F2010A017
HYDROGEN ENHANCED COMBUSTION: A STUDY ON SOOT FORMATION AND STRUCTURE IN ACETYLENE-AIR LAMINAR INVERSE DIFFUSION FLAMES
Hydrogen as a fuel additive to enhance combustion of hydrocarbon fuels like natural gas and diesel fuels has been studied by several investigators. Reduction of soot emissions from diesel engines is one of the major concerns in pollution control from vehicles. Use of hydrogen as a supplementary fuel in diesel engines has shown mixed results on soot reduction. To further understand the effect of hydrogen on soot formation in hydrocarbon-air diffusion combustion the present investigations were carried out. Soot formation and its structure with hydrogen addition in laminar diffusion flames of acetylene and air were investigated. Acetylene was taken as the principal fuel as it is considered one of the main precursors in soot formation mechanism. Inverse diffusion (IDF) and normal diffusion (NDF) flames were employed to study the effect of hydrogen on nascent and mature soot particles, respectively. Soot samples from the flame were collected on a transmission electron microscope (TEM) grid using thermophoretic sampling technique. Primary soot particle diameter, soot volume fraction, aggregate size and fractal dimension of soot particles were studied. From the TEM samples the images at 80000 (80 K) and 15000 (15 K) magnifications were obtained which were used for determination of primary particle diameter and soot volume fraction, respectively. Hydrogen was added up to 7.2 % by mass keeping acetylene flow rate constant. Hydrogen addition enhanced flame temperatures by up to 200 K. Addition of hydrogen showed significant reduction in primary particle size and soot volume fractions both for the NDF and IDF. In the IDF, with hydrogen addition primary particle size reduced from 18-26 nm. to 14-21 nm. In NDF, the particle size was larger due to more residence time available for surface growth and with hydrogen addition the particle size reduced from 30-40 nm to 26-34 nm. For all the flames, hydrogen addition reduced the soot volume fraction (SVF) by nearly 50%. SVF in NDF reduced from nearly 0.75 ppm to 0.4 ppm, and in IDF from 0.2 ppm to around 0.1 ppm. Again, the soot volume fraction in IDF was much lower than in the NDF. With hydrogen addition soot aggregate size was somewhat smaller. TEM photographs show that in general the nature of soot aggregates is chainlike and is not affected by hydrogen addition. Studies on IDF show that hydrogen has significant influence on initial soot formation and structure. Primary particles as well as aggregates are significantly smaller than without hydrogen. These findings suggest that large reductions in soot emissions can be obtained with hydrogen addition in diffusion combustion systems. In diesel engines, combustion phasing may have to be optimized for obtaining soot reduction with hydrogen addition.
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Session: IC Engines, Goals and Development