Fuel Staging for NO_x Reduction in Biomass Combustion:  Experiments and Modeling

Abstract

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.

High-Quality Solid Fuel Production from Leaf Litter of Urban Street Trees

“…Values for UM, MW and PC were above the threshold value of 0.6% DM, as proposed by Obernberger et al [12] for an unproblematic combustion process considering NOx emissions. Thus, technical measures such as air staging are necessary to prevent NOx emissions in combustion [25,26]. …”

Reduced chemical kinetic mechanisms for NOx emission prediction in biomass combustion

“…In general, the total NO x emission from biomass can be divided into fuel-NO x , thermal-NO x , and prompt-NO x . Fuel-N content is responsible for the majority of NO x emission from any biomass combustion plant, and the two main NO x precursors are NH 3 and HCN, which are the main products of fuel-N conversion after the thermal degradation [20,21]. The chemical mechanism that has been used in this study is, as mentioned above, an extended detailed chemical kinetic mechanism developed by combustion groups at the Technical University of Denmark (DTU) and the Norwegian University of Science and Technology (NTNU) [3,13,14,[22][23][24][25][26][27][28][29][30][31][32].…”

Experimental Investigation on NOx Reduction by Primary Measures in Biomass Combustion: Straw, Peat, Sewage Sludge, Forest Residues and Wood Pellets

“…In solid fuels fired systems, fuel-NO accounts for more than 80% of the total NO x and NO x is mainly produced by conversion of volatile nitrogen-containing species such as NH 3 and HCN, while remaining char-N oxidation in the reactor bed accounts for a minor part of the total NO [11,[17][18][19][20][21][22]. In addition, thermal NO x formation becomes important at temperatures above 1400 °C, which is far from the typical temperature range (800-1200 °C) of biomass combustion systems [16,23,24]. Experimental investigations are needed to characterize the release of nitrogen containing species from different solid biomass fuels to establish input for CFD modeling studies [25].…”