煤层自燃升温及燃烧过程的声波时空演化规律及产生机制研究

Spontaneous combustion of coal seams is a serious hazard in China. Due to the complexity in the formation, development and influencing factors of coal spontaneous combustion (CSC), the detection of hidden fire zone (source) of CSC remains a worldwide problem. The acoustic signal produced during CSC is a way for coal rock to accumulate and release energy when it undergoes thermal damage at a high temperature. To facilitate the sound wave detection and the location of hidden high-temperature fire zone (source) of CSC, a sound-temperature-gas test platform for large-scale CSC is established in the study first. Next, after the analysis and elimination of environmental interference sound waves, the response law and time-series characteristics of ultrasonic, acoustic and infrasound waves in the whole process of coal spontaneous heating and combustion are studied and explored. Moreover, the spectra of sound wave dominant characteristics in the whole CSC process are determined by means of Fourier transform and wavelet analysis. Meanwhile, the effective acoustic signal is extracted, and the sound wave generated by coal rock heating is distinguished from that generated by coal rock fracturing under load. Besides, the mechanisms of thermal deformation and fracturing and acoustic wave generation by piezoelectric and electromagnetic effects in the CSC process are revealed in detail through mesoscopic and microscopic tests, based on which a self-combustion thermal-force-acoustic constitutive model of CSC is established. Finally, the spatial characteristics of acoustic wave field, the characteristics of acoustic wave propagation and attenuation, and the wave source locating methods in the CSC process are studied according to the acoustic characteristics exhibited during the development of fire zone, which provides a basis for locating the hidden fire zone (source) of CSC in the field.

我国煤层自燃火灾危害严重。由于煤层自燃形成、发展以及影响因素的复杂性，煤层自燃隐蔽火区（源）探测仍是一个世界性难题。煤层自燃过程中的声波信号是煤岩升温至高温燃烧损伤集聚释放能量的一种方式，为解决声波探测及定位煤层自燃隐蔽高温火区（源）的问题，本项目首先拟建立大尺度煤层自燃声-温-气测试平台，对环境干扰声波进行分析及排除后，研究并揭示煤层自燃升温及燃烧全过程的超声波、声波及次声波的响应规律和时序多变特征；其次，采用傅立叶变换和小波分析，确定煤层自燃全过程的声波优势特征频谱，提取有效声波信号并区分煤岩升温和受载破裂产生声波的差异；再次，通过细-微观测试，精细化揭示煤层自燃热致变形破裂及压电和电磁效应产生声波机制，建立煤层自燃热-力-声本构模型；最后，基于煤层自燃火区发展过程的声波特征，研究煤层自燃过程的声波波场空间特征、传播衰减特性及寻源定位方法，为现场声波定位煤层自燃隐蔽火区（源）提供依据。

煤自燃严重威胁着煤矿的生产安全，高效快捷的探测预警煤自燃危险具有重要的意义，为研究应用声波法进行煤层自燃隐蔽高温火区（源）的探测预警，本项目围绕“煤层自燃升温及燃烧过程的声波时空演化规律及产生机制”为主线进行研究。首先，研究得到煤层自燃升温及燃烧过程声波时序特征；通过建立煤层自燃声-温-气实验系统，测试了煤层自燃升温及燃烧过程的次声波和声发射信号、气体和温度的变化；煤升温过程中次声波信号随着缓慢-加速-剧烈氧化升温呈现平稳、发展、快速增大的阶段变化特征，CO、C2H4的释放速率与次声波信号存在良好的对应性，通过皮尔逊系数计算得到声波信号与温度的相关系数均在0.7以上。其次，确定煤层自燃过程的声波优势特征频谱，不同频率的次声波随着温度的升高呈波动变化，这表明煤在升温过程中变形破裂的频次呈增大的变化，通过分形方法研究得到3种损伤破坏条件下煤所产生的声发射信号强度依次为：温度-压力耦合条件>单轴加载条件>单纯受热升温条件，通过提取有效声波信号并区分煤岩升温和受载破裂产生声波的差异。再次，通过跨尺度岩心扫描系统、原子力显微镜和扫描电镜实验，分析了煤升温过程中煤表面结构及其内部成分变化规律，得到了煤自燃热膨胀变形破裂过程中裂隙和裂纹的发展演化与产生声波的定性定量关系，并进一步从煤自燃过程中煤结构的物理与化学变化两个角度探究了煤升温时的声波产生机制。最后，理论分析煤层自燃过程的声波场空间特征、传播衰减特性及寻源定位方法，分析了温度对煤岩体声波传播速度的影响，得到声波信号沿R方向呈负指数变化规律，通过建立声波CT反演方法实现了煤自燃高温区域进行反演。本课题全面系统阐明了煤自燃过程中声波信号的时空演化规律及产生机制，为实现煤自燃的高效监测预警提供了理论支撑。