In this work, a CFD (Computational Fluid Dynamics) tool was developed to simulate the ash deposition on the heat transfer surfaces of furnaces. The effects of such slagging and fouling deposits on boiler operation and pollutants formation was investigated.Major particles physical transformations are reviewed in Chapter 2, while Chapter 3, encompasses an intensive literature review on the main mineral components in coals. Differentiating between mineral species is of great importance as minerals with low softening /melting temperature or eutectic points formed between different mineral inclusions facilitate deposition. For each individual mineral specie the known chemical and physical processes and effects with regard to deposition are presented in detail.The complexity and variability of the coal mineralogy points towards the use of chemical mechanisms that account for as many as possible chemical compounds and interactions. Reliable and accurate thermochemical data are therefore needed. For this purpose a mineral matter, coal and biomass and chemical activity databases were generated. Each mineral description is compiled into several subset databases. One subset refers to the polynomial format to calculate the mineral thermodynamic properties, e.g. enthalpy, entropy and specific heat capacity. The mineral database contains detailed descriptions for 200 individual mineral species, while the coal and biomass database contains 110 biomass and coal chemical analyses. The chemical reactions and their kinetic details sum up 70 entries, Chapter 4.Florean is a three-dimensional simulation program developed at the Institute for Fuel and Heat Technology. Besides conservation equations for mass, momentum and energy, FLOREAN has the capability to calculate chemical reactions and pollutants formation. The main conservation equations and program modelling capabilities are presented in Chapter 5. The EnSight Gold format and the ParaView post-processor are implemented for data analyses and visualisation.The basis of Lagrangian approach to two-phase flows is the simulation of particles trajectory by tracking the representative parcels of particles. The particles are initialised from a finite number of starting locations and each individual particle taken into consideration is tracked throughout the computational domain. The model is presented in details in Chapter 6. The novelty of this approach consists in dealing with the chemical aspects of mineral species in the particles. Each particle is considered a small chemical reactor, with the included mineral matter grains interacting with the surrounding gases. The chemical aspects of mineral transformation are modelled either by chemical equilibrium or kinetic models respectively, as presented in Chapter 4.The last chapter is dedicated to CFD code validation against practical findings (Chapter 7). For the validation two cases were considered. A relatively small test scale combustion chamber was fired with Hambach coal and ash deposition rate and analyses are compared with the CFD findings. The second case refers to a real size coal fired power plant. The CFD deposit predictions were compared with furnace wall thermal imaging instances. Good agreement exists between the experimental and modelled data in both cases. Finally a detailed parametric study is conducted to determine the effect of slagging and fouling deposit properties (thickness, porosity) on the boiler operation (heat flux to the walls and pollutants formation) and vice versa.