Contents lists available at SciVerse ScienceDirect
Fuel Processing Technology
j ourna l homepage: www.e lsev ie r .com/ locate / fuproc
Sintering characteristics of sewage sludge ashes at elevated temperatures
Liang Wang ⁎, Geir Skjevrak, Johan E. Hustad, Morten G. GrønliDepartment of Energy and Process Engineering, Norwegian University of Science and Technology, Kolbjørn Hejes vei 1A, 7491, Trondheim, Norway
In this work the sintering characteristics and mineral transformation behaviors of sewage sludge ash (SSA) atelevated temperatures were investigated by using ash fusion analyzer, X-ray fluorescence (XRF), X-ray dif-fraction (XRD) and scanning electron microscopy equipped with energy dispersive X-ray spectrometry(SEM/EDX). High initial fusion temperatures above 1100 °C were detected from the sewage sludge ashes(SSA 1 and SSA 2) with high Al contents. Corundum, quartz and calcium aluminum silicates were dominatingcrystalline phases identified from SSA 1 and SSA 2 sintered at elevated temperatures. For the SSA 3 with ahigh Fe content, low initial melting temperature of 994 °C was detected with observation of severe fusion be-havior from the ash sintering tests. SEM analysis revealed that SSA 3 melted completely into a more homo-geneous and continuous phase at high sintering temperatures. A significant amount of Fe bearing mineralphases and quartz (SiO2) was identified from the sintered SSA 3. Diffraction intensities of hematite(Fe2O3), quartz (SiO2) and alkali feldspar decreased with increasing sintering temperatures, suggesting inter-action and re-assemblage of these mineral phases. In combining the XRD and SEM/EDX analyses, it is believedthat formation of low melting temperature iron silicates is the main reason for sintering of SSA 3.
Due to the rapid urbanization throughout the world, the amountof waste water is increasing dramatically with more processing re-quirements [1–3]. Sewage sludge, as a main by-product of wastewater treatment process, is becoming a public issue due to the ever-increasing amount and risk to the environment [1]. Compared to con-ventional disposal ways, sewage sludge combustion is one of thepromising methods with several advantages [2,3]. With a relativehigh heating value close to the brown coal, energy could be recoveredfrom sewage sludge by means of combustion in different devices [2].Through the combustion, the volume of sewage sludge can be effec-tively reduced to a small amount stabilized ash. Most of toxic organicspecies can be destroyed at high enough combustion temperatureswith lower emissions. Heavy metals in sewage sludge are retainedin incineration residues and possibly handled with minimum envi-ronmental impacts. [1–3].
Currently sewage sludge is combusted directly in different kindsof incinerators or co-combusted with other fuels in combustors[2,3]. However, compared to other solid fuels such as coal, the sewagesludge has higher and varied ash contents about 20 to 50wt.% on adry basis, which result in heavy loads on ash handling and flue gascleaning system in a combustion device [2–4]. In particular, thechemical and mineral compositions in the sewage sludge ash have
rights reserved.
significant effects on their properties during combustion processes.Operational related problems have been often observed in combus-tion devices associated with sewage sludge ash sintering, agglomera-tion and slagging behaviors. These problems lead to declined energyproduction efficiencies, high maintenance costs and unscheduledshutdown of combustion systems [2–7]. Several studies have beencarried out on ash behaviors during combustion of the sewage sludge.Formation of molten ash layers and droplets on sewage sludge charresidue surfaces was reported by Cui et al [5], as two sewage sludgesamples combusted in an electrical heated furnace in a pure oxygenatmosphere at 900 °C. With the increasing combustion time, sewagesludge burn residues were covered by fused ash and agglomerated to-gether to form solid blocks [5]. Zhang et al studied combustion behav-iors of two kinds of sewage sludge in a drop tube reactor with a fixedtemperature of 1200 °C [6]. Molten ashes were observed from charparticle surfaces as the fed sludge passing the tube furnace with0.6 s residence time. It was suggested that interactions of major min-eral compounds in sludge, including calcium oxides, phosphorusoxide and aluminosilicates, led to formation of large molten agglom-erates [6]. Shao et al [7] reported agglomeration characteristics of thesewage sludge combustion in a bench-scale fluidized bed combustor.With higher combustion temperatures and using degraded sands asbed material, more severe agglomeration was observed with forma-tion of large sintered pieces and aggregates in the combustor. SEM-EDX and XRD analysis on collected agglomerates revealed that forma-tion of lowmelting points eutectics of Fe2O3 and SiO2 in bed materialsmight be a main initiator for bed agglomeration. It was followed bypossibly reactions of P, Mg and Ca from sewage sludge with Si from
VM, volatile matters; FC, fixed carbon; d, dry basis; daf, dry and ash free; HHV, higherheating value
a The O content was determined by difference.
89L. Wang et al. / Fuel Processing Technology 96 (2012) 88–97
sand particles to enhance further bed agglomeration [7]. Moreover,sintering characteristics of sewage sludge have been studied undervarying sintering temperatures and time scales [8–11]. However,aims of these studies were to evaluate properties of sintered sewagesludge and/or ash residues as ceramic production feedstock in termsof compressive strength and bulk density. Available literature resultsare quite dispersive and limited to deeply understand sinteringbehaviors of different sewage sludge ashes. In addition, differentmethods have been applied to recover phosphorus from the wastewater i.e. flocculation using precipitation chemicals such as iron sul-fate, aluminum sulfate and calcium sulfate [2,12–14]. The ash contentand chemical compositions of the sewage sludge are affected by utili-zation of different phosphorus precipitation agents. It results in dif-ferent ash chemistries and mineral transformation properties duringcombustion of the sewage sludge consequently [12–14]. To author'sknowledge, no work has been done to study effects of phosphorusprecipitation agents on sewage sludge ash chemical and mineralogi-cal composition transformation during sintering processes. A detailedknowledge of sewage sludge ash sintering characteristics, correlatedwith difference in their chemical and mineral compositions, need tobe clarified.
In this study, ashes produced from three sewage sludge sampleswere used as representatives for the investigation. The characteristicsand sintering behaviors of sewage sludge ashes were studied by XRF,ash fusion analyzer, XRD and SEM-EDX. Various indices based on ashchemical compositions were applied to predict sewage sludge ashfouling and slagging propensities. The results will be useful to achievea better understanding of sewage sludge ash sintering mechanismsand provide useful references for utilization of sewage sludge as anenergy source.
2. Materials and methods
2.1. Sample preparation and characterization
Sewage sludge samples (SS 1–SS 3) from three waste water plantswere used in this work. The SS 1 and SS 2 were obtained from theplants where aluminum sulfate (Al2(SO4)3) is applied for phosphorusparticipation during the waste water treatment. While the iron sul-fate (Fe2(SO4)3) is used in the plant where SS 3 was produced [13].Prior to the experimental work, all three received wet sewage sludgeswere dried in an oven at 105 °C to get constant weight, which werethen grounded to sizes less than 1 mm and stored in sealed con-tainers for further characterization. ASTM standards (E 872) were
Table 2XRF analysis results of sewage sludge ash (wt.%).
performed to determine volatile matter of all sewage sludge samples.Elemental compositions of dried sewage sludge were measured byconducting an elementary analyzer (Vario MACRO CHNS). Heatingvalue of each sludge sample was measured by a bomb calorimeter(IKA C5000). The ash content of each sludge sample was determinedby ASTM standard D 1102. Two grams of oven dried sample was putinto a porcelain crucible and heated in an electrical muffle furnaceat 550 °C for 5 h under oxidizing atmosphere. Difference betweenthe initial mass and the final mass was considered as ash content. Fu-sion behaviors of sewage sludge ashes (SSA) were examined with anash fusion analyzer (Carbolite CAF Digital). Residues collected afterash content tests (550 °C) were shaped into cubic specimens andheated from 25 °C to 1600 °C with a heating rate 6 °C/min under anoxidizing atmosphere. The external shape changes of each specimenwere recorded and characterized by following standard (ISO540:1995). Four characteristic temperatures were identified andlogged, including initial deformation temperature (IDT), softeningtemperature (ST), hemisphere temperature (HT), and flowing tem-perature (FT). To get reliable results, the ash fusion tests for eachash sample were performed three times, and average values are plot-ted together with deviations.
2.2. Ash sintering evaluation
To evaluate the sintering behaviors at elevated temperatures, eachsewage sludge ash produced at 550 °C was loaded into crucibles andheated at three final temperatures (900 °C, 1000 °C and 1100 °C) inthe same furnace, respectively. All samples were heated at desiredfinal temperatures for 1 h to ensure sufficient time for chemical inter-action and mineral phase transformation. After sintering treatment,all samples were cooled down to the room temperature for visual ob-servation. Then, the sintered ash residues were carefully collectedfrom the crucibles without destroying initial structures and storedfor further analysis.
2.3. Chemical composition and microstructure analysis
The main chemical compositions of the sewage sludge ashesobtained at 550 °C were determined by X-ray fluorescence (XRF)spectrometry (Bruker S8). The mineral compositions of sewagesludge ashes produced from different sintering temperatures wereanalyzed with an X-ray diffractometer (Bruker D8 Focus) equippedwith a SOL-XE detector. Operating conditions of the XRD were40 KV and 40 mA Cu Ka (λ=1.54 Å) radiation and step-scanned inthe 2θ range 10°–80°. Crystalline phases contained in each samplewere identified by instrument integrated data processing softwareTOPAS plus ICDD-PDF 2 database.
The morphology and microchemistry of sintered sewage sludgeashes were examined by a field emission scanning electron microsco-py (FE-SEM, Carl Zeiss Supra) equipped with an energy dispersiveX-ray spectrometer (EDX, Bruker). All ash residues produced fromsintering tests were collected and embedded into epoxy resin. Thesamples mounted in the resin were cut, grinded and polished to getsmooth cross-sections. The cross-sections were then coated withcarbon and analyzed by SEM-EDX with spot analysis and elementmapping methods.
Fig. 1. Fusion characteristic temperatures of sewage sludge ashes.
90 L. Wang et al. / Fuel Processing Technology 96 (2012) 88–97
2.4. Ash slagging and fouling prediction
Different correlations have been used for predicting slagging andfouling tendencies of solid fuel ashes. In general, the ash chemicalcompositions in solid fuels can be divided into two groups where Agroup includes acidic constitutes with high melting temperatures(SiO2+Al2O3+TiO2), while compounds belonging to B groups actas fluxing agents to decrease the ash melting temperature (Fe2O3+CaO+MgO+Na2O+K2O) [15]. For P rich fuels, P2O5 content isadded to group B, since it enhances the formation of low meltingpoints fraction in the fly ash [15,16]. Different indices were appliedin this study to predict slagging and fouling propensity of the sewagesludge ash, including the sum of basic constitutes in the ash (Rb),thefouling index related to ratio of basic to acidic constitutes (Rb/a), theslag viscous index (SR), and the fouling index (Fu). In addition, indexFs was used to evaluate the slagging propensity of sewage sludgeashes, which is based on the ash initial deformation temperature
110011501000950
10000
1000
100
10
1
0
Initial deformation temper
Fou
ling
and
slag
ging
indi
ces
R2 = 0.9934
Fig. 2. The relationships between ash initial deformation temperature and selected foulingsenting indices calculated from SSA1 to SSA 3).
and hemisphere temperature determined according to standardmethods [17]. All indices definition and calculation methods are listedin Appendix A.
3. Results and discussion
3.1. Fuel and ash characterization
The characteristics of three sewage sludge used in the presentwork are summarized in Table 1. The three sewage sludge sampleshave heating values from 17.5 to 19.4 MJ/Kg and significant highash contents above 40 wt.%, which indicates combustion could be afeasible way for energy recovering, but with large amount ash resi-dues to be handled. Table 2 presents the main chemical compositionsof three sewage sludge ashes and calculated fouling and slagging indi-ces as well. It can be seen that the main ash compositions in all sludgeashes are Al2O3, SiO2, P2O5, CaO and Fe2O3, with small amount of K2O,
1150 1200 1250ature (°C)
Rb
SR
Rb/a
Rb/a(+p)
Fouling index
Fs
Linear (Rb)
Linear (SR)
Linear (Fs)
Linear (Fouling index)
Linear (Rb/a)
Linear (Rb/a(+p))
R2 = 0.9998
R2 = 0.8967
R2 = 0.9329
R2 = 0.9499
R2 = 0.9313
and slagging indices (three makers with same pattern along the X axis direction repre-
Fig. 3. a. X-ray diffraction analysis of SSA 1 sintered at different temperatures. b. X-ray diffraction analysis of SSA 2 sintered at different temperatures. c. X-ray diffraction analysis ofSSA 3 sintered at different temperatures. 1 SiO2; 2 Al2O3; 3 AlPO4; 4 NaSi3AlO8; 5 Ca7Mg2P6O24; 6 Al2O3·2SiO2; 7 CaAl2Si2O8; 8 KSi3AlO8; 9 KAlSiO4; 10 Fe2O3; 11 CaSiO3.
91L. Wang et al. / Fuel Processing Technology 96 (2012) 88–97
Na2O and MgO. Both SSA 1 and SSA 2 contain high content of Al2O3,which is possibly caused by utilization of the precipitation chemicalAl2(SO4)3 during the waste water treatment. A large amount ofFe2O3 was detected from SSA 3, which is about four and three timeshigher than those of the other two sludge ashes.
3.2. Ash fusion tests and prediction
The ash melting behaviors of three sewage sludge ashes are pre-sented in Fig. 1. It shows that SSA 1 and SSA 2 have evidently higherash fusion characteristic temperatures than SSA 3, especially for HT
92 L. Wang et al. / Fuel Processing Technology 96 (2012) 88–97
and FT. For SSA 1 and SSA 2, larger temperature intervals from1100 °C to 1500 °C were identified between the IDT and FT. It indi-cates abundance of the high melting temperature matters in the SSA1 and SSA 2, which did not melt until a certain high temperatureand supported the skeleton structure of cubic ash specimens duringfusion tests [18,19]. SSA 3 has a low IDT value for 994 °C and flowedat around 1160 °C with a narrow fusion temperature range about166 °C. During ash fusion tests, swelling and fast shrinking of SSA 3cubic specimens were clearly observed. It indicates that the SSA 3melted as liquid phase under elevated temperatures. After fusiontests, the melted SSA 3 presented as a semi-transparent thin layeron the sample holder surface with depletion of solid ash particles.The ash fusion test results showed that SSA 3 has the highest sinteringand melting propensity, compared to the other two sewage sludgeashes.
Different fouling and slagging indices were calculated based onthe bulk chemical compositions of three sewage sludge ashes. Itshould be noted that these indices are normally applied for coalashes [15]. In this work, they are applied as complements for sewagesludge ash melting and sintering tests. As all calculated indices givenin Table 2, SSA 3 appears the most problematic one regarding to foul-ing and slagging propensity. Graphical representation of the indicesvalues against the ash initial deformation temperature is shown inFig. 2. Well fitted trend lines indicate clear relations between thesludge ashes' chemical compositions and melting behaviors. Initialdeformation temperatures of three sewage sludge ashes increasewith decreasing fluxing to acidic oxides ratio Rb/a, Rb/a(+p), and thesum of fluxing oxides Rb. The calculated Fs value of SSA 3 based onIDT and HT is lower than 1052 °C, which implies possible severe slag-ging of SSA 3 during combustion [17]. The interpretation of appliedslagging/fouling indices and ash properties also suggests that thesecoal ash study based indices have a good ability to predict the sewagesludge ash fouling and slagging tendency.
3.3. Mineral transformation with elevated sintering temperatures
To evaluate sintering behaviors, each ash sample produced at550 °C from the parent sewage sludge was heated 1 h at settled tem-peratures 900 °C, 1000 °C and 1100 °C, respectively. Significant differ-ences of morphology were observed between the three sewagesludge ashes after sintering evaluation. The SSA 1 kept the originalloose structure even after the 1100 °C sintering treatment. Through1 hour heating at 1000 °C, the SSA 2 sample showed a sign of particlesbonding with presence of small aggregates. Both amounts and sizes offormed aggregates were increased after SSA 2 sintered at 1100 °C. TheSSA 3 with 900 °C sintering treatment showed slight bonding struc-tures of ash particles. Severe ash sintering was observed from resi-dues of SSA 3 after heated at 1000 °C, which was covered bycrystalloid substances on the surface. With 1100 °C heating treat-ment, the SSA 3 flowed on the bottom of the crucible, and some por-tions were shrinked as liquid droplets. The visual observations of thesintered SSA samples agreed with the ash fusion tests and empiricalindices prediction that SSA 3 has the highest tendency to melt duringheating process.
The mineral phases contained in sewage sludge ash produced atdifferent temperatures were identified by X-ray powder diffractome-try (XRD) analysis. It should be noted that only main crystallinephases were observed by XRD in a sample. Amorphous materialsand crystalline phases with too little amount were not possibly di-rectly identified [20].
For the SSA 1 produced at 550 °C, main identified mineral phasesare quartz (SiO2) and corundum (Al2O3) as show in Fig. 3a. The quartz(SiO2) and corundum (Al2O3) remained as crystalline phases athigher sintering temperature with gradually increased intensities.Anorthite (CaAl2Si2O8) pattern was observed SSA 1 sintered at900 °C, which may due to reactions of CaO with Al2O3, SiO2 in the
temperature range from 900 °C to 1000 °C [10]. Large amount of co-rundum (Al2O3) identified from SSA 1 is caused by presence ofAl2(SO4)3 in sewage sludge, which decomposed during high temper-ature heating with Al2O3 left. Corundum (Al2O3) is an inert mineralphase with a significant high melting temperature more than2000 °C, which has the ability to enhance the both coal and biomassash sintering temperatures by a dilution effect [18–21].
The main mineral transformation in SSA 2 from 550 °C to 1100 °Cis presented in Fig. 3b. As compared with SSA 1, similar mineralphases were identified from SSA 2 sintered at different heating tem-peratures. Quartz (SiO2) and corundum (Al2O3) were two main min-eral phases observed from high temperature sintered products.Aluminum phosphate (AlPO4) was observed from XRD patterns dueto reaction between the Al2O3 and P2O5 as the sintering temperaturehigher than 900 °C [8]. A new aluminum silicate microcline (KSi3-AlO8) was observed from SSA 2 and the intensity of the phase in-creased with elevated sintering temperature. Identification of thesealkali aluminum silicates is possibly caused by different reasons (1)reactions between SSA self-contained aluminum silicates (i.e. zeo-lites) and alkali metals during the sintering process, (2) reactions be-tween SSA self-contained alkali metals with clay minerals fromcontaminated sand/soils during waste water sedimentation, and(3) introduction of aluminum silicates from contaminated sand/soilminerals. Zeolites are a group of dehydrated aluminum silicates,which are widely used as detergents for human laundry and adsor-bents for waste water purification process [13]. A large fraction of ze-olites in waste water remains in sewage sludge, which maycontribute to the formation of alkali aluminum silicates identified insintered sewage sludge ashes. The increasing of microcline (KSi3AlO8)peak in Fig. 3b implies continuous formation of this mineral phase athigher sintering temperatures. Same as SSA 1, corundum (Al2O3) andanorthite (CaAl2Si2O8) are two main mineral phases identified fromSSA 2 samples heated at 1000 °C and 1100 °C. As two well-known re-fractory phases, they can raise the solid fuel ash fusion temperatureand decrease sintering tendency [18,19,24]. Formation of these highmelting temperature phases may explain the slow fusion process ofSSA 1 and SSA 2 under heating.
The transformation of mineral compositions in the SSA 3 at elevatedtemperatures is shown in Fig. 3c. As compared with the SSA 1 and theSSA 2, significant difference can be observed from XRD patterns identi-fied from the sintered SSA 3. It was identified that hematite (Fe2O3),metakaolinite (Al2O3·2SiO2) andwollastonite (CaSiO3)weremain crys-talline phases in the sintered SSA 3 products except the quartz (SiO2).No corundum (Al2O3) was observed from SSA 3 XRD patterns. It indi-cates that utilization of the different precipitation chemicals has evidentimpacts on mineral compositions of sewage sludge ashes. The XRDpeaks of hematite (Fe2O3) identified at 2θ=28° and 2θ=36° decreasedwith increasing sintering temperature. Meanwhile, the peak of quartz(SiO2) identified at 2θ=26° decreased when sintering temperaturewas above 900 °C. It suggests transition of these mineral phases andthe partial melting due to phase assemblage. Iron oxides have beenreported to cause ash melting and fouling during sludge and coal ther-mal conversion processes [7,15,18,25–28]. Iron oxides can react withquartz to form eutectics having low melting points at temperaturerange 900 °C to 1000 °C, which are in amorphous phase and ‘invisible’for XRD analyses [26]. Quantum chemical calculation results confirmedthat the iron bearing silicates have a low thermal stability andmay startto melt under a relative low temperature [27]. In addition, iron oxidescan also reactwith clayminerals to form lowmelting point iron bearingaluminum silicates, whichmight contribute severe sintering of SSA 3 aswell [28]. Alkali aluminum silicates were also observed from SSA 3 pro-duced at 550 °C and 900 °C including Na-feldspar (NaSi3AlO8) and K-feldspar (KSi3AlO8). However, intensities of these mineral phases de-creasedwith elevated sintering temperatures. Decrease and eliminationof XRD peaks of Na-feldspar (NaSi3AlO8) and K-feldspar (KSi3AlO8) il-lustrated in Fig. 3c are probably related to melting of these phases to
93L. Wang et al. / Fuel Processing Technology 96 (2012) 88–97
liquid (slag) or react with iron oxides with formation of silicates[22,23,29]. This gives high melted fraction in SSA 3 and enhances thesintering degree.
3.4. SEM-EDX analysis
Sewage sludge ashes after sintering tests were also collected andexamined by SEM-EDX analysis. The backscattered electron images(BSE) were taken from a scanned area, which revealed better con-trast of elements distribution. In general, the heavy elements withgreater atomic numbers result in brighter areas in a BSE image.Figs. 4–6 illustrate representative BSE images captured from cross-sectioned sewage sludge ashes. In each BSE image, jet black regionsare void space filled by the epoxy resin, and rest of areas representash particles in different grain sizes and sintering behaviors at in-creasing temperatures. Fig. 4a–c presents an overview of crossed-sectioned SSA 1, and there is no evident agglomerates and bridgesformation under increasing sintering temperatures. As shown inFig. 5a and b, similar fiberlike areas shown in Fig. 4 can be observedfrom the SEM images of SSA 2 sintered at 900 °C and 1000 °C, butthese areas start to connect with each other to form more continu-ous phases shown in Fig. 5b. In Fig. 5c, the fiberlike areas decreasedramatically, which are replaced by a more compact morphology.Two ash grains in left corner of Fig. 5c are connected with a brightwhite zone. The bright white color in the zone indicates different el-ement compositions in comparison with surrounding gray areas.Fig. 6 shows evident sintering and melting processes of SSA 3 heat-ed under increasing treatment temperatures. In Fig. 6a, the areaswith loose structure indicate that SSA 3 keeps original microstruc-tures after 900 °C sintering treatment. Fig. 6b shows clear fusionand agglomeration of SSA 3 sintered at 1000 °C. Two zones can bedistinguished easily according to the different colors and brightness.The light white gray areas with a more continuous phase representthe molten portion of SSA 3. The darker gray patches illustrate theother materials that are connected or covered by the melted ash.Fig. 6c shows that SSA 3 heated at 1100 °C was melted completelywith presenting of continuous solidified phase. The voids withspherical shapes indicate bubbling and swelling of viscous ashmelt at the high sintering temperature [30]. It should be notedthat the scale bar increased from 20 to 200 μm in Fig. 6c, which rep-resents transition of SSA 3 from sintering stage to a molten fromand further shrinkage into a single liquid drop.
Themicrochemistry of three sewage sludge ashes sintered at 1100 °Cwas examined by EDX analyses. Extra EDX analyses were performed onthe residue from SSA 3 sintered at 1000 °C. The close-up views of hightemperature sintered ashes are presented in Figs. 7 to 10,wheremappingofmain elements is illustrated. In the samefigures, different sample spotsweremarked, and their chemical compositions in weight percentage arenormalized and presented in Table 3 in oxides form.
20 µm
a b
Fig. 4. Back-scattered electron images of cross-sectioned sewage sl
In Fig. 7, the EDX spot (spots 1 and 2) analysis results revealedthat fiberlike areas are rich in Al, P, Si, Fe and Ca with small amountsof K, Na, Mg and S. EDX element maps illustrated clear correlationsbetween elements Al, P, Ca and Si, which represent the distributionof the four most abundant elements in SSA 1. Chemical compositionsof spot 3 falling on the big grain in right upper corner of the BSEimage contain more than 95% Si with presence of Al, Ca, Na and K inlevels of 0.7–1.0%. This element distribution closes to normal sandchemical compositions [22,23]. In addition, a strong Si signal can beobserved in element maps, which has clear correlations with Al, Ca,Na and K in the same zone. Therefore, this big grain is a sand/soil par-ticle, which was possibly introduced in the sewage sludge during thewaste water sedimentation process [31].
Two spots (spots 1 and 2) were selected in the bright white zonebridging two ash grains in Fig. 8. EDX spot analysis results showedthat iron is the dominating element with over 50% in weight per-centage, with presence of relative high amounts of Si and Al. Thesimilar chemical compositions were detected from a zone (spot 6)in the corner of an ash grain, which shows bright white color asthe bridge section does. In agreement with spot analysis, the strongiron signal can be observed from the same zone in Fe element map.Therefore, the bright section represents the formation of iron alumi-num silicates. However, considering the low iron content in SSA 2,the amount of formed iron aluminum silicates might be too smallto be identified by XRD. In addition, if the formed iron aluminumsilicates were melted into glassy phase, they may be also invisiblefor XRD. Therefore, none iron aluminum silicates can be foundfrom XRD patterns obtained from SSA 2. In the zone with spot 5,Si, Al and K are main elements according to EDX spot analyses,with strong correlations between the three elements showed in el-emental maps. Combined spot chemical compositions with elemen-tal mapping, mineral association in this zone could be potassiumaluminum silicates.
Figs. 9 and 10 represent EDX spot analysis and elemental associa-tions for SSA 3 sintered at 1000 °C and 1100 °C, respectively. FromFig. 9, it is easy to distinguish three zones due to abundance of differ-ent elements. EDX analysis results (spots 2 and 3) revealed that Si isthe main element over than 97% in dark gray zones, with detection ofa small amount of Al, K, Fe, Na and Mg. Thereby these zones repre-sent sand/soil particles with irregular shapes, which are clearlyshown in the Si element map. The sand/soil particles are coveredand connected by the ash melts that are illustrated as rest areas inlight gray color. The EDX analysis (spot 1) showed that neck sectionsare enriched in Fe2O3 and SiO2, with a level of the former over than45% and that of latter at 18%. This elemental distribution indicates re-actions between the two elements and possible role of Fe in the for-mation of agglomerated bridges. For the melted ash portion (spot 4),an enrichment of iron (Fe2O3) was clearly observed and presented asthe similar level of silicon (SiO2) at around 20%. Other elementsdetected there include aluminum (Al) and phosphorus (P) with an
20 µm 20 µm
c
udge ash 1 sintered at (a) 900 °C, (b) 1000 °C and (c) 1100 °C.
a
20 µm 200µm
b c
20 µm
Fig. 6. Back-scattered electron images of cross-sectioned sewage sludge ash 3 sintered at (a) 900 °C, (b) 1000 °C and (c) 1100 °C.
20 µm
a
20 µm 20 µm
b c
Fig. 5. Back-scattered electron images of cross-sectioned sewage sludge ash 2 sintered at (a) 900 °C, (b) 1000 °C and (c) 1100 °C.
94 L. Wang et al. / Fuel Processing Technology 96 (2012) 88–97
amount of 15–20%, followed by Ca, Mg, K and Na at a level of 1–5%.As compared with sampling spot 4 in the melted ash zone, spot 1 ismuch closer to the sand particles representing the initial formationof fused matters. The concentrations of Fe and Si at spot 1 are consid-erably higher than those of spot 4, while the concentrations of otherelements (Al, P, Ca, K and Mg) at spot 1 are lower than those detectedat spot 4. The differences of element concentrations in spot 1 and 4imply the role of Fe (from sludge) and Si (from sand particles) in ini-tializing formation of sticky layers on sand particle surfaces, whichacted as a glue matter to promote agglomerating bridges formation.This observation is plausibly related to decreased intensities of hema-tite (Fe2O3) and quartz (SiO2) observed from XRD patterns (Fig. 3c),as a result of forming and fusion of low melting point iron silicates.These iron silicates were presented as viscous phase and caused sin-tering of ash grains. The similar eutectic formation mechanism wasreported in [32]. Presence of iron in peat ash caused severe bed ag-glomeration due to plausible interaction between peat ash and
Fig. 7. SEM-EDX spot analysis and elementa
quartz bed sand with formation of low melting point iron silicates[32]. For Fig. 9, it should be noted that silicon distribution in thescanned area does not correlate well with other elements, especiallyfor element Fe. It is due to extremely high absolute concentration ofSi in sand/soil particles, which results in Si signal in the rest of areatoo weak to be illustrated compared those from sand particles. InFig. 9, a zone where spot 5 falling on represents strong correlationsbetween Si, K, Al and Na shown in elements mapping. Togetherwith EDX analysis (spot 5), it is concluded that alkali aluminosilicatesmight be the main mineral phases in this zone. Fig. 10 shows SSA 3melted completely at 1100 °C in a continuous phase with clear obser-vation of two zones in the BSE image. The zone with brighter color isrelated with high Fe, Si and Al contents as presented in elementalmaps, which was confirmed by EDX spot analyses (spots 1 and 2).It suggests formation of iron silicates/aluminosilicates, which fusedin a continuous phase during the sintering process. In the zonewith darker gray color (spot 3), iron (Fe2O3) concentration is lower
Fig. 9. SEM-EDX spot analysis and elemental mapping of SSA 3 sintered at 1000 °C.
Fig. 8. SEM-EDX spot analysis and elemental mapping of SSA 2 sintered at 1100 °C.
95L. Wang et al. / Fuel Processing Technology 96 (2012) 88–97
and presented as same level of silicon (SiO2) and aluminum (Al2O3)at 20–25%. The P, Ca and Mg concentrations in spots 3 and 4 arehigher than those found at the spots 1 and 2. In addition, elementalmaps show relatively even distribution of these three elements. Itimplies mineral phases containing P, Ca and Mg dissolved in theiron rich melt. The strong correlations between Si, Al, K and Nashown in elemental maps and EDX spot analysis (spots 4 and 5)imply possible interactions between these elements and indicate for-mation of alkali aluminosilicates. Several small bright patches can beseen in Si element map, and some of them have no correlation withother elements. These patches represent sand particles that are
Fig. 10. SEM-EDX spot analysis and elementa
trapped by melted ash without further transformation. This observa-tion agree well with XRD pattern obtained from SSA 3 sintered at1100 °C, where quartz (SiO2) was still identified but with a lowintensity.
4. Conclusions
Application of phosphorus precipitate agents has considerable ef-fects on sewage sludge chemical compositions. The Al and Fe contentsin sewage sludge ashes studied in present work are significantly high
96 L. Wang et al. / Fuel Processing Technology 96 (2012) 88–97
due to use of precipitate chemicals (Al2(SO4)3 and Fe2(SO4)3) in dif-ferent waste water plants, respectively.
The SSA 1 and SSA 2 containing high content Al were character-ized with high melting temperatures and low sintering tendenciesunder elevated temperatures. The slow ash fusion process was attrib-uted to large amounts of inert Al2O3 in two ashes and formation ofhigh melting points mineral phases during ash sintering processes.
Compared to SSA 1 and SSA 2, the SSA 3 has an evidently higher Fecontent and a low melting temperature. During the ash sintering pro-cess, the iron bearing compound hematite is readily to react withquartz and aluminum silicates. The reaction products iron silicatesand iron aluminum silicates may melt at low temperatures, which ini-tiate and enhance the SSA 3 sintering and further fusion as heatingtemperature increased.
Acknowledgment
The financial support from the Research Council of Norway isgratefully acknowledged. Finally, we thank two reviewers for their in-sightful comments on our work.
Appendix A
Slagging and fouling indices used in this work:
(1) The sum of percentage of basic components (Rb):
Rb ¼ Fe2O3 þ CaOþMgOþ K2OþNa2O:
(2) The ratio of basic to acidic components in ash (Rb/a)
Rb=a ¼Fe2O3 þ CaOþMgOþ K2OþNa2O
Al2O3 þ SiO2 þ TiO2:
(3) The ratio of basic to acidic components in ash with consideringphosphorus in ash (Rb/a(+p))
[1] R. Wim, Sewage sludge as a biomass resource for the production of energy:overview and assessment of the various options, Energy & Fuels 22 (2008) 9–15.
[2] J. Werther, T. Ogada, Sewage sludge combustion, Progress in Energy and Combus-tion Science 25 (1999) 55–116.
[3] P. Stasta, J. Boran, B. Ladislav, S. Petr, O. Jaroslav, Thermal processing of sewagesludge, Applied Thermal Engineering 28 (2006) 1420–1426.
[4] M. Hartman, K. Svoboda, M. Pohorely, O. Trnka, Combustion of dried sewagesludge in a fluidized-bed reactor, Industrial and Engineering Chemistry Research44 (2005) 3432–3441.
[5] H. Cui, Y. Ninomiya, M. Masui, H. Mizuhoshi, T. Sakano, C. Kanaoka, Fundamentalbehaviors in combustion of raw sewage sludge, Energy & Fuels 20 (2005) 77–83.
[6] L. Zhang, M. Ito, A. Sato, Y. Ninomiya, T. Sakano, C. Kanaoka, M. Masui, Combusti-bility of dried sewage sludge and its mineral transformation at different oxygencontent in drop tube furnace, Fuel Processing Technology 85 (2004) 983–1011.
[7] J. Shao, D.H. Lee, R. Yan, M. Liu, X. Wang, D.T. Liang, T.J. White, H. Chen,Agglomeration characteristics of sludge combustion in a bench-scale fluidizedbed combustor, Energy & Fuels 21 (2007) 2608–2614.
[8] K.-L. Lin, K.-Y. Chiang, D.-F. Lin, Effect of heating temperature on the sinteringcharacteristics of sewage sludge ash, Journal of Hazardous Materials 128 (2006)175–181.
[9] G.R. Xu, J.L. Zou, G.B. Li, Effect of sintering temperature on the characteristics ofsludge ceramiste, Journal of Hazardous Materials 150 (2008) 394–400.
[10] X. Wang, Y. Jin, Z. Wang, R.B. Mahar, Y. Nie, A research on sintering characteristicsand mechanisms of dried sewage sludge, Journal of Hazardous Materials 160(2008) 489–494.
[11] Y-c. Huang, K-c. Li, Effect of reducing conditions on sludge melting process,Chemosphere 50 (2003) 1063–1068.
[12] S. Brett, J. Guy, G.K. Morse, J.N. Lester, Phosphorus Removal and RecoveryTechnologies, Environmental and Water Resource Engineering Section, ImperialCollege of Science, Technology andMedicine, London, 1997 ISBN: 0-948411-10-0.
[13] A. Pettersson, M. Zevenhoven, B.-M. Steenari, L.-E. Åmand, Application ofchemical fractionation methods for characterisation of biofuels, wastes derivedfuels and CFB co-combustion fly ashes, Fuel 87 (2008) 3183–3193.
[14] O. Atsushi, S. Junichi, K. Masaru, N. Toshihiro, X-ray fluorescence analysis ofsludge ash from sewage disposal plant, X-Ray Spectrometry 37 (2008) 544–550.
[15] R.W. Bryers, Fireside slagging, fouling, and high-temperature corrosion ofheat-transfer, Progress in Energy and Combustion Science 22 (1996) 29–120.
[17] D. Gregory, J. Maier, S. Gunter, Ash fusibility and compositional data of solid re-covered fuels, Fuel 89 (2010) 1534–1540.
[18] S.V. Vassilev, K. Kitano, S. Takedas, T. Tsurue, Influence of mineral and chemicalcomposition of coal ashes on their fusibility, Fuel Processing Technology 45(1995) 27–51.
[19] J.C. Van Dyk, Understanding the influence of acidic components (Si, Al, and Ti) onash flow temperature of South African coal sources, Minerals Engineering 19(2006) 280–286.
97L. Wang et al. / Fuel Processing Technology 96 (2012) 88–97
[20] T. Minna, D. Jouni, S.L. Risto, The role of amorphous material in ash on the ag-glomeration problems in FB boilers-a powder XRD and SEM-EDS study, Energy& Fuels 16 (2002) 871–877.
[21] M.J. Fernadez Llorente, P.D. Arocas, L. Gutierrez Nebot, J.E. Carrasco Garcia, The ef-fect of the addition of chemical materials on the sintering of biomass ash, Fuel 87(2008) 2651–2658.
[22] B.M. Steenari, O. Lindqvist, High-temperature reactions of straw ash and the anti-sintering additives kaolin and dolomite, Biomass and Bioenergy 14 (1998) 67–76.
[23] E. Lindström, M. Öhman, R. Backman, D. Broström, Influence of sand contamina-tion on slag formation during combustion of wood derived fuels, Energy & Fuels22 (2008) 2216–2220.
[24] M.B. Folgueras, R.M. Diaz, J. Xiberta, M.P. Garica, J. Juan Pis, Influence of sewagesludge addition on coal Ash fusion temperatures, Energy & Fuels 19 (2005)2562–2570.
[25] R. Yan, D.T. Liang, K. Laursen, Y. Li, L. Tsen, J.H. Tay, Formation of bed agglomera-tion in a fluidized multi-waste incinerator, Fuel 82 (2003) 843–851.
[26] B.M. Steenari, O. Lindqvist, V. Langer, Ash sintering and deposit formation inPFBC, Fuel 77 (1998) 407–417.
[27] Z. Zhang, X. Wu, T. Zhou, Y. Chen, N. Hou, G. Piao, N. Kobavashi, The effect of iron-bearing mineral melting behavior on ash deposition during coal combustion, Pro-ceedings of the Combustion Institute 33 (2011) 2853–2861.
[28] D.M. Mason, J.G. Patel, Chemistry of ash agglomeration in the U-GAS® process,Fuel Processing Technology 3 (1980) 181–206.
[29] A.P. Reifenstein, H. Kahraman, C.D.A. Coin, N.J. Calos, G. Miller, P. Uwins, Behav-iour of selected minerals in an improved ash fusion test: quartz, potassium feld-spar, sodium feldspar, kaolinite, illite, calcite, dolomite, siderite, pyrite andapatite, Fuel 78 (1999) 1449–1461.
[30] S.K. Gupta, T.F. Wall, R.A. Creelman, R.P. Gupta, Ash fusion temperatures and thetransformations of coal ash particles to slag, Fuel Processing Technology 56(1998) 33–43.
[31] L. Wang, G. Skjevrak, J.E. Hustad, M.G. Grønli, Effects of Sewage Sludge andMarble Sludge Addition on Slag Characteristics during Wood Waste PelletsCombustion, Energy & Fuels 25 (2011) 5775–5785.
[32] R. Heikkinen, R.S. Laitinen, T. Patrikainen, M. Tiainen, M. Virtanen, Slagging ten-dency of peat ash, Fuel Processing Technology 56 (1998) 69–80.
