Knowledge Management System Of Institute of process engineering,CAS
|导师||林伟刚 ; 李松庚|
|关键词||预氧化 降粘 热解 固体核磁共振|
Most of electricity in China is produced from coal. In order to realize clean and effective conversion of coal, Pingmei Group built a coal pyrolysis and cascade utilization pilot plant at Pingdingshan District, whose throughput could reach 600 thousand ton per year. However, plenty of local coal have strong caking property, which will result in difficulties in operation. The objective of this work aims at decaking of strong caking coal by pre-oxidation at low temperature and ensuring the stable operating of the pilot plant. At the moment, the process of decaking by pre-oxidation at low temperature draws great attention but knowledge of its mechanism is limited. Besides, effect of decaking via pre-oxidation at low temperature on pyrolysis behaviors of coal drew little attention. This research focused on the effect of oxidation conditions on coal’s caking property and pyrolysis products’ composition and yield. Mechanism of decaking by pre-oxidation at low temperature was also discussed. Pre-oxidation decaking experiments at low temperature with a fixed bed were carried out to find the effect of oxygen concentration and oxidation final temperature on coal’s caking property. It was found the increase of oxygen concentration was benefit for decaking. However, its effect on decaking became weak when the concentration was over 10%. At the final oxidation temperature of 150℃, the caking property of coal decreased slowly with the oxidation time, indicating that it was difficult for decaking at this temperature. At the oxidation temperature of 200~300℃, the coal’s caking property decreased rapidly with the oxidation time. The caking index decreased with time in a similar tendency at 250℃ and 300℃. 13C solid state NMR was carried out to characterize coal samples and explore decaking mechanism. It was found that various changes in coal’s molecule structure resulted in the decrease of caking index. The content of ether was increased by pre-oxidation at low temperature, which enhanced the cross-link of the branched chain and bridged bond in coal’s macromolecular structure. Thus the aromatic rings lost fluidity during pyrolysis, and diffusion of plastic mass was not enough to hold all the coal particles together. The shortening of aliphatic carbon chains and the increase of carbonyl content resulted in the decreasing of liquefied plastic mass, which was also benefit for decaking. Pyrolysis experiments within a fixed bed were carried out to investigate the effect of decaking on pyrolysis products’ composition and yields. It was found that the pyrolytic gas except for CO2/CO, char and tar would decrease to some extent after pre-oxidation at low temperature. The loss would increase with the oxidation time and final temperature. Around 60~70% of tar and 3~4% of char were lost when the caking index of strong caking coal sample decreased to 15~20. Besides that, H2 and hydrocarbon (CH4, C2~C5) suffered serious losses, while CO2 and CO increased. It was indicated that decaking strong caking coal to weak caking coal would reduce plenty of char, tar, CH4, H2, and hydrocarbon of C2~C5. To reduce the loss of volatile matter during decaking, a small percentage of weak caking coal was added into strong caking coal. The mixed coal was decaked by pre-oxidation at low temperature. It was found that the final caking index showed a good linear dependence on the blend mass ratio of weak caking coal after an hour’s oxidation in atmosphere containing 10% O2 at 200, 250, 300℃. Compared with counterpart results given by pure strong caking coal, mixed coal lost less pyrolysis products while its caking index was kept at similar level.
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