Two biomass gasification concepts, i.e. indirectly heated, fast internally circulating fluidised bed (FICFB) gasification with steam as gasifying agent and two-stage, directly heated, fixed bed Viking gasification are compared with respect to their performance as gas generators. Based on adjusted equilibrium equations, the gas composition and the energy requirements for gasification are accurately modelled. Overall energy balances are assessed by an energy integration with the heat cascade concept and considering energy recovery in a steam Rankine cycle. A detailed inventory of energy and exergy losses of the different process sections is presented and potential process improvements due to a better utility choice or feed pretreatment like drying or pyrolysis are discussed. While Viking gasification performs better as an isolated gas generator than state-of-the-art FICFB gasification, there is large potential for improvement of the FICFB system. Furthermore, a concluding analysis of the gasification systems in an integrated plant for synthetic natural gas production shows that FICFB gasification is more suitable overall due to a more advantageous energy conversion related to the producer gas composition. (C) 2009 Elsevier Ltd. All rights reserved.