The effect of changes in fuel calorific value on combustion was investigated on a single standard Siemens SGT-300 7.9 MW dry low emission (DLE) combustor in a high pressure rig at actual engine operating conditions. The change in fuel calorific value was achieved by blending CO2 and/or N-2 with Natural Gas. The lowest fuel calorific value was tested at a Wobbe Index (WI) of 23 MJ/m(3) which was associated with 35% CO2 (in volume) added to Natural Gas. The engine loads tested varied from 5% to full load. NOx and CO emissions were measured in addition to the frequency and amplitudes of pressure fluctuations and various temperatures. The impact of pilot split (ratio of pilot fuel to total fuel mass flow rate) and fuel composition on these was investigated. The results showed a noticeable benefit where the reduction in WI provided a reduction in NOx emissions. The amplitude of the pressure fluctuations was not an issue being controlled, where necessary, by pilot split. The test results showed that WI of 25 MJ/m(3) appeared critical to combustion performance compared to Natural Gas. Review of the literature and calculation of the flame properties attempted to provide explanations for the trends observed during testing. (C) 2012 Elsevier Ltd. All rights reserved.