Environ. Sci. Technol. 2008, 42, 9014–9030
Fuel-Mercury Combustion Emissions: An Important Heterogeneous Mechanism and an Overall Review of its Implications KEITH SCHOFIELD* Materials Research Laboratory, University of California at Santa Barbara, Santa Barbara, California 93106
Received May 23, 2008. Revised manuscript received October 9, 2008. Accepted October 20, 2008.
An extensive examination of combustion gases containing trace amounts of mercury shows unambiguously mercury’s propensity for heterogeneous chemistry. Although additional mechanisms for the oxidation chemistry of mercury have been implied by the continuing inadequacy of modeling attempts, details of the specific chemistry have remained unknown. Now it is shown that mercury can efficiently chemi-deposit onto surfaces encountered in practical combustors. If sulfur is present, condensed mercuric sulfate forms momentarily. This is then converted by gaseous HCl to HgCl2 that may sublime into the flow or be retained. This elusive and efficient noncatalytic mechanism most likely explains the observed fractional conversions to the dichloride observed in coal combustors. A receptive surface acts solely as an intermediary, facilitating the conversion while disguising its role. Without sulfur, a corresponding mechanism occurs but via HgO that is similarly converted to the dihalide. Such heterogeneous dynamics have significant repercussions for both full-scale combustors and bench-type experiments, which data have been reassessed and reviewed. Conclusions imply that observations concerning mercury will be system dependent and no two combustors can be exactly alike. The re-examination of prior work provides significant support for these conclusions. This fundamental understanding now lays a foundation for meaningful interpretations and program planning. It has indicated also the extreme care needed in sampling and monitoring the speciation of mercury in such combustion flows for reliable results. It now points to a simple low-cost surface-induced mitigation method for effectively converting the mercury in flue gases to the watersoluble dichloride. It is in essence no more than an optimization of the natural process that is currently occurring in combustors but to only limited degrees.
Introduction Coal and municipal waste combustion emissions, metal smelters, and medical vaccines now are the major remaining anthropogenic sources of mercury to which the world’s population is placed at environmental risk. The combustion chemistry of mercury’s flue gas emissions has followed an interesting path of development particularly over the past decade when significant efforts have been made to understand it to little avail. Nevertheless, attempts to establish * Corresponding author phone: +1-805-966-6589; fax: +1-805965-9953; e-mail:
[email protected]. 9014
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mitigation methods have continued unabated without any foundational underpinning of the actual nature of the chemistry. As a result the concept of accepting purely empirical methods, even neural networks (1, 2), and speculative engineering models (3, 4) or kinetic theories (5), that have no chemical validity is now quite pervasive. The present paper is a detailed validation and extension of previous initial efforts to establish a more fundamental understanding of the exact chemistry involved (6, 7). Herein is now summarized the results of a 5 year effort and almost 600 experiments to resolve mercury’s basic combustion chemistry. This new understanding has now permitted an extensive re-evaluation of the rich literature of previous results obtained in bench-type, slip-stream and full scale experiments. The mechanism that is proven beyond doubt to occur in flame gases with mercury levels on a parts per million by volume scale (ppmv), does now appear to be equally able to describe behavior at a parts per billion by volume level (ppbv). The new experimental approach that has been invoked is based on previous combustion/deposition experience gained from using well-characterized and controlled research flame techniques. Coupled to the numerous analytical methods now available for fully identifying trace quantities of materials in their solid phase, it has been possible to examine the chemical behavior of mercury in unprecedented detail. Although there were several earlier indications in the literature suggesting mercury’s propensity for heterogeneous chemistry, these have been largely disregarded due to the widespread presumption that this was predominantly a purely gas phase phenomenon. This, in spite of the fact that mercury has long been known to be captured in or on fly ash. As will be shown, it now appears very likely that its heterogeneous nature plays the dominant role and there is actually no reliable evidence in the literature supporting other mechanisms in coal combustors. The mitigation and even measurement of mercury emissions in coal burning facilities is a demanding task due to the low levels that are present in the flue gases. For most coals these normally are at concentrations in the flue gases of the order of 2 ppbv (or about 10 µg m-3 at 500 K). Consequently, there is an automatic ordering of Hg , Cl < S in coal combustors. Although mercury is one of the least abundant of the trace metals in coal, the volume of coal consumed globally is now on a scale of over 5 billion tons per year or more and increasing (China ∼2.7 billion, United States ∼1.7 billion, India ∼0.5 billion). As a result this poses a global emissions control problem that for effectiveness will require a simple, 10.1021/es801440g CCC: $40.75
2008 American Chemical Society
Published on Web 11/18/2008
minimal cost control method that can be embraced internationally. That is the goal of the present understanding and review, which now appears to suggest one such possible solution.
Previous Proposed Mechanisms Trace levels of any mercury compound, in any high temperature process, automatically volatize as elemental atomic mercury. This is due to the relatively low thermal stability of all solid phase mercury compounds, very few surviving above 500-600 °C (8). In the gas phase it also displays mainly weakly bound molecules and as a result does have a very limited range of chemical diversity compared to other elements. The bond strength of gaseous HgO now is considered to be very weak ( HgO Additionally, there is this option of modifying up the chain if the deposit is initially formed in a low environment of sulfur that later increases. As a result, in a chlorine-free flame, once formed, HgSO4 is totally stable over a specific temperature range. If a deposit is collected at one temperature for a known time and then the additions are stopped but the temperature is raised, then the observed quantity does decrease and the measured Hg:S content ratio can reproduce the curve shown in Figure 5. However, the agreement with the expected falloff shape is not exact and is seen to be a function of the deposit thickness. The closest agreement is when thin deposit films are involved. More details of this are discussed in the Supporting Information involving oxide deposits where the falloff curve is not as steep and is better suited for such studies. Consequently, it appears that the curve shapes in Figures 5 and 6 reflect a balance between deposition and dissociation rates. Based on this, their rates of falloff as a function of temperature can be used to identify an activation energy for the dissociation process in its onset region. Using the 320-420 °C data of Figure 5 implies a value of about 90 ( 10 kJ mol-1 for HgSO4 and 40 ( 10 kJ mol-1 for HgSO4 · 2HgO from Figure 6. It is difficult to relate such values to specific changes due to the nature of solid phase dissociation kinetics and the justification of using such Arrhenius type analyses in attempting to relate to specific chemistry (90-92). Although new physical approaches are still in development (93, 94) the specificity of a rate constant can be modified by innumerable morphological parameters. As reported for dissociation of the apparently simple HgO(s) molecule, in the 200-450 °C range the apparent activation energies can vary from close to 0 to 160 kJ mol-1, showing the difficulty
of interpretation (95). In the current deposition, the formation and possible dissociation process can be occurring simultaneously and any energy of formation undoubtedly may facilitate some redissociation. Thermogravimetric analysis measurements of bulk scale solid dissociation rates are necessarily obtained at temperatures above their threshold as they require a reasonable rate of weight loss. In the present case such measurements were made with HgSO4 for numerous sample weights from 1 to 10 mg at temperature of 475-650 °C. A sample would be held at one temperature and its rate of dissociation obtained. These data appeared to be best described by a zero mass-dependent model and implied an activation energy of 243 kJ mol-1. Such an example showed the difficulty that still exists with solid phase kinetics and that macroscopic bulk scale measurements can not be readily extrapolated to thin film conditions at lower temperatures. Effects of Flame Chlorine. The changes when chlorine is present in the combustion are quite profound. First, the rates of deposit indicated in Figure 5 were examined if exactly equal amounts of mercury and chlorine are present together with sulfur in the flame. At all probe temperatures, there is a dramatic reduction in the amount of deposit formed that immediately appears to be a halving. With increased chlorine such that Cl:Hg ) 2:1 in the flame then hardly any deposit is collected at all. Classical chemical textbooks indicate that mercury sulfate is vigorously attacked by chlorine but are none specific concerning the nature of the chlorine. As a result, subsequent experiments were undertaken in this program to initially obtain an HgSO4 deposit on the heatable probe. The flame was then extinguished but the deposit maintained at 200-250 °C while room temperature flows were passed over it of either gaseous Cl2 or HCl in nitrogen. HCl interacted with and removed the deposit extremely rapidly. However molecular Cl2 had no effect at all even after extended periods. It was obvious that the chlorine in a flame, which is predominantly HCl, is the reactive species interacting with the HgSO4 deposit and modifying its chlorine-free rate of deposit. When it is very carefully measured at 230 °C, the amount of deposit with and without chlorine, in the case of an Hg:Cl ratio of 1:1, was seen to be exactly halved to within a 2% accuracy. The most obvious explanation is a quantitative conversion of the sulfate deposit to HgCl2 with its vaporization back into the gas flows. The dichloride, generally referred to as a corrosive sublimate, has a significant volatility similar to that of elemental mercury (0.26 Pa, 2.6 ppmv, 50 °C; 14 Pa, 135 ppmv, 100 °C; 3000 Pa, 200 °C (96)) with a boiling point of 304 °C. To get such an exact fractional decrease in the quantity collected on the probe is rather convincing evidence in itself for a dominant and efficient formation of HgCl2. The alternate mercurous chloride, Hg2Cl2(s), now is known to fully dissociate on vaporization to HgCl2(s) and Hg(g) but has a vapor pressure about 30-fold smaller at these lower temperatures (97, 98). Its boiling point is 383 °C. For it to be playing a role, and be consistent with this quantitative halving of deposit, the surface would have to retain all the vaporizing Hg(g), which would appear unlikely. Further discussion concerning mercurous chloride, Hg2Cl2(s), may be found in the Supporting Information. An examination of the relative curve shapes in Figure 5 together with the deposit Hg:S ratios begins to give some insight into the mechanism. The data at e90 °C tends to be variable. It suggests either incomplete conversion or more probably some retention of mercury as HgCl2(s) along with sulfur that have been entrapped in the deposit. X-ray analyses of such samples provide no simple answer. As probe temperatures increase it is apparent that a deposit of HgSO4 has to be formed before it can be converted to HgCl2. Evidence for this is apparent from the ratios of the two curves for Cl ) 0 and Cl ) 1 in Figure 5. The horizontal red lines drawn
on the Cl ) 1 curve represent the expected magnitude of the deposit if the full extent of deposit is produced initially (as exemplified by the rate at 230 °C). This is then instantly followed by the deposit’s normal rate of dissociation coupled to the removal of half of the full depositing amount by HCl. In other words, the two removal processes are independent of one another and the increased chaotic disruption that is occurring on the surface does not modify either process. Consequently, neither can be said to be dominant but both are obviously kinetically efficient. An additional important implication is that the amount of Hg and HCl reaching and being held by the surface is equal to their concentration ratios in the gas phase and not related to their molecular weights or diffusion coefficients. In these flame gases the data also confirm that HgCl2(s) can not be formed directly in one step on the surface. This has also been implied previously by bench-type experiments in flow reactors containing Hg(g) and HCl(g) (99, 100). Also, that the HCl most likely converts the HgSO4 to HgCl2(s) that then sublimes rather than directly ablating it as HgCl2(g). Consequently, in these experiments, even though on a molecular basis a single monomolecular layer of HgSO4 is being produced on the surface approximately every five seconds, the data still reflect a very orderly behavior. The same type of data for a pure Schuetteite deposit is shown in Figure 6. Here again, although fewer datum points have been obtained, chlorine effectively halves the amount deposited. However, in this case, its thermal rate of dissociation initiates from a lower temperature and the two processes of conversion and dissociation occur over a wider temperature window. An interesting difference is that the red horizontal broken lines on the Cl ) 1 curve in Figure 6 in this case are most consistent with a full quota of HCl removal of mercury but only half the normal rate of dissociation in the absence of chlorine. The solid horizontal red lines are those levels expected if chlorine removes its quota of mercury and the normal full rate of thermal dissociation occurs. In other words, it appears that the dichloride formation may be very rapid in this instance and that the thermal dissociation rate is displaying a mass dependence of a smaller available deposit area or thickness. An examination as to what is actually occurring to the deposit composition as it is eroded by HCl is also seen in Figures 4 and 5 for the curves relating to Hg:Cl ) 1:1. In Figure 5, at the lower temperatures there is little change in the Hg:S ratio from its expected unity value for the cases of Cl ) 0 and Cl ) 1. It is essentially all HgSO4 that is being converted by the HCl with both Hg and S removed. At the higher temperature where some formation of Schuetteite is occurring, it is noted that HCl does remove both sulfates. However, in so doing, it modifies the composition and the overall Hg:S ratio. For example, at about 375 °C in Figure 5 a deposit with Cl ) 0 is seen to have about a 15% Schuetteite content (Hg:S ) 1.31:1) that is then reduced to 8% (Hg:S ) 1.16:1) following Cl ) 1 removal and thermal dissociation. This chlorine effect is illustrated in the most pronounced manner in Figure 4 when flame Hg:SO2 is in a 1:3 ratio. Under such conditions at about 200 °C, deposits approximate to being a 50/50 molecular mix of the two sulfates (Hg:S ) 2:1) but change with Cl ) 1 to being 66% HgSO4 and 34% HgSO4 · 2HgO. In other words, about 2-fold more Schuetteite is being removed or, because of its formula, 6-fold more mercury is removed as Schuetteite than it is as HgSO4. As a result HCl does appear to have some preference for removing Schuetteite over HgSO4. Consequently, as illustrated by the four comparisons of behavior with Cl ) 0 and Cl ) 1 shown in Figure 4, HCl removes either of the two pure sulfates as a whole but, if mixtures of the two occur, the rates of HCl with Schuetteite appear to be faster. Moreover, the Hg:S ratios indicate that if HCl attacks the HgSO4 · 2HgO molecule it VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. The efficient heterogeneous chemi-deposition mechanism of mercury from fossil fuel combustion flue gases onto relatively cool base-metal surfaces at about 150-400 °C that facilitates its conversion to the gaseous dichloride. converts the full molecule. This is as expected from its structure (79), its formula being more correctly written as Hg3O2(SO4). As a result, any attack on its mercury content naturally destroys the whole molecule. Figure 4 also provides some indication that Hg can not form the dichloride heterogeneously from HCl without the involvement of the sulfate. When the flame ratios of Hg:SO2 ) 1:0.4, the deposit is essentially all Schuetteite and there is little change seen in its composition in Figure 4 with the presence of Hg:Cl ) 1:1. However, if the chlorine rapidly and directly commandeered half the influx of mercury, the surface would sense that the free-Hg:SO2 influx was actually closer to a 1:1 ratio and would have produced a slightly modified Schuetteite/sulfate distribution as expected from Figure 4. The same can be said of the Hg:SO2 ) 1:3 data that do not reflect that of 1:6 on adding chlorine. Mercury’s Heterogeneous Mechanism in Flame Gases. As a result of a very extensive characterization, an elegant mechanism has been discovered whereby gaseous mercury resolves its otherwise very difficult gas phase conversion to gaseous HgCl2. As pictorially indicated in Figure 7, if a surface exists and conditions fall within the acceptable framework, then the mercury chemi-deposits efficiently with sulfur and forms a stable sulfate. This is facilitated by the twodimensionality of the surface and its associated condensed phase kinetics and thermodynamics. Gaseous HCl then vigorously converts the sulfate to the dichloride that then can sublime back into the gas phase. The surface acts solely as the facilitator of the mechanism, remaining unchanged and disguising its role in this manner. The gaseous mercury starts in the gas phase and ends in the gas phase as HgCl2 leaving little indication of its heterogeneous exploits. Due to the fact that two HCl molecules are required in the conversion of the sulfate it is likely that HgCl2(s) is initially formed that then is either retained on the surface or sublimes as the thermally very stable gaseous dichloride. The efficiency of the overall mechanism is seen as being controlled by two competing reactions that depend on the specific conditions. In the upper temperature region of the functioning window there will be competition between the rate at which the sulfate is thermally dissociating back to gaseous mercury and the rate of conversion to the dichloride. This is where chlorine concentration levels can exhibit a pivotal role in the process. Overall, the main rate-controlling step remains the influx of mercury to the surface. Conversion efficiency then depends mainly on temperature and chlorine concentrations. Downstream Exhaust Measurements. The experiments in the higher temperature flame gases show mercury’s great propensity for heterogeneous chemistry if surfaces are available in a specified temperature window. Such chemistry under those conditions is now irrefutable and must be accepted as a fundamental aspect of mercury’s nature. However, in practical combustors such relatively cool surfaces occur only downstream in the cooling flue gases, and mercury 9022
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concentrations are on a much lower scale. The question therefore is whether this same chemistry can occur in those cases or whether it requires hot gases as a driver. In order to confirm the occurrence of such a heterogeneous channel and establish its operational window with cooling gases, experiments were performed downstream in flames where the burned gases had cooled to e200 °C. For this purpose the internally heated stainless steel probe was used and positioned about 0.1 s downstream. At such a distance in propane flames, the exhaust gases are equilibrated and additionally cooled and diluted by radiation and air indrawn into the open air flame gases. Nevertheless, in spite of this dilution, as apparent even by eye, faint deposits of sulfate have been observed on the probe with surface temperatures of 150-250 °C. The width of this window is narrower than that seen in Figure 5 and indicates a possible onset temperature close to 150 °C that reflect a surface formation activation energy with cooler gases. Hg:S ratios confirmed the formation of the sulfates and no deposits were obtained if the flame gases also contained chlorine. Scaling Considerations and Relevance to Coal Combustion. In coal combustors the concentration levels in the cooling flue gas region can be about 1-10 ppbv Hg, with chlorine as HCl on a ppmv scale that can run from 1 to 1000 ppmv and SO2 ever present at levels of hundreds to several thousand ppmv. Consequently, the order is invariably S, Cl . Hg and the deposition mechanism outlined above will involve predominantly HgSO4 formation. Therefore, the relevant question remaining is whether the mercury sulfate intermediate can operate in a coal combustor where mercury levels are at a 1000-fold less concentration than in the present laboratory studies. There will undoubtedly be a temperature regime where HgSO4 is formed irrespective of the mercury concentration. However, how will scaling modify the width of the functioning temperature window? The strength of the scaling arguments has emerged by studying corresponding experiments involving HgO deposition in sulfur-free flames. This is better suited to answer such a question due to its less precipitous dissociation falloff region. As a result, these arguments are presented in the Supporting Information and discusses in more detail the HgO deposition system that closely parallels the behavior seen here with HgSO4. However, a brief summary of the results that shows the factors considered is of relevance here in consideration of full-scale conditions. Such chemi-deposition in a practical coal combustor will involve three controlling kinetic processes: a first order influx of atomic mercury to any surface it can find, a rate of thermal dissociation of the depositing HgSO4, and a rate of conversion of this to HgCl2 by HCl. In studies of ever smaller thicknesses of HgO deposits it was apparent that the rate of dissociation became first order in concentration dependence for thin layers and that the temperature location of the falloff curve as reflected in Figure 5 did not vary as would have been expected if the rate of deposition and the rate of dissociation were exhibiting differing mass (concentration) dependences. Consequently, the chemi-deposition should scale to ppbv mercury levels and exhibit an operational temperature range. Its actual width will be controlled by a formation activation energy at its lower temperature cutoff, and on the actual HCl concentration at its higher temperature onset. Because the thermal dissociation is controlled by an activation energy, its rate will exhibit a strong temperature exponent that is in direct competition with the conversion by HCl that exhibits a strong concentration component. Obviously, enhanced HCl levels will elevate the exact temperature onset for mercury oxidation but this will be tempered within a range by the exponent term of the dissociation. This is most probably the factor that explains the great sensitivity to HCl levels that is widely reported.
Evidential Support for Heterogeneous Mechanisms in the Previous Literature. In the last five years, a change of mindset and direction finally has started to occur with regards to mercury’s combustion chemistry. Although there had been significant indications in the literature for more than 20 years of mercury’s possible heterogeneous nature, this was implied in different research activities and had been totally disregarded. Medhekar et al. (101) in examining the reaction of gaseous Hg with molecular chlorine at 250 °C had found that this produced HgCl2 heterogeneously very efficiently on Inconel, quartz, steel, and even Teflon surfaces. Nevertheless, it was the combination of the inability of gas phase kinetic models to explain mercury’s behavior and the work of Hall et al. (102, 103) in the early-nineties, that forced investigators to begin to embrace the possibility of major heterogeneous mechanistic roles. This also helped to remove the incongruity with the previous long accepted occurrence of mercury absorption onto fly ash particles. Moreover, it was becoming apparent that fly ash absorption, although possibly initiated as a physical adsorption, was ending up with the mercury chemically bound in or on the ash (44). Huggins et al. (104) utilizing X-ray analysis concluded that none of their observations were consistent with the mercury being physi-sorbed. Dunham et al. (105) also made the interesting observation that while the ash samples oxidized mercury, only part of this was retained in the ash but that retained was in molecular form. Moreover, two recent studies have examined the thermal stability of the chemisorbed mercury in fly ash and have shown that 90% was removed within a minute of heating at 538 °C (106). The more detailed report of numerous fly ash samples from different sections of the ash collection train showed that mercury release actually occurred between 300 and 400 °C (107). Moreover, the release curve shape implies that of a single pure component in the ash. However, the release temperature range does not seem to agree with what might be expected for HgSO4 or HgO under such TGA conditions as illustrated by others (85, 108) and in the present work. The boiling point of HgCl2 is 304 °C, but in a normal thermogravimetric analysis, with a heating rate of about 20 °C/minute, HgCl2(s) vaporizes totally by 200-250 °C (108). Direct absorption experiments of HgCl2(g) onto coal chars have shown that it can be retained only to temperatures of 160 °C or slightly higher (109). Even if reduced by the char to Hg2Cl2(s) its dissociative vaporization to HgCl2(g) might be expected to be very similar to this mercuric halide (83, 98). This makes the interpretation of the results of Rubel et al. (107) difficult, and it might imply the presence of additional chemisorption binding of molecules that are formed within particles. This could introduce additional activation energy for retention of HgCl2 and modify its normal vapor pressure (96). Experimental chemical kineticists in particular have realized for some time now the significant difficulty of quantitatively studying homogeneous reactions of gaseous mercury without their results being dominated by heterogeneous effects (40-42, 65, 66, 110, 111). Also, in full- or pilot-scale combustors, more detailed observations in the cooling flue gases have noted in the past few years that oxidation of mercury can at times be evident across several of the devices that intercept the flow. These can be the air preheater (112, 113), the fabric or candle filter baghouses (114-118), and one comment even suggested a role for the duct walls while not expanding further on the implications of such a statement (119). Also, it is well established that sampling through stainless steel probes is unreliable for mercury speciation measurements (119, 120). At present, there is a hope that selective catalytic reactors (SCRs) used for NOx control might to some extent do double duty and also control mercury. Present studies are rather ambitious examining the numerous parameters in rather
complex slipstream facilities. As a result, the interpretations remain difficult. It is apparent that there is a sensitivity to temperature, chlorine levels, the presence of the NH3 additive and to deposition problems caused by alkali and alkaline earth sulfates. Modeling attempts by Niksa and Fujiwara (121) and Senior (122) still have to be seen as largely empirical and mathematical in nature as the chemistry involved has not been identified and remains highly speculative. Nevertheless, the heterogeneous role of the SCR’s catalyst is accepted without question. However, in the recent studies by Cao et al. (123, 124) it is rather noteworthy that 37% mercury oxidation occurred even in the empty slipstream reactor with no catalysts present by solely adding 300 ppmv of HCl to the gas flows. With additions of only 6 ppmv HBr the oxidation increased to 86%. It is strange that the implications of such results appeared of lesser interest than the goal of examining the enhancements seen with the SCR catalysts. In fact, with HBr, the performance was better without the SCR operating. The most recent bench-scale study under SCR conditions clearly identifies the pivotal role of HCl and the effectiveness of the catalytic surfaces in the oxidation of mercury. However, the injection of ammonia for the necessary NOx control is a severe interferent for mercury (125) as is the deactivation by pore blocking due to alkali and calcium sulfate deposition (126). In relation to coal combustors, the present work has quantified one noncatalytic heterogeneous mechanism, namely the formation of HgSO4 that then is converted to HgCl2 by HCl. As already mentioned, a second direct heterogeneous mechanism of Hg with Cl2 also has been previously reported (101). These are two efficient mechanisms and it would seem logical that either one or the other will be occurring in the SCR studies. There is little Cl2 in coal combustor flows but the Deacon process is a well established catalytic mechanism on certain metal oxides whereby, in the 300-450 °C range, HCl can be oxidized to Cl2 by O2 (127). However, it is sensitive to the catalyst and CuO is the favored oxide due to its reasonable efficiency. In the slipstream experiments of Cao et al. (123), direct spiking gas additions of Cl2 indicated that in the SCR it is the HCl and not Cl2 that is the chlorine species involved in mercury’s oxidation. It is a little surprising that the direct formation between Hg and Cl2 does not contribute, but any Cl2 formed might also be depleted by other species. Their data do seem to minimize a possible role for the Deacon mechanism. Because SCR’s function by adding traces of NH3, with which HCl can also react, and their normal operating temperature range is 340-370 °C, it is not surprising that levels of oxidation are small for low chlorine content coals and that HCl levels above 100 ppmv are needed to drive the chemistry. As seen in Figure 5, the SCR operates at temperatures that are approximating to the upper temperature range where HgSO4 formation and dissociation can occur, also explaining a need for higher levels of HCl. Unfortunately, the term catalytic oxidation has been freely repeated ever since the patent by Brooks (128) that suggested oxidizing mercury using a bed containing a commercial catalyst of most probably oxides of Mn, V, Cr, Pb, or Se. Unless the material modifies a comparable rate of reaction on a base-type material, it is imprecise to call it a catalyst. Consequently, although the potential for catalytic oxidation of Hg has been automatically accepted in recent years, whether the processes being examined are actually catalytic remains unproven. Presto et al. (63, 129) have recently summarized various commercial catalysts for their ability to oxidize mercury. Comparisons do not differ by large amounts and observed differences may arise from numerous other causes. The current program has confirmed comparable rather efficient rates of oxidation of mercury on platinum and on steel. The latter is not known to have a general catalytic VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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nature, and as the rates are already large, further catalysis is unlikely anyway. A recent Density Functional Theory study of mercury’s adsorption onto Ag, Au, Cu, Ni, Pt, Pd metal surfaces concludes that the interaction efficiency decreases in this order (130). However, this is a reflection more of mercury’s amalgamation ability, which is clearly not involved in this chemi-deposition chemistry. The data suggest that the presently described processes are all noncatalytic in nature and do not benefit from mercury’s amalgamation abilities. What the recent studies do clearly indicate is the importance of temperature, chlorine, sulfur, and residence times as the controlling parameters both in SCR studies of mercury oxidation (125, 131-135) and in other general flue gas observations (116, 136, 137). Catalysis quite often is of greatest value in promoting processes at lower temperatures than they would otherwise occur and this may be the case in some studies with mercury especially at temperatures below 200 °C. Recently, fixed bed testing has indicated that a V2O5 based catalyst can oxidize mercury at 135 °C without previously required ultraviolet activation (138). Clearly, in analyzing data on mercury, considerations have to be made whether amalgamation, true catalysis, or chemi-deposition processes are involved. The question of what happens on fly ash or injected sorbent particles is a large subject unto itself (44). However, the more interesting findings relevant to chemistry are well worth summarizing. The major observation is that mercury can be chemi-sorbed but the process is very inefficient, requiring, for example, C/Hg mass ratios of several thousands and a residence time of several seconds for small activated charcoal particle injection (44). This undoubtedly is due to the complex temperature/time history of the particles and the weak initiating interaction. The major factors are undoubtedly the limited time when a particle experiences the appropriate temperature window for possible interaction and the need for mercury/particle collisions that can be very constrained especially by a laminar flow. Also, any real catalytic effects that may stem from a surface’s ability to physically retain the atomic mercury for slightly different times. Because fly ash and direct particle injections experience different environments it might be appropriate to examine them separately. Fly ash has a composition that reflects its coal source and the efficiency of its combustion (139-143). It can have a wide range of variability that includes its morphology, particle size, surface area, and composition, and also its temperature/ time history through the combustion train. Such factors can introduce a dilemma for the interpretation of fly ash data. Moreover, it is not sufficient to characterize the performance of fly ash solely by collecting and analyzing the ash. The real question is not necessarily its retention efficiency, but how much additional oxidation of mercury has occurred due to the ash’s presence. As noted by Dunham et al. (105), the ash may be effective yet retain nothing. However, what is retained is seen to be oxidized and appears to be stable and not readily leachable (144-146). Without detailed mass balances of mercury it is difficult to fully interpret its effectiveness. This is exemplified by the continuing controversy concerning the apparent correlation between mercury capture rates and the amountofunburnedcarbonintheflyash(70,141-143,147-149). Moreover, as seen by the complexity in modeling a particle through combustion (150), questions can be raised concerning the actual surface temperature history of fly ash as a function of time and whether this is an aspect that is affected by the unburned carbon level. Fly ash experiences the full combustion course. As it cools it finally traverses a temperature regime where chemideposition can occur with oxidation but little or no retention, and then at lower temperatures also a physi-sorption region where oxidation and retention are observed (38). For this 9024
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reason, to a certain extent, low temperature bench-type experiments that try and simulate the behavior of fly ash in full-scale combustors are limited in scope as the ash no longer retains its prior exact history and virgin properties (105). Fly ash is chemically complex and can contain a variety of oxide and chloride elements that may aid mercury’s oxidation to sulfate or oxide with subsequent conversion to the dichloride (63, 129, 151). At lower temperatures it can also directly absorb the gaseous dichloride. Theoretically, there is a suggestion that it may be the partially oxidized carbons on the fly ash surface that initially manage to adsorb the atomic mercury onto the oxygen and not the carbon (152, 153). Mercury does have a rich organo-mercurial chemistry that does exhibit direct HgsC bonding (154, 155). Recently, the mercury fulminate molecule finally has been shown to have direct HgsC bonding in its Hg(CNO)2 structure but is rather unstable (156). However, such HgsC bonding does not readily occur in nature and it requires biochemical enzymic transformations to create the divalent mercury HgCH3+ ion that is the problem in aquatic environments (157). Even the synthesis of organo-mercury compounds is not straightforward and usually requires sodium amalgamation in the process to facilitate this somewhat reluctant bonding. As mentioned already, the thermal mercury release profiles from fly ash samples as a function of temperature are most intriguing (107). Pavlish et al. (44) also noted that, whereas fly ash spiked with HgCl2 desorbed this by 220 °C, if spiked with Hg, fly ash samples required temperatures closer to 350 °C. Whereas the corresponding profiles of coal samples clearly indicate mercury being released from a variety of compounds in the coal, that from fly ash suggests a single compound. As mentioned already, even this data is difficult to interpret. The release temperature seems to be possibly too low for HgSO4 and rather high for HgCl2. If the latter, in spite of its normal vapor pressure (96), the ash would have to retain it essentially to its boiling point (107, 109). Whereas fly ash transits the whole combustion environment, particle sorbent injection tends to be limited to the cooler flue gases that are e160 °C. Consequently, it is essentially an initial adsorption method. Because of the injection, the temperature history of the particles is uncertain but will necessarily be relatively cool. Obviously, it is of little value for the mercury to remain physically unchanged on particles as its re-emission would jeopardize disposal. Its value has to depend on the occurrence of low temperature conversion, and the initial retention time will be critical (158). As a result, activated charcoals known for their retention capability have become the mainstay for injection techniques and are invariably aided by capture on a fabric filter system that effectively lengthens the interaction time (159, 160). The stability and leachability of adsorbed mercury on activated charcoals appears to pose no disposal problems (161). As might be expected, pretreatment of the particles by impregnation with chlorides, bromides, and iodides (162-167), or sulfur is also beneficial (168-170). In bench scale testing of activated charcoal at 300 °C, Lachas et al. (171) noted that no mercury was ever detected without the presence of S or Cl. Although theory (153) implies that activated carbon infused with fluorine may have the greatest adsorption capacity for mercury it is to be noted that HgF2 differs from its fellow halides in being easily hydrolyzed by water to HgO(s) and HF. However, the HgO could then rapidly react with HCl producing HgCl2. Recent slipstream experiments with and without SCR catalysts in a 327-366 °C range have indicated in fact a possible ordering of HBr > HI > HCl > HF in their mercury oxidation abilities (124). The situation with bromine is of particular interest in that heterogeneously it appears to be more effective with mercury than chlorine. This is a little surprising as their
chemistries generally are very similar with bromides having slightly weaker bonded molecules than their corresponding chlorides. Like Cl2 (101), Br2 recently has been shown to react heterogeneously very rapidly with mercury on fly ash to produce HgBr2 (111), a molecule with vapor pressures similar to HgCl2 and with a boiling point of 322 °C. The bond strength of HBr (D°298K ) 366 kJ mol-1) is weaker than that of HCl (432 kJ mol-1). As a result, HBr will not be as dominant in the flue gases as is HCl. Although much less studied, it is generally considered that it may constitute a 50-98% contribution to the gaseous bromine, and Br and Br2 may be more evident. Yan et al. (172) attempted ambitious equilibrium calculations considering coal combustion (54 elements, 3200 compounds) for oxidizing conditions at 300-1800 K. Although such calculations are only meaningful accepting the various assumptions made, they do provide a rough guide as to what might be considered. They confirmed that HgBr2 can similarly be important in mercury’s emission chemistry and also reminded us that there are also many other elements in coal that also vie for chlorine or bromine. At present, reported experiments have either added bromine as a salt such as NaBr or CaBr2 to the fuel (173, 174), or as HBr directly into 300 °C flue gases (38). Preliminary results imply a significant improvement factor in the degree of mercury oxidation. However, it may be expected that less bromine will be lost through binding to other elements present than is chlorine. Whether bromine’s chemistry differs by the presence of higher levels of Br and possibly Br2 has to be seen. An impediment to bromine’s use in practice is one of added cost. However, it well illustrates again that heterogeneous interactions can be sufficiently rapid even with low levels of halogens if the required conditions are met. Treatment of activated charcoal with elemental sulfur is interesting in that it has been reported to react directly with mercury within the particle to form HgS even under oxygenrich flame conditions (168, 169). At the low flue gas temperatures where it is used, it will most probably remain as this. Being its natural ore, HgS is a most desirable end product as it is thermally stable and totally insoluble in water at normal conditions. Another laboratory scale experiment has even shown that Hg can be removed by (NH4)2SO4 particles at 178 °C possibly forming the sulfate by direct exchange (175). A recent development, “The Thief Process” (176), combines the benefits of fly ash and particle injection by withdrawing partially combusted coal particles from the furnace that can be a 50/50 mix of carbon and ash. This is then reinjected into the lower temperature flue gases where it shows a performance comparable to activated charcoals. Behavior is similar but the method offers significant economic advantages over other injection methods (177). Consequently, the chemistry as reported in the present studies appears to be relevant to fly ash behavior. For the lower temperature injected particles whether the same chemistry occurs but requires some catalysis requires further study. The extended interaction time once on the particle may be important. The problem raised on accepting an efficient heterogeneous mechanism for mercury oxidation is that it introduces significant questions and uncertainties concerning the interpretations of much of the past work where it was not considered. Moreover, it modifies the basic thinking that is involved in the analysis of any observations. To the present day, analyses still tend to revolve around homogeneous gas phase reactions and interactions with particles (111). Now, on re-examining previous slipstream or reactor studies there are clear indications that wall interactions had to play a pivotal role. Many such experiments report long memory effects after ending an addition which is a clear indication of this.
A disturbing consequence is that a heterogeneous role implies that no two systems will be alike as the surface areas and conditions will necessarily differ. Moreover, the sample cooling and analysis procedures for mercury are necessarily fraught with potential risks for perturbing the results and interpretations. A recent study injected liquid nitrogen spray into the flue gases obviously to circumvent such sample cooling difficulties (38). Also, as mentioned already, a very important recent discovery was that chlorine in the sample can interfere in some cases in the first impinger of a wet chemical analyzer for mercury speciation measurements. This has significant implications on the results of numerous prior studies some of which they have listed (53). All these prior results are biased to various degrees and reflect mercury’s oxidation in the analyzer rather than in the experiment. Such arguments now help to explain why various effects initially reported in bench-scale experiments with, forexample,NO,NO2,H2O,orFe2O3 additions(48,61,105,178-180) have not been confirmed in full-scale combustors (181). Use of amalgamation of mercury on gold, silver, or palladium metals has also been studied in bench and pilotscale experiments and met with limited success (100, 182). However, these did show an interesting factor. When these metals absorb mercury, either directly or if coated onto a fabric filter, the mercury requires Cl2 for oxidation. As illustrated in a recent comment by Liu (183), the current level of confusion and misunderstanding of the behavior of mercury in combustors remains quite appalling in spite of the huge expenditures. Moreover, the conceived obligation to present mechanisms that are not based on any understanding continues to spawn meaningless speculation (184). The ultimate dilemma is that it is obviously impossible to derive any detailed level of understanding solely from fullscale data because it is fraught with such innumerable variables. Moreover, it is also experimentally demanding and to some degree not fully possible to provide truly simulated data from bench-type experiments. Hopefully, the results presented herein will be of some help in guiding the design and planning of future studies and their interpretation.
An Implied Intrinsic Mitigation Method for Mercury Emissions from Coal Combustors After reassessing our current knowledge, it would seem logical to assume that only one major mechanism for mercury oxidation is occurring in all coal combustors but at present to varying extents. Moreover, that a lignite coal with chlorine levels in the flue gases as low as 1 ppmv or additions of equally low levels of bromine have been seen as creating a reasonable level of oxidation shows that mercury does have already a built-in mechanism in coal combustors. It appears to be simply missing the required facilities for initiation and efficiency in most cases. The present research has clearly illustrated mercury’s significant propensity for heterogeneous interactions in combustion systems and has discovered one elegant and efficient mechanism that can oxidize mercury to the dichloride. It is quite plausible that this is the mechanism currently endeavoring to occur in full-scale coal combustors. From this and other deposition studies it appears that low concentrations of species benefit significantly by utilizing the two-dimensional space of surfaces and the resulting change from gas phase to condensed phase kinetics and thermodynamicstoachieveotherwisedifficulttransformations. The mechanism tends to suggest that the enhancement of mercury’s oxidation might be accomplished by simply providing a large stationary surface area in the region of the cooling exhaust gases. It does not need to be catalytic, but needs to be located where the gases will maintain its surface temperature at that optimal for deposition (185). Currently, VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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it is the severe temperature, time, and gas/surface interaction constraints that are limiting the degrees of oxidation observed on fly ash or any other surface encountered. If designed such that the flows are made to specifically encounter such a stationary surface several times in transit, combined with the other supplemental fly ash and other contributions occurring in the system, a more complete conversion to the dichloride would appear plausible. The fact that certain coals, high in chlorine content, in combustors fit with fabric filter bags already display high levels of oxidation (90%) is seen as very encouraging. Hopefully this new level of understanding now lays a better foundation for the planned development of control methods that are simpler and of minor cost compared to currently pursued approaches.
Acknowledgments This work made use of the MRL Central Facilities supported by the MRSEC Program of the National Science Foundation under Award No. DMR05-20415. It was made possible by funds generously donated by the Robinson-Schofield Foundation for which I am most grateful, and without which this work would not have been realized. I also thank my old colleague, Dr. Martin Steinberg, now deceased, for having designed and constructed the internally heated probe that was very useful to this program. I also thank Professor Roger Millikan for his time and interest in taking digital photographs of these deposits.
(13)
(14)
(15) (16) (17) (18) (19)
(20) (21) (22)
Supporting Information Available Although sulfur-free combustion systems are of less interest to the commercial fossil-fuel community, their study herein has provided a much broader database of the general heterogeneous chemistry of mercury. Its inclusion shows the generality of these chemistries and provides additional underlying acceptability of the conclusions drawn. This material is available free of charge via the Internet at http:// pubs.acs.org.
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