Mechanistic modeling, numerical simulation and validation of slag-layer growth in a coal-fired boiler S. Balakrishnan a , R. Nagarajan a, * , K. Karthick b a Particle Science Laboratory, Department of Chemical Engineering, Indian Institute of Technology e Madras, Chennai 600036, India b Development Engineer, Research & Development, Bharat Heavy Electricals Ltd., Tiruchirapalli 620014, India article info Article history: Received 19 August 2014 Received in revised form 27 November 2014 Accepted 21 December 2014 Available online xxx Keywords: Slagging deposits Retraction phenomenon Bouncing potential Computational fluid dynamics Thermophoretic transportation Shedding abstract In a tangentially coal-fired boiler, for locations inside and near the combustor, heat-transfer by radiation is significant, and hence, ash particles arrive in molten state. The aim of the present study is to adopt a mechanistic modeling approach which incorporates energy-conservation principles to address slag-layer growth. In order to determine the outcome of molten ash impaction, a mechanistic bouncing potential model, incorporating the phenomenon of recoiling of molten ash droplets after impaction, is employed. The bouncing potential is a representation of the excess energy possessed by the recoiling splat, and is used to determine the outcome of molten ash impaction e to stick or to bounce. Computational fluid dynamics techniques, incorporating the effect of thermophoresis, are adopted to estimate the arrival rate of ash particles, and the bouncing potential model, as a user-defined function, is incorporated in the simulation package to determine the status of the droplets after impaction. Two coals of Indian origin are simulated for slag-layer growth for a period of 100 min. The simulation results, when compared with field data provided by BHEL-Trichy, indicate that the model qualitatively predicts the growth of slag- layers. It has been further inferred that smaller particles dominate deposit formation and its growth. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Deposition of ash particles on heat-transfer surfaces in a coal- fired boiler e termed as fouling and slagging e is one of the main reasons for unscheduled boiler shut-downs. Fouling and slagging deposits offer more resistance to heat-transfer, interfere with the aerodynamic flow of flue gas, reduce the available flow area, and enhance corrosion and erosion of boiler tubes [1]. Hence, devel- oping an understanding of and predictive ability for fly ash depo- sition are essential to avoid unscheduled power outages and to schedule boiler maintenance. Deposition of molten ash particles e termed as slagging - results in fused deposits, and is identified in the radiant sections of the boiler, where locations are directly exposed to flame radiation and are the hottest parts of the boilers. Though many studies have been published [2,3], a recent study by Weber et al. [3] made two significant observations regarding a lack of reliable predictions for ash deposition. First, they proposed that ash deposition needs to be addressed in a more scientific manner than employing a variable particle sticking probability whose value is typically tuned to match model predictions with measured values. Second, they noted that not enough publications are avail- able in literature that are based on computational fluid dynamic modeling to address slagging in a boiler. Such suggestions, and an urgent need for effective utilization of high-ash Indian coals, have motivated this study of the slagging propensity of Indian coals. In order to calculate sticking probability, conventionally, vis- cosity of arriving ash particles has been used as a relevant param- eter [4]. Typically, the actual viscosity is compared with a critical viscosity to arrive at a conclusion regarding fraction of ash particles that sticks [5]. The sticking probability in terms of viscosities is expressed as: h sticking ¼ m critical m actual m actual > m critical ¼ 1 m actual < m critical (1) The above expression implies that for viscosities lower than the critical viscosity, the sticking probability is 1; for viscosities greater than critical viscosity, the sticking probability is the ratio of critical to actual viscosities. * Corresponding author. Tel.: þ91 44 22574158, þ91 44 22578070; fax: þ91 44 22574152. E-mail address: nag@iitm.ac.in (R. Nagarajan). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy http://dx.doi.org/10.1016/j.energy.2014.12.058 0360-5442/© 2014 Elsevier Ltd. All rights reserved. Energy xxx (2015) 1e9 Please cite this article in press as: Balakrishnan S, et al., Mechanistic modeling, numerical simulation and validation of slag-layer growth in a coal-fired boiler, Energy (2015), http://dx.doi.org/10.1016/j.energy.2014.12.058