Removal of HCl in Flue Gases by Calcined Limestone at High

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Removal of HCl in flue gases by calcined limestone at high temperatures Geng-Min Lin, and Chien-Song Chyang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01830 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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Removal of HCl in flue gases by calcined limestone at high temperatures Geng-Min Lin, Chien-Song Chyang*

Department of Chemical Engineering, Chung Yuan Christian University, Chung Li District, Taoyuan City, Taiwan 32023, R.O.C.

* Corresponding author: Tel.: +886 3 2654119. Fax: +886 3 4636242. E-mail: [email protected], [email protected] (C.S. Chyang)

Abstract Experimental study of dry HCl removal from simulated combustion flue gases using calcined limestone (CaO) is reported. The study was conducted in a unique fixed-bed reactor coupled with an online Fourier transform infrared (FTIR) spectrometer to investigate the effects of the presence of SO2, CO2 and O2 on the chlorination reactivity of calcined limestone. The dechlorination efficiency, defined by the effluent HCl concentration, would be used to describe the HCl absorption history to gain a better understanding of the chlorination behavior. The experimental results indicated that the HCl uptake capacity remains less affected under various gas atmospheres at 650 °C although the chlorination is found to be faster when CO2 is present. At temperatures of 750 or 850 °C, the presence of SO2 or O2 significantly decreases the reactivity of the calcined limestone toward HCl. The concurrent


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sulfation of chlorides, such as CaCl2·H2O and/or CaClOH, causes the subsequent re-release of HCl to the gas phase, thus reducing the attainable extent of the chlorination reaction. The presence of O2 impedes the conversion of calcined limestone to chlorides, presumably through some type of reaction involving the dechlorination of the sorbent particles. Keywords: Chlorination; Calcined limestone; Dechlorination efficiency; Chlorides

1. Introduction

The control of hydrogen chloride (HCl) emissions from hot flue gases is a matter of great importance and interest not only due to their polluting and corrosive nature but also their potential to chlorinate carbon. Chlorine is found in many commonly used fuels and abundantly present in medical and hazardous solid wastes and is primarily released in the form of HCl that leaves the stack following fuel combustion or waste incineration processes. Organic chloride, mostly found in the form of plastics, e.g., PVC and PVDC, is the dominant source of HCl emissions during solid waste incineration and has a greater potential than inorganic chloride to generate HCl.1-6 HCl might even combine with alkali or alkaline earth metals to form troublesome compounds, forming deposits and thus causing corrosion in the equipment.7, 8 Another severe problem caused by the presence of chlorine in flue gases is the emission of toxic chlorinated organic compounds, especially polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Dioxins and dioxin-like compounds pose serious threats to both


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human health and the environment due to their highly toxic natures. The general consensus is that HCl plays a key role in forming dioxin compounds. Molecular chlorine, usually originating from the oxidation of HCl through fly ash or metal catalysts (known as the Deacon reaction), is a well-recognized precursor for the formation of chlorinated aromatic compounds.9-12 It was even suggested that the amounts of PCDD/Fs formed using HCl and Cl2 are fairly equal.13 This shows the direct or indirect importance of HCl in forming dioxin compounds. Hence, HCl can be regarded as one of the most troublesome species among the acidic gases, meaning that the removal of HCl from hot flue gases is imperative. One feasible solution to achieve such an aim is to feed sorbents into a hot furnace for the direct capture of HCl. Economically, calcium-based sorbents are considered to be one of the most promising candidates because of their advantages of being inexpensive and abundantly available. Technically, the fluidized bed combustor (FBC) is particularly suited for in situ dechlorination by the direct injection of sorbents into the hot furnace mainly due to its favorable removal conditions as well as characteristics of continuous operation. The addition of calcium-based sorbents into the hot furnace also has a beneficial effect on the reduction of dioxin compounds.14-17 The use of optimum operating conditions to reduce HCl emissions in flue gases would help in controlling the PCDD and PCDF formation and protecting the equipment against corrosion. At the combustion temperatures, the main calcium compound that reacts with HCl is


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calcined limestone. Previous studies have led to the conclusion that the reaction temperature is a key parameter that determines the removal efficiency of HCl by calcined limestone at high temperature.18-21 However, there is also a need to explore how other species in the flue gases affect the chlorination reactivity of the calcined limestone because the information available in the literature is very limited. The flue gas composition is one of the crucial parameters that determine the high-temperature chlorination behavior of calcium-based compounds, apart from the temperature sensitivity. Generally speaking, it would appear that the capture of HCl by CaO is more difficult in the flue gas atmosphere.22-25 At elevated temperatures, calcium-based solids can simultaneously react with acidic gases, and hence there probably exists an interaction or competing mechanism between the cocapture of acidic gases. Carbonation is one such competing reaction that is a result of the presence of CO2 in the gas atmosphere.18, 20, 26 The object of this work is to conduct experiments that would yield a better understanding of the influence that the gas atmosphere has on the chlorination reactivity of calcined limestone. Apart from HCl, SO2 is also commonly present in flue gases in cases such as co-combustion of fossil fuel and refuse or incineration of municipal solid waste (MSW). In the context of combustion, the simultaneous presence of O2 and CO2 is essential for simulating the flue gas. For these reasons, SO2, CO2 and O2 were chosen as indispensable and representative species in the synthetic flue gas. Due to the presence of various acidic gases


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including HCl, SO2 and CO2, which can be simultaneously react with the calcined limestone at elevated temperatures, it would be expected that consecutive or competing reactions probably occur during the chlorination experiments. In view of this, the best option to estimate the extent of chlorination is to analyze experimentally the HCl concentration variation as a function of time unlike the common method of monitoring the variation of the sorbent weight as a function of time. An online FTIR analyzer is particularly suited to continuously record the gaseous components discharged during the entire experiment, especially under conditions in which two or more reacting gases are involved in the gas–solid reaction. In the present work, the dechlorination efficiency would be used to describe the HCl absorption history to gain a better understanding of the chlorination behavior. In addition, the HCl uptake capacity (the amount of HCl absorbed per unit mass of sorbent) would be used in place of sorbent conversion to express the capture ability of the sorbent particles. 2. Experimental Experiments simulating in-furnace dechlorination by calcined limestone injection were carried out in a bench-scale reactor unit (shown schematically in Figure 1). A FTIR spectrometer was employed to simultaneously and continuously monitor the HCl (ppmv), SO2 (ppmv) and CO2 (%) concentrations in the exhaust as a function of time. All sample lines were of Teflon with 316 stainless steel fittings. Details of the experimental rig are given elsewhere.21 In the present work, the unique fixed-bed reactor featuring a highly efficient


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cooling of the exhaust gas as well as a low space requirement was made of quartz to prevent corrosion and oxidation problems. Two k-type thermocouples were installed at various locations of the reactor to monitor the reaction and exhaust temperatures, respectively, the first of which provided feedback to the temperature controller programmer that regulated the heating of the electric furnace. The temperature of the exhaust gases ranged from 32 to 36 ˚C when the reactor was operated at 650 - 850 ˚C. Possible leakage from the reactor joint as a result of the slight pressurization by the gases supplied from cylinders has been successfully limited to less than 0.5% of the total gas feed by applying a self-developed leak-proofing method, and leakage checks were conducted at the beginning of each experimental run. After each run, the entire system was held under a N2-gas sweep until no HCl could be detected to ensure a clean system for the next run. In this work, naturally occurring limestone (calcium carbonate, CaCO3) served as a lime precursor (calcium oxide, CaO). The solid sorbent was ground and screened to a particle size of 500 - 590 µm. Prior to each run, a weighed sample of limestone particles (0.1±0.001 g) was first calcined at 850 ˚C in N2 to obtain the calcined limestone needed for the experiment. The synthetic flue gases used in this study were mixed from gas cylinders of 1800 ppmv of HCl in N2, 1500 ppmv of SO2 in N2, 99.9999% O2, 99.9999% CO2, and 99.9999% N2. Each gas flow was regulated and controlled by a thermal mass flow controller (Bronkhorst, Netherlands). The synthetic flue gases comprised a combination of 491 ppmv HCl, 0 or 491


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ppmv SO2, 0 or 11 vol.% CO2, 0 or 10 vol.% O2, and the balance N2. The gas flow contained HCl and N2 in all the experiments; the influences of the gas atmosphere were investigated by introducing CO2, O2 and SO2 to the mixture. All of the gas species were premixed in a vessel before entering the fixed-bed reactor. Throughout the experiment, the total volumetric flow rate of the gas mixture was kept at 1000 sccm. To determine the effects of the gas atmosphere on the chlorination performance of the calcined limestone, control tests were first conducted with a gas mixture of 800 ± 8 mg/Nm3 (491 ppmv) of HCl in N2 at three representative temperatures (650, 750 and 850 ˚C) as reference experiments. Once the calcination process is complete, the reaction temperature will be brought to the desired value followed by the introduction of the reacting gas. In this work, the apparent and intrinsic chlorination behaviors of calcined limestone would be identified by analyzing the time-dependent dechlorination efficiency and HCl concentration variations, respectively. The dechlorination efficiency, defined by plots of the HCl concentration in the exhaust as a function of time (experimentally measured by the FTIR), is applied to describe the absorption behavior of HCl by the calcined limestone, despite the fact that there exists a competing mechanism for the cocapture of these acidic gases by the calcined limestone. The dechlorination curve as a function of time not only presents the absorption history but also indirectly reflects the uptake ability of the sorbent through the concept of a material balance. The history of the effluent concentration of the


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absorbate (HCl) with respect to time is identified as the apparent chlorination behavior. The performance of calcined limestone in HCl uptake determined from the gas-phase material balance is treated as the intrinsic chlorination behavior. The dechlorination efficiency is calculated by the equation:

Dechlorina tion efficiency (η HCl , %) =

Ce − Ct × 100% Ce

(1)

where the term Ct represents the measured outlet HCl concentration in the gas mixtures (mol/Nm3), and Ce represents the outlet equilibrium concentration of HCl in the gas mixtures (mol/Nm3). In this work, blank tests in which synthetic flue gas containing at least HCl and O2 was fed to the reactor without any sorbent indicated that the HCl concentration at the exhaust was somewhat less than that at the inlet. This means that the oxidation process of HCl to Cl2 inside the reactor can take place in the experiments with the presence of O2 at elevated temperatures.23 This is the reason why the outlet equilibrium concentration of HCl is regarded as the effective reactant concentration rather than the inlet concentration. 3. Results and discussion 3.1 Apparent behaviors of chlorination Figure 2 depicts the chlorination histories of the calcined limestone under the studied experimental conditions. It can be seen from this figure that the dechlorination efficiency declines rapidly to a certain value over the first few minutes. As was suggested by our previous study21, a much higher space velocity, defined as the volumetric rate of disposal of


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HCl-containing gas per unit of bulk sorbent, is thought to be responsible for this phenomenon. Following this rapid decline, an either linear or sigmoidal curve as a function of time can be observed, showing the real reaction history between the HCl and calcined limestone. Figure 2 (a) presents the results of the tests of independent chlorination (i.e., without the presence of CO2, SO2 or O2 in the gas stream). As seen from Figure 2 (a), the chlorination test of calcined limestone at 750 ˚C exhibits a superior dechlorination efficiency in the early stage of the reaction, and subsequently drops fairly quickly. The observed dechlorination efficiency at 650 ˚C, however, remains relatively stable. It is also noted from this figure that the final dechlorination efficiency at 650 ˚C does not reach the baseline level, which means that the chlorination processes are still proceeding at a very slow reaction rate. In a word, this can be attributed to the greater diffusion resistance of the CaCl2 layer. More specifically, the direct chlorination process is mainly determined by chemical reaction at temperatures above 750 °C; nevertheless, the chlorination process is almost governed by combination of the chemical reaction and the product layer diffusion at temperatures of 650-700 °C.20 The difference of diffusion limitation leads to the variation of apparent reaction rate observed. Figure 2 (b) shows the results of the chlorination test in the presence of CO2 and O2. The chlorination behavior at 650 ˚C is seen to be particularly influenced by the coexistence of 11 vol.% CO2 and 10 vol.% O2 in the gas atmosphere, compared with the case of independent chlorination in Figure 2 (a). First, it is clearly visible that the dechlorination efficiency at 650 ˚C has been


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improved in this case, making it much better even than that at 750 ˚C. Second, the time needed to achieve complete chlorination has also been greatly shortened, suggesting a faster reaction rate. Such a finding may not be surprising because, in our previous study, the presence of CO2 was found to promote the reaction rate of the chlorination process, particularly at a temperature of approximately 650 ˚C.20 At this temperature, the CO2 partial pressure used in this work is sufficient to achieve the carbonation of the calcined limestone, thus affecting the absorption behavior. Upon elevating the reaction temperature, the carbonation reaction would become thermodynamically unfavorable. From Figure 2 (b), it appears that the coexistence of CO2 and O2 have no evident effect on the apparent dechlorination behavior at temperatures of 750 ˚C or above. A close inspection of the curves in Figure 2 (b) reveals, however, a slight but not significant reduction in the dechlorination efficiency. Figure 2 (c) presents the results of chlorination experiments performed in a gas atmosphere that contains CO2, O2 and SO2. It would appear that, due to the presence of SO2 in the reacting gas, the dechlorination efficiency at 650 °C is somewhat less, but, on the whole, no apparent difference in reactivity is observed. However, a considerable reduction in the dechlorination efficiency is clearly seen at temperatures of both 750 and 850 °C. Interestingly, the dechlorination efficiencies turn negative for a certain period of time and then gradually return to the baseline level as the reactions proceed within the calcined


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limestone. A dechlorination efficiency lower than the baseline level implies that the effluent HCl concentration exceeds the inlet value. It is intuitive that the excess HCl concentration level (more than 491 ppm) does not satisfy the conditions of material balance. More precisely, from the perspective of the "gas-phase" material balance of HCl this result appears illogical because the chlorine output should not be greater than the output. However, if the overall chlorine material balance during the chlorination process, including gas-phase as well as solid-phase chlorine, is taken into account, such a phenomenon may not be impossible. Therefore, the most likely possibility is that HCl is re-released from solid chlorides through some type of dechlorination reaction. In this way, all the cases will turn out to be reasonable in terms of the material balance of chlorine. This unusually high effluent concentration of HCl has been demonstrated to be associated with the simultaneous sulfation behavior of calcined limestone.21 Figure 3 shows the corresponding SO2 emissions during the chlorination experiments of calcined limestone. The observed decrease in the SO2 concentration indicates that SO2 is being absorbed through the sulfation of chlorides that are initially formed. The sulfation process of chlorides provides the most reasonable explanation for not only the previously described excessive HCl concentration level but also the decrease in the SO2 concentration level. At temperatures of 700 °C or above, the simultaneous sulfation reactions greatly affect the chlorination behavior, suppressing the dechlorination efficiency. The sulfation reaction occurs not only with the fresh calcined limestone but also


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with the chlorides formed. The reason for the suppressed chlorination will be discussed later in the text. On the other hand, it was also found in the experiments with the presence of O2 at temperatures of 750 and 850 ˚C that the equilibrium concentration levels of the HCl in the effluent gas are less than the inlet HCl concentration (not shown in this figure). This loss can be ascribed to that the HCl, in all likelihood, is oxidized to yield molecular chlorine (Cl2) via a homogeneous reaction, which is also known as the Deacon reaction. It has been reported previously that the oxidation process of HCl could occur without any metal catalyst at temperatures above 600 °C, especially at HCl concentrations less than 1000 ppm.23 Elevating the reaction temperature can promote the oxidation of HCl to Cl2. In chlorination experiments with a gas atmosphere of SO2, CO2 and O2 in N2, the molar ratios of Cl2/HCl in the effluent gas at 650, 750 and 850 °C were evaluated to be 0.009, 0.036 and 0.115, respectively. Previous works identified the solid-phase product of the chlorination of calcium-based sorbents as calcium hydroxychloride (CaClOH), hydrated calcium chloride (CaCl2·H2O) or calcium hypochlorite [Ca(ClO)2], and it is suggested that CaClOH may ultimately be converted to either CaCl2·H2O or CaCl2.21, 27-35 In addition, it should be noted that many previous investigations usually assumed CaCl2 as the sole product to describe the reaction between HCl and calcium-based sorbents; nevertheless, the solid sorbent conversion based on this assumption would be underestimated by as much as 100% in some cases, depending


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mainly on the components of the complex product mixture. In this work, the following homoand heterogeneous reactions involving chlorine are suggested to be involved:

4HCl( g ) + O2 ( g ) → 2Cl2 ( g ) + 2H2O( g )

(2)

CaO ( s ) + HCl ( g ) → CaClOH ( s )

(3)

CaO( s ) + CO2 ( g ) → CaCO3 ( s )

(4)

CaCO3 ( s ) + HCl( g ) → CaClOH ( s ) + CO2 ( g )

(5)

CaCO3 ( s ) + 2 HCl( g ) → CaCl 2 ⋅ H 2O( s ) + CO2 ( g )

(6)

CaClOH( s ) + HCl( g ) → CaCl2 ⋅ H2O( s)

(7)

4CaClOH( s ) + O2( g ) → 4CaO( s) + 2Cl2( g ) + 2H 2O( g )

(8)

CaCl2 ⋅ H 2O( s) + O2( g ) → Ca(ClO) 2( s ) + H 2O( g )

(9)

CaCl2 ⋅ H2O(s) → CaCl2(s) + H2O( g )

(10)

As mentioned, the chlorides that are initially formed are subsequently consumed by a reaction with SO2, usually called the sulfation of chlorides, thus rereleasing the HCl to the gas phase. The sulfation of chlorides can occur at appropriate temperatures with the coexistence of SO2. In other words, the chlorides formed act as a sorbent to react with SO2, indicating an indirect sulfation or a suppressed chlorination process. The sulfation of chlorides, therefore, further gives rise to the negative dechlorination efficiency. In the present experiments, the sulfation reactions of chlorides are expressed as follows:

CaClOH ( s ) + SO2 ( g ) + 1 / 2O2 ( g ) → CaSO4 ( s ) + HCl( g )


(11)

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CaCl2 ⋅ H2O(s) + SO2( g ) + 1/ 2O2( g ) → CaSO4 (s) + 2HCl( g )

(12)

The reaction between chlorides and SO2 in the presence of oxygen gives the thermally stable solid product CaSO4 under the conditions being studied. From these two reactions, one can also see that the products of the chlorination of calcined limestone are not necessarily CaCl2 but rather CaCl2·H2O and/or CaClOH. Another substantial piece of evidence to support this statement is that in our previous studies, sulfation has been observed to continue even after the sorbent used for the sulfation tests had completely undergone the chlorination process, demonstrating that the calcined limestone reacts with HCl to preferentially form either CaCl2·H2O or CaClOH. 3.2 Intrinsic behaviors of chlorination 3.2.1 Uptake history An analysis of the absorption efficiency of the sorbent is essential for understanding the reactivity and also can serve as a reference for controlling the HCl emissions at high temperatures. The uptake ability of the sorbent used can be calculated by analyzing the time-dependent HCl concentration variations. Details of the analysis method have been described elsewhere.21 At any time t (less than teq), the moles of HCl that have reacted with the calcined limestone, Nt (mol), can be expressed as t =t

N t = υ m × (Ce × t − ∫ Ct dt ) t =0

(13)

where teq is the reaction time required for achieving complete chlorination and νm is the


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volumetric flow rate of the gas mixture (Nm3/min). The calculation results of the HCl uptake ability of the calcined limestone for different gas atmospheres and temperatures as a function of time are shown in Figure 4, in which the ordinate expresses the cumulative uptake of HCl. As has previously been suggested to generally be the case for the temperature dependence, the HCl uptake ability of the calcined limestone drops dramatically with the reaction temperature rising from 650 to 850 °C.18-20, 23, 36 The capture of HCl by calcined limestone is more difficult in the flue gas atmosphere at temperatures of 750 °C or above except for the test at 650 °C. The curves in Figure 4 (a) show a typical asymptotic behavior. In Figure 4 (a), the independent chlorination proceeds at a relatively slow rate over a prolonged reaction time (360 min), implying that the presence of CO2 in the gas phase can increase the chlorination rate.20 Sun et al. investigated the independent chlorination reaction under analogous conditions.37 Their results likewise revealed a much slower reaction rate at 650 °C. On the other hand, comparing the progress of chlorination in the presence and absence of SO2 in Figure 4 (a), it is clearly shown that the simultaneous presence of SO2 exerts a small inhibitory influence on the HCl uptake ability at 650 °C. Partanen et al. reported an analogous but more significant inhibition in chlorination when simultaneous sulfation occurred at 650 °C.25 The less favorable sulfation reactions of both the calcined limestone and the chlorides formed are considered to be responsible for this small inhibition of the chlorination.


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However, as illustrated in Figure 4 (b) and (c), the simultaneous sulfation greatly limits the attainable extent of the chlorination reaction. Additionally, the curves obtained in the experiments in the presence of SO2 are found to rise at first and then decline, especially at 750 °C. This is not surprising given that some of the thermodynamically unstable chlorides formed vigorously react with SO2 to rerelease HCl to the gas phase, causing a decline in the observed amount of captured HCl. Consequently, it is conceivable that the subsequent re-release of HCl via this indirect sulfation reaction induces an elevated HCl concentration level, i.e., the negative dechlorination efficiency in Figure 2 (c). The reaction temperature of 850 °C is thermodynamically favorable for SO2 removal but much less favorable for HCl removal. Less chlorides available for indirect sulfation would be formed under unfavorable temperature conditions, thereby resulting in a minor release of HCl. Figure 4 (c) confirms that the presence of SO2 could pronouncedly suppress the chlorination reactivity. 3.2.2 Uptake capacity The present experiments are aimed at yielding a better understanding of the influence of the gas atmosphere on the chlorination reactivity of calcined limestone, keeping in mind that using the conversion of CaO to CaCl2 to estimate the extent of chlorination may be inappropriate for most cases, mainly because CaCl2·H2O and/or CaClOH are often the solid products. The results obtained from the material balance analysis and the need for a reference for controlling the HCl emissions from flue gases led us to examine the uptake performance


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of the sorbent. For these reasons, a comparative evaluation of the ultimate uptake capacity (the amount of HCl absorbed per unit mass of sorbent, g HCl/g sorbent) at the temperatures and in the gas atmospheres being studied was conducted, and the results are illustrated in Figure 5. Judging from Figure 5 (a), the HCl uptake capacity remains less affected under various gas atmospheres at 650 °C, but a considerable difference is observed at temperatures of 750 °C or above. Although the gas atmosphere exerts only a small influence on the HCl uptake capacity at 650 °C, the previous analysis already indicated that the role of CO2 in the gas phase is beneficial in completing the chlorination faster. This is, to some extent, in agreement with the findings of our previous work20, which found that the chlorination of CaO is markedly accelerated in the presence of CO2. The simultaneous carbonation behavior accounts for the enhanced reaction rate, yet the detailed mechanism is not clear, indicating the need for further investigation. However, at the higher studied temperatures, the current partial pressure of CO2 is unlikely to achieve the carbonation; therefore, in the case of CO2 and O2 addition, the observed decrease in the HCl uptake capacity could be related to the presence of O2. Xie et al. reported that the HCl removal capacity was slightly reduced with the addition of 5 vol. % O2 and considered that the presence of O2 possibly impedes the conversion of CaO to CaClOH.33 One speculative explanation may be that the presence of O2 impedes the conversion of calcined limestone to CaClOH, thereby lowering the sorbent


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utilization. The uptake capacity is thus counteracted by reaction (8) resulting from the presence of O2. A further reduction in the HCl uptake capacity is observed when SO2 is present, especially at a temperature of 850 °C. On the one hand, a temperature of ~ 850 °C thermodynamically favors the direct sulfation reaction of calcined limestone, but on the other hand, the generated chlorides (not CaCl2) start to vigorously react with SO2 at or above 700 °C, leading to the indirect suppression of the chlorination. Thus, a very low chlorination reactivity is revealed at 850 °C (only approximately 0.15 g HCl/g sorbent). Figure 5 (a) demonstrates an analogous temperature dependence of the HCl uptake capacity under various gas atmospheres. It is worth mentioning that in all cases, the HCl uptake capacity decreases sharply at temperatures from 650 to 750 °C, while a smaller tendency to decrease is observed as the temperature continues to increase. Also of importance from the desulfurization point of view is to what extent the sulfation is affected under conditions in which chlorination occurs concurrently. Figure 5 (b) is a plot of the SO2 uptake capacity against the operating temperature. As illustrated in this figure, a linear, monotonic increase in the SO2 uptake capacity is observed upon elevating the temperature. Further inspection of the curves in Figure 5 (b) reveals that the effect of increasing the temperature from 650 to 750 °C on the uptake capacity is more pronounced than that when moving from 750 to 850 °C. The chlorination is much more predominant than the sulfation at a temperature of 650 °C, but the sulfation becomes more efficient at a


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temperature of 750 °C and even predominant at 850 °C. This suggests that there are two distinct temperature regimes for better SO2 and HCl capture. The HCl uptake capacity becomes worse as the sulfation of chlorides starts to vigorously occur at approximately 750 °C, thus increasing the extent of the SO2 uptake. Previously, Lawrence et al. observed that the spent sorbent that had completely undergone the sulfation process could further react with SO2 upon the addition of HCl. They also reported that the presence of HCl could greatly improve the SO2 uptake capacity at 825 °C.38 This is not surprising in view of the fact that the sulfation of chlorides takes place. The use of chlorides as additives was also demonstrated in the literature to have enhancing effects on the sulfation process.22, 39-41 On the other hand, it is also of importance to know the change of the overall uptake capacity (HCl/SO2) with the reaction temperature under conditions in which there exists the competitive capture of HCl and SO2. Hence, judging from the results revealed in Figure 5, one can know that although increasing the operating temperature favors the SO2 uptake, there still is a significant decline in the overall uptake capacity at temperatures from 650 to 750 °C. Both the sulfation of chlorides and the thermodynamically unfavorable temperature for the chlorination of calcined limestone are responsible for the sharply reduced uptake. It is interesting to note, however, that further increasing the operating temperature to 850 °C causes no pronounced effect on the overall uptake capacity. Essentially the same trends were reported in our previous study21, i.e., at or above 700 °C, the overall uptake capacity


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remained nearly unchanged, despite the differences in the reactivities of SO2 and HCl. Moreover, one should keep in mind that the sulfation of the generated chlorides is usually accompanied by the re-release of HCl into the gas phase. In summary, from the observations in Figure 5, one can conclude that at temperatures of 750 °C or above, the presence of SO2 has enhancing effects on the sorbent utilization, mainly due to the concurrent sulfation of chlorides and calcined limestone, yet it suppresses the HCl uptake efficiency. On the other hand, the presence of O2 is believed to be responsible for the slight decrease in the HCl uptake capacity, presumably through some type of reaction involving the dechlorination of the sorbent particles. 4. Conclusion This work on the chlorination of calcined limestone under conditions similar to those of combustion has shown the responsible reaction mechanisms to be fairly complex. The dechlorination curve provides further information that assists in analyzing the HCl absorption behaviors. A quantitative analysis of the uptake capacity clearly illustrates the absorption performance of the calcined limestone under various experimental conditions. This work suggested that CaCl2·H2O and/or CaClOH are often the solid products rather than CaCl2 during the high-temperature chlorination of calcined limestone, and they react with SO2 to greatly affect the chlorination behavior. In the experiments, the role of CO2 is beneficial in completing chlorination faster at a temperature of 650 ˚C; nevertheless, it exerts


Energy & Fuels

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hardly any effect on the chlorination behavior at temperatures of 750 ˚C or above. At temperatures of 750 to 850 ˚C, a considerable inhibitory effect on the chlorination efficiency of calcined limestone is observed when either SO2 or O2 is present. The presence of SO2 results in a higher overall capture efficiency, yet the HCl capture efficiency is found to be greatly inhibited. It turns out that the concurrent sulfation of chlorides causes the subsequent re-release of HCl to the gas phase, thus reducing the attainable extent of the chlorination reaction. The presence of O2 impedes the conversion of calcined limestone to chlorides, presumably through some type of reaction involving the dechlorination of the sorbent particles. In addition, in O2 atmospheres, a higher temperature favors the formation of molecular chlorine from HCl. Acknowledgments The funding of this research was supported by the Ministry of Science and Technology under Grant No. MOST 106-3113-E-033-001.


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Energy & Fuels

Nomenclature ηHCl

[%]

dechlorination efficiency

Ce

[mol/Nm3]

equilibrium concentration of HCl in the gas mixture

Ct

[mol/Nm3]

outlet concentration of HCl in the gas mixture

Nt

[mol]

moles of HCl that have reacted with the calcined limestone

νm

[Nm3/min]

volumetric flow rate of gas mixture

teq

[min]

reaction time required for achieving complete chlorination


Energy & Fuels

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References 1.

Wey, M.-Y.; Fang, T.-J. The effect of organic and inorganic chlorides on the formation of

HCl with various hydrogen containing sources in a fluidized bed incinerator. Environment International 1995, 21, 423-431. 2.

Wang, K.-S.; Chiang, K.-Y.; Lin, S.-M.; Tsai, C.-C.; Sun, C.-J. Effects of chlorides on

emissions of hydrogen chloride formation in waste incineration. Chemosphere 1999, 38, 1571-1582. 3.

Wang, Z.; Huang, H.; Li, H.; Wu, C.; Chen, Y.; Li, B. HCl Formation from RDF

Pyrolysis and Combustion in a Spouting-Moving Bed Reactor. Energy Fuels 2002, 16, 608-614. 4.

Wey, M.-Y.; Chen, J.-C.; Wu, H.-Y.; Yu, W.-J.; Tsai, T.-H. Formations and controls of

HCl and PAHs by different additives during waste incineration. Fuel 2006, 85, 755-763. 5.

Wey, M. Y.; Liu, K. Y.; Yu, W. J.; Lin, C. L.; Chang, F. Y. Influences of chlorine content

on emission of HCl and organic compounds in waste incineration using fluidized beds. Waste Manage. 2008, 28, 406-415. 6.

Zhu, H. M.; Jiang, X. G.; Yan, J. H.; Chi, Y.; Cen, K. F. TG-FTIR analysis of PVC

thermal degradation and HCl removal. J. Anal. Appl. Pyrolysis 2008, 82, 1-9. 7.

Liu, K.; Xie, W.; Li, D.; Pan, W.-P.; Riley, J. T.; Riga, A. The effect of chlorine and

sulfur on the composition of ash deposits in a fluidized bed combustion system. Energy Fuels


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2000, 14, 963-972. 8.

Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter, L. L. The implications of

chlorine-associated corrosion on the operation of biomass-fired boilers. Prog. Energy. Combust. Sci. 2000, 26, 283-298. 9.

Griffin, R. D. A new theory of dioxin formation in municipal solid waste combustion.

Chemosphere 1986, 15, 1987-1990. 10. Gullett, B. K.; Bruce, K. R.; Beach, L. O. Formation of chlorinated organics during solid waste combustion. Waste Management & Research 1990, 8, 203-214. 11. Gullett, B. K.; Bruce, K. R.; Beach, L. O.; Drago, A. M. Mechanistic steps in the production of PCDD and PCDF during waste combustion. Chemosphere 1992, 25, 1387-1392. 12. Takasuga, T.; Makino, T.; Tsubota, K.; Takeda, N. Formation of dioxins (PCDDs/PCDFs) by dioxin-free fly ash as a catalyst and relation with several chlorine-sources. Chemosphere 2000, 40, 1003-1007. 13. Addink, R.; Bakker, W. C. M.; Olie, K. Influence of HCl and Cl2 on the Formation of Polychlorinated Dibenzo-p-dioxins/Dibenzofurans in a Carbon/Fly Ash Mixture. Environ. Sci. Technol. 1995, 29, 2055-2058. 14. Takeshita, R.; Akimoto, Y. Control of PCDD and PCDF formation in fluidized bed incinerators. Chemosphere 1989, 19, 345-352.


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15. Tagashira, K.; Torii, I.; Myouyou, K.; Takeda, K.; Mizuko, T.; Tokushita, Y. Combustion characteristics and dioxin behavior of waste fired CFB. Chem. Eng. Sci. 1999, 54, 5599-5607. 16. Liu, W.; Zheng, M.; Zhang, B.; Qian, Y.; Ma, X.; Liu, W. Inhibition of PCDD/Fs formation from dioxin precursors by calcium oxide. Chemosphere 2005, 60, 785-790. 17. Li, Y.; Wang, H.; Jiang, L.; Zhang, W.; Li, R.; Chi, Y. HCl and PCDD/Fs emission characteristics from incineration of source-classified combustible solid waste in fluidized bed. RSC Adv. 2015, 5, 67866-67873. 18. Partanen, J.; Backman, P.; Backman, R.; Hupa, M. Absorption of HCl by limestone in hot flue gases. Part I: the effects of temperature, gas atmosphere and absorbent quality. Fuel 2005, 84, 1664-1673. 19. Chyang, C.-S.; Han, Y.-L.; Zhong, Z.-C. Study of HCl Absorption by CaO at High Temperature. Energy Fuels 2009, 23, 3948-3953. 20. Lin, G.-M.; Chyang, C.-S. Effect of CO2 on high temperature chlorination behavior of calcined limestone in an innovated fixed-bed reactor. Journal of the Taiwan Institute of Chemical Engineers 2016, 62, 60-67. 21. Lin, G.-M.; Chyang, C.-S. Simultaneous HCl/SO2 Capture by Calcined Limestone from Hot Gases. Energy Fuels 2016, 30, 10696-10704. 22. Matsukata, M.; Takeda, K.; Miyatani, T.; Ueyama, K. Simultaneous chlorination and sulphation of calcined limestone. Chem. Eng. Sci. 1996, 51, 2529-2534.


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23. Liu, K.; Pan, W. P.; Riley, J. T. A study of chlorine behavior in a simulated fluidized bed combustion system. Fuel 2000, 79, 1115-1124. 24. Lawrence, A. D.; Bu, J. The reactions between Ca-based solids and gases representative of those found in a fluidized-bed incinerator. Chem. Eng. Sci. 2000, 55, 6129-6137. 25. Partanen, J.; Backman, P.; Backman, R.; Hupa, M. Absorption of HCl by limestone in hot flue gases. Part III: simultaneous absorption with SO2. Fuel 2005, 84, 1685-1694. 26. Duo, W.; Kirkby, N. F.; Seville, J. P. K.; Kiel, J. H. A.; Bos, A.; Den Uil, H. Kinetics of HCl reactions with calcium and sodium sorbents for IGCC fuel gas cleaning. Chem. Eng. Sci. 1996, 51, 2541-2546. 27. Jozewicz, W.; Gullett, B. K. Reaction Mechanisms of Dry Ca-Based Sorbents with Gaseous HCl. Ind. Eng. Chem. Res. 1995, 34, 607-612. 28. Allal, K. M.; Dolignier, J. C.; Martin, G. Reaction Mechanism of Calcium Hydroxide with Gaseous Hydrogen Chloride. Rev. Inst. Fr. Pét. 1998, 53, 871-880. 29. Bodénan, F.; Deniard, P. Characterization of flue gas cleaning residues from European solid waste incinerators: assessment of various Ca-based sorbent processes. Chemosphere 2003, 51, 335-347. 30. Yan, R.; Chin, T.; Liang, D. T.; Laursen, K.; Ong, W. Y.; Yao, K.; Tay, J. H. Kinetic Study of Hydrated Lime Reaction with HCl. Environ. Sci. Technol. 2003, 37, 2556-2562. 31. Chin, T.; Yan, R.; Liang, D. T. Study of the Reaction of Lime with HCl under Simulated


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Flue Gas Conditions Using X-ray Diffraction Characterization and Thermodynamic Prediction. Ind. Eng. Chem. Res. 2005, 44, 8730-8738. 32. Partanen, J.; Backman, P.; Backman, R.; Hupa, M. Absorption of HCl by limestone in hot flue gases. Part II: importance of calcium hydroxychloride. Fuel 2005, 84, 1674-1684. 33. Xie, X.; Li, Y.; Wang, W.; Shi, L. HCl removal using cycled carbide slag from calcium looping cycles. Appl. Energy 2014, 135, 391-401. 34. Xie, X.; Li, Y.-J.; Liu, C.-T.; Wang, W.-J. HCl absorption by CaO/Ca3Al2O6 sorbent from CO2 capture cycles using calcium looping. Fuel Process. Technol. 2015, 138, 500-508. 35. Li, Y.; Wang, W.; Cheng, X.; Su, M.; Ma, X.; Xie, X. Simultaneous CO2/HCl removal using carbide slag in repetitive adsorption/desorption cycles. Fuel 2015, 142, 21-27. 36. Bie, R.; Li, S.; Yang, L. Reaction mechanism of CaO with HCl in incineration of wastewater in fluidized bed. Chem. Eng. Sci. 2005, 60, 609-616. 37. Sun, Z.; Yu, F.-C.; Li, F.; Li, S.; Fan, L.-S. Experimental Study of HCl Capture Using CaO Sorbents: Activation, Deactivation, Reactivation, and Ionic Transfer Mechanism. Ind. Eng. Chem. Res. 2011, 50, 6034-6043. 38. Lawrence, A.; Bu, J.; Gokulakrishnan, P. The interactions between SO2, NOx, HCl and Ca in a bench-scale fluidized combustor. Journal of the Institute of Energy 1999, 72, 34-40. 39. Davini, P.; DeMichele, G.; Ghetti, P. An investigation of the influence of sodium chloride on the desulphurization properties of limestone. Fuel 1992, 71, 831-834.


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40. Zhao, Y.; Lin, W.-C. Multi-functional sorbents for the simultaneous removal of sulfur and lead compounds from hot flue gases. J. Hazard. Mater. 2003, 103, 43-63. 41. Hu, G.; Dam-Johansen, K.; Wedel, S.; Hansen, J. P. Enhancement of the Direct Sulfation of Limestone by Alkali Metal Salts, Calcium Chloride, and Hydrogen Chloride. Ind. Eng. Chem. Res. 2007, 46, 5295-5303.


Energy & Fuels

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Figure captions Schematic of the bench-scale fixed-bed reactor system: (1) mass flow controller; (2) air purging pump; (3) cooler; (4) drying column; (5) blending Figure 1 chamber; (6) electrical heater; (7) fixed-bed reactor; (8) FTIR spectrometer; (9) NaOH solution; (10) thermocouple. Dechlorination efficiency of calcined limestone at three representative Figure 2

temperatures in a gas atmosphere that contains (a) HCl in N2; (b) HCl, CO2, and O2 in N2; (c) HCl, SO2, CO2, and O2 in N2. Corresponding SO2 emissions during the chlorination experiments of

Figure 3 calcined limestone Effect of the gas atmosphere on the absorption behavior of HCl by calcined Figure 4 limestone at (a) 650 ˚C; (b) 750 ˚C; (c) 850 ˚C. Uptake capacity comparison of calcined limestone (the mass of gas reactant Figure 5

absorbed per unit mass of sorbent) under experimental conditions used the (a) HCl uptake capacity; (b) SO2 uptake capacity


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Figure 1 Schematic of the bench-scale fixed-bed reactor system: (1) mass flow controller; (2) air purging pump; (3) cooler; (4) drying column; (5) blending chamber; (6) electrical heater; (7) fixed-bed reactor; (8) FTIR spectrometer; (9) NaOH solution; (10) thermocouple.


Energy & Fuels

dechlorination efficiency (%)

100 80

HCl = 491 ppmv N2 = balance

o

750 C o

850 C

60 o

650 C

40 20 0 -20

0

30

60

90

120

150

reaction time (min) (a)

100 dechlorination efficiency (%)

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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HCl = 491 ppmv CO2 = 11 vol.% O2 = 10 vol.% N2 = balance

80 o

o

60

o

750 C

650 C

850 C

40 20 0 -20

0

30

60

90

reaction time (min) (b)


120

150

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100 dechlorination efficiency (%)

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

HCl = 491 ppmv SO2 = 491 ppmv CO2 = 11 vol.% O2 = 10 vol.% N2 = balance

80 o

650 C

60 o

750 C

40 20

o

850 C

0 -20

0

30

60

90

120

150

reaction time (min) (c) Figure 2 Dechlorination efficiency of calcined limestone at three representative temperatures in a gas atmosphere that contains (a) HCl in N2; (b) HCl, CO2, and O2 in N2; (c) HCl, SO2, CO2, and O2 in N2.


Energy & Fuels

500 o

emitted SO2 concentration (ppmv)

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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650 C

400 o

750 C

300 o

850 C

200

HCl = 491 ppmv SO2 = 491 ppmv CO2 = 11 vol.% O2 = 10 vol.% N2 = balance

100 0

0

30

60 90 reaction time (min)

120

150

Figure 3 Corresponding SO2 emissions during the chlorination experiments of calcined limestone


4

20 o

650 C

HCl/CO2/O2 in N2

15 HCl/SO2/CO2/O2 in N2

10

HCl in N2

5

0

0

30

60

90

120

150

reaction time (min) (a)

4

moles of HCl reacted with sorbent (*10 )

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Energy & Fuels


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20 o

750 C

15 HCl in N2

10

HCl/CO2/O2 in N2

5 HCl/SO2/CO2/O2 in N2

0

0

30

60

90

reaction time (min) (b)


120

150

4

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

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20 o

850 C

15

10 HCl in N2

5

HCl/CO2/O2 in N2 HCl/SO2/CO2/O2 in N2

0

0

30

60

90

120

150

reaction time (min) (c) Figure 4 Effect of the gas atmosphere on the absorption behavior of HCl by calcined limestone at (a) 650 ˚C; (b) 750 ˚C; (c) 850 ˚C.


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1.5 ultimate HCl uptake capacity (g gas reactant/g sorbent)

Ultimate HCl uptake capacity

HCl in N2

1.2

HCl/CO2/O2 in N2

0.9

HCl/SO2/CO2/O2 in N2

0.6 0.3 0.0

650

750

850 o

temperature ( C) (a)

1.5 Ultimate SO2 uptake capacity

ultimate SO2 uptake capacity (g gas reactant/g sorbent)

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Energy & Fuels

1.2 0.9

HCl/SO2/CO2/O2 in N2

0.6 0.3 0.0

650

750

850 o

temperature ( C) (b) Figure 5 Uptake capacity comparison of calcined limestone (the mass of gas reactant absorbed per unit mass of sorbent) under experimental conditions used the (a) HCl uptake capacity; (b) SO2 uptake capacity.


Removal of HCl in Flue Gases by Calcined Limestone at High

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