李艳, 彭贤汉, 洪文鹏, 兰景瑞, 李浩然. 碳化木的水热制备及其光热水蒸发性能强化[J]. 农业工程学报, 2022, 38(18): 222-228. DOI: 10.11975/j.issn.1002-6819.2022.18.024
    引用本文: 李艳, 彭贤汉, 洪文鹏, 兰景瑞, 李浩然. 碳化木的水热制备及其光热水蒸发性能强化[J]. 农业工程学报, 2022, 38(18): 222-228. DOI: 10.11975/j.issn.1002-6819.2022.18.024
    Li Yan, Peng Xianhan, Hong Wenpeng, Lan Jingrui, Li Haoran. Hydrothermal preparation of carbonized wood for photothermal water evaporation performance enhancement[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2022, 38(18): 222-228. DOI: 10.11975/j.issn.1002-6819.2022.18.024
    Citation: Li Yan, Peng Xianhan, Hong Wenpeng, Lan Jingrui, Li Haoran. Hydrothermal preparation of carbonized wood for photothermal water evaporation performance enhancement[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2022, 38(18): 222-228. DOI: 10.11975/j.issn.1002-6819.2022.18.024

    碳化木的水热制备及其光热水蒸发性能强化

    Hydrothermal preparation of carbonized wood for photothermal water evaporation performance enhancement

    • 摘要: 针对目前太阳能驱动界面水蒸发技术材料成本高、制备过程复杂、蒸发速率较低等问题,该研究基于农林樟子松自然进化的多孔结构,采用操作简单的一步热水法制备了亲水性强、蒸发速率接近理论极限的多孔光热材料。基于光热材料的碳化程度与供水高度调节,结合太阳能驱动界面水蒸发试验,考查了不同碳化温度、碳化时间和光热材料高度对界面光热水蒸发特性的影响规律。基于微观结构、元素分布、毛细水输运与传热规律分析,重点阐述了碳化木强化界面水蒸发的潜在机理。结果表明,温和的碳化过程保留了原木的原始孔隙结构,但木纤维被大量碳微球覆盖,形成了更加粗糙的表面,增强原木对入射光的吸收能力,使蒸发表面的温度提升了2.3 ℃。碳化反应去除了原木的部分疏水性木质素和半纤维素,增加了亲水基团-OH的比例,因此芯吸时间为120 s时,水在碳化木中的爬升高度由4.2 mm增加至22.3 mm。碳化木的稳态蒸发速率相比水和原木分别提高了130%和28%。碳化温度和时间分别为200 ℃和8 h的碳化木获得了最大蒸发速率1.24 kg/(m2·h)。光热材料侧壁高度的增加降低了顶部蒸发面的供水速率和平均温度,因蒸发器热阻和热损失的减少而提升了蒸发速率。当侧壁高度为15 mm时,蒸发效率为66.2%,其等效蒸发速率为1.48 kg/(m2·h),与理论极限接近。该研究为农林废弃物资源化利用和太阳能驱动界面水蒸发过程热湿平衡调控理论的发展提供了重要参考。

       

      Abstract: Abstract: Interfacial photothermal evaporation is expected to resolve the water shortage, inconvenient water intake, and low water quality in most rural areas. However, the high material cost, complex preparation process, and low evaporation rate of the solar absorber are restricting the development of this novel technique. It has been reported that adjusting the water supply rate on the light-absorbing surface is an important approach to developing an efficient and continuous interfacial photothermal evaporator. In this work, the naturally evolved, porous, and photothermal material of pinus sylvestris was prepared with simple preparation, strong hydrophilicity, and evaporation rate close to the theoretical limit using a one-step hydrothermal method. Subsequently, the solar-driven interfacial water evaporation experiments were conducted to clarify the effects of carbonization degree and water supply height of the photothermal material on the interfacial evaporation performance. The surface temperature and evaporation reduction were considered at the different carbonization temperatures (100, 125, 150, 175, and 200℃), carbonization time (2, 4, 6, 8, 10, and 12 h), and the heights of the photothermal material (5, 10, and 15 mm). The underlying mechanism was then determined using the microstructure, element distribution, capillary water transport, and heat transfer. Importantly, the enhanced interfacial water evaporation was enabled by the carbonized wood. The results showed that the original pore structure of the log was retained with the rough surface formed by carbon microspheres after the mild carbonization process. There was a 2.3 ℃ temperature rise of the evaporation surface, particularly beneficial to the increase of light absorption. Moreover, the proportion of the C-H/C-C bond in the log (51.5%) was higher than that of the C-O-C/C-OH bond (37.7%), whereas, the proportion of the C-H/C-C bond (37.8%) in the carbonized wood was lower than those. It infers that the carbonization process greatly contributed to the log with more hydrophilic groups. This was because the parts of hydrophobic lignin and hemicellulose were removed during carbonization. Therefore, the climbing height of water in the carbonized wood increased from 4.2 to 22.3 mm during the climbing time (120 s). By contrast, the rising heights of water were 3.1 and 9.0 mm in the log and carbonized wood, respectively, indicating the weak capacity of transverse water transport. The data also agreed well with the classical Lucas-Washburn imbibition model. The steady-state evaporation rate of the carbonized wood-based evaporator increased by 130% and 28%, respectively, compared with the water- and log-configured evaporators. The maximum evaporation rate reached 1.24 kg/(m2?h) at the temperature of 200 ℃ after 8 h carbonization. It was noted that the excessive carbonization formed a bright and smooth surface, leading to the reflection loss of incident light with the reduced surface temperature of the evaporator. As a result, the evaporation rate dropped significantly. More importantly, the sidewall height of the material reduced the average temperature of the evaporation surface, resulting in the reduction of radiative and convective heat losses from the evaporator to the surroundings, thus improving the evaporation rate. Once the sidewall height was 15 mm, the evaporation rate reached 1.48 kg/(m2?h), and the corresponding solar-to-vapor conversion efficiency was 66.2%. This finding can also provide an important reference for the utilization of agricultural and forestry wastes, particularly for the heat-moisture balance during solar-driven interfacial water evaporation.

       

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