research article recovery of value-added products from...

Hindawi Publishing CorporationISRN Chemical EngineeringVolume 2013 Article ID 268947 6 pageshttpdxdoiorg1011552013268947

Research ArticleRecovery of Value-Added Products from HydrothermalCarbonization of Sewage Sludge

Pannarai Saetea and Nakorn Tippayawong

Department of Mechanical Engineering Faculty of Engineering Chiang Mai University Chiang Mai 50200 Thailand

Correspondence should be addressed to Nakorn Tippayawong ntippayawongyahoocom

Received 3 May 2013 Accepted 5 June 2013

Academic Editors D Cazorla-Amoros A S Chiang and A Ragauskas

Copyright copy 2013 P Saetea and N Tippayawong This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

This paper is about the conversion of wet waste stream into valuable products via thermal processing Hydrothermal carbonizationof sewage sludge was carried out at 200∘C and 21MPa in a closed reactor for 1ndash6 h Main products were in solid and liquid phasesThe resulting hydrochar was shown to have HC and OC ratios moving towards natural lignite improved energetic contentand adsorption property in terms of iodine number The aqueous solution was found to contain high concentration of plantfood nutrients especially nitrogen and potassium They may be desirable for subsequent fuel and chemical production as wellas applications in agriculture The study shows that valuable products can be generated successfully from sewage sludge usinghydrothermal carbonization

1 Introduction

Sewage sludge is generated as an inevitable by-product ofwastewater treatment activity Their production is expectedto rapidly increase with urbanization and industrial devel-opment In Thailand for example about 4000 tons per dayof dry treated sewage sludge solids is generated Sewagesludge consists of mostly offensive and toxic substances Itmust be disposed of or managed properly otherwise seriouseffects on humans and ecological systems will occur Severalmethods can be adopted for the management of sewagesludge such as landfill disposal incineration and utilizationin agriculture but each of these options has importantlimitations Both incineration and landfill are troubled bytheir lowly public image There are always concerns regard-ing associated costs and emissions Application of sewagesludge to agricultural lands is restricted due to possiblecontamination of the soil and vegetation as well as hazardousconsequences for animals and human The decline of thetraditional disposal routes for sewage sludge has created astrong demand for more cost effective and environmentallyacceptable alternativesThis has motivated the research com-munity to search for innovative and beneficial use of sewagesludge for years [1 2]

A very appealing method is hydrothermal carbonization(HTC) also known as subcritical water or hot compressedwater carbonization It is ideal for high moisture contentmaterial such as sewage sludge HTC can be described asa thermochemical process for converting an organic feed-stock into value-added products at moderate temperatures(180ndash350∘C) and pressures (2ndash10MPa) in the presence ofliquid water Hydrothermal degradation of organic matterand synthesis of basic chemicals and fuels have recentlygained considerable scientific interests [3ndash6] The use ofchar produced via HTC (ie hydrochar) has traditionallybeen focused on agronomic applications Presently it ispossible to find a wide range of uses mainly due to itsproperties and the diversity of materials that can be usedin their synthesis Consequently hydrochars are regardedas valuable materials for various industrial environmentaland agricultural applications Proposed uses of hydrocharinclude an adsorbent a carbon based smart material anenergy source and a soil amendment agent [2ndash5 7ndash18]Typically in comparison with pyrolysis chars hydrocharshave lower carbon content less aromatic structure and areless biologically stable [18] but the HTC process allows forhigher carbon yield and overall energy efficiency when wet

2 ISRN Chemical Engineering

biomass is usedThe significant benefits associated with HTChave led to a number of new research projects onwaste streamcarbonization [5 18 19] HTC has shown enormous potentialas a green waste conversion technique ultimately convertingwaste materials to value-added products while promotingcarbon recovery in the solid phase [5 18ndash20]

To date there remain relatively few studies evaluatingHTC of waste materials Reported works on sewage sludgewere even more limited Notable studies on hydrothermaltreatment of sewage sludge include Shanableh [21] Mummeet al [22] and Escala et al [23] However none of theseworks have reported extensively on the effect of reactiontime coupled to the mild HTC conditionThe main objectiveof the present work was to investigate the amenability ofsewage sludge for hydrothermal carbonization and to obtainhydrochar that is a solid energy carrier with a low energydemand The HTC process as well as the characteristics ofthe obtained hydrochar and the liquid filtrate product was thefocus of this experimental study

2 Materials and Methods

21 Raw Materials Sewage sludge was obtained from awastewater treatment plant of Chiang Mai University Thai-land It was collected from the decantation and the secondaryprocess and then subjected to anaerobic stabilization anddryingThewaste sample was dried at 105∘C for 4 h to achieveconstant weight and then comminuted and sieved into auniform size It was ground to particle diameter of less than1mm and stored in a desiccator for later use All chemicalsused were of analytical grade from J T Barker Co Ltd(Bangkok Thailand)

22 Carbonization Process The HTC experiments were car-ried out in a 1000mL pressure reactor equipped with anexternal resistance heater and internal sensors for pressureand temperature Dried sewage sludge (100 g) was dispersedin 300mL of distilled water contained in a round-bottomedflask Oxalic acid was used as a catalyst It was added to thereactor at 1 1 catalyst to feed mass ratio The mixture wasshaken vigorously to create a homogeneous suspension ItspH was about 6 It was left at room temperature overnightshaken again and then transferred to the reactor The heaterwas then switched on The reactor was heated up to 200∘C ata heating rate of about 5∘Cmin It was kept at this conditionfor 1 2 4 and 6 h after which the heater was turned off andthe heat insulation was dismounted The reactor was allowedto cool down for about 15 h to ambient condition

The solid (hydrochar) and liquid products were collectedand subsequently separated by filtration The hydrochar waswashed thoroughly with hot distilled water and dried in anoven for 4 h at 105∘C At least three tests were performedfor each condition The hydrochar samples obtained weredenoted according to carbonization time as H-1 H-2 H-4and H-6 respectively

23 Characterization of Products The raw material and thehydrochars were analyzed for their chemical compositionProximate analysis was conducted using a proximate analyzer

model TGA 701 according to ASTM E 870-82 E 871-82 E872-82 and D 1102-84 standards The LECO elemental ana-lyzers model CHNS-932 andmodel VTF-900 were employedfor ultimate analysis according to ASTMD 3176 and E 775-87standards The surface morphology was studied by scanningelectron microscopy Hydrochar imaging was carried outusing a JEOL JSM-5910 LV Scanning Electron MicroscopeDuring the SEM analysis char samples were selected andimaged randomly to minimize bias Magnifications between500x and 1000x were typically used The standard testmethod for the determination of adsorption characteristicsin terms of iodine number of hydrochars was used (ASTMD4607-94 2002)

Kjeldahl method molybdenum-bluemethod and atomicemission spectroscopy (Perkin Elmer ICP-AES model Opti-ma 3000) were used to determine the nutrient content of theliquid product The interesting elements in this study wereN P and K All analytical determinations were performed intriplicate and average results were presented

3 Results and Discussions

31 Hydrochars Characteristics of sewage sludge and itshydrochars were evaluated to study the changes of theraw material with respect to energy-related properties andchemical components Against dried sewage sludge Table 1shows solid yields ultimate analysis proximate analysis andheating value of the hydrochars

For the raw material it was observed that the driedsewage sludge had low fixed carbon but notably high frac-tions of ash (40) and volatile organic matter (gt50) Thehydrochar yields obtained from the HTC were dependenton the carbonization time in the 74ndash81 range Standarddeviations of the mean yields are also shownThe values herewere higher than those reported by Mumme et al [22] andEscala et al [23] whose yields of sludge hydrochars were36ndash72 and 56ndash69 respectively It should be noted thattreatment conditions were not exactly similar However thetrend was clear that higher recovery of solid product wasfound at shorter reaction time Increasing the carbonizationtime led to a slight decrease in yield due to larger over-all decomposition of biomass material or because carbonexisting in sewage sludge as volatile matter progressivelyreacted and diffused outThe increase in fixed carbon and thedecrease in volatile matter content between the rawmaterialsand hydrochars confirmed that carbonization took placeFor the first 60min only volatile matter at the surface wasremoved At longer reaction times volatile matter in theinterior of the particle would be removed but with moredifficulty than at the surface The fixed carbon content fromHTCwas increased from86 to 93 122 139 and 142 forH-1 H-2 H-4 and H-6 respectively A significant shift in fixedcarbon content was not expected due to an originally highash content and mild HTC conditions applied

Change in elemental composition of the solid materialwas observed as a result of carbonization (Table 1) HTC ofthe sewage sludge led to an increase in the carbon content ofthe solid residue from 49 (sewage sludge) to 56ndash58 in thehydrochar samples The high fraction of carbon was retained

ISRN Chemical Engineering 3

Table 1 Chemical characteristics and properties of dried sewage sludge and its hydrochars from HTC at 200∘C and 1ndash6 h

Sewage sludge H-1 H-2 H-4 H-6Yield ( ww) mdash 812 plusmn 04 800 plusmn 11 771 plusmn 09 736 plusmn 05

Elemental analysis ()C 493 576 561 581 571H 70 64 67 64 68N 84 42 38 42 45O 340 326 310 302 296S 13 11 10 11 10Proximate analysis ()Ash 400 506 497 487 484Volatile matter 515 398 379 374 372Fixed carbon 86 93 122 139 142HHV (MJkg) 206 228 230 234 237Energy densification mdash 111 112 114 115Energy recovery () mdash 913 901 887 854

in the hydrochar products while the rest mainly remainedin the organic compounds that dissolved in the aqueoussimilar to those reported in the literature [24 25] It shouldbe noted that gas generation from HTC was generally small(2ndash5) according to Escala et al [23] and Lu et al [26]Therewas slight reduction in the oxygen and hydrogen contentsThese variations are consistent with the formation of a lesscondensed material Chemical transformation of organicmaterial to carbon rich material or coalification process maybe presented using a van Krevelen diagram [3] Atomic HCand OC ratios for sewage sludge and its hydrochars wereplotted in Figure 1 along with other substances (celluloselignite and subbituminous coals) for comparison Pathwaysfor demethanation dehydration and decarboxylation werealso illustrated It was observed that the HC and OCratios decreased after HTC due mainly to dehydration anddecarboxylation Similar trendwas reported by Parshetti et al[27] for similarly mild HTC condition (150∘C 20min) Itwas likely that further carbonization was possible to increasethe extent of coalification At more severe HTC conditions(higher temperatures or longer times) the HC and OCratios of hydrochars were expected to approach the valuessimilar to those associated with lignite and subbituminouscoals A significant advantage of HTC process is its simplicitythat it only involves heating biomass in water in a confinedsystem Reaction temperature selected was 200∘C becausecarbonization at this condition was reported to be able tosufficiently generate product that is well qualified and similarto coal quality [24] Higher temperatures would increasereaction pressure significantly resulting in increased cost andcomplexity of processing equipment However it was clearthat at the mild HTC condition considered here reactiontimes of up to 6 h did not affect chemical composition ofthe final products significantly The hydrochars producedshowed only slight change in compositions for all conditionsconsidered

Analysis of hydrochar energetic content is also shownin Table 1 High heating value (HHV) was calculated based

08

1

12

14

16

18H

C at

omic

ratio

OC atomic ratio

Demethanation Dehydration

Decarboxylation

0 02 04 06 08 1

Figure 1 van Krevelen diagram of the sewage sludge (◼) and itshydrochars H-1 (∙) H-2 (998771) H-4 (998779) and H-6 (I) includingdata of cellulose (◻) lignite (Q) and subbituminous (loz) coals forcomparisonThe arrows represent demethanation dehydration anddecarboxylation pathways

on Dulongrsquos formula Energy densification was defined asan HHV ratio between the hydrochar and the startingmaterialThe calculatedHHVand energy densification factorwere found to increase with increasing carbonization timeshowing ranges of 228ndash237MJkg and 111ndash115 respectivelyThis observation was in line with the increase in carboncontent of the hydrochars as both HHV and carbon arean indication of the degree of coalification obtained Withrespect to the energy recovery factor defined as a ratiobetween energy content in the hydrochar divided by that inthe sewage sludge it was found to be high ranging from 85to 91

At a fixed reaction temperature reaction time wasexpected to have an influence on hydrochar microscopicproperties Images of representative sewage sludge andhydrochar are shown in Figure 2 From a close inspection


(a)

(b)

Figure 2 SEM images of (a) dried sewage sludge and (b) resultinghydrochars

with SEM a clear difference in surface morphology betweenthe hydrochar and the sewage sludge was observed showingthe rupture of structure due to HTC process The hydrocharappeared to be in brownish black color suggesting that it wasnot completely carbonized It also showed amore uneven androugher surface than the raw material The breakdown of thestructure may be contributed to full or partial degradation ofremaining lignocellulosic components of the raw material

Iodine number is normally used as an indication ofadsorption ability of a particular material Iodine numberof dried sewage sludge was found to be 93 plusmn 4mggFigure 3 shows the results of iodine adsorption capability ofhydrochars The HTC process proved to develop porosityof dried sewage sludge A high iodine number of about222 plusmn 12mgg was found at 1 h It decreased with increasingthe reaction time The HTC temperature used here was notenough to drive the inner volatile matter and tar out Tar andvolatile matter in the pores may block the diffusion of thereacting agent into the structureThus only limited numbersof pores were developed Hence as HTC time increasediodine absorption capacity decreased dramatically becausesmall pore structure was destroyed resulting in larger poresAs a result the ability to absorb iodine decreased Lowporosity of hydrochar implicated here was consistent withthose reported by Parshetti et al [27] and Mumme et al[22] The high temperature and long reaction times were

6

180

200

220

240

Iodi

ne n

umbe

r (m

gg)

Time (h)1 2 3 4 5

Figure 3 Effect of HTC time on iodine adsorption ability of hydro-chars

Table 2 Properties of liquid by-products from HTC at 200∘C and1ndash6 h

L-1 L-2 L-4 L-6N (mgL) 2392 plusmn 10 2393 plusmn 12 2396 plusmn 11 2419 plusmn 15P (mgL) 804 plusmn 8 809 plusmn 11 811 plusmn 6 813 plusmn 10

K (mgL) 1516 plusmn 12 1516 plusmn 9 1517 plusmn 8 1519 plusmn 11

unfavorable for the porous structure of the char The porousstructure cracked and the pores might be partially blockedas a result of the softening and melting of the materialconstituents leading to a poor surface property [24]

32 Liquid Filtrate By-Product It is generally known thatwater plays a significant role as a solvent and reactant inthe HTC process The liquid phase is expected to contain ahigh load of organics and inorganics It may be recycled asa nutrient solution to agricultural lands The liquid filtrateobtained was completely sterile after the HTC at 200∘C Aslight drop in pH of the aqueous phase was observed afterHTC reaction It is noted here that original pH of deionizedwater was neutral The liquid phase was found to be acidicwhich can be explained by the formation of a variety oforganic acids that typically occur during the HTC processIn this work analysis of the liquid product from HTC ofsewage sludgewas carried out for nutrient content Results areshown in Table 2The liquid phase was found to contain highvalues of potassium and nitrogen while the phosphorus wasrelatively low These findings were in qualitative agreementwith those by Escala et al [23] Longer reaction time at thismild HTC condition did not appear to affect the recoveryof nutrient content significantly Further investigation oncontents of heavy metals is needed if this liquid product isto be beneficial for reuse in agriculture

4 Conclusion

In this work hydrothermal carbonization of sewage sludgehas been carried out The waste can be thermally converted


into carbonaceous solid product with high yields of 74ndash81The resulting hydrochar had a higher energetic content andbetter adsorption capability than the starting raw materialLonger carbonization times increased the hydrocharrsquos fixedcarbon and carbon content but decreased its yields and iodineadsorption ability The sterile liquid product contained highnutrients This work demonstrated that the HTC processoffers attractive and alternative technique for the conversionof sewage sludge to value-added products Furtherworksmaybe performed to gain better understanding of the underlyingprocess to characterize related properties and to identifyapplications for these products

Acknowledgments

The authors would like to thank the Department of ScientificServices Ministry of Science and Technology and the MaeMoh Laboratory Electricity Generating Authority of Thai-land for technical support

References

[1] S Krigstin and M Sain ldquoFractionation of dry recycled paper-mill sludge to higher value componentsrdquo Journal of BiobasedMaterials amp Bioenergy vol 1 no 3 pp 315ndash322 2007

[2] K M Smith G D Fowler S Pullket and N J D GrahamldquoSewage sludge-based adsorbents a review of their productionproperties and use in water treatment applicationsrdquo WaterResearch vol 43 no 10 pp 2569ndash2594 2009

[3] M Sevilla and A B Fuertes ldquoThe production of carbon ma-terials by hydrothermal carbonization of celluloserdquoCarbon vol47 no 9 pp 2281ndash2289 2009

[4] M M Titirici and M Antonietti ldquoChemistry and materialsoptions of sustainable carbon materials made by hydrothermalcarbonizationrdquo Chemical Society Reviews vol 39 pp 103ndash1162010

[5] A Funke and F Ziegler ldquoHydrothermal carbonization ofbiomass a summary and discussion of chemical mechanismsfor process engineeringrdquo Biofuels Bioproducts and Biorefiningvol 4 no 2 pp 160ndash177 2010

[6] H A Ruiz R M Rodriguez-Jasso B D Fernandes A AVicente and J A Teixeira ldquoHydrothermal processing as analternative for upgrading agriculture residues and marinebiomass according to the biorefinery concept a reviewrdquo Renew-able amp Sustainable Energy Reviews vol 21 pp 35ndash51 2013

[7] Q Wang H Li L Chen and X Huang ldquoMonodispersed hardcarbon spherules with uniform nanoporesrdquo Carbon vol 39 no14 pp 2211ndash2214 2001

[8] X Cui M Antonietti and S-H Yu ldquoStructural effects of ironoxide nanoparticles and iron ions on the hydrothermal car-bonization of starch and rice carbohydratesrdquo Small vol 2 no6 pp 756ndash759 2006

[9] M-M Titirici A Thomas and M Antonietti ldquoBack in theblack hydrothermal carbonization of plant material as anefficient chemical process to treat the CO

2

problemrdquo NewJournal of Chemistry vol 31 no 6 pp 787ndash789 2007

[10] M M Titirici A Thomas S-H Yu J-O Muller and MAntonietti ldquoA direct synthesis of mesoporous carbons withbicontinuous pore morphology from crude plant material by

hydrothermal carbonizationrdquoChemistry ofMaterials vol 19 no17 pp 4205ndash4212 2007

[11] D Cao Y Sun and G Wang ldquoDirect carbon fuel cell funda-mentals and recent developmentsrdquo Journal of Power Sources vol167 no 2 pp 250ndash257 2007

[12] R Demir-Cakan N Baccile M Antonietti and M-M TitiricildquoCarboxylate-rich carbonaceous materials via one-stephydrothermal carbonization of glucose in the presence ofacrylic acidrdquo Chemistry of Materials vol 21 no 3 pp 484ndash4902009

[13] J P Paraknowitseh A Thomas and M Antonietti ldquoCarboncolloids prepared by hydrothermal carbonization as efficientfuel for indirect carbon fuel cellsrdquo Chemistry of Materials vol21 no 7 pp 1170ndash1172 2009

[14] M Sevilla and A B Fuertes ldquoChemical and structural prop-erties of carbonaceous products obtained by hydrothermalcarbonization of saccharidesrdquo Chemistry vol 15 no 16 pp4195ndash4203 2009

[15] B Hu K Wang L Wu S-H Yu M Antonietti and M-MTitirici ldquoEngineering carbon materials from the hydrothermalcarbonization process of biomassrdquo Advanced Materials vol 22no 7 pp 813ndash828 2010

[16] M C Rillig MWagner M Salem et al ldquoMaterial derived fromhydrothermal carbonization effects on plant growth and arbus-cular mycorrhizardquo Applied Soil Ecology vol 45 no 3 pp 238ndash242 2010

[17] M Sevilla A B Fuertes and R Mokaya ldquoHigh densityhydrogen storage in superactivated carbons from hydrother-mally carbonized renewable organic materialsrdquo Energy andEnvironmental Science vol 4 no 4 pp 1400ndash1410 2011

[18] J A Libra K S Ro C Kammann et al ldquoHydrothermalcarbonization of biomass residuals a comparative review of thechemistry processes and applications of wet and dry pyrolysisrdquoBiofuels vol 2 no 1 pp 71ndash106 2011

[19] ND Berge K S Ro JMao J R V FloraMA Chappell and SBae ldquoHydrothermal carbonization ofmunicipal waste streamsrdquoEnvironmental Science and Technology vol 45 no 13 pp 5696ndash5703 2011

[20] I-H Hwang H Aoyama T Matsuto T Nakagishi and TMatsuo ldquoRecovery of solid fuel from municipal solid wasteby hydrothermal treatment using subcritical waterrdquo WasteManagement vol 32 no 3 pp 410ndash416 2012

[21] A Shanableh ldquoProduction of useful organic matter from sludgeusing hydrothermal treatmentrdquo Water Research vol 34 no 3pp 945ndash951 2000

[22] J Mumme L Eckervogt J Pielert M Diakite F Rupp and JKern ldquoHydrothermal carbonization of anaerobically digestedmaize silagerdquo Bioresource Technology vol 102 no 19 pp 9255ndash9260 2011

[23] M Escala T Zumbuhl Ch Koller R Junge and R KrebsldquoHydrothermal carbonization as an energy efficient alternativeto established drying technologies for sewage sludge a feasibil-ity study on a laboratory scalerdquo Energy amp Fuels vol 27 pp 454ndash460 2013

[24] S M Heilmann H T Davis L R Jader et al ldquoHydrothermalcarbonization of microalgaerdquo Biomass and Bioenergy vol 34no 6 pp 875ndash882 2010

[25] S M Heilmann L R Jader M J Sadowsky F J Schendel MG von Keitz and K J Valentas ldquoHydrothermal carbonizationof distillerrsquos grainsrdquo Biomass and Bioenergy vol 35 no 7 pp2526ndash2533 2011


[26] X Lu B Jordan and N D Berge ldquoThermal conversion ofmunicipal solid waste via hydrothermal carbonization compar-ison of carbonization products to products from current wastemanagement techniquesrdquoWaste Management vol 32 no 7 pp1353ndash1365 2012

[27] G K Parshetti S K Hoekman and R BalasubramanianldquoChemical structural and combustion characteristics of car-bonaceous products obtained by hydrothermal carbonization ofpalm empty fruit bunchesrdquo Bioresource Technology vol 135 pp683ndash689 2012

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Chemical EngineeringInternational Journal of Antennas and

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Navigation and Observation



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biomass is usedThe significant benefits associated with HTChave led to a number of new research projects onwaste streamcarbonization [5 18 19] HTC has shown enormous potentialas a green waste conversion technique ultimately convertingwaste materials to value-added products while promotingcarbon recovery in the solid phase [5 18ndash20]

To date there remain relatively few studies evaluatingHTC of waste materials Reported works on sewage sludgewere even more limited Notable studies on hydrothermaltreatment of sewage sludge include Shanableh [21] Mummeet al [22] and Escala et al [23] However none of theseworks have reported extensively on the effect of reactiontime coupled to the mild HTC conditionThe main objectiveof the present work was to investigate the amenability ofsewage sludge for hydrothermal carbonization and to obtainhydrochar that is a solid energy carrier with a low energydemand The HTC process as well as the characteristics ofthe obtained hydrochar and the liquid filtrate product was thefocus of this experimental study

2 Materials and Methods

21 Raw Materials Sewage sludge was obtained from awastewater treatment plant of Chiang Mai University Thai-land It was collected from the decantation and the secondaryprocess and then subjected to anaerobic stabilization anddryingThewaste sample was dried at 105∘C for 4 h to achieveconstant weight and then comminuted and sieved into auniform size It was ground to particle diameter of less than1mm and stored in a desiccator for later use All chemicalsused were of analytical grade from J T Barker Co Ltd(Bangkok Thailand)

22 Carbonization Process The HTC experiments were car-ried out in a 1000mL pressure reactor equipped with anexternal resistance heater and internal sensors for pressureand temperature Dried sewage sludge (100 g) was dispersedin 300mL of distilled water contained in a round-bottomedflask Oxalic acid was used as a catalyst It was added to thereactor at 1 1 catalyst to feed mass ratio The mixture wasshaken vigorously to create a homogeneous suspension ItspH was about 6 It was left at room temperature overnightshaken again and then transferred to the reactor The heaterwas then switched on The reactor was heated up to 200∘C ata heating rate of about 5∘Cmin It was kept at this conditionfor 1 2 4 and 6 h after which the heater was turned off andthe heat insulation was dismounted The reactor was allowedto cool down for about 15 h to ambient condition

The solid (hydrochar) and liquid products were collectedand subsequently separated by filtration The hydrochar waswashed thoroughly with hot distilled water and dried in anoven for 4 h at 105∘C At least three tests were performedfor each condition The hydrochar samples obtained weredenoted according to carbonization time as H-1 H-2 H-4and H-6 respectively

23 Characterization of Products The raw material and thehydrochars were analyzed for their chemical compositionProximate analysis was conducted using a proximate analyzer

model TGA 701 according to ASTM E 870-82 E 871-82 E872-82 and D 1102-84 standards The LECO elemental ana-lyzers model CHNS-932 andmodel VTF-900 were employedfor ultimate analysis according to ASTMD 3176 and E 775-87standards The surface morphology was studied by scanningelectron microscopy Hydrochar imaging was carried outusing a JEOL JSM-5910 LV Scanning Electron MicroscopeDuring the SEM analysis char samples were selected andimaged randomly to minimize bias Magnifications between500x and 1000x were typically used The standard testmethod for the determination of adsorption characteristicsin terms of iodine number of hydrochars was used (ASTMD4607-94 2002)

Kjeldahl method molybdenum-bluemethod and atomicemission spectroscopy (Perkin Elmer ICP-AES model Opti-ma 3000) were used to determine the nutrient content of theliquid product The interesting elements in this study wereN P and K All analytical determinations were performed intriplicate and average results were presented

3 Results and Discussions

31 Hydrochars Characteristics of sewage sludge and itshydrochars were evaluated to study the changes of theraw material with respect to energy-related properties andchemical components Against dried sewage sludge Table 1shows solid yields ultimate analysis proximate analysis andheating value of the hydrochars

For the raw material it was observed that the driedsewage sludge had low fixed carbon but notably high frac-tions of ash (40) and volatile organic matter (gt50) Thehydrochar yields obtained from the HTC were dependenton the carbonization time in the 74ndash81 range Standarddeviations of the mean yields are also shownThe values herewere higher than those reported by Mumme et al [22] andEscala et al [23] whose yields of sludge hydrochars were36ndash72 and 56ndash69 respectively It should be noted thattreatment conditions were not exactly similar However thetrend was clear that higher recovery of solid product wasfound at shorter reaction time Increasing the carbonizationtime led to a slight decrease in yield due to larger over-all decomposition of biomass material or because carbonexisting in sewage sludge as volatile matter progressivelyreacted and diffused outThe increase in fixed carbon and thedecrease in volatile matter content between the rawmaterialsand hydrochars confirmed that carbonization took placeFor the first 60min only volatile matter at the surface wasremoved At longer reaction times volatile matter in theinterior of the particle would be removed but with moredifficulty than at the surface The fixed carbon content fromHTCwas increased from86 to 93 122 139 and 142 forH-1 H-2 H-4 and H-6 respectively A significant shift in fixedcarbon content was not expected due to an originally highash content and mild HTC conditions applied

Change in elemental composition of the solid materialwas observed as a result of carbonization (Table 1) HTC ofthe sewage sludge led to an increase in the carbon content ofthe solid residue from 49 (sewage sludge) to 56ndash58 in thehydrochar samples The high fraction of carbon was retained







08

1

12

14

16

18H

C at

omic

ratio

OC atomic ratio


Decarboxylation

0 02 04 06 08 1





(a)

(b)




6

180

200

220

240

Iodi

ne n

umbe

r (m

gg)

Time (h)1 2 3 4 5







4 Conclusion




Acknowledgments


References










2
























RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation















08

1

12

14

16

18H

C at

omic

ratio

OC atomic ratio


Decarboxylation

0 02 04 06 08 1





(a)

(b)




6

180

200

220

240

Iodi

ne n

umbe

r (m

gg)

Time (h)1 2 3 4 5







4 Conclusion




Acknowledgments


References










2
























RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










(a)

(b)




6

180

200

220

240

Iodi

ne n

umbe

r (m

gg)

Time (h)1 2 3 4 5







4 Conclusion




Acknowledgments


References










2
























RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation











Acknowledgments


References










2
























RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation














RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









research article recovery of value-added products from...

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