基于简单碰撞理论煤粉燃烧动力学模型的研究-PART Ⅲ:氧气可达比表面积

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1200K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;1200～1600K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1600K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.

STUDY ON REACTION KINETICS MODEL OF PULVERIZED COAL COMBUSTION BASED ON SCT--PART Ⅲ: OXYGEN ACCESSIBLE SPECIFIC SURFACE AREA

According to the magnitude of average distance of oxygen molecular freedom motion at different temperature, comparing oxygen molecular diffusion rate of three kinds of diffusions through micropores, mesopores and macropores with combustion rate (CR) of pulverized coal char (pcc), it is shown that CR is less than transition diffusion rate, and less one or more orders than molecular diffusion rate on particle surface within 2000 K. When the temperature is less than 1200 K, oxygen molecules can pass through the pores of diameters less than 15-28 nm, Knudsen diffusion rate (KDR) is more from one to five orders than CR, and burning occurs on the internal surface; When the temperature is between 1200 K and 1600 K, CR is all most equal to KDR, the critical pores diameters are 28-38 nm, and firing occurs on the external and internal shallow surface; When the temperature is more than 1600 K, KDR is less one order than CR, controlled by Knudsen diffusion for the pores diameter less than 38-50 nm. The oxidation of pcc chiefly occurs on the oxygen accessible surface of the critical pores diameters of Knudsen diffusion more than 10-50 nm.