A detailed two-dimensional numerical model for biomass pyrolysis has been developed. The model solves for conservation of mass, momentum, energy, and species within a two-dimensional wood particle and includes competitive pyrolysis reactions, shrinkage of the solid particle, and moisture content. The model is validated using the experimental data of J. M. Hoerning for wood pyrolysis, and is the first detailed model to be validated using a range of particle sizes and temperatures. In the validation process, char conductivity was found to increase with the size of the char layer rather than with temperature, indicating that char cracking may be a more important process that pore-to-pore radiation. The two-dimensional model is used to predict pyrolysis times for square particles ranging from 0.1 mm x 0.1 mm to 5 cm x 5 cm and temperatures ranging from 800K to 2000K. These runs were repeated with rectangular particles with a 2:1 aspect ratio using both grain direction and cross-grain direction particles. Pyrolysis times can vary by 30% depending on the grain direction. Selected two-dimensional results were compared with one-dimensional results. The model showed that the predicted pyrolysis times can differ by over 50% between one-dimensional and two-dimensional models. The amount of gas released from the particle over time by one-dimensional and two-dimensional models can differ dramatically for most pyrolysis situations, indicating that existing one-dimensional models should only be used for situations that are truly one-dimensional. The results indicate the limitations of most existing models comes from assuming that the particle can be modeled as a one-dimensional particle, rather than from a lack of good property data. Multi-dimensional pyrolysis models will be needed to adequately predict pyrolysis for most situations.