ARTICLE pubs.acs.org/est
Synchrotron-Based XANES Speciation of Chromium in the Oxy-Fuel Fly Ash Collected from Lab-Scale Drop-Tube Furnace Facun Jiao,†,‡ Niken Wijaya,† Lian Zhang,†,* Yoshihiko Ninomiya,‡ and Rosalie Hocking§ †
Department of Chemical Engineering, Monash University, Clayton, GPO Box 36, Victoria 3800, Australia Department of Applied Chemistry, Chubu University, 1200 Matsumoto-Cho, Kasugai, Aichi 487-8501, Japan § Monash Centre for Synchrotron Science and School of Chemistry, Monash University, Clayton, Victoria 3800, Australia ‡
bS Supporting Information ABSTRACT: Speciation of chromium (Cr) in the fly ash collected from oxyfiring of Victorian brown coal has been reported for the first time to address the potential formation of toxic Cr(VI) and the variation of the quantities of Cr(III)-bearing species with flue gas composition. Synchrotron-based X-ray absorption near-edge structure (XANES) was employed for Cr speciation. Apart from a pure O2/CO2 mixture (27/73, v/v) versus air, the O2/CO2 mixtures doped with SO2, HCl, and steam individually or together to simulate real flue gas have also been tested. Under all of the conditions tested here, the fractions of Cr(VI) in the fly ashes are insignificant, constituting no more than 5% of the total Cr. The test of Cr-doped brown coal in pyrolysis further confirmed that the Cr(VI) formation preferentially occurred through a local oxidation of Cr(III) at the oxygen-containing functions sites within coal matrix, rather than through an oxidation by external bulk O2. This reaction is also highly temperature-dependent and slower than the interaction between Cr(III) and other metals such as iron oxide. Increasing temperature to 1000 °C inhibited the oxidation of Cr(IIII) to Cr(VI). Shifting the combustion gas from air to O2/CO2 exerted little effect on the Cr(VI) formation. Instead, the formation of iron chromite (FeCr2O4) was facilitated in O2/CO2, probably due to a strong reducing microenvironment formed by the CO2 gasification reaction within the char matrix. The accumulation of HCl in flue gas favored the vaporization of chromium as gaseous chloride/oxychloride, as expected. The coexistence of SO2 inhibited this phenomenon by promoting the formation of sulfate. The presence of steam was even beneficial for the inhibition of water-soluble Cr sulfate through stabilizing the majority of Cr into alumino-silicate which is in the slagging phase.
’ INTRODUCTION Emission of Cr from coal combustion is of environmental concern, as its hexavalent state, Cr(VI), is highly water-soluble and toxic, which has been classified as a group A inhalation carcinogen by the US Environmental Protection Agency (EPA).1,2 Conversely, the trivalent state, Cr(III), is one of the essential trace micronutrients for the carbohydrate metabolism in low doses.3 With the increase in the awareness of the public in the environmental impact of coal utilization, it is vital to reveal the fate of Cr during the combustion of coal, particularly in any new process of burning coal in different oxidizing gases instead of air. Oxy-fuel combustion is such a technology, which uses the high-purity oxygen mixed with recycled flue gas (RFG) for coal combustion to deliver a CO2-rich flue gas for direct sequestration and/or storage.4 The speciation of Cr in coal combustion-derived ash depends on its original mode of occurrence and flue gas composition.5 Extensive studies had been carried out to specify Cr in different coals and their ashes derived from conventional air-firing.611 Through the use of synchrotron-based XANES, the mode of occurrence of Cr in coal has been confirmed to exhibit a form that is closely associated with organic macerals in a form of amorphous r 2011 American Chemical Society
Cr(III) oxyhydroxide, i.e., CrOOH, 12,13 and/or Cr(III) combined with illite.14,15 In contrast, Cr in ash is predominantly present as Cr(III) incorporated into alumino-silicate matrix of a slag phase. With respect to the fraction of toxic Cr(VI), it is estimated to account for up to 5 wt% of the total Cr in the fly ash samples collected from the air-firing power plants burning the eastern USA coals.13 The similar finding has been achieved for the proportion of Cr(VI) in the ashes collected from the Australian bituminous coalfired power plants.16 The enrichment of total Cr and Cr(VI) in the Australian bituminous coal fly ash was also confirmed showing an inverse relationship with ash particle size.16 The larger Cr (VI) fraction in the total Cr were also observed in different cases, including 926% for the fly ash collected from several western U. S. coals,7 1019% Cr(VI) for the ash from Israel power stations,6 and 728% for cofiring biomass and bituminous coals.17 To date, no experimental studies have been conducted to address the emission and speciation of Cr during oxy-fuel Received: February 16, 2011 Accepted: June 13, 2011 Revised: June 12, 2011 Published: June 13, 2011 6640
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Table 1. Flue Gas Compositions Tested in This Work and the REs of Cr under Individual Conditions of Coal Combustion a concentration of Cr, wt% flue gas composition
term
a
coarse
fine
RE of Cr (Aver. ( STD)
air
pure air
0.1590
0.105
0.85 ( 0.18
oxy
27% O2/73% CO2
0.1240
0.103
0.80 ( 0.06
oxy-HCl (250)
27% O2/73% CO2 doped with 250 ppm HCl
0.1440
0.130
0.49 ( 0.27
oxy-HCl (500)
27% O2/73% CO2 doped with 500 ppm HCl
0.1340
ND
0.37 ( 0.31
oxy-SO2 (250)
27% O2/73% CO2 doped with 250 ppm SO2
0.1660
ND
0.58 ( 0.37
oxy-SO2 (500)
27% O2/73% CO2 doped with 500 ppm SO2
0.2030
0.109
0.97 ( 0.20
oxy-SO2 (1000) oxy-SO2 (500)-HCl (250)
27% O2/73% CO2 doped with 1000 ppm SO2 27% O2/73% CO2 doped with 250 ppm HCl and 500 ppm SO2
0.1090 0.1370
0.116 0.142
1.20 ( 0.13 0.68 ( 0.05
oxy-steam
27% O2/20% steam/53% CO2
0.108
0.124
0.95 ( 0.26
oxy-steam-HCl (250)
27% O2/20% steam/53% CO2 doped with 250 ppm HCl
0.168
ND
0.55 ( 0.05
oxy-steam-SO2 (500)
27% O2/20% steam/53% CO2 doped with 500 ppm SO2
0.116
ND
0.98 ( 0.30
0.154
1.04 ( 0.14
oxy-steam-SO2 (500)-HCl (250) 27% O2/20% steam/53% CO2 doped with 250 ppm HCl and 500 ppm SO2 0.106
Note, the flow rates for flue gases remained constantly at 10 L/min (STD) for all the conditions.
combustion. Although the influence of CO2 instead of N2 on Cr species at high temperature is insignificant from the perspective of thermodynamic equilibrium,18 coal combustion is a process in which plenty of complicated subprocesses including mass/heat transfer and chemical reactions are involved. Particularly, during oxy-fuel combustion, the char-CO2 gasification plays an important role on char conversion and the microenvironment within char matrix.19,20 The preferentially formed carbon monoxide (CO) has proven to increase the melting and coalescence propensity of minerals and even the vaporization extent of volatile metals.21 Moreover, it is noteworthy that, upon the recirculation of flue gas, the impurities such as HCl, SO2/SO3 and even steam can be gradually accumulated in a furnace,22 which, in turn, could affect the redox propensity of Cr during coal combustion.16 To address these knowledge gaps, the oxy-fuel combustion of a low-rank brown coal, namely Victorian brown coal, has been conducted in a lab-scale drop-tube furnace (DTF) to examine the Cr emission and speciation. For a low-rank coal, its abundant maceral-bound Cr(III) and oxygen-containing functional groups presumably favor the Cr(VI) formation during air combustion.23 If that is the case, the high oxygen percentage (thus high partial pressure in the combustor) used for oxy-fuel combustion, e.g., 2736% in CO2, would be further beneficial for the formation of Cr(VI). In this regard, a systematic investigation has been made throughout this work to address the influence of a variety of gas compositions including pure air and O2/CO2 mixture with and without the doping of the impurities including HCl, SO2, and H2O. For speciation, the synchrotron-based XANES has been employed to quantitatively determine the oxidation states of Cr both in coarse ash particles and inhalable fine particulates less than 5 μm in size. Apart from raw coal combustion, the pyrolysis of raw coal and Cr-doped coal in pure N2 and CO2 was also conducted to examine if a pure CO2 stream is really influential on Cr speciation.
can be seen, the ash content in the coal tested is as low as 2.1 wt%. X-ray fluorescence spectroscopy (XRF) analysis indicates a concentration of approximately 26 ppm (mg/kg) for Cr in this coal sample. Coal Combustion, Char/Ash Characterization. Coal combustion was carried out in a lab-scale DTF at a nominal gas residence time of 4 s, a furnace temperature of 1273 K, and a coal feeding rate of 0.5 g/min. Schematic of the DTF used is illustrated in Figure S1 of the SI. The details of the coal combustion procedure were explained in the SI section. In brief, two major gases were tested: pure air versus an O2/CO2 mixture with 27% O2. The latter gas has proven to match air in terms of volatile flame temperature and coal burnout rate.24 Moreover, as tabulated in Table 1, three impurity gases including HCl of 0500 ppmV, SO2 of 01000 ppmV, and steam of 20% (v/v), were dosed either individually or together into the O2/CO2 mixture and examined systematically. The exit O2 content is approximately 10% to ensure the completion of coal burnout under the conditions tested here. Ash particles were collected through the use of a flask and a Waterman silica microfiber thimble filter installed downstream the DTF reactor. The coarse particles larger than 5.0 μm, namely coarse fraction hereafter, dropped by gravity into the flask underneath the reactor, whereas those smaller than 5.0 μm, namely fine ash fraction, were entrained by flue gas and deposited into a high-purity thimble filter installed after the flask. All of the samples were characterized by a precalibrated XRF for Cr quantification (see Figure S2 of the SI) and XANES for Cr speciation. The relative enrichment factor (RE) was used to quantify the variation of Cr mass loss (i.e., vaporization extent) 25 with flue gas composition, which was calculated according to the following:
’ EXPERIMENTAL SECTION
Where, Cash and CHTA refer to the concentrations of Cr in an overall ash sample (coarse plus fine) and the high-temperatureash (HTA) of the raw coal, respectively. The symbol, Yash, denotes the overall ash yield based on the mass of HTA fed with the raw coal together into the furnace.
Coal Properties. An air-dried low-rank Victorian brown coal with a particle size of 105153 μm were tested, the properties of which are listed in Table S1 of the Supporting Information, SI. As
RE ¼
6641
Cash Yash CHTA 100
ð1Þ
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Table 2. Percentages of Cr (III)-Bearing Species in Raw Coal, Washed Coal Residues, Pyrolyzing Chars, and the Ash Samples Collected from Coal Combustion in Air versus Oxy (27% O2/73% CO2) CrCl3
organic Cr
FeCr2O4
Cr-Silicate
raw coal
69
31
water-washed coal nitric acid-washed coal
54 46
46 54
N2 Char
10
CO2 Char
7
22
19
49
17
30
ND
46
ND
53
47
ash collected from air combustion
coarse fine
ND
ND
74
26
ash collected from coal combustion in 27% O2/73%CO2
coarse
ND
ND
65
35
fine
ND
ND
83
17
The Cr K-edge XANES spectra were collected from the beamline 20B with a blending magnet at the 2.5 GeV Photon Factory in the High Energy Accelerator Research Organisation (KEK), Tsukuba, Japan. The spectra were recorded in the fluorescence detection mode at room temperature, stepping from about 100 below to 1000 eV above the edge of Cr (5989 eV). For the curve-fitting of XANES spectra, the least-squares fitting methods were used to quantify the proportions of Cr(VI) and individual Cr(III)-bearing species. The pre-edge range of 410 eV was deconvoluted to extract the percentage of Cr(VI) peaking at 3.54.5 eV,6 as detailed in Figure S3 of the SI. In contrast, for the Cr (III)-bearing species, a principal component analysis (PCA) combined with least-squares fitting (LSF) through the SixPack package has been used. The standard compounds in Figure S4 of the SI were used.
’ RESULTS AND DISCUSSION Chromium in the Raw Brown Coal. Figure S5 of the SI illustrates the Cr K-edge spectra for Cr in the raw coal and its washed residues. The corresponding curve-fitting results are tabulated in Table 2. A large fraction of CrCl3 has been confirmed in the raw coal. Such a finding is in line with that has been reported for Cr in sewage sludge,26 but has not been mentioned for black coal. This species is also sparingly soluble in water, and hence, its removal upon water washing is insignificant, as indicated by a remaining proportion of 54% for CrCl3 in water-washed residue versus 69% in the raw coal. Moreover, as approximately 46% CrCl3 still remained intact upon nitric acid washing, it is indicative that more than half of this species is locked firmly within coal matrix and is thus difficult to mobilize. Another key species for Cr (III) in the raw coal is organic Cr, the spectrum of which shows the similarity with that has been reported for Cr in the organic/light fraction of black coal.12 It is most probably present as amorphous CrOOH bound tightly with organic maceral via oxygen-containing functional groups such as carboxylic acid.5 Due to the protection effect of coal carbon, few of it is removable upon acid leaching. Volatility of Cr during Oxy-Fuel Combustion. Cr is a partial volatile metal, exhibiting an RE factor of 0.91 in bottom ash and 1.75 in ultrafine ash collected downstream of the ESP during coal combustion in air.17 Without the addition of HCl to the flue gas, the RE of Cr achieved in this work ranged from 0.80 to 1.20 on a basis of the total Cr in the HTA, as demonstrated in Table 1. This indicates a slight loss of Cr during the combustion of this coal when no extra HCl was introduced into flue gas. In addition,
since the concentrations of Cr in coarse and fine ash fractions are rather comparable, it is thus suggestive that, the gaseous Cr vapors, if formed by vaporization at the flame zone, were mostly condensed and evenly distributed into different sizes. In contrast, when the extra HCl was doped into flue gas, the concentration of Cr in fine ash fraction dropped to a value lower than the detection limit of the XRF used here, whereas the presence of Cr in the coarse ash was little changed. Its lower RE factors are a clear sign of the promoted vaporization of Cr through chlorination. Codoping steam with HCl affected little on the recovery of Cr vapors. Adding SO2 with HCl together resulted in a remarkable increase in the RE factor of Cr from 0.49 ( 0.27 in the oxy-HCl (250) case to 0.68 ( 0.05 in the oxy-SO2 (500)-HCl (250) case. A further introduction of steam to flue gas is more influential, resulting in the closure of the mass balance of Cr in the oxysteam-SO2 (500)-HCl (250) case. Clearly, the coexistence of SO2 and steam together promoted the condensation of Cr chloride back into solid particles, through sulfation, and/or the incorporation of Cr vapors into refractory minerals. Fractions of Cr(VI) in the Samples Tested. Irrespective of the experimental condition, the fraction of Cr(VI) in the total Cr of any ash sample was found no larger than 5%, the limit for quantification of the curve-fitting method adopted here, as demonstrated in Table 3. Such an observation is broadly consistent with the literature observation that has been achieved for most of the air-fired bituminous coal cases. It however raises confusion as to why the abundant organic-bound Cr in low rank coal and excessive O2 in oxy-fuel combustion did not promote the Cr(VI) formation. One probable explanation is the kinetic control for the oxidation of Cr(III), which requires a long residence time and/or a slow heating rate as that has been confirmed for the presence of approximately 20% Cr(VI) in the high-temperature-ash (HTA) of an American sub-bituminous coal.6 Another probable explanation is the deactivation of the oxygen-containing functional groups such as carboxylic acids at the furnace temperature studied here. In other words, the oxidation of Cr(III) to Cr(VI) is more likely a process which is triggered by the inherent oxygen within coal matrix, rather than the external bulk O2 in flue gas. To prove this hypothesis, coal pyrolysis in pure N2 (99.999%) and CO2 (99.995%) was further conducted. Approximately 600 ppm Cr was also doped into the raw coal through ion-exchanging (see the SI) to increase the signal-to-noise ratio of XANES spectra and the curve-fitting accuracy to about 0.1%. As evident in Figure 1 and Table 4, the Cr(VI) formation was indeed favored during coal pyrolysis at 6642
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Table 3. Results from Least-Squares Fitting of Cr XANES Pre-Edge of Victorian Brown Coal Ash Samples fine ash
coarse ash peak A (Cr (III))
peak B (Cr (VI))
peak A
peak B
Cr (VI) % position, eV height position, eV height
Cr (VI) %
position, eV
height
position, eV
height
air
1.0
0.043
3.5
0.026
1
1.15
0.01
3.7
0.013
1
oxy
1.7
0.039
3.5
0.033
2
1.5
0.032
3.8
0.027
2
oxy- HCl (250)
1.8
0.026
4.5
0.019
1
1.7
0.029
3.6
0.016
1
oxy- HCl (500)
0.9
0.037
2.4
0.056
5
1.9
0.042
4.1
0.045
3
1.2
0.021
4
0.022
2
oxy-SO2 (250) oxy-SO2 (500)
1.3
0.033
3.9
0.049
4
oxy-SO2(1000) oxy-SO2 (500)- HCl (250) oxy-steam
1.4 1.2
0.049 0.029
3.9 3.7
0.067 0.041
5 3
1.1
0.031
3.5
0.028
2
1.6
0.059
3.7
0.065
5
1.3 1.6
0.049 0.028
4.1 3.6
0.036 0.017
2 1
oxy-steam-HCl (250)
1.7
0.013
4.2
0.028
3
1.2
0.047
3.5
0.019
1
oxy-steam-SO2 (500)
2.1
0.043
3.8
0.035
2
1.0
0.025
4.3
0.031
3
oxy-steam- SO2 (500) HCl (250)
1.2
0.047
4.1
0.045
3
1.4
0.041
3.8
0.016
1
Table 4. Fractions of Cr-Bearing Species in the Cr-Doped Brown Coal and Its Char Samples Cr
Cr
(VI) (acac)3 Cr-doped coal
Figure 1. The Cr K-edge spectra for Cr-doped brown coal and its char samples.
800 °C, which should occur through the oxidation of Cr(III) at the inherent oxygen-containing functional sites as there is little gaseous O2 added into the system. This process is also highly temperature dependent, showing a maximum yield for Cr(VI) at 800 °C. Further increasing the furnace temperature to 1000 °C decreased the fraction of Cr(VI) of the total Cr either in N2 or CO2, clearly demonstrating the deactivation of the oxygencontaining functional groups through a rapid decomposition or the intensified interaction between Cr(III) and other metals at the oxygen-containing sites. The char-CO2 gasification derived carbon monoxide could trigger these reactions, considering the brown coal tested here is highly reactive in CO2.27 The results here also confirmed that the oxidation of Cr(III) to Cr(VI) is a very slow process when compared with the interactions between Cr(III) and other metals such as iron at 1000 °C. Once the Cr(III) is fixed into a stable species within char matrix, the introduction of excess O2 for char combustion affected little on its oxidation. Otherwise, more than 5% Cr(VI) of the total Cr
Organic Cr
CrCrCl3 silicate FeCr2O4
0
31
40
29
0
0
600 °C N2 char 600 °C CO2 char
0.8 1.4
19.8 22.7
29.8 26.6
24.8 21.7
14.9 21.7
9.9 5.9
800 °C N2 char
4.7
21.0
12.4
26.7
21.9
13.3
800 °C CO2 char
6.3
17.8
16.9
24.4
22.5
12.2
1000 °C N2 char
3.4
24.2
21.3
9.7
7.7
33.8
1000 °C CO2 char
3.8
22.1
20.2
11.5
6.7
35.6
should be formed in the fly ashes collected from raw coal combustion. The use of CO2 instead of N2 showed slight but remarkable difference for the fraction of Cr(VI) of the total Cr in the chars obtained at 800 °C in Table 4. The O2 impurity in the CO2 bottle gas, although it cannot be ruled out completely, is seemingly not the major issue causing this phenomenon. It is more likely that, the CO2 chemically assisted the oxidation of Cr(III) through a chemical reaction proposed elsewhere.16 This reaction should be much slower than the local oxidation of Cr(III) within char matrix and even insignificant once the furnace temperature is over 800 °C. Regarding the conditions of adding HCl to flue gas, the gaseous Cr(VI) species such as CrO2Cl2 were very likely formed, little of which were however condensed into solid particles, and hence, no Cr(VI) was detected in the ash samples derived from the oxy-HCl cases in Table 3. Cr(III) in the Pure Air and Oxy (27% O2/73% CO2) Ashes. The XANES spectra for Cr in the ash samples generated from these two pure cases are depicted in Figure S6 of the SI. The quantitative analysis results are also tabulated in Table 2 to compare with the results for raw coal and pyrolyzing char samples. Irrespective of gas composition, Cr-silicate and FeCr2O4 spinel are the two major condensable species formed both in coarse and fine ash particles, indicating the continuation of the stabilization of the original organically bound Cr and CrCl3 by 6643
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Table 5. Presence of Cr in Brazilian Lignite and Its Char and Ashes Organic Cr
Cr-silicate
FeCr2O4
raw coal
40
54
6
CO2 char overall air-ash
25 ND
45 72
30 28
overall oxy-ash
ND
51
49
refractory minerals with the progressing of coal combustion. The preferential formation of FeCr spinel, either in coarse or fine ash particles, is in agreement with the above discussion on the inhibition of Cr(VI) formation during the Cr-doped coal pyrolysis at 1000 °C (see Table 4). The reaction between Cr(III) and iron oxide should occur quickly and even in situ at the oxygen-containing functional sites within coal matrix. Apart from Cr, iron in the brown coal sample used here has also proven to ionically bound as ion-exchangeable divalent (Fe2+) cation and oxyhydroxide nanoparticles with a size of approximately 50 Å in the closed voids.28 The limited size of the closed voids should greatly limit the agglomeration of FeCr spinel, which thus bear a very fine particle size and were readily shed away from burning char surface. The thermodynamic equilibrium modeling using Factsage elsewhere also confirmed the preferential formation of FeCr spinel over the other species from 350 to 1550 °C.17 With respect to Cr-silicate, its preference in coarse ash fraction is an indicator of the encapsulation of the mobile Cr by the silicondominant slag, either at the high-temperature flame zone or in the cool flue gas where the deposition of the saturated Cr vapor on the surface of preexisting silicon particles is available. Silicon has proven to be the dominant metal in the coarse ash particles (see Table S2 of the SI).22 The mobile Cr here should be those derived from the vaporization of the original CrCl3 and the dissociation of organic Cr from the oxygen-containing functional groups. The use of O2/CO2 mixture is seemingly favorable for the formation of FeCr2O4 spinel. The abundant reducing gases such as carbon monoxide (CO) formed surrounding coal surface could still be the major cause for this phenomenon. As demonstrated by the thermodynamic modeling results in Figure S7 of the SI, a slight increase in the concentration of CO on char surface has the potential to significantly promote the formation of FeCr spinel through reducing more trivalent iron to divalent state. For another brown coal collected from Brazil, consisting of 34 wt% fixed carbon, 31 wt% volatile matter and 30.9 wt% ash and approximately 30 mg/kg Cr, the similar observation has also been confirmed in Figure S8 of the SI and Table 5. Influence of HCl and SO2 on Cr(III) Speciation. Cr species in the ash samples collected from oxy-HCl and oxy-SO2 cases are illustrated in Figures S9 and S10 of the SI, respectively. The quantitative fitting results for Cr(III) species are illustrated in Figure 2. A stable increase in the fraction of Cr-silicate is a clear sign of the enhanced vaporization of Cr upon the increase of HCl content in flue gas, through the chlorination of the original organic Cr. The resulting chloride was partially encapsulated into the silicon-dominant ash skeleton when diffusing out of the burning char particles. Reduction in the FeCr spinel is in agreement with this observation, as less Cr was available to interact with iron locally in the vicinity of the oxygen-containing functional sites. The loss of iron through chlorination, as confirmed in a companion study elsewhere,22 is also a major cause
Figure 2. Fractions of Cr-silicate and FeCr2O4 in the coarse and fine ash fractions upon the addition of HCl to O2/CO2 mixture.
Figure 3. Fractions of FeCr2O4 and Cr2(SO4)3 in fine ash fraction upon the addition of SO2 to O2/CO2 mixture.
resulting in the pronounced reduction in the FeCr spinel fraction. Changes to the quantities of Cr(III)-bearing species in fine ash particles generated in the O2/CO2 mixture doped with SO2 are plotted in Figure 3. The increase in the fraction of Cr2(SO4)3 upon the addition of SO2 to flue gas should mainly occur at the flue gas cooling stage, as the thermodynamic equilibrium modeling has proven the preferential sulfation of Cr at the temperatures below 700 °C.23 Apparently, once the original Cr vaporized into gaseous species, the introduction of SO2 into flue gas inhibited the heterogeneous interaction between gaseous Cr and solid ash particles. As a consequence, few of Cr-silicate was formed, and the relative fraction of FeCr spinel was reduced as well. Such a change is apparently influential in terms of ash toxicity, since the FeCr spinel is virtually insoluble in aqueous fluids whereas Cr(III) sulfate is readily soluble. The difference in Cr solubility alone would likely determine a significant difference in the toxicity of Cr.29 Influence of Steam on Cr(III) Speciation. As demonstrated in Figure S11 of the SI and Table 6, the partitioning of Cr-silicate and FeCr2O4 between two ash fractions was altered noticeably upon the introduction of steam to flue gas. The increase in the fraction of Cr-silicate implies that steam could chemically assist in the interaction between Cr and quartz (SiO2) and/or aluminum oxide (Al2O3) forming stable slagging species, similar to that has been confirmed for the interaction between NaCl and SiO2.30,31 6644
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Table 6. Influence of Steam on the Fractions of Cr (III)-Bearing Compounds in the Ashes Generated from Oxy-Fuel Combustion fine ash
coarse ash Cr-silicate
FeCr2O4
oxy
35
oxy-steam
46
oxy-SO2 (500) oxy-steam-SO2 (500)
21 27
31 44
48 29
oxy-steam-HCl (250)-SO2 (500)
41
39
20
Cr-silicate
FeCr2O4
65
17
83
54
29
From the thermodynamic equilibrium perspective, the presence of 20% steam in flue gas, as demonstrated in Figure S12 of the SI, favors the vaporisation of Cr as CrO2OH(g), CrO3(g), and CrO2(OH)2(g) at 600 °C onward, even when 250 ppmV HCl coexisted in flue gas. Since the mass balance of Cr in the oxysteam case was very close to that for the oxy case, it is inferable that the vaporized oxide and/or hydroxide of Cr were readily incorporated into the molten slag when diffusing out of char particle surface. In contrast, the addition of 500 ppmV SO2 to the wet flue gas favored the homogeneous sulfation of the gaseous oxide/hydroxide, thereby leading to the formation of condensable sulfate rather than Cr-silicate, particularly in the fine ash fraction. A further introduction of 250 ppmV HCl to flue gas reduced the fractions of Cr2(SO4)3 and FeCr spinel, suggesting the slower sulfation rate of CrCl3 than the gaseous oxide and/or hydroxide when abundant steam is present in flue gas. However, as the mass balance of Cr was nearly closed for the case of the coexistence of steam, SO2, and HCl in flue gas, and the fraction of Cr-silicate was increased to a similar level with that for dry and clean O2/CO2 mixture, it is evident that the gaseous CrCl3, once vaporized, was quickly incorporated by silicon oxide into the molten slag. This reaction should preferentially occur at the relatively high temperatures where the sulfation reaction is unfavorable. Implication of These Findings. This work has for the first time reported the partitioning of chromium during the combustion of Victorian brown coal in air and under oxy-fuel mode. Victorian brown coal is the single largest source meeting over 85% of the electricity needs of the State of Victoria in Australia. Oxy-fuel combustion is deemed as one of the most promising processes to mitigate its notoriously high carbon footprint in the foreseeable future. As has been confirmed, either in the pure N2 or CO2 or in the O2/CO2 mixtures with an exit O2 content of about 10% in flue gas, the toxic Cr(VI) is insignificant in the ashes collected from conventional air combustion and oxy-fuel combustion with the recirculation of either dehydrated and clean fuel gas or a dirty stream contaminated by abundant steam, HCl and SO2. Such a conclusion is apparently applicable to the real boilers with an exit O2 content of 25%. However, it has to be cautioned that the recycling of HCl favors the vaporization of Cr(III) species to form fugitive gases which are mostly Cr (VI)-bearing species. This risk is not minimized/ eliminated unless extra SO2 is also present in the combustor. Steam has also proven to inhibit the vaporization of Cr and its sulfation propensity. The use of dried brown coal and dehydrated flue gas reduces the fraction of Cr stabilized into the slag alumino-silicate, thereby potentially increasing the risk with respect to the formation of water-soluble sulfate and gaseous chloride.
Cr2(SO4)3
10
Cr2(SO4)3
71 47 61
53 39
67
23
In terms of the Cr(VI) fraction in the total Cr, the noticeable discrepancy observed between the pyrolyzing char of the Crdoped brown coal and raw coal combustion ashes confirmed the importance of the local microenvironment on the oxidation of Cr(III). The inherent oxygen-containing functional groups, rather than the external bulk O2, is the key oxidant promoting the formation of Cr(VI), which, however, occurs in a slow rate relative to that for the interaction between Cr(III) and other metals particularly iron oxide at the oxygen-containing functional sites. The bituminous coals apparently have fewer oxygencontaining functional groups, if not negligible. The distinctively different microenvironment of the char matrix of different lowrank coals may also explain why the Cr(VI) fractions in their ashes vary randomly, as shown in this work and elsewhere.6,7,17 The remarkable quantity of Cr(VI) in the char obtained from the pyrolysis of Cr-doped brown coal at 800 °C is also interesting, which is a clear sign of the preferential formation of Cr(VI) through local oxidation at low temperatures. This is opposed to the theoretical modeling results which only show the probable formation of Cr(VI) such as CrO3(g) above 1200 °C. The similar discrepancy has also be noticed elsewhere,27 strongly indicating the insufficient database for Cr speciation at high temperatures. Further research through the use of pure compounds and Crdoped chars derived from differently ranked coals are underway to clarify both thermodynamic and kinetic characteristics of the Cr(VI) formation.
’ ASSOCIATED CONTENT
bS
Supporting Information. In this section, the details for raw coal property, Cr doping procedure, coal combustion experiment, Cr quantification and speciation methods, XANES spectrum of all ash samples, and thermodynamic equilibrium modeling conditions and results were described. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: +61-3-9905-2592, Fax: +61-3-9905-5686, E-mail: lian.zhang@ monash.edu.
’ ACKNOWLEDGMENT This work is supported by the Australian Research Council (ARC) Future Fellowship Grant (FT0991010), Grant-in-aid for Scientific Research on Priority Areas (B), (20310048), Ministry of Education, Science, Sports and Technology, Japan and the Linkage Infrastructure, Equipment and Facilities Program of the 6645
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Environmental Science & Technology ARC (LE0989759). We also acknowledge Dr. Michael Cheah at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, for XANES operation support.
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