 to 15% (w.b.), and die temperature inthe range of 90 to 130 °C on briquette physical qualityattributes, such as moisture content, unit density immediatelyafter compaction (density-1), unit density after 2 weeks ofstorage (density-2), and durability rating (DR).
Materials and Methods
Four types of agricultural biomass residues (i.e., barley, cano-la, oat, and wheat straws) were collected from a farmer in theCentral Butte area of Saskatchewan, Canada in September2008 in the form of bales typically having dimensions of0.45×0.35×1.00 m . Adapa et al.  used these strawsin their composition investigation (shown in Table 1). Table 1indicates that the composition of agricultural straws varieswith feedstock variety. The straw samples were ground ini-tially using a chopper (fabricated by Bioprocessing Lab,Department of Agricultural and Bioresource Engineering,University of Saskatchewan ) with no screens and furtherground using a hammer mill (Serial no. 6 M13688; GlenMillsInc., Maywood, NJ) using screen openings of 19.1, 25.4, and31.75 mm . The ground straw samples were further usedfor briquetting studies. Each of these ground straw sampleswere conditioned to a moisture content of 9, 12, and 15 %
Table 1 Chemical composition of barley, wheat, oat, and canola straws
Composition (% DMa) Barley Canola Oat Wheat
Protein 3.62 6.53 5.34 2.33
Fat 1.91 0.69 1.65 1.59
Starch 0.11 0.34 0.12 2.58
Lignin 17.13 14.15 12.85 13.88
Celluloseb 33.25 42.39 37.60 34.20
Hemicellulosec 20.36 16.41 23.34 23.68
Ash 2.18 2.10 2.19 2.36
aDM dry matterb Cellulose percentage is calculated indirectly from percentage ADF andlignin (%ADF -%lignin) c Hemicellulose percentage is calculated indirectly from percentage NDFand ADF (%NDF–%ADF) 
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(w.b.), respectively, placed in Ziploc bags and stored in controlstorage set at 4 °C (52 % relative humidity). The moisturecontent of the straw samples was determined using ASABEstandard S358.2 .
In order to investigate the effects of process variables onphysical quality attributes (feedstock moisture content, densi-ty immediately after compaction (density-1) and after 2 weeksof storage (density-2), and durability rating (DR) of biomassbriquette), experiments were designed at three levels of com-pression pressure (7.5, 10, and 12.5 MPa), die temperature(90, 110, and 130 °C), feedstock moisture content (i.e., 9, 12,15 %, w.b.), and hammer mill screen size (19.05, 25.4, and31.75 mm), while keeping the retention time of choppedbiomass in the die for 30±2 s. Table 2 indicates the actualand coded levels of the variables selected based on Box-Behnken experimental design. Combination of the four inde-pendent variables at three levels yielded 27 experimentalcombinations that were repeated ten times. Using the fourground biomass samples (canola, barley, wheat, and oat),1,080 experimental briquetting runs in total were conducted.
Hydraulic Briquette Press
A laboratory hydraulic press (Model 10HP, Serial A4569,Curtis Hoover Ltd., Edmonton, Alberta, Canada) reported byMani et al.  was used in the briquetting experiments of theselected agricultural biomass residues (Fig. 1). The pressconsisted of an upper and lower hydraulic-driven ram movinginside an electrically heated die. This briquetting machine hasa cylindrical die, an upper ram that moves down through die,and a lower ram that is held in place and acts as support. Thedie is connected to a heater that heats the chopped biomass tobe compacted. Six type “T” thermocouples were installed onthe die to measure and control its temperature. Five out of thesix thermocouples were connected to the data acquisitionsystem, while the sixth one was connected to a heater withtemperature controller (Chromalox Temperature Control, Cat.No. AR-5529 [Edwin L. Wiegan Co., Pittsburgh, PA]). The
die diameter was about 30 mm; the length of the briquettevaried depending on the mass of the feed material and com-pression pressure used for briquetting. Two pressure transduc-ers, one measuring the top hydraulic-driven ram (upper ram)(Daytronic Model 502–30000 (range 0–3000 psig),Daytronic, Dayton, OH) and the other measuring the bottomhydraulic-driven ram (lower ram) (Schaevitz, Model P1021-0005 (range 0–3,500 psig), Schaevitz, Hampton, VA) wereused. Both pressure transducers were calibrated to measurecompression pressures in the range of 0–17.24 MPa (0–2,500 psi). The amplifier linked to the compartment is usedto get an output voltage in the range of 0–5 V. The amplifierproduces 1 V for each 3.45 MPa (500 psi). A displacementtransducer was used to measure the upper ram displacement.
Figure 2 indicates the process flow diagram for makingbriquettes using agricultural straws. The agriculturalstraws were initially chopped and later ground using ahammer mill. Tumuluru et al.  has discussed, in detail,the chopper and hammer mill used for grinding of theagricultural straws used in the present briquetting study.The moisture content of the ground biomass was furtheradjusted to the desired levels per experimental design.The mass of moisture adjusted straw samples used formaking briquettes was 10±0.05 g. Three preset loads of5,298, 7,065, and 8,831 N (corresponding to compressionpressures of 7.5, 10, and 12.5 MPa, respectively) wereused to compress the samples in the die. The lower ram,when held in place, acts as support and does not movedown before ejection. The upper ram was moved at aspeed of 0.04 m/s; once the desired preset load wasattained, the upper ram was retained in place for 30±2 sin order to avoid a spring-back of the compressed bio-mass. The compression pressure of the upper ram wasgreater than the lower ram during residence time becausethere are other forces involved, such as frictional force toextrude the briquette. The ejection speed used was0.013 m/s. Thereafter, the briquette was ejected out ofthe barrel by moving down the upper and lower rams.Ten briquette replicates were made using each of thetreatment combinations. After compression, the dimen-sions (i.e., height and diameter) and mass of all of thebriquettes were measured to determine the initial unitdensity (density-1). Samples were then stored for 2 weeksin Ziploc bags at room temperature of 20 °C. This storageperiod will allow the briquettes to reach a final unitdensity with greater dimensional stability. After 2 weeksof storage, the briquettes were further evaluated for bri-quette moisture content, final unit density (density-2), anddurability rating.
Table 2 Actual and coded levels of briquetting process conditions
Independent variables Code Coded levels
−1 0 1
Die temperature (°C) x1 90 110 130
Feedstock moisture content (%) x2 9 12 15
Compression pressure (MPa) x3 7.5 10 12.5
Hammer mill screen size (mm) x4 19.1 25.4 31.75
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Physical Quality Attributes
The unit densities of each of the briquette samples immedi-ately after compaction (density-1) and after 2 weeks of storage(density-2) were calculated based on ASABE StandardS269.4  by direct measurement. Durability rating (DR)and briquette moisture content were measured after 2 weeks ofstorage. The briquette moisture content after the durability testwas determined based on ASABE Standard S358.2 .Durability rating (DR) represents the measure of shear andimpact forces that briquettes could withstand in the course ofhandling, storage, and transportation processes [6, 12, 23].The briquette DR was tested based on ASABE StandardS269.4  by tumbling ten briquettes (representing the tenreplicates of each combination) in a tumbling durability testerfor 3 min at 40 rpm. According to this procedure, for particleswith a mass of more than 20 %, the average original particle
mass are designated “CSM” (cube size material). If a sampledoes not produce any CSM in the output sample, the durabilityrating is zero.
Experimental Data Analysis
In general, response surface models are used in process andproduct development in chemical and biological engineeringproblems [24, 25]. A second-order response surface modelwas developed for the briquetting process variables like dietemperature (x1), feedstock moisture content (x2), compres-sion pressure (x3), and hammer mill screen size (x4).Equation 1 indicates the response surface model developedfor briquette quality attributes like briquette moisture content,unit density immediately after compaction (density-1), unitdensity after 2 weeks of storage (density-2), and durabilityrating (DR).
Fig. 1 Laboratory-scale briquettepress
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y ¼ b0þX
bijxixj þ ε ð1Þ
where y is the dependent variable (observed); xi and xj arethe coded independent variables; b0, bi, and bj are coefficients;n is the number of independent variables; and ∈ is a randomerror . Statistica , a statistical software, was used todevelop the response surface models. Analysis of variance(ANOVA) which is used to examine the difference betweengroup means for full second-order response surface model,and was used to understand the statistical significance oflinear, quadratic, and interactive effect of process variables. Major limitations of response surface methodology(RSM) to optimize a process accurately when the system iscomplex and has a high degree of interaction between processvariables.
Optimization using evolutionary algorithms like geneticalgorithm (GA), which are computational methods, aregaining importance to solve complex processes. GAs areadaptive heuristic search algorithms that mimic the processesof biological evolution [28–31]. GAs have been successfullyused for highly complex and nonlinear systems where theobjective is to either maximize or minimize function [28,30]. Shankar and Bandyopadhyay , Shankar et al. ,and Tumuluru et al.  successfully used GA to optimize theextrusion process conditions for physical properties, volumet-ric flow rates, and proximate composition of the extrudatesduring single screw extrusion. In the present study, the GAsoftware developed by Shankar and Bandyopadhyay , and
Shankar et al.  in C and VC++ language was used.Response surface models developed for the briquettequality attributes were used as the objective functionsto find the optimum process conditions which can min-imize briquette moisture content and maximize density-1, density-2, and DR. In the present problem, a fixedpopulation of 100, fixed number of iterations of 100(which is the stopping criteria of the algorithm), andcrossover and mutation probabilities of 0.98 and 0.02were used .
Tumuluru et al.  discussed in detail the densities ofbarley, wheat, canola, and oat straws ground using ham-mer mill fitted with screen sizes of 19.1, 25.4, and31.75 mm. The bulk densities of these ground strawssamples were in the range of 36 to 67, 37 to 58, 48 to58, and 40 to 58 kg/m3, respectively. Experimental resultsindicated that briquetting of these agricultural straw sam-ples has increased the unit density by about 8–12 times atdifferent briquetting process conditions (Table 3). Theaverage briquette unit densities (density-1: immediatelyafter compaction and density-2: after 2 weeks of storage),briquette moisture content, and durability rating (DR)values of barley, canola, oat, and wheat straw briquettes,based on Box-Behnken experimental design, are listed inTable 3. The combination of the four independent processvariables (die temperature, compression pressure, hammermill screen size, and feedstock moisture content) produceddifferent quality briquettes. The standard deviation valuesobserved for briquette moisture content and density-1 anddensity-2 were in the range of 0.01 to 0.05 % (w.b.) and20 to 50 kg/m3, for the four different feedstocks studied.The observed experimental results indicated that feedstockmoisture content played a major role in the biomassbriquetting process. Briquette densities decreased with in-creasing feedstock moisture content. Briquettes producedat a low feedstock moisture content of 700 kg/m3
for all straw samples, whereas briquettes at 15 % (w.b.)produced the lowest unit densities with more cracks ob-served on the surface. The change in briquette unit densityafter 2 weeks of storage for barley, canola, oat, and wheatstraw briquettes were in the range of 1.3 to 9, 0 to 2.7, 0to 6.1, and 0 to 11.4 %, respectively. The observed DRvalues were higher at a lower feedstock moisture contentand higher die temperature. Figure 3 shows typical bri-quettes manufactured using canola, wheat, oat and barleystraw samples.
Hammer mill (31.75, 25.4, 19.05-mm screen) Stage-2
Initial moisture measurement
Overnight storage in a control environment (4 °C, RH-52%)
Initial density immediately after compaction (Density-1)
Briquetting with heat addition
Stored for 2 weeks at room temperature (20 °C)
Physical properties (briquette moisture content, final briquette
density (Density-2), and durability rating)
Chopper (no screen) Stage-1 grinding
Fig. 2 Process flow chart for briquetting of agricultural straws
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Response Surface Analysis
Regression equations developed for the physical quality attri-butes of the briquettes (briquette moisture content, initial unitdensity immediately after compaction (density-1), final unitdensity after 2 weeks of storage (density-2), and durabilityrating (DR)) in terms of die temperature, compression pres-sure, feedstock moisture content, and hammer mill screen sizein real values are provided in Table 4. To understand thesignificant effect of linear, quadratic, and interactive effect ofprocess terms in the response surface model with respect toquality attributes (density-1, density-2, DR, and briquette mois-ture content), ANOVAwas performed. Response surface plots
were drawn using the regression equations (see Table 4) devel-oped for briquette quality attributes (moisture content, density-1,density-2, and DR). Response surface plots were drawn bychanging two process variables, whereas the other two variableswere kept constant at the center point of the experimental design.
Briquette Moisture Content
Moisture content (x2) of the four feedstocks had a positivecorrelation and had an extremely significant (p
and wheat straw briquettes, but it had no significant effect onbarley and oat straw briquettes. Compression pressure (x3)also played a significant effect on wheat and oat briquettes atp0.01) thefeedstocks, whereas compression pressure was significant forcanola (Fig. 6), barley, and oat (figures not shown) and screensize for wheat straw briquettes. Density-2 values in the range of650 to 700 kg/m3 were maintained after 2 weeks of storage,when briquettes were prepared at a low initial feedstock mois-ture content of 9 % (w.b.) and higher compression pressure of12.5 MPa for canola, barley, and oat straw briquettes. In thecase of wheat straw briquettes, however, a low initial feedstock
Fig. 3 Wheat, barley, canola, andoat straw briquettes produced atdifferent experimental conditions
394 Bioenerg. Res. (2015) 8:388–401
moisture content of 9 % (w.b.) and larger screen size of31.75 mm resulted in final bulk density values of 600 to650 kg/m3 after 2 weeks of storage.
ANOVA indicated that die temperature (x1) had a positivecorrelation with briquette durability rating (DR), while feed-stock moisture content (x2) had a negative correlation. Thehigh die temperature (x1) used in producing briquettes resultedin better DR, which made the briquettes withstand the shear,
impact, tumbling, and rotational forces during DR testing. Thehigh die temperature assisted in the local melting of thebiomass binding constituents (such as lignin), which subse-quently leads to the mechanical interlocking of the biomassparticles . These results confirmed the high values of DRin Table 3, where a high die temperature (130 °C) withappropriate combinations of other material and process vari-ables resulted in high-durability briquettes. Die temperature(x1) and feedstock moisture content (x2) significantly affected(p
significant effect on the DR values of barley, canola, andwheat straw briquettes, but had significant effect (p95 %
were predicted at high die temperature of 130 °C and a lowerfeedstock moisture content of 9 % (w.b.). In the case of canolastraw briquettes, both high and low feedstock moisture con-tents yielded highly durable briquettes at high die temperature.Lowering the die temperature and increasing the feedstockmoisture content reduced the DR values. For barley strawbriquettes, feedstock moisture content was the influencingprocess variable, where at 9 % (w.b.), it resulted in maximizing
Fig. 4 Moisture content ofcanola straw briquette withrespect to feedstock moisturecontent and die temperature
Fig. 5 Density-1 (unit densityimmediately after compaction) ofcanola straw briquette withrespect to compression pressureand feedstock moisture content
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DR values to >90 % in the temperature range of 90 to 130 °Cusing medium screen size of about 24 mm. The responsesurface plot analysis of the experimental data indicated trendsof the process variables which can maximize density-1, den-sity-2, and DR and minimize the briquette moisture contentbut could not precisely pinpoint the optimized processconditions.
The response surface models developed were further opti-mized using genetic algorithm to identify the process condi-tions to maximize density-1, density-2, and DR and minimizethe briquette moisture content (Table 5). Higher die tempera-tures of >122 °C maximized density-1, density-2, and DR andminimized the briquette moisture content, except for canolawhere lower die temperature of about 93 °C was found to be
Fig. 6 Density-2 (unit densityafter 2 weeks of storage) of canolastraw briquette with respect tocompression pressure andfeedstock moisture content
Fig. 7 Durability rating of canolastraw briquette with respect to dietemperature and feedstockmoisture content
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sufficient. Low feedstock moisture content of about 9% (w.b.)maximized density-1, density-2, and DR values and mini-mized the briquette moisture content for all the feedstocks,except for wheat and canola where medium (11.81 %, w.b.)and high (14.52 %, w.b.) feedstock moisture content werenecessary. Medium to high compression pressures (10–12.5 MPa) maximized the unit densities of all feedstocks.Large hammer mill screen sizes of >30 mm maximized den-sity-1, density-2, and DR and minimized the briquette mois-ture content for wheat and canola, for barley and oat mediumhammer mill screen size particle size (23.51 mm) and lower(19.28 mm) maximized the DR values.
Grover and Mishra  reported that low feedstock moisturecontent (8 to 10 %, w.b.) is optimal for manufacturing ofbiomass briquette free of cracks. The right amount of moisturein the biomass acts as a facilitator of natural binding agentsand as a lubricant [3, 6]. Many studies have indicated thatfeedstock moisture content below 8 % (w.b.) or above 12 %(w.b.) would lead to lower-quality pellets or briquettes in
terms of density and durability [11, 16, 36–39]. In general,an increase in feedstock moisture content to >15 % couldresult in a decrease in pellet or briquette densities [10, 35,40–43]. Tumuluru et al. , in their recent studies, indicatedthat pelletization at a feedstock moisture content of 38 %(w.b.) decreased the density by about 50 % compared topellets made at a low feedstock moisture content of 10 %(w.b.). In general, a high feedstock moisture content of 15 %(w.b.) results in expansion of the briquette volume causing adecrease in the density . However, the percentage decreasein density depends on the type of biomass pelleted orbriquetted. Therefore, a feedstock moisture content of about10 % (w.b.) is considered optimal to obtain highly dense anddurable briquettes, which has been corroborated by the presentfindings. High briquette moisture content may not be desirablein thermochemical conversion applications because it reducesthe thermal efficiency due to heat being used to drive off thewater. The storage stability of high moisture briquettes isreduced because it hastens the product’s biological degrada-tion and increases the dry matter losses. Lowering the bri-quette moisture to
transportation losses. The present results indicated that canolastraw briquetted better at low feedstock moisture content andhigh die temperature to produce briquettes with high initialand final densities when compared to wheat, barley, and oatstraws. The reason for this may be due to the higher proteincontent in the canola straw, which may have resulted in theformation of protein-fat complexes in the presence of heat andcompression pressure, resulting in better binding.
In almost all cases, there was a decrease in the unit densityof the briquettes after 2 weeks of storage. The reason for thiswas due to the dimensional instability, which was more sig-nificant in the axial direction (height) than in the lateraldirection (diameter), which in some cases tend to be verymarginal depending on the combination of the material andoperating variables. A similar expansion trend was also re-ported by Mani et al.  on the compaction of corn stoverand by Al-Widyan et al.  on the stability of olive cakebriquettes. Dimensional stability is mainly dependent on feed-stock moisture content and compression pressure. The presentstudy indicated that a change in density or decrease in dimen-sional stability is more significant at a high feedstock moisturecontent of 15 % (w.b.) and lower compression pressure of7.5 MPa. The high feedstock moisture content and low-compression pressure may not have provided the enablingcondition for mechanical interlocking and the binding effectof the particles. The present results suggest that briquettesmade with canola straw have the lowest change in density,indicating a higher dimensional stability during storage. Thepositive correlation of the screen size indicates that largehammer mill screen size may result in briquettes with highdensity as much as when an appropriate combination withfeedstock moisture content, die temperature, and compressionpressure is made.
At high die temperature (130 °C) and low feedstock mois-ture (9 to 12 %, w.b.), briquettes with a high DR wereproduced. Present results show that large hammer mill screensizes resulted in higher durability due to better interlocking ofparticles during the briquetting process. Also, high die tem-peratures associated with the briquetting process may result inthe melting points of some constituents (such as lignin) beingapproached, alongside local melting of the materials, resultingin the binding of the particles. The present study indicates thata combination of low feedstock moisture (9 %, w.b.) and highcompression pressure and die temperature and large hammermill screen size resulted in higher durability rating values.However, in some cases, the durability rating decreased at ahigh compression pressure (12.5 MPa) due to high feedstockmoisture content (15 %, w.b.), which then resulted in surfacecracks. Mani et al.  reported similar observations wherehigh feedstock moisture content (15 %, w.b.) was predomi-nantly responsible for low durability rating in briquettes. Inthis study, the lowest DR values are produced at 15 % (w.b.)moisture, except for canola, where low and high feedstock
moisture content favored high durability values at high dietemperatures of >120 °C.
This investigation indicates that low to high compressionpressures of about 7.5 to 12.5 MPa can produce better qualitybriquettes only if an optimal combination with other indepen-dent variables are selected. This implies that energy costsincurred by high-compression pressure application duringthe densification process can be reduced if biomass withoptimal feedstock moisture and particle size and die temper-atures are used. According to Mani et al. , compressionpressure has the highest effect on the total energy consump-tion. The screen size of the grinder plays a role in thebriquetting process. However, its effect is not as significantas feedstock moisture content, compression pressure, and dietemperature. This study indicated that using large hammermill screen size in appropriate combination with other materialand process variables could help to produce good qualitybriquettes, which might reduce the production cost. Maniet al. , Kaliyan and Morey , and Tumuluru et al. reported that the energy consumption for grinding increasesexponentially as the screen size becomes smaller. Because abriquette is used for direct combustion, a large screen size (25to 32 mm) can be used for briquette formation, while avoidingthe high cost of grinding. In the present study, the optimizedprocess conditions identified for briquetting of different feed-stock are limited for the experimental design followed. Asmentioned earlier, the response surface methodology is a verypopular technique that has been used extensively in differentchemical processes to understand process variable effect andtheir interaction effect. With that being said, the optimizedprocess conditions identified in this paper for the responsesurface models can be further used for scale-up of thebriquetting process for the feedstocks studied using evolution-ary operation (EVOP) method as proposed by Box and Hunter.
Canola straw produced briquette with a higher initial unitdensity (density-1) and durability rating (DR) compared tothe other three biomass feedstocks. Good quality briquettes, interms of unit density and durability, can be produced with lowtomedium feedstockmoisture contents of 9 to 12% (w.b.) andhigh die temperature (130 °C) and high compression pressure(12.5 MPa). ANOVA indicated that die temperature was pos-itively correlated with DR and did not significantly affect theunit density, while compression pressure was positively cor-related with the briquette unit density. Regressions equationsdeveloped for barley, wheat, oat, and wheat straw briquetteproperties (moisture content, unit density immediately aftercompaction (density-1), unit density after 2 weeks of storage(density-2), and DR) have adequately described the process
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based on the coefficient of determination values. Analysis ofthese equations using RSM and GA indicated that a lowerfeedstock moisture content of about 7.5 to 9 % (w.b.), higherdie temperature of about 120 to 130 °C, a medium to largerhammer mill screen size of 25 to 31.75 mm, and low to highcompression pressure of 7.5 to 12.5 MPa minimizes thebriquette moisture content to 700 kg/m3. A maximum DRof >90 % is achievable at high die temperature of 130 °C, alow to medium feedstock moisture content of 9 to 11% (w.b.),low to high compression pressure of 7.5–12.5 MPa, and alarge hammer mill screen size of about 31.75 mm, except forcanola where a low compression pressure of 7.5–8.5 MPa andfor oat, small screen size of about 19 mm are desirable formaximizing the DR values.
Acknowledgments This research was supported financially, in part, bythe Cellulosic Biofuel Network of Agriculture andAgri-Food Canada andDepartment of Energy, Office of Energy Efficiency and RenewableEnergy under DOE Idaho Operations Office Contract DE-AC07-05ID14517. The authors would like to acknowledge Louis Roth and BillCrerar of University of Saskatchewan, and David L. Combs of IdahoNational Laboratory’s R&D Publications Support Team for the graphicsassistance.
Conflict of Interest & U.S. Department of Energy Disclaimer Nocompeting financial interests exist. This information was prepared as anaccount of work sponsored by an agency of the US government. Neitherthe US government nor any agency thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legal liabilityor responsibility for the accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, or represents thatits use would not infringe privately owned rights. References herein toany specific commercial product, process, or service by trade name,trademark, manufacturer, or otherwise, do not necessarily constitute orimply its endorsement, recommendation, or favoring by the USgovernment or any agency thereof. The views and opinions of the authorsexpressed herein do not necessarily state or reflect those of the USgovernment or any agency thereof.
Open Access This article is distributed under the terms of the CreativeCommons Attribution License which permits any use, distribution, andreproduction in any medium, provided the original author(s) and thesource are credited.
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Impact of process conditions on the density and durability of wheat, oat, canola, and barley straw briquettesAbstractIntroductionMaterials and MethodsFeedstocksExperimental PlanHydraulic Briquette PressBriquetting ProcessPhysical Quality AttributesExperimental Data Analysis
ResultsResponse Surface AnalysisBriquette Moisture ContentUnit Density: Immediately After Compaction (Density-1), and After 2&newnbsp;Weeks of Storage (Density-2)Durability Rating