Corn-Stalk Chemical Looping Combustion Using tert-Butanol Waste

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Corn-stalk chemical looping combustion using tert-butanol waste solution Wu Qin, Jianye Wang, Qiang Gao, Liguo Jiao, Xinnong Chen, Shubo Chen, Kaijun Jia, Xianbin Xiao, Zong-Ming Zheng, Jin Zhao, Lu Liu, and Changqing Dong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03948 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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Energy & Fuels

Corn-stalk chemical looping combustion using tertbutanol waste solution Wu Qin*,†, Jianye Wang†, Qiang Gao†, Liguo jiao†, Xinnong Chen†, Shubo Chen†, Kaijun Jia†, Xianbin Xiao†, Zongming Zheng†, Jin Zhao‡, Lu Liu*,†, and Changqing Dong*,† †National

Engineering Laboratory for Biomass Power Generation Equipment & Beijing Key Laboratory of Energy Safety and Clean Utilization, School of Renewable Energy, North China Electric Power University, Beijing 102206, China; ‡State Grid Energy Conservation Services CO., Ltd. Beijing, China.

ABSTRACT: Inadequate fixed carbon conversion is the main culprit for low efficiency of solid fuel chemical looping combustion (CLC). To improve the efficiency, we report here a strategy to sufficiently converse fixed carbon during corn stalk CLC by introducing ter-butanol solution to generate adequate oxidants (H2O and CO) in cascade reaction systems. Ter-butanol solution CLC was performed at the first reaction stage using Fe2O3/Al2O3 as oxygen carrier (OC). Then, the products of the first reaction stage acts as oxidizer to drive gasification and combustion of corn stalk in the following reaction stage at 850℃ under different peroxidation coefficients (Ω), which is a proportional coefficient of the actual amount of Fe2O3/Al2O3 used in reaction to the theoretically calculated amount required for complete oxidation of fuel. We demonstrated that terbutanol solution can directly promote the gasification of corn stalk and tune the oxidation state of the reduced OC for oxidizing the fuel gas during CLC process, which not only transfers oxygen but catalyzes corn stalk gasification. Reaction stoichiometry and thermodynamics analysis further implied that the reported ter-butanol solution – corn stalk cascaded CLC is thermodynamically

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feasible. Organic waste solution - biomass CLC, serves the double end of organic waste treatment and solid CLC promotion. 1. INTRODUCTION Large amounts of organics, including small molecule organic matters, fats, heterocyclic aromatic hydrocarbons, fused aromatic hydrocarbons and other complex organic substances, exists in industrial wastewater,1 which pollute environment and endangers human being. Numerous methods have been developed to treat these organic pollutants. Differing from adsorption and membrane separation to concentrate organic waste water, oxidation processes can degrade the organic pollutants, which are more attractive. For example, advanced oxidation processes (AOPs) produce highly reactive sulfate radicals (SO42−), hydroxyl radicals (·OH), or both, to decompose organic compounds,2 such as organic pollutants in industrial and municipal wastewater,3 pulp and papermaking wastewater,4 and landfill leachate,5 as well as chlorinated organic and etheric organics in water or sediment.6-9 AOPs often require transition metal-containing catalyst, together with light, electricity or ultrasound as active energy, to drive the reactions, 10-14 which is hence difficult to achieve large-scale industrialization. Nevertheless, combustion can act as another oxidation technique to completely decompose the organics into CO2, CO, NOx, SOx, etc. and release heat for realizing resource utilization (waste-to-energy),15-17 which can be expected for large-scale industrialization. However, fuel gas from the traditional combustion technique causes secondary pollution, and the further treatment of the flue gas leads to additional cost. Chemical looping combustion (CLC)18-20 uses mostly the oxide of Fe, Ni, Cu, Co, Mn et al.21 as oxygen carrier (OC) to transport oxygen to oxidize fuel into mainly CO2 and H2O, avoiding generation of thermal NOx and the mixing of N2 in flue gas. In the CLC system, fuel (CxHyOz) is oxidized by OC into CO2 and H2O in the fuel reactor (FR), and synchronously the OC is reduced.


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Energy & Fuels

The reduced OC is transported to the air reactor (AR), where it is oxidized by O2 from the input air. Then the oxidized OC will be transferred to the FR for the next reaction cycle. CLC can be performed for different kinds of fuels including gas, liquid, and solid fuel. In comparison of the relatively simple gas CLC process22 and liquid fuel CLC process23, the solid fuel CLC, especially biomass and coal CLC, undergo pyrolysis, volatile oxidation, semi-coke gasification, and oxidation reactions.24 Solid fuel CLC suffers from slow gasification rate of char, and the insufficient conversion of fixed carbon due to lack of oxidation agent.25 In addition, it should be noted that the ash in the solid fuel plays a two-side function for the CLC. On the one hand, the ash in the solid fuel can deposit on the surface of OC particles decreasing the specific surface area hence hindering adsorption and reaction,26 On the other hand, the elements Na, K, and Ca in ash can activate OC to improve its reactivity for chemical looping reactions, which has been confirmed previously. 27-28 Traditionally, CO2 and H2O can act as oxidation agent for gasification of coal and biomass. Therefore, the product of organic waste solution CLC together with the large amount of water vapor can act as the oxidation agent to theoretically drive the gasification of biomass and coal during CLC processes. However, the characterization and possibility of solid CLC driven by organic waste solution remain unknown. In this work, ter-butanol solution and corn stalk were selected as the probe organic waste solution and solid fuel to perform CLC experiments. The products population, carbon conversion, and reaction stoichiometry were analyzed to reveal the characterization of corn stalk CLC driven by ter-butanol solution. The detailed thermodynamics analysis of the reduction process and the oxidation process of the two-stage CLC reaction system further verify the possibility of self-heat balance. Results show that multi-stage liquid-solid CLC processes cannot only treat the organic


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pollutant but promote solid fuel CLC, which throws light on harmless treatment of organic waste solution and novel applications of CLC technology. 2. MATERIALS AND METHODS 2.1 Materials. The iron-based OC Fe2O3/Al2O3 was prepared by supported Fe2O3 (60wt.%) on Al2O3 (40wt.%) using the impregnation method.29 The detailed process begun with weighing an appropriate amount of Fe(NO3)3·9H2O, which was poured into ionic water in a beaker with stirring to obtain the saturated Fe(NO3)3 solution. Then Al2O3 powder was added to the saturated solution. The mixture was continuously stirred. After turned into a paste, the mixture was dried at 70 ° C for 6 hours. The dried mixture was then transferred to a combustion boat, which was placed in a muffle furnace and calcined at 850 ° C for 1.5 hours. After the muffle furnace is cooled down, the calcined material ground into powder with diameter of 60~100 meshes. Then the prepared Fe2O3/Al2O3 was obtained. The phase composition and structure of Fe2O3/Al2O3 were characterized by X-Ray diffraction (XRD) and scanning electron microscopy (SEM). 2.2 Fixed-bed Setup and Procedure. A complete CLC system includes a FR and an AR to perform oxidation of fuel and oxidation of the reduced OC, respectively. However, for many research works concerning CLC, a single fixedbed was usually set up to perform the oxidation of fuel and oxidation of the reduced OC indirectly under switch between oxidizing gas and carrier gas. As comparison, a two-stage fixed-bed reaction system (Ⅱ-S) and a three-stage fixed-bed reaction system (Ⅲ-S) were developed for performing the CLC experiments. The schematic arrangement of Ⅲ-S is shown in Figure 1. The main reactor is a quartz tube of 900 mm length and 18 mm inner diameter. In the middle part of the reactor, there are three reaction stages include ter-butanol CLC, corn stalk gasification, and the syngas


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CLC, denoted as ①, ②, and ③, respectively. However, Ⅱ-S is similar to Ⅲ-S with two reaction stages ter-butanol CLC and in situ gasification chemical-looping combustion (iG-CLC) of corn stalk corn set in the middle part of the reactor denoted as ① and ②, respectively. For Ⅱ-S, ② refers to the mixing of the corn stalk and OC for an iG-CLC process.

Figure 1 The schematic arrangement of Ⅲ-S.

To perform CLC experiments, corn stalk and OC were placed on quartz wool fixed on a porous dam-board in the middle part of the main reactor. Then N2 was blown into the reactor with continuous heating. After the temperature (T) in the reactor reach 850 ºC, the reduction reaction process was conducted, for which 2.4% ter-butanol solution, prepared by dissolving ter-butanol in deionized water, was continuously injected into the reactor for 30 min (tin) from the top side by an injection pump (Longer pump LSP 01-1A) under input of N2 at a rate of 160 mL/min (𝑟𝑁2(𝑖𝑛)). The product gases were tested by the gas chromatograph (Agilent Micro GC 490). During the reduction


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reaction process, Fe2O3/Al2O3 was reduced. After the reduction process finished, the reduced Fe2O3/Al2O3 was completely oxidized by air at the rate of 650 mL/min (𝑟𝑎𝑖𝑟) for 30 min for characterizing its regeneration properties. The detailed experimental conditions are listed in Table 1. Herein, 2.4% ter-butanol solution is continuously injected into the reactor while corn stalk is fixed in the reactor, because we not only study the CLC characteristics of ter-butanol wastewater, but also study the driving characteristics of its products in corn stalk CLC. For future industrial applications, a realistic fluid bed reaction system should be developed for such mixed fuel CLC. Table 1 Experimental conditions. Species

Operation parameters

T

850 ºC 3.43 g

Fe2O3/Al2O3 for ① of Ⅱ-S and Ⅲ-S Fe2O3/Al2O3 for ② of Ⅱ-S and for ③ of Ⅲ-S

3.111 g, 4.356 g, 4.978 g, 5.6 g, 6.222g

corn stalk 2.4% ter-butanol wastewater

0.3 g 5mL 60 mL/min 0.167 mL/min 30 min

𝑟𝑁2(𝑖𝑛) Flow rate of 2.4% ter-butanol wastewater tin

𝑟𝑎𝑖𝑟

650 mL/min

2.3 Data Evaluation. Proximate and ultimate analysis of corn stalk are listed in Table 2. Table 2 The proximate and ultimate analysis of corn stalk. Monitoring project Fixed carbon Volatiles Moisture Ash Total carbon Total hydrogen Total oxygen Total sulfur

Notation Far Var Mar Aar Car Har Oar St,ar

Units wt.% wt.% wt.% wt.% wt.% wt.% wt.% wt.%

Corn stralk 12.94 67.42 10.29 9.35 41.14 5.94 32.54 0.17


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Total nitrogen Nar wt.% 0.58 According to Car+Har+Oar+Nar+Sar+Aar+Mar=100%, the oxygen content of corn stalk can be obtained. Then, formula of the corn stalk can be written as C1.686H2.919O.

According to Table 2, the following parameters are calculated. The dry syngas generated from CLC combustion are mainly H2, CH4, CO, and CO2. The total concentration for the i gas species (corresponding to H2, CH4, CO, and CO2, respectively) in the dry basic syngas, Ci, was calculated using the following equation: t

𝐶i =

∫0v ∙ yidt t

∫0vdt

(1)

where yi is the volume ratio of the i species in dry syngas, t the total CLC reaction time, and v the total volume flow rate for the outlet dry syngas. v can be obtained by the following equation: 𝐹in

𝑣 = 𝑦N2

(2)

where Fin and 𝑦N2correspond to the volume flow rate of the inlet N2 and the volume fraction of N2 in dry syngas, respectively. The total volume for the generated i species, Vi, was evaluated by the integral equation: 𝑡

𝑉𝑖 = ∫0𝑣 ∙ 𝑦𝑖𝑑𝑡

(3)

The carbon dioxide capture rate, ηc, is defined as the ratio of the generated carbon dioxide to the theoretical amount of carbon dioxide according to the total moles of carbon (𝑛c) of ter-butanol and corn stalk. Based on 𝑛c and the ideal gas molar volume (Vm = 24.45 L/mol), ηc was approximately quantitatively estimated using the formula:


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𝑡

𝜂c =

∫0𝑣 ∙ 𝑦𝑐𝑜2𝑑𝑡 𝑉m × 𝑛𝑐

(4)

The carbon conversion (Xc) is defined as the ratio of carbon amount in the generated syngas to to total carbon amount in the fuel, which is calculated using formula as follows: 𝑡

𝑋c =

∫0𝑣 ∙ (𝑦CH4 + 𝑦CO + 𝑦CO2)𝑑𝑡 24.45 × 𝑛𝑐

(5)

The gas yield, Gv, is defined as the gas production rate per unit mass of corn stalk. According to nitrogen balance between inlet gas and outlet gas of the CLC reaction system, Gv can be calculated by the following formula: 𝑡

𝐺v =

∫0𝑣 ∙ (𝑦H2 + 𝑦𝐶𝐻4 + 𝑦𝐶𝑂 + 𝑦𝐶𝑂2)𝑑𝑡 𝑚𝑠𝑡𝑎𝑙𝑘

(6)

where mstalk is the total mass of corn stalk used in the CLC reaction system. 3. RESULTS AND DISCUSSION 3.1 Ter-butanol Solution CLC at the First Reaction Stage. For the first reaction stage ①, high concentration of CO2 and H2O from ter-butanol CLC would benefit the gasification of biomass tar at the second reaction stage ②, since CO2 and H2O would act as efficient oxidation agent.30 Therefore, ter-butanol CLC experiments were performed under different peroxidation coefficients Ω, which is defined a proportional coefficient of the amount of OC used in reaction to the theoretically calculated amount required for complete oxidation of fuel.31 Herein, CLC experiments were performed for the given amount of ter-butanol under three Ω (1.6, 2.0, and 2.4) by changing the amount of Fe2O3/Al2O3. Figure 2 depicts the gas yield of terbutanol CLC under various  at 850℃. According to the ter-butanol CLC results, rather small concentration of CO and CH4 could be detected, while the highest amount of CO2 and relatively


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Energy & Fuels

low H2/CO2 can be realized at the Ω of 2.0. The results may relate to that the CLC processes involve an equilibrium between hydrogen production and hydrogen combustion, relating to the reactions as, C4H10O + Fe2O3  CO2 + H2O + Fe2O3-x 3FeO + H2O→ Fe3O4 + H2 3Fe + 4H2O→ Fe3O4 + 4H2

(R1) (R2) (R3)

CLC differs from the traditional combustion using O2 as the oxidation agent, partial reduction usually happens to Fe2O3, and the oxidation state decreases from surface to bulk.32 The catalytic gasification of ter-butanol by the reduced Fe2O3, together with reaction between the reduced Fe2O3 and water in the ter-butanol solution, favors the generation of hydrogen.33 Therefore, high amount of generated hydrogen can be observed under Ω = 1.6. However, when Ω reaches 2.4, the reduced Fe2O3 would maintain relatively high oxidation state. Less oxidation of the reduced Fe2O3 by water happened, while partial generated hydrogen was oxidized by OC of high oxidation state, leading to relatively lower hydrogen amount under Ω = 2.4 than under Ω = 1.6 and Ω = 2.0. Moreover, the calculated Xc for the case of 2.0 is 98.6%, while 83.2% and 77% for the cases of 1.6 and 2.4, respectively. The low Xc under Ω = 1.6 may relate to the insufficient combustion of ter-butanol. However, the low Xc under Ω = 2.4, may result from low catalytic activity of the reduced OC at high oxidation state (seen Figure S2 in the supporting information), leading to incomplete pyrolysis of ter-butanol. According to Figure 2, 2.0 could be considered as the relatively optimized

 for ter-butanol CLC at the first reaction stage ①.


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Energy & Fuels

0.5 CO2 H2

0.4 gas yield (L/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.3 0.2 0.1 0.0

1.6

2.0 peroxidation coefficient

2.4

Figure 2. Gas yield of ter-butanol CLC under various .

3.2 Corn Stalk CLC Driven by Ter-butanol in the Ⅲ-S. Based on the results of the above experiment, ter-butanol driving corn stalk CLC in the Ⅲ-S was performed at 850℃. In the Ⅲ-S, except for the first ter-butanol CLC process (R1 and R2), the second stage relates to products of the first stage driving corn stalk gasification, and the third stage is the combustion of gas products generating from the second stage. Following the first ter-butanol CLC process (R1 and R2), CO2 as well as the generated H2O and the H2O in the ter-butanol solution, can then act as gasification agent to drive gasification of corn stalk at the second stage, mainly corresponds to the reactions R4 and R5: CO2 + C (biomass char)  2CO

(R4)

H2O + C (biomass char)  CO + H2

(R5)


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Further, the combustible gases are then combusted at the third stage, mainly the following reactions R6-R10: CO + 3Fe2O3  CO2 + 2Fe3O4

(R6)

CO + Fe3O4  CO2 + 3FeO

(R7)

CO + FeO  CO2 + Fe

(R8)

H2 + 3Fe2O3  H2O + 2Fe3O4

(R9)

H2 + Fe3O4  H2O +3FeO

(R10)

Flue composition profiles during the Ⅲ-S under different  of 1.0, 1.4, 1.6, 1.8, and 2.0 at 850℃, are collected in Figure 3. As can be observed in Figure 3, the concentration of CO2 increases with the increase of Ω, since more lattice oxygen provided at higher Ω such as 1.6, 1.8 and 2.0 favors carbon oxidation into CO2 while lower Ω (1.0 and 1.4) leads to more CO and CH4. Ω plays a key role for corn stalk CLC in the Ⅲ-S. According to Figure 3, Ω > 1.6 shows relatively low concentration of CO and CH4, as well as the high concentration of CO2, which implies more complete oxidation under high Ω. However, Xc related to corn stalk CLC driven by ter-butanol driving in the Ⅲ-S under various

 are further discussed, as shown in Figure 3 (f). It can be observed from Figure 3 (f), the unconverted carbon of corn stalk decreases with the increased of . At the same time, when the third stage  ranged from 1.0 to 2.0, Xc increased from 79.05% to 96.35%, with the extremum of 97.43% at  = 1.8. According to Figure 3 (f), the calculated Xc for  = 1.0, 1.4, 1.6, 1.8, 2.0, is 79.1%, 80.2%, 85.2%, 97.4%, and 96.4%, respectively. While compared to the case of  = 1.8, the relatively high  at 2.0 may relate to slightly higher oxidation state of the reduced OC, and hence exhibiting a bit lower catalytic activity for corn stalk gasification process, which shows a


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Energy & Fuels

certain similarity to the relation between the activity and oxidation state for catalytic ter-butanol gasification system as shown in the supporting information. Therefore,  = 1.8 shows a little higher Xc than  = 2.0. Based on Figure 4 and Figure 5, we take 1.8 as the relatively optimal  for the third reaction stage ③ in the Ⅲ-S.

24 22 20 18 16 14 12 10 8 6 4 2 0

(b) H2 CH4

Flue gas concentration (%)


(a)

CO CO2

0

20

40

60

80

100 120 140 160 180

24 22 20 18 16 14 12 10 8 6 4 2 0

Time (min)

24 22 20 18 16 14 12 10 8 6 4 2 0

CO CO2

0

20

40

60 80 100 Time (min)

120

H2 CH4 CO CO2

0

20

40

60

80 100 120 140 160 180 200 Time (min)

H2 CH4


CH4

140

160

(f) H2 CH4

Unconverted carbon (g)

(e)

(d) 24 22 20 18 16 14 12 10 8 6 4 2 0

H2


(c) 24 22 20 18 16 14 12 10 8 6 4 2 0


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CO CO2

CO CO2

0

20

40

60 80 100 Time (min)

0.12

120

140

160

1.0 1.4 1.6 1.8 2.0

0.10 0.08 0.06 0.04 0.02 0.00

0

20

40

60

80 100 Time (min)

120

140

160

0

20

40

60 Time (min)

80

100

120

Figure 3. Flue composition profiles for corn stalk CLC driven by ter-butanol the Ⅲ-S under various  of a) 1.0,


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b 1.4, c) 1.6, d) 1.8, and e) 2.0 at 850 ºC. And f) the unconverted carbon with time.

3.3 Corn Stalk CLC Driven by Ter-butanol in the Ⅱ-S. As a comparison to Ⅲ-S, experiments were further performed the Ⅱ-S. ① of Ⅱ-S and ① of Ⅲ-S have the same reaction corresponding to ter-butanol CLC process, ② of Ⅱ-S related to the iG-CLC process for the mixture of corn stalk and Fe2O3. Figure 4 (a) illustrates the total Xc for Ⅱ-S under different , where Xc for Ⅲ-S are also given as a comparison. For Ⅱ-S, the calculated Xc under  = 1.6, 1.8, and 2.0, is 90.6%, 99.5 %, and 97.7%, respectively. The  = 1.8 favors the CLC reactions with the highest Xc. Comparing to the two other cases of 1.6 and 2.0,  = 1.8 can almost completely convert corn stalk during CLC process. Flue composition profiles for the  =1.8 case in Ⅱ-S is collected in Figure 4 (b), high concentration of H2 and CO2 is realized. However, for Ⅲ-S, the calculated Xc under  = 1.6, 1.8, and 2.0, is 85.23%, 97.43%, and 96.35%, respectively. Comparing the detailed Xc for Ⅱ-S and Ⅲ-S, we observed that Xc for Ⅱ-S is higher than that Xc for Ⅱ-S and Ⅲ-S under each . The enhancement of Xc for Ⅱ-S may result from that OC-catalyzed gasification of corn stalk, since OC usually acts as an efficient catalyst during steam reforming reactions.34-36 In addition, elements (such as Ca and Al) in corn stalk would alter the properties of the OC at its different oxidation state to avoid bed agglomeration and deposit formation promoting combustion.26,37, 38


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Energy & Fuels

(a)

(b)

99.5% 97.4%

96.5%

0.95 90.6% 0.90

0.85

0.80

85.2%

1.6

25

97.7%

||-S |||-S

1.8 Peroxidation coefficiency

2.0


1.00

Xc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H2

20

CH4 CO CO2

15 10 5 0

0

20

40 60 Time (min)

80

Figure 4. a) Comparison of total Xc for Ⅱ-S and Ⅲ-S under different  and T = 850 ºC. b) Flue composition profiles for the  =1.8 case in Ⅱ-S.

For both Ⅱ-S and Ⅲ-S, 1.8 can be hailed the relatively optimized  for the CLC reactions, which is only a quantitatively conclusion. Actually, the detailed reaction mechanism for the CLC processes of corn stalk and ter-butanol solution would be rather complex. The ① reaction mechanisms of the Ⅱ-S are the same to those of the Ⅲ-S mentioned above. According to iGCLC,24 the reaction mechanisms for ② of the Ⅱ-S would correspond to the following reaction formulas, Corn stalk  Volatile matter + Cchar

(R11)

H2O + Cchar  H2 + CO + ash

(R12)

Cchar +CO2  CO + ash

(R13)

Fe2O3 + Volatile matter + CO + H2  Fe2O3-x +CO2 + H2O

(R14)

3.4 Effect of Ter-butanol Solution on Corn Stalk CLC in Ⅱ-S and Ⅲ-S.


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To confirm this promoting effect of ter-butanol solution for corn stalk CLC in Ⅱ-S and Ⅲ-S, experiments with and without the ter-butanol CLC reaction stage ① in the Ⅱ-S and Ⅲ-S were compared in Figure 5. As can be observe in Figure 5, for both Ⅱ-S and Ⅲ-S, the reaction stage ① can obviously promote the corn stalk CLC to generate more H2, CH4, CO, and CO2, leading to higher Xc and ηc, while compared to the cases without turt-butanol solution CLC stage ① . According to Figure 5, an increase of 40.6% and 47.6% of CO2, 86.6% and 88.3% of CH4, together with a slightly increase of CO, were observed for Ⅲ-S (the lower) and Ⅱ-S (the upper) respectively, when ter-butanol solution was present. However, in our tert-butanol waste solution driving corn stalk CLC system, the absolute amount of corn stalk and tert-butanol is calculated to be 1.03 × 10-2 mol and 1.61 × 10-3 mol. The two quantities differ almost an order of magnitude. The noticeable increase of carbon-containing molecule (especially CO2 and CH4) mainly derive from corn talk. Therefore, it can be concluded that without the presence of ter-butanol solution, great part of fixed carbon in corn stalk hasn’t been converted. Therefore, it can be concluded that CLC of ter-butanol solution can directly promote the gasification of corn stalk and indirectly affect the oxidation of fuel gas during CLC process through tuning the oxidation state of the reduced OC. The results with the absence of ter-butanol CLC reaction stage ① also suggest that the mixture of corn stalk and Fe2O3/Al2O3 for a iG-CLC process in Ⅱ-S promotes CLC reaction favoring the generation of CH4, CO and CO2 showing higher concentration and higher Xc than those for the Ⅲ-S with the separation of gasification process ② and oxidation process ③. Two major reasons would contribute to these results. Firstly, as the above conclusion goes, the presence of calcium/aluminum can enhance the restraint to agglomeration of OC active component and deposit formation.34 Secondly, mixture of OC and corn stalk increases the contact area between OC and


15

Energy & Fuels

corn stalk, which promotes the in situ gasification of corn talk and the direct oxidation of the gasification gas molecules.34-36 10 8 with ① without ①

6

Total flue concentration (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 29

4 2 0 10

H2

8

CH4

CO

CO2

CO

CO2

with ① without ①

6 4 2 0

H2

CH4

Figure 5 Effect of ter-butanol solution on corn stalk CLC in Ⅱ-S (the upper) and Ⅲ-S (the lower) under  =1.8 and T = 850 ºC.

3.5 Analysis of Reaction Stoichiometry for the Ⅱ-S. Grading reaction system and variables alter the rate of Xc and related reactions. According to the population of carbon compounds with time during CLC of corn stalk driven by ter-butanol, unconverted carbon in every CLG process can be calculated as: Cr = Cbiomass − (Ctotal − Cbutanol)

(6)

where, Cr is the unreacted carbon in corn stalk, Cbiomass is the total carbon content in corn stalk, Ctotal is the total carbon amount for the whole CLC reaction system, and Cbutanol is the carbon amount from CLC of ter-butanol.


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Energy & Fuels

Under the influence of OC, the change rate of carbon content in corn stalk with time, rc, can be further discussed with the following calculations: 𝑑𝐶𝑟

𝑟𝑐 = 𝑚𝑑𝑡

(7)

where Cr is unreacted carbon in corn stalk, t the time of CLC reaction, and m the content of Fe2O3/Al2O3 used in the reaction system. 𝑟𝑐 with time (t) for the Ⅲ-S and Ⅱ-S under different  are compared in Figure 6 a and b, respectively. For Ⅲ-S, with the CLC reaction start, 𝑟𝑐 increases quickly with t, then decreases, which would corresponds to oxidation of ter-butanol into CO2 and H2O at the first stage ①, CO2 and H2O driving corn stalk gasification into mainly CO, H2, CH4, and CO2 at the second stage ② , then the oxidation of the fuel gas (CO, H2, and CH4) by OC at the third stage ③, for which the related reaction mechanisms R1 to R 6 are given above. Generally, for II-S, 𝑟𝑐 increases and then decreases, and the peak value appears when the reaction time is about 40 min, which is similar with the case for Ⅲ-S. However, after 40 min, 𝑟𝑐 curves for Ⅲ-S show longer tail than those for II-S, suggesting that 𝑟𝑐 for II-S is higher than for Ⅲ-S under various  cases shown in Figure 6. Gradation reaction based on II-S favors the CLC processes of corn stalk driven by ter-butanol solution.


17

-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

r (min )

Energy & Fuels

10 8 6 4 2 0 10 8 6 4 2 0 10 8 6 4 2 0

Page 18 of 29

  

|||-S ||-S

  

|||-S ||-S

|||-S ||-S

  

0

10

20

30

40

50

60

70

80

90 100 110 120

t (min)

Figure 6. Comparison of rcarbon for a) the III-S and b) the II-S under different 

3.6 Thermodynamics Analysis for II-S. 3.6.1 Ter-butanol CLC Process ① As discussed above,  around 1.8 can realize relatively high Xc of 99.5 % with high 𝑟𝑐. Therefore, thermodynamics analysis was performed for the II-S under the case of  = 1.8. For the first reaction step ① of the II-S, the content of the products could be obtained by integrating the distribution of the reaction products with time during ter-butanol CLC process. The volume content of CO2, CH4, and CO was 12%, 2.5%, and 0.13%, respectively. Because the CO content is small, we would ignore the CO term in the combustion reaction equation. After balancing the equation, the H2 term is not appeared, since ter-butanol CLC lead to the generation of H2O and CO2, while the generation of H2 is from the reaction between H2O and the reduced iron oxide. The CLC reaction equation could be present as follows, 28Fe2O3+3(3-y)C(CH3)3OH  10(3-y)CO2(g) + 2(3-y)CH4(g) + 28Fe2Oy +11(3-y) H2O

(R14)


18

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Energy & Fuels

With CLC between ter-butanol and Fe2O3, the iron valence state changes with time, and the enthalpy of OC is involved in the changing valence state. According to the generated CO2 and CH4 with time, electron transfer from carbon atoms could be calculated, hence charge populating on Fe in the Fe2O3 could be evaluated. Electron transferred from C element and Fe element, 𝑛(𝑒𝑐+ ) and + 𝑛 (𝑒𝐹𝑒 ) were calculated as:

𝑛(𝑒𝑐+ ) =

𝑛

V(𝐶𝑂2)

V(𝐶𝐻4)

𝑉𝑚

𝑉𝑚

+ (𝑒𝐹𝑒 )

=

𝑛1 +

(8)

𝑛2

𝑛(𝑒𝐶+ )

(9)

𝑛(𝐹𝑒)

where 𝑛1 denotes the mole conversion of ter-butanol into CO2, 𝑛2 ter-butanol into CH4. 𝑛(𝐹𝑒) is the total mole amount of Fe atom in the used Fe2O3. Then, a fitted function can be obtained to reveal the relation between valence of Fe in Fe2Oy and reaction time (t) as follows: y = 3 + 0.0088 t - 0.0023 t2 + 0.0002 t3

(10)

According to equation (9), the calculated final average valence of Fe is 1.93, which is a little lower than the oxidation state of FeO. To perform thermodynamics analysis, more related physical parameters

39

are needed, which

are listed in Table 3 where ∆𝑓H𝜃𝑚 denotes standard molar enthalpy and C𝑚,𝑝 denotes the molar specific heat capacity. In Table 3, what needs special explanation is that the 𝐶𝑚,𝑝 of FeO and Fe2O3 is fitting by 𝐶𝑚,𝑝 data at different temperature from the thermodynamic database of Aspen Plus V8.2 with property method of solids. And according to 𝐶𝑚,𝑝 of Fe, FeO, Fe3O4, and Fe2O3, 𝐶𝑚,𝑝 of non-stoichiometric iron oxide intermediates can be obtained. Table 3 Related physical parameters


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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Physical parameters ∆𝑓H𝜃𝑚(Fe, 298 K) ∆𝑓H𝜃𝑚(Fe3O4, 298 K) ∆𝑓H𝜃𝑚(FeO, 298 K) ∆𝑓H𝜃𝑚(Fe2O3, 298 K) ∆𝑓H𝜃𝑚(C2H6O, g, 298 K) ∆𝑓H𝜃𝑚(CO2, 298 K) ∆𝑓H𝜃𝑚(CO, 298 K) ∆𝑓H𝜃𝑚(CH4, 298 K) ∆𝑓H𝜃𝑚(H2O, g, 298 K) C𝑚,𝑝(FeO, 298 K-1173 K) C𝑚,𝑝(Fe, 298 K-1173 K) C𝑚,𝑝(Fe2O3, 298 K-1173 K) C𝑚,𝑝(C4H9OH, 298 K-1500 K) C𝑚,𝑝(CO2, 298 K-1500 K) C𝑚,𝑝(CO, 298 K-1500 K) C𝑚,𝑝(CH4, 298 K-1500 K) C𝑚,𝑝 (H2O, 298 K-1500 K) C𝑚,𝑝(O2, 298 K-1500 K) C𝑚,𝑝(C1.686H2.919O)

Unit kJ/mol kJ/mol kJ/mol kJ/mol kJ/mol kJ/mol kJ/mol kJ/mol kJ/mol J/(mol·K) J/(kg·K) J/(mol·K) J/(mol·K) J/(mol·K) J/(mol·K) J/(mol·K) J/(mol·K) J/(mol·K) J/(mol·K)

Page 20 of 29

Value 0 -1118.4 -272 -824.2 -235.1 -393.509 -110.525 -74.81 -241.818 Cp = -5×10-6T2 + 0.0195T + 44.711 Cp = 0.45×103 -7 3 Cp=3×10 T -0.0006T2+0.4689T+6.8279 Cp=0.145T+104.12 Cp=26.75+42.258×10-3×T-14.25×10-6×T2 Cp=26.537+7.6831×10-3×T-1.172×10-6×T2 Cp=14.15+75.496×10-3×T-17.99×10-6×T2 Cp=30+10.7×10-3×T-2.022×10-6×T2 Cp=36.16+0.845×10-3×T-0.7494×10-6×T2 63.43

With these physical parameters in Table 3, the reaction enthalpy for R(14), ∆𝑟𝐻𝑚(850℃), was calculated to be 706.06 kJ/mol, implying that it is an endothermic process. Since the total amount of ter-butanol is 0.0016 mol, the total heat necessary for ter-butanol CLC process is 1.14 kJ. Except for the oxidation of ter-butanol during CLC process, water in the solution was also heated from room temperature to 1123K, the change of enthalpy needs to be considered. The change of enthalpy was calculated to be 19.89 kJ/(0.27 mol water) in the solution, which is far higher than the reaction enthalpy for the oxidation of ter-butanol. Therefore, the total heat for oxidation of terbutanol and heat adsorption by water in the solution is 21.03 kJ. 3.6.2 Corn Stalk CLC Process ② For given amount of Fe2O3 and corn stalk C1.686H2.919O, the reaction equation can be obtained by integrating the volume of product gas with t to calculate the related molar quantity, which is shown as


20

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Energy & Fuels

4.54  1.686 Fe2O3(s) + 2(3-y) C1.686H2.919O(s) = 3.372  (3-y) CO2(g) + 4.54  1.686 Fe2Oy(s) + 5.838(3-y) H2O(g)

(R15)

CLC of C1.686H2.919O leads to the generation of CH4, CO and CO2, the molar quantity of which was obtained by integrating their total volume with t. In addition, valuation analysis can directly show that when one C atom in C1.686H2.919O converts into CO or CO2, C atom lost 4e or 6e, respectively. And C atom obtains 2e while converts into CH4. Besides, since H2 generated from ter-butanol pyrolysis has no relationship with the change of valence state of OC, hydrogen elements for generation of H2 can only be assigned to H2O or C1.686H2.919O. According to the ratio of the reaction components at different t, the fitted function between the valence charge of Fe in Fe2O3 (y) and t is as: y = 3 + 0.0130t - 0.0031t2 + 0.0003t3

(11)

According to equation (12), the calculated y is 1.24 lower than that (1.93) during the first terbutanol CLC step, and lower than the oxidation state of FeO. Therefore, the reaction energy of equation (6) at 850℃, ∆𝑟𝐻𝑚(850℃), was calculated to be 1185.956 KJ/mol according to Kirchhoff theory. Since the total amount of ert-butanol is 0.0016 mol, the total heat needed for this step is 7.23 KJ. 3.6.3 Oxidation of the Reduced Fe2O3 Herein, further discussed reaction enthalpy for oxidation reaction of the reduced Fe2O3 that comes from both the ter-butanol CLC process and the corn stalk CLC process. The reaction equation is as 5.736Fe2Oy(s)+5.736(1.5-0.5y)O2(g)  5.736Fe2O3(s)

(R16)


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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

where y relates to the 0.013 mol Fe2O1.89 from the ter-butanol solution CLC step, and 0.021 mol Fe2O1.24 from the corn stalk CLC step. The standard molar enthalpy of oxygen is zero, so the number of measurements does not influent the data. Using the parameters given above in Table 3, the total reaction energy ∆𝑟𝐻𝑚(1123K) was calculated to be -2288.954 KJ/mol. Based on the amount of iron oxide used during the two CLC processes, the total calculated heat released from the oxidation stage is -38.44 KJ. The enthalpy change for every reaction stage of the complete chemical looping process is listed in Table 4. Accordingly, total heat needed for the ter-butanol CLC stage includes the enthalpy change for oxidation of ter-butanol and water heating, which is calculated to be 21.03 kJ. Total heat needed for the corn stalk CLC stage is 7.23 KJ. However, for a complete CLC course, it refers not only to fuel oxidation process in FR, but the reduced Fe2O3 oxidation process in AR. Herein, oxidation of the reduced Fe2O3 releases a total heat of 38.44 KJ. Total energy from the complete CLC process (including the oxidation of fuel and the oxidation of the reduced Fe2O3) could be calculated as 38.44 − 7.23 − 21.03 = 10.18 kJ. The result shows that reaction enthalpy of oxidation stage in AR can meet the energy needing for the reduction reaction of fuel in FR, implying the feasibility of corn stalk CLC driven by ter-butanol solution. Then the hot flue gas from the fuel reactor and air reactor could be utilized to for power generation or heating. However, operational experience in iG-CLC has shown that it is not possible to reach complete oxidation of the reducing small molecules, such as H2, CO, and CH4, which makes an additional oxygen polishing step necessary.40 Herein, the generated H2, CO, and CH4, can be separated from CO2 for further applications. Table 4 Total heat H(total) for each process.

ter-butanol CLC stage

H(total) (KJ) 21.03


22

Page 23 of 29

Corn stalk CLC stage Oxidation of the reduced Fe2O3 stage

7.23 -38.44

3.7. Structural Characterizations. XRD patterns and SEM images for the fresh Fe2O3/Al2O3 and the regenerated Fe2O3/Al2O3 are illustrated in Figure 7 (a) and (b), respectively. The XRD pattern shows sharp peaks corresponding to the (012), (104), (110), (113), (024), (116), (214), and (300) facets of Fe2O3 without the presence of XRD peaks for Al2O3.31,41 XRD result suggests that the crystalized Fe2O3 is well dispersed on the support Al2O3. After the prepared Fe2O3/Al2O3 was reduced by fuel molecule, the reduced Fe2O3/Al2O3 could be regenerated by oxidation using air. The XRD pattern for the regenerated Fe2O3/Al2O3 shows peaks almost the same as to those of the fresh Fe2O3/Al2O3. The surface structure of the fresh and the regenerated Fe2O3/Al2O3 is rough and porous, as shown in Figure 7 (b). EDS results show almost no carbon deposition on the reduced Fe2O3/Al2O3 related to the case of =1.8, which verifies the close of carbon balance.

b

a (214) (300)

(116)

(024)

(110) (113)

(104)

(012)

The fresh Fe2O3/Al2O3 Indensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Fresh Fe2O3/Al2O3

The regenerated Fe2O3/Al2O3

20

30

40

50

60

70

80

90

2 

Regenerated Fe2O3/Al2O3

Figure 7. XRD patterns a) and SEM images for the fresh Fe2O3/Al2O3 and the regenerated Fe2O3/Al2O3 b).


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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

4. CONCLUSIONS We demonstrate that the ter-butanol solution can enhance gasification and combustion of corn stalk in the cascaded CLC systems. Participation of ter-butanol in the III-S under  of 1.8 at 850℃ showed an increase of Xc from 79.05% to 97.43%, while to 99.5% for the II-S. The enhancement is attributed to the generation of sufficient oxidant agents (mainly H2O and CO2) from ter-butanol solution CLC, which drastically promotes the gasification of corn stalk. In addition, direct interaction between corn stalk and OC in the Ⅱ-S lead to higher rc and the final Xc than those for Ⅲ-S, which can be attributed to that OC acts as catalyst to catalyze gasification of corn stalk, and elements (such as Ca and Al) in corn stalk alter the properties of the OC to avoid agglomeration and deposit formation. Further, thermal equilibrium analysis verified the feasibility of organic solution – solid fuel CLC. Ter-butanol solution driving stalk gasification / combustion provides a new route for treating organic waste solution and energy conversion. ■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Generated fuel gas for ter-butanol pyrolysis experiments catalyzed by the reduced Fe2O3/Al2O3 at 500 ℃. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Wu Qin), [email protected] (Lu Liu) or [email protected] (Changqing Dong). ■ ACKNOWLEDGMENTS


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Energy & Fuels

The authors wish to thank the National Natural Science Foundation of China (51776071), the Fundamental Research Funds for the Central Universities (2018ZD08, 2018MS034, 2016YQ07), and the State Grid Science and Technology Program (SGJN0000ASJS1700136). ■ REFERENCES (1) Van den Berg, M.; Birnbaum, L.; Bosveld, A. T.; Brunström, B.; Cook, P.; Feeley, M.; Giesy, J. P.; Hanberg, A.; Hasegawa, R.; Kennedy, S. W.; Kubiak, T.; Larsen, J. C.; van Leeuwen, F. X.; Liem, A. K.; Nolt, C.; Peterson, R. E.; Poellinger, L.; Safe, S.; Schrenk, D.; Tillitt, D.; Tysklind, M.; Younes, M.; Waern, F.; Zacharewski, T. Environ. Health Perspect. 1998,

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