Chemical Looping Hydrogen Generation Using Synthesized Hematite

Subscriber access provided by UNIV OF DURHAM

Article

Chemical looping hydrogen generation using synthesized hematitebased oxygen carrier co-modified by potassium and copper Lulu Wang, Laihong Shen, Weidong Liu, and Shouxi Jiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01190 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

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

Chemical looping hydrogen generation using synthesized hematite-based oxygen carrier co-modified by potassium and copper Lu-lu WANG，，Lai-hong SHEN*, Weidong LIU, Shouxi JIANG （Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education，School of Energy and Environment，Southeast University, Nanjing 210096, China） * Corresponding author: Tel.: +86 83795598; E-mail address: [email protected] (L. Shen).

Abstract Hematite has become a promising oxygen carrier (OC) due to its abundance and low cost for chemical looping hydrogen generation (CLHG). However, the poor redox reactivity, low yield and purity of hydrogen are the main issues using hematite as OC. In order to solve the problems, a synthesized oxygen carrier based on iron ore co-modified by copper and potassium was first proposed for CLHG to increase the reduction reactivity and hydrogen yield. Experiments were conducted in a batched fluidized bed reactor to evaluate the performance of the synthesized OC. The results demonstrated that the adding of potassium and copper elevated the reduction reactivity. The reduction reactivity was in the sequence of 5Fe1.67Cu10K > 5Fe1Cu10K > 5Fe0.625Cu10K > 5Fe1Cu5K > 5Fe1Cu0K > hematite. As compared with

hematite,

the

oxygen

transport

conversion

increased

70.11%

using

5Fe1.67Cu10K. The reduction reactivity enhancement was attributed to the self-diffusion and pores formation via adding potassium as well as the high reactivity and oxygen transport conversion of copper loading. K2Fe4O7 and CuFe2O4 were detected in the synthesized OCs by XRD analysis, which were active phases for reduction. Moreover, the high oxygen transport conversion and reactivity revealed the deep reduction of iron oxides. The hydrogen yield increased 2.1 times on account of the existence of potassium and copper. Meanwhile, the hydrogen production rate was improved. Additionally, 850 oC was suitable for CLHG in consideration of reaction rate and low melting point of additive. The hydrogen purity was up to 99.9%, indicating that copper and potassium play significantly synergistic roles on

ACS Paragon Plus Environment

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

suppressing carbon deposition. Therefore, the synthesized oxygen carrier based on iron ore co-modified by potassium and copper was suitable for CLHG. Keywords: chemical looping hydrogen production; iron ore; co-modified by copper and potassium; carbon deposition; hydrogen yield

1. Introduction It is generally known that the concentration of carbon dioxide (CO2) regarded as the staple greenhouse gas, has been increasing greatly in the recent decades leading to the global warming [1]. The atmospheric emission of CO2 could be primarily due to the combustion of fossil fuel such as coal and petroleum [2]. Regarding to efficiently reducing the production of CO2 and pollutant discharge, hydrogen is considered as the sole alternative energy carrier of fossil fuels and is becoming more attractive. The abundant features, ubiquitous resource and environmental friendly effect determine hydrogen to be the perpetual resource for applying. According to the logical sources of hydrogen, several commercial technologies from fossil fuel are commonly used for hydrogen production. The dominating technology to produce H2 on industry scale is the steam methane reforming (SMR). The SMR process is complicated to generate pure hydrogen with additional procedure like H2 purification and pressure swing adsorption (PSA), and of high cost to capture CO2 [3]. It is inevitable to generate CO2 during the other processes for hydrogen production from fossil fuel as well. Therefore, it would be necessary to discover a clean, cheap and feasible way to produce pure hydrogen with capturing CO2. Chemical looping (CL) is a novel technology to solve the issue of CO2 capture with no extra energy consumption. In contrast to the other techniques, CL has higher efficiency and the exhausts of fuel reactor are CO2 of high concentration and H2O which can be condensed and removed, thereby capturing CO2 easily without supererogatory energy. Some CL-based technologies for producing hydrogen have been proposed. Chemical-looping reforming (CLR), a novel technology was first proposed to generate hydrogen from the partial oxidization of methane by Mattisson


Page 2 of 31

Page 3 of 31

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

and Lyngfelt in 2001 [4]. However, the production was always syngas and also needed PSA to increase the cost of pure hydrogen production. In comparison with CLR, chemical looping hydrogen generation (CLHG) is a simple process to generate hydrogen directly originating from CLC, without water-gas shift and PSA. Similar to the CLC system, the CLHG system, is composed of three interconnected reactors, which are a fuel reactor, an air reactor and a steam reactor. Metal oxides have reduction reaction with gaseous fuel such as CO and syngas, taking ‫݁ܨ‬ଶ ܱଷ as an example of metal oxide according to the reactions: CO + 3‫݁ܨ‬ଶ ܱଷ → ‫ܱܥ‬ଶ + 2‫݁ܨ‬ଷ ܱସ

( R1 )

CO + ‫݁ܨ‬ଷ ܱସ → ‫ܱܥ‬ଶ + 3‫ܱ݁ܨ‬

( R2 )

CO + ‫ܱܥ → ܱ݁ܨ‬ଶ + ‫݁ܨ‬

( R3 )

The reduced metal oxide is then transported to the steam reactor, and steam is split with the reduced oxygen carrier to produce H2 according to the reaction: Fe + ‫ܪ‬ଶ ܱ → ‫ܪ‬ଶ + ‫ܱ݁ܨ‬

( R4 )

‫ ܱ݁ܨ‬+ ‫ܪ‬ଶ ܱ → ‫ܪ‬ଶ + ‫݁ܨ‬ଷ ܱସ

( R5 )

And the particles not fully oxidized are back to the air reactor for regeneration according to R6: 4‫݁ܨ‬ଷ ܱସ + ܱଶ → 6‫݁ܨ‬ଶ ܱଷ

( R6 )

The property of oxygen carrier is the cornerstone of chemical looping technology. The indispensable properties are of high redox reactivity as well as eminent attrition resistance. A number of researches are conducted on metal oxides such as Ni- [5, 6], Cu- [7-9], Fe-based [9-11] oxygen carrier for chemical looping combustion (CLC). CuO-based carriers have efficient oxygen carrier capability, and they have peculiar characters in chemical looping oxygen uncoupling, whereas CuO results in sintering or agglomeration because of the fairly low melting point. Fe-based oxygen carrier is an attractive choice because of the low cost, environmental compatibility, and high stability, whereas comparatively low redox reactivity is the major defect, especially lower for CH4. Similar to the studies on OC for CLC, the most suitable oxygen carrier for CLR is NiO with better selectivity towards H2 [12]. Q. Zafar and T. Mattisson have investigated different kinds of oxygen carrier, and have determined that CuO,


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

Mn2O3 and Fe2O3 using SiO2 as a support are not suitable because the fuel conversion is so poor with a large amount of unreacted CH4 contained in the exit gas [13]. Most of the oxygen carriers for CLC and CLR are not appropriate to be employed directly to CLHG, because the oxygen carrier must have the ability to split water to produce hydrogen besides favorable thermodynamic properties, low costs and benign environmental influences. For instance, Ni and Co are thermodynamically difficult to react with the steam to produce hydrogen, however the Ni and Co system with the support of ferrite could be more suitable [14]. Magnetite is also studied in the reduction of gas fuel and oxidation of steam to form H2 by Svoboda [15], but the formation of undesired soot is a serious problem. Various studies reported the oxides of Mn, Cr, Cu, Fe, Sn, W and Ce were used as potential candidates for hydrogen generation [16-20]. Thus, the iron oxide is selected as the suitable one for CLHG taking equilibrium hydrogen concentration, physical strength, and melting points into account. Liu illustrates that iron ore modified by K is appropriate in the CLHG owing to the fact that K facilitates the rate of reduction and hydrogen production [21]. However, the poor reactivity of hematite and carbon deposition on the surface of OC are the main problems in CLHG process. Cu-based OC has higher reactivity than hematite, but there are some arguments on whether CuO can be used as oxygen carrier for hydrogen production. Son had found that the fully reduced particle had oxidation with water to generate 3.7 L H2 per kilogram of metal oxide at 573-723 K using CuO/Al2O3 as oxygen carrier [17]. On the contrary, according to the thermodynamic equilibrium constant, it is too difficult for Cu to produce hydrogen with steam. Even so, it was demonstrated in a number of studies that copper oxides possibly improve the reduction reactivity and even the extent of the reduction when Cu-Fe oxygen carrier was used. Siriwardane prepared CuO-Fe2O3-alumina as oxygen carrier for CLC and a catalyst for methane steam reforming to produce pure hydrogen [22]. The preparation of oxygen carrier is so troublesome that it’s relatively difficult for a large scale. Compared with Fe2O3, iron ore should be more widely used as oxygen carrier, however the reactivity should be improved. There were few researches on the CLHG process using hematite combined with copper. Potassium is a good catalyst used


Page 4 of 31

Page 5 of 31

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

commonly in industry and is a promoter for pore formation. Copper oxide is of high oxygen transport capacity and can efficiently increase the reduction reactivity. In order to increase the reactivity and decrease carbon deposition on the oxygen carrier with high pure hydrogen generation, this work proposed a new kind of oxygen carrier based on iron ore co-modified by potassium and copper to investigate the performance during the period of reduction and hydrogen production. In this paper, it is focused on the characteristics of the oxygen carrier based on iron ore co-modified by the nitrates of potassium and copper in the CLHG process. Experiments were conducted in a batched fluidized bed reactor to discuss the effects of the loading of potassium and copper, as well as the reaction temperature, on the reduction reactivity and hydrogen production yield. Carbon deposition properties, and cyclic stability of redox reactivity were analyzed in the experiments.

2. Experimental 2.1 Oxygen carrier preparation The synthesized oxygen carriers were prepared by wet impregnation. The properties of iron ore (Nanjing Steel Manufacturing Company) as oxygen carrier were shown in Table 1. The particles were firstly sieved on the mesh of 0.3-0.4 mm and then placed in a vacuum container. The ore was immersed in the solution of Cu(NO3)2 added with KNO3 for 20 hours. After that, the obtained solution was stirred at 90 oC in the thermostatic magnetic stirring apparatus for 3 hours and dried in an oven until complete evaporation of the moisture. Finally, the precursor was calcined in a muffle oven at 900 oC for 7 hours to enhance the mechanical strength. The synthesized oxygen carrier was secondly chosen to be in the range of 0.3-0.4 mm. The oxygen carriers,

named

5Fe1Cu0K,

5Fe1Cu5K,

5Fe1Cu10K,

5Fe1.67Cu10K,

5Fe0.625Cu10K were produced for the experiment. The numbers before Fe and Cu represented the molar ratio of Fe to Cu, and the digits before K were the mass ratio of potassium loading, thus for 5Fe1Cu5K, the molar ratio of Fe to Cu was 5 and the mass ratio of KNO3 was 5%. The mass ratio of KNO3 was defined as:


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

ܺ୏୒୓య =

Page 6 of 31

݉୏୒୓య ݉୦ୣ୫ୟ୲୧୲ୣ

Table 1 – Properties of iron ore as oxygen carrier particles Iron ore 3

Bulk density (kg/m )

1.8×103

Particle size (mm)

0.3-0.4 2

BET surface area (m /g)

1.9089

3

Pore volume (mm /g)

11.8

X-ray photoelectron spectroscopy (XPS) was usually used to analyze the real composition over particle surfaces. The prepared oxygen carriers and the elemental compositions of the oxygen carrier surfaces analyzed by X-ray fluorescence (XRF) were summarized in Table 2. The atomic percent suspected the enrichment of potassium and copper loading on the hematite surface. Table 2 – XPS results of the fresh oxygen carrier materials Molar ratio

Mass ratio

(Fe:Cu)

(KNO3:hematite)

Atomic percent (%)

sample

Hematite

Fe

Si

Al

Cu

K

39.24

1.78

2.12

0

0

5Fe1Cu0K

5:1

0

34.99

1.55

1.68

8.02

0

5Fe1Cu5K

5:1

5:100

33.92

1.67

1.94

7.44

2.25

5Fe1Cu10K

5:1

10:100

31.91

1.6

2.31

7.02

3.14

5Fe1.67Cu10K

3:1

10:100

31.57

1.49

2.41

11.13

3.08

5Fe0.625Cu10K

8:1

10:100

32.18

2.03

1.8

4.39

4.09

Moreover, the elemental mapping of the fresh oxygen carrier 5Fe1Cu10K was shown in Fig.1 as an example, and the image displayed the uniform distribution of Fe (Green), Cu (Blue) and K (Red) on the surface of the synthesized oxygen carrier. It demonstrated that the impregnation method was beneficial for the uniformity and dispersion of the active phase. It was well known that the impregnation method showed higher selectivity of CO, H2 and CH4, resulting in a high reaction efficiency [7].


Page 7 of 31

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

Fig. 1 - Elemental mapping of fresh synthesized 5Fe1Cu10K OC (Green: Fe; Blue: Cu; Red: K)

2.2 Experimental procedure Experiments were conducted in a batched fluidized bed reactor made of quartz as shown in Fig.2. The experimental system includes a gas control unit, a reaction unit, a gas treatment and analysis unit and a temperature control unit. The different gases introduced into the reactor were measured by mass flow controllers, such as N2, O2 and CO. Besides those, deionized water was imported by a TBP-50A constant flow-type pump and steam was generated by heater bands to evaporate the water flow. The reaction unit contained a vertical tube with I.D. 32 mm, heated by the electrical furnace, and there was a distribution plate in the middle of the reactor. The temperature was controlled by a thermocouple with an automatic temperature-rising program. The reactor temperature maintained steadily at the settled value during the reduction and hydrogen production procedure. At last, the gases were collected in a bag for offline analyzing the content of CO, CO2, O2 and H2 by a NGA2000 type gas analyzer (EMERSON Company, USA). The exhaust gases had to go through the steam condenser and filters for purifying before gas analysis.


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

Fig. 2 - Schematic diagram of the batched fluidized bed reactor

In every case, oxygen carrier was added from top of the reactor and fell on the porous plate when the whole reactor was heated in a N2 atmosphere. Each sample was assured to contain equal quality iron ore. The total flow rate of the fluidizing gas was 2L/min which was sufficient to fluidize the bed inventory. The experiments consisted of full oxidation, reduction and hydrogen generation. When the reactor temperature kept stable, 5% O2 with the supporting gas (N2) was introduced. The target temperature was lower than the reduction temperature to avoid temperature rise, because the oxidation reaction is exothermic. The oxidation duration was approximately 20 minutes to ensure full oxidation. The inlet gas was switched to N2 and 7.5% CO for 60 minutes when the reduction began. Meanwhile gas bag was prepared for sampling the outlet gas every two minutes. Hydrogen was produced while the reductive oxygen carrier was partially oxidized by steam (0.5 g/min) with the supporting gas N2. The yield and purity of hydrogen were analyzed by collecting outlet gas via bags every two minutes. Between every two step, purging with N2 (2 L/min) for 10 minutes was needed for the residual gas elimination. If cyclic experiments were needed to be conducted, after the purging, repetition of the whole processing would start with the reduction step.


Page 8 of 31

Page 9 of 31

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

Different cases were carried out to test the effects of the loading of potassium and copper, and the reaction temperature. The diverse loading was for exploring the roles of copper and potassium on CLHG processing. And the reaction temperature ranging from 800 oC to 900 oC was investigated the influence on oxygen transport conversion and hydrogen production.

2.3 Data evaluation The total outlet gas flow (qout) is calculated by the mass balance of N2: ௤ొమ ᇲ

‫ݍ‬୭୳୲ =

௑ొ మ

=

௤ొమ ᇲ ଵି௑ిో ି௑ిోమ ି௑ౄమ

(1)

where ‫୒ݍ‬మ ᇱ is the inlet flow rate of N2. And ܺେ୓ , ܺେ୓మ , ܺୌమ are the gas concentration of CO, CO2 and H2 in the outlet gases measured by flue gas analyzer, whose units are %. The superscript ` is the sign of inlet gas. The CO2 generation rate with time, ݂େ୓మ (‫)ݐ‬, is defined by the concentration of oxidative product CO2.

݂େ୓మ (‫ܺ = )ݐ‬େ୓మ ∙ ‫ݍ‬୭୳୲

(2)

The oxygen transport conversion, ߱୓ , is defined by the mass ratio of oxygen transported from OC to the fuel: ୑

ో ߱୓ = ଶଶ.ସ×௠

ోి

௧

∙ ‫׬‬଴ ‫ݍ‬୭୳୲ ∙ ܺେ୓మ ∙ d‫ × ݐ‬100%

(3)

where ݉୓େ is the input mass of the different oxygen carriers, and MO is the molar mass of oxygen. The H2 production, ܸୌమ , is defined by hydrogen volume during the hydrogen production stage: ௧

ܸୌమ = ‫׬‬଴ ܺୌమ ∙ ‫ݍ‬୭୳୲ ∙ d‫ݐ‬

(4)

The hydrogen purity (ωୌమ ) is evaluated by the ratio of hydrogen volume to the additional volume of hydrogen and carbonaceous gas: ೟

ω ୌమ =

೟

‫׬‬బ ௑ౄమ ∙௤౥౫౪ ∙ୢ௧

‫׬‬బ (௑ౄమ ା௑ిో ା௑ిోమ )∙௤౥౫౪ ∙ୢ௧

× 100%


(5)

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

3. Results and discussions 3.1 Effect of potassium and copper loading on oxygen carrier reduction 5Fe1Cu0K, 5Fe1Cu5K, 5Fe1Cu10K and hematite were used to evaluate the effect of potassium loading on oxygen carrier reduction at 850 oC in the experiments. The oxygen carriers should be guaranteed that the equal content of iron ore was 28 g. The reduction period was 60 min. Fig. 3 displays the CO2 generation rate of different potassium loading amount as a function of reaction time. It can be seen obviously that the addition of K and Cu lead to more CO2 generation, indicating an obvious reactivity rise in comparison with hematite. That was owing to the active phases formed between the foreign K, Cu and hematite [23-28]. To analyze the separate effect of K, the effect of different amounts of potassium on increasing the CO2 production of hematite was in the sequence of 5Fe1Cu10K > 5Fe1Cu5K > 5Fe1Cu0K. The curves tendency was nearly the same, for 5Fe1Cu5K and 5Fe1Cu10K. There was a slow and steady decline after the peak of CO2 generation rate, but some was slightly different from 5Fe1Cu0K. The case 5Fe1Cu0K with no potassium had a steep decline after reaching a maximum of CO2 generation. However, the oxygen carrier with potassium 5Fe1Cu10K maintained generating CO2 of high concentration after 60 minutes. The reactivity enhancement was likely due to the migration of alkali metal K [29]. The self-diffusion of K, from the external region to the internal part of oxygen carrier, leaded to the formation of porous facilitating the gas diffusion and resulted in maintaining the high reactivity [23-25, 30]. Secondly, some active ferrites were formed such as CuFe2O4 and K2Fe4O7 in the synthesized OC of higher reactivity with CO. The adding of K also had a catalytic effect on weakening the strength of the bond of Fe-O and lowering the activation energy markedly, thereby increasing the reduction rate [31]. What’s more, from the four curves, adding copper only had an influence on reactivity in the former stage, and the reactivity enhancement due to the amount of potassium was more obvious in the latter part.


Page 10 of 31

Page 11 of 31

5Fe1Cu0K 5Fe1Cu5K 5Fe1Cu10K Hematite

0.12 0.10

2

fCO /L·min-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

0.08 0.06 0.04 0.02 0

10

20

30

40

50

60

time/min

Fig. 3 - Effect of potassium loading on CO2 generation rate at 850 oC

Fig. 4 represents the effect of potassium loading on oxygen transport conversion of OC to evaluate the reduction behavior during the reduction period for 60 min at 850 oC. The high oxygen transport conversion means the high reactivity oxygen carrier exhibits. Among the four oxygen carriers, 5Fe1Cu10K had the largest oxygen transport conversion, suggesting an increase in reactivity during the reduction. It can be clearly seen that the oxygen transport conversion after adding K was as 1.3-1.5 times as 5Fe1Cu0K, while 5Fe1Cu0K was just a little higher than hematite indicating the enhancement of potassium was more apparent than copper. The pore promoter K increased the gas-solid reaction contact area, and the lattice oxygen transported from the oxygen carrier to the fuel generating CO2, which was the reason why 5Fe1Cu5K and 5Fe1Cu10K had high oxygen transport conversion [26]. Cu and K were foreign metal for the hematite, and the Krikendall effect could happen in the oxygen carrier [30, 32]. And alkali-rich phase behaved as catalyst to improve the reduction of hematite by lowering the activation barrier [30]. What’s more, not only K but also Cu exists the diffusion phenomenon, and the different diffusion rates bring about the structure change and result in the pore formation inside the oxygen carrier or enlargement near the original vacancies [32].


Energy & Fuels

12 10 8 ω 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

Page 12 of 31

6 4 2 0 Hematite

5Fe1Cu0K 5Fe1Cu5K Samples

5Fe1Cu10K

Fig. 4 - Effect of potassium loading on oxygen transport conversion during the reduction for 60 min at 850 oC

Synthesized iron ore samples with different molar ratio of Fe to Cu, including 5Fe1Cu10K, 5Fe1.67Cu10K and 5Fe0.625Cu10K, were also investigated in order to understand the effect of copper loading on the reduction reactivity at 850 oC. Fig. 5 shows the oxygen transport conversion for 5Fe1Cu10K, 5Fe1.67Cu10K and 5Fe0.625Cu10K during the 60 minutes of reduction time. From the columns, the oxygen transport conversion ߱୓ decreased with the increasing molar ratio of Fe to Cu. For 5Fe0.625Cu10K, the oxygen transport conversion was the smallest, and the mixing ratio of copper was less than 10 wt %. The oxygen transport conversion for 5Fe1.67Cu10K was 11.87%, the highest of the three oxygen carriers. That can be explained by three reasons: firstly, copper oxides of high oxygen transport capacity were easily reduced, and the reactivity was improved with the increasing content of copper; secondly, as the addition of copper increased, more active substances like CuFe2O4 generated; thirdly, Cu can improve the deep reduction of Fe2O3, which was benefit for hydrogen production [28]. The result agreed with Yang’s conclusion that when the mixing ratio of copper ore with iron ore was 10-20 wt %, the effects of copper could be sufficiently utilized [33].


Page 13 of 31

12 10 8 ω 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

Energy & Fuels

6 4 2 0 5Fe0.625Cu10K

5Fe1Cu10K Samples

5Fe1.67Cu10K

Fig. 5 - Effect of copper loading on oxygen transport conversion at 850 oC

The BET surface areas of the investigated oxygen carriers were summarized in Table 3. The BET data showed that the surface areas of the fresh synthesized oxygen carrier decreased as compared to those of hematite, whereas the surface areas increased after reactions. Therefore, the adding of potassium and copper on hematite had been proven to improve the pore structure and enhance the gas-solid contact areas during redox reactions. Table 3 - BET surface areas of the investigated oxygen carriers Hematite

5Fe1Cu0K

5Fe1Cu5K

5Fe1Cu10K

5Fe0.625Cu10K

5Fe1.67Cu10K

Fresh (m2/g)

1.9089

1.3575

1.0152

0.9395

1.1451

0.6451

Used (m2/g)

0.9992

1.9629

2.0955

2.7766

2.1881

2.8395

Furthermore, not only the physical surface areas but also the chemical mechanism played an important role in improving the reactivity. Fig.6 shows XRD profiles of the fresh raw and synthesized OC, taking 5Fe1Cu0K and 5Fe1Cu10K as examples. Both the synthesized OC showed the presence of CuFe2O4 after adding copper. K2Fe4O7 was detected when potassium was introduced according to the reaction (R7).

‫ܭ‬ଶ O + 2‫݁ܨ‬ଶ ܱଷ → ‫ܭ‬ଶ ‫݁ܨ‬ସ ܱ଻

(R7)

CuFe2O4 and K2Fe4O7 had been proven to be more active than hematite and the alkali-rich phase could also behave as a catalyst to lower the activation barrier [23-28, 30, 33], which was the reason for the reactivity enhancement after introducing foreign copper and potassium.


Energy & Fuels

Hematite

Fe2O3 CuFe2O4

K Fe O 2 4 7

Intensity/a.u.

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 31

5Fe1Cu0K

5Fe1Cu10K

10

20

30

50

40

60

70

2θ θ/degree

Fig.6 - XRD patterns for Hematite, 5Fe1Cu0K and 5Fe1Cu10K

The results demonstrated that the adding of potassium and copper evidently increased the reduction rate and the oxygen transport conversion. It meant that the reduction of iron oxides was deeper. Reduction to Fe would be advantageous to hydrogen production, since the capacity for producing hydrogen over the transition of Fe3O4 to Fe was as four times as that over the transition of Fe3O4 to FeO. However, the transition of Fe3O4 to Fe was possible to contribute to the sintering of oxygen carrier particles, and the reactivity fell down. The deactivation of the synthesized oxygen carrier was not observed. Thus, the synthesized oxygen carrier modified with potassium and copper generated more Fe, which was beneficial to the yield of hydrogen production.

3.2 Effect of potassium and copper loading on hydrogen production After the study of potassium and copper on the reduction reactivity, the prepared synthesized oxygen carrier based on iron ore were investigated in order to understand the effect of potassium and copper on the hydrogen production yield at 850 oC, and the hydrogen production stage was 30 min.


Page 15 of 31

H2 concentration as a function of time for hydrogen production with oxygen carriers of different potassium loading at 850 oC is shown in Fig. 7. H2 concentration increased with the incremental KNO3 loading. For the four cases, 5Fe1Cu10K generated the most hydrogen with the same reduction time, indicating it had the most metallic iron. The ability of oxygen carrier to produce hydrogen was closely associated with the active phase FeO and Fe in the oxygen carrier particles [15]. During the reduction period, the oxygen carrier with more K loading had higher oxygen transport conversion, inferring to deepen the reduction degree and generate more hydrogen. The H2 concentration of hematite, 5Fe1Cu0K, 5Fe1Cu5K and 5Fe1Cu10K reached a peak of 8.5%, 10.5%, 12.5% and 15.3%, respectively. The results were a little different from K-modified hematite. For K-modified hematite, H2 peak concentration did not raise rapidly until the K loading amount was 10% [21]. But for K-Cu-modified hematite, the peak value had an even rise. The H2 peak concentration for 5Fe1Cu10K was 1.8 times of hematite, while that of 10%K-hematite was just 1.3 times. It can be concluded that the hydrogen yield increase was not only in relation to the adding of potassium, but also the existence of copper. Furthermore, it took twelve minutes to reach the maximum for hematite, while for the other three it took ten minutes. Fig.7(b) showed the total H2 volume released using hematite, 5Fe1Cu0K, 5Fe1Cu5K and 5Fe1Cu10K as oxygen carriers. It was to be noted that the hydrogen production yield increased with the increasing potassium loading. Therefore, the synthesized OC improved not only the hydrogen production rate but also the hydrogen yield. (b) 4.5

(a) 18 5Fe1Cu0K 5Fe1Cu5K 5Fe1Cu10K Hematite

16 14

4.0 3.5 3.0

10

2.5 VH /L

12

2

2

XH /%

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

8

2.0

6

1.5

4

1.0

2

0.5 0

5

10

15

20

time/min

25

30

0.0 Hematite

5Fe1Cu0K 5Fe1Cu5K Samples

5Fe1Cu10K

Fig. 7 - Effect of potassium loading on H2 concentration and H2 volume at 850 oC


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

(a) H2 concentration and (b) H2 volume

Fig. 8(a) illustrates the effect of copper loading on H2 concentration during the hydrogen production. H2 concentration as a function of reaction time was a convex curve. Hydrogen generated as soon as the steam was introduced to the reactor, but it took some time to keep the steam flow stable. At the first two minutes, H2 concentration was rather low because of the dilution by nitrogen. H2 concentration reached up to the peak 16.2% at 8th min, 15.3% at 10th min and 14.7% at12th min, for 5Fe1.67Cu10K, 5Fe1Cu10K and 5Fe0.625Cu10K, respectively. The peak of H2 concentration for 5Fe1.67Cu10K was the highest. Fig. 8(b) presents the effect of copper loading on total H2 volume. It was clear that the amount of hydrogen generated increased with the copper loading. As the copper loading increased from 8.78% to 19.89%, the hydrogen production volume increased 17.4%. The highest hydrogen yield was 3.88 L for 5Fe1.67Cu10K. The improvement of hydrogen production rate can be estimated that the addition of copper enlarged the pore structure during the reduction, which was helpful for steam to have reaction with the reductive oxygen carrier and elevated the H2 production yield [34]. Additionally, the yield of hydrogen for 5Fe1.67Cu10K was highest because Fe2O3 in the oxygen carriers was deepest reduced to form the largest amount of FeO and Fe. Cu was beneficial for deepening the reduction extent of Fe2O3 [28]. The more content of Cu, the deeper reduction it would have. Consequently, there was more existence of FeO and Fe in the reducing product leading to more hydrogen generation. Furthermore, the results of H2 volume agreed with the results of the oxygen transport conversion of OC. The hydrogen purity values of the three oxygen carriers were higher than 99.9%.


Page 16 of 31

Page 17 of 31

(b) 4.5

(a) 18

5Fe1Cu10K 5Fe1.67Cu10K 5Fe0.625Cu10K

16 14

4.0 3.5 3.0

10

2.5 V H /L

12

2

2

XH /%

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

8

2.0

6

1.5

4

1.0 0.5

2 0

5

10

15

20

25

30

0.0 5Fe0.625Cu10K

time/min

5Fe1Cu10K

5Fe1.67Cu10K

Samples

Fig. 8 - Effect of copper loading on H2 concentration and H2 volume at 850 oC (a) H2 concentration and (b) H2 volume

3.3 Effect of reaction temperature Reaction temperature is one of the vital impact factors on oxygen carrier reactivity and hydrogen production yield. Although 5Fe1.67Cu10K processed the best reduction reactivity and hydrogen production capacity, its probability of sintering was high. 5Fe1Cu10K was used to investigate the effect of reaction temperature in CLHG process at 800-900 oC in consideration of reducing the extent of sintering or agglomeration at high temperature. Fig. 9(a) shows the oxygen transport conversion at varied reaction temperatures. According to the figure, ω୓ was not monotonically increasing with temperature. It was noted that ω୓ at 850 oC was higher than that at 800 oC, indicating that the reaction behavior was promoted with the rising temperature in the normal fluidization state without agglomeration, which was coincident with the theory that chemical reaction rate increased with the increasing temperature. However, there was an apparent decrease of ω୓ at 900 oC. The CO2 generation rate at 900 oC was also less than the others, and appeared an abrupt decline after a striking rise when the reduction had been carried out for 10 minutes from Fig.9(b). That was likely due to the agglomeration of oxygen carrier particles at a high temperature 900 oC.


Energy & Fuels

(b) 0.12

(a) 12.0

800 oC 850 oC 900 oC

0.11

11.5

0.10 0.09

2

fCO /L·min-1

11.0

ω 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

Page 18 of 31

10.5

0.08 0.07

10.0

9.5 800

850

900

0.06 0

10

reaction temperature/oC

20

30

40

50

60

time/min

Fig. 9 - Effect of reaction temperature on oxygen transport conversion and CO2 generation rate for 5Fe1Cu10K (a) oxygen transport conversion (b) CO2 generation rate

In order to prove that the reactivity decline was owing to agglomeration, some pictures of used oxygen carrier particles were taken from macroscopic perspectives after the experiments at 800 oC, 850 oC and 900 oC. As the images of Fig.10 demonstrated, the agglomeration of particles and sintering on the surface did not happen until the reaction temperature reached 900 oC. The used oxygen carrier after experiments at 800 oC and 850 oC remained disperse and granular particles, while the particles after experiments at 900 oC were sintered and agglomerated into larger particles. The thermal sintering may begin at approximately 70% of the melting point [20]. The synergetic roles of Cu and K resulted in the severe agglomeration because their melting point are both low. Gu [23, 24] also have studied the enhancement of potassium modified iron ore as oxygen carrier in the chemical looping combustion and found the surface sintering when the adding amount of K loading was 10% and higher. It had been concluded that hematite could alleviate the agglomeration of copper in the temperature range of 900-1000 oC. Wang [35] had indicated that the properties of reduction and oxidation reaction were improved with the increasing reaction temperature from 750 oC to 900 oC using Fe45Cu15M40 as oxygen carrier. Chen [36] had proposed that higher temperature was beneficial for hydrogen production with Al2O3-supported iron oxides to obtain more metallic Fe. But according to this experimental data, the existence of potassium made the suitable reaction temperature below 900 oC.


Page 19 of 31

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

Fig. 10 - Macroscopical images of used 5Fe1Cu10K OC (a) 800 oC (b) 850 oC (c) 900 oC

Fig.11 shows the scanning-electron microscope (SEM) images of 5Fe1Cu10K at 900 oC before and after reactions. It can be significantly observed that the microscopic morphology of oxygen carriers changed after reaction at 900

o

C. The fresh

synthesized OC was regular and presented the crystal structure, whose distribution was disperse and uniform. However, the used one did not have regular structure, whose edges and corners were indistinct. And from the SEM images, the pore structure decreased dramatically after the cycles at 900

o

C because of the

agglomeration. Therefore, the reactivity deterioration of oxygen carrier was due to the surface sintering and agglomeration occurred at 900 oC.

Fig. 11 - SEM images of 5Fe1Cu10K OC at 900 oC (a) fresh 5Fe1Cu10K OC and (b) used 5Fe1Cu10K OC

Fig. 12 displays H2 concentration trends at different temperatures. At the beginning of steam oxidation, approximately 4 minutes, H2 concentration production rate increased with the reaction temperature. However, the H2 concentration of 900 oC


Energy & Fuels

did not remain rising as a result of the agglomeration during the reduction period. The agglomeration caused the inactivation of oxygen carrier and prevented the gas-solid contact. The maximal H2 concentration was 14.2% at 900 oC, lower than the others. It was interesting that H2 concentration of 800 oC had the highest peak, and the total H2 volume was 3.56 L for 800 oC, a bit higher than 850 oC. As Fig. 13 shows, the reaction equilibrium constant of Fe3O4 to FeO increased with temperature, so the higher temperature leaded to more oxygen carrier mass loss. But the reaction equilibrium constant of FeO to Fe3O4 by steam oxidation decreased with the rising temperature. As a consequence, the hydrogen yield was limited by the temperature, even though the deep reduction production FeO was enough. There was a kind of competitive relationship between mass loss and reaction temperature. It can be inferred that under the situation when there was slight difference between the oxygen transport conversion for 800 and 850

o

C, the reaction temperature played a

predominate role on hydrogen production. On the other hand, the reaction equilibrium constant of Fe to FeO by steam oxidation to generate hydrogen increased with the drop of temperature. The low temperature made the reaction of Fe to FeO easier and H2 concentration higher, and that was the possible explanation for the highest peak and most hydrogen production at 800 oC. What’s more, the curve of 800 oC sharply fell after the peak and in the latter part of the reaction time, the H2 concentration was rather low. On the contrary, the H2 concentration of 850 oC declined slowly, and higher than 4% at the 28th minutes. It appeared that the production of deep reduction was significant and the effect of temperature was remote in the latter stage of hydrogen production. 20 800 oC 850 oC 900 oC

16

12 2

XH /%

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 31

8

4

0

5

10

15

20

25

time/min


30

Page 21 of 31

Fig. 12 - Effect of reaction temperature on H2 concentration for 5Fe1Cu10K

Fe + H

2.0

2

1.6

CO Fe 3O 4 +

K

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

O(g) = Fe O+

H2 (g)

+ CO 2(g) (g) = 3FeO

1.2

3FeO 0.8

0.4

+HO 2 ( g) =

FeO + CO(g) = 750

800

Fe O 3 4 + H2

(g)

Fe + CO (g) 2 850

900

950

Temperature/ oC

Fig. 13 - Effect of temperature on equilibrium constant

3.4 Cyclic reactions properties To determine the ability of synthesized oxygen carrier to generate hydrogen, cyclic redox experiments of 5Fe1.67Cu10K at 850 oC as a sample were conduct in the batched fluidized bed reactor as well. Fig.14 shows the CO2 generation rate during the different cycle. Throughout the 60 minutes of the first cycle, oxygen carrier possessed high CO2 generation rate. However, after 40 minutes in the second and third cycle, the CO2 generation rate kept low. It can be found that the reduction reactivity of the first cycle was pronounced better than the second and third cycle. That was explained by the fact that the reactant in the first cycle was different from the second and third cycle. After the first cycle, the reduced oxygen carrier could not be fully oxidized by the steam, so it was not possible to return to Fe2O3 and CuO. The similar tendency of CO2 generation rate in the second and third cycle demonstrated that the reactivity maintained stable during the cyclic redox experiments, which was vital for the application in the chemical looping hydrogen production.


Energy & Fuels

First Second Third

0.12 0.10 0.08

2

fCO /L·min-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

Page 22 of 31

0.06 0.04 0.02

0

10

20

30

40

50

60

time/min

Fig. 14 - Effect of cycle number on CO2 generation rate of 5Fe1.67Cu10K at 850 oC

Fig.15 displays the oxygen transport conversion of 5Fe1.67Cu10K after the reduction reaction and the total H2 volume during the hydrogen production in several cycles. As the following chart shows, ߱୓ declined 41.27% for the second time, compared to the case of first cycle. The primary reason mentioned before was that iron was not fully oxidized with oxygen or air, and the partial oxidation by steam could only reach the state of Fe3O4. Therefore, the oxygen transport capacity and reactivity for the following cycle was not nearly as high as the first cycle. Besides that, the existence of copper also had effect on the oxygen transport conversion. In the reduction period of the first cycle, CuO as well as CuFe2O4 was reduced to Cu, and that contributed the oxygen transport due to the good oxygen usage and high reaction rate. But in fact reduced Cu was difficult to be oxidized by steam at 850 oC, therefore it was almost impossible to detect CuO after the hydrogen production period. Therefore, the dramatic drop of the oxygen transport conversion resulted from the different initial state of oxygen carrier. The fresh oxygen carrier for the first cycle composed of Fe2O3 and CuFe2O4, while the used oxygen carrier for the following cycles composed of Fe3O4 and Cu. However, the oxygen transport conversion of the second cycle was nearly as same as the ninth one, except for the first cycle. The reductive properties of 5Fe1.67Cu10K remained steady in the nine-cycle experiments. With respect to the hydrogen production, as the Fig. 15 represented, the accumulated H2 volume was nearly the same during the ninth redox cycles. The hydrogen production was associated with the extent of reduction and the content of reduction product FeO and Fe. It can be referred that the reductive extent of iron oxides was


Page 23 of 31

alike. Although the reaction of Fe3O4 to FeO and Fe was hard to react, an hour of reduction time was enough to react as deep as possible for the cyclic experiments. And despite the oxygen transport conversion of oxygen carrier decreased after the first cycle, H2 volume still kept high and steady. XPS was used to analyze the potassium and copper loading on the fresh and used 5Fe1.67Cu10K surfaces after nine cyclic experiments. The atomic percent of copper decreased to 10.46%, while that of potassium decreased to 2.94%. The XPS results demonstrated that a small amount of potassium and copper had been lost during the cyclic experiments. However, the reduction reactivity and hydrogen production still kept stable during the redox reactions. In general, the reactivity and the performance of generating hydrogen of oxygen carrier appeared constant during the cyclic experiments. 5.0 12 4.5

11 10

4.0 VH

2

3.5

8 7

2

ωO/%

9

VH /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

Energy & Fuels

ωO 3.0

6 2.5 5 4

1

2

3 Cycle number

6

9

2.0

Fig. 15 - Effect of cycle number on oxygen transport conversion and hydrogen production of 5Fe1.67Cu10K at 850 oC

3.5 Analysis of carbon deposition Besides the hydrogen yield, the purity of hydrogen is worthy of attention for hydrogen production when iron based oxygen carrier was used for CLHG process. Carbonaceous fuels, such as CO, are available to produce carbon and adhere to the particle. Boudouard reaction (R8) is one of the most common side reactions causing carbon deposition and it also possibly exists in the chemical looping hydrogen production process.

2CO → ‫ ܥ‬+ ‫ܱܥ‬ଶ

( R8 )

It had been found that the reductive product of FeO and Fe had a catalytic role on the carbon deposition [37, 38].The carbon deposited on the oxygen carrier was


Energy & Fuels

oxidized to CO and CO2 with steam during the hydrogen production period leading to the low purity of hydrogen, according to:

C + ‫ܪ‬ଶ ܱ → ‫ ܱܥ‬+ ‫ܪ‬ଶ

( R9 )

CO + ‫ܪ‬ଶ ܱ → ‫ܱܥ‬ଶ + ‫ܪ‬ଶ

( R10 )

Fig.16 shows the effluent gas concentration of raw hematite and 5Fe1.67Cu10K. For both hematite and 5Fe1.67Cu10K, carbonaceous gas including carbon monoxide and carbon dioxide was detected during the hydrogen production process, indicating carbon deposition happened in the reduction reaction. Compared to raw hematite, it was obvious that H2 concentration was higher, and CO and CO2 concentrations were both lower for 5Fe1.67Cu10K. The sum of carbonaceous gas concentration for 5Fe1.67Cu10K decreased dramatically when adding the copper and potassium. Therefore, the addition of copper and potassium was useful for suppressing carbon deposition, coinciding with the conclusion from previous studies [21, 39, 40]. In addition, it was not possible to observe CO or CO2 until the reaction time was 10 minute using raw hematite as oxygen carrier; however for 5Fe1.67Cu10K, only after 8 minutes there was no carbonaceous gas. 0.07

XCO+CO ,Hematite

16

0.06

XCO+CO ,5Fe1.67Cu10K

14

2

2

XH ,Hematite

0.05

2

XH ,5Fe1.67Cu10K

12 10

0.03

8

0.02

6

0.01

4

2

0.04

0.00

2

XCO+CO /%

2

XH /%

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 24 of 31

2 0

5

10

15 time/min

20

25

30

Fig. 16 - Comparison of effluent gas concentration for hematite and 5Fe1.67Cu10K at 850 oC

The explanations of high H2 concentration using the synthesized oxygen carriers modified by potassium and copper had been discussed in the former section. In addition, the carbon deposited on oxygen carrier retarded the connection between steam and oxygen carrier, slowing down the reaction rate, and decreasing the hydrogen production. The deposited carbon was then oxidized by steam. Meanwhile, hydrogen was produced according to (R9) and (R10). But the hydrogen production


Page 25 of 31

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

from that reactions was fractional as compared to the hydrogen from water splitting by reductive iron. The hydrogen generation using iron ore was limited by the carbon deposit on the surface of oxygen carrier particles. Thus, the hydrogen concentration using the synthesized oxygen carrier adding with potassium and copper was higher because the amount of carbon deposition was less than that using iron ore. In general, the carbon deposition is disadvantageous to hydrogen production. The results confirmed that the adding of potassium and copper played a role in suppressing carbon deposition. There were some inferences to be drawn for the inhibition on carbon deposition for the synthesized oxygen carriers. O2 produced from CuFe2O4 oxygen uncoupling at the beginning of reduction as R11 had reaction with the deposited carbon immediately in the reduction stage, so there was less deposited carbon when the hydrogen production began [41]. ଵଵ଴଴୏ିଵଶ଴଴୏

4‫݁ܨݑܥ‬ଶ ܱସ ሱۛۛۛۛۛۛۛۛۛሮ 4‫ܱ݁ܨݑܥ‬ଶ + 2‫݁ܨ‬ଶ ܱଷ + ܱଶ

( R11 )

As well known that carbon deposition was affected by the acidity of the surface, potassium addition made the surface less acidic to inhibit the rate of carbon formation [42]. The reactivity of OC was promoted by copper and potassium, resulting in less CO and more CO2 around the surface, so the carbon deposition was prohibited because Boudouard reaction is reversible. Several interpretations were also provided to explain that the time we detected the carbonaceous gas for OC with potassium and copper last shorter. Firstly, Cu is beneficial to facilitate the gasification of the deposited carbon with steam resulting in removing the carbon deposition [40]. Secondly, it had been largely reported that copper changed the structure of carbon filaments [43-45]. The formation of carbon layer on the oxygen carrier particle is different after adding copper, and the desired carbon layer seemed to be easily removed. On the other hand, potassium also had been applied to be a promoter to suppress the carbon deposition. The results being reported indicated that potassium had the resistance to the carbon deposition, and promoted the gasification of the carbon on the surface of particle, as well as reduced the structure of carbon layer from filament or whisker to amorphous carbon [46-48]. In conclusion, the synthesized OC


Energy & Fuels

adding copper and potassium not only suppressed the carbon deposition but also promoted the gasification of carbon. Fig.17 displays the hydrogen purity of raw hematite and 5Fe1.67Cu10K. The hydrogen purity of 5Fe1.67Cu10K was significantly increased. For the whole hydrogen production time, the total hydrogen purity of 5Fe1.67Cu10K was 99.95%, much higher than that of raw hematite. The purity of hydrogen satisfied the requirement for hydrogen fuel cells. 100

99

ωH /%

98

2

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 26 of 31

97 Hematite 5Fe1.67Cu10K 96

0

5

10

15

20

25

30

time/min

Fig. 17 - Comparison of hydrogen purity for hematite and 5Fe1.67Cu10K at 850 oC

4. Conclusions The characteristics of hematite modified by potassium and copper using impregnation method were investigated in a batched fluidized bed reactor in the chemical-looping hydrogen generation process. The results indicated that adding of potassium and copper had significant effects on enhancing the reduction reactivity as well as elevating the hydrogen production. The reactivity increased with the loading of either potassium or copper. It was likely due to the formation of the active ferrites such as CuFe2O4 and K2Fe4O7 by introducing foreign ions. The diffusion and catalysis of potassium and the high reactivity of copper oxides made contributions to promote the reduction reactivity. With respect to hydrogen production, H2 concentration evenly increased with the potassium and copper loading due to the synergetic effect on improving the extent of reduction and the catalytic effect on the hydrogen production process. As the results demonstrated, 5Fe1.67Cu10K exhibited the best reduction reactivity and ability to generate hydrogen. Compared to raw hematite, its oxygen


Page 27 of 31

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

transport conversion increased 1.7 times and hydrogen volume increased 2.1 times. In consideration of reaction temperature, it is interesting to conclude that temperature did not have a monotonously improving influence on the reactivity and hydrogen production. Although hematite co-modified by potassium and copper alleviated the agglomeration, 850 oC was suitable for reduction limited with the low melting point of potassium and copper, and 800 oC was appropriate for hydrogen production. The synthesized oxygen carrier maintained steady reactivity and stable ability to generate hydrogen in the cyclic experiments. Furthermore, carbon deposition was suppressed remarkably after hematite was co-modified by copper and potassium leading to a pronounced increase in hydrogen purity. All the hydrogen purity of the synthesized OC was more than 99.9% which satisfied the requirement for hydrogen fuel cells. In conclusion, iron ore co-modified by potassium and copper was feasible for application as oxygen carrier in the CLHG process.

Acknowledgements We gratefully acknowledge the support of this research work by the National Key R&D Program of China (2016YFB0600801) and the National Natural Science Foundation of China (Grant Nos. 51561125001 and 51476029).

References [1]

[2] [3]

[4]

[5]

[6]

T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, et al., "Climate change 2013: The physical science basis," ed: Cambridge University Press Cambridge, UK, and New York, 2014. F. A. Bazzaz, "The response of natural ecosystems to the rising global CO2 levels," Annual review of ecology and systematics, pp. 167-196, 1990. Y.-N. Wang and A. E. Rodrigues, "Hydrogen production from steam methane reforming coupled with in situ CO 2 capture: conceptual parametric study," Fuel, vol. 84, pp. 1778-1789, 2005. T. Mattisson and A. Lyngfelt, "Applications of chemical-looping combustion with capture of CO2," Second Nordic Minisymposium on CO2 Capture and Storage, Göteborg, Sweden, 2001. L. Shen, J. Wu, Z. Gao, and J. Xiao, "Characterization of chemical looping combustion of coal in a 1kW th reactor with a nickel-based oxygen carrier," Combustion and Flame, vol. 157, pp. 934-942, 2010. H. Stainton, A. Ginet, K. Surla, and A. Hoteit, "Experimental investigation of CLC coal combustion with nickel based particles in a fluidized bed," Fuel, vol.


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

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

101, pp. 205-214, 2012. L. F. de Diego, F. Garcı́a-Labiano, J. Adánez, P. Gayán, A. Abad, B. M. Corbella, et al., "Development of Cu-based oxygen carriers for chemical-looping combustion," Fuel, vol. 83, pp. 1749-1757, 2004. B. M. Corbella, L. De Diego, F. García, J. Adánez, and J. Palacios, "The performance in a fixed bed reactor of copper-based oxides on titania as oxygen carriers for chemical looping combustion of methane," Energy & Fuels, vol. 19, pp. 433-441, 2005. B. Wang, H. Zhao, Y. Zheng, Z. Liu, R. Yan, and C. Zheng, "Chemical looping combustion of a Chinese anthracite with Fe2O3-based and CuO-based oxygen carriers," Fuel Processing Technology, vol. 96, pp. 104-115, 2012. A. Tong, S. Bayham, M. V. Kathe, L. Zeng, S. Luo, and L.-S. Fan, "Iron-based syngas chemical looping process and coal-direct chemical looping process development at Ohio State University," Applied Energy, vol. 113, pp. 1836-1845, 2014. P. Gayán, M. A. Pans, M. Ortiz, A. Abad, L. F. de Diego, F. García-Labiano, et al., "Testing of a highly reactive impregnated Fe2O3/Al2O3 oxygen carrier for a SR–CLC system in a continuous CLC unit," Fuel Processing Technology, vol. 96, pp. 37-47, 2012. M. Ryden and A. Lyngfelt, "Using steam reforming to produce hydrogen with carbon dioxide capture by chemical-looping combustion," International Journal of Hydrogen Energy, vol. 31, pp. 1271-1283, 2006. Q. Zafar, T. Mattisson, and B. Gevert, "Integrated hydrogen and power production with CO2 capture using chemical-looping reforming redox reactivity of particles of CuO, Mn2O3, NiO, and Fe2O3 using SiO2 as a support," Industrial & engineering chemistry research, vol. 44, pp. 3485-3496, 2005. K. Svoboda, A. Siewiorek, D. Baxter, J. Rogut, and M. Pohořelý, "Thermodynamic possibilities and constraints for pure hydrogen production by a nickel and cobalt-based chemical looping process at lower temperatures," Energy Conversion and Management, vol. 49, pp. 221-231, 2008. K. Svoboda, G. Slowinski, J. Rogut, and D. Baxter, "Thermodynamic possibilities and constraints for pure hydrogen production by iron based chemical looping process at lower temperatures," Energy Conversion and Management, vol. 48, pp. 3063-3073, 2007. K. Svoboda, A. Siewiorek, D. Baxter, J. Rogut, and M. Punčochář, "Thermodynamic possibilities and constraints of pure hydrogen production by a chromium, nickel, and manganese-based chemical looping process at lower temperatures," Chemical Papers, vol. 61, 2007. S. R. Son, K. S. Go, and S. D. Kim, "Thermogravimetric analysis of copper oxide for chemical-looping hydrogen generation," Industrial & Engineering Chemistry Research, vol. 48, pp. 380-387, 2008. F. Li, H. R. Kim, D. Sridhar, F. Wang, L. Zeng, J. Chen, et al., "Syngas chemical looping gasification process: oxygen carrier particle selection and


Page 28 of 31

Page 29 of 31

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

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29] [30]

[31]

performance," Energy & Fuels, vol. 23, pp. 4182-4189, 2009. P. Gupta, L. G. Velazquez-Vargas, and L.-S. Fan, "Syngas redox (SGR) process to produce hydrogen from coal derived syngas," Energy & Fuels, vol. 21, pp. 2900-2908, 2007. K.-S. Kang, C.-H. Kim, K.-K. Bae, W.-C. Cho, S.-H. Kim, and C.-S. Park, "Oxygen-carrier selection and thermal analysis of the chemical-looping process for hydrogen production," International Journal of Hydrogen Energy, vol. 35, pp. 12246-12254, 2010. W. Liu, L. Shen, H. Gu, and L. Wu, "Chemical Looping Hydrogen Generation Using Potassium-Modified Iron Ore as an Oxygen Carrier," Energy & Fuels, vol. 30, pp. 1756-1763, 2016. R. Siriwardane, H. Tian, and J. Fisher, "Production of pure hydrogen and synthesis gas with Cu–Fe oxygen carriers using combined processes of chemical looping combustion and methane decomposition/reforming," International Journal of Hydrogen Energy, vol. 40, pp. 1698-1708, 2015. H. Gu, L. Shen, J. Xiao, S. Zhang, T. Song, and D. Chen, "Iron ore as oxygen carrier improved with potassium for chemical looping combustion of anthracite coal," Combustion and Flame, vol. 159, pp. 2480-2490, 2012. H. Gu, L. Shen, Z. Zhong, X. Niu, H. Ge, Y. Zhou, et al., "Potassium-Modified Iron Ore as Oxygen Carrier for Coal Chemical Looping Combustion: Continuous Test in 1 kW Reactor," Industrial & Engineering Chemistry Research, vol. 53, pp. 13006-13015, 2014. H. Ge, L. Shen, H. Gu, and S. Jiang, "Effect of co-precipitation and impregnation on K-decorated Fe2O3/Al2O3 oxygen carrier in Chemical Looping Combustion of bituminous coal," Chemical Engineering Journal, vol. 262, pp. 1065-1076, 2015. S. Milić, N. Colović, M. Antonijević, and F. Gaál, "A Thermoanalytical Study of the Solid State Reactions in the K2CO3-MxOy Systems. Evidence for a kinetic compensation effect," Journal of Thermal Analysis and Calorimetry, vol. 61, pp. 229-238, 2000. X. Niu, L. Shen, S. Jiang, H. Gu, and J. Xiao, "Combustion performance of sewage sludge in chemical looping combustion with bimetallic Cu–Fe oxygen carrier," Chemical Engineering Journal, vol. 294, pp. 185-192, 2016. R. Siriwardane, H. Tian, T. Simonyi, and J. Poston, "Synergetic effects of mixed copper–iron oxides oxygen carriers in chemical looping combustion," Fuel, vol. 108, pp. 319-333, 2013. H. R. Paneth, "The Mechanism of Self-Diffusion in Alkali Metals," Physical Review, vol. 80, pp. 708-711, 1950. J. Bao, Z. Li, and N. Cai, "Promoting the Reduction Reactivity of Ilmenite by Introducing Foreign Ions in Chemical Looping Combustion," Industrial & Engineering Chemistry Research, vol. 52, pp. 6119-6128, 2013. P. Legutko, T. Jakubek, W. Kaspera, P. Stelmachowski, Z. Sojka, and A. Kotarba, "Strong Enhancement of deSoot Activity of Transition Metal Oxides by Alkali Doping: Additive Effects of Potassium and Nitric Oxide," Topics in


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

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

Catalysis, 2016. H. J. Fan, U. Gösele, and M. Zacharias, "Formation of nanotubes and hollow nanoparticles based on Kirkendall and diffusion processes: a review," small, vol. 3, pp. 1660-1671, 2007. W. Yang, H. Zhao, K. Wang, and C. Zheng, "Synergistic effects of mixtures of iron ores and copper ores as oxygen carriers in chemical-looping combustion," Proceedings of the Combustion Institute, vol. 35, pp. 2811-2818, 2015. M. H. Khedr, A. A. Farghali, and A. A. Abdel-Khalek, "Microstructure, kinetics and mechanisms of nano-crystalline CuFe2O4 reduction in flowing hydrogen at 300–600°C for the production of metallic nano-wires," Journal of Analytical and Applied Pyrolysis, vol. 78, pp. 1-6, 2007. S. Wang, G. Wang, F. Jiang, M. Luo, and H. Li, "Chemical looping combustion of coke oven gas by using Fe2O3/CuO with MgAl2O4 as oxygen carrier," Energy & Environmental Science, vol. 3, p. 1353, 2010. S. Chen, Q. Shi, Z. Xue, X. Sun, and W. Xiang, "Experimental investigation of chemical-looping hydrogen generation using Al2O3 or TiO2-supported iron oxides in a batch fluidized bed," International Journal of Hydrogen Energy, vol. 36, pp. 8915-8926, 2011. W. Jozwiak, E. Kaczmarek, T. Maniecki, W. Ignaczak, and W. Maniukiewicz, "Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres," Applied Catalysis A: General, vol. 326, pp. 17-27, 2007. S. Takenaka, M. Serizawa, and K. Otsuka, "Formation of filamentous carbons over supported Fe catalysts through methane decomposition," Journal of Catalysis, vol. 222, pp. 520-531, 2004. K.-S. Kang, C.-H. Kim, W.-C. Cho, K.-K. Bae, S.-W. Woo, and C.-S. Park, "Reduction characteristics of CuFe2O4 and Fe3O4 by methane; CuFe2O4 as an oxidant for two-step thermochemical methane reforming," International Journal of Hydrogen Energy, vol. 33, pp. 4560-4568, 2008. K.-S. Cha, H.-S. Kim, B.-K. Yoo, Y.-S. Lee, K.-S. Kang, C.-S. Park, et al., "Reaction characteristics of two-step methane reforming over a Cu-ferrite/Ce– ZrO2 medium," International Journal of Hydrogen Energy, vol. 34, pp. 1801-1808, 2009. B. Yu, P. Zhang, L. Zhang, J. Chen, and J. Xu, "Studies on the preparation of active oxygen-deficient copper ferrite and its application for hydrogen production through thermal chemical water splitting," Science in China Series B: Chemistry, vol. 51, pp. 878-886, 2008. M. Carlsson, "Carbon Formation in Steam Reforming and Effect of Potassium Promotion," Johnson Matthey’s international journal of research exploring science and technology in industrial applications, p. 313, 2015. C. Bernardo, I. Alstrup, and J. Rostrup-Nielsen, "Carbon deposition and methane steam reforming on silica-supported Ni  Cu catalysts," Journal of catalysis, vol. 96, pp. 517-534, 1985. N. Rodriguez, M. Kim, and R. Baker, "Deactivation of copper nickel-catalysts due to changes in surface composition," Journal of catalysis, vol. 140, pp.


Page 30 of 31

Page 31 of 31

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

[45]

[46]

[47]

[48]

16-29, 1993. Y. Li, J. Chen, L. Chang, and Y. Qin, "The doping effect of copper on the catalytic growth of carbon fibers from methane over a Ni/Al 2 O 3 catalyst prepared from Feitknecht compound precursor," Journal of Catalysis, vol. 178, pp. 76-83, 1998. T. Horiuchi, K. Sakuma, T. Fukui, Y. Kubo, T. Osaki, and T. Mori, "Suppression of carbon deposition in the CO 2-reforming of CH 4 by adding basic metal oxides to a Ni/Al 2 O 3 catalyst," Applied Catalysis A: General, vol. 144, pp. 111-120, 1996. J. Juan-Juan, M. Román-Martínez, and M. Illán-Gómez, "Effect of potassium content in the activity of K-promoted Ni/Al 2 O 3 catalysts for the dry reforming of methane," Applied Catalysis A: General, vol. 301, pp. 9-15, 2006. F. Frusteri, L. Spadaro, F. Arena, and A. Chuvilin, "TEM evidence for factors affecting the genesis of carbon species on bare and K-promoted Ni/MgO catalysts during the dry reforming of methane," Carbon, vol. 40, pp. 1063-1070, 2002.


Chemical Looping Hydrogen Generation Using Synthesized Hematite

Recommend Documents