Nanostructure and Oxidation Reactivity of Nascent Soot Particles in Ethylene/Pentanol Flames

Yaoyao Ying, Chenxuan Xu, Dong Liu, Bo Jiang, Pengfei Wang, Wei Wang
2017 Energies  
As byproducts of the combustion process of hydrocarbon fuels, soot particles are difficult to remove, and they can greatly harm human health and pollute the environment. Therefore, the formation and growth processes of the soot particles has become a study focus of researchers. In this paper, the nanostructure and oxidation reactivity of carbonaceous particles collected from ethylene inverse diffusion flames with or without the additions of three pentanol isomers (1-pentanol,
more » ... 3-methyl-1-butanol, and 2-methyl-1-butanol) were investigated in detail. The nanostructure and oxidation characteristics of nascent soot particles were characterized using high resolution transmission electron microscopy (HRTEM), X-ray diffractometry (XRD) and thermogravimetric analysis (TGA). It was found that the nascent soot cluster of pure ethylene flame had a loose structure, while the additions of pentanol isomers made the soot agglomerates more compact and delayed the growth of graphitic structures. The pentanol isomer additions also contributed to a higher disorder of the crystallite arrangement in the soot nanostructure. According to the TGA experiments, the results showed that the addition of pentanol isomers enhanced the oxidation reactivity of soot particles, which could help to reduce soot particle emissions. charge-coupled device (CCD) camera and a reconstruction model to reconstruct the three dimensional fields of temperature and concentration based on a regional reconstruction method. In the past few decades, there have been a large number of experimental and theoretical studies showing that polyacetylene, ionic substances and polycyclic aromatic hydrocarbons (PAHs) may be the gaseous molecular precursors of soot. As the research further developed, PAHs have been accepted as the main precursors of soot particles [1, [4] [5] [6] 10 ]. An early study focusing on the gaseous precursors of particles was performed by Dobbins et al. [11] . They studied the chemical evolution of the combustion in the center of the normal diffusion flame of ethylene by means of laser microprobe mass spectrometry, and found the molecular forms of PAHs in the precursors of the soot particles. Wang et al. [12] studied the growth mechanism and synergistic effect of PAHs in counterflow flames of hydrocarbon fuels and improved the formation mechanism of PAHs. The results showed that a synergistic effect was more obvious in larger PAH molecules, which was proved by experimental observations. The physicochemical properties of soot particles depend largely on the combustion conditions. A recent study found that the blended fuel based on the different fuel physical and chemical characteristics had good emission characteristics and high economic performance [13]. Compared with the pure hydrocarbon fuel combustion, the premixed combustion of the premixed oxygenated biomass fuel showed a beneficial effect on the primary particle emissions [14, 15] . Furthermore, the influence of various additives in diffusion flames on soot formation has been extensively studied [16] [17] [18] . Zelepouga et al. [19] investigated the effect of acetylene and PAHs addition on soot formation in non-premixed, axisymmetric, co-flowing methane flames with oxygen and oxygen-enriched air, and found that all additives promoted soot formation and the PAHs addition was more significant. Oxygenate-containing biofuels, such as bio-alcohols, can be produced from cellulosic biomass through biochemical conversion processes [20] . These bio-alcohols are shown to have an effect on reducing particulate emissions [21, 22] . The effects of ethanol as a fuel additive on the particulate emissions of diesel engines have been widely studied [23] [24] [25] . Venkateswarlu et al. [26] used 2-methyl-1-propanol as an additive for biodiesel and diesel-blended fuel, and found that 2-methyl-1-propanol addition significantly decreased the combustion emissions of CO and soot. Moreover, higher carbon chain bio-alcohols, such as butanols and pentanols, have received much interest. Pentanol has similar physical properties as diesel fuel and higher energy density and cetane number than ethanol and butanol [27] . As a result, pentanol seems to be a more promising fuel additive to reduce soot formation in practical applications. Wei et al. [28] compared the combustion and exhaust characteristics of different mixed proportion (10%~30%) of diesel/pentanol mixed fuels in a four cylinder diesel engine. The results showed that adding pentanol could decrease the mass concentration and number concentration of soot particles. Li et al. [29] revealed that soot emissions decreased obviously with the addition of pentanol to diesel and biodiesel fuels in a single-cylinder direct-injection diesel engine. Rajesh Kumer and Saravanan [30] investigated the effects of blending n-pentanol with diesel on the performance and emission characteristics of a diesel engine with exhaust gas recirculation (EGR), and they found that a simultaneous decrease in smoke emissions and NO x could be attained by the combination of pentanol/diesel blends and a medium EGR rate. Zhu et al. [31] conducted a study on a diesel engine fueled with biodiesel-pentanol blends to obtain the combustion, gaseous and particulate emissions under different engine loads. The results revealed that the particle mass and number concentrations were reduced when pentanol was added to biodiesel. The above review shows that the pentanol additive had an effect on soot reduction and investigations on the application of pentanol have mainly concentrated on diesel engines, especially on its influence on particulate emissions. However, there are very few detailed studies concerning soot nanostructure and oxidation characteristics of nascent soot particles in the combustion of pentanol-doped blended fuels from the basic scientific view and fundamental flame experimental devices. Therefore, the main goal of this study was to investigate the effect of pentanol addition on soot nanostructure and oxidation reactivity based on the fundamental inverse diffusion flame platform
doi:10.3390/en10010122 fatcat:o3nldprkozcghh4vvlgmbbctu4