Characteristics of flame heat release and soot emissions of methane and ethylene laminar diffusion flames with dimethyl ether addition
Session chaired by Pr. Christine Rousselle
ABSTRACT Biofuels could contribute significantly to decarbonisation of the transport and power generation sectors and offer the potential of pollutant emission reduction. In 2018, the World Health Organisation revealed that about 4.2 million deaths around the world are attributed to air pollutions [1]. Prolonged exposure to particles may cause cardiovascular and respiratory illness. Consequently, following the rise in the public’s general awareness regarding not only the environmental protection, but more importantly the need for alternate energy sources, decreasing the current levels of greenhouse gas emissions has become the main area of concern, creating a higher demand for relevant research. The utilization of cleaner fuels, such as biofuels, as an alternative to conventional fuels is one of the solutions as it offers similar energy content with fewer emissions. It has been suggested that mixing dimethyl ether (DME) or inert gas with hydrocarbon biofuel reduces the emissions particularly soot [2-4]. However, there is still lack of understanding of biofuel flame structure and the relationship between structure, heat release and emissions. The objective of this study is to understand the characteristics of soot emissions and flame heat release rate (HRR) for different DME mixtures with hydrocarbon biofuels. In this work, the effects of DME and nitrogen (N2) addition to methane (CH4) and ethylene (C2H4) fuels on combustion characteristics of flame heat release rate, soot emissions and flame temperature were investigated experimentally and numerically in a co-flow non-premixed laminar flame. The co-flow burner used in this work has a similar structure to the Yale co-flow burner [5]. The HRR and soot volume fraction were measured experimentally using CH*, OH* and C2* chemiluminescence and planar two-colour soot pyrometry technique, respectively. The CH*, OH* and C2* were used particularly to find the correlation between their chemiluminescence and HRR as well as to investigate the soot signal’s effect on the measurements of HRR. Numerical calculations of all flame conditions were also performed using COSILAB software which was implemented with a detailed chemical kinetic mechanism (Mech_56.54, [6]) comprising of 113 species and 710 reactions to investigate the destruction and production rates of species including OH*, OH, CH*, CH, HRR, CH3, C3H3, C2H2 and flame temperature. The validated mechanism (Mech 56.54) was used because it includes the chemiluminescence reactions, allowing the comparison with the experimental results to further investigate the characteristics of soot emissions and HRR for different DME mixtures. Constant fuel stream volumetric flow rate was maintained throughout all test conditions. The effect of DME on combustion characteristics was also studied in flames having equal power output and total stream volumetric flow rate by adding nitrogen N2 (dilution). It has been observed from the current experimental results (from direct images) that the soot radiation appearance in methane flame became initially stronger with little addition of DME (25%). However, when DME mixture ratio was increased by more than 25%, the soot radiation appearance methane and flame became weaker, indicating either less soot concentration or reduced soot temperature. Whereas, numerical results suggested that C2H2 (soot precursors) peak value was observed to be reduced with 25% DME mixture ratio, nevertheless, the area under C2H2 profile increased. Furthermore, it was noticed numerically that DME/CH4 flame mixture temperature increased by increasing DME mixing ratio, unlike with the CH4/N2 mixture conditions in which the addition of N2 reduced the flame temperature. The addition of DME and/or N2 to methane or ethylene flames were found to decrease the soot concentration and HRR both numerically and experimentally. Numerical results showed that soot concentration and HRR were affected more by the addition of N2 in comparison to the addition of DME to methane or ethylene flames. The addition of N2 to DME/air mixture reduced the HRR, soot concentration and flame temperature significantly. The CH*, OH* and C2* chemiluminescence results of equal power output conditions showed a decrease in their intensities as more DME and N2 are added simultaneously to methane flame, indicating lower HRR and soot concentration. Whereas, numerical results illustrated that the area under curves for all species profiles remained unchanged throughout all flames having similar power output. This study contributes towards collecting and analysing missing data that are related to the relationship between the biofuels flame structure and emissions. Keywords: biofuel, co-flow diffusion flame, dimethyl ether, heat release rate, soot emissions. Acknowledgment: The authors would like to acknowledge EPSRC David Clarke Fellowship for the funding support of the research project [Grant Ref: EP/S017259/1]. Reference: Can be found in the PDF file.
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