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Catalytic Effects of CeO2/Fe2O3 and Inherent Mineral Matters on Anthracite Combustion Reactions and Its Kinetic Analysis Xuzhong Gong, and Shu Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02568 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017
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Energy & Fuels
Catalytic Effects of CeO2/Fe2O3 and Inherent Mineral Matters on Anthracite Combustion Reactions and Its Kinetic Analysis
Xuzhong Gonga,b,
Shu Zhangc,*
a) National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China; b) University of Chinese Academy of Sciences, Beijing 100049, PR China; c) College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, Jiangsu, PR China;
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ABSTRACT Effects of external and internal catalysts on anthracite combustion reactivity and kinetics were investigated using the method of differential thermal analysis (DTA). It was found that the combustion reactivity of both raw and demineralized anthracites was all apparently improved by the introduction of CeO2 and Fe2O3. When CeO2 and Fe2O3 were used in raw anthracite, the combustion starting time from DTA curve was advanced from 1470 s to 1312 s and 1325 s respectively, compared to the improvements from 1285 s to 1089 s and 1055 s while adding the same amounts of CeO2 and Fe2O3 into demineralized anthracite. The addition of catalysts has increased the combustion rate for raw anthracite much more significantly than that for demineralized anthracite. RDTA analysis (change the reference material from Al2O3 to raw anthracite) showed a direct effect of catalysts on its combustion, which broadly agreed with the results of DTA study. The inherent mineral matter at high contents could potentially prevent the catalytic activity of CeO2. The kinetic study has demonstrated that the combustion reactions of all samples could see the two-stage scheme phenomena with different values of apparent activation energy and different fitting models. The addition of CeO2 and Fe2O3 has reduced the activation energy for demineralized anthracite more than that for raw anthracite. KEYWORDS: DTA; Reactivity; Catalytic combustion; Anthracite; Kinetics;
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1. INTRODUCTION Differential thermal analysis (DTA) has been widely used as a very important tool for measuring exothermic or endothermic characteristics of samples at different temperatures
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. Coal combustion that is a phenomenon of strong exothermic
reactions between oxygen and hydrocarbons could be thus well characterized by DTA 4
. Specifically, the changes in heat release due to the changes in combustion
conditions (e.g. with/without catalysts) could be reflected by DTA curves 5,6. Catalytic combustion of pulverized coal has been considered as a clean and efficient combustion technology to increase combustion reactivity 7-9, and improve combustion efficiency 10-12. Although alkali and alkaline earth metal compounds are popularly used as catalysts for coal combustion
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, they have shown unbearable negative effects on
operations, such as fouling, slagging and corroding issues. In order to avoid the above shortcomings, CeO2 and Fe2O3 are therefore selected as catalysts for coal combustion in this work. As we know, they have been substantially applied in catalytic areas due to their unique physical and chemical structures
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. CeO2 and Fe2O3 have not only
higher catalytic activity, but also show other advantages, including low toxicity and nearly zero corrosive effects. CeO2 and Fe2O3 have demonstrated their excellent catalytic effects on anthracite combustion reactions based on our previous studies 11,16. On the other hand, some researches showed that inherent mineral matters of coal hindered the combustion reactivity
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, while others suggested that there were
excellent catalytic effects of mineral matters on coal conversion
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researches also indicated that the inherent mineral matters could inhibit the catalytic gasification or combustion reactions due to the interactions between catalyst (K2CO3) and clay mineral25,26. However, it was not clear if the interaction between CeO2/Fe2O3 and inherent mineral matters could influence the catalytic combustion reaction of anthracite. Therefore, there is a need to compare the catalytic performance of CeO2 and Fe2O3 for coal combustion in the presence and absence of the inherent mineral matters, exploring the insights into the possible interactions between externally-added and inherently-existing inorganics, and thus its effects on combustion reaction. The anthracite is the main feedstock used for industrial gasification and combustion, such as PCI for blast furnaces27,28. Additionally, in our previous results, compared with other type coals, such as lignite and bituminous coal, we found that catalysts showed more significant catalytic effects on anthracite combustion11,16. A type of Chinese anthracite was thus used as experimental sample in the present work, and its catalytic combustion reactivity and kinetics were investigated using DTA method. To examine effects of mineral matters on coal combustion reactivity and the potential interactions between mineral matters (ash) and catalyst, the combustion experiments were conducted using demineralized anthracite and “synthesized coal” in the presence and absence of catalysts. To measure the direct effects of catalysts on anthracite combustion reactions, raw anthracite was placed in the reference crucible and anthracite loaded with catalysts was used in the sample crucible in DTA tests. 2. EXPERIMENTAL SECTION 2.1 Materials
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The sample used was a type of Chinese anthracite which was pulverized and sieved to the range of 100~150 µm for this study. To prepare ash-free coal sample, 10g coal sample was firstly mixed with 150 ml of 6 N HCl and stirred for 24 h at room temperature. After filtering, 150 ml of HF (40%) was added to HCl-washed coal which was also stirred for 24h. Then the coal was further washed with boiling deionized water till no chloride ion was detected, followed by moisture evaporation at 80 oC in a vacuum oven for 24h. After the acid treatment, the content of ash in the coal is below 1 wt %, which has negligible catalytic effects on coal conversion based on previous study. Proximate and ultimate analyses for the anthracite are conducted and shown in Table 1. About 5 g of the mixture of anthracite with CeO2 or Fe2O3 (10 % by mass) was always manually blended for 20 min in a mortar. The purity of CeO2 and Fe2O3 were over 99% with particle sizes of
