The main objective of the present work is to model the impact of the presence of potassium chloride on the slow pyrolysis of cellulose in terms of mass loss profiles, product yields formation and gas speciation. In the present work, we propose to modify the cellulose sub-mechanism of the Bio-PoliMi kinetic model by introducing a chemical reaction that considers the potassium chloride catalytic effect during pyrolysis. The kinetic parameters and reaction coefficients of the introduced reaction were fitted to the experimental results using a two-step fitting procedure. The experimental results were obtained from thermogravimetric and pyrolysis tests, up to a temperature of 973 K using a heating rate of 5 K/min, for five cellulose samples doped with different potassium chloride concentrations (pure sample, 0.5 wt%, 1.09 wt%, 2.0 wt% and 3.0 wt%). The experimental results show that the presence of potassium chloride affects both primary and secondary pyrolysis, affecting the product yields and the gas composition. The modified mechanism proposed in the present work is able to predict with good accuracy the thermogravimetric profiles as well as the total product yields of char, gas and tar. Furthermore, the mechanism is able to predict reasonably well the release characteristics of CO and CO2 gaseous species, even though the final yield is under-predicted. As for CH4 and H-2 species, the mechanism is not able to follow their experimental trends since the original mechanism cannot capture the secondary pyrolysis of the cellulose.