Simulation study on in-situ pyrolysis behavior of tar-rich coal under overburden stress
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摘要:
富油煤原位热解油气转化为提高国内油气自主保障和推动煤炭绿色低碳产业提供了新思路，原位地层条件约束下富油煤热解行为与常规地面热解不同，造成油气产出规律差异显著，但相关研究甚少。通过模拟不同覆岩应力下富油煤原位热解过程，利用低温氮气吸附实验、X-射线衍射和高分辨率透射电子显微镜等手段研究地层应力对煤体热解宏观膨胀变形、微观孔隙结构演化和半焦分子结构的影响，探讨不同应力荷载对富油煤热解的作用机制。结果表明：随着应力荷载的增强，富油煤原位热解特性呈现出2个明显的阶段性特征。低应力荷载阶段(0～10 MPa)，由于煤样缺乏有效的径向围限压力，导致轴向应力的增强不断压裂煤体，从而增强了孔隙连通性和热解流体释放能力，一方面有利于在对流过程中形成大孔，即 > 50 μm热解大孔数量显著增多；另一方面降低了热解流体二次反应几率，进而提高焦油产率、促进煤分子结构有序化生长，表现为面网间距逐渐减小、堆砌度和延展度逐渐增大，镜质组随机反射率不断提高。然而，在高应力荷载阶段(> 10 MPa)，煤体被不断压实、裂隙闭合，内部热解流体运移受到显著抑制，一方面在较弱对流下更易形成2～25 μm相对较小孔隙；另一方面强化了热解流体的二次反应程度，从而使焦油产率降低、产气量和半焦产率提高，X-射线衍射和高分辨率透射电镜数据显示持续高压滞流造成煤基质膨胀变形不利于热解半焦芳香结构秩理化生长。
Abstract:The oil and gas conversion of tar-rich coal in-situ pyrolysis provides a new idea to improve the domestic oil and gas independent guarantee and push the green and low-carbon industry of coal. The behavior of tar-rich coal pyrolysis under crustal stress is distinct from conventional ground pyrolysis, leading to significant differences in oil and gas output law, but there are few relevant studies. Tar-rich coal pyrolysis simulation experiments under different overburden stresses were conducted, and the influence of the crustal stress on pyrolysis deformation, pore structure evolution, and molecular structure disparities was analyzed using low-temperature N2 adsorption, X-ray diffraction, and high-resolution transmission electron microscopy. The effect mechanism of the different stress loadings on the pyrolysis of tar-rich coals was explored. The results showed the tar-rich coal in-situ pyrolysis properties exhibited two stages as the stress loading increased. During the low-stress loading stage (0～10 MPa), the lack of effective radial confining pressure on the coal samples resulted in the enhancement of axial stress constantly fracturing the coal, improving pore connectivity and pyrolysis fluid release ability. On the one hand, it was conducive to the formation of large pores during convection, and the number of > 50 μm pyrolysis macropores increased significantly; on the other hand, it was conducive to the reduction of the chance of the secondary reaction of pyrolysis fluid, which led to the improvement of tar yield and the growth of the coal molecular structure, which was reflected in the gradual decrease in interlayer spacing, the gradual increase in stacking height and lateral size, and the increase in random vitrinite reflectance from the optical properties. However, the compaction of coal and the closure of fissures at the high-stress loading stage ( > 10 MPa), inhibit internal pyrolysis volatile migration; on the one hand, it is easier to form a relatively small pore size of 2-25 μm under the weak convection; on the other hand, it strengthens the degree of secondary reaction of pyrolysis fluids, leading to a reduction in the tar yield and an increase in the gas and semi-coke yields. Furthermore, X-ray diffraction and high-resolution transmission electron microscopy data indicated that the swelling deformation of the coal matrix caused by the continuous high-pressure stagnant flow was not favorable to the orderly growth of pyrolysis semi-coke aromatic structure.
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Keywords:
- tar-rich coal /
- in-situ pyrolysis /
- stress loading /
- product distribution /
- oil and gas conversion
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图 1 富油煤原位热解模拟装置原理示意
Figure 1. Schematic diagram of the physical simulation equipment for tar-rich coal in-situ pyrolysis
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图 3 不同应力荷载下热解煤体位移变化
Figure 3. Displacement variation in coal during pyrolysis under different stress loadings
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图 4 不同应力荷载下煤热解膨胀变形特性
Figure 4. Pyrolysis swelling and deformation properties under different stress loadings
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图 5 显微组分的形貌特征及大孔( > 2 μm)孔径分布
Figure 5. Morphological characteristics of macerals and pore size distribution of macropore( > 2 μm)
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图 8 原煤与半焦样品的XRD衍射图谱
Figure 8. XRD diffraction patterns of raw coal and semi-coke samples
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图 9 应力荷载与XRD结构参数的关系
Figure 9. Relationship between stress loading and XRD structural parameters
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图 10 试样的芳香条纹尺寸分布
Figure 10. Frequency distribution of size of aromatic rings for samples
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图 11 试样的原始 HRTEM图像
注：以15°为区间的芳香条纹伪彩色图像和与之对应的方向玫瑰花图.a1～a3：原煤；b1～b3：0 MPa；c1～c3：10 MPa；d1～d3：30 MPa
Figure 11. Original HRTEM micrographs
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图 12 择优取向度与定向化程度的关系
Figure 12. Relationship between preferred orientation degree and ordering degree
表 1 原煤煤质特征数据
Table 1 The date of quality characteristics of raw coal
工业分析/% 元素分析/% 煤岩分析/% FCad Mad Ad Vdaf Std Cdaf Hdaf Odaf Ndaf V I L M 54.27 6.26 3.88 39.77 0.27 81.71 5.26 11.78 0.98 54.33 42.00 0.34 3.33 
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表 2 高分辨率透射电子显微镜中晶格条纹类型
Table 2 Classification of lattice fringes in a high resolution transmission electron microscope
芳香条纹尺寸 芳香条纹长度/nm 最小值 最大值 平均值 归类 萘 0.28 0.49 0.39 0.30～0.54 2×2 0.49 0.71 0.60 0.55～0.74 3×3 0.74 1.13 0.93 0.75～1.14 4×4 0.98 1.56 1.27 1.15～1.44 5×5 1.23 1.98 1.60 1.45～1.74 6×6 1.47 2.41 1.94 1.75～2.04 7×7 1.72 2.84 2.28 2.05～2.44 8×8 1.96 3.26 2.61 2.45～2.84
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表 3 热解半焦样品的煤质特征
Table 3 Characteristics of semi-coke samples derived from pyrolysis
应力载荷/MPa 元素分析 H/C Roran/% Std Cdaf Hdaf Ndaf 0 0.14 86.39 2.04 0.89 0.283 3.91 5 0.12 87.16 2.04 1.07 0.281 3.93 10 0.14 88.01 2.04 0.98 0.278 3.98 15 0.08 87.56 2.05 0.91 0.281 3.95 20 0.12 86.74 2.05 0.99 0.284 3.89 25 0.08 89.25 2.13 1.01 0.286 3.84 30 0.14 88.81 2.19 1.11 0.296 3.86
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