Abstract
Recent studies have described and evidenced the enhancement of fundamental combustion parameters such as laminar flame speed due to the catalytic influence of H2O with heavily carbonaceous syngas mixtures. In this study, the potential benefits of these subtle changes in water loading and hence reaction pathways are explored in terms of delayed lean blowoff, and primary emission reduction in a premixed turbulent swirling flame (Ø = 0.6–0.8), scaled for practical relevance. Chemical kinetic models initially confirm that H2O has a substantial impact on the employed fuel behaviour; increasing flame speed by up to 60% across an experimental range representative of fluctuation in atmospheric humidity (∼1.8 mol%). OH* chemiluminescence and OH planar laser induced fluorescence (PLIF) were employed to analyse the changes in heat release structure resulting from the experimental addition of H2O vapour to the combustor. Equivalent concentrations of liquid H2O were introduced into the central recirculation zone of the premixed flame as an atomised spray, to investigate the influence of phase changes on the catalytic effect. Near the lean stability limit, H2O addition compresses heat release to shorten the elongated flame structure. Whereas with a stable and well-defined flame structure, the addition triggers a change in axial heat release location, causing the flame front to retract upstream toward the burner outlet. Higher quantities of two-phase flow were combined to explore the possibility of employing the spray as a stabilising mechanism, effectively dampening the observed influence of humidity. The chemical enhancement induced by the controlled supply was shown to reduce the lean blowoff stability limit, enabling an increase in additional air flow of almost 10%. However, the catalytic effect of H2O diminishes with excessive supply and thermal quenching prevails. There is a compound benefit of NOx reduction from the use of H2O as a flame stabiliser with the practically-relevant syngas: First NOx production decreases due to thermal effect of H2O addition, with potential for further reduction from the change in lean stability limit; leanest experimental concentrations reduced by up to a factor of four with two-phase flow at the highest rates of supply. Hence, the catalytic effect of H2O on reaction pathways and reaction rate predicted and observed in the laminar environment, is shown to translate into practical benefits in the challenging environment of turbulent, swirl-stabilised flames.
Highlights
Carbonaceous converter gas typically comprises 50– 80% CO, 10–18% CO2, 1–3% H2 in a balance of N2 [1,2]
Water has been shown to provide two competing influences on premixed flame propagation; first it acts as a diluent and the reaction is slowed by a reduction in adiabatic flame temperature [4]
The Abel inverted images suggest a shortening of the flame structure, as peak heat release intensities move from the boundary confinement, chemical timescales are reduced locally and the CRZ contracts, trends that are evident in the OH planar laser induced fluorescence (PLIF) results: Fig. 12(a) gives a comparison between the average Ø = 0.8 spray and vapour experiments with the laser plane fixed in an equivalent position; ∼13 mm downstream of the a burner exit nozzle
Summary
Carbonaceous converter gas typically comprises 50– 80% CO, 10–18% CO2, 1–3% H2 in a balance of N2 (mol%) [1,2]. D.G. Pugh et al / Combustion and Flame 177 (2017) 37–48 addition of water vapour, as concentrations varied by amounts representative of change in atmospheric humidity [4]. Pugh et al / Combustion and Flame 177 (2017) 37–48 addition of water vapour, as concentrations varied by amounts representative of change in atmospheric humidity [4] This level of fluctuation would be expected to provide a significant impact on the behaviour of converter gas in practical application
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