Measurement and modeling the coefficient of restitution of char particles under simulated entrained flow gasifier conditions

Inefficiencies in plant operations due to carbon loss in flyash, necessitate control of ash deposition and the handling of the slag disposal. Excessive char/ash deposition in convective coolers causes reduction in the heat transfer, both in the radiative (slagging) section and in the low-temperature convective (fouling) heating section. This can lead to unplanned shutdowns and result in an increased cost of electricity generation. CFD models for entrained flow gasification have used the average bulk coal composition to simulate slagging and ash deposition with a narrow particle size distribution (PSD). However, the variations in mineral (inorganic) and macerals (organic) components in coal have led to particles with a variation in their inorganic and organic composition after grinding as governed by their Particle Size Distribution (PSD) and mineral liberation kinetics. As a result, each particle in a PSD of coal exhibits differences in its conversion, particle trajectory within the gasifier, fragmentation, swelling, and slagging probability depending on the gasifier conditions (such as the temperature, coal to oxygen ratio, and swirling capacity of the coal injector).;Given the heterogeneous behavior of char particles within a gasifier, the main objective of this work was to determine boundary conditions of char particle adhering and/or rebounding from the refractory wall or a layer of previously adhered particles. In the past, viscosity models based on the influence of ash composition have been used as the method to characterize sticking. It is well documented that carbon contributes to the non-wettability of particles. Therefore, it has been hypothesized that viscosity models would not be adequate to accurately predict the adhesion behavior of char. Certain particle wall impact models have incorporated surface tension which can account the contributions of the carbon content to the adhesive properties of a char particle. These particle wall impact models also predict the coefficient of restitution (COR) which is the ratio of the rebound velocity to the impacting velocity (which is a necessary boundary condition for Discrete Phase Models). However, particle-wall impact models do not use actual geometries of char particles and motion of char particles due to gasifier operating conditions. This work attempts to include the surface geometry and rotation of the particles.;To meet the objectives of this work, the general methodology used for this work involved (1) determining the likelihood of particle becoming entrapped, (2) assessing the limitations of particle-wall impact models for the COR through cold flow experiments in order to adapt them to the non-ideal conditions (surface and particle geometry) within a gasifier, (3) determining how to account for the influence of the carbon and the ash composition in the determination of the sticking probability of size fractions and specific gravities within a PSD and within the scope of particle wall impact models, and (4) using a methodology that quantifies the sticking probability (albeit a criterion or parameter) to predict the partitioning of a PSD into slag and flyash based on the proximate analysis.;In this study, through sensitivity analysis the scenario for particle becoming entrapped within a slag layer was ruled out. Cold flow educator experiments were performed to measure the COR. Results showed a variation in the coefficient of restitution as a function of rebound angle due rotation of particles from the educator prior to impact. The particles were then simply dropped in "drop" experiments (without educator) to determine the influence of sphericity on particle rotation and therefore, the coefficient of restitution. The results showed that in addition to surface irregularities, the particle shape and orientation of the particle prior to impacting the target surface contributed to this variation of the coefficient of restitution as a function of rebounding angle. Oblique particle impact measurements and images suggested the possibility of particles simultaneously rolling and sliding due to non-sphericity.;This work has identified the importance of characterizing particle orientation due to rotational motion in all three Cartesian coordinates prior to impact in addition to characterizing simultaneous sliding and rotation in oblique impact for non-spherical particles. A sticking probability based on the critical velocity was developed to provide consistency between CFD models and an industrial friendly model to predict partitioning of slag and flyash. Based on the results of this model developed in this work, flyash was shown to be reduced by reducing the average particle size. In summary, the connection between the physics of char particles impacting the wall of a gasifier and their ash as well as carbon composition has been comprehensively investigated in this study. (Abstract shortened by UMI.).