Experimental study on nano-sized particles produced in flames burning methane

Natural Gas is one of the primary sources of energy for many of our activities; moreover natural gas is mostly composed of methane, therefore it is of great interest to study the combustion of methane and its byproducts in different combustion systems. During the last few years it has been shows that primary particles with typical sizes smaller than three nanometers are easily produced close to the flame front of rich premixed flames and in non-premixed combustion systems [1-3]. These clusters, which are probably composed by polymers of polycyclic aromatic hydrocarbons, may growth for effect of particle-particle coagulation process and for the addition of molecules from the gas-phase and lead to the formation of graphitic-like structures with typical size larger than ten nanometres, defined as soot or elemental carbon. The characterisation of the kinetic of the formation of the particles emitted from combustion requires the development of experimental techniques able to detect particles in the size range between 2-50 nm accounting also for their different chemical composition. At this purpose, in this paper we present an experimental study of Nanoparticles of Organic Carbon (NOC), with a typical size range of 1 - 10 nm, and soot particles, with sizes in the range 10 - 100 nm, formed in methane flames at atmospheric pressure. An optical diagnostic method based on the interaction of the fifth harmonic of a Nd:YAG laser with combustiongenerated aerosols was performed to measure the concentration profiles of both NOC and soot particle with high spatial resolution [4]. The high energy of the selected harmonic enhances fluorescence from aromatic chromophoric groups and also allows soot particles to heat up and emit incandescent radiation [5]. Optical measurements are complemented by Differential Mobility Analysis (DMA) measurements, in selected conditions, which allow the measurement of the size distribution functions of the formed particles. The experimental data, have been performed in different flame conditions: laminar premixed, laminar diffusion and turbulent diffusion flames. In the present work we compare the amounts of particulates from the methane combustion, with respect to other gaseous fuels such as ethylene in similar flame conditions. Finally, the experiments were performed on three different systems, to the aim to study the effects of premixing, diffusion and turbulence on the particles formation in methane combustion