Characterisation of fuel bound nitrogen in the gasification process and the staged combustion of producer gas from the updraft gasification of softwood pellets

This paper presents experimental results derived from test runs performed with a laboratory-scale updraft fixed-bed gasifier coupled to a combustion chamber to produce data for the characterisation of fixed-bed gasifier operation and to investigate the release behaviour and the conversion of fuel-bound nitrogen during gasification and subsequent staged combustion of the producer gas using softwood pellets. Spatial temperature profiles and the composition of the producer gas of the gasifier have been measured for different air flow rates. In addition, the concentrations of relevant nitrogenous gas species including tars have been measured in the producer gas and at different positions in the combustion chamber. The air flow rate has a significant influence on the composition of the producer gas and the temperature profile of the packed bed of the gasifier. Results show that during updraft fixed bed gasification almost the entire fuel-bound nitrogen is released as N bound in tars from the packed bed and is then subsequently released as HCN, NO, NH₃ and N₂ as a result of tar cracking during combustion. This strong N-fixation in the tars was not expected and is of great relevance concerning NO_x formation during combustion of the producer gas.