This paper examines the performance of a large swirl burner/furnace system in terms of its ability to efficiently burn simulated gases of very low calorific value. Earlier work has demonstrated that the combustion behaviour and temperature levels produced by waste gas from production of carbon black can be simulated approximately by mixtures of natural gas diluted with substantial amounts of excess air. The system comprises a 2 MW swirl burner/furnace system that can be operated either non-premixed with axial fuel injection on the centreline (for comparative purposes), or piloted-premixed with up to 60% of the natural gas introduced axially on the centre line in order to pilot the flame and enable the flame detector to monitor the flame front. The results are discussed in terms of the system aerodynamics, and in particular the influence of coherent structures on the flame stabilisation process, temperature, power, excess air levels, mode of fuel entry and emissions of NOx and CO. In particular it is shown that gases of very low calorific value (down to 0.17 MJ m(-3); lambda = 22) can be efficiently burnt when small amounts of piloting or support fuel are introduced axially on the centre line. Although very low NOx emission levels can be achieved with premixed or simulated low-CV gas consumption, some penalty accrues in terms of CO emission levels-which are typically around 150 ppm as distinct from 50 ppm with non-premixed combustion or for simulated low-CV gases (with a CV of >1 MJ m(-3)). This probably arises from the very low power levels the unit was operated for combustion of very low-CV gas, giving rise to low exhaust temperatures and insufficient residence time for full oxidation of the CO.