This study investigates the effect of combustion conditions on ash transformations and deposition in a utility boiler fired with a sub-bituminous coal. To mitigate ash deposition problems experienced in the boiler, adjustments were made to operating conditions by increasing the primary air to the boiler and rejection rate of heavy minerals from the mills. Ash deposits before and after the adjustments were collected from different locations in the boiler and analysed for their textural (SEM imaging), chemical (XRF), and mineralogical (XRD) characteristics. Ash deposition is found to follow a non-equilibrium process, and the deposits formed are highly non-uniform. Before the adjustments, the ash deposits were firmly bonded to heat transfer surfaces with high glass contents, suggesting that the deposits had experienced extensive melting. The presence of ferrous iron in the superheater and reheater deposits indicates that these deposits were formed under less oxidising conditions. After the adjustments, the deposits are friable and contain lesser amounts of glass. Ferric iron is observed in these samples, indicating that the deposits experienced a more oxidising environment. The observations of variable amounts of K, Ca, Fe, and P in the aluminosilicate glass phases in both sets of deposits indicate that K, Ca, Fe, and P-containing minerals are the main fluxing agents responsible for ash deposit growth. The differences in the distribution of the Si, Al, and Fe species in the samples suggest that ash partitioning has occurred in the boiler during pf combustion.