Thermodynamic evaluation and optimization of the (NaCl + Na2SO4 + Na2CO3 + KCl + K2SO4 + K2CO3) system

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Abstract

A complete, critical evaluation of all phase diagrams and thermodynamic data was performed for all condensed phases of the (NaCl + Na2SO4 + Na2CO3 + KCl + K2SO4 + K2CO3) system, and optimized parameters for the thermodynamic solution models were obtained. The Modified Quasichemical Model in the Quadruplet Approximation was used for modelling the liquid phase. The model evaluates first- and second-nearest-neighbour short-range order, where the cations (Na+ and K+) were assumed to mix on a cationic sublattice, while anions (CO32-,SO42-,andCl-) were assumed to mix on an anionic sublattice. The thermodynamic properties of the solid solutions of (Na,K)2(SO4,CO3) were modelled using the Compound Energy Formalism, and (Na,K)Cl was modelled using a substitutional model in previous studies. Phase transitions in the common-cation ternary systems (NaCl + Na2SO4 + Na2CO3) and (KCl + K2SO4 + K2CO3) were studied experimentally using d.s.c./t.g.a. The experimental results were used as input for evaluating the phase equilibrium in the common-cation ternary systems. The models can be used to predict the thermodynamic properties and phase equilibria in multicomponent heterogeneous systems. The experimental data from the literature are reproduced within experimental error limits.

Introduction

Combustion of biomass fuels is often associated with the formation of inorganic alkali compounds that may cause problems for the combustion devices. The build-up of deposits lowers the efficiency of heat exchangers and may cause plugging in flue gas channels. Deposits are also associated with corrosion of heat exchangers, and the formation of a liquid phase is often connected to corrosion of alloys. Alkali chlorides are considered to be central to several of the operational problems that are encountered when firing biomass fuels. Potassium and chlorine are among the most common ash-forming elements in many biomass fuels. KCl has higher volatility than other inorganic alkali compounds present in biomass combustion, such as alkali sulphates, carbonates and silicates. Gaseous KCl can condense on heat exchanger surfaces in the combustion devices, initiating the build-up of deposits. Alkali chlorides also form low-melting mixtures with other alkali compounds, which may cause further build-up of the deposits due to the increased stickiness of the ash. In the pulp and paper industry, black liquor is combusted to produce heat and electricity and to recycle the process chemicals. The process chemicals are sodium-based compounds, which are removed from the recovery boiler in a liquid state, consisting mainly of Na2CO3 and Na2S. The formation of NaCl and KCl is known to cause problems in the recovery boiler, usually with plugging and fouling of the flue gas channels. The concentration of alkali metals in the black liquor is considerably higher than in wood-derived fuels or fuels from agricultural waste. The chemical behaviour of mixtures containing alkali chlorides at high temperatures is of importance in understanding the possible formation and build-up of harmful deposits in biomass-fired power plants and kraft recovery boilers. Thermodynamic modelling is an important tool for predicting the melting behaviour of alkali compounds in combustion conditions [1], [2].

In the present study, the thermodynamic properties of the condensed phases in the (NaCl + Na2SO4 + Na2CO3 + KCl + K2SO4 + K2CO3) system have been evaluated and optimized. The binary systems involving chloride and sulphate or chloride and carbonate were optimized in this study on the basis of experimental thermodynamic and phase equilibrium data from the literature. The other binary systems have been evaluated and optimized in preceding studies [3], [4], [5]. The higher-order systems were also evaluated and optimized in the present study, and thermal analysis was performed to obtain experimental data for the melting behaviour in the (NaCl + Na2SO4 + Na2CO3) and (KCl + K2SO4 + K2CO3) subsystems.

The present study is a continuation of previous thermodynamic evaluations of other alkali salt systems [4], [5], [6]. The liquid phase in the (Na + K + S) system [6], the (Na2SO4 + K2SO4 + Na2S2O7 + K2S2O7) system [4], and the (Na2CO3 + Na2SO4 + Na2S + K2CO3 + K2SO4 + K2S) system [5] was modelled with Modified Quasichemical Model in the Quadruplet Approximation. The thermodynamic data for the stoichiometric phases and solution phases are consistent for all the systems. All calculations and optimizations were performed using the FactSage software package [7]. The Optisage module was used for the optimization of the solution parameters.

Section snippets

Thermodynamic data for pure compounds

The thermodynamic data (ΔH298.15K,S298.15K,andCp) for the condensed stoichiometric compounds of the (NaCl + Na2SO4 + Na2CO3 + KCl + K2SO4 + K2CO3) system have been reported previously [3], [4], [5]. The thermodynamic data for NaCl and KCl were taken from the evaluation by Chartrand and Pelton [3], and are based on data in the compilation of Barin et al. [8]. The thermodynamic data of the compounds Na2CO3, Na2SO4, K2CO3, and K2SO4 were taken from the evaluations by Lindberg et al. [4], [5], and are

Thermodynamic model for the solid solutions

Common-anion (Na + K) salts such as sulphates, pyrosulphates, carbonates and chlorides tend to form complex solid solutions. Solid solutions are also formed in the common-cation systems (Na2SO4 + Na2CO3) and (K2SO4 + K2CO3). Several reciprocal solid solutions exist in the (Na2SO4 + Na2CO3 + K2SO4 + K2CO3) system, where the solid solution with a hexagonal crystal structure is the solid phase in equilibrium with the reciprocal liquid phase. The thermodynamic properties of the solid solutions in the (Na2SO4 + Na

Thermodynamic model for the liquid phase

For the (NaCl + Na2SO4 + Na2CO3 + KCl + K2SO4 + K2CO3) liquid solution, the thermodynamic model must take into account two cations, Na+ and K+, and three anions, CO32-,SO42-,andCl-, being distributed on a cationic and an anionic quasi-sublattice. The liquid phase is therefore a multicomponent reciprocal solution. In reciprocal molten salt solutions large deviations from ideal mixing can occur due to the strong first-nearest-neighbour (cation–anion) interactions. Simultaneously, strong

(NaCl + Na2SO4)

The phase diagram of the (NaCl + Na2SO4) system has been measured by thermal analysis [17], [18], [19], [20], [21] and visual-polythermal methods [22], [23], [24], [25], [26], [27], [28]. It is a simple binary system with no solid solution or intermediate phases. No measurements of the thermodynamic properties of the liquid phase have been reported. The measured liquidus and solidus temperatures from the experimental studies are in good agreement with each other. The optimized liquid phase

Common-cation ternary systems

The liquidus temperatures of the common-cation ternary systems (NaCl + Na2SO4 + Na2CO3) and (KCl + K2SO4 + K2CO3) have been measured by Bergman and Sementsova [22] using visual-polythermal methods. No ternary solid phases were reported and the precipitating solid phases were the alkali chlorides and the hexagonal solid solution containing carbonate and sulphate. In the present study additional measurements on the solidus temperatures of both of the common-cation ternary systems were conducted using

(NaCl + Na2SO4 + KCl + K2SO4)

The phase relations in the ternary reciprocal system has been studied with thermal analysis [17], [18], [19] and visual-polythermal methods [25], [28], [41]. The studies only report the liquidus temperatures, except for Jänecke [17], who measured the thermal events between room temperature and the liquidus temperature. Only Akopov and Bergman [25], [28] and Jänecke [17] reported the phase relations in the whole compositional range, while the other studies [18], [19], [41] measured liquidus

Multicomponent (NaCl + Na2SO4 + Na2CO3 + KCl + K2SO4 + K2CO3) system

Bergman and Sementsova have measured the liqudus temperature in the multicomponent reciprocal (NaCl + Na2SO4 + Na2CO3 + KCl + K2SO4 + K2CO3) system using visual-polythermal methods [32], [36], [41]. Sementsova and Bergman [36] made the measurements in the (Na2CO3 + K2SO4 + KCl) subsystem, Bergman and Sementsova [41] measured in the (NaCl + K2SO4 + K2CO3) and (KCl + Na2SO4 + Na2CO3) systems, while Bergman and Sementsova [32] measured the liquidus of additional sections in the (NaCl + Na2SO4 + Na2CO3 + KCl + K2SO4 + K2CO3)

Discussion

The (solid + liquid) equilibrium in the (NaCl + Na2SO4 + Na2CO3 + KCl + K2SO4 + K2CO3) system has been studied extensively. The thermodynamic database of the system obtained in the present study reproduces the (solid + liquid) equilibrium in binary, ternary and quaternary systems within the experimental error limits. However, additional experimental studies are recommended for further understanding of the phase relations of the multicomponent system. No experimental data on the thermodynamic properties of

Conclusions

A critical evaluation of all available thermodynamic and phase diagram data for the (NaCl + Na2SO4 + Na2CO3 + KCl + K2SO4 + K2CO3) system has been made. The liquid phase was modelled using the Modified Quasichemical Model in the Quadruplet Approximation. All chloride-sulphate and chloride-carbonate binary systems have been evaluated and optimized in this study. The solidus temperatures of the (NaCl + Na2SO4 + Na2CO3) and (KCl + K2SO4 + K2CO3) systems were measured with d.s.c./t.g.a., and the thermodynamic data

Acknowledgments

The authors thank Peter Backman for performing the d.s.c./t.g.a. experiments and Linus Perander for giving comments on an early version of the manuscript. This work was supported by the Fondation de l’École Polytechnique de Montréal and the Nordic Energy Research. The work has also been a part of the activities of the Åbo Akademi Process Chemistry Centre funded by the Academy of Finland. Additional support obtained from our industrial partners Andritz Oy, Foster Wheeler Energia Oy,

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