Biomass fuels are being increasingly used for domestic heating and power generation to cut down the net CO2 emission to the atmosphere. In most cases, those fuels are thermally-thick under packed-bed combustion conditions. In this paper, a double-mesh numerical scheme is proposed and implemented to simulate the detailed combustion processes for a biomass fuel with sizes ranging from 5 mm to 50 mm. Bench-top experiments were also carried out to validate the theoretical simulation. Under the specific conditions of investigation, it is found that a bed packed with particles over 35 mm can develop a temperature gradient over 400 degrees C inside the particles at the flame front, and significant overlapping of moisture evaporation, devolatilization and char burn-out is observed in the bed-height direction; CH4 emission over the bed top is more focused on the central part of the combustion period for larger particles; CO level in the flue gases increases with decreasing particle sizes and the opposite is true for H-2 emission. The overall air to fuel stoichiometric ratio for the whole combustion period increases significantly with increasing particle sizes, from 0.57 (fuel-rich) at 5 mm to 1.2 (fuel-lean) at 35 trim, but for the constant stage during combustion, the range of ratio narrows to 0.32-0.35.