The co-firing technology of coal with biomass has been implemented to enhance the usage of biomass in power generation, thus reducing the release of greenhouse gas emissions. This study deals with the fireside issues, namely ash-related issues that arise during co-combustion of coal and biomass takes place. Ash release from biomass can lead to ash deposition problems such as fouling and slagging on surfaces of power generation boilers. The scope of this work includes the development of a conceptual model that predicts the ash release behaviour and chemical composition of inorganics in coal and biomass when combusted. An advanced analytical method was developed and introduced in this work to determine the speciation of biomass. The method known as pH extraction analysis was used to determine the inorganic speciation in three biomass samples, namely wood chips, wood bark and straw. The speciation of biomass and coal was used as an input to the model to predict the behaviour and release of ash.