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Transformation of Organically Bound Chromium during Oxy-coal Combustion: the Influence of Steam and Mineral Xiaoyu Li, Hui Dong, Juan Chen, Chun-mei Lu, Guangqian Luo, and Hong Yao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03123 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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Transformation of Organically Bound Chromium during Oxy-coal Combustion: the Influence of Steam and Mineral Xiaoyu Li,1 Hui Dong,1 Juan Chen,1* Chunmei Lu1, Guangqian Luo2, Hong Yao2

1:

School of Energy and Power Engineering, Shandong University, Jinan, 250061, China 2

: State Key Laboratory of Coal Combustion, Huazhong University of Science & Technology, Wuhan, 430074, China

*: Corresponding authors: Tel: +86-531-88392414, Fax: +86-531-88392701, E-mail: [email protected]

A manuscript for Energy & Fuels


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Highlights 1) CaO and Fe2O3 have high reactivity with Cr compared to limestone and kaolin. 2) Steam inhibited CaO capturing Cr during oxy-coal combustion. 3) Steam enhanced Fe2O3 reacted with Cr during oxy-coal combustion. 4) The coexistence of H2O and CaO remarkably promoted toxic Cr(VI) formation. 5) H2O offset the passive effect of HCl on Cr recovery when H2O coexistence with HCl.


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Abstract The influence of steam and minerals on Cr transformation during oxy-coal combustion has been examined in a drop tube furnace at 1273 K. Extra 500 ppmV HCl and 1000 ppmV SO2 were also added to flue gas to examine the competition of H2O, HCl and SO2 on Cr behaviors during oxy-coal combustion upon the recirculation of impurities in flue gas. It was found that CaO and Fe2O3 exhibited high capability on capturing Cr-bearing vapors during combustion compared to limestone and kaolin. Changing air to oxy-coal combustion obviously weakened the role of CaO, whereas promoted reaction between Fe2O3 and Cr vapors. The existence of H2O in flue gas facilitated the oxidation of trivalent Cr(III) to high valance Cr vapors in a short second time scale. CaO, particularly in the co-existence of H2O during coal combustion, remarkably enhanced the toxic Cr(VI) fraction in solid ash. H2O affected the behavior of minerals reacted with Cr. CaO was inhibited and Fe2O3 was enhanced by steam to react with Cr during oxy-coal combustion. The retention of Cr in ash reduced with H2O increase from 5% to 20% during oxy-coal combustion. HCl significantly promoted the release of Cr through chlorination reaction. The presence of H2O offset the negative effect of HCl on Cr recovery when H2O was coexistent with HCl in flue gas. SO2 favored the retention of Cr via sulfation reaction to form Cr2(SO4)3 condensed into solid ash. Moreover, it was found Cr vapors preferred to react with HCl than SO2 during combustion. Keywords: Chromium, Steam, Oxy-coal combustion, Mineral.


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1. Introduction Cr is one of the heavy metals usually present in fly ash derived from coal combustion, leading to a wide array of negative impacts on the environment and human health, caused by the carcinogenic and toxic hexavalent chromium1-3, which is highly watersoluble and mobile. The hexavalent Cr(VI) can cross cell membranes and be reduced to Cr(III) to react with hydrogen peroxide forming highly damaging hydroxyl radical.4 Unlike inorganic bonding chromium which is difficult to vaporize and mainly fixed directly in the bottom ash, the organic chromium combined with coal matrix through the oxygen functional groups5, is easy to be gasified and released to flue gas, performs a series of complex homogeneous and heterogeneous reactions in coal combustion process, also easily be oxidized to toxic hexavalent Cr(VI). With the improvement in the awareness of people in the environmental pollution, it is urgent to reveal the migration and control of Cr, especially organically bound Cr, during coal combustion.

Oxy-coal combustion is a promising low-emission process for reducing the carbon footprint of coal.6 The flue gas is recirculated to mix with high-purity oxygen instead of air for combustion to enhance the concentration of CO2 in flue gas for sequestration and/or storage7. However, a large portion of recycled flue gas results in the accumulation of gaseous impurities such as H2O, HCl and SO2 in the furnace. The massive CO2 promotes char gasification to produce locality reducibility atmosphere8, 9. The gas environment changing, in turn, is supposed to alter the behaviors of heavy metal Cr. The overwhelmingly dominant CO2 and carbon monoxide (CO) derived from char-CO2 gasification8, 9 are supposed to impact the redox of Cr. In addition, it has been confirmed in our previous work that the impurity gases including SO2 and HCl have effect on Cr transformation as well.10, 11 Regarding simulated steam in the


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furnace during oxy-fuel combustion, thermodynamic equilibrium calculations have proven that the overall trend of chromium transition is to a higher state through the formation of CrO2(OH)2 and CrO2(OH)10,

12, 13

, the kinetic behavior and detail

reaction paths of Cr-bearing species at the presence of H2O are still unknown.

Through doping Cr into coal prior to combustion to mimic the organically bound Cr and increase the signal-to-noise ratio for the XANES analysis, the vaporization propensity of the predominant organically bound Cr was clarified11. It was found that CaO has a high reactivity with Cr vapors, but leads to a high fraction of toxic Cr(VI) formation in solid ash. The effect of HCl in flue gas on the reactions between CaO and Cr species has also been clarified14. Besides, steam is one of the most important gas components during oxy-fuel combustion. However, the influence of steam on chromium captured by CaO is still unclear. Limestone is widely used in desulfurization in the combustion process, especially during fluidized bed combustion, which has a calcination process in the combustion process, and whether this process would affect the capability on capturing Cr is still unrevealed. Moreover, kaolin is confirmed to effectively capture the metallic vapors of Pb, Cd, Na and control the emission of PM1 at high temperatures7,

15-18

. It’s however not clear if kaolin is

applicable in terms of capture Cr vapors during combustion. It was found Fe2O3 reacted with chromium to form Fe-chromite at high temperatures14. However, the influences of steam on Fe2O3 capture Cr species and their effect on Cr retention/toxicity (i.e. Cr(VI) fraction) during oxy-coal combustion still haven’t been reported. Furthermore, though some studies have been carried out to investigate the effect of HCl and SO2 on Cr transformation, limited data are available on Cr speciation with the coexistence of H2O during oxy-coal combustion. ACS Paragon Plus Environment

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This study aims to investigate the influence of steam on the capabilities of different minerals in terms of capture Cr gaseous species and their effects on Cr retention and toxicity (i.e. Cr(VI) fraction) during oxy-coal combustion through a laboratory-scale drop tube furnace (DTF) to mimic the real pulverized-coal boiler. The effects of minerals employed including CaO, Fe2O3, limestone and kaolin on capturing Cr vapors were also compared between air and oxy-coal combustion. The kinetic behaviors of Cr at the presence of H2O were simulated through CHEMKIN package to offer some insights into the transformation of the Cr-bearing species. Apart from H2O, the additions of extra 500 ppmV HCl and 1000 ppmV SO2 to flue gas were also conducted to examine the competition effect of H2O, HCl and SO2 on Cr speciation and retention in solid ash during oxy-coal combustion upon the recirculation of impurities in flue gas.

2. Experimental Section 2.1 Coal Properties A low-rank coal with a particle size of 45 ∼ 105 µm was employed for experiment. The proximate and ultimate analysis of the coal and the composition of ash were listed in Table 1. The occurrence of chromium in the employed low-rank coal as shown in Figure S1 was investigated through the European Community Bureau of Reference (BCR) extraction method19, which is adapted from the Tessier method20. It consists of 58 % organically bound Cr and 40 % residual state chromium stay in the mineral crystal lattice, which is difficult to vaporize and enriches directly to the solid ash during coal combustion.


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In order to mimic the organically bound Cr and increase the concentration of Cr for X-ray photoelectron spectroscopy (XPS) analysis, chromium was doped through ionexchangeable method onto the coal. The chromium glycine (Cr-Gly), a kind of organic chromium compound, was synthesized and used for Cr doping onto the coal. The preparation of Cr-Gly and the Cr doping process were descripted in detail in the supporting information (SI). The concentration of Cr is 1263 ppm (mg/kg) in the resulting Cr-doped coal, which is exclusively trivalent Cr(III) as organically bound species.10 The added Cr(III) is bonding to the oxygen-containing functional groups such as carboxylic acid through ion-exchanging, bearing a high similarity with the inherent maceral-associated chromium which is organically bound cation21.

2.2 Coal Combustion and Cr quantification The combustion of Cr-doped low-rank coal was carried out in a lab-scale electrically heated drop tube furnace (DTF) to mimic a real pulverized-coal fired boiler. Schematic of the DTF used was illustrated in Figure 1 and operation procedure was explained in detail elsewhere11, 22. The furnace temperature was set at 1273 K and the coal was fed at 0.3 g/min carried by 1 L/min primary gas and fed through a watercooled injector into the inner chamber of a quartz reactor. In the meantime, 9 L/min secondary gas was preheated to the furnace temperature through the annulus between the inner and outer chamber of the reactor, and then feeding into the inner chamber to mix with the primary gas and coal particles at the tip of injector. Two major gas compositions were tested: air versus 27% O2 balanced by CO2 with an identical total gas flow rate of 10 L/min controlled by the mass flowmeter. The nominal gas residence time in the furnace was 4 s. Various volume fractions of H2O including 5%,


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8%, 10% and 20% generated through a steam generator were dosed individually into the secondary gas at the bottom of the reactor for O2/CO2 combustion experiment.

Four kinds of solid minerals including calcium oxide, iron trioxide, kaolin and limestone were added to the Cr-doped coal physically before experiment at a mass percentage of 5 wt% to investigate their effect on scavenging Cr vapors. The compositions and Cr concentrations of kaolin and limestone employed here were presented in Table S1. As can be seen, the concentrations of Cr in kaolin and limestone were 73 ppm and 33 ppm respectively. Besides, 12 ppm of Cr in calcium oxide and 20 ppm of Cr in iron trioxide were detected by the inductively coupled plasma atomic emission spectrometry (ICP-AES). Furthermore, the influences of steam on the capability of minerals in terms of capture Cr vapors have been examined. Two impurity gases containing HCl of 500 ppmV and SO2 of 1000 ppmV were also dosed into the O2/CO2 mixture to investigate their competitive effects with steam on chromium transformation during combustion. The exit O2 content in the DTF experiments remained approximately 10% to ensure a high burnout rate of coal, which varied from 89.6 % to 99.7 % in this study. Ash particles were collected separately into two parts: coarse particles larger than 5.0 µm, namely coarse ash, dropped by gravity into the flask underneath the reactor, and particles smaller than 5.0

µm, namely fine ash, were entrained by flue gas and deposited into a thimble filter installed after the flask. The concentration of chromium in the solid ash was detected by the inductively coupled plasma atomic emission spectrometry (ICP-AES) after microwave digestion. And the species of Cr in ash collected was analyzed via X-ray photoelectron spectroscopy (XPS).


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In order to quantify the variation of Cr retention rate in solid ash23, 24 under different conditions, a relative enrichment factor (RE) was used and calculated according to

RE=(

Ccoarse ash Ccoal

×

Ycoarse ash 100

+

Cfine ash Ccoal

×

Yfine ash 100

)×100%

(1)

Where, Ccoarse ash and Cfine ash refer to the Cr concentrations in coarse and fine ash samples, respectively. Ccoal denotes the concentration of Cr in the Cr-doped coal. The symbols, Ycoarse ash and Yfine ash represent the coarse ash and fine ash yield based on the mass of Cr-doped coal into the drop tube furnace.

2.3 Dynamic Simulations Both of the DTF experiments and the thermodynamic analysis can indicate the composition of the Cr-bearing species but can’t offer comprehensive insights into the transformation of the Cr-bearing species. The CHEMKIN package was used for kinetic simulations, which is a program to solve complex chemical reaction problems, including 29 reactor modules that address the needs of modeling complex systems. In this study, a mechanism file within 47 elementary reactions and 21 species indicated in Table S2 was arranged as an input file. A closed homogeneous reactor module was chosen to simplify the actual combustion situation. Simulations were carried out at a temperature of 1273 K at 0.1 MPa for the whole calculation process. In the gas feed, the concentrations of H2O, O2 and CrOOH were set according to that in the DTF experiment.

3. Results and Discussions


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3.1 The capture of Cr by various minerals under air and oxy-coal combustion During combustion, the organically bound Cr rapidly decomposed and vaporized to form the Cr(III)-bearing vapors such as CrOOH(g), as discussed in literature11, 21. With the increase of oxygen and particle temperature during char oxidation/combustion, the vaporized Cr vapor was partly condensed into ash. To clarify the capability of ashforming minerals on capturing Cr-bearing vapors during combustion, four kinds of additives including calcium oxide, limestone, Fe2O3 and kaolin were employed to mix with the Cr-doped coal for combustion in DTF at 1273 K under air and oxy-fuel conditions. The resulting RE factors of Cr in solid ash were depicted in Figure 2. As can be seen, regarding air combustion in panel (a), adding either CaO or Fe2O3 to coal obviously favored the retention of Cr in solid ash particles, indicating the occurrence of the interaction between Cr(III) hydroxide vapors and solid additives according to the reactions Eqs (1) and (2) below. Besides, limestone exhibited lower capability on capturing Cr vapors compared to CaO, which however was even much better than that for Si-Al-based mineral of kaolin. For oxy-fuel combustion as shown in panel (b), only CaO and Fe2O3 indicated high capability for capture Cr vapors. The additives of limestone and kaolin had little effect on Cr retention in the solid ash under oxy-fuel combustion. CaO+2CrOOH(g)→ CaCr2O4+H2O

Fe2O3+4CrOOH(g)→ 2FeCr2O4+0.5O2(g)+2H2O

∆G = - 698.525 KJ

(1)

∆G = - 1100.036 KJ

(2)

The fractions of chromium captured by four different additives under air and oxy-fuel combustion were compared in Figure 3. Obviously, changing air to oxy-fuel combustion weakened the role of CaO on capture Cr-bearing vapors, decreasing the


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captured fraction of Cr from 85 % for air case to 68 % for oxy-fuel combustion. This should be attributed to the carbonation of calcium oxide under high CO2 partial pressure during oxy-fuel combustion. The influence of CO2 was further confirmed by a lower Cr capture percent of only 1.4 % when adding limestone to Cr-doped coal under oxy-fuel combustion, comparing to that of 25.3 % during air combustion. The decomposition of limestone was inhibited by the high partial pressure of CO2 in oxyfuel combustion25. However, regarding the addition of Fe2O3, oxy-fuel combustion caused an increase in the capture fraction of Cr from 70 % in air condition to 77 % in oxy-fuel combustion. The Cr recovery was basically dominated by the direct reaction of Fe2O3 with vaporized Cr via the reaction Eq. (2). The overwhelmingly dominant CO2 and carbon monoxide (CO) derived from char-CO2 gasification reaction during oxy-fuel combustion were supposed to alter the valence state of iron, reducing Fe3+ into Fe2+ (such as FeO), which readily reacted with the gaseous Cr vapors through the reaction Eq. (3) proposed below and promoted Cr retention in the solid ash. Kaolin exhibited little capability on Cr capture either in air or in oxy-fuel combustion. FeO+2CrOOH(g)→ FeCr2O4+H2O ∆G = - 614.762 KJ

(3)

3.2 The influence of H2O on Cr transformation during oxy-coal combustion 3.2.1 The influence of H2O on Cr gaseous species at high temperature The kinetic analysis through the CHEMKIN package was conducted to understand the effect of H2O on the dynamic behavior of chromium gaseous species under the combustion temperature of 1273 K. Figure 4 showed the transformation of Cr vapors with 5% H2O doping into flue gas. It can be seen that the reactions between steam and Cr(III) hydroxide vapors occur. The presence of H2O facilitates the oxidation of


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CrOOH to CrO(OH)2 in a quite short time scale. And CrO(OH)2 further transforms to a higher valent state, the pentavalent CrO2(OH), which finally converts to hexavalent CrO2(OH)2 or CrO3. As shown in Figure 4, the gas of CrO(OH)2 reaches a peak at approximately 0.1 s and then transformed to CrO2(OH). 74 % of CrOOH is oxidized to CrO2(OH) at 1273 K in 4 s which is the gas residence time for drop tube furnace experiment. These two formed Cr vapors of CrO2(OH) and CrO(OH)2 react with CaO via reactions Eqs (4) and (5). 4CrO2(OH)(g)+4CaO+O2(g)→4CaCrO4+2H2O(g)

∆ G = - 468.679 KJ

(4)

2CrO(OH)2(g)+2CaO+O2(g)→2CaCrO4+2H2O(g)

∆ G = - 324.906 KJ

(5)

3.2.2 The influence of H2O on Cr retention and speciation in ash during oxy-coal combustion The combustion experiments of Cr-doped coal with doping different percent of H2O were conducted in drop tube furnace at 1273 K to clarify the influence of H2O on Cr transformation and retention by different minerals in coal itself. The variation of Cr retention rate in solid ashes collected from oxy-coal combustion with different H2O concentrations was demonstrated in Figure 5. Clearly, the recovery of Cr in solid ash was significantly increased from approximately 19.5 % for 27% O2/CO2 combustion (oxy) to 38 % for the case of oxy-5%H2O and 35.6 % for oxy-8%H2O at 1273 K, indicating steam promoted the interactions between Cr-bearing species and mineral grains in the coal. The speciation of Cr and their fractions in ash analyzed through XPS curve-fitting were indicated in Figure 6. The corresponding XPS spectra for coarse and fine ashes were illustrated in Figure S2 and Figure S3 of SI. Comparing with Cr-doped coal combustion in 27% O2/CO2 atmosphere, it can be seen that doping


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H2O caused a remarkable decrease of trivalent CaCr2O4 and an increase of hexavalent CaCrO4 in the solid ash. In the meantime, K2Cr2O7 was also increased upon doping H2O. The improved hexavalent CaCrO4 and K2Cr2O7 should be attributed to the oxidation of Cr species by H2O, which affected the interaction of Cr vapors with minerals in coal, promoting the formation of chromate during combustion. Besides, the fraction of FeCr2O4 was obviously improved from 21.4 % to 32.4 % for oxy-coal combustion with 8% H2O. On the basis of previous discussions, it can be known that during oxy-coal combustion with steam doping, the oxidation of pyrite should be inhibited by the increased reducing gas including carbon monoxide (CO) and H2 derived from char-H2O gasification, which resulted in the existence of low valence iron oxide such as FeO, promoting the capture of Cr-bearing vapors to form FeCr2O4 according to the reaction Eq. (3). Clearly, apart from oxidation of Cr vapors, H2O also had influences on the behavior of minerals including Fe- and Ca- minerals, which will be discussed in details in the following part.

3.2.3 The influence of H2O on Cr capture by minerals during oxy-coal combustion Experiments to clarify the impact of steam on the capability of minerals in terms of capture Cr were conducted through adding extra CaO or limestone or Fe2O3 to Crdoped coal during DTF combustion. To mimic the dry recycle oxy-fuel combustion process, 8 % H2O was doped into O2/CO2 atmosphere to investigate its impact on Cr retention and speciation. The retention of Cr with and without H2O was illustrated in Figure 7. For Cr-doped coal combustion only, doping 8% H2O enhanced Cr retention in the solid ash from 19.5 % to 35.6 %. From Figure 7, it can be concluded that doping H2O to flue gas inhibited the capability of CaO in terms of capture Cr-bearing


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vapors, contributing to a remarkable decrease in the Cr retention ratio from 86 % for Cr-doped coal with CaO addition to 52 % for doping H2O to Cr-doped coal + CaO. Steam enhanced the carbonation of CaO during oxy-fuel combustion by formation of the unstable intermediate product of Ca(HCO3)2 in the reaction process26, the activation energy of which is much lower than that of the reaction between CaO and CO2. By contrast, regarding limestone, the retention of Cr was increased significantly upon doping H2O to flue gas during oxy-fuel combustion. As shown in Figure 7, with steam doping, the retention of Cr in solid ash by the addition of limestone was enhanced to 48 %, which reached a similar Cr retention level of 52 % for adding CaO to coal. On one hand, the decrease of CO2 partial pressure by doping steam promoted the decomposition of limestone to CaO under oxy-fuel combustion. On the other hand, steam increased sorbent reactivity by changing its morphology, shifting the pore volume to larger pores, resulting in a structure which could provide a better gas-solid contact in the internal voids27. Moreover, for Fe2O3, steam favored its effect on capture Cr-bearing vapors under oxy-coal combustion. It is due to the enhanced ferrous iron formation from Fe3+ reduced by carbon monoxide (CO) and H2 derived from char-H2O gasification28 when doping H2O to oxy-coal combustion.

Table 2 illustrated the chromium speciation and their fractions (through XPS curvefitting) in coarse and fine ashes collected from Cr-doped coal, Cr-doped coal + CaO and Cr-doped coal + CaO + 8% H2O under oxy-coal combustion in DTF at 1273 K. As can be seen in Table 2, Ca-Cr compounds including trivalent CaCr2O4 and hexavalent CaCrO4 were the dominating chromium compounds in the ash collected from Cr-doped coal combustion, occupying 38.04 % of total Cr in coarse ash and 46.58 % of total Cr in fine ash. The addition of CaO remarkably favored the fractions


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of both trivalent CaCr2O4 and hexavalent CaCrO4 in either coarse ash or fine ash due to the reactions between Cr-bearing vapors and CaO. Besides, chromium bonding with Na, Fe in the coal and Cr2(SO4)3 were accordingly reduced with CaO addition as shown in Table 2. Moreover, after doping steam to Cr-doped coal + CaO case, the formed CaCr2O4 and CaCrO4 in ash were both reduced (the generation of CaCrO4 was decreased although its fraction increase), providing evidence supporting the inhibited effect of steam on CaO. Steam promoted CaO transformation to the unstable intermediate products of Ca(OH)2 and Ca(HCO3)2, which reduced the reaction extent between CaO and Cr-bearing vapors. Furthermore, it can be seen from Table 2, the fraction of FeCr2O4 was clearly improved after doping steam to Cr-doped coal + CaO case, attributing to the promoted effect of H2O on iron oxide capture Cr vapors under oxy-coal combustion. Apparently, CaO enhanced the fraction of toxic hexavalent Cr(VI) in ash, especially together with steam in the flue gas, the Cr(VI) fraction of the total Cr in ash was reached over 40 % as shown in Table 2.

However, as illustrated in Figure 5, with the fraction increase of H2O doped into flue gas from 5 % to 20 %, the recovery of Cr in solid ash was decreased. It was attributed to the effect of H2O on CaO which was formed from quick decomposition of CaCO325 (mineral grains in coal) during combustion. H2O promoted the transformation of CaO to the unstable intermediate products of Ca(OH)2 and Ca(HCO3)2 during oxy-coal combustion, which reduced the reaction extent between CaO and Cr-bearing vapors. Upon doping 8 % H2O, FeCr2O4 accounted for 30 % of the total Cr in Figure 6 and only 27.45 % Ca-Cr compounds (calcium chromite plus calcium chromate) were formed. It can be speculated that, with steam increase, the fraction of Ca-Cr compounds would further decrease.


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3.3 Competition effect of H2O, HCl and SO2 on chromium transformation during oxy-coal combustion Apart from H2O, either 500 ppmV HCl or 1000 ppmV SO2 doped to the O2/CO2 mixture was further conducted to clarify the competition effect of H2O, HCl and SO2 on Cr transformation and retention during oxy-coal combustion upon the recirculation of impurities in flue gas. The results for Cr retention were depicted in Figure 8. The Cr species fractions in coarse and fine ashes from XPS spectra fitting were illustrated in Table 3. The corresponding XPS spectra were illustrated in Figure S4 and Figure S5 of SI. As shown in Figure 8, different with the increase of Cr retention by H2O, doping HCl to flue gas significantly reduced Cr retention from 31 % for oxy-8% H2O case to 17.5 % for oxy-8%H2O-500ppmHCl in coarse ash. The presence of HCl in flue gas favored the formation of gaseous Cr species consisting primarily of CrOxCly such as CrO2Cl229, 30 and some CrCl3, which have a lower boiling point and a lower reaction rate with minerals than the corresponding oxide/hydroxide. These Cr gaseous species mostly escaped the collection system as fugitive species rather than condensing into solid ash particles. As indicated in our previous investigation11, the existence of HCl in flue gas (without H2O) reduced the retention of Cr in ash, however, when HCl was coexistent with H2O in flue gas, the Cr retention was even a little higher than that of the oxy case (without HCl and H2O) (see Figure 8), supporting the presence of H2O offset the effect of HCl on Cr recovery. Moreover, the addition of SO2 (oxy-8%H2O-1000ppm SO2) further promoted Cr retention in either coarse ash or fine ash, comparing to oxy-8% H2O case. It was attributed to the performed sulfation reaction between chromium vapors and SO2, which contributed to the Cr2(SO4)3 formation and condensation into coarse ash and fine ash. As can be seen


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from Table 3, doping SO2 remarkably improved Cr2(SO4)3 fraction from 11.66 % in coarse ash and 10.24 % in fine ash for oxy-8% H2O case to approximately 40 % in either coarse or fine ash for oxy-8% H2O-1000ppm SO2 condition. Furthermore, it indicated that doping SO2 reduced the formation of toxic hexavalent Cr(VI) species such as CaCrO4 and K2Cr2O7 formed through gas-to-solid reactions, which is slower than the gas-to-gas reactions between SO2 and Cr vapors.

Regarding coexistence of H2O, HCl and SO2, namely oxy-8%H2O-500ppmHCl1000ppmSO2, as indicated in Figure 8, comparing with oxy-8%H2O-1000ppmSO2, adding 500ppm HCl led to Cr retention decreased from 37.7 % to 19.3 % in coarse ash, and from 12.5 % to 6.3 % in fine ash. Obviously, the retention of Cr for oxy8%H2O-500ppmHCl-1000ppmSO2 case was quickly dropped to the retention level with oxy-8%H2O-500ppmHCl, which was much lower than that for oxy-8%H2O1000ppmSO2. The comparison between these three cases provided strong evidence on the preferential reaction of Cr vapors with HCl compared to SO2. For the speciation of Cr in ash, as can be seen from Table 3, HCl inhibited the formation of Cr2(SO4)3. Moreover, the fraction increase of Ca-Cr and Na-Cr compounds with the presence of HCl indicated the formed primary gaseous species of CrCl3 and CrO2Cl2 reacted with minerals grains in coal (such as CaO) via the reactions Eq. (6) and Eq. (7). Whereas the lower mass of Ca-Cr and Na-Cr compounds comparing to oxy-8%H2O1000ppmSO2 further supported their lower reaction rate with minerals than the corresponding chromium oxide/hydroxide. CrO2Cl2(g)+2CaO→CaCrO4+CaCl2 2CrCl3 (g)+4CaO →CaCr2 O4 +3CaCl2


∆ G= - 88.549 KJ

(6)

∆G=-563.338KJ

(7)

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4. Conclusions The influence of steam and minerals on Cr transformation during oxy-coal combustion has been examined in a drop tube furnace at 1273 K. Besides, the additions of extra 500 ppmV HCl and 1000 ppmV SO2 to flue gas were also conducted to examine the competition of H2O, HCl and SO2 on Cr behaviors during oxy-coal combustion upon the recirculation of impurities in flue gas. It was found that CaO and Fe2O3 exhibited high capability on capturing Cr-bearing vapors during combustion compared to limestone and kaolin. Changing air to oxy-coal combustion obviously weakened the role of CaO, whereas promoted Fe2O3 reaction with Cr vapors. The existence of H2O promoted the retention of Cr in solid ash during oxycoal combustion. Steam facilitated the oxidation of trivalent Cr(III) to high valance Cr vapors. CaO, particularly in the co-existence of H2O during coal combustion, remarkably enhanced the toxic Cr(VI) fraction in solid ash. H2O affected the behavior of minerals reacted with Cr. CaO was inhibited and Fe2O3 was enhanced by steam to react with Cr during oxy-coal combustion. With H2O fraction increase from 5% to 20% during oxy-coal combustion, the reduction of Cr retention in ash was mainly attributed to the negative effect of H2O on CaO capture Cr. HCl significantly promoted the release of Cr through chlorination reaction. The presence of H2O offset the negative effect of HCl on Cr recovery when H2O was coexistent with HCl in flue gas. SO2 favored the retention of Cr via sulfation reaction to form Cr2(SO4)3 condensed into solid ash. Moreover, it was found Cr vapors preferred to react with HCl compared with SO2 during combustion.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51406062), the International Science and Technology Cooperation Program of China (2015DFA60410) and the National Natural Science Foundation of China (No. 51776112). The Fundamental Research Funds of Shandong University (2016TB001) is also acknowledged.

Supporting Information In this section, the preparation of Cr-Gly, the Cr doping procedure, the compositions of kaolin and limestone, the elementary reactions for kinetic simulations and the XPS spectra of Cr in ash samples were described.


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Page 21 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

References: (1) Baruthio, F. Biological Trace Element Research 1992, 32, 145-153. (2) Dayan, A. D.; Paine, A. J. Human & Experimental Toxicology 2001, 20 (9), 439451. (3) Huggins, F E; Shah, N; Huffman, G P; et al. Fuel Processing Technology 2000, 63(2), 79-92. (4) Costa, M.; Klein, C. B. Critical Reviews in Toxicology 2006, 36 (2), 155-163. (5) Huggins, F E; Najih, M; Huffman, G P. Fuel 1999, 78(2), 233-242. (6) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Progress in Energy and Combustion Science 2005, 31 (4), 283-307. (7) Chen J., Yao, H.; Zhang, P.; Xiao, L.; Luo, G.; Xu, M. Proceedings of the Combustion Institute 2011, 33, 2837-2843. (8) Rathnam, R. K.; Elliott, L. K.; Wall, T. F.; Liu, Y.; Moghtaderi, B. Fuel Processing Technology 2009, 90, 797-802. (9) Saastamoinen, J. J.; Aho, M. J.; Hämäläinen, J. P.; Hernberg, R.; Joutsenoja, T. Energy & Fuels 1996, 10, 121-133. (10) Jiao, F.; Wijaya, N.; Zhang, L.; Ninomiya, Y.; Hocking, R. Environmental Science & Technology 2011, 45, 6640-6646. (11) Chen, J.; Jiao, F.; Zhang, L.; Yao, H.; Ninomiya, Y. Environmental Science & Technology 2012, 46, 3567-3573. (12) Ebbinghaus, B B. Combustion & Flame 1993, 93(1-2), 119-137. (13) Roy, B; Wei, L C; Bhattacharya, S. Fuel 2013, 114(2), 135-142.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(14) Chen, J.; Jiao, F.; Zhang, L.; Yao, H.; Ninomiya, Y. Journal of Hazardous Materials, 2013, 261: 260-268. (15) Gale, T. K.; Wendt, J. O. L. Proc. Combust. Inst. 2005, 30, 2999-3007. (16) Yao, H.; Naruse, I. Proc. Combust. Inst. 2005, 30, 3009-3016. (17) Yao, H.; Mkilaha, I S N.; Naruse, I. Fuel 2004, 83, 1001-1007 (18) Davis, S. B.; Gale, T. K.; Wendt, J. O. L. Aerosol Sci. Technol. 2000, 32, 142151. (19) Rauret, G; Rubio, R; LÓPEZ-SÁNCHEZ, J F. International Jounal of Environmental Analytical Chemistry 1989, 36(2), 69-83. (20) Tessier, A; Campbell, P G C; Bisson, M. Analytical Chemistry 1979, 51(7), 844851. (21) Galbreath, K. C.; Toman, D. L.; Zygarlicke, C. J.; Pavlish, J. H. Energy Fuels 2000, 14(6), 1265-1279. (22) Zhang, L.; Jiao, F.; Binner, E.; Chen, L.; Bhattacharya, S.; Ninomiya, Y.; Li, C.Z. Proceedings of the Combustion Institute 2011, 33, 2795-2802. (23) Ruud, M. Fuel Processing Technology 1994, 39, 199-217. (24) Selçuk, N.; Gogebakan, Y.; Gogebakan, Z. Journal of Hazardous Materials 2006, 137, 1698-1703. (25) Chen, J; Yao, H; Zhang, L. Fuel 2012, 102, 386-395. (26) Yu, J; Zeng, X; Zhang, G; Zhang, J; Wang, Y; Xu, G. Environ. Sci. Technol. 2013, 47, 7514-7520.


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(27) Kavosh, M.; Patchigolla, K; Anthony, E. J.; Oakey, J E. Applied Energy 2014, 131, 499-507. (28) Hecht, E S; Shaddix, C R; Molina, A; Haynes, B S Proc Comb Inst 2011, 33, 1699-1706. (29) Linak, W P; Wendt, J. O. L. W. Progress in Energy and Combustion Science 1993, 19(2), 145-185. (30) Guo, B.; Kennedy, I. M. Combustion and Flame 2001, 126(1-2), 1557-1568.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 1 The properties of the low-rank coal Proximate analysis, air-dried, wt%

Ultimate analysis, air-dried, wt%

M

V

A

FC

C

H

N

S

O*

Total Cr ppm

13.62

33.92

12.22

40.24

55.44

4.37

0.55

1.07

12.74

35

Ash composition, wt% SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

TiO2

SO3

P2O5

7.83

7.68

2.99

29.49

8.65

0.17

10.43

0.40

24.72

0.03

*

: by difference


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Table 2 The fractions of Cr species in collected ash from Cr-doped coal combustion with and without CaO addition Fraction of Cr(VI) in the ash %

Fraction of Cr in the ash % Ash sample NaCrO2

FeCr2O4

Cr2(SO4)3

CaCr2O4 MgCr2O4

CaCrO4

K2Cr2O7

Cr（VI)

Cr-doped coal coarse ash

15.25

23.67

15.13

28.72

9.32

7.91

17.23

Cr-doped coal fine ash

14.50

14.84

15.75

33.53

13.05

8.33

21.38

Cr-doped coal+CaO coarse ash

0.13

9.68

0.02

61.99

21.01

7.17

28.18

Cr-doped coal+CaO fine ash

15.11

6.59

0.41

44.94

22.48

10.48

32.96

Cr-doped coal+CaO +8%H2O coarse ash

1.04

18.47

0.03

36.87

30.15

13.44

43.59

Cr-doped coal+CaO +8%H2O fine ash

8.56

19.33

0.01

29.24

28.29

14.55

42.84


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 3 The fractions of Cr species in collected ash from Cr-doped coal combustion doping with H2O, HCl and SO2 Fraction of Cr(VI) in the ash %

Fraction of Cr in the ash % Ash sample NaCrO2

FeCr2O4 Cr2(SO4)3

CaCr2O4 CaCrO4 MgCr2O4

K2Cr2O7

Cr（VI)

Oxy-8%H2O coarse ash

13.45

29.72

11.66

12.71

14.57

17.89

32.46

Oxy-8%H2O fine ash

12.16

32.22

10.24

8.21

20.45

16.72

37.17

Oxy-8%H2O-1000ppmSO2 coarse ash

10.69

25.56

40.86

12.49

2.00

8.41

10.41

Oxy-8%H2O-1000ppmSO2 fine ash

8.05

23.89

39.66

11.41

2.03

14.97

16.99

Oxy-8%H2O-500ppmHCl1000ppmSO2 coarse ash

15.78

27.03

17.64

19.71

6.90

12.93

19.84

Oxy-8%H2O-500ppmHCl1000ppmSO2 fine ash

17.95

26.77

11.28

21.91

10.52

11.57

22.09


Page 27 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure Captions Figure 1 Schematic drawing of the DTF reactor facility employed in this study. Figure 2 The retention of Cr in solid ash collected from DTF combustion with four different additives under (a) air combustion and (b) oxy-coal combustion. Figure 3 The fraction of Cr captured by four different additives under air and oxy-coal combustion. Figure 4 The transformation of Cr-bearing species in the Cr-O2-H2O system at 1273 K. Figure 5 Variation of Cr retention rate in ashes collected from oxy-coal combustion upon the increase of H2O concentration. Figure 6 The fraction of Cr species in the ash collected from oxy-coal combustion with and without 8% H2O Figure 7 Influence of H2O on Cr retention by CaO, limestone and Fe2O3 under oxycoal combustion. Figure 8 Variation of Cr retention in ashes during oxy-coal combustion upon doping H2O, HCl and SO2


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1 Schematic drawing of the DTF reactor facility employed in this study


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120

80 60 40

120

(a)

100 RE of Cr, %

100 RE of Cr. %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(b)

80 60 40

20

20

0

0

Figure 2 The retention of Cr in solid ash collected from DTF combustion with four different additives under (a) air combustion and (b) oxy-coal combustion.


Energy & Fuels

95

air oxy-fuel

% of Cr captured

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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75

55

35

15

-5 Cr-doped coal+CaO

Cr-doped coal+limestone

Cr-doped coal+Fe₂O₃

Cr-doped coal+kaolin

Figure 3 The fraction of Cr captured by four different additives under air and oxycoal combustion


Page 31 of 35

100 Fraction of Cr-bearing species, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

80 CrO₃ CrOOH CrO(OH)₂ CrO₂(OH) CrO₂(OH)₂

60

40

20

0 0

7.88E-06

0.003

t, s

0.539

3.139

5.739

Figure 4 The transformation of Cr-bearing species in the Cr-O2-H2O system at 1273 K


Energy & Fuels

50

40 RE of Cr, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

30

20

10

0

Figure 5 Variation of Cr retention rate in ashes collected from oxy-coal combustion upon the increase of H2O concentration.


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Page 33 of 35

40 Fraction of Cr-bearing species, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Oxy Oxy-8%H₂O

30

20

10

0

Figure 6 The fraction of Cr species in the ash collected from oxy-coal combustion with and without 8% H2O


Energy & Fuels

120 without 8%H₂O 100

RE of Cr, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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with 8%H₂O

80 60 40 20 0 Cr-doped coal

Cr-doped coal+CaO

Cr-doped coal+limestone

Cr-doped coal+Fe₂O₃

Figure 7 Influence of H2O on Cr retention by CaO, limestone and Fe2O3 under oxycoal combustion


Page 35 of 35

50 40 RE of Cr, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Coarse ash Fine ash

30 20 10 0

Figure 8 Variation of Cr retention in ashes during oxy-coal combustion upon doping H2O, HCl and SO2


Graphic for abstract:

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