Study on the migration characteristics of sodium and chlorine in

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Study on the migration characteristics of sodium and chlorine in chemical looping process of ZhunDong coal with hematite oxygen carrier Huijun Ge, Laihong Shen, Tao Song, and Shangyi Yin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02774 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Study on the migration characteristics of sodium and chlorine in chemical looping process of ZhunDong coal with hematite oxygen carrier Huijun Ge 1, Laihong Shen 1*, Tao Song 2, Shangyi Yin 2 1

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, 2 Sipailou, Nanjing 210096, China

2

School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing 210042, China

Corresponding Author * Tel: +86-25-8379 5598. Fax: +86-25-5771 4489. Email: [email protected]

ABSTRACT: ZhunDong coal is the largest coal resource in China but characterized by high sodium and chlorine content. Chemical looping combustion with hematite as oxygen carrier is an attractive alternative technology for ZhunDong coal conversion without severe slagging and fouling problems. However, the influence of chemical looping process for the migration characteristics of sodium and chlorine is not clear. Especially with the presence of hematite, it is bound to have a great influence on the migration of sodium and chlorine. For better controlling slagging and fouling problems caused by sodium and chlorine migration, it is necessary to study the migration paths of sodium and chlorine in chemical looping process. Experimental results and analysis show that the migration characteristics and path of sodium and chlorine in chemical looping process are different at different temperature range. However, some common grounds could be found. The releasing form of chlorine in chemical looping process is similar, which is released mainly in the form of HCl gas phase as well as a little of NaCl gas phase. On the aspect of sodium migration, sodium is mainly reserved as solid phase, i.e. Na2O•3SiO2 and Na2O•nAl2O3•mSiO2.

KEYWORDS: ZhunDong coal; chemical looping combustion with hematite; migration characteristic; sodium and chlorine

1

ACS Paragon Plus Environment

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1. Introduction Chlorine is an important non-metallic element in the process of plant growth, which plays an active role in plant photosynthesis and also participates in cell division activities. Research on chlorine in the thermal power engineering is now concentrated on the study of biomass fuels. Studies have shown that, in the process of biomass fuels combustion, the migration of chlorine is often accompanied by the migration of alkali metals. On the one hand, chlorine has the effect of conveying alkali metals in the combustion process, which helps the alkali metals to migrate from the interior of biomass particle to biomass particle surface, thus accelerating the release of alkali metal into the gas phase in the form of chloride

1, 2.

In addition to the forms of chloride in alkali

metals, some chlorine will also be released in the form of HCl in the gas phase. Both of the gaseous products can cause serious corrosion problems to the heating surface, and the alkali metal chloride can even deteriorate the heat transfer efficiency 3. In order to prevent such problems, washing alkali metals and adding some additives are two effective measures. Washing alkali can remove a large number of alkali metal in the biomass fuel, which decreases the volatilization production of alkali metal in the biomass combustion process. Adding additives, such as alumina and kaolin, can transfer alkali metal chloride into aluminosilicate of high melting point instead of molten material of low melting point

4, 5.

However, the two

methods did not fundamentally solve the problem of corrosion caused by chloride. Although the release of alkali metal chloride is controlled effectively, more chlorine will be released in the form of HCl, which is still a hidden trouble for heating-surface corrosion. In addition, even if aluminosilicate of high melting point can be formed, some chlorine element will still react with the alkali metal aluminates to form the gaseous alkali metal chloride 6. Nowadays, the researches on chlorine diffusion in coal fuel are not many. ZhunDong coal is the largest coal resource in China but characterized by high sodium and chlorine content. According to the practical operation condition in ZhunDong coal plants, severe slagging and fouling problem often exists on the heating surface. Chemical looping combustion with hematite as oxygen carrier, as shown in Figure 1, is an attractive alternative technology for ZhunDong coal conversion without severe slagging and fouling problems

7, 8.

The combustion process takes place via indirect contact,

that is, oxygen (from air) is transferred by hematite (Fe2O3) to ZhunDong coal. The product stream exiting from the fuel reactor is ideally composed of CO2 and water. After water condensation, almost pure CO2 can be obtained. The reduced hematite (Fe3O4/FeO) is transported to the air reactor for regeneration by air and then start a new cycle in the fuel reactor. On the one hand, the catalytic effect of sodium in ZhunDong coal solves the slow coal gasification rate in Chemical looping combustion. On the other hand, with the presence of silica and 2


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alumina in the hematite, it is found that a high melting point sodium aluminosilicate, Na2O•Al2O3•6SiO2 instead of low melting point sodium silicates is formed in the fly ash, mitigating the severe slagging and fouling problems 7, 8. However, the influence of chemical looping process for the migration release of sodium and chlorine is not clear. Especially with the presence of hematite, it is bound to have a great influence on the migration of sodium and chlorine. For better controlling slagging and fouling problems caused by sodium and chlorine migration, it is necessary to study the migration paths of sodium and chlorine in chemical looping process. Chemical looping system is composed of two interconnected fluidized bed reactors, i.e. air reactor and fuel reactor. Among them, the reactions in fuel reactor are more complex and the migration of sodium and chlorine is more important for the chemical looping process. As shown in Figure 2, the reaction in fuel reactor includes fuel pyrolysis, gasification and redox reaction with hematite. Therefore, in this paper, the migration paths of sodium and chlorine for every process in fuel reactor are discussed in detail. It is worthwhile to note that there is some difference between the actual chemical looping operating conditions and the testing conditions in this paper. The experimental setup is tube furnace instead of actual chemical looping setup and the coal samples selected are prepared NaCl-located ZhunDong coal instead of natural coal sample. However, the testing parameters are almost similar to actual chemical looping parameters, including fluidization agent, the ratio of steam to N2, and the ratio of coal to hematite 7, 8. 2. Experimental section 2.1. Coal sample preparation The ultimate analysis, proximate analysis and ash composition analysis of ZhunDong coal are listed in Table 1. Sodium in ZhunDong coal mainly exists in form of water-soluble sodium 9. And the anions in ZhunDong coal are mainly water-soluble anions, such as Cl-, SO42- and HCO3-. Among them, most sodium in ZhunDong coal is in the form of NaCl, as well as a small amount of Na2SO4 and NaHCO3 10. To study the migration characteristics of sodium and chlorine in the chemical looping process, the interference of Na2SO4 and NaHCO3 must be excluded. Therefore, the experimental coal sample is prepared by loading NaCl solution after acid washing. The specific preparation process is as follows

10:

(a) firstly, the ZhunDong raw coal is washed using dilute sulfuric acid (0.2 mol/L),

which is soaked for 20 h at room temperature; (b) then it is filtered and flushed by de-ionized water until without SO42- ion; (c) the granule is dried 8 hours under 105 oC and then impregnated with NaCl solution for 24 h; (d) at last, it is dried under 105 oC and the NaCl-located ZhunDong coal sample is prepared. The mass fraction of Na in the prepared coal sample is fixed at 0.3%. 2.2 Experimental setup and procedure 3


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2.2.1 Tube furnace and Fourier infrared analyzer experiment In order to detect the gaseous products, a combination of tube furnace and Fourier infrared analyzer is chosen, as shown in Figure 3. The interior of tube furnace is a quartz tube (60mm × 6mm × 1000mm), which can be heated through surrounding silicon carbide rod. In the center of the quartz tube is a porcelain boat, where the prepared coal sample is placed. And the temperature of the furnace can be set by control cabinet and tested through thermocouple. This paper investigates three processes, that is, coal pyrolysis, gasification and redox reaction with hematite. The experimental procedures are different for each reaction. All of the ZhunDong coal samples are NaCl-located ZhunDong coal sample, and the quality fractions of Na in the are all fixed at 0.3%. The specific experimental step for coal pyrolysis is as follows: (1) The tube furnace is set to the required temperature in N2 atmosphere, which is then automatically heated; (2) When the tube furnace reaches the setting temperature, start Fourier infrared analyzer; (3) Keep N2 atmosphere unchanged, and the porcelain boat with 0.5g prepared ZhunDong coal sample is placed into the tube furnace rapidly; (4) Since then, the reaction has began. Meanwhile, the Fourier infrared analyzer records the content of gaseous HCl per 4 seconds in the gaseous products. (5) The reaction time is set to 2 min, then the temperature control system is turned off and the tube furnace is cooled in N2 atmosphere, and finally the corresponding coke sample is obtained. This is the experimental steps for coal pyrolysis, and the specific experimental steps for other reactions in ZhunDong coal chemical looping process will be introduced in each following Results and Discussion section. In addition, it is noted that the NaCl(g) content is inferred by released HCl content and sodium solid-phase analysis by three-step extraction experiment. 2.2.2 Three-step extraction experiment of coke Three-step extraction experiment of coke obtained from tube furnace experiment (section of 2.1.1) is carried out. To obtain enough coke (about 1g) from tube furnace experiment, each tube furnace experiment must be performed several times. The purpose of the extraction experiment is to confirm forms of sodium in coke so that to infer sodium migration characteristic. The specific experimental steps are as follows 9: (1) Selecting 1g coke from tube furnace experiment, and adding 100ml distilled water, and then stirring under 60 oC water temperature for 24h, which is water-extract fluid;

4


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(2) After drying the filter residue from water-extract fluid, pouring the filter residue into 1 mol/L ammonium acetate solution and getting ammonium acetate-extract fluid; (3) After drying the filter residue from ammonium acetate-extract fluid, inputting the filter residue into 1 mol/L dilute hydrochloric acid and getting dilute hydrochloric acid-extract fluid; (4) The sodium in the filter residue from dilute hydrochloric acid-extract fluid is insoluble sodium. (5) The detection of soluble sodium forms in each extract fluid is carried out with the ion emission spectrometer (Optima 5300 DV icp-oes) from PE company. And the insoluble sodium content is from the difference between total sodium content (XRF analyzing) and soluble sodium content. 3. Results and discussion 3.1 Migration characteristics of sodium and chlorine during the pyrolysis of ZhunDong Coal 3.1.1 The release of HCl during ZhunDong coal pyrolysis process Firstly, the tube furnace experiment of ZhunDong coal sample under N2 atmosphere is carried out, and the experiment respectively is proceed at the pyrolysis temperature of 400, 600, 800 and 1000 oC. The changes of HCl concentrations in the exit gas are shown in Figure 4. 400 and 600 oC are considered as relatively low temperature and 800 and 1000 oC are relatively high temperature. According to Figure 4, it can be found that the change of HCl concentration in each temperature shows a similar trend, which increases to the maximum with time and then drops sharply. However, the HCl concentration at high temperature (800 and 1000 oC) is not in an order of magnitude with HCl concentration at low temperature (400 and 600 oC). The HCl concentration at low temperature is much higher than that at high temperature. Lots of literatures

11-17

about potassium migration

features in the biomass combustion process pointed out that the migration characteristics of potassium in the process of biomass combustion are different when the temperature is above or below 500 oC. At low temperatures, the precipitation of potassium has nothing to do with chlorine content, and more potassium is deposited on the biomass microporous surface in the form of KCl, K2SO4 and K2CO3. Besides, chlorine in biomass is mainly released into the gas phase in the form of HCl. At high temperature, more KCl solid phase will be evaporated into KCl gas phase directly so that the HCl concentration correspondingly decreases. Considering the similarity of alkali metal migration, and the theoretical analysis for potassium fits well with the results in Figure 4, so the inference is also considered to be applicable to the migration characteristics of NaCl in ZhunDong coal, especially for the gas-phase chlorine migration. At 400 oC, HCl concentration reaches a maximum of 1800 ppm at 36s. As stated in Experimental Section, the experimental ZhunDong coal samples are NaCl-loaded samples, so the 5


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Na and Cl in the coal are mainly in the form of NaCl. When the pyrolysis temperature is 400 oC, due to the HCl concentration detected in the exit gas is relatively high, therefore, it is inferred that NaCl solid phase is no directly evaporated to the NaCl gas phase and more chlorine from NaCl is released in the form of HCl. When the pyrolysis temperature reaches 600 oC, HCl concentration is also relatively high, but less than that under 400 oC. HCl concentration reaches a maximum of 1450 ppm at 32s. With the increase of pyrolysis temperature from 400 to 600 oC, more NaCl solid phase is directly transferred to NaCl gas phase although the amount of vaporized NaCl is not much. Therefore, a part of Cl cannot be released in the form of HCl and released HCl concentration is decreased. When the pyrolysis temperature rises to 800 and 1000 oC, HCl concentration drops to 0~200 ppm dramatically. The HCl concentration at 800 oC is slightly higher than that at 1000 oC, and the maximum values are 175 ppm and 100 ppm respectively. Higher temperature is more conducive to NaCl solid phase to be evaporated as NaCl gas phase. Therefore, HCl concentration releasing out of the reactor drops relatively at higher pyrolysis temperature. 3.1.2 The migration morphology of sodium during ZhunDong coal pyrolysis process After tube furnace experiment, the residual coke is conducted with three-step extraction experiment to analyze the migration morphology of sodium during the pyrolysis of ZhunDong coal. The experimental results are shown in Figure 5. Sodium quality fraction in the NaCl-loaded coal sample during tube furnace experiment is 0.3%, which is known as water-soluble sodium. Besides, the preparation process of NaCl-loaded coal sample includes dilute sulfuric acid washing, and a bit of insoluble sodium still remain in the coal sample. Therefore, the total amount of sodium is slightly larger than 3000 μg/g, including water-soluble sodium of 3000 μg/g and a bit of insoluble sodium. It's important to note that the insoluble sodium in the ZhunDong coal is sodium silicate or sodium aluminosilicate 18-20. According to Figure 5, with the increase of pyrolysis temperature, the content of all soluble sodium has a slight increase, and then declines sharply when temperature exceeds 600 oC. At 400 oC,

the content of all soluble sodium is 2850 μg/g, which is slightly less than 3000 μg/g. This shows

that only a small amount of sodium is released in gas phase under 400 oC, and most sodium remain in coke after pyrolysis reaction. From Figure 4, chlorine during pyrolysis reaction is mainly in the form of HCl rather than NaCl gas phase. Therefore, it can be inferred that most of sodium in ZhunDong coal exsits in coal coke in the form of solid or molten phase at 400 oC, and a lot of chlorine is released in the form of HCl. At 600 oC, the content of soluble sodium in each form is slightly higher than corresponding content at 400 oC. According to the result in Figure 4, when the pyrolysis temperature is 600 oC, 6


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HCl concentration detected in the exit gas declines in comparison with 400 oC, so it can be inferred that some NaCl in coal is released in the form of gas phase, thus resulting in a decline of HCl concentration. However, Figure 5 shows a rise of the content of soluble sodium in coke after pyrolysis reaction. It can be hypothesized that some insoluble sodium is converted to soluble sodium through the pyrolysis reaction at 600 oC, so that the soluble sodium in coke shows a small increase. When the pyrolysis temperature rises to 800 oC and 1000 oC, the content of soluble sodium in coke decreases sharply. Combined with the results in Figure 4, higher pyrolysis temperature can prompt NaCl solid phase to be evaporated to NaCl gas phase, thus the HCl concentration falls sharply. When more sodium is evaporated in the form of NaCl gas phase, the content of soluble sodium that staying in the coke shows a large decline, which is consistent with the results of Figure 5. In addition, some researches

12-17

find that NaCl is more easy to react with SiO2 and Al2O3 to

produce insoluble Na2O•nAl2O3•mSiO2 at high temperature, so the content of soluble sodium will also decrease. 3.1.3 The migration path of chlorine and sodium during ZhunDong coal pyrolysis process Based on the HCl release and sodium solid-phase analysis (Figure 6) 7, 8, the migration path of chlorine and sodium during ZhunDong coal pyrolysis process is inferred and shown in this section. Figure 7 shows the migration path at lower pyrolysis temperature (400 oC and 600 oC). When pyrolysis temperature is 400 oC, most chlorine in raw coal is released in the form of HCl gas phase, and a little chlorine in combination with sodium stay in coke in the form of solid or molten NaCl. Besides, there is also solid Na2O•3SiO2 detected in coke after pyrolysis process 7. When pyrolysis temperature is 600 oC, a small amount of NaCl gas phase appears in the exit gas. However, most chlorine in raw coal is still released in the form of HCl gas phase. On the other hand, most sodium is preserved in coke in the form of molten NaCl and solid Na2O•3SiO2. Figure 8 shows the migration path of chlorine and sodium at higher pyrolysis temperature (800 oC

and 1000 oC). When the pyrolysis temperature is 800 oC, the gas composition is the same as that

at 600 oC, which is the mixture of HCl gas phase and NaCl gas phase. However, HCl concentration decreases and NaCl gas phase concentration increases in comparison with 600 oC. To be specific, nearly all of chlorine is converted to gas phase, and only trace amounts of chlorine still exist in coke after pyrolysis process of 800 oC. At the same time, solid Na2O•3SiO2 and Na2O•nAl2O3•mSiO2 are also detected in coke 8. The production of Na2O•nAl2O3•mSiO2 is due to the reaction between the NaCl gas phase and the SiO2 and Al2O3. When the pyrolysis temperature is 1000 oC, similarly, almost all of chlorine is converted to HCl gas phase or NaCl gas phase, but HCl concentration decreases further and NaCl concentration 7


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increases in comparison with 800 oC. The molten Na2O is found in coke, which is converted from some Na2O•nAl2O3•mSiO2. In addition, the disappearance of Na2O•3SiO2 in comparison with 800 oC

is due to that higher temperature is conducive to the reaction between NaCl gas phase with SiO2

and Al2O3 together instead of SiO2 alone. 3.2 Migration characteristics of sodium and chlorine during the gasification of ZhunDong coal In this section, the gasification reaction of ZhunDong coal is analyzed in tube furnace and the residual solid phase after gasification reaction is detected through three-step extraction experiment . The specific tube furnace experimental step is same with pyrolysis reaction, but the N2 atmosphere is converted to steam/N2 atmosphere. In addition, pyrolysis process is inevitable in the gasification process, which is relatively shorter, however, in order to make the entire migration path more specific and clear, the pyrolysis process is extended deliberately and the pyrolysis process is considered as long as enough. In other words, pyrolysis and gasificaiton occur in sequence during the gasification of ZhunDong coal. 3.2.1 The release of HCl during ZhunDong coal gasification process Figure 9 shows the change of HCl concentration in the exit gas during the gasification process at different temperatures. Compared with pyrolysis reaction (Figure 4), it is found that the changing trends of HCl concentration are similar, which increase first and then decrease over time. And HCl concentration at low temperature (400 oC and 600 oC) is much higher than the HCl concentration at high temperature (800 oC and 1000 oC). In order to finely analyze the gasification process, it can be considered that a short pyrolysis process exists before steam gasification occurs. When the gasification temperature is 400 oC, HCl concentration reaches a maximum of 2250 ppm after 40s. The pyrolysis process of ZhunDong coal has been studied before, and it is found that sodium morphology in residual solid is mainly composed of solid or molten NaCl and Na2O•3SiO2. The solid or molten NaCl will react with steam in the gasification process. Therefore, HCl concentration in the gasification process is improved compared to the pyrolysis process. When the gasification temperature is 600 oC, HCl concentration also increases compared to the pyrolysis process in Figure 4. Basically, after pyrolysis reaction, molten NaCl or NaCl gas phase will react with steam, which prompts the increase of HCl concentration. In addition, for gasification reaction, the peak (1600 ppm) of HCl concentration at 600 oC is less than the peak (2250 ppm) at 400 oC, and HCl concentration at 600 oC is evidently less than that at 400 oC after 36 s. This is because partially molten NaCl will be converted into NaCl gas phase directly, which run out reactor without reacting with steam, resulting in a decrease of HCl concentration. However, it can be also found that, at the initial period of the reaction, HCl concentration at 600 oC is higher than that at 400 oC. It can be speculated that, in the initial stage, reaction rates between either molten NaCl or 8


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NaCl gas phase and steam are improved at higher temperature. However, as consumption of NaCl slow down in the later stages of the gasification process, HCl concentration decreases sharply. In addition, a small number of molten Na2O and NaOH will be converted directly into corresponding gas phases instead of NaCl gas phase under 600 oC. When the gasification temperature is 800 oC, although HCl concentration is higher than that in the pyrolysis process (about 50 ppm), the increase is limited. More NaCl solid phase will be evaporated to NaCl gas phase at 800 oC. However, the reaction between NaCl and steam is not high under 800 oC, so the majority of NaCl gas phase is still released. In other words, chlorine is mainly released in the form of NaCl. When the gasification temperature is 1000 oC, the changing curve of HCl concentration is similar to 800 oC. HCl concentration is also improved compared with the pyrolysis process, while the increase is very small and limited. The increase of HCl concentration is due to the reaction between NaCl gas phase and steam. However, the reaction rate at 1000oC is higher than 800 oC, so the increase of HCl concentration at 1000 oC is slightly bigger. In addition, Na2O is more possible to be generated at 1000 oC compared to 800 oC, so it can be concluded that a small amount of Na2O solid phase from pyrolysis process at 1000 oC will react with steam, and some NaOH gas phase is released in export of reactor. 3.2.2 The migration morphology of sodium during ZhunDong coal gasification process After tube furnace experiment, the residual solid is conducted with three-step extraction experiment to analyze the migration morphology of sodium during the gasification process of ZhunDong coal. The experimental results are shown in Figure 10. It can be found that the content of total soluble sodium and soluble sodium of any form decreases with the gasification temperature. The comparison of the morphology of soluble sodium between pyrolysis and gasification reaction is presented in Figure 11. The content of soluble sodium for each temperature after pyrolysis process is higher than that after gasification process, which indicate that more sodium in ZhunDong coal is released in gaseous form in the gasification process. In addition, the differences between the contents of sodium after two reaction processes are different, reaching the maximum and the minimum when the temperature is 600 oC and 800 oC correspondingly. 400 oC is relatively low temperature, and only some NaCl solid phase or molten NaCl is conversed to molten Na2O or NaOH instead of gas phases. Therefore, the total content of soluble sodium does not appear larger decline. When the gasification temperature is 600 oC, main morphology of sodium existed in residual solid is similar to that at 400 oC. However, higher temperature will lead partial molten Na2O or NaOH into the gas phases, finally resulting in the decrease of soluble sodium content. 9


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In addition, it has been pointed out previously that Na2O•3SiO2 and Na2O•nAl2O3•mSiO2 are two main solid states in residual solid when the pyrolysis temperature is 800oC. Based on XRD analysis and the small difference of sodium content between pyrolysis and gasification reaction at 800oC, it can be inferred that Na2O•3SiO2 and Na2O•nAl2O3•mSiO2 are also two main occurrence forms in residual solid after gasification reaction. When the gasification temperature is 1000 oC, Na2O solid phase exists in residual solid and reacts with steam generating NaOH gas phase. Therefore, the difference of sodium content between pyrolysis and gasification reaction is slightly higher than that at 800 oC. 3.2.3 The migration path of chlorine and sodium during ZhunDong coal gasification process Based on the HCl release and sodium XRD analysis, the migration path of chlorine and sodium during ZhunDong coal gasification process is inferred and shown in this section. It has been to noted that, in order to make the entire migration path more specific and clear, the pyrolysis process is extended deliberately and the pyrolysis process is considered as long as enough. Figure 12 shows the migration path at 400 oC. When the gasification temperature is 400 oC, HCl gas phase will be released during the pyrolysis process, while the coke mainly consists of NaCl solid phase or molten NaCl and Na2O•3SiO2 solid phase. With that NaCl solid phase or molten NaCl react with steam in gasification process, HCl concentration shows a increase, and molten Na2O or NaOH become two main solid phases in residual solid. Figure 13 shows the migration path at 600 oC. When the gasification temperature is 600 oC, HCl gas phase and a small amount of NaCl gas phase will be released during the pyrolysis process. And the main solid phases in coke are molten NaCl and Na2O•3SiO2 solid phase. In the process of gasification, NaCl gas phase will react with steam to produce Na2O or NaOH gas phase, so HCl concentration increases and NaCl concentration decreases at the exit. In addition, the molten NaCl in the coke will reacted with steam so that increasing further HCl concentration. The molten NaCl is also transformed into molten Na2O or NaOH. Figure 14 shows the migration path at 800 oC. When the gasification temperature is 800 oC, during the pyrolysis process before gasification reaction, chloride is mainly released in the form of NaCl gas phase accompanied with a small amount of HCl gas phase. In addition, Na2O•3SiO2 and Na2O•nAl2O3•mSiO2 are two main kinds of solid states existing in coke after pyrolysis reaction. In the process of gasification reaction with steam, Na2O•3SiO2 and Na2O•nAl2O3•mSiO2 almost do not react with steam, while only NaCl gas phase will react with steam leading to the increase of HCl concentration and appearance of Na2O or NaOH gas phase. Figure 15 shows the migration path at 1000 oC. When the gasification temperature is 1000oC, chlorine is mainly released in the form of NaCl gas phase during the pyrolysis process before 10


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gasification reaction. In the process of gasification reaction with steam, NaCl gas phase will react with steam to produce Na2O or NaOH gas phase, so HCl concentration increases and NaCl concentration decreases at the exit. In addition, there is a small amount of Na2O gas phase produced by the decomposition of Na2O•nAl2O3•mSiO2 in coke. Meanwhile, Na2O gas phase can react with steam to produce NaOH gas phase. 3.3 Migration characteristics of sodium and chlorine in chemical looping combustion process of ZhunDong Coal 3.3.1 Thermodynamic equilibrium analysis of possible reactions in fuel reactor and air reactor In the process of chemical looping combustion with hematite oxygen carrier, ZhunDong coal goes through three stages in fuel reactor in turn: pyrolysis, gasification and redox reaction with hematite. Through the analysis of migration characteristics of sodium and chlorine in the pyrolysis and gasification process, the chlorine in ZhunDong coal is mainly released in the form of HCl gas phase and a small amount of NaCl gas phase. Therefore, the reactions between HCl and hematite oxygen carrier are main consideration in the redox reaction process. The main ingredient in hematite is Fe2O3, which also appears in Fe3O4, FeO and even Fe during the redox reaction process with CO and H2. A series of competitive reactions of HCl with Fe2O3, Fe3O4, FeO and Fe in the fuel reactor are shown as follows. Fe2O3 + 6 HCl(g) → 2 FeCl3 + 3 H2O(g) Fe3O4 + 8 HCl(g) →2 FeCl3 + FeCl2 + 4 H2O(g)

(R1) (R2)

FeO + 2 HCl(g) → FeCl2 + H2O(g)

(R3)

2 Fe + 6 HCl(g) →2 FeCl3 + 3 H2(g)

(R4)

Fe + 2 HCl(g) → FeCl2 + H2(g)

(R5)

With related thermodynamic parameters, the equilibrium constants of above competitive reactions within a temperature range of 200-1000 oC are shown in Figure 16, which is calculated with HSC Chemistry 5.11. The equilibrium constants of all reactions, except R16, increased with the temperature. The equilibrium constants of the five reactions decrease with temperature, indicating that the increase of temperature is not conducive to the positive progress of these five reactions. In addition, the equilibrium constants of (R3) and (R5) are greater than those of the other three reactions, which means that one mole FeO or Fe can easily react with two moles HCl to produce FeCl2. However, as the temperature rises, the equilibrium constant decreases, and when the temperature exceeds 800oC, the equilibrium constant is less than 0, which means that the two reactions are difficult to carry out at high temperature. As for (R1), (R2) and (R4), when the temperature exceeds 200oC, the equilibrium constant of each reaction is less than or close to 0, which indicates that all three reactions are difficult to carry out. 11


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Therefore, it is concluded that when the temperature in fuel reactor is 200 to 1200 oC, a large number of HCl released by pyrolysis and gasification process of ZhunDong coal is difficult to react with iron oxide. The results will be verified in the following tube furnace experiment. The reacted hematite in fuel reactor will be transported into the air reactor for regeneration. Although it is infered that HCl is difficult to react with iron oxide to generate iron chloride, there may be a small part of iron chloride produced in fuel reactor and transferred to air reactor. Therefore, it is still necessary to study the possible reactions happened in air reactor as equations of (R6) ~ (R9), which can affect the regeneration of oxygen carrier. The equations of (R6) ~ (R9) are the reactions of iron chloride FeCl3 or FeCl2 with the O2 in the air reactor. 4 FeCl3 + 3 O2(g) → 2 Fe2O3 + 6 Cl2(g)

(R6)

4 FeCl2 + 3 O2(g) → 2 Fe2O3 + 4 Cl2(g)

(R7)

4 FeCl2+ O2(g) →2 FeCl3 + 2 FeOCl

(R8)

4 FeOCl + O2(g) → 2 Fe2O3 + 2 Cl2(g)

(R9)

Similarly, the thermodynamic equilibrium analysis for the equations of (R6) ~ (R9) are conducted, as shown in Figure 17. It can be found that equilibrium constants of four reactions all decrease with temperature. However, the equilibrium constants at most temperature point are all bigger than 1, and the equilibrium constants of three kinds of reaction of (R6) ~ (R8) are much bigger than 1. It shows that FeCl3 and FeCl2 are easier to be oxidized into Fe2O3 by O2, so as to realize the regeneration of hematite oxygen carrier. Although the regeneration of oxygen carrier is not a problem, it can be found that Cl2 will appear in the exit of air reactor, which will have a bad effect on the environment. Therefore, synthesizing the above analysis, it can be concluded that no matter whether iron oxide in hematite is reacted to iron chloride in fuel reactor, the oxygen carrier can be regenerated with the circulation of bed materials. 3.3.2 The release of HCl during the chemical looping process of ZhunDong coal with hematite in FR In this section, the experiment of ZhunDong coal with hematite in tube furnace reactor was carried out. 0.5 g ZhunDong coal loaded of NaCl and 20 g hematite particle were mixed for the tube furnace experiment, which simulated the chemical looping process of ZhunDong coal and hematite in fuel reactor. The reaction temperature is set to 400, 600 , 800, and 1000 oC respectively. In comparison with pyrolysis and gasification rate, the solid-solid reaction rate of ZhunDong coal with hematite is relatively slow. Therefore, pyrolysis and gasification reaction proceeded prior to the redox reaction during the reaction process of ZhunDong coal with hematite in fuel reactor.

12


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Energy & Fuels

Figure 18 shows the change of HCl concentration along with time in the outlet gas at different temperatures. Similar with the process of pyrolysis and gasification, the change curves of the HCl concentration with time increase first and then decrease. Besides, HCl concentrations at relatively low temperatures (400 oC and 600 oC) are always greater than those at relatively high temperature (800oC and 1000oC). According to the above analysis in Section 3.3.1, when the temperature is between 400 oC and 600 oC, HCl and Fe-based oxide have the reaction possibility, although the equilibrium constant is very small. Once there is any reaction between HCl and Fe-based oxide likeas (R1) ~ (R5), HCl concentration will show a decline. This is the reason why the HCl concentration appears a slightly decrease at 400oC and 600oC during chemical looping process. When the temperature is between 800oC and 1000oC, according to the above analysis in Section 3.3.1, HCl will not react with Fe-based oxide. However, NaCl gas phase can react with Al2O3 and SiO2 in hematite at high temperatures in accordance with the equation of (R10), which will lead to slight rise of HCl concentration. In addition, Na2O and NaOH igas phase may also react with Al2O3 and SiO2 in hematite in accordance with the equation of (R11) ~ (R17). 2 NaCl + 3 SiO2 + H2O → Na2O•3SiO2 + HCl

(R10)

Na2O + 3 SiO2 → Na2O•3SiO2

(R11)

2NaOH + 3 SiO2 → Na2O•3SiO2 + H2O

(R12)

Na2O + Al2O3 + 2 SiO2 →Na2O•Al2O3•2SiO2

(R13)


(R14)


(R15)

Na2O•3SiO2 + Al2O3 →Na2O•Al2O3•2SiO2+ SiO2

(R16)

Na2O•3SiO2 + Al2O3 + 3 SiO2 →Na2O•Al2O3•6SiO2

(R17)

3.3.3 The migration morphology of sodium during the chemical looping process of ZhunDong coal with hematite in FR Three-step extraction experiment is then conducted to analyze the migration morphology of sodium during the chemical looping process of ZhunDong coal with hematite in fuel reactor. The experimental results are shown in Figure 19. It can be found that the variation trend of sodium content is similar to that for gasification process. The content of total soluble sodium and soluble sodium of any form decreases with the gasification temperature. The difference of soluble sodium content between gasification and chemical looping process is shown in Figure 20. The difference between gasification and chemical looping process is less than that between pyrolysis and gasification process (Figure 11), especially at relative low temperature.

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From Figure 20, the differences of the soluble sodium content between the gasification and chemical looping process at different temperatures are different. When the temperature is 400oC, after the redox reaction in fuel reactor, the content of soluble sodium is slightly less than that after the gasification reaction. This is mainly due to the reaction between a small amount of molten Na2O and NaOH solid phase with Al2O3 and SiO2 in hematite, just like the equations of (R11) ~ (R17). Besides, reaction time is enough and some insolvable silicon aluminate is produced, which leads to the decline of soluble sodium content. When the temperature is 600 oC, after the redox reaction in fuel reactor, the content of soluble sodium is slightly higher than that after gasification reaction. This is mainly due to the reaction between NaCl, Na2O and NaOH gas phase with SiO2 in hematite like as equations of (R10) ~ (R12), resulting in a slight increase of the content of soluble sodium. The possibilities of the equations of (R13) ~ (R17) are not very large for the NaCl, Na2O and NaOH gas phase at this temperature. Meanwhile, the reaction time for gas phase is shorter, and the soluble sodium can not be converted to insoluble sodium. When the temperature is 800 oC and 1000 oC, the addition of soluble sodium content after redox reaction does not increase further. At high temperature, although more sodium gas phase is generated and will react with SiO2 and Al2O3 in hematite to be fixed, the equations of (R11) ~ (R17) are also promote at high temperatures and some soluble sodium is transformed into insolvable sodium. Therefore, the content of soluble sodium does not appear a large increase after chemical looping process. 3.3.4 The migration path of chlorine and sodium during the chemical looping process of ZhunDong coal with hematite in FR Based on the HCl release and sodium XRD analysis, the migration path of chlorine and sodium during chemical looping process in fuel reactor is inferred and shown in this section. The prediction of the migration path in the chemical looping process of ZhunDong coal in fuel reactor is made one by one in order of the pyrolysis, gasification, and redox reaction. Figure 21~ Figure 24 show the migration path at different temperatures. When the temperature is 400 oC, chloride migration path is shown as follows: first of all, in the pyrolysis process, most of chlorine is released in the form of HCl gas phase, and some chloride is conserved in solid phase in the form of solid or molten NaCl; then, in the gasification process, some solid or molten NaCl will react with steam and is converted into HCl gas phase; in the chemical looping process, small amount of HCl gas phase will react with the Fe-based oxide and be fixed in the form of FeCl3 solid phase, while most of chlorine will be released in the form of HCl gas phase.

14


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Energy & Fuels

The migration path of sodium at 400 oC is shown as follows: at first, most of sodium will be stored in the form of solid or molten NaCl and Na2O•3SiO2 during the pyrolysis process; in the gasification process, the solid or molten NaCl will react with the steam to be transformed into the molten Na2O and NaOH; in the chemical looping proces, the molten Na2O or NaOH will react with Al2O3 and SiO2 in hematite to form some solid phases such like Na2O•3SiO2 and Na2O•nAl2O3•mSiO2. When temperature is 600 oC, chloride migration path is shown as follows: in the pyrolysis process, most of the chlorine is released in HCl gas phase (much) and NaCl gas phase (little), and a little of chloride is conserved in solid phase in the form of molten NaCl; in the gasification process, the partial melting and gaseous NaCl will react with steam to form HCl gas phase; in the chemical looping process, a small amount of HCl gas phase can react with Fe-based oxide to form of FeCl3 solid phase, while some chlorine is release in the form of HCl gas phase (much) and NaCl gas phase (little). The migration path of sodium at 600 oC is similar to that at 400 oC. The biggest difference is that, in the chemical looping process, some NaCl, Na2O and NaOH gas phases will react with Al2O3 and SiO2 in hematite and more sodium is fixed in the form of Na2O•3SiO2 and Na2O•nAl2O3•mSiO2 solid phase. When the temperature is 800 oC, chlorine migration path is shown as follows: firstly, in the pyrolysis process, most of chlorine is released in the form of HCl gas phase (little) and NaCl (much); in the gasification process, NaCl gas phase will react with steam to form HCl gas phase, resulting in high HCl concentration and low NaCl concentration; in the chemical looping process, due to the slow reaction rate between HCl and Fe-based oxide, as well as only part of NaCl gas phase will react with Al2O3 and SiO2 in hematite, chlorine is mainly released in the form of HCl and NaCl gas phase. The migration path of sodium at 800 oC is shown as follows: firstly, during the transient pyrolysis process, most of the sodium is released in the form of NaCl gas phase , and a small number of sodium is converted to solid Na2O•3SiO2 and Na2O•nAl2O3•mSiO2; in the chemical looping process, due to the presence of Al2O3 and SiO2 in hematite, Na2O•3SiO2 is transferred into Na2O•nAl2O3•mSiO2 gradually. When the temperature is 1000 oC, the migration paths of chlorine and sodium are similar to correspondingly path at 800 oC. The only difference is that there is a big probability of the reaction between gaseous NaCl, Na2O and NaOH with Al2O3 and SiO2 in the chemical looping process. Therefore, the HCl concentration at the exit is higher and NaCl concentration is lower at 1000 oC. In addition, more sodium is fixed in the solid phase in the form of Na2O•nAl2O3•mSiO2 at 1000 oC. 15


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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4. Conclusions In order to investigate the the migration paths of sodium and chlorine in the process of Zhundong coal chemical looping with hematite, the whole process is divided into fuel pyrolysis, gasification and redox reaction. And the migration paths in each reaction process are studied and presented in detail. At different temperature ranges, chlorine shows different migration paths in chemical looping process. However, the releasing form at the exit is same, which is mainly in HCl gas phase (majority) and NaCl gas phase (minority). Sodium also shows different migration paths for different temperature ranges in chemical looping process, but the final product is always mainly composed of a little of NaCl gas phase and much solid Na2O•3SiO2 and Na2O•nAl2O3•mSiO2. Acknowledgement This work was funded by Natural Science Foundation of Jiangsu Province (BK20180388), National Postdoctoral Program for Innovative Talents (BX201700049), and China Postdoctoral Science Foundation (2017M621584) and National Natural Science Foundation of China (Grant Nos. 51561125001, 51476029). References (1) Baxter, L.; Gale, T.; Sinquefield, S. Influence of ash deposit chemistry and structure on physical and transport properties, Developments in Thermochemical Biomass Conversion: Springer Netherlands, 1997; pp 1247-1262. (2) Baxter, L.; Miles, T.; Jenkins, B. The behavior of inorganic material in biomass-fired power boilers: field and laboratory experiences. Fuel Process. Technol. 1998, 54, 47-78. (3) Ma, X. The new development on the study of problems with alkali metals during straw combustion. Water Conservancy & Electric Power Machinery. 2006, 28, 28-34. (4) Nielsen, H.; Frandsen, F.; Dam-Johansen, K. The implications of chlorine-associated corrosion on the operation of biomass-fired boilers. Prog. Energy Combust. Sci. 2000, 26, 283-298. (5) Bakker, R. University of California Press: Davis, 2000; Vol. 61-09, Section B, pp 4841. Biomass fuel leaching for the control of fouling, slagging, and agglomeration in biomass power generation.

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(6) McKendry, P. Energy production from biomass (Part 3): gasification technologies. Bioresour. Technol. 2002, 83, 55-63. (7) Ge, H.; Shen, L.; Gu, H.; Song, T. Combustion performance and sodium transformation of high-sodium ZhunDong coal during chemical looping combustion with hematite as oxygen carrier, Fuel. 2015, 159, 107-117. (8) Ge, H.; Shen, L.; Gu, H.; Song, T. Combustion performance and sodium absorption of ZhunDong coal in a CLC process with hematite oxygen carrier. Appl. Therm. Eng. 2016. 94, 40-49. (9) Zhang, J.; Han, C.; Yan, Z. The varying characterization of alkali metals (Na, K) from coal during the initial stage of coal combustion. Energy Fuels. 2001, 15, 786-793. (10) Xu, Y. The transformation of inorganic sodium and It’s effect on the products during the pyrolysis of Zhundong coal [D]. Harbin Institute of Technology. 2015. (11) Yu, C.; Zhang, W. Inorganic material emission during biomass pyrolysis. J. Fuel Chem. Technol. 2000, 28, 420-425. (12) Björkman, E.; Strömberg, B. Release of chlorine from biomass at pyrolysis and gasification conditions. Energy Fuels. 1997, 11, 1026-1032. (13) Knudsen, J.; Jensen, P.; Lin, W. Secondary capture of chlorine and sulfur during thermal conversion of biomass. Energy Fuels. 2005, 19, 606-617. (14) Cheng, J.; Zhou, J.; Liu, J. Sulfur removal at high temperature during coal combustion in furnaces: a review. Prog. Energy Combust. Sci. 2003, 29, 381-405. (15) Olsson, J.; Jäglid, U.; Pettersson, J. B. Alkali metal emission during pyrolysis of biomass. Energy Fuels. 1997, 11, 779-784. (16) Jensen, A.; Dam-Johansen, K.; Wójtowicz, M. TG-FTIR study of the influence of potassium chloride on wheat straw pyrolysis. Energy Fuels. 1998, 12, 929-938. (17) Knudsen, J.; Jensen, P.; Dam-Johansen, K. Transformation and release to the gas phase of Cl, K, and S during combustion of annual biomass. Energy Fuels. 2004, 18, 1385-1399.

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(18) Wen, Q. Wang, C.; Che. D. Alkali Metal Occurrence Mode and Its Influence on Combustion Characteristics in ZhunDong Coals. J. Combust. Sci. Technol. 2014, 20(3), 216-221. (19) Liu, J.; Wang, Z.; Xiang. F. Modes of occurrence and transformation of alkali metals in Zhundong coal during combustion. J. Fuel Chem. Technol. 2014, 42(3), 316-322. (20) Edgcombe, L. J. State of Combination of Chlorine in Coal. Extraction of Coal with Water. Fuel. 1956, 35(1), 38-48.

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Energy & Fuels

Fe2O3

Air

Oxygen depleted air

CO2 H2O

Fuel Reactor

Air Reactor

Fe3O4/FeO

ZhunDong coal

Steam

Figure 1 Drawing of chemical looping combustion process using hematite as oxygen carrier (oxygen carrier: hematite; gasificaton agent: steam; fuel: ZhunDong coal)

HCl NaCl Cl2

HCl NaCl Cl2

Cl

SiO2 Al2O3

CO2/H2O

Fe2O3

NaCl NaOH Na2SO4 Cl2

Na

Raw coal

Fe3O4 FeO

Pyrolysis

Redox raction with hematite

Gasification

Outside

Figure 2 The schematic diagram for the migration paths of sodium and chlorine in fuel reactor of chemical looping process with hematite as oxygen carrier

Figure 3 Tube furnace and Fourier infrared analyzer unit (quartz tube: 60mm × 6mm × 1000mm) 19


Energy & Fuels

250

2000

400 C 600 oC 800 oC 1000 oC

1600

200

1200

150

800

100

400

50

0

0

20

40

60

80

100

HCl concentration（ ppm（

o


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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0 120

Time (s)

Figure 4 HCl concentration over time in the pyrolysis process of ZhunDong coal at different temperatures

Figure 5 Forms of sodium after the pyrolysis of coke at various temperatures

20


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A

650

o

A

C

650

A—NaCl B—SiO2

B

B

B B

A

A

B A

A

A

B

A

A

800 C A—NaCl B—SiO2

CC

B

AC B

A

B

A

B

A

A

A

o

950 C A—NaCl B—Al2O3

A

C—NaAl2O4 A

B

10

B 20

B

30

B A

C 40

B

C

A CC

50

60

A 70

A

BA

C—Na2Si3O7

A

C

A—NaCl B—Al2O3 B

950 oC A—NaCl B—SiO2

C

A

o

800 A

A B

A

A

o

C—Na2Si3O7

C

C

A

B A

B

o

A—NaCl B—Al2O3

80

2-theta (degree)

A

B

A 90

10

20

C 30

C 40

A 50

A

B A 60

70

80

90

2-theta (degree)

Figure 6 XRD analyzing for the products from the reaction (A) between NaCl(s) and SiO2 and (B) between NaCl(s) and Al2O3 at different temperatures

Cl

majority

majority

Cl

HCl(g)

HCl(g) much NaCl(g) little

or

in m ity

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

Na

Raw coal

minority

NaCl(s) NaCl(salgd) Na2O 3SiO2(s)

Na

Pyrolysis (400 oC)

Raw coal

majority

NaCl(salgd) Na2O 3SiO2(s)

Pyrolysis(600 oC)

Figure 7 Migration path of chlorine and sodium in the pyrolysis process at lower temperature (400 oC

and 600 oC)

21


Energy & Fuels

majority

Cl


m

m aj o

aj or

rit

y

ity

ce

Na2O 3SiO2(s) Na 2O nAl2O3 mSiO2(s) minority

Raw coal

HCl(g) little NaCl(g) much

tra

ce

Na

majority

Cl

tra

Na

Pyrolysis(800 oC)

Na2O(salgd) Na 2O nAl2O3 mSiO2(s) minority

Pyrolysis(1000 oC)

Raw coal

Figure 8 Migration path of chlorine and sodium in the pyrolysis process at higher temperature (800 and 1000 oC)

2400

400 oC 600 oC 800 oC 1000 oC

2000

1600

250

200

150 1200 100 800 50

400

0

0

20

40

60

80

100


oC


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

Page 22 of 28

0 120

Time (s)

Figure 9 HCl concentration over time in the gasification process of ZhunDong coal at different temperatures

Figure 10 Forms of sodium after the gasfication of coke at various temperatures 22


Page 23 of 28

Content of soluble sodium（ μg/g（

3500

pyrolysis reaction gasification reaction difference between two reactions

3000 2500 2000 1500 1000 500 0

400

600

800

1000

Temperature（ oC（

Figure 11 Comparison of soluble sodium content in residual solid after pyrolysis and gasification at different temperatures

majority

HCl ↑

HCl(g)

Cl

HCl(g)

Cl →

H

Cl

ity or

in m

Na

Raw coal

NaCl(s) NaCl(salgd) Na2O 3SiO2(s)

Na2O(salgd) NaOH(salgd) Na2O 3SiO2(s)

Pyrolysis (400 oC)

Gasification (400 oC)

Figure 12 Migration path of chlorine and sodium in the gasification process at 400 oC

majority


majority

Raw coal

NaCl↓

HCl(g) much NaCl(g) little Na2O(g) trace NaOH(g) trace

H

minority

Na

HCl ↑

Cl

Cl

Cl →

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


NaCl(salgd) Na2O 3SiO2(s)


Pyrolysis (600 oC)

Figure 13 Migration path of chlorine and sodium in the gasification process at 600 oC 23


Energy & Fuels


majority

Cl

HCl(g) much NaCl(g) little Na2O(g) little NaOH(g) little

HCl↑

m

aj

or it

y

ce

tra

NaCl↓

Na minority

Na2O 3SiO2(s) Na2O nAl2O3 mSiO2(s)

Na2O 3SiO2(s) Na2O nAl2O3 mSiO2(s)


Pyrolysis (800 oC)

Raw coal


majority

Cl

HCl(g) trace NaCl(g) much

HCl(g) much NaCl(g) little Na2O(g) little NaOH(g) little

HCl↑ NaCl↓

m

aj o

rit

y

ce

tra

Na minority

Raw coal

Na2O(salgd) Na2O nAl2O3 mSiO2(s)

Na2O nAl2O3  mSiO2(s)

Pyrolysis (600 oC)



10

Fe2O3+6HCl(g)=2FeCl3+3H2O(g) FeO+2HCl(g)=FeCl2+H2O(g) Fe3O4+8HCl(g)=2FeCl3+FeCl2+4H2O(g) Fe+2HCl(g)=FeCl2+H2(g) 2Fe+6HCl(g)=2FeCl3+3H2(g)

8 6 4

Log(Kp)

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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2 0 -2 -4 -6 -8

-10

200

400

600

800

1000

1200


Figure 16 Influence of temperature on equilibrium constant of (R4) ~ (R8) 24


Page 25 of 28

4FeCl3+3O2(g)=2Fe2O3+6Cl2(g) 4FeCl2+3O2(g)=2Fe2O3+4Cl2(g) 4FeCl2+O2(g)=2FeCl3+2FeOCl 4FeOCl+O2(g)=2Fe2O3+2Cl2(g)

30 25

Log(Kp)

20 15 10 5 0 200

400

600

800

1000

1200


Figure 17 Influence of temperature on equilibrium constant of (R9) ~ (R12)

2400

1600

250

200

150 1200 100

800

50

400

0

0

20

40

60

80

100


400 oC 600 oC 800 oC 1000 oC

2000


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

0 120

Time (s)

Figure 18 HCl release over time during chemical looping process of ZhunDong coal in fuel reactor at different temperatures

25


Energy & Fuels

Figure 19 Forms of sodium after the chemical looping reaction of coke at various temperatures

Content of soluble sodium（ μg/g（

3500

gasification reaction redox reaction in FR difference between two reactions

3000 2500 2000 1500 1000 500 0 400

600

800

1000


Figure 20 Comparison of soluble sodium content in residual solid after gasification and chemical looping process at different temperatures

majority

HCl(g)

Cl

HCl↑

Cl H

Fe

Pyrolysis (400 oC)

3

Cl →

Cl

Raw coal

NaCl(s) NaCl(salgd) Na2O 3SiO2(s)

HCl(g)

→ Cl

ity or

Na

HCl↓

HCl(g)

in m

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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Na2O(salgd) NaOH(salgd) Na2O 3SiO2(s) Gasification (400 oC)

Na2O 3SiO2(s) majority Na2O  nAl2O3 mSiO2(s)minority FeCl3 trace Redox reaction in FR (400 oC)

Figure 21 Migration path of chlorine and sodium in the chemical looping process at 400 oC 26


Page 27 of 28

HCl(g) much NaCl(g) little Na2O(g) trace NaCl↓ NaOH(g) trace

Cl

NaCl(g) little

H Cl Cl → NaCl(salgd) Na2O 3SiO2(s)


Pyrolysis (600 oC)


majority

Raw coal


NaCl↓ 3 Cl Fe → → C l Na

minority

Na

HCl↓

HCl↑

majority HCl(g) much

Na2O 3SiO2(s) much Na2O  nAl2O3 mSiO2(s) little FeCl3 trace

Redox reaction in FR (600 oC)

Figure 22 Migration path of chlorine and sodium in the chemical looping process at 600 oC

majority

Cl

HCl(g) little NaCl(g) much

ce

tra

HCl(g)much NaCl(g)little Na2O(g)little NaCl↓ NaOH(g)little HCl↑

HCl↑ NaCl↓

HCl(g)much NaCl(g)little

m

aj or

a→

ity

N

Na minority

Raw coal

Na2O 3SiO2(s) Na2O nAl2O3 mSiO2(s)

Na2O 3SiO2(s) Na2O nAl2O3 mSiO2(s)

Pyrolysis (800 oC)



Redox reaction in FR(800 oC)


majority

Cl

HCl(g) trace NaCl(g) much

tra ce

HCl↑ NaCl↓

HCl(g)much NaCl(g)little Na2O(g)little NaOH(g)little

HCl↑ NaCl↓

HCl(g)much NaCl(g)little

m

aj

or

a→

ity

N

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

Na

minority

Raw coal

Na2O(salgd) Na2O nAl2O3 mSiO2(s)

Pyrolysis (1000 oC)




Redox reaction in FR (1000 oC)


27


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

Page 28 of 28

Table 1 Proximate analysis, ultimate analysis and ash composition analysis of ZhunDong coal Coal

ZhunDong coal

Proximate analysis/(wt%, ar)

Ultimate analysis/(wt%, ar)

M

V

FC

A

C

H

O

N

S

12.15

28.54

53.90

5.41

65.69

4.98

16.33

0.85

1.78

Ash composition analysis/(wt%, ar) SiO2

Al2O3

CaO

MgO

Fe2O3

Na2O

K2O

SO2

P2O5

10.58

12.60

24.27

8.20

19.91

7.50

0.42

13.75

2.03

28


Study on the migration characteristics of sodium and chlorine in

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