Energy Fuels 2010, 24, 2918–2923 Published on Web 04/23/2010
: DOI:10.1021/ef100001h
Co-firing of Single, Binary, and Ternary Fuel Blends: Comparing Synergies within Trace Element Partitioning Arrived at by Thermodynamic Equilibrium Modeling and Experimental Measurements A. George,* M. Larrion, D. Dugwell, P. S. Fennell, and R. Kandiyoti Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom Received January 1, 2010. Revised Manuscript Received March 28, 2010
The suitability of thermodynamic equilibrium modeling as a predictive tool in assessing the partitioning of trace elements in co-fired fuel blends and the synergistic effects involved in these processes were evaluated. The relationships between modeled results and experimental data obtained by the combustion of three separate fuels (Polish coal, sewage sludge, and straw) as well as their binary and ternary blends in a laboratory-scale suspension-firing reactor have been examined. The percentage of trace elements retained in the combustion ash as a proportion contained in the initial fuel was calculated, as well as the partitioning of each trace element between the bottom and sinter ash. Synergistic influences during the co-firing of fuel blends in both the modeled and empirical data were appraised for 13 trace elements, considered to be of primary toxicological importance, viz. As, Be, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Se, V, and Zn. Elements that are thermodynamically predicted to form solid oxides (Be, V, Cr, and Mn) were generally found to be retained in the ash. Elements that were predicted to form gaseous oxides (Se) appeared to volatilize in reasonable agreement with thermodynamic calculations. For elements predicted to form a gaseous chloride (Zn and Pb), there was competition between chloride formation and oxidation. Cd and Mo were predicted to form volatile hydroxides. These appeared to be kinetically constrained. As and Ni oxidize to form solid species with minor components in the fuel, such as Mg or Fe, with which they are already associated. Hg was found to be volatile as an atomic species, and a positive retention synergy was found for Hg when combusting the ternary blend.
This paper reports work examining the fate of trace elements during co-firing of biomass and waste fuels with coal, in fluidized-bed combustors, focusing on the effect of co-firing on the primary emissions of trace metals in the hot raw gas leaving the combustor. It would be expected that most of the inventory of trace metals would be removed subsequently in a commercial plant, as the gas is cooled and cleaned during gas cleanup (wet scrubbers, bag filters, and electrostatic precipitators). However, the trends in primary emissions of trace elements define the task of the gas conditioning system necessary to bring gaseous emissions down to acceptable levels and allow for the subsequent toxicity hazard in the solid waste streams to be assessed and controlled (determining whether bottom or fly ash can be used in building materials or how they are otherwise disposed, e.g., to land-fill). Here, trace element contents were measured in ashes leaving a laboratory-scale suspension-firing reactor, which permits close simulation of the conditions within a fluidized-bed combustor; these measurements are compared to the initial trace element contents of the fuels and detailed thermodynamic modeling. The raw fuels and the ash residues after combustion are subjected to both open and closed acid digestion, followed by inductively coupled plasma-mass spectrometry or -atomic emission spectrometry (ICP-MS or ICP-AES, respectively), to ascertain the fate of the trace metal inventory in the original fuel. A dedicated spectrophotometric method has been used to determine the Hg content of the raw fuels and solid ashes directly. The experimental methods are detailed below.
Introduction In addition to the major constituents carbon, hydrogen, oxygen, nitrogen, and sulfur, all fuels, whether of fossil, biomass, or waste origin, contain a large number of elements at the trace level (defined as less than 1000 ppm on a mass basis). Most elements can be identified in coals by analytical techniques capable of detecting at or below the 1 ppm level. Many, particularly heavy metals, have known toxic or carcinogenic properties, e.g., As, Cd, Hg, Pb, and Zn;1-3 some accumulate on ultrafine particulates, which can be assimilated by the human lung. Therefore, the fate of the trace element inventory (partitioning between the bottom and fly ashes or within exhaust gases, as either genuine gaseous species or adsorbed onto sub-micrometer particles) in the fuel fired to a combustor has become an issue in recent decades, as greater awareness of their environmental and human health impact has developed. Additionally, all solid streams will eventually be emitted to the environment, rendering them a potential hazard with respect to trace element leaching to the aqueous environment or bio-uptake and accumulation in plant matter. *To whom correspondence should be addressed. E-mail: ageorge@ lbl.gov. (1) Mahata, J.; Basu, A.; Ghoshal, S.; Sarkar, J. N.; Roy, A. K.; Poddar, G.; Nandy, A. K.; Banerjee, A.; Ray, K; Natarajan, A. T.; Nilsson, R.; Giri, A. K. Mutat. Res., Genet. Toxicol. Environ. Mutagen. 2003, 534, 133–143. (2) Rose, S.; Shea, J. A. Dev. Environ. Sci. 2007, 5, 99–131. (3) United States Environmental Protection Agency (EPA). Toxicological review of trivalent chromium. National Center for Environmental Assessment, Office of Research and Development, Washington, D.C., 1998. r 2010 American Chemical Society
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: DOI:10.1021/ef100001h
George et al. Table 1. Ultimate and Proximate Analyses of Initial Fuels and the Amounts of Minor Components Presenta moisture ash volatile fixed carbonb sulfur chlorine gross calorific value (kJ/kg) net calorific value (kJ/kg) calorific value (daf )b volatile matter percentage (daf )b carbon hydrogenc nitrogen oxygenb SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O Mn3O4 P2O5 SO3
Figure 1. Schematic diagram of the suspension-firing reactor.
Experimental Section Figure 1 depicts the suspension-firing reactor used to conduct the combustion experiments required for this study, described in detail by Miller et al.4,5 Polish coal, sewage sludge, and straw were combusted individually. Binary mixtures of Polish coal and sewage, in ratios of 8:2 and 7:3 by lower heating value, were also combusted, with the final fuel blend being a ternary mixture of polish coal, sewage, and straw in a blend ratio of 7:3:3. Fuel particles (100-200 μm) were combusted in an upward flow of preheated air in an electrically heated, vertical, quartz reactor tube (1260 mm length and 46 mm inner diameter). The reactor tube was situated concentrically inside a two-section electric resistance furnace: the lower (2 kW) heater preheated incoming combustion air, and the upper (1 kW) heater maintained roughly isothermal conditions in the reaction zone. Particles were constrained to circulate within a limited (upper) section of the quartz reactor tube by a quartz-sintered disk (2 μm), providing a physical upper boundary, and a conical restriction, providing a lower boundary because of the high superficial velocity of the gas at the neck. Fuel, conveyed by an air stream, was admitted to the top of the suspension reactor, from a quartz feed hopper, via a rotary metering valve and down a water-cooled (5 mm inner diameter) quartz feeder tube protruding below the quartz sintered disk into the hot suspension reaction zone. Six tiny “cups” metered the fuel, with the feed rate (typically about 1 g/min) being controlled by an electric motor. The fuel feeding system (i.e., the valve rotation speed) was specially calibrated for each fuel/blend but permitted nearly continuous, steady fuel feeding; run times of 10-20 min were typical. Exhaust gases passed through the quartz sinter, through a bed of quartz fiber, catching fine particulates escaping the sinter-disk, and then out through a side arm, for subsequent gas analysis, if required. All experiments were at 900 °C, with an air/fuel ratio, relative to stoichiometric conditions, of 1.2. Approximately 90% of the combustion air was injected into the base of the reactor, with the remainder used to pneumatically convey the fuel. Proximate and ultimate analyses of the fuels used, together with the mass fractions of the minor species within the
Polish coal
sewage
straw
3.4 9.7 31 55.9 1.47 0.3 29200 28158 33600 35.7 72.56 4.2 1.22 8.45
14.2 22.8 54.5 8.5 1.18 0.06 15940 14615 25300 86.5 36.98 4.62 5.03 15.2
9.4 5.5 67.5 17.6 0.17 0.29 16730 15484 19660 79.3 41.4 5.19 0.73 37.6
31 13.8 5.5 1.3 18.1 3.1 0.7 1.2 0.1 19.7 1.4
55.3 1.2 0.5 0.1 8.1 2.1 1.7 16.5 0.1 3.3 4.3
Minor Components in the Ash 43.2 17.4 7.8 0.6 7.7 5 1.2 1.5 0.1 0.3 7.6
a All values are in mass % unless stated otherwise. b Calculated using determined values by difference. c Corrected for moisture content.
fuels, are presented in Table 1.6 As expected, coal contained the most fixed carbon and least volatile matter and moisture and had the highest calorific value. The main points of interest with regard to the amounts of major and minor elements present are the relatively low sulfur content of the straw, the high chlorine contents of the Polish coal and straw, the high potassium and SiO2 (but low alumina) contents of the straw, and the high phosphorus content of the sewage sludge. Also, the sewage sludge has a large amount of ash contained within it, and a large proportion of this is CaO. Quantification of the release of trace elements from combustion is by the measurement of the trace element retention in the “bottom ash” recovered from the reactor walls and the catch-pot and the finer “sinter-ash” recovered by washing the quartz sinter. Two parameters are used to express trace element partitioning behavior. Percentage of retention (mass of trace elements recovered in ashes/mass of trace elements in raw fuel) is the percentage (by weight) of a trace element present in the fuel remaining in the total combustion ash. A low value indicates significant loss of the element from the reactor as genuine vapor species, as aerosol, or on fine particulates (approximately less than 2 μm) and, thus, potential release to the atmosphere. Such measured losses represent a worst-case scenario, in that appreciable condensation of volatilized elements might be expected in a commercial plant, because exhaust gases encounter lower temperatures during their passage through the gas-cleaning system. Relative enrichment (concentration of trace elements in fine ash/concentration of trace elements in coarse ash) is the ratio between the concentration of the trace element in the sinter ash and that in the bottom ash. A value significantly greater than 1 suggests initial volatilization, followed by condensation, of the trace element on the cooler sinter surfaces. In coal combustion, the sinter is around 40 °C cooler than the nominal reactor temperature; for biomass, this difference increases to around 70 °C.5 Relative enrichment provides qualitative information about the propensity of the more volatile trace elements to vaporize under reaction conditions and then condense on finer ash particles, with their greater surface area/mass ratios, as the gas (and particulate inventory) cools down.
(4) Miller, B. B.; Dugwell, D. R.; Kandiyoti, R. Fuel 2002, 81, 159–171. (5) Miller, B. B.; Kandiyoti, R.; Dugwell, D. R. Energy Fuels 2002, 16, 956–963. (6) Richaud, R.; Lachas, H.; Collot, A. G.; Mannerings, A. G.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, 77, 359–368.
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most trace elements (particularly Mn, Cu, Zn, and Pb), followed by the coal, with the straw being the “cleanest” fuel. Data are presented as a percentage of retention in Table 3. The relative enrichment of elements in the fly ash is presented in Table 4. Also shown in Table 3 are the results of thermodynamic calculations of the expected retention of elements in the combined sinter and bottom ashes; these are determined from the sum of the trace elements present in all species, which are solids or liquids at 900 °C for the calculated equilibrium concentration in the suspension-firing reactor and are discussed below. Table 3 shows that, when fired singly, Polish coal shows greater retention of trace elements in the ash than sewage sludge for Cu, Ni, Mn, Se, and Hg but lower retention of Cd, Cr, Mo, Zn, and As, with the retentions of Mo and As being particularly poor in the Polish coal, as opposed to the sewage sludge. The addition of sewage sludge to coal at the 7:3 level is generally beneficial or neutral for retention of trace elements in the ash, except for Cu (100-65%) and Zn (75-47%). However, any benefits in the percentage of retention are mitigated by the fact that the sewage sludge contains a higher initial trace element inventory. The addition of straw to obtain a 7:3:3 mixture gives retention in excess of 80% for Be, Cu, Mn, Ni, V, and Mo (excellent compared to just 40% for coal alone). Cr remains similar at 70%, but there is a reduction of Zn retention (60-45%), As retention (93-70% but better than the retention for coal alone at 52%), Pb retention (78-47%) and Cd retention (53-29% compared to coal at 43%). Hg shows a surprisingly high retention in the ternary mixture at 20%. Table 4 shows that there is little evidence of evaporation followed by condensation, because measured values for relative enrichment in the fly ash are generally less than 2. Both As and Pb show values fractionally above 2.0 for the cases of coal alone, straw alone, and the 8:2 binary mixture of coal plus sludge. Se shows values above 2.0 for both coal/ sludge blends, with the 3.24 value for the 8:2 blend being the highest recorded. In the case of the tertiary blend, all values are low, with Hg being the highest at only 1.46. The overall conclusion is that little evaporation-condensation occurs, although it should be noted that the sinter temperature is only 40-70 °C cooler than the reaction temperature of 900 °C. Thermodynamic Modeling. Solid fuels are mixtures of a large number of organic and mineral compounds that undergo complex transformations during combustion, potentially involving hundreds of reactions. Little or no kinetic data exist for most of the reactions involving trace elements, preventing investigation of trace element behavior through kinetic modeling. The application of thermodynamic equilibrium modeling provides some insight into the likely behavior of trace elements contained in the fuel combusted. Thermodynamic calculations on trace element behavior during coal combustion have been performed by Frandsen et al.,9 Linak and Wendt,10 and Mojtahedi.11 Trace element behavior has been investigated in coal/sewage sludge systems by Reed et al.12 and in municipal solid waste combustion by Durlak et al.13 In general, these studies have shown that
Table 2. Trace Element Contents of Initial Fuels
Be V Cr Mn Ni Cu Zn As Se Mo Cd Hg Pb
Polish coal (mg/kg)
sewage sludge (mg/kg)
straw (mg/kg)
approximate error ((%)
0.73 19.27 11.65 39.72 12.43 13.08 88.91 2.49 3.49 0.75 0.51 1.02 18.45
0.40 15.61 49.72 248.87 31.63 564.22 636.96 6.37 5.15 11.28 1.29 2.40 118.03
0.00 2.24 10.18 21.58 4.29 7.02 87.74 0.29 0.05 0.82 0.14 0.58 3.32
8 8 8 8 8 8 8 15 15 8 8 5 8
Results for the 13 heavy metals studied (As, Be, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Se, V, and Zn) are expressed in tabular form for percentage of retention and relative enrichment in Tables 2 and 3, respectively. The statistical errors in the percentage of retention (derived from the standard deviation of six repetitions of each measurement, with two different runs with the suspension-firing reactor, followed by three independent digestions of the ashes) are generally below 10%.
Analytical Methods All of the elements studied other than Hg have been quantified by ICP-MS or ICP-AES after the acid digestion of the samples. Details of ash trace element quantification using these analytical techniques, including detailed information on instrument calibration, have been presented elsewhere,6-8 but aspects of this type of analysis are described below. First, the fuels and ashes are digested. For the less volatile elements (all except As, Se, and Hg), an open “wet-ashing” digestion method was employed, involving pre-ashing in the case of raw fuels (to eliminate carbonaceous material), heating the sample with concentrated H2SO4 in platinum crucibles, followed by a 12 h ashing step in a muffle furnace. Further heating with a mixture of hydrofluoric and percholoric acids releases trace elements into solution. The final solid residue is dissolved in nitric acid prior to analysis. To quantify As and Se, the ashes are digested directly by nitric acid in a closed bomb within a microwave oven. This method is less severe, and elements held in silicates are not released, with leaching rather than total digestion being achieved. Hg cannot be analyzed by ICP-MS because of its loss during both digestion methods. The use of a dedicated spectrophotometric method, with direct solid sample injection, removes the need for pretreatment. The LECO AMA 254 uses the principle of mercury amalgamation with gold and is described in detail by Richaud et al.6 Results Trace element partitioning data for both fuel sets are shown in Tables 2-4 for the raw fuels alone (Table 2), as well as the binary and tertiary fuel blends. Data for 13 heavy metals are presented, viz. As, Be, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Se, V, and Zn. Table 2 shows that the sewage sludge contains the
(9) Frandsen, F.; Dam-Johansen, K.; Rasmussen, P. Prog. Energy Combust. Sci. 1994, 20, 115–138. (10) Linak, W. P.; Wendt, J. O. L. Fuel Process. Technol. 1994, 39, 173–198. (11) Mojtahedi, W. Combust. Sci. Technol. 1989, 63, 209–227. (12) Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2001, 15, 1480–1487. (13) Durlak, S. K.; Biswas, P.; Shi, J. J. Hazard. Mater. 1997, 56, 1–20.
(7) Richaud, R.; Lachas, H.; Healey, A. E.; Reed, G. P.; Haines, J.; Mason, P.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 2000, 79, 1077–1087. (8) George, A.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2008, 21, 728–734.
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Table 3. Percentage of Retention of Elements in the Combined Sinter and Bottom Ashesa Polish coal (%)
Be V Cr Mn Ni Cu Zn As Se Mo Cd Hg Pb
sewage sludge (%)
straw (%)
PC/SS (8:2) (%)
PC/SS (7:3) (%)
PC/SS/ST (7:3:3) (%)
exp
ther
exp
exp
exp
ther
exp
exp
ther
97.4 97.1 56.6 97.8 84.0 99.7 75.5 52.1 13.2 40.2 43.3 9.52 77.7
77.4 0.0 79.2 99.2 99.3 96.3 0.0 94.4 0.0 0.0 0.0 0.0 0.0
99.8 96.7 75.1 73.7 79.6 81.6 80.5 90.8 10.4 93.4 52.2 1.06 77.4
95.9 81.9 52.7 67.1 59.0 98.1 44.4 89.8 37.6 64.3 19.7 38.4 69.2
90.2 91.2 69.6 94.2 91.1 78.7 72.1 79.9 5.29 91.8 54.3 4.80 78.1
87.4 0.0 99.3 99.8 100.0 100.0 98.8 52.8 0.00 0.00 0.00 0.00 0.00
89.8 99.5 68.9 87.2 80.8 64.9 59.9 93.9 5.36 81.6 53.1 2.01 76.7
99.9 86.0 72.4 89.9 91.0 84.8 46.7 70.3 5.19 84.5 29.0 20.8 47.2
74.2 86.8 99.4 99.3 85.6 96.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0
dominant gas-phase species BeH2O2 V4O10 CrO3 Mn2O3 ZnCl2 AsO2 SeO2 MoO4H2 Cd(OH)2 Hg PbCl2
a exp, experimental measurements; ther, thermodynamic calculations. Particularly poor agreement between the thermodynamics and experiment are highlighted in bold. Also shown is the dominant gas-phase species (from thermodynamic calculations) containing each trace element (calculations showed that these were the same for all fuel blends; hence, only one column is shown).
phase is modeled as an ideal gas, and the condensed phases are treated as pure substances, with an activity of unity. The Gibbs free energy of the system is then minimized subject to the constraints of stoichiometry. Selected data files have been 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 one or more trace elements. Detailed description of the thermodynamic equilibrium modeling methods adopted is available elsewhere.5,15 Species unlikely to play a significant role in the real system must be rejected, and this is still a matter of judgment. The formation of compounds requiring the interaction of two trace elements is unlikely; thus, they are excluded. However, the interaction between minor and trace elements has been considered because, although dilute in the fuel, minor elements comprise a significant portion of the residual ash. Comparison between Experimental Measurements, Thermodynamic Modeling, and the Results from Literature. For a number of trace elements (particularly those mainly present within the sewage sludge), our results are compared to the work of Fuentes et al.,17 who applied leaching agents of three different types to sewage sludge and showed that trace elements were present in four different classes of material: (1) exchangeable fraction associated with carbonated phases, (2) reducible fraction or fraction associated with Fe and Mn oxides, (3) oxidizable fraction or bound to organic matter, and (4) residual fraction. The most important class for this study is the residual fraction, because this fraction is likely to be held within mineral matter and will hence react extremely slowly. Experimental results are compared below, for each element, to the results from thermodynamic modeling and recent literature. Beryllium. Beryllium is predicted, for all fuel mixtures except the ternary mixture, to be solid Al2BeO, which agrees well with the experimental data. This is clearly reasonable, given the large statistical association noted in Wang et al.16 between Be and Al for coals and the fact that the majority (>80%) of the Be is contained within the coal for all blends.
Table 4. Relative Enrichment of Elements in Fly Ash
Be V Cr Mn Ni Cu Zn As Se Mo Cd Hg Pb
Polish coal
sewage sludge
1.31 1.22 1.41 1.00 1.51 1.08 1.01 1.47 0.55 0.74 1.40 1.12 1.27
1.31 1.02 1.38 1.07 1.51 1.41 1.69 2.03 1.00 1.05 1.21 1.05 2.00
PC/SS straw (8:2) 1.14 1.07 1.02 1.01 1.17 1.08 1.73 2.20 1.02 1.08 1.55 1.43 2.20
1.38 1.06 1.01 1.05 1.09 0.92 1.75 2.28 3.24 1.01 1.61 1.01 2.26
PC/SS (7:3)
PC/SS/ST (7:3:3)
1.41 1.21 1.08 1.01 1.28 1.00 1.72 1.20 2.30 1.04 1.34 1.04 1.73
1.45 1.35 1.14 0.98 1.46 0.93 1.69 1.21 1.11 1.08 1.07 1.36 1.19
thermodynamic equilibrium modeling is valuable for predicting the fate of trace elements. However, it has become apparent that great care must be exercised in interpreting model output, because many anomalies between model prediction and experimental data have been observed, with some of them a result of fundamental limitations inherent in the thermodynamic modeling approach. Modeling assumes that the system is at chemical equilibrium at a particular temperature and pressure, with no account being taken of any kinetic hindrance to the chemical reaction. Furthermore, incomplete mixing may cause local species concentrations to deviate significantly from the average values used in modeling. Thermodynamic databases are incomplete, so that not all possible species (or reactions) may be modeled satisfactorily. Furthermore, the mode of occurrence of the trace elements in the solid fuel, which may influence volatility, is not considered. Potentially volatile elements held within refractory silicates (or otherwise mineralized) may not be released to take part in the equilibration process. Conversely, low volatility metals bonded organically to the fuel matrix, i.e., as porphyrins, may be released to the vapor phase as the particles of fuel heat up. The MTDATA computer code14 was used for calculations, using the Scientific Thermodata Group Europe (STGE) database, with a carefully selected subset of data for elements of interest for particular simulations. The gas
(15) Miller, B. B.; Kandiyoti, R.; Dugwell, D. R. Energy Fuels 2003, 17, 1382–1391. (16) Wang, J.; Yamada, O.; Nakazato, T.; Zhang, Z. G.; Suzuki, Y.; Sakanishi, K. Fuel 2008, 87, 2211–2222. (17) Fuentes, A.; Llorens, M.; Saez, J.; Isabel Aguilar, M.; Ortuno, J. F.; Meseguer, V. F. Bioresour. Technol. 2008, 99, 517–525.
(14) Davies, R. H.; Dinsdale, A. T.; Gisby, J. A.; Robinson, J. A. J.; Martin, S. M. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2002, 26, 229–271.
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emission is an interplay between the oxidation of zinc to ZnO (mp 2521 K) and much slower thermodynamically preferred reactions with Cl (because of the lower [Cl] in the gaseous phase). Straw, when added to the binary sludge/coal mixture, adds large quantities of Cl, increasing the kinetics of the Zn/Cl reactions and causing greater volatilization, in agreement with recent work.18 Interestingly, Fuentes et al.17 shows that Zn is generally found to be mainly (>80%) available (i.e., not in the mineral fraction) in sewage sludge. This may be the reason why it is more easy to volatalize Zn in oxidizing than reducing conditions:18 If all Zn were bound within crystalline material, it might be expected that the retention under both sets of conditions would be similar. Arsenic. For As, the situation is complicated. Detailed modeling (not presented here) shows that, for coal alone, sludge/coal, and the ternary blend, there is a sharp transition from solid arsenates (As2MnO8 for coal and As2MgO8 for sewage sludge and the ternary blend) to the gaseous arsenic oxides, AsO and AsO2, as the temperature increases. The predicted transition temperatures are 1200, 1173, and 1070 K for coal alone, the binary blend, and the ternary blend, respectively. Thermodynamically, As in the ternary blend should be completely gaseous, and As in the binary blend should be partially in the gas phase, at the reactor temperature. Similarly, the lowest measured ash retention value for As, of 52% for the coal alone, in not in accordance with the prediction of virtually 100% solid. The work of Huggins et al.19 indicates that As is most likely to be present in the coal as some form of arsenate or as arsenopyrite (FeAsS). Previous work20 shows that roasting of FeAsS at moderate temperatures of up to 973 K rapidly liberates 100% of the As. The low retention of As in the coal alone may be because As rapidly volatilized but did not have time to react to AsO2 and recondense on the ash during its transition through the reactor. The initial form of arsenic for sewage sludge was probably an arsenate or monomethylarsonic acid (MMAA) or dimethylarsinic acid (DMAA).21 The reason for the unexpectedly high retention of As in the ternary blend (in comparison to the equilibrium prediction of completely volatile As) is that, at equilibrium, the high quantity of silica within the straw preferentially reacts with Mg (and some Ca to form a variety of solid-phase species, including most abundantly CaMgSi2O6), thus preventing the formation of solid As2MgO8. Of course, the predicted reactions with extremely stable SiO2 species would be very slow; therefore, they do not happen in practice, leaving free Mg to react with gaseous arsenic oxides, before condensing to form solid arsenates. Selenium. Thermodynamically, Se is predicted to oxidize to gaseous SeO2. The observed low retention of Se in the ash agrees well with this, and it is clearly reasonable for oxidizing conditions. This agrees well with the observations of Sekine et al.,18 who observed almost complete volatilization of Se under oxidizing conditions at temperatures of ∼1200 K, whereas the retention was ∼55% during pyrolysis. Molybdenum. A very large proportion of molybdenum in any of the blends (>80%) actually originated from the sewage sludge. Thermodynamic modeling calculates that Mo2O4H2 is the preferred gas-phase species; this is clearly
For the ternary mixture, the prediction is for Be to be gaseous Be(OH)2. The high measured retention within the ash indicates that the formation of the hydroxide is probably kinetically limited; therefore, the Be remains as aluminate. Vanadium. Kinetic constraints can probably prevent the formation of a molecule as complex as gaseous V4O10, which is thermodynamically predicted to be the most abundant V-containing species under the conditions simulated. Previously,5,12 we have shown that if V4O10 is excluded from thermodynamic calculations and V2O5 is constrained to be the most complicated compound formed, vanadium mainly remains as a mixture of solid phases, such as FeO 3 V2O5, CaO 3 V2O5, 2CaO 3 V2O5, V2O5, and VO2, in accordance with the experimental results. Chromium. Cr is thermodynamically expected to be present as CrO3 in both the gaseous and solid phases. Incomplete volatilization was observed, broadly agreeing with the thermodynamics, which yielded a slightly higher percentage of retention. Additionally, much of Cr in any blend comes from the sewage sludge. Fuentes et al.17 indicates, using the Community Bureau of Reference (BCR) sequential extraction method, that a substantial proportion of this was probably mineralized, concurring well with the thermodynamic predictions in this study. Manganese. Mn in the sewage sludge dominated the blends. In all cases, Mn was predicted to oxidize to solid Mn2O3. The high recovery of Mn indicated that this was reasonable, particularly given the relatively high concentration of O2 present. Nickel. Both the sewage sludge and coal were important sources of Ni, except when straw was burned alone. Greater than 80% retention was observed in all cases, except in the combustion of straw alone. Thermodynamically, Ni should form solid Fe2NiO4. In fact, Wang et al.16 shows that there is a moderate association of the amount of Fe found with the amount of Ni in coals, indicating that thermodynamics may correctly predict the solid phase (at least for the coal). For sewage sludge, Fuentes et al. indicates mineralization (i.e., incorporation of trace elements into solid-phase crystalline matter, making it more difficult to subsequently react) of 40-60% of the Ni, again indicating a probable high retention. Copper. The vast majority (>90%) of the copper in each blend is originally in the sewage sludge. Thermodynamically, Cu should be in the form of solid Cu(AlO2)2. It seems more likely that the copper rapidly oxidizes to CuO (non-volatile at 1173 K). Retention of copper in the ash was generally high but was lowest (64.9%) for the blend of sewage sludge and Polish coal. It is difficult to explain the low retention for this particular blend. Fuentes et al.17 found that only a small proportion of the copper in sludge was mineralized. Zinc. Thermodynamically, Zn is expected to form mainly volatile ZnCl2 for the Polish coal fired singly and also for the ternary blend, because both fuel mixtures contain large quantities of Cl compared to the amount of Zn present [sewage sludge/coal contains just less than 2 mol of Cl/mol of Zn, and the dominant Zn species is expected to be nonvolatile Zn(AlO2)2]. Experimentally, zinc is sparingly volatile in all cases. In fact, our results agree well with recent work,18 which indicated low volatility of Zn under combustion conditions at temperatures of up to 1200 K. The zinc
(19) Huggins, F. E.; Huffman, G. P.; Kolker, A.; Mroczkowski, S. J.; Palmer, C. A.; Finkelman, R. B. Energy Fuels 2002, 16, 1167–1172. (20) Dunn, J. G.; Chamberlain, A. C. Miner. Eng. 1997, 10, 919–928. (21) Carbonell-Barrachina, A. A.; Jugsujinda, A.; Burlo, F.; Delaune, R. D.; Patrick, W. H. Water Res. 2000, 34, 216–224.
(18) Sekine, Y.; Sakajiri, K.; Kikuchi, E.; Matsukata, M. Powder Technol. 2008, 180, 210–215.
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: DOI:10.1021/ef100001h
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unlikely to form in this system. Instead, the most likely reaction is the oxidation of Mo (in the coal, possibly present as MoS2 co-mineralized with pyrite22) to MoO2 or MoO3, both of which are solid at 1173 K. It is interesting that the retention of Mo is lower for the coal, possibly indicating that Mo is held in a different form within the sewage sludge. Cadmium. Cadmium is also assumed to volatilize as the dihydroxide. The retention of cadmium is higher than thermodynamically expected, in all cases, possibly because solid CdO rapidly forms, prior to the slower reaction to Cd(OH)2 and volatilization. Mercury. Mercury is predicted to be completely volatilized to the vapor phase, and for the most part, this is found to be the case. Very low retention of Hg by the sewage sludge indicates that Hg is not mineralized. Interestingly, the amount of Hg retained in the ash during combustion of the blend is substantially greater than that which would be expected from the simple addition of the amounts retained by each fuel singly (>10% of the total added, doubling the total amount retained). This positive synergy with respect to retention needs further investigation. Lead. Thermodynamics indicate the formation of volatile PbCl2 in all cases. However, a high retention of lead is observed in the ash. In all blends, the majority of the lead measured (>70%) is derived from the sludge. Fuentes et al.16 found that lead was generally found to be ∼60-90% contained within the residual (mineralized). If it is assumed that this material does not readily react with Cl and, hence, volatalize, the observed retention of ∼50-80% is reasonable. Following a similar argument to that above for zinc, it
may be that the large amount of Cl in the straw helps to volatalize the accessible Pb. Conclusions It is important to acknowledge the difference between small-scale laboratory experiments and the operation of the commercial plant. Data on trace element partitioning behavior derived from this work should be viewed as being indicative of trends only; it is unlikely that absolute values of trace metal concentrations measured in the suspensionfiring reactor will be exactly replicated on a full-scale plant. However, some general conclusions can be drawn. Elements that are thermodynamically predicted to form solid oxides (Be, V, Cr, and Mn) are generally found to be retained in the fuel; it is probable that there are few kinetic constraints to oxidation under combustion conditions. Some elements that are predicted to form gaseous oxides (Se) appear to volatilize in reasonable agreement with thermodynamic calculations. If an element is predicted to form a gaseous chloride (Zn and Pb), there is competition between chloride formation and oxidation; some volatilization occurs, enhanced by the increased addition of Cl, but such volatilization is not assured. The formation of volatile hydroxides (Cd and Mo) appears to be kinetically constrained, and these trace elements may instead form non-volatile oxides. As and Ni probably oxidize to form solid species with minor components in the fuel, such as Mg or Fe, with which they are already associated. The level of Cu escape was found difficult to explain without further experiments. As expected, Hg was found to be volatile as an atomic species. In this study, thermodynamic modeling proved to be a useful tool in assessing the speciation of trace elements post-combustion and would help to assess the quanitity and phase of these elements in the combustion process.
(22) Spears, D. A.; Borrego, A. G.; Cox, A.; Martinez-Tarazona, R. M. Int. J. Coal Geol. 2007, 72, 165–176.
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