生物炭改性黏土气体突破机理微-细观研究及在土质气体屏障中的应用

Landfill gas control requires high gas holding capacity of landfill covers. However, conventional landfill covers normally have poor performance in preventing landfill gas emission, thereby they need to be urgently improved. Biochar has a big potential application in enhancing the performance of landfill gas barriers since it has a special pore structure and good gas adsorption capacity. This project aims to investigate gas emission characteristics of biochar-amended clay and reveal the mechanism of gas breakthrough in biochar-amended clay barriers and provide optimized design method for the biochar-amended clay barriers. To achieve the above objectives, understanding how the application of biochar into clay changes gas emission in clay and the action mechanism, developing mathematical model for gas emission in biochar-amended clay, determining the key controlling parameters for biochar-amended clay barriers are the key scientific challenges for the design of clay gas barriers. This project comprehensively adopts laboratory element tests, column physical model tests, theoretical study and numerical modelling. The project proposes a comprehensive study on gas breakthrough mechanism of biochar-amended clay from perspectives of macroscopic experiments and microscopic pore structure, respectively. Furthermore, this project also proposes to develop gas mathematical model in unsaturated soil considering the effect of biochar application and evaluate the performance of the biochar-amended clay barriers in preventing gas emission. The outputs from this project can enrich the theory of gas flow in unsaturated soils and provide solid theoretical guidance for the design of landfill gas barriers.

填埋气逸散防控要求覆盖屏障具有良好的闭气性能，然而传统土质覆盖屏障闭气性能不佳，亟需改良。生物炭因其独特的孔隙结构和气体吸附特性，对改良土质气体屏障潜力巨大。本课题拟获取生物炭改性黏土的渗气特性，揭示生物炭改性黏土覆盖屏障气体突破机理，提出生物炭改性黏土覆盖屏障的优化设计方法。为此，明确生物炭的掺入改变黏土渗气特性的规律及作用机理、建立生物炭改性黏土气体渗流数学模型、确定生物炭改性黏土覆盖屏障的关键调控参数，是实现黏土覆盖屏障闭气功能设计的关键和亟需解决的问题。本项目综合采用室内单元试验、土柱模型试验、理论模型构建、数值模拟等研究方法开展研究。拟从宏观试验和微细观结构分析两个角度对生物炭改性黏土气体突破机理展开系统深入的研究，构建考虑生物炭影响的非饱和土气体渗流数学模型，综合评估生物炭改性黏土气体屏障的闭气性能。研究成果能丰富非饱和土气体渗流理论，为填埋场气体屏障的科学设计提供理论依据。

填埋场覆盖层是垃圾填埋场封场后阻止填埋气逸出的最后一道屏障，如何改良传统土质覆盖层，进而提高其闭气性能，最大限度地减少填埋气的排放是当前解决垃圾填埋气恶臭扰民问题的关键。生物炭在解决全球生态环境安全与农业可持续发展等方面的作用已获得普遍认可，对改良土质覆盖屏障潜力巨大。本项目综合土单元特性试验、土柱物理模型试验、微细观分析和数值模拟手段，对生物炭改性土气体运移特性和突破机理展开系统深入研究，综合评估了生物炭改性土气体屏障的闭气性能。研究结果表明，掺入细颗粒的生物炭增大了土体持水能力，减小了土体透气系数。随着生物炭掺量增加，土体有效含水率增加，土体近饱和透水系数和透气系数均显著减小。从微细观角度分析原因发现，掺入细颗粒生物炭显著减小了土体峰值孔径和大孔隙，从而使土体进气值增大。在干燥蒸发过程，掺入细颗粒（小于0.25mm）生物炭能使土体保持较高的持水量，其中掺入20%生物炭降低土体累积蒸发量和覆盖层气体逸出速率最显著。基于本项目研究结果，建议在填埋场覆盖层建设中掺入细颗粒生物炭，并且在保证覆盖层稳定性的前提下，可尽量加大覆盖层倾角，以此来降低气体逸出速率。本课题研究结果不仅对发展非饱和土气体渗流基础理论有科学意义，也对填埋场恶臭气体防控有实际意义，可为垃圾填埋场终场覆盖层的后续科学化建设提供指导意见。