Chemical Looping Hydrogen Generation Using Potassium-Modified

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Chemical looping hydrogen generation using potassium-modified iron ore as oxygen carrier Weidong Liu, Laihong Shen, Haiming Gu, and LiFeng Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02280 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 14, 2016

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Chemical looping hydrogen generation using potassium-modified iron ore as oxygen carrier Weidong Liu, Laihong Shen*, Haiming Gu, Lifeng Wu

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

Abstract

Chemical looping hydrogen generation (CLHG) consists of an oxidation process, a reduction process and a hydrogen generation process. Achieving deep reduction of the oxygen carrier is the challenge for the CLHG process. In this paper, experiments on CLHG using K-modified iron ore as oxygen carrier and CO as fuel were carried out in a laboratory scale fluidized bed reactor. High temperature improved the reduction reactivity. But at the same reduction condition, higher temperature didn’t improve the hydrogen generation process, which means higher temperature mainly benefited the reduction process, and then elevated hydrogen generation in a CLHG process. Adding KNO3 improved the rate of reduction and hydrogen generation. With the KNO3 loading in iron ore increasing from 0 % to 10 %, not only the carbon conversion was accelerated in the reduction process, but also the hydrogen generation. High KNO3 loading in iron ore also can maintain longer reaction times. 10 % K-modified iron ore could decrease carbon deposition. The SEM analysis and the cycling experiments indicated that adding K could keep the porous structure of the oxygen carrier and K1

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modified iron ore was a stable catalyst in the CLHG process.

Keywords Chemical looping hydrogen generation, Oxygen carrier, Iron ore, Potassium-modified

1. Introduction

Hydrogen, a clean energy carrier, is expected to be the most important energy source. In consideration of its combustion product, water, which has no negative impact on the environment, the demand for H2 in refineries, metallurgy fuel cells is considered to increase in the future. The steam-iron process was one of the earliest methods for producing hydrogen which can get almost pure hydrogen with simple purification. However, with the emergence of steam-methane reforming technology and its low conversion rate of the reducing gas, the steam-iron process was limited eventually. Currently, hydrogen is produced mainly by the steam methane reforming (SMR) process1. Coal, the most carbon-rich and consequently the most polluting carbonaceous fuel, remains relatively abundant, so coal gasification is also an important method for hydrogen generation2-4. However, the hydrogen generation by reforming or gasifying carbonaceous fuels leads to the emission of carbon dioxide. Therefore, it is necessary to take carbon capture into account in the process of hydrogen generation5-8. Chemical-looping reforming (CLR) is a technology developed from CLC9-11, which was proposed by Mattisson and Lyngfelt12. However, the selection of oxygen carrier suitable for CLR is crucial for the development of the technology. The studies13-16 on CLR have proven that NiO, due to its strong catalytic properties, is a suitable oxygen 2


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carrier. Based on the unique design of CLC, Chemical-Looping hydrogen generation (CLHG), emerging as a promising technology that facilitates CO2 inherent separation at low cost, has arisen during last years. Compared to the CLR, the CLHG is suitable for other fuel gas, like CO. Frequently, a CLHG system is composed by three interconnected fluidized bed reactors, the air-reactor (AR), the fuel-reactor (FR) and the steam-reactor (SR), and the oxygen carrier circulating between them5. The proposed routine is as follows: the oxygen carrier, usually a metal oxide, is reduced in the fuel-reactor by gaseous fuels such as syngas or natural gas; the reduced metal oxide then reacts with steam to produce hydrogen in an oxidizing reactor called stream-reactor, which replaces the air-reactor of general CLC process12; At last, the metal returns back to the air-reactor where it is regenerated. The schematic drawing of the Chemical-Looping hydrogen generation process using Fe2O3 as oxygen carrier is shown in Figure 1.

CO 2+CO

Fuel Reactor

CO

N2+O2

Air Reactor

Air

Steam Reactor

Fe3O4

H2

Steam

Fig 1. The schematic drawing of the Chemical-Looping Hydrogen generation process using hematite as oxygen carrier and CO as fuel 3


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Many tests17-21 have been conducted in laboratory batch reactors, for instance in thermo-gravimetric analyzer (TGA) or laboratory moving-bed reactor to examine the reactivity of the prepared oxygen carriers. Similarly, the selection of oxygen carrier is also a tough problem for CLHG. The studies on CLHG have proven that most of the suitable metal oxides for CLC or CLR, such as Ni- and Cu-based oxygen carriers which have poor reactivity with steam to produce hydrogen gas, are not appropriate to be oxygen carriers in direct hydrogen generation22-24. Puneet Gupta et al.25 evaluated metal oxides of Ni, Cu, Cd, Co, Mn, Sn and Fe for the syngas redox process based on thermodynamic equilibrium limitations. From their studies, only Fe- and Sn-based metal oxides are suitable for steam conversions. However, according to the low melting point of Sn (231 ˚C), Sn is not appropriate for the process of CLHG in view of high stability of the solids at reaction temperatures. It was found that iron oxide in hematite state Fe2O3 showed high reactivity and was a suitable oxygen carrier for CLC. Therefore, oxides of iron (especially Fe2O3) are expected to the most suitable oxygen carriers for CLHG. Although iron oxide is thought to be the suitable oxygen carrier, obtaining more about the iron oxide’s behavior in the chemical-looping hydrogen generation is necessary for the design of CLHG system. Four states of iron-based oxygen carrier (hematite Fe2O3, magnetite Fe3O4, wustite FeO and metallic iron Fe) circulate through these three reactors26-28. In the FR, the oxygen-rich state hematite (Fe2O3) is reduced to the oxygen-lean state wustite (FeO), magnetite (Fe3O4) or metallic iron (Fe) by fuel gas (taking CO as an example) following reaction (R1-R3): 3Fe2O3 + CO → 2Fe3O4 + CO2，∆H900˚C=35.3 kJ/mol

(R1)

Fe3O4 + CO → 3FeO + CO2，∆H900˚C =10.1 kJ/mol

(R2)

4


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FeO + CO → Fe + CO2，∆H900˚C =-16.3 kJ/mol

(R3)

At full conversion of the fuel gas, the product stream exiting from the FR is ideally composed of CO2 and water, which once condensed, yields an almost pure CO2 steam to be captured. Then the reduced FeO or Fe is transported to the SR and oxidized to Fe3O4 by water steam and produce hydrogen according to reaction (R4-R5): Fe + H2O → FeO+ H2，∆H900˚C =-16.8 kJ/mol

(R4)

3FeO + H2O → Fe3O4 + H2，∆H900˚C =-43.2 kJ/mol

(R5)

The generated hydrogen from SR will be captured with the unreacted steam condensation. Fe3O4 introduced to AR is regenerated to Fe2O3 for a new cycle following reaction (R6): 4Fe3O4 + O2 → 6Fe2O3，∆H900˚C =-493.2 kJ/mol

(R6)

The AR exit stream contains N and some excess O. The reaction (8) is strongly exothermic to sustain the thermal balance of the whole system. The net reaction in CLHG is reaction (R7): CO + αH2O + (1-α)/2 O2 → CO2 + αH2

(R7)

α is related with the reduction degree of iron-based oxygen carrier. α=0 means that there’s no FeO producted. α=8/9 means that all the Fe2O3 in the iron-based oxygen carrier was reducted to Fe. In addition, Bohn et al.17 concluded that reducing FeO to Fe in the reducing process was poor with reactivity of the iron oxide declining. Cleeton et al.29 also suggested that the Fe3O4-Fe transition was extremely slow and it took much more time than the Fe2O3-Fe3O4 transition. Meanwhile, in the FR of CLC, the reduced solid is Fe3O4 and any deeper reduction of Fe3O4 would lead to unconverted fuel gas exhausting from the FR because of thermodynamic equilibrium. Unfortunately, the oxygen carrier must be reduced to FeO or Fe in CLHG. Thus, it is necessary to modify the conventional fuel5


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reactor of CLC for the need of CLHG. The studies described above were based on theoretical models and the experiments were limited in cycles of Fe3O4-FeO-Fe or performed in a packed bed. Some studies30, 31

focus on the full Fe2O3-Fe3O4-FeO-Fe-FeO-Fe3O4-Fe2O3 cycle for hydrogen

generation in a fluidized bed. But the Fe2O3-Fe3O4-Fe2O3 process, which adds the reduction time, actually is not necessary in a laboratory scale fluidized bed reactor, as we applied below. Researches32-35 have proved alkali metal can facilitate the reactivity of oxygen carrier. To improve the reactivity, Gu36,

37

conducted experiments using potassium

modified iron ore as oxygen carrier. The study showed that adding potassium actually could improve the iron ore reactivity, but also sintered the particles surface and the K loading in the iron ore should not be higher than 10 %. According to Milić38, potassium ferrites, e.g., KFe11O17, K2Fe10O16, and K2Fe4O7 would be formed during the calcinations process at 950 ˚C. Zhang et al.39 evaluated different potassium salt to modified the hematite, their study indicated that KNO3 has the best catalytic effect, then K2CO3, K2SO4, K3PO4 in order.

FeO/Fe

CO 2+CO

Fuel Reactor

CO

Steam

Steam Reactor

H2

Fe3O4 Fig 2. The schematic drawing of the Chemical-Looping Hydrogen generation process without oxidation process

6


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In this paper, hematite Fe2O3 was used as the oxygen carrier and CO as fuel in a fluidized bed to investigate the feasibility of the CLHG process. The aim of this work is to simplify the CLHG system based on traditional CLC system which only contains reduction process and hydrogen generation process, as shown in Figure 2. Effects of reduction time, operation temperature, K loading and cycling experiments on the hydrogen generation were discussed.

2. Experiment

2.1 Preparation for the oxygen carrier

In this work, an Australian hematite (ρ = 4472 kg/m3) was selected. The original Australian hematite was mainly composed of Fe2O3, about 83.25 wt. %, as shown in Table 1, based on the X-ray fluorescence (XRF) measurement. Tab.1 Chemical composition of the original Australian hematite

Fe2O3

SiO2

Al2O3

Other

83.25

7.06

5.32

4.37

KNO3 was used to obtain element K and the potassium-modified iron ore was made by impregnation method. The mass fraction of KNO3 was defined as: f KNO3 =

mKNO3 miron ore

(1)

the mass fractions of KNO3 were set to 0 %, 3 %, 6 % and 10 %. Add the aqueous solution of KNO3 to a container in which the iron ore particles were placed. Keep the container stable at room temperature for 20 h, and then the mixture was dried at 120º C in air atmosphere until no water existing. Finally, the mixture oxygen carrier was 7


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put into a muffle furnace for calcination in air atmosphere at 950 ºC for 3 h. Select particle size between 0.2 mm to 0.25 mm for using in the experiments. Figure 3 shows the PSD and the D (0.5) was 0.223 mm. The SEM results in Figure 4 indicated both the original and the 10 % K-modified iron ore had a porous structure, but slight sintering appeared on the surface of the 10 % KNO3-decorated iron ore. Grains on the surface of the 10 % KNO3-decorated iron ore particles were bigger, which would weaken the diffusion of CO into the core of oxygen carrier particles and the gas-solid reactions. The BET results showed the BET surface area, pore volume and pore size of the 10 % KNO3-decorated iron ore were lower (Table 2).

Fig.3 Particle Size Distribution of the K-modified oxygen carrier particles

Original iron ore

Iron ore loaded with 10 % KNO3

Fig 4. SEM images of the fresh and K-modified oxygen carrier 8


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Tab.2 BET analysis of the K-modified and fresh oxygen carrier

Sample

BET Surface Area m2/g

Pore Volume mm3/g

Pore Size nm


0.9

3.6

16.4

original iron ore

1.8

12.0

25.7

2.2 Experimental setup and procedure

The experiments were conducted in a laboratory scale fluidized bed reactor as shown in Figure 5. The system consists of a reactor, a temperature control system, a gas intake system, a steam generator system, a gas analysis system and a gas purifying system, including a steam cooler and filters. The reactor contained a straight stainless steel tube (i.d. = 32 mm) with a porous media as a distributor plate located at the bottom, and it was electrically heated by a furnace. The reaction temperature was monitored by two K-type thermocouples, one between the tube and the heater and the other inside the tube. In the experiments,

Valve 2 Reservoir Valve 1 Filter

Thermocouple and Temperature Controller T

Vent Emerson

Mass Flow Controller

N2

Drier

Vent

C O: 0-100 % C O2: 0-100 % C H4: 0-1 0% O2: 0-2 5% H2: 0-5 0%

Thermocouple T

Inlet Flow

Mass Flow Controller

Pump

T Steam Generator

distilled water

CO

Fig 5. Schematic diagram of the experimental system 9


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distilled water was introduced by a micro-pump and preheated to 160 ˚C by a steam generator. The steam was carried by the gas from the gas intake system into the reaction chamber, and the outlet flow can be calculated using the N2 mass balance method. The high pure N2 (99.99 %) and CO (99.99 %) were introduced and measured by mass flow controllers. In each case, a batch sample of 45 g oxygen carrier particles was placed on the porous media, and then the reactor was heated in N2 atmosphere at 2 L/min to the reaction temperature of 750, 800, 850 and 900 ˚C. The gas velocity, ug, was 0.15-0.17 m/s in the range of 750 ˚C to 900 ˚C. The minimum fluidized velocity, umf, of the particles, whose size was the D (0.5) (D = 0.223 mm), was 0.039 m/s at 750 ˚C. The gas flow was sufficient to fluidize the oxygen carrier. The laboratory scale fluidized bed reactor was in all process of a CLHG cycle a. Reduction of oxygen carrier When the reaction temperature reached a steady condition, the inlet gas was switched to a mixture of N2 (1.75 L/min) and CO (0.25 L/min). Simultaneously, the reduction of oxygen carrier started and gas sampling began. The reduction duration was set to 60 min. b. Reactor purifying N2 purge stage (2 L/min) purged the reactor for 10 min between the reduction and hydrogen generation steps. c. Hydrogen generation When the reaction temperature reached a steady condition, the inlet gas was switched to a mixture of N2 (1.5 L/min) and steam (0.4 g/min). And the hydrogen generation reaction of oxygen carrier and vapor started and gas sampling began. d. Reactor purifying 10


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N2 purge stage (2 L/min) purged the reactor for 10 min between the hydrogen generation and reduction steps. The different experimental cases are defined by temperature, KNO3 loading and cycle. The reaction temperature are set to 750, 800, 850 and 900 ºC. KNO3 loading fKNO3 was set to 0 %, 3 %, 6 % and 10 %. In the cycle experiments, ten times of experiments were conducted at same conditions.

2.3 Data evaluation

2.3.1. Outlet gas molar flow rate The volume rate of inlet N2, VN2 , was fixed, then the molar flow rate of inlet N2,

nN2 , was fixed and the gas concentrations at the outlet were measured by the gas analyzer. The molar flow rate of all the outlet gas, nout , could be calculated by N2 mass balance method:

nout =

nN 2

(2)

1- X CO - X CO2 - X H 2

Where XCO, XCO2, and XH2 were the measured gas concentrations of CO, CO2 and H2 in the outlet gases, respectively.

2.3.2 Purity of H2 To evaluate the purity of H2, the gas yield of carbonaceous gases, only contained CO and CO2, was defined as the ratio of carbonaceous gases in the products during the hydrogen generation reaction stage.

f CO +CO2 =

∑n (X out

∑n (X out

H2

CO

+ X CO2

)

+ X CO + X CO2

)

11


(3)

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The total carbonaceous gases volume is defined as:

(

VCO +CO2 = 22.4*∑ nout X CO + X CO2

)

(4)

As same as above, f H 2 was defined as the purity of H2:

f H2 =

∑n

out

∑n (X out

H2

X H2

+ X CO + X CO2

)

(5)

The total H2 volume is defined as:

VH 2 = 22.4*∑ nout X H 2

(6)

2.3.3 Conversion of iron ore The Fe2O3-Fe3O4-FeO-Fe-FeO-Fe3O4-Fe2O3 cycle actually was a process that iron ores losing an atom of O during reduction process and getting an atom of O during the hydrogen generation and oxidation processes. Oxygen from the iron ore oxidized CO to CO2. The mass of the lost atom of O can be evaluated the existing form of Fe. The conversion of iron ore at anytime can be defined as:

f Fe =

16* ∑ nout X CO2 22.4* ( m0 - m f

)

*100%

(7)

m0 is the original mass of Fe2O3 in the iron ore, m f is the final mass of Fe generated from all the Fe2O3 in the iron ore. Actually, if we treat reactions (R1-3) as a step-by-step reaction chain, there was some particular f Fe . When all of the Fe2O3 generated Fe3O4, f Fe was 10.7 %. When all of the Fe3O4 generated FeO, f Fe was 33 %.

3. Results and discussion

3.1. Effect of temperature on the KNO3-modified iron ore 12


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In this section, the iron ore loaded with 10 % KNO3 was used to investigate the effect of temperature, and the experiments using the original iron ore were also conducted for comparison. 35

750℃ 800℃ 850℃ 900℃

30

H2 Concentration (%)

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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25 20 15 10 5 0 0

10

20

30

40

50

60

Time (min)

Fig 6. Effect of temperature on H2 concentration using the iron ore loaded with 10 % KNO3 at different reduction temperatures

Figure 6 shows that high temperature improved elevated hydrogen generation reactivity. During the hydrogen generation process, the reaction rate increased with the temperature rising and the peak concentration moved upward. The peak hydrogen concentrations were 25 %, 27 %, 29 %, 32 % for both the reduction and hydrogen generation temperatures of 750, 800, 850 and 900 ˚C, respectively. The H2 volumes were 3.2, 3.3, 3.8 and 4.7 L at temperatures of 750, 800, 850 and 900 ˚C, respectively. The H2 volume at 900 ˚C increased 50 % compared with the H2 volume at 900 ˚C. Higher temperatures tend produce higher hydrogen concentration and higher hydrogen generation. The effect of temperature on the redox process was conducted at temperatures of 750, 800, 850 and 900 ˚C. The reduction process lasted 60 min. From Figure 7, the reactivity of iron oxides increased with the rising of the operation temperature in the 13


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reduction stage. Moreover the reaction (R2) is endothermic, so elevated temperature benefited the reduction to FeO. 750℃ 800℃ 850℃ 900℃

CO2 Concentration (%)

10

8

6

4

2

0 0

10

20

30

40

50

60

Time (min)

Fig 7. Effect of temperature on CO2 concentration using the fresh iron ore

10

20

30

40

d

50

60

C

B

B

c

70

80

90

A:Fe2O3 B:Fe3O4 C:FeO

C C

Relative itensity (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

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B

D:K2Fe4O7

E:K2Fe22O34

C C

B

B C

B

CB

B

B

C

B C

b A

A

A D E ED E AE

a

A

A

A A E A

A AE A A A A A A

A A A

10

20

A 30

40

A

A

AA

A

50

60

A A AAA AA A 70

80

90

2θ (Degree)

Fig 8. XRD analysis of the oxygen carrier a. Original iron ore; b. Iron ore loaded with 10 % KNO3

c. the reduced iron ore; d. the reduced iron ore loaded with 10 % KNO3

The XRD analysis in Figure 8 shows the original iron ore consisted of Fe2O3 after calcination. The element of K in the iron ore loaded with 10 % KNO3 was existing in 14


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K2Fe4O7 and K2Fe22O34, which weaken the chemical bond of Fe-O and improved the reduction reactivity. Compared to the reduced iron ore, the reduced iron ore loaded with 10 % KNO3 has more FeO and less Fe3O4. Adding K is an effective method to achieve deep reduction of the oxygen carrier. Therefore in the subsequent hydrogen generation process, hydrogen generation increased. So the temperature had effect on H2 concentration by two way: a. higher temperature benefited the reduction, and then increased the hydrogen generation; b. higher temperature can directly increase the hydrogen generation.

Figure 9 shows the effect of temperature on H2 concentration using the iron ore loaded with 10 % KNO3 at the same reduction temperature, 750 ˚C. The trends are quiet different with the trends in Figure 6. H2 concentration at different hydrogen generation temperatures were almost same. The inflection at 36 min mainly because of the reaction had switched from reaction (R4) to reaction (R5). In (R4), 1 mole Fe can produce 1 mole H2, but (R5) needs 3 mole FeO to produce 1 mole H2.

30

750℃ 800℃ 850℃ 900℃

25


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20

15

10

5

0 0

10

20

30

40

50

60

Time (min)

Fig 9. Effect of temperature on H2 concentration using the iron ore loaded with 10 % KNO3 at the same reduction temperature 15


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The peaks in Figure 9 can indicate that higher hydrogen temperature can increase hydrogen generation, but the effects are slight. In order to obtain and maintain high temperature, the cost was not worthy to obtain a little increase. High temperature mainly benefited the reduction process, then increase the subsequent hydrogen generation process. The relationship between the thermodynamic equilibrium constants of reactions (R4-5) within a temperature range of 750-900 ˚C are shown in Figure 10. The equilibrium constants of both reactions decreased with temperature. This suggested that higher temperature promoted the reaction process toward the reverse direction. In addition the equilibrium constant was independent of temperature. The equilibrium constant of reaction (R5) was a negative, which meant high temperature would lead the reaction (R5) going on to the reverse direction. It was not necessary to conduct the hydrogen generation experiments at high temperature. Overall, high reduction temperature, like 900 ˚C, and suitable hydrogen generation temperature, like 750 ˚C, were effective and economic for chemical looping hydrogen generation.

1.0 0.8 0.6 0.4

Fe + H2O = FeO + H 2

0.2

LogKp

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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0.0 -0.2

3FeO + H 2O = Fe3O4 + H 2

-0.4 -0.6 -0.8 -1.0 750

800

850

900

Temperature (℃ )

Fig 10. Effect of temperature on thermodynamic equilibrium constant 16


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3.2 Effect of KNO3 loading

Figure 11 shows the concentrations of H2 reached a maximum of 24.1 %, 25 %, 26.1 % and 32.1 % at KNO3 loading of 0 %, 3 %, 6 % and 10 %, respectively. The difference was that the H2 concentration peak using iron ore with 10 % K loading is much higher than others. But for the cases using iron ore with 3 % and 6 % K, the H2 concentration peaks were a little higher than that using iron ore an oxygen carrier. It indicated that there is a competitive relation between K loading and the amount of reaction. For the case using iron ores with 3 % and 6 % K loading, the reaction was the key to promote reaction (R4). But for the case using iron ore with 10 % K loading, enough K loading was the key to promote reaction (R4). The fast reaction rate before 14 min mainly because of a lot of reactants and K loading. The reaction after 14 min could be treated as the main reaction. All the H2 concentrations started to decrease to a stable stage at 14 min. The main reaction stage maintained for about 16 min for the iron ore without K loading. When the iron ores

35

0%KNO3 3%KNO3 6%KNO3 10%KNO3

30


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25 20 15 10 5 0 0

10

20

30

40

50

60

Time (min)

Fig 11. Effect of KNO3 loading on H2 concentration at 900ºC

17


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modified by KNO3 were used, the main reaction could maintain longer time. The main reaction could maintain for 22, 28 and 32 min when using iron ores with K loading of 3 %, 6 % and 10 %, respectively, which mean iron ore with more K loading has better reactivity and longer working life.

3.3 Carbon deposition

Carbonaceous gases, like CO and CH4, can be deposited on surface of the particles during the reduction stage40, which called Boudouard Reaction (R8): 2CO CO2+C

(R8)

The carbon deposited particles can contribute to producing CO or CO2 in hydrogen generation process, lowering purity of H2, reaction (R9-10). H2O+C →H2+CO

(R9)

H2O+CO →H2+CO2

(R10)

Tab.3 Effect of time on the conversion of iron ore and 10 % K-modified iron ore

Sample Conversion Carbonaceous Gas/L

10.7 % 0

iron ore 33 % 0.058

36.7 % 0.097

10 % K-modified iron ore 10.7 % 33 % 47.9 % 0 0.034 0.075

In Table 3 , all the Fe2O3 generated Fe3O4 need 12 min for the iron ore, but only need 10 min for 10 % K-modified iron ore, when fFe is 10.7 %. This illustrated that 10 % K-modified iron ore has better reactivity. But for both two cases, there is no carbonaceous gas generated, which mean carbon deposition didn’t occur in the reduction stage of Fe2O3 generated Fe3O4. When all the Fe3O4 generated FeO, when f Fe is 33 %, it took 51 min for the iron ore, but only took 34 min for 10 % K-modified iron ore, 33 % less than the former. 18


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Carbon deposition occurred in the stage of Fe3O4 generate FeO. But for the case of using 10 % K-modified iron ore, generated carbonaceous gas was only 0.034L, 41 % less than former. At 60 min, all reaction was in the stage of FeO to Fe, the carbonaceous gas volume had huge increase. The conversion of iron ore in experiment using iron ore was 37 %, the carbonaceous gas volume was 0.097, nearly twice of that at 51 min, but the conversion of iron ore only increase 3.7 %. At 60 min, The conversion of iron ore modified by 10 % K, which has been proved have better reactivity, was 48 %, still higher than that using original iron ore. The carbonaceous gas volume was 0.075L, more than twice of that at 51 min. That illustrated the stage of FeO to Fe has very serious carbon deposition. But using 10 % K-modified iron ore can weaken carbon deposition. No matter at the same conversion of iron ore, or at the same time, the 10 % K-modified iron ore need less reaction time, deep conversion of iron ore and less carbonaceous gas generated.

3.4 Effect of cycle

The cycle experiments were conducted to investigate the stability of the KNO3 modified iron ore, and the iron ore loaded with 10 % KNO3 was used. The experiments using the original iron ore were also conducted for comparison. As the first cycle began from the reaction (R1) of Fe2O3 to Fe3O4, the later nine reaction began from the reaction (R2) of Fe3O4 to FeO. Here would not discuss the first cycle.

Figure 12 shows the effect of cycle on the H2 volume at 900ºC. During 10 cycles, the H2 volume for the cases using original iron ore decreased from 2.1L to 2L in the initial three cycles and the change was slight. Correspondingly, H2 volume for the 19


Energy & Fuels

original iron ore decreased to 1.6 L in the finial cycle, decreased 24 %. When using the KNO3-modified iron ore, the H2 volume increased to 3.5 L, increased about 66 % compared to using the original iron ore. And the H2 volume stabilized at about 3.5 L during all the 10 cycles. It got the maximum of 3.6 L at the fourth cycle, and the minimum of 3.3 L at the tenth cycle. KNO3-modifieded iron ore had better reactivity and could maintain the high reactivity with the experiments going on.

0%KNO3 10%KNO3

4

3

H2 Volume/L

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

1

0 1

2

3

4

5

6

7

8

9

10

11

Cycle Number

Fig 12. Effect of cycle

Original iron ore


Fig 13. SEM images of the oxygen carrier after 10 cycles 20


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Energy & Fuels

The SEM results in Figure 4 and 13 were opposite. In Figure 14 the iron ore with 10 % KNO3 has better porous structure after 10 cycles. There were still small grains on the particle surface of the original iron ore (Figure 13). No evident sintering was found, and the grains and pores on the particle surface of original iron ore became bigger. But the porous structure was still better than the iron ore with 10 % KNO3 after calcination. The iron ore loaded with 10 % KNO3 displayed porous surface. The grains and pores were smaller than that of the iron ore before the cycles, which was opposite with the results in CLC27-28. The reaction (R4) and (R5) are slight exothermic reaction, but the reaction (R6) is severe exothermic reaction. In CLC, the severe heat release could cause sintering. Sintering was found in both of the reduction process and the oxidation process. The porous structure of the iron ore loaded with 100 % KNO3 after 10 cycles maybe because of the adding of K, the exact reason needs subsequent research. But it’s clear that K-modified iron ore had a stable catalytic effect in the CLHG process.

4 Conclusion

To Achieve deep reduction of the oxygen carrier is the challenge for the CLHG process. Potassium salt was added to modify the iron ore (an Australian hematite). Experiments on CLHG using the K-modified iron ore as oxygen carrier and CO as fuel were carried out in a laboratory scale fluidized bed reactor. The results indicate that compared with the original iron ore, the K-decorated iron ore promoted the reaction rate. With the KNO3 loading in iron ore increasing from 0 % to 10 %, not only the carbon conversion was accelerated in the reduction process, 21


Energy & Fuels

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but also the hydrogen generation. Larger H2 peak was obtained when the iron ore loaded with higher KNO3 loading was used. High KNO3 loading in iron ore also can maintain longer reaction time. Higher temperature didn’t increase the hydrogen generation. At the same reduction condition, higher temperature didn’t improve the hydrogen generation process, which means higher temperature mainly benefited the reduction process, and then elevated hydrogen generation in a CLHG process. Obvious carbon deposition was observed in the CLHG process no matter using or not using potassium-modified iron ore as oxygen carrier. Carbon deposition was not found in the reaction of Fe2O3 to Fe3O4. Carbon deposition mainly occurred in the stage of FeO to Fe. Using 10 % K-modified iron ore could weaken carbon deposition. The cycling experiments show H2 volume nearly maintained at a constant and Kmodified iron ore had a stable catalytic effect in the cycling process. The XRD results indicate K2Fe4O7 and K2Fe22O34 in the iron ore loaded with 10 % KNO3 weaken the chemical bond of Fe-O and improved the reduction reactivity. The SEM results show that the iron ore loaded with 10 % KNO3 displayed slight sintering after calcination and better porous surface after cycles than the original iron ore. Adding K could keep the porous structure of the oxygen carrier and K-modified iron ore was a stable catalyst in the CLHG process. Overall, the studies suggest that the KNO3-decorated iron ore is an effective measure to improve the CLHG process and further investigations are necessary.

*Corresponding Author Name: Laihong Shen E-mail: [email protected] 22


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Acknowledgment

This work was supported by the National Natural Science Foundation of China (51561125001, 51476029, 51276037).

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Chemical Looping Hydrogen Generation Using Potassium-Modified

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