煤生物降解地球化学表征及商业性生物气化的地质控制

Biodegradation of organic matter in the subsurface is well documented in shale sediments and oil reservoirs but much less understood for coals. Although large plays of secondary biogenic methane in many coal fields prove that coal derived products can support substantial, in situ generation of biogenic gas, surprisingly little is known about the fraction of coal ultimately converted to methane and factors controlling the process. High resolution (cm scale) geochemical characterization of coals and coal biodegradation simulation experiments will be performed in the present study to understand the mechanisms responsible for the biodegradation of coal and accurately quantify mass fraction removed by in situ methanogenesis. The purpose of our study is to establish proper biodegradation assessment system for coal and explore the possibility of industry application. Bioconversion of coal to methane is of great interest as an underexploited source of clean energy and has many economic and environmental advantages over more conventional processes that could make low-grade ores economically viable. This process coupled with CO2 sequestration has great potential real time methane production and greenhouse gas emission. However, the feasibility of in-situ coal bioconversion to methane by methanogens has not been demonstrated yet. We will make our best effort to tackle the fundamental issues related to the process.

在页岩沉积物和油藏中发生的生物降解已被大量解释，而煤生物降解的研究程度则很低。尽管许多煤田中的次生生物气被认为与煤的生物降解有关，但煤中到底哪些组分被降解并转变成甲烷及控制这个过程的主要因素至今仍未有清楚认识。本项目拟通过对已知生物降解煤田样品高分辨率的地球化学表征及煤生物降解产气模拟实验来深化对煤生物降解机理的认识及准确定量煤被产甲烷菌降解和转化的质量馏分，确认煤的降解标志，建立煤生物降解评价体系并前瞻煤的生物气化前景。煤的生物气化不仅可以获取洁净能源，还可与二氧化碳地质埋存相结合，大大降低能源开发对环境的影响，这对现有煤层气开发及目前尚无经济价值煤层的利用都有极其重要意义，但实现这个过程现实可行性尚未得到科学论证，本项目拟开展有益探索。

煤炭是世界上最丰富的化石能源之一，也是温室气体排放的主要来源。生物气化或微生物强化煤层气开发是近年来的研究热点，因为与直接燃烧相比，其二氧化碳排放量少。通过微生物将煤生物气化可以通过添加微生物或养分来加快转化效率，这个过程可以在原地（废弃或不可开采的煤层）也可以在异地（煤炭废物或开采出的煤）中进行。尽管该技术为获取不可开采的煤炭资源的能源潜力提供了创新且有吸引力的替代方案，但采用这种策略仍然面临着多重挑战。煤中烃类与油藏中观察到的生物降解变化相同，但保留的特征似乎完全不同。在芳烃（烷基萘和烷基菲）被严重地生物降解后，仍会有少量正构烷烃出现在GC-MS上，因此，多数地球化学家得出芳香烃比饱和烃优先降解的结论。事实上，煤生物降解的敏感性不仅取决于其分子结构，还取与物理状态（游离态与吸附态）和潜在反应物的溶解度有关。单纯从GC-MS图谱上不能确切反映生物降解的影响，而每种化合物的绝对浓度更可靠。类似于油层中生物降解发生在油/水界面附件，煤的生物降解与地层水位置或砂岩含水层或割理水的流动有关，而烃与水的接触情况在规模和频率上与储油层完全不同，水在大多数煤中可能会受到限制，并且其分布不像油水过渡带中那样连续，煤中裂缝和割理系统的变化可能会刺激烃类生物降解，形成多个微梯度。毫无疑问，渗透率在生物气化和煤层气生产中起关键作用。煤的渗透率随孔隙压力的增加而增加，但随围压的增加而降低。由于溶剂吸附而引起的煤基质膨胀会显着降低渗透率，但溶剂注入可能会在煤生物转化过程中增加煤层孔隙压力，进而导致有效应力降低和渗透率增加。为煤的生物转化量身定制溶剂和营养物配方必须考虑溶剂使用的影响。总体上看，天然煤炭的生物降解速度都太慢，目前尚没有明确的商业前景。提高煤的渗透性，以允许更多地接触生物和养分，去除抑制气化的干扰，是提高生物气化效率一条明路。