Environ. Sci. Technol. 2007, 41, 5330-5335
Influence of a Modification of the Petcoke/Coal Ratio on the Leachability of Fly Ash and Slag Produced from a Large PCC Power Plant M A R I A I Z Q U I E R D O , * ,† O R I O L F O N T , † NATALIA MORENO,† XAVIER QUEROL,† FRANK E. HUGGINS,‡ ESTHER ALVAREZ,† SERGI DIEZ,† PEDRO OTERO,§ JUAN CARLOS BALLESTEROS,§ AND ANTONIO GIMENEZ§ Institute of Earth Sciences “Jaume Almera”, CSIC, Lluis Sole Sabaris s/n. 08028 Barcelona, Spain, CME/CFFS, University of Kentucky, 105 Whalen Building, 533 S. Limestone Street, Lexington, Kentucky 40506-0043, and ENDESA GENERACIO ÄN S.A., Ribera del Loira, 60, planta 2, sector E, 28042, Madrid, Spain
Co-firing of coal with inexpensive secondary fuels such as petroleum coke is expected to increase in the near future in the EU given that it may provide certain economic and environmental benefits with respect to coal combustion. However, changes in the feed fuel composition of power plants may modify the bulk content and the speciation of a number of elements in fly ash and slag. Consequently, leachability of these byproducts also can be modified. This study is focused on identifying the changes in the environmental quality of co-fired fly ash and slag induced by a modification of the petcoke/coal ratio. Petcoke was found to increase the leachable content of V and Mo and to enhance the mobility of S and As. However, with the exception of these elements, the addition of this secondary fuel did not drastically modify the bulk composition or the overall leachability of the resulting fly ash and slag.
Introduction Given that the use of different fuel blends may reduce the production costs of energy, co-firing of coal with inexpensive co-matter (such as biomass, sewage sludge, or petroleum coke) is expected to increase in the near future in the EU. Petcoke is economically attractive as it is a widely available low-cost fuel with an increasing production due to the higher demand for crude oil (1-4). In addition to the economic and environmental benefits, the use of a given proportion of petcoke in the fuel blend could simplify ignition and increase flame stability (2, 5). There have been a number of studies on the possible problems arising from the use of petcoke in co-firing. The main concerns identified have been the additional production of NOx, CO2 and particularly SO2 (1, 3-7), but flue gas desulfurization (FGD) facilities have helped to reduce emis* Corresponding author phone: +34934095410; fax: +34934110012; e-mail:
[email protected]. † Institute of Earth Sciences “Jaume Almera”, CSIC. ‡ CME/CFFS, University of Kentucky. § ENDESA GENERACIO Ä N S.A. 5330
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sions, allowing a partial replacement of coal by this sulfurenriched fuel. Other key questions to be addressed include (1) the behavior of petcoke, (2) the modified volatility of some metals and metalloids due to a higher Cl, S, and F content of the flue gas, and (3) the increase in the bulk content of specific trace pollutants (V, Ni, Mo, Cu, Hg, As, Sb, Zn, or Pb) and unburned carbon in co-fired solid byproducts (2, 3, 5, 8-9). The partial replacement of coal by alternative fuels in pulverized coal combustion (PCC) power plants may not only modify the emission volume or the bulk content of a number of elements in the combustion byproducts, but also their fate and speciation. The potential change of the mode of occurrence of elements may impact the leachable yields of fly ash and slag since the leaching behavior depends on the contents of elements and their speciation (10). Regarding the environmental quality of co-fired solid byproducts, the leaching properties of PCC fly ash are already well characterized since this topic has been widely studied (10-19, among others). However, such leaching properties may be significantly modified by the introduction of other combustibles. The degree of influence of co-firing on the leachability of the resulting solid byproducts remains to be resolved given the relevance of this issue in waste management. Indeed, management strategies are often based to leachability, since it provides more reliable data on the potential soil or groundwater pollution. For this reason the environmental quality of a material is related to the retention ability of trace pollutants rather than on their content. Nowadays, a large proportion of fly ash and slag is economically exploited, mainly as a cement raw material and for replacing natural aggregates in construction, but co-firing could render them valueless for these applications. In Europe, a number of PCC power plants have already started to add petcoke to the feed fuel blend with proportions reaching 40%. Furthermore, the partial replacement of coal by petcoke is expected to be tested in a number of power plants. This means that the environmental implications of using petcoke as a complementary fuel are not a hypothetical concern but a real problem to be addressed without delay. The main aims of this study are (1) identification of possible changes of the bulk content of elements of environmental concern in fly ash and slag resulting from cofiring of coal with petroleum coke, (2) determination of the leaching properties of these byproducts, and (3) assessment of the influence that an increase of the petcoke/coal ratio of feed fuel may exert on the leachability of inorganic constituents and, consequently, on the environmental quality of the co-fired fly ash and slag.
Materials and Methods Four samples were selected for this study from a power plant located in the northwest of Spain. This power plant is currently fed with a fuel blend of 4% petroleum coke with 96% blended coal (24% imported bituminous coal and 72% Spanish anthracite). The petcoke proportion was increased to 24% with the aim of investigating the maximum petcoke content that can be added to the fuel blend without imposing drastic technological changes in the boiler. This limitation does not provide much operational flexibility but, otherwise, changes in the operational conditions and combustion design would severely increase the costs of energy. Thus, two samples of fly ash and two samples of slag were collected from the power plant in February 2005, corresponding to two different petcoke/coal ratios. Fuel blends, fly ash, and slag samples were acid-digested by using a special two-step digestion method devised for the 10.1021/es063002d CCC: $37.00
2007 American Chemical Society Published on Web 06/28/2007
TABLE 1. Proximate, Ultimate, and Elemental Analysis of the Two Feed Fuel Blendsa %
moist
VM
ash
C
H
N
S
O
HHV (MJ/kg)
LP blend HP blend
1.06 0.71
12 10.6
26.20 21.48
63.5 70.2
2.56 2.63
1.21 1.25
1.30 2.00
4.16 3.44
21.84 23.89
%
Al2O3
CaO
Fe2O3
K2O
MgO
MnO
Na2O
P2O5
SiO2
SO3
TiO2
LP blend HP blend
8.40 5.07
0.99 0.75
2.60 1.44
1.23 0.66
0.61 0.37
0.02 0.01
0.27 0.15
0.19 0.16
18.28 10.98
1.50 3.32
0.34 0.21
mg/kg
Cl
F
As
Cd
Cr
Cu
Hg
Mo
Ni
Pb
Sb
Se
V
Zn
LP blend HP blend
604 923
273 193
37 20
0.5 0.3
74 47
37 23
0.18 0.11
4 6
56 276
34 25
5 3
18 11
101 593
52 35
a
LP, HP: low and high petcoke proportion, respectively; moist: moisture; VM: volatile matter; HV: heat value.
analysis of trace elements in coal and combustion wastes by Querol et al. (20). Another hydrofluoric acid digestion described by Thompson and Walsh (21) was carried out to determine the silica contents. The fly ash and coal international reference materials NBS 1633b and SARM 19, respectively, were also digested to check the accuracy of the analytical and digestion methods. The determination of Cl and F was performed by means of a sample fusion following the methods described by Chakrabarti (22) and Sager (23), respectively. To determine the mobility of elements the compliance leaching test EN 12457-2 (24) was applied. This EN method is a single-batch leaching test performed at a liquid to solid ratio (L/S) of 10 L/kg with 24 h of agitation time and deionized water as leachant. Although the EN 12457-2 procedure requires a size reduction of coarse samples to e4 mm, slag particle size was reduced down to 1 mm with the aim of comparing fly ash and slag leachability, thus minimizing the variations that could be attributed to a difference in specific surface area of particles. This milling should be taken into consideration since leachable concentrations in slag may be overestimated as a consequence of the higher surface area. In all cases, analyses were performed in duplicate. Major, minor, and trace element concentrations in solid samples and leachates were determined by means of inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry (ICP-AES), and high-performance liquid chromatography (HPLC). Hg concentration in solid samples and leachates was analyzed with a mercury analyzer LECO AMA 254 (by means of atomic absorption spectrometry with the Au amalgam technique). The contents of C, H, and N were determined by means of conventional elemental analyzers. Since prior studies revealed that petroleum coke is an important source of V and Ni, the speciation of these elements was studied to gain a better understanding and interpretation of their leaching behavior. To this end, XAFS (X-ray absorption fine structure) spectroscopy analyses were performed.
Results and Discussion The composition of the feed fuel blend (Table 1) may exert a certain influence on the characteristics of fly ash and slag, namely: the bulk content, the partitioning during combustion, and the mobility of elements. Influence on the Bulk Content of Elements. The contents of elements in the selected co-fired slag and fly ash obtained for two petcoke/coal ratios were determined (Table 2, complete data in Table S1 of the Supporting Information). Moreover, the results were compared with the usual ranges of concentration for these elements in 23 EU PCC fly ash (18), as shown in Figure 1. The comparison points out that the bulk content of a number of elements, including the
major elements, does not differ significantly in fly ash but shows fairly similar concentrations, since most co-fired fly ash values fall within the ranges of 23 EU PCC fly ash and close to the percentile 25 and 75 range (p25-p75). This means that a higher petcoke proportion does not severely modify the overall composition of the resulting fly ash. Regarding slag, no similar database was found in order to compare the influence of the partial replacement of coal by petcoke on the bulk content of elements. The consequences of increasing the petcoke proportion are only significant in the case of very few elements. As shown in Figure 1, fly ash from co-combustion of coal with petcoke is characterized by V and Ni contents markedly higher than those typically found in PCC fly ash, which is consistent with other studies (2, 4, 8, 25-27). The contents of V and Ni, largely supplied by petcoke (Table 1), increased about 1 order of magnitude in the high-petcoke fly ash with respect to the low-petcoke fly ash. Furthermore, a moderate enrichment in Mo in the high-petcoke fly ash was determined. It should be noted that a low petcoke proportion gave rise to fly ash with contents of the aforementioned elements resembling PCC EU fly ash. This indicates that the contents of V, Ni, and Mo are only sensitive to the addition of relatively high proportions of this secondary fuel. The contents of As and Cr are similar in the two fly ash samples, with values clearly in the upper range of PCC fly ash even for the low petcoke/ coal ratio. These elements are not mainly supplied by petcoke (Table 1), which suggests that the coal contribution may be significant. On the other hand, although this particular petcoke was found to supply S (Table 1), the increase in the S content in the resulting co-fired solid byproducts is relatively moderate given that both the low-petcoke and high-petcoke fly ash fall in the p25-p75 PCC EU range. Apart from the aforementioned elements, the addition of petroleum coke does not entail other significant variations in the content of minor and trace elements. Consequently, it can be concluded that the bulk contents of elements in fly ash are not very sensitive to the addition of a low proportion of petcoke to the blend since only high proportions have a significant impact. The high-petcoke slag was clearly enriched in V and Ni and moderately enriched in Mo with respect to the lowpetcoke sample. In contrast, S is depleted when the proportion of petcoke in the feed fuel blend increases, which will be discussed below. Nevertheless, with the exception of the elements listed above, no clear distinction can be drawn between low-petcoke and high-petcoke slag. Influence on the Partitioning of Elements. Petroleum coke supplies constituents that may exert some influence on the volatility of elements and consequently on their partiVOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Bulk content of a number of elements in the low-petcoke and high-petcoke fly ash (dotted and continuous lines, respectively), and range of contents (in pale gray) and percentile 25 and 75 range (in dark gray) of 23 EU PCC fly ash (18).
TABLE 2. Bulk Content and Leachable Content According to EN 12457-2 Leaching Test of a Number of Elements in the Co-Fired Slag and Fly Ash low petcoke slag
Si Al Ca K Mg Na Fe Ti S
As B Ba Cd Cl Cr Cu F Hg Mo Ni Pb Sb Se V Zn
high petcoke fly ash
bulk %
leachable mg/kg
24.75 12.65 2.12 2.97 1.18 0.52 6.33 1.07 0.01
89 45 76 50 18 22 13 1 27
slag
bulk %
leachable mg/kg
24.03 13.69 2.03 3.04 1.10 0.63 5.16 1.24 0.20
bulk %
bulk %
leachable mg/kg
6 80 2098 285 39 168 0.1 1 1868
25.04 12.57 2.11 3.00 1.20 0.60 6.36 1.06 0.01
69 33 45 49 18 19 9 1 19
23.85 12.71 1.97 3.00 1.09 0.61 4.85 1.19 0.35
9 32 3073 587 337 315 0.1 1 3190
bulk mg/kg
leachable mg/kg
bulk mg/kg
leachable mg/kg
bulk mg/kg
leachable mg/kg
bulk mg/kg
leachable mg/kg
23 58 1486 1 597 252 128
