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Further insight into the formation and oxidation of CaCr2O4 during solid fuel combustion Hongyun Hu, Mengya Shi, Yuhan Yang, Huan Liu, Mian Xu, Junhao Shen, and Hong Yao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05538 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018
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Further insight into the formation and oxidation of CaCr2O4 during solid fuel
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combustion
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Hongyun Hua, Mengya Shia, Yuhan Yanga, Huan Liua,b, Mian Xua, Junhao Shena,
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Hong Yao a,b,*
5
a
6
Huazhong University of Science and Technology, Wuhan 430074, China
7
b
8
Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering,
Department of New Energy Science and Engineering, School of Energy and Power
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*
Corresponding author. Tel & Fax: +86-27-87545526(O) E-mail:
[email protected] (Prof. Hong Yao) 1
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Abstract
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The control of toxic chromate (Cr6+) formation is still a significant challenge in
12
solid fuel combustion. In particular, the mechanism of chromium transformation from
13
Cr3+ to chromate or other unoxidized forms remains unclear. The present study
14
confirms the formation of a significant unoxidized Cr-containing compound
15
CaCr2O4(Cr3+) during solid fuel combustion. Experiments were conducted, for the
16
first time, to clarify the mechanism of CaCr2O4 oxidation, which is quite different
17
from Cr2O3 oxidation. The findings demonstrate that CaCr2O4 was formed at
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temperatures above 1200 K, through rapid decomposition of CaCrO4 or slow and
19
direct interaction between CaO and Cr2O3. Compared to Cr2O3, CaCr2O4 could be
20
oxidized at lower temperatures under the influence of free CaO. In the absence of free
21
CaO, the oxidation of CaCr2O4 was minimal; however, in the presence of CaSO4,
22
calcium in the form of CaCr2O4 participated in the oxidation of CaCr2O4. Thus,
23
chromium in the form of CaCr2O4 was more likely to be oxidized when
24
CaCr2O4-containing fly ash was reheated. Fortunately, CaCr2O4 showed slight
25
basicity on the surface, allowing it to react with acidic gases. Accordingly, measures
26
were proposed to suppress the oxidation of CaCr2O4 by stimulating the reactions
27
between CaCr2O4 and acidic substances, like SO2 and Si/Al-compounds. These
28
compounds competed with chromium at high temperatures to react with calcium in
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the fly ash and in CaCr2O4. As a result, the unoxidized chromium was transformed
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into highly stable Cr2O3 or Ca3Cr2 (SiO4)3.
2
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TOC/Abstract Art
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Hated Ash
Cr2O3/Ca3Cr2 (SiO4)3
Cr5+/Cr6+
Reheating
+ Si/Al-compounds CaSO4
+ CaO
Oxidation Suppression (I)
Oxidation (II) Oxidation (I)
SO2
CaSO4 CaCr2O4
Ash Residues
Combustion
+ CaO
Decomposition CaCrO4
Cr2O3
+ CaO + CaO
Solid Fuel
CaSO4 Oxidation Suppression (II)
Organic-Cr / Cr2O3
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3
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Introduction
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Chromium is widely distributed in coal, municipal solid waste, sewage sludge
35
and other solid fuels 1, 2. In China, a large amount of chromium is released within flue
36
gas into atmosphere during solid fuel combustion 3, which is a great threat to human
37
health. Even so, most of the chromium is transferred into ash residues, predominantly
38
as trivalent chromium (Cr3+) 2. More importantly, trivalent chromium can partly
39
oxidize to form more toxic hexavalent chromium (Cr6+), which is classified as a group
40
A inhalation carcinogen 4. Compared with Cr3+, Cr6+ is more easily leached from ash
41
residues, leading to serious water and/or soil pollution 5. Therefore, a series of
42
stringent policies were implemented by the Chinese government for emission control
43
of Cr6+ 6.
44
Generally, the predominant form of chromium in solid fuels is Cr3+, and it is
45
difficult to oxidize during fuel combustion 7, 8. However, a large fraction of Cr6+ was
46
found in the fly ash during the combustion of solid fuels that contained high contents
47
of alkali and/or alkaline earth metal compounds 1, 9. Among these substances, CaO is
48
confirmed to facilitate Cr3+ oxidation over a wide temperature range 10, 11. Conversely,
49
CaO is widely used as a sorbent for the capture of acid gases, such as SO2, and toxic
50
trace elements, such as As2O3, from flue gas
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conditioner for dewatering of sewage sludge
52
ubiquitous use, CaO is frequently introduced into solid fuels, and it is essential to
53
understand the oxidation of Cr3+ in the presence of CaO.
54
12, 13
. CaO is also commonly used as a
14
. Because of its prevalence and
To date, several studies have been conducted to understand the mechanisms 4
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involved in Cr3+ oxidation in the presence of CaO. The oxidation of Cr3+ was found to
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be a slow process, and most chromium still existed as Cr3+ in the ash produced in the
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combustion of coal and waste sludge 8, 11. Nevertheless, the unoxidized Cr3+ may
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transfer into various species
59
distinguished these unoxidized species or examined their potential oxidizing capacity.
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When ash residues containing unoxidized Cr3+ are reheated at high temperature, the
61
oxidation of unoxidized Cr3+ compounds likely determines the Cr6+ concentration in
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the products, such as in the case of fuel combustion in a circulating fluidized bed or in
63
the case of ash thermal treatment for the production of building materials.
5, 15
, and to our knowledge, few studies have
64
Our previous study found that CaCr2O4 was an important intermediate in
65
chromium oxidation in the presence of CaO 16. Unlike Cr2O3, CaCr2O4 is composed
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of both chromium and calcium. The details regarding the oxidation performance of
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chromium in CaCr2O4 are still unclear and are surely different from that of Cr2O3,
68
according to our former experiments. On the other hand, it is possible that the calcium
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in CaCr2O4 participated in chromium oxidation, and its behavior might differ from
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that of free CaO. In addition, free CaO could react with acid gases and/or acidic
71
inorganic components, such as Si/Al-compounds, in the ash
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compete with chromium oxides to react with free CaO. As a result, the amount of free
73
CaO available for chromium oxidation decreases. Based on these theories, Mao et al.
74
limited the potential of CaO to promote Cr2O3 oxidation by converting free CaO into
75
phosphate minerals 18. Similarly, Low et al. suppressed the oxidation of Cr2O3 through
76
scavenging of free lime by reaction with kaolinite
19
17
. These substances
. Therefore, the fate of calcium
5
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likely determined the oxidation of CaCr2O4, and measures could be taken to inhibit
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CaCr2O4 oxidation based on our understanding of calcium behavior in the process.
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The aim of this work is to investigate the formation of CaCr2O4 in the Ca-Cr-O
80
system and to evaluate the oxidation of CaCr2O4 in simulated flue gas. In this study,
81
CaCr2O4 was synthesized and well characterized. Then, the oxidation mechanism of
82
CaCr2O4 was determined, considering the effects of free CaO and CaSO4. The fate of
83
calcium in CaCr2O4 was also explored, especially in reactions with SO2 and
84
Si/Al-compounds. Based on our findings, several measures were proposed to suppress
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the oxidation of CaCr2O4 during reheating of CaCr2O4-containing fly ash.
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Experimental Procedures
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Materials. Analytical grade reactants, including CaO, Cr2O3, and CaSO4, were
88
used as raw materials for the investigation of chromium oxidation and for the
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preparation of test samples: CaCrO4 (Cr6+), Ca3(CrO4)2 (Cr5+) and CaCr2O4 (Cr3+)).
90
To determine the fate of CaCr2O4 during the thermal treatment of fly ash, ash residues
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were collected from a municipal solid waste incineration plant. Water-washed ash and
92
acid-washed ash samples were obtained after washing the raw fly ash with deionized
93
water and dilute acid solution, respectively. The properties of the ash samples are
94
given in Table S1 and Figure S1 (shown in the Supporting Information). To prepare
95
the mixed samples, all reactants were thoroughly mixed and finely ground prior to the
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reaction.
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Oxidation of Chromium Oxides. The oxidation of chromium oxides was
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conducted in a horizontal tube furnace, and a thermogravimetric analyzer and in situ 6
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high temperature X-ray powder diffraction (XRD) instrument were used to
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characterize the oxides. The testing temperature, heating rate and atmosphere were
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well controlled, and the detailed conditions for chromium oxidation in each sample
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are summarized in Table S2.
103
After the reactions, some Cr-products, like CaCrO4, were directly obtained after
104
cooling. However, for CaCr2O4, some by-products were removed to obtain high purity
105
CaCr2O4. As stated in the following discussion section, CaCr2O4 was formed either
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through the decomposition of oxidized CaCrO4 (labeled as CaCr2O4-I) or through the
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interaction between CaO and Cr2O3 (labeled as CaCr2O4-II). Based on the leaching
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abilities of various Cr-products
109
CaCrO4 by dissolving the formed Ca3(CrO4)2 with deionized water. Similarly,
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CaCr2O4, formed through the interaction between CaO and Cr2O3, was purified by
111
removing excessive CaO from the products with diluted HCl solution.
20
, CaCr2O4 was separated from the decomposed
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Characterization of Cr-products. To determine the crystalline structures of the
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Cr-products, both traditional and in situ high temperature XRD were employed. The
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microstructure and surface basicity of the samples were determined by scanning
115
electron microscope (SEM) and CO2 temperature-programmed desorption (CO2-TPD)
116
techniques, respectively. To get more details of the Cr-samples, transmission electron
117
microscope (TEM) observations were conducted. Before analysis, the samples were
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thoroughly dispersed over a standard glass slide and pressed by a copper grid coated
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with amorphous carbon film.
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After CaCr2O4 reacted with CaSO4, the formed chromate was leached out of the 7
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products with deionized water, due to the high leaching ability of chromate. The
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oxidation of CaCr2O4 was evaluated by measuring the concentration of chromium
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using microwave plasma-atomic emission spectroscopy (MP-AES). However, after
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the reaction of Cr2O3 with CaO in N2, the distribution of the formed CaCr2O4 and
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unreacted Cr2O3 in the product could not be distinguished because of their similar
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leaching abilities. The transformation of Cr2O3 into CaCr2O4 was evaluated according
127
to the contents of Ca and Cr in the products, as detected by X-ray fluorescence (XRF).
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Specifically, the fraction of chromium in the form of CaCr2O4 was calculated based
129
on the content of Ca in the sample, and the remaining chromium was in the form of
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Cr2O3.
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Results and Discussion
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Formation of CaCr2O4. Thermogravimetric experiments on the decomposition
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of CaCrO4 were conducted in carrier gas containing various concentrations of oxygen,
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and the TG/DTG curves are shown in Figure 1. The concentration of oxygen played
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an important role in the decomposition of CaCrO4. The initial temperature of CaCrO4
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decomposition decreased with decreasing O2 concentration. In an oxygen-depleted
137
atmosphere (pure N2), the initial decomposition temperature was as low as 1186 K.
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Figure S2 presents the in situ XRD patterns of the products formed during the
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decomposition of CaCrO4 in air. It was clear that CaCrO4 decomposed to form
140
Ca3(CrO4)2(Cr5+) and CaCr2O4 (CaCr2O4-I, Eq. (1)). According to Eq. (1), 50% of
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chromium was transformed into CaCr2O4 in the decomposition of CaCrO4.
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4CaCrO4 → Ca3(CrO4)2 + CaCr2O4 + 2O2 8
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Alternatively, the formation of CaCr2O4 might occur through the direct
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interaction between CaO and Cr2O3. From TG-DSC curves (not shown here), a weak
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endothermic peak was observed between 1250 K and 1350 K, which was assigned to
146
the formation of CaCr2O4 (CaCr2O4-II, Eq. (2)). To obtain more detailed information,
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the crystalline structures of the CaO/Cr2O3 mixtures were recorded via in situ high
148
temperature XRD during the heating process under vacuum. The heating program is
149
given in Figure S3. Figure 2 shows the patterns of the products formed at each
150
temperature. The results suggest that the formation of CaCr2O4 through direct
151
interaction between CaO and Cr2O3 was more difficult than via decomposition of
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CaCrO4. The direct formation of CaCr2O4 began at higher temperature (around 1300
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K), and the process was quite slow, as evident by the lack of peaks for CaCr2O4 even
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after heating at 1323 K for 10 min.
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CaO + Cr2O3 → CaCr2O4
(2)
156
To limit the effect of physical contact between the reactants on CaCr2O4
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formation, the interactions between CaO and Cr2O3 were conducted with excess CaO.
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Figure 3 shows the fractions of formed CaCr2O4 as a ratio of total chromium after
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thermal treatment at temperatures ranging from 1223 K to 1423 K. A similar
160
phenomenon for the slow formation of CaCr2O4 was observed. Additionally, the
161
formation of CaCr2O4 generally took place at higher temperature. At each temperature,
162
limited amounts of CaCr2O4 formed in a short reaction time, and the formation of
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CaCr2O4 was remarkably stimulated by increasing the reaction time from 1 to 5 min.
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Further increasing the reaction time from 5 to 10 min resulted in a continuous increase 9
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in the CaCr2O4 fraction in the products obtained at 1223 and 1323 K. However, few
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changes were found for CaCr2O4 formation at 1423 K. After heating at 1423 K for 10
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min, about 10% of chromium still remained unreacted, which was mainly caused by
168
insufficient contact between CaO and Cr2O3 or other unknown factors.
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According to the results, the formation of CaCr2O4 occurred in the combustion of
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solid fuel either through the rapid decomposition of CaCrO4 or through the slow and
171
direct interaction between CaO and Cr2O3. Several studies also stated the formation of
172
CaCr2O4 in solid fuel combustion fly ash
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regarding CaCr2O4 formation, CaCr2O4 was separated from each product by
174
dissolving chromate and excess CaO. After purification, CaCr2O4 was characterized.
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Although the samples showed similar XRD patterns, the microstructures differed in
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various CaCr2O4 products (shown in Figure S4). More specifically, CaCr2O4-I was
177
needle/rod-shaped, while CaCr2O4-II was granular. As shown in Figure 4, Cr2O3,
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CaCr2O4-I and CaCr2O4-II had interplanar distances of 2.52, 2.96 and 2.45 Å,
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corresponding to the crystal planes (110), (130) and (021), respectively. Different
180
microstructure characteristics of CaCr2O4 might lead to various oxidation
181
performances.
15, 21
. To get more detailed information
182
Oxidation of CaCr2O4. Figure S5 depicts the TG/DTG curves for the oxidation
183
of three Ca-Cr mixtures: CaO with Cr2O3, CaCr2O4-I or CaCr2O4-II. The initial
184
oxidation temperatures progressed in the following order, Cr2O3 (973 K) > CaCr2O4-I
185
(823 K) > CaCr2O4-II (773 K), indicating that the oxidation of CaCr2O4 could take
186
place at lower temperatures than the oxidation of Cr2O3. However, the oxidation of 10
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CaCr2O4 was slower, and this was likely due to the various forms of calcium in the
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reactants. It is assumed that calcium in free CaO and CaCr2O4 play different roles in
189
the oxidation of chromium. However, the oxidation of CaCr2O4-I and CaCr2O4-II had
190
slight difference in the oxidation capacities. In addition, a sharp mass-loss peak
191
(centered at approximately 1323 K) was recorded in each curve, which was attributed
192
to the decomposition of oxidized chromate.
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To further clarify the mechanism involved in the oxidation of CaCr2O4, the XRD
194
patterns of the oxidized products, in terms of heating temperature and Ca/Cr molar
195
ratio, were obtained, and the results are displayed in Figure S6. At a molar ratio of 1:1
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Ca (including calcium from CaCr2O4 and added free CaO) to Cr, most chromium
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remained unoxidized, especially at 1373 K. The results suggest that calcium in the
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form of CaCr2O4 could not play the same role as free CaO in the promotion of
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chromium oxidation. Additionally, the oxidation of CaCr2O4 was mainly determined
200
by free CaO, which was further confirmed by evaluating the oxidation of CaCr2O4
201
mixed with varied free CaO content at 1373 K. From the TG results, shown in Figure
202
5, few changes were found regarding the mass of the sample in the absence of free
203
CaO, demonstrating that calcium in CaCr2O4 did not promote the oxidation of
204
chromium under these experimental conditions. The oxidation of CaCr2O4 was
205
observed upon the addition of free CaO in the sample, which was significantly
206
enhanced by increasing the free CaO content. Apart from the contents of free CaO, the
207
oxidation of CaCr2O4 was strongly affected by temperature. The forms of oxidized
208
chromium differed at various temperatures. As demonstrated by Figure S6, CaCr2O4 11
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was oxidized to CaCrO4 (Cr6+) at 1173 K (Eq. (3)) while at 1373 K CaCr2O4 formed
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thermally stable Ca3(CrO4)2 (Cr5+) (Eq. (4)). Compared to the formation of CaCrO4,
211
higher free CaO content was needed for the formation of Ca3(CrO4)2.
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CaCr2O4 + CaO + 1.5O2 → 2CaCrO4
(3)
213
CaCr2O4 + 2CaO + O2 → Ca3(CrO4)2
(4)
214
In the fuel combustion process, free CaO probably reacted with acid gases and
215
was rapidly transformed into corresponding Ca-salts 22. CaSO4 was widely distributed
216
in solid fuels and was also the dominant Ca-compound in the fuel combustion.
217
Therefore, chromium oxides were more likely to interact with CaSO4 rather than free
218
CaO, and the oxidation of CaCr2O4 under the influence of CaSO4 should be further
219
investigated. Figure 6 compares the TG/DTG curves for the oxidation of Cr2O3 or
220
CaCr2O4 affected by CaSO4. For the mixture of CaCr2O4 and CaSO4, one mass-gain
221
peak was observed between 1000 K and 1200 K, which was probably related to the
222
oxidation of chromium. In addition, the mass-loss peak around 1350 K was attributed
223
to the decomposition of oxidized products. In contrast, the oxidation of Cr2O3 was
224
minimal, and the mass of the Cr2O3-containing sample slowly decreased with
225
increasing heating temperature. A significant mass-loss peak was observed at 1323 K,
226
and this was likely caused by the release of chromium in the form of Cr2(SO4)3 23, due
227
to the interactions between Cr2O3 and CaSO4 at this high temperature.
228
To further confirm the oxidation of CaCr2O4 in the presence of CaSO4, CaCr2O4
229
was heated under air in a horizontal tube furnace after mixing with various contents of
230
CaSO4 to obtain certain Ca/Cr ratios. The oxidation of CaCr2O4 was evaluated by 12
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measuring highly soluble chromate and the percentage of oxidized chromium at
232
different temperatures, and these results are presented in Figure 7. Without the
233
addition of CaSO4, the oxidation of CaCr2O4 was minimal. After CaSO4 addition, a
234
small fraction of CaCr2O4 was oxidized at 873 K, and the oxidation of CaCr2O4 was
235
strongly stimulated by increasing temperature. About 40% of the chromium in
236
CaCr2O4 was oxidized after heating at 1173 K at a molar ratio of 1:1 Ca to Cr.
237
Interestingly, an increase in the CaSO4 content slightly affected the amount of
238
oxidized CaCr2O4. The XRD patterns of the products (given in Figure S7) show that
239
CaCrO4 was the main oxidized compound, and the unoxidized chromium was
240
transformed into Cr2O3. It is worth noting that CaSO4 remained the predominant
241
crystalline structure in the products. Therefore, CaSO4 was not instrumental in the
242
oxidation of CaCr2O4, but CaSO4 played a significant role in this process, which
243
promoted the transformation of CaCr2O4 into CaCrO4 and Cr2O3 (Eq. (5))
244
2CaCr2O4 + 1.5O2 → 2CaCrO4 + Cr2O3
(5)
245
Inhibition of CaCr2O4 Oxidation. As stated before, CaCr2O4 was more easily
246
oxidized than Cr2O3, even in the absence of free CaO. Therefore, conversion of
247
CaCr2O4 into Cr2O3 is a promising way to lower the oxidation capacity of chromium.
248
However, the problem lies in the removal of calcium from CaCr2O4. It is well known
249
that free CaO is a strong alkaline substance that tends to react with acid gases and
250
other acidic compounds at high temperature
251
might have a similar reaction performance to free CaO, which is mostly determined
252
by the surface basicity of CaCr2O4.
13, 17
. Calcium in the form of CaCr2O4
13
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For comparison, the basic sites on the surface of CaCr2O4, Cr2O3 and CaO/Cr2O3
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mixtures (molar ratio 1:1) were determined by CO2-TPD, and the resulting profiles
255
are given in Figure S8. A broad desorbed peak of CO2 was clearly observed for Cr2O3
256
in the temperature range of 573 to 1073 K, which was mainly assigned to strong basic
257
sites (related to unsaturated O2- ions) 24, 25. The strong basic sites stimulated reactions
258
with acidic compounds at high temperature. After the addition of free CaO, a large
259
amount of CO2 evolution was found at temperatures between 673 and 873 K, which
260
was related to great moderate basic sites (corresponding to M-O pairs)
261
contrast, apart from the strong basic sites, CaCr2O4 contained some moderate basic
262
sites on the surface, as evident by the several weak desorption peaks at low
263
temperatures. As a result, CaCr2O4 might show similar performance to CaO in a
264
reaction with acidic compounds.
25, 26
. In
265
Figure S9 shows the XRD patterns of CaCr2O4 after reaction with SO2. It was
266
found that CaCr2O4 could react with SO2 by forming Cr2O3 and CaSO4 at 1173 K and
267
1373 K (Eq. (6)). However, the reaction was strongly suppressed at 973 K.
268
Furthermore, the oxidation of CaCr2O4 was hardly observed, even in the presence of
269
formed CaSO4, demonstrating that the sulphonation of CaCr2O4 occurred more easily
270
than the oxidation of CaCr2O4.
271
CaCr2O4 + SO2 + 0.5O2 → CaSO4 + Cr2O3
(6)
272
Similarly, the heating temperatures had an obvious effect on the interactions
273
between CaCr2O4 and Si/Al-compounds. Figure 8 displays the XRD patterns of
274
thermally treated mixtures containing CaCr2O4 and Si/Al-rich fly ash. At 973 K and 14
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1173 K, Si/Al-compounds hardly reacted with calcium compounds including CaCr2O4.
276
Meanwhile, calcium compounds in ash, such as calcite, stimulated the oxidation of
277
CaCr2O4 by forming CaCrO4, especially in the Ca-rich water-washed fly ash (shown
278
in Table S1). In the acid-washed fly ash, calcite was removed and the remaining
279
calcium was mainly in the form of CaSO4. The formation of CaCrO4 in the
280
acid-washed fly ash further confirmed the promotion of CaCr2O4 oxidation under the
281
influence of CaSO4. Nevertheless, most chromium remained unoxidized in the form
282
of CaCr2O4 after thermal treatment with acid-washed fly ash.
283
In most cases, increasing heating temperature strongly enhanced the interactions 27, 28
284
between Si/Al-compounds and calcium
285
CaCr2O4 was transformed into calcium aluminosilicates (Eq. (7)), and unoxidized
286
chromium was in the form of Cr2O3 in the acid-washed fly ash. In the Ca-rich
287
water-washed fly ash, CaCr2O4 reacted with Si/Al-compounds, and some intermediate
288
products formed in the heating process (Eq. (8)). As a result, a fraction of the
289
chromium
290
Si/Al-compounds in fly ash competed with CaCr2O4 to react with Ca-compounds at
291
1373 K, which effectively suppressed the oxidation of CaCr2O4.
292 293 294
was
transformed
into
. After the reaction, calcium from
Ca3Cr2
(SiO4)3
(Eq.
CaCr2O4 + Si/Al-compounds → CaAl2SiO2 + Cr2O3 Ca-compounds +SiO2 → CaSiO3 CaCr2O4 + 2CaSiO3 +SiO2 → Ca3Cr2(SiO4)3
(9)).
Therefore,
(7) (8) (9)
295
Environmental Implications. Few studies have reported on the formation and
296
oxidation of CaCr2O4 in ash residues during solid fuel combustion. This is largely 15
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because of the low concentration of chromium in ash samples and limited analytical
298
methods. The findings in this study demonstrate that CaCr2O4 was probably formed
299
through the decomposition of CaCrO4 and/or the direct interaction between CaO and
300
Cr2O3. Compared to Cr2O3, CaCr2O4 was more easily oxidized to form more toxic
301
chromate species (Cr6+ or Cr5+), and this reaction could take place at low temperature,
302
around 773 K. Therefore, measures should be taken to suppress the formation of
303
CaCr2O4 during fuel combustion.
304
In solid fuel samples containing low free CaO content, the reaction between CaO
305
and Cr2O3 predominantly generated CaCr2O4. However, the reaction was very slow,
306
and decreasing the reaction time could effectively inhibit the formation of CaCr2O4.
307
Furthermore, considering the increased CaCr2O4 formation at high temperatures,
308
another promising way for the control of CaCr2O4 formation is to perform fuel
309
combustion at temperatures below 1200 K.
310
In Ca-rich fuel, chromium was probably oxidized to form calcium chromate
311
during combustion 1, 29. An effective method to suppress the oxidation of chromium is
312
to decrease the free CaO content, by separating Ca-rich components from the fuel or
313
enhancing the interactions of Si/Al- or P-compounds with free CaO 18, 19. In addition,
314
increasing the operating temperature could partly decrease the fraction of chromate in
315
ash residues. This effect was attributed to the rapid decomposition of oxidized
316
CaCrO4. As a result, a large fraction of unoxidized chromium was transformed to
317
CaCr2O4. Then, control of CaCr2O4 oxidation was the key issue, especially for ash
318
residue treatment via heating. When ash samples were reheated, CaCr2O4 showed 16
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great oxidation capacity for the formation of toxic chromate. Even in the absence of
320
free CaO, the oxidation of CaCr2O4 could occur in the presence of CaSO4, which was
321
widely prevalent in the fly ash during fuel combustion and/or desulfurization of flue
322
gas.
323
According to the results in this study, CaCr2O4 contained some basic sites on its
324
surface to support the reactions with acid gases or other acidic compounds. The
325
addition of sulfur-rich materials into the CaCr2O4-containing ash residue enhanced the
326
transformation of CaCr2O4 to CaSO4 and Cr2O3 at high temperature. In this way, SO2
327
produced in the heating process was captured in situ, and the oxidation of chromium
328
was suppressed. Another practical approach was to mix CaCr2O4-containing ash with
329
Si/Al-rich ash before thermal treatment. In this case, calcium from CaCr2O4 was
330
transformed into aluminosilicates, and the chromium remained unoxidized. However,
331
it is worth noting that Ca-compounds in ash might stimulate the oxidation of CaCr2O4,
332
especially at low temperatures like 973 K and 1173 K. Therefore, the operating
333
temperature should be well controlled to suppress CaCr2O4 oxidation by enhancing
334
the interactions between CaCr2O4 and Si/Al-compounds.
335
In summary, in order to control Ca-chromate formation in fuel combustion,
336
attention should be paid to the ash forming process as well as the ash treatment
337
process. The findings from this study broaden our understanding of the formation of
338
CaCr2O4 as well as its thermal behavior. Moreover, the control strategies for
339
chromium oxidation during fuel combustion could be optimized based on these
340
findings. 17
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Acknowledgments
342
The present work was supported by the National Natural Science Foundation of
343
China (51606075, 51406062, 51506064) and the General Financial Grant from the
344
China Postdoctoral Science Foundation (Grant, 2016M592330). We appreciate the
345
experimental measurements provided by the Analytical and Testing Center of
346
Huazhong University of Science and Technology.
347
References
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Figure captions
438
Figure 1. TG-DSC curves for the decomposition of CaCrO4 in terms of various
439
concentration of oxygen in the carrier gas
440
Figure 2. In situ high temperature XRD patterns of the mixture of CaO and Cr2O3
441
during thermal treatment under vacuum
442
Figure 3. The formation of CaCr2O4 during reactions between CaO and Cr2O3 in N2
443
at a molar ratio of Ca and Cr 4:1
444
Figure 4. TEM images and crystallites size measurements of Cr3+-compounds
445
Figure 5. Thermogravimetric analysis of CaCr2O4 oxidation at 1373 K with the
446
addition of various contents of free CaO
447
Figure 6. TG curves for the oxidation of Cr2O3 and CaCr2O4 under the influence of
448
CaSO4
449
Figure 7. Percentages of oxidized chromium in the heated mixtures of CaCr2O4 and
450
CaSO4 at various mole ratios of Ca and Cr (ND, not detected)
451
Figure 8. XRD patterns of the products from the reactions between CaCr2O4 and (a)
452
water-washed ash, (b) acid-washed ash
453
23
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TG / %
100 98
16 1337 K
TG curve
12
96
8
94
4
-1
20
In N2 with 20% O2
DSC / mW mg
102
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DSC curve
TG curve
96
6
94
4
92 2
90
DSC curve 0
In pure N2
6 1186 K
TG curve
4
TG / %
88 100 98 96 94 92 90 88 86 84
454
-1
8
DSC / mW mg
TG / %
98
1304 K
2
-1
100
0
In N2 with 5% O2
DSC / mW mg
92
DSC curve 0 373 473 573 673 773 873 973 1073 1173 1273 1373
Temperature / K
455
Figure 1. TG-DSC curves for the decomposition of CaCrO4 in terms of various
456
concentration of oxygen in the carrier gas
24
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2 2 1 2
2
Intensity / A.U.
1 3
1.CaO 2.Cr2O3 3.CaCr2O4
4
2
2
4. Experimental carrier
2 1
11
1323 K- 10min 1323 K-1min 1273K-1min 1223 K-1min 1173 K-1min
10 457
20
30
40
50 60 2-Theta / °
70
80
90
458
Figure 2. In situ high temperature XRD patterns of the mixture of CaO and Cr2O3
459
during thermal treatment under vacuum
460
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Cr(CaCr2O4)/Total Cr (%)
100 1223 K 1323 K 1423 K
80 60 40 20 0 1
461
3 5 Heating time / min
10
462
Figure 3. The formation of CaCr2O4 during reactions between CaO and Cr2O3 in N2
463
at a molar ratio of Ca and Cr 4:1
26
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Cr2O3 464
CaCr2O3-I
CaCr2O3-II
Figure 4. TEM images and crystallites size measurements of Cr3+-compounds
465
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108 CaCr2O4 without CaO
106
CaCr2O4 with CaO n(Ca/Cr) 1:1 CaCr2O4 with CaO n(Ca/Cr) 1.2:1
104 TG / %
CaCr2O4 with CaO n(Ca/Cr) 1.5:1
102 100 98 96
Heating to 1373 K in Argon
Adding 20 % O2
94 0 466
10
20
30 40 50 60 Retention time / min
70
80
90
467
Figure 5. Thermogravimetric analysis of CaCr2O4 oxidation at 1373 K with the
468
addition of various contents of free CaO
469
28
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100.5
TG / %
99.0
TG curve for the mixture of CaSO4 and CaCr2O4
97.5 96.0 94.5 93.0 373
470
TG curve for the mixture of CaSO4 and Cr2O3
573
773 973 Temperature / K
1173
1373
471
Figure 6. TG curves for the oxidation of Cr2O3 and CaCr2O4 under the influence of
472
CaSO4
473
29
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Percentages of oxidized chromium / %
Environmental Science & Technology
474
50 Without CaSO4 addition n(Ca):n(Cr) 1:1 n(Ca):n(Cr) 2:1
40 30 20 10 0
ND 873
ND 1023 Temperature / K
ND 1173
475
Figure 7. Percentages of oxidized chromium in the heated mixtures of CaCr2O4 and
476
CaSO4 at various mole ratios of Ca and Cr (ND, not detected)
30
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(a)
1.CaCrO4 2.CaCr2O4 3.Ca3Cr2(SiO4)3 4.Quartz SiO2
63 55 4 3 5 4 36
5 56
5.Cr2O3 6.Gehlenite CaAl2SiO7
3
55
1373 K
1 4
1
Intensity / A. U.
1 1
4 5
4
1
4 1 41
41 1
1173 K
1 4 1 4
4 2
10
20
1
1 41 5
30
1
40
(b)
Intensity / A. U.
50 60 2-Theta /°
70
80
90
4.Quartz SiO2 5.Cr2O3
4 5 3 4
24 1 4
4 2 1 20
973 K
1.CaCrO4 2.CaCr2O4 3.Anorthite CaAl2Si2O8
4
10
4
30
32 5
5
4
22
2
22 1
2
40
4
4
1373K
4
4
4
1173K
4
4
4
973K
50 60 2-Theta /°
70
80
90
477
Figure 8. XRD patterns of the products from the reactions between CaCr2O4 and (a)
478
water-washed ash, (b) acid-washed ash
31
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