Adapting the Mgo-CO2 Working Pair for Thermochemical Energy Storage by Doping with Salts

energies

Article Adapting the MgO-CO2 Working Pair for Thermochemical Energy Storage by Doping with † Salts: Eﬀect of the (LiK)NO3 Content

Seon Tae Kim 1,* , Haruka Miura 1, Hiroki Takasu 1, Yukitaka Kato 1, Alexandr Shkatulov 2 and Yuri Aristov 1,3

1 Tokyo Institute of Technology, 2-12-1-N1-22, Ookayama,¯ Meguro-ku, Tokyo 152-8550, Japan; [email protected] (H.M.); [email protected] (H.T.); [email protected] (Y.K.); [email protected] (Y.A.) 2 Department of Applied Physics, Eindhoven University of Technology, De Rondom 70, 5612 AP Eindhoven, The Netherlands; [email protected] 3 Boreskov Institute of Catalysis, Ac. Lavrentiev av. 5, Novosibirsk 630090, Russia * Correspondence: [email protected] The original paper was presented in: Shkatulov, A.I., Kim, S.T., Kato, Y., Aristov, Yu.I. Adapting the † MgO-CO2 working pair for thermochemical energy storage by doping with salts. Riehl, R., Preißinger, M., Eames, I., Tierney, M., Eds. In Proceedings of the Heat Powered Cycles Conference 2018, Bayreuth, Germany, 16–19 September 2018; ISBN 978-0-9563329-6-7.

Received: 30 April 2019; Accepted: 11 June 2019; Published: 13 June 2019

Abstract: The MgO-CO2 working pair has been regarded as prospective for thermochemical energy storage (TCES) due to its relatively high heat storage capacity, low cost, and wide availability. This study is aimed at the optimization of the molar salt content, α, for the MgO modified with the eutectic mixture of LiNO3 and KNO3 (Li0.42K0.58NO3) which was earlier shown to provide high conversion, ∆x, in heat-storage/release processes at 300–400 ◦C. The composites that have different salt content were prepared and carbonation kinetics was investigated under various conditions (carbonation temperature, Tcarb., is 290–360 ◦C and CO2 pressure, P(CO2), is 50–101 kPa). Significant accelerating effect was revealed at α 0.05, and the ∆x value was maximized at α = 0.10–0.20. The largest ≥ conversion of 0.70 was detected at α = 0.10 and Tcarb. = 350 ◦C that corresponds to the specific useful heat (Qcomp.) is 1.63 MJ/kg-composite. However, the salt content of 0.20 ensures the high conversion, ∆x = 0.63–0.67 and Qcomp. = 1.18–1.25 MJ/kg-composite in the whole temperature range between 290 and 350 ◦C. The (LiK)NO3/MgO composite with an optimal salt content of 0.20 exhibits reasonable durability through cyclic experiment at 330 ◦C, namely, the stabilized reacted conversion ∆x = 0.34 (Qcomp. = 0.64 MJ/kg-composite). The studied (Li0.42K0.58)NO3 promoted MgO-CO2 working pair has good potential as thermochemical storage material of middle temperature heat (300–400 ◦C).

Keywords: thermochemical energy storage; magnesium oxide; magnesium carbonate; salt modification; eutectic mixture

1. Introduction Storage of thermal energy by means of chemical reactions with high heat eﬀect, or thermochemical energy storage (TCES), is a promising approach to convert primary energy source due to high thermal energy storage capacity and long-term storage at ambient temperature without insulation [1–3]. To the date, chemical reactions that can be neatly applied to TCES at medium temperatures (200–500 ◦C) to store heat from industrial and renewable (e.g., solar energy) sources are not numerous. Aside from large-scale industrial processes such as ammonia production [4] there are only a few solid-gas reactions

Energies 2019, 12, 2262; doi:10.3390/en12122262 www.mdpi.com/journal/energies Energies 2019, 1210, 2262x FOR PEER REVIEW 2 2of of 13 temperatures. They are dehydration of Mg(OH)2 [5], Ca(OH)2 [6], dehydrogenation of MgH2 and somethat were other extensively hydrides [7]. studied speciﬁcally for TCES at medium temperatures. They are dehydration of Mg(OH)The 2carbonation[5], Ca(OH) 2of[6 MgO,], dehydrogenation Equation (1), ofthat MgH has2 theand high some reaction other hydrides enthalpy, [7]. 116.4 kJ/mol, and suitableThe equilibrium carbonation temperature, of MgO, Equation Teq = 388 (1), °C that (at P has(CO the2) = high 101 kPa), reaction was enthalpy, considered 116.4 for kJTCES/mol, since and thesuitable late 70s equilibrium [8]. temperature, Teq = 388 ◦C (at P(CO2) = 101 kPa), was considered for TCES since the late 70s [8]. MgO(s) + CO2 (g) ↔ MgCO3 (s), ∆ 𝐻 = −116.4 kJ/mol (1) MgO + CO MgCO , ∆0H = 116.4 kJ/mol,, (1) (s) 2 (g) ↔ 3 (s) r − Figure 1 shows the principle of a MgO/CO2/sorbent TCES system. For heat storage, MgCO3 is Figure1 shows the principle of a MgO /CO2/sorbent TCES system. For heat storage, MgCO3 decomposed by supplied heat energy, Qdec., at temperature, Tdec., of 300–350 °C (the backward reaction is decomposed by supplied heat energy, Qdec., at temperature, Tdec., of 300–350 ◦C (the backward in Equation 1).The products (MgO and CO2) are separated and decomposed CO2 moves into the reaction in Equation (1)). The products (MgO and CO2) are separated and decomposed CO2 moves sorbent reactor with generating heat of sorption, Qs; the heat is stored in a chemical form for into the sorbent reactor with generating heat of sorption, Qs; the heat is stored in a chemical form for theoretically infinite time (Figure 1a). In the heat release process, the CO2 desorbed by desorption theoretically inﬁnite time (Figure1a). In the heat release process, the CO 2 desorbed by desorption heat, Qdes., is supplied to MgO reactor then generates heat energy, Qcarb., (the forward reaction in heat, Qdes., is supplied to MgO reactor then generates heat energy, Qcarb., (the forward reaction in Equation 1) at temperature, Tcarb., of ~400 °C (Figure 1b). Usually, carbonation pressure, Pcarb., and Equation (1)) at temperature, Tcarb., of ~400 ◦C (Figure1b). Usually, carbonation pressure, Pcarb., and desorption temperature, Tdes., are higher than decarbonation pressure, Pdec., and temperature of desorption temperature, Tdes., are higher than decarbonation pressure, Pdec., and temperature of sorption heat, Ts, respectively. sorption heat, Ts, respectively.

(a) CO2 (Pdec.)

Waste heat, Qdec. Sorption heat, Qs (Tdec. ≅ 330℃) (Ts)

MgCO3 Sorbent

(b) CO2 (Pcarb. > Pdec.)

Useful heat, Qcarb. Desorption heat, Qdes. (Tcarb. ≅ 400℃) (Tdes. > Ts)

Sorbent MgO + CO2

Figure 1. Principle of MgO/CO2/sorbent TCES system. (a) Heat storage process by MgCO3 decarbonation. Figure 1. Principle of MgO/CO2/sorbent TCES system. (a) Heat storage process by MgCO3 (b) Heat output process by MgO carbonation. decarbonation. (b) Heat output process by MgO carbonation. Although there are many thermodynamic advantages to MgO carbonation, TCES systems using Although there are many thermodynamic advantages to MgO carbonation, TCES systems using this reaction have rarely been reported in reviews dedicated to TCES [9–12]. The reason is a strong this reaction have rarely been reported in reviews dedicated to TCES [9–12]. The reason is a strong irreversibility of reaction as the interaction of MgO with CO2 is kinetically hindered [13,14] that makes irreversibility of reaction as the interaction of MgO with CO2 is kinetically hindered [13,14] that makes a return of the stored heat practically impossible. Due to this, the first detailed study of the MgO-CO2 a return of the stored heat practically impossible. Due to this, the first detailed study of the MgO-CO2 working pair has appeared only recently [15]. The authors made a brief screening of salt additives working pair has appeared only recently [15]. The authors made a brief screening of salt additives to to pure MgO with the aim to find a salt which increases the oxide reactivity towards carbonation. pure MgO with the aim to find a salt which increases the oxide reactivity towards carbonation. They They used a powerful salt modification approach earlier suggested and tested for TCES at middle used a powerful salt modification approach earlier suggested and tested for TCES at middle temperatures in many papers such as LiCl for hydration of MgO [16], metal nitrates for doping of temperatures in many papers such as LiCl for hydration of MgO [16], metal nitrates for doping of MgO MgO [17,18], and Li for hydration of CaO [19,20]. This approach was also successfully applied to [17,18], and Li for hydration of CaO [19,20]. This approach was also successfully applied to synthesize synthesize MgO-based sorbents for intermediate-temperature CO2 capture [21] and high-temperature MgO-based sorbents for intermediate-temperature CO2 capture [21] and high-temperature heat storage heat storage systems [9]. systems [9]. In total, eight salts with MgO (salt content 10 mol%) were prepared and investigated in [15]. It was found that CH3COOLi and Li0.42K0.58NO3 considerably promote the MgO carbonation. The

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In total, eight salts with MgO (salt content 10 mol%) were prepared and investigated in [15]. It was found that CH3COOLi and Li0.42K0.58NO3 considerably promote the MgO carbonation. The eutectic mixture Li0.42K0.58NO3 of LiNO3 and KNO3 was chosen for further study as the most efficient and thermally stable additive. It allowed the decarbonation process to be efficiently carried out at T > 330 ◦C and the specific useful heat of 1.6 MJ/kg-MgO in composite to be reached at 360 ◦C for 300 min of carbonation. This paper addresses a detailed investigation of the (Li0.42K0.58NO3)/MgO composites (referred to as (LiK)NO3/MgO). The composites that have different salt content are prepared and their feasibility was evaluated under various conditions: Tcarb. = 290–360 ◦C and P(CO2) = 50–101 kPa. Additionally, the specific useful heat was estimated to find an optimal salt content and cyclic experiments were also conducted to confirm the durability of material.

2. Experimental

2.1. Materials Preparation The composite materials of lithium nitrate (LiNO , 99.0% Reakhim), potassium nitrate (KNO , 3 ≥ 3 99.5%, Reakhim), magnesium hydroxide (Mg(OH) , 99.0% Reakhim) with diﬀerent mixing molar ≥ 2 ≥ ratio between Li0.42K0.58NO3 salt and Mg(OH)2 were prepared as follows. Certain amounts of dried (160 ◦C) KNO3 and LiNO3 were mixed with 1.0 g of Mg(OH)2. After this, some water was added to dissolve the salts and to form slurry. This slurry was dried at 65–70 ◦C by using a rotary evaporator under reduced pressure and vigorous stirring to ensure uniform distribution of the salt throughout the material. The (LiK)NO3/MgO material was prepared by dehydration of (LiK)NO3/Mg(OH)2 in situ. The molar salt content in the composites, α [-], was deﬁned by the following equation, Equation (2).

n(Li0.42K0.58NO3) n(Li0.42K0.58NO3) α = = (2) n(Li0.42K0.58NO3) + n(Mg(OH)2) n(Li0.42K0.58NO3) + n(MgO) where n [mol] is the amount of a component. The (LiK)NO3/MgO composites were prepared with the salt content of 2, 5, 10, 20, and 30 mol% (α = 0.02, 0.05, 0.10, 0.20, and 0.30).

2.2. Materials Characterization

The dynamic study of carbonation of the (LiK)NO3/MgO composites was carried out by using a TG analysis system TGD-9600 (Advance RIKO, Inc.) equipped with a flow gas control system which allowed measurements in CO2 or Ar atmosphere (Figure2); total gas flow rate is 100 CCM for 101 kPa of total pressure. For the kinetic study of carbonation, (LiK)NO3/Mg(OH)2 composites powder was loaded into the platinum cell (D:8 mm, H:10 mm) in order to secure same amount of MgO in composites (~30.6 mg of MgO), then heated to 350 ◦C (5 K/min) and completely dehydrated at this temperature for 30 min. Then, the measuring cell was heated or cooled to the desired temperature Tcarb. in Ar flow and switched to the flow of CO2 which initiated carbonation. Carbon dioxide was supplied to the reaction chamber for 300 min. To evaluate the carbonation reactivity of (LiK)NO3/MgO composite, reacted mole fraction, x [-], was calculated from the balance-measured sample mass change:

∆m x = (3) n0 + MCO2 where ∆m is the sample mass change [kg], n0 is the initial amount of MgO in the sample [mol], and

MCO2 is the molecular weight of CO2 [kg/mol]. The heat output performance, Qcomp. [kJ/kg-composite] and Wcomp. [kW/kg-composite], of LiK(NO)3/MgO composites were also as:

0 ∆r H x Qcomp. = · (1 α) (4) Mcomp. · − Energies 2019, 10, x FOR PEER REVIEW 4 of 13 Energies 2019, 12, 2262 4 of 13

d𝑄comp. 𝑊 = comp. dQcomp. (5) W = d𝑡 (5) comp. dt where the reaction enthalpy0 ∆𝐻 = 116.4 kJ/mol, Mcomp. [kg/mol] is the molar weight of where the reaction enthalpy ∆r H = 116.4 kJ/mol, Mcomp. [kg/mol] is the molar weight of (LiK)NO3/MgO (LiK)NO3/MgO composite, and t [s] is the process time. composite, and t [s] is the process time.

Mass Flow Regulator Ball valve Controller 3-way valve

MFC

CO2 MFC Sample

MFC

Figure 2. Schematic diagram of a TGA system (TGD-9600, Advance RIKO, Inc., Kanagawa, Japan). Figure 2. Schematic diagram of a TGA system (TGD-9600, Advance RIKO, Inc., Kanagawa, Japan). 3. Results and Discussion 3. Results and Discussion The interaction of pure MgO with CO2 is kinetically hindered; therefore, namely the MgO carbonationThe interaction has to be of accelerated pure MgO to with make CO a return2 is kinetically of the stored hindered; heat feasible. therefore, In thisnamely work, the we MgO have optimizedcarbonation the has salt to content be accelerated keeping into mind, make ﬁrsta return of all, of dynamic the stored features heat offeasible. the MgO In carbonationthis work, we reaction. have optimized the salt content keeping in mind, first of all, dynamic features of the MgO carbonation 3.1.reaction. Carbonation Kinetics for Various Salt Contents

3.1. CarbonationFor a general Kinetics comparison for Various of the Salt new Contents (LiK)NO 3/MgO composites, for which salt content changes between 2 and 30 mol%, the isothermal carbonation was studied at 300 ◦C and 350 ◦C and P(CO2) = 101 kPa.For Thea general pure MgOcomparison undergoes of the no new carbonation (LiK)NO3/MgO at T composites,350 C (Figure for which3b) salt and content only little changes effect carb. ≤ ◦ ° ° wasbetween observed 2 and at 30α =mol%,0.02; the however, isothermal significant carbonation acceleration was studied of carbonation at 300 C and was 350 revealedC and atP(COα 20.05.) = 101 kPa. The pure MgO undergoes no carbonation at Tcarb. ≤ 350 °C (Figure 3b) and only little ≥effect The carbonation conversion, ∆x, reaches 0.15–0.66 and 0.33–0.70 at 300 ◦C and 350 ◦C, respectively. Itwas demonstrates observed at a highα = 0.02; sensitivity however, of the significant composite acceleration reactivity to of the carbonation content of dopingwas revealed salt at lowat α contents. ≥ 0.05. The carbonation conversion, Δx, reaches 0.15–0.66 and 0.33–0.70 at 300 °C and 350 °C, respectively. It At tcarb. < 180 min, faster and reliable carbonation performance is observed for α= 0.20 compared with demonstrates a high sensitivity of the composite reactivity to the content of doping salt at low other materials. In contrast, for α= 0.10, good performance is observed at 350 ◦C which significantly contents. At tcarb. < 180 min, faster and reliable carbonation performance is observed for α = 0.20 reduced at lower reaction temperature. For α= 0.30, the carbonation performance is always lower. compared with other materials. In contrast, for α = 0.10, good performance is observed at 350 °C Carbonation performance for composites with α = 0.10 and 0.20 at various CO2 partial pressure which significantly reduced at lower reaction temperature. For α = 0.30, the carbonation performance and temperature was also investigated (Tcarb. = 300 ◦C and 350 ◦C, PCO2 = 50 kPa, 70 kPa, and 101 kPa). is always lower. At Tcarb. = 300 ◦C, the composite with α = 0.20 exhibits superior carbonation performance than that with α = 0.10 (Figure4a). The carbonation reactivity at α = 0.20 and partial CO2 pressure of 50 kPa is even higher than that of α = 0.10 at 101 kPa (Figure4b). At 350 ◦C, the composites with α = 0.10 and 0.20 exhibit similar carbonation reactivity at any CO2 partial pressure (Figure4b).

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0.7 0.7 (b) (a)

0.6 α = 0.02 0.6 α = 0.05

[-] α = 0.10

[-] 0.5 x

x 0.5 α = 0.20

α = 0.30 Energies 2019, 12, 2262 0.4 5 of 13 Energies0.4 2019, 10, x FOR PEER REVIEW 5 of 13

0.3 0.3 α = 0.30 α = 0.20 0.7 0.7 α = 0.05 α = 0.10 (b) (a) 0.2 0.2 α

= 0.02 fraction, mole Reacted Reacted mole fraction, 0.6 α = 0.02 0.6 α = 0.05 0.1 0.1 o o Tcarb. = 300 C [-] α = 0.10 Tcarb. = 350 C [-] 0.5 Pcarb. = 101 kPa x Pure MgO Pcarb. = 101 kPa x 0.5 α = 0.20 0 0 α = 0.30 0.40 50 100 150 200 250 300 0.40 50 100 150 200 250 300 Carbonation time, t [min] Carbonation time, t [min] carb. carb. 0.3 0.3 α = 0.30 α = 0.20

Figure 3. Carbonation dynamics ofα = the0.05 (LiK)NOα = 0.10 3/MgO composites: isothermal experimental results 0.2 0.2 ° ° α 2 at (a) 300 C and (b) 350 C under P =(CO 0.02 ) = 101 kPa. fraction, mole Reacted Reacted mole fraction,

0.1 0.1 T = 300oC T = 350oC Carbonation performance for compositescarb. with α = 0.10 and 0.20 at various CO2 partialcarb. pressure Pcarb. = 101 kPa Pure MgO Pcarb. = 101 kPa and temperature was also investigated (Tcarb. = 300 °C and 350 °C, PCO2 = 50 kPa, 70 kPa, and 101 kPa). 0 0 0 50 100 150 200 250 300 At Tcarb. = 3000 °C, 50 the composite 100 150 with 200 α = 2500.20 exhibits 300 superior carbonation performance than that Carbonation time, t [min] Carbonation time, t [min] with α = 0.10 (Figure 4a). The carbonationcarb. reactivity at α = 0.20 and partial CO2 pressurecarb. of 50 kPa is ° even higherFigure than 3. Carbonationthat of α = dynamics0.10 at 101 of thekPa (LiK)NO (Figure3/MgO 4b). composites:At 350 C, isothermalthe composites experimental with resultsα = 0.10 at and Figure 3. Carbonation dynamics of the (LiK)NO3/MgO composites: isothermal experimental results 0.20 exhibit(a) 300similar◦C and carbonation (b) 350 ◦C under reactivityP(CO2 )at= any101 kPa.CO2 partial pressure (Figure 4b). at (a) 300 °C and (b) 350 °C under P(CO2) = 101 kPa. 0.7 0.7 (a) (b) Carbonation performance for composites with α = 0.10 and 0.20 at various CO2 partial pressure 0.6 0.6 α = 0.10 and temperature was also investigated (Tcarb. = 300 °C and 350 °αC,= 0.20 PCO2 = 50 kPa, 70 kPa, and 101 kPa).

At Tcarb.[-] 0.5 = 300 °C, the composite with α = 0.20 exhibits[-] 0.5 superior101 carbonationkPa performance than that x x 70 kPa with α = 0.10 (Figure 4a). The carbonation reactivity at α = 0.20 and partial CO2 pressure of 50 kPa is 50 kPa even higher0.4 than that of α = 0.10 at 101 kPa (Figure 4b).0.4 At 350 °C, the composites with α = 0.10 and

2 0.20 exhibit0.3 similar carbonation reactivity at any CO partial0.3 pressure (Figure 4b). 0.7 0.7 0.2 (a) 0.2 (b) 101 kPa Reacted mole fraction, fraction, mole Reacted fraction, mole Reacted α = 0.10 0.6 70 kPa 0.6 α = 0.10 0.1 0.1 α = 0.20 α = 0.20 50 kPa

[-] [-] 101 kPa 0.5 o 0.5 o x Tcarb. = 300 C x Tcarb. = 350 C 0 0 70 kPa 0 50 100 150 200 250 300 0 5050 kPa 100 150 200 250 300 0.4 0.4 Carbonation time, t [min] Carbonation time, t [min] carb. carb. Figure0.3 4. Carbonation dynamics of α = 0.10 (empty symbols)0.3 and 0.20 (bold symbols) composite under Figure 4. Carbonation dynamics of α = 0.10 (empty symbols) and 0.20 (bold symbols) composite under various CO2 partial pressure: (a) 300 ◦C and (b) 350 ◦C. 0.2 0.2 various CO2 partial pressure: (a) 300 °C and101 (kPab) 350 °C. Reacted mole fraction, fraction, mole Reacted fraction, mole Reacted 70 kPa The acceleration of carbonationα = 0.10 is observed because the eutectic salt melts at 125 C[22] and acts 0.1 0.1 ◦ asThe a reactionacceleration medium of carbonation in which bothα = 0.20is COobserved50and kPa MgO because are dissolved the eutectic [21 ].salt The melts dissolution at 125 °C of solid[22] and MgO acts in 2 o o Tcarb. = 3002+C 2 Tcarb. = 350 C as a thereaction molten medium0 salt and in its which dissociation both CO MgO2 and MgMgO +areO dissolved(Figure0 5 )[21]. helps The to overcomedissolution the of high solid energy MgO → − in theof molten the MgO salt0 lattice and 50 its re-construction dissociation 100 150 MgO that 200 results → 250Mg in2+ + the 300 O2 e− ff(Figureective0 activation 5) helps 50 to 100 of overcome the 150MgO towardthe 200 high 250 reaction energy 300 Carbonation time, t [min] Carbonation time, t [min] carb. carb. of thewith MgO CO lattice2 [23]. re-construction At α = 0.02, volume that of results the liquid in th phasee effective is small activation and the promotionof the MgO eff towardect is negligible; reaction withthe CO carbonation2 [23]. At α of= 0.02, MgO volume was activated of the whenliquidα phaseis greater is small than 0.05and andthe thepromotion reactivity effect was maximizedis negligible; at Figure 4. Carbonation dynamics of α = 0.10 (empty symbols) and 0.20 (bold symbols) composite under the carbonationα = 0.10–0.20. of TheMgO reactivity was activated decreases when at larger α is greaterα, probably, than 0.05 because and thethe liquid reactivity salt filmwas on maximized the MgO various CO2 partial pressure: (a) 300 °C and (b) 350 °C. 2+ 2 at α =surface 0.10–0.20. becomes The reactivity thicker that decreases makes the at pathlarger for α, diprobably,ffusion of because CO2, Mg the liquidand O salt− longer. film on Also, the moreMgO melting salt surrounding MgO promotes ionization before crystallization2+ of MgCO2− 3. surfaceThe becomes acceleration thicker of that carbonation makes the is pathobserved for diffusion because theof CO eutectic2, Mg salt and melts O atlonger. 125 °C Also, [22] andmore acts melting salt surrounding MgO promotes ionization before crystallization of MgCO3. as a reaction medium in which both CO2 and MgO are dissolved [21]. The dissolution of solid MgO in the molten salt and its dissociation MgO → Mg2+ + O2− (Figure 5) helps to overcome the high energy of the MgO lattice re-construction that results in the effective activation of the MgO toward reaction

with CO2 [23]. At α = 0.02, volume of the liquid phase is small and the promotion effect is negligible; the carbonation of MgO was activated when α is greater than 0.05 and the reactivity was maximized at α = 0.10–0.20. The reactivity decreases at larger α, probably, because the liquid salt film on the MgO surface becomes thicker that makes the path for diffusion of CO2, Mg2+ and O2− longer. Also, more melting salt surrounding MgO promotes ionization before crystallization of MgCO3.

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Energies 2019, 12, 2262 6 of 13 Energies 2019, 10, x FOR PEER REVIEW 6 of 13 CO2 CO 2 Liquefied (LiK)NO3 (Tmp : 125℃) CO 2- Liquefied (LiK)NO3 CO2 3 Mg2+ CO 2+ 2 (Tmp : 125℃) Mg 2- O2- 2- 2- CO CO2- 2 CO O CO3 2 CO 3 2+ 3 Mg CO2 Mg2+ 2- CO 2- O 2- 2- 3 Solid COMgO2 CO 3 O (Tmp : 2,852℃) Solid MgO (Tmp : 2,852℃) Figure 5. Carbonation mechanism of (LiK)NO3/MgO [23].

FigureFigure 5. 5. CarbonationCarbonation mechanism mechanism of of (LiK)NO (LiK)NO33/MgO/MgO [23]. [23]. At 350°C, all the kinetic curves are S-shaped with the induction period which significantly reducesAtAt if 350 the°◦C, C,salt all thecontentthe kinetic kinetic increases curves curves are toare S-shaped 10 S-shaped mol% with andwith the theninduction the inductionremains period almostperiod which constant significantlywhich significantly (Figure reduces 3b). Accordingreducesif the salt ifto content the the salt mentioned increases content to mechanismincreases 10 mol% to and of10 then themol% salt remains andmelting, almostthen theremains constant induction almost (Figure period constant3b). Accordingmay (Figure be attributed to 3b). the to Accordingmentionedthe low solubility to mechanism the mentioned of CO of the2 inmechanism salt the melting, liquid of thesalt the induction film.salt melting, Alternatively, period the may induction bethis attributed period period tomay may the indicate lowbe attributed solubility a slow of CO in the liquid salt film. Alternatively, this period may indicate a slow formation of critical formationto the 2low of solubilitycritical nuclei of CO of2 MgCOin the liquid3 phase salt formed film. Alternatively,in the molten thissalt becauseperiod may of insufficient indicate a slow super- saturation.formationnuclei of Indeed, MgCOof critical3 phasethe nuclei duration formed of MgCO of in this the3 phase moltenperiod formed saltstrongly because in the depends molten of insu saltffioncient thebecause system super-saturation. of insufficient deviation Indeed,fromsuper- the equilibriumsaturation.the duration (see Indeed, of the this nextthe period duration Section). strongly of this depends period on st therongly system depends deviation on the from system the equilibriumdeviation from (see the the equilibriumnext Section). (see the next Section). 3.2. Carbonation Kinetics under Various Reaction Temperatures 3.2.3.2. Carbonation Carbonation Kinetics Kinetics under under Various Various Reaction Reaction Temperatures Temperatures ToTo elucidate elucidate the the effect effect of temperaturetemperature on on the the carbonation carbonation dynamics, dynamics, composites composites with αwith= 0.05–0.20 α = 0.05– To elucidate the effect of temperature on the carbonation dynamics,° composites with α = 0.05– 0.20were were examined examined under under isothermal isothermal conditions conditions at atT Tcarb.= =290–360 290–360 C,C, PP(CO(CO2)) == 101101 kPa kPa and and ttcarb = =300 0.20 were examined under isothermal conditions atcarb. Tcarb. = 290–360 °◦C, P(CO2)2 = 101 kPa and tcarb carb= 300 min (Figure 6). min300 min(Figure (Figure 6). 6).

0.7 0.7 0.7 0.7 (b) (a) (b) (a) 0.6 0.6 0.6 0.6 290oC 290oC α o o [-] α = 0.20 o(290 C) 300 C o [-] = 0.20 (290 C) [-] 0.5 300 C x

[-] 0.5 x 0.5 x

x 0.5 o o o 330 C o α = 0.05 (300o C) α = 0.20 o(300 C) 330 C α = 0.05 (300 C) α = 0.20 (300 C) o o o o 350350C C α = 0.05 (330o C) α = 0.20 o(330 C) 0.4 0.40.4 α = 0.05 (330 C) α = 0.20 (330 C) 0.4 o o 360360C C o o α =α 0.05 = 0.05 (350 (350oC) C) α = 0.20α = 0.20(350 o(350C) C)

0.30.3 0.30.3

0.20.2 0.20.2 Reacted mole fraction, fraction, mole Reacted Reacted mole fraction, fraction, mole Reacted Reacted mole fraction, fraction, mole Reacted Reacted mole fraction, fraction, mole Reacted

0.10.1 0.10.1 α =α 0.05= 0.05 & 0.20 & 0.20 α = 0.10 o α = 0.10 Pure MgO (350 C) o Pcarb. = 101 kPa Pcarb. = 101 kPa Pure MgO (350 C) Pcarb. = 101 kPa Pcarb. = 101 kPa 0 0 0 0 0 50 100 150 200 250 300 00 50 50 100 100 150 150 200 200 250 250 300 0 50 100 150 200 250 300 Carbonation time, t [min] Carbonation time, t [min] Carbonation time, t carb.[min] Carbonation time,carb. t [min] carb. carb.

FigureFigure 6. 6. CarbonationCarbonation kineti kineticscs of of (LiK)NO (LiK)NO33/MgO/MgO at at (a) (a) αα == 0.100.10 and and (b) (b) αα == 0.050.05 and and 0.20 0.20 at at various various Figure 6. Carbonation kinetics of (LiK)NO3/MgO at (a) α = 0.10 and (b) α = 0.05 and 0.20 at various carbonationcarbonation temperatures. temperatures. carbonation temperatures. ForFor all all tested tested materials, materials, the the kinetics kinetics at at low low temperature temperature (290–300 (290–300 °C)◦C) are are convex convex curves, curves, so so that that For all tested materials, the kinetics at low temperature (290–300 °C) are convex curves, so that thethe synthesis synthesis rate rate is is maximal maximal at at tt = 00 and and gradually gradually decreases decreases in in time. time. It It is is quite quite different different from from the the the synthesis rate is maximal at t = 0 and gradually decreases in time. It is quite different from the carbonationcarbonation at at 350 350 °C◦C (Figures (Figures 33bb andand4 4b),b), whichwhich initialinitial raterate isiszero zeroand and the the process process starts starts after after a a carbonationcertaincertain induction induction at 350 period. period.°C (Figures This This fact fact3b andmay may 4b),denote denote which changing changing initial carbonationcarbonation rate is zero mechanism mechanism and the process or or its its rate-limiting rate-limiting starts after a certainstepstep with withinduction temperature. temperature. period. Kinetic This Kinetic fact curves curves may atdenote at 330 330 °C◦ changingC are are intermediate intermediate carbonation between between mechanism the the two two orboundary boundary its rate-limiting cases. cases. stepTheThe with appearanceappearance temperature. andand prolongation prolongationKinetic curves of theof atthe induction 330 induction °C are period intermediateperiod are accompanied are accompanied between with the the with two significant theboundary significant increase cases. Theincreasein appearance the final in the conversion finaland conversionprolongation∆x (300 Δ min):x (300of fromthe min): induction 0.15 from to 0.330.15 period forto 0.33α = arefor0.05, αaccompanied = and 0.05, from and 0.37 from with to 0.37 0.70 the to at 0.70significantα = at0.10. α increase=For 0.10. the Forin latter the the final material,latter conversion material, further furthe Δ enlargementx (300r enlargement min): of from the of induction0.15 the toinduction 0.33 period for period α is = observed0.05, is observed andat from 360 at ◦0.37 C360 along to°C 0.70along with at α = 0.10.withthe ∆ Forthex-reduction the Δx latter-reduction to material, 0.62 to (Figure 0.62 furthe 6(Figurea).r enlargement Thus, 6a). the Thus, closer of the the closer carbonationinduction the periodcarbonation temperature is observed temperature to the at equilibrium 360 °Cto alongthe withequilibrium the Δx-reduction temperature to at0.62 P(CO (Figure2) = 101 6a). kPa Thus, (388 °C) the [14], closer the longerthe carbonation the induction temperature period. Besides to the equilibrium temperature at P(CO2) = 101 kPa (388 °C) [14], the longer the induction period. Besides

Energies 2019, 12, 2262 7 of 13 Energies 2019, 10, x FOR PEER REVIEW 7 of 13 temperaturethis factor, the at Pcomplex(CO2) = 101temperature kPa (388 ◦ dependenceC) [14], the longer of the the carbonation induction period.dynamics Besides can be this due factor, to the complexinterplay temperaturebetween other dependence influencing of factors, the carbonation such as dynamics can be due to the interplay between other- the influencing temperature factors, effect such on asthe CO2 and MgO solubilities in the melt. It is known that the CO2 solubility in a KNO3 melt passes a maximum at 375 °C and reduces at the higher temperature, - the temperature effect on the CO2 and MgO solubilities in the melt. It is known that the CO2 whereas the CO2 solubility in a LiNO3 melt gradually increases with temperature [24]; solubility in a KNO3 melt passes a maximum at 375 ◦C and reduces at the higher temperature, - the dependence of the CO2, Mg2+ and O2− diffusivities on temperature, as their diffusion can be whereas the CO solubility in a LiNO melt gradually increases with temperature [24]; responsible for very2 slow approaching3 the equilibrium. 2+ 2 - the dependence of the CO2, Mg and O − diffusivities on temperature, as their diffusion can be For the material with α = 0.20, the final conversion is almost constant at 290 °C ≤ Tcarb. ≤ 350 °C, whereasresponsible the synthesis for very rate slow remarkably approaching enhances the equilibrium. at low temperature (Figure 6b). This material providesFor thethe material high conversion with α = 0.20,(0.63–0.67) the final together conversion with isthe almost good constant carbonation at 290 dynamicsC T and 350can beC, ◦ ≤ carb. ≤ ◦ whereasrecommended the synthesis for TCES rate in remarkably this temperature enhances range. at low temperatureIt is worthy (Figureto mention6b). This that material the carbonation provides theconversion high conversion of 0.65–0.70, (0.63–0.67) reported together in this with paper, the is good close carbonation to the maximal dynamics one’s and ever can reported be recommended for MgO- forbase TCES materials in this doped temperature with salts: range. e.g., It the is worthy CO2 uptake to mention 15.7 mmol/g that the at carbonation 340 °C and conversion 16.8 mmol/g of 0.65–0.70,at 300 °C reportedwas reported in this for paper, MgO isdoped close towith the LiNO maximal3-(NaK)NO one’s ever3 [25] reported and (LiNaK)NO for MgO-base3 [26], materials respectively. doped with salts: e.g., the CO2 uptake 15.7 mmol/g at 340 ◦C and 16.8 mmol/g at 300 ◦C was reported for MgO 3.3. Characterization of the Materials by SEM doped with LiNO3-(NaK)NO3 [25] and (LiNaK)NO3 [26], respectively. The morphology of the composites was observed through scanning electron microscopy (SM- 3.3. Characterization of the Materials by SEM 200, TOPCON Inc.) with an acceleration voltage of 7 kV. The SEM images taken at 5,000 magnification indicateThe morphologythe hexagonal of themorpholo compositesgy of was MgO observed crystals through (0.5–1.5 scanning μm size) electron before microscopy carbonation. (SM-200, At TOPCONincreasing Inc.) salt withcontent, an acceleration the crystal border voltage becomes of 7 kV.The fuzzier SEM due images to a taken thick at salt 5000 layer magnification surrounding indicate MgO the(Figures hexagonal 7a,c,e,g). morphology This fact could of MgO support crystals the (0.5–1.5reason whyµm size)too much before salt carbonation. decreases the At reactivity. increasing After salt content,carbonation, the crystal the particles border becomesof the product, fuzzier that due tois a thickmixture salt (MgCO layer surrounding3 + MgO + salt), MgO are (Figure smaller7a,c,e,g). and Thispoorly fact crystallized could support with the respect reason to why the tooinitial much (MgO salt decreases+ salt) mixture the reactivity. (Figures After7b,d,f,h). carbonation, This can thebe particlescaused by of the the salt product, melting that and isa coating mixture of (MgCO the (MgCO3 + MgO3 + MgO)+ salt), crystals are smaller surface. and The poorly morphology crystallized of withcomposite respect with to the α initial= 0.20 (MgOcertainly+ salt) differs mixture from (Figure the other7b,d,f,h). materials This canduebe to causedlarger bystructural the salt meltingfeatures andformed coating by the of salt the coating (MgCO (Figure3 + MgO) 7f). crystalsThis cansurface. create a better The morphology network for ofCO composite2 diffusion with and facilitateα = 0.20 certainlyreaction of di ffCOers2 fromwiththe the other dissolved materials MgO, due and, to larger therefore, structural to be features a reason formed for the by thegood salt dynamic coating (Figureperformance7f). This of this can creatematerial. a better network for CO 2 diffusion and facilitate reaction of CO2 with the dissolved MgO, and, therefore, to be a reason for the good dynamic performance of this material.

(a) α = 0.05 (Before carbonation) (b) α = 0.05 (After carbonation)

Figure 7. Cont.

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(e) α = 0.20 (Before carbonation) (f) α = 0.20 (After carbonation)

(g) α = 0.30 (Before carbonation) (h) α = 0.30 (After carbonation)

FigureFigure 7.7. SEM images of materials ((αα = 0.05, 0.10, 0.20, and 0.30) before and after carbonation atat 350350 ◦°CC forfor 300300 min.min.

3.4. Evaluation of the Speciﬁc Useful Heat 3.4. Evaluation of the Specific Useful Heat

The specific useful heat and power related to unit mass of composite, Qcomp. and Wcomp., are The specific useful heat and power related to unit mass of composite, Qcomp. and Wcomp., are estimated from the measured reacted mole fraction by Equations (4) and (5). At 300 C, the (LiK)NO /MgO estimated from the measured reacted mole fraction by Equations 4 and◦ 5. At 300 °C,3 the composite with α= 0.20 exhibits larger specific useful heat Qcomp. = 1.23 MJ/kg-composite than the (LiK)NO3/MgO composite with α = 0.20 exhibits larger specific useful heat Qcomp. = 1.23 MJ/kg- compositecomposite than with theα= composite0.10 (Figure with8a), α even = 0.10 if (Figure it contains 8a), even less MgO.if it contains At 350 less◦C, MgO. the latter At 350 material, °C, the α = 0.10, has higher Qcomp. value of 1.63 MJ/kg-composite (Figure8b). Both values are superior to those latter material, α = 0.10, has higher Qcomp. value of 1.63 MJ/kg-composite (Figure 8b). Both values are earliersuperior measured to those for earlier other measured salt/MgO compositefor other sa forlt/MgO TCES system,composite namely, for TCES 1.04 MJsystem,/kg-composite namely, and1.04 1.17 MJ/kg-composite for the CaCl2 and LiBr modified MgO-H2O TCES material respectively [27,28]. MJ/kg-composite and 1.17 MJ/kg-composite for the CaCl2 and LiBr modified MgO-H2O TCES material respectively [27,28].

Energies 2019, 10, x FOR PEER REVIEW 9 of 13 Energies 2019, 12, 2262 9 of 13 Energies 2019, 10, x FOR PEER REVIEW 9 of 13 2 T = 300oC 2 (a) carb. o P = 101 kPa (b) Tcarb. = 350 C 2 carb. P = 101 kPa T = 300oC 2 carb. (a) carb. o (b) Tcarb. = 350 C α = 0.02 Pcarb. = 101 kPa α = 0.02 Pcarb. = 101 kPa α = 0.05 α 1.5 α = 0.02 = 0.05 α = 0.10 1.5α = 0.02 α = 0.05 α = 0.10 1.5 α = 0.20 α = 0.05 α = 0.10 1.5 α = 0.20 α = 0.30 α = 0.10 α = 0.20 α α = 0.30 α = 0.30 = 0.20 α 1 1 = 0.30 1 1 [MJ/kg-composite] [MJ/kg-composite] [MJ/kg-composite] [MJ/kg-composite] comp. comp. Q Q comp.

0.5 comp. 0.5 Q 0.5 Q 0.5

Pure MgO Pure MgO 0 0 0 0 50 100 150 200 250 300 0 0 50 100 150 200 250 300 0 50 100 150 200 250 300 0 50 100Carbonation 150 time, 200 t [min] 250 300 Carbonation time, t [min] Carbonation time, t [min]carb. Carbonation time, t [min] carb. carb. carb. Figure 8. Specific useful heat, Qcomp., as a function of the carbonation time at (a) 300 °C and (b) 350 °C FigureFigure 8. 8.SpecificSpeciﬁc useful useful heat, heat, QQcomp.comp., as, as a afunction function of of the the carbonation carbonation timetime at at ( a()a 300) 300◦C °C and and (b ()b 350) 350◦C °C calculated from Figure 3. calculatedcalculated from from Figure Figure 3.3.

TheThe change change of maximumof maximum speciﬁc specific useful useful heat, Qheat,comp.-max Qcomp.-max, according, according to mixing to molemixing ratio mole is ratio is The change of maximum specific useful heat, Qcomp.-max, according to mixing mole ratio is displayed in Figure9. All mixing mole ratio of (LiK)NO 3/MgO3 composite exhibits similar reaction displayeddisplayed in Figure in Figure 9. All 9. mixingAll mixing mole moleratio ratioof (LiK)NO of (LiK)NO3/MgO /MgOcomposite composite exhibits exhibits similar similarreaction reaction tendency under every temperature; the overall Qcomp.-max value has increased with the increasing tendencytendency under under every every temperature; temperature; the overall the overall Qcomp.-max Qcomp.-max value valuehas increased has increased with the with increasing the increasing reaction temperature, Figure9a. It is also conﬁrmed that α of 0.05 and 0.10 (LiK)NO3/MgO composites reaction temperature, Figure 9a. It is also confirmed that α of 0.05 and 0.10 (LiK)NO3/MgO composites reactionare sensitive temperature, to reaction Figure temperature 9a. It is also change, confirmed on the that other α of hand, 0.05α andof0.20 0.10 and (LiK)NO 0.30 composites,3/MgO composites show are sensitive to reaction temperature change, on the other hand, α of 0.20 and 0.30 composites, show arestable sensitive results, to reaction Figure9b. temperature change, on the other hand, α of 0.20 and 0.30 composites, show stablestable results, results, Figure Figure 9b. 9b. 2 2 2 2 (a) (a) Pcarb. = 101Pcarb. kPa= 101 kPa (b) (b) Pcarb. = 101 kPaPcarb. = 101 kPa tcarb. = 300tcarb. min= 300 min tcarb. = 300 mintcarb. = 300 min α = 0.02 α = 0.02 α = 0.05 α = 0.05 α = 0.10 α = 0.10 1.5 1.5 α 1.5 1.5 = 0.20 α = 0.20 α = 0.30 α = 0.30

1 1 1 1 [MJ/kg-composite] [MJ/kg-composite] [MJ/kg-composite] [MJ/kg-composite] comp.-max comp.-max 290oC Q comp.-max Q

0.5comp.-max 0.5 290oC

o Q Q 0.5 300 C 0.5 300oC 330oC o 350oC 330 C 350oC 0 0 00 0.05 0.1 0.15 0.2 0.25 0.3 2900 300 310 320 330 340 350 0 0.05 0.1α [−] 0.15 0.2 0.25 0.3 290Temperature 300 310 [ oC] 320 330 340 350 o α [−] Temperature [ C] Figure 9. Changes of maximum specific useful heat, Qcomp.-max: (a) Qcomp.-max by salt content in the Figure 9. Changes of maximum speciﬁc useful heat, Q :(a) Q by salt content in the Figure 9. Changes of maximum specific usefulcomp.-max heat, Qcomp.-maxcomp.-max: (a) Qcomp.-max by salt content in the composite, α, and (b) Qcomp.-max by carbonation temperature, Tcarb.. composite, α, and (b) Qcomp.-max by carbonation temperature, Tcarb.. composite, α, and (b) Qcomp.-max by carbonation temperature, Tcarb.. 3.5.3.5. Evaluation Evaluation of ofthe the Specific Speciﬁc Power Power of of Heat Heat Release Release 3.5. Evaluation of the Specific Power of Heat Release FigureFigure 10 10 shows shows thethe heat heat output output rate, rate,Wcomp. Wcomp., of ,α of= 0.10α =and 0.10 0.20 and composites 0.20 composites under various under reaction various reactiontemperature. Figuretemperature. At10 initial shows At time initial the of carbonation, timeheat ofoutput carbonation, both rate, composites W bothcomp. ,composites of show α = high 0.10 Wshow comp.and high values0.20 W compositescomp. that values are increased thatunder are various increasedasreaction reaction as temperature. reaction temperature temperature decreased,At initial decreased, time but thisof carbonation, order but isthis changed order both afteris composites changed 50 min; after carbonationshow 50 high min; W under carbonationcomp. values high that are temperature exhibits better W values. The highest W value of α = 0.20, 0.68 kW/kg-composite, underincreased high temperature as reaction exhibits temperaturecomp. better W decreased,comp. values. butThecomp. highestthis order Wcomp. is changedvalue of α after = 0.20, 50 0.68 min; kW/kg- carbonation is higher by a factor of ~2 than that of α = 0.10 composite, 0.40 kW/kg-composite, at 290 C. The composite composite,under high is higher temperature by a factor exhibits of ~2 than better that W ofcomp. α = values. 0.10 composite, The highest 0.40 W kW/kg-composite,comp. value◦ of α = 0.20,at 290 0.68 °C. kW/kg- with α = 0.20 shows generally higher Wcomp. performance than α = 0.10 composite at low temperatures Thecomposite, composite iswith higher α = 0.20by a showsfactor ofgenerally ~2 than higherthat of Wα comp.= 0.10 performance composite, than 0.40 αkW/kg-composite, = 0.10 composite atat 290 °C. lowThe temperatures composite (≤ with 330 °C). α = Since0.20 showsits high generally specific useful higher heat W capacity,comp. performance Qcomp., and than output α = rate, 0.10 W compositecomp., at underlow various temperatures reaction (≤ 330conditions, °C). Since α its= high0.20 specificis suggested useful as heat optimized capacity, mixing Qcomp., andmole output ratio rate,for Wcomp., (LiK)NOunder3/MgO various composite. reaction conditions, α = 0.20 is suggested as optimized mixing mole ratio for (LiK)NO3/MgO composite.

Energies 2019, 12, 2262 10 of 13

(Energies330 2019C)., Since10, x FOR its PEER high REVIEW speciﬁc useful heat capacity, Qcomp., and output rate, Wcomp., under various10 of 13 Energies≤ 2019, ◦10, x FOR PEER REVIEW 10 of 13 reaction conditions, α = 0.20 is suggested as optimized mixing mole ratio for (LiK)NO3/MgO composite.

0.8 0.8 0.8 (a) α = 0.10 0.8 (b) α = 0.20 P = 101 kPa P = 101 kPa (a)0.7 α = 0.10carb. 0.7 (b) carb.α = 0.20 P = 101 kPa P = 101 kPa 0.7 carb. 0.7 carb. 0.6 0.6 0.6 0.6 0.5 0.5

0.5 0.5 290oC 0.4 290oC 0.4 300oC 300oC 330oC290oC 0.4 290330oCoC 0.4 [kW/kg-composite] 0.3 [kW/kg-composite] 0.3 350oC300oC 300350oCoC o

comp. comp. 330 C 330oC

[kW/kg-composite] [kW/kg-composite] o 0.3 W 0.2 W 0.30.2 350 C 350oC comp. comp.

W 0.2 0.1 W 0.20.1

0.1 0 0.10 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Carbonation time, t [min] Carbonation time, t [min] 0 carb. 0 carb. 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Carbonation time, t [min] Carbonation time, t [min] Figure 10. Comparison of heatcarb. output rate, Wcomp., under various reaction temperature:carb. (a) α = 0.10, (b) α = 0.20. Figure 10. Comparison of heat output rate, Wcomp., under various reaction temperature: (a) α = 0.10, Figure(b )10.α = Comparison0.20. of heat output rate, Wcomp., under various reaction temperature: (a) α = 0.10, (4.b )Material α = 0.20. Durability on Cyclic Experiment 4. Material Durability on Cyclic Experiment The durability of the (LiK)NO3/MgO (α = 0.20) is investigated through a cyclic experiment 4. Material(FigureThe Durability 11). durability The reacted on of theCyclic conversion (LiK)NO Experiment 3value,/MgO Δ ( xα , =is 0.20)increased is investigated for 2 cycles throughand becomes a cyclic stabilized experiment with (Figureincreasing 11). number The reacted of cycles; conversion Δx is value,converged∆x, is to increased 0.34 and for that 2 cyclesvalue andis equivalent becomes stabilizedto 0.64 MJ/kg- with The durability of the (LiK)NO3/MgO (α = 0.20) is investigated through a cyclic experiment increasingcomposite. number It is also of cycles;found that∆x is decarbonation converged to 0.34 for and 150 thatmin value is not is enough equivalent to completely to 0.64 MJ/kg-composite. decarbonize (Figure 11). The reacted conversion value, Δx, is increased for 2 cycles and becomes stabilized with Itthe is (LiK)NO also found3/MgO that (α decarbonation= 0.20) composite. for 150The mindifference is not of enough reacted to mole completely fraction, decarbonize x, during cyclic the increasing number of cycles; Δx is converged to 0.34 and that value is equivalent to 0.64 MJ/kg- (LiK)NOexperiment3/MgO are (shownα = 0.20) in composite.Figure 12. Carbonation The difference reactivity of reacted is mole significantly fraction, xincreased, during cyclic after experiment1st cycle by composite.aregas showndiffusivity It is in also Figure enhancement.found 12. Carbonation that decarbonation Then reactivity carbonation isfo significantlyr 150 reactivity min is increased notand enough reacted after 1stto conversion cyclecompletely by gas values di decarbonizeffusivity are the (LiK)NOenhancement.gradually3/MgO getting Then( αstabilized = carbonation0.20) composite.during reactivity initial The 5 cyclesand difference reacted (Figure conversionof12a); reacted Δx value valuesmole was fraction,are changed gradually x from, duringgetting 0.58 tocyclic experimentstabilized0.44. However, are during shown there initial in is Figure no 5 cycles particular 12. (Figure Carbonation change 12a); in∆x Δ valuereactivityx values was after changedis significantly 10th cycle from (Figure 0.58 increased to0.44. 12b), However, especially after 1st there cycle18th by gas diffusivityiscycle no particularand 20th enhancement. cycle change showed in ∆x samevaluesThen reacted aftercarbonation 10th conversion cycle reactivity (Figure values 12 (b), Δandx especially= 0.34). reacted 18th conversion cycle and 20th values cycle are graduallyshowed getting same reactedstabilized conversion during valuesinitial ( ∆5x cycles= 0.34). (Figure 12a); Δx value was changed from 0.58 to

0.44. However, there is no particular change5th in Δx values10th after 10th15th cycle (Figure20th 12b), especially 18th 0.7 cycle and 20th cycle showed same reacted conversion values (Δx = 0.34).

0.6

5th 10th 15th 20th [-] 0.7 0.5 x

0.6 0.4

[-] 0.3 0.5 x

0.2

Reacted mole fraction, fraction, mole Reacted 0.4

0.1 Pcarb. = 101 kPa 0.3 Pdecb. = 0 kPa (Ar atmosphere) tcarb. and tdecb. = 150 min 0 0.2 0 20406080100

Reacted mole fraction, fraction, mole Reacted Carbonation time, t [hrs] carb. 0.1 Figure 11. Cyclic experimental result for (LiK)NO3/MgO compositePcarb. = 101 kPa (α = 0.20) at 330 ◦C. 3 Figure 11. Cyclic experimental result for (LiK)NO /MgO compositePdecb. = 0 kPa (α (Ar = atmosphere)0.20) at 330 °C. tcarb. and tdecb. = 150 min 0 0 20406080100 Carbonation time, t [hrs] carb.

Figure 11. Cyclic experimental result for (LiK)NO3/MgO composite (α = 0.20) at 330 °C.

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0.6 0.6 (a) (b) 1.00 1.00 0.5 0.5 Q Q [-] [-] 0.80 comp. 0.80 comp. x 0.4 x 0.4 [MJ/kg-composite] [MJ/kg-composite]

0.60 0.60 0.3 0.3

1st 7th Cycle No. [-] 0.40 Cycle No. [-] 0.40 0.2 0.2 2nd 1st 10th 7th

2nd 10th Reactedmole fraction, 3rd Reactedmole fraction, 15th 0.1 3rd 0.20 0.1 15th 0.20 4th 18th 4th Pcarb. = 101 kPa 18th Pcarb. = 101 kPa Pdecb. = 0 kPa (Ar atmosphere) Pdecb. = 0 kPa (Ar atmosphere) 20th 5th 5th tcarb. and tdecb. = 150 min 20th tcarb. and tdecb. = 150 min 0 0 0 0 0 50 100 150 0 50 100 150 Carbonation time, t [min] Carbonation time, t [min] carb. carb. Figure 12. Change in the reacted mole fraction during the cyclic experiment: (a) 1st–5th cycles and (b) Figure7th–20th 12. cycles.Change in the reacted mole fraction during the cyclic experiment: (a) 1st–5th cycles and (b) 7th–20th cycles. 5. Conclusions

5. ConclusionsThe carbonation dynamics of (LiK)NO3/MgO composites, that have various molar salt content (α = 0.02, 0.05, 0.10, 0.20, and 0.30) was investigated for effective utilizing of the medium temperature The carbonation dynamics of (LiK)NO3/MgO composites, that have various molar salt content heat (300–400 C). A set of carbonation dynamic experiments were conducted under various reaction (α = 0.02, 0.05, 0.10,◦ 0.20, and 0.30) was investigated for effective utilizing of the medium temperature temperature T = 290 C–360 C and CO partial pressure P(CO ) = 50–101 kPa to find the optimal heat (300–400 °C). A set◦ of carbonation◦ dynamic2 experiments were2 conducted under various reaction salt content. temperature T = 290 °C–360 °C and CO2 partial pressure P(CO2) = 50–101 kPa to find the optimal salt The carbonation reactivity is revealed at α> 0.05 and reached a maximum at α= 0.10–0.20. content. The carbonation mechanisms are suggested and SEM measurements are also conducted for understanding The carbonation reactivity is revealed at α > 0.05 and reached a maximum at α = 0.10–0.20. The carbonation behavior of the (LiK)NO3/MgO composites. The specific useful heat, Qcomp., was calculated carbonation mechanisms are suggested and SEM measurements are also conducted for from the measured reacted mole fraction, ∆x. Despite the largest Qcomp. = 1.63 MJ/kg-composite was understanding carbonation behavior of the (LiK)NO3/MgO composites. The specific useful heat, detected at α = 0.10 and Tcarb. = 350 ◦C, α = 0.20 was suggested as the optimal salt content because it Qcomp., was calculated from the measured reacted mole fraction, Δx. Despite the largest Qcomp. = 1.63 ensures high and stable Qcomp. values of 1.18–1.25 MJ/kg-composite in the whole temperature range MJ/kg-composite was detected at α = 0.10 and Tcarb. = 350 °C, α = 0.20 was suggested as the optimal between 290 ◦C and 350 ◦C. To confirm the durability of the optimized (LiK)NO3/MgO composite salt content because it ensures high and stable Qcomp. values of 1.18–1.25 MJ/kg-composite in the whole (α = 0.20), cyclic experiments are conducted at 330 ◦C. The reacted conversion became stabilized at ∆x = temperature0.34 (Qcomp. =range0.64 MJbetween/kg-composite). 290 °C Theand studied 350 °C. (Li 0.42ToK 0.58confirm)NO3 promotedthe durability MgO-CO of 2theworking optimized pair (LiK)NOhas good3/MgO potential composite for thermochemical (α = 0.20), storagecyclic ofexperiments middle temperature are conducted heat (300–400 at 330◦ C).°C. The reacted conversion became stabilized at Δx = 0.34 (Qcomp. = 0.64 MJ/kg-composite). The studied (Li0.42K0.58)NO3 Author Contributions: Conceptualization, A.S.; Investigation, H.M.; Project administration, Y.K.; Resources, H.T. promoted MgO-CO2 working pair has good potential for thermochemical storage of middle and A.S.; Supervision, Y.K. and Y.A.; Writing—original draft, S.T.K.; Writing—review & editing, Y.A. and S.T.K. temperature heat (300–400 °C). Funding: This research received no external funding. Author Contributions: Conceptualization, A.S.; Investigation, H.M.; Project administration, Y.K.; Resources, Acknowledgments: The authors deeply appreciate the Tokyo Tech World Research Hub Initiative (WRHI) for its H.T.research and A.S.; support. Supervision, Y.K. and Y.A.; Writing – original draft, S.T.K.; Writing – review & editing, Y.A. and S.T.K. Conflicts of Interest: The authors declare no conflicts of interest. Funding: This research received no external funding. Nomenclature Acknowledgments: The authors deeply appreciate the Tokyo Tech World Research Hub Initiative (WRHI) for itsTCES research support. thermochemical energy storage TG thermogravimetric Conflicts of Interest: The authors declare no conflicts of interest. SEM scanning electron microscopy Nomenclaturen amount of a chemical compound [mol] n initial amount of MgO in a composite [mol] TCES0 thermochemical energy storage Mcomp. molar mass of a composite [g/mol] TG thermogravimetric Qcomp. specific useful heat [kJ/kg-composite] SEM scanning electron microscopy Tcarb. carbonation temperature [◦C] nT eq amountequilibrium of a transitionchemical temperature compound [ ◦[mol]C] nT0mp initialmelting amount point [ ◦ofC] MgO in a composite [mol] Mcomp. molar mass of a composite [g/mol] Qcomp. specific useful heat [kJ/kg-composite]

Energies 2019, 12, 2262 12 of 13

Wcomp. heat output [kW/kg] X reacted mole fraction, [-] A molar fraction of salt in a composite [-] 0 ∆r H reaction enthalpy [kJ/mol]

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