In internally circulating fluidised bed, there occurs gas bypassing from draught to annulus region and vice versa. The effect of gas bypassing on product gas composition in both annulus and draught tube is studied using a model proposed in literature. The same modelling is applied to estimate the number of circulations a particle should undergo to achieve a specified conversion and the total gas yield and solid circulation rate inside the reactor are calculated at different temperatures for different conversion levels. The effect of coal feed rate, particle size, oxygen to coal mass ratio and steam to coal mass ratio on the number of circulations, total gas yield and solid circulation rate is studied. The optimum conditions to achieve 95% conversion with minimum number of circulations and maximum gas yield per kg of coal per pass are obtained. The conversion and product gas composition are studied for a mixture of particles and compared with uniform size particles. The product gas composition depends strongly on operating parameters such as oxygen/coal mass ratio and steam/coal mass ratio and gas bypassing. © 2019, © 2019 Indian Institute of Chemical Engineers.