ARTICLE pubs.acs.org/EF
An Experimental Comparison of the Sooting Behavior of Synthetic Jet Fuels Meghdad Saffaripour, Parham Zabeti,† Mohammadreza Kholghy, and Murray J. Thomson* Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada, M5S 3G8 ABSTRACT: An experimental study is conducted for the laminar, atmospheric pressure, sooting, coflow diffusion flames of prevaporized Jet A-1 and four synthetic jet fuels to compare their sooting characteristics and flame structures. Soot volume fraction, species concentration, and temperature profiles are measured, using the laser extinction measurement method, gas chromatography, and fine wire thermocouples, respectively. To evaluate the sooting tendency of the fuels, their smoke point heights are measured using the ASTM D1322 method and compared. The synthetic jet fuels under investigation are (1) Fully Synthetic Jet Fuel (FSJF), which is a coal-to-liquids (CtL) kerosene plus some coal tar derived material from Sasol, (2) FischerTropsch Synthetic Paraffinic Kerosene (FT-SPK), which is a gas-to-liquids (GtL) product from Shell, (3) SPK plus naphthenic cut (50% by volume), and (4) SPK plus hexanol (20% by volume). The threshold sooting index (TSI) of the proposed surrogate mixtures for the fuels are calculated and compared to the TSI of the real fuels to evaluate the performance of the surrogates in predicting the sooting behavior of the actual fuels. The surrogate mixtures underpredict the sooting tendency of the Jet A-1, FSJF, and SPK + naphthenic cut, although they correctly capture the trend. It is shown that the soot concentration and the TSI values are strongly dependent on the aromatic content of the fuels. Fuels with the largest soot concentrations and sooting indexes, in decreasing order, are Jet A-1, FSJF, SPK + naphthenic cut, FT-SPK, and SPK + hexanol. There are minor differences between the species concentration and temperature profiles of the heavily sooting flames (Jet A-1, FSJF, and SPK + naphthenic cut) and the lightly sooting flames (FT-SPK and SPK + hexanol), caused by the lower contribution of aromatics to the formation of light aliphatic species and the higher radiative heat transfer by soot particles in the heavily sooting flames. It is demonstrated that the soot levels in flames are proportional to benzene concentrations but not to acetylene levels. Thus, only aromatic-based inception models can capture the differences in soot formation for jet fuels.
1. INTRODUCTION With the growing concerns over climate change as well as the price and availability of petroleum-based fuels, the aviation industry is considering the use of alternative fuels for aircraft. Presently, the synthetically produced jet fuels, along with the hydro-processed bioderived oils, are the only alternative fuels that can replace conventional jet fuels with no modifications to the engine design. Synthetic jet fuels are derived from biomass, coal, or natural gas by the FischerTropsch (FT) process. The main properties of synthetic fuels are very similar to conventional fossil fuels, thereby allowing full substitution or high blending.1 A better understanding of the sooting behavior of synthetic aviation fuels is required, because of the substantial role of soot particles in transferring heat to the combustion chamber walls and emission of smoke at the engine’s exhaust. Non-premixed laminar coflow flames include a coupling between transport and chemistry and allow the study of soot formation in a multidimensional structure. Therefore, they are the preferred configuration for studying the sooting characteristics of jet fuels at a fundamental level. In the present work, an experimental study is performed on the non-premixed, laminar, atmospheric pressure, and sooting coflow flames of the following five jet fuels: 1. Jet A-1, which is a widely used petroleum-based fuel 2. Fully Synthetic Jet Fuel or FSJF, a FischerTropsch coal-toliquids (CtL) product plus some coal tar derived material from Sasol 3. FischerTropsch Synthetic Paraffinic Kerosene or FTSPK, a gas-to-liquids (GtL) kerosene from Shell. FT-SPKs can also be derived from biomass. 4. SPK plus naphthenic cut (50% by volume) 5. SPK plus hexanol (20% by volume) r 2011 American Chemical Society
A detailed analysis of the physical and chemical properties of the above synthetic jet fuels was conducted by Pidol et al.2 FSJF is the only fuel to pass all the ASTM D7566 Jet A-1 requirements. It is a fully synthetic jet fuel and consists of 50% synthetic paraffinic kerosene, derived from the FT process, and 50% severely hydrogenated coal tar kerosene. FT-SPK, stemming fully from the FT synthesis, does not pass the Jet A-1 requirements for density, aromatic content, distillation, and thermal stability. However, it complies with the ASTM D7566 criteria for synthetic paraffinic kerosenes and using it as a blending base, with up to 50% by volume, is permitted. The addition of 50% by volume of naphthenic compounds to SPK was also studied. Naphthenic compounds are representative of a sample that comes from a coal or a biomass liquefaction process. As it is very difficult to source this kind of compound, a commercial solvent base was assumed to be representative of such future products. Almost all the properties of SPK + naphthenic cut pass the Jet A-1 requirements, mainly in terms of energy content per unit volume, cold flow properties, minimum aromatic content, and density. Adding oxygenated compounds to the fuel drastically affects the key fuel properties; hence, these compounds cannot be blended in substantial amounts. However, they have a favorable impact on particulate matter emissions. Blending high carbon number alcohols with SPK improves some of the properties such as flash point and water solubility. Detailed characterization results for the candidate fuels are presented in Table 1. Information about other Received: August 10, 2011 Revised: November 7, 2011 Published: November 09, 2011 5584
dx.doi.org/10.1021/ef201219v | Energy Fuels 2011, 25, 5584–5593
Energy & Fuels
ARTICLE
Table 1. Chemical Composition of the Jet Fuels under Study in the Present Worka hydrocarbon groups (wt %) fuel
NP
Jet A-1b
22
14
2
25
5
37
0
10
FT-SPKc SPK + naph.c
28 15
63 30
0 0
