Engineering Characteristics of Tanjung Bin Coal Ash
Engineering Characteristics of Tanjung Bin Coal Ash Muhardi Senior Lecturer in Faculty of Engineering, Universitas Riau, Indonesia e-mail: [email protected]
Aminaton Marto, Khairul Anuar Kassim, Ahmad Mahir Makhtar Professors in Faculty of Civil Engineering, Universiti Teknologi Malaysia, Malaysia; e-mail: [email protected], [email protected], [email protected]
Lee Foo Wei Lecturer in Faculty of Engineering and Green Technology, University Tunku Abdul Rahman, Malaysia; e-mail: [email protected]
Yap Shih Lim Geotechnical Engineer in Meinhardt(s) Pte Ltd, Singapore e-mail: [email protected]
ABSTRACT Tanjung Bin power station is one of the four coal power plants in Malaysia, producing180 tons/day of bottom ash and 1,620 tons/day of fly ash from 18,000 tons/day of coal burning. This paper focuses on the some engineering properties of coal ash (fly ash and bottom ash) from Tanjung Bin power station (e.g. grain size, specific gravity, compaction, shear strength, permeability and compressibility) In addition, morphology, mineralogy and chemistry of coal ash are studied using scanning electron microscope (SEM), x-ray diffraction (XRD and x-ray fluorescence (XRF) Tanjung Bin coal ashes were compacted at 95% of optimum moisture content, sealed and cured for 0, 7, and 28 days before they were analyzed for morphological and mineralogical analyses. Morphological analysis showed that the number of irregular shaped particles increased confirming change in material type with curing period. From mineralogical analysis, the crystalline compounds present in Tanjung Bin coal ash were quartz, mullite, magnetite, hematite, and calcium oxide. From chemical analysis, Tanjung Bin fly ash is classified as class F in which fly ash has low lime, less than 10%. Its low specific gravity, freely draining nature, ease of compaction, good frictional properties, high shear strength and low compressibility can be gainfully exploited in the construction of embankments, roads, reclamation and fill behind retaining structures. KEYWORDS: Fly ash, Bottom ash, Engineering characteristics, Tanjung Bin power station
INTRODUCTION Coal has been projected as an important resource fuel in the forthcoming in Malaysia. It is projected that the installed capacity on the coal power plant in the year 2010 will be 7,200 MW (about 40% of the total), requiring about 22.5 million tons of coal, that is for 8,200 MW capacities (Mahmud, 2003) Currently, there are four coal power plants in Malaysia namely, Tanjung Bin (2,100 MW), Jimah (1,400 - 1117 -
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MW), Sultan Salahuddin Abdul Aziz / Kapar (2,420 MW) and Sultan Azlan Shah / Manjung (2,100 MW) power plants, as shown in Figure 1. Fly ash and bottom ash are two of the coal waste products. Other waste products are slag and flue gas desulfurization (FGD) It has been reported that the Tanjung Bin power plant alone needs about 18,000 tons/day of coal to generate electricity. As a result of large utilization of coal, a large volume of coal ash as waste material will be produced. The large quantity of coal ash will be a considerable disposal concern to power plants companies due to the increase requirement for ash storage space. Hence, this will increase the expenses as there will be the need to obtain large areas. Due to this, the power plants companies will be a social and environmental problem because of the magnification of disposal areas and the increased disposal expenses will be finally transferred to end users. For that reason, the utilization of coal ash in construction industry, in particular which requires large quantity materials such as in embankment construction, is greatly shows potential to answer the disposal problem of coal ash. This paper focuses on the characterization of fly ash and bottom ash collected from Tanjung Bin power station in Malaysia that includes the investigation of morphological, mineralogical, physical and mechanical properties. Information regarding to chemical properties of coal ash is required before these materials can be safely and effectively utilized. The physical and mechanical properties, in particular, are important parameters affecting the behavior of coal ash in various engineering applications. Information concerning the morphology and mineralogy are important for addressing the potential environmental impacts associated with coal ash utilization and disposal (Abbas, 2002)
Figure 1: Location of coal-fired power plants in Malaysia (Mahmud, 2003)
TESTING PROGRAM The experimental programs are designed to first characterize the samples of coal ash material taken from the power plant in Tanjung Bin, Malaysia. In order to characterize the particle shape and surface textures of coal ash particles and to gain some insight of the behavior of coal ash material, microscopic examinations were conducted on the samples of coal ash using scanning electron microscopy (SEM) ZEISS SUPRA 35-VP. The X-ray diffraction (XRD) using Siemens Diffraktometer D5000 was used to investigate the mineralogy properties of coal ash. X-ray fluorescence (XRF) Bruker AXS Model S4 Pioneer had been used for the identification of chemical content of coal ash. For physical characterization, the coal ash was subjected to specify gravity and particle size distribution and permeability tests. For mechanical characterization, the laboratory work was included to compaction, permeability, shear strength and compressibility tests.
RESULTS AND DISCUSSION Morphology Characteristics From SEM analysis, the electron micrographs of coal ashes are found to be as shown in Figure 2 and 3 for fly ash and bottom ash, respectively with increasing in curing period. Tanjung Bin fly ash appears to be composed of powder like particle with dark grey colour. Under the micrograph, fly ash is wellrounded, spherical in shapes, round glassy spheres, number of irregular shaped particles and their surface appear to be very smooth. Some particles are approximately hollow spheres with thin wall. The effects of increased age and water adding in fly ash have also been examined under SEM. As the curing age of specimen increases, the number of irregular shaped particles increases confirming change in material type. Besides that, it is found that the particle size has also increased from 10 μm to 24 μm with increasing in curing period. This is probably due to the pozzolanic reaction occurred as explained by Kaniraj and Gayatri (2004) According to Ghosh and Subbarao (2007), iron rich spherical particles showed different morphology with low and high calcium fly ash. Figure 3a and 3b is SEM photomicrographs of the Tanjung Bin bottom ash particles (finer than 0.075 mm) which were taken at the magnifications of 500. The bottom ash particles in this size range appeared to be in three types: fine fractions of shattered bottom ash particles, large spherical fly ash like particles, and agglomerates of bonded fly ash particles. However, the majority of particles are appeared to be the first type. Some bottom ash particles appeared to be combined with fly ash particles, in which they are observed to be loosely held to the surfaces of larger particles. After the compaction, water was observed presented in the particles of bottom ash, the particles became smaller due to crushing effects. Fine fractions of shattered and irregular bottom ash particles are predominant in the compacted specimens for the 7 and 28 days. There are also some agglomerates bonded particles existed after 7 days compacted specimen due to the crystal growth and little pozzolanic activities, as shown in Figure 3c and 3d. Angularity affects the particle interlock, and a rough surface texture restraints movement of one particle on another. According to Huang (1990), those materials with greater angularity and rough surface texture are preferred in highway bases and sub-bases. In addition, bottom ash particles are irregular and angular in terms of shape and had a rough, gritty surface textures. Bottom ash particles are free from dust, clean and shinny. Some large particles are crushable due to the internally and externally porous and popcorn likes behaviour. Particles agglomerations had also been observed in bottom ash and these agglomerates ranged from lightly cemented to strongly bonded (Kim et al., 2005)
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(a) Pure
(b) 0 day
(c) 7 days
(d) 28 days
Figure 2: SEM photomicrographs of Tanjung Bin fly ash
(a) Pure
(b) 0 day
(c)
(d) 28 days
7 days
Figure 3: SEM photomicrographs of Tanjung Bin bottom ash
Mineralogy Characteristics Result of XRD analysis shows that the crystalline compounds present in fly ash were quartz, mullite, aluminium oxide, hematite and calcium oxide. This is in conformity with the results of other XRD studies which have indicated the phase compositions of the Indian fly ashes as quartz, mullite, hematite, magnetite, calcium oxide and glass. It is also observed that the chemical compounds of fly ash disappeared with the increase in curing period. This might be due to the chemical compounds which have reacted or soluble with/into water (Kaniraj and Gayatri, 2004) Result of XRD analysis for pure bottom ash shows that mullite, silicon oxide, and silicon phosphate are the predominant crystalline form substances. After compaction, silicon oxide became the main crystalline substances at 0, 7 and 28 days curing period. The intensity peak of the crystalline substances is sharper, and shows a rapid increase in the 7 and 28 days curing period for bottom ash samples compared to at 0 day and pure samples. Mineralogical examination showed that silica is present partly in the crystalline forms of quartz and in combination with the alumina as mullite. The iron appears partly as the - 1121 -
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oxide magnetite and hematite. Some of the crystalline compounds such as silicon phosphate, calcium aluminium oxide, calcium oxide and magnetite are found to disappear after compaction and their disappearance state increases with the increase in curing period. This might be due to the chemical reactions with water after curing for some periods of times or mineralogy changes after oven dried before XRD testing (Huang, 1990)
Chemical Characteristics It is important to ascertain the chemical composition of the coal ash because of the change with time leading to changes in mechanical properties. The chemical composition of the coal ash samples was investigated using XRF analysis. The percentage of the chemical contents for Tanjung Bin coal ash using XRF analysis is given in Table 1. Table 1: Chemical content of Tanjung Bin coal ash using XRF analysis Chemical contents SiO2
Fly ash 51.80%
Bottom Ash 42.70%
Al2O3
26.50%
23.00%
Fe2O3
8.50%
17.00%
CaO
4.81%
9.80%
K 2O
3.27%
0.96%
TiO2
1.38%
1.64%
MgO
1.10%
1.54%
P 2O 5
0.90%
1.04%
Na2O
0.67%
0.29%
SO3
0.60%
1.22%
BaO
0.12%
0.19%
The major of contents of Tanjung Bin coal ash (reported as oxides) were silica, alumina, iron and calcium oxide. Smaller percentage of magnesium, kalium, barium, potassium, sodium, and titanium oxides were also found. As can be seen from this table, Tanjung Bin fly ash has relatively low lime (CaO) content, less than 10 percent. According to ACCA (2003) and ASTM C 16 (2004), class of Tanjung Bin fly ash is class F which is ash is typically obtained from bituminous and anthracite coals and consists primarily of an alumina silicate glass, with quartz, mullite, and magnetite also present.
Physical Characteristics Specific Gravity From the result, Tanjung Bin fly ash has low apparent specific gravity with average value of 2.3, if compare to natural soils with specific gravity in the range of 2.5 to 2.7. As reported by some researchers
in previous studies (Misra, 2000; Sato and Nishimoto, 2001; Sahu, 2001; Kim, 2003; ASTM C-618, 2004; Pandian, 2004; Basak et al., 2004; Prabakar et al., 2004; Kaniraj and Gayathri, 2004; Das and Yudbhir, 2006 and Edil et al., 2006), the specific gravity of fly ash lies between 1.73 and 2.71. The Tanjung Bin fly ash is considered in a range from the previous studies. Tanjung Bin bottom ash has low specific gravity which is 1.99. The low specific gravity of bottom ash is explained by its low iron oxide contents. According to Das and Yudhbir (2006), for iron content greater than 10%, the specific gravity value is directly proportional to iron content but for lime content greater than 15%, the specific gravity value is more irrespective of iron content. In addition, the specific gravity of fly ash has more range with the variety value of and average lower than natural soils due to different chemical content and particle structure. For bottom ash with slighter quantities of porous and popcorn-like particles, it generally shows a higher specific gravity which is as high as 2.8 while a porous or hollow ash may present a specific gravity as low as or even lower than 1.6. For dry bottom ash, the reported specific gravity varies from 2.0 to 2.6 with an average of 2.35 while for wet bottom ash; it ranges from 2.6 to 2.9 with an average of 2.75. Theoretically, wet bottom ash tends to have a higher specific gravity than dry bottom ash due to its dense nature (Kim et al., 2006)
Particle Size Distribution Figure 4 shows the grain size distributions for fly ash and bottom ash from Tanjung Bin power plant. Generally, the fly ash was well graded, ranging from mostly fine silt to fine sand sizes. A majority of the sizes occurred in a range between 0.6mm and 0.001 mm. Bottom ash gradations from the two specimens were quite similar and exhibit well graded size distribution. Their sizes ranging from fine gravel to fine sands sizes and the majority sizes occurred in a range between 20 mm and 0.075 mm. Grain size distribution obtained indicated bottom ash as coarse grained materials. The average coefficient of uniformity, cu for bottom ash is about 16.56 while the average coefficient of curvature, cc is 1.01. According to the Unified Soil Classification System (USCS), bottom ash is classified as well graded sand and according to the classification by AASHTO system, bottom ash fall in the A-1 group and classified as A-1-a.
Figure 4: Particle size distribution of fly ash and bottom ash - 1123 -
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M Mechanical Characteristics Compaction Behaviour Figures 5 and 6 show the plots ts oof dry density versus the moisture content for fly ash as and bottom ash, respectively from standard proctorr ttest. The results show that the optimum moisture content con (w(opt)) of fly ash and bottom ash are 19.75% an and 21.5% with maximum dry density (γd(max)) of 1.53 1. Mg/m3 (15.01 kN/m3 and 13.1 Mg/m3 (12.85 kN//m3) respectively. In the previous study, typical values val for maximum dry density and the optimum moistu sture content for Western Pennsylvania fly ash typicall lly range from 11.9 to 18.7 kN/m3 and from 13 to 32 %, respectively. Maximum dry density and thee optimum o moisture content for West Virginia bottom aash typically range from 11.6 to 18.4 kN/m3 and d from f 12 to 34 %, respectively. It can be concluded th that the compaction characteristics vary from source ce to source. This is due to different low specific gravity ity and a high air void content. Normally ash contain ains 5-15% air voids at maximum dry density (Kim, 2003 003) The γd, max values of the fly ash and bottom ash are lower than the γd, max of the sandy soils in the range ge from 17-20 kN/m3. The low γd, max of fly ash and bottom bo ash are main benefit, especially where a structur tural fill using fly ash and bottom ash are constructe cted on low bearing capacity foundation such as soft soil oils.
Compaction curves of Tanjung Bin fly ash Figure 5: C
Figure 6: Compaction curves of Tanjung Bin bottom ash
Permeability The permeability of Tanjung Bin fly ash was measured for samples compacted at 95% relative compaction through falling head permeability test. The permeability obtained was 4.87 x 10-9 m/s, which indicates that the fly ash exhibits the permeability approximately corresponding to that of the fine sand, mixtures of sandy silt and clay or silts, according to USCS. It means that it has poor drainage characteristics. The coefficient of permeability of fly ash depends upon the grain size and pozzolanic activity. Hydraulic conductivity is primarily influenced by the nature of void system between the particles. As shown before, the Tanjung Bin fly ash has the particle size ranging between 0.6 mm to 0.001 mm. Besides that, large specific surfaces of fly ash would cause more resistance to flow of water through the voids. The degree of saturation has an effect on the permeability of a particular material if the material is about 85% saturated. At this situation, air bubbles occur within the pore space of the soil and cause blocking of the seepage channel, reducing the permeability (Pandian, 2004) The permeability of Tanjung Bin bottom ash was measured by a constant head test using a permeameter. The measured coefficient of permeability is 1.72 x 10-4 m/s which indicates the bottom ash exhibits the permeability approximately corresponding to that of the clean sands/gravel mixtures, and is comparable to those of well graded sand and gravel soils. Huang (1990) did a series of permeability tests on Indiana bottom ashes and found that the fines included in bottom ash had a foremost effect on the permeability and hence the permeability decreased as the fine contents increased. Bottom ash particles have voids much larger than fly ash particles. Therefore, its ability to allow the flow of water through it is greater than fly ash. Since the gradation of bottom ash and sand are similar, they tend to exhibit similar permeability. The drainage characteristics associated with the above permeability values are considered good. Hence, compacted bottom ash tested could provide good drainage, which is suitable to be used as backfill materials in road embankment construction.
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Consolidation Behaviour Compressibility characteristics of coal ash depend on its initial density, degree of saturation, selfhardening characteristics and pozzolanic activity. Partially saturated ashes are less compressible compared to fully saturate ones (Pandian, 2004) Result of consolidation test gave the coefficient of consolidation (cv) of Tanjung Bin fly ash varies from 0.0946 to 0.4825 m2/year. From that results, the value of coefficient of consolidation is relative same in the range 0.1 to 0.5 m2/year. Pandian (2004) reported the value of coefficient of consolidation for Indian fly ash varies from 0.04 to 3648 m2/year. The value of Tanjung Bin fly ash was low compared to the results reported. This indicated any structures constructed on such fills did not suffer large deformation when fly ash is properly compacted. The compression index (cc) of the Tanjung Bin fly ash and bottom ash were 0.15 and 1.54, respectively. Comparison with others results; Khaniraj and Gayathri (2004) determine the values of cc to be 0.072. Pandian (2004) has reported the cc vary from 0.068 to 0.301 for compacted at optimum moisture content (OMC) conditions for Indian fly ash. The value of Tanjung Bin fly ash was included in the range of Indian fly ash results. However, when comparing with the other result, Pandian (2004) reported the cc of bottom ash vary from 0.057 to 0.484. The value of Tanjung Bin bottom ash is higher than Indian bottom ash. According to Huang (1990), bottom ash is slightly more compressible than the sand. There are two factors contribute to this finding. Firstly, angularity and rough surface texture of bottom ash particles become overstressed during movement and rearranging of the particles and thus break to allow compression. Secondly, the popcorn-like and weak particles tend to break at a relative low stress level causing in increased deformations.
Strength Behaviour An important engineering property that is necessary for using coal ash in many geotechnical applications is its strength. Tables 2, 3 and 4 summarize the strength parameters obtained under different test conditions. Table 2: Strength parameters from direct shear tests (peak values) Materials Fly ash
Condition Compacted
φp (degrees) 23
cp (kPa) 12
Fly ash
Compacted and saturated
26
3
Bottom ash
Compacted
32
3.8
Bottom ash
Compacted and saturated
31
0
Table 3: Drained strength parameters from triaxial tests Materials Fly ash
Condition Compacted and saturated
φcd (degrees) 41
ccd (kPa) 25
Bottom ash
Compacted and saturated
46
0
Table 4: Undrained strength parameters from triaxial tests Materials Fly ash
Condition Compacted and saturated
φcu (degrees) 41
ccu(kPa) 34
Bottom ash
Compacted and saturated
44
0
Comparisons between the as-compacted and soaked fly ash samples in direct shear tests show that the shear strength goes down due to the disappeared capillary suction. The reduction in the strength, however, was generally significant. In addition, the peak shear stress includes a component of cohesion which is either an apparent cohesion due to the effects of suction or a true cohesion due to pozzolanic activity. Any cohesion due to pozzolanic activity destroyed during the shearing process when peak condition is reached (Tri Utomo, 1996) The results obtained for drained triaxial tests for Tanjung Bin fly ash, compared with Thomas (2002), Lav et al.(2006), Kim (2003) is slightly higher as compared to others due to saturation stage were different and effect of suction that any cohesion due to pozzolanic action is destroy during shearing. Comparing undrained strength between Tanjung Bin fly ash and the results obtained from Pandian (2004) has ranges between 0 kPa for cohesion and 200 to 410 for friction angle. There were large limitations of the data due to various sources of coal, stockpile or condition PFA, degree of saturation before testing. Results of direct shear test of Tanjung Bin bottom ash is quite similar to results from Indian bottom ash (Pandian, 2004) for peak friction angle and slightly higher for peak cohesion. Peak cohesion and friction angle of Indian bottom ash were vary from 0.1-0.2 kPa and 310 to 370 for compacted condition, respectively and for compacted and saturated condition, peak cohesion and friction angle were vary from 0 kPa and 300 to 370, respectively. Comparing undrained strength between Tanjung Bin bottom ash and the results obtained from Pandian (2004) has ranges between 0-27 kPa for cohesion and 240 to 350 for friction angle.
CONCLUSIONS The detailed investigations carried out on Tanjung Bin coal ash show that fly ash and bottom ash has good potential for use in construction industry, especially for geotechnical applications. Its low specific gravity, freely draining nature, ease of compaction, good frictional properties, high shear strength and low compressibility can be gainfully exploited in the construction of embankments, roads, reclamation and fill behind retaining structures. This not only solves the problems associated with the disposal of fly ash and bottom ash (like requirement of precious land and environmental pollution) for power plants but also reduced expenses of electricity for end users.
ACKNODWLEDGEMENT The authors would like to thank the Ministry of Higher Education (MOHE), Malaysia for funding this research by Fundamental Research Grant Scheme (FRGS) Vote No.78220 and Universiti Teknologi Malaysia for providing the laboratory testing facilities.
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REFERENCES 1.
Abbas, G. (2002) Handbook of Pollution Control and Waste Minimization. Published, CRC Press.
2.
ACAA (American Coal Ash Association) (2003) Fly Ash Facts for Highway Engineers. Technical Report ACAA, USA.
3.
ASTM (American Standard Testing Method) (2004) ”Standard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete.” ASTM, USA.
4.
Basak, S., Bhattacharya, A. K. and Paira, S.L.K. (2004) ”Utilization Fly Ash in Rural Road Construction in India and its Cost Effectivenes.” Electronic Journal of Geotechnical Engineering (EJGE)
5.
Das, S.K. and Yudbhir (2006) ”Geotechnical Properties of Low Calcium and High Calcium Fly Ash.” Journal of Geotechnical and Geological Engineering, Vol. 24, p 249-263.
6.
Edil, T.B., Acosta, H.A. and Benson, C.H. (2006)”Stabilizing Soft Fine-Grained Soils with Fly Ash.” Journal of Material in Civil Engineering, Vol. 18, No. 2.
7.
Ghosh, A. and Subbarao, C. (2007) Strength Characteristic of Class F Fly Ash Modified with Lime and Gypsum. Journal of Geotechnical and Geoenvironmental Engineering ASCE, July 2007, pp757765.
8.
Huang H.W. (1990) The Use of Bottom Ash in Highway Embankments, Subgrade and Subbases. Joint Highway Research Project, Final Report, FHWA/IN/JHRP-90/4 Purdue University, W. Lafayette, Indiana.
9.
Kaniraj, S.R. and Gayathri, V. (2004) ”Permeability and Consolidation Characteristics of Compacted Fly Ash.” Journal of Energy Engineering, Vol. 130, No. 1.
10. Kim, B.J., Prezzi, M. and Salgado, S. (2005) Geotechnical Properties of Fly and Bottom Ash Mixtures for Use in Highway Embankments. Journal of Geotechnical and Geoenvironmental Engineering, ASCE. 11. Kim, B.J., Yoon, S.M. and Balunaini, U. (2006) Determination of Ash Mixture Properties and Construction of Test Embankment –Part A. Joint Transportation Research Program, Final Report, FHWA/IN/JTRP-2006/24 Purdue University, W. Lafayette, Indiana. 12. Kim, B. (2003) Properties of Coal Ash Mixtures and their Use in Highway Embankments. PhD Thesis, Purdue University, Indiana, USA. 13. Lav, A.H., Lav, M.A. and Goktepe, A.B. (2006) Analysis and Design of a Stabilized Fly Ash as Pavement Base Material. Istanbul Technical University, Faculty of Civil Engineering, Turkey.
14. Mahmud, H.O. (2003) Coal - Fired Plant in Malaysia. The 15th JAPAC International Symposium 19 September 2003, Tokyo. 15. Misra, A. (2000) ”Utilization of Western Coal Fly Ash in Construction of Highways in Midwest.” Final Report, University of Missouri, Kansas City, USA. 16. Pandian, N.S. (2004) ” Fly Ash Characterization with Reference to Geotechnical Aplications.” Journal of Indian of Institute of Science, Vol. 84, p 189-216. 17. Prabakar, J., Dendorkar, N. and Morchhale, R.K. (2004) ”Influence of Fly Ash on Strength Bahavior of Typical Soils.” Journal of Construction and Building Materials, Vol. 18, p 263-267. 18. Sahu, B.K. (2001) ”Improvement in California Bearing Ratio of Various Soils in Botswana by Fly Ash.” International Ash Utilization Symposium 22 – 24 October 2001, Lexington Kentucky, USA. 19. Sato, A. and Nishimoto, S. (2001) Effective Reuse of Coal Ash as Civil Engineering Material. International Ash Utilization Symposium 22 – 24 October 2001, Lexington Kentucky, USA. 20. Sear, L.K.A. (2001) The Properties and Use of Coal Fly Ash. Thomas Telford Ltd, London, UK. 21. Thomas, Z. (2002) Engineering Properties of Soil Fly Ash Sub-grade Mixtures. Iowa State University, Department of Civil Engineering, USA. 22. Tri Utomo, S.H. (1996) The Effects of Time on Properties of Pulverised Fuel Ash. PhD Thesis, University of Newcastle upon Tyne, UK.
© 2010 ejge
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Aminaton Marto, Khairul Anuar Kassim, Ahmad Mahir Makhtar Professors in Faculty of Civil Engineering, Universiti Teknologi Malaysia, Malaysia; e-mail: [email protected], [email protected], [email protected]
Lee Foo Wei Lecturer in Faculty of Engineering and Green Technology, University Tunku Abdul Rahman, Malaysia; e-mail: [email protected]
Yap Shih Lim Geotechnical Engineer in Meinhardt(s) Pte Ltd, Singapore e-mail: [email protected]
ABSTRACT Tanjung Bin power station is one of the four coal power plants in Malaysia, producing180 tons/day of bottom ash and 1,620 tons/day of fly ash from 18,000 tons/day of coal burning. This paper focuses on the some engineering properties of coal ash (fly ash and bottom ash) from Tanjung Bin power station (e.g. grain size, specific gravity, compaction, shear strength, permeability and compressibility) In addition, morphology, mineralogy and chemistry of coal ash are studied using scanning electron microscope (SEM), x-ray diffraction (XRD and x-ray fluorescence (XRF) Tanjung Bin coal ashes were compacted at 95% of optimum moisture content, sealed and cured for 0, 7, and 28 days before they were analyzed for morphological and mineralogical analyses. Morphological analysis showed that the number of irregular shaped particles increased confirming change in material type with curing period. From mineralogical analysis, the crystalline compounds present in Tanjung Bin coal ash were quartz, mullite, magnetite, hematite, and calcium oxide. From chemical analysis, Tanjung Bin fly ash is classified as class F in which fly ash has low lime, less than 10%. Its low specific gravity, freely draining nature, ease of compaction, good frictional properties, high shear strength and low compressibility can be gainfully exploited in the construction of embankments, roads, reclamation and fill behind retaining structures. KEYWORDS: Fly ash, Bottom ash, Engineering characteristics, Tanjung Bin power station
INTRODUCTION Coal has been projected as an important resource fuel in the forthcoming in Malaysia. It is projected that the installed capacity on the coal power plant in the year 2010 will be 7,200 MW (about 40% of the total), requiring about 22.5 million tons of coal, that is for 8,200 MW capacities (Mahmud, 2003) Currently, there are four coal power plants in Malaysia namely, Tanjung Bin (2,100 MW), Jimah (1,400 - 1117 -
Vol. 15 [2010], Bund. K
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MW), Sultan Salahuddin Abdul Aziz / Kapar (2,420 MW) and Sultan Azlan Shah / Manjung (2,100 MW) power plants, as shown in Figure 1. Fly ash and bottom ash are two of the coal waste products. Other waste products are slag and flue gas desulfurization (FGD) It has been reported that the Tanjung Bin power plant alone needs about 18,000 tons/day of coal to generate electricity. As a result of large utilization of coal, a large volume of coal ash as waste material will be produced. The large quantity of coal ash will be a considerable disposal concern to power plants companies due to the increase requirement for ash storage space. Hence, this will increase the expenses as there will be the need to obtain large areas. Due to this, the power plants companies will be a social and environmental problem because of the magnification of disposal areas and the increased disposal expenses will be finally transferred to end users. For that reason, the utilization of coal ash in construction industry, in particular which requires large quantity materials such as in embankment construction, is greatly shows potential to answer the disposal problem of coal ash. This paper focuses on the characterization of fly ash and bottom ash collected from Tanjung Bin power station in Malaysia that includes the investigation of morphological, mineralogical, physical and mechanical properties. Information regarding to chemical properties of coal ash is required before these materials can be safely and effectively utilized. The physical and mechanical properties, in particular, are important parameters affecting the behavior of coal ash in various engineering applications. Information concerning the morphology and mineralogy are important for addressing the potential environmental impacts associated with coal ash utilization and disposal (Abbas, 2002)
Figure 1: Location of coal-fired power plants in Malaysia (Mahmud, 2003)
TESTING PROGRAM The experimental programs are designed to first characterize the samples of coal ash material taken from the power plant in Tanjung Bin, Malaysia. In order to characterize the particle shape and surface textures of coal ash particles and to gain some insight of the behavior of coal ash material, microscopic examinations were conducted on the samples of coal ash using scanning electron microscopy (SEM) ZEISS SUPRA 35-VP. The X-ray diffraction (XRD) using Siemens Diffraktometer D5000 was used to investigate the mineralogy properties of coal ash. X-ray fluorescence (XRF) Bruker AXS Model S4 Pioneer had been used for the identification of chemical content of coal ash. For physical characterization, the coal ash was subjected to specify gravity and particle size distribution and permeability tests. For mechanical characterization, the laboratory work was included to compaction, permeability, shear strength and compressibility tests.
RESULTS AND DISCUSSION Morphology Characteristics From SEM analysis, the electron micrographs of coal ashes are found to be as shown in Figure 2 and 3 for fly ash and bottom ash, respectively with increasing in curing period. Tanjung Bin fly ash appears to be composed of powder like particle with dark grey colour. Under the micrograph, fly ash is wellrounded, spherical in shapes, round glassy spheres, number of irregular shaped particles and their surface appear to be very smooth. Some particles are approximately hollow spheres with thin wall. The effects of increased age and water adding in fly ash have also been examined under SEM. As the curing age of specimen increases, the number of irregular shaped particles increases confirming change in material type. Besides that, it is found that the particle size has also increased from 10 μm to 24 μm with increasing in curing period. This is probably due to the pozzolanic reaction occurred as explained by Kaniraj and Gayatri (2004) According to Ghosh and Subbarao (2007), iron rich spherical particles showed different morphology with low and high calcium fly ash. Figure 3a and 3b is SEM photomicrographs of the Tanjung Bin bottom ash particles (finer than 0.075 mm) which were taken at the magnifications of 500. The bottom ash particles in this size range appeared to be in three types: fine fractions of shattered bottom ash particles, large spherical fly ash like particles, and agglomerates of bonded fly ash particles. However, the majority of particles are appeared to be the first type. Some bottom ash particles appeared to be combined with fly ash particles, in which they are observed to be loosely held to the surfaces of larger particles. After the compaction, water was observed presented in the particles of bottom ash, the particles became smaller due to crushing effects. Fine fractions of shattered and irregular bottom ash particles are predominant in the compacted specimens for the 7 and 28 days. There are also some agglomerates bonded particles existed after 7 days compacted specimen due to the crystal growth and little pozzolanic activities, as shown in Figure 3c and 3d. Angularity affects the particle interlock, and a rough surface texture restraints movement of one particle on another. According to Huang (1990), those materials with greater angularity and rough surface texture are preferred in highway bases and sub-bases. In addition, bottom ash particles are irregular and angular in terms of shape and had a rough, gritty surface textures. Bottom ash particles are free from dust, clean and shinny. Some large particles are crushable due to the internally and externally porous and popcorn likes behaviour. Particles agglomerations had also been observed in bottom ash and these agglomerates ranged from lightly cemented to strongly bonded (Kim et al., 2005)
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(a) Pure
(b) 0 day
(c) 7 days
(d) 28 days
Figure 2: SEM photomicrographs of Tanjung Bin fly ash
(a) Pure
(b) 0 day
(c)
(d) 28 days
7 days
Figure 3: SEM photomicrographs of Tanjung Bin bottom ash
Mineralogy Characteristics Result of XRD analysis shows that the crystalline compounds present in fly ash were quartz, mullite, aluminium oxide, hematite and calcium oxide. This is in conformity with the results of other XRD studies which have indicated the phase compositions of the Indian fly ashes as quartz, mullite, hematite, magnetite, calcium oxide and glass. It is also observed that the chemical compounds of fly ash disappeared with the increase in curing period. This might be due to the chemical compounds which have reacted or soluble with/into water (Kaniraj and Gayatri, 2004) Result of XRD analysis for pure bottom ash shows that mullite, silicon oxide, and silicon phosphate are the predominant crystalline form substances. After compaction, silicon oxide became the main crystalline substances at 0, 7 and 28 days curing period. The intensity peak of the crystalline substances is sharper, and shows a rapid increase in the 7 and 28 days curing period for bottom ash samples compared to at 0 day and pure samples. Mineralogical examination showed that silica is present partly in the crystalline forms of quartz and in combination with the alumina as mullite. The iron appears partly as the - 1121 -
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oxide magnetite and hematite. Some of the crystalline compounds such as silicon phosphate, calcium aluminium oxide, calcium oxide and magnetite are found to disappear after compaction and their disappearance state increases with the increase in curing period. This might be due to the chemical reactions with water after curing for some periods of times or mineralogy changes after oven dried before XRD testing (Huang, 1990)
Chemical Characteristics It is important to ascertain the chemical composition of the coal ash because of the change with time leading to changes in mechanical properties. The chemical composition of the coal ash samples was investigated using XRF analysis. The percentage of the chemical contents for Tanjung Bin coal ash using XRF analysis is given in Table 1. Table 1: Chemical content of Tanjung Bin coal ash using XRF analysis Chemical contents SiO2
Fly ash 51.80%
Bottom Ash 42.70%
Al2O3
26.50%
23.00%
Fe2O3
8.50%
17.00%
CaO
4.81%
9.80%
K 2O
3.27%
0.96%
TiO2
1.38%
1.64%
MgO
1.10%
1.54%
P 2O 5
0.90%
1.04%
Na2O
0.67%
0.29%
SO3
0.60%
1.22%
BaO
0.12%
0.19%
The major of contents of Tanjung Bin coal ash (reported as oxides) were silica, alumina, iron and calcium oxide. Smaller percentage of magnesium, kalium, barium, potassium, sodium, and titanium oxides were also found. As can be seen from this table, Tanjung Bin fly ash has relatively low lime (CaO) content, less than 10 percent. According to ACCA (2003) and ASTM C 16 (2004), class of Tanjung Bin fly ash is class F which is ash is typically obtained from bituminous and anthracite coals and consists primarily of an alumina silicate glass, with quartz, mullite, and magnetite also present.
Physical Characteristics Specific Gravity From the result, Tanjung Bin fly ash has low apparent specific gravity with average value of 2.3, if compare to natural soils with specific gravity in the range of 2.5 to 2.7. As reported by some researchers
in previous studies (Misra, 2000; Sato and Nishimoto, 2001; Sahu, 2001; Kim, 2003; ASTM C-618, 2004; Pandian, 2004; Basak et al., 2004; Prabakar et al., 2004; Kaniraj and Gayathri, 2004; Das and Yudbhir, 2006 and Edil et al., 2006), the specific gravity of fly ash lies between 1.73 and 2.71. The Tanjung Bin fly ash is considered in a range from the previous studies. Tanjung Bin bottom ash has low specific gravity which is 1.99. The low specific gravity of bottom ash is explained by its low iron oxide contents. According to Das and Yudhbir (2006), for iron content greater than 10%, the specific gravity value is directly proportional to iron content but for lime content greater than 15%, the specific gravity value is more irrespective of iron content. In addition, the specific gravity of fly ash has more range with the variety value of and average lower than natural soils due to different chemical content and particle structure. For bottom ash with slighter quantities of porous and popcorn-like particles, it generally shows a higher specific gravity which is as high as 2.8 while a porous or hollow ash may present a specific gravity as low as or even lower than 1.6. For dry bottom ash, the reported specific gravity varies from 2.0 to 2.6 with an average of 2.35 while for wet bottom ash; it ranges from 2.6 to 2.9 with an average of 2.75. Theoretically, wet bottom ash tends to have a higher specific gravity than dry bottom ash due to its dense nature (Kim et al., 2006)
Particle Size Distribution Figure 4 shows the grain size distributions for fly ash and bottom ash from Tanjung Bin power plant. Generally, the fly ash was well graded, ranging from mostly fine silt to fine sand sizes. A majority of the sizes occurred in a range between 0.6mm and 0.001 mm. Bottom ash gradations from the two specimens were quite similar and exhibit well graded size distribution. Their sizes ranging from fine gravel to fine sands sizes and the majority sizes occurred in a range between 20 mm and 0.075 mm. Grain size distribution obtained indicated bottom ash as coarse grained materials. The average coefficient of uniformity, cu for bottom ash is about 16.56 while the average coefficient of curvature, cc is 1.01. According to the Unified Soil Classification System (USCS), bottom ash is classified as well graded sand and according to the classification by AASHTO system, bottom ash fall in the A-1 group and classified as A-1-a.
Figure 4: Particle size distribution of fly ash and bottom ash - 1123 -
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M Mechanical Characteristics Compaction Behaviour Figures 5 and 6 show the plots ts oof dry density versus the moisture content for fly ash as and bottom ash, respectively from standard proctorr ttest. The results show that the optimum moisture content con (w(opt)) of fly ash and bottom ash are 19.75% an and 21.5% with maximum dry density (γd(max)) of 1.53 1. Mg/m3 (15.01 kN/m3 and 13.1 Mg/m3 (12.85 kN//m3) respectively. In the previous study, typical values val for maximum dry density and the optimum moistu sture content for Western Pennsylvania fly ash typicall lly range from 11.9 to 18.7 kN/m3 and from 13 to 32 %, respectively. Maximum dry density and thee optimum o moisture content for West Virginia bottom aash typically range from 11.6 to 18.4 kN/m3 and d from f 12 to 34 %, respectively. It can be concluded th that the compaction characteristics vary from source ce to source. This is due to different low specific gravity ity and a high air void content. Normally ash contain ains 5-15% air voids at maximum dry density (Kim, 2003 003) The γd, max values of the fly ash and bottom ash are lower than the γd, max of the sandy soils in the range ge from 17-20 kN/m3. The low γd, max of fly ash and bottom bo ash are main benefit, especially where a structur tural fill using fly ash and bottom ash are constructe cted on low bearing capacity foundation such as soft soil oils.
Compaction curves of Tanjung Bin fly ash Figure 5: C
Figure 6: Compaction curves of Tanjung Bin bottom ash
Permeability The permeability of Tanjung Bin fly ash was measured for samples compacted at 95% relative compaction through falling head permeability test. The permeability obtained was 4.87 x 10-9 m/s, which indicates that the fly ash exhibits the permeability approximately corresponding to that of the fine sand, mixtures of sandy silt and clay or silts, according to USCS. It means that it has poor drainage characteristics. The coefficient of permeability of fly ash depends upon the grain size and pozzolanic activity. Hydraulic conductivity is primarily influenced by the nature of void system between the particles. As shown before, the Tanjung Bin fly ash has the particle size ranging between 0.6 mm to 0.001 mm. Besides that, large specific surfaces of fly ash would cause more resistance to flow of water through the voids. The degree of saturation has an effect on the permeability of a particular material if the material is about 85% saturated. At this situation, air bubbles occur within the pore space of the soil and cause blocking of the seepage channel, reducing the permeability (Pandian, 2004) The permeability of Tanjung Bin bottom ash was measured by a constant head test using a permeameter. The measured coefficient of permeability is 1.72 x 10-4 m/s which indicates the bottom ash exhibits the permeability approximately corresponding to that of the clean sands/gravel mixtures, and is comparable to those of well graded sand and gravel soils. Huang (1990) did a series of permeability tests on Indiana bottom ashes and found that the fines included in bottom ash had a foremost effect on the permeability and hence the permeability decreased as the fine contents increased. Bottom ash particles have voids much larger than fly ash particles. Therefore, its ability to allow the flow of water through it is greater than fly ash. Since the gradation of bottom ash and sand are similar, they tend to exhibit similar permeability. The drainage characteristics associated with the above permeability values are considered good. Hence, compacted bottom ash tested could provide good drainage, which is suitable to be used as backfill materials in road embankment construction.
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Consolidation Behaviour Compressibility characteristics of coal ash depend on its initial density, degree of saturation, selfhardening characteristics and pozzolanic activity. Partially saturated ashes are less compressible compared to fully saturate ones (Pandian, 2004) Result of consolidation test gave the coefficient of consolidation (cv) of Tanjung Bin fly ash varies from 0.0946 to 0.4825 m2/year. From that results, the value of coefficient of consolidation is relative same in the range 0.1 to 0.5 m2/year. Pandian (2004) reported the value of coefficient of consolidation for Indian fly ash varies from 0.04 to 3648 m2/year. The value of Tanjung Bin fly ash was low compared to the results reported. This indicated any structures constructed on such fills did not suffer large deformation when fly ash is properly compacted. The compression index (cc) of the Tanjung Bin fly ash and bottom ash were 0.15 and 1.54, respectively. Comparison with others results; Khaniraj and Gayathri (2004) determine the values of cc to be 0.072. Pandian (2004) has reported the cc vary from 0.068 to 0.301 for compacted at optimum moisture content (OMC) conditions for Indian fly ash. The value of Tanjung Bin fly ash was included in the range of Indian fly ash results. However, when comparing with the other result, Pandian (2004) reported the cc of bottom ash vary from 0.057 to 0.484. The value of Tanjung Bin bottom ash is higher than Indian bottom ash. According to Huang (1990), bottom ash is slightly more compressible than the sand. There are two factors contribute to this finding. Firstly, angularity and rough surface texture of bottom ash particles become overstressed during movement and rearranging of the particles and thus break to allow compression. Secondly, the popcorn-like and weak particles tend to break at a relative low stress level causing in increased deformations.
Strength Behaviour An important engineering property that is necessary for using coal ash in many geotechnical applications is its strength. Tables 2, 3 and 4 summarize the strength parameters obtained under different test conditions. Table 2: Strength parameters from direct shear tests (peak values) Materials Fly ash
Condition Compacted
φp (degrees) 23
cp (kPa) 12
Fly ash
Compacted and saturated
26
3
Bottom ash
Compacted
32
3.8
Bottom ash
Compacted and saturated
31
0
Table 3: Drained strength parameters from triaxial tests Materials Fly ash
Condition Compacted and saturated
φcd (degrees) 41
ccd (kPa) 25
Bottom ash
Compacted and saturated
46
0
Table 4: Undrained strength parameters from triaxial tests Materials Fly ash
Condition Compacted and saturated
φcu (degrees) 41
ccu(kPa) 34
Bottom ash
Compacted and saturated
44
0
Comparisons between the as-compacted and soaked fly ash samples in direct shear tests show that the shear strength goes down due to the disappeared capillary suction. The reduction in the strength, however, was generally significant. In addition, the peak shear stress includes a component of cohesion which is either an apparent cohesion due to the effects of suction or a true cohesion due to pozzolanic activity. Any cohesion due to pozzolanic activity destroyed during the shearing process when peak condition is reached (Tri Utomo, 1996) The results obtained for drained triaxial tests for Tanjung Bin fly ash, compared with Thomas (2002), Lav et al.(2006), Kim (2003) is slightly higher as compared to others due to saturation stage were different and effect of suction that any cohesion due to pozzolanic action is destroy during shearing. Comparing undrained strength between Tanjung Bin fly ash and the results obtained from Pandian (2004) has ranges between 0 kPa for cohesion and 200 to 410 for friction angle. There were large limitations of the data due to various sources of coal, stockpile or condition PFA, degree of saturation before testing. Results of direct shear test of Tanjung Bin bottom ash is quite similar to results from Indian bottom ash (Pandian, 2004) for peak friction angle and slightly higher for peak cohesion. Peak cohesion and friction angle of Indian bottom ash were vary from 0.1-0.2 kPa and 310 to 370 for compacted condition, respectively and for compacted and saturated condition, peak cohesion and friction angle were vary from 0 kPa and 300 to 370, respectively. Comparing undrained strength between Tanjung Bin bottom ash and the results obtained from Pandian (2004) has ranges between 0-27 kPa for cohesion and 240 to 350 for friction angle.
CONCLUSIONS The detailed investigations carried out on Tanjung Bin coal ash show that fly ash and bottom ash has good potential for use in construction industry, especially for geotechnical applications. Its low specific gravity, freely draining nature, ease of compaction, good frictional properties, high shear strength and low compressibility can be gainfully exploited in the construction of embankments, roads, reclamation and fill behind retaining structures. This not only solves the problems associated with the disposal of fly ash and bottom ash (like requirement of precious land and environmental pollution) for power plants but also reduced expenses of electricity for end users.
ACKNODWLEDGEMENT The authors would like to thank the Ministry of Higher Education (MOHE), Malaysia for funding this research by Fundamental Research Grant Scheme (FRGS) Vote No.78220 and Universiti Teknologi Malaysia for providing the laboratory testing facilities.
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