Fuel Processing Technology 124 (2014) 243–248

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Fuel Processing Technology

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An experimental study on binderless briquetting of Chinese lignite:Effects of briquetting conditions

Baolin Sun a, Jianglong Yu a,b,⁎, Arash Tahmasebi a, Yanna Han a

a Key Laboratory for Advanced Coal and Coking Technology of Liaoning Province, School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, Chinab Chemical Engineering, University of Newcastle, Callaghan, New South Wales 2308, Australia

⁎ Corresponding author at: Key Laboratory for AdvancedLiaoning Province, School of Chemical Engineering, UniveLiaoning, Anshan 114051, China. Tel.: +61 2 40333902.

E-mail address: jianglong.yu@newcastle.edu.au (J. Yu)

http://dx.doi.org/10.1016/j.fuproc.2014.03.0130378-3820/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 September 2013Received in revised form 26 February 2014Accepted 11 March 2014Available online 28 March 2014

Keywords:Binderless briquettingLigniteCompressive strengthWater contentHydrogen bond

Technology development of lignite upgrading involving drying and binderless briquetting is important to theefficient utilization of lignite resources. In this study, the effects of briquetting conditions, i.e., briquettingpressure, temperature, coal properties and pretreatment, on the compressive strength of briquettes weresystemically investigated using a lab-scale briquetting testing rig. The experimental results indicated that thecompressive strength of briquettes increased with increasing the compression pressure. An optimum strengthwas obtained at the briquetting temperature of 150 °C. There also existed an optimum moisture content of14–16% at which the highest compressive briquette strength was achieved. The increase in the amount of–125 μm size fraction in the feed sample increased the briquette strength. FTIR analysis showed that theoxygen-containing functional groups have a significant influence on the compressive briquette strength due tothe ability of forming more hydrogen bonds.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Low-rank coals account for nearly half of the global coal reserves [1].Lignite resources in China are more than 130.3 billion tons, forming13% of the total world coal reserves. Lignite is playing an increasinglyimportant role in supplying primary energy in developing countriessuch as China [2]. Generally, lignite coals have low heating value andhigh moisture and oxygen content. Some lignite coals also featurehigh ash contents. The high moisture content in lignite (25–60%) [3]results in low calorific value, low efficiency, high transportation cost,and high CO2 emissions [4]. The heating value of lignite can be signifi-cantly increased through drying and briquetting. Briquetting can alsodecrease the spontaneous combustion tendency of lignite [5,6]. There-fore, better understanding of lignite drying and binderless briquettingis critical for development of lignite utilization technologies.

Briquetting pressure and temperature, and coal moisture contentare important parameters in the binderless briquetting process [7].Lignite briquetting technologies include the stamping briquetting tech-nology, the roll briquetting technology, the screw extrusion briquettingtechnology, and the ring forming technology [8]. Residual moisture incoal reduces internal friction, making coal particles easy to move andcausing lubrication effect for briquetting [9]. Previous studies havereported that there is an optimum moisture content (about 15%) for

Coal and Coking Technology ofrsity of Science and Technology

.

lignite binderless briquetting [7,10,11]. It has also been reported thathigh quality briquettes can be obtained by decreasing coal particle size[12]. Ellion and Trommer [13] postulated that the highest briquettestrength is obtained when fine particles fill the interstices of larger par-ticles and maximum contact surface between them is achieved. Theyalso reported that the compressive strength increased with increasingbriquetting pressure up to 150 MPa and further increasing the pressuredid not have any significant effect on compressive strength [13].Demirbas and Sahin [14] reported the same trend for biomassbriquetting. Another important factor affecting the briquette strengthis the briquetting temperature. Paul et al. [15] suggested that thestrength is enhanced by increasing the temperature.

Some properties of the lignite such as elasticity and plasticity have asignificant effect on binderless briquetting. Increasing the elasticity orreducing the plasticity of coal is beneficial to briquetting [16]. Iyengaret al. [17] proposed that hydrogen bonds play a key role in briquettingand are responsible for the compressive strength of briquettes. Theacidic functional groups such as free carboxylic and phenolic groupscan form hydrogen bonds [13].

Most of the briquetting studies in literature are reported on high-rank coals. Lignites, due to high moisture content, are difficult tobriquette. The effects of operating parameters on the compressivestrength of lignite are reported to some extent in literature. However,to the authors' knowledge a systematic study on the lignite briquettingmechanism and the effects of operating parameters has not beenreported in literature. Furthermore, low-rank coals are rich in oxygenfunctional groups which have an important role on hydrogen bond for-mation. However, the role of these functionalities on the briquetting

244 B. Sun et al. / Fuel Processing Technology 124 (2014) 243–248

process is not clear. In this study, the effects of operating parameterssuch as briquetting pressure, coalmoisture content, briquetting temper-ature, particle size distribution, and oxygen-containing functional groupon briquette strength are systematically studied.

2. Experimental

2.1. Coal sample

Chinese Hulunbeir lignite supplied by Shenhua Coal Co. from InnerMongolia was used in briquetting tests and was assigned as “HL”.The properties of the as-received coal are shown in Table. 1. The coalhas high moisture content on as received basis (27.51%). The raw coalwas crushed and screened into different particle sizes (R1 b 125 μm,R2 = 125–425 μm, R3 = 425–1000 μm).

2.2. Drying and binderless briquetting

Raw samples with different particle size distribution were driedunder nitrogen atmosphere at 150 °C in a fluidized-bed dryer setupcomprising of a quartz reactor, an electrically heated furnace, and a tem-perature controller. The moisture content of the lignite was controlledby the drying residence time. The samples with different moisturecontent (7.10, 10.7, 12.88, 14.85, 18.58, 26.05, and 22.05%) after dryingwere fed into a mould for briquetting. The schematic diagram forbriquetting and compressive strength test is shown in Fig. 1. A heatingjacket was used to control the temperature during briquetting. Thebriquettes were disc shaped with the diameter of 20 mm and heightof 20 mm. For each measurement, at least three briquettes wereprepared and the results reported here are the averages of threemeasurements. The experimental error ranged between ±7% of themean value.

2.3. Measurement of compressive strength of binderless briquettes

Measurements of the compressive strength of the briquettes weredone using an automatic compressive strength tester as shown inFig. 1. The maximum pressure before the briquettes fractured wasrecorded as the briquette strength. In order to eliminate the effect ofbriquette size on the recorded strength, the strength was expressed asthe maximum crushing stress derived from the maximum crushingload [7].

2.4. Oxidation experiments and FT-IR analysis

The oxidized samples were used to study the effect of hydrogenbonding on binderless briquetting of lignite. Hydrogen peroxidesolution (30 vol.%) was used for oxidation of lignite. 10 g of HL lignitesamples was added into 100 ml of hydrogen peroxide solution in a250 ml round bottom flask. Oxidation was allowed for 12 h at thetemperature of 30 °C which was controlled by electrically heated ther-mostatic water bath. After oxidation, the samples were filtrated anddried at 80 °C in vacuum drying oven for 24 h. At least three sampleswere prepared under these conditions and subjected to furthermeasurements and the results reported in this paper are the averagesof three measurements.

Table 1Proximate and ultimate analyses of coal samples used in this study.

Proximate analysis (wt.%) Ultimate analysis (wt.%)

Mt Mad Aad Vad Cdaf Hdaf Odafa Ndaf Sdaf

27.51 14.88 12.12 37.1 72.28 5.89 20.59 0.93 0.22

ad—air dried; daf—dry ash free.a By difference.

The total amount of acidic functional groups was determined basedon the amount of Ba (OH)2 solution reactedwith the oxidized sample at30 °C. After reaction, the filtrate produced was washed by deionizedwater and both the solid sample and the solution after water washingwere collected. The HCl solution was added to the collected solutionafter oxidation. The NaOH solution was used for titration of the excessHCl and phenothalin was used as indicating agent. A blank test was car-ried out in parallel. The chemical reaction is shown in Eq. (1). The totalamount of acidic functional groups of oxidized sample was calculatedfrom Eq. (2).

Ba OHð Þ2 þ 2HA→BaA2↓þ 2H2O ð1Þ

ntotal ¼C V−V0ð Þ

mð2Þ

where ntotal (mol/g) is the total amount of acidic functional groups, C(mol/L) is the concentration of the standard NaOH solution, V (L) isthe volume of the NaOH solution, V0 (L) is the volume of the NaOHsolution in blank test, and m is weight of the sample.

The carboxyl group content of oxidized samples was measuredbased on the amount of the calcium acetate solution reacted with thecoal sample during ion exchange experiments. After filtration andwater washing, the standard NaOH solution was added to the filtrate.A blank test was also carried out in parallel. The chemical reaction isshown inEq. (3). The carboxyl groups of oxidized samplewere calculatedfrom Eq. (4).

Ca CH3COOð Þ2 þ 2RCOOH→Ca RCOOð Þ2 þ 2CH3COOH ð3Þ

nCOOH ¼C

0V

0−V0

0� �

mð4Þ

where nCOOH (mol/g) is the total amount of acidic functional groups,C' (mol/L) is the concentration of the standard NaOH solution, V' (L) isthe volume of the standard NaOH solution in tests, V'0 (L) is the volumeof the standard NaOH solution in the blank test, and m is weight of thesample.

2.5. FTIR spectroscopic analysis

Infrared (IR) spectra of rawand oxidized coal sampleswere obtainedusing a Thermo Fisher Nicolet IS5 mid-FTIR spectrometer. KBr pelletswere prepared by grinding 1.0 mg of coal sample with 100 mg KBr.The IR spectra of the lignite sample in the 4000–400 cm−1 regionwere studied by curve-fitting analysis using a commercially availabledata-processing program (OriginPro, OriginLab Corporation). Theassignment of the bands in the infrared spectra was made accordingto the literatures [18–22]. Initial approximation of the number ofbands and peak positions was obtained by examining second deriva-tives of the spectral data. Gaussian and Lorentzian functions wereused as mathematical functions for band shapes at hydroxyl, aliphatichydrogen and carbonyl stretching regions [18,21,23,24]. The initial setof the peak parameters was left optimized until convergence of thedata was achieved.

3. Results and discussion

3.1. Effects of briquetting parameters on briquette strength

3.1.1. Effect of briquetting pressureIt has been reported in the literature that a compressive strength of

350 kPa is sufficient to ensure the survival of the briquettes under thepressures likely to be encountered in handling and transportation [7].

Fig. 1. Experimental setup used for briquetting and compressive strength tests.

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The effect of briquetting pressure on the briquette with particle sizedistribution R1:R2:R3 = 3:1:1 and briquetting at room temperature isdemonstrated in Fig. 2. It can be seen that the compressive strengthwas monotonically increased with briquetting pressure. At lowbriquetting pressure, the contact between the coal particles was notsufficient to obtain high strength due to the gaps between particlesand their high plasticity. When pressure was further increased, finerparticles were generated as a result of friction between the particles.Fine particles filled the gaps between the larger particles, increasingthe contact area. It has also been reported that higher pressures canenhance the formation ofmechanical interlocking between the particles[25,26]. Under high pressure, densification of particles increased adhe-sion, forming intramolecular bonds in the contact area and decreasingthe distance between the particles. The forces such as Van der Waals'force, liquid bridge force, capillary force, etc. increase with the decreaseof the distance between particles resulting in higher compressivestrength of briquettes [27].

3.1.2. Effect of briquetting temperatureThe effect of briquetting temperature on the compressive strength of

briquette at different moisture contents with particle size distributionR1:R2:R3 = 3:1:1 and briquetting pressure of 216.56 MPa is demon-strated in Fig. 3. It can be seen that the compressive strength increasedwith temperature and reached itsmaximumat briquetting temperatureof 150 °C. Further increasing the temperature decreased the briquettestrength. This trend was observed for samples with different moisturecontents. Briquettes produced at 150 °C had a smooth and shiny surface

Fig. 2. Compressive strength of binderless briquettes as a function of briquetting pressureat moisture contents of: (a) 14.85%; (b) 18.58%; (c) 26.05%; (d) 22.05%; (e) 12.88%;(f) 10.70%; (g) 7.10% (with particle size distribution R1:R2:R3 = 3:1:1 and briquetting atroom temperature).

with no cracks. Coal tar pitch has a softening point of around 70 °C [28].However, heat treatment (e.g. drying) can significantly increase thesoftening point of pitch to around 150–175 °C as a result of removal oflow-boiling volatiles [29]. Therefore, it can be concluded that whenbriquetting at 150 °C, the pitch in the coal structure can soften and actas the bonding material for briquetting. When the temperature wasfurther increased, the moisture in lignite was completely removed dueto the long residence time. Hydrogen bond strength also decreaseswith increasing temperature. As a result, the hydrogen bond intensityand concentration decreased which in turn led to the decrease in thecompressive strength of briquette produced at temperatures above150 °C.

3.2. Effects of coal properties on briquette strength

3.2.1. Effect of moisture contentThe compressive strength of briquette as a function ofmoisture con-

tent under different briquetting pressureswith particle size distributionof R1:R2:R3 = 3:1:1 at room temperature is shown in Figs. 4 and 5. Therelationship between compressive strength and moisture content wasdifferent under different briquetting pressures. When the briquettingpressure was 140 MPa, the compressive strength increased rapidlywith increasing the moisture content to 15% and leveled out thereafter.At higher briquetting pressure (165.6, 191.1, and 216.6 MPa), therewasan optimummoisture content at which briquettes showed a maximumcompressive strength. The optimum moisture content was 14–16% forHL coal under current experimental conditions. Similar trends werereported in the literature [7,27,30]. The variation in compressive strength

Fig. 3. Compressive strength of binderless briquettes as a function of briquetting temper-ature at moisture contents of: (a) 13.39%; (b) 16.74%; (c) 9.56% (with particle sizedistribution of R1:R2:R3 = 3:1:1 and briquetting pressure of 216.56 MPa).

Fig. 4. Compressive strength of binderless briquettes as a function of moisture content at140.13 MPa (with particle size distribution of R1:R2:R3 = 3:1:1 at room temperature).

Fig. 6. Effect of particle size on compressive strength (moisture content of 27.51%,briquetting pressure of 216.56 MPa, at room temperature).

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at differentmoisture contents under different briquetting pressures canbe attributed to the changes in the distance between the particles atdifferent briquetting pressure, which affects the bonding forcesbetween the particles.

Hydrogen bond has a significant effect on the compressive strengthof the briquette [13,17]. Water is absorbed in multiple layers on thelignite surface [31]. The first layer is absorbed by hydrogen bondingbetween thewatermolecules and the active sites on theparticle surface.The other layers aremainly absorbed through long-distance attractions.The energy of the hydrogen bonding is larger than the long-distanceattraction forces. At 14–16% moisture contents, the monolayer wateris complete. At this moisture content hydrogen bonding is at itsmaximum strength, resulting in the highest compressive strength ofbriquettes. With further increasing the water content, multilayerwater is formed and the long-distance forces replace hydrogen bondingresulting in the decrease in the compressive strength of briquettes.

3.2.2. Effect of particle size distributionParticle size distribution is an important parameter in the

briquetting process [13]. The effect of particle size distribution oncompressive strength at moisture content of 27.51% and briquettingpressure of 216.56 MPa at room temperature is shown in Fig. 6. Theeffect of mass percentage of particles smaller than 125 μm in briquette

Fig. 5. Compressive strength of binderless briquettes as a function ofmoisture content andbriquetting pressure of (a) 165.61 MPa; (b) 191.08 MPa; (c) 216.56 MPa (with particlesize distribution of R1:R2:R3 = 3:1:1 at room temperature).

on its compressive strength was investigated. The proportions of othersize fractions in briquette were equal. For example, when 50% of theparticles were less than 125 μm, the 125–425 and 125–1000 μm frac-tions were both 25%. As can be seen, the compressive strength wasmonotonically increased with the increase of the proportion of particlessmaller than 125 μm. Kaliyan and Morey [32] also reported that thequality of the pellets made from biomass increased with the increaseof the percentage of the small size particles. With the proportion ofsmall particles increasing, small particles fill in the gaps between thelarge particles which subsequently increase the contact area betweenthe particles. The increased contact area allows the formation of stron-ger bonds between the particles resulting in an increased compressivestrength. Another reason may be that at smaller particle sizes, thedistance between the particles is shorter which increases the intensityof the forces formed between the particles resulting in higher compres-sive strength [33]. The particles in the briquette can form force chains.The total length of the force chains depends on the number of the con-tact forces and the particle size [34]. At smaller particle sizes, the lengthof the force chains increases resulting in higher compressive strength.

As a result of higher surface area in smaller particles, the contents ofhydroxyl groups, especially those of OH\π, OH\OH and OH\etherincrease [35]. Hydroxyl groups can form hydrogen bonds between theparticles. The increase in the intensity of hydrogen bonding resulted inthe enhanced compressive strength of briquettes. The effect of hydro-gen bonding on briquette strength is discussed in detail in the followingsection.

3.2.3. Effect of oxygen-containing functional group content on briquettestrength

In low-rank coals, a considerable amount of oxygen is present in theform of carboxyl and phenolic functional groups [36]. These groups canform hydrogen bonds between the coal particles. Hydrogen bonds havea significant effect on briquette strength [17]. The effect of oxygenfunctional groups was evaluated by comparing the briquetting behaviorof raw coal with the oxidized sample. The changes in acidic functionalgroups after oxidation are shown in Fig. 7. It can be seen that the carboxyland phenolic hydroxyl group content increased after oxidation by H2O2.The increase in oxygen-containing functional groups was attributed tohydrolysis of ester during oxidation which resulted in the productionof carboxylic acid, alcohol and phenol. Aliphatic hydrogen groups suchas\CH2 and \CH3 were formed as a result of dissociation of aromaticrings during oxidation. At later stages of oxidation, these groups werethemselves oxidized to from \OH and \C_O which in turn weretransformed into \COOH. Hence, the oxygen-containing functionalgroup content of lignite after oxidation was increased.

Fig. 7. The acidic group content after H2O2 oxidation.Fig. 9. Changes in hydroxyl (\OH), carboxyl (\COOH), and carbonyl (C_O) Absorptionas a result of oxidation.

247B. Sun et al. / Fuel Processing Technology 124 (2014) 243–248

The compressive strength of the briquette made from oxidized coalwas 16.8 MPa which was higher than that of raw coal at 10.4 MPa (atroom temperature with the briquetting pressure of 216.6 MPa andmoisture content of 11.0%). The significant increase in the compressivestrength of oxidized coal was attributed to the increase in hydrogenbonds which was induced at higher oxygen-containing functionalgroups concentration.

The IR spectra of raw and oxidized coal are shown in Fig. 8. Changesin absorption of functional groups in 3000–2800 cm−1 regions (aliphaticC\H stretching) were investigated. Absorption of five bands in thealiphatic hydrogen absorption region attributed to asymmetric methyl(\CH3) and methylene (\CH2\) stretching (2953 and 2922 cm−1,respectively), symmetric methyl (\CH3) and methylene (\CH2)stretching (2865 and 2850 cm−1, respectively), and methane (C\H)stretching (2895 cm−1) was studied in detail. The absorption ofaliphatic C\H stretching increased after the oxidation (Fig. 8). Thiswas attributed to dissociation of aromatic rings by H2O2 to formaliphatic C\H.

Hydrogen bonds, one of non-covalent associative interactions, play akey role to keep themacromolecular structure of low-rank coals; there-fore, utilization of low-rank coals (including briquetting) is affected bythe hydrogen bonding. Oxygen functional groups (carbonyl, carboxyland hydroxyl groups) are responsible for formation of hydrogenbonds. The 3650–3000 cm−1 zone was curve-fitted to a series ofsix bands corresponding to OH\N (acid/base structures at around3050 cm−1), tightly bound cyclic OH tetramers (3200 cm−1),

Fig. 8. Comparison of IR spectra of raw coal and oxidized coal: (a) oxidized coal; (b) rawcoal.

OH\ether O hydrogen bond (3300 cm−1), self-associated n-mers (nN 3) (around 3410 cm−1), OH\π hydrogen bonds (3510 cm−1), andfree OH (3610 cm−1). The 1850–1500 cm−1 zone was also curve-fitted to a series of 7 bands corresponding to carboxyl groups andquinines (1740–1650 cm−1), aromatic carbon (around 1610 cm−1)and carboxylate and aromatic ring stretch groups (1570–1480 cm−1).Fig. 9 summarizes changes in hydroxyl (\OH), carboxyl (\COOH),and carbonyl (C_O) absorption as a result of oxidation. It can be seenthat the COO−, \OH and \C_O contents increased after oxidationwhich in turn enhanced the formation of hydrogen bonds and briquettestrength as discussed above. Aliphatic C\H groups play an importantrole during low temperature oxidation [37,38]. The aliphatic C\Hgroups were oxidized to form \COOH and C_O. The \COOH andC_O bonds (resonance-assisted H-bonds) are strong H-bonds whichplay an important role in briquette compressive strength [39,40]. Thealiphatic C\H groups were oxidized to form \OH; hence, the contentof hydroxyl structures was increased.

4. Conclusions

The compressive strength of the briquettes was greatly influencedby the process parameters. The compressive strength increased withincreasing briquetting pressure. The relationship between compressivestrength and moisture content was different under different bindingpressures. At higher briquetting pressures, there existed an optimummoisture content of 14–16% for HL lignite at which briquettes had thehighest compressive strength. The compressive strength also increasedwith increasing the amount of small particle size fraction (b125 μm) inthe coal sample. The optimumbriquetting temperature was found to be150 °C and further increasing the briquetting temperature led to adecrease of the compressive strength.

The effect of hydrogen bonding on briquette strength was alsoinvestigated, showing that the briquettes produced from oxidized coalhave a higher strength compared to that of un-oxidized coal. This wasattributed to a higher density of hydrogen bond association as a resultof larger amount of oxygen-containing functional groups in oxidizedcoals. These results were confirmed by FTIR analysis.

Acknowledgments

This study was supported by the Natural Science Foundation ofChina (21176109, 21210102058, and U1361120).

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