Syngas production by chemical looping gasification of biomass with steam and CaO additive

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Syngas production by chemical looping gasification of biomass with steam and CaO additive Yuting Wu, Yanfen Liao*, Guicai Liu, Xiaoqian Ma Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization, School of Electric Power, South China University of Technology, Guangzhou, 510640, PR China

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abstract

Article history:

In the present work chemical looping gasification (CLG) of biomass, with introduction of

Received 22 May 2018

steam and CaO, was conducted in a fixed bed to produce syngas. The addition of steam

Received in revised form

facilitated both the reforming reaction and water gas shift reaction to produce more

26 August 2018

gaseous products, especially H2. Depending on gas yield and gasification efficiency, the

Accepted 30 August 2018

requirement of steam was sufficient for CLG at steam flow n ¼ 0.6 r/min. It was also

Available online xxx

confirmed that CaO had a positive effect in gasification and the addition in the ratio of CaO/

Keywords:

absorbent at low temperature but as catalyst above 700  C. However, the catalysis of CaO

Biomass

received the major attention because of its availability for high syngas yield (1.367 m3/kg)

Chemical looping gasification (CLG)

and efficiency (carbon conversion efficiency was 89.28% and gasification efficiency was

Steam

88.81%) at 800  C. CaO was found to retard the sintering and porosity reduction of Fe2O3, the

C ¼ 1 was adequate for syngas production. Moreover, CaO mainly played a role as CO2

CaO additive

oxygen carrier, by covering Fe2O3 and forming calcium ferrites during multiple redox cycles. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction As an innovative gasification technology, chemical looping gasification (CLG) of biomass has a unique advantage in biomass energy conversion efficiency. Compared to conventional gasification, CLG uses the lattice oxygen of oxygen carrier for the partial oxidation of biomass, thus obtains a higher efficiency of 10e25% and facilitates the generation of H2 and CO enriched syngas, which can be applied in the fields such as fuel cells, hydrogen gas stations, etc. [1e5]. Fe-based oxygen carrier, as the most common one, has been proved to be acceptable for CLG [6,7], but its reactivity is not ideal for syngas production of biomass, especially when H2 enriched

gas is expected. Modified Fe2O3 by other metal oxide additive is investigated, and CaO is found to be a cheap and environmental friendly metal oxide. CaO-based gasification research [8,9] showed that the introduction of CaO into gasification process was beneficial to drive the chemical reaction balance forward hydrogen production, by capturing and removing CO2 from the reaction system continuously. Hu and Hao [8] found that in the gasification process of wet biomass with CaO, CaO played the dual role both as the catalyst and CO2 absorbent, and that its catalyst effect on the water gas shift reaction (CO þ H2O/CO2 þ H2) was stronger than that on the steam reforming of methane (CH4 þ H2O/CO þ H2). Siefert, Shekhawat [10] used CaO for in-situ CO2 capture in the steam-coal

* Corresponding author. E-mail address: [email protected] (Y. Liao). https://doi.org/10.1016/j.ijhydene.2018.08.197 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wu Y, et al., Syngas production by chemical looping gasification of biomass with steam and CaO additive, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.197

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gasification experiments and found that the gasification kinetics rate still increased after CaO being reused in six cycles of gasification and calcination. In CaO-CLG (chemical looping gasification with CaO additive) process, H2 can be produced continuously with in-situ CO2 capture and CaO is regenerated through cyclic process [11]. Udomsirichakorn, Basu [12] revealed that 30% higher concentration of H2 and triple amount of H2 was yielded in CaO-CLG compared to sand-based CLG. Tar content was obtained at the lowest yield in CaO-CLG with CO2 concentration reducing significantly. Besides, it was also discovered that CaO played the role of catalyst as well as CO2 sorbent in CaO-CLG [13,14]. This catalytic ability was confirmed during the pyrolysis of biomass with CaO addition. It was also found that CaO had strong activity and selectivity in the catalytic cracking of oxygen-containing functional groups, long-chain alkanes and aromatic components [15,16]. The CLG temperature of the carbonaceous fuel is generally in the range of 800  C through 1000  C. As the temperature increases, the gasification efficiency and the conversion efficiency are improved [17,18]. While the optimal temperature of CO2 absorption by CaO is about 600e700  C, and the absorption performance will weaken with the rising temperature [19,20]. Moreover, due to the high alkali metal content in biomass ash, oxygen carrier in CLG process is faced with the ash deposition and sintering problem, especially in high temperature [21]. Therefore, it is significant to investigate the reactivity of biomass CLG with CaO additive, and the effect on the sintering of oxygen carrier, so as to find out the appropriate temperature range for a high syngas conversion efficiency and reduction of sintering. The aim of this work is to investigate the reaction performance of rice straw CLG in a fixed bed. With Fe2O3 as oxygen carrier, the effect of steam, CaO/C molar ratio and temperature on the gas composition and biomass conversion efficiency was studied. And the multiple redox cycles experiments were conducted, in order to evaluate the function of CaO on the performance of oxygen carrier.

Experimental Materials Rice straw used in this work was collected from Guangzhou city, which was common in Guangdong Province. Fe2O3 powder (analytical reagent) was prepared as oxygen carrier. All the samples were dried for 24 h at 105  C, then were crushed with a pulverizer and passed through an 80 mesh sieve. CaO powder was calcined for 3 h at 900  C to ensure the purity before being used as CO2 sorbent. All samples were

stored in a desiccator before experiments. The proximate analysis (dry basis) and ultimate analysis (dry basis) of biomass feedstock were obtained according to ASTM standard, and the heating value was measured by calorimeter, which are listed in Table 1. It can be seen that the volatile content (73.44% on dry basis) is quite high in rice straw, which benefits the decomposition and gasification. While the high oxygen content means the gaseous products will have more CO and CO2 [22e24].

Experimental setup and procedure The gasification experiments were carried out in a U-shaped fixed-bed reactor. The schematic diagram of the experimental setup was shown in Fig. 1. The system mainly consisted of gas supply section, reaction section, condensation section, gas collection section and detection section (gas chromatography (GC)). During the experiment process, argon was used as the carrier gas (50 ml/min) and the steam was generated by evaporation of water which was fed into reactor continuously by constant flow pump. Fe2O3 and CaO were mixed with 0.2 g rice straw mechanically. Then they were filled in a copper mesh basket and were hung at the top of the U-type reactor (a 14 mm inside diameter and 378 mm high quartz tube). The vertical U-type reactor was heated in an electric furnace controlled by thermocouple. Before experiments, the sample was hung in the quartz tube nozzle by a copper wire. In order to heat sample rapidly, the U-type reactor was put in the furnace 10 min firstly, and after it reached the target temperature, the copper wire was released from the tube nozzle and the sample fell quickly into the constant-temperature region with the wire. In the whole process, sealing of the reactor was kept and the mixture of argon and steam was purged to maintain the reaction atmosphere. To ensure rice straw fully reacted with Fe2O3 and CaO, the reactants remained in the reactor for 30 min. The volume of the collected gaseous products was determined through the drainage method, and then all the gas were collected into the gas sampler. The composition of gaseous products was analyzed by GC, which was calibrated with standard gas to quantify the concentration of CO, H2, CO2, CH4 and C2Hm. In order to ensure the reliability of the experiment data, all the conditions were carried out three times.

Thermodynamics analysis and data evaluation The process of CLG with steam and CaO is complicated. The major reactions taking place are shown as follows [1]: Water gas: C þ H2 OðgÞ/H2 þ CO DH > 0

(1)

Table 1 e Proximate and ultimate analysis of rice straw (dry basis). Proximate analysis (wt%)

Ultimate analysis (wt%) a

Volatiles

Ash

Fixed carbon

C

H

N

S

O

73.44

10.09

16.47

39.70

6.01

0.95

0.24

43.01

a

Qnet (MJ/kg) 14.986

Calculated by difference.

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Heating Constant Section flow pump

Sample

3

GC Gas sampler

Flow controller Cooler

Ar Quartz tube Furnace

Thermocouple

Fig. 1 e Schematic diagram of U-shaped tube fixed-bed reactor.

Water gas shift: CO þ H2 OðgÞ/H2 þCO2

DH < 0

(2)

Steam reforming of methane: CH4 þH2 OðgÞ/3H2 þ CO DH >0 (3) CH4 þ2H2 OðgÞ/4H2 þCO2

DH > 0

(4)

Reforming of Tar: Tar þ H2 O/CO þ H2 þ CO2 þCm Hn þ …

(5)

Boudouard: C þ CO2 /2CO DH > 0

(6)

CaO carbonation: CaO þ CO2 /CaCO3 CaCO3 calcination: CaCO3 /CaO þ CO2

DH < 0 DH > 0

(7) (8)

The gaseous products in biomass CLG mainly contain CO, H2, CO2, CH4, C2H4, C2H6, and other components are less, thus only the main products are considered, and C2H4, C2H6 are referred to as C2Hm. In order to analyze the effect of experimental conditions on the gasification process, the following parameters are introduced for comparative analysis. Gas yield（Gv, m3/kg）is given by： Gv ¼

Vg mb

(E1)

where Vg denotes the volume of gas product in the standard state, and the parameter mb is the quantity of experimental biomass (dry basis). Carbon conversion efficiency（hc, %）is calculated as： hc ¼

GCO þ GCO2 þ GCH4 þ 2GC2 Hm  12  100% 22:4  fC

(E2)

where GCO , GCO2 , GCH4 , GC2 Hm denote the gas yield of each gas component respectively, and the parameter fC is the carbon content in the biomass dry basis. Gasification efficiency（h, %）is calculated by the following formula： h¼

Qg  100% Qnet

(E3)

where Qg denotes as the lower calorific value of gas product and Qnet denotes the lower calorific value of raw material.

Results and discussion Effect of steam flow on syngas production The effect of steam flow on CLG was investigated in this work. In our research about microalgae CLG to produce combustible gas, it was found that the Fe2O3/C molar ratio of 0.2 could reach the maximum gasification efficiency [25], thus the samples were mixed in accordance with the molar ratio of C: Fe2O3: CaO ¼ 1: 0.2: 1. The steam flow (n) was varied from 0 to 1.2 r/min (1 r/min ¼ 0.048 g/min) at 850  C. Fig. 2 showed the gas yield variation at different steam flow. The low syngas yield was obtained in the absence of steam (n ¼ 0 r/min), the gaseous products in biomass CLG was mainly produced from two paths, the primary fast crack of large molecular and the partial oxidation of pyrolysis products by Fe2O3. The main pyrolytic gas products were H2, CO, CO2 and CH4, which could reach 0.5e0.55 m3/kg [26]. Fe2O3 acted as oxygen source and improved the gas products to 0.7e0.8 m3/ kg [27], due to the partial oxidation of tar and char as shown below. TarþFe2 O3 /H2 þH2 O þ CO þ CO2 þFeO þ Fe3 O4 C þ 3Fe2 O3 /CO þ 2Fe3 O4

DH > 0

(9) (10)

But the biomass CLG without steam still got only 67.85% gasification efficiency. With the addition of steam, the main gas products H2 and CO2 were notably improved. H2 yield was elevated rapidly to 0.646 Nm3/kg at n of 0.3 r/min, then increased slowly to 0.75 Nm3/kg. CO2 yield continuously increased from 0.178 Nm3/kg to 0.375 Nm3/kg, while CO yield decreased from 0.374 Nm3/kg to 0.214 Nm3/kg. It was normally considered that H2 was mainly from the ring crack and the recombination of aromatic ring during the pyrolysis and gasification of biomass [28e30]. With steam addition, the reactions between volatiles and steam (including water gas shift, reforming of tar and methane as shown in reaction (1)e(5)) facilitated the production of most of gaseous products, especially for H2. However, excess steam addition might restrict H2 production leading to the slow rate of increase in gas yield. The explanation would be shown below.

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Fig. 2 e Effect of steam flow on gas components.

The increasing of steam flow facilitated the decline of CO, even though the water gas reaction and reforming reactions (reaction (1), (3), (5)) contributed to the production of CO. It was indicated that in the steam atmosphere, the water gas shift (reaction (2)) was more significant than others. Furthermore, CH4 and C2Hm had low yield so the influence of steam flow change on them was not obvious. In Fig. 2 (b), gas yield presented increase trend as steam flow increased from 0 to 1.2 r/min. However, carbon conversion efficiency and gasification efficiency exhibited a tendency to increase firstly and then had trivial change. And in Fig. 2 (a), H2 yield has similar trend. The results appeared to suggest that excess steam addition would be adverse to the conversion of biomass. On one hand, excess steam would absorb a lot of heat resulting in unevenness of temperature distribution in reactor and reduce the activity of the water gas shift reaction [1,31]. On the other hand, steam addition would increase the gas velocity in reactor. It meant that residence time of gas in reactor decreased and gas-solid reaction was weakened, which caused less carbon in biomass transferring into syngas. According to the above conclusions, the appropriate addition amount of steam was in favor of H2 production. And it was infeasible to apply a too-large steam amount which could reduce syngas quality and increased the system energy consumption. Therefore, steam flow n ¼ 0.6 r/min was suitable to get high gas yield and efficiency of CLG.

Effect of CaO/C molar ratio on syngas production To research the effect of CaO on CLG, experiments were conducted in condition of CaO/C valuing for 0, 0.25, 0.5, 1.0 and 1.5 at 650  C and 850  C. Meanwhile, Fe2O3/C ¼ 0.2 and n was 0.6 r/min. The variation of syngas yield and CLG efficiency were shown in Fig. 3. The results showed that H2 yield increased dramatically, while CO and CO2 were decreased with the addition of CaO at 650  C. The results were attributed to the carbonation effect of CaO. CaO carbonation led to the decrease of CO2 yield, which pushed reactions (2) and (4) to the direction of H2 production, resulting in the increase of gasification efficiency. As for CO, it decreased at the pyrolysis stage due to the restriction on the ether bond cracking by CaO at low temperature [15,25,32]. Also, the absorption of CO2 led to the reduction of CO yield, due to the right shift of the water gas shift occurred in a decrease of CO2. As presented in Fig. 3 (b),

CO2 gas yield changed slightly but H2 and CO increased with the addition of CaO at 850  C. It indicated that CaO could hardly absorb CO2 but mainly played a catalytic role at high temperature. This result could be verified and explained in Section Effect of reactor temperature. With the increase in CaO/C molar ratio, H2 and CO2 yield varied slightly, indicating that CaO addition in the ratio of CaO/C ¼ 1 was sufficient for the gasification process. Furthermore, excess CaO would cover the surface of the reactant such as the one condition shown in Fig. 4. From Fig. 4, it was remarkable to see that the surface of oxygen carrier was covered by Ca element after gasification, which enhanced the CaO effect on gasification, and weakened the excess oxidation effect of Fe2O3. The coverage of CaO on the surface helped to improve the selectivity of gasification process but might degrade the performance of oxygen carrier and the gasification of char. In conclusion, CaO/C ¼ 1 was adequate for syngas production.

Effect of reactor temperature In order to evaluate the effect of temperature on the reactivity of CaO and Fe2O3 oxygen carrier, experiments were conducted in the absence (CaO/C ¼ 0) and presence (CaO/C ¼ 1) of CaO at different temperatures. Meanwhile, Fe2O3/C ¼ 0.2 and n was 0.6 r/min. Fig. 5 showed the variation of gas components as the temperature rose from 600  C to 850  C. The gas yields with and without CaO were increased significantly with the increasing temperature, indicating that temperature influenced CLG process remarkably regardless of the introduction of CaO. Pyrolysis of biomass was a strong endothermic process so high temperature promoted the release of volatiles. Furthermore, high temperature was conducive to the secondary decomposition of macromolecule volatiles and promoted the conversion of tar and char to produce syngas. As shown in Fig. 5 (a), CO2 yield was significantly decreased with the addition of CaO, indicating that CaO played a role as CO2 absorbent when the temperature was below 650  C. While the temperature rose from 650  C to 850  C, CO2 yield increased sharply and even exceed that without CaO. One reason was that the carbonation of CaO weakened in high temperature. In addition, the catalytic cracking of CaO by CaO also produced CO2 [15]. At this point, H2 yield was always higher than that without CaO. It could be

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Fig. 3 e CLG profiles at different CaO/C.

Fig. 4 e Ca and Fe distribution on the surface of used oxygen carrier (800  C, CaO/C ¼ 1.5).

Fig. 5 e Effect of temperature on gas components.

explained that at high temperature, the catalytic dehydrogenation effect of CaO was significant on the reforming of tar and hydrocarbon to produce H2 [8]. It was remarkable that there was no difference between H2 gas yield with or without CaO at 850  C. It was because CaO as catalyst at high temperature mainly affected the dehydrogenation of aromatic compounds, but most of aromatic compounds had cracked before dehydrogenation at high temperature [14]. As regards CO, when the temperature was below 750  C, the gas yield of CO was markedly lower than that without CaO, but it was slightly higher than that without CaO at the temperature higher than 750  C. It could be explained that the ether bond cracking was restricted by CaO at low temperature. And at high temperature, carbonation of CaO at the heating process

and calcination of CaCO3 deferred the release of CO2 and raised the temperature reaching the requirement of Boudouard reaction (6), which enhanced CO generation [25]. As shown in Fig. 5 (b), gas yield, carbon conversion efficiency and gasification efficiency of CLG with CaO were mounting with increasing temperature, and the carbon conversion efficiency was 92.59% and gasification efficiency was 94.27% at 850  C. When the temperature was 800  C, the efficiency of CLG with CaO was markedly higher than that without CaO, and carbon conversion efficiency was 89.28% and gasification efficiency was 88.81%. Combining the results of Fig. 5 (a), it was found that the catalytic performance of CaO was outstanding and the effect of carbonation was weak at high temperature. CaCO3 was decomposed to CaO and CaO

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catalyzed the cracking of oxygen-containing functional groups to produce more H2 and CO, resulting in the improvement of CLG efficiency at high temperature. In order to detect the effect of reactor temperature, the XRD analysis of used oxygen carrier were conducted. In Fig. 6, the crystalline phase of oxygen carrier with CaO after reduction was shown, and the result indicated that Fe2O3 was mainly converted to Fe3O4 at different temperatures. It was attributed to the fact that steam could provide oxygen for CLG as a gasification agent and avoided the deep reduction of oxygen carrier. As shown previously [33,34], FeO and Fe, which produced from reduction of Fe2O3, could be partial oxidized by H2O to obtain H2 and Fe3O4, and this process was also used for pure H2 production. CaO was converted to CaCO3 and Ca(OH)2 when the temperature was below 650  C, but CaCO3 was decomposed and more CaO was generated from 700  C to 850  C. The results were consistent with the results in Fig. 5. It was noteworthy that there was no new phase generating between Fe2O3 and CaO in the gasification process at different temperature and the diffraction intensity of CaO was high at the temperature above 700  C. It indicated that CaO could keep original phase to perform its function in gasification. Generally, the initiation temperature of the decomposition of Ca(OH)2 was about 400  C at 101 kPa [35]. And CaCO3 decomposed completely at about 727  C [8]. But in the XRD results, the phase of Ca(OH)2 could detected until the reaction temperature was about 750  C, it could be attributed that CaO absorbed the steam leaving at the reactor after gasification reaction. Moreover, the steam partial pressure was high in the steam-rich atmosphere which prevented the decomposed of Ca(OH)2 and accelerated the calcination of CaCO3 [36]. Although CO2 absorption was not obvious and carbonation-calcination looping could not achieve in CLG at 800  C, CaO still achieved its catalytic function on promoting syngas production and improving CLG efficiency. Considered above results, it was optimum to control the temperature at 800  C to ensure the oxygen carrier had a good reaction performance and got the high efficiency of CLG.

Fig. 6 e XRD patterns of used oxygen carrier and CaO at different temperatures.

Analysis of multiple redox cycles In order to investigate the stable performance of oxygen carrier in redox cycles, ten redox cycle experiments were carried out with steam flow n ¼ 0.6 r/min at 800  C. Considered of the reaction of copper mesh basket and air at the oxidation stage, the mixture sample of straw, Fe2O3 and CaO was mixed corresponding to the molar ratio of CaO: Fe2O3: C ¼ 1: 0.2: 1 and filled in a quartz tube with inner diameter of 10 mm instead of copper mash basket. The method of reduction was the same as above. After reduction, the atmosphere was switched to air with 200 ml/min and began the oxidation stage, which was lasted for 30 min. The solid residue was taken out from the reactor and remixed with 0.2 g straw as the sample of the next redox cycle. The variation of syngas and CLG efficiency with increasing cycle number was shown in Fig. 7. CO2, CH4 and C2Hm gas yield were almost invariable in ten redox cycles. CO increased slightly in the initial 3 cycles while barely changed in later cycles. On the contrary, H2 had an increasing trend and was from 0.605 Nm3/kg to 0.661 Nm3/kg in ten redox cycles. At the same time, the gasification efficiency and carbon conversion efficiency fluctuated in 89.4% ± 2.3% and 88% ± 1.9%, respectively. The results were similar with previous research [37]. It could be inferred that the reactivity of the oxygen carrier was impaired and the lattice oxygen declined with the increase of cycle numbers. H2 and CO were oxidized by lattice oxygen in reduction stage, therefore, when the activity of lattice oxygen declined the reaction was restricted and H2 and CO increased slightly. Besides, sintering and ash deposition on the surface of the oxygen carrier resulted in the decrease in specific surface area and pore volume, which would be confirmed later. Inactivation of CaO after several cycles was probably another reason for the variation of syngas production. The reaction of CaO and tar/char would lead to the inactivation of CaO [38]. Some research [39,40] found the presence of H2O and CO2 accelerated sintering and porosity reduction of CaO. The surface reduction in CaO was impressible to steam and the CaO pore size was enlarged in atmosphere containing CO2, which implied that CO2 played a role in sintering when it reached the decomposition temperature of CaCO3 [39]. In spite of the inactivation, the addition of CaO was sufficient to catalyze the production of CO2 so CO2 showed slight change instead of decrease in redox cycles. To explain the reactivity variation of the oxygen carrier in redox cycles, the characterization analysis of the fresh oxygen carrier and the oxidized oxygen carrier in redox cycles were performed by SEM, XRD and BET. The surface morphology of the fresh and oxidized oxygen carrier was shown in Fig. 8. The fresh oxygen carrier surface was characterized by abundant small particles and had a porous structure. There was remarkable difference between their morphology with and without CaO in redox cycles. As shown in Fig. 8 (b) and (c), the agglomeration of oxygen carrier in redox cycles without CaO was serious. Compared to that without CaO, the view in Fig. 8 (d) showed the particles with addition of CaO were overall uniform and loose, but the porous structure visibly degraded over the first redox process. Although the serious agglomeration and a loss of porosity,

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Fig. 7 e Syngas as a function of cycle number.

Fig. 8 e SEM images of the oxygen carrier in multiple redox cycles. which led to the decline of the oxygen carrier performance, could be observed in Fig. 8 (e), the oxygen carrier performance with CaO was better than that without CaO after 10 cycles. The results were in accordance with the study [41] and attested that the addition of CaO was advantageous to retard sintering phenomenon and improved the stability of oxygen carrier. The XRD patterns of oxygen carrier after different redox cycles were shown in Fig. 9. After redox cycles, Fe2O3 was hard to be detected and CaO was the main phase on the surface of oxygen carrier. A new phase Ca2Fe2O5, which was formed by the reaction of 2CaO þ Fe2O3 / Ca2Fe2O5 [42], was detected and the diffraction intensity increased with cycle times increasing. Combined with the results in Fig. 6, CaO and Fe2O3 were mixed mechanically at the first gasification process and Fe2O3 was converted into Fe3O4 which could not react with CaO to generate Ca2Fe2O5. But Fe3O4 was oxidized to Fe2O3 and reacted with CaO long enough in the oxidized process resulting in the formation of Ca2Fe2O5. It was found that Ca2Fe2O5 had high reactivity with solid fuel but low reactivity with syngas, and performed good regeneration in chemical looping partial oxidation of solid fuel [25,43]. Some studies had

Fig. 9 e XRD patterns of used oxygen carrier with CaO after redox cycles.

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Table 2 e Pore structure analysis of the oxygen carrier in multiple redox cycles. Samples

Fresh oxygen carrier CaO Used oxygen carrier (after 1st redox cycle) Used oxygen carrier (after 10th redox cycle)

BET area Total pore Average pore (m2/g) volume diameter (cm3/g) (nm) 2.644 5.209 2.007

0.0061 0.0118 0.0111

9.27 9.03 22.19

1.621

0.0108

26.61

found that partial active oxides were hindered by CaO in CaOdecorated CLG [41]. According to aforementioned results, it was concluded that CaO covered the surface of oxygen carrier in redox cycles and the reduction of oxygen carrier surface would weaken the contact between Fe2O3 and volatiles, resulting in the increase of H2 and CO. But the coverage of CaO could remit the sintering phenomenon of oxygen carrier and the formation of calcium ferrites also improved the oxygen carrier reactivity. The pore structure of the oxygen carrier analysis shown in Table 2 was found to be destroyed upon cycling. After redox cycles, the BET area and the total pore volume of used oxygen carrier decreased while the average pore diameter increased markedly. Integrated with the SEM results, the particles were getting large and the inner structure of the particles was destroyed, leading to the decrease of small pores and the formation of large pores. Therefore, the decrease of pore number resulted in the decrease of the BET area and the total pore volume. According to the results above, H2 and CO increased and other syngas components barely changed in multiple redox cycles. It was attributed to the reduction of the oxygen carrier surface area through the coverage of CaO and the catalysis of CaO. There was a new phase, Ca2Fe2O5, which was in favor of syngas production, formed in the oxygen carrier oxidizing process. The sintering of CaO, which was connected with the high temperature or atomic rearrangement in carbonationcalcination looping [44], led to a reduction of the oxygen carrier porosity and reactivity after 10 cycles. However, CaO was still considered beneficial to retard the agglomeration of oxygen carrier by coverage and forming Ca2Fe2O5 upon cycling.

Conclusions Chemical looping gasification (CLG) of biomass with addition of Fe2O3 oxygen carrier, steam and CaO for syngas production was conducted in a fixed bed. The influences of steam addition on CLG were studied and it was found that H2 yield and gasification efficiency were notably improved with proper amount of steam. The effects of CaO at 650  C and 850  C were evaluated as well. The results showed that the addition of CaO had a favorable impact for H2 yield and CaO/C ¼ 1 was optimum for syngas production. Besides, it was notable that excess CaO had no significant effect on CLG, but formed a layer on the surface of the oxygen carrier and hindered its performance. The effects of gasification temperature on CLG were determined with specific focus on syngas yield,

gasification efficiency and oxygen carrier performance. The increase in reactor temperature was beneficial to syngas production. In terms of syngas yield and gasification efficiency, controlling the temperature at 800  C could ensure the utmost performance of CaO and oxygen carrier, and achieved ideal syngas yield and gasification efficiency. Moreover, it was found that CaO played a role as CO2 absorbent below 700  C and as catalyst at higher temperature. In the multiple redox cycle tests, the gasification performance was almost stable in ten redox cycles. From SEM, XRD and BET results, it was attested that the addition of CaO could remit the sintering phenomenon and improved the reactivity of oxygen carrier by covering the surface of oxygen carrier and forming calcium ferrites. In conclusion, the addition of steam and CaO significantly promoted syngas production in CLG of biomass.

Acknowledgement This investigation was supported by National Natural Science Foundation of China (51676074), Guangdong Natural Science Foundation (2015A030311037), Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization (2013A061401005), Key Laboratory of Efficient and Clean Energy Utilization of Guangdong Higher Education Institutes (KLB10004).

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