Combustion chemistry and fuel-nitrogen conversion in a laminar premixed flame of morpholine as a model biofuel

Abstract

The present study has been motivated by the need to understand and predict fuel-nitrogen conversion in the combustion of biomass-derived fuels. Within that broader context, an earlier related publication (Lucassen et al., Proc. Combust. Inst. 32 (2009) 1269–1276) has investigated morpholine (C 4H 9NO, 1-oxa-4-aza-cyclohexane) as a model oxygen- and nitrogen-containing biofuel, and species identification was presented for a slightly fuel-rich Φ = 1.3 (C/O = 0.41) laminar premixed morpholine-oxygen-argon flame at 40 mbar. To attempt a more detailed insight into the flame structure and combustion mechanism, the present contribution has now combined photoionization (PI) and electron ionization (EI) molecular-beam mass spectrometry (MBMS) to determine absolute mole-fraction profiles of numerous major and intermediate species with up to 6 heavy atoms. In general, PI-MBMS and EI-MBMS results were found in good agreement. The results reveal formation of a number of intermediates that may contribute to harmful emissions, including aldehydes and several nitrogen-containing compounds in percent-level concentrations. Both NH 3 and HCN pathways are seen to contribute to NO formation. To identify reaction pathways for this detailed experimental analysis, development of a flame model was started, considering a combustion mechanism for cyclohexane and analogous fuel-breakdown reactions for morpholine by addition of necessary thermodynamic, transport and kinetic parameters. The present model captures relevant features of the morpholine flame quite well, including HCN, N 2, and NO, and it can serve as a nucleus for further development of detailed combustion models for fuel-nitrogen conversion from model biofuel compounds.