This work presents detailed measurements of fouling deposits of a 100 kW (nominal thermal input) pilot-scale combustor operated over different tests with different combustion settings. After the tests, the heat exchangers of the convective section were removed for deposit examination. The results offer information regarding the fouling behavior during periods that lasted more than 20 h, starting from clean tubes. These long periods yielded valuable information about the bulk deposit buildup and final shapes as well as the effects of the oscillating flow patterns that occur in the staggered-plate tube banks. Special consideration was given to the deposit thickness profile around the tubes, final deposit shapes, and deposit mechanisms. Two different sub-bituminous coal test campaigns have been considered in this study. Typical bulky deposits were observed for a Wyoming coal (first test), where the deposits were about 4-6 mm in the tube winds and about 1 mm in the lees. These results were significantly different for an Indonesian coal (second test), where, in most of heat exchange surfaces, agglomerations caused by direct vapor condensation on the surface were observed, especially at gas temperatures below 470 degrees C. Combinations of very different deposition mechanisms (inertial impaction, vapor/aerosol condensation, and thermophoresis) were observed in the fouled material.