The formation of NOx and soot in oxygen-enriched suspension flames

Conference poster, 2018

This work discusses the formation of NOx and soot in propane flames and how the formation is influenced by the oxygen content in the oxidizer, both in oxy-fuel combustion and in oxygen-enriched air combustion. Experiments were conducted in the Chalmers 100 kW unit applying oxygen-enriched air and a continuous increase in NOx and soot formation was observed when the feed gas oxygen content was increased up to a specific value (around 30%); at that feed gas oxygen content the soot formation increased by order of magnitudes while the NOx emission decreased significantly, see Figure 1. A similar shift in flame characteristics was observed in oxy-fuel combustion but at a higher oxygen content (≈40%). This work aims to examine how NOx and soot chemistry relate and we discuss plausible explanations for the sudden change in the concentration of these two pollutants. According to the literature, there are three main chemical mechanisms by which soot and NOx may interact: 1) gas-phase interactions at a primary precursor level, involving competition for radicals as well as reactions of NOx with hydrocarbon radicals from primary aromatic precursors, 2) reactions of nitrogen included in the polycyclic aromatic hydrocarbon (PAH) clusters with simultaneous PAH-NO reduction or indirect influence of NOx on the PAH oxidation and growth processes, and, 3) heterogeneous interactions between soot particles and NOx through adsorption of NOx on the active sites in soot surface or through soot-catalyzed reactions.

The present work is based on combustion modeling. Detailed reaction mechanisms, obtained from the literature, that are relevant to the gas phase nitrogen chemistry and soot formation during combustion in flames, were implemented to investigate possible interactions and their sensitivity to combustion conditions. Various cases were modelled, cases that are characterized by different reactor inlet oxygen concentrations, residence times, temperature conditions and mixing rates. The modeling results show that temperature and inlet oxygen concentration are important to the radical pool and, thus, to the rates of the key-reactions in soot and nitrogen oxide formation. An increasing soot formation in the flame region was observed when the local temperature and the inlet oxygen concentration were increased in the model. The observed NOx reduction is attributed mainly to competition for radicals on a precursor level and to heterogeneous reduction on the soot.