Study suggests GTL-naphtha-gasoline-ethanol blends can function as well as gasoline with lower emissions
Results of a study by a team from the University of Birmingham (UK) and Shell Global Solutions suggest that blends of gasoline with gas-to-liquids (GTL) naphtha can perform comparable combustion and full power output to conventional gasoline, with less than 2% difference in normalized ISFC (indicated specific fuel consumption) and gaseous emissions similar to, if not lower than that of conventional gasoline. A paper on their study is published in the journal Fuel.
The GTL Fischer-Tropsch process produces GTL diesel (the cleaner combustion and emissions qualities of which have been well studied), GTL naphtha, GTL kerosene, GTL normal Paraffin and GTL base oils. GTL naphtha mainly contains C4 to C11 hydrocarbons with a high proportions of straight chain paraffins. Although it has a consistent quality and near-zero sulfur and heavy metals, GTL naphtha has a low octane rating, making it unsuitable for blending in gasoline. (GTL naphtha currently is used as an alternative high-quality feedstock for plastics.) However, that low octane rating can be addressed by using ethanol as an octane booster.
The introduction of bio-ethanol as a blending component has made the octane rating of GTL naphtha a less limiting factor because ethanol has a high octane rating. However, currently there is little knowledge available about the performance of gasolines containing GTL naphtha in spark ignition engines.
In this study, four gasoline fuels containing up to 23.5 vol.% GTL naphtha, three of which were close to being EN228 compliant, were tested in an AVL state-of-art single cylinder gasoline research engine. … The focus was on the assessment of full load combustion characteristics and emissions of these new gasoline fuels with GTL naphtha.—Wang et al.
The reference fuel was a typical EN228 compliant gasoline; the four test fuels had similar octane rating with Fuel A (the reference). Fuel B contained 7.3 vol.% GTL naphtha but no ethanol. Fuels C-E were blends of various refinery streams, GTL naphtha (12.8–24 vol.%), and ethanol (5–20 vol.%). Fuels B-D were almost EN228 compliant; however Fuel E had an oxygen content of 7.2 wt.%, which exceeded the EN228 upper limit of 3.7 wt.%.
Two modern engine configurations, a boosted direct injection (DI) and a port fuel injection (PFI), were used. The tests were conducted under full load conditions in the engine speed range of 1000–4500 rpm.
In addition to the findings on combustion and gaseous emissions, the researchers also found that compared to the conventional gasoline, lower particulate emissions were observed in gasoline fuels containing up to 15.4 vol.% GTL naphtha and 10 vol.% ethanol.
In this study, due to the limited amount of GTL naphtha available and the time constrain, less than six repeats were conducted for each fuel; therefore, no robust statistical significance analysis can be provided. Additional repeat tests on this engine and further tests on a wider range of engines/vehicles would be required to generalize the validity of these findings.
Chongming Wang, Jasprit Chahal, Andreas Janssen, Roger Cracknell, Hongming Xu (2017) “Investigation of gasoline containing GTL naphtha in a spark ignition engine at full load conditions,” Fuel, Volume 194, Pages 436-447 doi: 10.1016/j.fuel.2017.01.042
Rockstroh, T., Floweday, G., and Wilken, C. (2016) “Options for Use of GTL Naphtha as a Blending Component in Oxygenated Gasoline,” SAE Int. J. Fuels Lubr. 9(1):191-202 doi: 10.4271/2016-01-0879