富油煤地下热解是一种可有效提取煤中油气资源的绿色低碳技术，是煤炭清洁低碳利用的前沿方向。煤中纳米孔隙是地下热解过程中吸附和解吸焦油气的主要空间，因此探明地下热解条件下纳米孔隙结构特性演化规律，是提高富油煤地下热解工艺油气产率的关键科学问题。利用中国散裂中子源(CSNS)-小角中子散射(SANS)技术，模拟煤层地下热解环境，定量表征富油煤在无围限压力条件下不同加热速率与温度下纳米孔隙的散射强度、平均孔径与分形维数演变，结合低温吸附(BET)、热重实验(TG)、扫描电镜(SEM)等多种物理表征方法，揭示富油煤地下热解过程中纳米孔隙结构特性演化规律。研究结果表明：在地下热解过程中，纳米孔隙的平均孔径随温度的升高逐渐增大，在热解干燥脱气阶段(< 300 ℃)发育较缓慢，在活泼阶段(300～500 ℃)纳米孔隙发育最为剧烈，增长幅度为57.1%，在随后二次脱气阶段发育再次减缓；在低−中−高温(≤800 ℃)条件下热解没有引起富油煤表面分形维数D_s的明显变化。基于SANS散射光谱，分析发现热解过程中富油煤纳米孔隙分布呈现各向同性特征，表明热解反应不会影响本实验样品中纳米孔隙发育的方向性。对比不同加热速率(5 ℃/min与20 ℃/min)，实验发现加热速率对地下热解过程中富油煤纳米孔隙平均孔径与分形维数影响较小；较慢加热速率下富油煤的最终整体散射强度更高，纳米孔隙结构发育更充分，更有利于地下热解反应的进行。此外，与BET等常规表征方法相比，CSNS-SANS技术可检测到样品中的闭孔，且实验条件更符合原位环境，实验结果更具可靠性。

Underground pyrolysis of tar-rich coal is a green and low-carbon technology that effectively extracts oil and gas resources from coal. It represents the forefront of clean and low-carbon utilization of coal. The nano-pores present in coal play a vital role in the adsorption and desorption of tar and gas during the underground pyrolysis process. Therefore, understanding the evolution of nano-pore structure under underground pyrolysis conditions is a key scientific issue for enhancing the oil and gas yield of tar-rich coal through this process. To address this, the China Spallation Neutron Source (CSNS) - Small-Angle Neutron Scattering (SANS) technique was employed to simulate the in-situ pyrolysis environment of coal seams. This allows for the quantitative characterization of the scattering intensity, average pore size, and fractal dimension evolution of nano-pores in tar-rich coal in different heating rates and temperatures under unconfined pressure conditions. The complementary physical characterization methods such as physical adsorption (BET), thermogravimetric analysis (TG), and scanning electron microscopy (SEM) were also utilized. The research findings reveal the evolution of nano-pore structure characteristics during the underground pyrolysis of tar-rich coal. The research results demonstrate that during the underground pyrolysis process, the average pore size of nano-pores gradually increases with the rise in temperature. Nanopore development is relatively slow during the pyrolysis drying and degassing stage ( < 300 ℃), while the most significant growth occurs during the active stage (300-500 ℃), resulting in a 57.1% increase. Subsequently, nanopore development slows down during the secondary degassing stage. Importantly, it was observed that the low-medium-high temperature (≤800 ℃) pyrolysis conditions does not cause any significant changes in the surface fractal dimension (Ds) of tar-rich coal. Based on SANS spectroscopy analysis, it was discovered that the distribution of nano-pores in tar-rich coal during the pyrolysis process exhibits isotropic characteristics. This suggests that the pyrolysis reaction does not impact the directional development of nanopores in this experimental samples. Furthermore, a comparison between different heating rates (5 ℃/min and 20 ℃/min) reveals minimal influence on the average pore size and fractal dimension of nano-pores during underground pyrolysis of tar-rich coal. Notably, under slower heating rates, tar-rich coal exhibits higher overall scattering intensity, indicating more substantial development of the nano-pore structure. This, in turn, facilitates the progress of the underground pyrolysis reaction. In addition to these findings, the CSNS-SANS technique proves advantageous over the conventional characterization methods such as BET. It enables the detection of closed pores within the samples and provides experimental conditions that closely resemble the in-situ environment, thereby ensuring more reliable results.