Article pubs.acs.org/EF
Biomass Char Direct Chemical Looping Gasification Using NiOModified Iron Ore as an Oxygen Carrier Zhen Huang, Fang He,* Yipeng Feng, Kun Zhao, Anqing Zheng, Sheng Chang, Guoqiang Wei, Zengli Zhao, and Haibin Li CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), Guangzhou 510640, People’s Republic of China ABSTRACT: Chemical looping gasification (CLG) is considered as a novel gasification technology because gas-phase oxygen of the gasifying medium can be replaced by lattice oxygen of the oxygen carrier. The gasifying mediums (e.g., pure O2 and steam) used as the oxygen source can apparently improve the char conversion in traditionally biomass gasification. Similarly, the objective of this study is to investigate char CLG with the oxygen carrier as an individual oxygen source. A NiO-modified iron ore oxygen carrier was prepared by the impregnation method coupled with ultrasonic treatment. The characteristics of the oxygen carrier were analyzed by an X-ray diffractometer (XRD) and H2 temperature-programmed reduction (H2-TPR). The formation of spinel-type nickel iron oxide NiFe2O4 can evidently enhance the reactivity of the oxygen carrier. The reduction of the oxygen carrier by biomass char was investigated using thermogravimetric analysis (TGA) together with a fixed-bed reactor under an inert atmosphere. TGA tests show that the reactivity of the oxygen carrier increased with the increase of NiO loading. An optimal mass ratio of char/oxygen carrier is determined at 4:6 with the aim of obtaining a maximum reaction rate. The presence of spinel-type nickel iron oxide NiFe2O4 apparently improved the reaction rate of char gasification. The fixed-bed gasification results show that CO was generated faster than other components because carbon was partially oxidized and H2 was quickly consumed by the lattice oxygen [O] of the oxygen carrier. A relatively high carbon conversion of 55.56% was obtained in the char CLG, in comparison to that of char pyrolysis (5.52%). The lattice oxygen [O] of the oxygen carrier was fully consumed by biomass char. Moreover, biomass char was catalytically pyrolyzed becausee the deep reduction products (metallic iron and nickel) can act as catalysts for char pyrolysis. XRD analysis shows that the oxygen carrier was deeply reduced into Fe (Ni) alloy and Fe3C species during the reduction stage of char CLG. However, the regenerated oxygen carrier after oxidation can be recycled for char CLG on the basis of XRD and scanning electron microscopy (SEM) analyses.
1. INTRODUCTION Chemical looping gasification (CLG) is a novel gasification technology, which derives from chemical looping combustion (CLC).1−3 It is achieved by the oxygen carrier circulating between the fuel reactor (FR) and the air reactor (AR) to provide the oxygen and heat source needed for fuel gasification. The target product in the CLG is synthesis gas rather than heat for the CLC because of the lower ratio of fuel/oxygen carrier.4 The conversion rate of solid fuels in the CLG highly depends upon the fraction of volatiles in the fuel; thus, biomass is wellsuited for chemical looping utilization.5 Additionally, biomass is a renewable resource that is considered carbon-neutral during the utilization process.6,7 Hence, the process of biomass CLG is a very attractive and promising approach for biomass highefficient utilization. A simplified principle for CLG of biomass was represented below (Figure 1). In the FR (reduction stage), biomass is first pyrolyzed into three phase products of gas, liquid, and solid at high temperatures and then the intermediates of biomass pyrolysis contact the oxygen carrier, with the reduction reactions occurring between them. On the one hand, the consumption of pyrolysis products can promote biomass pyrolysis. On the other hand, the reaction products (i.e., CO2 and H2O), which derive from the total oxidation of the fuels, can act as a gasifying agent, enhancing the biomass and char gasification. In the AR (oxidation stage), the reduced oxygen © XXXX American Chemical Society
carrier is reoxidized to its initial state by combustion air. In comparison to traditional biomass gasification technologies, CLG of biomass has several potential advantages as follows:8,9 First, the recycling of the oxygen carrier can provide the oxygen needed for biomass gasification, saving the cost to produce pure oxygen. Second, the heat for the endothermic reduction reactions is provided by the circulating solids coming from the AR at high temperatures. Finally, the gas lower heating value (LHV) is elevated because lattice oxygen is more prone to partially oxidize the fuels in comparison to gas-phase oxygen. Additionally, the oxygen carrier can also act as a catalyst for tar cracking to reduce the tar content in the produced gas of biomass gasification.10,11 The feasibility of the biomass direct chemical looping (BDCL) scheme was discussed on the basis of the perspective of energy conversion efficiencies and economic analysis, which demonstrate that the BDCL system integrated with carbon capture and storage (CCS) to receive carbon credit is capable of producing electricity at a competitive price to the current fossil fuel processes.12 Process simulation of the BDCL based Special Issue: 4th (2013) Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 1, 2013 Revised: November 14, 2013
A
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Figure 1. Schematic of CLG of biomass with iron ore as an oxygen carrier.
Figure 2. Schematic layout of the fixed-bed reactor: (1) flow meter, (2) thermocouple, (3) alumina crucible, (4) alundum tube, (5) heating furnace, (6) temperature controller, (7) gas washing bottle, (8) dryer, and (9) gas chromatograph (GC).
conversion processes.13 In the previous work,9 CLG of biomass was investigated in a bubbling fluidized-bed reactor using iron
on Aspen Plus was performed. The results show that the BDCL process is significantly more efficient than conventional biomass B
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In the study, an iron ore from Australia was used as an original oxygen carrier. On the basis of the chemical analysis, as shown in Table 2, the original iron ore mainly consisted of around 90 wt % Fe2O3 as the reactivity component and 6.78 wt % SiO2 as the inert component. The oxygen carrier of NiO-modified iron ore was prepared by the impregnation method. The required amounts of Ni(NO3)2·6H2O (reagent grade) were weighed at a desired stoichiometric ratio and put into a beaker, and then an amount of deionized water was added to make a nitrate aqueous solution. After that, the prepared aqueous solution was transferred to another beaker, where the iron ore powder was placed. The mass ratio of Ni(NO3)2· 6H2O/iron ore was set to 0.05, 0.25, and 0.5, respectively. The mixture was placed in an ultrasonic container for 5 h and dried at 120 °C under an air atmosphere. After that, the sample was calcined at 1200 °C in a muffle furnace for 3 h to obtain the oxygen carrier in the most oxidized state. Finally, the resulting product was ground again to obtain the fresh oxygen carrier sample. On the basis of the X-ray fluorescence (XRF, AXIOSmAX-PETRO) measurement, the mass ratio of NiO/ oxygen carrier of three samples is detected to 1, 6, and 10%. 2.2. Experimental Setup and Procedure. A thermogravimetric analyzer (Netzsch STA409C/PC) was used to investigate the reactivity on biomass char with the oxygen carrier under an argon atmosphere. In each run, about 15 mg of char and oxygen carrier mixtures (which were thoroughly mixed with agate mortar and pestle) was heated in an alumina crucible from 30 to 1200 °C with a ramp of 20 K/min. The flow rates of purge and shielding gases were both kept at 20 mL/min. To collect more reacted sample for phase analysis and better evaluate the performance of CLG of biomass char, a previously mentioned fixed-bed reactor (Figure 2) was used in the study. The system consists of a corundum tube reactor, a temperature controller, an alumina crucible, a gas washing bottle, a dryer, and a gas analysis unit. The reactor with a length of 1000 mm and an inner diameter of 40 mm was heated by a globar furnace. The procedures of char CLG are similar to those of char preparation. After reduction of the oxygen carrier, the stream of argon was switched into an oxygen-poor air stream (9% O2 in N2) to recover the oxygen carrier to the initial state. The flue gases were passed through a gas washing bottle to lower the temperature and wipe off impurities and a dryer to remove water, and then they were introduced into sampling bags for GC analysis. The reduction stage of char CLG lasted for 15 min at 1200 °C to obtain a high reaction rate, and the oxidization period was sustained for 45 min at 850 °C to prevent the sintering because of overoxidation in the study. The fresh and reacted oxygen carrier particles were analyzed by a series of characterization methods. An XRD (X’Pert Pro MPD) using Cu Kα radiation (40 kV and 40 mA) was used to analyze the crystal structure of fresh and reacted samples. The samples were scanned at a scanning rate of 2°/min from 2θ = 5° to 80° with a step of 0.02°. The surface morphology and characteristics of the samples were performed by scanning electron microscopy (SEM) on a Hitachi S4800 instruments. The reactivity of the oxygen carrier samples was evaluated with a TPR on a PULSAR TPR/TPD of Quantachrome Instruments. The TPR experiments were conducted in flowing of 5.0 vol % H2 balanced with helium at a flow rate of 20 cm3/min from room temperature to 1000 °C with a heating rate of 8 °C/min, and then the sample was kept isothermally at 1000 °C for 1 h. The composition of the outlet gas was monitored by means of a quadrupole mass spectrometer.
ore as an oxygen carrier under an inert atmosphere. The gas yield of 1.06 N m3/kg biomass and carbon conversion of 82.23% was obtained at a temperature of 840 °C and a Fe2O3/ C molar ratio of 0.23. Obviously, the good contact of gas−solid phases facilitates the reactions between gas products of biomass pyrolysis and solid oxygen carrier. For the liquid products (tar), it can be catalyzed cracking in the presence of the Fe-based oxygen carrier because the active metallic iron can mediate C− C and C−H bond cleavage.14 The inadequate contact of solid− solid phases might limit the reaction rate between solid products, namely, char and oxygen carrier. However, the gasphase oxygen is introduced to promote char conversion in the traditional biomass gasification.15,16 Similarly, the introduction of the oxygen carrier is intended to enhance the char conversion for elevating the carbon conversion of biomass CLG. Additionally, iron ore is a promising candidate of the oxygen carrier because of its low cost and environmentally friendliness,17,18 but the low reactivity is a challenge for its application.19,20 Although the cost of NiO is higher than iron ore, the reactivity of NiO is much higher than that of iron ore. Some literature reported that the oxygen-transfer capacity of NiFe2O4, which was generated via the reaction between NiO and Fe2O3 at high temperatures, was higher than that of NiO.21,22 Moreover, it was reported that a ratio (1:2) of NiO/ Fe2O3 was cost-effective for a 1−10 MW system of CLC.22 Therefore, the objective of this study is to evaluate the feasibility of CLG of biomass char using NiO-modified iron ore as an oxygen carrier. In the present work, a NiO-modified iron ore oxygen carrier was prepared by the impregnation method coupled with ultrasonic treatment and then characterized by means of an Xray diffractometer (XRD) and H2 temperature-programmed reduction (H2-TPR). The reactivity of biomass char with the oxygen carrier was investigated using a thermogravimetric analysis (TGA) reactor together with a fixed-bed reactor under an inert atmosphere.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The char used in the study was prepared by pyrolysis of biomass (pine from Guangdong, China) in a fixed-bed reactor, which is shown in Figure 2. The procedures of prepared char are briefly described as follows. Biomass was first crushed and sieved into particles with a size range of 250−425 μm. Second, biomass particles were loaded into an alumina crucible and placed at the cold side of the alundum tube. After that, the crucible was quickly placed in the heated zone of the reactor when the reactor temperature reached 800 °C. Then, biomass particles were pyrolyzed under isothermal conditions for 13 min. Of course, the air in the reactor was replaced by argon before biomass pyrolysis. At last, the crucible was rapidly pushed into the other cold area of the reactor, and the char sample was quenched under a stream of argon. The ultimate and proximate analyses of biomass char are listed in Table 1. In the study, the char sample was ground to powder to mix better with oxygen carrier particles.
3. RESULTS AND DISCUSSION 3.1. XRD Analysis of Fresh Oxygen Carriers. The oxygen carrier particles were analyzed by an XRD to identify the crystalline phase structure. XRD patterns of the original and NiO-modified iron ore oxygen carriers are depicted in Figure 3. It is observed that there are only two phases, i.e., SiO2 and Fe2O3, in the original iron ore oxygen carrier. These two phases are presented in the NiO-modified iron ore oxygen carriers as well. Besides, a phase of spinel-type nickel iron oxide NiFe2O4
Table 1. Ultimate and Proximate Analysis of Biomass Char (wt %, Air-Dried Basis) ultimate analysis
a
proximate analysis
C
H
N
S
Oa
ash
volatile matter
fixed carbon
86.31
2.27
0.14
0.01
6.23
5.04
15.24
79.72
By difference. C
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Table 2. Elemental Composition Analysis of the Original Iron Ore (wt %) item
S
Fe2O3
SiO2
Al2O3
CaO
MgO
Na2O
K2O
P
content
0.024
89.53
6.78
1.28
0.13
0.043
0.027
0.13
0.047
Figure 4. H2-TPR profiles of different oxygen carrier samples. Figure 3. XRD patterns of original and NiO-modified oxygen carriers. S, SiO2; N, NiO; NF, NiFe2O4; and other peaks for Fe2O3.
NiO. Another shoulder, which is similar with the first peak of iron ore, is attributed to a transformation from hematite to magnetite; at the same time, the NiO phase is reduced to metallic nickel in this stage. The third peak, which is the most intense peak with a defined maximum at 755 °C, is assigned to the reduction of spinel-type nickel iron oxide NiFe2O4. The last hydrogen consumption peak shifting to a higher temperature at 950 °C is mainly associated with magnetite stepwise reduced to metallic iron; simultaneously, the reduction products of the third stage, which are Ni−FeO solid solution, are fully converted into Fe (Ni) alloy and α-Fe in this stage. The area under the TPR curve can represent the consumption of hydrogen because the TCD signal is linearly related to the H2 concentration of flue gas. Therefore, it is apparent that the hydrogen consumption of NiO-modified iron ore is more than that of iron ore. Additionally, the onset temperature of the peak can qualitatively describe the catalyst reducibility and oxygen mobility within the oxide.23 For the main reduction peak of two oxygen carriers, the reduction temperature of NiO-modified iron ore (755 °C) is lower than that of iron ore (903 °C). These two aspects together indicated that the reactivity of the NiO-modified oxygen carrier is higher than that of iron ore. Given the XRD test results, the reason is certainly associated with the presence of spinel-type nickel iron oxide NiFe2O4 in the NiO-modified iron ore. 3.3. TGA Tests for the Reduction of the Oxygen Carrier by Biomass Char. 3.3.1. Effect of NiO Loading on the Reduction of the Oxygen Carrier by Char. The effect of NiO loading on the reduction of the oxygen carrier by char was illustrated in Figure 5. For the reduction of NiO-modified iron ore by char, it is apparent that both the mass loss and the mass loss rate increase with the increase of NiO loading. Additionally, it is noted that the larger weight loss peak is gradually forwarded with the NiO loading, and it is located at 1060 °C with the maximum mass loss rate of 1.44 wt %/min when the amount of NiO loading attains 10 wt % in the oxygen carrier. There is only one sharp weight loss peak located at 880 °C with the maximum mass loss rate of 2.20 wt %/min for the pure NiO reduction by char. The results imply that the increase of NiO loading can obviously improve the reactivity of the oxygen
is observed in the modified oxygen carrier samples. It is attributed to the reaction as follows: Fe2O3 + NiO → NiFe2O4
(1)
The relative content of the crystal phase in the samples can be represented by the relative intensity of the main peak of the XRD patterns. The most intense reflection peaks of the Fe2O3 phase (IFe2O3) and the NiFe2O4 phase (INiFe2O4) are located at 2θ = 33.23° and 35.73°, respectively. According to the XRD spectra, the ratio of INiFe2O4/IFe2O3 of three modified oxygen carriers is equal to 0.62, 0.90, and 1.27 in accordance with the order of the increase of the amount of NiO loading. It is wellunderstood that the intensity of the NiFe2O4 phase increases with the amount of NiO loading on the basis of reaction 1. Additionally, a phase of NiO is also observed, and its peak presents some overlap with the peak of NiFe2O4 in the 6 and 10% NiO loading oxygen carriers. It may be attributed to uneven impregnation during the process of preparation of the oxygen carrier. The structural change of the oxygen carrier is conducive to its reactivity. The details will be discussed later. 3.2. TPR Tests. To investigate the effect of structure changes on the reducibility of the oxygen carriers, H2-TPR tests were conducted over the original iron ore oxygen carrier and the NiO-modified iron ore oxygen carrier (10 wt % NiO loading). As seen in Figure 4, it is evident that the TPR curve of the original iron ore shows a sharp reduction peak with a defined maximum at 576 °C, followed by a more intense broad peak starting at a temperature range from 681 to 1000 °C with a defined maximum at 903 °C. The first sharp peak corresponded to hematite reduced to magnetite, whereas the broad peak is associated with a stepwise reduction of magnetite, which was first converted to wustite and then reduced to metallic iron. For the sample of NiO-modified iron ore, the TPR curve presents four reduction peaks. A lower temperature shoulder located at 250 °C may be related to the reduction of NiO because it was also presented in the TPR profile of pure D
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Figure 5. Effect of NiO loading on the reduction stage of char CLG.
Figure 6. Effect of the oxygen carrier/char mass ratio on the CLG of biomass char. Oxygen carrier = 10 wt % NiO-loading iron ore.
carrier. However, the amount of NiO loading should be wellcontrolled in terms of its high cost and environmental hazard. On the basis of the above analysis, iron ore with loaded 10 wt % NiO is an appropriate oxygen carrier candidate for char gasification. Hence, the reactivity on char with a 10 wt % NiOloading iron ore oxygen carrier was investigated in detail in the study. 3.3.2. Effect of the Mass Ratio of Oxygen Carrier/Char on the Reduction of the Oxygen Carrier by Char. The effect of the mass ratio of oxygen carrier/char (i.e., 7:3, 6:4, 5:5, and 4:6) on the reduction of the oxygen carrier was performed in the TGA reactor under an argon atmosphere, as shown in Figure 6. The residual of four mixtures is approximately 80.14, 73.52, 74.41, and 77.56 wt % at the end of 1200 °C, respectively. According to section 3.3.3 (as discussed later), the residual of the baseline experiment, namely, char pyrolysis, is 93.37 wt % in the study. Hence, the mass loss of four mixtures, which is caused by oxygen carrier reduction, is about 14.89, 19.85, 17.30, and 12.50% among the temperature range from room temperature to 1200 °C, respectively. As seen in Figure 6b, it is evidently that the mixture of the maximum mass loss rate is the sample of 6:4. The above views suggest that the reaction rate of the sample of 6:4 proceeds the fastest among the four samples. The lower mass ratio of oxygen carrier/char means less amount of reactive oxygen release for char gasification; thus, the reaction rate of the oxygen carrier by
char is suppressed. Therefore, the reaction rate of the sample of 4:6 and 5:5 is relatively slower than that of the sample of 6:4. On the basis of the “fuel-induced oxygen release” mechanism,24 plenty of oxygen carrier/char contacts facilitate the char gasification. The contacts between the oxygen carrier and char of the sample of 7:3 are less than those of the sample of 6:4 because of the low char content; thus, the reaction rates of the sample of 7:3 are restrained. A similar result was reported by the literature.19 According to the above analysis, the reaction rate of the sample of 6:4 proceeds the fastest among four samples; hence, the mass ratio of oxygen carrier/char was fixed at 6:4 to investigate the effect of other factors on the reduction stage of char CLG in the study. 3.3.3. Effect of Different Oxygen Carriers on the Reduction of the Oxygen Carrier by Char. The experiments of original iron ore reduction by char and NiO-modified iron ore reduction by char under an argon atmosphere were conducted by TGA. Meanwhile, the baseline experiment of char pyrolysis, where the oxygen carrier was replaced by Al2O3, was also carried out. The results are shown in Figure 7. The mass loss of the baseline experiment is slight, and it is only for 6.63% within the temperature range from room temperature to 1200 °C. There is no appreciable weight loss peak for the baseline experiment. Because of structural changes of reducing species, E
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loss of NiO-modified iron ore reduction by char is 7.83 wt % more than that of pure iron ore reduction by char. The underlying concern is that the addition of NiO can improve the reactivity of iron ore. As mentioned above, the XRD pattern of NiO-modified iron ore presents three species other than SiO2, which is the reason for the presence of two weight loss peaks. The smaller weight loss peak located at about 850 °C corresponds to a transform from spinel-type nickel iron oxide NiFe2O4 to Ni-FeO solid solution; additionally, Fe2O3 and NiO species are reduced into Fe3O4 and metallic nickel species here, respectively. The larger weight loss peak appears at 1050 °C, which is ascribed to the further reduction of Ni−FeO solid solution by char, and the final products of Fe (Ni) alloy and α-Fe are obtained in the stage. At the same time, magnetite is further reduced to metallic iron, and a species of Fe3C may appear here. In comparison to the DTG curve of iron ore reduction by char, the weight loss peaks of NiO-modified iron ore are larger. It is ascribed to the addition of NiO causing the structural changes, improving the reactivity of the decorated oxygen carrier. The reactivity of the oxygen carrier can also be evaluated by a temperature value (Tmax), in which reaction rates achieve a maximum.29 A higher Tmax suggests a poor reactivity of the oxygen carrier. As shown in the DTG curves, the value of Tmax of NiO-modified iron ore reduction by char is at 1050 °C; however, it exceeds 1200 °C for the iron ore reduction by char. Hence, it again demonstrates that the doping of NiO to form composite is conducive to the increase of reactivity. The result is in good agreement with some of the literature,30−33 where it was reported that the addition of various metal oxides (e.g., NiO, CuOx, LiO, and TiO2) can overcome the low reduction kinetics of hematite (Fe2O3). The investigation of Shimokawabe et al.34 further indicated that metal oxides with an oxidation number of +2 (NiO and CuO) form metal ferrites (MFe2O4, where M is Ni or Cu) with higher reduction kinetics. 3.4. CLG of Char in a Fixed-Bed Reactor. To evaluate the reactivity of the oxygen carrier well, the experiment on CLG of biomass char with NiO-modified iron ore (10 wt % loading) as an oxygen carrier was conducted in a fixed-bed reactor. At the same time, the baseline experiment, i.e., char pyrolysis, was also performed to compare to the char CLG. On the basis of TGA experimental results, the mass ratio of oxygen carrier/char was selected at 6:4 (the oxygen carrier is 0.75 g, and char is 0.5 g). The oxygen carrier was replaced by alumina in the baseline experiment. The experimental results are shown in Figure 8. Biomass char is mainly composed of fixed carbon on the basis of proximate analysis, and its major volatiles are evolved in the earlier stage. Hence, the pyrolysis of biomass char, namely, char carbonization, is a hard process, and the residual volatiles will be released at a temperature enough high. The reaction rate of the carbonization process achieves a maximum at 5 min because of the delayed heat transfer between char samples and argon; subsequently, it rapidly decreases in the study, as seen in Figure 8a. It is interesting to note that the rate of H2 generation proceeds the fastest during the carbonization. The reason could be explained by the fact that the formation of polycyclic aromatic hydrocarbon originating from aromatics with less benzene ring is enhanced during the aromatization process (i.e., the char possess graphite structure), facilitating the generation of H2.35,36 The evolution of the reaction rate of char CLG is similar to that of char carbonization, but it is evidently enhanced because the oxygen carrier is conducive to char conversion; especially, reaction 2 obviously promotes the char
Figure 7. Effect of the oxygen carrier on the reduction stage of CLG of char.
the kinetics of reduction of hematite (Fe2O3) is considered a complex gas−solid reaction.25,26 Magnetite (Fe3O4) can be completely reduced to wustite (Fe1 − xO) before its reduction to metal iron (Fe) when the reduction temperature oversteps 570 °C.27,28 Thus, above 570 °C, the reduction path of hematite can be described as follows: Fe2O3 → Fe3O4 → Fe1 − xO → Fe. There is three apparent weight loss peaks for a typical hematite reduction DTG curve, as seen in Figure 7b. The first mass loss peak indicative for a transition to magnetite is located at about 800 °C. Subsequently, the reduction of hematite proceeds to the second transition at 1100 °C, corresponding to a stepwise reduction from magnetite to wustite. As the reduction temperature exceeds 1200 °C, wustite is further reduced to metallic iron for the last as well as the maximum mass loss stage. Within this temperature range, the residual of the mixture is 81.34 wt %. It suggests that the char reacted with lattice oxygen of hematite causes 12.03 wt % mass loss of the mixture after considering the baseline experiment. For the NiO-modified iron ore reduction by char, the thermogravimetry (TG) curve demonstrates an ultimate weight loss of 26.49 wt % and the differential thermogravimetry (DTG) curve presents two weight loss peaks. The results indicate that char and oxygen atoms of 19.86 wt % interacted to generate gas products escaping from the TGA system after subtracting the effect of the baseline experiment. The weight F
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slight and has a little increase in char CLG. It may be attributed that the process of alkylation, aromatization, and polycondensation is strengthened because of the presence of the oxygen carrier. C + [O] → CO
(2)
CO + [O] → CO2
(3)
H 2 + [O] → H 2O
(4)
After the reduction of the oxygen carrier, the regeneration of the reduced oxygen carrier was conducted under an oxygenpoor air atmosphere. As seen in Figure 8c, the CO 2 concentration gradually decreases with the increase of the reaction time, whereas the O2 concentration presents an opposite variation trend. After the oxidation of 40 min, the CO2 concentration drops to zero and the O2 concentration tends to be constant. It means that the residual carbon is fully converted and lattice oxygen has been recovered, which is accordance with the XRD spectra of regenerated oxygen carrier particles. The crystalline phase structure of reduced and regenerated oxygen carrier samples was analyzed by XRD, as shown in Figure 9. It is observed that three phases and three sharp peaks
Figure 9. XRD patterns of reduced and regenerated NiO-modified oxygen carriers.
are present in the XRD spectra of the reduced sample. A sharp peak corresponds to the formation of Fe3C. It is ascribed to the following reaction:
Figure 8. Process of char CLG with a NiO-modified oxygen carrier and baseline test.
3Fe2O3 + 11C → 9CO + 2Fe3C
(5)
Another two sharp peaks are identified as the Fe (Ni) alloy phase, which is due to the oxygen carrier withstanding severe reduction. The XRD analysis indicates that lattice oxygen [O] is not detected in the reduced oxygen carrier, which means that lattice oxygen [O] is fully converted into gas products via char gasification. The presence of various phases is related to the imbalance of the heat and mass transfer of the system. In comparison to the fresh oxygen carrier sample (Figure 3), a few differences were observed in the regenerated sample. The ratio of INiFe2O4/IFe2O3 of the regenerated sample is lower than that of the fresh sample. It is attributed to the fact that the calcination temperature is not high enough for regeneration of the reduced sample. The result is in accordance with the literature.31 Overall, the spinel-type structure of the sample is still
conversion. At the same time, a large amount of H2 is consumed fast via reaction 4 because the reactivity of H2 with the oxygen carrier is better than that of CO with the oxygen carrier in the chemical looping process.37,38 Hence, these two aspects together result in the generation of CO proceeding the fastest in the char CLG. The lattice oxygen [O] of the oxygen carrier provides an oxygen source for char conversion; thus, partial oxidation of carbon (reaction 2) is the dominated reaction during the reduction stage of char CLG. Hence, the CO content of char CLG is apparently higher than that of char pyrolysis, as shown in Figure 8b. At the same time, the CO2 content increases and the H2 content decreases in char CLG in comparison to char pyrolysis because of reactions 3 and 4. The CH4 content is G
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Figure 10. SEM images of the fresh and regenerated oxygen carrier particles: (left) fresh oxygen carrier particles and (right) regenerated oxygen carrier particles.
Table 3. Material Balance Analysis of Two Modes input mass (g)
output mass (g)
run
char
oxygen carrier
total
gas
char carbonization char CLG
0.50 0.50
0.75 0.75
1.25 1.25
0.42 0.07
t
∫0 nxoutlet,CO2dt nC,char
(6)
t
XO = 1 −
∫0 n(x inlet,O2 − xoutlet,O2 − xoutlet,CO2)dt nO,OC
solid
total
recovery (%)
0.81 1.16
1.23 1.23
98.40 98.40
be noted that the oxygen conversion in the oxygen carrier attains 98.89% according to eq 7, which suggests that the lattice oxygen [O] was almost completely generated into gas products. It is well in accordance with the XRD analysis. For the baseline experiment, the carbon conversion is calculated for only 5.52% according to the displacement method. The result further indicates that the char conversion is evidently strengthened because of the presence of the oxygen carrier, which provides the oxygen source for char conversion. Moreover, an important fact is that lattice oxygen [O] in the oxygen carrier sample of 0.75 g is at a maximum consumed carbon of about 0.15 g, whereas the amount of carbon converted into the gas product is approximatly 0.16 g on the basis of the displacement method. It means that a portion of char is catalytic pyrolyzed during the reduction stage of char CLG because the further reduction products (metallic iron and nickel) are used as a catalyst. The results further indicated lattice oxygen [O] in the oxygen carrier was fully consumed by char, which is well in agreement with the XRD analysis and oxygen conversion (XO). Additionally, the material balance analysis of two modes was performed, as shown in Table 3. The calculation of the mass of gas products is referred to in the literature,9 and the mass of liquid products is relatively low enough to be neglected. The recovery is defined as the ratio of output mass and input mass before and after reaction in each run. It is found that the recovery of both char carbonization and char CLG is 98.40%, which suggests that each run is reliable; thus, the data are reasonable. The reaction temperature has to be held at high temperature to achieve a high reaction rate under an inert atmosphere. A high temperature may cause several problems. On the one hand, a high temperature easily causes sintering, resulting in devitalization of the oxygen carrier. On the other hand, a lot of energy would be needed for sustaining a high temperature. However, some weak oxidizing agents, i.e., CO2 and H2O, derived from biomass volatiles and the products of the oxygen carrier with reducing gases, were presented in the actual CLG of biomass char. It is well-known that CO2/H2O facilitates the
maintained, and lattice oxygen is recovered after a successive redox reaction. Thereby, it indicates that the NiO-modified iron ore sample seems to be a promising candidate as an oxygen carrier for char CLG. The shape and morphological features of fresh and regenerated oxygen carrier particles were characterization by SEM, as shown in Figure 10. The surface of fresh oxygen carrier particles is loosely covered with granules with a size below 5 μm and exhibits a porous structure that is beneficial to the diffusion of reactant gases into the core of oxygen carrier particles, enhancing the reactions between them. Some small grains gather and grow after successive redox reactions; in addition, slight sintering is observed. However, in general, the pore structure is well-maintained, and no substantially change is occurred for the regenerated oxygen carrier sample. In conclusion, it suggests that the NiO-modified iron ore sample used as an oxygen carrier can be recycled for char CLG on the basis of XRD and SEM analyses. The carbon conversion (XC) of char and oxygen conversion (XO) of the oxygen carrier are calculated as follows respectively: XC = 1 −
liquid
(7)
where n denotes the molar flow rate of flue gas, xoutlet,CO2 and xoutlet,O2 are the concentrations of CO2 and O2 during the oxidation stage of char CLG, respectively, xinlet,O2 is the O2 concentration of the reactor inlet, which holds at a constant of 9%, t is the reaction time of the oxidization stage of char CLG, which is sustained at a constant of 45 min, and nC,char and nO,OC are the initial amounts of carbon in the char and oxygen in the oxygen carrier, respectively. It is calculated that the carbon conversion attains about 55.56% on the basis of eq 6. It should H
dx.doi.org/10.1021/ef401528k | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
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biomass char conversion at a relatively low temperature. Hence, it is more significant to investigate the cyclic performance of the oxygen carrier at a relatively low temperature (e.g., 840 °C) under a weak oxidizing atmosphere. The relevant experiments have been proposed, and the experimental results will be reported in the future.
4. CONCLUSION A NiO-modified iron ore oxygen carrier was prepared by the impregnation method coupled with ultrasonic treatment and then characterized by means of XRD and H2-TPR. The reactivity of biomass char with the oxygen carrier was investigated using a TGA reactor together with a fixed-bed reactor under an inert atmosphere. The formation of spinel-type nickel iron oxide NiFe2O4 can evidently improve the reactivity of the oxygen carrier, which facilitates char conversion. The reactivity of the oxygen carrier increases with the increase of NiO loading, and an optimal mass ratio of char/oxygen carrier is determined at 4:6, aiming to obtain a maximum reaction rate. A continuous char CLG test demonstrated that CO was generated at the highest rate because of partial oxidization of carbon and rapid consumption of hydrogen. The carbon conversion is apparently elevated from 5.52% of the baseline experiment to 55.56% of char CLG because of the presence of the oxygen carrier. The lattice oxygen [O] of the oxygen carrier was fully consumed by biomass char. Moreover, biomass char was catalytically pyrolyzed by deep reduction products of the oxygen carrier (i.e., Fe and Ni). XRD analysis shows that the oxygen carrier sample was reduced to Fe (Ni) alloy and Fe3C species. The structure of the regenerated oxygen carrier sample has no substantial changes on the basis of XRD and SEM analyses. It indicates that NiO-modified iron ore as an oxygen carrier can be recycled for char CLG.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-02087057721. Fax: +86-02087057737. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (51076154). This work was also supported by the Science and Technology Project of Guangdong (2010B010900047) and the “12th Five Years” National Science and Technology Support Program (2011BAD15B05).
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dx.doi.org/10.1021/ef401528k | Energy Fuels XXXX, XXX, XXX−XXX
