Summary
The worldwide demand of fuels has been intensified in recent years and is expected to continue growing. To satisfy these energy requirements and diversify the source of fuels, the energy industry has to face the challenge of using alternative feedstocks in order to produce transportation fuels like jet or diesel. Direct Coal Liquefaction (DCL) process enables liquid yields higher than indirect liquefaction via Syngas and FT process (typically 3.5 bbl/T coal for the best available DCL processes compared to 2,5 bbl/T coal for indirect process, on a dry ash free basis including hydrogen production and upgrading in both cases). In order to obtain high quality transportation fuels, raw coal liquid effluents derived from Direct Coal Liquefaction need to be severely upgraded using hydrotreating and hydrocracking.
This work focuses on the characterization of physical and chemical properties and composition of jet fuel cuts obtained by DCL before and after hydroprocessing, and shows that high quality jet product could be obtained using appropriate hydrocracking conditions.
Introduction
The worldwide demand of fuels has been intensified in recent years and is expected to continue growing. To satisfy these energy requirements and diversify the source of fuels, the energy industry has to face the challenge of using alternative feedstocks in order to produce transportation fuels like jet or diesel. Direct Coal Liquefaction (DCL) process, such as Axens H-CoalTS process, enables liquid yields higher than indirect liquefaction via Syngas and FT process (typically 3.5 bbl/T coal for the best available DCL processes compared to 2,5 bbl/T coal for indirect process, on a dry ash free basis including hydrogen production and upgrading in both cases).
This work focuses on the characterization of physical and chemical properties and composition of jet fuel cuts obtained by DCL before and after hydroprocessing, and shows that high quality jet product could be obtained using appropriate hydrocracking conditions.
Materials and Methods
A full boiling range effluent (C5-450°C) from DCL was separated into different distillation fractions which were then characterized using complementary analytical techniques.
In our work, the upgrading of direct coal liquefaction effluents was carried out under severe hydrocracking (HCK) operating conditions, in a high-pressure fixed-bed reactor, under a total pressure higher than 100 bar and at relatively moderate temperatures. The liquid hourly space velocities (LHSV) are among the lower ones of HCK processes. The catalyst used in these experiments was a base metal hydrotreating / hydrocracking catalyst which was sulfided in situ. A comparative experiment in high pressure hydrotreating conditions was also performed.
After reaction, effluents were cooled, condensed and separated into a gas phase and a liquid phase. Liquid phase effluents were analyzed by means of a physical distillation (ASTM D2892) and characterized using standard petroleum analysis and also using a multidimensional gas chromatography device (2D-GC or GCxGC) equipped with a flame-ionization detector and cryogenic system[1].
Results and Discussion
The chemical composition of raw DCL effluent, including kerosene and diesel fractions, was found to be much more complex compared to conventional petroleum fractions.
Tableau 1 : Raw Direct Coal Liquefaction effluent characterisation
Tableau 2 : Characterisation of Kerosene and Diesel cuts obtained by DCL
Compared to crude oil cuts, products obtained from direct coal liquefaction are highly condensed (polynaphthenes and polyaromatics), with a very low amount of paraffinic compounds, resulting in poor combustion properties. The heteroelements analyses are also unusual with very high nitrogen and oxygen contents but low sulfur content. In order to achieve the specifications of transportation fuels, these products have to undergo a further purification and upgrading treatment that can be performed in a hydrotreatment / hydrocracking process at very high severities.
2D-GC is a powerful analytical tool which give quantitative chemical families analyses. 2D-GC is now widely used in IFP New Energy for naphtha[2], kerosene, diesel[3,6,8,9] or vacuum distillate[4,5,7,10] cuts characterization. In our study, 2D-GC was used to optimize upgrading conditions in regards to petroleum analyses, by understanding structure changes that are occurring during hydrotreating and hydrocracking.
Figure 1 : GCxGC analysis of 200-250°C cut obtained by DCL
Figure 2 : GCxGC analysis of 200-250°C cut after Hydrotreating DCL effluent
Tableau 3 : GCxGC analysis of kerosene cuts before and after hydrotrating
Hydrotreating only allows partial aromatics hydrogenation resulting in a slight density decrease. Mono-aromatics and tri-naphthenes content remains high while paraffins content is very low which does not result in a significant smoke point improvement with a typical value to be around 17 mm for a hydrotreated kerosene.
Tableau 4 : GCxGC analysis of 200-250°C cut after Hydrocracking DCL effluent
Tableau 5 : 2D-GC analysis of kerosene cuts before and after hydrocracking
Hydrocracking allows aromatics hydrogenation which results in naphthenes content increase. The structure of hydrocracked effluent is highly poly-naphthenic but the amount of tri-naphthenes is reduced when compared to a hydrotreated product. The decreases of tri-naphthenes can be explained by ring opening reactions that occur during hydrocracking. The paraffin content increases when the initial boiling point decreases.
Hydrocracking reactions allow a significant density increase of the kerosene cut. However, with typical cut points for kerosene such as 180-250°C, specific gravity remains slightly higher than the upper limit of specific gravity specification of 0.84. This is related to the high naphthenes content as these compounds have important densities compared to the corresponding paraffins with the same carbon atoms number. However, the density can be decreased by incorporating some heavy naphtha in the kerosene pool. This operation is not detrimental to the kerosene quality and particularly the flash point which remains around 50-55°C which is well above the Jet A1 specifications of 38°C.
Tableau 6 : Petroleum analyses of kerosene cut after hydrocracking
Petroleum analyses showed that the kerosene cut complies with all Jet A1 or JP-8 specifications, i.e. density, flash point, freezing point, viscosity, smoke point, specific energy, lubricity, thermal stability.
Aromatics content after HCK is very low (less than 10 % wt, typically less than 5%), nitrogen and sulfur levels after HCK are extremely low (less than 1 ppm wt).
With an initial boiling point around 135-150°C, the flash point is in the range 50-55°C and well above the specification which allow the refiner to have more flexibility between naphtha and kerosene production.
The most severe hydrocracking conditions allow to obtain a kerosene that exhibits good combustion properties. Typical smoke point values are around 22-24 mm for a kerosene obtained after hydrocracking whereas the smoke point of the kerosene cut before upgrading is only in the range of 12-16 mm.
In addition, Cetane Number of kerosene after hydrocracking is typically around 35-40 which allows this cut to be partially sent to the diesel pool. The diesel cut obtained by hydrocraking a DCL effluent has been previously studied[11,12] and also exhibits good combustion properties with Cetane Number higher than 50.
The mainly naphthenic structure and the related density presents the advantage of a significantly higher volumetric heat of combustion (+ 5%) when a compared to the average value calculated from a World fuel sampling program[13]. This results in a lower fuel consumption and a higher autonomy of jets.
Conclusions
Detailed molecular analysis is mandatory to explain the good combustion qualities of Hydrocracked DCL products. Comprehensive 2D-GC is a powerful analytical tool which give quantitative chemical families analyses.
Taking into account the good smoke point, it seems that naphthenes structure found in hydrocracked DCL kerosene can exhibit good combustion properties contrary to what is generally admitted for petroleum fractions.
Kerosene cut obtained after DCL effluent hydrocracking complies with all Jet A1 or JP-8 specifications.
Direct Coal Liquefaction in conjunction with optimized hydrocracking can therefore be considered as a key process to provide alternative ultra low sulfur transportation fuels with excellent cold properties, high volumetric heat of combustion increasing the autonomy and good combustion characteristics, with an optimized high liquid yield on coal.
Source: Bret Strogen - University of California, Berkeley
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