In order to examine the cause of occasional slagging problems, a dedicated 7-day sampling and data collection were carried out on two 300-MWe units of Mae Moh power plant owned by the Electricity Generating Authority of Thailand. The properties of feed lignite varied from low (similar to 11% in ash) to medium (similar to 18% in ash) CaO content during the sampling period. The elemental content was measured for collected fly ashes, bottom ashes and slags, and compared with those of the feed lignite. The analysis and operational data obtained during the 7-day sampling period were compared with those obtained from the additional laboratory-scale investigation of high Ca lignites and those obtained from one of the units during another period when slagging problem was experienced. The study found that, apart from Ca, the concentration of three other elements and their compounds -Fe, Al and Si, plays a very important role in slag formation and its control while these lignites are used in large utility units. The flue exit gas temperatures (FEGT) of the unit experiencing slagging were also found to approach the initial deformation temperatures (IT) of the lignite ash. The solution appears to be the monitoring of the concentration of four elements, i.e. oxides of Ca, Fe, Al and Si in the feed lignite, and in ensuring through proper mining or blending that concentration of CaO, Fe(2)O(3) and Al(2)O(3) is kept < 20%, similar to 15% and > 15%, respectively, at all times. Also, it is important that the FEGT during operation is kept sufficiently below the IT of the ash of feed lignite at all times. (c) 2008 Elsevier Ltd. All rights reserved.