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1 Introduction 1.1 Overview of Coal Petrology Coal petrology is a discipline that studies coal as an organic rock, examining its properties, changes, and applications. It considers coal to be an organic rock with complex and variable properties, composed of various components with different characteristics mixed together in different structures, rather than a pure substance; it introduces the concepts of active components and inert components, and classifies and quantitatively analyzes the microscopic components into vitrinite, semivitrinite, inertinite, chitin, as well as minerals. Certain physical and chemical properties of coal itself, as well as the coalification process it undergoes – such as density, elemental composition, the processes involved in coal formation, and geological age – are closely related to two indicators: the microscopic composition of coal rocks and the vitrinite reflectance. Applied coal petrology involves taking into account the fact that coal is not a pure substance, and using various conventional research methods to study the impact of various components in coal as well as their interactions on the properties of coal; it also examines the effect of coals with different degrees of metamorphism and their interactions on the properties of blended coals. 1.2 Coking coal blending technology The shift from coking with a single type of coal to coking using a mixture of various coals represents a significant advancement in the coking industry; modern coke ovens almost always use mixtures of different coals for coking. Coal blending technology, as a field of scientific research, continues to develop, but over the past few decades it has largely remained at a qualitative, empirical level. As metallurgical technology imposes increasingly stringent requirements on coke quality, empirical coal blending can no longer meet the needs of coke production, as it is unable to fundamentally explain the abnormal phenomena that occur during coal blending for coke production, nor can it achieve a transition from qualitative to quantitative approaches. In this regard, coal petrology, as one of the fundamental theories of modern coking, although its development is not yet complete or mature, is more scientific and advanced than the current classification of raw coal due to its renewed understanding of coal and the feasibility of its theories. With the advancement and refinement of coal-rock theory, as well as the development of coal blending technology, it is now universally recognized that scientific coal blending is inseparable from coal petrology. At present, among the coal blending technologies developed around the world, those that have undergone thorough validation and demonstrate good performance all rely on coal petrology as their foundation. In the 1980s, coal-rock blending technology in China began to develop rapidly. Predicting coke quality using coal petrological approaches and methods, and using this to guide coal blending, is a significant achievement resulting from over 50 years of development in coal petrology, as well as an important scientific outcome for the coking industry. Currently, coal petrology is widely applied in the research and production of coal. In the coking industry, coal petrology is being widely accepted as a useful theory and is gradually applied in production practices. 2 Basic Principles of Coal Rock Blending Based on coal petrology theory and a thorough understanding of coal, several recognized basic principles have been established for the blending of coal rock materials. 2.1 Coal is a heterogeneous material. Each type of coal is a natural mixture of various components, which is why the vast majority of coals do not meet the requirements for coking on their own. To this end, coal and rock researchers use microscopic observation of the dynamic changes that occur in coal during heating, classifying those components that can melt and form active bonds during heating as cohesive active components; those that cannot melt or form active bonds are classified as non-cohesive inert components. The vitrinite and chitin groups are active components, the liptinite group is an inert component, and the semivitrinite group is an amphoteric component. 2.2 Heterogeneity in the mass of various active components in coal The reflectance distribution diagram of the vitrinite fraction demonstrates this. For any single type of coal, since the coalification processes experienced by its various active components are concentrated and relatively close to each other, and their degree of metamorphism is similar as well, a normal distribution is observed in the reflectance distribution graph of the vitrinite group. 2.3 Both inert components and active components are essential in coal blending. A lack or excess of either can be detrimental to coking, leading to a decline in coke quality. Coal blending, which has certain requirements regarding coke quality, is actually a combination of different active components along with an appropriate amount of inert components. Coal petrology determines the properties of a coal, primarily based on the vitrinite reflectance and vitrinite content, and is unrelated to the current coal classification systems. 2.4 Coking process This process is not one in which coal particles melt together to form homogeneous coke; rather, it involves the connection of coal particles through interfacial reactions and bonding, with both chemical reactions and physical bonds taking place. Crystal phase analysis of coke shows that the inert components become integrated with the molten active components through chemical reactions and physical bonding, and further polymerize into coke lumps as a result of the pyrolysis and polycondensation reactions of organic substances occurring parallelly inside and outside the coal particles. Based on the above viewpoints, the coal-rock blending technology is theoretically fairly mature, but to date, no specific method that is widely applicable and highly effective has been developed. 3 Several representative coal-rock blending methods Through decades of development, this theory has been continuously refined and improved. Various research institutions and manufacturers have developed several representative coal-rock blending systems or mathematical models, some of which have yielded fairly satisfactory results. 3.1 Amosov and Spiro methods Proposed in 1957 by I.N. Amosov and others from the former Soviet Union, and improved by Spiro and others in the United States. In the mid-1960s, Hideo Kimura and others in Japan made further improvements, which were applied at Nippon Steel in 1974. Based on the coal rock composition and vitrinite reflectance, this method divides the microscopic components of coal rocks into two categories: Active components = vitrinite + chitin + 1/3 semivitrinite; Inactive components = fibrite + 2/3 semivitrinite + mineral components. Two indices are proposed: the strength index SI and the compositional balance index CBI. SI=∑ai*/∑x CBI=(100-∑*)/∑*/∑bi. Where: a is the strength index; b is the optimal combination value for vitrinite and inert components with a reflectance of i‰; i represents the coal rank, ranging from 3 to 21; x is the percentage content. 3.2 United States Bethlehem Steel Corporation Act: Proposed by Thompson in the United States; the principle is the same as before, but the approach differs. Implemented at Bethlehem Steel in the United States. For vitrinite with reflectances ranging from 0.8% to 1.8%, equal reflectance curves were drawn based on the content of inert components and ASTM coke stability, while semivitrinite was treated as if it contained inert components. 3.3 Takuhisa Miyazu of Japan: By introducing Giezel’s maximum fluidity MF and using the vitrinite reflectance R0max as an indicator of metamorphism, an MOF diagram was developed to guide coal blending. It is believed that MF