Energy & Fuels 2008, 22, 1641–1649
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Leaching Hierarchies in Co-combustion Residues A. George,* D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering, Imperial College London, Prince Consort Road, London SW7 2BY, United Kingdom ReceiVed September 18, 2007. ReVised Manuscript ReceiVed January 21, 2008
The leaching propensities from co-combustion residues of 10 trace elements have been evaluated. The elements assessed were Be, V, Cr, Zn, As, Se, Cd, Ba, Hg, and Pb. Eight fuels varying from coal blends to coal and secondary fuel mixtures to ternary mixtures were co-combusted in two reactor configurations (a fixed-bed “hot-rod” reactor and a muffle furnace) and at two temperatures (850 and 950 °C). It was thus possible to assess the effect of fuel type, reactor configuration, and temperature on the leaching propensity of trace elements and establish potential extent-of-leaching hierarchies. The trace-element content in each fuel was determined, as was the trace element in the resulting postcombustion ash. The ash was subjected to a miniaturized toxicity characteristic leaching procedure (TCLP) developed for this study, and the trace element content in the leachate was analyzed. This leaching test gives a good measure of the potential toxicity of leachates from landfill. Percentage retentions of elements in the ashes and leachates have subsequently been calculated. Mercury and selenium were almost completely volatilized during combustion and, therfore, were largely absent from the ashes, in all cases. For the other trace elements, it was not possible to establish a hierarchy of relative traceelement retention. Retention was primarily a function of the combustion method, with no clear effect of temperature retention being observed. The measured trace-element retentions have been compared to those predicted by thermodynamic equilibrium modeling, using the MTDATA software. The model successfully predicted the measured values in many cases; however, many anomalies were also noted. This suggests that thermodynamic modeling is best used for interpreting experimental data rather than as a stand-alone design tool. From trace-element analysis in the leachates, an extent-of-leaching hierarchy could be established. The elements that underwent low degrees of leaching were Zn, Hg, Pb. Those that underwent low to moderate leaching were Be, Cr, and Cd, and those that were leached to a great extent were V, As, Se, and Ba. This hierarchy was observed for all fuels and conditions studied. Leaching was found to be a strong function of the combustion temperature and combustion method. The type of fuel used in this study did not appear to influence the degree of leaching. When assessing the potential toxicity of leachate from co-combustion residues, Zn, Hg, and Pb may be deemed of least concern, while a greater emphasis should be placed in mitigating the release of the remaining elements.
Introduction Trace metals found in ashes have the propensity to leach into any aqueous medium with which the ash comes into contact. If this leachate is emitted to the natural environment, as can be the case from landfills, there is also a danger of human exposure to the harmful metals contained therein.1 Ashes from coal combustion are deemed to be nontoxic and can generally be landfilled.2 If the fuel is a mixture of coal and a secondary fuel, however, the residue is deemed either toxic or is dealt with on a case-by-case basis, depending upon local regulations.3–5 Much landfilling of combustion residues was performed when measures were not in place to prevent the leachate entering the environment, and in these cases, retrospective analysis of the toxicity of leachate is required. While much progress has been made in reducing the amount of leachate runoff via modern * To whom correspondence should be addressed. E-mail: a.george1@ ic.ac.uk. (1) Ghosh, A.; Sáez, E.; Ela, W. Effect of pH, competitive anions and NOM on the leaching of arsenic from solid residuals. Sci. Total EnViron. 2006, 363 (1-3), 46. (2) European Waste Catalogue (EWC). 1994. (3) Landfill Regulations (England and Wales). 2004, SI number 1375. (4) United States Environmental Protection Agency (U.S. EPA). 40 CFR Part 258sCriteria for Municipal Solid Waste Landfills. (5) European Union (EU) Directive 2002/95/EC.
landfill designs, these still do not preclude failure and subsequent leaks.6 It is therefore important to assess the potential for trace elements to be leached in all combustion residues earmarked for disposal in any type of landfill. This study aims to assess the degree of metal leaching from ashes obtained from a range of fuels, thereby gaining an understanding into which factors in the combustion process affect leaching. A total of 10 trace elements deemed harmful were studied, namely, Be, V, Cr, Zn, As, Se, Cd, Ba, Hg, and Pb. This research attempts to quantify the partitioning of these trace elements during the combustion of eight different fuels. The roles of fuel type, temperature, and reactor configuration on this partitioning during the combustion process were studied. The leaching behavior of elements from the resulting combustion residues as a function of the above variables could therefore be assessed. Experimental Protocol The experimental protocol may be categorized in three parts. First, the content of the aforementioned trace elements initially (6) Lee, G. F.; Jones-Lee, A. Deficiencies in subtitle D landfill liner failure and groundwater pollution monitoring. Presented at the NWQM National Conference Monitoring: Critical Foundations to Protect Our Waters; U.S. EPA, Washington, D.C., 1998.
10.1021/ef700560e CCC: $40.75 2008 American Chemical Society Published on Web 02/23/2008
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Table 1. Fuels Studied and Ratios of Mixtures Used fuel number 1 2 3 4 5 6 7 8
fuel type South Africa coal 1997 South Africa coal 1999 Freyming coal slurry/South Africa coal 1999 Freyming schlamms/Freyming coal slurry South Africa coal 1997/wet saw dust Freyming coal slurry/dry saw dust South Africa coal 1997/undried bark Freyming schlamms/Freyming coal slurry/petroleum coke
fuel ratio
7:3 4:1 3:1 3:7 1:3 2:7:1
present in the fuels was assessed. Trace-element content in the solid residues was ascertained by inductively coupled plasma-mass spectrometry (ICP-MS) for all elements, with the exception of mercury. Mercury contents were determined using a “LECO AMA 254” mercury analyzer.7 Second, combustion tests were carried out on the fuels and the trace elements remaining in the resulting ash analyzed. The combustion tests were carried out using two reactor configurations: a “hot-rod” reactor8 and a muffle furnace. Experiments were conducted using temperatures of 850 and 950 °C with each combustor configuration. Analyses were conducted to quantify trace-element concentration in the ash, via ICP-MS, and converted to percentage retentions. Third, the ash was submitted to leaching tests via a miniaturized version of the toxicity leaching characteristic procedure (TCLP)9 able to deal with small sample masses. The resulting leachate was analyzed for trace-element content. Results obtained as concentrations in the leachate were converted to the percentage of trace element in the ash removed by leaching.
Experimental Procedure The fuels studied were supplied by Centre d’Etudes et Recherches des Charbonnages de France (CERCHAR). The fuels fall into three categories: coal alone, coal and secondary fuel blends, secondary fuel blends alone, blended in different proportions as shown in Table 1. One of the secondary fuels used is referred to as schlamms. Schlamms is a term used to define fine tailings obtained from coal preparation. Water obtained from de-ashing coal for commercial preparation is decanted and filtered, and the high-water-content residue remaining is referred to as schlamms. The product has a higher ash content than commercial coal and a fine particle size of less than 300 µm. The fuels will henceforth be referred to in the text by their fuel number as shown in Table 1. Table 2 depicts the ultimate and proximate analyses of the fuels. Fuel Preparation for Trace-Element Analysis. The sample size range of the fuels, obtained by sieving, was 106–150 µm. The fuels were then blended mechanically by measuring the required proportions in a flask and rotating the flask for 30 min between two rollers at a rate of 150 revolutions min-1. For experimental consistency, trace-element analysis on the fuels and combustion tests were conducted on a dry fuel basis. To remove moisture, fuels were vaccuum-dried at 50 °C for the 24 h prior to analysis or combustion tests. For fuels 5 and 7, which partly consisted of wet saw dust and undried bark, respectively, moisture was considered an inherent fuel characteristic to be studied. These fuels did not undergo the preliminary drying phase. (7) Charpenteau, C.; Seneviratne, R.; George, A.; Millan, M.; Dugwell, D. R.; Kandiyoti, R. Screening of low cost sorbents for arsenic and mercury capture in gasification systems. Energy Fuels 2007, 21 (5), 2746–2750. (8) Pindoria, R. V.; Chatzakis, I. N.; Lim, J. -Y.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Hydropyrolysis of sugar cane bagasse: Effect of sample configuration on bio-oil yields and structures from two bench-scale reactors. Fuel 1999, 78 (1), 55. (9) George, A.; Dugwell, D. R.; and Kandiyoti, R. Development of a miniaturized technique for measuring the leachability of toxic trace elements from coal-biomass co-combustion ash residues. Energy Fuels 2007, 21 (2), 728–734.
Evaluation of Initial Trace-Element Content in Fuels. The trace-element content in the fuels was ascertained by acid digestion followed by ICP-MS analysis. Two differing digestion processes were required to determine both the volatile and less volatile elements being studied. To quantify the amount of the nonvolatile trace elements in the fuels, a predigestion step to breakdown the organo-metallic links was used, followed by an acid digestion capable of breaking down the alumino-silicates in the ash. An opendigestion method as described in previous studies10 was employed. This method is however not suitable for the more volatile elements, As and Se. The fuels therefore underwent an additional digestion in a closed microwave system. The method used, although not capable of breaking down all of the sample, was found to give accurate readings of As and Se based on comparisons to reference materials.11 It is not possible to measure mercury in the digestion methods described above. Hg was therefore measured directly using a dedicated analyzer, the “LECO AMA 254” mercury analyzer. The instrument allows for the analysis of solid as well as liquid samples, without pretreatment, via a spectrophotometric method based on the fact that mercury absorbs at a wavelength of 254 nm. The premise of the method is combustion and decomposition of the sample causing mercury to be released and trapped on a gold amalgam. The mercury is subsequently released, and absorption at 254 nm is measured through two consecutive cells. All reagents used in this study were ARISTAR-grade; the water used to dilute reagents was deionized to a resistivity of 18.2 mΩ or distilled water in the case of the leaching tests. The ICP-MS analyses in this study were performed using a Plasma Quad 2+ ST VG Elemental from Fisons Instruments. Combustion Tests. Once the trace-element content in the fuels in Table 1 was determined, combustion tests were carried out on each fuel. Two combustion configurations were used in this study viz. a “hot-rod” reactor12 modified from a design by O’Brien13 and a muffle furnace. The hot-rod reactor is a tubular reactor consisting of a rod fashioned from Incoloy 800 HT, a wrought nickel-ironchromium base alloy strengthened by the addition of molybdenum, aluminum, and titanium. It is 350 mm in length and 10 mm internal diameter. To prevent contamination from the reactor walls, a quartz tube, internal diameter of 6 mm, was inserted into the reactor. A high-quality quartz wool, Saffil, was placed in the center of the quartz tube to hold the sample in place. Sample sizes of up to 100 mg were used. Electrical energy is supplied to the reactor via two water cooled copper electrodes, which are clamped in place to the reactor wall. The sample was placed equidistantly between the two electrodes. The temperature is measured via two external thermocouples placed at the extremities of the reactor body, just above the electrodes. The external thermocouples are placed directly on the reactor wall, and the reactor is insulated. Another thermocouple placed inside the reactor measures temperatures at the sample point. The heating rate was set to 10 K s-1 and was controlled at 0.1 s intervals. The sweep velocity used in all experiments was 0.1 ms-1. The experiments carried out in the “hot-rod” reactor were replicated in a muffle furnace, operated at atmospheric pressure and the same temperatures as the “hot-rod” reactor. A maximum sample size of 2 g was used in the muffle furnace. The differences between the muffle furnace and the hot rod were the absence of a gas flow in the furnace, with the sample heating rate being 1 K s-1 for the (10) Lachas, H.; Richaud, R.; Jarvis, K. E.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Determination of 17 trace elements in coal and ash reference materials by ICP-MS applied to milligram sample sizes. Analyst 1999, 124 (2), 177. (11) Richaud, R.; Herod, A.; Kandiyoti, R. Comparison of trace element contents in low-temperature and high-temperature ash from coals and biomass. Fuel 2004, 8 (14–15), 2001. (12) Megaritis, A.; Messenböck, R. C.; Collot, A. G.; Zhuo, Y.; Dugwell, D. R.; Kandiyoti, R. Internal consistency of coal gasification reactivities determined in bench-scale reactors: Effect of pyrolysis conditions on char reactivities under high-pressure CO2. Fuel 1998, 77 (13), 1411. (13) O’Brien, R. J. Tar production in coal pyrolysis. Ph.D. Thesis, Department of Chemical Engineering, Imperial College, London, U.K., 1986.
Leaching Hierarchies in Co-combustion Residues
Energy & Fuels, Vol. 22, No. 3, 2008 1643
Table 2. Proximate and Ultimate Analyses of Fuels Provided by CERCHAR fuel number material moisture ash volatile matter sulfur chlorine carbon hydrogen nitrogen fuel number material moisture ash volatile matter sulfur chlorine carbon hydrogen nitrogen
1
2
3
4
South Africa coal 1997 3.2 15.9 22.3 0.7 0.02 66.9 3.3 1.6
South Africa coal 1999 5.0 14.9 21.8 0.6 0.03 66.7 3.2 1.6
Freyming coal slurry/South Africa coal 1999 2.4 40.7 17.8 0.6 0.07 46.0 2.4 1.0
Freyming schlamms/Freyming coal slurry 2.7 30.5 23.3 0.9 0.2 54.5 3.1 0.7
5
6
7
8
South Africa coal 1997/wet saw dust 5.2 11.7 33.6 0.4 0.03 63.4 3.6 1.3
Freyming coal slurry/dry saw dust 2.5 38.9 20.5 0.5 0.07 45.9 2.6 1.0
South Africa coal 1997/undried bark 5.2 19.0 29.2 0.5 0.04 55.9 3.3 1.2
Freyming schlamms/Freyming coal slurry/petroleum coke 2.0 40.3 16.8 1.48 0.1 45.6 2.5 0.9
muffle furnace instead of 10 K s-1 and a larger sample capacity of 3 g in the muffle furnace. To conduct an error analysis and to obtain enough ash for trace element analysis, each experiment for each fuel needed to be repeated several times. The number of times an element needed to be repeated depended upon the ash content of the fuel. Experiments for each fuel were repeated a minimum of 6 times. Evaluation of Trace-Element Content in Ashes. The ash was collected after each combustion experiment, and a portion was taken to undergo the digestions described for ICP-MS analysis and analysis with the Leco AMA 254. The remaining ash was set aside for leaching tests. The procedure followed to digest the ashes was the same as for the fuels, without the preliminary step required for breaking down organic matter. To conduct error analysis and verify repeatability, the analysis for each ash was repeated at least 6 times. Leaching Experiments and Evaluation of Trace-Element Content in Leachate. Once the trace-element concentrations in the fuels and their combustion ashes were determined, leaching tests were conducted to ascertain the concentration of trace elements in the leachate that might be produced from land filling this ash. A test appropriate to simulating leachate escaping from landfill is the United States Environmental Protection Agency (U.S. EPA) TCLP.14,15 The required sample size required for this test was 3 orders of magnitude higher than the available masses obtained from combustion experiments in this study. A miniaturized TCLP9 specifically developed for this study was therefore employed. In this instance, a minimum sample size of 100 mg could be used. Very high repeatability was obtained with leaching experiments; therefore, each experiment was repeated 3 times. This liquid phase is then further filtered, and a portion is sampled for the Hg analysis. The remaining leachate was refrigerated. All leachate samples were introduced into the ICP-MS apparatus within 24 h of their preparation. Thermodynamic Equilibrium Modeling. The interpretation of experimental data relating to the degree of retention of trace elements in combustion residues has been augmented by thermodynamic modeling using the MTDATA software. Thermodynamic equilibrium modeling of trace-element partitioning behavior has been carried out using the MTDATA computer (14) Blackburn, W. B.; Show, I.; Taylor, D. R.; Marsden, P. J. Collaborative study of the toxicity characteristics leaching procedure (TCLP). Report number EPA/600/4-87/045, U.S. EPA, Washington, D.C., 1986; U.S. EPA contract 68-03-1958. (15) Newcomer, L. R.; Blackburn, W. B.; Kimmell, T. A. Performance of the toxicity characteristic leaching procedure. Wilson Laboratories, S-Cubed, U.S. EPA, Washington, D.C., 1986.
code16 developed by the National Physical Laboratory, Teddington, London, U.K. This code is based on the principle of Gibbs freeenergy minimization to calculate equilibrium species compositions for a variety of physical situations. Thermodynamic data have been supplied by the Scientific Thermodata Group Europe (STGE) Pure Substance Database. The gas phase is modeled as an ideal gas, and the condensed phases are modeled as pure substances, with an activity of unity. The MULTIPHASE module has been run to simulate conditions over the temperature range from 900 to 1400 K in 20 K steps, at atmospheric pressure, with various system compositions. Carefully selected data files were prepared for systems containing the major elements (for this purpose, C, H, N, O, S, and Cl) and the minor elements (Na, K, Mg, Ca, Al, Fe, Si, and P) together with up to five trace elements of interest. The system composition for thermodynamic equilibrium calculation is entered on an elemental basis. A more detailed description of the use of MTDATA to predict trace-element partitioning during co-combustion is given by Miller et al.17 Errors. Preliminary tests with a standard reference material were conducted prior to analyzing all samples. Only when these tests gave a value within 5% of the known reference value were the subsequent sample results deemed quantitatively satisfactory. When this was the case, a standard deviation was calculated between each set of data points, yielding the error. Trace-element concentrations in the fuels and ashes were calculated with errors below 10% in all cases, except Cd, As, and Se, where errors were always less than 15%. Leaching results were always repeatable to within 5%.
Results Table 3 shows the initial trace-element concentrations in the fuels before combustion. Trace-element contents are typical of the levels found in the kinds of fuels studied, e.g., levels of Hg around 1 ppm and below and levels of Ba in the several hundreds of parts per million range. Ba is abundant enough to be considered a minor element in Fuel 7. Fuels 1 and 2 are SA coals of the same origin, differing only in the year that they were mined. They have very similar trace-element contents in (16) Davies, R. H.; Dinsdale, A. T.; Gisby, J. A.; Robinson, J. A. J.; Martin, S. M. MTDATAsThermo-dynamic and phase equlibrium software from the National Physical Laboratory. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2002, 26 (2), 229. (17) Miller, B. B.; Dugwell, D. R.; Kandiyoti, R. Trace element emissions from co-combustion of secondary fuels with coal: A comparison of bench-scale experimental data with predictions of a thermodynamic equilibrium model. Energy Fuels 2002, 16, 956.
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Table 3. Initial Trace Element Concentration in the Fuels Be V Cr Zn As Se Cd Ba Hg Pb concentration concentration concentration concentration concentration concentration concentration concentration concentration concentration (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) fuel fuel fuel fuel fuel fuel fuel fuel
1 2 3 4 5 6 7 8
3.14 3.12 3.74 2.42 2.51 3.26 8.09 2.98
41.6 44.9 89.0 76.4 44.3 90.4 139 442
80.4 79.1 278 94.5 19.0 630 149 126
78.4 66.5 143 105 62.0 126 210 166
1.58 2.74 8.68 12.2 2.60 10.8 12.6 11.8
the majority of cases, implying homogeneity of this coal with respect to trace-element content. South Africa 1999 coal contains more than twice the concentration of As, Se, and Cd compared to the South Africa 1997 coal. The type of handling and storage of this coal may lead to the loss of these trace elements over time. Fuel 7 contains the highest amounts of 5 of the 10 trace elements studied (Be, Zn, As, Cd, and Ba) and higher quantities of Pb and Cr and can therefore be considered the richest fuel in trace elements. Fuel 7 is a mixture of one part South African coal and three parts wood bark. The wood bark in this instance is the contributing factor in the high trace element quantities in fuel 7. Fuels 3 and 6, overall, have a generally higher trace element inventory than the other fuels, while fuel 8 contains comparatively high quantities of V and Pb. Trace-Element Retention in Ashes. The percentage of traceelement retention in the ashes after combustion in the muffle furnace (MF) and the “hot-rod” reactor (HR) at temperatures of 850 and 950 °C were calculated from the above results and concentrations found in the ashes. Results for each trace element are described below and compared to thermodynamic model predictions made for the purposes of this study18 using the MTDATA software. Beryllium in Ash. Beryllium retention in the ashes varied from being completely retained to a minimum of 12.1%. Thermodynamic modeling predicted that retention would not vary between 850 and 950 °C. For the unblended coals, fuels 1 and 2, nearly 100% retention was predicted, with the formation of beryllium oxides and silicates, BeO, BeO · Al2O3, and BeO · SiO2. When errors are taken into account, this is in agreement with the results obtained. For fuels where woody material has been added, beryllium retention is predicted to decrease significantly, where 100% woody fuel would cause 100% beryllium volatilization via the formation of Be(OH)2 (g). For fuels 5 and 7, beryllium is volatilized at around 60 and 40%, respectively, and this is accurately predicted. For fuel 6, however, which contains 70% dry saw dust, the beryllium retained is higher than anticipated. This may be due to the small amount of BeO forming in the gaseous phase condensing because of the lack of carrier gas in the muffle furnace. For Fuels 3, 4, and 8, it was predicted that beryllium would be almost completely retained (95%). This was the case for fuels 3 and 4 but not for fuel 8, where experiments showed approximately 65% retention for both temperatures and reactors. From the results obtained both experimentally and through modeling, for the temperature ranges studied, beryllium retention is not a function of the temperature. Vanadium in Ash. The predicted profile for vanadium retention was similar to that of beryllium. Vanadium was predicted to be 95–100% retained for the South African coals (18) George, A. Leaching properties of co-combustion residues. Ph.D. Thesis, Department of Chemical Engineering, Imperial College, London, U.K., 2005.
1.57 3.34 2.67 5.28 3.31 4.17 0.34 4.71
0.25 1.01 0.44 0.21 0.14 0.80 1.06 0.43
453 453 656 379 370 554 1535 392
0.55 0.50 1.37 0.08 1.54 0.61 0.13 0.20
20.4 20.5 54.5 55.4 22.9 46.4 98.6 159
in fuel 1 and 2. The solid oxides of vanadium predicted to form were VO2, V2O5, FeO · V2O5, CaO · V2O5, and 2CaO · V2O5. Experimental results concurred with these predictions. For the nonwoody fuel blends, predictions also showed that vanadium would be retained to a slightly lesser extent (80–95%), and this was borne out experimentally for the muffle furnace but not for the “hot-rod”, where retentions dropped to around 50%. Fuels 5-7 showed vanadium retentions from 55 to 80%; modeling showed that vanadium could be more mobile because of its different mode of occurrence in the wood. The 100 °C temperature differential did not seem to affect the proportion of vanadium retained for any of the eight fuels. Chromium in Ash. Thermodynamic modeling predicts that solid Cr2O3 will form at the temperatures studied (up to a temperature of 1100 °C) with small quantities of CrO3 gas forming (from 830 °C) for the unblended fuels 1 and 2, giving a retention in the 60–80% region. The results concur with these predictions with retentions of 57.6–84.6%, except for a lower value of 46% in the fuel 1 “hot-rod” ash produced at 850 °C. For fuels 5-7, CrO3 gas was predicted to form at 870 °C, thus reducing the chromium retention to the region of 60%. As with the other elements, chromium retention did not appear to be a function of the temperature for any of the fuels except for fuels 1 and 7 (South Africa coal 1997 and a blend of South Africa coal 1997 and wood bark). In these instances, it appeared that there was a 30% decrease in chromium retention when the combustion temperature was increased. For fuel 7, this may be explained by the fact that more CrO3 gas is expected to form as the temperature increases. For fuel 1, the reduced retention may be due to the fact that CrO2 gas formed does not condense to form Cr2O3 as predicted, because of the lower levels of chromium present, reducing the probability of molecules meeting and reacting. Zinc in Ash. The zinc compounds predicted to form after combustion were ZnO in both solid and gas phases and ZnO · Al2O3 and 2ZnO · SiO2 in the solid phase. ZnO · Al2O3 and ZnO are predominantly predicted to form at temperatures of up to 950 °C. Retention was predicted to be near 100% for all of the fuels. However, retentions were a broad range from 28.8% for fuel 7 to nearly 100% for fuel 4. “Hot-rod” ashes showed a much lower retention than muffle furnace ashes. The lowerthan-predicted retention is also particularly pronounced for fuels with higher organic content. An explanation for these lower retentions may be that most of the zinc present in the blended fuels is present in the organic matrix. This concurs with previous studies (Kendric et al. and Pendias et al.), where it was stated that zinc is bound to lighter organic enzymes and vitamins present in woody material. Contrary to this, thermodynamic predictions state that in these instances the zinc released (in this instance as ZnO gas) will condense on the ash particles and form a solid compound. For the fuels where there is a large degree of volatilization and a sweep gas, as in the case of the
Leaching Hierarchies in Co-combustion Residues
“hot-rod” experiments, not all of the ZnO gas predicted to condense may do so. The results for zinc retention at 950 °C are lower than the results for 850 °C for all of the fuels, except fuels 1 and 7. This is borne out to some extent by thermodynamic predictions, which state that between 850 and 950 °C retention falls from 100% to approximately 90%. However, when experimental errors are taken into account, temperature dependence of zinc retention may not be confidently stated. Arsenic in Ash. Thermodynamic modeling predicted that between 60 and 100% of the arsenic would be retained as calcium and magnesium arsenates [Ca3(AsO4) and Mg3(AsO4)] at the temperatures studied. This did not agree with the results obtained, where arsenic was shown to be volatilized by as much as 95% for fuels 3 and 4 (although most other fuels showed arsenic volatilization to be above 50%). These fuels contain high quantities of Freyming coal slurry. It may be suggested that most of the arsenic is present in the organic form in this fuel, thus contributing to a high degree of arsenic volatility. Temperature dependence within the 100 °C differential studied was not predicted by thermodynamic modeling nor was it exhibited empirically. Selenium in Ash. Selenium was present in small amounts in the original fuel, and selenium retention was low for all of the fuels studied (1.0-19.1%). Fuels 1–3 showed a selenium retention between 6 and 10%, with the exception of fuel 1 exhibiting a selenium retention of 19% after combustion at 850 °C. For fuels 4–8, more than 96% of selenium was volatilized at both temperatures studied. Thermodynamic predictions suggest that all of the selenium should be emitted after combustion as SeO or SeO2 above 650 °C for all of the fuels studied. The results obtained for fuel 1 and to a lesser extent fuels 2 and 3 do not concur with this prediction. Possible modes of occurrence of selenium in coals are in pyrites or sulfides. It is therefore possible that selenium is not released from this mode of occurrence or that a compound is formed, which may not be included in the model databases. The existence of an unaccounted for compound would also explain the fact that the temperature appears to have an affect on selenium retention between 850 and 950 °C. Other than results obtained for fuel 1, where retention decreased from 19.1 to 10.8%, the temperature did not influence selenium volatility at the combustion temperatures studied. Mercury in Ash. Trace-element concentrations for mercury were comparatively low in all of the fuels studied, ranging from 0.1 to 1.5 ppm. The retention of mercury was also low for all of the fuels, with maximums of 8.3 and 8.0% for fuels 3 and 4, respectively. For the remaining fuels, mercury volatility was higher than 96%. Thermodynamic predictions showed mercury volatilization to be 100% for all fuels at temperatures above 650 °C, with 98% being elemental mercury, with a maximum of 2% in the HgO form. The higher values of mercury retention for fuels 3 and 4 are not predicted in simulations. In previous studies, Miller measured a mercury retention of 20% for a Polish coal during combustion in a suspension-firing reactor. It was postulated in this instance that the mercury was retained because of capture by unburnt carbon (Hassett and Eylands). In the case of Miller, this theory was rebutted after investigation using thermogravimetric analysis (TGA) analysis of the ash showed no unburnt carbon present. In this investigation, the sample was shown to be 100 ( 0.1% ash. However, although the quantity of carbon seems negligible, even a few tens of percent of carbon will give a large excess of carbon relative to the few atoms of mercury, allowing the mercury to be recaptured.
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Temperature dependence is not exhibited with regard to the mercury released for fuels 1, 2, 5, and 6. Temperature effects on mercury retention do appear to exist for fuels 3, 4, 7, and 8. Because these were the four fuels that showed a higher degree of mercury retention, the hypothesis for an unknown compound forming, which is released to a greater extent at the higher temperature, is strengthened. Cadmium in Ash. Cadmium was the element that deviated most from the predictions of thermodynamic modeling. For all of the fuels studied, no cadmium species in the solid phase were predicted to exist above 850 °C. However, empirical results showed retention varying from 8.1% (fuel 1, “hot-rod”) to 97.6% (fuel 4, muffle furnace). The higher retentions were systematically found in the muffle furnace ashes. CdSO4 solid was predicted to exist up to temperatures of 710 °C for fuels 1-4 and 8, with CdO · SiO2 from 690 to 750 °C. For the remaining fuels, it was predicted that CdO · SiO2 would form for temperatures up to 850 °C. Above this temperature, it was predicted that the gases Cd, CdOH, CdCl2, and Cd(OH)2 would form in various proportions depending upon the temperature. These predictions also concurred with those from previous studies (Miller, Lachas, and Scotto). Given that chorine concentrations were low in the fuels, simulations were conducted with the exclusion of chlorine. Without the possibility of Cl compounds forming, it was found that sulfates were indeed favored to form at temperatures higher than 800 °C. This therefore does not explain the existence of cadmium in the solid form at the given temperatures, and it must be concluded that this must be due to compounds forming that are not included in the databases used. In the muffle furnace ashes, where retentions were always higher than in the corresponding “hot-rod” ashes, cadmium may have been condensing on the ash because of the lack of carrier gas. Cadmium retention was shown to be a function of the temperature for fuels 3, 6, and 8, but no temperature dependence was shown for the other fuels at the temperatures studied. Barium in Ash. It is shown that there were high retentions of barium for fuels 7 and 8 up to 95%, taking into account the experimental error range, but reduced amounts for the remaining fuels, in the range of 50–80%. Barium was predicted to be completely retained for fuels 1-4 and 8, with the production of BaSO4, BaO · SiO2, or BaO · Al2O3. The addition of woody material to the fuels reduced the BaO · Al2O3 content, but around 85% of barium should nevertheless be retained. This decrease indicates that the mode of occurrence of barium in petroleum coke and bark must favor its retention, while for the other fuels, barium is predicted to possibly recondense but does not do so. Lead in Ash. For the unblended fuels, it was determined via thermodynamic modeling that lead should be completely retained for temperatures up to 870 °C, after which PbCl and PbO gases were favored to form. In the solid state, lead was predicted to be present mostly as PbSO4. For the blended fuels, which included woody matter, it was predicted that a far smaller percentage of PbSO4 would form (in the region of 60–70%) and that lead dichloride gas would be predominantly favored at temperatures up to 1000 °C. The results for the muffle furnace ashes show a retention range of 55.8-100%, with no variation between temperatures. Retention in the “hot-rod” ashes varied from 2.2 to 92.9%, with temperature dependence being more prominent. Fuels 3, 6, and 8 retentions concur with predictions (all in the region of 65%), while all of the other fuels vary largely from predictions. This may be explained by low levels of chlorine present in the fuels ( (Be, Cr) > (Zn, Hg, Cd, Pb) The trace elements in the first group above were leached by over 70% for all CERCHAR fuels, at the conditions specified. The final group was for elements that showed a weak leaching propensity (from 0 to 35%); the middle group, in this case limited to beryllium and chromium, were elements that showed an intermediate degree of leaching. For the “hot-rod” reactor, although there was a large reduction in the extent of leaching compared to the muffle furnace, a relative leaching hierarchy among the trace elements could be established, for both 850 and 950 °C. This leaching hierarchy was similar to that in the muffle furnace and is shown below. (V, As, Se, Ba) > (Be, Cd) > (Zn, Hg, Cr, Pb)
For the hot-rod reactor, retentions are generally below 50%, except for Se, which exhibited a high degree of leaching throughout. The degree to which metals are leached relative to each other is similar for all of the temperatures, combustion configurations, and fuels studied, with the exception of chromium and cadmium. It may nevertheless be stated that Cr and Cd are never more than moderately leached, in all cases. Trace-Element Leaching as a Function of the Combustion Temperature. Table 6 shows how trace-element leaching from ashes is affected by an increase in the combustion temperature at which they were produced. There is a change in leaching quantity with a change in the temperature for 43.1% of the sample set. The quantity of trace element leached decreased in 35.6% of instances. Leaching of Be, V, Cr, Zn, and Cd was more susceptible to an increase in temperature than the other elements. Fuel type was not a controlling variable, where temperature increases affected leaching. It may therefore be inferred that the mode of occurrence of an element in the fuel is the factor that creates this temperature dependency. In certain instances, the trace-element leaching increased with an increasing temperature. This occurred in 7.5% of cases, all of them for Cr. This may be due to chromium being chemically released from the ash matrix at 950 °C but not physically removed. Elements physically bound to the ash would be more easily leached than those chemically bound, hence accounting for an increase in the amount leached at higher temperatures. The degree to which trace element leaching varied with temperature was not constant for a particular trace element, combustion method, or fuel. Trace-Element Leaching as a Function of the Combustion Method. There was a consistent trend in the leaching of trace metals from ashes produced in the muffle furnace and “hotrod”. Except for the elements Se and Hg (which were initially present in very small quantities), there was always more trace element leached from the muffle furnace ashes than from the “hot-rod” ashes. This may be attributed to the fact that no carrier gas passed through the muffle furnace. When fuels were combusted in the muffle furnace, the trace elements were not swept out of the reaction zone, and could more readily condense onto the ash as the reactor cooled. Trace elements physically condensed on the ash were susceptible to leaching and largely account for the extra leaching observed for muffle furnace ashes (Table 7). Summary Influence of the Combustion Temperature on TraceElement Retention. There was no significant temperature dependence observed with the 100 °C temperature differential from 850 to 950 °C. This was true in approximately 90% of the experiments carried out. Where temperature dependence was observed, this could not be correlated to a particular trace element. Influence of the Combustion Method on Trace-Element Retention. Trace-element retention did appear to be a function of the combustion method. The trace-element retention changed in 72% of cases when the combustion method was changed. Trace-element retention decreased in 88% of these cases when the combustion method was changed from the muffle furnace to the “hot-rod” reactor. In the cases where an increase in traceelement retention was observed, this was predominantly for the element arsenic. Influence of the Fuel Type on Trace-Element Retention. No link could be established between the type of fuel used and
Leaching Hierarchies in Co-combustion Residues
trace-element retention, despite there being large differences in initial trace-element content. This implies that trace elements are bound to all of the fuels studied in a similar fashion. Matching of Trace-Element Retention Measurements with Model Prediction. A comparison of the measured traceelement retentions with those predicted by thermodynamic equilibrium modeling reveal a good match in many cases; however, many anomalies were also noted. This suggests that thermodynamic modeling is best used for interpreting experimental data rather than as a stand-alone design tool. This concurs with observations of other researchers.17 Influence of the Temperature on Trace-Element Leaching. Temperature increases affected trace elements leaching much more than they affected their retention in the ash after combustion. The leaching of trace elements in the fuels studied was affected by the temperature change of 100 °C in 43.1% of cases. Influence of the Combustion Method on Trace-Element Leaching. In cases where the combustion method was the variable studied, the trace element leaching was consistently less from the “hot-rod” reactor ashes than from the muffle furnace ashes. The reduction was by a consistent amount between 20 and 35%. This was the case for all trace elements, except selenium and mercury, at both temperatures studied.
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Influence of the Fuel Type on Trace-Element Leaching. The fuel type did not discernibly affect the degree of traceelement leaching. Conclusions From the above results, it is possible to observe that for the fuel set and combustion conditions studied, trace elements exhibit consistent leaching behavior. It was possible to establish an extent-of-leaching hierarchy for all fuels, temperatures, and combustion configurations. All trace elements are leached to the same extent relative to each other, except for Cd and Cr, which are nevertheless never more than moderately leached. A study of a wider range of fuels and conditions would be required to validate this as a comprehensive hierarchy. For the purposes of environmental stewardship, this hierarchy may be used to understand the potential toxicity of leachates, with the highly leached elements (V, As, Se, and Ba) being of most concern. Zn, Hg, and Pb may be deemed of least concern, while greater emphasis should be placed in mitigating the release of the remaining elements. The hierarchy established in this study would of course need to be used in conjunction with absolute trace-element contents in the ashes being evaluated. EF700560E
