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Characterization of some coal combustion solid residues
Fuel 78 (1999) 613–618
Characterization of some coal combustion solid residues L. Armesto*, J.L. Merino CIEMAT, Avda Complutense 22, 28040 Madrid, Spain Received 6 August 1997; received in revised form 27 July 1998; accepted 3 September 1998
Abstract The solid combustion residues generated during fluidized bed coal combustion and pulverized coal differ as a result of different operating conditions. This paper summarizes a study carried out on the combustion residues arising from two types of fluidized bed combustors, both pressurized bubbling and atmospheric circulating (Escatron power plant and Ciemat pilot plant, respectively), and a pulverized coal power plant. The influence of the combustion system on the characteristics of the residues and the problems related to their disposal, from the point of view of leaching characteristics, were studied. The results show that the main components of the solid residues from fluidized bed coal combustion are those arising from the sulphation reaction and by-products of this reaction, while the inorganic constituents of coal are the main components from pulverized coal combustion residues. Resulting from this composition the most important components, as regards to concentration, are calcium and sulphate in the leachates from fluidized bed combustion residues, while the leachates from pulverised coal combustion residues contain only the elements from the mineral matter. One other interesting observation is the high concentration of free-lime in circulating fluidized bed combustion residues as compare to the pressurized bed combustion residues. 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: Fluidized beds; Coal combustion; Ashes
1. Introduction Coal is the most abundant and widely dispersed fossil energy resource in the world and the largest single energy resource used for power production. However, there is a conflict between the requirements for increased energy production on the one hand and less pollution on the other. Increasingly stringent environmental legislation in many countries demands cost-effective and environmentally acceptable technologies. To meet these criteria, clean coal technologies are being developed. The fluidized bed combustion (FBC) is one such technology [1]. Various fluidized bed designs were developed. If the fluidization velocity is taken as the criterion for classification, these designs may be divided into bubbling and circulating types, while if the operating pressure were used for this purpose the types of fluidized bed would be atmospheric and pressurized. FBC technologies may provide performance, economic, and environmental advantages compared with conventional coal combustion. FBC systems offer the benefits of fuel flexibility, internal emissions control during combustion
and low combustion temperatures resulting in little ash fusion and minimizing nitrogen oxide formation [2]. The use of in-bed desulphurization in FBC systems results in the production of large quantities of residues, particularly when a high sulphur coal is burnt. The properties of these residues are different from those produced by pulverized coal combustion and are dependent on the operating conditions. On the other hand, the chemical and physical characteristics of these residues will have an effect on their environmental impact, disposal or utilization. The aim of the work described in this paper was to study the influence of different technologies on the chemical and physical characteristics of the solid residues obtained. To accomplish this objective, solid residues from a pilot scale circulating fluidized bed combustor (CFBC) (0.3 Mwt), a demonstration pressurized fluidized bed combustor (PFBC) (80 Mwe) and a pulverized coal (PC) thermal power unit were examined from the point of view of chemical, physical and leading characteristics.
2. Experimental * Corresponding author. Tel: ⫹34 91 346 60 00; fax: ⫹34 91 346 60 05. E-mail address: [email protected] (L. Armesto)
The solid combustion residues studied were generated in three combustion systems: Escatron PFBC plant [3],
0016-2361/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(98)00164-1
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L. Armesto, J.L. Merino / Fuel 78 (1999) 613–618
Table 1 Coal analysis Coal type
Lignite (LI)
Proximate Analysis (wt% db) Ash Volatile matter Fixed carbon Ultimate analysis (wt% db) C S N H O HHV (kcal/kg)
Anthracite (A)
32.6 44.0 23.4
32.5 7.8 59.7
65.0 12.9 1.0 5.4 15.7 4513
90.6 3.0 1.2 2.9 2.3 5332
Ash analysis SiO2 (wt%) Al2O3 (wt%) Fe2O3 (wt%) Na2O (wt%) MgO (wt%) TiO2 (wt%)
Lignite (LI) 48.6 8.60 3.64 0.99 1.82 0.60
Anthracite (A) 44.70 28.80 10.11 2.11 2.67 1.10
K2O (%) Cr (ppm) Ni (ppm) Sr (ppm) V (ppm) Zn (ppm)
Compostilla PC power plant [4] and Ciemat CFBC pilot plant [5]. The Escatron plant uses limestone as SO2 sorbent and lignite (LI) as fuel. The Compostilla power plant utilizes as fuel mixtures of different anthracitic coals from the mining area near the power plant (A). In order to study the influence of different technologies on the solid combustion residues characteristics, both types of coal, using limestone as sorbent, were burned in the Ciemat pilot plant. Table 1 shows the main characteristics of coals including the main components of their ashes and Table 2 shows the main operating conditions of the combustion tests (average values).
Lignite (LI) 1.69 46 132 1273 66 132
Anthracite (A) 4.60 156 162 490 227 475
LI2B) and another one from the particulate control systems (LI1F, A1F, LI2C). The CFBC system has a baghouse for particulate control removal and the PFBC system had two cyclones for particulate control. The pulverized coal (PC) system generated only one type of solid combustion residue from the particulate control system (A2F). The chemical analyses of the major and minor components were determined by ‘Plasma ICP-MS’, with a preliminary preparation of acid digestion. The crystalline phases of ashes were analyzed by powder X-ray diffraction (XRD). Table 3 shows the chemical composition of the different residues obtained and Table 4 shows their mineralogical composition. 3.2. Particle size distribution
3. Methodology and results 3.1. Chemical and mineralogical composition The PFBC and CFBC systems generated two major waste streams, one from overflow of the bed (LI1B, A1B and Table 2 Operating conditions: average values Combustion system
ACFBC
Coal type T (⬚C) Vf (m/s) Pressure (MPa) Ca/S (molar) O2 (wt%)
LI 850 6.5 — 2.5 4.2
A 850 6.3 — 3.0 4.2
PFBCC
PC
LI 850 1.0 1 2.5 4.0
A 1200 — — — 6.0
The particle size distribution is an important parameter for many utilization options of these residues, such as the replacement of aggregates in civil engineering uses. The particle size distribution of the residues from the particle control systems was determined by COULTER COUNTER, and from the bed ashes by screening using sieves (0.065–3.0 mm). Figs. 1 and 2 show the particle size distribution of residues from the particle systems. 3.3. Leaching tests The coal combustion residues are subject to weathering and other natural processes during storage, disposal or utilization. Contaminants can be leached from the residues and may be released to groundwater and surface waters.
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Table 3 Chemical composition of different residues
Ct (wt%) Cinor (wt%) SO3 (wt%) SiO2 (wt%) Al2O3 (wt%) Fe2O3 (wt%) CaO (wt%) MgO (wt%) Na2O (wt%) K2O (wt%) TiO2 (wt%) Ba (ppm) Cr (ppm) Cu (ppm) Mn (ppm) Ni (ppm) Sr (ppm) V (ppm) Zn (ppm)
ACFB LI1B
LI1F
A1B
PFBC A1F
LI2B
LI2C
PC A2F
0.6 0.6 31.0 8.7 2.5 0.7 48.O 0.6 0.1 0.3 0.01 140 37 12 100 19 315 61 76
3.4 0.8 14.7 22.6 4.7 2.0 31.3 0.9 0.5 0.7 0.09 71 15 23 136 ⬍ 10 445 10 77
2.0 0.1 17.8 24.4 8.9 2.6 26.2 0.9 0.4 1.6 0.15 310 42 49 167 25 26 54 73
21.6 0.1 12.7 20.9 10.1 5.1 19.9 1.3 0.6 1.7 0.15 598 49 44 216 52 51 71 115
5.7 5.3 22.6 6.8 2.6 1.5 44.1 0.4 0.1 0.3 0.02 65 14 ⬍8 78 ⬍ 10 130 10 47
4.5 4.0 16.9 18.4 5.8 3.3 38.0 0.3 0.4 0.8 0.11 132 34 12 104 18 135 53 64
4.1 0.3 0.5 48.4 23.8 9.4 2.0 0.1 0.8 4.2 1.00 610 100 29 200 40 50 33 69
Different leaching tests were developed to determine the interaction of residues with the surrounding environment: laboratory batch tests, laboratory column tests and field tests using lysimeters. In this work batch column leaching tests were carried out. Although batch tests provide a rapid method of assessment, column tests were developed to provide a more representative idea of leaching under natural conditions. There are many different batch leaching test procedures. In this work the leaching test protocol required by Spanish legislation [6], a modification of the EP batch tests, was used. In these tests the procedure followed 100 g of crushed material (⬍4 mm) mixed with deionized water in a liquid:solid ratio of 16:1; a solution of 0.5 M acetic acid was added, until pH 5 was reached, but with a maximum quantity of solution added of 400 ml. Resulting from the alkaline character of the residues it was not possible to reach this pH in spite of having added the maximum quantity. Therefore, it was necessary to repeat extractions on the filtered solid obtained in the previous extraction until pH 5 was obtained. In the column tests, residues were placed in a column and subjected to a flow of liquid through the material. Deionized water was used as the leaching media. Leaching can be
Table 4 Mineralogical composition LI1B LI1F A1B A1F LI2B LI2C A2F
Anhydrite, a-quartz, lime, calcite Anhydrite, a-quartz, lime Anhydrite, a-quartz, lime Anhydrite, a-quartz, lime Calcite, anhydrite, lime, a-quartz Calcite, anhydrite, a-quartz, lime a-quartz
controlled by compacting the material in the column and by increasing the flow rate of the liquid. The samples used in the leaching tests were a mixture of bed and the particle control systems residues in proportion according to their discharges from the plant (reference as Al, LI1, A2 and LI2) using a liquid/solid ratio (L/S) of: 2,1 for LI2 and A2, 0,8 for LI1 and 3 for the A1 sample. Table 5 shows the main characteristics of the first (L-1) and the last (L-5) leachates obtained for different solid combustion residues samples studied in the batch leaching tests. Table 6 shows the main leachates components obtained in the column tests.
4. Discussion 4.1. Chemical and mineralogical composition The main components of the solid residues from coal combustion in fluidized beds, using limestone as a sorbent of the SO2 formed during the combustion process, are those arising from the sulphation reaction (CaSO4) and the byproducts of this reaction (CaO, CaCO3). However, the main components of the solid residues from the pulverized coal system are coal ash and unburnt carbon [7]. On the other hand, anhydrite is the major phase present in CFBC residues, while the PFBC bottom residues (LI2B) have calcite as the major phase. The concentration of freelime (CaO) is much higher for the CFBC residues than for PFBC residues. This is because of the existence of different mechanisms of the desulphurization process in the two FBC systems. Thus, in the atmospheric beds, CaCO3 calcines to form CaO, the CaO then reacts with SO2 forming CaSO4. However, in PFBC, the calcination of CaCO3 is normally
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The PC combustion residues presented poorly crystallized phases, so that only a-SiO2 was detected by XRD. Resulting from the high CaO content in the FBC residues, the use of these in applications involving water is not feasible, since the formation is possible of aluminosulfates such as ettringite and the hydration of anhydrite to produce important volume [8] changes. On the other hand, the use of such residues, in the manufacturing of cements and concretes is also not possible, due fundamentally to their high SO3 contents. However, the PC residues, as regards to their composition, can be used in the manufacturing of cements. 4.2. Particle size Fig. 1. Particle size distribution of solid residues from PFBC and CFBC particle control system.
inhibited resulting from excessive partial pressures of CO2. Therefore, in PFBC processes, the desulphurization reaction would proceed according to the following reaction: CaCO3 ⫹ SO2 ⫹ 1=202 X CaSO4 ⫹ CO2 The rest of the components are very similar for PFBC and CFBC residues. The PC residues have as main components SiO2, Al2O3, K2O and Fe2O3. All these components are from the inorganic constituents of coal. On the other hand, minor component (Ba, Cr, Cu, Mn…) contents in the PC residues (A2F) are higher than in the CFBC residues (A1F). There are several possible reasons for this, it could result from the high temperature used in PC combustion leading to the volatilization of any components of the coal mineral matter, so that these components are present in the flue gas and consequently can condense in the cooler parts of the power plant such as the particle control system. Consequently solid residues collected in this system contain trace elements. The dilution effects caused by addition of sorbent in the fluidized bed combustion system are also important.
Resulting from the different coal particle size utilized in the different combustion system, the particle size distribution in the solid combustion residues studied and the distribution of the solids residues, between the particle control systems and the overflow of the bed are very different. While the residues generated in the PC combustion systems are all collected in the particle control system, in the case of PFBC and CFBC facilities the 60%–65% of solids generated are collected in these particle control systems. The residues generated in PC combustion systems are smaller than those generated in the fluidized systems. On the other hand, the particle size of the solids collected from the particle control systems is smaller in PFBC facility than in the CFBC installation, as a result of the different fluidization rates of the two facilities and higher efficiency of the particle control systems of the pressurized fluidized bed plant. The fluidization velocity used in PFBC system is lower than the fluidization velocity used in the CFBC system. Then the particle size eluted from the combustor in the PFBC system is lower than the particle size of solid eluted in the CFBC system. The particle size distribution of solid residues has a marked influence on the geotechnical properties for disposal and some applications. 4.3. Leaching tests
Fig. 2. Particle size distribution of residues from the CFBC and PC particle control system.
4.3.1. Batch leaching tests In all cases, the main components of the leachates are Ca ⫹2 and SO 4 . The sulphate and calcium concentrations in leachates obtained with PC residues are lower than in fluidized bed residues. This is caused by the different composition of the residues. On the other hand, the trace elements concentration is very low, being lower than the analytical detection limit. One interesting observation is the presence of elements such as Al and Fe, in the leachates of FBC residues when the pH is lower than 6. The results obtained in the batch leaching tests show that only the sulphate concentration is higher than the EU (European Union) limit, therefore it is necessary to control it.
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Table 5 Characteristics of leachates from the batch leaching tests (mg/l) SO 4
Ca
K
Na
Mg
Cr
Sr
Zn
AI
Fe
Cl
225 10
14 0.3
⬍ 10 ⬍ 10
614 ⬍4
⬍ 0.5 0.6
0.8 ⬍ 0.3
⬍ 0.3 ⬍ 0.3
⬍ 0.5 ⬍ 0.5
⬍ 0.3 ⬍ 0.3
⬍5 5
CFBC solid residues (A1) L1 1380 675 L5 1676 410
4.9 2.1
12.0 ⬍ 10
⬍4 ⬍4
⬍ 0.5 ⬍ 0.5
2.3 0.5
⬍ 0.3 ⬍ 0.3
⬍ 0.5 6.5
⬍ 0.3 0.6
7 ⬍5
PFBC solid residues (LI2) L1 1500 2675 L5 1400 1475
14.0 3.9
14 ⬍ 10
49 13
⬍ 0.5 ⬍ 0.5
3.5 3.1
⬍ 0.3 0.35
⬍ 0.5 110
34 24
⬍5 ⬍5
CFBC solid residues (LI1) L1 1340 3050 L5 1500 550
5.3 3.2
⬍ 10 ⬍ 10
⬍4 17
⬍ 0.5 ⬍ 0.5
4.0 1.2
⬍ 0.3 ⬍ 0.3
⬍ 0.5 15
⬍ 0.3 3
⬍5 ⬍5
PC solid residues (A2) L1 246 L2 2.3
5. Conclusion
4.3.2. Column leaching tests The permeability of the fluidized bed residues decreased gradually as leaching proceeded while the permeability of columns that contain PC residues remained more constant. This limits the amount of leachates that could be collected before the columns became blocked. One of the most important characteristics of the leachates obtained is its pH, from highly alkaline for the CFBC samples (12,3–13, 1), and decreasing in alkalinity for the PFBC samples (9,4) to the PC samples (8,1), resulting from their different lime content. The most important components, as regards the concentration, in the leachates from residues obtained in fluidized bed systems are calcium and sulphate, their concentration remaining practically constant in the leachates throughout the tests. The sulphate concentration is high in the case of PFBC samples (LI2), while the calcium concentration is high in CFBC residues (LI1) as a result of the different concentration of these components in the solid combustion residues. The leachates obtained from PC solid residues contain more of the trace elements than the samples obtained from fluidized beds. The concentrations of the main components in the leachates from PC samples decrease quickly while in the case of fluidized bed samples, the decrease is slow or remains constant throughout the leaching tests.
The study shows that the combustion operation conditions strongly influence the solid combustion residue characteristics. In this sense, the residues obtained in a fluidized bed combustor (FBC) using limestone as the SO2 sorbent, have as main components anhydrite (CaSO4), lime (CaO) and calcite (CaCO3), while the main components of the pulverized coal combustor (PC) are the inorganic constituents of the coal. The PFBC residues differ from CFBC residues in that calcite does not calcine at the pressures typical of PFBC, since the free lime is less in these residues. Because of these characteristics, the pH and calcium concentration are higher in the leachates from CFBC residues than in those from the PFBC residues. One other interesting conclusion is that the use of CFBC residues in applications involving water is not possible because of their high CaO content, while the PFBC residues can not be used in cement manufacture as a result of their high SO3 content. Resulting from the combustion temperature utilized in PC system, the solid residues obtained presented higher trace elements content than CFBC and PFBC residues. The concentrations of these elements in the leachates from all cases are below the EU limits. One other interesting observation is that permeability of the fluidized bed residues decreased gradually as leaching
Table 6 Main components extraction obtained in the column leaching tests (mg/l) Sample
pH
SO 4
Ca
K
Na
Mo
Cl
A1 LI1 A2 LI2
12.0–11.2 12.0–12.3 8.1–9.0 9.4–9.7
2419–1236 1770–1367 3550–48 2500–2200
980–580 2800–1400 285–25 710–395
630–130 2700–1150 720–11 920–550
170–44 1875–890 685–5.3 435–195
0.86– ⬍ 0.3 9.7–3.2 16–0.07 6.2–3.3
275–20 487–88 6.1– ⬍ 1 221–6
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proceeded while the permeability of the PC residues remained constant. Acknowledgements The authors, would like to thank ENDESA and ECSC for their collaboration and financial support. References [1] Nilsson C, Clarke LB. IEACR/73 IEA Coal Reseach London, 1994.
[2] Smith IM. IEACR/21 Coal research London, 1994. [3] Gestion de cenizas de calderas de lecho fluido presurizadas y atmosfe´ricas, Madrid, Spain: CIEMAT, 1996. [4] Plumed A, Can˜adas L, Otero P, et al. Coal and Science and Technology 1995;24:1783. [5] Armesto L, Cabanillas A, Otero J. In: 13th International Conference on Fluidized Bed Combustion, vol. 2, 1995. p. 1455. [6] Ley 20/1986 de 14 Mayo. Basica de Residuos To´xicos y Peligrosos (B.O.E. num 120, de 20 Mayo 1986). [7] Leccuyer L, Leduc M, Lefevre R, Ausset P. In: 14th International Conference on Fluidized Bed Combustion, vol. 1, 1997. p. 529. [8] Conn R, Selakumar K. In: 14th International Conference on Fluidized Bed Combustion, vol. 1, 1997. p. 507.
Characterization of some coal combustion solid residues L. Armesto*, J.L. Merino CIEMAT, Avda Complutense 22, 28040 Madrid, Spain Received 6 August 1997; received in revised form 27 July 1998; accepted 3 September 1998
Abstract The solid combustion residues generated during fluidized bed coal combustion and pulverized coal differ as a result of different operating conditions. This paper summarizes a study carried out on the combustion residues arising from two types of fluidized bed combustors, both pressurized bubbling and atmospheric circulating (Escatron power plant and Ciemat pilot plant, respectively), and a pulverized coal power plant. The influence of the combustion system on the characteristics of the residues and the problems related to their disposal, from the point of view of leaching characteristics, were studied. The results show that the main components of the solid residues from fluidized bed coal combustion are those arising from the sulphation reaction and by-products of this reaction, while the inorganic constituents of coal are the main components from pulverized coal combustion residues. Resulting from this composition the most important components, as regards to concentration, are calcium and sulphate in the leachates from fluidized bed combustion residues, while the leachates from pulverised coal combustion residues contain only the elements from the mineral matter. One other interesting observation is the high concentration of free-lime in circulating fluidized bed combustion residues as compare to the pressurized bed combustion residues. 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: Fluidized beds; Coal combustion; Ashes
1. Introduction Coal is the most abundant and widely dispersed fossil energy resource in the world and the largest single energy resource used for power production. However, there is a conflict between the requirements for increased energy production on the one hand and less pollution on the other. Increasingly stringent environmental legislation in many countries demands cost-effective and environmentally acceptable technologies. To meet these criteria, clean coal technologies are being developed. The fluidized bed combustion (FBC) is one such technology [1]. Various fluidized bed designs were developed. If the fluidization velocity is taken as the criterion for classification, these designs may be divided into bubbling and circulating types, while if the operating pressure were used for this purpose the types of fluidized bed would be atmospheric and pressurized. FBC technologies may provide performance, economic, and environmental advantages compared with conventional coal combustion. FBC systems offer the benefits of fuel flexibility, internal emissions control during combustion
and low combustion temperatures resulting in little ash fusion and minimizing nitrogen oxide formation [2]. The use of in-bed desulphurization in FBC systems results in the production of large quantities of residues, particularly when a high sulphur coal is burnt. The properties of these residues are different from those produced by pulverized coal combustion and are dependent on the operating conditions. On the other hand, the chemical and physical characteristics of these residues will have an effect on their environmental impact, disposal or utilization. The aim of the work described in this paper was to study the influence of different technologies on the chemical and physical characteristics of the solid residues obtained. To accomplish this objective, solid residues from a pilot scale circulating fluidized bed combustor (CFBC) (0.3 Mwt), a demonstration pressurized fluidized bed combustor (PFBC) (80 Mwe) and a pulverized coal (PC) thermal power unit were examined from the point of view of chemical, physical and leading characteristics.
2. Experimental * Corresponding author. Tel: ⫹34 91 346 60 00; fax: ⫹34 91 346 60 05. E-mail address: [email protected] (L. Armesto)
The solid combustion residues studied were generated in three combustion systems: Escatron PFBC plant [3],
0016-2361/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(98)00164-1
614
L. Armesto, J.L. Merino / Fuel 78 (1999) 613–618
Table 1 Coal analysis Coal type
Lignite (LI)
Proximate Analysis (wt% db) Ash Volatile matter Fixed carbon Ultimate analysis (wt% db) C S N H O HHV (kcal/kg)
Anthracite (A)
32.6 44.0 23.4
32.5 7.8 59.7
65.0 12.9 1.0 5.4 15.7 4513
90.6 3.0 1.2 2.9 2.3 5332
Ash analysis SiO2 (wt%) Al2O3 (wt%) Fe2O3 (wt%) Na2O (wt%) MgO (wt%) TiO2 (wt%)
Lignite (LI) 48.6 8.60 3.64 0.99 1.82 0.60
Anthracite (A) 44.70 28.80 10.11 2.11 2.67 1.10
K2O (%) Cr (ppm) Ni (ppm) Sr (ppm) V (ppm) Zn (ppm)
Compostilla PC power plant [4] and Ciemat CFBC pilot plant [5]. The Escatron plant uses limestone as SO2 sorbent and lignite (LI) as fuel. The Compostilla power plant utilizes as fuel mixtures of different anthracitic coals from the mining area near the power plant (A). In order to study the influence of different technologies on the solid combustion residues characteristics, both types of coal, using limestone as sorbent, were burned in the Ciemat pilot plant. Table 1 shows the main characteristics of coals including the main components of their ashes and Table 2 shows the main operating conditions of the combustion tests (average values).
Lignite (LI) 1.69 46 132 1273 66 132
Anthracite (A) 4.60 156 162 490 227 475
LI2B) and another one from the particulate control systems (LI1F, A1F, LI2C). The CFBC system has a baghouse for particulate control removal and the PFBC system had two cyclones for particulate control. The pulverized coal (PC) system generated only one type of solid combustion residue from the particulate control system (A2F). The chemical analyses of the major and minor components were determined by ‘Plasma ICP-MS’, with a preliminary preparation of acid digestion. The crystalline phases of ashes were analyzed by powder X-ray diffraction (XRD). Table 3 shows the chemical composition of the different residues obtained and Table 4 shows their mineralogical composition. 3.2. Particle size distribution
3. Methodology and results 3.1. Chemical and mineralogical composition The PFBC and CFBC systems generated two major waste streams, one from overflow of the bed (LI1B, A1B and Table 2 Operating conditions: average values Combustion system
ACFBC
Coal type T (⬚C) Vf (m/s) Pressure (MPa) Ca/S (molar) O2 (wt%)
LI 850 6.5 — 2.5 4.2
A 850 6.3 — 3.0 4.2
PFBCC
PC
LI 850 1.0 1 2.5 4.0
A 1200 — — — 6.0
The particle size distribution is an important parameter for many utilization options of these residues, such as the replacement of aggregates in civil engineering uses. The particle size distribution of the residues from the particle control systems was determined by COULTER COUNTER, and from the bed ashes by screening using sieves (0.065–3.0 mm). Figs. 1 and 2 show the particle size distribution of residues from the particle systems. 3.3. Leaching tests The coal combustion residues are subject to weathering and other natural processes during storage, disposal or utilization. Contaminants can be leached from the residues and may be released to groundwater and surface waters.
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Table 3 Chemical composition of different residues
Ct (wt%) Cinor (wt%) SO3 (wt%) SiO2 (wt%) Al2O3 (wt%) Fe2O3 (wt%) CaO (wt%) MgO (wt%) Na2O (wt%) K2O (wt%) TiO2 (wt%) Ba (ppm) Cr (ppm) Cu (ppm) Mn (ppm) Ni (ppm) Sr (ppm) V (ppm) Zn (ppm)
ACFB LI1B
LI1F
A1B
PFBC A1F
LI2B
LI2C
PC A2F
0.6 0.6 31.0 8.7 2.5 0.7 48.O 0.6 0.1 0.3 0.01 140 37 12 100 19 315 61 76
3.4 0.8 14.7 22.6 4.7 2.0 31.3 0.9 0.5 0.7 0.09 71 15 23 136 ⬍ 10 445 10 77
2.0 0.1 17.8 24.4 8.9 2.6 26.2 0.9 0.4 1.6 0.15 310 42 49 167 25 26 54 73
21.6 0.1 12.7 20.9 10.1 5.1 19.9 1.3 0.6 1.7 0.15 598 49 44 216 52 51 71 115
5.7 5.3 22.6 6.8 2.6 1.5 44.1 0.4 0.1 0.3 0.02 65 14 ⬍8 78 ⬍ 10 130 10 47
4.5 4.0 16.9 18.4 5.8 3.3 38.0 0.3 0.4 0.8 0.11 132 34 12 104 18 135 53 64
4.1 0.3 0.5 48.4 23.8 9.4 2.0 0.1 0.8 4.2 1.00 610 100 29 200 40 50 33 69
Different leaching tests were developed to determine the interaction of residues with the surrounding environment: laboratory batch tests, laboratory column tests and field tests using lysimeters. In this work batch column leaching tests were carried out. Although batch tests provide a rapid method of assessment, column tests were developed to provide a more representative idea of leaching under natural conditions. There are many different batch leaching test procedures. In this work the leaching test protocol required by Spanish legislation [6], a modification of the EP batch tests, was used. In these tests the procedure followed 100 g of crushed material (⬍4 mm) mixed with deionized water in a liquid:solid ratio of 16:1; a solution of 0.5 M acetic acid was added, until pH 5 was reached, but with a maximum quantity of solution added of 400 ml. Resulting from the alkaline character of the residues it was not possible to reach this pH in spite of having added the maximum quantity. Therefore, it was necessary to repeat extractions on the filtered solid obtained in the previous extraction until pH 5 was obtained. In the column tests, residues were placed in a column and subjected to a flow of liquid through the material. Deionized water was used as the leaching media. Leaching can be
Table 4 Mineralogical composition LI1B LI1F A1B A1F LI2B LI2C A2F
Anhydrite, a-quartz, lime, calcite Anhydrite, a-quartz, lime Anhydrite, a-quartz, lime Anhydrite, a-quartz, lime Calcite, anhydrite, lime, a-quartz Calcite, anhydrite, a-quartz, lime a-quartz
controlled by compacting the material in the column and by increasing the flow rate of the liquid. The samples used in the leaching tests were a mixture of bed and the particle control systems residues in proportion according to their discharges from the plant (reference as Al, LI1, A2 and LI2) using a liquid/solid ratio (L/S) of: 2,1 for LI2 and A2, 0,8 for LI1 and 3 for the A1 sample. Table 5 shows the main characteristics of the first (L-1) and the last (L-5) leachates obtained for different solid combustion residues samples studied in the batch leaching tests. Table 6 shows the main leachates components obtained in the column tests.
4. Discussion 4.1. Chemical and mineralogical composition The main components of the solid residues from coal combustion in fluidized beds, using limestone as a sorbent of the SO2 formed during the combustion process, are those arising from the sulphation reaction (CaSO4) and the byproducts of this reaction (CaO, CaCO3). However, the main components of the solid residues from the pulverized coal system are coal ash and unburnt carbon [7]. On the other hand, anhydrite is the major phase present in CFBC residues, while the PFBC bottom residues (LI2B) have calcite as the major phase. The concentration of freelime (CaO) is much higher for the CFBC residues than for PFBC residues. This is because of the existence of different mechanisms of the desulphurization process in the two FBC systems. Thus, in the atmospheric beds, CaCO3 calcines to form CaO, the CaO then reacts with SO2 forming CaSO4. However, in PFBC, the calcination of CaCO3 is normally
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The PC combustion residues presented poorly crystallized phases, so that only a-SiO2 was detected by XRD. Resulting from the high CaO content in the FBC residues, the use of these in applications involving water is not feasible, since the formation is possible of aluminosulfates such as ettringite and the hydration of anhydrite to produce important volume [8] changes. On the other hand, the use of such residues, in the manufacturing of cements and concretes is also not possible, due fundamentally to their high SO3 contents. However, the PC residues, as regards to their composition, can be used in the manufacturing of cements. 4.2. Particle size Fig. 1. Particle size distribution of solid residues from PFBC and CFBC particle control system.
inhibited resulting from excessive partial pressures of CO2. Therefore, in PFBC processes, the desulphurization reaction would proceed according to the following reaction: CaCO3 ⫹ SO2 ⫹ 1=202 X CaSO4 ⫹ CO2 The rest of the components are very similar for PFBC and CFBC residues. The PC residues have as main components SiO2, Al2O3, K2O and Fe2O3. All these components are from the inorganic constituents of coal. On the other hand, minor component (Ba, Cr, Cu, Mn…) contents in the PC residues (A2F) are higher than in the CFBC residues (A1F). There are several possible reasons for this, it could result from the high temperature used in PC combustion leading to the volatilization of any components of the coal mineral matter, so that these components are present in the flue gas and consequently can condense in the cooler parts of the power plant such as the particle control system. Consequently solid residues collected in this system contain trace elements. The dilution effects caused by addition of sorbent in the fluidized bed combustion system are also important.
Resulting from the different coal particle size utilized in the different combustion system, the particle size distribution in the solid combustion residues studied and the distribution of the solids residues, between the particle control systems and the overflow of the bed are very different. While the residues generated in the PC combustion systems are all collected in the particle control system, in the case of PFBC and CFBC facilities the 60%–65% of solids generated are collected in these particle control systems. The residues generated in PC combustion systems are smaller than those generated in the fluidized systems. On the other hand, the particle size of the solids collected from the particle control systems is smaller in PFBC facility than in the CFBC installation, as a result of the different fluidization rates of the two facilities and higher efficiency of the particle control systems of the pressurized fluidized bed plant. The fluidization velocity used in PFBC system is lower than the fluidization velocity used in the CFBC system. Then the particle size eluted from the combustor in the PFBC system is lower than the particle size of solid eluted in the CFBC system. The particle size distribution of solid residues has a marked influence on the geotechnical properties for disposal and some applications. 4.3. Leaching tests
Fig. 2. Particle size distribution of residues from the CFBC and PC particle control system.
4.3.1. Batch leaching tests In all cases, the main components of the leachates are Ca ⫹2 and SO 4 . The sulphate and calcium concentrations in leachates obtained with PC residues are lower than in fluidized bed residues. This is caused by the different composition of the residues. On the other hand, the trace elements concentration is very low, being lower than the analytical detection limit. One interesting observation is the presence of elements such as Al and Fe, in the leachates of FBC residues when the pH is lower than 6. The results obtained in the batch leaching tests show that only the sulphate concentration is higher than the EU (European Union) limit, therefore it is necessary to control it.
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Table 5 Characteristics of leachates from the batch leaching tests (mg/l) SO 4
Ca
K
Na
Mg
Cr
Sr
Zn
AI
Fe
Cl
225 10
14 0.3
⬍ 10 ⬍ 10
614 ⬍4
⬍ 0.5 0.6
0.8 ⬍ 0.3
⬍ 0.3 ⬍ 0.3
⬍ 0.5 ⬍ 0.5
⬍ 0.3 ⬍ 0.3
⬍5 5
CFBC solid residues (A1) L1 1380 675 L5 1676 410
4.9 2.1
12.0 ⬍ 10
⬍4 ⬍4
⬍ 0.5 ⬍ 0.5
2.3 0.5
⬍ 0.3 ⬍ 0.3
⬍ 0.5 6.5
⬍ 0.3 0.6
7 ⬍5
PFBC solid residues (LI2) L1 1500 2675 L5 1400 1475
14.0 3.9
14 ⬍ 10
49 13
⬍ 0.5 ⬍ 0.5
3.5 3.1
⬍ 0.3 0.35
⬍ 0.5 110
34 24
⬍5 ⬍5
CFBC solid residues (LI1) L1 1340 3050 L5 1500 550
5.3 3.2
⬍ 10 ⬍ 10
⬍4 17
⬍ 0.5 ⬍ 0.5
4.0 1.2
⬍ 0.3 ⬍ 0.3
⬍ 0.5 15
⬍ 0.3 3
⬍5 ⬍5
PC solid residues (A2) L1 246 L2 2.3
5. Conclusion
4.3.2. Column leaching tests The permeability of the fluidized bed residues decreased gradually as leaching proceeded while the permeability of columns that contain PC residues remained more constant. This limits the amount of leachates that could be collected before the columns became blocked. One of the most important characteristics of the leachates obtained is its pH, from highly alkaline for the CFBC samples (12,3–13, 1), and decreasing in alkalinity for the PFBC samples (9,4) to the PC samples (8,1), resulting from their different lime content. The most important components, as regards the concentration, in the leachates from residues obtained in fluidized bed systems are calcium and sulphate, their concentration remaining practically constant in the leachates throughout the tests. The sulphate concentration is high in the case of PFBC samples (LI2), while the calcium concentration is high in CFBC residues (LI1) as a result of the different concentration of these components in the solid combustion residues. The leachates obtained from PC solid residues contain more of the trace elements than the samples obtained from fluidized beds. The concentrations of the main components in the leachates from PC samples decrease quickly while in the case of fluidized bed samples, the decrease is slow or remains constant throughout the leaching tests.
The study shows that the combustion operation conditions strongly influence the solid combustion residue characteristics. In this sense, the residues obtained in a fluidized bed combustor (FBC) using limestone as the SO2 sorbent, have as main components anhydrite (CaSO4), lime (CaO) and calcite (CaCO3), while the main components of the pulverized coal combustor (PC) are the inorganic constituents of the coal. The PFBC residues differ from CFBC residues in that calcite does not calcine at the pressures typical of PFBC, since the free lime is less in these residues. Because of these characteristics, the pH and calcium concentration are higher in the leachates from CFBC residues than in those from the PFBC residues. One other interesting conclusion is that the use of CFBC residues in applications involving water is not possible because of their high CaO content, while the PFBC residues can not be used in cement manufacture as a result of their high SO3 content. Resulting from the combustion temperature utilized in PC system, the solid residues obtained presented higher trace elements content than CFBC and PFBC residues. The concentrations of these elements in the leachates from all cases are below the EU limits. One other interesting observation is that permeability of the fluidized bed residues decreased gradually as leaching
Table 6 Main components extraction obtained in the column leaching tests (mg/l) Sample
pH
SO 4
Ca
K
Na
Mo
Cl
A1 LI1 A2 LI2
12.0–11.2 12.0–12.3 8.1–9.0 9.4–9.7
2419–1236 1770–1367 3550–48 2500–2200
980–580 2800–1400 285–25 710–395
630–130 2700–1150 720–11 920–550
170–44 1875–890 685–5.3 435–195
0.86– ⬍ 0.3 9.7–3.2 16–0.07 6.2–3.3
275–20 487–88 6.1– ⬍ 1 221–6
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proceeded while the permeability of the PC residues remained constant. Acknowledgements The authors, would like to thank ENDESA and ECSC for their collaboration and financial support. References [1] Nilsson C, Clarke LB. IEACR/73 IEA Coal Reseach London, 1994.
[2] Smith IM. IEACR/21 Coal research London, 1994. [3] Gestion de cenizas de calderas de lecho fluido presurizadas y atmosfe´ricas, Madrid, Spain: CIEMAT, 1996. [4] Plumed A, Can˜adas L, Otero P, et al. Coal and Science and Technology 1995;24:1783. [5] Armesto L, Cabanillas A, Otero J. In: 13th International Conference on Fluidized Bed Combustion, vol. 2, 1995. p. 1455. [6] Ley 20/1986 de 14 Mayo. Basica de Residuos To´xicos y Peligrosos (B.O.E. num 120, de 20 Mayo 1986). [7] Leccuyer L, Leduc M, Lefevre R, Ausset P. In: 14th International Conference on Fluidized Bed Combustion, vol. 1, 1997. p. 529. [8] Conn R, Selakumar K. In: 14th International Conference on Fluidized Bed Combustion, vol. 1, 1997. p. 507.