Pergamon Prog. Energy Combust. Sci. Vol. 22, pp. 189-200, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All fights reserved 0360-1285[96 $29,00 PII: S0360-1285(96)00004-4 ASH FOULING IN COAL-FIRED UTILITY BOILERS. MONITORING AND OPTIMIZATION OF ON-LOAD CLEANING A. Valero and C. Cortbs Centro de Investigacirn del Rendimiento de Centrales El~ctricas (CIRCE), Department of Mechanical Engineering, University of Zaragoza, Maria de Luna 3, 50015 Zaragoza, Spain Abstract--Even though considerable advances have been made in the fields of boiler design and coal characterization, ash deposition on heat transfer surfaces continues to be a significantproblem in existing conventional utility boilers. A cost effectiveway to deal with this difficulty is the continuous monitoring of fouling tendencies. These techniques have become a widespread practice in coal-fired power stations as a tool for operation optimization. In spite of that, tittle information has been given about design criteria and case-study experiences. In this paper, the fundamentals, concepts and application principles are reviewed, attempting a systematic analysis of current developments and open questions. The main operational measure related to ash fouling is on-load cleaning by mechanical means. Based on a fouling monitoring system, improvement of sootblowing procedures can lead to significant savings, although a rigorous approach is not a trivial issue. The paper reviews the state of the art, as well as several stages in cleaning optimization. Finally, an attempt is made to formulate possible perspectives. Copyright © 1996 Elsevier Science Ltd. Keywords: coal-fired boilers, fouling, slagging, on-line monitoring, efficiencyoptimization. CONTENTS 1. Introduction 189 2. Overview on Boiler Slagging and Fouling 191 3. Fouling Monitoring 192 3.1. Measurement of Heat Fluxes 192 3.2. Monitoring of Furnace Slagging 194 3.3. Boiler Heat Transfer Calculations 196 4. Cleaning Optimization 197 5. Conclusions 199 Acknowledgement:s 199 References 199 l. INTRODUCTION In recent years, significant advances have been made in the field of coal characterization. One of the goals has been the quantitative prediction of ash deposition phenomena in coal-fired boilers. The limited, empirical indices derived from standard ash analysis and physical tests 1'2 have given way to elaborate models based on computer-controlled scanning electron microscopy and other advanced analytical techniques. 3'4 Although these methods are complex and expensive 4 and their validation is still under progress, 5 present results are encouraging enough to expect outstanding advances in the near future. Nevertheless, from an industrial perspective, the problem is far from having been solved. Electric utilities around the world own about 900GW, of installed capacity for coal-fired generation,3 most of it consisting of pulverized coal units ranging from 50 to 1300 MW¢. The average date these units were built was in the seventies. This means that many plants must carry on with past design practices, which proved to be inadequate in many instances, particu- larly in the highly empirical subject of anticipating boiler fouling. A well-known example is the sizing of the combustion chamber. During the scaling-up in the 60s, the logical trend was to maintain the heat release rates per unit area with larger boiler capacity. In this manner, ash fusibility was not properly addressed, but this only became apparent after recurring experiences of furnace fouling due to high flame temperature. The trend changed during 1970-80, 6 the price being a quantity of undersized furnaces bound to operate for at least 20 years. Furthermore, even if boiler designs had been faultless, there is nowadays another factor to take into account. In order to meet environmental and economic restrictions, utilities should often consider modification of the fired coal, i.e. washing, blending or complete change of supply. Although design rules promptly evolved to incorporate differences in coal ash character, 6'15 it is impossible to conceive a plant to burn efficiently any kind of fuel. Switching coals has raised the question of predicting the impact of coal quality variations,7'8 and a salient issue is, again, 189