Present study is to develop a 200,000 kcal/h class 13-step-grate wood pellet firing boiler. A series of numerical simulations regarding combustion load and heat transfer characteristics inside the combustion chamber of a wood pellet boiler has been conducted. We have considered combustion load ranging from 56% to 130%. The results show that a high temperature zone exists at the exit of the 1st chamber for all combustion loads considered. The flame is stretched to the exit of the 2nd chamber. Combustion gas is flowing along the partition, and a recirculation zone is formed at the top of partition. Compared to experimental data, numerical simulations predict the temperature distribution for partial load operation.