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 NOx formation during combustion of the producer gas.

► Gasification process and conversion of fuel-bound nitrogen have been investigated.
► During gasification almost entire fuel-bound N is released as N bound in tars.
► N bound in tars is released as HCN, NO, NH3 and N2 as result of tar cracking.
► NH3 and HCN are partly oxidised to NO and NO2 in the combustion chamber.
► Results are highly relevant for design and modelling of low-NOx combustion chambers.