Understanding the physical phenomena of pyrolysis and representing them with an appropriate mathematical model is essential in the design of pyrolysis reactors and biomass gasifiers. Description of the chemical processes of pyrolysis is coupled to an unsteady state, one dimensional, variable property model of transport phenomena, including heat convection, conduction and radiation, volatiles and gas transport by diffusion and convection and momentum transfer. In this study, a generalized reference model (Model I) incorporating all the above effects is proposed. This is further improved by proposing two simplified models (Models II and III) incorporating additional assumptions. A finite difference pure implicit scheme utilizing a Tri-Diagonal Matrix Algorithm (TDMA) is employed for solving the heat transfer and mass transfer model equations. A Runge-Kutta fourth order method is used for the chemical kinetics model equations. Simulations are performed considering different geometries of equivalent radius ranging from 0.0001 to 0.017 m and temperatures ranging from 303 to 2800 K. The results obtained using these improved models are in excellent agreement with the experimental data, much better than the agreement with earlier models reported in the literature. The improved validated model, which is best suited for wide ranges of operating conditions, is utilized to investigate the influence of particle size, particle shape, product distribution, conversion time and heat of reaction.