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Catalytic Gasification Activity of Na2CO3 and Comparison with K2CO3 for a High-aluminium Coal Char Yongwei Wang, Zhiqing Wang, Jiejie Huang, and Yitian Fang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01537 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015
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Catalytic Gasification Activity of Na2CO3 and Comparison with K2CO3 for a High-aluminium Coal Char Yongwei Wang1,2, Zhiqing Wang1, Jiejie Huang1∗, Yitian Fang1 1
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China
2
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
A Manuscript submitted to Energy & Fuels
Short Running Title:
Catalytic Gasification Activity of Na2CO3 and
Comparison with K2CO3 for a High-aluminium Coal Char
∗
Corresponding author. Tel.: +86 0351 2021137. E-mail address:
[email protected] (J. Huang) 1 ACS Paragon Plus Environment
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Catalytic Gasification Activity of Na2CO3 and Comparison with K2CO3 for a High-aluminium Coal Char Yongwei Wang1,2, Zhiqing Wang1, Jiejie Huang1, Yitian Fang1 1
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China
2
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
ABSTRACT: The catalytic performance of Na2CO3 for high-aluminium coal gasification was specially investigated, and the effect of the ash constituents in Sunjiahao (SJH) high-aluminium coal char on the catalytic activity of Na2CO3 and comparison with that of K2CO3 was also studied through the steam gasification experiments of SJH char catalyzed by Na2CO3 or K2CO3 carried out using a TGA and a fixed bed reactor at 800 oC respectively. The results indicate that the steam gasification rates of SJH char and acid-treated SJH char catalyzed by Na2CO3 is higher than that by K2CO3 with equal mass of catalyst. Na2CO3 can be used as the catalyst for the high-aluminium coal gasification at 800 oC. The effect of the ash components on the catalytic activity of Na2CO3 is larger than that of K2CO3 in the catalytic steam gasification of SJH high-aluminium coal char at 800 oC. The X-ray diffraction (XRD) patterns of the steam gasification ashes of SJH char impregnated by 2 ACS Paragon Plus Environment
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Na2CO3 and/or K2CO3 reveal that the main composition is aluminosilicate that is proved no catalytic activity. For the catalytic steam gasification of acid-treated SJH char, the catalytic action of Na2CO3 has better catalysis that attributes to the superior mobility and poor volatility of Na2CO3.
KEYWORDS: gasification; catalytic activity; Na2CO3; K2CO3; high-aluminium coal char
1. INTRODUCTION As an efficient and clean utilization method of coal, gasification has been developed rapidly in China. However, most existing gasification technologies have several disadvantages, for example, severe gasification conditions, high oxygen consumption, high capital investment and unattractive economics1, which impede the application of novel coal gasification technologies. With the growing demand of natural gas, much attention has been paid on catalytic coal gasification2-13, due to its extraordinary advantages: (I) catalytic gasification can strikingly lower the reaction temperature10, so that the investment cost of gasification would be lowered; (II) it can remarkably enhance the gasification reaction rate10, thus elevating the gasification efficiency; (III) moreover, catalytic gasification is in favor of the production of methane, which is regarded as one of the desirable methods to produce synthetic or substitute natural gas (SNG).11 Catalytic gasification, as a result, is one of the promising coal gasification technologies. 3 ACS Paragon Plus Environment
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Although catalytic coal gasification has been developing for several decades, it is not put into industrial application yet. One of the major reasons for this result is the poor recovery of the gasification catalyst, which is usually considered to be attributed to the deactivation of the gasification catalyst resulted from the interaction between the catalyst and the mineral matter in coal. Wang et al.8 investigated the transformation of potassium catalyst during the coal steam gasification and indicated that the interaction of K2CO3 with alumina in the coal minerals could give rise to the deactivation of K2CO3 catalyst. Their conclusion suggested that there existed a linear relationship between the molar number of the deactivated potassium catalyst and the content of alumina. Zhang et al. also found that the addition of bauxite led to the deactivation of K2CO3 by reacting with K to generate the water-insoluble kaliophilite in the catalytic steam gasification of lignite.12 Therefore, the alumina in the coal mineral matter plays an important role in the catalyst deactivation during the course of the alkali catalyzed coal gasification. With regard to the recovery of the catalyst used in the catalytic gasification, a few researchers have used various extraction methods to improve its recovery. Among the different extraction processes reported by Sheth et al.13, H2SO4 extraction is the best recovery method of the gasification catalyst. Unlike the solvent extraction methods, Kim et al.14 reclaimed the catalyst from the catalytic gasification residue on the basis of size and ferromagnetism. Unfortunately, the results of the forementioned catalyst recovery processes are not satisfying. It is generally considered that the most suitable reactor for the catalytic coal gasification is a fluidized bed reactor.15-17 Nonetheless, slagging which may cause 4 ACS Paragon Plus Environment
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defluidization and a forced shutdown, is one of the operational problems usually encountered in the industrial fluidized bed coal gasifier. Although the alkali carbonates can obviously improve the gasification efficiency, it can aggravate the problem of agglomeration.9,12 The agglomerated tendency is closely related to the ash fusion point of the coal which is usually proportional to the content of alumina in the coal ash.12,18 The higher the alumina content is, the higher the ash fusing point of the coal is, thus reducing slagging risk. Hence, high-aluminium coals may be fit for the fluidized bed catalytic coal gasification.19 However, high-aluminium coals contain rich alumina which may react to the alkali catalyst to form water-insoluble aluminosilicate in the catalytic coal gasification, leading to the deactivation of catalyst.8,12 If some alumina can be extracted from the catalytic gasification residue of the high-aluminium coal during the recovery of the gasification catalyst, the cost of the catalyst recovery may be lowered. Thus, more attention should be paid to investigating the catalytic gasification process of high-aluminium coals. As an important industrial chemical raw material, Na2CO3 reserves are larger than that of K2CO3, and the price of Na2CO3 is about as one quarter as that of K2CO3. Meanwhile, Na2CO3 is usually selected as the raw material to extract alumina from coal fly ash in industrial application.20, 21 Hence, it is possible to reduce the cost of the gasification catalyst and profitably extract alumina from the catalytic gasification residue if Na2CO3 is used as the catalyst for the catalytic gasification of high-aluminium coals.
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However, little work has been published about the catalytic steam gasification of high-aluminium coals. Therefore, in order to understand the catalytic performance of Na2CO3 and the interaction between ash components and Na2CO3, the catalytic steam gasification of SJH coal char was studied using a TGA and a fixed bed reactor. K2CO3 is usually selected as the catalyst for coal gasification, so that the comparison to K2CO3 is necessary. It is expected that this study can provide some guidance for the catalytic gasification of high-aluminium coals.
2. EXPERIMENTAL 2.1 Samples and reagent Sunjiahao bituminous coal from Inner Mongolia, a high-aluminium coal, was collected as the coal sample. The coal sample was ground and sieved to a particle size of less than 120 µm. Then the sample was dried in an oven at 105 oC for 12 h, and then stored in a desiccator for further use. The anhydrous salts of sodium carbonate and potassium carbonate (99.8% purity) were purchased from Tianjin Hengxing Chem. Co., Inc. and used as received. The sodium aluminosilicate (NaAlSiO4) was bought from Henan Wanda Chem. Co., Inc. and used as received. The proximate and ultimate properties of SJH parent coal, SJH char and acid-treated SJH char are listed in Table 1. The ash compositions of SJH original coal, SJH char and acid-treated SJH char are presented in Table 2. 2.2 Preparation of SJH coal char
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The coal sample was pyrolyzed at 800 oC in a fixed bed reactor under a continuous nitrogen atmosphere (150 mL/min). The procedure is described briefly as follows: the crucible with 10 g of coal sample (
