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Transcript of Novel index - R1 Finaleprints.nottingham.ac.uk/40535/1/A novel index for... · The Rice Husk had...
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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:[email protected]
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Howeverthisreductioninthepeaktemperature isnotobvious inFigure2abecauseof its216
lowvolatilecontentofYC.217
218
219
220
a.
b
c
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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
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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
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(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
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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
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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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