Effect of Sodium Removal on Chemical Looping Combustion of High

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Effect of Sodium Removal on Chemical Looping Combustion of High-sodium Coal with Hematite as Oxygen Carrier Jingchun Yan, Huijun Ge, Shouxi Jiang, Haiming Gu, Tao Song, Qingjie Guo, and Lai-hong Shen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00044 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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Effect of Sodium Removal on Chemical Looping Combustion of High-sodium Coal with Hematite as Oxygen Carrier Jingchun Yana, Huijun Gea, Shouxi Jiang*a, Haiming Gub, Tao Songc, Qingjie Guod, Laihong Shena a

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China b School c d

of Energy and Power Engineering, Nanjing Institute of Technology, Nanjing 211167, China

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

State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan, 750021, China

Abstract Chemical looping combustion (CLC) technology has been proved to be an effective method to cope with severe ash-related problems occurred during utilization of high-sodium Zhundong coal (ZD). While alkali metals in coal have significant impacts on coal conversion process, it is still remained unclear how different occurrence modes of alkali metals influence thermal conversion of ZD during a CLC process. Here, a method using sequential extraction to remove different occurrence modes of sodium from ZD is presented. And the effects of cycle numbers, gasification agents and different occurrence modes of sodium on CLC performance of ZD were investigated in a laboratory-scale fluidized bed reactor. The data indicate that water-soluble sodium is the predominant occurrence form in ZD. The H2O-soluble sodium plays a prohibitive role in a typical CLC reduction process, while the CH3COONH4-soluble sodium and HCl-soluble sodium have a distinct catalytic effect on promoting combustion performance during CLC of ZD. The BET and SEM analysis show that the surface structure of ZD with different extraction level changed slightly. It indicates that removal of different modes of sodium plays a dominant role in the combustion performance of ZD during a CLC process. The main reduced phase of oxygen carrier after use was Fe3O4 and a certain amount of Na2O·Al2O3 ·SiO2 with high melting point was also detected. The results implied that the CLC with hematite as oxygen carrier was suitable approach for ZD conversion without severe ash-related problems. Key words 1 / 29

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Chemical Looping Combustion; Sodium-Rich Coal; Hematite Oxygen Carrier; Sodium removal Highlights A sequential extraction process was used to remove sodium in coal. The H2O-soluble sodium poses an inhibition effect on the char gasification process. The CH3COONH4-soluble and HCl-soluble sodium promote CLC performance of ZD. Na2O·Al2O3·SiO2 with high melting point was detected in oxygen carrier after use.

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1. Introduction Coal, which has the characteristics of low price, abundant supply, diversified category and insusceptible feature, is of crucial importance to the industrial revolution in the world. Particularly, coal is playing a dominant role in the electricity generation of the world, especially in developing countries such as China and India [1,2]. By 2017, the total amount of the proved coal reserves in China was 138819 Mtoes, accounting for 13.4% of the world proved reserves. The China’s coal consumption in 2017 was 1892.6 Mtoes, 70% of which was directly burnt for electricity generation [3-4]. Coal will still be in an indispensable position due to its considerable proved and prospective reserves in next decades. Zhundong coalfield, recently discovered in Junggar basin located in Western China, is by far the largest integrated coalfield with an estimated large reserve amount of 390 Gt, around 40 percent of the whole Chinese coal reserves [5-6].

This coalfield is arousing increasing interest of many researchers on account of its

potentially huge reserves. Hence, massive investigations of Zhundong coal characteristics have been widely proposed [7-13]. Zhundong coal (ZD) is featured with high volatile matter, medium moisture content, and low content of ash, sulfur and trace element, which takes on characteristics of typical lignite [7]. It is widely used as fuel for industrial application in in-situ boilers operating nearby the coalfield. However, owing to its unique property of high content of sodium, severe high temperature corrosion, ash slagging and fouling problems occurred on the heating surface of industrial boilers, giving rise to a reduced effectiveness of the heat transfer. Li et al.

[8]

investigated ash deposition during

combustion of ZD in a drop tube furnace and found that the deposits were abundant in low melting point compounds including anhydrite, lime and nepheline etc., while refractory mineral phases like quartz and mullite were depleted, causing significant sintering and fusion. Wang et al. [9] reported that albite (NaAlSi3O8) and sodium sulfate (Na2SO4) with a low melting point were found at low deposition temperature. Both the formation of low-melting point compounds and the condensation of gaseous alkalis led to slagging and fouling on the convection heat exchanger surfaces. Wang et al. [10] also achieved similar results that the condensing and deposition of sodium and calcium sulfates exerted great influences on the slagging and ash deposition on convection heating surfaces. Moreover, many investigations have been carried out on ash deposition and alkali metals transformation during combustion of ZD coal [11-13]. These researches promote the emergence of settlement methods that prevent fouling and 3 / 29


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slagging when using fuels with high concentration of alkali and alkaline earth metal (AAEM). Previous studies have found that additives such as aluminium silicates based additives, could capture alkali metal species in the form of refractory materials with high melting points. Thus, several methods including fuel blending and binding with additives have been proposed to mitigated ash-related problems [14-21]. Mann et al. [15] used kaolin as an alkali absorbent in a pressurized fluidized combustor and as much as 93% of sodium released from the combustion of lignite could be absorbed. Vuthaluru et al. [16] found that adding 3 wt.% bauxite into raw coal would appear to show a better performance concerning slagging and fouling propensity compared with use of the raw coal alone. Ge et.al

[17]

found that the fouling problem can be alleviated when using

hematite as oxygen carrier during chemical looping combustion process, because the intrinsic supports such as SiO2 and Al2O3 played roles as additives to inhibit a high sintering rate. To alleviate the high temperature corrosion, Broström et al. [18] injected ammonium sulfate in a full scale CFB and the analysis of deposits showed that corrosive alkali chlorides in the flue gas were significantly decreased. Steenari et al. [19] investigated the effects of limestone powder on the ash melting behavior. The results showed that the limestone powder made most potassium-rich phosphates transform to phosphates with calcium and magnesium, and the sintering temperature was increased by at least 100 oC. Moreover, combustion of biomass in a fluidized bed reactor with phosphoric acid as a fuel additive was also investigated

[20, 21].

Introduction of more

phosphorus in the ash residues may convert the available fuel ash basic oxides into phosphates and thus prevented the formation of K-silicates with low melting point. Among the above methods, chemical looping combustion (CLC) is an innovative technology with inherent CO2 capture, and large energy penalties of gas separation are avoided during CLC of coal [22]. As depicted in Fig. 1, the CLC process is achieved via a metal oxide (MexOy) called oxygen carrier circulating between two major reactors, i.e. the air reactor (AR) and the fuel reactor (FR) to supply the required oxygen. The redox reactions concerning oxygen carriers during cycles occur according to R1 and R2, respectively.

(2n + m)MexOy + CnH2m → (2n + m)MexOy - 1 + nCO2 + mH2O - Q2

(R1)

2MexOy - 1 + O2 → 2MexOy + Q1

(R2)

The commonly used oxygen carriers include CuO, Fe2O3, NiO, CoO and Mn3O4. Considering the high make-up costs, poor stability and mechanical properties for MexOy as oxygen carrier in their pure state, natural ores containing metal oxides which 4 / 29


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are inherently bound to materials like Al2O3, SiO2 and TiO2 are used as a superior alternative

[23].

As mentioned above, the introduction of aluminium silicates based

additives proves to be effective for controlling ash-related problems. It is easy to understand, as a consequence, that its own supports such as Al2O3 and SiO2 in oxygen carries occupy an important role in alleviating the fouling and sintering problems from high-sodium coal. Fuel

CO 2, H2O

Fuel Reactor

MexOy

MexOy-1

Air Reactor

Air

N2, O2

Fig. 1 Schematic of chemical looping combustion technology Previous publications have involved the macroscopical combustion performance of CLC of ZD. However, the researches on transformation behaviors of alkali metal species in ZD coal during a CLC process are rarely reported. Alkali metals present various forms in coal. In terms of sodium in ZD coal, it exhibits four occurrence modes including soluble salts (e.g. NaCl), organic state (e.g. carboxylate), insoluble silicate or aluminosilicate (e.g. Na-Al-Si) and acid-soluble inorganic species [24, 25]. A sequential extraction process by successive leaching with chemicals has been widely used to determine the occurrence mode of alkali metals in coal. Four types of alkali metals are qualified as H2O-soluble, CH3COONH4-soluble, HCl-soluble and insoluble form according to the commonly-used leaching agents, i.e. H2O, CH3COONH4 and HCl [26, 27].

The degree to which alkali metals in coal affect ash-related problems varies with

the occurrence modes of alkali metals. The present work focuses on the effect of various occurrence modes of alkali metals, mainly sodium on the combustion performance of high-sodium coal, ZD for example, during a CLC process. The method of sequential 5 / 29


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extraction was adopted to prepare coal samples containing various forms of sodium. The characteristics of coal particles with different extraction level and oxygen carriers were also investigated.

2. Material and method 2.1 Sample preparation ZD, which was obtained from Zhundong coalfield, Xinjiang Province, China, was chosen as solid fuel. The individual samples of ZD raw coal were sieved to yield a size range of 600-800 𝜇m, which were then dried in a dry oven at 120 oC for 24 h and stored in sealed containers for further application and disposal. The proximate and ultimate analysis of ZD raw coal were given by the following Table 1. Table 1 Proximate and ultimate analysis of ZD raw coal Proximate analysis (wt.%, ad)

ZD

Ultimate analysis (wt.%, ad)

M

V

FC

A

C

H

O

N

S

14.58

28.05

53.03

6.34

64.18

4.302

9.751

0.50

0.347

An X-ray fluorescence spectrometer (XRF, ZSX Primus Ⅱ, Rigaku, Japan) is used to analyze the compositions of ZD ash, which was obtained in a muffle oven at 850 oC for 3h. The results are showed as oxides in Table 2. It is observed that ZD features with high alkali contents especially sodium. Table 2 Elemental composition analysis of ZD ash (wt.%)

ZD

CaO

SiO2

SO3

Fe2O3

Al2O3

Na2O

MgO

TiO2

K2O

Others

21.55

16.56

13.99

14.05

9.88

9.22

5.75

1.75

0.51

6.74

An extensively used method of sequential extraction, as stated by Benson [28], was adopted to prepare coal samples containing various forms of alkali. As seen from Table 2, unlike biomass, the potassium content in ZD coal is so little that the influence of it can be neglected. Thus, the removal of sodium is mainly considered here. The flow diagram is shown in Fig. 2. Firstly, samples of ZD coal with a particle size range of 600-800 𝜇m on a dry basis were added into deionized water, then heated at 60 oC in water bath equipment for 24 h with magnetic stirring. The filtered coal samples after water scrubbing were dried in a dry oven at 120 oC for 24 h, sieved again and marked as water-washing ZD (WW-ZD). The ratio of ZD raw coal to deionized water was solid sample of 1 g to solution of 30 mL. Subsequently, samples of WW-ZD were extracted with 1.0 mol·L-1 ammonium acetate (CH3COONH4) at a temperature of 60 oC for 24 h. 6 / 29


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The filtered coal samples after drying in the oven at 120 oC for 24 h and sieving were denoted as ammonium acetate washing ZD (AAW-ZD). The ratio of solid-to-liquid was set as WW-ZD of 1g to ammonium acetate solution of 25 mL. The solid samples after CH3COONH4 extraction were dropped into 1.0 mol·L-1 hydrochloric acid (HCl) at the same temperature as prior experiments, i.e. 60 oC for 24 h. The remain solid samples obtained by drying and sieving were labeled as hydrochloric acid washing ZD (HAWZD). The ratio of solid-to-liquid was also set as AAW-ZD of 1g to hydrochloric acid solution of 25 mL. In each process, the filtrate or solution was transferred to a 500-mL volumetric flask, diluted to volume and analyzed by Atomic Absorption Spectrophotometer (TAS-990, Beijing Purkinje General Instrument Co.,Ltd., China) to determine the contents of different occurrence modes of sodium. The HCl-ZD samples were digested by Digested with HCl-HNO3-HF-HClO4 solution and diluted to volume before insoluble sodium was analyzed. AAS analysis

Dry ZD coal

Deionized water (Solid-to-liquid ratio 1g:30ml)

Stir at 60oC for 24 hours. Then stand for 24 hours and filter

Filtrate Residue

Dried at 120oC for 24 hours and sieved

AAS analysis

WW-ZD

1.0 mol·L-1 CH3COONH4 solution (Solid-to-liquid ratio 1g:25ml)


Filtrate Residue


AAS analysis

-1

AAW-ZD

1.0 mol·L hydrochloric acid (Solid-to-liquid ratio 1g:25ml)

HAW-ZD


Digested with HCl-HNO3-HF-HClO4 system and diluted

Filtrate Residue

Stir at 60oC for 24 hours


Solution

AAS analysis

Fig. 2 Flow diagram of sequential extraction procedure

Natural iron ore, which was supplied by Nanjing steel manufacturing company, was selected as oxygen carrier in this work. Before use, the large pieces of ore were crushed and calcined in a muffle oven at 950 oC for 3h to improve the mechanical properties, after which the calcined particles were sieved to the size range of 100-300 𝜇 m for use as fresh oxygen carrier. The chemical compositions of fresh oxygen carrier 7 / 29


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based on X-ray Fluorescence analysis (XRF) are listed in Table 3. The bulk density of oxygen carrier is 1967 kg/m3, hence the critical fluidization velocity of oxygen carrier under ordinary pressure is calculated as 0.145 m/s. ZD and iron ore are of different density features. The purpose of preparing coal and oxygen carrier particles with different diameters is to reduce the entrainment of coal particles out from the reactor. Table 3 Elemental composition of the fresh hematite oxygen carrier (wt.%) Compositions

Fe2O3

SiO2

Al2O3

CaO

P2O5

TiO2

K2O

SO3

Others

Contents

83.25

7.06

5.33

0.23

0.29

0.09

0.03

0.25

3.47

2.2 Experimental setup Experiments were conducted in a batch fluidized bed reactor, as illustrated in Fig. 3, to investigate the effect of various occurrence modes of sodium in coal on CLC performance of ZD. The reactor consists of a straight quartz tube with a height of 570 mm and a diameter of 32 mm within an electric heating furnace. A porous distributor is installed 450 mm from the bottom to load oxygen carrier. In each batch experiment, a sample of 30 g fresh oxygen carrier with a size range of 100-300 𝜇m was added on the distributor after the reactor was heated up to the set temperature of 900 oC. Afterwards, a mixture stream of N2 and O2 with flow rates of 1.75 L/min and 0.1 L/min was introduced into the reactor from the bottom to ensure the oxygen carrier to be completely oxidized before coal was added. The inlet flow was switched to a mixture of N2 (1.75L/min) and gasification agent i.e. steam or CO2 when the temperature remained stable. When the flow rate of steam was set as 0.5g/min, the fluidization number was around 2.5. The coal sample (1 g) was added from the top of the reactor after the temperature reached stability again. The time starts when coal particles were added into the reactor. The outlet gas was firstly cooled, then dried and filtered before being collected by gas bag per 1 min for offline analysis in EMERSON gas analyzer. For multiple cycle experiments, the reduction period lasted for at least 25 min and oxygen carrier particles were oxidized for 15 min each cycle. Before coal was introduced, the system was purged with nitrogen for O2 replacement. Once coal particles were added into the reactor, intensive heat and mass transfer happened between solid fuel and oxygen carrier particles followed by a series of reactions. The devolatilization of coal takes place as soon as coal particles were instantaneously heated up to reactor temperature, as R3 shows. Char is gasified by gasification agents of steam and CO2 (R4, R5). The gaseous products of devolatilization 8 / 29


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as well as char gasification subsequently react with oxygen carrier, mainly Fe2O3 to yield CO2 and water (R6-R8). When the atmosphere is changed to oxidation state at the end of reduction process, the reduced Fe3O4 is oxidized back to Fe2O3 and completes regeneration (R9). Devolatilization of coal: (R3)

Coal → Volatiles (CO, H2, CH4) + Char + Ash Char gasification: C + H2O → CO + H2

(R4)

C + CO2 → 2CO

(R5)

CO + 3Fe2O3 → 2Fe3O4 + CO2

(R6)

H2 + 3Fe2O3 → 2Fe3O4 + H2O

(R7)

CH4 + 12Fe2O3 → 8Fe3O4 + CO2 + 2H2O

(R8)

Reduction reactions:

Oxidation reaction: (R9)

4Fe3O4 + O2 → 6Fe2O3 Charging Chute

Thermocouple and Temperature Controller CO 2

H2O

N2

O2

T

Emerson CO: 0-100% CO2: 0-100% CH4: 0-10% O 2: 0-25% H 2: 0-50%

Pump Mass Flow Controller

1 ml/min TBP 50A

Steam Generator

Vent

Vent

T Cooler Drier Filter Inlet Flow

Furnace

T Thermocouple

Fig. 3 Schematic layout of batch fluidized bed reactor

2.3 Data evaluation The volume flow rate of inlet nitrogen was constant and the volume fractions of outlet gases (Xi,out, i=CO, CO2, CH4, H2, O2) were analyzed by the gas analyzer. During the reduction stage, the molar flow rates of outlet gas component (ni,out, i=CO, CO2, CH4, H2, O2) can be calculated based on the nitrogen balance method: 9 / 29


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𝑋𝑖,𝑜𝑢𝑡 ∙ 𝐹𝑖𝑛,𝑁2

𝑛𝑖,𝑜𝑢𝑡 = 22.4 × (1 ― ∑𝑋

𝑖,𝑜𝑢𝑡

)

(1)

where 𝐹𝑖𝑛,𝑁2 is the flow rate of inlet N2. The cumulative molar flow rates of outlet gases are defined as: 𝑡

𝑛𝑖 = ∫𝑜𝑛𝑖,𝑡𝑑𝑡

(2)

The volume fractions of carbonaceous gases can be calculated as: 𝑓𝑖 =

𝑛𝑖 𝑛𝐶𝑂 + 𝑛𝐶𝑂2 + 𝑛𝐶𝐻4

(3)

where ni (i=CO, CO2, CH4) are the cumulative molar flow rates of carbonaceous gases derived from the Eq. (2). The carbon conversion rate can be calculated as: 𝑥𝐶 = 𝑛𝐶𝑂,𝑜𝑢𝑡 + 𝑛𝐶𝑂2,𝑜𝑢𝑡 + 𝑛𝐶𝐻4,𝑜𝑢𝑡 ― 𝑛𝐶𝑂2,𝑖𝑛

(4)

The carbon conversion efficiency at time t of the reduction stage is the fraction of total carbon contained in the fuel converted to carbonaceous gases, which is defined as below: 𝑡

𝑋𝐶,𝑟𝑒𝑑 =

∫𝑡 𝑥𝐶𝑑𝑡 0

(𝑚𝑓𝑢𝑒𝑙 × 𝜑𝐶,𝑓𝑢𝑒𝑙)/𝑀𝐶

(5)

with 𝑚𝑓𝑢𝑒𝑙 the mass of solid fuel initially added in the reactor, 𝜑𝐶,𝑓𝑢𝑒𝑙 the carbon content of the solid fuel and 𝑀𝐶 the molar weight of C.

3. Results and discussion 3.1 The occurrence modes of sodium in ZD The occurrence modes of sodium in ZD sample based on the AAS analysis are presented in Fig. 4. The specific contents are shown on the top of each column.

10 / 29


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60

Relative content of sodium in ZD/% (gg-1)

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

H2O-soluble

1350

CH3COONH4-soluble HCl-soluble Insoluble

50 40 30 20 10 0

423

359 168

H2O-soluble

CH3COONH4-soluble

HCl-soluble

Insoluble

Fig. 4 Occurrence modes of sodium in ZD

It can be seen that the H2O-soluble sodium makes up the largest proportion of alkali metal sodium in ZD, with around 58.7% of total sodium content. The sodium in CH3COONH4-soluble and HCl-soluble form accounts for 15.6% and 7.3% respectively, while the remaining 18.4% is attributed to the insoluble sodium. Despite slight difference, the analysis results are basically in agreement with those obtained from previous works [29-31]. It should be noted that data of sodium occurrence modes obtained by sequential extraction method are more accurate than those directly from X-ray Fluorescence analysis. Because a portion of sodium will loss during ZD ashing process, and XRF is a nonquantitative analysis method.

3.2 Effect of gasification agent The selection of gasification agent exerts an important influence on the CLC of solid fuel. The concentrations of gaseous products for ZD as a function of time for a typical reduction cycle with different gasification agents at 900oC, are depicted in Fig. 5 (a) and (b). When CO2 was used as gasification agent, it is clearly seen from Fig. 5 (a) that the concentration of CO increases up to a maximum of 5.94% at the first minute. After that, the concentration curve of CO2 shows a decrease trend due to the process of gasification reaction R5. After the peak value, the reduction process takes about 30 minutes before the CO concentration goes down linearly to zero. The initial concentration peaks of CH4 and H2 are thought to be principally resulted from the release of volatiles during the rapid devolatilization process, after which almost no CH4 and H2 are generated.

11 / 29


Energy & Fuels

10

18 16

8

12 10

ZD (1g)

Hematite (30g)

CO2 (366 ml/min)

T=900℃

8

Gas concentrations/%

14


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CO CO2 CH4 H2

6 4

ZD (1g)

Hematite (30g)

Steam (0.5g/min)

T=900℃

6

CO CO2

4

CH4 H2

2

2 0

0 0

2

4

6

8

10 12 14 16 18 20 22 24 26 28 30

0

2

4

6

8

10

12

14

16

18

20

22

24

Time/min

Time/min

(a)

(b)

Fig. 5 Concentrations of gaseous products during CLC process of ZD with CO2 or steam as gasification agent

It can be noticed from Fig. 5 (b) that a completely different combustion performance of ZD is obtained when gasification agent is switched to steam. The concentrations of carbonaceous gases show a single peak after initial one minute and then fall to zero rapidly. The initial peaks of CO, H2 and CH4 are mainly from volatiles, while the formation of CO2 is also attributed to the reaction between pyrolysis products and oxygen carrier in addition to the pyrolysis gases and gasification intermediates. In gasification process, water plays a role of contributor of oxygen and hydrogen element, which is believed to be responsible for the greatly increase yields of hydrogen. Moreover, compared with results achieved under CO2 atmosphere, the reaction time is shortened by nearly half (15 min). It may also be noticed that the concentration of H2 is with two peaks. The lower peak at initial stage is due to pyrolysis of coal, similar to the previous discussion. And the second one appearing at 5th minute is believed to be related to the Water-gas shift (WGS) reaction between CO and steam, which would be discussed in detail in section 3.3.2. In consideration of the better gasification results, steam is used as gasification agent in all the following experiments. 3.3 Combustion performance of ZD with different extraction level 3.3.1 Gas concentrations In an effort to compare the combustion performance for ZD with different extraction level during a CLC process, experiments were conducted with ZD, WW-ZD, AAW-ZD and HAW-ZD as solid fuel and results are shown in Fig. 6 (a)-(d). Similar variation trends are observed for gas concentrations with ZD, WW-ZD and AAW-ZD, while evident difference can also be noticed. As is presented in the elliptical dashed frame in figures, all the concentration change curves of main gaseous products during 12 / 29


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initial 4 minutes reach a peak after one minute and then start to decrease. However, the peak values of gaseous products for ZD coal through different treatment are various. The concentration of CO2 for WW-ZD increases to peak at 11.9%, while those for ZD and AAW-ZD reach 9.88% and 8.56%, respectively. A similar tendency occurs to the concentration profiles of CO and H2 for these three types of ZD coal. This is an interesting phenomenon beyond our expectation which is worth further exploration. 12

10


8

Hematite (30g)

Steam (0.5g/min)

WW-ZD (1g)

Hematite (30g)

Steam (0.5g/min)

T=900℃

10

T=900℃

6


ZD (1g)

CO CO2 CH4

4

H2 2

0

8

CO CO2

6

CH4 4

H2

2 0

0

2

4

6

8

10

12

14

16

18

20

22

24

0

2

4

6

8

Time/min

12

14

16

18

20

22

24

(b)

10 AAW-ZD (1g)

Hematite (30g)

Steam (0.5g/min)

T=900℃

5

4

6

CO CO2

4

CH4


8

10

Time/min

(a)


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

H2 2

0

HAW-ZD (1g)

Hematite (30g)

Steam (0.5g/min)

T=900℃

CO CO2

3

CH4

2

H2

1

0

0

2

4

6

8

10

12

14

16

18

20

22

24

0

5

10

15

20

25

30

35

40

45

50

Time/min

Time/min

(c)

(d)

Fig. 6 Concentrations of gaseous products during CLC process of ZD (a), WW-ZD (b), AAW-ZD (c), HAW-ZD (d) with steam as gasification agent

As stated above, the initial stage included two main processes, i.e. pyrolysis of coal and gasification of char. For a better understand of the influence mechanism of these two processes, experiments that separated the two processes were performed under identical experimental conditions. The only difference was that steam was not added into the reactor until the pyrolysis process was over. The concentrations of gaseous products as a function of time are shown in Fig. 7 (a)-(d). It is not hard to find from Fig. 7 (a) and (b) that the pyrolysis curves for ZD and WW-ZD before steam is added are almost identical. It may be concluded therefore that the H2O-soluble sodium has nearly no effect on the pyrolysis process of coal. However, it can be clearly seen 13 / 29


Energy & Fuels

from Fig. 6 (a) and (b) that the relative fractions of carbonaceous gases for WW-ZD in the initial stage of CLC process are higher than ones for untreated ZD, which indicates that higher conversion rate of char for ZD without H2O-soluble sodium is obtained. Furthermore, similar results can be observed from the rear stage of char gasification process in Fig. 7 (a) and (b). Thus, it could reasonably be inferred that H2O-soluble sodium exerts an inhibition effect on the char gasification process for ZD. This could be resulted by a combination of diffusion process of H2O-soluble sodium, thermodynamics and kinetics of chemical reactions, and changes in morphology and structure interior and exterior surface of coal particles. During the preliminary thermal conversion stage of coal, the release of H2O-soluble alkali metal species into gas phase has been verified by experiments and documented in many references [8, 26, 27, 32]. In this process, high content of H2O-soluble sodium is firstly carried to the surface of coal particle from pore structure by evaporated water in coal, and is then volatilized into the gas phase. The overall process is endothermic, which is believed to be partly responsible for the lower fractions of carbonaceous gases for ZD. Besides, the transformation of sodium among various occurrence modes has been found in some researches. Most of the H2O-soluble sodium was released into gas phase while part of the remainder was converted into an insoluble form

[25].

The stable form of Na in the

char was transferred to other forms during char gasification via the pore opening and a series of chemical reactions, which were also endothermic processes [33]

were in agreement with what Sugawara et al.

[27].

The results

found. Given the above, the

endothermic effect during the early stage of char gasification process where H2Osoluble sodium is prone to transform into other occurrence modes, is also adverse to the char gasification process of ZD. Moreover, the surface morphologic structure changes of ZD after water washing could make a difference, which is worthwhile to be further investigated. 8

8

CO CO2

Hematite (30g) T=900℃

6

H2

4

Steam (0.5g/min)

3 2 1 0 -1

CO CO2

Hematite (30g)

T=900℃

6

CH4

5

WW-ZD (1g)

7


ZD (1g)

7


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 14 of 29

CH4 H2

5 4

Steam (0.5g/min)

3 2 1 0

0

5

10

15

20

25

30

35

-1

40

0

5

Time/min

10

15

20

Time/min

(a)

(b) 14 / 29


25

30

35

40

Page 15 of 29

AAW-ZD (1g)

T=900℃

4

3.5

CO CO2

Hematite (30g)

CH4

CO CO2

Hematite (30g)

T=900℃

H2 3

HAW-ZD (1g)

3.0


5


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

Steam (0.5g/min)

2

1

CH4

2.5

H2 2.0 Steam (0.5g/min) 1.5 1.0 0.5

0

0.0 0

5

10

15

20

25

30

35

40

0

5

Time/min

10

15

20

25

30

35

40

Time/min

(c)

(d)

Fig. 7 Concentrations of gaseous products in the processes of coal pyrolysis and char gasification for ZD (a), WW-ZD (b), AAW-ZD (c), HAW-ZD (d). Steam was injected into the reactor at the beginning of the 23rd minute for ZD, WW-ZD and AAW-ZD, and the 9th min for HAW-ZD.

Different from H2O-soluble sodium, CH3COONH4-soluble sodium in ZD exerted influences on both pyrolysis and char gasification process, which could be seen from Fig. 6 (c) and Fig. 7 (c). The concentrations of carbonaceous gases obviously decrease for ZD after being extracted with ammonium acetate. It may also be noticed that the concentration of CO2 for AAW-ZD shows a slower decrease rate compared with those for ZD and WW-ZD. One possible reason is that CH3COONH4-soluble sodium has a positive impact on coal pyrolysis and char gasification process. In the first minute, ZD that was not treated with ammonium acetate solution contained more CH3COONH4soluble sodium. Hence, the pyrolysis and char gasification rates are accelerated and the amount of remaining char is reduced, which results in the rapid decline of CO2 and CO concentrations. In the case of AAW-ZD with little CH3COONH4-soluble sodium, the consumption of carbon in the initial one minute is relatively less. The yields of gaseous products during the later process thus increase. However, the overall carbon conversion rate in the initial few minutes is still lower than ones for ZD and WW-ZD, which could be observed from Fig. 8.

15 / 29


Energy & Fuels

100

80 ZD WW-ZD AAW-ZD HAW-ZD

60 75

40

73.196

70

69.407

68.76

65

Carbon conversion/%

Carbon conversion/%

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 16 of 29

20

66.104 63.98

62.757

60

60.709

55

56.241

ZD WW-ZD AAW-ZD

54.19 52.138

50 45 44.463

40 39.439

0

35

2

3

4

5

Time/min

0

5

10

15

20

25

30

35

40

45

50

55

60

Time/min

Fig. 8 carbon conversion efficiency during typical reduction process of CLC for four types of ZD coal.

Similar variation trends exist in gas concentrations for ZD, WW-ZD and AAWZD in spite of some clear differences. However, it is interesting that the gas concentration profiles for HAW-ZD is entirely different. The formation rates of gaseous products are rather slow during the whole gasification process. Both the carbon conversion efficiency and carbon conversion rate for HAW-ZD significantly decrease according to Fig. 8. It can be seen that the time taken to attain 60% carbon conversion varies from about 3 minutes for ZD to approximately 20 minutes for HAW-ZD. The CLC process of HAW-ZD is prolonged due to the extraction of HCl-soluble sodium in coal. Conclusions may be draw that the existence of HCl-soluble sodium, same as CH3COONH4-soluble sodium, is beneficial to the CLC of ZD. The reason for the results is thought to be related to the existence forms organic sodium in coal structure. The CH3COONH4-soluble sodium exists in the form of carboxylate, while the HCl-soluble sodium is bound to the oxygen and nitrogen functional groups in coordination form [34]. The organic sodium existing in the macromolecular structure of coal may promote the fracture of basic structural units and bridged bonds, which is conducive to improve the combustion characteristic of coal. Besides, it also implies that not all occurrence modes of sodium in coal have a catalytic effect on char gasification process. It can be concluded from the above results that organic sodium species in the form of CH3COONH4-soluble sodium and HCl-soluble sodium have a distinct catalytic effect on char gasification process, while H2O-soluble sodium is catalytically inactive. 3.3.2 Water-gas shift process 16 / 29


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Energy & Fuels

From Fig. 6 and Fig. 7, it is noteworthy that the concentration profile of H2 for HAW-ZD is different from ones for the other three types of coal. Apart from the initial concentration peak during pyrolysis process, no H2 is generated in the whole CLC process of HAW-ZD. Nevertheless, there are two peaks of H2 concentration in the CLC process of ZD, WW-ZD and AAW-ZD, at the first minute and around the 6th minute. As for the latter peak, there is a basic chemical reason that can best be illustrated by the following reactions R10 and R11. Water-gas shift (WGS) equilibrium: CO(g) + H2O(g) → CO2(g) + H2(g) ∆H0900 = ― 33.132 kJ/mol ∆G0900 = 2.571 kJ/mol

(R10)

∆H0800 = ― 34.116 kJ/mol ∆G0800 = ― 0.515 kJ/mol Reverse reaction of methanation: (R11)

CH4 + H2O → CO + 3H2

But in view of the slight amount of CH4 produced during devolatilization process, the effect of R11 on the H2 concentration is negligible. The WGS reaction dominates the subsequent formation of H2. The reaction between CO and steam is exothermic in term of chemical thermodynamics. And the Gibbs free energy at 900 oC is greater than zero, which indicates that the CO is difficult to react with steam to produce H2 at high temperature. However, some researchers have proved that iron-based materials could act as high-temperature shift catalysts to promote the WGS reaction [35, 36]. And Fe3O4 was further confirmed the active phase which was responsible for the WGS reaction. Coincidentally, the main composition of natural hematite that is used as oxygen carrier during CLC process is Fe2O3. When solid fuel is added into the reactor, Fe2O3 contained in the oxygen carrier is reduced by gaseous products to Fe3O4, which is deemed to trigger the WGS reaction. It has, of course, been known for a long time that alkali metals promote the WGS reaction rate. In 1981, Sato et al.

[37]

found an improvement in the

water gas shift rate when doped Pt-TiO2 with Na ion. Klier

[38]

also highlighted the

promoting influence of alkali dopants and Campbell et al. [39] observed a promotion of the WGS activity of Cu by Cs ions. Our previous works have also proven that the introduction of foreign AAEM ions (K+, Na+, Ca+) into hematite can improve the WGS rate in the temperature range of 800 to 920 oC during the CLC process [40-42]. For HAWZD, a major proportion of sodium has been extracted and thus the promotion effect of alkali metals is alleviated. Moreover, the reaction rate between reductive gases and oxygen carrier is slow so that the relatively high oxidation degree of hematite is another 17 / 29


Energy & Fuels

significant disincentive to the WGS reaction. 3.3.3 Effect of redox cycles The cycle experiments were conducted to investigate the effect of different occurrence modes of sodium in ZD on the stability of hematite. Fig. 9 gives the effect of redox cycle on the yields of carbonaceous gases at 900 oC. As seen from Fig.9 (a) and (b), similar variation tendencies of carbonaceous gases yield for ZD, WW-ZD and AAW-ZD were observed. The yield of CO for ZD increased from 21.7% to 25.9% during the initial three cycles and then basically stabilized at around 24.5% in spite of light fluctuations. Correspondingly, the yield of CO2 first decreased from 76.9% to 72.6% in the initial three cycles and then maintained at nearly 74.5%. When ZD was treated by sufficient washing, the stable yield of CO after initial three cycles kept at 23.2% and the yield of CO2 maintained correspondingly at about 75.8%. Similar trends of carbonaceous gases yield were obtained for ZD after CH3COONH4 extraction. The yield of CO first increased from 24.2% to 26.9% during initial four cycles and then kept about 26.2%. Accordingly, the stable value of CO2 yield for AAW-ZD was approximately 71.9%, which showed a slight decrease compared with that for ZD. It is clear that H2O-sodium exerted an inhibiting effect on the conversion of carbonaceous gases into CO2, while CH3COONH4-soluble sodium has a catalytic effect in promoting the CO2 yield. 80

80 70

70

fCO2, WW-ZD

fCO2, ZD

60

60

50

50

fCO, fCO2, fCH4 (%)

fCO, fCO2, fCH4 (%)

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 18 of 29

40 fCO, WW-ZD

30 fCO, ZD

20

30

1

2

3

4

5

fCH4, AAW-ZD

fCH4, WW-ZD

6

7

8

9

0

10

fCO, ZD

fCO, AAW-ZD

10 fCH4, ZD

fCO2, ZD

40

20

10 0

fCO2, AAW-ZD

1

2

Cycle number

3

4

5

fCH4, ZD

6

Cycle number

(a)

(b)

18 / 29


7

8

9

10

Page 19 of 29

80 70 fCO2, ZD

60

fCO, fCO2, fCH4 (%)

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

fCO2, HAW-ZD

50 fCO, HAW-ZD

40 30 20

fCO, ZD

10 0

fCH4, AAW-ZD

1

2

3

4

fCH4, ZD

5

6

7

8

9

10

Cycle number

(c) Fig. 9 Effect of redox cycle on yields of carbonaceous gases for (a) WW-ZD, (b) AAW-ZD and (c) HAW-ZD compared with ZD.

Interestingly, the yield of CO2 for HAW-ZD was relatively low during the whole CLC process, which is seen from Fig.9 (c). The CO2 yield decreased from the maximum of 55.3% to the minimum of 50.3% at the 7th cycle and then stabilized at about 50.5%. The source of CO2 comes from two generation paths, i.e. reaction between reduced carbonaceous gases and oxygen carrier and the WGS reaction. On the one hand, the undesired lower conversion of carbonaceous gases to CO2 reflects the weakened catalytic effect of oxygen carrier, which is probably due to the low content of sodium in HAW-ZD. However, the difference between HAW-ZD and the other three types of ZD is so enormous as to be beyond the impact of sodium in ZD. As stated above, the WGS reaction is almost nonexistent when HAW-ZD is used as fuel. Hence, it could be reasonably inferred that inhibition of WGS reaction leads to the high CO concentration. Besides, changes in morphology and structure of coal particles after hydrochloric acid washing may also play a role in the decrease of carbonaceous conversion efficiency, which will be discussed in the section 3.4. 3.4 Characterization of coal and oxygen carrier For the purpose of deeply investigating the cause of combustion performance change for ZD treated in different ways, the morphological and structural features of coal and oxygen carrier particles were characterized by Brunauer-Emmett-Teller (BET, Micrometric ASPA 2020), Scanning Electron Microscope (SEM, Hitachi S-4800) and X-Ray Diffraction (XRD, SHIMADZU). 3.4.1 Surface morphology of coal The images of SEM for ZD with different extraction level are shown in Fig. 10. The magnification of 10,000× was used to analyze the surface characteristics of coal particles. The surface micro-pores increase for WW-ZD, which can be observed in Fig. 19 / 29


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 20 of 29

10 (B). The H2O-soluble sodium is considered to be absorbed on the surface and pore structure of particles

[27, 43].

Hence, water-washing treatment resulted in an increasing

number of pores with micro sizes. These changes provide favorable conditions for the diffusion of steam into the interior of WW-ZD particles, thus promoting the gasification process. As seen from Fig. 10 (C) and (D), the surface of ZD particles with acid treatment is relatively smooth, especially for HAW-ZD. This structure may be adverse to the diffusion of steam into the interior of char particles and the CLC process for HAW-ZD is thus weaken. (A) ZD

(B) WW-ZD

(D) HAW-ZD

(C) AAW-ZD

Fig. 10 SEM images of ZD (A), WW-ZD (B), AAW-ZD (C) and HAW-ZD (D)

For further investigation, the surface areas and pore structures of ZD with different extraction level are analyzed by BET and results are listed in Table 4. Despite changes occur in average surface area, total pore volume and average pore diameter, the differences among four types of coal are too minor to result in the significant changes in reaction performance. Therefore, removal of different modes of sodium plays a dominant role in the combustion performance of ZD during a CLC process. Table 4 BET surface areas of ZD with different extraction level

ZD

as, BET

Total pore volume

Average pore diameter

(m2/g)

(10-3 cm3/g)

(nm)

1.4432

0.63759

1.8168

20 / 29


Page 21 of 29

WW-ZD

1.5058

0.62244

1.6534

AAW-ZD

1.4037

0.67745

1.9752

HAW-ZD

1.3719

0.71669

1.9863

3.4.2 Phase characterization of oxygen carrier The phase analysis is imperative to understand the side reactions involving sodium during the CLC process. The crystalline phase compositions of fresh and used oxygen carrier particles after 10 cycles for ZD with different extraction level were examined by XRD, as shown in Fig. 11 and Fig. 12. The XRD data indicate that the primary minerals in the fresh hematite are Fe2O3 (iron oxide) and SiO2 (quartz). After 10 redox cycles, the XRD patterns of reduced oxygen carrier samples for four types of coal are almost the same. The dominant phase corresponding to Fe3O4 is observed as well as a small quantity of SiO2 and Al2O3. However, differences are shown in minor and trace compositions. It can be seen from Table 5 that minor composition of Na2O·Al2O3·2SiO2 (nepheline) and trace of Na3Fe5O9 present in oxygen carrier with ZD as coal, while only trace of Na2O · Al2O3 · 2SiO2 is observed in oxygen carrier samples with ZD after treatment. 7000

Fresh hematite

A

6000 A Fe2O3 B SiO2

5000

Intensity/CPS

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

A

4000 3000

A A

2000

A

A

A A

1000 B

A

A

B

A

A

AA A

0 10

20

30

40

50

60

70

80

90

2/

Fig. 11 The XRD pattern of fresh hematite oxygen carrier

21 / 29


Energy & Fuels

9000

9000 8000

7000

7000

6000

6000

Intensity/CPS

Intensity/CPS

(a) ZD

C

8000

5000 4000 3000

C

C

2000

C

C

1000

C

C 0

10

20

30

40

50

60

4000 3000

C

C

70

C 0

10

20

(c) AAW-ZD

C

C

DA

0 80

C

C

30

40

C

50

60

70

80

2/

9000

8000 7000

5000

1000

C C C

2/

9000

(b) WW-ZD

C

2000

C

A

0

(d) HAW-ZD

C

8000 7000

C

6000

Intensity/CPS

6000

Intensity/CPS

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 29

5000 4000 3000

C C

2000 1000

C

C 0

10

20

30

C

A

0 40

4000 3000

1000

C 60

70

80

C

C

0

10

C

C

C

B

0 50

C

C

2000

C

C

5000

20

C

DA 30

40

50

60

70

80

2/

2/

A Fe2O3

B SiO2

C Fe3O4

D Al2O3

Fig. 12 The XRD patterns of oxygen carriers for (a) ZD, (b) WW-ZD, (c) AAW-ZD and (d) HAW-ZD collected after 10 cycles

Table 5 XRD analyses of samples of oxygen carrier with ZD, WW-ZD, AAW-ZD and HAW-ZD as fuel after 10 redox cycles.

Sample

Coal type

Oxygen carrier

Mineralogical composition Dominant

Minor

Trace

1

ZD

Hematite

Fe3O4

Na2O·Al2O3·2SiO2

Na3Fe5O9

2

WW-ZD

Hematite

Fe3O4

-

Na2O·Al2O3·2SiO2

3

AAW-ZD

Hematite

Fe3O4

-

Na2O·Al2O3·2SiO2

4

HAW-ZD

Hematite

Fe3O4

-

Na2O·Al2O3·2SiO2

As stated above, some researchers have proved that the introduce of additives containing aluminium silicate was effective to absorb sodium into sodium aluminosilicates during high-sodium coal combustion process, especially under steam atmosphere

[14-21, 44].

It was inferred that the presences of Na2O · Al2O3 · 2SiO2 and

Na3Fe5O9 are formed from reactions between the vaporized H2O-soluble sodium salts and minerals in hematite. Sodium species with low-melting point are first vaporized into gas phases as sodium salts such as NaCl or Na2SO4. Then gas-solid reactions 22 / 29


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Energy & Fuels

between these sodium salts and silica take place leading to the formation of silicates and aluminosilicates with high melting point (reactions R12 and R13). Subsequently, the intermediate products further react with silica and aluminium oxide to produce Na2O·Al2O3 ·2SiO2 or nepheline with a higher melting point of 1526 oC under high temperature (reaction R14). According to the investigation reported by Punjak et al. [45], Na2O·Al2O3·2SiO2 may also be generated by reactions between NaCl and Al2O3·2SiO2 or meta-kaolinite under steam atmosphere, which is shown as reaction R15. The metakaolinite in the reaction was possibly created from the decomposition of anhydrite (reaction R16) [46]. For oxygen carrier sample with ZD as fuel, trace of Na3Fe5O9 is also detected. The presence of the mineral is likely caused by the reaction of sodium species and the main component of hematite (reactions R17 and R18) [47]. It not hard to find that the existence of H2O-soluble sodium salts is critical for these reactions to take place. For WW-ZD, AAW-ZD and HAW-ZD, a majority of H2O-sodium was extracted resulting in the decrease of nepheline formation. Meanwhile, the formation of sodium aluminosilicates, Na2O · Al2O3 · 2SiO2, also provides potent evidence for the case that the CLC technology is indeed an attractive and suitable approach to mitigated ash-related problems during combustion of ZD. 2NaCl + 3SiO2 + H2O → Na2O ∙ 3SiO2 + 2HCl

(R12)

Na2SO4 + 3SiO2 → Na2O ∙ 3SiO2 + SO2 + 1 2O2

(R13)

Na2O ∙ 3SiO2 + Al2O3 → Na2O ∙ Al2O3 ∙ 2SiO2 (nepheline) + SiO2

(R14)

2NaCl + Al2O3 ∙ 2SiO2 + H2O → Na2O ∙ Al2O3 ∙ 2SiO2 + 2HCl

(R15)

Al2Si2O5

> 723K

Na2CO3 + Fe2O3

Al2O3 ∙ 2SiO2 + 2H2O

(R16)

923 - 1023K

(R17)

3Na2Fe2O4 + 2Fe2O3

Na2Fe2O4 + CO2

> 1023K

2Na3Fe5O9

(R18)

4 Conclusion CLC with hematite as oxygen carrier was regarded as an attractive technology for high-sodium coal conversion with advantages of alleviating ash-related problems. For a better understand of the effect of different sodium occurrence modes in high-sodium coal on combustion performance during CLC process, a series of experiments were conducted in a fluidized bed reactor. Conclusions can be drawn as follows: 23 / 29


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



An extensively used method of sequential extraction was used to prepare coal samples containing various forms of sodium. The AAS results showed that H2Osoluble sodium was the dominant sodium form in ZD with around 58.7% of total sodium content, while the sodium in CH3COONH4-soluble and HCl-soluble forms accounted for 15.6% and 7.3%, respectively. The remaining 18.4% is attributed to the insoluble form of sodium.



With respect to concentrations of gaseous products during a typical reduction CLC process, the data indicated that different occurrence modes of sodium exert different influences on coal devolatilization and char gasification processes. The H2O-soluble sodium nearly has no effect on the pyrolysis process of coal, but it may pose an inhibition effect on the char gasification process. Poor combustion characteristic was obtained for ZD after being extracted by ammonium acetate and hydrochloric acid, especially for HAW-ZD. It indicated that organic sodium species in the form of CH3COONH4-soluble sodium and HCl-soluble sodium had a distinct catalytic effect on char gasification process. Moreover, features of change in H2 concentration profiles revealed that hematite doped with sodium contained in ZD promoted the WGS reaction. During 10 redox cycles, lower conversion of carbonaceous gases to CO2 was obtained for HAW-ZD due to its low content of sodium. Overall, the extraction of H2O-soluble sodium from ZD was beneficial to improve the combustion performance during CLC of ZD, while the elimination of organic sodium in ZD had the opposite effect.



The SEM and BET analysis results showed that the morphology and surface structures of ZD with different extraction level slightly changed. Nevertheless, these changes were too minor to result in the significant changes in coal reaction performance. It indicated that the removal of different modes of sodium plays a dominant role in the combustion performance of ZD during a CLC process. The XRD analyses of oxygen carriers after 10 cycles showed that the main reduced phase was Fe3O4 with simultaneous formation of a certain amount of Na2O·Al2O3 ·SiO2 with steam as gasification agent. The results implied that the CLC technology was indeed an attractive and suitable approach to mitigated ashrelated problems during combustion of ZD, especially when ZD was coped with extraction methods.

Acknowledgements The authors gratefully acknowledge the support of this research work by National 24 / 29


Page 24 of 29

Page 25 of 29 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

Key R&D Program of China (2016YFB0600801), the Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (2017-K04) and the National Natural Science Foundation of China (Grants 51476029, 51276037, 51561125001 and 51406035).

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Effect of Sodium Removal on Chemical Looping Combustion of High

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