Characterization of an Australia hematite oxygen carrier in chemical looping combustion with coal

International Journal of Greenhouse Gas Control 11 (2012) 326–336

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Characterization of an Australia hematite oxygen carrier in chemical looping combustion with coal Tao Song a , Jiahua Wu b , Haifeng Zhang a , Laihong Shen a,∗ a b

Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China Dongfang Electric Corporation Central Research Institute, Chengdu 611731, Sichuan, China

a r t i c l e

i n f o

Article history: Received 23 December 2011 Received in revised form 29 June 2012 Accepted 28 August 2012 Available online 31 October 2012 Keywords: CO2 capture Interconnected fluidized beds Chemical looping combustion Oxygen carrier Hematite

a b s t r a c t Chemical looping combustion (CLC) is a promising technology to capture CO2 at low cost with solid fuels. This paper presents a continuous operation chemical looping process with a natural Australia hematite. To fulfill this, the prototype of a 1 kWth interconnected beds was employed equipping with a stable coal feeding device. In this prototype, a spouted fluidized bed was used as a fuel reactor and a fast fluidized bed as an air reactor. The oxygen from air was transferred to the fuel by the solid hematite that circulated between the interconnected fluidized bed reactors. Experimental results indicated that the Australia hematite showed a stable reactivity and resistant to agglomeration and to attrition ability. At a fuel reactor temperature 950 ◦ C, a little CH4 was measured and there were neither hydrocarbons heavier than CH4 nor tars in the exit of the fuel reactor. The carbon conversion efficiency was about 81.2%, and the loss rate of this hematite oxygen carrier due to attrition is about 0.0625%/h. XRD results showed that the active phase Fe2 O3 of the hematite oxygen carrier was mostly reduced to Fe3 O4 phase by coal gasification products in the fuel reactor, and only small part of oxygen carrier is further reduced to FeO. No tendency of decreased reactivity of the hematite oxygen carrier was observed during 10 h of operation. This hematite has a good behavior as an oxygen carrier, suitable for use in CLC with coal. Further, with regards to improve the carbon conversion efficiency, a few of Ni-based particles were mechanically mixed with the hematite particles to improve the coal gasification rate. The effect of Ni-based particles addition on the gas conversion and char conversion were evaluated. Also, the possible catalytic mechanism was discussed. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction Greenhouse gas emissions have been received increased scrutiny because of their perceived relation to global warming. CO2 as the primary greenhouse gas deeply affects the climate of the earth. Global warming due to CO2 emissions into the atmosphere has been a public concern and one potential solution is to implement CO2 capture and storage (CCS) technology (Liu et al., 2010). Therefore, the countries around the world continue to progress toward restrictions of greenhouse gas emissions, especially for CO2 . Fossil fuel consumption is the major source of anthropogenic CO2 emission. CO2 capture from power generation is a great challenge for research and engineers. At present, there is an increasing interest in the chemical looping combustion (CLC) as a way to produce relatively pure CO2 that can be readily captured. A detailed review of progress concerning over the development of CLC and chemical looping reforming technologies was given by Adánez et al. (2012). A well-accepted approach is

∗ Corresponding author. Tel.: +86 25 8379 5598; fax: +86 25 5771 4489. E-mail address: [email protected] (L. Shen).

to conduct chemical looping process in two fluidized-bed reactors connected by solid transportation lines. To achieve this process, the prototype designed usually consists of two separate reactors: an air reactor (AR) and a fuel reactor (FR). Between these two reactors oxygen is transported by an oxygen carrier, which is often a metal oxide, thereby avoiding direct contact between fuel and air. In this way, the nitrogen from the air leaves the system from the air reactor, whereas the flue gas from the fuel reactor consists of only CO2 and water. After water condensation, almost pure CO2 can be obtained, and then compressed into liquid for storage. Coal is a cheaper and more abundant resource than other fossil fuels. It will become even more important when oil and gas become scarcer while coal is relatively abundant (Feng et al., 2007) and at the same time being a reliable fuel for power production (Wall, 2007). With the present development of CLC technology, most investigations have been turned to use solid fuels most for coal directly application to CLC. That is to introduce the coal directly to the fuel reactor where the oxygen carrier is reduced by the fuel, thus avoiding separate gasification and separation steps. In this condition, gasification could be undertaken in CO2 or mixtures of CO2 and steam. With respect to industrial application solid fuel CLC technology, the key of a CLC system is the oxygen carrier. In

1750-5836/$ – see front matter. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijggc.2012.08.013

T. Song et al. / International Journal of Greenhouse Gas Control 11 (2012) 326–336

Nomenclature CLC AR FR FO,ARtot FAR,in FAR,out XO2 ,AR XCO2 ,AR

chemical looping combustion air reactor fuel reactor total oxygen consumption in the air reactor total gas flow entering the air reactor (m3 /h) total gas flow at the outlet of the air reactor (m3 /h) O2 concentration in the flue gas of the air reactor (%) CO2 concentration in the flue gas of the air reactor (%) FC,AR total flow of oxygen consumption for char combustion in the air reactor (m3 /h) FC,FR total carbonaceous gas flow leaving the fuel reactor (m3 /h) FC,coal carbon flow of coal feed (m3 /h) FN2 ,in total gas flow of N2 entering the fuel reactor (m3 /h) XCO2 ,FR CO2 concentration in the flue gas of the fuel reactor (%) CO concentration in the flue gas of the fuel reactor XCO,FR (%) XCH4 ,FR CH4 concentration in the flue gas of the fuel reactor (%) CO flow leaving the fuel reactor (m3 /h) FCO,FR FCH4 ,FR CH4 flow leaving the fuel reactor (m3 /h) FCO2 ,FR CO2 flow leaving the fuel reactor (m3 /h) FC,fly ash carbon flow of fly ash (m3 /h) Cchar,FR carbon flow from the gasified char in the fuel reactor (m3 /h) Cchar,efficient carbon flow from the effective char introduced (m3 /h) carbon flow coming from the volatile matter fed CVol (m3 /h)

Greek letters ˚ oxygen concentration of the gas flow entering the air reactor, chosen as 0.21 in this study moles of oxygen needed to convert the fuel com˚O pletely, per moles of carbon conversion carbon conversion efficiency (%) carbon capture efficiency (%) CC char char gasification efficiency (%) ˝OD oxygen demand (%)

order to avoid a high sintering rate, metal oxide as an oxygen carrier should be synthetically manufactured by supporting it on an inert high-temperature material such as SiO2 , Al2 O3 or Y-stabilized ZrO2 (YSA), which improves significantly the reactivity, durability, and lifetime of oxygen carrier (Corbella et al., 2006; Zafar et al., 2006). Except for a high chemical reactivity in the two stages of reduction and oxidization of the oxygen carrier, the manufacture cost of oxygen carrier is another important factor affecting CLC industrial application. A low-cost CaSO4 has attracted attention as an oxygen carrier in CLC. Ding et al. (2011) investigated a binder-supported CaSO4 oxygen carrier with CH4 as a fuel and obtained an optimal extrusion condition. However, there are still some challenges for the use of the CaSO4 oxygen carrier in CLC, such as the release of sulfur leading to the loss of oxygen carried capacity. Currently, iron ore as an oxygen carrier has been developed as an attractive and suitable material for CLC industrial application with a low cost. Since 2008, some investigations with iron ore as an oxygen carrier are performed at Chalmers University of Technology from Sweden (Berguerand and Lyngfelt, 2008a,b, 2009a,b; Leion et al.,

327

2008; Linderholm et al., 2011; Ryden et al., 2010; Azis et al., 2010), C.S.I.C. from Spain (Adánez et al., 2010; Cuadrat et al., 2011a,b,c; Abad et al., 2011), Stuttgart University from Germany (Bidwe et al., 2011) and Southeast University from China (Gu et al., 2011, 2012; Xiao et al., 2010a,b; Zhang et al., 2011; Song et al., 2011a). Hematite as one of the iron ores takes up more than 60% of iron ore reserve in the world. It has been one of the most important ore in the industrial application. Hematite is mainly composed of Fe2 O3 , which is the active phase that behaves as an oxygen carrier. Some inert materials, such as SiO2 and Al2 O3 , are likely to improve the reactivity and durability of the oxygen carriers (Song et al., 2011a). In our research team, Xiao et al. (2010b) investigated its reaction performance using coal as fuel under a pressured condition. Results show a good reaction performance and no sintering on the surface of the hematite. Gu et al. (2011) found that a high carbon capture efficiency can be achieved with a sawdust as fuel using a natural hematite at a fuel temperature of 800 ◦ C. Mattisson et al. (2001) investigated the characterization of a hematite oxygen carrier and results showed that the sufficient reaction rates for reduction and oxidation were shown by employing the hematite oxygen carrier. Up to present, less investigation has been carried out using hematite oxygen carrier for a long-term continuous operation to detect its reaction performance, durability, and lifetime. The advantages of solid fuels direct CLC technology are now widely recognized and many key problems have been gradually solved. The solid fuel gasification rate was accelerated at the expense of gasification products (Keller et al., 2010; Leion et al., 2009). However, the coal gasification rate is still the time limiting step in CLC with solid fuels (Leion et al., 2009; Dennis et al., 2010; Brown et al., 2010). To accelerate the gasification of solid fuels is a key to increase the solid fuels conversion in the fuel reactor, CO2 capture efficiency and the total combustion efficiency. With respect to the solid fuels gasification, to improve the fuel reactor temperature may be a suitable method. However, a high temperature pushes the sintering process of oxygen carrier, which can be cause the deactivation of oxygen carriers. The metallic nickel is well known as a good catalyst for coal gasification (Tomita et al., 1983; Kurbatova et al., 2011). Also, when Ni-based oxygen carriers are used in CLC process, it shows good reaction ability in the previous studies (Adánez et al., 2006; Abad et al., 2007a). Ryden et al. (2010) investigated the reaction performance of ilmenite in a batch fluidized-bed reactor using CH4 as fuel. It was found that a 14% combustion efficiency is improved with an addition of NiO content of 0.6 wt.%. NiO addition to the oxygen carrier is due to the highest reactivity with CH4 . There were also some investigations carried out to utilize the catalytic effect of Ni in CLC process, which is limited to be employed in CLC with gaseous CH4 as a fuel. Mattisson et al. (2008) found that almost complete conversion of CH4 into CO2 and H2 O can be achieved with a very small amount of NiO material, and that the reaction proceeds with CO and H2 as intermediates. The same investigation was found by Johansson et al. (2006a). It was found that addition of as little as 1 wt.% NiO to a sample of Fe2 O3 improved the capacity of the sample to convert CH4 into CO2 and H2 O considerably. In this work, some experiments were carried out to use an Australia hematite as an oxygen carrier for a 10 h continuous operation in a 1 kWth interconnected fluidized-bed reactor at atmospheric pressure. The reactor allows coal gasification with simultaneous CO2 enrichment in one bed (FR) and the regeneration of the oxygen carrier in the other bed (AR). Also, to be used for multiple cycles, the hematite oxygen carriers are required to have excellent chemical and mechanical properties. Therefore, the reacted oxygen carrier was characterized by SEM, BET and XRD, and the mechanical strength of the hematite was discussed. Further, with regards to improve the carbon conversion efficiency, a few of Ni-based particles were mechanically mixed with the hematite

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T. Song et al. / International Journal of Greenhouse Gas Control 11 (2012) 326–336

Table 1 The surface characterization and pore volume of the materials used in this work at different calcination temperature. BET (m2 /g)

Raw hematite Fresh hematite

BJH (m3 /g)

5.06 1.89 0.57

0.03 0.015 0.005

21.3 10.2 9.1

Table 2 Elementary analysis of the hematite used in this work. Elements

Fe

Si

Al

Ca

Mg

K

P

wt.%

54.62

7.52

2.22

0.44

0.22

0.21

0.078

Elements

Cr

Zn

O

S

Mn

Ti

wt.%

0.0062

0.0048

34.52

0.064

0.049

0.042

particles to examine if the reactivity could be improved, when the solid coal was used as fuel introduced to the continuous operation system. 2. Experimental 2.1. Oxygen carrier materials and coal type A natural hematite from Australia was used as oxygen carrier particles supplied by Nanjing Steel Manufacturing Company. Thermal treatment for the iron ore used before as an oxygen carrier is necessary and it can improve the mechanical strength of the particles. However, the temperature of the thermal treatment is significant. Firstly, an investigation was performed at different thermal treatment temperature of 970 ◦ C and 1100 ◦ C for this hematite, and the results are summarized as Table 1. The pore structure properties of the raw and reacted samples were measured by nitrogen adsorption/desorption isotherms at 77 K with a Micromeritics instrument ASAP 2020. The surface area and pore volume were calculated from the Brunauer–Emmett–Teller (BET) equations and Barrett–Joyner–Halenda (BJH) method, respectively. As shown in Table 1, it can be observed that the improvement of mechanical strength is at the expense of the BET decrease itself. The BET decreases significantly from 5.06 m2 /g for the raw hematite to 1.89 m2 /g at 970 ◦ C and 0.57 m2 /g at 1100 ◦ C. In this work, the samples calcined at 970 ◦ C was defined as a fresh oxygen carrier and used. Based on XRF (X-ray Fluorescence, ARL-9800, Switzerland) analysis, the hematite was mainly composed of 78 wt.% Fe2 O3 , 16.1 wt.% SiO2 , and the elementary analysis of this hematite is shown in Table 2. The bulk density for the particles was 1967 kg/m3 . The particle size fraction of this hematite used is summarized in Table 3. Also, the photographs of solid hematite particles before and after thermal treatment are presented in Fig. 1. In the continuous experiments, the particles inventory of the hematite oxygen carrier employed was about 1.97 kg.

Table 3 Particle size fraction of the fresh hematite particles. Particle diameter (␮m)

Mass distribution (%)

300–350 200–300 100–200 80–100 54–80 <54

20.37 47.87 30.97 0.51 0.28 0

dpore (nm)

Calcining Temperature (◦ C)

Time (h)

970 1100

3 3

The nickel-based particles were prepared using co-precipitation and have been used in our previous study as an oxygen carrier (Shen et al., 2010a,b; Song et al., 2011b). It was composed of 20.0 wt.% NiO, 39.0 wt.% NiAl2 O4 , and 41.0 wt.% Al2 O3 after calcination. The bulk density of the Ni-based particles was 1250 kg/m3 . The particles added to the hematite were 3 wt.% and 10 wt.%, respectively. In order to prevent stratification between the two particles due to difference of density, the particle size of the Ni-based particles was 300–450 ␮m. As for NiAl2 O4 in the nickel-based particles, although some investigations (Zhang et al., 2000; Li and Chen, 1995) showed that the reduction of NiAl2 O4 occurs at a high temperature of 720–930 ◦ C using the TPR (temperature programmed reduction) in H2 , it seemed that the spinel was more difficult to reduce than NiO. According to the experiments in a 10 kWth CLC prototype after 100 h continuous operation by Johansson et al. (2006b), the reduction of NiAl2 O4 spinel was not occurred. Therefore, in the presence of NiAl2 O4 in the oxygen carrier samples, its influence on the reaction performance of hematite mixed with NiO can be ignored. The crushing strengths of Australia hematite and Ni-based particles were 3.8 N of particles sized 200–300 ␮m and 4.1 N of particles sized 300–450 ␮m, respectively. They were determined using a Shimpo FGN-5 crushing strength apparatus. The crushing strength was defined as the force needed to fracture a particle and the values presented here were averages of 30 measurements. The coal sample used in the study was a typical Chinese coal, Shenhua bituminous coal (SH), China. The proximate analysis and ultimate analysis of coal sample is presented in Table 4. The coal particles were sieved to a size range of 200–450 ␮m for use. 2.2. The 1 kWth continuous CLC prototype The objective of the interconnected fluidized bed experiments is to convert solid bituminous coal to CO2 and H2 O during continuous operation. The schematic diagram of the 1 kWth prototype used for the experimental test is shown in Fig. 2. The prototype is composed of a fast fluidized bed as an air reactor, a cyclone, a spout-fluid bed as a fuel reactor and a loop-seal. The fast fluidized bed is a circular column of 20 mm in inner diameter and 1600 mm in height. The spout-fluid bed is a rectangular bed, with a cross section of 50 mm × 30 mm, and a height of 1000 mm. The loop-seal connects the spout-fluid bed with the fast fluidized bed and is fluidized by steam stream to prevent the contamination of the flue gas between the two reactors. It is a rectangular bed with a cross section of 34 mm × 30 mm, and a height of 370 mm. In the fast fluidized bed oxygen carrier particles are entrained to the top of the bed by a stream of air, transported to the spout-fluid bed through the cyclone, and then back-passed to the fast fluidized bed through the

Table 4 Proximate, ultimate analysis of coal. Proximate analysis

wt.%, ad

Ultimate analysis

wt.%, daf

Moisture Volatile Ash Fixed carbon Low heating value (MJ/kg)

6.01 35.10 4.76 54.13 27.1

C H O N S

69.57 4.30 13.81 1.03 0.52

T. Song et al. / International Journal of Greenhouse Gas Control 11 (2012) 326–336

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Fig. 1. The photograph of solid hematite particles used.

loop-seal. This makes up the external circulation of oxygen carrier particles in the interconnected fluidized beds. In this work, the total air (Nanjing Tongguang Gases Co., Ltd.) flow of the fast fluidized bed was at 0.84 m3 /h (20 ◦ C). The steam supply for the spout-fluid bed and the loop-seal were at 0.12 kg/h and 0.21 kg/h, respectively. The high-purity nitrogen stream (Nanjing Tongguang Gases Co., Ltd., 99.99% N2 ) for the pneumatic medium for conveying coal particles into the bottom of the spout-fluid bed was at a flow of 0.27 m3 /h (20 ◦ C). In the whole experiments the coal was fed at a same rate of 0.1 kg/h. The fuel of SH bituminous coal addition corresponds to 0.75 kWth . The two reactors were electrically heated in an oven, which supplies heat for start-up and compensates heat loss during operation. Thermocouples and pressure drop transducers locate at different

points of the prototype displaying the current operating conditions at any time. The outlets from the air reactor and the fuel reactor were induced with a suction pump to an ice-water cooler where the steam was condensed and removed. The exhaust gas was sampled by gas bags for offline analysis, and the compositions of the flue gas including O2 , CO2 , CO, CH4 and H2 of the two reactors was analyzed by a NGA2000 type gas analyzer (EMERSON Company, USA). Gas chromatograph (Agilent 6890N) was used to detect the C2 –C4 of the two reactors. An X-ray diffractometer (XRD, Rigaku Co.) using Cu K␣ radiation was employed to analyze the oxygen carrier samples. The samples were scanned in a step-scan mode with a step size of 0.02◦ over the angular 2 range of 10–90◦ . Also, the morphological features of oxygen carrier samples were characterized by a field-emission scanning electron microscope (SEM, Japanese Electronics Company). The gas leakage between the reactors must be minimized to prevent carbon dioxide from leaking to the air reactor decreasing the efficiency of carbon dioxide capture. In this system, the possible gas leakages are: (i) from the air reactor to the loop-seal and to the fuel reactor, (ii) from the air reactor to the cyclone and to the fuel reactor, (iii) from the fuel reactor to the loop-seal and to the air reactor, (iv) from the fuel reactor to the cyclone, (v) from the loop-seal to the fuel reactor, and (vi) from the loop-seal to the air reactor. In the 1 kWth prototype, the gas leakages of the situations above have been investigated. The results show that the gas leakage pathway of (i–iii) can be completely avoided in the presence of the loop-seal. The loop-seal prevents carbon dioxide from leaking to the air reactor, which could achieve the highest carbon dioxide capture efficiency 98%. 2.3. Data evaluation During the continuous operation, several parameters were measured during operation: (1) (2) (3) (4)

The temperature in the air reactor and fuel reactor. The pressure drops in the system. The fluidizing gas flows for the system. The O2 , CO, H2 , CH4 , C2 –C4 and CO2 concentrations from the flue gas of the two reactors.

Table 5 shows the technology assessment and calculation equations (Eqs. (1)–(4)) used in this work. The total oxygen consumption in the air reactor is defined as FO,ARtot including oxygen needed for particle oxidation and char combustion. Fig. 2. Configuration of the 1 kWth interconnected fluidized beds. a–g, pressure tapping points.

FO,ARtot = FAR,in × ˚ − FAR,out × XO2 ,AR

(5)

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T. Song et al. / International Journal of Greenhouse Gas Control 11 (2012) 326–336

Table 5 Technology assessment and calculation equations used in this work. Technology assessment

Calculation equations

Carbon conversion efficiency

conversion =

FC,FR + FC,AR FC,coal

Carbon capture efficiency Char gasification efficiency

˝OD =

Oxygen demand

2FCH4 ,FR + 0.5FCO,FR ˚O (FCO2 ,FR + FCO,FR + FCH4 ,FR )

The CO2 and O2 concentrations in the flue gas of the air reactor, noted as XCO2 ,AR and XO2 ,AR , are given based on the measured data. The gas flow in the air reactor outlet is noted as FAR,out and expressed as: FAR,out = FAR,in × (1 − ˚) ×

1 1 − XCO2 ,AR − XO2 ,AR

(6)

FC,AR is the total carbonaceous gas flow leaving the air reactor, which was also defined as a value of oxygen consumption for char combustion in the air reactor. FC,AR = FAR,out × XCO2 ,AR

(7)

The conversion efficiency of carbon in fuel reactor (conversion ) is used to illustrate the conversion extent of carbon in the coal into the carbonaceous gases. The carbon capture efficiency (CC ) is the ratio of coal conversion to the carbonaceous gases in the fuel reactor, when the effective coal used, excluding the carbon remaining in the fly ash. These two parameters are defined as Eqs. (1) and (2). FC,FR is the total carbonaceous gas flow leaving the fuel reactor, which is obtained on the basis of the known flow of nitrogen added into the fuel reactor (FN2 ,in ). The measured concentrations of gaseous products in the flue gas of the fuel reactor are diluted by the flow of nitrogen added into the fuel reactor. FC,FR = FN2 ,in ×

(1)

FC,FR CC = FC,FR + FC,AR FCO2 ,FR + FCO,FR + FCH4 ,FR − CVol Cchar,FR char = = Cchar,efficient FCO2 ,FR + FCO,FR + FCH4 ,FR + FCO2 ,AR − CVol

XCO2 ,FR + XCO,FR + XCH4 ,FR 1 − (XCO2 ,FR + XCO,FR + XCH4 ,FR )

(8)

where XCO2 ,FR , XCO,FR and XCH4 ,FR represent CO2 , CO and CH4 concentrations in the flue gas of the fuel reactor, respectively. The carbon measured in the gases coming from the fuel reactor and the air reactor is less than the carbon present in the introduced coal, because there is elutriation of char in the fly ash. The carbon flow of fly ash (FC,flyash ) is obtained on the basis of the carbon balance in the system. It is the difference between the carbon flow of coal feed (FC,coal ) and the total carbonaceous gas flow leaving the two reactors as follows: FC,flyash = FC,coal − FC,FR − FC,AR

(2) (3) (4)

3. Results and discussion 3.1. Results of continuous operation at a fuel reactor of 950 ◦ C The test involved 10 h of stable operation. Fig. 3 displays the typical time series of bed pressure drops both in the spout fluid bed and in the high velocity fluidized bed. Most of oxygen carrier particles employed in the experiment remained in the spout-fluid bed, which was favorable to a series of reactions for CLC of coal in the spout-fluid bed. Agglomeration of particles in fluidized beds is a potential risk for difficulties in operation and in worst case for process failure. Generally, it can be expected that agglomerations could more easily appear in a small scale system with low velocities, compared to a large-scale system where gas velocities could be more than an order of magnitude larger (Berguerand and Lyngfelt, 2008a). In this prototype, under the positive effect of the jet flow in the fuel reactor with a spouted-bed, no defluidization due to the agglomeration was found in the continuous experiments. The outlet of the fuel reactor was mainly composed of oxidized CO2 , CO and some CH4 as not fully oxidized products of char gasification and volatile matter. Also, there were neither hydrocarbons heavier than CH4 nor tars in the fuel exit of the fuel reactor. Fig. 4 shows the concentration profiles of CO2 , CO and CH4 in the flue gas of the fuel reactor during 10 h of operation at the fuel reactor temperature 950 ◦ C. During the initial stage till to 200 min, the exit concentrations of the fuel reactor of CO and CH4 slightly increase, and the CO2 concentration decreases. After 200 min continuous operation, the profiles of stable concentrations are presented in Fig. 4. The same results were obtained by Moldenhauer et al. (2011) using minerals and industrial by-products as oxygen carriers. Initially the particles

(9)

The carbon of effective coal is the sum of all carbon containing species measured in the outlet streams of both fuel reactor and air reactor. In order to investigate the char conversion characterization in CLC process, the char gasification efficiency (char ) in the fuel reactor is defined as Eq. (3). CVol represents the carbon in the volatile matter calculated according to the proximate and ultimate analysis of the coal. A total oxygen demand for the fuel reactor gases, ˝OD (Eq. (4)), is defined as the fraction of oxygen lacking to achieve a complete combustion of the fuel reactor product gas in comparison to the oxygen demand of the effective introduced coal, O2 demandcoal, eff . ˚O is the moles of oxygen needed to convert the fuel completely per moles of carbon in the fuel. It is calculated using fuel analysis in Table 4. For the Shenhua coal studied here, the value is 1.11.

Fig. 3. Typical time series of bed pressure drops in both the high velocity fluidized bed and the spout-fluid bed. Pge, Pfe, Pad and Pcd were the pressure drop of the points of g-e, f-e, a-d and c-d.

4.0 3.5

28

CO2

26

3.0

24

2.5

22

2.0

20 18

1.5

CO

16

1.0

14

0.5 0.0

12

CH4 0

100

200

300

400

10 600

500

Time (min) Fig. 4. Gas composition of the fuel reactor exit gas on a dry basis versus time at the fuel reactor temperature 950 ◦ C.

Gas concentration of air reactor (vol.%)

underwent a considerable alteration process during which they grew bigger, more porous and as a result became more reactive. After this initiation period they were stable in respect of structural stability and reactivity (Moldenhauer et al., 2011). Indeed, an activation of this material with increasing the number of redox cycles was found and stabled after 10 cycles (Xiao et al., 2010a). After the initiation of 200 min continuous operation, no tendency of decreased reactivity of the hematite oxygen carrier is observed during the experiments with coal at the fuel reactor temperature 950 ◦ C, showing a good long-term reactivity of this hematite oxygen carrier. CO concentration is in the range of 1.0–1.25 vol.%, CH4 concentration 0.18–0.25 vol.%, and CO2 concentration around 25.01–27.64 vol.%. The selectivity toward the production of CO2 is stable, indicating a good regeneration ability of the hematite oxygen carrier. There is less residual char along with the oxygen carrier particles coming into the air reactor. The CO2 concentration of the flue gas from the air reactor is about 1.45–1.90 vol.%, and the O2 concentration is about 5.80%, as shown in Fig. 5. The carbon conversion efficiency is about 81.2% according to Eq. (1). The carbon contained in the fly ash was about 18.8%. In fact, the char elutriation from the fuel reactor is related to the residence time of char particles in the fuel reactor. Song et al. (submitted for publication) found that there was some NOx

Particle diameter (␮m) >150 100–150 80–100 54–80 <54 Actual loss (g) (dp < 54 ␮m) Operation time (h) Loss rate (%/h)

Mass (g)

Mass distribution (%)

0 3.11 4.76 11.79 30.31

0 6.23 9.53 23.60 60.65

30.31 10 0.0625

formation in the air reactor due to char combustion. In the air reactor, the char combustion rate is larger than the one of oxygen carrier regeneration by the air. That is, in the bottom of the air reactor, some NOx formation occurs due to unconverted char combustion. If sufficient residence time for the char gasification in the fuel reactor is provided, both the limited NO emissions from the air reactor and a highly efficient use of coal can be achieved. 3.1.1. Mechanical strength The oxygen carrier requires high reactivity and mechanical strength for a long time use under the cyclic redox reaction in the interconnected fluidized beds of high temperatures (Baek et al., 2011). Because of a high velocity, the main oxygen carrier loss due to the attrition occurs in the fast fluidized bed (air reactor). Table 6 shows the elutriated oxygen carrier particles due to attrition in the exit of the air reactor, including the particle diameter, particle mass and mass distribution. The small amount ash from char combustion in the exit of the air reactor was negligible. As presented in Table 6, the particle size of the powder leaving the air reactor is mainly less than 54 ␮m. The loss rate of fine particles is about 0.0625%/h. The crushing strengths of used Australia hematite of particles sized 200–300 ␮m was measured, equaling to 3.2 N. Generally, with respect to the different iron ores used in CLC process, the physical and chemical characterization including mechanical strength, reaction performance should be considered. The hematite used in this work from Australia showed a good attrition resistance, suitable for use in CLC process. The lifetime of the oxygen carrier corresponding to the loss of fines can be calculated approximately as: tlife =

1 (h) mass loss rate (%/h)

(10)

The mass loss rate of fine particles was about 0.0625%/h, which translates to a life time of this oxygen carrier of 1600 h.

10

8 O2 6

4 CO2

2

0

331

Table 6 Elutriated oxygen carrier particles due to attrition.

30 Gas comp. of CO 2 in fuel reactor ( vol.% )

Gas comp. of CO and CH 4 in fuel reactor ( vol.% )

T. Song et al. / International Journal of Greenhouse Gas Control 11 (2012) 326–336

0

100

200

300

400

500

600

Time (min) Fig. 5. Gas composition of the air reactor exit gas on a dry basis versus time at the fuel reactor temperature 950 ◦ C.

3.1.2. SEM and BET analysis of oxygen carriers The magnifications of 100× and 20,000× were selected to analyze the surface micrographs of fresh and reduced oxygen carriers. As shown in Fig. 6(a), the surfaces of fresh oxygen carrier particles are clearly rougher, and covers with big grains of a size around 2–3 ␮m. After 10 h tests in the 1 kWth CLC prototype with coal, the surface changes to a more coarse texture with the development of cracks and fissures in the particle. As shown in Fig. 6(b), the larger pores are replaced by some smaller pores, indicating the growth of grains. The porous surface of the particles facilitates the diffusion of reactant gases into the core of oxygen carrier particles, enhancing the reactions between gas and oxygen carrier particles. The BET surface area of the fresh solid particles with a size range of 2–3 ␮m is at 1.89 m2 /g. After 10 h continuous reaction, it is clearly found that the BET surface area of 1.41 m2 /g with a size range of 0.5–1 ␮m, indicating the effect of sintering at this operation temperature and the split of the larger grains. Song et al. (2011a) investigated the reduction characterization of the hematite by H2 using SEM–EDS.

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Fig. 6. SEM images for fresh and reduced hematite oxygen carriers (a) fresh and (b) reduced.

It was found that the inert materials of SiO2 or Al2 O3 are good mediums for resistance against contact between the liquid phase and the active grains, which suppresses the extensive sintering of the oxygen carrier particles.

3.1.3. XRD analysis Fig. 7 shows the XRD spectra of fresh oxygen carrier, the reduced oxygen carrier of the fuel reactor, and the oxidized oxygen carrier of the air reactor respectively. The samples were scanned in a step-scan mode with a step size of 0.02◦ over the angular 2 range of 10–90◦ . There are five phases, Fe2 O3 , Fe3 O4 , FeO, Fe2 (SiO4 ) and SiO2 , in the three samples, as indicated in Fig. 7. The contents of Fe2 O3 , FeO, Fe2 (SiO4 ) and Fe3 O4 in the samples can be characterized by the relative intensity of the major peak. On the basis of the XRD spectra, the IFe3 O4 /IFe2 O3 , IFeO /IFe2 O3 , IFe2 (SiO4 ) /IFe2 O3 ratios of the reduced oxygen carrier of the fuel reactor equal to 5.61, 1.75 and 0.74, respectively. In the air reactor, the value of IFe3 O4 /IFe2 O3 equals to 0.12. It indicates that the active phase Fe2 O3 of the hematite oxygen carrier is mostly reduced to Fe3 O4 phase by coal gasification products in the fuel reactor, and only small part of oxygen carrier is further reduced to FeO. The formation of Fe2 (SiO4 ) may be due to the reaction of 2FeO + SiO2 → 2Fe2 (SiO4 ). Compared with the results obtained by Song et al. (submitted for publication) using an anthracite as fuel, due to a low gasification rate, the hematite oxygen carrier was only reduced by the anthracite gasification products to the phase of Fe3 O4 , whereas the further reduction of hematite oxygen carrier by coal gasification products is observed with the bituminous coal. Therefore, the reduction products of hematite oxygen carrier in the fuel reactor are related to the coal type. If the coal is easily gasified, the hematite oxygen carrier particles will be further reduced by coal gasification products.

However, if the hematite particles are reduced too far, during the experiments the particles formed agglomerates in the fuel reactor is the biggest problem. It may cause fluidization conditions deteriorated. Although small part of oxygen carrier was further reduced to FeO, no defluidization was found in the continuous experiments. The reduced oxygen carrier can be well regenerated in the air reactor as shown in Fig. 7. The effect of the presence of Fe2 (SiO4 ) in the reduced oxygen carrier on the performance of oxygen carrier is unknown. However, the Fe2 (SiO4 ) can be decomposed in the air reactor via the reaction of 2Fe2 (SiO4 ) + O2 → 2Fe2 O3 + 2SiO2 . A good candidate of oxygen carrier for CLC should have high oxygen transfer capacity, be suitable under a significant number of successive reduction–oxidation cycles, be resistant to agglomeration and to attrition, have a high sintering temperature, be cheap and environmental friendly (Stainton et al., 2012). According to the investigation in this work, this kind of hematite shows a good reaction characterization after long time continuous operation, suitable for use as an oxygen carrier for industry application. 3.2. Ni-based particles addition Although a steady CO2 capture was achieved during the above continuous experiments only with the hematite oxygen carrier, with regards to improve the carbon conversion efficiency, a few of Ni-based particles were mechanically improved with the hematite particles to access the carbon conversion as well as carbon capture efficiency. In this section, all tests involved 3 h of stable continuous operation. No difficulties inherent to the process itself were observed. 3.2.1. CH4 conversion Generally, NiO has a considerably high reactivity with CH4 . Also, when reduced by a fuel, NiO is converted directly into metallic Ni,

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333

Fig. 7. XRD spectra of fresh and reacted hematite oxygen carriers. (a) Reduced oxygen carrier, (b) oxidized oxygen carrier and (c) fresh oxygen carrier. Table 7 Exit gas concentrations of both fuel reactor and air reactor (all the data were shown using the average values). Fuel reactor

100 wt.% hematite oxygen carrier 97 wt.% hematite + 3 wt.% Ni-based particles 90 wt.% hematite + 10 wt.% Ni-based particles

Air reactor

CO

CO2

CH4

CO2

O2

1.15 1.22 1.12

26.3 27.2 28.1

0.22 0.19 0.16

1.7 1.3 1.02

5.8 5.1 4.2

which is well known to catalyze decomposition of CH4 and other hydrocarbons (Ryden et al., 2010). When coal was used as fuel in this study, the exit gas concentrations both of the fuel reactor and air reactor with the addition of Ni-based particles in the hematite oxygen carrier particles are summarized and presented in Table 7. The data obtained by using the Australia hematite as an oxygen carrier is also shown. All the data is shown using the average values during the continuous operation. CH4 in the exit gas flow of the fuel reactor is mainly from the coal pyrolysis. As shown in Table 7, for hematite as bed material, the CH4 concentration is 0.22%. When using hematite improved with 3 wt.% and 10 wt.% Ni-based particles, the CH4 came from exit gas of the fuel reactor decreases to 0.19% and 0.16%, indicating the improved CH4 conversion in the presence of Ni-based particles. However, the decreasing trend for CH4 conversion is much slight. Considering the experimental error with the kind of gas analysis equipment used here, it can be concluded that when coal is used as fuel at a fuel reactor of 950 ◦ C, the small addition of NiO has a weak effect on the CH4 evolution. 3.2.2. Char conversion efficiency Cuadrat et al. (2011c) and Berguerand and Lyngfelt (2008a) showed that in the fuel reactor char gasification products react fast with the oxygen carrier, which means most reducing gases from the exit of the fuel reactor coming from the volatile matter. Fig. 8 shows the char gasification efficiency versus fuel reactor temperature in comparison with the results obtained by Shen et al. (2010a) using 100% Ni-based oxygen carrier. The char gasification efficiency is temperature dependent, which is consistent with the results obtained by Cuadrat et al. (2011c) and Berguerand and Lyngfelt (2008a,b) using an ilmenite oxygen carrier. Also, When using hematite improved with 3 wt.% and 10 wt.% Ni-based particles, the char gasification efficiency increases from 85.8% without Ni addition to 89.2% and 91.8%, respectively. That is, more gases are released since char was being faster gasified. Also, the presence of

oxygen carrier in the system has shown to help with the decomposition and later oxidation of tars and higher hydrocarbons than CH4 , since no of these species were formed when using this hematite and mixed oxygen carriers. The same results were obtained with an ilmenite by Cuadrat et al. (2011c). In the presence of Ni-based particles, the point at which char gasification efficiency increases significantly seems to be correlated with the catalytic effect of Ni, leading to the amount of residual char coming into the air reactor and char elutriation decrease. Therefore, the carbon capture efficiency and carbon conversion efficiency at a fuel reactor temperature 950 ◦ C are improved, as shown in Table 8. The oxygen demand due to the unburnt gases present in the fuel reactor with the addition of Ni-based particles is also shown. It can be seen that the oxygen demand decreases because of the increase in the ratio of Ni-based particles. This can be explained through the

Fig. 8. Char gasification efficiency versus fuel reactor temperature.

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Table 8 Carbon capture efficiency (CC ), carbon conversion efficiency (conversion )) and oxygen demand (˝OD ) with the addition of Ni-based particles.

100 wt.% hematite oxygen carrier 97 wt.% hematite + 3 wt.% Ni-based particles 90 wt.% hematite + 10 wt.% Ni-based particles

CC (%)

conversion (%)

˝OD (%)

87.2 90.3 92.6

81.2 86.1 87.6

3.3 3.1 2.7

raise of the gasification rate: there is more production of the gasification products, which must react with hematite oxygen carrier particles. This fact suggested that the improved reaction performance with the addition of Ni-based particles is comparable to the increase of the gasification rate. Focusing on the char gasification and combustion in the fuel reactor, steam gasification two independent reactions (gasification and water gas shift) can take place. The following reaction system was considered: C + H2 O → CO + H2 CO + H2 O → CO2 + H2

gasification watergasshift

It is known that Ni is a good catalyst for steam gasification reaction in the metallic state (Haga and Nishiyama, 1988). Zhao et al. (2010) investigated the catalytic steam gasification of natural coke in a fluidized bed. It was found that Ni-based catalyst increased gasification rate of the natural coke effectively. The improved char conversion efficiency is found with a little addition of Ni-based particles into the fuel reactor, as shown in Table 8, indicating the positive effect of Ni-based particles both on the conversion of hydrocarbons and the carbon conversion. Also, it is interesting that the CO concentration increased to 1.22% with 3 wt.% Ni-based particles and decreased to 1.12% with 10 wt.% addition in comparison with the one of 1.15% without Ni-based particles addition, as shown in Table 7. Previous experimental evidence has been found that water gas shift reaction is relatively fast at temperatures involved in CLC systems using Ni-based oxygen carrier (Abad et al., 2010), whereas the gas composition is far from water gas shift equilibrium condition using Fe-based oxygen carrier (Abad et al., 2007b). Therefore, the water gas shift reaction may be promoted with more NiO contained particles addition in the hematite oxygen carrier. 3.2.3. Possible catalytic mechanism With respect to improve the solid fuel conversion, Teyssie et al. (2011), Schwebel et al. (2012) and Cuadrat et al. (2011b) investigated the influence of Ca addition in CLC with ilmenite as an oxygen carrier. Results showed that the carbon conversion was improved by the addition of Ca. Yu et al. (2012) investigated the effects of alkali carbonates addition on the reduction rate of coal char with Fe2 O3 oxygen carrier, and the feasibility of coal char direct CLC with alkali carbonate impregnated Fe2 O3 oxygen carrier. It was found that the high reduction rate of coal char with Fe2 O3 oxygen carrier can be achieved with alkali carbonates addition. In our previous work (Gu et al., 2012), we have investigated the reaction performance of K2 CO3 impregnated iron ore oxygen carrier in coal CLC process. The results indicated that compared with the original iron ore, the K2 CO3 -decorated iron ore promoted the reaction rate and shorten the time to obtain reaction balance. However, as for the catalytic mechanism, all of the investigations were presented less clear. The reason is that in comparison with the typical catalytic coal gasification, the catalysts should be atomically dispersed throughout the char, which is not the case in chemical looping catalytic combustion process. According to our previous work, even the catalysts are not atomically dispersed throughout the char, the catalytic effect of K2 CO3 impregnated iron

ore on coal conversion in CLC process can be observed. Thus, it is supposed that in CLC process the coal particles are surrounded by hematite oxygen carriers and Ni catalysts. The contacts among the particles of coal, oxygen carriers and catalysts are inevitable. The contacts make the efficient use of the catalytic effect of Ni particles. As we known, coal is composed by a preponderance of aromatic structures with three-, four-, and five-fused benzene rings and other structures with a single benzene ring. When coal is used in the CLC process, the catalytic effect of Ni on coal gasification may be through two ways. The first one was to help to break the benzene rings to form some big molecules. The big molecules were converted to small ones, which was easily oxidized by oxygen carrier particles. The second one was to catalytic the reaction of the small molecules conversion, such as water gas shift reaction. However, for the catalytic mechanism of Ni, it is difficult to state the obvious catalytic mechanism. This interesting topic is needed to be deeper analyzed in a future work. 3.2.4. Sulfur deactivation The sulfur compound in the fuel reactor is mainly of H2 S and SO2 . In our previous study (Shen et al., 2010a,b), the sulfur behavior was investigated using the same NiO oxygen carrier in CLC process. García-Labiano et al. (2009) investigated the fate of sulfur using a NiO based oxygen carrier. In the presence of sulfur contained in the coal, the gaseous sulfur may react with metal or metal oxide to form metal sulfides or sulfates in the fuel reactor, which produced an oxygen carrier deactivation. However, the sulfides are transported to the air reactor where SO2 is produced as final gas product (GarcíaLabiano et al., 2009; Shen et al., 2010a). Also, the oxygen carrier can recover their initial reactivity after certain time without sulfur addition. This conclusion was also obtained by Forero et al. (2010) using a copper based oxygen carrier. Certainly, the coal with low sulfur content is well suggested as fuel used in CLC process. Further, the active phase of Fe2 O3 in the hematite also can oxidize the H2 S to SO2 , which can alleviate the deactivation of catalyst of Ni particles. 4. Conclusion 10 h continuous operation was performed in a 1 kWth CLC unit with a natural Australia hematite. Also, the Ni-based particles addition attempted to improve the system performance was investigated. This Australia hematite as an oxygen carrier showed a stable reactivity as well as a good ability of resistant to agglomeration and attrition. For the base case conducted at a fuel reactor temperature of 950 ◦ C, the carbon conversion efficiency was about 81.2%, and the loss rate of this hematite oxygen carrier due to attrition was about 0.0625%/h. No tendency of decreased reactivity of the hematite oxygen carrier was observed during 10 h of operation with coal at the fuel reactor temperature 950 ◦ C, indicating a good long-term reactivity of this hematite oxygen carrier. XRD results showed that the active phase Fe2 O3 of the hematite oxygen carrier was mostly reduced to Fe3 O4 phase by coal gasification products in the fuel reactor, and only small part of oxygen carrier is further reduced to FeO. The reduction products of hematite oxygen carrier in the fuel reactor were related to the coal type. If the coal was easily gasified, the hematite oxygen carrier particles will be further reduced by coal gasification products. There was some Fe2 (SiO4 ) formed, which was likely to have no influence on the activation of the hematite oxygen carriers. SEM results showed that there was no obvious sintering on the surface of the reduced oxygen carriers, which was ascribed to the suppression effect on extensive sintering of the inert materials of SiO2 or Al2 O3 in the hematite particles. Adding some Ni-based particles to the hematite improved the char conversion rate. The char gasification efficiency increased by

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