In　this　paper,　a　model　that　couples　a　reduced　alkali　kinetic　mechanism　for　alkali　sulfate　formation　during　biomass　combustion　with　an　ash　deposition　model　using　computational　fluid　dynamics　(CFD)　techniques　is　presented.　Starting　with　a,　detailed　gas　phase　kinetic　mechanism　for　alkali　chemistry,　a　systematic　reduction　procedure　has　been　performed　using　sensitivity　analysis　to　reduce　the　reaction　mechanism　to　a　level　that　can　be　implemented　into　a　CFD,　calculation.　This　reduced　mechanism　has　been　validated　against　the　full　reaction　mechanism　under　the　temperature　conditions＇　that　typically　occur　in　biomass　combustion　furnaces.　Both　mechanisms　are　compared　to　the　experimental　data　obtained　using　a　plug　flow　reactor　reported　in　the　literature.　An　ash　deposition　model　taking　into　considerations　the　ash　sticking　probability　and　the　condensation　of　potassium　salts　has　been　developed.　The　reduced　mechanism　and　the　deposition　model　developed　are　implemented　into　a　CFD　model　to　predict　ash　depositions　in　a　10　MWth　biomass　grate　furnace.　The　model　has　adequately　reproduced　the　deposit　position　and　shape　as　observed　in　the　test　furnace.