Binderless, All-Lignin Briquette from Black Liquor Waste: Isolation, Puriﬁcation, and Characterization

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Article Binderless, All-Lignin Briquette from Black Liquor Waste: Isolation, Puriﬁcation, and Characterization

Yati Mardiyati 1,*, Emia Yoseva Tarigan 1, Pandji Prawisudha 2, Silvia Mar’atus Shoimah 1, Raden Reza Rizkiansyah 1 and Steven Steven 1

1 Material Science and Engineering Research Group, Faculty of Mechanical and Aerospace Engineering, Institut Teknologi Bandung, Jl.Ganesha 10, Bandung 40132, Indonesia; [email protected] (E.Y.T.); [email protected] (S.M.S.); [email protected] (R.R.R.); [email protected] (S.S.) 2 Energy Conversion Research Group, Faculty of Mechanical and Aerospace Engineering, Institut Teknologi Bandung, Jl.Ganesha 10, Bandung 40132, Indonesia; [email protected] * Correspondence: [email protected]

Abstract: Lignin isolated from black liquor waste was studied in this research to be utilized as binderless, all-lignin briquette, with a caloriﬁc value in the range of 5670–5876 kcal/kg. Isolation of lignin from black liquor was conducted using the acid precipitation method. Sulfuric acid, citric acid, and acetic acid were used to maintain the pH level, which varied from 5 to 2 for the precipitation process. The inﬂuence of these isolation conditions on the characteristic of lignin and the properties of the resulted briquette was evaluated through the Klasson method, proximate analysis, ultimate

analysis, Fourier Transform Infrared (FTIR), adiabatic bomb calorimeter, density measurement, and Drop Shatter Index (DSI) testing. The ﬁnding showed that the lignin isolated using citric acid

Citation: Mardiyati, Y.; Tarigan, E.Y.; maintained to pH 3 resulted in briquette with 72% fixed carbon content, excellent 99.7% DSI, and a Prawisudha, P.; Shoimah, S.M.; calorific value equivalent to coal-based briquette. Rizkiansyah, R.R.; Steven, S. Binderless, All-Lignin Briquette from Keywords: acid precipitation; binderless briquette; black liquor waste; lignin Black Liquor Waste: Isolation, Purification, and Characterization. Molecules 2021, 26, 650. https:// doi.org/10.3390/molecules26030650 1. Introduction Inevitable depletion of non-renewable and limited fossil fuel in the future, such as Academic Editors: petroleum, coal, natural gas, etc., is an issue, since 80% of world’s energy generation Łukasz Klapiszewski and was still dependent on the utilization of these fossil fuels [1–3]. The concern over this Teofil Jesionowski problem and advancing demand for energy with the increase of global human population Received: 20 December 2020 and development of commercial and industrial activities, are raising awareness on the Accepted: 23 January 2021 Published: 27 January 2021 development of alternative energy source. Application of rapidly generated byproducts like municipal solid waste or renewable resources such as biomass are some alternatives that

Publisher’s Note: MDPI stays neutral were researched as fuel for energy generation in the future [1]. Some biomass, for example, with regard to jurisdictional claims in spruce sawdust [4], water hyacinth [5,6], coffee residue [7], eucalyptus leaves [7], and published maps and institutional afﬁl- lignin [8] was studied to be developed into fuel in previous studies. Lignin is a naturally iations. occurring polymer that exists in plants. Lignin could be obtained through extraction from plants or the more readily available black liquor. Black liquor is a byproduct of the kraft papermaking process that yields as dry weight around 170 million tons annually [9]. Black liquor is harmful to the environment as it could contain compounds such as methyl mercaptan and dimethyl sulﬁde [10]. To overcome this Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. problem, some measure was conducted to prevent the buildup of black liquor waste in This article is an open access article an environmentally harmful manner. Utilization as fuel in a recovery boiler to generate distributed under the terms and electricity and steam in pulp mills was presently applied as energy conservation and one conditions of the Creative Commons of the viable options to reduce the black liquor waste. Direct utilization of black liquor Attribution (CC BY) license (https:// as fuel usually occurs in concentrated form, with a 65–70% solid content [11,12]. It has a creativecommons.org/licenses/by/ typical energy value as a fuel of around 3225.5–3891 kcal/kg, which allows providing up 4.0/). to 12% of the energy required in the industry mentioned above [11,12].

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Black liquor consists of organic and inorganic constituents. The organic constituent is the part that contributes to the generation of energy during its combustion. Existence of inorganic constituents that could reach as high as 40–50% become the main problem in the direct utilization of black liquor as fuels, as it is rendered into molten smelt and low melting point ash [11]. Accumulation of ash and smelt inside the boiler greatly reduces its thermal efficiency and also generates a corrosive environment that could affect the lifespan of metal-based equipment [11]. Isolation of its organic constituent, the actual fuel in black liquor, could be a viable pathway to greatly improve its application as fuel. Lignin is the largest organic constituent within black liquor, which covers around 38–47% of its dry mass, depending on the type of wood [9,10]. Up to 80% of lignin in black liquor could be recovered [12]. Lignin molecules consist of a large number of carbon atoms, contributing to its outstanding potential as a biofuel. In addition to being a biofuel, lignin is also reported to have potential utilization in some other applications such as electrochemical sensors, biosensors, fillers, antioxidants, bacterial growth suppressants [13], wastewater adsorbents [14], packaging, tissue engineering, and abrasive tools [15]. The application of lignin in biofuel application is drawing attention, as it has a heat value of around 5975.1—6333.7 kcal/kg, almost two times that of the concentrated black liquor [11]. The high energy potential of lignin makes it a promising substance to be developed into green energy material. Lignin-based fuel could be considered as green, as it is obtained from renewable raw materials like trees and plants, and especially from black liquor waste in the paper and pulp industry. Lignin can be isolated from black liquor by reducing the pH of the system. Sulfuric acid is regarded as effective for use in the lignin precipitation process, as it could bring the system pH to as low as 2 [16]. A lower pH is required to allow protonation on phenolic and carboxylic functional groups in lignin, and allow it to separate from black liquor [16]. However, sulfuric acid utilization has some drawbacks, such as the release of harmful gases that could lead to an environmental problem and which is carcinogenic to humans [17,18]. The utilization of a weaker acid is considered to be viable to overcome these problems. Lignin is reported to be used as a binder in briquette application [1,2,19–22]. Previous research studies utilized lignin as a binder for anthracite-based briquette [23], organic municipal solid waste [24], and wood [4,25]. It was also reported that lignin can serve both as a binder and fuel, improving both the physical and heat properties of the briquette. Briquette is densified particles or loose fuel materials. Briquetting involves pressure to compact the particles or loose fuel materials into smaller volume agglomerates that are capable of remaining compressed [1]. It has some advantages over direct utilization of biomass as fuel, such as a higher heat content and a more compact size, which gives ease of storage space for use and transport [26]. Briquettes are usually produced using coal or biomass such as agricultural waste, with binder occasionally added to the system. Binder is needed in some cases where the material cannot form a strong densified form with a role to facilitate inter-particulate bonding, thus allowing the densified form to be formed [1,21]. The organic-based binder is preferred to the inorganic-based binder, even though the latter form briquette with a higher compressive strength, compaction ratio, and is more hydrophobic [1]. This is because the addition of inorganic substances tends to increase the ash content and burn out temperature, and reduces the calorific value of the resulted briquette [1,26]. Despite their flexibility, binder-based briquettes have some disadvantages such as non-uniform combustion properties and decrease in compacting properties at high temperatures, as the influence of different thermal behavior between the binder and fuel material [19,20]. Utilization of the binderless briquette, a type of briquette compacted without binder, is more advantageous in these cases, although material that could be used in this type of briquette is limited as it should be able to form a strong attraction between each particle. Lignin, which is usually used as the binder, should be capable of being compacted into an all-lignin, binderless briquette. This capability was possible since it has both significant Molecules 2021, 26, x FOR PEER REVIEW 3 of 14

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Lignin, which is usually used as the binder, should be capable of being compacted into an all-lignin, binderless briquette. This capability was possible since it has both significant properties as fuel and the ability to bind with each other through a hydrogen bond, which is contributed from the hydroxy group (-OH)(-OH) inin lignin.lignin. In this this research, research, lignin lignin was was isolated isolated and and purified purified from from black black liquor liquor by using by using the acid the precipitationacid precipitation method method and was and utilized was utilized as a binderless, as a binderless, all-lignin all-lignin briquette. briquette. Isolation Isolation of lig- ninof lignin was conducted was conducted using citric using acid citric and acid acetic and acid, acetic a more acid, environment a more environment friendly friendlyalterna- tivealternative to the stronger to the stronger sulfuric sulfuricacid. Citric acid. and Citric acetic and acid acetic was acidreported was reportedto have the to ability have the to precipitateability to precipitate lignin [17]. lignin Effect [ 17of]. pH Effect and oftype pH of and acid type on ofthe acid purity on theof obtained purity of lignin obtained and thelignin properties and the of properties briquette of were briquette evaluated. were evaluated.To the best Toof theour bestknowledge, of our knowledge, the utilization the ofutilization lignin into of lignina single into component a single component binderless binderless briquette briquette was not wasreported, not reported, which mostly which appliedmostly appliedas a binder. as a binder.Evaluation Evaluation of briquette of briquette properties properties was conducted was conducted throughthrough density measurement,density measurement, proximate proximate and ultimate and ultimate analysis, analysis, calorific calorific value valuetest, an test,d drop and dropshatter shatter test. test. 2. Results and Discussion 2. Results and Discussion 2.1. Effect Effect of pH and Acid Type on the Lignin Isolation Process Isolation of lignin lignin from from black black liquor liquor was was co conductednducted by by the the acid acid precipitation precipitation method. method. Sulfuric acid, citric acid, and acetic acid were utilized to reduce the pH level of black liquor and allow lignin to precipitate. The The pH level of black liquor was reduced from basic to acid level, using the correspondingcorresponding acids.acids. The precipitation condition was maintainedmaintained throughout all all levels levels of of pH pH from from 5 5until until the the lowest lowest pH pH possible, possible, with with the theaddition addition of the of acids.the acids. Figure1 1 shows shows the the effect effect of of various various pH pH levels leve andls and acid acid type type on theon yieldthe yield of the of isolated the iso- latedlignin. lignin. The yield The yield of the of lignin the lignin samples samples that werethat were precipitated precipitated using using acetic acetic acid, acid, citric citric acid, acid,and sulfuric and sulfuric acid increasedacid increased with with the decrease the decrease of pH of level. pH level. The highestThe highest yield yield obtained obtained was was20.83%, 20.83%, which which was was precipitated precipitated using using sulfuric sulfuric acid acid at at pH pH 2. 2. The The yield yield waswas comparablecomparable to previous previous studies studies that that could could isolate isolate lignin lignin from from black black liquor, liquor, with with a yield a yield of around of around 16– 23%16–23% [27]. [27 The]. The precipitation precipitation using using acetic acetic acid acid and and citric citric acid acid with with pH pH 2 2 could could not be achieved, duedue toto itsits limited limited acid acid strength. strength. At At pH pH 4, the4, the three three acids acids resulted resulted in a similarin a similar yield yieldpercentage. percentage. This indicated This indicated that the that pH the level pH had level a significanthad a significant effect than effect the than type the of acidtype onof acidthe yield on the of yield precipitated of precipitated lignin, which lignin, was which in line was with in line previous with previous studies thatstudies stated that that stated the thatyield the of ligninyield of increased lignin increased with a decrease with a decrease in the pH in of the the pH precipitation of the precipitation process [16 process]. The [16].lower The the lower pH level, the pH the level, higher the the higher H+ ion the concentration H+ ion concentration in the solution. in the Increasesolution. of Increase H+ ion ofinducing H+ ion inducing protonation protonation of negatively-charged of negatively-charged phenolic group phenolic of lignin group and of lignin render and its surfacerender itscharge surface neutral charge [28 neutral]. Neutralization [28]. Neutralization of lignin surfaceof lignin charge surface reduces chargethe reduces repulsive the repul- force sivebetween force moleculesbetween molecules and leads and the leads intermolecular the intermolecular attraction attraction to become to become dominant. dominant. Those Thoseconditions conditions cause lignincause moleculeslignin molecules to aggregate to aggregate and precipitate. and precipitate.

22 Sulfuric 21 Citric Acetic 20

Yield (%) 17

2345 pH Figure 1. Yield of the isolated lignin in various pH and acid types.

The isolation process through precipitation was followed by washing in boiling water to purify the isolated lignin from impurities. The purity of precipitated lignin was evaluated

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Molecules 2021, 26, 650 4 of 14 The isolation process through precipitation was followed by washing in boiling water to purify the isolated lignin from impurities. The purity of precipitated lignin was evaluated using the Klasson method. The purity increased with the decrease of pH during the using the Klasson method. The purity increased with the decrease of pH during the precipitation process, as presented in Figure 2. The highest lignin purity was 87%, which precipitation process, as presented in Figure2. The highest lignin purity was 87%, which resulted from isolation using sulfuric acid in pH level 2. Lignin impurities in the precipi- resulted from isolation using sulfuric acid in pH level 2. Lignin impurities in the precipitate tate were caused by the presence of several substances in the black liquor that attached to were caused by the presence of several substances in the black liquor that attached to the lignin, which could include inorganic and organic substances. The main inorganic the lignin, which could include inorganic and organic substances. The main inorganic substances that exist in black liquor are sodium sulfide, sodium carbonate, sodium sulfate, substances that exist in black liquor are sodium sulfide, sodium carbonate, sodium sulfate, sodium hydroxide, sodium thiosulfate, and sodium chloride [29]. These substances were sodium hydroxide, sodium thiosulfate, and sodium chloride [29]. These substances were difficultdifficult to to dissolve dissolve in in water. water. Some Some possible possible organic organic substance substance impurities impurities were were extractives, extractives, aceticacetic acid, acid, formic formic acid, acid, and and methanol. methanol. These These substances substances are are contained contained in black in black liquor liquor in relativelyin relatively high high amounts, amounts, which which was was around around 25–35% 25–35% for extractives; for extractives; 5% for 5% acetic for acetic acid; acid; 3– 5%3–5% for forformic formic acid; acid; and and 3% 3% for for methanol methanol [29]. [29 ].Acid Acid soluble soluble lignin lignin is is not not considered considered to to be be includedincluded as as the the final final lignin lignin content content because because its its smaller smaller molecular molecular weight weight makes makes it it more more difficultdifficult to to recover. recover. The The recovery recovery of of these these frac fractiontion of of lignin lignin would would increase increase the the production production cost,cost, if if applied applied on on a a larger larger scale. scale.

90 sulfuric 88 citric acetate 86

80 Purity (%) Purity 78

72 2345 pH FigureFigure 2. 2.TheThe purity purity of ofisolated isolated lignin lignin in invarious various pH pH and and acid acid types. types.

2.2.2.2. Effect Effect of of Acid Acid Type Type on on the the Proximate Proximate Value Value of of Isolated Isolated Lignin Lignin TheThe result result of of the the proximate proximate analysis analysis is is pres presentedented in in Table Table 1.1. Proximate Proximate analysis analysis result result consistsconsists of of moisture moisture content, content, volatile volatile matter, matter, ash ash content, content, and and fixed ﬁxed carbon. carbon. In In this this study, study, wewe used used SNI SNI 4931–2010 4931–2010 as as a a standard standard reference reference to to evaluate evaluate the the quality quality of of lignin lignin briquette briquette createdcreated in in this this research research [30]. [30].

TableTable 1. 1. ProximateProximate and and ultimate ultimate analysis analysis test test results results of of precipitated precipitated lignin. lignin.

ProximateProximate Analysis Analysis Ultimate Ultimate Analysis Analysis Moisture Volatile Ash Fixed Moisture Volatile Fixed Car- Carbon Hydrogen Nitrogen Oxygen Sulfur Sample Content Matter AshContent Con- Carbon Carbon Hydrogen Nitrogen Oxygen Sulfur Sample Content Matter bon (%) (%) (%) (%) (%) (%) (%) tent(%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) LSA2LSA2 2.542.54 19.9819.98 0.110.11 77.28 77.28 58.7558.75 6.136.13 0.41 0.41 31.531.5 3.48 3.48 LSA3 3.06 23.26 0.18 73.5 58.52 6.29 0.36 32.14 2.59 LCA3LSA3 3.593.06 22.0523.26 0.170.18 74.19 73.5 72.4358.52 4.136.29 0.68 0.36 19.9032.14 2.59 2.66 LAA3LCA3 7.713.59 12.8022.05 0.100.17 79.39 74.19 59.8072.43 6.444.13 0.26 0.68 31.0419.90 2.66 2.18 LAA3 7.71 12.80 0.10 79.39 59.80 6.44 0.26 31.04 2.18 The result showed that lignin isolated using sulfuric acid at pH 2 (LSA2) had the The result showed that lignin isolated using sulfuric acid at pH 2 (LSA2) had the lowest moisture content. Evaluation of the moisture content of the samples, as presented lowestin Figure moisture3, showed content. that all Evaluation samples had of the a moisture moisture content content lower of the than samples, that required as presented in the inSNI Figure 4931-2010 3, showed standard. that all samples had a moisture content lower than that required in the SNI 4931-2010 standard.

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14 1414 Carbonized Coal Briquette - Class B (SNI 4931-2010) 12 CarbonizedCarbonized Coal Coal Briquette Briquette - Class- Class B B (SNI (SNI 4931-2010) 4931-2010) 1212

10 1010

8 88

6 66 Moisture Content (%) Moisture Content (%) 4 Moisture Content (%) 44

2 22

0 00 LSA2LSA2 LSA3 LSA3 LCA3 LCA3 LAA3 LAA3 LSA2 LSA3 LCA3 LAA3 Figure 3. Moisture content of lignin LSA2, LSA3, LCA3, and LAA3 compared to carbonized coal FigureFigure 3. 3. Moisture MoistureMoisture content content content of of of lignin lignin lignin LSA2, LSA2, LSA2, LSA3, LSA3, LSA3, LCA3, LCA3, LCA3, and and and LAA3 LAA3 LAA3 compared compared compared to to carbonized tocarbonized carbonized coal coal coal briquette from the SNI 4931–2010 requirement. briquettebriquette from from the the SN SNISNI I4931–2010 4931–2010 requirement. requirement.

TheThe lowest lowest volatile volatile content content of of the the precipitated precipitated lignin lignin resulted resulted from from isolation isolationisolation using usingusing aceticacetic acid acid to toto pH pHpH 3 3 (LAA3), (LAA3), reachedreached reached asas as low low low as as as 12.80%, 12.80%, 12.80%, as as as presented presented presented in in Figurein Figure Figure4. The4. 4. The The presence pres- pres- enceenceof volatile of of volatile volatile content content content was was was possibly possibly possibly caused caused caused by by by impurities impurities impuritiesremaining remaining remaining afterafter after the the purification puriﬁcationpurification process.process. Results Results Results also also also showed showed that that the the volati volati volatilelele content content of of LSA3 LSA3 and and LCA3 LCA3 were were slightly slightly aboveabove the thethe allowed allowed volatilevolatile volatile contentcontent content by by by the the the SNI SNI SNI 4931–2010 4931–2010 4931–2010 standard. standard. standard. Utilization Utilization Utilization of acetic of of acetic acetic acid acidacidat pH at at 3pH pH for 3 3 precipitationfor for precipitation precipitation of lignin of of lignin lignin also also resultedalso resulted resulted in the in in lowestthe the lowest lowest ash content,ash ash content, content, which which which was was onlywas 0.10%. Overall evaluation of the ash content of all samples was in good agreement with onlyonly 0.10%. 0.10%. Overall Overall evaluation evaluation of of the the ash ash co contentntent of of all all samples samples was was in in good good agreement agreement the SNI 4931–2010 standard requirement, as presented in Figure5. withwith the the SNI SNI 4931–2010 4931–2010 standard standard requir requirement,ement, as as presented presented in in Figure Figure 5. 5.

30 3030

25 2525 Carbonized Coal Briquette - Class B (SNI 4931-2010) CarbonizedCarbonized Coal Coal Briquette Briquette - Class- Class B B (SNI (SNI 4931-2010) 4931-2010) 20 2020

15 1515

10 1010 Volatile Content (%) Volatile Content (%) Volatile Content (%)

5 55

00 0 LSA2LSA2 LSA3 LSA3 LCA3 LCA3 LAA3 LAA3 LSA2 LSA3 LCA3 LAA3 Figure 4. VolatileVolatile content content of of lignin lignin LSA2, LSA2, LSA3, LSA3, LCA3 LCA3,, and and LAA3 LAA3 with with carbonized carbonized coal coal briquette briquette FigureFigure 4. 4. Volatile Volatile content content of of lignin lignin LSA2, LSA2, LSA3, LSA3, LCA3 LCA3, ,and and LAA3 LAA3 with with carbonized carbonized coal coal briquette briquette SNI (4931–2010). SNISNI (4931–2010). (4931–2010).

Figure 5. Ash content of the lignin LS2, LS3, LCA3, and LAA3 compared to the carbonized coal FigureFigure 5. 5. Ash AshAsh content content content of of ofthe the the lignin lignin lignin LS2, LS2, LS2, LS3, LS3, LS3, LCA3, LCA3, LCA3, and and and LAA3 LAA3 LAA3 compared compared compared to to the the to carbonized thecarbonized carbonized coal coal coal briquette SNI 4931–2010. briquettebriquette SNI SNI 4931–2010. 4931–2010.

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The fixed carbon content of the precipitated lignin samples, as compared to the carbonized coal briquette of SNI 4931–2020 are shown in Figure 6. The result showed that Molecules 2021, 26, x FOR PEER REVIEWthe resultant lignin for all samples had a fixed carbon content above the SNI6 4931–2010of 14 Molecules 2021, 26, 650 6 of 14 standard.

The fixed carbon content of the precipitated lignin samples, as compared to the carbonizedThe fixed coal carbon briquette content of SNI of 4931–2020 the precipitated are shown lignin in samples,Figure 6. asThe compared result showed to the that car- thebonized resultant coal briquettelignin for of all SNI samples 4931–2020 had are a fixed shown carbon in Figure content6. The above result the showed SNI that4931–2010 the re- standard.sultant lignin for all samples had a fixed carbon content above the SNI 4931–2010 standard.

Figure 6. Fixed carbon of LS2, LS3, LAS3, and LAA3 samples compared to the carbonized coal briquette SNI 4931–2010.

Summary of proximate analysis evaluation fitted to SNI 4931–2010 standard is pre- sentedFigureFigure in 6. 6. FixedFigureFixed carbon carbon 7. The of of LS2, result LS2, LS3, LS3, showed LAS3, LAS3, and andthat LAA3LAA3 the samples LSA2 samples comparedand compared LAA3 to the lignin to carbonized the carbonizedbriquettes coal coal samples briquettebriquette SNI SNI 4931–2010. were in good agreement with the SNI standard 4931–2010, as it had a moisture content, volatileSummarySummary content, of ofand proximate proximate ash content analysis analysis under evaluation evaluation the maximum fitted ﬁtted to to SNI SNIallowed 4931–2010 4931–2010 value standard standard and a fixedis is pre- pre- carbon contentsentedsented abovein in Figure Figure the 77. .minimum TheThe resultresult showedrequirement.showed thatthat thethe LSA2LSA2 andand LAA3LAA3 ligninlignin briquettesbriquettes samplessamples were in good agreement with the SNI standard 4931–2010, as as it it had had a a moisture content, content, volatile content, and and ash content under the maximum allowed value and a fixed ﬁxed carbon contentcontent above above the the minimum minimum requirement.

FigureFigure 7. 7.SummarySummary of of proximat proximatee analysis analysis result. result. Figure2.3. Effect 7. Summary of Acid Type of proximat on Ultimatee analysis Analysis result. of Isolated Lignin 2.3. Effect of Acid Type on Ultimate Analysis of Isolated Lignin The ultimate test result that consisted of the mass percent of carbon, hydrogen, nitro- 2.3. Effect of Acid Type on Ultimate Analysis of Isolated Lignin gen,The oxygen, ultimate and sulfurtest result in the that isolated consisted lignin, of is the presented mass percent in Figure of8. carbon, The highest hydrogen, carbon nitrogen,content oxygen,The was ultimate and obtained sulfur test fromresult in the that utilizationisolated consisted lignin, of of citric the ismass acid presented atpercent pH 3 duringinof carbon,Figure precipitation, hydrogen,8. The highest whichnitro- carbon gen, oxygen, and sulfur in the isolated lignin, is presented in Figure 8. The highest carbon contentreached was 72.43%. obtained from the utilization of citric acid at pH 3 during precipitation, which content was obtained from the utilization of citric acid at pH 3 during precipitation, which reachedreached 72.43%. 72.43%. TheThe percentage percentage of hydrogenhydrogen and and oxygen oxygen contained contained in LCA3 in LCA3 was lowerwas lower than LSA2,than LSA2, LSA3,LSA3, and and LAA3.LAA3. This waswas probably probably because because citric citric acid acid had hada better a better ability ability to dissolve to dissolve impuritiesimpurities and and consistedconsisted ofof various various polysaccharide polysaccharide degradation degradation products products that had that a hadhigh a high

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Molecules 2021, 26, 650 7 of 14 amount of hydrogenamount and of oxygenhydrogen in theirand oxygenstructures. in their To confirm structures. this Tohypothesis, confirm thisFTIR hypothesis, FTIR characterizationcharacterization was conducted towas evaluate conducted functional to evaluate groups functional present ingroups the samples. present in the samples.

Figure 8. Effect ofFigure FigureAcid Type 8.8. EffectEffect on Ultimate ofof AcidAcid Type TypeAnalysis onon UltimateUltimate of LSA2, AnalysisAnalysis LSA3, LCA3, ofof LSA2,LSA2, and LSA3, LSA3,LAA3. LCA3,LCA3, andand LAA3.LAA3.

FTIR spectra of ThetheFTIR lignin percentage spectra samples of the of hydrogenare lignin presented samples and in oxygen are Figure presented contained 9. The inabsorbance Figure in LCA3 9. Thepattern was absorbance lower than pattern LSA2, of the FTIR spectrumLSA3,of the andFTIRrepresents LAA3. spectrum Thisthe vibrrepresents wasational probably themodes becausevibr ofational the citric samples. modes acid had ofThe athe bettersamples samples. ability The to dissolvesamples showed a broadimpurities showedband centered a broad and consistedat band 3400 centered cm of−1, various which at 3400 attributed polysaccharide cm−1, which to the attributed degradationO-H stretching to the products vibra- O-H stretching that had a vibra- high tion in lignin. Theamounttion band in lignin. ofaround hydrogen The 2900 band andcm around−1oxygen and 2850 2900 in cmtheir cm−1− 1indicated structures.and 2850 the cm To −vibration1 conﬁrmindicated of this theC-H hypothesis, vibration of FTIR C-H stretching in thecharacterizationstretching methyl and in themethylene wasmethyl conducted grouand p,methylene torespectively. evaluate grou functional Thep, respectively. band groups around The present 1700 band cm in around− the1 samples. 1700 cm−1 FTIR spectra of the lignin samples are presented−1 in Figure-1 9. The absorbance-1 pattern of was assigned towas the vibrationassigned toof theC=O vibration stretching. of C=OThe 1600stretching. cm , 1512 The cm1600, cmand−1 ,1450 1512 cm cm-1 , and 1450 cm-1 the FTIR spectrum represents the vibrational modes of the samples. The samples showed bands indicatedbands the C=C indicated stretching the inC=C aromatic stretching ring. in The aromatic result showedring. The that result LCA3 showed had athat LCA3 had a a broad band centered at 3400 cm−1, which attributed to the O-H stretching vibration higher number ofhigher aromatic number groups, of aromatic as compared groups, to asLSA3 compared and LAA3. to LSA3 On theand other LAA3. hand, On the other hand, in lignin. The band around 2900 cm−1 and 2850 cm−1 indicated the vibration of C-H the lignin puritythe of LCA3lignin waspurity lower of LCA3 than wasLSA3 lower but was than slightly LSA3 but higher was than slightly LAA3. higher This than LAA3. This stretching in the methyl and methylene group, respectively. The band around 1700 cm−1 proved that someproved of the that aromatic some ringsof the contained aromatic ringsin LCA3 contained were not in LCA3aromatic were rings not that aromatic rings that was assigned to the vibration of C=O stretching. The 1600 cm−1, 1512 cm−1, and 1450 cm−1 made up lignin. madeThese up aromatic lignin. groupsThese aromatic were probably groups impurities were probably resulting impurities from the resulting deg- from the deg- bands indicated the C=C stretching in aromatic ring. The result showed that LCA3 had radation of lignin,radation such ofas lignin,phenols such and asse veralphenols other and aromatic several otherderivatives. aromatic The derivatives. C-O The C-O a higher number of aromatic groups, as compared to LSA3 and LAA3. On the other stretching of syringil,stretching guacyl, of syringil, and phenolic guacyl, groups and phenolic could be groups indica tedcould by bepeaks indica at ted1327 by peaks at 1327 hand, the lignin purity of LCA3 was lower than LSA3 but was slightly higher than LAA3. cm−1, 1267 cm−1, cmand−1 ,1215 1267 cm cm−1−,1 ,respectively. and 1215 cm The−1, respectively. absorbance Thepeak absorbance in 1100 cm peak−1 and in 1035 1100 cm−1 and 1035 This proved that some of the aromatic rings contained in LCA3 were not aromatic rings cm−1 indicated thecm C-H−1 indicated deformation the C-H of thedeformation aromatic ringof the and aromat C-O stretchingic ring and in C-O secondary stretching in secondary that made up lignin. These aromatic groups were probably impurities resulting from alcohol. thealcohol. degradation of lignin, such as phenols and several other aromatic derivatives. The Comparison of the atomic ratio H:C and O:C in the lignin LSA2, LSA3, LCA3, and C-O stretchingComparison of of syringil, the atomic guacyl, ratio and H:C phenolic and O:C groups in the couldlignin be LSA2, indicated LSA3, by LCA3, peaks and at LAA3 in the Van Kravelen diagram is presented in Figure 10. The result showed that 1327LAA3 cm in− 1the, 1267 Van cm Kravelen−1, and 1215diagram cm− 1is, respectively.presented in TheFigure absorbance 10. The peakresult in showed 1100 cm that−1 LSA2, LSA3, and LAA3 were included in the peat category, whereas lignin LCA 3 ap- andLSA2, 1035 LSA3, cm− 1andindicated LAA3 thewere C-H included deformation in the ofpeat the category, aromatic ringwhereas and C-Olignin stretching LCA 3 ap- in proached the coal category. secondaryproached the alcohol. coal category.

Figure 9.Figure The results 9.FigureThe of results normalized 9. The of results normalized FTIR of charnormalized FTIRacterization characterization FTIR of char LS3,acterization LCA3, of LS3, and LCA3, of LAA3. LS3, and LCA3, LAA3. and LAA3.

Comparison of the atomic ratio H:C and O:C in the lignin LSA2, LSA3, LCA3, and LAA3 in the Van Kravelen diagram is presented in Figure 10. The result showed that LSA2,

Molecules 2021, 26, 650 8 of 14

Molecules 2021, 26, x FOR PEER REVIEW 8 of 14 Molecules 2021, 26, x FOR PEER REVIEW 8 of 14

LSA3, and LAA3 were included in the peat category, whereas lignin LCA 3 approached the coal category.

Figure 10. Position of the resultant Briquette on the Van Kravelen diagram (adapted and modified Figure 10. Position of the resultant Briquette on the Van Kravelen diagram (adapted and modiﬁed Figurefrom [31]). 10. Position of the resultant Briquette on the Van Kravelen diagram (adapted and modified fromfrom [31]). [31]). 2.4.2.4. Effect Effect of of Acid Acid Type Type on Caloric Value 2.4. Effect of Acid Type on Caloric Value TheThe calorific valuevalue ofof thethe lignin lignin LSA2, LSA2, LSA3, LSA3, LCA3, LCA3, and and LAA3 LAA3 compared compared to coalto coal bri- briquettes,quettes,The accordingcalorific according value to the to SNIofthe the 4931–2010SNI lignin 4931–2010 LSA2, standard LSA3,standard presented LCA3, presented in and Figure LAA3 in 11 Figure. Basedcompared 11. on Based Figure to coal on11 , briquettes,Figurethe highest 11, accordingthe calorific highest valueto calorific the resulted SNI value 4931–2010 in ligninresulted st LSA2,andard in lignin which presented LSA2, was around whichin Figure was5876 11. around kcal/kg. Based 5876 Theon Figurekcal/kg.overall 11, result The the overall showed highest result that calorific all showed lignin value briquettethat resulted all lignin prepared in briquette lignin in this LSA2, prepared research which werein wasthis equivalent researcharound to5876were the kcal/kg.equivalentcarbonized The to coaloverall the briquette, carbonized result accordingshowed coal briquette, that to theall lignin SNI according 4931–2010 briquette to standard.the prepared SNI 4931–2010 in this research standard. were equivalent to the carbonized coal briquette, according to the SNI 4931–2010 standard. 7000 7000 Carbonized Coal Briquette - Class B (SNI 4931-2010) 6000 Carbonized Coal Briquette - Class B (SNI 4931-2010) 6000 5000 5000 4000 4000 3000 3000 2000

Calorific Value (kcal/kg) 2000 Calorific Value (kcal/kg) 1000 1000 0 LSA2 LSA3 LCA3 LAA3 0 LSA2 LSA3 LCA3 LAA3 FigureFigure 11. 11. CalorificCaloriﬁc Value Value of the Resultant Lignin Briquettes. Figure 11. Calorific Value of the Resultant Lignin Briquettes. 2.5.2.5. Effect Effect of of Acid Acid Type Type on Dens Densityity and the Drop Shatter Index (DSI) 2.5. Effect of Acid Type on Density and the Drop Shatter Index (DSI) TheThe results results of of density measurements measurements and and th thee Drop Drop Shatter Index (DSI) of the lignin LSA2,LSA2,The LSA3, LSA3, results LCA3, LCA3, of density and and LAA3 LAA3 measurements are are shown shown and in in Table the Drop 22.. DSIDSI Shatter ofof allall ligninIndexlignin briquettes(DSI)briquettes of the showedshowed lignin LSA2,interestinginteresting LSA3, results, results,LCA3, withand with LAA3a a value value are above above shown 99%, 99%, in Table with DSI2. DSI of ofLSI all 3 lignin briquette briquettes even achieving showed 100%. All briquettes prepared in this research showed a higher DSI than in commercially interesting100%. All briquettesresults, with prepared a value in above this research 99%, with showed DSI ofa higher LSI 3 briquetteDSI than ineven commercially achieving available biomass briquette, which was around 96% [32]. This ﬁnding showed that lignin 100%.available All briquettesbiomass briquette, prepared which in this was research around showed 96% [32]. a higher This findingDSI than showed in commercially that lignin could be used to form excellent briquettes even without a binder and was very promising availablecould be biomassused to form briquette, excellent which briquettes was around even 96%without [32]. a This binder finding and wasshowed very that promising lignin to be utilized as a binderless briquette. couldto be utilizedbe used asto forma binderless excellent briquette. briquettes even without a binder and was very promising to be utilizedDensity asmeasurement a binderless showed briquette. that the lignin briquette had a density of around 1200 to 1225Density kg/m measurement3. The highest showed briquette that density the lignin was briquette 1225 kg/m had3 anda dens reachedity of aroundin the LSA31200 tosample. 1225 kg/m The 3density. The highest of all obtained briquette briquette density was higher1225 kg/m than3 andthe commercialreached in thelignin LSA3 bri- sample.quette, whichThe density was around of all obtained 900 to 1000 briquette kg/m3 was[32]. higher That condition than the commercialconfirmed its lignin excellent briquette,DSI value which among was thearound others. 900 to 1000 kg/m3 [32]. That condition confirmed its excellent DSI value among the others.

Molecules 2021, 26, 650 9 of 14

Table 2. DSI and Density measurement of the lignin briquette.

Sample DSI (%) Density (kg/m3) LSA2 99.8 1190 LSA3 100 1225 LCA3 99.7 1184 LAA3 99.7 1200

Density measurement showed that the lignin briquette had a density of around 1200 to 1225 kg/m3. The highest briquette density was 1225 kg/m3 and reached in the LSA3 sample. The density of all obtained briquette was higher than the commercial lignin briquette, which was around 900 to 1000 kg/m3 [32]. That condition conﬁrmed its excellent DSI value among the others. The overall result showed the potential of the resultant binderless, all-lignin briquette to be utilized for energy generation. Lignin briquette also had the potential to be developed into a supercapacitor, if the higher carbon content could be attained, creating another economical value for the resulted lignin briquette.

3. Materials and Methods 3.1. Materials The kraft black liquor used in this research was obtained from a pulp and paper industry in Indonesia, which utilized wood from Acacia manginum, Acacia crasicarpa, and Pinus sylvetris. The processed pulp had a kappa number around 16 in the digester and 10, after the oxygen delignification process. Analytical sulfuric acid with 97% purity, citric acid anhydrate, and acetic acid, respectively, was purchased from the SMART LAB, PT. Brataco and Rofa Laboratorium, Bandung, Indonesia. Deionized water (DI) used for the lignin purification was purchased from the Chemistry Study Program, ITB, Indonesia. Chemicals used were utilized without further purification.

3.2. Method 3.2.1. Isolation of Lignin from Kraft Black Liquor Lignin from the kraft black liquor was isolated through the acid precipitation process. First, black liquor was dissolved in demineralized (DM) water. Black liquor and water were maintained with a weight ratio of 1:12. Sulfuric acid (H2SO4) (6M) was added gradually to the diluted black liquor solution, until the pH level was maintained at 2. A similar process was conducted with precipitation pH maintained to 3, 4, and 5. Citric acid anhydrate 50% and acetic acid 99% were also similarly used for precipitation with final precipitation pH levels varying between 3, 4, and 5. The precipitate was filtered from the solution using filter paper and was washed using deionized (DI) water, until a neutral pH was achieved. Lignin precipitates were dried and weighed periodically, until their constant masses were reached. Dried lignin was dispersed with a weight ratio of 1:300 to deionized water in a reflux flask. Finally, lignin was filtered, dried, and weighed periodically, until their constant masses were obtained. Table3 showed the designation of samples prepared in this research, based on the type of acid used and the pH level was maintained during precipitation.

3.2.2. Briquetting Process The briquetting process was carried out using a hydraulic press and punching tools that consisted of a lower punch, a die, and an upper punch. Briquetting was conducted in the Energy Conversion Lab, Institut Teknologi Bandung, Indonesia. The pressure was maintained to 500 bar, as a result of the calculated minimum pressure required to form a lignin briquette with a certain briquette strength. The briquetting was conducted using a 3.5-tonne hydraulic jack to obtain a 30-mm diameter cylindrical briquette. The briquetting pressure in this process was higher than the conventional briquetting process, with a binder that usually required 100 bar to produce a briquette. A typical binderless Molecules 2021, 26, x FOR PEER REVIEW 10 of 14

Table 3. Sample Designation.

Code Acid type pH LSA2 2 LSA3 3 Sulfuric acid LSA4 4 Molecules 2021, 26, 650 LSA5 5 10 of 14 LCA3 3 LCA4 Citric acid 4 briquettingLCA5 process, such as that used in Komarek, usually5 requires 500–1000 bar to produce a binderlessLAA3 briquette [33]. The resultant lignin briquettes3 are shown in Figure 12. LAA4 Acetic acid 4 Table 3. SampleLAA5 Designation. 5

3.2.2. BriquettingCode Process Acid Type pH The briquettingLSA2 process was carried out using a hydraulic press and2 punching tools LSA3 3 Sulfuric acid that consistedLSA4 of a lower punch, a die, and an upper punch. Briquetting was 4 conducted in the Energy ConversionLSA5 Lab, Institut Teknologi Bandung, Indonesia. The 5 pressure was maintained to 500 bar, as a result of the calculated minimum pressure required to form a LCA3 3 lignin briquetteLCA4 with a certain briquetteCitric strength. acid The briquetting was conducted 4 using a 3.5-tonne hydraulicLCA5 jack to obtain a 30-mm diameter cylindrical briquette. 5The briquetting pressure in LAA3this process was higher than the conventional briquetting 3process, with a binder that LAA4usually required 100 bar toAcetic produce acid a briquette. A typical 4binderless briquetting process,LAA5 such as that used in Komarek, usually requires 500–1000 5 bar to produce a binderless briquette [33]. The resultant lignin briquettes are shown in Figure 12.

Figure 12. Lignin briquette of (a)) LSA2,LSA2, ((b)) LSA3,LSA3, ((cc)) LCA3,LCA3, andand ((dd)) LAA3.LAA3.

3.2.3. Evaluation of Lignin Yield The yield ofof ligninlignin waswas calculatedcalculated fromfrom thethe ratioratio betweenbetween thethe resultedresulted drydry puriﬁedpurified ligninlignin ((bb)) toto the black liquor used in the process (a). Measurement was repeated threethree timestimes forfor eacheach variation.variation. The percentagepercentage ofof yieldyield waswas calculatedcalculated usingusing EquationEquation (1).(1). b 𝑏 LigninLignin yield yield= = × × 100% (1)(1) a 𝑎

3.2.4. Evaluation of Lignin Purity Evaluation of lignin purity isolated from black liquor was conducted using the Klasson lignin method, according to the TAPPI T-222 OM-02 [34]. Testing was conducted in Metallurgical and Material Engineering Laboratory, Material Engineering Study Program, Institut Teknologi Bandung, Bandung, Indonesia. The testing process used 1 g of lignin (a) that dissolved in 15 mL of 72% sulfuric acid and was stirred using a 200-rpm magnetic stirrer for 2 h. The dissolved sample was diluted until the sulfuric acid concentration reached 3% and boiled for 4 h. The precipitate was separated from the solution using the Whatman 42 ﬁlter paper. The residue was then washed with DI water to neutral pH, dried, Molecules 2021, 26, 650 11 of 14

and weighed periodically to a constant weight (b). Testing was repeated three times for each variation. The percentage of lignin calculated using Equation (2).

b Lignin purity = × 100% (2) a

3.2.5. Evaluation of Moisture Content The moisture content of lignin was evaluated according to the ASTM D 3173 [35]. Moisture content was determined by measuring the mass loss of the sample after heating. Lignin sample was weighed (a) and placed into a crucible. The sample was dried in an oven heated to 104 ◦C–110 ◦C for 1 h. The sample was cooled in a desiccator for 1 h. The dried sample was weighed to determine its ﬁnal mass (b). The sample was measured 3 times for each variation. The percentage of moisture content was determined using Equation (3).

(a − b) Moisture content = × 100% (3) a

3.2.6. Evaluation of Ash Content Ash content measurement was carried out according to the ASTM D 3174 [36]. Lignin was weighed to 1 g (a) and put into a capsule and then heated in a furnace at 450–500 ◦C. The temperature was increased to 700–750 ◦C for 1 h and then increased to 950 ◦C for 2 h. The residue left after heating was cooled and weighed (b). The sample was measured 3 times for each variation. Ash content was calculated using Equation (4).

b Ash content = × 100% (4) a

3.2.7. Evaluation of Volatile Matter Volatile matter content was evaluated according to the ASTM D 3175 [37]. Lignin was weighed to 1 g (a) and placed into a covered platinum crucible. The crucible was put into an inert furnace and heated at 950 ◦C for 7 min. The sample was cooled in a desiccator after heating. The ﬁnal mass (b) was weighed and the weight loss percentage was determined using Equation (5). (a − b) Weight loss = × 100% (5) a The percentage volatile matter was calculated as the difference between weight loss percent and moisture percent. The sample was tested 3 times for each variation.

3.2.8. Evaluation of Fixed Carbon Fixed carbon content calculated using Equation (6).

Fixed carbon (%) = 100 − (moisture,% + ash,% + volatile,%) (6)

3.2.9. Evaluation of Gross Caloriﬁc Value The caloriﬁc value was determined by burning the sample using an adiabatic bomb calorimeter, according to the ASTM D 5865 [38]. The sample was tested in the Public Services Agency of Mineral and Coal, Bandung, Indonesia.

3.2.10. Drop Shatter Index (DSI) Testing Drop shatter test was conducted according to ASTM D-440-86 [39]. Initial mass (a) of the briquette was weighed, then the sample was placed into the testing machine and dropped from 1.8 m above a steel plate. The dropped briquette was returned to the machine and the process was repeated one more time. Dropped briquette and its crumbled Molecules 2021, 26, 650 12 of 14

parts were sieved. Mass of briquette remaining in the sieve counted as the ﬁnal mass of briquette (b). DSI was calculated using Equation (7).

Drop Shatter Index (DSI), % = (b/a) × 100 (7)

3.2.11. Density Measurement The density of the briquette was calculated from the ratio between mass to volume of the briquette. The volume of the briquette was calculated from the dimension of the briquette, measured using a Vernier caliper. The briquette density was calculated using Equation (8). mass of briquette Density of briquette, g/cm3 = (8) volume of briquette

3.2.12. Ultimate Testing Ultimate testing was conducted on the basis of ASTM D3176 [40]. Testing was conducted in the Public Services Agency of Mineral and Coal, Bandung, Indonesia.

3.2.13. FTIR Characterization The lignin powder was pelletized using KBr and its spectra were recorded in the range of 4000 cm−1 to 650 cm−1, using Shimadzu Prestige 21. The FTIR measurement was conducted in the Chemistry Study Program, Institut Teknologi Bandung, Indonesia.

4. Conclusions The result of this study demonstrated the capability of lignin isolated from black liquor to be used as binderless, all-lignin briquette. Briquette prepared from lignin isolated using citric acid show promise to be utilized for the application mentioned above, as it had a high ﬁxed-carbon content, excellent DSI, and a decent caloriﬁc value that was equivalent to coal-based briquette.

Author Contributions: Conceptualization, Y.M.; Supervision, Y.M.; formal analysis, S.S.; resources, S.S.; project administration, Y.M. and S.S.; investigation, E.Y.T.; methodology, Y.M., P.P., and E.Y.T.; writing—original draft preparation, S.M.S., and R.R.R.; writing—Review and Editing, Y.M., S.M.S., and R.R.R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to institutional restrictions. Conﬂicts of Interest: The authors declare no conﬂict of interest. Sample Availability: Samples of the compounds are available from the authors.

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