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Document Type

Original Study

Subject Areas

Mechanical Engineering

Keywords

Co-firing; Exhaust emissions; Coaxial burners; Swirling flames; Diffusion flames.

Abstract

The current stringent environmental regulations, the depletion of fossil fuels and the emergence of oil biofuels and synthetic gases had motivated combustion scientists to search for alternative technologies that meet efficient burning, flexible use of multiple fuels and conform to a clean environment. In that respect, several developments had emerged including, low swirl combustion for partially premixed mixtures, β€œMild” combustion for diffusion flames. The present experimental study introduces a new concept that combines separate admissions of fuel and air with a central recirculation zone following the diffusion combustion mode, partial premixing within a premix chamber resulting in the creation of a uniform and dispersed reaction zone having a uniform distributed temperature with diminished temperature peaks, leading to low NOx and CO emissions. In that respect, this work investigates experimentally the flame flow field of cofiring diesel and LPG in terms of the combustion characteristics and exhaust emissions at a fixed loading condition of 20 kW while varying equivalence ratio (πœ™) between 0.6 and 0.9. Three cofiring cases of liquid fuel/gaseous fuel ratios (LGR) of 90/10, 80/20, and 70/30 based on energy share are compared to the standalone diesel combustion. A coaxial burner is designed and manufactured with a central pressure jet atomizer housed inside a vane swirler. This set is coaxially surrounded by an annulus gaseous hub feeding twelve angularly spaced gaseous jets. A coaxial vane swirler is fitted at the outer annulus area. A conical premix chamber is mounted at the burner gun exit to direct the outer swirling air stream to the flame core. The burner is coaxially fitted to a water-cooled horizontal cylindrical combustion chamber. At a higher equivalence ratio (πœ™ = 0.9), increasing the LPG ratio shows a clear reduction in CO emissions compared to standalone diesel combustion. However, at a lower equivalence ratio (πœ™ =0.6), the LPG ratio has a minor influence on the CO level. On the Contrary, NOX concentration reaches its peak value with the increase of the gaseous fuel share in energy. The cumulative heat transfer to the combustor walls is always higher in the co-firing test cases due to the relatively higher radiation and convection heat transfer modes. Optimal conditions are achieved at LGR=70/30, exhibiting the lowest CO emissions (~ 30 ppm), adequate NOx levels (~15 ppm) and the highest cumulative heat flux (~55% of input load at πœ™ =0.9) among all the test cases

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