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Energy & Fuels 2007, 21, 744-755
Effect of Inorganic Matter on Trace Element Behavior during Combustion of Coal-Sewage Sludge Blends M. Bele´n Folgueras,*,† R. Marı´a Dı´az,‡ Jorge Xiberta,† and Manuela Alonso† Department of Energy and Department of Chemical Engineering, UniVersity of OViedo, Independencia 13, 33004 OViedo, Spain ReceiVed October 26, 2006. ReVised Manuscript ReceiVed January 6, 2007
The effect of inorganic matter on the volatility of hazardous elements As, Be, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Se, Tl, and V during coal-sewage sludge combustion was studied. For this purpose, two bituminous coals from the Asturian Central Basin and sewage sludge treated with FeCl3 and lime were used. The combustion of sludge-coal blends was performed in a laboratory furnace at 800, 900, 1000, and 1100 °C. The results have shown that the inorganic matter of materials used in the sludge-coal blends affects the behavior of the above metals significantly. Thus, the high Cl contents of the sludge can increase the volatility of some trace elements (Cd, Cu, and Pb) because of chloride formation, but the retention of part of these elements in ashes by Fe and Si lessens the hazardous influence of Cl. The higher the Si contents of the coal, the higher the Cd and Pb retention during combustion. Another element that plays an important role in the retention of trace elements is Ca (especially Ca from sludge additives that is finely distributed), which can retain As in ashes and decrease Se volatility.
1. Introduction Because of the necessity of removing great amounts of sewage sludge, whose production will rise as a result of the application of the 98/15/EC Directive, combustion of this residue and combustion of sludge-coal blends (also named cocombustion) will be increasingly important. This practice gives the opportunity to take advantage of the energetic potential of this waste, and therefore a double function is achieved. This technique has two clear advantages. On one hand, there exists wide experience in combustion technology, which can be applied to this residual fuel, and on the other hand, the existing facilities are flexible enough to burn a great variety of fuels, including wastes. Although this means that there are no technological barriers for the combustion or cocombustion of sludge, other types of implications, such as environmental or operational ones, may limit the use of this potential energetic resource. Among other aspects, trace element emissions must be carefully controlled during sludge combustion or cocombustion. In answer to the concern about trace element emissions, the European Union has imposed the application of the 2000/76/EC Directive on all member states. This directive restricts the emissions of the trace elements As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Tl, and V for facilities of waste incineration. Moreover, the U.S. Clean Air Act Amendments (1990) identified 11 trace elements, normally present in coal (As, Be, Cd, Co, Cr, Hg, Mn, Ni, Pb, Sb, and Se), as hazardous air pollutants. Therefore, dedicating more research effort to the knowledge of hazardous elements’ behavior during combustion is necessary. There are a lot of papers that analyze trace element behavior and show the potential risk of their emissions for coal,1-6 while fewer are * Corresponding author tel.: +34-98-5104333; fax: +34-98-5104322; e-mail:
[email protected]. † Department of Energy. ‡ Department of Chemical Engineering. (1) Clarke, L.; Sloss, L. Trace Element-Emissions from Coal Combustion and Gasification; IEA Coal Research [IEACR/49]: London, 1992.
written on other energetic resources, such as residual biomass or its blends with coal.7-11 Trace element behavior (volatilization or retention in ashes) during combustion is a complex phenomenon that depends on temperature, chemical and physical interactions between the components, and the mode of occurrence of the trace elements in the materials. The knowledge of such behavior can be very useful in order to retain certain elements and avoid their emissions. Vassilev et al.12 found that kaolinita and coals (2) Querol, X.; Fernandez-Turiel, J. L.; Lopez Soler, A. Trace Elements in Coal and Their Behaviour during Combustion in a Large Station. Fuel 1995, 74 (3), 331-343. (3) Bool, L. E.; Helble, J. J. A Laboratory Study of the Partitioning of Trace Elements during Pulverized Coal Combustion. Energy Fuels 1995, 9, 880-887. (4) Senior, C. L.; Bool, L. E., III; Morency, J. R. Laboratory Study of Trace Element Vaporization from Combustion of Pulverized Fuel. Fuel Process. Technol. 2000, 63, 109-124. (5) Galbreath, K. C.; Toman, D. L.; Zygarlicke, Ch. J.; Pavlish; J. H. Trace Element Partitioning and Transformations during Combustion of Bituminous and Subbituminous U.S. Coals in a 7-kW Combustion System. Energy Fuels 2000, 14 (6), 1265-1279. (6) Xu, M.; Yan, R.; Zheng, Ch.; Qiao, Y.; Han, J.; Sheng, Ch. Status of Trace Element Emission in a Coal Combustion Process: A Review. Fuel Process. Technol. 2003, 85, 215-237. (7) Folgueras, M. B.; Dı´az, R. M.; Xiberta, J.; Prieto, I. Volatilization of Trace Elements for Coal-Sewage Sludge Blends during Their Combustion. Fuel 2003, 82, 1939-1948. (8) Cenni, R.; Frandsen, F.; Gerhardt, T.; Spliethoff, H.; Hein, K. R. G. Study of Trace Element Partitioning in Pulverized Combustion. Waste Manage. (Amsterdam, Neth.) 1998, 18, 433-444. (9) Lopes, M. L.; Abelha, P.; Lapa, N.; Oliveira, J. S.; Cabrita, I.; Gulyurtlu, I. The Behaviour of Ashes and Heavy Metals during the CoCombustion of Sewage Sludges in a Fluidised Bed. Waste Manage. (Amsterdam, Neth.) 2003, 23, 859-870. (10) Amand, L.-E.; Leckner, B. Metal Emissions from Co-Combustion of Sewage Sludge and Coal/Wood in Fluidized Bed. Fuel 2004, 83, 18031821. (11) Kouovu, P.; Backman, R. Estimation of Trace Element Release and Accumulation in the Sand Bed during Bubbling Fluidised Bed CoCombustion of Biomass Peat and Refuse-Derived Fuels. Fuel 2003, 82, 741-753.
10.1021/ef060536r CCC: $37.00 © 2007 American Chemical Society Published on Web 02/22/2007
Effect of Inorganic Matter on Trace Element BehaVior
enriched in these minerals may be used as sorbents for the retention of Pb, Sb, and Cu in refuse-derived char ash from municipal solid waste. Gullet and Raghunathan13 showed that hydrated lime, limestone, and kaolinite were effective sorbents for the capture of As, Cd, and Pb by assays carried out on a pilot scale at 1000-1300 °C. The addition of dried sewage sludge to the coal can interfere in the combustion process and increase toxic trace element emissions, because of both the differences in coal and sludge composition and higher trace elements’ concentrations in sludge. Moreover, some inorganic additives such as FeCl3 and lime added to sludge in the wastewater treatment plant can affect to the behavior of inorganic matter of coal significantly during cocombustion and, therefore, trace element volatilization. In a previous work,7 trace metal volatilization in the cocombustion of a bituminous coal with a high ash yield with dry sludge treated with FeCl3 and lime has been analyzed, differences in trace element volatility between coal and sludge-coal blends being found. In the sludge, most of the elements that can form volatile compounds with chlorine were volatilized (Ag, Cu, Li, Rb, Cs, Pb, Cd, and Tl). In the sludge-coal blends, the elements Ag, Pb, Rb, and Tl were volatilized, but Cu, Cd, Cs, and Li were not. These differences are mainly related to interactions of trace elements with inorganic matter. The aim of this work is to shed light on the joint effects of chlorine and inorganic species, such as Si, Fe, Al, Ca, and P, on the behavior of trace elements during combustion at different temperatures of sludge-coal blends. The elements Si, Fe, Ca, Al, and P of the sludge-coal blends may promote the retention of some trace elements by means of chemical interactions, depending on their concentration in the sample, while the high content of Cl in the sludge may induce the formation of volatile chlorides in the sludge-coal blends, which could increase emissions of some trace elements. The volatility of the trace elements As, Be, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Se, Tl, and V was studied by means of fixed combustion in an electric furnace with continuous low airflow in the temperature range 800-1100 °C. For the tests, two bituminous coals and sludge treated with FeCl3 and lime and their blends were used. 2. Experimental Section The sludge treated with the additives FeCl3 and lime was taken from a wastewater treatment plant in Asturias. The two bituminous coals from San Nicola´s (SN) and Montsacro (MS) pits have different ash yields (10.2 and 17.2 wt %, respectively) and different concentrations of major elements. Trace elements in materials were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) and ICP-atomic emission spectrometry (ICP-AES). After aqua regia digestion, the elements As, Cd, Cu, Pb, and Sb were analyzed by ICP-MS, while the elements Hg and Se and Tl were analyzed by both ICP-MS and ICP-AES. The minimum detection limits of the technique applied were 0.1 ppm for As and Se; 0.02 ppm for Sb and Tl; 0.01 ppm for Cd, Cu, and Pb; and 5 ppb for Hg. The elements Be, Co, Cr, Mn, Ni, and V were digested by a HClO4-HNO3-HCl-HF solution and analyzed by ICP-MS. The minimum detection limits of the technique applied were 2 ppm for Mn; 1 ppm for Be, Cr, and V; 0.2 ppm for Co; and 0.1 ppm for Ni. The chloride (12) Vassilev, S. V.; Braekman-Danheux, C.; Laurent, Ph.; Thiemann, T.; Fontana, A. Behaviour, Capture and Inertization of Some Trace Elements during Combustion of Refused-Derived Char from Municipal Solid Waste. Fuel 1999, 78, 1131-1145. (13) Gullet, B. K.; Raghunathan, K. Reduction of Coal Based Metal Emissions by Furnace Sorbent Injection. Energy Fuels 1994, 8, 10681076.
Energy & Fuels, Vol. 21, No. 2, 2007 745 Table 1. Chlorine and Sulfur Concentration in the Materials and Major and Minor Elements in Their Ashes at 800 °C analysis ash yield volatile matter Stotal Cl
SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5
coal SN
coal MS
Proximate Analysis (wt %, db) 10.2 17.2 35.1 26.3
sewage sludge 44.5 53.7
Cl and S in Materials (wt %, db) 0.74 1.09 0.12 0.013
0.50 1.1
Major and Minor Elements of Ashes (Expressed as Oxides, wt %, db) 29.22 37.02 20.09 23.63 20.96 22.03 3.53 2.85 10.70 6.14 0.42 0.40 2.64 2.69 0.64 0.75 0.26 0.16
16.19 6.78 14.38 3.98 47.39 0.13 0.52 0.42 5.57
concentration was determined by both the Eschka and Volhard methods, while the sulfur concentration was determined by the Eschka method. Every sample was analyzed at least three times. From materials, 10 wt % and 50 wt % sludge-coal blends were obtained, and their trace elements, chlorine, and sulfur concentrations were calculated from those of the components (sludge and coal) taking into account the proportions of sludge and coal in each blend. To study the trace element behavior of sewage sludge compared to that of coal, as well as coal-sludge blends during combustion, the samples were ground to a particle size less than 200 µm. Afterward, the fixed combustion of samples of 3.0 g was conducted in a laboratory electric furnace under an atmosphere of dynamic air, in the temperature range from 800 to 1100 °C. The rate of heating until the final combustion temperature was 10 °C/min; when this temperature was reached, it was maintained for 1 h. The experimental procedure has been explained in a previous work.7 Major and minor elements in ashes obtained at the temperatures tested from the materials and their blends were analyzed by ICPAES after a LiBO2 fusion. The minimum detection limits of the technique applied were as follows: (a) 0.01 wt % for MgO, CaO, Na2O, TiO2, and P2O5; (b) 0.02 wt % for SiO2; (c) 0.03 wt % for Al2O3; and (d) 0.04 wt % for Fe2O3 and K2O. Trace elements in these ashes were analyzed by the methods previously described for the unburnt samples. The minerals of the materials and their blends, as well as their ashes at different temperatures, were established (qualitative analyses) by X-ray diffraction (XRD). The diffractograms were made using a Phillips PW 1710 X-ray powder diffractometer and Cu KR radiation (using a graphite monochromator). Diffraction intensities were recorded in the 2Θ range 5-65°.
3. Results and Discussions 3.1. Characterization of Materials. There are important differences in the ash yield and the composition of materials (major, minor, and trace elements). In Table 1, ash yield, volatile matter, and Cl and S concentrations in the coals and the sludge together with major and minor element concentrations in their ashes at 800 °C are shown. As can be seen in Table 1, the concentration of chlorine in the sludge is much higher than that in the coals (>10 times) as a consequence of FeCl3 addition to this material in the wastewater treatment plant. On the contrary, S is quite low in the sludge. With regard to major and minor elements in ashes, sludge contains more P2O5 and CaO but less Fe2O3 and SiO2. The coals SN and MS have lower ash yields and lower SiO2 concentrations than Soton (ST) coal, used in a previous work.7 For the sludge and the coals, Table 2 shows the average concentrations of the 14 trace elements studied together with
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Folgueras et al.
Table 2. Trace Elements of Materials (ppm) and Their Comparison with Clarke’s Mean and Range1
a
element
sewage sludge7
coal SN
coal MS
Clarke’s mean
Clarke’s range
As Be Cd Co Cr Cu Hg Mn Ni Pb Sb Se Tl V
19.3 ( 1.4 1 0.72 ( 0.17 4.0 ( 0.4 23 ( 3 103.76 ( 5.18 1.371 ( 0.072 378 ( 4 17.0 ( 0.5 68.51 ( 2.46 2.3 ( 0.7 0.9 ( 0.1 0.05 ( 0.01 23 ( 3
4.5 ( 0.8
