Fragmentation and attrition of coal char particles undergoing collisions during combustion at temperatures from 900 to 1100 K
✍ Scribed by Peter M. Walsh; Tong Li
- Publisher
- Elsevier Science
- Year
- 1994
- Tongue
- English
- Weight
- 655 KB
- Volume
- 99
- Category
- Article
- ISSN
- 0010-2180
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✦ Synopsis
Yields of small char particles, formed from larger particles by attrition and fragmentation during combustion, were measured at temperatures characteristic of fluidized bed combustors. The small particles were generated by burning char in a furnace on a vibrating screen. Particles passing through 1.3 × 1.3-mm openings were collected outside the furnace and weighed after complete destruction of the parent particles. The use of the screen isolates the process of fragment formation from the process of elutriation, and allows greater control of collision energy and frequency than is possible in a fluidized bed. Pittsburgh Seam char initially 5 mm in diameter was burned at temperatures of 900, 1000, and 1100 K in the presence of 2, 5, 10, and 21 mol% oxygen. By varying the vibration frequency and energy of collisions, the yield of fines was varied from 0 to 40 wt% of the fixed carbon in a batch feed. The yield of fines decreased with increasing temperature, decreased with increasing oxygen, and increased with simultaneous increases in the energy and frequency of collisions. The results were consistent with a model for parallel attrition and fragmentation, in which attrition is viewed as purely mechanical, proportional to collision frequency, and proceeding at a rate independent of combustion. Fragmentation was seen as occurring at a critical value of the char porosity, at a rate controlled by the removal of char by combustion. The critical porosity depends upon collision energy, rather than collision frequency. The model reproduced important features in the dependence of the fine particle yield on vibration frequency, collision energy, oxygen concentration, and temperature, in both diffusion and chemical rate-controlled combustion regimes, under conditions at which attrition made a greater contribution to small particles than fragmentation, and vice versa.