The purpose of this study was to evaluate the effect of fine ash particles and alkali metals on the decreasing heat flux at the initial stages of ash deposition which has a significant effect on heat transfer characteristics during the whole ash deposition process. For this purpose, a series of slagging tests were conducted in a 1.5 MWth pilot plant. The heat transfer characteristics of four bituminous and three sub-bituminous coals were studied using a slagging probe set in the high temperature zone simulating the water wall in actual boilers. The heat flux through a slagging probe was calculated and the decreasing rate of heat flux per unit weight fraction of ash (dq/dt-ash) was compared for each coal. Deposits on the slagging probe were sampled and analyzed after each test run. Analysis results showed that particle fractions with a diameter less than 10 mu m (R-10under) of bituminous coals varied in wide range: from 5 to 45 wt.%, while those of sub-bituminous coal did not change greatly: from 16 to 30 wt.%. In terms of alkali components, the inner layer of deposits of bituminous coals tends to enrich Al, Fe and Ca. In the case of sub-bituminous coals, refractory compounds of Si, Al and Ti, alkali compounds of Na, K, Fe and P. To evaluate the influence of factors on the decreasing rate of heat flux at the initial stage of ash deposition quantitatively, a multiple regression analysis of heat flux data (dq/dt-ash) was performed using the following parameters: R-10under, and also Na2O and K2O values in each parent coal. Calculation using a regression equation agreed well with experimental data. (C) 2012 Elsevier Ltd. All rights reserved.