Adapting the MgO-CO2 working pair for thermochemical energy storage by doping with salts

Energy Conversion and Management 185 (2019) 473–481

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Adapting the MgO-CO2 working pair for thermochemical energy storage by doping with salts

T

⁎

A.I. Shkatulova, , S.T. Kimb, H. Miurab, Y. Katob, Yu.I. Aristova,b a b

Boreskov Institute of Catalysis, Ac. Lavrentiev av. 5, Novosibirsk 630090, Russia Tokyo Institute of Technology, 2-12-1-N1-22, Ōokayama, Meguro-ku, Tokyo 152-8550, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Thermochemical energy storage Magnesium oxide Magnesium carbonate Salt modification Doping

In this work, the MgO-CO2 working pair has been adapted for thermochemical energy storage (TCES) at medium temperatures by the MgO modification with inorganic salts to promote the TCES dynamics. Brief screening of modifying salts showed that lithium acetate (LiOAc) and mixed lithium-potassium nitrate (Li0.42K0.58NO3) additives could considerably promote the MgO carbonation at P(CO2) ≤10 bar and T ≥300 °C. The de- and recarbonation kinetics, as well as cycling stability of the doped MgO/MgCO3, was reported to outline possible TCES operating conditions (T = 280 °C–380 °C, P(CO2) = 0–1 bar). The heat storage capacity of the salt-promoted MgO was estimated to be 1.6 GJ/m3. A concept of chemical heat pump utilizing the MgO-CO2 working pair was discussed. The salt-promoted MgO-CO2 working pair was concluded to be promising for TCES.

1. Introduction The envisaged transition of society towards carbon-neutral and more efficient energy technologies [1] is unimaginable without powerful tools of thermal energy management as around two thirds of all the primary energy produced is dissipated as heat [2]. Thermal energy storage is among such tools, helping to reduce a mismatch between energy supply and demand. This may be the reason for the remarkable growth of a number of publications in the area of thermal energy storage and transformation for the last decade. Thermal energy can be stored for the later use in sensible, latent or chemical forms [3]. Thermochemical energy storage (TCES) is an emerging, yet underexploited, technology which provides a high storage density and long storage duration in comparison to sensible and latent heat storage. To this day, development of materials for TCES is mainly focused at utilization of low-temperature heat (< 200 °C) bearing in mind such applications as space heating, hot tap water and air conditioning [4–7]. A fewer effort is aimed at utilization of heat at medium (200–500 °C), and high temperatures (> 500 °C) despite about 25% of waste heat is dissipated at T > 200 °C in industrial processes, cogeneration systems and other [8]. Reactions and processes proposed and studied for TCES at medium temperatures are not numerous. They include dehydrogenation of ammonia [9], cyclohexane [10], and MgH2 [11], dehydration of Mg(OH)2 [12] and Ca(OH)2 [13] along with several other reactions [14]. Most of

⁎

Corresponding author. E-mail address: [email protected] (A.I. Shkatulov).

https://doi.org/10.1016/j.enconman.2019.01.056 Received 16 October 2018; Accepted 9 January 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

these reactions suffer from low heat storage density, kinetic problems or material degradation and other factors hindering their application. Therefore, the development of novel advanced materials for TCES at medium temperatures is an important and challenging task. Metal carbonates can be used as a heat storage media by TCES due to their endothermic decomposition accompanied by a release of CO2, or de-carbonation: MCO3 = MO + CO2. The metal oxide MO (solid) and CO2 (gas) are separated from each other and heat is stored in a chemical form for a theoretically infinite time. To release the stored heat one has to supply CO2 to the metal oxide and carry out the reverse exothermic reaction of carbonation. TCES by metal carbonates was proposed in several works, starting with that of Wentworth and Chen in 1976 [15]. In further works, only CaCO3 and PbCO3 were considered for TCES due to favorable reaction rates [14,16–19]. Magnesium carbonate, MgCO3, is only briefly mentioned in some reviews dedicated to TCES due to lack of the data on its application for TCES. The equilibrium

MgCO3(s) = MgO(s) + CO2(g)

o Δr H298 = 116.4 kJ /mol

can be characterized by the turning temperature at which

Tturn =

Δr H ∘ (Tturn ) Δr S ∘ (Tturn )

≈

∘ Δr H298 ∘ Δr S298

(1) ΔrGo298

= 0: (2)

According to the data on free formation enthalpies and entropies [20], for reaction (1) in bulk ΔrHo298 = 116.4 kJ/mol and

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(Reakhim, ≥99.5%), Li2CO3 (Reakhim, ≥99.9%), K2CO3 (Reakhim, ≥99.0%) and Na2CO3 (Reakhim, ≥99.8%) and euthectic mixture of LiNO3 and KNO3, namely, Li0.42K0.58NO3 (further referred to as (LiK) NO3). The euthectic mixture was prepared in advance by co-crystallization of both nitrates in the defined proportions from a water solution. 2.2. Materials characterization The study of carbonation of the new Salt/MgO materials was carried out by using a Rubotherm magnetic suspension thermobalance. The initially prepared sample Salt/Mg(OH)2 was sealed into a reaction chamber. The dehydration was carried out under vacuum at 400 °C. Then, the temperature was lowered to 300 °C and carbon dioxide was supplied to the reaction chamber for several minutes to reach the pressure 10 bar. The weight signal was corrected for the buoyancy effect with the use of a blank measurement. The parameter α60, that is the conversion of MgO to MgCO3 after 60 min of 10 bar CO2 treatment, was chosen as a characteristic of the carbonation performance. It was defined as:

Fig. 1. The equilibrium CO2 pressure over a bulk MgCO3 at various temperatures in the lgP – 1/T coordinates.

ΔrSo298 = 176.0 J/mol/K which yields Tturn ≈ 661 K = 388 °C. Accounting for the temperature dependency of ΔrHo and ΔrSo gives Tturn = 398 °C at P(CO2) = 1 bar [21]. Above this temperature, decarbonation prevails while at a lower temperature the reverse process occurs (Fig. 1). The high value of the reaction enthalpy along with the absence of deliquescence and swelling/ shrinking phenomena on macroscopic level [22], as well as non-corrosiveness, make the MgO-CO2 working pair attractive for TCES. Unfortunately, reaction (1) demonstrates irreversibility, i.e. the interaction of MgO with CO2 is kinetically hindered [23] that makes the release of the stored heat practically impossible. Perhaps, due to this fact, no assessment of the salt-promoted MgO-CO2 system for TCES was reported to this day. A promising approach which could enhance the reactivity of TCES materials is doping them with a salt. Previously, this approach was shown to be fruitful for Mg(OH)2 and Ca(OH)2 as TCES materials [24,25]. The doping may be carried out with nitrates of alkali metals and some other salts [26,27]. In many cases, the doping decreased the dehydration temperature of hydroxide and increased the hydration rate of oxide. It has recently been shown that MgCO3 doping with nitrates may also affect its de-carbonation/re-carbonation [22,23]. In this work, we made a brief screening of salt additives to pure MgO aiming at finding a proper additive that increases the oxide reactivity towards carbonation. For the best additives, de-carbonation and re-carbonation kinetics at various temperatures and pressures were studied to map the set of conditions (T, P(CO2)) under which the doped MgCO3 can be efficiently used for TCES. The materials and their transformations were characterized by HRTEM and XRD in situ. The heat storage capacity was evaluated and a concept of chemical heat pumps utilizing the MgO-CO2 working pair was discussed.

α60 =

[m (60) − m 0 ] m 0 ωMgO / , MCO2 MMgO

(3)

where m0 is the initial mass of Salt/MgO, [m(60) – m0] represents the mass gain by t = 60 min due to CO2 sorption, ωMgO is the weight content of MgO in the material (ωMgO is different for every sample as only a molar content of the salt was fixed), MCO2 and MMgO are the molar masses of CO2 and MgO, respectively. The heat released by t = 60 min of carbonation per 1 g of the MgO was estimated as

Q60 =

∘ α60·Δr H268 , MMgO

(4)

where the reaction enthalpy ΔrHo298 = 116.4 kJ/mol. Non-isothermal carbonation of a chosen material was carried out by using thermogravimetric analysis method (TG-9600P, Advance RIKO, Inc.) in a closed system. Experiments were conducted at the temperatures range from room temperature to 500 °C (10 °C min−1) under two different CO2 pressures (1 and 2 bar), and around 10 mg of the composite was used for each experiment. The carbonation and de-carbonation kinetics, as well as the cycling stability, were studied by using a TGA setup (Advance RIKO TGD-9600) equipped with a flow gas control system which allowed measurements in CO2 or Ar atmosphere (total pressure 1 bar). The stability in consecutive carbonation/de-carbonation cycles is the important property of TCES materials. To evaluate it, the Salt/Mg(OH)2 was loaded into a platinum cell and decomposed at 400 °C in Ar flow. Then thirteen consecutive cycles were carried at T = 330 °C and P = 1 bar by switching the purge gas from Ar to CO2 and backwards. For the kinetic study of carbonation, a sample of the cycled Salt/ MgO (about 50 mg) was loaded into the platinum cell and heated to the desired temperature Tcarb in Ar flow. When the temperature and the sample weight had been stabilized, the measuring cell was switched to the flow of pure CO2 which initiated carbonation (Fig. 2a). After the carbonation, CO2 flow was switched back to Ar flow, and the temperature was raised to 380 °C to ensure a complete de-carbonation before the next carbonation stage. For the de-carbonation kinetic study, the cycled Salt/MgO was preliminarily carbonated for 6 h at P(CO2) = 1 bar at T = 330 °C (Fig. 2b). Then, CO2 flow was switched to pure Ar flow to initiate decarbonation, and the temperature was quickly (< 1–2.5 min) raised/ dropped to the target de-carbonation temperature Tdec. After each decarbonation kinetic measurement, the sample was fully de-carbonated in Ar flow for the next carbonation measurement at the various target temperature.

2. Experimental 2.1. Materials preparation The materials Salt/MgO (salt content 10 mol. %) were prepared by dehydration of Salt/Mg(OH)2 composites which were synthesized by mixing of dry salt and Mg(OH)2. The salts were dried in oven at 160 °C overnight before the mixing. In order to distribute salt uniformly, 30–40 ml of water per 1 g of Mg(OH)2 was added to the mixture. The slurry was subsequently dried under vigorous stirring and decreased pressure using a rotary evaporator. The salts were chosen based on our previous results [26]: CH3COOLi (LiOAc, Sigma Aldrich, ≥97.0%), CH3COOK (KOAc, Sigma Aldrich, ≥99.0%), KNO3 and NaNO3 474

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Fig. 2. Schemes of kinetic experiments for carbonation (a) and de-carbonation (b).

The samples were loaded into the XRK-900 (Anton Paar) reaction chamber that allowed heating and gas control during the experiment. De-hydration of the initial Salt/Mg(OH)2 was carried out in a flow of dry helium (300 ml/min) at 100, 250 and 300 °C. After de-hydration, pure CO2 was supplied to the chamber at 300 °C. After 1 h, the CO2 flow was switched to helium, and the temperature was raised to 400, 450 and 500 °C thus allowing the carbonate to decompose. High resolution transmission electron microscopy (HR-TEM) images were obtained by using a JEM-2010 electron microscope (JEOL, Japan) with a lattice-fringe resolution of 0.14 nm and at an accelerating voltage of 200 kV. Samples to be examined by HR-TEM were prepared on a perforated carbon film mounted on a copper grid by placing slurry of the material in dry hexane on the film followed by evaporation of hexane. 3. Results 3.1. Choice of the doping salt

Fig. 3. Illustration of the effect of salt additives on the MgO carbonation reactivity and the heat released.

The pure MgO obtained by dehydration of the initial Mg(OH)2 cannot be carbonated at 300 °C and P(CO2) = 10 bar: its conversion α60 after 60 min of carbonation does not exceed 0.01 (Fig. 3). This may be due to kinetic impediments [23]. Indeed, low basicity of MgO along with non-polarity of CO2 molecule are responsible for the high activation barrier of the reaction O2− + CO2 = CO32−. Moreover, such a reaction would involve an increase in the molar volume by a factor of 1.73 which could as well create diffusion hindrance. Lowering the temperature does not make the carbonation to proceed despite bringing the reaction even further from the equilibrium. The low reactivity is also observed for MgO doped with the additives of Li2CO3 and KNO3. The mixtures of MgO with Na2CO3, K2CO3, and KOAc (Ac = CH3CO) can be carbonated, however, the parameter α60 does not exceed 0.1 and the total heat released during 60 min of carbonation, Q60, is < 300 J/g-MgO which is comparable to the latent heat of phase-change materials [30] but is quite low for TCES. The additives LiOAc, Li0.42K0.58NO3 = (LiK)NO3, and NaNO3 considerably increase the conversion α60 and the released heat Q60 which exceeds 1800 J/g-MgO in the case of lithium acetate (Fig. 3). For further study, we have chosen (LiK)NO3 and LiOAc since they exhibit the highest carbonation conversion. Stability of the salt under the cycle operating conditions is of high importance for TCES materials as they must be stable in multiple consecutive heat “storage/release” cycles. To evaluate the stability of the two candidate salts – LiOAc·2H2O and (LiK)NO3 – heating in Ar flow is carried out. Lithium acetate loses hydrate water at 100 °C and then, apparently, decomposes upon heating at 310–330 °C (yielding Li2CO3 and acetone as the main products [31]) since the weight loss is 46.2%

The oxide fraction carbonated by the moment t, αc(t), was calculated by using the expression:

α c (t ) =

[m (t ) − m 0 ] m 0 ωMgO / MCO2 MMgO

(5)

For de-carbonation kinetics, the conversion was determined by a similar expression:

αd (t ) =

[m 0 − m (t )] m 0 ωMgCO3 / , MCO2 MMgCO3

(6)

where m0 is the initial sample mass after carbonation (αc ∼ 0.4–0.45), ωMgCO3 is the mass content of MgCO3 in the sample after the carbonation step (Fig. 2b), MMgCO3 is the MgCO3 molar mass. The heat released during the carbonation by the moment t was calculated as follows:

Qcarb (t ) =

∘ α c (t )·Δr H298 . MMgO

(7)

X-ray diffraction analysis in situ was performed by using a D8 Advance X-ray diffractometer (Bruker) equipped with a LynxEye detector maintaining Bragg-Brentano parafocusing geometry. The radiation of Cu Kα1 and Kα2 was used for the experiments while Cu Kβ line was filtered by a Ni monochromator. The XRD patterns were collected in the 2θ range 10-80° with a step of 0.05° and counting time of 2 s in each point. The analysis of the experimental data was carried out by a Topaz v. 4.2 software and with use of the ICDD database. 475

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Fig. 4. Thermogravimetric tests on the thermal stability for pure lithium acetate dihydrate LiOAc⋅2H2O (a) and mixed lithium-potassium nitrate (LiK)NO3 (b).

(Fig. 4a). On the contrary, (LiK)NO3 shows no indication of decomposition to nitrite as the weight loss was only 3.0% at 500 °C (Fig. 4b). Thus, for the further study, we selected the material (LiK)NO3/MgO as the more stable one.

3.2. De-carbonation conditions and the cycling stability To determine suitable conditions of carbonation and de-carbonation, the (LiK)NO3/ MgO composite was heated up to 500 °C with a constant rate of 5 K/min at P(CO2) = 1 and 2 bar. The material exhibited the onset of weight gain at 254–261 °C due to carbonation (Fig. 5). The weight gain accelerates upon further heating and ends at 395 °C at P(CO2) = 1 bar and 420 °C at 2 bar. These temperatures are 5–10 °C lower than the calculated equilibrium temperatures (Fig. 1) which indicates that the salt doping slightly shifts the equilibrium towards lower temperatures favoring the decomposition. A similar effect was previously described for hydroxides [32]. At higher temperature, the loss of the mass due to de-carbonation of (LiK)NO3/MgCO3 is observed. The driving force of the reaction plays a crucial role in its dynamic behavior [33]. For some gas–solid transformations, a superheating/ subcooling of 50–100 °C is necessary to carry out the reaction in practice [32]. Thus, the carbonation temperature 330 °C, which is 68 °C lower than the equilibrium temperature Tturn = 398 °C at P (CO2) = 1 bar, is chosen for the study of cycling stability. It is found that the carbonation conversion increased for 5 cycles and reached 0.43–0.45 (Fig. 6). Since the cycled sample shows a higher carbonation conversion, it was chosen for further kinetic study.

Fig. 6. Cycling stability of (LiK)NO3/MgO over 13 carbonation/de-carbonation cycles. T = 330 °C, P = 1 bar.

3.3. Characterization by X-Ray Diffraction and high resolution transmission electron microscopy At room temperature, the X-Ray Diffraction (XRD) pattern of (LiK) NO3/Mg(OH)2 contains intense reflexes of Mg(OH)2 along with the minor reflexes of the salt dopant (Fig. 7a). The dopant exhibits only reflexes of α-KNO3 phase (PDF-2 #04-012-4420). No signals from lithium compounds were detected which may indicate that the major part of the doping salt does not occur in crystalline form. Moreover, in the initial samples there are signals from hydromagnesite (PDF-2 # 04013-7631) which presents in minor quantities. Heating of the material up to 100 °C in He flow causes LiNO3 to crystallize and part of KNO3 also crystallizes in β-form. At 250 °C reflexes of MgO appear indicating decomposition of Mg(OH)2 and hydroxocarbonate. Full decomposition occurs at 300 °C along with the salt melting since only MgO reflexes can be observed. After the decomposition, the He flow was switched to CO2 flow and after 1 h reflexes of MgCO3 (PDF-2 # 00–008-0479) were observed (Fig. 7b). After carbonation, the flow was again switched to He and material was heated up to 400 °C. At these conditions, MgCO3 was fully decomposed. After cooling down, traces of Li2CO3 phase crystallize and after waiting for a longer time LiNO3 and β-KNO3 phases are observed (Fig. 7c). The initial, decomposed and cycled materials were characterized by HRTEM (Fig. 8). The initial sample consists of hexagonal platelets of Mg (OH)2 covered with the salt in accordance with the literature [32]. After

Fig. 5. Weight change of (LiK)NO3/MgO in the course of heating in the CO2 atmosphere. 476

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Fig. 7. X-Ray Diffraction patterns of (LiK)NO3/Mg(OH)2 and products of its decomposition (a) carbonation and de-carbonation (b) at various temperatures. The decarbonated sample was cooled down to room temperature in He flow (c).

Fig. 8. HRTEM images of initial (a), dehydrated (b) and cycled (c) (LiK)NO3/MgO. 477

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decomposition, the form of the crystals is not retained as observed in the case of pure Mg(OH)2-MgO transition [34]. Instead, the decomposed material consists of intergrown cubic crystallites of MgO forming agglomerates of irregular shape (Fig. 8b). After cycling, these agglomerates are grown into irregular hexagonal crystals of magnesium carbonate (Fig. 8c). Thus, according to the XRD data, the carbonation of MgO occurs already at P(CO2) = 1 bar and T = 300 °C, the salt dopant being in a molten state. The phase of the mixed salt is not observed. Instead, the crystalline salt dopant is present in the form of LiNO3, α- and β-KNO3 phases. The salt is conserved in the material after de-carbonation at 500 °C. The HRTEM data indicate the hexagonal morphology of MgCO3 formed after carbonation/ decarbonation cycling from the material with irregularly-shaped morphology. This fact could suggest that the molten salt phase assists the MgCO3 crystal growth. 3.4. Carbonation and decarbonation kinetics The carbonation kinetics of (LiK)NO3/MgO was studied for the material subjected to 13 cycles since such material shows higher carbonation conversion as compared to the first 5 cycles (Fig. 6). At 290–320 °C, an induction period of 20–25 min is observed. The initial rates are almost the same, however, the final carbonation conversion gradually increases at higher temperatures from 0.30 to 0.45 (Fig. 9). At 360 °C, both the induction period and the final carbonation conversion further increase. At 365 °C, the final conversion is slightly lower than at 360 °C. No carbonation is observed at 370 °C. Thus, the final conversion reaches a maximum of 0.54 at 360 °C after 6 h of carbonation. This conversion corresponds to the amount of heat released per 1 g of the MgO Qcarb = 1570 J/g-MgO (or 1260 J per 1 g of the composite) which is of high interest for TCES (Fig. 9). The complex dependence of the carbonation rate on temperature is a manifestation of the complicated carbonation mechanism. The whole process consists of several consecutive stages [28,29,35,36]:

Fig. 10. Decarbonation kinetics of (LiK)NO3/MgCO3 at various temperatures in Ar flow.

molten nitrate goes through a maximum for KNO3 if one increases the temperature [37]. This resembles the observed maximum of the conversion which may be explained by the increase of the CO2 solubility at 290–360 °C and its decrease at T > 360 °C. Moreover, at T > 360 °C the reaction systems approach the equilibrium temperature (< 398 °C) which further reduces the reaction rate. The de-carbonation also exhibits an induction period at T = 290–310 °C (Fig. 10). At T > 330 °C the induction period disappears, and a zero-order kinetic equation can describe the process. The de-carbonation rate increases with temperature. The material can be fully de-carbonated at T > 330 °C already after 1 h which may be of interest for TCES.

• Melting of the salt (T = 125 °C) to form a thin liquid film on the surface of MgO. • Dissolution of CO and MgO in the film (MgO dissociates into Mg and O ). • Reaction O + CO = CO in the liquid phase. • Crystallization of MgCO from the melt due to oversaturation of the

4. Discussion

melting

4.1. Heat storage density and maximal power

2+

2

2−

2−

Amount of the heat stored per unit mass or volume plays a crucial role when considering thermochemical systems. The gravimetric and volumetric heat storage capacities (GSC and VSC) can be defined as follows:

2− 3

2

3

solution. This is a complex process that includes stages of nucleation and growth.

GSC (t ) =

Because of this mechanism, the induction period may be attributed to the low solubility of CO2 in the liquid film. The solubility of CO2 in

∘ α (t )·Δr H298 ωMgO , MMgO

VSC (t ) = GSC (t )·ρ ,

(8) (9)

where α is the conversion of heat storage or release reaction, ρ is the material density. The bulk density ρbulk and the real density ρreal give GSCbulk and GSCreal, respectively. As (LiK)NO3/MgCO3 can be readily de-carbonated at T > 300 °C (Fig. 10), the carbonation conversion αc was chosen for the GSC estimation since this parameter limits the amount of CO2 that the material can exchange. The maximal value of αc under the tested operating conditions was taken as 0.55. The real measured density for the loose powder of (LiK)NO3/MgO is ρreal = 1000 kg/m3 while the theoretical bulk density was assumed to be ρbulk = 3580 kg/m3. Using these parameters, the GSC was estimated to be 1600 kJ/kg-MgO while VSCbulk and VSCreal were found to be 5.8 GJ/m3 and 1.6 GJ/m3, respectively. These values of heat storage capacity are typical for thermochemical materials and surpass the ones for PCMs (Table 1). The heat storage/release power is also among the parameters important for TCES. In this work, the maximal heat release and storage powers were assessed on the basis of the kinetic study using the

Fig. 9. Carbonation kinetics of (LiK)NO3/MgO in open system at P (CO2) = 1 bar. 478

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Table 1 Gravimetric and volumetric heat storage capacities of (LiK)NO3/MgO in comparison to selected systems operating at medium temperatures. For chemical reactions, the heat storage capacity is calculated on the assumption of full conversion with the exception of the MgO-CO2 reaction (αc = 0.6). Reaction or material

Tstorage, oC

GSC, kJ/kg

VSCbulk, GJ/m3

ρreal, kg/m3

VSCreal, GJ/m3

Ref.

MgO + CO2 = MgCO3 PbO + CO2 = PbCO3 CaO + H2O = Ca(OH)2 MgO + H2O = Mg(OH)2 Mg + H2 = MgH2 KNO3 K0.52Na0.48NO3

300 400 390 250–300 305 334 223

1400 395 1864 2040 3125 95 125

5.8 3.70 6.15 4.53 6.07 0.20 0.27

1000 900 445 150–700 855 2110 2180

1.1 0.355 0.83 0.28–1.30 2.67 0.20 0.27

– [38,39] [40] [32,41] [42] [43] [43]

Fig. 11. Maximal power of carbonation and de-carbonation of (LiK)NO3/MgO at various temperatures.

following expressions: realse Wmax =

dα c ∘ Δr H298 dt

storage Wmax =−

dαd ∘ Δr H298 dt

Fig. 13. Thermodynamic representation of possible chemical heap pump cycles based on MgO-CO2 working pair and popular low-temperature CO2 sorbents.

(10)

Carbonation – the heat release stage – can be efficiently (400–500 W/kg) carried out at T = 320–340 °C and P(CO2) = 1 bar (Fig. 11). Decarbonation – heat storage – can be efficiently (> 300 W/ kg) carried out at T > 330 °C. These power levels are promising for heat storage. Performance of this material on a reactor level, e.g. in a bed, requires further investigation to account for heat and mass transfer in the bed. Thus, doping MgO with (LiK)NO3 makes this material feasible for TCES at medium temperatures regarding both gravimetric and volumetric heat storage capacities as well as the maximal power of heat storage and release. However, the considered system requires supply or removal of CO2(g) in order to operate. The storage of CO2 could be

Table 2 Possible candidates for coupling with MgO-CO2 working pair and their characteristics based on the literature data. Working pair

Tdes, oC

Ts, C

a, mmol-CO2/g

COP

Ref.

Mg-MOF-74 – CO2 Zeolite 13X – CO2 Zeolite NaY – CO2 Activated carbon AC-PPy

100 73 120 56

20 20 30 25

5 2.5 2 2

1.8 1.8 2.2 2.76

[46] [47] [48] [49]

Fig. 12. An operation scheme of a chemical heat pump (heat storage mode) utilizing the MgO-CO2 working pair. 479

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1.6 MJ/kg-MgO and 1.6 GJ/m3, respectively. Along with a high maximal heat storage/release power these parameters are promising for TCES. Coupling the MgO-CO2 working pair with some generic lowtemperature CO2 sorbents may be a basis for a chemical heat pump with COP = 1.8–2.8 which upgrades heat from 56 to 120 °C to 320–350 °C utilizing external heat at T > 330 °C. Thus, the results showed that magnesium oxide can be adopted for TCES by doping with a proper salt.

arranged in a pressurized vessel and its removal during the de-carbonation of MgCO3 could be carried out by mechanical pumping. Such realization, however, requires additional work to be used for pumping which reduces the system efficiency. Chemical binding of the released CO2 makes the mechanical work unnecessary and leads to the concept of a chemical heat pump [44]. 4.2. MgO-CO2 working pair for chemical heat pumping

Declaration of interests

A simple thermally-driven chemical heat pump (CHP) consists of two mutually connected vessels containing sorbents that can bind a gaseous species, for instance, carbon dioxide. In this paper, we consider a CHP with MgO-CO2 as a working pair. The operation cycle of such CHP is intermittent, and under storage mode it comprises the three steps (Fig. 12): 1) de-carbonation of MgCO3 using waste heat Qdec at T = Tdec accompanied by sorption of the released CO2 by a sorbent S at T = Tsorb and generation of the heat Qsorb; 2) storage of heat in chemical form by isolating the two vessels from each other; 3) release of the stored heat Qcarb at Tcarb by supplying lowtemperature heat Qdes at Tdes thus desorbing CO2 from S∙nCO2. This classic CHP cycle can be thermodynamically represented by two isosteres and two isobars (Fig. 13). The temperatures Tdec and Tcarb lie within the interval 290–380 °C as shown above while Tdes and Ts may vary depending on the nature of the second working pair S-CO2. Here we propose several S-CO2 working pairs which could be coupled with the MgO-CO2 to compose the CHP and evaluate its theoretical COP. Among many options including chemical CO2 binding (e.g. with the use of LiOH, NaOH, CaO etc.), we consider only adsorption (Table 2) since CO2 can be recovered in this case using low-temperature heat at Tdes < 200 °C [45]. An important parameter of such CHP characterizing its first-law efficiency is the Coefficient Of Performance or COP:

COP =

Qcarb = Qdes

α c ·Δr H ∘ 1 a

a

∫0 Δiso H ∘ (a) da

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, (11)

where a is the amount of CO2 sorbed per unit mass of the sorbent S, ΔisoHo is the isosteric heat of ad-/desorption for CO2 on S. The obtained COP values lie within the interval 1.8–2.8 which may be of interest for chemical heat pumping. The materials in Table 2 can ad/adsorb CO2 at ambient temperature Ts = 20–30 °C. The desorption temperature Tdes for the considered adsorbents does not exceed 120 °C and may be lowered at the expense of the amount CO2 exchanged in a cycle. This makes possible the use of low-temperature heat (including solar energy) for driving the heat pump. Thus, in the light of our measurements and literature data on low-temperature CO2 sorbents, the MgO-CO2 working pair may be of interest for chemical heat pumping. 5. Conclusions In this work, the MgO-CO2 working pair was adapted for thermochemical energy storage by using a salt modification approach. A brief survey of salt additives (Fig. 2) showed that CH3COOLi and Li0.42K0.58NO3 considerably promote the MgO carbonation. The additive Li0.42K0.58NO3 was chosen for further study as the most thermally stable. The carbonation kinetics of Li0.42K0.58NO3/MgO, the de-carbonation kinetics of Li0.42K0.58NO3/ MgCO3 and the cycling stability of the composite were studied under tentative conditions of TCES cycles. The material Li0.42K0.58NO3/MgO showed an increase in the carbonation conversion for the first 5 cycles and then a stable de-carbonation conversion over the last 8 cycles. The carbonation kinetics was studied at 280–360 °C. It was found that the de-carbonation process may be efficiently carried out at T > 330 °C. The gravimetric and maximal volumetric heat storage capacities estimated for Li0.42K0.58NO3/MgO were 480

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