Due to the higher oxygen content and lower heating value, the amount of biomass required in a combined cycle, where it is used as supplementary fuel, to meet a given energy demand is such that the biomass consumes almost all of the oxygen remaining from gas turbine combustion process under certain conditions. This situation requires additional air for biomass combustion thus reducing the cycle efficiency and the net work output rate while increasing CO2 emissions. Three conditions at which the oxygen is completely consumed are identified based on alterations in net fuel utilization. The first condition is linked to fuel utilization, which is observed to be significantly affected by variations in temperatures at three locations in the combined cycle (air temperature entering the gas turbine combustion chamber, gas turbine inlet temperature and HRSG inlet temperatures). The second condition relates to the characteristics of the feedstock (oxygen content of the biomass and heating value of natural gas). The heat loss due to combustion of natural gas and biomass is the third condition that affects oxygen availability. The current work assesses these conditions in order to identify the proper condition at which no additional air is required for supplementary firing of biomass. (C) 2009 Elsevier Ltd. All rights reserved.