The combustion of herbaceous biomass can lead to deposit formation on the walls of the combustion chamber and on boiler tubes. These deposits are being caused by fly ash. The aim of the present investigation was to identify the mechanisms of deposit formation, to analyse the chemical and morphological properties of the deposit, and to propose measures for deposit reduction. Special attention was given to the deposit formation from flue gas having been cleaned from large particles. It is typically found in boilers from moving bed furnaces. Experiments were performed in the flue gas of a turbulent flow reactor by sampling the depositions on a cooled cylinder after having removed large particles on an obstacle. Furthermore, depositions from the boilers of two moving bed furnaces were collected. The investigations show that depositions similar to those in moving bed furnaces were achieved in the turbulent flow reactor when large particles were removed before sampling. The deposits from herbaceous biomass were identified as accumulations of fine particles in the nanometer range. The chemical properties of the deposits are those of KCI, the major compound in the deposit. Among the different formation mechanisms thermophoresis was revealed to be the crucial process for deposit formation under the investigated conditions.