An extended overview of the phase-mineral transformations of organic and inorganic matter that occur during biomass combustion was conducted. Some general considerations and particularly problems associated with the composition of biomass and biomass ash (BA) and behaviour of biomass during burning were discussed initially. Then, reference peer-reviewed data plus own investigations were used to organise and describe systematically the above topics. It was demonstrated that the phase composition of BA is polycomponent, heterogeneous and variable and includes: (1) mostly inorganic matter (IM) composed of non-crystalline (amorphous) and crystalline to semi-crystalline (mineral) constituents; (2) subordinately organic matter (OM) consisting of char and organic minerals; and (3) some fluid matter associated with both IM and OM. Approximately 291 phases or minerals were identified in BA. These species have primary, secondary or tertiary origin in the combustion residue and they are generated from natural (authigenic and detrital) and technogenic phases or minerals originally present in biomass. Afterwards, common issues related to the composition, occurrence, transformation and origin of common constituents in biomass and BA such as: (1) OM, namely cellulose, hemicellulose, lignin, char and other organic phases plus organic minerals; and (2) IM such as silicates, oxides and hydroxides, phosphates, sulphates (plus sulphides, sulphosalts, sulphites and thiosulphates), carbonates (plus bicarbonates), chlorides (plus chlorites and chlorates), nitrates, glass, amorphous (non-glass) material and other inorganic phases; were described and compared to coal ash. As a final point, a systematization of physico-chemical transformations during biomass combustion is given. It was found that the original OM and IM in biomass during combustion transform: (1) initially to devolatilization of OM and burning of combustible gases and char with formation of intermediate and less stable oxalates, nitrates, chlorides, hydroxides, carbonates, sulphates and inorganic amorphous (non-glass) material; (2) subsequently to more stable silicates, phosphates and oxides; (3) then to melting accompanied by dissolution of the refractory minerals; with increasing combustion temperatures in the system; and (4) followed by crystallisation of melt and formation of glass accompanied by some salt condensation and hydroxylation, hydration and carbonation of newly formed phases during cooling of BA. Finally, some post-combustion transformations of the newly formed minerals and phases to stable during weathering species among silicates, hydroxides, phosphates, sulphates, carbonates, chlorides and nitrates also occur due to their hydration, hydroxylation and carbonation by moisture and CO2 in the air through storage of BA. Certain major associations related to the occurrence, content and origin of elements and phases were identified in the BA system and they include: (1) Si-Al-Fe-Na-Ti (mostly glass, silicates and oxyhydroxides); (2) Ca-Mg-Mn (commonly carbonates, oxyhydroxides, glass, silicates and some phosphates and sulphates); and (3) K-P-S-Cl (normally phosphates, sulphates, chlorides, glass and some silicates and carbonates). These associations were applied for classification of BAs to four types and six sub-types. It was found that such systematic relationships have a key importance in both fundamental and applied aspects related to innovative and sustainable processing of biomass and BA. The ash formation mechanisms and ash fusion behaviour, as well as some indications of potential technological problems and environmental risks during combustion of biomass types and sub-types and application of their BAs will be described in Part II of the present work. (C) 2013 Elsevier Ltd. All rights reserved.