In 1997 AE Energietechnik implemented in the course of a EU-THERMIE funded demonstration project, together with some partners, a biomass gasifier in Zeltweg, Austria. This plant operates as a Circulating Fluidised Bed Reactor with a hot low-calorific product gas produced and transported into an existing coal-fired boiler. The thermal capacity (fuel input) is up to 20 MWth (design 10 MWth), compared to a thermal capacity of 344 MWth of the PC-boiler, a coal substitution of approximately 5%. The plant started commercial operation in December 1997 and, after a large measuring program, very promising operational records are available. In addition, some gasification tests with alternative fuels (waste wood, different plastics) were carried out. They have also produced some interesting results and proved the plant's suitability for such a purpose. This concept can be seen as a very economic way to increase the share of renewable fuels in fossil-fired power plants where the equipment is originally not designed for such fuels.