Determination of size and porosity of chars during combustion of biomass particles
Determination of size and porosity of chars during combustion of biomass particles
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DOI：
10.1016/j.combustflame.2022.112182
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发表时间：
2022-08
影响因子：
4.4
通讯作者：
Yuan Yao;A. Panahi;M. Schiemann;Y. Levendis
中科院分区：
文献类型：
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作者：
Yuan Yao;A. Panahi;M. Schiemann;Y. Levendis
本研究的重点是碳质焦的大小和总孔隙率（孔体积），源于高加热速率和高温热解和/或燃烧的生物质。重点是焙烧生物质焦。首先，已知质量的单个生物质颗粒的炭残留物的孔隙率被确定，基于骨架密度的假设值，并通过比较实验观察到的温度-时间的历史与其燃尽时间的数值预测。计算出几种生的和焙烧的生物质颗粒的平均炭孔隙率（有效孔隙率）在92- 95%的范围内。此后，这些推导出的孔隙率值再次输入到模型中，以计算其他生物质颗粒前体的焦炭的尺寸，其初始尺寸和质量是未知的。将这样的生物质颗粒筛分至不同的标称尺寸范围。这一次，除了孔隙率之外，上述尺寸范围内的生物质颗粒的代表性时间-温度曲线也被输入到模型中。生物质颗粒是高度不规则的，具有大的纵横比，并且在许多情况下，它们在高加热速率和升高的温度下熔融和球化。知识的初始大小的焦炭，挥发性火焰熄灭后，需要建模其非均相燃烧阶段。为此目的，数值预测与测量的焦炭尺寸从捕获的焦炭和实时高速，高放大倍率的电影摄影观察其燃烧的扫描电子显微镜获得的一般协议。结果表明，所研究的生物质类型的生成的半焦是高度多孔的大空腔。在快速热解时，炭的平均初始尺寸在用于对其高度不规则的母体生物质颗粒进行尺寸分类的筛的筛目尺寸的中值的50-60%的范围内。
This research focused on the size and overall porosity (pore volume) of carbonaceous chars, originating from high-heating rates and high-temperature pyrolysis and/or combustion of biomass. Emphasis was given to torrefied biomass chars. First, the porosity of char residues of single biomass particles of known mass was determined, based on an assumed value of skeletal density and by comparing experimentally observed temperature-time histories with numerical predictions of their burnout times. The average char porosities (effective porosities) of several raw and torrefied biomass particles were calculated to be in the range of 92–95%. Thereafter, these deduced porosity values were input again to the model to calculate the size of chars of other biomass particle precursors, whose initial size and mass were not known. Such biomass particles were sieve-classified to different nominal size ranges. This time, besides the porosity, representative time-temperature profiles of biomass particles in the aforementioned size ranges were also input to the model. Biomass particles are highly irregular with large aspect ratios and, in many cases, they melt and spherodize under high heating rates and elevated temperatures. Knowledge of the initial size of the chars, upon extinction of the volatile flames, is needed for modeling their heterogeneous combustion phase. For this purpose, numerical predictions were in general agreement with measurements of char size obtained from both scanning electron microscopy of captured chars and real-time high-speed, high-magnification cinematographic observations of their combustion. Results showed that the generated chars of the examined biomass types were highly porous with large cavities. The average initial dimension of the chars, upon rapid pyrolysis, was in the range of 50–60% the mid-value of the mesh size of the sieves used to size-classify their highly irregular parent biomass particles.