Thermomechanical analysis of laboratory ash, combustion ash and deposits from coal combustion

88 (2007) 1099–1107www.elsevier.com/locate/fuproc

Fuel Processing Technology

Thermomechanical analysis of laboratory ash, combustion ash anddeposits from coal combustion

Yinghui Liu a,⁎, Rajender Gupta b, Liza Elliott a, Terry Wall a, Toshiro Fujimori c

a Cooperative Research Centre for Coal in Sustainable Development, Department of Chemical Engineering,University of Newcastle, EB Building, Callaghan, NSW 2308, Australia

b Chemical Engineering, the University of Alberta, Canadac Ishikawajima-Harima Heavy Industries Co., Ltd. (IHI), Tokyo, Japan

Abstract

Mineral impurities in coal form ash and a part of the fly ash form deposits in pulverized coal-firing furnaces. Understanding of thetransformation of mineral matter in coals to flyash and deposit formation has improved knowledge and helped industrial engineers better handleash-related problems. In Australia, ash fusibility tested in accordance to the standardized procedure or measured by Thermo-Mechanical Analysis(TMA) has been widely used to compare and predict slagging potential of various coals. The current study aims at obtaining an understanding ofthe sensitivity of TMA analysis to the physical, chemical and morphological properties of coal combustion deposits. In the study combustionresidues including ash prepared in a laboratory oven at a temperature of 815 °C, flyash collected in a pilot scale furnace and deposits collectedfrom a utility furnace generated from one Australian coal are used for TMA analysis. Ash samples with various levels of iron content wereobtained from different milling performance, ash samples with various silicon and alumina contents were prepared by mixing ash with quartz,kaolinite and bauxite. Results indicated that TMA measurements on coal ashes are very sensitive to iron content and can be used to indicate ironrelated slagging problems in pf-fired boilers. For ash deposits, both the physical properties such as their homogeneous/heterogeneous nature andash chemistry affect TMA measurement.© 2007 Elsevier B.V. All rights reserved.

Keywords: Thermomechanical analysis; Ash; Deposit

1. Introduction

Mineral impurities in coal form ash and a part of the fly ashform deposits in a pulverized coal-firing furnace. Ash depo-sition on furnace walls in pf fired boilers is termed slaggingwhen it occurs in the high temperature areas of furnaces directlyexposed to flame radiation, and fouling in other regions such astubes in the convection section of the boiler. A number ofengineering indices are being used to evaluate the potential ofcoal ash to form deposits in furnaces. These indices are derivedfrom various techniques which include the ash fusibility

⁎ Corresponding author.E-mail address: liu_yinghui@msn.com (Y. Liu).

0378-3820/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.fuproc.2007.06.028

temperature (AFT) test, ash composition analysis and ash vis-cosity measurements.

The AFT test has been the most accepted method by industryto evaluate fusibility of coal ashes prepared at 815 °C in labo-ratory. It provides four characteristic temperatures for laboratoryash as it is heated at approximately 5 °C to 10 °C/min from1000 °C to 1600 °C. Initial deformation temperature (IDT) isthe temperature at which the first rounding of the tip of thespecimen occurs. Sphere temperature (ST) is the temperature atwhich the height of the specimen is equal to the width of thebase. Hemisphere temperature (HT) corresponds to thetemperature at which the height of the specimen is equal tohalf the width of the base, its shape being approximatelyhemispherical. The flow temperature is the temperature atwhich the height of the fused ash specimen is 1.5 mm [1].

Determination of AFT temperatures in standardized test innature is continuous observations by a human operator on

Table 1Ash analysis of coal samples

Coal A Coal B Coal C Coal D Coal E

Silicon as SiO2 56.3 52 55.6 50.3 47.6Aluminium as Al2O3 30.7 29.6 31 29.8 28.6Iron as Fe2O3 6 13 6.2 13.4 16.0Calcium as CaO 1.1 1.2 1.03 1.7 1.64Magnesium as MgO 0.74 0.91 0.53 1.1 1.12Sodium as NaO 0.08 0.12 0.21 0.08 0.37Potassium as K2O 0.34 0.29 0.24 0.22 0.11Titanium as TiO2 2 2 2.3 1.8 2.22Manganese as Mn3O4 0.12 0.28 0.16 0.29 0.29Phosphorus as P2O5 0.05 0.06 b0.01 0.07 0.09Sulfur as SO3 1 0.94 1.23 0.75 1.77

Fig. 2. Geometry and sampling location of boiler.

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changes in shape of an ash specimen when it is heated under aspecified heating rate. To eliminate the effect of operators andthus to improve repeatability and reproducibility of ashfusibility test, some alternative ash fusibility tests includingthermomechanical analysis (TMA), improved ash fusion test(IAFT) have been developed in Australia to directly measureash fusion behaviour by its mechanical characteristics,electrical conductance or by an computer controlled camera.[3,7,8,10] TMA developed in the CSIRO, Australia as analternative fusibility test for laboratory coal ashes has beenused to compare and predict slagging potential of variouscoals.[5,10,12]. TMA measurements provide two types ofinformation: firstly it gives ram penetration into the sampleexpressed as a function of temperature; secondly it indicatescharacteristic temperatures expressed as “peaks” the tempera-tures of rapid penetration, with the height of the peak beingproportional to the maximum change in penetration withchange in temperature. The peaks are obtained from the firstderivative of penetration trace.

Fig. 1. Chemical compositions of coal ash an

The TMA operating procedure has been standardised for ashconfiguration (sample mass and sample compression), heatingrate, and TMA ram configuration (geometry and load of pene-trating ram).[10]. The effects of ash chemistry on TMA mea-surements have been systematically studied at the University ofNewcastle. Ash chemistry described by the amount of basicoxide, nature of basic oxide and the ratio of SiO2/Al2O3 deter-mines major peaks events in TMA measurements. The ashfusion behaviour can be correlated and interpreted with variouseutectic temperatures from phase diagrams SiO2–Al2O3–X,where X=FeO, CaO, MgO, K2O, Na2O. [4,6,12] Possible

d blends formed with various minerals.

Fig. 3. Photographs of clinker A sampled from ash hopper and clinker Bobtained from OAF level respectively.

Table 2Chemical analysis for clinker samples

Clinker A1(fire side)

Clinker A2(middle)

Clinker A3(tube side)

Clinker B1(OAP)

Clinker B2(SAP)

Silicon asSiO2

54.3 54.6 54.7 60.6 61.4

Aluminiumas Al2O3

25.9 25.8 26 26.8 24.9

Iron asFe2O3

16.9 17 16.3 8.2 9.9

Calcium asCaO

0.63 0.62 0.64 0.7 0.7

Magnesiumas MgO

0.56 0.6 0.62 0.52 0.52

Sodium asNaO

b0.05 b0.05 b0.05 b0.05 b0.05

Potassiumas K2O

0.22 0.23 0.2 0.24 0.22

Titanium asTiO2

1.4 1.4 1.4 1.8 1.6

Manganeseas Mn3O4

0.31 0.32 0.31 0.17 0.21

Phosphorusas P2O5

0.04 0.6 0.04 0.04 0.04

Sulfur asSO3

0.03 0.04 b0.02 b0.02 b0.02

Strontiumas SrO

b0.02 b0.02 b0.02 b0.02 b0.02

Barium asBaO

b0.02 b0.02 b0.02 b0.02 b0.02

Zinc as ZnO b0.02 b0.02 b0.02 b0.02 b0.02Vanadium asV2O5

0.03 b0.02 b0.02 0.03 0.03

Chromiumas Cr2O3

0.06 0.03 0.03 0.03 b0.02

Copper asCuO

b0.02 0.05 b0.02 b0.02 b0.02

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reactions between ash and crucible material also have beenstudied for crucible materials as molybdenum (Mo), platinum(Pt), carbon (C), zirconia (ZrO2), alumina (Al2O3), and boronnitride (BN).[2]

The thermal conditions at which ash is prepared was alsoconsidered as an important factor to influence TMA fusibilitymeasurement. Fusibility of combustion ash obtained from fur-naces in several Australian power plants experienced flametemperatures as high as 1500 °C was compared with that oflaboratory ash prepared at 815 °C. TMA results for laboratoryashes indicated the first peak at lower temperature than thecombustion ashes, which is explained by differences inmineralogical composition such as illite. [13] The TMAperformance for deposit formed from a pilot scale furnacewas used to predict porosity of ash deposits. [9]

In the present study, the TMA fusibility of laboratory ash,combustion ash and deposits formed from an Australian thermalcoal has been studied. The research work aimed at comparingTMA performance for different coal combustion products andgaining an insight into how properties of coal combustion

products affect TMA measurement. In this study, effect ofphysical, chemical and morphological properties of ash and slagdeposit has been considered.

2. Experimental

2.1. Sample

Table 1 presents the ash analysis of the coal samples usedin the study. The coal is produced from a mine located inQueensland, Australia. This coal was selected because of itssimple ash composition with SiO2, Al2O3 and Fe2O3 aredominant oxides present in coal ashes. With proper coalpreparation and handling, levels of iron containing minerals inas-mined coal can be reduced to control slagging. Thus there isvariation between coals used in the power station from time totime. Coal A to E are typical coals obtained from different timeperiods. Coal A and coal C have relative low iron contents ofaround 6% Fe2O3 in ash and coal B and coal D have relative highiron content of around 13% Fe2O3 in ash. Coal E has the highestiron content of 16.0% Fe2O3 in ash. Except for differences in ashchemistry, there are some differences in particle sizes of coals.Coal A and coal B are coarser than coal C and coal D.

Fig. 4. Schematic diagram of TMA apparatus and ash sample assembly.

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The laboratory ashes are prepared from coals A–D at 815 °Cin a laboratory muffle furnace in accordance to AustralianStandard for proximate analysis. The purpose of TMA mea-surement for these four coals is to study the sensitivity of TMAmeasurement to iron levels. The compositions of these four coalashes are presented in “Lower Fe2O3” category in Fig. 1. Ashchemistry also indicates the SiO2/Al2O3 ratio for the coal A–Dis in the range of 1.7–1.8, which is close to that of coal ash E ofSiO2/ Al2O3 1.7.

Ash of coal E is obtained from a pilot scale combustionfurnace which experienced a flame temperature as high as1400 °C. [11] Ash of coal E is then mixed with grinded finequartz, kaolinite and bauxite at various proportions to obtainfinal samples containing 2.5%, 15% and 50% correspondingmineral respectively. In this way, effect of SiO2 by quartzaddition, effect of Al2O3 by bauxite addition and effect ofcombined SiO2 and Al2O3 with a fixed SiO2/Al2O3 ratio bykaolinite addition on TMA performance have been studied. Thechemical compositions of raw coal ash, pure mineral added andtheir mixtures with various proportions are presented in Fig. 1,in which coal ash E is represented by the red spot in the ternarydiagram and pure quartz is indicated by hollow blue dot on thetop apex of the ternary diagram, mixtures of raw coal ash andquartz are represented by hollow blue dots lying between rawcoal ash and quartz apex, and the distance of the dot to raw coalash and quartz is determined by rule of lever. Bauxite is locatedin the Al2O3 corner apex in the diagram, mixtures of bauxite andcoal ash are represented by dots lying between the coal ash andbauxite. And so on for kaolinite addition.

Two ash deposit samples formed from burning low ironlevel coals and high iron level coals respectively were sam-pled from 450 MW boiler in a power station in Queensland,Australia. Thus deposit designated as clinker A was sampledwhen coal B was used in the power station and smaller piecesof deposits designated as clinker B was obtained when coal Cwas burning in the furnace. The geometry of the furnace isillustrated in Fig. 2, which shows the furnace is of front andrear wall firing configuration with three rows of burners on thefront wall and two rows of burners on the rear wall. Over fireair (OFA) is also used in the furnace to reduce NOx emission.Clinker Awas sampled at the bottom of ash hopper and clinkerB was sample at OFA level. The photographs of clinker A andclinker B are shown in Fig. 3. The chemical analysis results ofclinker samples are tabulated in Table 2. Received clinkersamples were firstly dried in air, then crushed and grinded intofine powders with size less than 90 microns prior to TMAmeasurement.

2.2. TMA analysis

TMA measurements were carried out in a SETARAMTMA92 Thermo Mechanical Analyser. A schematic diagramof the apparatus is shown in Fig. 4. Approximately 50 mg ofsample was placed into a molybdenum crucible and thencompacted with a pressure of 260 KPa, after which a pene-trating ram was inserted into the crucible containing ash. Aschematic diagram of the TMA crucible, ash pellet and pene-

tration ram assembly is shown in magnified zone in Fig. 4. Theentire assembly was then placed into the TMA instrument andpurged with high purity argon for 15 minutes. The sampleassembly was heated from ambient conditions to 500 °C at arate of 50 °C/min and reside at this temperature for 20 min,after that the sample is heated at a rate of 5 °C/min until atemperature of 1600 °C. A load of 100 g is applied to thepenetrating ram resulting in a pressure of 140 kPa at theinterface between the penetrating ram and sample surface. Asthe sample is heated the penetrating ram sinks into the ash,which will eventually contact the base of the crucible when theslag fully flows into the annulus between the crucible andpenetrating ram. The output from the TMA consists of rampenetration into the sample, expressed as a percentage of theoriginal height of the sample, at a specific temperature. Thefirst derivative of the penetration gives the rate of change ofpenetration of the ram.

2.3. SEM analysis of clinkers

Scanning electron microscopy (SEM) analysis was used toobserve the morphology of slag deposits under different

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magnifications. Due to the large sizes of the clinkers, severalsamples were randomly selected from one clinker block, fromeach sample several SEM images can be obtained. A combi-nation of a serial of low magnification images (50 times) couldgive a good overview for the bulk sample.

3. Results

3.1. Effect of ash chemistry on TMA measurements

TMA results obtained for coal ash with various additivestypes and amounts are shown in Fig. 5 where the TMA pene-tration starts at 800 °C and reached 20% penetration at tem-perature of 1150 °C, after that there is an accelerated penetrationtakes place until 1300 °C reaching 80% penetration, finally thepenetration profile levelled off until it reaches final penetration.This is a characteristic TMA profile for coal ashes containing ironas the only major basic oxide.

Fig. 5. TMA results for coal ashes with additives. (a) Effect of 2.5% additives. (b) Effeprofiles.

When ash chemistry of the coal was modified by addingother minerals such as quartz, kaolinite and bauxite, resulted inTMA traces are shown for 2.5%, 15% and 50% additivesrespectively in Fig. 5 (a) (b) and (c). Fig. 5 (a) shows thataddition of 2.5% quartz, kaolinite and bauxite does not affectTMA characteristics significantly. All the three TMA tracesfollow the same trend as raw coal ash, though coal ashes withadditives start fusing at a temperature 20 °C low than raw coalash. This limited change in fusibility behavior can be ex-plained by the FeO–SiO2–Al2O3 ternary phase diagramshown in Fig. 1, coal ashes with 2.5% additives are close toraw coal ash and will not demonstrate significant changes inliquidus temperature.

Fig. 5 (b) demonstrates the TMA traces when 15% quartz,kaolinite and bauxite were added into the coal ash respectively.Comparatively, coal ash added with quartz shows sluggishfusion compared to coal ash without additives; coal ash addedwith kaolinite and bauxite shows fusion start at a lower

ct of 15% additives. (c) Effect of 50% additives. (d) Effect of iron levels on TMA

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temperature than raw coal ash. For all the TMA traces the majorpenetration event happens at temperature range between1100 °C and 1300 °C. According to liquidus temperaturesshown in Fig. 1 addition of quartz will not affect liquidustemperature of the mixture and addition of kaolinite and bauxiteincrease liquidus temperature of the mixture.

Fig. 5 (c) illustrates the effect of adding 50% additives onTMA measurement. It can be seen that compared with rawcoal ash, ashes with 50% additives do not present sig-nificant penetration between 1100 °C and 1300 °C which is

Fig. 6. TMA profiles for cl

characteristic in raw coal ash TMA trace. Ash added withquartz shows sluggish fusion compared with ash addedwith kaolinite and bauxite. Liquidus temperature determinedfrom phase diagram shows addition of kaolinite and bauxiteincrease liquidus temperature by 110 °C and 185 °Crespectively.

Fig. 5 (d) compares the TMA traces of coal ashes of high andlow iron levels. Results indicate the major penetration eventstarts at 1100 °C. For high iron coal ashes the major penetrationevent finishes at 1300 °C and for low iron coal ashes it is

inker A and clinker B.

1105Y. Liu et al. / Fuel Processing Technology 88 (2007) 1099–1107

observed gradual penetration until final penetration. The charac-teristics of TMA traces for low iron coal ashes are similar to thatof raw coal ashed added with 50% additives.

3.2. Effect of deposit morphology on TMA measurements

TMA traces for deposit samples Clinker A and Clinker B areshown in Fig 6. For Clinker A, an extremely rapid onset ofmelting is noted at a temperature of about 1200 °C, with anincrease in penetration of 80% as the temperature increases by100 °C. For Clinker B1 this onset temperature is 1375 °C andfor B2 it is 1325 °C, with a less severe extent of subsequent

Fig. 7. Panorama for

penetration. Before rapid onset of melting of clinker B, gradualpenetration happens, indicating mineral phases of the clinkerschange in this temperature range.

The differences between Clinker A and Clinker B can beexplained by differences in iron contents and homogeneous/heterogeneous nature. Clinker A has iron content of 16% andclinker B has iron content of around 9% expressed as Fe2O3. Itis expected high iron deposit melts at lower temperature thanlower iron deposit.

SEM images shown in Fig. 7 indicate the morphology ofclinker A and clinker B. Clinker A consists of hollow bubblesand continuos phase. The shape of bubbles is close to spherical,

Clinker A and B.

Fig. 7 (continued ).

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which is expected to form from surface tension in viscous slagat high temperature. In continuos phase some crystals areobserved with composition close to mullite. All these infor-mation indicate clinker A exists in the molten stage. SEMimages of clinker B show quite different characteristics fromclinker A. This contains bubble like clinker A, but continuosphase has not coalesced. This indicates that clinker B is in atransition through sintering stage to a molten form.

4. Conclusions

From this study it is concluded that:

1. TMA measurements on coal ashes is very sensitive toiron content in coal ash and can be used to quantitative-ly compare and predict iron related slagging in pf-firing boilers.2. Addition of kaolinite, bauxite or quartz increases themelting point of ash. However, addition of quartz into coalash has greater effect on increasing the ash fusion thankaolinite and bauxite.3. Reduction of iron content in coal ash is more effective thanto dilute coal ash with mineral additives.4. For ash deposits both physical properties such as theirhomogeneous/heterogeneous nature and ash chemistry affectTMA measurement.

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