Abstract: Reducing formation of gaseous species, which contain chlorine, potassium and sodium, can effectively abate deposit formation, slagging, corrosion and harmful pollutant emission during thermal conversion of biomass fuels. The occurrence modes, transformation routines and release characteristics of those species, especial gaseous species have been investigated by thermodynamical equilibrium analysis technique for 5 biomass fuels, such as straw, bark, woodchips, waste wood and olive residues from 400K~1600K during pyrolysis and gasification with excess air coefficient as 0.4 and 0.8. The calculation results show that chlorine element exists as species of KCl(s), HCl(g), KCl(g), (KCl)2(g) and NaCl(g) for 5 biomass fuels during pyrolysis and gasification. The solid chlorinecontained species will be gradually transformed into gaseous when reaction temperature is higher than 800K~1000K. Alkali metal elements, potassium and sodiumin biomass fuels will be released as gaseous species above 900K. A little KCN(g) and NaCN(g) are transformed during pyrolysis. The amount of gaseous species KCl(g), K(g) and Na(g) will decrease and transform into NaCl(g), KOH(g) and NaOH(g) above 1000K during gasification. The optimum gasification temperatures for the lowest gaseous release of species containing chlorine and alkali metal elements are 800K and 900K, respectively. Hence, developing mildtemperature gasification technology is an alternative to abate the common problems occurred in biomass gasification reactors.