We investigated the influence of firing biogas on the performance and operating characteristics of gas turbines. Combined heat and power systems based on two different gas turbines (simple and recuperative cycle engines) in a similar power class were simulated. A full off-design analysis was performed to predict the variations in operations due to firing biogas instead of natural gas. wide range of biogas compositions differing in CH4 content was simulated. Without consideration of operating restrictions on the compressor and turbine, using biogas was predicted to augment the power output in both engines. Power output increased as CH4 content decreased. The main reason is the increase in turbine power due to increased fuel flow. Gas turbine efficiency increased with decreasing CH4 content in the simple cycle engine, but decreased in the recuperative cycle engine. Net efficiency including the fuel compression power consumption decreased with decreasing CH4 content even in the simple cycle engine. The heat recovery also increased by firing biogas. However, the increased turbine flow was accompanied by a surge margin reduction of the compressor and overheating of the turbine blade. These two problems were more severe in simple cycle gas turbines and as the ambient temperature increased. The turbine blade temperature and the compressor surge margin could be recovered to the reference values by either under-firing or compressor air bleeding, which are effective for blade temperature control and surge margin control, respectively. However, satisfaction of both restrictions by a single modulation caused excessive power and efficiency losses. An optimal combination between under-firing and air bleeding would minimize the performance penalty. (c) 2011 Elsevier Ltd. All rights reserved.