Bioresource Technology 101 (2010) 234–238

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Bioresource Technology

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Compaction of poultry litter

M. Bernhart a, O.O. Fasina a,*, J. Fulton a, C.W. Wood b

a Department of Biosystems Engineering, Auburn University, Auburn, AL 36849, United Statesb Department of Agronomy and Soils, Auburn University, Auburn, AL 36849, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 October 2008Received in revised form 3 August 2009Accepted 4 August 2009Available online 3 September 2009

Keywords:DensityStorageBreakage forceSpecific energy

0960-8524/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.08.030

* Corresponding author. Address: 214 Tom Corleyneering Department, Auburn University, Auburn, AL 3334 844 3574; fax: +1 334 844 3530.

E-mail address: fasinoo@auburn.edu (O.O. Fasina).

Poultry litter, a combination of accumulated chicken manure, feathers and bedding materials, is a poten-tial feedstock for bioenergy and other value-added applications. The use of this waste product has beenhistorically limited to within few miles of the place of generation because of its inherent low density.Compaction is one possible way to enhance the storage and transportation of the litter. This study there-fore investigates the effect of moisture content (19.8–70.7%, d.b.) and pressure (0.8–8.4 MPa) on the com-paction characteristics of poultry litter.

Results obtained showed that the initial density of densified poultry litter, energy required for compac-tion and the strength of the densified material after 2 months of storage were significantly (P < 0.05)affected by moisture content and pressure applied during compaction. The density of the compactedmaterial was only affected by pressure applied during compaction after 2 months of storage. The specificenergy required to produce the densified material varied from 0.25 to 2.00 kJ/kg and was significantlyless than the energy required to produce pellets from biological materials. The results obtained fromthe study can be used for the economical design of on-farm compaction equipment for poultry litter.

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1. Introduction

Compactibility is defined as the ability of a bulk solid to beagglomerated into a compact form of specified strength. This dif-fers from compressibility in that compressibility involves the abil-ity of a bulk material to decrease in volume under pressure(Leuenberger and Jetzer, 1984). Compressibility studies are there-fore useful in diagnosing flow problems in bulk solids while themain focus of compactibility studies is to enhance storage and han-dling properties of bulk solids through increased density of thematerial (Demirbas, 1999).

Poultry litter is a low density material (bulk density less than500 kg/m3 – Bernhart and Fasina, 2009). This makes it costly totransport poultry litter from production sites to areas where itcould be effectively utilized for value-added applications such asin soil fertilization and bioenergy/bioproduct applications (Colleyet al., 2006). Biosecurity is also a concern when poultry litter isto be transported over long distances and when using shared trans-portation equipment. Poultry litter is dusty, and is known to con-tain pathogens such as Escherichia coli (Nandi et al., 2004). Thus,during transportation, pathogen infected dust maybe released intothe atmosphere. This in turn could lead to the spreading of poultry-

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related diseases in locations where the litter is transportedthrough. This biosecurity problem could be considerably mini-mized if the poultry litter is densified because of the possiblereduction in dispersion of dust and dust-laden microorganismsduring transportation of the litter.

Traditionally, agricultural materials are densified into pellets,cubes, and bales. Pellets are the densest of these agglomerates.Therefore, they require the highest amount of input energy (19–90 kJ/kg) during manufacturing (Tabil and Sokhansanj, 1996; Kali-yan and Morey, 2006; Colley, 2006). This high energy input makesit uneconomical and impractical for broiler farmers to purchaseand operate a pellet mill. Similar to the production of pellets, pres-sure agglomeration is used to manufacture cubes except that lowerpressures are used. Production of cubes is presently limited to for-age crops such as alfalfa. As the name implies, cubes are usually inthe form of a square cross section, manufactured from choppedbiomass and typically vary from 12.7 to 38.1 mm dimensionally(Sokhansanj and Turhollow, 2004).

Baling is a process that combines compression and packing (ty-ing with twines or wrapping) operations. It is typically used forgrassy or fibrous-like materials that are stringy in nature (Badger,2003). Bale dimensions are roughly 44 in by 49 in (1026 by1040 mm; Badger, 2003). Poultry litter is not stringy and thereforethe low-cost volume reduction method that can be developed forpoultry litter is limited to pressure agglomeration. The objectiveof this study is to optimize the compaction of poultry litter for effi-cient transportation and off-site utilization. This will be achieved

M. Bernhart et al. / Bioresource Technology 101 (2010) 234–238 235

by determining (a) effect of moisture content on the minimumpressure required to manufacture densified poultry litter and (2)energy required to compact poultry litter.

2. Methods

2.1. Sample preparation

The poultry litter sample (with wood shavings as the beddingmaterial) used in this study was obtained from Breezy BottomFarm, Altoona, AL. The heating value and composition of the poul-try litter sample have been published elsewhere (Fasina, 2006).

Experimentation was carried out on poultry litter at moisturecontents between 19.8% and 70.7% (dry basis). This was becausepreliminary study showed that the minimum moisture content atwhich poultry litter will compact was 19.8% (dry basis). To adjustthe moisture content of a sample to the desired level, a calculatedquantity of water was added and mixed with poultry litter in amixer for 15 min. The samples were stored in an air tight containerfor 24 h to allow moisture equilibration to take place. The moisturecontent of each sample was then verified by a moisture analyzer(Model IR-200, Denver Instruments, Arvada CO). All moisture con-tents are reported in dry basis unless otherwise noted.

700

800

900

1000

1100

1200

1300

4 5 6 7 8 9

Dens

ity (k

g/m

3 )

Pressure (MPa)

19.8% 27.7% 31.9%

35.3% 40.8% predicted

Fig. 1. Effect of initial moisture content (dry basis) and pressure on the density ofdensified poultry litter (first experiment).

2.2. Compaction

The compaction apparatus consisted of a 27.3 mm diameter die.The die had a length of 135.2 mm and was composed of two semi-circular halves that were held together by clamps. Before each test,the inside of each half was lubricated with vegetable oil to mini-mize the amount friction between the poultry litter and the die,and to ease the removal of the compacted litter from the die. Thetwo halves were then clamped together and the die was filled withpoultry litter. A plunger (25.6 mm diameter, 124.6 mm length) at-tached to the crosshead of a texture analyzer (model TA-HDi, Sta-ble Microsystems, Surrey, UK) was used to compress the sample ata speed of 1 mm/s until the desired force was reached. The com-pacted sample was held in a creep mode (i.e. constant force) fora period of 60 s after the desired force was reached.

Two sets of compaction experiments were carried out. The firstset of experiments was carried out on samples at moisture con-tents of 19.8%, 27.7%, 31.9%, 35.3%, and 40.8% (dry basis). The sam-ples at these moisture contents were exposed to compactionpressures of 4.2, 5.0, 5.9, 6.7, 7.5 and 8.4 MPa. The results from thisexperiment were then used to design the second set of experi-ments (see Section 3) that involved compacting poultry litter sam-ples at higher moisture contents (46.4%, 54.1%, 60.5%, 70.7%, drybasis). The goal of the second experiment is to reduce the pressureat which poultry litter can be densified by increasing moisture con-tent. Therefore, samples were compacted at pressures of 0.8, 1.7,2.5, 3.4, 4.2, 5.0 and 5.9 MPa for the second set of experiments.

Force and deformation data were automatically acquired andstored by the software provided by the manufacturer of the textureanalyzer. The force–deformation data was used to calculate the en-ergy expended during the compaction process. Once compactionwas completed, the agglomerated sample was removed from thedie and stored in the laboratory (temperature of 22 �C and a rela-tive humidity of 45%) for a period of 2 months. The agglomeratedsamples were exposed to atmospheric air in the laboratory duringthe storage period to mimic the way the process will be handled atthe farm level and to prevent the growth of microorganisms (dueto high moisture content of samples after compaction).

A digital vernier caliper (Solar ABSOLUTE Digimatic Caliper,Model CD-S6”C, Mitutoyo Corporation, Japan) was used to measurethe length and the diameter, while a digital balance accurate to

0.001 g (Model AR3130, Ohaus Corp., Pinebrook, NJ) was used tomeasure the mass of freshly made and stored densified material.The densities of the samples were calculated from the ratio of massto volume (obtained from length and diameter measurements). Inaddition, the strengths of the densified samples from the secondset of experiment (i.e. those that were adjusted to moisture con-tents of 46.4–70.7%) were also obtained by means of a 12.7 mmdiameter round probe (Model TA-23, Texture Technologies, Scars-dale, NY). This was carried out by placing a densified sample on aflat plate in its natural position (i.e. radial dimension was in thesame direction as that of the compressive force). The probe (at-tached to the crosshead of the texture analyzer) was subsequentlyused to compress the sample at a speed of 1 mm/s. The force re-quired to rupture the sample was obtained from the maximumforce in the force–deformation curve. All of the procedures out-lined in the previous section were carried out in duplicate.

2.3. Data analysis

Regression analysis was performed using the proc reg functionin SAS statistical software package (Version 9.1, SAS Institute Inc.,Cary, NC, 2002–2003) and plotted with the experimental datausing Microsoft Excel (Microsoft Office XP Professional, 2005). Sig-nificance testing was carried out using proc anova function in theSAS statistical package.

3. Results and discussion

3.1. Density of poultry litter compacts

Results from the first set of experiment showed that poultry lit-ter could not be agglomerated when applied pressure was less than5.0 MPa within the moisture content range of 19.8% and 40.8%(d.b.). Beyond this pressure level, the density of compacts in-creased with increase in applied pressure and moisture content(minimum of 787 kg/m3 and maximum of 1196 kg/m3) (Fig. 1).According to Pietsch (2002), the most common binding mechanismof wet agglomeration (such as what was carried out in this study)is liquid bridges that were developed from free water at the coor-dination points between the particles forming the agglomerate.Moisture therefore acted as binder during the agglomerationprocess.

Eq. (1) was fitted to the experimental density values as a func-tion of moisture content and pressure.

0

200

400

600

800

1000

1200

0.84 1.7 2.5 3.4 4.2 5 5.9 6.7

Den

sity

(kg/

m3 )

Pressure (MPa)

before storageafter storage

Fig. 3. Effect of 2 months of storage on density of compacted poultry litter.Compaction was carried out at moisture content of 60.5% (dry basis).

236 M. Bernhart et al. / Bioresource Technology 101 (2010) 234–238

qcompact ¼ 159:994þ 20:343 �M þ 187:075 � LnðrÞ; R2 ¼ 0:907

ð1Þ

19.8 6M 6 40.8%, dry basis; 5.0 6 r 6 8.4 MPa.Mani et al. (2006) found that moisture content and pressure sig-

nificantly affected the compression of switchgrass, wheat strawand barley straw into pellets. The pellet density obtained from thiswork varied from 600 to 1136 kg/m3. Similarly, Demirbas (1999)found a logarithmic relationship between applied pressure andresulting density of briquettes manufactured from waste paperand wheat straw mixtures and applied pressure. The author alsofound that briquetting density increased with moisture content.Even though considerable higher pressures were used in thesestudies (300–800 MPa), the briquette densities obtained (50–850 kg/m3) were lower than the densities obtained in this study.We suspect this is due to the much lower moisture content ofthe samples (7–18%, dry basis).

The second set of compaction tests was conducted at highermoisture values (46.4–70.7%, dry basis) and lower pressure levels(0.8–5.0 MPa) with the hypothesis that less energy would be re-quired to compact poultry litter at higher moisture contents, sincemoisture is acting as a binder during the compaction process. Theresults obtained showed that this hypothesis is true as poultry lit-ter could be agglomerated at pressure as low as 0.84 MPa. In addi-tion, it was found that it was not practically possible to applypressures greater than 5.0 MPa on poultry litter that were condi-tioned to moisture contents of 60.5% and higher because duringthe agglomeration process, the poultry litter began to exhibit fluidlike (or slurry) behavior and therefore could not be contained inthe die. Fig. 2 shows that within the pressure range of 0.8 and5.9 MPa, the density of the high moisture content compacted sam-ples increased in a logarithm fashion with applied pressure andmoisture content (Eq. (2)).

qcompact ¼ 574:570þ 7:363 �M þ 125:778 � LnðrÞ; R2 ¼ 0:831

ð2Þ

46.4% 6M 6 70.7%, dry basis; 0.8 6 r 6 5.9 MPa.

3.2. Storage effect on density of compacts

During the 2 months of storage, the densified poultry litter sam-ples lost moisture to an average moisture content of 10.4 ± 0.4%(dry basis). Consequently, this resulted in changes in the densityof the samples (Fig. 3). Despite the fact that moisture content

Fig. 2. Effect of initial moisture content (dry basis) and pressure on the density ofdensified poultry litter (second experiment).

and pressure significantly affect the density to which the samplescan be compacted (Figs. 1 and 2), this was not the case for samplesthat have been stored for 2 months under laboratory conditions.Statistical testing (P < 0.05) showed that within the moisture rangeof 19.8–70.7% (dry basis) only pressure had a significant effect onthe density of the stored densified samples (Fig. 4). The averagedensity values at the various moisture contents were thereforeused to obtain logarithmic relationship between sample density(kg/m3) and the applied pressure (MPa) (Eq. (3)).

qcompact ¼ 592:190þ 101:241 � LnðrÞ; R2 ¼ 0:710 ð3Þ

A drawback of adding moisture to poultry litter to enhance itscompactibility is the possibility for pathogens to grow on the wetdensified litter after being compacted on-farm. However, our studyshowed that the densified material readily loses the added mois-ture within a month of compaction. It should be mentioned thatin practice, farmers often temporarily store poultry litter outdoorswhere they are easily rained upon (Collins, 2009). Therefore, theaddition of moisture to the sample is similar to the natural processthat poultry litter experience during normal storage at the on-farmlevel.

Fig. 4. Logarithmic relationship between density of compacted poultry litter andapplied pressure after 2 months of storage. Notice that this relationship isirrespective of moisture content of poultry litter at the time of compaction. Thevalues in the legend are dry basis moisture content of poultry litter.

Fig. 5. Effect of moisture content (%, dry basis) and pressure on breakage force fordensified poultry litter after 2 months of storage.

M. Bernhart et al. / Bioresource Technology 101 (2010) 234–238 237

3.3. Breakage force of densified poultry litter

As stated in the materials and methods section, breakage forcestudy was carried out only on samples that were obtained from thesecond set of experiments. Fig. 5 shows that the average force re-quired to rupture the densified poultry litter samples (after2 months of storage) vary from 16.6 N to 357.0 N. This ruptureforce range is similar to that required to rupture pellets (McMullenet al., 2005) despite the fact that pellets are manufactured at signif-icantly higher pressure (>50 MPa). Despite the fact that the mois-ture had no significant effect on density of the compactedsamples after 2 months of storage, the force (N) required to rupturethese samples was significantly influenced by moisture content ina linear fashion and by pressure (MPa) in an logarithmic fashion(Eq. (4)). This again confirms that moisture acts as a natural binderduring the agglomeration process. Pietsch (2002) attributed thenatural binder effect of moisture on the fact that the coating ofmoisture on particle surfaces improves the natural adhesion of par-ticles to each other.

Fbreakage ¼ 50:917þ 0:462 �M þ 108:220 � LnðrÞ; R2 ¼ 0:707

ð4Þ

46.4% 6M 6 70.7%, dry basis; 0.8 6 r 6 5.9 MPa.

Fig. 6. Effect of moisture content (%, dry basis) and pressure on the specific energyrequired to compact poultry litter.

3.4. Specific energy for poultry litter compaction

The specific energy required to form agglomerates of poultry lit-ter ranged from 0.190 kJ/kg to 1.763 kJ/kg within the moisture con-tent range of 19.8–70.7% (dry basis) and applied pressure range of0.84–5.9 MPa (Fig. 6). This is significantly lower than the specificenergy required to manufacture pellets from biomass feedstock(typically 19–90 kJ/kg; Colley et al., 2006). By substantially increas-ing the moisture content, it is therefore feasible to reduce the en-ergy required to compact biomass feedstocks by nine fold. Therequired energy (kJ/kg) increased with applied pressure (MPa)and decreased with moisture content (%) in a linear fashion as fol-lows (Eq. (5)):

E ¼ 0:479� 0:0108 �M þ 0:204 � r ð5Þ

4. Conclusion

It can be concluded that both the pressure applied and themoisture content of the poultry litter at the time of compactionsignificantly affect the density of the compacted samples and theenergy required for the compaction. After 2 months of storage,the densities of the compacted samples were only significantly af-fected by the amount of pressure applied during compaction whilethe moisture content no longer had any significant affect. However,the force required to rupture the densified samples after 2 monthsof storage was significantly affected by both the moisture contentand the pressure applied. It was found that when the moisture con-tent of poultry litter was increased to as high as 60.5%, the energyrequired to compact poultry litter in comparison to the energy re-quired for pelleting was reduced by nine fold. Based on the resultsfrom this study, it is recommended that poultry litter be com-pacted at a moisture content of 60.5% and a pressure of 5.0 MPa.We anticipate that the results from this study will be used in thedesign of economical and simple on-farm compaction equipment.

Acknowledgement

The authors gratefully acknowledge funding from the followingsources: Alabama Legislature Funded Initiative on Value-AddedUtilization of Poultry Litter, AALGA (Alabama Agriculture LandGrant Alliance) and the Southeastern Sun Grant Center.

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