The paper presents slagging and fouling properties of ashes when hard coal was fired in a pulverized coal-fired boiler (230 t/h capacity) and the change of fouling indices of resulting fly ash when halloysite fireside additive was used. During reported full-scale study the pulverized coal-fired boiler was fired with hard coal with the addition of pulverized halloysite injected directly into furnace through the designed spray-nozzles in the upper part of boiler furnace. Chemical analysis of fuel properties and collected from the ESP (electrostatic precipitator) fly ash samples was carried out to obtain the following information: determine chemical properties of coal and its ash produced during combustion in order to determine its propensity for slagging versus reference study conducted without the use of additive identify the slagging and fouling indicators when halloysite additive was used. The tests results are intended to determine the effectiveness of the pulverized halloysite injection to the furnace in terms of reducing slagging and fouling of heating surfaces, in particular the evaporator and convective section of the boiler. The required halloysite feeding is determined by comparing the change in chemical composition (ash modification effectiveness) between fly ashes produced in the reference and test case so as it is corrected according to the fuel slagging properties (fuel slagging index). (C) 2015 Elsevier Ltd. All rights reserved.