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Energy & Fuels 2007, 21, 2846-2852
Performance Synergies between Low-Temperature and High-Temperature Fischer-Tropsch Diesel Blends Delanie Lamprecht* Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, P.O. Box 1, Sasolburg 1947, South Africa
Luis P. Dancuart Sasol Technology (Pty) Ltd, P.O. Box 1, Sasolburg 1947, South Africa
Kaveer Harrilall Sasol Technology Fuels Research, P.O. Box 1, Sasolburg 1947, South Africa ReceiVed March 12, 2007. ReVised Manuscript ReceiVed June 21, 2007
The highly paraffinic related fuel properties such as high H:C ratio, high cetane number, and low density together with the virtually zero-sulfur and very low aromatics content of low-temperature Fischer-Tropsch (LTFT) derived synthetic diesel contribute to its clean combustion performance. The low density and low aromatic content however results in LTFT diesel having a lower volumetric heating value than conventional diesel and elastomer compatibility concerns where the diesel is to be used in a mixed fuel scenario. Blending LTFT diesel with another synthetic derived fuel with similar, good fuel properties, but which contains monoaromatics, such as the coal derived high-temperature Fischer-Tropsch (HTFT) diesel, could unlock potential performance synergies in the fuel properties of such blends. The Fischer-Tropsch (FT) diesel blends have an ultralow sulfur content, high cetane number, and good cold flow properties and are stable under specified storage and oxidizing conditions. With the presence of about 25% mono-aromatics in HTFT diesel, the shrinking nature of swollen elastomer seals typically found in diesel fuel injection systems is less when exposed to the FT blend than when exposed to neat LTFT diesel. The HTFT diesel improves the volumetric heating value and fuel economy of the LTFT diesel in such an FT blend with comparable particular matter and hydrocarbon exhaust emissions. Although the blend with HTFT diesel increases the volumetric heating value, it decreases the specific heating value (mass basis). These alternative diesel fuel blends therefore provide future fuel characteristics that are compatible with current infrastructure and technology.
1. Introduction There has been considerable discussion within the European Union (EU) since the late 1980s on strategies and programs to improve air quality. One area under assessment is that related to the use of transportation fuels such as diesel. The EU passenger vehicle exhaust emission regulations and fuel specifications subsequently became tighter with the 2005 emission limits (EURO 4) for carbon monoxide (CO), hydrocarbon plus oxides of nitrogen (HC + NOx), and particulate matter (PM), being 0.50, 0.30, and 0.025 g/km, respectively.1 Undoubtedly, a fuel with low sulfur and aromatic content would produce lower PM emissions. Although the sulfur content does not influence NOx emissions directly, its elimination from the fuel enables the use of NOx after-treatment methods in new vehicles. The Californian Air Resources Board (CARB) specified diesel and Swedish Environmental Class 1 (EC1) diesel are examples of fuels with a low sulfur and low polycyclic aromatic hydrocarbon (PAH) content that are available in the market. Compared to CARB diesel and Swedish EC1 diesel, low-temperature Fis-
cher-Tropsch (LTFT) diesel, also referred to as gas-to-liquid (GTL) diesel, produced the lowest regulated and unregulated exhaust emissions.2 The comparative study2 was conducted without optimizing the 1999, 5.9 L, turbocharged, direct injection Cummins engine to take advantage of the unique properties of the fuels tested. The Sasol Slurry Phase Distillate (Sasol SPD) LTFT process is a well-known technology in which carbon monoxide and hydrogen are reacted over an iron or cobalt containing catalyst to produce a mixture of straight chain paraffinic hydrocarbons ranging from methane to heavy waxes, with smaller amounts of olefins and oxygenates. The mixture is sometimes referred to as FT syncrude. This LTFT process includes three major processing steps. The first step is reforming which converts natural gas to synthesis gas (H2 and CO) using well-established technology. As an alternative to natural gas, synthesis gas can also be produced by gasification of coal or biomass. These processes are sometimes referred to as coal-to-liquid (CTL) or biomassto-liquid (BTL). While the GTL process term refers to schemes
* E-mail:
[email protected]. Tel.: +27 16 960-4285. Fax: +27 11 522-1009. (1) CONCAWE Motor Vehicle emission regulations and fuel specificationss part 1, summary and annual 1999/2000 update; report no. 1/01, 2001; p 1.
(2) Fanick, E. R.; Schubert, P. F.; Russell, B. J.; Freerks, R. L. Comparison of emission characteristics of conventional, hydrotreated and Fischer-Tropsch diesel fuels in a heavy duty diesel engine. SAE Tech. Pap. Ser. 2001, 2001-01-3519.
10.1021/ef0701283 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/31/2007
Synergies between Fischer-Tropsch Diesel Blends
Energy & Fuels, Vol. 21, No. 5, 2007 2847
based on natural gas, i.e., methane, to obtain the synthesis gas, the quality of the synthetic products is essentially the same once the synthesis conditions and the product workup are defined. The second step is the conversion of synthesis gas into FT syncrude by using the Sasol SPD slurry reactor technology developed and commercialized by Sasol in 1993. Reactors for the production of FT syncrude using the LTFT process include slurry bed or tubular fixed bed reactors, with operating temperatures in the range of 210-260 °C. The LTFT catalyst may comprise active metals such as iron or cobalt. While each catalyst will give its own unique product slate, in all cases the syncrude product contains some waxy, highly paraffinic species. Other products from this unit include a gas stream consisting of light hydrocarbons, a small amount of unconverted synthesis gas and FT reaction water as a byproduct. The syncrude is then upgraded in the third step to middle distillate fuels using a hydroconversion process which, in the case of the Sasol SPD process, is completed using the Chevron isocracking technology. In this step, the heavy waxes are hydrocracked and olefins and oxygenates are hydrogenated while the lighter fractions of the FT syncrude are saturated to paraffins at the same time. The highly paraffinic LTFT diesel that is produced by the Sasol SPD process has a high hydrogen to carbon ratio, low density, and a cetane number greater than 70 and contains ultralow levels of sulfur with an almost zero aromatic hydrocarbon content.3 These excellent fuel properties contribute to the clean combustion of the LTFT diesel but to the loss of volumetric heating value. LTFT diesel exhibits a range of cold flow properties, depending on the degree of hydroprocessing. Similarly to conventional diesel obtained from severe hydrotreatment done to remove sulfur and aromatic components, as is the case with Swedish EC1 diesel,4 the hydroprocessed LTFT diesel has poor lubrication properties. It is now known that it is not the reduction in the sulfur and aromatic compounds of hydroprocessed fuels that reduces fuel lubricity but rather the reduction of other natural surface-active polar compounds in the fuel.5-8 However, LTFT diesel may be dosed with any one of a variety of lubricity improver additives to attain an acceptable level of lubricity performance. Diesel fuel can come into contact with a range of elastomeric materials in fuel injection equipment. The change in the physical properties of elastomers exposed to diesel fuel depends, among other things, on the polarity of the diesel and on the base polymeric units of the elastomer. With the hydrocarbon monomer units of, for example, standard nitrile butadiene (NBR) seals, a copolymer of butadiene and 2-propenenitrile, having almost the same polarity as the diesel fuel, the diesel penetrates the elastomer, causing it to swell. Some elastomers found in fuel injection equipment will therefore swell when in contact with diesel fuel to an extent
associated with the aromatic content of the fuel.9 This has led to a concern over the effectiveness of elastomeric material in the market where more and more environmentally adapted fuel with reduced aromatics is used. An elastomer that has been exposed to high aromatic fuel and then to low aromatic, severely hydrotreated fuel may cause leaching of absorbed aromatics, causing it to shrink. If the elastomer is still pliable, this shrinkage will not cause a leak, but an aged elastomer will loose its elasticity and a leak may occur. It is therefore not the low aromatic hydrocarbon diesel that causes fuel system leaks but the combination of a change from higher to lower aromatics fuel. The above was confirmed with the aging of new NBR in LTFT diesel and US No. 2-D diesel without preconditioning.10 With no change in the fuel environment, the NBR did not shrink. Halogenation or copolymerisation of elastomer seals, for example Viton (a copolymer of hexafluoropropylene and vinylidene fluoride), changes the polarity of the elastomer such that it is more unlikely for the diesel fuel to penetrate the elastomer. The power output for diesel engines with the same injection pump settings would typically improve from 0.4% to 1.6% for every 0.01 kg/L increase in hydrocarbon fuel density.11 Aromatic compounds have a much higher density and volumetric heating value than naphthenes or paraffins with the same carbon number.12 The density of the paraffinic LTFT diesel, which is lower than conventional diesel, implies a lower power output at equivalent volumetric fuel consumption rates. The high-temperature Fischer-Tropsch (HTFT) process then again operates at higher temperatures (310-340 °C) than the LTFT process.13 Known reactor types for the production of syncrude in this case are the circulating bed or the fixed fluidized bed systems, often referred to as the Synthol processes.13 More specifically, the latter is also known as the Sasol Advanced Synthol reactor. The typical catalyst for HTFT synthesis is iron based. This process is completed through various steps which may include natural gas reforming or gasification of coal to produce synthesis gas (H2 and CO). This is followed by the HTFT conversion of synthesis gas. One of the products from this synthesis is an olefinic syncrude fraction, also known as Synthol light oil (SLO).13 Products from the HTFT process have a lower molecular weight than those derived from the LTFT process and, as an additional distinction, contain a higher proportion of unsaturated species. This SLO is fractionated into naphtha and distillate fractions. The distillate fraction of SLO is further hydrotreated and distilled to produce two distillates boiling in the diesel range: a light hydrotreated distillate (also referred to as distillate hydrotreater or DHT diesel) and a distillate selective cracked (DSC) heavy diesel. The HTFT derived DHT diesel, which constitutes about 70 vol % of the HTFT diesel pool produced commercially at Sasol Synfuels in Secunda, South Africa, also contains ultralow sulfur levels and has a cetane number greater than 50 and a density greater than 0.800 kg/L that meets current European national
(3) Schaberg, P. W.; Myburgh, I. S.; Botha, J. J.; Roets, P. N.; Viljoen, C. L.; Dancuart, L. P.; Starr, M. E. Diesel Exhaust Emissions using Sasol Slurry Phase Distillate Process Fuels. SAE Tech. Pap. Ser. 1997, 972898. (4) Mozdzen, E. C.; Wall, S. W.; Byfleet, W. D. The No-Harm Performance of Lubricity Additives for Low Sulphur Diesel Fuels. SAE Tech. Pap. Ser. 1998, 982571. (5) Wei, D.; Spikes, H. A. The lubricity of diesel fuels. Wear 1986, 111, 217-235. (6) Wang, J. C.; Cusamo, C. M. Predicting lubricity of low sulphur diesel fuels. SAE Tech. Pap. Ser. 1995, 952564. (7) Lacey, I. P.; Westbrook, S. R. Diesel fuel lubricity. SAE Tech. Pap. Ser. 1995, 950248. (8) Barbour, R. H.; Rickeard, D. J.; Elliot, N. G. Understanding Diesel Lubricity. SAE Tech. Pap. Ser. 2000, 2000-01-01918.
(9) Cusano, C. M.; Stafford, R. J.; Lucas, J. M. Changes in elastomer swell with diesel fuel composition. SAE Tech. Pap. Ser. 1994, 942017. (10) Morgan, P. M.; Viljoen, C. L.; Roets, P. N.; Schaberg, P. W.; Myburgh, I. S.; Botha, J. J.; Dancuart, L. P. Some comparative Chemical, Physical and Compatibility properties of Sasol Slurry Phase Distillate diesel fuel. SAE Tech. Pap. Ser. 1998, 982488. (11) Heinze, P. Engine performance and emissions with future type diesel fuels. Presented at the Institution of Mechanical Engineer International Conference on Petroleum Based fuels and Automotive Applications, 1986; Paper No. C306/86. (12) Bacha, J.; Blondis, L.; et al. CheVron Diesel Fuels Tech. ReV. 1998, (FTR-2). (13) Dry, M. E. Sasol Fischer-Tropsch Processes. Appl. Catal. 1983, 2, 167-213.
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Figure 1. Block flow diagram of the LTFT and HTFT processes. Table 1. Selected Fuel Properties of LTFT-HTFT DHT Diesel Blends analysis color density at 15 °C distillation IBP T10 T50 T95 FBP flash point viscosity at 40 °C CFPP water sulfur acid number cetane no. oxidation stability bromine number total aromatics HFRR WSD SL BOCLE load
units kg/L °C °C °C °C °C °C cSt °C vol % mg/kg mg KOH/g mg/100 mL g Br/100 g mass % µm g
method
HTFT DHT
15% LTFT
30% LTFT
50% LTFT
70% LTFT
85% LTFT
LTFT
ASTM D1500 ASTM D4052 ASTM D86
1 0.809 184 208 239 363 385 78 2.14 0 0.003 3 0.004 57 0.5 9.4 23.9 547 4400
1 0.803 180 205 242 359 385 74 2.11 -1 0.003 2 0.005 59 0.5 8.2 20.3 549 2800
1 0.797 166 200 242 351 379 72 2.10 -3 0.003 2 0.003 61 0.5 6.7 16.8 552 2800
1 0.789 159 195 243 343 367 66 2.07 -6 0.003
