Understanding of the release mechanism of potassium from biomass is crucial to address the detrimental ash-related issues that are suffered widely in biomass boilers. However, currently available experimental methods only allow the analysis of the overall amount of potassium release; the profiles of released potassium species are still unknown. This study develops a new two-step kinetic model, which integrates the potassium release along with biomass devolatilisation and the reactions of potassium species in the environment of near real biomass combustion. The model enables the quantification of potassium species released from biomass during combustion over a broad range of temperatures. The modelling results indicate that the majority forms of released potassium are KOH and KCl, their amounts could reach as high as 2.6 × 10−5 mol/g biomass and 4.3 × 10−5 mol/g biomass, respectively. With the temperature increasing up to 1400 K, K2SO4 suddenly becomes the third significant potassium species with an amount of 1.8 × 10−5 mol/g biomass. Intermediate species are greatly involved in the release of potassium, at the initial stage, KCl and KOH are very likely to be homogeneously combined as (KCl)2 and (KOH)2; while KO and KO2 are two most important intermediate species that involved in the potassium transitions throughout the biomass combustion process. Those intermediate species, however, show a significant effect on the formations of KCl and KOH, but less on the formation of K2SO4. The developed model is capable to forecast the potassium release from various types of biomass feedstocks, thus promoting solutions that can effectively address alkali-related biomass combustion challenges.