Addition　of　sludge　into　biomass　is　an　alternative　method　to　capture　alkali　metals　and　obtain　biomass　good　combustion　conditions.　The　ash　fusion　characteristics　of　sewage　sludge　(SS),　petrochemical　sludge　(PS),　wheat　straw　(WS),　and　series　of　the　SS/PS　and　WS　blends　in　different　blending　ratios　were　investigated　in　this　study.　The　mineral　matter　transformations　during　the　co-firing　of　SS/PS　with　WS　were　examined.　Results　indicate　that　the　addition　of　SS/PS　can　raise　the　ash　fusion　temperatures　of　WS　ash,　and　the　ash　fusion　temperatures　of　most　SS/PS–WS　ashes　are　higher　than　those　of　WS　ash.　The　SS/PS-WS　ashes　have　different　mineral　matters　and　microstructures　in　different　blending　ratios　after　the　co-firing　of　SS/PS　with　WS.　It　is　found　that　the　mineral　matters　in　the　SS–WS　ash　are　silicates,　silicon　dioxide　and　ferric　oxide　with　high　melting　points,　while　the　matters　in　the　PS–WS　ash　are　phosphate,　manganese　aluminate,　silicon　dioxide,　and　aluminum　nitride　with　high　melting　points.　The　results　obtained　in　this　study　are　not　only　useful　for　biomass-fired　power　stations　looking　for　alternative　co-firing　fuels　but　reduce　ash　deposit　formation　and　agglomeration　in　biomass-fired　boiler.　©　2020　Elsevier　Ltd