Spent Isopropanol Solution as Possible Liquid Fuel for Moving Bed

Article pubs.acs.org/EF

Spent Isopropanol Solution as Possible Liquid Fuel for Moving Bed Reactor in Chemical Looping Combustion Ping-Chin Chiu,

†

Young Ku,*,

†

Hsuan-Chih Wu,

†

Yu-Lin Kuo,‡ and Yao-Hsuan Tseng

†

†

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan

‡

ABSTRACT: Isopropanol (IPA) solution as liquid fuel for chemical looping combustion (CLC) with Fe2O3/Al2O3 oxygen carriers was investigated by a moving bed reactor. CO and H2 were the fuel gases generated by IPA solution in the moving bed reactor prior to combustion with oxygen carriers. The yields of fuel gas generated from IPA solution were determined to modify the equation of oxygen carrier-to-fuel ratio for moving bed operation. The IPA conversion and CO2 yield in the outlet stream from moving bed reactor reached 100% as oxygen carrier-to-fuel ratio was higher than 7.94 at 900 °C. Heat balance for input and output processing capacities is approximately achieved by theoretical estimation. By analysis of system processing capacity, 70% of output processing capacity is available for energy use of external facility. By analysis of processing efficiency, aqueous solution as liquid fuel would cause considerable heat consumption for water evaporation, especially for dilute solution. Chemical looping combustion is suggested to be an alternative treatment technology with positive processing capacity for spent organic solvent as heat is well recovered in liquid fuel CLC system.

1. INTRODUCTION Isopropanol (IPA) solution is extensively consumed by semiconductor and liquid crystal display (LCD) industries on cleansing wafers and panels in fabrication process.1 Concentrated spent IPA solution may be recovered for reuse; however, solution containing less than 20 vol % IPA is considered less valuable for recovery. Consequently, thermal treatment is typically applied to treat dilute IPA solution, and a huge amount of heat is consumed by evaporating great amount of water. Spent IPA solutions containing 3 to 20 vol % IPA are typically treated at more than 850 °C for complete combustion in incinerators or kilns with considerable auxiliary fuels.2−4 Chemical looping combustion (CLC) is considered to be an alternative to provide high conversion of dilute organic solutions without additional fuel. Chemical looping combustion using metal oxides (also called oxygen carriers) as chemical intermediates to provide with oxygen for fuel combustion5 is being developed recently because of inherent CO2 separation as well as power generation.6,7 A chemical looping reactor system comprises a fuel reactor and an air reactor for reduction and oxidation of oxygen carriers, respectively, as shown in Figure 1. The oxygen carrier (MexOy) is reduced by carbonaceous fuel (CnHm) to generate CO2 and H2O, as described by reaction 1 in the fuel reactor. The reduced oxygen carrier (MexOy−1) is oxidized by air to regenerate MexOy in the air reactor, as described by reaction 2, which is an exothermic reaction that provides heat to external facilities.

Figure 1. Scheme of chemical looping combustion.

The operating condition of chemical looping system is typically in the range from 800 to 1000 °C and from 1 to 5 atm, according to previous studies.8,9 Oxygen carriers applied for CLC should provide excessive oxygen carrying capacity, high reaction rate, great mechanical strength, and long-term recyclability.10 Iron-, copper-, nickel-, manganese-, and cobalt-based oxides are typical materials used as oxygen carriers for the study of CLC.11 However, sintering and attrition of these oxygen carriers during chemical looping operation are considered to be the main concerns to reduce their reactivity and recyclability. Therefore, the coupling of oxygen carriers with support materials, such as Al2O3, TiO2 and NiAl2O4, is employed for improving the life of oxygen carriers for long-term operation.12 Among these oxygen carriers, iron-

CnH 2m + (2n + m)MexOy → nCO2 + mH 2O + (2n + m)MexOy − 1

(1)

2MexOy − 1 + O2 → 2MexOy © 2013 American Chemical Society

Received: July 2, 2013 Revised: October 7, 2013 Published: October 31, 2013

(2) 657

dx.doi.org/10.1021/ef4012438 | Energy Fuels 2014, 28, 657−665

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Figure 2. Schematic diagrams of (a) fluidized bed reactor and (b) moving bed reactor.

moved from the bottom to the top of the reactor to combust with oxygen carriers. Thus, the design of moving bed reactor with counter-flow pattern of oxygen carrier, and fuel is thermodynamically feasible by using Fe2O3 as oxygen carrier to yield over 99% of CO2 in the effluent stream because high pCO2/pCO and pH2O/pH2 ratios are achieved.22,23 Moreover, higher oxygen capacity in iron-based oxygen carriers can be utilized in the moving bed system than that in the fluidized bed system, since the residence time of oxygen carriers in moving bed reactor is longer than fluidized bed reactor.24,25 The design of moving bed reactors with counter-flow pattern demonstrated superior conversion efficiency of fuel for syngas combustion as well as higher oxygen utilization of iron-based oxygen carriers; therefore, moving bed reactor is selected as fuel reactor for IPA combustion in this study. The fuels, such as natural gas, coal, petroleum coke, and biomass combusted by CLC are frequently studied by various researchers17,26−31 and compared in the previous studies;20,33 however, only few studies on liquid fuel combustion are reported.33,34 Liquid fuels may provide advantage for higher energy intensity than gas fuels to reduce storage capacity for similar processing capacity. Besides, fuel gas generated by liquid fuel evaporation is obtained more quickly than that generated by gasification of solid fuels, leading to higher fuel conversion than solid fuels because liquid fuel containing 100% of volatile matter that evaporated to be fuel gas without gasification reactant. However, evaporation of liquid fuel is necessary prior to feeding into the fuel reactor of CLC system resulted in external heat consumption. With regard to the CLC system,

based oxygen carriers are considered as potentially for practical CLC applications because of their high oxygen carrying capacity, high melting point, good mechanical strength, and relatively low cost.12,13 For CLC, oxygen carriers have to be continuous circulated between the fuel and the air reactors; moreover, the gas leakage for both reactors should be avoided. Based on the mode of oxygen carrier transportation in the fuel reactor, moving bed and fluidized bed systems are the two major reactor systems developed for the application of CLC. A fluidized bed system usually composes of a fuel reactor, an air reactor, an air compressor, loop seals, cyclones, and a cold trap, as illustrated in Figure 2a.8 The oxygen carrier particles are well-mixed with fuel by fluidizing gas for gas/solid fuel combustion in the fuel reactor. The reduced oxygen carrier particles are flown to a loop seal that oxygen carrier particles transported to the air reactor by pressurized inert gas as well as oxidation of oxygen carrier particles. The pressurized air for circulation of particles is typically operated for over 10 times of terminal velocity of oxygen carrier particle.14 The attrited particles are separated from the air reactor by a cyclone, and then, the remaining oxygen carrier particles flow back to the fuel reactor through a loop seal for next cycle. The fluidized bed chemical looping systems in different designs, such as circulating fluidized bed, dual circulating fluidized bed, and spouted fluidized bed, were operated and studied extensively.15−20 The scheme of moving bed reactor is shown in Figure 2b, the fresh or regenerated oxygen carriers, typically in the size between 1.5 and 3 mm in particle diameter, are fed into the top of the moving bed fuel reactor and are moved through the reactor.21,22 Fuels are 658


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Figure 3. Schematic diagram of the moving bed reactor system for IPA combustion. hematite (99.9%, China Steel) and alumina (99%, Chin Jung). The ratio of Fe2O3 and Al2O3 was 60/40 in weight percentage to prepare the Fe 2 O3 /Al 2 O3 particles. Fe 2 O3 and Al2 O 3 particles with approximately 1 μm in diameter were mixed with deionized water at room temperature. The well-mixed Fe2O3/Al2O3 slurry was dried at 80 °C for 6 h to form particles; the particles were subsequently pulverized and screened for sizes between 1.2 and 1.4 mm. The Fe2O3/Al2O3 particles were then sintered at 1300 °C for 2 h in a muffle furnace. The crush strength of prepared Fe2O3/Al2O3 oxygen carrier was analyzed by a texture machine (TA.XT plus) according to ASTM D4179-01. The reactivity and recyclability of Fe2O3 and Fe2O3/Al2O3 oxygen carriers were analyzed by a Netzsch STA 449F3 TGA. Oxygen carriers (200 mg) were loaded in an alumina crucible for TGA analysis, and the temperature of TGA chamber was increased with a ramping rate of 20 °C/min in N2 atmosphere and eventually kept at 900 °C. The 200 mL/min reducing gas composed of 10% H2 (99.999%), 10% CO (99.999%), and 80% N2 (99.995%) was introduced into TGA chamber for 10 min to reduce oxygen carriers. After the reduction process, 200 mL/min N2 was introduced for 5 min to sweep residual reducing gas in the TGA chamber. After purging, 200 mL/min air was then introduced for 6 min to oxidize the reduced oxygen carriers. The procedure of reduction and oxidation was replicated for 20 cycles to determine the reactivity and recyclability of oxygen carriers. The degree of reduction of oxygen carriers by thermogravimetric analysis is described by following equation:

heat generated in the air reactor is needed to transfer to the fuel reactor and evaporator to reach heat balance of the whole process. IPA solution is selected as liquid fuel for estimation of liquid fuel CLC system on treatment efficiency and heat flow analysis IPA solution due to waste solvent traditionally treated with considerable heat consumption by supplying auxiliary fuel to maintain the temperature more than 850 °C for complete combustion.2−4 However, a huge amount of heat is consumed by water evaporation resulting in low benefits and high costs for the treatment. Therefore, CLC is aimed to be an alternative technology to provide high conversion without additional fuel for IPA treatment in this study. In this study, 20% IPA solution was used as fuel for combustion with iron-based oxygen carriers in a moving bed reactor. IPA conversion and gas yields were determined with varied oxygen carrier feeding rate at 900 °C by a moving bed reactor. Heat flow analysis of CLC system for IPA combustion was employed to evaluate the heat balance of input and output processing capacities and the processing efficiency of CLC system.

2. MATERIAL AND METHODS 2.1. Preparation and Characterization of Fe 2O 3 /Al 2O 3 Particles. The oxygen carriers used in this study were composed of 659


Energy & Fuels X red =

mo − m(t ) mo − mr

Article

ηIPA =

(3)

3 × FIPA,in

× 100 (6)

where FCO2, FCO, and FCH4 are the outlet molar flow rate of CO2, CO, and CH4, respectively. The CO2 yield (YCO2) is defined as the fraction of CO2 in the outlet stream from the fuel reactor after steam condensation described by following equation:

where mo is weight of fully oxidized oxygen carrier; mr is weight of fully reduced oxygen carrier; m(t) is weight of oxygen carrier after reduction period of t. 2.2. Moving Bed Reactor System. Moving bed reactor system employed in this study is shown in Figure 3, the fuel reactor was made of a 76.20 mm inner diameter (ID) SS310 tube with a proportionalintegral-derivative (PID)-controlled heating element covering 900 mm of reactor height. Two 6.35 mm ID SS310 tubes were connected at the top and the bottom of the reactor for fuel inlet and flue gas outlet, respectively. The oxygen carrier inlet connected to the upper section of the fuel reactor was composed of a 25.40 mm ID SS310 tube and a screw conveyor. The dimension of outlet section was identical to the inlet for moving out the reduced oxygen carriers. The outlet stream from the moving bed reactor was cooled by a cold trap to condense and was afterward analyzed by a gas chromatography equipped with a thermal conductivity detector (GC-TCD, China Chromatography 2000) to detect carbon dioxide, carbon monoxide, methane, hydrogen, and oxygen. For empty bed operation, the temperature of the reactor was operated at 800, 850, and 900 °C, and 20% IPA solution, which prepared by mixing of 2-propanol (99 vol %, Fisher) and deionized water, was injected into the reactor by a syringe pump. The flow rate of IPA solution was controlled by the syringe pump from 0.44 to 4.36 mmol/min. The injected IPA solution was vaporized immediately in a chamber covered by a heat element at 150 °C, and the evaporated IPA was carried by N2 into the fuel reactor. For moving bed operation, 2.9 kg of Fe2O3/Al2O3 oxygen carriers were packed in the moving bed reactor with 220 mm of bed height. The flow rate of Fe2O3/Al2O3 oxygen carriers feed into the moving bed reactor was operated between 11.5 and 29.7 g/min. The oxygen carrier-to-fuel ratio (ϕ) is the ratio of oxygen provision by oxygen carrier and oxygen demand by fuel gas in the fuel reactor at a specific temperature, defined as Fe2O3 molar flow rate versus oxygen demand from fuel gas as described by eq 4. Therefore, ϕ should be theoretically at least greater than unity to provide excess oxygen carriers for complete combustion of fuel gas in realistic system.

ϕ=

FCO2 + FCO + FCH4

YCO2 =

FCO2 FCO2 + FCO + FCH4 + FH2

× 100 (7)

3. RESULTS AND DISCUSSION 3.1. Properties of Fe2O3/Al2O3 Oxygen Carriers. The crystalline phases of reduced oxygen carrier are Fe3O4, FeO, and Fe in sequence of degree of reduction according to the XRD patterns in our previous study.35 However, for the alumina supported hematite oxygen carrier, FeO was found to react with Al2O3 to be FeAl2O4 in the reduced oxygen carriers. In accordance with literature, Fe2O3 was reported to be reduced to FeO during the operation of counter-flow moving bed reactor.22 Hence, 1/3 oxygen in Fe2O3 are assumed to be consumed for fuel gas combustion in a moving bed reactor. Therefore, for CO, H2, and CH4 combustion with Fe2O3, the stoichiometric coefficient (b), which represented oxygen carrier demand for fuel combustion, are 1/3, 1/3, and 4/3, respectively. The crush strength of oxygen carriers ranged of 1.2 to 1.4 mm was approximately 22 N that possessed proper mechanical strength for operation in the moving reactor. Besides, the attrition rate was around 27% examined by ASTM 4058−96 for particle size ranged of 1.2 to 1.4 mm. As depicted in Figure 4,

FFe2O3 bFFuel

(4)

where FFe2O3 is the actual molar flow rate of oxygen carriers; FFuel is the molar flow rate of fuel gas which combusted with oxygen carriers; b is the stoichiometric coefficient of fuel gas combusted with Fe2O3/Al2O3 oxygen carriers. Based on literature and our previous study, Fe2O3 is reduced to FeO through moving bed operation, indicating that 1/3 mol of iron-based oxygen carriers consumed to generate 1 mol of CO2 or H2O by CO or H2, respectively.22,25,35 Moreover, 4/3 mol of ironbased oxygen carriers are consumed to generate 1 mol of CO2 and H2O by CH4. Thus, the stoichiometric coefficients of CO, H2 and CH4 are 1/3, 1/3, and 4/3 for complete combustion with Fe2O3/Al2O3 oxygen carriers by moving bed operation, respectively. For Fe2O3/ Al2O3 oxygen carriers, the molar flow rate, FFe2O3, is calculated by eq 5:

FFe2O3 =

Figure 4. Degree of reduction of Fe2O3 and Fe2O3/Al2O3 particles using syngas as reducing gas conducted by TGA for 20 redox cycles at 900 °C.

x Fe2O3ṁ OC MFe2O3

(5)

the Xred for TGA experiments with Fe2O3/Al2O3 particles using syngas as reducing gas was maintained at approximately 40% for 20 successive redox cycles. However, the Xred of Fe2O3 was decayed rapidly after second redox cycle and was maintained at 5% after 10 cycles, indicating that the reactivity of Fe2O3 was significantly improved with Al2O3 support. Avoidance of agglomeration of Fe2O3/Al2O3 particles was leading to better performance on reactivity and recyclability than Fe2O3 reported in our previous study.36 For Fe2O3/Al2O3 particles, formation of FeAl2O4 by reduced FeO and Al2O3 was characterized by X-

where ṁ OC is mass flow rate of Fe2O3/Al2O3 oxygen carriers; xFe2O3 is the fraction of Fe2O3 contained in the oxygen carrier; MFe2O3 is 159.69 g/mol as molecular weight of Fe2O3. The ratio of Fe2O3 and Al2O3 was 60/40 in weight percentage for the prepared Fe2O3/Al2O3 particles; thus, xFe2O3 is 0.6. The IPA conversion (ηIPA) is the conversion of inlet IPA to carbonaceous gases, such as CO, CO2, and CH4 in the fuel reactor, and each IPA molecule is decomposed to three carbonaceous compounds after conversion. The calculation of fuel conversion (ηIPA) for IPA is described as 660


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outlet stream possibly because methane reforming reaction is the major reaction to generated fuel gases for experiments conducted above 800 °C. Therefore, syngas would be the fuel gas generated by IPA solution to combust with oxygen carriers. The evaporated IPA was decomposed to CO2, CO, and H2, and their yields in the outlet stream were linearly increased with increasing IPA flow rate in the empty bed reactor at 900 °C as illustrated in Figure 6. The yields of CO, H2, and CO2 in the

ray diffraction (XRD), and FeAl2O4 possessed higher melting point than FeO to mitigate agglomeration of particles at 900 °C.36,37 Fe2O3/Al2O3 particles that were examined by crush strength and TGA demonstrated proper crush strength and reasonable reactivity, indicating Fe2O3/Al2O3 particles were preliminary validated as oxygen carrier for moving bed operation. 3.2. IPA Decomposition in Empty Bed Reactor. It is preferred that liquid fuels are evaporated and possibly decomposed before they are combusted by CLC. Effects of reaction temperature and feeding flow rate on the fuel gas composition after IPA decomposition were investigated in an empty bed reactor. The evaporated IPA may be decomposed to form CH4 and CO, as described by reaction 8 in the moving bed reactor above 800 °C. CH4 and CO can then react with steam to carry out methane reforming and water−gas shift reactions, as described by reactions 9 and 10, respectively. IPA decomposition: C3H 7OH → 2CH4 + CO ΔHID,900 = 4.40 kJ/mol

(8)

methane reforming reaction: CH4 + H 2O ↔ CO + 3H 2 ΔHMR,900 = 226.22 kJ/mol

(9)

water−gas shift reaction: CO + H 2O ↔ CO2 + H 2 ΔHWGS,900 = − 33.13 kJ/mol

(10) Figure 6. Empty bed experiments for IPA decomposition with the flow rate from 0.44 to 4.36 mmol/min at 900 °C.

The gas components in the outlet stream from empty bed reactor were composed of CO, H2, and CO2, as illustrated in Figure 5. The gas composition in the outlet stream was

outlet stream are denoted by YCO, YH2, and YCO2. The yields of CO, H2, and CO2 were determined by linear regression fitting regarding to their molar flow rates versus IPA molar flow rate described by eqs 11, 12, and 13, respectively. Thus, YCO, YH2, and YCO2 were determined to be 1.25, 6.57, and 1.75, respectively. FCO = YCOFIPA,in

(11)

FH2 = YH2FIPA,in

(12)

FCO2 = YCO2FIPA,in

(13)

The H2/CO ratio was 3:1 through methane reforming reaction described by eq 9; however, more H2 was generated by the water−gas shift reaction described by eq 10, whereas the amount of CO was decreased. Therefore, the amount of H2 was consequently reached approximately 5 times greater than CO based on H2 and CO gas yields. The molar flow rate of fuel gas, FFuel, is described by eq 14, which is sum of FCO and FH2 for using IPA solution as fuel.

Figure 5. Empty bed experiments for IPA decomposition with 2.18 mmol/min of IPA inlet flow rate at 800, 850, and 900 °C.

FFuel = YgFIPA,in

corresponded to the proposed mechanism in accordance with reactions 8−10. The concentration of H2 was around 70% and was not varied with reaction temperature. The concentration of CO was slightly increased whereas the concentration of CO2 was decreased for experiments conducted at 900 °C, indicating the water−gas shift reaction was inhibited and the endothermic methane reforming reaction was more favorable at higher temperatures. Besides, methane was not being detected in the

(14)

The fuel gas yield, Yg, which is sum of YCO and YH2, is determined as 7.82, indicating each IPA molecule can be transformed to approximately 8 CO/H2 molecules for combustion with oxygen carriers. According to mole balance for eqs 8 and 9, 8 mol of CO/H2 by theoretical calculation would be generated by 1 mol of IPA, close to Yg determined as 7.82. 661


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to provide sufficient oxygen carriers to completely combust with fuel gas; thus, 100% CO2 yield was achieved as ϕ above 7.94. For each mole of IPA molecules, approximate 8 mol of Fe2O3/Al2O3 oxygen carrier is consumed for complete combustion, because each IPA molecule can be transformed to around 8 molecules of CO/H2 for combustion according to the determined fuel gas yield (Yg) by empty bed experiments. Hence, the condition for complete combustion of the fuel gas generated from IPA solution was obtained through the moving bed experiments. 3.4. Analysis of Heat Balance for IPA Combustion. Heat generated by CLC system should compensate the heat consumption for liquid evaporation in advance of feeding into fuel reactor; therefore, heat flow diagram is employed to analyze the heat balance and processing efficiency for IPA combustion in CLC system. The heat flow diagram for IPA combustion is illustrated in Figure 8. The inlet IPA solution is evaporated in an evaporator prior to feeding into the fuel reactor in CLC system. Therefore, the heat demands for evaporating IPA (ΔQIPA) and H2O (ΔHH2O) are estimated for calculating processing capacity of IPA at 100 °C (Qin,100°C) by following equation:

For moving bed operation, oxygen carrier-to-fuel ratio described by eq 4 is rearranged by combining with FFuel and FFe2O3 described by eqs 14 and 5, respectively, and rewritten as eq 15 for IPA solution combustion at 900 °C. The MFe2O3 and xFe2O3 are 159.69 g/mol and 0.6 for preparation of oxygen carrier. b is 1/3 because H2 and CO are the major fuel gases for combustion. Yg is 7.82 based on the empty bed experiments. Therefore, ϕ is determined by operating parameters, mass flow rate of oxygen carriers (ṁ OC) and IPA inlet flow rate (FIPA,in) by eq 15 for IPA combustion at 900 °C. ⎛ x Fe O ṁ OC ⎞ ⎛ 1 ⎞ ⎛ ṁ ⎞ 1 ⎟⎟ ⎜ ⎟ ϕ = ⎜⎜ 2 3 = 1.44 × 10−3⎜⎜ OC ⎟⎟ ⎝ FIPA,in ⎠ ⎝ MFe2O3 ⎠ ⎝ b ⎠ (YgFIPA,in) (15)

3.3. IPA Combustion in Moving Bed Reactor. IPA combustion was conducted by feeding Fe2O3/Al2O3 oxygen carriers into the moving bed reactor to provide oxygen for combustion of fuel gases generated from IPA. The feeding rate of oxygen carriers for complete combustion was determined with the flow rate operated from 11.50 to 29.70 g/min; thus, ϕ was in the range of 3.79 to 9.80. The IPA conversion (ηIPA) reached 100% for all the conditions of moving bed experiments, as shown in Figure 7. However, the CO2 yield was not reach

Q in,100 ° C = Q in,25 ° C + ΔQ H O + ΔQ IPA 2

(16)

The input IPA processing capacity is calculated as Q in,25 ° C = ΔHC,IPA × FIPA,in

(17)

where ΔHC,IPA is enthalpy of combustion for IPA that is −1830 kJ/mol at 25 °C and 1 atm;38 and FIPA,in is 4.36 mmol/min feeding into the moving bed reactor in this study. Thus, Qin,25°C is calculated as −133.0 J/s. The heat demand of H2O evaporation is calculated as ΔQ H O = (Cp,H2OΔT1 + ΔHH2O)FH2O,in 2

(18)

where Cp,H2O is specific heat capacity of water with 0.075 kJ/ mol·K; ΔT1 is the temperature difference from 25 to 100 °C; ΔHH2O is latent heat of water evaporation with 40.68 kJ/mol; and FH2O,in is 74 mmol/min feeding into the moving bed reactor in this study. Thus, ΔQH2O is estimated as 57.1 J/s for water evaporation. The heat demand of IPA evaporation is calculated as ΔQ IPA = (Cp,IPA(l)ΔT2 + ΔHIPA + Cp,IPA(g)ΔT3)FIPA,in Figure 7. Moving bed experiments for combustion of IPA with Fe2O3/ Al2O3 oxygen carriers with oxygen carrier-to-fuel ratio (ϕ) from 3.79 and 9.80 at 900 °C.

(19)

where Cp,IPA(l) and Cp,IPA(g) are the specific heat capacities of liquid and gaseous IPA with 0.158 and 0.105 kJ/mol·K, respectively; ΔT2 and ΔT3 are the temperature differences from 25 to 82.3 °C and 82.3 to 100 °C for heating of liquid and gaseous IPA, respectively; ΔHIPA is latent heat of IPA

100% as ϕ lower than 7.94 due to incomplete combustion of CO and H2. The feeding rate of oxygen carriers was increased

Figure 8. Heat flow diagram for IPA combustion by using 4.36 mmol/min of IPA and 74 mmol/min of H2O as fuel. 662


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evaporation with 40.06 kJ/mol; and FIPA,in is 4.36 mmol/min. Thus, ΔQIPA is estimated as 3.7 J/s for IPA evaporation, and Qin,100°C is obtained as 193.8 J/s. For estimation of output processing capacity, the amounts of H2 and CO generated by IPA in this study are used for calculation. The iron-based oxygen carriers operated in moving bed reactor was suggested reduced to majorly FeO.22,25,36 In this study, H2 and CO were the major fuel gases generated by IPA, thus reductions of Fe2O3 by H2 and CO in the fuel reactor are suggested by reactions 20 and 21, respectively.13

achieved for IPA combustion in CLC system as illustrated in Figure 8. The heat for water evaporation is greatly consumed in advance of feeding into CLC system; therefore, ΔQH2O and ΔQIPA is considered essential for IPA solution in terms of heat consumption. System processing capacity (ΔQsys) is calculated as 135.9 J/s by eq 27, indicating 30% of Qout,100°C would be consumed by evaporation of IPA and H2O and 70% of Qout,100°C is available for energy use of external facility by combustion of 20 vol% of IPA solution in CLC.

ΔHrxn,H2 = 27.5 kJ/mol

H 2 + Fe2O3 → 2FeO + H 2O

Q sys = Q out,100 ° C − (ΔQ H O + ΔQ IPA ) 2

(20)

As indicated by the heat flow diagram in Figure 8, the heat generated by CLC system should compensate the heat demands of IPA and H2O evaporation to avoid negative system processing capacity. For analysis of heat consumption of water evaporation, the theoretical calculation of ΔQH2O for IPA solution is estimated for 5−95% of water content and the IPA inlet flow rate is 4.36 mmol/min as constant for each water content to evaluate processing efficiency (ηP) varied by water content as the following equation:

CO + Fe2O3 → 2FeO + CO2 ΔHrxn,CO = − 4.7 kJ/mol

(21)

In order to estimate output processing capacity for a CLC system, oxidation of FeO with O2 in the air reactor is referred to previous study for theoretical calculation described by reaction 22.13 O2 + 4FeO → 2Fe2O3

(27)

ΔHrxn,O2 = − 553.6 kJ/mol (22)

ηP =

Thus, Qout,900°C for IPA combustion by moving bed reactor in CLC system is calculated by the following equation:

Q sys (Q in,100 ° C)

(28)

The ηP decreased slightly with increasing water content and decreased dramatically for higher water content as illustrated in Figure 9. Furthermore, ηP is estimated with positive value as

Q out,900 ° C = FCOΔHrxn,CO + FH2ΔHrxn,H2 + FO2ΔHrxn,O2 (23)

where ΔHrxn,CO, ΔHrxn,H2, and ΔHrxn,O2 are the enthalpies of reaction for reactions 20, 21, and 22, respectively; FCO and FH2 are 5.48 and 27.82 mmol/min obtained in this study, respectively; FO2 for oxidation from FeO to Fe2O3 is theoretically determined as 16.65 mmol/min. Thus, the output processing capacity at 900 °C, Qout,900°C, for IPA combustion is −141.3 J/s at 900 °C. For comparing with input and output processing capacity, the output processing capacity at 100 °C (Qout,100°C) is selected as the state of processing capacities to exclude the reactions of IPA decomposition and IPA reforming due to lack of experimental data from 100 to 900 °C for estimation in this study. Therefore, Qout,100°C is calculated as the following equation: Q out,100 ° C = Q out,900 ° C + ΔQ̂ CO + ΔQ̂ H O 2

2

(24)

where ΔQ̂ CO2 and ΔQ̂ H2O are the heat demands of CO2 and H2O, which completed combusted by IPA, decreased from 900 to 100 °C calculated as eqs 25 and 26. ΔQ̂ CO = ΔĤ CO2FCO2,out

(25)

ΔQ̂ H O = ΔĤ H2O(g)FH2O,out

(26)

2

2

Figure 9. Heat analysis on water evaporation with varied water content of IPA solution combusted by CLC at 900 °C.

water content lower than 90%. Besides, heat is possibly insufficient as process heat loss is included in realistic CLC system for very dilute solution as fuel in CLC. Therefore, heat recovery for liquid fuel combustion would be essential on designing a liquid fuel CLC system. As the heat is insufficient for the entire system, cocombustion with high carbon content liquid fuel is suggested for realistic operation that would be easily to enhance ηP without additional device. Consequently, CLC is suggested to be an alternative treatment technology with positive processing capacity for spent organic solvent.

where ΔĤ CO2 and ΔĤ H2O(g) are enthalpies of CO2 and H2O with −39.98 and −30.80 kJ/mol from 900 to 100 °C, respectively; and FCO2,out and FH2O,out are 13.08 and 17.44 mmol/min as 4.36 mmol/min of IPA is completely combusted. Thus, ΔQ̂ CO2 and ΔQ̂ H2O are calculated as −8.7 and −46.9 J/s, respectively. Thus, Qout,100°C is calculated as −196.7 J/s. Qin,100°C estimated by theoretical calculation is closed to Qout,100°C that calculated based on results in this study; therefore, heat balance for input and output processing capacities is approximately 663


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5. CONCLUSIONS CO and H2 were the fuel gases generated by IPA solution in the moving bed reactor. Due to methane reforming reaction was favored at 900 °C, the concentration of H2 was higher than CO and CO2. The amounts of CO and H2 generated by IPA solution were estimated to modify the equation of oxygen carrier-to-fuel ratio (ϕ) for moving bed operation. Combustion of IPA solution as fuel for CLC was preliminary feasible in the moving bed reactor. CO2 yield and IPA conversion were reached 100% as ϕ was higher than 7.94 at 900 °C. Heat balance for input and output processing capacities is approximately achieved as Qin,100°C is closed to Qout,100°C for IPA combustion in CLC system. By analysis of system processing capacity, 70% of Qout,100°C is available for energy use of external facility by combustion of 20 vol % of IPA solution in CLC. By analysis of ηP, aqueous solution as liquid fuel would cause considerable heat consumption for water evaporation, especially for dilute solution. Therefore, heat recovery in CLC system is needed to be considered as designing a liquid fuel CLC system. Consequently, chemical looping combustion is suggested to be an alternative treatment technology with positive processing capacity for spent organic solvent.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Grant NSC-100-3113-E-007005 from the National Science and Technology ProgramEnergy and Grant EPA-100-U1U4-04-003 from Environmental Protection Administration in Taiwan. The authors appreciated China Steel Corp. for providing hematite powders for the preparation of oxygen carriers.

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ABBREVIATIONS

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Acronyms

CLC = chemical looping combustion IPA = isopropanol

ṁ OC = [g/min] mass flow rate of oxygen carrier ηIPA = [%] IPA conversion ηP = [%] processing efficiency of liquid fuel CLC system FFuel = [mmol/min] molar flow rate of fuel gas FFe2O3 = [mmol/min] molar flow rate of oxygen carrier FIPA,in = [mmol/min] molar flow rate of inlet IPA FH2O,in = [mmol/min] molar flow rate of inlet H2O FCH4 = [mmol/min] molar flow rate of CH4 FCO2 = [mmol/min] molar flow rate of CO2 FCO = [mmol/min] molar flow rate of CO FH2 = [mmol/min] molar flow rate of H2 YCO2 = yield of CO2 YCO = yield of CO YH2 = yield of H2 Yg = sum of yields of fuel gases Qin,100°C = [J/s] processing capacity of IPA at 100 °C Qin,25°C = [J/s] processing capacity of IPA at 25 °C Qout,900°C = [J/s] output processing capacity by CLC at 900 °C Qout,100°C = [J/s] output processing capacity by CLC at 100 °C Qsys = [J/s] system processing capacity ΔQIPA = [J/s] heat demand of IPA evaporation ΔQH2O = [J/s] heat demand of H2O evaporation ΔQ̂ CO2 = [J/s] heat demands of CO2 decreased from 900 to 100 °C ΔQ̂ H2O = [J/s] heat demands of H2O decreased from 900 to 100 °C ΔHC,IPA = [kJ/mol] enthalpy of combustion of IPA ΔHH2O = [kJ/mol] latent heat of water evaporation ΔHIPA = [kJ/mol] latent heat of IPA evaporation ΔHrxn,CO = [kJ/mol] enthalpy of reaction for CO combusted with Fe2O3 ΔHrxn,H2 = [kJ/mol] enthalpy of reaction for H2 combusted with Fe2O3 ΔHrxn,O2 = [kJ/mol] enthalpy of reaction for O2 combusted with FeO ΔĤ CO2 = [kJ/mol] enthalpy of CO2 decreased from 900 to 100 °C ΔĤ H2O = [kJ/mol] enthalpy of H2O decreased from 900 to 100 °C Cp,H2O = [kJ/mol·K] specific heat capacity of water Cp,IPA(l) = [kJ/mol·K] specific heat capacity of liquid IPA Cp,IPA(g) = [kJ/mol·K] specific heat capacity of gaseous IPA

REFERENCES

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Symbols

pCO2 = [psi] partial pressure of CO2 pCO = [psi] partial pressure of CO pH2O = [psi] partial pressure of H2O pH2 = [psi] partial pressure of H2 Xred = [%] degree of reduction mo = [mg] weight of fully oxidized oxygen carrier mr = [mg] weight of fully reduced oxygen carrier m(t) = [mg] weight of oxygen carrier after a specific reduction time ϕ = oxygen carrier-to-fuel ratio b = stoichiometric coefficient of fuel gas combusted with oxygen carrier xFe2O3 = fraction of Fe2O3 in oxygen carriers MFe2O3 = [g/mol] molecular weight of Fe2O3 664


Energy & Fuels

Article

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