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  • Fuel 207 (2017) 365–372

    Contents lists available at ScienceDirect

    Fuel

    journal homepage: www.elsevier .com/locate / fuel

    Full Length Article

    Interactions between molten salts and ash components during Zhundong coal gasification in eutectic carbonates

    http://dx.doi.org/10.1016/j.fuel.2017.06.079 0016-2361/� 2017 Published by Elsevier Ltd.

    ⇑ Corresponding author. E-mail address: hongyunhu@hust.edu.cn (H. Hu).

    Junhao Shen a, Hongyun Hu a,⇑, Mian Xu a, Huan Liu a, Kai Xu a, Xiuju Zhang a, Hong Yao a, Ichiro Naruse b a State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China bDepartment of Mechanical Science & Engineering, Nagoya University, Nagoya 464-8603, Japan

    h i g h l i g h t s

    � The distribution patterns of mineral matter in Zhundong coals were characterized. � The fate of Zhundong coal ashes was studied in the high-temperature molten salts. � Alkali/alkaline earth metals were predominantly dissolved in the molten salts. � Si/Al was separated from the molten salts including formed Li/K-aluminosilicates. � The thermal properties of the molten salts were inevitably affected by coal ashes.

    a r t i c l e i n f o

    Article history: Received 14 March 2017 Received in revised form 17 June 2017 Accepted 19 June 2017 Available online xxxx

    Keywords: Molten salt gasification Zhundong coal Ash forming Mineral matter Thermal property

    a b s t r a c t

    Molten salt gasification provides a promising way for the combined usage of solar energy and coal, during which molten salts act as a heat storage and transfer medium. However, the interactions between coal ash components and molten salts might have undesired effects on the molten salt gasification. The pre- sent study investigated the distribution of mineral matters in typical Zhundong coals and the fate of coal ashes in the high-temperature Li2CO3-Na2CO3-K2CO3 eutectic system. The results showed that alkali and alkaline earth metals (AAEMs) were widely distributed in Zhundong coals in the forms of NaCl, CaCO3, CaSO4 and organic matter, which were predominantly dissolved in the molten salts. After the dissolution of these AAEMs, the melting temperature and enthalpy of the molten salts were changed, while the vis- cosity of the mixtures was hardly affected in melting phase. On the other hand, Si/Al-compounds were also of high content in some Zhundong coals, existing as quartz, kaolin and amorphous species. Some of these compounds tended to react with molten salts, by forming Li-silicate or Li/K-aluminosilicates, which were precipitated out of the system. Meanwhile, the suspension of the unreacted Si/Al- compounds increased the viscosity of molten salts. Additionally, interactions between coal ashes and molten salts brought about the decrease in thermal diffusivity and thermal conductivity of the eutectic system as well as the increase in specific heat capacity.

    � 2017 Published by Elsevier Ltd.

    1. Introduction

    In order to meet the large demand of energy and to solve the severe environmental pollution problems, it is encouraged to use renewable energy and to take advantage of traditional energy in a clean way [1,2]. Based on this idea, molten salt gasification offers a promising way for the combined usage of solar energy and solid fuels [3]. In the process, molten salts act as heat storage medium for concentrated solar energy, because of their high thermal stabil- ity and high heat capacity [4]. On the other hand, molten salts were

    used as catalytic medium and heat carrier for the gasification of solid fuels [5,6], meanwhile as in-situ capture agent of polluting gases like HCl and H2S [7].

    Located in northwest China, Xinjiang province is rich in both coal and solar energy resources [8,9]. Solar energy therein is cur- rently not exploited much due to its instability and intermittence [10]. And most of the coal in Xinjiang province (like Zhundong coals) contains a high concentration of alkali and alkaline earth metals (AAEMs), which causes serious fouling and slagging prob- lems in the traditional boiler [11,12]. Molten salt gasification tech- nology, of tremendous benefit to the storage of solar energy in concentrated mode and the conversion of solar energy into chem- ical energy, could well solve the problems mentioned above [5,13].

    http://crossmark.crossref.org/dialog/?doi=10.1016/j.fuel.2017.06.079&domain=pdf http://dx.doi.org/10.1016/j.fuel.2017.06.079 mailto:hongyunhu@hust.edu.cn http://dx.doi.org/10.1016/j.fuel.2017.06.079 http://www.sciencedirect.com/science/journal/00162361 http://www.elsevier.com/locate/fuel

  • 366 J. Shen et al. / Fuel 207 (2017) 365–372

    Simultaneously, AAEMs in coal could be stabilized in the molten salts and partly substituted for raw molten salts. However, molten salts could react not only with carbonaceous substances in the gasification process, but also with ash compounds [14]. The inter- actions between the molten salts and the ash components might change the properties of the molten salts, thus in turn affecting the carbon conversion. Therefore, understanding the behavior of ash compounds in high-temperature molten salts is essential for using Zhundong coals through molten salt gasification.

    Apart from AAEMs, Zhundong coal ashes mainly consisted of Si, Al, and certain amount of Fe, S and Cl [15]. The characteristics of the ash varied among different mining areas [16]. And the distribu- tion patterns of mineral matter determined the ash forming behav- ior as well as the ash thermal behavior in high-temperature molten salts [16,17]. Martín-Aranda et al. [14] investigated the fate of ash components in the LiCl/KCl eutectic mixture, at 873 K with a heat- ing rate of 15 �C/min. It was found that [14] some minerals were dissolved into the molten salts while inert components were sep- arated from the molten salts and remained unreacted. The dissolv- ing of the ash compounds changed the composition of molten salts, affecting its melting temperature, viscosity and other properties [18,19]. Moreover, inert ash matrix in the fine scale could be sus- pended in the molten salts, which might play the similar role as nano particles in the nanofluid, by increasing the specific heat capacity and thermal conductivity of the molten eutectic [20]. Usu- ally, alkali metals molten carbonates were widely used in molten gasification/pyrolysis of coal, biomass and waste solids at temper- atures ranging from 500 �C to 900 �C, which could be defined as the following Eqs. (1)–(3) [3,5,6,21,22]. However, few researches have been reported to clarify the mechanisms regarding the interactions between molten salts and fuel ashes.

    M2CO3 þ 2C ¼ 2Mþ 3CO ð1Þ

    2Mþ CO2 ¼ M2Oþ CO ð2Þ

    M2Oþ CO2 ¼ M2CO3 M ¼ Li;Na and K ð3Þ The present study aims to investigate the behavior of Zhundong

    coal ashes in the ternary eutectic (Li2CO3-Na2CO3-K2CO3). More- over, the distribution patterns of the mineral matter in the coal samples were observed through various ash forming processes. On this basis, several typical ash components were chosen to react with the molten salts to further understand the fate of Zhundong coal ashes in the eutectic carbonates. Changes in the properties of the molten salts were also investigated after reacting with the coal ashes or typical ash components.

    2. Experimental

    2.1. Materials

    Six kinds of Zhundong coals were used in the present study, labelled as Coal 1# to 6#. The samples were ground and sieved to the desirable particle size (45–106 lm). Various ash samples were prepared in a muffle furnace by heating the coals, at 400 �C with a heating rate of 10 �C/min (denoted as LTA), at 815 �C with a slow heating rate of 15 �C/min (denoted as SHTA), or at 815 �C with a rapid heating rate (denoted as RHTA), respectively.

    For the preparation of the molten salts, analytical grade reac- tants, including Li2CO3, Na2CO3 and K2CO3, were firstly physically mixed at a mass ratio of 32.1:33.4:34.5. Then, the mixtures were put in a corundum crucible and heated at 800 �C to form a single phase liquid. After maintaining for 6 h, the molten salts were cooled to room temperature and finely ground in an agate mortar.

    Other reactants used in the experiments, like CaCO3, CaSO4, NaCl, quartz and kaolin, are of analytical grade.

    2.2. Experimental procedures

    Fig. SM-1 shows the schematic of the interactions between mol- ten salts and coal ashes. Before heating process, the ash samples were thoroughly mixed with the molten salts at a mass ratio 1:9. Then the mixtures were heated in a muffle furnace from room tem- perature to 800 �C, with a heating rate of 10 �C/min. After reaction for 2 h, the products were cooled to room temperature with a cool- ing rate of about 20 �C/min and collected. Typically, the products mainly consisted of two different layers. Part of the products were precipitated and separated from the molten salts, forming the lower-layer. The rest was remained in the liquid phase and formed the upper-layer product during the cooling process. Subsequently, products in various layers were separated and characterized.

    In order to further illustrate the interactions between molten salts and Zhundong coal ashes, the main components in the coal ashes, like CaCO3, CaSO4, NaCl, quartz and kaolin, were chosen to react with molten salts in the same way as coal ashes, and the products were collected and analyzed.

    2.3. Analytical methods

    To measure the distribution of elements in the products formed during the interactions between coal ashes and molten salts, the upper layer was dissolved by diluted HNO3 and the lower layer

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