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Journal of Cleaner Production 200 (2018) 588e597
Contents lists available at ScienceDirect
Journal of Cleaner Production
journal homepage: www.elsevier.com/locate/jclepro
Hydrogen production from vegetable oil via a chemical looping
process with hematite oxygen carriers
Guo-Qiang Wei a, b, c, d, Wei-Na Zhao e, Jun-Guang Meng b, c, d, Jie Feng a, *,
Wen-Ying Li a, **, Fang He b, c, d, Zhen Huang b, c, d, Qun Yi a, Zhen-Yi Du a, Kun Zhao b, c, d,
Zeng-Li Zhao b, c, d, Hai-Bin Li b, c, d
a
Training Base of State Key Laboratory of Coal Science and Technology Jointly Constructed by Shanxi Province and Ministry of Science and Technology,
Taiyuan University of Technology, Taiyuan 030024, China
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), Guangzhou 510640, China
c
CAS Key Laboratory of Renewable Energy, Guangzhou 510640, China
d
Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China
e
Guangdong Mechanical & Electrical College, Guangzhou 510515, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2018
Received in revised form
24 July 2018
Accepted 27 July 2018
Available online 30 July 2018
Hydrogen production from vegetable oil via a chemical looping process with hematite oxygen carriers
has been carried out in a fixed-bed reactor at 1023.15e1173.15 K. The lattice oxygen release process and
crystalline transformation for the oxygen carrier in the reaction process are investigated. Results indicate
that the maximum H2 composition achieved in the chemical looping hydrogen stage is over 91.72% when
hematite oxygen carriers consisting of Fe2O3, Al2O3 and SiO2 are used. The active component Fe2O3 in
oxygen carriers represents three reduced peaks corresponding to the crystalline form transition from
Fe2O3 to Fe3O4. Up to 96.13% of the lattice oxygen in Fe2O3 can be consumed in the reduction process. The
deep reduction of FeO to Fe represents a lower reaction rate with the exhaustion of the lattice. The
maximum carbon conversion efficiency of vegetable oil of 79.10% when the ratio of water to oil is 1.2 and
hydrogen-rich gas with a highest concentration of 91.72% are achieved in the chemical looping reforming
stage and H2 production stage, respectively. The impurities in the gas are ascribed to the carbon deposition and steam reaction in the chemical looping process. The micrographs of fresh and used oxygen
carriers present irregular and blocky structures, sintering is not observed after the multi-cycle reaction
process. The crystalline form and reaction activity remain stable after 20-cycle experiments, suggesting
that the hematite oxygen carrier is a promising candidate for hydrogen generation from vegetable oil via
the chemical looping process.
© 2018 Elsevier Ltd. All rights reserved.
Keywords:
Vegetable oil
Oxygen carriers
Chemical looping reforming
Chemical looping hydrogen
High purity H2 production
1. Introduction
The output of waste cooking oil (WCO) increases rapidly with the
development of the economy and society, which will cause environmental pollution and threaten human physical health if these
oils are discharged unreasonably (Mehrasbi et al., 2017). On the
other hand, energy crises and emissions are besetting many countries in the world. The WCO containing triglyceride and fatty acid
fractions is a promising feedstock alternative to produce energy or
chemicals, which can relieve the crisis and emissions to some extent
* Corresponding author.
** Corresponding author.
E-mail addresses: fengjie@tyut.edu.cn (J. Feng), ying@tyut.edu.cn (W.-Y. Li).
https://doi.org/10.1016/j.jclepro.2018.07.263
0959-6526/© 2018 Elsevier Ltd. All rights reserved.
(Morais et al., 2010; Senthur Prabu et al., 2017). There are various
applications for the WCO as a raw material in producing soap,
glycerin and synthetic plasticizers (Zheng et al., 2018). Additionally,
another application for the WCO is to synthesize biodiesel though
esterification and transesterification (Fereidooni et al., 2018;
Stephen and Periyasamy, 2018). This process presents special advantages such as renewability, environmental properties and
compatibility with engines and fuel standards (Moecke et al., 2016;
Nabi et al., 2017). However, there are technical problems that must
be overcome before further industrialization. In view of the water,
free fatty acids, diglycerides, triglycerides and other impurities in
the WCO, complex preprocessing is needed to meet the requirements of hydrodeoxygenation, decarbonylation, decarboxylation and thermal cracking reactions (Wang et al., 2017b).
G.-Q. Wei et al. / Journal of Cleaner Production 200 (2018) 588e597
589
CaHbCO$ ¼ CaHb$ þ CO$
(5)
CH$ þ 5H$ ¼ CH4 þ H2
(6)
Furthermore, some homogenous catalysts or acidic catalysts must
be adopted in the catalytic thermal cracking of WCO and biodiesel
production process (Chuah et al., 2017; Tran et al., 2016; Ueki et al.,
2018). In addition, the conductivity and thermal efficiency of the
WCO are insufficient, which may damage the reaction apparatus
(7)
due to overheating (Li et al., 2017). Improving the thermal conductivity of the deficient WCO becomes another priority before
biodiesel production (Borugadda and Goud, 2016). So, some complex equipment and technologies are needed to separate, purify and
catalyze WCO raw material in the biodiesel production process
(Asikin-Mijan et al., 2017; de Mello et al., 2017; Lani et al., 2017),
which increase the cost and technical complexity, as well as the
environmental risk (Eguchi et al., 2015). Additionally, the storage
stability of biodiesel must also be a focus due to the high amount
biodiesel gas blended in the production. It is very necessary to use
further amounts of antioxidant additives to minimize biodiesel
€lczmann et al., 2016). Consequently, the conversion
degradation (Po
path of the WCO with low cost, high efficiency and low environmental risk needs to be developed as a more promising alternative.
In regard to H2 production, there are various research efforts
examining steam reforming for fossil fuels, such as petroleum, coal
and natural gas, because this is a simple and mature technique that
has a better economic benefit (Shahbaz et al., 2017; Zhao et al.,
2017). Nevertheless, some disadvantages are also exhibited such
as the high cost associated with hydrogen separation, greenhouse
gas emissions and non-regeneration. To solve the energy, environmental and sustainable development problems, chemical
looping reforming (CLR) coupled with chemical looping hydrogen
(CLH) was proposed to produce syngas and high purity hydrogen
from WCO at a lower cost with less pollution. This process uses
WCO as the feedstock instead of fossil fuels, such as coal, petroleum
and CH4, revealing advantages for solving environmental problems
and producing clean energy simultaneously. Additionally, it provides a new industrial application of WCO. With the consistently
growing global demand for fats and vegetable oils over the past
decade, it is evaluated that the worldwide consumption of fats and
vegetable oils will increase by 25% and reach 178 Mt in 2025.
Approximately 20% of these oils is disposed of after the cooking
process, which is the source of waste cooking oil (Alqaralleh et al.,
2016; Lombardi et al., 2018). Consequently, the waste cooking oil
CLR coupled with CLH process could have great potential in industrial application for syngas and high purity hydrogen production due to its lower cost, less pollution and convenient scale-up.
The CLR and CLH processes are performed separately in the integrated system, which is composed of a steam reactor (SR), a fuel
reactor (FR) and an air reactor (AR). The CLR process is carried out
initially in the FR, where the WCO is pyrolyzed into small molecular
intermediates following reactions (1) to (7).
CmHnCOOCaHb ¼ CmHnO$ þ CaHbCO$
(1)
CaHbCO$ ¼ CaHb-1CO$ þ H$
(2)
CaHCO$ ¼ CaH$ þ CO$
(3)
CaH$ ¼ Ca-1H$ þ C$
(4)
WCO þ MemOn ¼ H2 þ CH4 þ CO þ CO2 þ … þ MemOn-d
(8)
MemOn-d þ H2O ¼ MemOn-dþε þ H2
(9)
MemOn-dþε þ air ¼ MemOn þ N2
(10)
Then, the intermediate products further react with oxygen
carriers to generate the final syngas. Apparently, the oxygen carriers (MemOn) supply active lattice oxygen to promote the macromolecular intermediate cracking to generate syngas and shift the
reaction equilibrium to higher carbon conversion. The oxygen
carriers are correspondingly reduced to lower valence states, as
exhibited in equation (8). Following the above steps, the reduced
oxygen carriers are transported into the SR to perform the CLH
reaction, accompanied by highly purified H2 production. The oxygen carriers are oxidized partially to recover lattice oxygen,
described by equation (9). To recover lattice oxygen completely
after the CLH process, an air oxidized process is implemented in the
AR according to equation (10). The oxygen carriers recover their
original form, and the reactions continuously proceed with the
oxygen carriers circulating in three reactors.
Obviously, a key role associated with heat and oxygen transfer in
the CLR and CLH processes is assigned to oxygen carriers. The lattice
oxygen transport and heat conductivity capacity of the oxygen
carries show significant effects on the chemical looping process.
Thus far, oxygen carriers including metal oxides, perovskite and
nonmetallic oxides have been investigated in chemical looping
combustion (CLC), CLR, CLH, etc. (Liu et al., 2017; Wei et al., 2017).
Though Ni- and Cu-based oxygen carriers indicate better reactivity,
Fe-based oxygen carriers are much preferred in the chemical
looping process due to their moderate reactivity, higher melting
point, non-toxicity and low cost (Huang et al., 2016; Sajen et al.,
2016; Wang et al., 2017a). Furthermore, Fe-based oxygen carriers
are suitable for the CLR and CLH processes, which has been
demonstrated in CH4 reforming and H2 production (Xiao et al.,
2014; Zeng et al., 2015). A CLH experiment for H2 generation was
performed in a 25 kWth reactor by Fan's group with synthetic Febased oxygen carriers, and more than 99.99% purity H2 with 100%
carbon capture was achieved in the reaction process (Fan et al.,
2008). Additionally, the mixed oxygen carriers indicate better
reactivity, anti-sintering and anti-carbonization due to the metallic
synergistic effect, whereas the higher cost restricts further largescale application (Huang et al., 2014). Hematite oxygen carriers
are ideal candidates for CLR and CLH processes due to their lower
application cost. In addition, hematite displays a high performance
in other similar processes such as tar catalytic cracking or bio-oil
CLH processes.
Therefore, instead of using fossil fuels, such as coal, petroleum
and CH4, the chemical looping coupling process was proposed to
produce syngas and high purity hydrogen from waste cooking oil by
adopting a hematite oxygen carrier with a lower cost and less
590
G.-Q. Wei et al. / Journal of Cleaner Production 200 (2018) 588e597
pollution.
2. Experiment and apparatus
2.1. Materials
To exclude interference factors and investigate the reaction
mechanism, vegetable oil from the local market in Guangzhou
(China) was used as a model of WCO. The proximate and ultimate
analysis results of the vegetable oil were as follows (mass fraction):
99.94%volatile matter, 0.06%ash, 76.32%C, 10.91%H, 12.63%O, 0.1%N,
and 0.05%S, which were based on a dry basis. The average molecular formula can be calculated as CH1.22O0.12 according to the
analysis results, ignoring N and S. The lower heating value of the
vegetable oil was 38.59 MJ/(kg, db).
Natural hematite samples from China were first pulverized and
screened to 0.18e0.25 mm particles, which were next calcined at
950 C in a muffle furnace for 4 h to obtain the oxygen carriers for
testing in the chemical process. The mass composition of different
elements in hematite oxygen carrier analyzed by X-ray fluorescence (XRF) were shown to be as follows: 62.78%Fe, 32.27%O, 2.83%
Si, 1.49%Al, 0.18%Mg, 0.1%P, 0.17%Ca, 0.07%Zn, 0.04%Na, 0.04%Mn,
0.03%Ti, 0.02%K, 0.02%Ni, etc.
2.2. Method and apparatus
Thermogravimetric analysis (TGA) measurements (STA409C/PC,
NETZSCH) combining with temperature-programed reduction
(TPR) analysis were used to investigate the reaction activity and
transfer rate of active lattice oxygen in hematite particles in a H2
atmosphere. The hematite particles were first ground into powder,
and then, the TGA test was performed with a 25 mg hematite
powder sample over the temperature range of 303.15e1273.15 K.
The heating rate of the reaction zone was kept at 20 K/min, and the
reducing gas mixture with volume fraction of 5%H2 and 95%N2 was
introduced at a flow rate of 100 mL/min. In addition, 30 mL/min of
N2 was used as a protective gas.
To further study the reaction mechanism of the CLR coupled with
CLH process of vegetable oil and oxygen carriers, fixed-bed reactor
experiments were carried out after the TGA test. Before the experiments, the design and selection of experimental parameters
needed to be performed. Temperature plays a key role in improving
the reaction activity and rate of oxygen carriers for the chemical
looping process. Importantly, the oxygen carriers are prone to sintering at high temperature. Additionally, high temperature produces a negative effect on the CLH reaction due its exothermic
characteristic according to thermodynamic analysis. Therefore, a
temperature range of 1023.15e1173.15 K is adopted in this experiment to achieve a good fuel conversion rate while avoiding sintering. In addition, reaction time is another important factor affecting
fuel conversion in the chemical looping process. It is obvious that a
longer reaction time can promote fuel conversion efficiency; however, the reaction rate of oxygen carriers will decrease with the
lattice oxygen consistently being consumed. Based on our previous
research, the reaction time of 46 min is suitable for the CLR or CLH
process with hematite oxygen carriers in a fixed-bed reactor. A
schematic drawing of the experiment is displayed in Fig. S1 in the
supporting information. It consisted of a fixed-bed reactor, a gas
condensing and cleaning unit, a gas analysis unit and a computer
control unit. The fixed-bed reactor included an electric furnace, a
quartz tube with a porous distributor and a thermocouple. The reaction temperature and gas flow rate were controlled by thermocouple and mass flow controllers, respectively. Two liquid
chromatography (HPLC) micropumps were adopted to inject
deionized water and vegetable oil, separately. The experimental
procedure is shown in the flowchart of Fig. 1. Before the experiments, a moderate amount of silica wool was spread out on the
distributor of the quartz tube, and then, 2 g of oxygen carriers were
put into the quartz tube. The reaction temperature was set at
1023.15e1173.15 K, and N2 was used to replace the air atmosphere at
a flow rate of 100 mL/min. The oil was injected at a flow rate of
0.1 mL/min, and deionized water was introduced at a variable flow
rate. Liquid production collected from the condensing unit was
small and therefore ignored. The reduced oxygen carriers were
recovered in the original crystal form by introducing an air atmosphere after the CLR and CLH reactions. Finally, the production gas
was purified and dried with isopropanol and an allochroic silica gel
desiccant. An Agilent 7890A gas chromatograph was used to analyze
the composition of gas production.
2.3. Characterization of oxygen carriers
X-ray diffraction (XRD, X'Pert PRO MPD) with Cu Ka radiation
(40 kV, 40 mA) was used to characterize the crystallinity of the oxygen carriers in the range of 10e80 . The scan rate and step size were
kept at 2 /min and 0.0167, respectively. To analyze the elemental
composition of samples, X-ray fluorescence (XRF, AXIOSMAXPETRO) was employed. Additionally, the evolution of the microstructure and BET surface area for N2 physisorption were investigated by field emission scanning electron microscopy (Hitachi
S4800) and a Micromeritics ASAP 2010, respectively. Before the experiments, the samples were kept at 573 K under a vacuum condition
for 3 h to eliminate the air in the oxygen carrier particles. In addition,
an oxygen carrier sample of 35 mg was analyzed by H2 temperatureprogrammed reduction (TPR, CPB-1, Quantachrome) to investigate
the reactivity and lattice oxygen release performance of the hematite
oxygen carrier. The flow rate of the gas mixture (5%H2þ95%N2) was
120 mL/min, and the heating rate was maintained at 10 K/min.
2.4. H2 production evaluation
Based on N2 balance, the gas relative compositions (Ci) of produced syngas and H2 were calculated with Eq. (11).
Z
ci ¼ Z
0
t
0
t
xi dt
v xCO þ xCO2 þ xH2 þ xCH4 þ xC2 H4 þ xC2 H6 dt
(11)
where xi denotes the volume fraction of species i, i indicates the
compositions and v is the volume flow of syngas.
The carbon conversion efficiency (hc) in the system was defined
as the ratio of the carbon element converted into gaseous products
from the oil fed into the fixed-bed reactor. It was calculated by Eq.
(12):
12 Vco2 þ Vco þ VCH4 þ 2VC2 H4 þ 2VC2 H6 GV
100%
hc ¼
22:4 ð303=273Þ Mc
(12)
where Gv (m3/kg) and Mc (%) are the gas yield and carbon fraction
in oil, respectively. Vco, Vco2, VCH4, VC2H4 and VC2H6 (%) represent the
volume compositions of CO, CO2, CH4, C2H4 and C2H6 in the outlet
gas of the fixed-bed reactor.
The lower heating value (LHV, kJ/Nm3) of the gas products was
calculated with Eq. (13):
LHV ¼ 126VCO þ 108VH2 þ 359VCH4 þ 635VC2 Hm
(13)
where VCO, VH2, VCH4 and VC2Hm are the volume fractions of CO, H2,
G.-Q. Wei et al. / Journal of Cleaner Production 200 (2018) 588e597
591
Fig. 1. Flowchart of the CLR and CLH experimental procedures.
CH4 and C2Hm in the flue gas, respectively.
3. Results and discussion
3.1. Characterization of hematite oxygen carriers
XRD analysis was used to characterize the crystalline form of
fresh oxygen carriers, as shown in Fig. 2. Obviously, the main
components of the hematite oxygen carriers were Fe2O3, Al2O3 and
Fig. 2. XRD analysis of the fresh oxygen carriers.
SiO2, which were verified with the Joint Committee on Powder
Diffraction Standards (JCPDS) card. It is apparent that Fe2O3 in oxygen carriers can supply lattice oxygen in the CLR and CLH processes. To avoid sintering or aggregation in the redox reactions, SiO2
and Al2O3 played a positive role in the oxygen carriers (Bao et al.,
2016; Zhu et al., 2017). In addition, some impurities (MgO, CaO,
etc.) in the hematite oxygen carriers were detected at the same
time, which were too small to be marked on the pattern.
TPR analysis of oxygen carriers was performed at
303.15e1373.15 K. The results are displayed in Fig. 3. There were
Fig. 3. TPR analysis of the oxygen carriers.
592
G.-Q. Wei et al. / Journal of Cleaner Production 200 (2018) 588e597
three reduction peaks in the TPR profile at 671.68, 770.18 and
1154.52 K. Based on the XRD analysis, it was found that Fe2O3 was
the primary active component in hematite oxygen carriers. Correspondingly, the reduction peak at 671.68 K was ascribed to the
crystal form transition from Fe3þ (Fe2O3) to Fe3þ$Fe2þ (Fe3O4) in the
oxygen carriers, and the adsorbed oxygen was consumed by the
reaction as follows (Meshkani and Rezaei, 2015):
Fe2O3 þ H2 ¼ 2FeO þ H2O
(14)
2H$ þ [O$] ¼ H2O
(15)
Moreover, it was observed the transition peak at 770.18 K corresponded to the reaction of Fe3O4 to FeO (Fe3O4þH2 ¼ 3FeO þ H2O),
and the reduction peak at 1154.52 K, which was assigned to the
generation of metallic iron, followed reaction (16).
FeO þ H2 ¼ Fe þ H2O
(16)
TPR analysis showed that the lattice oxygen and adsorbed oxygen in hematite samples exhibited better reactivity with H2.
To further verify the reaction rate and limitation of the hematite
sample, a TG-DTG (thermogravimetric analysis-differential thermal
gravity) analysis was carried out from 303.15 to 1223.15 K with the
mixture atmosphere of 5%H2 and 95%N2 (volume fraction). As
displayed in Fig. 4, the total weight loss of hematite oxygen carriers
reached 25.86%, which is ascribed to the reaction of active lattice
oxygen [O] and hydrogen (Reaction 15). Since the total oxygen
content in hematite oxygen carrier was 32.27% according to the XRF
data, the weight loss analysis indicated that 80.14% of oxygen in the
oxygen carriers was involved in the reduction reaction. Moreover,
96.13% active lattice oxygen in Fe2O3 was consumed in the reduction process, which suggested a larger active lattice oxygen capacity
and better reactivity for the hematite oxygen carriers. Correspondingly, the DTG profile was depicted as a function of reaction
time. Three reduction peaks were observed in the entire reaction
process at 673.15, 773.15 and 1123.15 K, which were assigned to the
reaction stages of Fe2O3 / Fe3O4, Fe3O4 / FeO and FeO / Fe. At
the beginning of reduction, the maximum weight loss rate of Fe2O3
/ Fe3O4 was 0.0043%/s, indicating lower lattice oxygen consumption rate. As the reaction proceeded, the conversion rate of
Fe3O4 increased rapidly to 0.0142%/s due to the FeO phase generation. Finally, the deep reduction of FeO / Fe presented a lower
reaction rate with the exhaustion of the lattice.
Fig. 4. TG-DTG analysis of the oxygen carriers.
3.2. Fixed-bed reactor investigation on the CLR coupled with CLH
process
3.2.1. Gas composition evolution as a function of temperature in the
CLR process
The syngas composition evolution as a function of temperature
is shown in Fig. 5. It was observed that the H2 component increased
with rising temperature, while the CH4, C2H4, C2H6 and CO2 displayed a decreasing tendency in the researched temperature range
(1023.15e1173.15 K). In particular, the CO concentration first
increased and then decreased slightly. There were some factors that
can explain these phenomena. First, the high temperature was
beneficial to the pyrolysis reaction of vegetable oil, as shown in
equations (1)e(7). This meant that more carbon/hydrogen free
radicals (such as CaHb-1CO$, H$, CO$ and CaH$) were released during
vegetable oil decomposition to generate intermediate pyrolysis
gases (Chang et al., 2017). In this stage, the content of low molecular
weight gas (such as H2, CO, CH4 and C2H4) increased to promote the
CLR reaction with oxygen carriers (equation (8)). Second, the high
temperature promoted the endothermic reactions in the CLR process, listed in equations (17), (20)e(22), (24) and (26), which
consumed H2, CO2, CH4, CO, C2H4 and C2H6 to generate more H2 and
CO. These reactions contributed to the increase of H2 and CO concentrations in the syngas. Meanwhile, other gas components
decreased with the consumption in the CLR process. In addition, the
high temperature depressed the exothermic reactions (equations
(18), (23) and (25)) in the CLR process, which reduced the amount
of CO generated. Therefore, as these reactions went to equilibrium,
CO displayed an upward tendency and then tended to be stable. By
contrast, the H2 composition increased with rising temperature,
while that of the other gases decreased gradually.
3H2 þ Fe2O3 ¼ 2Fe þ 3H2O △H1173.15K ¼ 61.79 kJ
(17)
3CH4 þ Fe2O3 ¼ 2Fe þ3CO þ 6H2 △H1173.15K ¼ 738.88 kJ
(18)
CO2 þ C ¼ 2CO △H1173.15K ¼ 550.42 kJ
(19)
CO2 þ CH4 ¼ 2CO þ2 H2 △H1173.15K ¼ 258.82 kJ
(20)
1.5C2H4 þ Fe2O3 ¼ 2Fe þ 3 CO þ 3H2 △H1173.15K ¼ 413.68 kJ
(21)
C2H6 þ Fe2O3 ¼ 2Fe þ 3CO þ 4.5H2 △H1173.15K ¼ 629.71 kJ
(22)
Fig. 5. Syngas composition evolution in the CLR process of vegetable oil.
G.-Q. Wei et al. / Journal of Cleaner Production 200 (2018) 588e597
CO þ H2O ¼ CO2 þ H2 △H1173.15K ¼ 33.13 kJ
(23)
C þ H2O ¼ CO þ H2 △H1173.15K ¼ 135.71 kJ
(24)
3CO þ Fe2O3 ¼ 2Fe þ 3CO2 △H1173.15K ¼ 37.61 kJ
(25)
CH4 þ 2H2O ¼ CO2 þ 4H2 △H1173.15K ¼ 192.56 kJ
(26)
Correspondingly, the carbon conversion efficiency of vegetable
oil in the CLR process under different temperatures is listed in
Table 1. It was found that a high temperature can obviously enhance
the carbon conversion in the CLR process based on the endothermic
reactions between lattice oxygen and the pyrolysis products from
vegetable oil, promoting reaction equilibrium toward the production direction. It seemed that a high temperature benefited the CLR
process, while the maximum carbon conversion efficiency of
57.04% was achieved at 1173.15 K, rather than 1223.15 K. This was
ascribed to carbon deposition in the reaction process. Theoretically,
with an increase of the temperature, the vegetable oil was prone to
cracking on the surface of oxygen carriers to generate carbon
deposition with the catalytic effect of the oxygen carriers at the
reduced state (Wang et al., 2017b). The carbon deposition increased
sharply at 1223.15 K, leading to a decrease in carbon conversion
efficiency in the gas production. Meanwhile, exothermic reaction
(19), which can consume the carbon deposition, was depressed at
high temperature. Additionally, the dimerization reaction of free
radical to oil promoted a decrease in the carbon conversion efficiency at high temperature.
3.2.2. Effect of water/oil on gas composition in the CLR process
To improve the composition of the production gas, deionized
water was introduced into the CLR reaction system at 1173.15 K. The
effect of water/oil ratio (W/O, volume) on the gas composition is
shown in Fig. 6. As indicated in the curves, the H2 and CO compositions in the syngas increased obviously with the increasing of
water/oil ratio, whereas the contents of CH4, C2H4 and CO2 declined
steadily in the reaction process. This was ascribed to the water-gas
shift reaction, the water gas reaction and the steam reforming reaction listed in equations (23), (24) and (26). These reactions were
enhanced to generate CO and H2 with the introduced steam.
Moreover, the produced CO2 was further consumed by the Boudouard reaction (19), leading to a gradual decline of the CO2 concentration. Additionally, the introduced water contributed to the
declining concentration of C2H4 and C2H6 via reactions (27) and
(28), which produced more CO and H2.
C2H4 þ 2H2O ¼ 2CO þ 4H2 △H1173.15K ¼ 56.07 kJ
(27)
C2H6 þ 2H2O ¼ 2CO þ 5H2 △H1173.15K ¼ 90.49 kJ
(28)
As shown in Table 2, the carbon conversion efficiency increased
promptly with increasing steam-to-oil ratio in the CLR process. The
maximum carbon conversion efficiency of 79.10% was achieved at a
W/O of 1.2. It was obvious that steam contributed to the carbon
conversion of vegetable oil in the CLR process. These reactions
Table 1
Carbon conversion efficiency as a function of temperature.
Temperature/K
Carbon conversion/%
1023.15
1073.15
1123.15
1173.15
1223.15
30.88
53.12
54.80
57.04
52.81
593
Fig. 6. Syngas composition under different W/O ratios in the CLR process.
Table 2
Carbon conversion efficiency under different W/O ratios.
W/O ratio/mL/mL
Carbon conversion/%
0.1
0.2
0.5
0.7
1.0
1.2
50.22
52.26
56.21
64.31
76.71
79.10
promoted the carbon conversion in the CLR process by enhancing
the generation of CH4, C2H4, CO and CO2, similar to reactions (23),
(24), (26), (27) and (28). H2O was helpful in the CLR processing of
vegetable oil at a lower steam-to-oil ratio, while it showed negative
effects on the following CLH process. This was ascribed to partial
oxidization of oxygen carriers by the steam, which decreased the
production of H2 in the next reaction stage. The reactions for the H2
production are represented in equations (29) and (30).
3Fe þ 4H2O ¼ Fe3O4 þ 4H2 △H1173.15K ¼ 96.64 kJ
(29)
3FeO þ H2O ¼ Fe3O4 þ H2 △H1173.15K ¼ 46.11 kJ
(30)
3.2.3. H2 generation as a function of reaction temperature in the
CLH process
Following the CLR process of vegetable oil, the CLH stage for H2
production was investigated by splitting deionized water with oxygen carriers in a fixed-bed reactor. Fig. 7 reveals the H2 production
as a function of reaction temperature. It was observed that the H2
production increased obviously from 1023.15 to 1173.15 K and then
decreased at 1223.15 K. Accordingly, the composition of the gas
production decreased from 91.72% to 89.05%. The highest H2 concentration of 91.72% was achieved at 1023.15 K, and the maximum
yield of 501.06 mL/g H2-rich gas with 89.89% H2 concentration was
obtained at 1173.15 K, corresponding to a 450.41 mL/g pure H2
yield. As described in equations (29) and (30), the H2 was generated
from the reactions between reduced oxygen carriers and H2O.
Moreover, the impurity gas was ascribed to the carbon deposited in
the CLR stage of vegetable oil processing and subsequent reactions
between H2O and deposited carbon in the CLH stage. Specifically,
the carbonaceous gas was generated from reactions (24) and (31).
These endothermic reactions were enhanced to produce more
carbonaceous gas with increasing temperature, resulting in the
594
G.-Q. Wei et al. / Journal of Cleaner Production 200 (2018) 588e597
value-added proposition for the CLH process.
3.3. Successive cycle tests for the CLR coupled with CLH process
Fig. 7. H2 production and composition as a function of reaction temperature.
To investigate the multi-cycle reaction performance of hematite
oxygen carriers, the CLR, CLH and air oxidation three stage reactions were alternatively performed in the fixed-bed reactor at
1173.15 K. The carbon conversion efficiency hc in the CLR stage, H2
concentration in the CLH stage and O2 composition in the air oxidization stage are exhibited in Table 3. It was observed that the
carbon conversion of vegetable oil in the CLR stage fluctuated in the
range of 55.14%e60.51%, indicating a stable lattice oxygen output of
the oxygen carriers after a 20-cycle reaction. Accordingly, the H2
production concentration varied from 89.93% to 92.44% in the
multi-cycle reaction process. To recover the lattice oxygen of oxygen carriers, the air oxidization stage was implemented after the
CLH process, and the oxygen concentration was stable in the scope
of 13.78%e14.68%. The reaction between oxygen carriers and O2 can
be represented as reaction (32).
4Fe3O4 þ O2 ¼ 6Fe2O3 △H1173.15K ¼ 102.57 kJ
decline of the H2 gas concentration.
C þ 2H2O ¼ CO2 þ 2H2 △H1173.15K ¼ 102.57 kJ
(31)
Though these side reactions produced undesirable carbonaceous gas, they also generated H2, suggesting partial advantageous
factors for H2 production. To increase the purity of H2, the carbon
deposition needs to be minimized. There are some strategies to
improve the carbon deposition in the vegetable oil reforming stage.
On the one hand, it is better to perform the experiments in a
circulating fluidized bed, which can restrain the carbon deposition
increase via intense collisions between OC particles (Wei et al.,
2015). A challenge involved in this experiment is the energy penalty and technical complexity. On the other hand, some steam is
beneficial to increasing the carbon conversion efficiency and
reducing the carbon deposition in the CLR stage. Nevertheless, the
introduction of H2O has a negative impact on the reduced oxygen
carriers, which leads to the decline of H2 production in the
following H2 production stage. Finally, it seems that an effective
method is to design and develop anti-carbon oxygen carriers,
which can solve the carbon deposition fundamentally (Bimbela
et al., 2017).
In addition, although small amounts of carbonaceous gas impurities are mixed in the production, a higher H2 volume concentration of 91.7% is available from the chemical looping process than
that from other H2 production processes, such as biomass steam
gasification (40.8%) (Xiao et al., 2017), catalytic steam reforming of
bio-oil (70.8%) (Bimbela et al., 2017) and supercritical water gasification of coal (less than 60%) (Jin et al., 2015), indicating a higher
(32)
Moreover, carbonaceous gas was still observed in the outlet gas,
suggesting that the carbon deposition was not eliminated
completely after the CLH stage. On the whole, the 20-cycle reactions displayed a better cyclic reaction performance of the hematite oxygen carriers.
3.4. XRD pattern of fresh and regenerated oxygen carriers
To further investigate the crystalline form evolution of oxygen
carriers in the reaction process, XRD analysis of oxygen carriers at
different reaction cycles are exhibited in Fig. 8. As shown in the XRD
patterns, the fresh hematite oxygen carriers were mainly composed
of Fe2O3, Al2O3 and SiO2. After five multi-cycle reactions, the
diffraction peak of SiO2 was minimized, while the Fe2O3 characteristic peaks were consistent with those of the fresh oxygen carriers, indicating that the lattice oxygen carriers were fully
recovered. The minimization of the SiO2 peak was ascribed to mass
loss in the redox cycle consumption. With the cyclic reaction
running, the Fe2O3 characteristic peaks remained in line with those
of the fresh oxygen carriers after 20 cyclic reactions, which displayed the stable cyclic reaction performance of hematite oxygen
carriers.
3.5. Morphology changes of hematite oxygen carriers
The micrograph of fresh and multi-cycle reaction hematite oxygen carriers is illustrated in Fig. 9. An irregular blocky structure
Table 3
Results of successive cycle tests for treatment of vegetable oil by the coupled CLR and CLH process.
Cycle
1
2
3
5
7
9
11
15
20
CLR stage
CLH stage
Air oxidization stage
Carbon conversion
%
Reforming gas heating value
MJ/m3
H2 concentration
%
O2 composition
%
Carbonaceous gas
%
57.04
55.14
54.99
59.83
58.28
59.38
59.95
56.58
60.51
26.70
26.17
28.57
29.49
26.66
30.45
28.93
28.56
27.81
89.93
92.63
91.10
90.60
90.80
91.16
91.44
92.40
90.58
14.68
14.18
14.04
13.78
13.44
14.13
14.43
14.24
13.47
1.10
1.93
1.30
1.57
1.90
1.44
0.89
1.16
1.32
G.-Q. Wei et al. / Journal of Cleaner Production 200 (2018) 588e597
595
occurred. Correspondingly, the porous gaps evolved into porous
channels. In the subsequent cyclic reaction processes, the particle
size decreased, and agglomeration phenomena increased. Additionally, the porous channel structure was maintained after 20
cyclic reactions. Apparent sintering did not occur in regenerated
hematite oxygen carriers. Additionally, the crystalline form and
reaction activity remained stable according to experimental results
given in Fig. 8 and Table 3, suggesting that the hematite oxygen
carrier had good potential for the CLR coupled with CLH process.
4. Conclusions
Fig. 8. XRD patterns of the oxygen carriers at different reaction cycles.
with particle sizes of 5e10 mm and porous gaps was distributed in
the fresh oxygen carriers. Obviously, the porous structure was
beneficial to reactant gas diffusion into oxygen carriers, enhancing
the reaction process (Das et al., 2018). After five cycles, the oxygen
carriers' particle sizes reduced to 2e3 mm, and agglomeration
A chemical looping coupling process is proposed and proven to
produce syngas and high purity hydrogen from vegetable oil
instead of fossil fuels by adopting hematite oxygen carriers with a
lower cost and less pollution. In the reduction process of hematite
oxygen carries, the reaction follows the path of Fe3þ / Fe3þ$Fe2þ
/ Fe2þ / Fe0, achieving the maximum conversion rate in the
second reaction stage. High temperature and water have a positive
effect on the CLR process, while excessive steam depresses deep
reduction of oxygen carriers in the CLR process, decreasing H2
production in the subsequent CLH stage. The optimized reaction
temperature for the CLR stage is 1173.15 K, corresponding to 57.04%
carbon conversion efficiency for vegetable oil. The maximum yield
of 501.06 mL/g for H2-rich gas with an 89.89% H2 concentration is
obtained at 1173.15 K in the CLH stage, corresponding to a
450.41 mL/g pure H2 yield. The concentration of H2 production
Fig. 9. SEM analysis of the oxygen carriers for multi-cycle tests: (a) fresh oxygen carriers; (b) 5 cycles; (c) 10 cycles; and (d) 20 cycles.
596
G.-Q. Wei et al. / Journal of Cleaner Production 200 (2018) 588e597
achieved from the CLH process varies from 89.93% to 92.44% in a
multi-cycle reaction process.
Acknowledgements
The authors gratefully acknowledge the financial support of the
National Key Research and Development Program of China (Grant
No. 2016YFB0901401), the National Natural Science Foundation of
China (Grant No.: U1610221; 51776133), the Science and Technology Projects of Guangdong (Grant No.: 2015A020215023), Science
and Technology Projects of Guangzhou (Grant No.: 201707010202),
Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (Grant No.: Y709jj1001), and
Training base of State Key Laboratory of Coal Science and Technology Jointly Constructed by Shanxi Province (Grant No.:
MKX201701) and Ministry of Science and Technology, Taiyuan
University of Technology (Grant No.: MKX201701).
Notation list
AR
C2H4
C2H6
CaO
CH4
CLC
CLH
CLR
CO
CO2
db
DTG
Fe2Fe2O3
Fe3þ
Fe3O4
FR
H
H2
HPLC
LHV
MemOn
MgO
O
O2SR
TG
TPR
W/O
WCO
XRD
XRF
hc
air reactor
ethylene
ethane
calcium oxide
methane
chemical looping combustion
chemical looping hydrogen
chemical looping reforming
carbon monoxide
carbon dioxide
dry basis
differential thermal gravity
Ferrous ion
iron oxide
Ferric ion
ferroferric oxide
fuel reactor
hydrogen radical
hydrogen
high performance liquid chromatography
lower heating value
metal oxide (oxygen carrier)
magnesium oxide
oxygen radical
oxygen ion
steam reactor
thermo gravimetric
H2 temperature programmed reduction
water/oil ratio
waste cooking oil
X-ray diffraction
X-ray fluorescence
carbon conversion efficiency
Appendix A. Supplementary data
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.jclepro.2018.07.263.
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