Validation attempts of proposed particle-level growth mechanism of ash agglomerates in fluidized bed combustors

Aditi B. Khadilkar, Peter L. Rozelle, Sarma V. Pisupati

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Increase in particle size occurs during several processes that use fluidized beds. While it is desirable in processes such as granulation and pellet-making its occurrence can cause undesirable operational issues in combustion and gasification processes. Models that incorporate interdependencies and variations in both bed-hydrodynamics and binder chemistry are required for accurate predictions of agglomerate growth kinetics. Penn State has developed a unique twoparticle collision-based model that combines these two aspects. The model is tested with polydisperse fluidized bed combustion and gasification systems. In fluidized bed gasification and combustion systems, agglomeration occurs by sticking of fuel ash particles that are wetted by slag-liquid. The non-uniform temperature, gaseous atmosphere and heterogeneity in ash chemical composition need to be accounted for in the prediction of the slag-liquid binder formation and chemistry. Additionally, the poly-dispersity of bed ash, particle velocities and collision frequencies are the important physics-based factors that affect particle growth rate. In the Penn State model, thermodynamic equilibrium calculations are used to estimate the amount of slag-liquid in the system, and the changes in particle collision frequencies are accounted for by continuously tracking the number density of the various particle sizes. The particle number density is also affected by the ash chemistry as solid particles melt to form slag. Computational fluid dynamics modeling is used in conjunction to obtain the initial granular physics inputs to the model. Thus, the heterogeneities in ash chemistry and granular physics are integrated in this model. The model development was presented previously and the present study focuses on validation of the modeling results and the methodology developed. For this purpose, ash agglomerates were generated in a laboratory-scale fluidized bed combustor at Penn State. Ash agglomerates were produced by operating the reactor at a superficial gas velocity close to the minimum fluidization velocity of the particles, using rejects from a bituminous coal with about 82 % ash content, under oxidizing conditions. Agglomerate samples were also obtained from a fluidized bed combustor at Canada. Polished cross-sections of these agglomerates were studied using scanning electron microscopy with energy disperse x-ray scattering (SEM-EDX) to relate the particle-level slag formation and sticking to the chemical composition. FactSage simulations of the slag-forming components were used to estimate the agglomeration temperatures. The results of this study indicated that agglomerate growth in fluidized bed combustors (FBC) is initiated at the particle-level by low-melting components rich in iron- and calcium-based minerals. Although the bulk ash chemical composition does not indicate potential for agglomeration, study of particle-level heterogeneities revealed that agglomeration can begin at lower temperatures than the FBC operating temperatures of 850 °C. After initiation at the particle-level, more slag is observed to form from alumino-silicate components at slightly higher temperatures caused from changes in the system, and agglomerate growth propagates in the bed. A post-mortem study of ash agglomerates using SEM-EDX helped to identify stages of agglomerate growth.

Original languageEnglish (US)
Title of host publicationInternational Congress on Energy 2016, ICE 2016 - Topical Conference at the 2016 AIChE Annual Meeting
PublisherAIChE
Pages560-570
Number of pages11
ISBN (Electronic)9781510834200
Publication statusPublished - Jan 1 2016
EventInternational Congress on Energy 2016, ICE 2016 - Topical Conference at the 2016 AIChE Annual Meeting - San Francisco, United States
Duration: Nov 13 2016Nov 18 2016

Publication series

NameInternational Congress on Energy 2016, ICE 2016 - Topical Conference at the 2016 AIChE Annual Meeting
Volume1

Other

OtherInternational Congress on Energy 2016, ICE 2016 - Topical Conference at the 2016 AIChE Annual Meeting
CountryUnited States
CitySan Francisco
Period11/13/1611/18/16

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All Science Journal Classification (ASJC) codes

  • Renewable Energy, Sustainability and the Environment
  • Energy Engineering and Power Technology

Cite this

Khadilkar, A. B., Rozelle, P. L., & Pisupati, S. V. (2016). Validation attempts of proposed particle-level growth mechanism of ash agglomerates in fluidized bed combustors. In International Congress on Energy 2016, ICE 2016 - Topical Conference at the 2016 AIChE Annual Meeting (pp. 560-570). (International Congress on Energy 2016, ICE 2016 - Topical Conference at the 2016 AIChE Annual Meeting; Vol. 1). AIChE.