Advances in the understanding of the formation and chemistry of the plastic layer during coke-making: A comprehensive review

When they are heated in coke ovens, coking coals become softened, coalesce into theremoplastic matter and then resolidify into a solid porous coke after the thermoplastic range. These stages take place layer by layer in the coke oven due to the temperature gradients. A coal charge in an industrial coke oven undergoes a thermoplastic transformation which corresponds to the formation of a plastic layer. The plastic layer is a foam-like thin layer consisting of the molten vitrinite and liptinite, inert coal components (i.e. inertinite macerals) and the volatile matter that is entrapped inside the layer. This is considered an important stage for the evolution of pore structures of a coke, which are closely related to the coke quality. This paper provides an overview of the current research in the operature literature into the thermoplasticity of coal, with a particular focus on the physical and chemical structure transitions inside the plastic layer using a variety of experimental techniques and the relevant phenomena such as IGP. The paper also analysed various factors affecting the formation and nature of the plastic layer such as coal rank, maceral compositions and rheological properties and coking conditions. The review included the most recent studies on plastic layers by the authors on pore structures analysed by using Synchrotron Micro-CT and chemical structure changes by employing FTIR techniques. The knowledge gaps and future research needs are identified and suggested.