The thermochemical conversion of biomass by Rash pyrolysis or liquefaction produces a crude liquid that can be used directly to substitute for conventional fossil fuels or upgraded to a higher quality fuel. Both the crude and upgraded products may be utilised for power generation. A computer program is used to model the flash pyrolysis of biomass with subsequent upgrading and refining of the crude liquid products to produce higher value and more marketable liquid fuel products. The structure and scope of the program, the hydrotreating model and the development of an energy self-sufficient flash pyrolysis model are detailed. The program is used to assess and compare the economic and technical opportunities for upgrading crude pyrolysis liquids into the higher quality fuels demanded by more advanced technologies.