Effect of mineral transformation on the surface tension, viscosity, and size fraction of char particles

Controlling ash deposition and handling slag disposal in an entrained flow gasifier is a general concern. Excessive Ash/char deposition in the convective section is an issue since it can lead to unplanned shutdowns until the deposits are cleared. Excess amount of char captured in the slag can render the slag useless for the cement industry. Therefore, the behavior of coal must be analyzed from the stage of injection to its final form (fly ash, component of slag, or bottom ash). To characterize this behavior of coal within the entrained flow gasifier, the Discrete Phase Model will be used. The Discrete Phase Model is a computational model that uses Eulerian flow to represent the gas phase but employs the Lagrangian method to determine the trajectories of the solid phase particles. As a requirement of the Discrete Phase Model, the boundary conditions for the particle phase must be characterized through the coefficient of restitution. The coefficient of restitution (COR) is defined as the ratio of the rebounding velocity to the impacting velocity. However, in literature describing ash and particulate behavior, the impacting velocity is also known as the deposition velocity. For an entrained flow gasifier, boundary conditions for the COR would have to be established for the refractory wall of a gasifier as well as the developing slag. Through an extensive literature review of particle wall collision models, the surface tension was found to be a main component that determines whether a particle adheres or rebounds for a wall or substrate. The surface tension has often been used to determine the force of adhesion in particle-wall collision models. Once the surface tension, particle size fraction, and density of a particle are known, a force of energy balance is prescribed in determining the deposition velocity prior to impact. Surface tension models developed by Hanoa and Tanaka on the basis of the Butler's equation for binary solutions could prove useful-if the molar fractions of the mineral composition of the slag (and char particles) are known. The main challenge in employing this surface tension model and any one particle wall collision model (that includes the force of adhesion) within the entrained flow gasifier is the mineral transformation of the parent coal. Issues in quantitatively characterizing mineral transformations include describing surface reactions inclusive of mineral distribution at the surface, differentiating the behavior between extraneous and inherent minerals, and incorporating the effects of the reducing environment in the gasifier on the transformation of minerals. Determining the reactions at the surface would require an understanding of the surface area to volume ratio of the ash particles in interest. Meanwhile the inherent mineral matter undergoes transformation at the particle temperature. This requires the temperature profile for the inherent mineral matter that is separate from that of the extraneous mineral matter - for which the temperature is governed by the gaseous medium. As for the effects of a reducing environment, the existence of hydrogen and carbon monoxide in lieu of water and carbon dioxide can lead to more reactive components for the mineral matter. Aside from mineral reactions agglomeration, fragmentation, and coalescence also play a role. Because the magnitude of forces determining the deposition velocity is dependent on the particle size and gravity, these factors must also be addressed. This paper will discuss a method to determine an approximation for the surface tension, viscosity, and resulting size fraction over the expected residence time in an entrained flow gasifier. Agglomeration, fragmentation, and coalescence will also be discussed.