Slow and medium speed marine engines employ Heavy Fuel Oil (HFO) as a major source of fuel. Heavy fuel oil is primarily composed of high molecular weight hydrocarbons which cannot evaporate, but are pyrolized at high temperatures. Pyrolysis produces volatile gases and polymers through thermal cracking and polymerisation respectively. The objective of the present work is to understand the effect of heating rates and chemical kinetics on droplet’s pollutant formation tendency during the combustion. Baert’s pyrolysis model based on chemical kinetics for thermal cracking and polymerisation rate is applied to a single droplet. Results of this pyrolysis model show that polymer formation within a droplet is dependent on droplet heating rate and composition. A small droplet produces more polymers compared to a big droplet at the same ambient condition which is contradictory to the expectancy. Therefore, Baert’s pyrolysis model cannot apply to spray combustion modelling because in an actual engine spray, droplets are small and heating rates are high due to combustion. Furthermore, it is observed in Baert’s pyrolysis model results that the polymerisation process starts prior to the thermal cracking. This order of thermal cracking and polymerization is contradictory to the experimental evidence. Consequently, several pyrolysis models are studied and a suitable modified model is proposed. Results of the modified pyrolysis model did not show any significant dependency of polymer formation on droplet heating rate and it showed thermal cracking beginning earlier than polymerisation. This modified pyrolysis model is applied to the spray combustion behaviour study of HFO.