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

Open Access
Author:
Gibson, La
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 02, 2013
Committee Members:
  • Sarma V Pisupati, Dissertation Advisor
  • Sarma V Pisupati, Committee Chair
  • Dr Yaw Yeboah, Committee Member
  • Robert John Santoro, Committee Member
  • Derek Elsworth, Committee Member
  • Dr Lawrence Shadle, Special Member
Keywords:
  • Coefficient of Restitution
  • Gasification
  • Critical Velocity
  • Population Model
  • Ash Deposition
  • Sticking Probability
Abstract:
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. Calculations also showed that the COR due to viscoelasticity is most sensitive. Therefore, the critical velocity was derived from a viscoelastic particle wall impact model based upon the yield strength and a variable termed the plastic loss factor. However, by setting the plastic loss factor equal to the COR, trivial solutions were obtained in the derivation of critical velocities where the COR had to equal zero in order for the particle to stick. Therefore, the damping ratio was set to a value of 1 to indicate critical damping while the COR was set to zero to independently solve for the plastic loss factor. By solving for the plastic loss factor, critical velocities were determined for particles in each specific gravity and size fraction used in this study. An alternative “rules based method” based upon the contact angle and the temperature of critical velocity was also used to determine a sticking probability. With the exception of some of the larger size fractions, there was a better agreement between the sticking probabilities based on the critical velocities and the sticking probabilities calculated using the “rules-based-criteria” than the “rules-based-criteria” and the conventional model (in which only the temperature of critical velocity was used). Capture efficiencies of these particles were calculated using sticking probabilities and impact efficiencies. The range of values of the capture efficiencies determined through the rules-based-criteria were similar to the range of values reported in previous experimental work concerning ash and char deposition. Conventional viscosity models only predicted a significant variation in the adhesion between particles of different specific gravities not particle sizes. By using the “rules-based-criteria”, the influence of the particle size fractions was also discerned in addition to that of the specific gravities within the PSD. With the influence of unburnt carbon accounted for, the particles from “lighter” specific gravity fractions (SG1 and SG2) among the largest size fractions contributed the most to the flyash whereas, the “heavier” specific gravity fractions (with the exception of SG4, SF1) contributed the most to the slag. Therefore, by reducing the largest size fractions and increasing the smallest size fractions, syngas increased incrementally, flyash decreased incrementally, and slag increased marginally. 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.