A transient unimodel for reacting pellets is considered with various modes of heat and mass transfer and structural changes for wood pyrolysis. Pellet breakup was found to be possible from strength calculations. This leads to an increase in the number of pellets and a decrease in the resistance to heat and mass transfer. The pressure and temperature buildup within 2.7 mm thick pellets was measured for wood pyrolysis/combustion experimentally. The bimodal wood pyrolysis was analyzed, and the rate constant and activation energy were found. Pellet breakup may also be used as a transient catalytic process where the catalysts become smaller as they break up in time.