Thermal Investigation of a Doped Alkali-Metal ... - ACS Publications

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Thermal Investigation of a Doped Alkali-Metal Carbonate Ternary Eutectic for Direct Carbon Fuel Cell Applications Michael J. Glenn, Jessica A. Allen, and Scott W. Donne* Discipline of Chemistry, University of Newcastle, Callaghan NSW 2308, Australia ABSTRACT: The carbonate eutectic mixture of Li2CO3, K2CO3, and Na2CO3 is commonly used as an electrolyte within the direct carbon fuel cell. Here, seven different minerals common to the ash content of Australian bituminous coals (anatase TiO2, SiO2, CaCO3, CaSO4, Fe2O3, FeS, and kaolin) were used to modify the ternary carbonate eutectic to explore the thermodynamics of the carbonate melting process. Thermal effects were examined using differential thermal analysis, where it has been shown that dissolution of the contaminant leads to liquid-phase disruption, the extent of which varies with dopant type. Furthermore, modeling of the melting process carried out using different heating rates allowed determination of the activation energy for melting in the presence of the various contaminants, where it was shown that the contaminants can dramatically affect the activation energy and, subsequently, the kinetics of the melting process.

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INTRODUCTION Energy Supply. Modern society is very dependent on a reliable supply of energy, the demand for which has been forecasted to increase dramatically into the future. Current predictions are that it will increase from 1.2 × 106 TW in 2000 to over 2.5 × 106 TW per annum by 2050.1,2 The main cause of this increase is a growing global population, which is estimated to be 12 billion by 2050.1,2 At present, the majority of energy used is derived from the combustion of fossil fuels such as coal, oil and natural gas. Into the future this is not a viable option for many reasons. First, from an environmental perspective, combustion of fossil fuels leads to significant emissions of greenhouse gases, such as carbon dioxide, as well as various SOx and NOx gases, and particulates.3 Present technologies for post-combustion capture of CO2 are energy-intensive, which reduces process efficiencies.4 Second, fossil fuel-based energy production is very inefficient, with the most efficient coal-fired power stations operating at only ∼40% thermal efficiency.5 Finally, fossil fuels are also a limited resource, with current estimates predicting they will be available, at most, for the next few hundred years. As such, there is a dire need for more sustainable, renewable, and environmentally friendly energy technologies to shift the burden of energy production away from fossil fuels.6 Many options for energy production are available at different stages of development; however, currently, none are able to directly accommodate the complete shift from fossil fuel-based energy to renewable energy. As a result, in the future, energy production will be more distributed and smaller scale in nature, rather than the very centralized large-scale fossil fuel-based power stations seen in operation today.7 In the absence of such immediate replacement options, what is required are technologies that bridge the gap between fossil fuel-based technologies and renewable technologies. One such transitional option is the direct carbon fuel cell (DCFC). The Direct Carbon Fuel Cell (DCFC). The basic reaction of the DCFC is identical to that used in a coal fired power station; i.e., © XXXX American Chemical Society

C(s) + O2 (g ) → CO2 (g )

(ΔH = − 394 kJ/mol at 25 °C) (1)

However, in the DCFC, this reaction is carried out electrochemically in a fuel cell configuration by splitting up the overall reaction into its associated redox reactions; i.e., C + 2O2 − → CO2 + 4e−

O2 + 4e− → 2O2 −

(anode)

(cathode)

(2) (3)

The intrinsic benefit of such a process is that there is only one energy transformation step involved; i.e., chemical to electrical, which means the efficiency of the process is quite high. Thermodynamically, this process is ∼100% efficient, but practically, it is ∼80% efficient,8 which is at least double the efficiency of a coal-fired power station. Energy that is not directly converted to electrical energy is converted to heat, because of resistive losses in the electrical components and the overpotential causing efficiency losses. The heat released during operation can be used to maintain the operating temperature of the fuel cell (500−700 °C).9 Therefore, only a minor amount of heat generated from external sources is required to maintain system operation, the required amount varying in accordance with cell thermal insulation and arrangement. Literature studies have indicated that the inclusion of minerals commonly found in anthracitic coals in the fuel can have a catalytic effect on the oxidative kinetics of the DCFC anode reaction.10 Minerals added included a selection of metal oxides,11 semimetal oxides,10 and clays12 that have been applied in a slurry-type DCFC arrangement where the carbon fuel is in close contact with the molten carbonate. Hypotheses put forth to explain the enhanced performance of carbon oxidation in the presence of these impurities suggest that cationic metals draw oxide ions to the carbon surface from the carbonate. Oxide ions act as an intermediate in the carbon oxidation pathway; hence, Received: May 6, 2015 Revised: July 13, 2015

A

DOI: 10.1021/acs.energyfuels.5b01027 Energy Fuels XXXX, XXX, XXX−XXX

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temperatures, potentially reducing energy consumption and start-up costs, increasing operational efficiency.

the cationic metals are catalytic. In addition, the increased concentration of oxide ions due to the solubilization of oxide containing impurities has also been cited as improving kinetics.11 This is beneficial to DCFC operation as it enhances oxidation rate and, therefore, the specific power of the system.11 DCFC Electrolytes. The molten carbonate electrolyte, which is the focus of this work, is typically a mixture of Li2CO3, Na2CO3, and K2CO3 to obtain an electrolyte with a lower melting point (MP) compared to the individual alkalimetal carbonates.13 The ternary system of carbonates has a eutectic composition of 43.5 mol % Li2CO3, 31.5 mol % Na2CO3, and 25.0 mol % K2CO3, with a reported melting temperature of 397 °C.13 The ternary eutectic melting point is lower than that of the three binary eutectics. It has been reported in the literature that the electrochemical performance of graphite in certain binary systems is superior to the ternary systems.14 In the DCFC, the alkali-metal carbonate electrolyte acts as an ionic shuttle to facilitate the oxidation processes that occur in the anode compartment, meaning that the overall reaction involves a carbonate species; i.e., C + 2CO32 − → 3CO2 + 4e−

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Carbonate Preparation. A ternary carbonate mixture was prepared from compounds with reagent-grade purity (>99%). Lithium, sodium, and potassium carbonates were obtained from Sigma−Aldrich and before use were dried in an alumina crucible at 150 °C for 2 h to remove water. Carbonate mixtures were made up to 200 g; they were prepared by first mixing 43.5% Li2CO3, 31.5% Na2CO3, and 25.0% K2CO3 (mol %), using a mortar and pestle. To ensure thorough mixing, the sample was passed through a ball mill (Fritsch, Pulverisette). Milling was carried out over four intervals of 10 min, in both clockwise and counterclockwise directions at 200 rpm, with a 2 min rest period between milling periods. Following this, the mixture was heated up to 600 °C in air in order to initially melt and, hence, homogenize the mixture. The melt was kept at 600 °C for 4 h, then cooled to ambient temperature, after which it was removed from the crucible, broken up, and ground to a fine powder using a mortar and pestle. Seven impurities common to coals sourced from New South Wales (NSW; Australia) mines12 were added to the unmodified carbonate mixture at a concentration of 5 wt %. The impurities examined were Fe2O3, FeS, TiO2, CaCO3, CaSO4, SiO2, and kaolin, which were all obtained from Sigma−Aldrich and before use were ground for 5 min to the consistency of a free-flowing powder using a mortar and pestle. They were then passed through a 300 μm sieve before being mixed thoroughly with the eutectic using a mortar and pestle prior to analysis. Differential Thermogravimetric Analysis (DTA). All thermal analysis experiments were carried out using a Perkin−Elmer Diamond TGA/DTA instrument. The mass of the sample used for the TGA/ DTA analysis was within the range of 12−16 mg. In order to achieve acceptable thermal conductivity between the sample and the thermocouple, aluminum crucibles were used. The TGA/DTA experiment consisted of heating and cooling cycles oscillating around the melting/freezing point of the eutectic. In all experiments the thermal behavior of the sample was monitored relative to a similar mass of α-Al2O3, which was used as the reference material. To ensure that the instrument was behaving correctly, analysis of calcium oxalate monohydrate decomposition was conducted at various heating rates and compared with literature data.17 Prior to each test, the sample was placed in the TGA/DTA Instrument and held for 1 min at 25 °C with a nitrogen flow rate of 20.0 mL/min in order to equilibrate at the initial conditions. The sample was then heated at 15 °C/min from 25 to 150 °C, where it was held for 10 min to ensure complete removal of water from the sample. After this equilibration period, the sample was again heated up to 600 °C at 20 °C/min, where it was held for 10 min. The sample was then cooled at 20 °C/min back down to 350 °C, where it was held for 10 min. The sample was subsequently heated and cooled six times across the temperature range of 350−600 °C at 5 °C/min, with equilibration times of 10 min inserted between heating and cooling ramps. This cycling was conducted to further ensure homogenization, as well as ensure reproducibility of the results, hence the steady state response of the carbonate mixture. Following homogenization, experiments were carried out at multiple heating rates for the pure eutectic and also for all contaminants at the 5 wt % concentration level in order to enable determination of the kinetic parameters by a multivariate nonlinear regression method, specifically the Friedman method.18 For application of this method, the samples were heated at rates of 2, 5, 10, and 20 °C/min consecutively up to 600 °C and held for 10 min for thermal equilibration, before being cooled back down to 350 °C, and held again for 10 min (full procedure is depicted in Figure 1). Mass was also monitored during this time, to ensure that no thermal decomposition of the carbonate ensued.

(4)

Carbonate ions consumed in eq 4 are replenished by combining the oxide ion with the CO2 to form carbonate; i.e., CO2 + O2 − → CO32 −

EXPERIMENTAL METHODS

(5)

With the use of coal as a fuel in the DCFC, another key role of the carbonate eutectic is as a reservoir for the impurities present in the coal. A very common feature of coal is the presence of inorganic impurities (ash) in the material at varying levels.15 These ash species are nonelectroactive and therefore are not consumed in the operation of the DCFC, and will accumulate within the electrolyte. Ash species that are common in coals include SiO2 (quartz), Al2O3, TiO2, Fe2O3, FeS, CaCO3, CaSO4,12 and the impact of impurities present in the molten carbonate electrolyte is essentially unknown. Studies aimed at using molten salts for high-temperature solar thermal applications used a similar eutectic composition of Li2CO3−Na2CO3−K2CO3 in the proportions 32.1−33.4− 34.5 (wt %).16 It was found that the unmodified eutectic gave rise to a single melting peak in a differential thermal analysis (DTA) experiment, signifying the melting of a single phase material. Interestingly, the addition of NaNO3, KCl, and NaOH lowered the melting point of the alkali-metal carbonate eutectic even further.16 These literature results were confirmed by the use of thermodynamic modeling software, as well as experimentally; however, the reasons for this complex thermal behavior were left to further study. This Work. The purpose of this work is to examine the effect that various coal ash impurities have on the thermal properties of the molten carbonate electrolyte. This will be examined here using differential thermal analysis of the melting process that the eutectic undergoes in the presence of common coal impurities. Specifically, the focus is on how the thermodynamics and kinetics of melting vary in the presence of impurities, also indicating whether the impurities are soluble or not in the molten carbonate electrolyte. These impurities are of particular importance for DCFC operation, because they are expected to be in close contact with the molten carbonate when present in coal fuels and their presence may lower operating B


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with the melting and freezing processes in the temperature window explored. An example of the DTA signal for a complete melt−freeze cycle is shown in Figure 3, in this case, for the

Figure 1. Temperature program used for thermal analysis.

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RESULTS AND DISCUSSION Gravimetric and Differential Thermal Analysis Results. The mass change associated with the melting and freezing cycles was found to be negligible, as shown in the thermogravimetric analysis (TGA) plot in Figure 2. This was an expected result since, at these temperatures, no thermal decomposition was expected,19 beyond the initial loss of water from the materials at low temperatures. Therefore, the main focus was on the differential thermal analysis (DTA) response relative to the heat transfer associated

Figure 3. Differential thermogravimetric analysis (DTA) result for the melting and freezing of the unmodified eutectic. Heating and cooling rate: 5 °C/min; N2 purge at 20 mL/min.

unmodified carbonate eutectic mixture. This figure shows an expected negative (endothermic) peak for melting and a positive (exothermic) peak for freezing. Note here that the melting of the eutectic is spread over a relatively large temperature range (370−405 °C) that encompasses the previously reported melting temperature of 397 °C13 and emphasizes the relatively slow kinetics of the melt process. Furthermore, the melting and freezing peaks are offset by 10− 20 °C, because of a lag in nucleation during the freezing half cycle.12 Both peaks are also superimposed on a sloped linear background, which represents changes in the heat capacity of the carbonate mixture with changes in temperature and phase; i.e., the sample has an increasing heat capacity as the temperature is increased. For further analysis, a linear background correction of the DTA data was used to focus specifically on the melting of the carbonate mixture. In addition, the DTA data was normalized with respect to the sample mass (μV/mg) post-dehydration at ∼220 °C. At this temperature, structural water in the case of certain contaminants such as kaolin was completely removed.20 This thereby enabled a direct comparison between different sample responses. The background correction was aimed at isolating the endothermic (melting) peak in the DTA response, although, for samples containing SiO2, two distinct peaks separated by ∼40 °C were observed. In this case, the peaks were normalized separately in such a way that the heating domain was limited to where the DTA response returned to baseline levels. Normalized response curves for different contaminant concentrations of the impurities are shown in Figure 4. Heating of the pure eutectic gave rise to a single DTA peak, which was expected for a well-homogenized system where

Figure 2. Mass percent of unmodified eutectic and selected contaminants added at the 5 wt % level. All other modified eutectic samples exhibited similar behavior. C


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Figure 4. Background-corrected and normalized DTA response (μV/mg) for the ternary Li, Na, and K carbonate eutectic deliberately contaminated with selected impurities at the 5 wt % level. Heating rates of 2, 5, 10, and 20 °C/min. Note that the DTA Response axis has been extended for the unmodified eutectic, and the temperature axis has been extended for the eutectic modified with SiO2 to display the second peak (inset).

thorough mixing between the individual carbonate salts had occurred, representing a single melting process.9 Upon the introduction of contaminants, the system behavior varied considerably. Three distinct variations to the pure eutectic were noted; i.e., (i) changes in the melting temperature where the DTA response peak moved to higher or lower temperatures, (ii) DTA peak shape changes, and (iii) changes

in the area under the curve. The nature of these variations was brought about by changing the type of contaminant. DTA Peak Shifts and the Thermodynamics of Melting. The melting points (MP) reported in Table 1 were identified as the onset of the DTA peak upon heating. The 5 °C/min scan was chosen as a reference; this was done (i) to be consistent when comparing samples and (ii) because this heating rate was slow enough to allow for sufficient thermal equilibration. It is D


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°C.22 Although it is difficult to determine the reason for this observed deviation in these early works, it is likely that homogenization played an important role in reproducible melting point observations, as presented here. For the samples where the eutectic has been doped, the importance of thermal cycling becomes even greater when assigning a MP for these quaternary mixtures. An example can be seen in the case of added kaolin (Figure 5), where it can be clearly seen that a kaolin-doped eutectic requires six melt/ freeze cycles to settle upon a stable MP value. The first melt occurs at ∼395 °C, then the value drops, plateauing at 378 ± 1 °C between cycles 2−5, before dropping again, equilibrating at a value of 374 ± 1 °C for cycles 6−11. Shifting of the DTA peak was noted with the addition of several contaminants. The addition of kaolin and CaCO3 resulted in a general shift toward lower melting temperatures; impurities such as Fe2O3, FeS, TiO2 and CaSO4 had little to no appreciable effect on the melting temperature, while SiO2 increased the melting temperature (see Table 1). To examine these observations further, it is worthwhile to summarize the key thermodynamic points associated with the process of melting. Melting is the process involved with the phase transition from solid to liquid forms of the substance under study. The melting temperature (Tm) is the temperature (at constant pressure, P) for which the chemical potential of the solid (μs) and liquid (μl) forms of the substance become equal; i.e.,

Table 1. Melting Point, Mole Percentage of Additive (at the 5 wt % Level), and the Activation Energy at α = 0.5 for the Unmodified Eutectic and the Eutectic Modified with a Variety of Additives additive

mole percent of additive

melting point (°C)

activation energy, at α = 0.5 (kJ mol−1)

none CaCO3 CaSO4 Fe2O3 FeS kaolin SiO2 (peak 1) SiO2 (peak 2) TiO2

0 5.2 3.1 3.3 5.9 1.8 8.4 8.4 6.4

387 376 381 387 383 374 401 449 383

675 2117 746 2072 2058 675 1986 1191 3381

worth mentioning that the unmodified eutectic MP in this work was identified as 10 °C lower than the commonly cited literature value (397 °C).13 A total of 11 melt/freeze cycles were performed on each sample; this protocol was employed (i) to produce homogeneous mixture, and (ii) to ensure reproducibility of the results collected. For the unmodified eutectic (i.e., a ternary mixture), it was found that three melt cycles were required before the system settled upon a reproducible melt temperature of 387 ± 1 °C (see Figure 5). During the first two cycles, the

μs = μl

(6)

This condition highlights the equilibrium between the solid and liquid forms of the substance; i.e., equilibrium is established with both solid and liquid phases present at Tm. At temperatures around the melting temperature, the chemical potential of the system is dictated by the dominant phase; i.e., at lower temperatures, the chemical potential of the solid dominates, whereas at higher temperatures, the liquid dominates. In a plot of chemical potential versus temperature (at constant pressure, P), the change in chemical potential is dictated by the molar entropy (Sm) of the dominant phase; i.e.,

⎛ ∂μ ⎞ ⎜ ⎟ = −S m ⎝ ∂T ⎠ P

(7)

Note that the molar entropy for a substance is always a positive value (cf. Third Law of Thermodynamics), and for solids is always less than that of the corresponding liquid. Therefore, the change in chemical potential with temperature for the solid phase in Figure 6 is always less than that of the corresponding liquid. The mechanism by which the melting temperature changes involves changes to the chemical potential of either the solid or liquid phases, as a result of the transition from pure forms to those contaminated by the added impurities. The most common case is the dissolution of a solute into a liquid solvent to form a solution with a lower chemical potential (stabilization) which, as suggested in Table 1, leads to a reduction in the melting temperature for certain solutes. While this might be the most common scenario, certainly there are many other possibilities, including destabilization of the solution relative to the pure solvent (an increase in chemical potential), and changes to the chemical potential of the solid state through the formation of a solid solution, either to stabilize or destabilize the resultant mixture (see Figure 6).

Figure 5. Melting point of the unmodified eutectic and for the eutectic + 5 wt % kaolin for each heating−cooling cycle, including those with a constant heating−cooling rate (10 °C/min) and variable heating− cooling rates (20, 10, 5, and 2 °C/min, in that order).

mixture melted at ∼401 °C, and then at 388 °C, respectively. The MP of the first cycle was observed at a value similar to that reported by Janz et al., where there is no suggestion that thermal cycling was carried out, or any indication of the importance that it might have.13 However, two earlier publications have reported melting point values for the carbonate eutectic at lower MP values of 380 °C21 and 390 E


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clear that any deviation from the eutectic melt temperature is evidence of impurity interactions, which most likely result from dissolution. Unfortunately, a more comprehensive hypothesis detailing the cause of these results currently cannot be inferred from the DTA response alone. In the liquid phase, any changes to the chemical potential arise as a result of the interaction between the soluble contaminant species and the liquid eutectic. The first point to note is that, given the changes in melting temperature that result from the addition of contaminants, a dissolution of these impurities into the molten eutectic is implied. Beyond this, the nature of the interaction between the ions resulting from the dissolution of the contaminant with the eutectic will determine the changes in chemical potential of the solution, relative to the pure eutectic. In effect, this can be thought of as a disruption of the liquid phase upon dissolution, therefore implying that dissolution has taken place to some extent upon the addition of all of the impurities, barring CaSO4 and TiO2, where a relatively small deviation from the eutectic melting point is observed. DTA Peak Shape Alteration. Another phenomenon shown clearly in Figure 4 is the change in shape of the normalized DTA signal for melting of the carbonate eutectic in the presence of all impurities. The origin of this DTA peak shape changing may reside in many different processes that can occur within the eutectic. To begin, the eutectic itself is an example of how different species within a mixture can influence the melting process. For example, the presence of Li2CO3, Na2CO3, and K2CO3 in the appropriate molar ratio leads to a significant depression in the melting temperature, compared to the unmodified individual carbonate species. The combined presence of these individual carbonate species influences the behavior of the overall mixture to the point that its melting temperature can be depressed by over 300 °C. In the particular case of this eutectic, there are no chemical transformations involved, but rather a distribution of the Li+, Na+, and K+ cations among the available sites within the carbonate crystal structure that causes a depression in the melting temperature.23 With the dispersion of cations in the carbonate structure, melting of this phase then becomes a single-step process. The observation of several melting peaks can be discussed in terms of several possible scenarios, including insoluble impurities, soluble impurities, and reaction of the impurity within the carbonate eutectic. (i). Insoluble Impurity. In this extreme case, the impurity is not soluble in the eutectic. The integrated area under DTA signal would be diminished by an amount corresponding to the mole fraction of the impurity present (see Table 2), since there is no apparent interaction between the carbonate eutectic and the impurity species. For all impurity species examined, this is not the case. It is clear from changes in the DTA signal that at least some dissolution of the added impurities has taken place, and the magnitude of the normalized area under the curve provides an indication of the extent of this dissolution (see Table 2). It is also worth noting that CaSO4 and TiO2 gave rise to signals most resembling the pure eutectic, thereby implying that minimal dissolution has taken place. In this case, what is suggested is that the TiO2 particles largely precipitate from the eutectic. (ii). Soluble Impurity. In this case the impurity is completely soluble in the molten carbonate eutectic. In the solid state, before melting occurs, the impurity may be present as either a separate solid phase, i.e., the original solid impurity, or as an impurity dispersed on the ionic level throughout the carbonate

Figure 6. Thermodynamics of melting highlighting possible mechanisms through which the melting point can vary, depending on the composition of the liquid and solid phases.

In the case where CaCO3 was introduced, the decrease in melting temperature is likely due to (i) destabilization of the solid phase leading to an increase in its chemical potential, or (ii) a decrease in the chemical potential of the liquid phase, or indeed a combination of the two processes. In the solid state, the pure eutectic is likely to exist as a solid solution of the Li+, Na+, and K+ cations distributed uniformly throughout the carbonate framework, rather than as three separate solid phases, hence the single DTA melt peak (Figure 4). This is because the basic structure of these solid phases is similar, which would enable interdispersion of the cationic species. With the addition of CaCO3 at the relatively low levels that have been considered here, the solid solution of cationic species may not be disrupted to the point where a separate solid phase would be formed. Nevertheless, based on the observation of a decrease in melting temperature, the apparent effect of this addition of Ca2+ ions to the solid solution is to increase the chemical potential of the solid phase. Interestingly, the addition of most impurities also had the effect of lowering the melting point (see Table 1), thereby implying a similar effect on the solid state. In contrast, Fe2O3 had no effect on the melting point, and SiO2 actually raised the melting point considerably (by ∼14 °C). In the case of Fe2O3 it may be that insufficient material had dissolved in order to detect a difference by comparing melting points. Specifically, what is meant is that Fe2O3 has the greatest molecular weight of the selected dopants (159.69 g/ mol); therefore, given that the mixtures were standardized in terms of weight percentage, there were actually fewer moles of Fe2O3 present by comparison to the other samples. With regard to the case of SiO2, this implies that the chemical potential of the solid state is reduced, relative to that of the unmodified eutectic; i.e., the solid eutectic is stabilized in the presence of these contaminants. The exact cause of this change is not known at this time; however, it is believed to be based on the interaction between the contaminant and the eutectic in the solid state, either as a multiphase system or a solid solution. It is F


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that the impurity may undergo a phase transition or even a reaction in the molten carbonate eutectic. Whether this be a decomposition reaction that the impurity undergoes by itself, or a reaction between the impurity and the carbonate species, the end result is likely to be similar to the previous case where the impurity is dispersed either as a separate or single-phase species within the solid carbonate. Since no mass loss is observed for the contaminants investigated here (Figure 2), any products formed must be solids. Furthermore, none of the impurities undergo phase transitions in the examined temperature range (360−600 °C). It is difficult to determine whether or not any reactions have taken place from analyzing the DTA response alone, hence, this possibility is speculative at this time. Enthalpy Change with Melting. The area was calculated under the DTA response curve (μV K mg−1), to determine an indicative value for the melt process enthalpy, which is endothermic. In all cases, the introduction of impurities into the melt causes a decrease in the area under the curve, which is a reduced DTA response (see Table 2). Contributions from the eutectic melting to the area under the curve might be expected to be reduced by an amount equivalent to the mole fraction displaced due to the presence of impurities. This was not observed when consulting Tables 1 and 2, suggesting some synergistic effect between the dopants and the eutectic melt, specifically, dissolution of the impurities, followed by disruption of the liquid phase, as discussed in the previous section. This effect was most notably observed for SiO2; other impurities that had a similar effect were kaolin, FeS, and CaCO3. These species were all previously determined to undergo dissolution when added to the eutectic, as evidenced by a shift in the position of the eutectic melt peak (see Table 1), and peak splitting in the case of SiO2 and kaolin (see Figure 4). This effect was less pronounced for the addition of other impurities, especially in the cases of CaSO4 and TiO2. This suggests that the former species are relatively insoluble and therefore tend to precipitate out. As expected, the integrated area under the DTA response curve increases as the scan rate increases. It is important to note that the equivalent enthalpy change is involved in the melt process, irrespective of the scan rate. In the case of the slower scan rates the melt process simply takes place over a longer time interval. This can be seen in Figure 7, where a plot of the DTA response versus time gives rise to a sharper, narrower peak for high scan rates; however, the area under the curve remains essentially equivalent, irrespective of the scan rate (see Table 3). Multivariate Nonlinear Regression Method for Determination of Kinetic Parameters. The theoretical basis for the determination of kinetic parameters associated with the melting process is given by the expression

Table 2. Integrated Area under the Baseline-Corrected DTA Response Curve for the Unmodified Eutectic and Each of the Contaminants Added at the 5 wt % Concentration Level Normalized Area (μV K mg−1)

sample

heating rate = 2 °C min−1




eutectic CaCO3 CaSO4 Fe2O3 FeS kaolin SiO2 (peak 1) SiO2 (peak 2) TiO2

19.06 2.53 5.51 1.46 1.67 0.41 1.88 0.29 3.89

52.17 5.25 9.54 6.10 3.84 5.17 3.06 0.48 10.25

104.28 10.08 15.98 10.99 6.85 9.37 6.21 0.99 14.66

185.22 14.45 31.83 20.70 9.29 10.51 8.98 1.41 20.50

structure.24 Dissolved impurities increase the system entropy so that the system then becomes a quaternary mixture instead of a tertiary mixture. Alternatively, this can be thought of as freezing-point depression, where the presence of dissolved impurities lowers the free energy required to achieve a melt state.23,25 When there is a separate solid impurity phase present, the processes that need to occur include melting of the eutectic, followed by dissolution of the impurity in the molten eutectic, presuming that the impurity has a higher melting temperature, compared to the carbonate eutectic. For impurities investigated in this study, this is the case. This would be expected to produce at least two peaks in the DTA response: one for melting and another for dissolution. Alternatively, the impurity may be dispersed throughout the solid eutectic as a single-phase species. Here, the DTA response upon melting may not be straightforward. Examples of where peak splitting occurs are in the case of kaolin and SiO2. In both of these cases, the profile of the peak and distance between the peaks varies considerably. In the case of added kaolin, the first peak occurs at 382 °C and the second can be described as a shoulder peak at 392 °C; hence, both peaks were normalized together. For SiO2, there is greater peak separation, with the first peak occurring at ∼409 °C and the second at ∼452 °C. At this point, it is believed that the reason for the separation between the first peak and the second peak reflect the heat required to bring about the dissolution of SiO2. What is also interesting is that the temperature at which the second peak occurs in the case of SiO2 increases with the scan rate (Figure 4). This is likely because dissolution in this case is kinetically limited by thermal diffusion through the eutectic. At slower scan rates, the applied heat can diffuse throughout the system and is more evenly dispersed. This kinetic behavior is different from the eutectic melt response, which is not limited by the availability of heat (see Figure 4) and, hence, is most likely an activation-limited process. Splitting of the DTA response peak was a phenomenon that clearly shows liquid-phase disruption. This is most likely attributable to the energetics of dissolution; i.e., that SiO2 dissolution requires more energy. Other species do not display a clear melt peak, despite showing evidence for dissolution according to other metrics, the reason being that this is a very coarse determination of dissolution; this means that peak splitting will become visible only in cases where dissolution is occurring to a great extent. (iii). Reaction of the Impurity within the Carbonate Eutectic. In addition to dissolution, there is also the possibility

β

⎛ E ⎞ dα = Af (α) exp⎜ − A ⎟ ⎝ RT ⎠ dT

(8) −1

where β is the heating rate (K min ), α the fractional extent of conversion for the thermal process being examined, T the temperature (K), A a pre-exponential factor (min−1), f(α) a function describing the nature of the thermal process under study, EA the activation energy of the process (J mol−1), and R the gas constant (R = 8.3143 J K−1 mol−1). Determination of the kinetic parameters (EA and A) can be carried out for a single experiment, assuming that a detailed understanding of the thermal process is known; i.e., f(α) is known with certainty. G


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Figure 7. Normalized DTA response (μV/mg) versus time for the ternary carbonate eutectic mixture with heating rates of 2, 5, 10, and 20 °C/min.

Figure 8. Extent of conversion (α) versus temperature for the eutectic modified with 5 wt % TiO2 using heating of 2, 5, 10, and 20 °C/min.

Table 3. Integrated Area under the Baseline-Corrected DTA Response Curve for the Unmodified Eutectic heating rate (°C min−1)

normalized area (μV min mg−1)

2 5 10 20

0.88 1.04 0.91 1.38

More often though, a model-less approach is taken whereby multiple datasets for the same process are collected at different heating rates with the data then analyzed by plotting ln(β dα/ dT) vs 1/T, from which EA can be determined from the slope of the plot. This is known as the Friedman method, and it allows for kinetic parameter determination in the absence of a thermal model. The Friedman method26 was applied to baseline-corrected DTA data, collected at heating rates of 2, 5, 10, and 20 °C/min, measured in succession on the same sample. A plot of the extent of conversion (α) versus temperature T (K) was first constructed (Figure 8), where α takes on values between 0 and 1. α was calculated numerically by integration of the DTA response, with respect to temperature, using the trapezoidal method. Following this, all of the calculated α values were divided by the total area, thereby normalizing α at each value of n on a scale between 0 and 1. At specific values of α, the activation energy is obtainable from a Friedman plot, as shown, for example, in Figure 9 in this case for the heating of the pure eutectic. A sound linear fit was obtained for all such plots in this work, with R2 values falling within the range of 0.92−0.99. An advantage of using the Friedman method is that it does not assume any kinetic order for the melting or dissolution processes apparent in this study. In addition, the calculation of kinetic parameters is based on data obtained from multiple experiments using the same sample; hence, greater confidence

Figure 9. Friedman plot (for various extents of conversion (α)), allowing for the determination of activation energy (J/mol) without assuming an overall kinetic model for the process. The R2 value for each dataset falls within the range of 0.92−0.99.

can be placed in the reproducibility of any conclusions that are drawn.18,27 Activation Energy Versus Extent of Conversion. A plot of activation energy (EA) versus extent of conversion (α) is shown in Figure 10. This figure shows that most samples have an activation energy which falls within the range of 500−2200 kJ/mol over the extent of conversion. Most often, EA was relatively constant, except for both low and high extents of conversion. Furthermore, the activation energy for the H


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Figure 10. Plot of activation energy (kJ/mol) versus extent of conversion (α) for the eutectic and the eutectic modified with 5 wt % of selected impurities.

literally implies that kinetics of melting are sluggish, meaning that it has the ability to store substantial energy.16 It can also be seen from Figure 10 that the profile of the plot of EA vs α varies in accordance with the type of contaminant used. Some mixtures start with a relatively high activation energy, which decreases as the conversion or reaction approaches completion. Samples such as the unmodified eutectic, and the eutectic modified with kaolin, TiO2, and CaCO3, conform to this trend. The other common profile seen in the case of SiO2 (both first and second peaks), Fe2O3, and FeS may be described as displaying some degree of fluctuation over the range of conversion; however, comparing start to finish, the activation energy does not vary appreciably.

unmodified eutectic, as well as the eutectic modified with kaolin tended to decrease as the extent of conversion increased. Perhaps the most obvious exception is the sample where 5 wt % of TiO2 was added to the eutectic, in which case the activation energy rose as high as 8500 kJ/mol, after which it decreased across the extent of conversion. This may be caused by the relatively high specific heat capacity of TiO2 (anatase); i.e., ∼70 J K−1 mol−1,28 in which case the TiO2 is acting as a heat sink, thereby sequestering heat that would otherwise contribute to the eutectic melting phenomenon. This specific finding may prove useful for its application as a thermal fluid. A useful thermal fluid is qualified as one which is resistant to thermal decomposition at higher temperatures and has a high specific heat capacity. A high activation energy, as in the case of TiO2, I


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Energy & Fuels In the former case, the activation energy starts high and decreases as the conversion nears completion, which is the expected result for the melting of a homogeneous material.29 Heat enters the sample material through the walls of the aluminum crucible, which has good thermal conductivity, therefore providing uniform heat distribution at the edges of the sample mass. At this early stage, the EA value is expected to be high as the sample melts from the periphery inward, toward the center. For the latter examples, where the EA value remains essentially unchanged, the presence of undissolved impurities or the dissolution process consuming heat may contribute. In both cases, the mechanism of heat flow has been interrupted. What is likely is that the effect is synergistic and, in actuality, both contribute to the result that is observed. It can be seen that, for a value of α = 0.5, the corresponding activation energy of the unmodified eutectic was 675 kJ mol−1 (see Table 1). This is relatively low, compared to most of the doped systems, except for the eutectic modified with kaolin (675 kJ mol−1) and CaSO4 (675 kJ mol−1). In the case of the CaSO4-modified eutectic, this was somewhat expected as dissolution was found to occur only in a limited manner. Furthermore, CaSO4 is expected to have a relatively low specific heat capacity in the temperature range of 360−430 °C. The melt kinetics of the kaolin-modified eutectic appears relatively similar to that of the eutectic, suggesting that kaolin dissolution does not greatly affect the rate of melting. This is particularly interesting, considering that added kaolin has previously been demonstrated in this work to have considerable impact on the thermodynamic parameters, such as melt enthalpy. For the remainder of the impurities added to the eutectic, the activation energy decreased into the range of 1986−2117 kJ mol−1, except for the addition of TiO2, which, of course, recorded a much higher value (3381 kJ mol−1).

It was found that the addition of TiO2 to the eutectic caused the activation energy to be much higher (∼8500 kJ mol−1). This was thought to be due to the high specific heat capacity of TiO2, which effectively acted as a heat sink that was sequestering heat. Therefore, this finding may prove useful in the field of thermal fluids. These findings are also relevant when considering large-scale DCFC operation where characteristics such as a low MP, low enthalpy of melting, high heat capacity, and facile melt kinetics enhance operational efficiency from a thermal perspective. Therefore, an additive such as kaolin would be an ideal candidate to induce these beneficial effects.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +61 2 4921 5477. Fax: +61 2 4921 5472. E-mail: scott. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Coal Innovation NSW is acknowledged for providing the funding to carry out this research. M.J.G. acknowledges the University of Newcastle for providing a Ph.D. scholarship.

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REFERENCES

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CONCLUSIONS It was found that impurities added to a ternary carbonate eutectic have many significant effects on melting behavior. Specifically, the baseline-corrected DTA data were analyzed in three different ways in order to assess these effects; i.e., comparing the integrated area under the DTA response peak to reflect the enthalpy of the melting process, the melting temperature, and DTA peak splitting. It was found that the addition of all impurities examined at the 5 wt % concentration level caused a reduction in the integrated area under the DTA response peak. Most impurities cause a decrease in the melting temperature, except for Fe2O3, which has no appreciable effect in this regard, and SiO2, which actually increases the melting temperature by ∼40 °C. The addition of kaolin and SiO2 cause DTA peak splitting, with the emergence of the second peak being assigned to dissolution of the added impurity. From the DTA response, it was therefore inferred that all impurities display some degree of dissolution into the eutectic, although the extent of that dissolution varies for each material. The changes in activation energy, as a function of the extent of conversion, showed two distinct profiles: either the activation energy decreased as conversion approached unity, or there was no appreciable change over the extent of conversion range. The former result is the one that is expected for a homogeneous system, while the latter implied some synergistic effect that is caused by the presence of impurities interrupting normal heat flow. J


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