Stricter emission limits and low prices for fossil fuels are considerable challenges for biomass combustion technologies. Solutions are provided with an innovative boiler concept developed within the last years. Due to its design, the boiler can be operated in different modes as a combi-boiler for solid and gaseous fuels. In the standard mode, solid biomass is used as fuel. Switching to the fuel staging mode enables the reduction of higher NOx emissions. The experiments demonstrate that reduction rates of up to 50% are possible. In low load times or if it makes sense from an economic point of view, the boiler can also be operated with gas only. Mainly the results from combustion experiments in fuel staging mode are discussed. Based on these results, a model has been designed to develop a control unit and to support the upscaling process.