1

Anovelindexforthestudyofsynergisticeffectsduringtheco-processingofcoalandbiomass1

JumokeM.OLADEJO1,StephenAdegbite1,ChengHengPang2,HaoLiu3,AshakM.Parvez1,TaoWu2,4,*2

1DepartmentofChemicalandEnvironmentalEngineering,TheUniversityofNottinghamNingboChina,Ningbo3

315100,China4

2Municipal Key Laboratory of Clean Energy Conversion Technologies, TheUniversity ofNottinghamNingbo5

China,Ningbo315100,China6

3DepartmentofArchitectureandBuiltEnvironment,TheUniversityofNottingham,NottinghamNG72RD,The7

UK8

4NewMaterialsInstitute,TheUniversityofNottinghamNingboChina,Ningbo315100,China9

Correspondingauthor:tao.wu@nottingham.edu.cn10

11

Abstract12

In this study, synergistic interaction between coal and biomass and its intensity was13

investigated systematically using a low rank coal and its blends with different biomass14

samplesatvariousblendingratios.Thecatalyticeffectsofmineralsoriginatedfrombiomass15

werealsostudied.Itwasfoundthatsomeofthemineralsexistingintheashderivedfrom16

oat straw catalysed the combustions process and contributed to synergistic interactions.17

However, for the coal and rice husk blends, minimal improvements were recorded even18

when the biomass and coal blending ratio was as high as 30 wt%. Biomass volatile also19

influencedtheoverallcombustionperformanceoftheblendsandcontributedtosynergistic20

interactionsbetweenthetwofuelsintheblends.Basedonthesefindings,anovelindexwas21

formulatedtoquantifythedegreeofsynergisticinteractions.Thisindexwasalsovalidated22

usingdataextracted from literatureandshowedhighcorrelationcoefficient. Itwas found23

that at a blending ratio of 30 wt% of oat straw in the blend, the degree of synergistic24

interaction between coal and oat straw showed an additional SF value of 0.25with non-25

catalyticandcatalyticsynergisticeffectcontributing0.16(64%)and0.09(36%)respectively.26

2

Thisindexcouldbeusedintheselectionofrighttypeofbiomassandproperblendingratios27

forco-firingatcoal-firedpowerstations,whichintendtoimprovecombustionperformance28

ofpoorqualitycoalbyenhancingsynergisticinteractionsduringco-processing.29

Keywords–Fuelcharacterisation;synergisticinteraction;performanceindex;synergyindex;30

thermogravimetricanalysis31

1.0Introduction32

Thelowcostandcarbonleannatureofbiomassmakeitapromisingenergyalternativefor33

themitigationofCO2emissions[1,2].However, thetechnical,economicandsocio-ethical34

issues associatedwith the large-scale utilization of biomass have hindered its large-scale35

development [3, 4]. One of the feasible solutions to mitigate these issues is to cofire36

biomasswithcoal.Thisapproachhasbecomeageneralpracticeinwesterncountriesas it37

offers significant social and environmental benefits such as energy security, energy38

sustainability,greenhousegasemissionreduction,andeconomicdevelopments[1].39

In the past few decades, extensive research has been carried out in understanding the40

suitability of coal/biomass blends in various thermochemical conversion processes [5-7].41

Synergisticeffectwasobservedforsomeblends[1,8]whileinsignificantadditivebehaviour42

wasalsoobservedforsomeotherblends[9,10].Thesynergyobservedincoal/biomassfuel43

blends was mainly attributed to both catalytic and non-catalytic synergistic effect of44

biomass constituents and their influence on the coal during co-firing. The non-catalytic45

synergistic effect is mainly associated with the high volatile content in biomass while46

catalyticsynergisticeffect isdictatedbyAlkaliandAlkaliEarthMetals (AAEMs) inbiomass47

which have catalytic impacts on the reactivity of chars derived from coal [11, 12].48

Nonetheless,eventhoughallbiomasshaveAAEMspecies,thepresenceofsynergyandits49

3

intensityisdependentonthephysical/chemicalpropertiesofthefuels,especiallytheAAEM50

contents[13].51

Todate,muchefforthasbeenmadetounderstandtheinfluenceofAAEMsonthecatalytic52

influence on co-processing of biomass with coal. Many researchers have studied the53

catalyticperformanceofashderivedfromhightemperatureashingprocess(≥550°C)or54

someashelements,suchasK,CaandSi [14].However,someAAEMspeciesarenormally55

releasedatverylowtemperatures(<500°C)[15].Thereforetheuseofhightemperatureash56

ascatalystdidnotshowthecatalyticeffectofAAEMsoriginatedfrombiomass.Sofar,not57

much work has been carried out to show the catalytic effect of minerals in biomass. In58

addition,althoughsynergistic interactions[1,8]havebeenstudiedgreatly inthepast few59

decades, there is notmuch effort beingmade to distinguish the contribution of catalytic60

effectandnon-catalyticeffectontheoverallsynergisticinteractionsoccurring,needlessto61

say there is a reliable approach to quantify synergistic interactions and the contribution62

fromcatalyticandnon-catalyticfactors.63

Thispaperfocusesonthesynergisticinteractionsbetweencoalandbiomassintheblends.64

Thermogravemetric analysis (TGA) was conducted to understand the catalytic effects of65

minerals(AAEMs)frombiomassandthenon-catalyticeffectsofvolatilemattersontheco-66

processingofbiomasswithcoal.Anovelindicatorwasthereforeproposedtoevaluatethe67

extentofsynergisticinteractionsaswellastoquantifythecontributionofcatalyticandnon-68

catalyticeffectstotheseinteractions.69

4

2.0Experimental70

2.1CoalandBiomassSamples71

Onecoalandtwotypesofbiomasswereusedinthisresearch.Thecoal,Yunnan(YC),was72

obtained fromFuyuan town (YunnanProvince, China),which ismainly used for industrial73

process heating especially inwine-making industry. The biomass samples,Oat Straw (OS)74

andRiceHusk(RH),werechosentorepresentagriculturalwasteandagro-industrialresidue75

respectivelyduetotheirabundanceglobally.76

2.1.1SamplePreparation77

The samples were prepared following standard procedures described elsewhere (BS EN78

14780andISO13909)[16,17].Allthesampleswereinitiallyreducedtoasizesmallerthan79

500µmusinga cuttingmill (RetschSM2000,Germany), and furthermilled tobe smaller80

than106µmusingaRetschSM200mill.Eachbiomasswasblendedwiththecoalinthree81

massfractions,i.e.,10,30and50wt%.82

2.2Proximate,UltimateandHeatingValueAnalyses83

Proximateanalysiswasperformedusingthethermo-gravimetricanalyser(TGA)(STA449F384

Netzsch,Germany)whileultimateanalysisof thesampleswasconductedusingaPE240085

SeriesIICHNS/OAnalyzer(PerkinElmer,USA).InaTGAtest,approximately5–10mgofthe86

samplewasplacedinanaluminacruciblefollowingatestingproceduredescribedelsewhere87

[18,19].Forultimateanalysis,approximately1.5mgofsamplewasplacedinaplatinumfoil88

pan. Thehigherheating value (HHV)of a samplewasmeasuredusingan IKACalorimeter89

C200 (IKA, USA), which utilized approximately 1.0 g of the sample. All experimentswere90

repeatedatleastthreetimeswiththeaveragevalueusedasthefinalvalue.91

2.3MineralCompositionofFuel92

5

MineralcompositionoftheunblendedfuelswasdeterminedbyusinganX-rayFluorescence93

(XRF)spectrometer,theprocedureadoptedisdescribedelsewhere[20].94

2.4ThermalAnalysis95

Combustioncharacteristicsof individual fuelsandtheirblendsweremeasuredfollowinga96

non-isothermal method, which was amended from elsewhere [21, 22]. In the test, the97

samplewasheated in air (80 vol%Nitrogenand20 vol%Oxygen) from50 to900 °Cat a98

heatingrateof20°Cmin-1andagasflowrateof50mlmin-1.Characterisationofpyrolysis99

wasalsoconductedusingthesametechniqueunderpurenitrogenatmosphere(>99.9%).All100

experimentswererepeatedatleastthreetimestoensurerepeatabilityandaccuracy.101

The initiation temperature (IT) is the temperatureatwhich0.3wt%mass loss rateof the102

samplewasachievedafterthereleaseofmoisture,whichisnormallyusedasanindication103

of the start of fuel decomposition. In fuel characterisation, the peak temperature (PT) is104

considered inversely proportional to the reactivity/combustibility of the fuel, which was105

determined as the temperature where the weight loss (!"!") of the sample reached its106

maximum.Theburnout temperature (BT) represents theend temperatureof theburning107

process,whichwas determined as the temperaturewhen the rate of burnout (mass loss108

rate)decreasedtolessthan1wt%min-1onweightbasis.Theignitiontemperatureatwhich109

thefuelburnsspontaneouslywithoutexternalheatsourcewasalsoobtainedbasedonthe110

methodadoptedbymanyothers[23].111

2.4PerformanceIndices112

Theignition(Zi)andcombustion(S)indexofthefuelandtheirblendswerecalculatedbased113

ontheEquations(1)and(2)[23].114

6

Z! =!"!" !"#!!!!"#

× 10! (1)115

S = !"!" !"#

!"!" !"!!!!!

× 10! (2)116

Where:117

(!"!")!"#isthemaximumrateofmassloss(%min-1);118

(!"!")!"istheaveragerateofmassloss(%min-1);119

t!"#isthetimeatwhichthepeakmasslossrateisattained(min);120

t!istheignitiontime(min);121

T!istheignitiontemperature(°C);122

T!istheburnouttime(min).123

2.5LowTemperatureAshing124

The low temperature ashing of biomass samples was performed using a PR300 Plasma125

Cleaner (Yamato Scientific, Japan). This device was used to burn off the carbonaceous126

componentsofthesampleatlowtemperatures(lessthan150⁰C)underwhichthepresence127

ofmineralsinbiomassremainsunchanged.Theplasmawasgeneratedatapowerof200W.128

Approximately 0.5 g of a sample was loaded on a glass crucible, placed in the ashing129

chamber, and exposed to pure oxygen at a flow rate of 100mlmin-1 to ensureminimal130

reflectionoftheplasmabeam.Eachashingexperimentrequired30hoursforthecomplete131

burningofcarbonaceousmaterials.132

7

2.6CatalyticEffectofBiomass-DerivedAsh133

To understand the influence of minerals from biomass on combustion process, low134

temperature ash of each biomass was blended with Yunnan coal at a blending ratio135

equivalentto30wt%biomassinblend.Theintrinsicreactivityoftheseblendswascarried136

out.137

3.0 ResultsandDiscussion138

3.1 Proximate,UltimateandHeatingValueAnalyses139

Resultsofultimateandproximateanalysesof thesamplesareshown inTable1.Thecoal140

sample showed the highest heating value, which suggests that the blending of coal with141

biomass of lower energy content would normally lead to the reduction in combustion142

temperatureinexistingutilityboilers[24].TheRiceHuskhadverysimilarsulphurcontent143

(0.4wt%)butsignificantlydifferentashcontent(21.2wt%)comparedwithOatStraw.Oat144

Strawhadthehighestvolatilematter(72.1wt%).145

Table1–Ultimateandproximateanalysisofsamples146

RiceHusk(RH) OatStraw(OS) YunnanCoal(YC)

Ultimateanalysis(wt%,daf)

Carbon 50.1 47.5 86.2

Hydrogen 7.4 6.8 5.1

Nitrogen 1.7 2.3 1.0

Sulphur 0.4 0.3 1.1

Oxygen(bydifference) 40.4 43.2 6.6

LHV(MJKg¯¹) 19.6 17.6 33.5

Proximateanalysis(wt%)

Moisture 4.1 4.0 4.5

8

VolatileMatter(VM) 62.8 72.1 27.2

FixedCarbon(FC) 11.9 17.4 57.3

Ash 21.2 6.5 11

Mineral composition of the samples is illustrated in Table 2. The biomass samples had147

relatively lowsulphurcontent,whichhelpsmitigatetheenvironmental impactsassociated148

withtheemissionofsulphuroxides(SOx).Normally,thereactionofAAEMsoriginatingfrom149

biomasswithSOxcouldleadtotheformationofsulphateswhichcontributestothecapture150

ofgasphasesulphur[25].Thisisanimportantadvantageofco-firingofcoalwithbiomass,151

especiallyforcoalsofrelativelyhighsulphurcontent,suchasYunnancoal.152

Normally, alkali metals, such as potassium (K) and sodium (Na), and alkali earth metals153

(AAEMs),suchascalcium(Ca)andmagnesium(Mg),areknowntohavecatalyticeffectto154

thethermaldecompositionof fuels [26].Table2showstheelementalcompositionof low155

temperatureashderived fromall samples studied. TheOS,RHandYChadhighAAEMof156

61.6wt%,26.9wt%and25.5wt%respectively.Thehighpotassiumcontent intheOSand157

RHandthehighcontentofcalciuminYCsuggesttheirlikelihoodofenhancingcombustion158

performance. Another interesting element that has been known to aid the release and159

activationof these catalyticAAEMs isCl,whichwas veryhigh inOS (24.2wt%) [27]. This160

furthersupportsthehighpotentialincatalyticeffectwhenOSisblendedwithYC.However,161

it was reported that the enhancement could be weakened by the reaction between the162

catalyticminerals, suchasAAEMs,with silicates and/or alumina-silicates [28]. Thismeans163

that the high Si content in RH (45%) and YC (26.2%)might hinder the catalytic effects of164

AAEMs. Nonetheless, the potential of enhanced catalytic effects of the YC and OS fuel165

blendsremainedpositiveduetothehighAAEM-to-Siratio.However,thehighSicontentof166

9

RHmightstillhindersuchimprovementsforYCandRHblends.Inaddition,itmustbenoted167

that agglomeration and clinkering may arise when a biomass fuel has high Na and K168

contentsasobservedinOSduetotheformationofstickylowtemperaturemeltsofsilicate169

eutectics[29].170

Table2–Mineralcompositionofthesamples(wt%)171

Elements RiceHusk(RH) OatStraw(OS) YunnanCoal(YC)

Fe 5.4 1.5 21.0

K 20.2 47.4 4.8

Si 45.0 8.8 26.2

P 15.8 3.1 -

Ca 4.6 14.2 20.7

S - - 20.5

Cl 5.0 24.2 -

Na 0.6 - -

Al 0.3 0.3 1.4

Ti - 3.8

Mg 1.5 - -

Mn 1.6 - -

Br

0.4 1.6

3.2 IntrinsicReactivity172

3.2.1 ReactivityofIndividualFuels173

ThethermaldecompositioncurvesofOS,RHandYCisshowninFigure1withkeyfeatures174

extracted and summarized in Table 3. It is evident that YChadonemajor decomposition175

stagewitha strongpeak for charburnoutwhile thebiomass sampleswere featuredwith176

twomainmasslossstagesrepresentingthedecompositionoforganiccompoundsinthefuel.177

For the biomass samples, the first stage in the range of 144 – 420 °C represented the178

10

decomposition of hemicellulose, cellulose and partial decomposition of lignin [30]. The179

secondstage representedmainlycharburnoutaswellas thedecompositionof ligninand180

fellintherangeof378-518°C.181

ItisshowedthatthedegradationofOSandRHbeganat144and166°Crespectively.Both182

samplesexhibitedaninitialslowmasslossfrominitiationtillabout255°Cduetotheslow183

decompositionoflignincontent.Whentemperaturewasraisedabovethispoint,themass184

lossrateincreasedrapidlyandreachedthepeaktemperaturesof299and309°C,forOSand185

RHrespectively,attributedmainlytothedecompositionofhemicelluloseandcellulose.186

AsshownintheDTGcurveofRH,themasslossrateincreasedimmediatelyafterthefirst187

reactionzone,whileforOS,aflatmasslossregionwasobservedbeforethesecondreaction188

zonewhichshowedasharpincreaseinDTGrate.ThissuggestsalowerreactivityoftheOS189

charparticlesandhighermasslossrateathighertemperatures.190

191

Figure1:DTGcurvesoatstraw,ricehuskandYunnancoal192

11

The difference in the 2nd stage reactivity could be linked to the catalytic influence of the193

mineralcontentsofOS. Itwasfound[31]thatcatalyticeffectofpotassiumcontributedto194

the clear distinction of the two devolatililization peaks and shifted the first peak195

temperature to a lower temperature. In this study, it is believed that thehighpotassium196

content in OS (as shown in Table 2) enhanced the complete decomposition of lighter197

volatilespeciesandthereleaseofmorevolatiles,whichsubsequentlyledtotheformation198

ofmoreporouscharwithhigheroverallburnoutreactivity.199

YC decomposed at a temperature range between 329 and 605 °C with its only peak200

appearing at 535 °C and exhibited a more synchronized mechanism of thermal201

decomposition. In this study, the comparison of the combustion and pyrolysis profiles202

showedthat83wt%(RH)and97wt%(OS)oftotalvolatilesinbiomasssampleswereburnt203

during the first reaction stage as an indication of its homogenous (gas-phase) ignition204

mechanism.ThisisrelativelyunclearforYCduetoitssingularpeakasitsdegradationcurve205

couldbeindicativeofthesimultaneouscombustionofbothvolatilesandcharoverawider206

temperaturerange.207

From the pyrolysis profiles shown in Figure 2, it is evident that the devolatilization of YC208

occurredathighertemperatures(355–571°C)comparedwithOS(146-489°C)andRH(168209

–486°C).ThelowpyrolysisrateandthehightemperaturerequiredforYCcouldsignifyan210

increaseinresistivityofvolatilereleaseintheorganicstructure.Thepyrolysismasslossrate211

of the biomass samples remained close to that of the combustion profile, which can be212

explainedbytheirhighcombustibilityandreactivityofthevolatilematter[32].213

Thedecreasingpeaktemperaturesofthebiomasssamplesduringcombustionasshownin214

Figures2b-csuggestedtheenhancedreactivityduringcombustioncomparedwithpyrolysis.215

12

Howeverthisreductioninthepeaktemperature isnotobvious inFigure2abecauseof its216

lowvolatilecontentofYC.217

218

219

220

a.

b

c

13

Figure2:Pyrolysisandcombustionprofilesof(a)Yunnancoal,(b)RiceHusk,and,(c)Oat221

Straw222

Normally, higher oxygen content of the biomass samples is an indicator of their high223

reactivity [33].Among these three fuels studied, themost reactive fuel isOSwithoxygen224

contentof43.2wt%asshowninTable1.Thehighoxygencontentandhighoxygen/carbon225

ratioledtotheformationofcharwithhigherreactivity[19].Likewise,thehighvolatileand226

lowfixedcarboncontentofbiomassresultedintheyieldofasmallamountofhighlyporous227

char,whichsubsequentlycontributedtothehighoverallreactivityofthefuel.228

For RH, OS and YC, the ratio of volatile matter to fixed carbon, another indicator of229

combustionreactivity,is5.3,4.2and0.48respectively.Thisratioisanindicatorofthefuel’s230

volatility, a ratio >4 suggests homogenous oxidation of the volatileswhile a ratio smaller231

than1 indicatesheterogeneousgas-solid reactions [22]. Therefore, the combustionofRH232

andOSwaspredominantlythegaseousphaseoxidationof itsvolatileswhile forYC itwas233

thesimultaneousoxidationofbothvolatilesandchar.234

Table3–CombustioncharacteristicsofRicehusk,Yunnancoalandtheirblends235

Property RH

50wt%YC+

50wt%RH

70wt%YC+

30wt%RH

90wt%YC+

10wt%RHYC

InitiationTemperature(°C)

166 192 222 222 329

FirstR

eaction

Zone

Temperaturerange(°C)

166-370 192-369 222-356 286-608 329-605

PeakTemperature(°C)

309 308 313 532 535

14

Totalmassloss(wt%)

49.3 24.4 13.1 85 88

Averagemasslossrate(wt%min-1)

4.8 2.8 1.9 5.3 6.4

Maximummasslossrate(wt%min-1)

11.7 4.9 3.2 13.4 15.5

Second

ReactionZo

ne

Temperaturerange(°C)

378-503 378-601 364-601

PeakTemperature(°C)

411 519 531

Totalmassloss(wt%)

18.8 53 66.5

Averagemasslossrate(wt%min-1)

3 4.8 5.6

Maximummasslossrate(wt%min-1)

4.1 7.8 11.1

BurnoutTemperature(°C)

503 601 601 608 605

ResidualWeightatburnout(wt%)

26.7 18.4 16.5 13.1 11.9

Table4–CombustioncharacteristicsofOatstraw,Yunnancoalandtheirblends236

OS

50wt%YC+

50wt%OS

70wt%YC+

30wt%OS

90wt%YC+

10wt%OS

YC

InitiationTemperature(°C) 144 162 201 244 329

FirstR

eactionZo

ne

Temperaturerange(°C) 144-420 162-346 201-345 244-334 329-605

PeakTemperature(°C) 299 299 301 305 535

Totalmassloss(wt%) 65 27.4 13.1 6 88

Averagemasslossrate 4.7 3 1.8 1.3 6.4

15

(wt%min-1)

Maximummasslossrate(wt%min-1)

13.9 7.2 2.9 1.8 15.5

Second

ReactionZo

ne

Temperaturerange(°C) 432-518 349-564 353-583 339-591

PeakTemperature(°C) 474 456 483 515

Totalmassloss(wt%) 17.6 58.1 68.7 79.9

Averagemasslossrate(wt%min-1)

4.1 5.4 5.9 6.3

Maximummasslossrate(wt%min-1)

6.8 8.7 11.1 13.9

BurnoutTemperature(°C) 518 564 583 591 605

ResidualWeightatburnout(wt%)

11.8 11.2 14.2 10.5 11.9

3.2.2CombustionCharacteristicsoftheBlends237

CombustioncharacteristicsoftheYC/RHblendsarepresentedinFigure3andTable3.238

239

Figure3:DTGcurveofYunnancoal/Ricehuskblends240

16

Generally speaking, theblends featured twopeaks.However, the first peakwasnot fully241

developed for theblendwith10wt%RH.Aspreviouslydescribedbyothers [34], the first242

peaktemperatureoftheblendwassimilartothefirstpeaktemperatureofthebiomass,i.e.243

rice husk (309°C), while the second peak temperature and burnout temperature were244

similar to thepeak (535 °C ) andburnout temperatures (605 °C)of theYunnanCoalwith245

minimaldeviations.Themaximumrateofdegradationofthefirstpeakincreasedwiththe246

increaseintheRH,whileforthesecondstage,theratereducedwiththeincreaseinRH.This247

occurrencewasdue to thecombustionofbiomassvolatilesprevailing in the first reaction248

zone, while the coal char burning dominated the second reaction zone [35]. It was also249

observedthattheresidualweightatburnouttemperatureincreasedwiththeincreaseinRH250

duetothehighashcontent,whichmightpresentextrabarrierforheatandmasstransfer.251

SimilartoYC/RHblends,theYC/OSblends,asshowninFigure4andTable2,hadtwodistinct252

peaks.However,therewasanoticeabledecreaseinthepeaktemperaturewiththeincrease253

inoatstraw.SincethepeakandburnouttemperatureofOSwere lowerthanthoseofYC.254

This reduction in the2ndpeak temperature indicated improved combustion reactivity as a255

result of synergistic interactions between coal and biomass as shown in in Figure 4b. To256

further prove the presence of synergy, the experimental resultswere comparedwith the257

theoreticalvaluescalculatedusingtheweightedsumofthepurefeedstock[36].Theresult258

obtained for the oat straw blend showed distinct shift of the 2nd reaction stage towards259

lower temperatures compared with theoretical values. However, the theoretical and260

experimentalvaluesofricehuskblendsweresimilarwhentheblendingratiowasbelow30261

wt%,whilefor50wt%,theshifttowardslowertemperaturesdidbecomenoticeable.262

17

Synergistic interactionscanbeassociatedwithcatalytic and/ornon-catalyticmechanisms.263

Thelatterinvolvestheformationoffreeradicalsandhydrogentransferfrombiomasstocoal264

while the former is based on catalytic effect of alkali and alkali earth metals present in265

biomassorcoal[37].Consequently,thesynergyobservedinYC/OSblendscouldbepartially266

attributedtothecatalyticeffectofmineralmattersinoatstrawduetoitshighalkalimetal267

content, a common occurrence in herbaceous biomass [14, 31, 38]. This could be268

supplemented by the non-catalytic improvement caused by the interactions of biomass269

volatileswithcoalcharaswellasthedifferencesinmorphology[39].Thereleaseofvolatiles270

from biomass could result in the formation of free radicals during thermal reaction to271

promote the breakdown of the dense and heat-resistive coal structural components272

(polycyclicaromatichydrocarbonbondedbyaromaticrings)atlowertemperatures[40,41].273

Therefore,thehigherhydrogen-carbonmoleratio(H/C)ofbiomassinblendscontributedto274

the improvementsobserved in coal decomposition [42, 43]. Theexistenceof synergywas275

quitecontentious,assynergisticinteractionisnotobservedinallcoal/biomassblendsinthe276

firstplace.Howeversynergisticenhancementwasobservedincoalandbiomassfuelblends277

evenafterthedemineralisationofitsbiomass,eliminatingcatalyticeffectofashastheonly278

causeofsynergy[31].279

Atlowblendingratios,suchas10wt%to30wt%,peaktemperatureofeachreactionzone280

was dominated by the fuel fraction with the higher mass loss. This mechanism of281

decompositionisanindicationofindependentdecompositionofbothfuels,whichsuggests282

the additive behaviours instead of synergistic interactions between YC and RH, which is283

similar to what was reported by others [44]. However, slight reduction in the peak284

temperature of the second reaction zone was observed in the 50 wt% RH blend, which285

18

suggestedsomeinteractionsbetweenYCandRH.Thiscanonlybeattributedtotheincrease286

involatilesavailablefrom50%RHanditsimpactsonnon-catalyticsynergymechanism.287

288

289

Figure4:DTGcurveofYunnancoal/oatstrawblends290

Taking into account the findings for both biomass blends, it can be concluded that the291

presence of synergy, its extent and the mechanism are dependent on biomass types,292

blendingratioandproperties.293

3.3 IgnitionTemperature294

The ignition temperatures (Ti) of the fuels are shown in Table 5,whichwere determined295

followingthemethoddescribedelsewhere[8].Theeaseofignitionofthebiomasssamples296

isaconsequenceoftheirhighvolatilecontent(>80wt%)asshowninTable1.Theignition297

temperature of YC was almost 200 °C higher than those of the biomass samples, which298

mightbeduetothehetero-homogeneousignitionmodeofthiscoal[11].299

19

Howeverthemainmasslossoftheblendswascharacterisedbythe2ndpeaktemperature,300

whichmore accurately depicted the effect of biomass addition on the oxidation of coal.301

Hence,a“triggertemperature”wasalsoextractedfromtheTGAprofilesforthe2ndreaction302

stage to characterise the ignition of the char oxidation, which is the temperature303

correspondingtothe intersectionof thetangent lineof the initialmass losscurve(before304

thesharpdropinmass)andthetangentlinethatisdrawnattheintersectionofthevertical305

linethroughthe2ndPeaktemperatureandthemasslosscurve[23].306

TheignitionpointsofYC/RHandYC/OSblendswereslightlyhigher(<20°C)thantheignition307

temperatureoftheOS(256°C)andRH(266°C)intheblend.Thissuggestsweakinteractions308

between fuels in the blends. The 10 wt% RH blend remained close to the ignition309

temperatureofYCduetotheimmaturefirstpeakasseeninFigure3a.However,thetrigger310

temperatures reduced significantly compared with that of YC (459 °C). The changes in311

trigger temperature with blending ratio are shown in Figure 5 with the dotted lines312

representing predictions based on additive behaviours. The blends exhibited a slow drop313

between10and30wt%,followedbyasharpdropinthistemperaturebetweenthe30and314

50wt%.The50wt%OSblendsexhibitedthelargesttemperaturedecrease.Thesechanges315

inignitionparameteraretheresultofinteractionsbetweenindividualfuelsintheblends.316

Table5–Ignitiontemperatureofindividualfuelsandtheirblends317

SampleMainIgnition

Temperature(°C)

CharTrigger

Temperature(°C)

100%RH 266

90wt%YC+10wt%RH 451 451

70wt%YC+30wt%RH 272 438

50wt%YC+50wt%RH 268 386

20

100%OS 256

90wt%YC+10wt%OS 271 439

70wt%YC+30wt%OS 256 403

50wt%YC+50wt%OS 259 301

100%YC 459

Thechartriggertemperatureswerealsocomparedwiththeoreticalvaluescalculatedfrom318

ignition temperatures of the parent fuels assuming that additive property applies. These319

calculatedvaluesarepresentedasthedashedlinesinFigure5.ForYC/RHblends,theactual320

trigger temperature was higher than predicted values. For YC/OS blends, the change of321

triggertemperatureof10-30wt%blendswererelativelylinearwhileforthe50wt%blend,322

itexhibitedsomeimprovementsandleadtoalowertemperature.323

The changes in the ignition and char trigger temperatures were believed to be the324

consequenceoftheinteractionsbetweentheorganicelementsofthedifferentfuelsinthe325

blend [8,23].Thisnon-catalytic synergymightbe linked to the increase involatilematter326

contentoftheblendsduetobiomassaddition.327

328

Figure5:Evaluatingadditivetrendinsecondreactionzonetriggertemperature329

21

Toquantifytheinfluenceofblendingonignitionandcombustionperformance,theignition330

(Zi) and combustion (S) index of individual fuels and their blends were calculated using331

Equations(1)and(2)andarepresentedinTable6.AsshowninTable6,OSshowedthebest332

ignitionpropertywhileYCwasthemostdifficulttoignite.Theignitionindexincreasedwith333

theincreaseinbiomasspercentageforalloatstrawblendsandthe30wt%RHblend,thisis334

in linewith thedecrease in ignition temperatureand ignition time.This suggests that the335

ignitionpropertiesofYunnancoalwereimprovedbyblendingwithoatstraworricehuskat336

certain blending ratios due to the interaction between the fuels. However, insignificant337

improvementinignitionindexwasobservedfor10wt%and50wt%RHblends.338

AscanbeseeninEquation(2),thecombustionindexwasdependentonpeakmasslossrate,339

peakandIgnitiontime;henceforthe10wt%RHblend,ithadsimilarignitionandpeaktime340

withYC.However,thepeakmasslossratewasreduced.Similarly,thereductioninthepeak341

masslossrateofthe2ndreactionzoneofthe50wt%RHblendhinderedtheincreaseinthe342

ignitionindex.IncomparisontotheOSblend,thistrendforRHblendscouldbeassociated343

withthehighashcontentofRH,whichreducedtheamountoforganicmatteravailablefor344

interactionwithYC.Itcanbeseenthatamongtheblends,the10wt%OSblendhadthebest345

ignition indexwhile the10wt%RHblendhadtheworst.This isconsistentwithwhatwas346

found forcoaland tobacco residuesblends,anearly linear increase in ignition indexwith347

increaseinbiomassduetothehighvolatilecontentofthebiomass[53].348

Table6–PerformanceParametersofIndividualfuelsandtheirblends349

RH OS YC

90%YC+10

wt%RH

70%YC+30

wt%RH

50wt%YC+

50wt%RH

90%YC+10

wt%OS

70%YC+30

wt%OS

50wt%YC+

50wt%OS

Zi(%/min³) 8.410.93.1 2.8 4.2 3.0 5.4 5.0 4.1

S 1.4 1.8 0.8 0.6 1.1 0.7 1.6 1.2 1.0

22

(%²/°C³min²)

The combustion index also suggested that OS was the most reactive. Improvement in350

combustion performance was observed for the 30 wt% RH and all YC/OS blends. The351

reduction in combustion index observed in the 10 wt% RH blend can be explained by352

Equation(2).Asmentionedearlier,the10wt%RHblendwasfeaturedwithasinglereaction353

stageat286–608°C.Thisindicatesalongerresidencetimerequiredforattainingadesired354

burnoutincomparisonwithYunnancoalwhichburntoutcompletelybetween329–605°C.355

Thissuggeststheabsenceofimprovementinthecombustionperformancefor10wt%RHin356

comparisonwithYunnanCoal.Thereductionsinthemaximum(7.8wt%min-1)andaverage357

mass loss rate (3.8wt%min-1) of the 50wt%RHblenddue to its double peaks couldbe358

interpretedasareductionintheoverallfuelreactivityifthiscombustionindexwasusedfor359

comparison.ThisisexplainedbythehighvalueofthemasslossratesforYCwithapeakof360

15.5wt%min-1andameanmasslossof6.4wt%min-1.361

Normally, the combustibility of any fuel is inversely proportional to the maximum362

decompositionratetemperature[45].Similarly,thedecreaseinthe2ndpeaktemperatureof363

theOSblendsillustratedanimprovementincombustionperformance.Theenhancementin364

the burnout of the fuels was represented by the small decrease in the burnout365

temperatures with the maximum decrease of 6.7% for the 50 wt% OS blend, which is366

consistentwithwhatwasreportedbymanyothers[34,46].367

23

368

Figure6:Changesinperformanceindexoffuelblends369

Althoughthecombustionand ignition indices (asshown inFigure6)canbeusedtoshow370

theinteractionsbetweenindividualfuelsduringco-processing,theaccuracyoftheseindices371

may be compromised due to the split of the weight loss into two reaction zones as the372

averageandmaximumweightlossreducesmorerapidlywiththeincreaseinblendingratio373

comparedwithtimeandtemperature.Therefore,thereisaneedtodevelopanovelindex374

totakeintoaccountthetworeactionzonesorthereactionzoneexhibitingmoresynergistic375

characteristics, thereby improve its reliabilityandensure the resultsare representativeof376

theentirecombustionprocess.377

3.4CatalyticEffectofBiomassMinerals378

In this study, the influence of the minerals from biomass on the co-combustion379

characteristicsoftheblendswasstudied.LowtemperatureashofOatStrawandRicehusk380

wereblendedwithYCtocomparewiththecurveobtainedfor30wt%biomassand70wt%381

YC,asshowninFigure7andTable7.The70wt%YCand30wt%biomasswaschosenasa382

reference due to the improvement in ignition and combustion indexwere noticed in the383

performanceindexforbothYC/OSandYC/RHblends(asillustratedinFigure6).384

24

Theresultsclearlyshowedchangesinthecharacteristicsofthe30wt%OSashblendwhich385

led toa lower ignition temperatureof403 °C,a lowerpeak temperatureof486 °Canda386

lowerburnouttemperatureof575°Ccomparedwiththoseof100%YC.ThePTandBTvary387

significantlyfromtheadditivedata.388

389

390

Figure7:DTGcurvesofexperimentalandtheoreticaldataof100%YCand(a)30wt%Oat391

Strawash&30wt%OatStraw;(b)30wt%RiceHuskash&30wt%RiceHusk392

a

.

b

25

Thevariationsintheignition,peakandburnouttemperaturescouldbeexplainedtosome393

extentbythecatalyticeffectoftheAAEMsoriginatedfrombiomasssuchasOatStraw.The394

catalyticeffectofAAEMswasfoundtobeinorderofNa>K>Ca[47].AsshowninTable2395

OS contained significant amount of AAEMs, which was greater than that of RH. The396

investigationonthethermalbehaviourofthelowtemperatureashderivedfromOatStraw397

(asillustratedinFigure8)showedamasslossof17.6wt%inatemperaturerangeof473-398

573 °C, which peaked at 552 °C. This mass loss was attributed mainly to the release of399

volatile AAEMs compounds at high temperatures such as K+, KCl and or KOH. The initial400

volatileinorganicreleasetemperature(552°C)islowerthantheburnouttemperatureofOS401

(518 °C), which suggests that AAEMs acted as catalyst for the burnout of OS. Even at a402

temperaturehigher than573°C, therewas still significantamountofAAEMs remainingas403

catalystforYCcharcombustion(burnouttemperatureis605°C)asonly17.6wt%massloss404

uponheatingwhilethe initialmassfractionofpotassiumforOS lowtemperatureashwas405

47.4wt% (as shown inTable 2). This is consistentwithwhatwas reported [15] that the406

release of a small fraction (<20wt%) of the organically bonded alkalimetals occurred at407

temperatures up to 800 °C. In this study, the high potassium and calcium content in OS408

explainsthereductionintheburnouttemperatureof30wt%OatStrawashblendfrom605409

to575°C.ThisreductioninburnouttemperaturewasalsoevidentinalltheYC/OSblends.410

ThehighAAEMscontent inbothOSandRHcontributed to the reduction incharburnout411

temperatureofthefuelblends.412

Table7–CombustioncharacteristicsofYCblendedwithlowtemperatureashofbiomass413

Sample

Ignition

Temp

(°C)

Peak

Temp

(°C)

Total

degradation

(wt%)

Average

degradation

(wt%/min)

Maximum

degradation

rate(wt%/min)

Burnout

Temp

(°C)

26

YunnanCoal 459 535 88 6.4 15.5 605

70%YC+30wt%

RiceHuskash454 529 61.7 4.2 10.1 601

70%YC+30wt%

OatStrawash403 486 64.2 5.2 10.0 575

414

Figure8:DTGofOatStrawlowTemperatureash415

The peak temperature and burnout temperature of the 30 wt% RH Ash blend was416

comparablewiththatof100wt%YC.Likewise,theignitiontemperatureofthe30wt%RH417

ash blend was similar to that of 100 wt% YC. This confirmed the absence of catalytic418

improvement when YC was blended with RH ash. Therefore, the synergistic interactions419

observed for RH (reductions in trigger temperature as shown in Table 3) could not be420

attributed to catalytic synergy for RH and are closely related to non-catalytic effects421

primarilylinkedtohighvolatilecontentandsubsequentlyhighcharporosity.422

Nonetheless,resultsofthe30wt%oatstrawandcoalblenddidnotdistinguishtheeffectof423

volatilesandmineralsalthoughitprovedtheexistenceofstrongsynergybetweenthetwo424

fuels.Therefore,ashderivedfromoatstraw(equivalentto2wt%ash)wasusedtoreveal425

27

theinfluenceofmineralsoriginatedfromoatstrawonco-firing.APTof528°CandaBTof426

588°CwereobservedasshowninFigure9,whichwere7°Cand17°Clowerthanthoseof427

thecoalrespectively.Thisdemonstratedamodestcatalyticofthemineralsinoatstraw.As428

previouslymentioned,thePTandBTwere483°Cand583°Crespectivelyforthe30wt%OS429

blend, it can thereforebe concluded that for30wt%OSblend, the significant synergistic430

effect could be attributed to the non-catalytic synergy by the organic content of the oat431

strawandthecatalyticactivityofash.432

433

Figure9:DTGofexperimentalandtheoretical30wt%oatstrawand2wt%oatstrawash434

blend435

3.5SynergyIndicator436

Inthisstudy,itisclearthatfactors,suchasbiomassblendingratio,biomassashproperties,437

volatile content, contributed to strong synergistic interactionsbetweencoal andbiomass.438

Each factor affects the synergy observed in the blends to some degree. To select proper439

biomassforco-processingwithcoalandtodeterminetheproperblendingratiotoenhance440

synergistic interaction and therefore improve overall combustion performance, there is a441

28

needtodevelopanovelindex,whichcanalsobeusedtoevaluatethedifferentimpactsof442

catalyticandnon-catalyticeffects.443

Synergyindex[48]proposedbyotherswassolelyafunctionofthereactiontimetoreach95%444

conversionwherelargermagnitudeoftheindexindicatesgreaterdegreeofsynergy.Inthis445

study, it is clear that themain synergistic improvement include the reduced2ndpeakand446

burnout temperatures, as shown in the line chart in Figure 10. These observations have447

been linked partially to the catalytic effects of biomass inorganic content as described in448

section3.4andsecondarily,tothenon-catalyticeffectsofbiomassorganics(highvolatiles449

and char structure). Therefore, the three characteristic factors, i.e., peak temperature,450

burnout temperature and time to peak of the second reaction stage, which have direct451

influence on combustion performance, are used as the parameters for the novel synergy452

index.453

454

Figure10:Improvementsinpeakandburnouttemperatureswithbiomassblending455

Consequently,theextentofsynergistic interactionbetweenthecoalandbiomassfuelcan456

bequantifiedby the formulationof a synergy factor (SF)basedon thepeakandburnout457

29

temperaturesofthesecondreactionzoneaswellasthetimetaken. Inthisstudy,anovel458

synergyfactorwasdeveloped,whichisexpressedasEquations(3).459

SF = !"!"#!"!"!"#$

(3) 460

WhereSIisasynergyindicator(°C-3min-1/2)andcanbecalculatedusingEquation(4):461

SI = !

!!!!!.!!!!!!

× 10! (4)462

Where,t!!!isthetimedifferencebetweenthestartandpeakofthesecondreactionzone463

(min);T!isthepeaktemperature(⁰C);T!istheburnouttemperature(⁰C).464

Using this index, a comparison baseline was created using the result extracted from the465

theoreticalblendsmodels todeterminewhether fuelblendestablishesamoresynergistic466

effect (SF > 1.15) or additive behaviour (0.8 ≤ SF ≤ 1.15). However, a value of SF ≤ 0.8467

suggestsdeterioratedcombustionperformanceafterblending.Thesynergyfactorsforthe468

YunnancoalandbiomassblendsdiscussedaboveareshowninFigure11.469

Itcanbeseenthatthesynergyfactorincreasedwiththeincreaseinblendingratio;however470

therateofincreasewithbiomassblendratioweredifferentfordifferentblends.Forthe30471

wt%biomassblends(asshowninFigure6),themostsignificantsynergisticeffectoccurred472

for 70 wt% Yunnan coal + 30 wt% oat strawwith a synergy factor of 1.50. The 70 wt%473

Yunnancoal+30wt%ricehuskblendshowedadditivebehaviourandhadanSFof1.13.For474

the 10 wt% biomass blends, the YC/RH blend exhibited additive behaviour, which was475

mainlyduetotheinsufficientamountofAAEMstocatalysecombustionprocess.Basedon476

theseresults,itcanbeconcludedthatforblendswith30wt%ofbiomass,oatstrawshowed477

moresignificantlyenhancedreactivitythanthatofricehusk.ThehighSFofthe50%OSash478

30

blendwasduetotheexistenceofsignificantamountofcatalyticspeciesresultingingreater479

enhancementincombustion.480

In this study, assuminganadditivebehaviour, for 70%YCand30wt%OS , the SF is of a481

valueof1.15.TheSFof70wt%YCand2wt%OSashwasfoundtobe1.24,whichsuggests482

thatcatalyticsynergyresultedinaSFchangeby0.09.ThedifferencebetweentheSFvalue483

(S.F=1.40)of70%YCand30wt%OSandthatof70wt%YCand2wt%OSash(SF=1.24)484

couldbeattributedtovolatileeffect(non-catalyticsynergy)andresultedinaSFchangeof485

0.16.Likewise,thenon-catalyticsynergydetectedinthe50wt%RHblend(SF=1.26)could486

be attributed to its volatile content, which affected reaction time, and characteristic487

temperature at higher blend ratio, and resulted in an increase in SF by 0.11 due to non-488

catalyticsynergy.489

490

Figure11:TheSynergyindicatorofYunnancoalblends491

Table8–ValidationofSynergyFactorsusingReportedData492

Biomassblendingratio/SynergyFactor

31

Biomasstypes 0% 10% 30% 50%

AustralianCoal(AC)blends[49]

Gumwood(GW) 1.00 1.19 1.27 1.43

Poplar(PP) 1.00 1.16 1.26 1.38

Rosewood(RW) 1.00 1.18 1.34 1.57

MengxiCoal(MC)blends[49]

Gumwood(GW) 1.00 1.18 1.35 1.41

Poplar(PP) 1.00 1.26 1.39 1.38

Rosewood(RW) 1.00 1.31 1.80 1.67

AustralianCoal(AC)blends[50]

OatStraw(OS) 1.00 1.11 1.36

Printedcircuitboard(PCB) 1.00 1.03 1.23

Rubber 1.00 0.95 1.02

Polystyrene(PS) 1.00 1.39 1.40

Figure 11 shows the synergy factor as a function of the biomass content of the blend493

(regression function R2 value ≥0.96). This is in line with past notions that the synergistic494

interactionthatoccurredinfuelblendsisafunctionoftheorganicandinorganiccontentof495

the biomass, hence proportional to the portion of biomass introduced into the blend.496

Nonetheless, theextentof enhancement remaineddependenton the constituentsof the497

biomasssampleused.498

Inorder toverify this index,combustiondataofAustralianandMengxicoalwithbiomass499

blendswerecollatedfromliterature[49,50],whichareillustratedinTable8.Basedonthe500

SF values, for all coal and biomass blends, significant synergistic interactions exist. As for501

Australian Coal and Rubber, due to the lack of AAEMs in Rubber, which led to lack of502

catalytic effects for combustion process, there was no noticeable synergistic effects503

(SF<1.15) being observed. However, for Australian Coal and PCB blends, at high blending504

ratio, catalytic effects became obvious, which led to significant synergistic interactions505

(SF>1.15) at higher blending ratios (30wt%). These findings are consistentwithwhat the506

32

authors found in their study and therefore proved the validity of the synergy factor507

proposedinthisstudy.508

It was also reported [14] that the presence of an optimal improvement level for all fuel509

blends, beyond which synergy was independent of the biomass blending ratio, which510

suggests that the improvementof theblendedfuelsmightplateauorevendecreaseafter511

certainblendingratio[14].ThisisalsoconfirmedbythelowerSFvaluesfor50wt%Poplar512

/MC and 50 % Rosewood/MC blends compared with the blends with only 30 wt% of513

biomass514

4.0 Conclusions.515

Inthisstudy,theco-firingofYunnancoalwithAAEMs-richOatStrawdemonstratedstrong516

synergisticinteractionbythereductionsin2ndPeakandburnouttemperatures.Itisfound517

thatAAEMsfrombiomassactedascatalystsforcoalcombustion,enablingcatalyticsynergy,518

which is biomass dependent. Non-catalytic synergistic interactions were also evident at519

higherblendingratios,whichwasmainlyattributedtothehigheramountofvolatiles.520

Anovelsynergyfactor(SF),whichshowedagoodcorrelationcoefficient,wasproposedto521

quantifythesynergisticeffectsandtodistinguishcatalyticeffectfromnon-catalyticeffect.522

Thisindexcanbeusedasatooltopredictsynergisticeffectduringco-processing,whichisof523

significantimportanceforoptimizingblendingratioforexistingboilersandforthedesignof524

new co-firing plant to avoid operation issues. This index also offers opportunities for525

selecting proper biomass for co-firing with poor quality coal to enhance the overall526

combustionperformance.527

Acknowledgement528

33

PartofthisworkwassponsoredbyNingboBureauofScienceandTechnologyunderitsInnovation529

TeamScheme(2012B82011)andMajorR&DProgramme(2012B10042).TheInternationalDoctoral530

InnovationCentreisalsoacknowledgedfortheprovisionofafullscholarshiptothefirstauthor.531

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