A computer model (ABC-code) has been constructed to describe the steady one-dimensional aerosol dynamics in combustion processes for gas and particle interactions at post-combustion conditions. The aerosol general dynamic equation has been solved numerically using a discrete-nodal point method for describing the particle size distribution. In the present model we consider those mechanisms that will affect the dynamics of alkali species after volatilisation, i.e. nucleation, condensation, coagulation, chemical reactions and deposition. The main features of the ABC code have been explained and an example calculation has been carried out for simulation of aerosol dynamics and alkali vapour behaviour in a real scale pulverised coal fired boiler. The results show that for predicting the gas phase concentrations of alkali species at different temperatures it is important to know the volatilisation of sodium, potassium, chlorine and sulphur and the formation rate of alkali sulphates in the gas phase. The initial ash size distribution determines the distribution of condensed alkalis between sub- and supermicron particle modes. The choice of the homogeneous nucleation model has some importance for the calculated submicron number size distribution. The effect of heterogeneous nucleation to initiate condensation on ash particles was negligible. The predictions of our simulation are in a qualitative agreement with the experimental results. The utilisation of the present code will lead to a better understanding of aerosol behaviour which will be of great importance for the control of toxic flue gas emissions, slagging and fouling in commercial boilers.