Organic Petrology applied to materials and coal by-products characterization

Optical microscopy is a very unique tool to find out about the structural order of carbonaceous materials and coal by-products. It has two main utilization ways as an instrument to distinguish and quantify different phases in a polished surface and as a record of the reflectance of a given material. Using one or both capabilities there are significant contributions to different aspects of coal utilization and materials science, which are the main objective of this review. The most traditional applications deal with the characterization of fuels for any utilization process (maceral and vitrinite reflectance analysis), in which coal petrographic techniques are the only ones able to identify and characterize the individual components of a blend. The next step is the contribution to the study of transformed coal products. In this way coke petrology has developed a complete optical texture classification system in which coke texture can be related to coke properties such as reactivity or mechanical strength. The contribution to coal combustion through the characterization of chars and identification of the parent material in fly ashes is also relevant and no less important is the following of hydrogenation processes. Half way between material and coal science are the subjects related to coal tar and petroleum pitches based cokes for different applications. In this case organic petrology can be used not only to quantify the amount of different phases but also to follow through the reflectance the structural transformation. In any process in which pitches act as binder, optical microscopy is a useful tool to quantify the components and to study the interfaces. Due to the versatility of C–C bonds, the structure of carbon materials can vary from the disordered isotropic optical texture to the well-ordered graphitic-like texture. These transformations can be followed by the measurement of anisotropy. While it is true that optical microscopy has much less resolution than other microscopy techniques such as HRTEM it has the possibility to account for sample heterogeneity which is very difficult to attain with other high resolution instruments. Some other contributions of organic petrology to materials science in relation to mechanical properties are also reviewed