With increasing use of biomass in combustion processes, the reduction of the related NO x emissions which originate mainly from the fuel nitrogen becomes more and more important. Efficient primary measures for NO x reduction are staged combustion techniques. Air staging has been investigated earlier and has found its way into practice. Since fuel staging has not been applied with nonpulverized biomass yet, the aim of the present work was to investigate the potential of fuel staging for NO x reduction in fixed bed systems. For this purpose, a prototype understoker furnace of 75 kW thermal input with two fuel beds in series was developed. Experiments were performed with wood chips (low nitrogen content) and UF-chipboards (high nitrogen content) to investigate the influences of the main process parameters, i.e., stoichiometric ratio, temperatures, residence time, and fuel properties on the conversion of fuel nitrogen to N-species. The most important parameters were found to be the temperature and the stoichiometric ratio in the reburn zone. The potential of fuel staging was measured and compared with air staging and unstaged combustion. The experiments show that low NO x emissions are already achievable with fuel staging at lower temperatures than with air staging, i.e., 900−1000 °C, and at a stoichiometric ratio of 0.85 in the reduction zone. The NO x reduction achieved under optimum conditions for UF-chipboard as main fuel was 78% which is higher than with air staging, where 72% NO x reduction was measured. For wood chips both measures attained about 66%. The nitrogen conversion during air and fuel staging has also been simulated using a furnace model based on ideal flow patterns as perfectly stirred reactors and plug flow reactors. A detailed reaction mechanism including the nitrogen chemistry (GRI-Mech 2.11) was implemented. The trends found with this model are in good agreement with the experiments and they indicate that even higher NO x reduction may be reached with improved process design. The investigations show that fuel staging is a promising technology for NO x reduction also for fixed bed biomass furnaces.

Emissions of nitrogen oxides from combustion processes should bc avoided since they contribute to the formation of acid rain and photoehemical smog. Following this aim, various primary measures have been developed to reduce the formation of nitrogen oxides during the combustion process. Staged combustion, meaning air Staging and fuel Staging, is known to be a suitable primary measure to minimize the NOx emissions caused by the nitrogen in the fuel. Research about air Staging for non-pulverized biomass combustion has already been done, whereas about fuel Staging no studies are published yet.

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