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根据不同温度下氧分子平均自由程的大小,比较了小孔、中孔和大孔中三种扩散速率与煤焦表面燃烧速度的大小。研究表明: 2000 K以内,颗粒表面分子扩散速率比氧化反应速率大1个数量级以上,过度扩散速率不小于氧化速率。温度小于1200 K时,燃烧速率比Knudsen扩散速率小1-5个数量级,扩散孔径小于15-28 nm,反应主要在内外表面进行;1200-1600 K时,燃烧速率与Knudsen扩散速率相当,扩散临界孔径28-38 nm,反应在外表面及浅层内表面进行;温度1600 K以上时, Knudsen扩散速率比燃烧速率小1个数量级,孔径38-50 nm以下内表面上碳的氧化速度受扩散控制。煤焦的氧化主要发生在Knudsen扩散临界孔径10-50 nm以上的氧气可达表面上。
According to the average free path of oxygen molecules at different temperatures, the three kinds of diffusion rates in pore, mesopore and macropore were compared with the surface burning rate of coal char. The results show that: within 2000K, the molecular diffusion rate of particle surface is more than one order of magnitude higher than the oxidation reaction rate, and the over diffusion rate is not less than the oxidation rate. When the temperature is less than 1200 K, the combustion rate is one to five orders of magnitude less than the Knudsen diffusion rate and the diffusion pore size is less than 15-28 nm. The reaction proceeds mainly on the inner and outer surfaces. At 1200-1600 K, the combustion rate is comparable to the Knudsen diffusion rate, The pore diameter is 28-38 nm, and the reaction proceeds on the outer surface and the shallow inner surface. Knudsen diffusion rate is one order of magnitude less than the combustion rate above 1600 K. The oxidation rate of carbon on the inner surface below the pore diameter of 38-50 nm is controlled by diffusion. The oxidation of coal char mainly occurs on oxygen accessible surfaces with a Knudsen diffusion critical pore size above 10-50 nm.