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低阶煤氧化制腐植酸本质是实现芳香环之间桥连的—CH2—、—C—O—、—O—等活泼化学键的选择性断裂,生成硝基、羟基等含氧官能团。电化学氧化法具有条件温和、反应易于控制、产品纯度高、氧化电位电位连续可调等优点[24]。
煤浆电解早期被发现可降低电解水阳极反应电位,进而可降低电解水制氢能耗,其原理如式(1)~式(2)所示,主要用于降低电解水制氢能耗。当采用煤为原料时,反应阴极和阳极的反应如式(3)~式(4)。
$$ \begin{split} &\text{电解水总反应:}{{\text{H}}_{2}}\text{O}\to \text{2}{{\text{H}}_{2}}\uparrow +{{\text{O}}_{2}}\uparrow\\ &{{\text{E}}^{0}}\text{=1}\text{.23}\ {{\text{V}}_{\text{RHE}}} \end{split} $$ (1) $$ \begin{split} &\text{电解煤浆总反应:{\text{C}}+}{{\text{H}}_{2}}\text{O}\to \text{2}{{\text{H}}_{2}}\uparrow +\text{C}{{\text{O}}_{2}}\uparrow\\ &{{\text{E}}^{0}}\text{=0}\text{.21}\ {{\text{V}}_{\text{RHE}}} \end{split} $$ (2) $$ 阴极反应:4{{\text{H}}_{2}}\text{O}+4{{\text{e}}^{-}}\to 4\text{O}{{\text{H}}^{-}}+2{{\text{H}}_{2}}\uparrow $$ (3) $$ 阳极反应:\text{Coal+4O}{{\text{H}}^{-}}\text{-4}{{\text{e}}^{-}}\to \text{Coa}{{\text{l}}_{\text{ox}}}、腐植酸等+2{{\text{H}}_{2}}\text{O} $$ (4) $$ 总反应:\text{Coal+2}{{\text{H}}_{2}}\text{O}\to \text{Coa}{{\text{l}}_{\text{ox}}}、腐植酸等+2{{\text{H}}_{2}}\uparrow $$ (5) 上述反应的本质是利用阳极产生的氧化介质(•OH)与低阶煤反应,或者煤颗粒与电极直接碰撞来实现煤的氧化,替代原本的阳极析氧反应。•OH进攻煤大分子结构桥连—CH2—、—C—O—、—O—等活泼化学键,生成硝基、羟基等含氧官能团,产生氧化煤(Coal ox)、腐植酸等氧化产品。
由于电解过程的氧化介质氧化能力(氧化电位)强,可以将含有10%左右含量腐植酸的油页岩原样氧化,提取出总腐植酸产率可以达到90%[25]。即便是无烟煤、烟煤也能产生相当产率的腐植酸[26-27],这在传统空气/O2、HNO3氧化制腐植酸过程是无法实现的。
在煤浆电解制氢过程,希望煤炭尽量彻底氧化,以便降低能耗。而对于制取腐植酸过程,则需要实现适度氧化来获得腐植酸(酸性)或腐植酸盐(碱性)。因而,只有强化煤浆电化学氧化,且合适地控制煤浆氧化程度是提高腐植酸产率的根本。围绕这一目标,研究人员针对煤质、电解液、阳极材料和操作参数(电位、浆液浓度等)方面展开了广泛研究。
煤浆的电化学氧化可用于降低制氢能耗[28]以及煤炭氧化制腐植酸[29]。由于酸性电解液可在一定程度促进煤中矿物质溶解,Fe3+/2+价态循环有利于提高煤浆电解制氢的电流密度,所以在酸性电解液主要用于电解制氢[30]。同样由于Fe3+/2+价态循环导致腐植酸容易过度氧化,酸性电解液电解煤浆制腐植酸收率不高[31](约10%)。由此可见,酸性介质对制取的腐植酸产品还存在很大的提升空间。
碱性电解液可将腐植酸转化成稳定的腐植酸盐,减少了腐植酸的过度氧化,因而早就有了研究。1986年Lalvani[32]采用三电极体系和恒电位法研究了褐煤在碱性介质中的氧化过程。制取腐植酸的实验流程如图5所示,制取的腐植酸的产率达到70%以上。
提高浆液浓度会降低体系的导电性,维持电流密度不变时必须提高阳极电位,从而导致阳极表面产生更多的氧化介质,一方面导致腐植酸过度氧化,另一方面阳极电位提高,析氧反应也更容易发生。
电极材料也是影响煤浆电解制腐植酸的重要原因,1932年Lynch[33]研究了(Cu、Ni、Pb、Pt)四种阳极电极材料,发现阳极产物中检测有腐植酸、O2、CO2和微量的CO。使用Cu电极的效果最佳,制取腐殖酸的速率较大,可以使反应停留在制腐植酸阶段,使用Pt电极,腐植酸会继续氧化成CO2。采用不同的电极材料和电解条件可以控制煤电解氧化阶段,从而得到期望的产物。
Research Progress on Electro-Chemical Oxidation of Low-Rank Coal to Humic Acid
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摘要:
目的 随着可再生能源的大力发展,“双碳”背景下低阶煤的清洁高效利用不仅提高了资源利用率、经济价值,更具有重大的社会意义。褐煤、风化煤等低阶煤是煤基腐植酸的主要来源,腐植酸因具有弱酸性、氧化还原性、生理活性等特性,用途广泛。然而直接提取方式难以实现煤中腐植酸的高效提取,通常需经氧化预处理来提高腐植酸的产率。 方法 文章首先简述了腐植酸的性质和应用;然后,介绍了传统的化学氧化的工艺过程、原理和特点,综述了新型低阶煤电化学氧化方法及优势;最后,对低阶煤电化学制腐植酸进行了展望,以期为低阶煤制取腐植酸提供参考。 结果 电化学氧化法具有条件温和、反应易于控制、产率高、减少环境污染等优点,是一种比较有前景制备腐植酸的方法。 结论 采用可再生电力驱动的电化学氧化实现低阶煤中腐植酸的高效提取是实现低阶煤非燃料高附加值利用的途径之一。 Abstract:Introduction With the vigorous development of renewable energy, clean and efficient utilization of low-rank coal not only improves resource utilization and economic value but also has great social significance. Low-rank coals such as lignite and weathered coal are the main sources of coal-based humic acid, humic acid is widely used because of its weak acidity, redox, and physiological activity, however, direct extraction is difficult to achieve efficient extraction of humic acid in coal, which usually requires oxidation pretreatment to improve the yield of humic acid. Method Firstly, the properties and applications of humic acid were introduced; then, the process, principle, and characteristics of traditional chemical oxidation were introduced, and the new low-rank coal electrochemical oxidation methods and advantages were summarized. Finally, the prospect of electro-chemical humic acid production from low-rank coal was prospected to provide a reference for the preparation of humic acid from low-rank coal. Result Electrochemical oxidation method has the advantages of mild conditions, easy control of reaction, high yield, and reduction of environmental pollution. It is a relatively promising method for the preparation of humic acid. Conclusion The efficient extraction of humic acid from low-rank coal by electrochemical oxidation driven by renewable power is one of the ways to achieve high value-added utilization of non-fuel in low-rank coal. -
Key words:
- low-rank coal /
- humic acid /
- development situation /
- electro-chemical oxidation
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图 3 H2O2氧化条件对HA产率的影响[20]
Fig. 3 Effect of H2O2 oxidation conditions on HA yield
表 1 化学氧化与电化学氧化的对比
Tab. 1. Comparison of chemical oxidation and electrochemical oxidation
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