Abstract Coal particles experience a transition from a reducing to oxidizing environment in the near-burner region of pulverized coal (pc) boilers. For the first time, we report a fundamental study of ignition of a coal-particle stream experiencing a flame environment that transitions from a reducing to an oxidizing environment (termed reducing-to-oxidizing environment). High-speed videography is used to observe the particles in situ, and scanning electron microscopy is used to characterize the sampled particles. The effects of particle size on ignition are presented for four size bins (63–74 µm, 75–89 µm, 90–124 µm and 125–149 µm) for PRB subbituminous coal at two nominal gas temperatures (1300 K and 1800 K). An oxidizing environment with 20% molar oxygen composition is used as base-case. In contradistinction to single particle studies where particles are reported to ignite heterogeneously at higher temperatures, this study shows that coal streams ignite homogeneously, irrespective of particle size, in the oxidizing environment. By changing nominal gas temperature from 1300 K to 1800 K, ignition time decreases, on average, by a factor of five for each of the particle size bins. For both gas temperatures, the trend in ignition delays as particle size changes is non-monotonic. However, at 1800 K nominal gas temperature, ignition delays are independent of particle size in the reducing-to-oxidizing environment and ignition delays are doubled on average when compared to those in the oxidizing environment. It is more noticeable at the lower gas temperature of 1300 K that homogeneous ignition of coal streams is oxygen-dependent below 90 µm particle size and temperature-dependent above 90 µm. In general, ignition delay is determined by volatile release rate (controlled by the particle temperature) and the local oxygen concentration. Micrographs of particles also confirm that ignition and char burnout times are longer in the reducing-to-oxidizing environments than those in the oxidizing environments.

中文翻译：

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粒径和还原氧化环境对煤流点火的影响

摘要 在煤粉 (pc) 锅炉的近燃烧器区域，煤颗粒经历了从还原环境到氧化环境的转变。我们第一次报告了对经历火焰环境的煤颗粒流点火的基础研究，该环境从还原环境过渡到氧化环境（称为还原到氧化环境）。高速摄像用于原位观察颗粒，扫描电子显微镜用于表征采样颗粒。在两种标称气体温度（1300 K 和 1800 K）下，PRB 次烟煤的四种尺寸分级（63–74 µm、75–89 µm、90–124 µm 和 125–149 µm）的颗粒大小对点火的影响. 具有 20% 摩尔氧组成的氧化环境用作基本情况。与据报道颗粒在较高温度下不均匀点燃的单颗粒研究不同，该研究表明，煤流在氧化环境中均匀点燃，而不管颗粒大小。通过将标称气体温度从 1300 K 更改为 1800 K，对于每个粒度仓，点火时间平均减少 5 倍。对于两种气体温度，随着颗粒尺寸的变化，点火延迟的趋势是非单调的。然而，在 1800 K 标称气体温度下，点火延迟与还原氧化环境中的颗粒大小无关，与氧化环境中的点火延迟相比，点火延迟平均增加了一倍。更值得注意的是，在 1300 K 的较低气体温度下，煤流的均质点火在 90 µm 粒径以下取决于氧气，而在 90 µm 以上则取决于温度。通常，点火延迟由挥发物释放速率（由颗粒温度控制）和局部氧浓度决定。颗粒的显微照片还证实，还原到氧化环境中的点火和炭烧尽时间比氧化环境中的要长。