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Secondary catalytic effect of circulating ash on the primary volatiles from slow and fast pyrolysis of coal Yaqing Zhang, Dengfeng Pan, Xuan Qu, and Peng Liang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03310 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018
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Secondary catalytic effect of circulating ash on the primary volatiles from slow and fast pyrolysis of coal Yaqing Zhanga, Dengfeng Panb,c, Xuan Qub, Peng Lianga,* a
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, PR China b State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China c University of Chinese Academy of Sciences, Beijing 100039, PR China
Abstract The lab-scale fixed-bed pyrolyzer by circulating ash as heat carrier was used to study the secondary catalytic effect of circulating ash on the primary volatiles from slow and fast pyrolysis of coal. Different mass ratios of ash to coal were layered in the pyrolyzer. Combined with the simulated distillation analysis, gas chromatograph-mass spectrum (GC-MS) and element analysis, the yields and compositions of gas, tar, and semicoke were analyzed. The results show that circulating ash catalyzes the cracking of primary volatiles generated by both slow and fast coal pyrolysis. CO2 is very sensitive to the proportion of circulating ash, it mainly comes from the breaking of –C–O– bonds with low energy in tar. After adding circulating ash, the gas yield increases, while the tar quality is improved despite the reduction in tar yield. Circulating ash is beneficial to the cracking of oxygen-containing, nitrogen-containing, and sulfur-containing bonds in asphalt. The components with carbon numbers of ≥26 in tar can be upgraded, and then results in the growing yields of ≤C25 components. Circulating ash can increase the tar saturation in the secondary catalytic reaction. This study can provide basic data for the effect of secondary catalytic as well as the heating rate on the coal pyrolysis by circulating ash as solid heat carrier. * Corresponding author. Tel.: +86 13678890728; fax: +86 532 86057718. E-mail address:
[email protected] (P. Liang) 1
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Keywords: Slow and fast pyrolysis; Coal volatiles; Circulating ash; Secondary catalytic 1. Introduction China is rich in low- and middle-rank coal resources which have high volatiles.1 The clean and efficient utilization of coal has always been a concern.2,3,4 In recent years, the western provinces in China such as Inner Mongolia, Shaanxi, and Xinjiang, etc. have developed the moving-bed pyrolysis technology by solid heat carrier for the use of these high-volatile coals.5 Coal pyrolysis technology by solid heat carrier is such a process that solid is preheated by other heat sources to serve as a heat carrier and then the solid heat carrier mixes with coal to provide heat for coal pyrolysis. Compared with the external thermal pyrolysis, the solid heat carrier is in direct contact with coal so that coal can be heated rapidly and uniformly. High tar yield can be obtained to make up for the shortage of oil, and semicoke with a high calorific value can be used for combustion. Ceramic ball, quartz sand, semicoke, and circulating ash are usually used as solid heat carriers in industry.6,7 However, extra heat is needed for quartz sand and ceramic balls to achieve the desired temperature. When semicoke is used as heat carrier, it is easy to be pulverized during the mixing process because of its low hardness. Nowadays, circulating fluidized bed (CFB) combustion has been developed greatly, so that the circulating ash as heat carrier is particularly valued.8−11 The ash from CFB has a high temperature of ~900 °C, which can be used to heat the coal directly without extra heat. Furthermore, semicoke generated by solid heat carrier pyrolysis can be used as fuel for CFB boilers. Therefore, the staged conversion for coal resources can be achieved. The circulating ash contains a certain amount of minerals which has catalytic effect on the coal pyrolysis. There is an important, inevitable, and real existence scientific problem in the moving-bed pyrolyzer by ash heat carrier that circulating ash not only acts as a heat carrier but also affects the distribution of pyrolysis products.12,13 In the moving-bed pyrolyzer, the circulating ash distributes 2
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uniformly around the coal particles. The release of coal pyrolysis volatiles undergoes two catalysis stages. (1) Primary catalysis stage: coal particles contain some minerals that have catalytic cracking ability of tar. Therefore, when the volatiles are released out from the coal particles and then flow upwards in the reactor, the generated volatiles will be catalyzed by the minerals insides coal and on the surface of above coal particles. The catalytic effect of volatiles in this stage is defined as the primary catalysis effect of volatiles. (2) Secondary catalysis stage: when the volatiles escape from the surface of coal particles, they are catalyzed by minerals in circulating ash. The catalytic effect of volatiles in this stage is defined as the secondary catalysis effect of volatiles. The catalytic cracking of pyrolysis volatiles is widely reported by many researchers. However, their researches are subjective and adjustable, mainly for the upgrade of coal tar,14 modification of catalysts,15 deactivation and regeneration of catalysts,16 etc. In the specific progress of coal pyrolysis by solid heat carrier, the unavoidable scientific problem of catalytic effect of circulating ash on pyrolysis volatiles has been less studied. Many studies17,18 have published the catalytic effect of inherent minerals in coal on the volatiles, which is the primary catalysis stage of ash to coal pyrolysis volatiles has been widely reported. Ahmad et al.19 studied the influence of inherent minerals in coal particles on the pyrolysates yields of Pakistani coals which have high mineral contents. They removed the inherent minerals from Pakistani coals by acid wash method, and found that the yield of total volatiles decreased. The previous researches of Wornat et al. 20 and Sathe et al. 21 indicated that the alkali and alkaline-earth metals (AAEM) in coal decreased the yields of tar, aromatics and some other unsaturated hydrocarbons. Otherwise, the release of aliphatic components was promoted by AAEM during coal pyrolysis. In order to determine the effect of major coal minerals on yields and compositions of pyrolysis products, Roets et al.22 added 5 wt% quartz, kaolinite, pyrite, dolomite, or calcite to an acid washed bituminous coal. It was found that the mineral activity increased in the order of quartz
