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Study finds blending gasoline with GTL improves efficiency and lowers emissions in compression-ignition PPC mode

A team from Eindhoven University of Technology, The Netherlands, in collaboration with Bengt Johansson at King Abdullah University of Science and Technology, Saudi Arabia, reports that blending gasoline with high cetane number (CN) gas-to-liquids (GTL) fuel appears to be a promising route to improve not only efficiency, but also PM and NOx emissions, particularly when operating in partially premixed combustion (PPC) mode.

Interestingly, they observed, this benefit is notwithstanding the high aromaticity of the gasoline compared to GTL. In open access paper on the work is published in the journal Fuel, they suggest that this “GTLine” approach promises to be a cost-effective way to accommodate GTL in a world in which low temperature combustion concepts, such as PPC, appear to be really taking off.

PPC can be defined as a combustion process in a compression ignition engine, whereby a temporal separation exists between the fuel injection and combustion events, thereby implying that a long ignition delay (ID) is the single most important prerequisite. A shared conclusion found in all these studies is that PPC is most readily realized with a low CN fuel, else prohibitively high EGR rates are likely necessary to adequately extend the ID.

In our previous work, we evaluated the performance of two Fischer–Tropsch (FT) GTL diesel fuels with CNs of 52 and 72 in various combustion modes from high (i.e., conventional diffusion) to low (i.e., PPC) temperature regimes. The results demonstrated that, at equal fuel aromaticity, a high CN yields no discernible benefit in terms of PM-NOx trade-off in high temperature combustion modes and even becomes a serious handicap when transitioning into the low temperature regime.

… While future GTL plant owners might take notice of aforementioned insights, producers of high CN GTL diesel must necessarily add low CN compounds in order to decrease the CN. … A well known approach to lower the CN of diesel fuel in general is to blend it with low CN fossil or biofuels. … one practical and relatively low cost fuel looks particularly promising, gasoline. Commonly referred to as Dieseline in literature, this mix of fossil fuels already has a proven track record as a PPC fuel. What is more, there exist, in Europe at least, a surplus of gasoline, thereby partially compensating for both the limited availability of GTL as well as its relatively high cost price.

—Reijnders et al.

The team carried out its experiments on a modified 12.6 liter HDDI DAF XE 355C in-line six diesel engine. The first cylinder is used as a test cylinder and is equipped with a dedicated in- and outlet manifold, EGR circuit and fuel injection system. Cylinders 2 and 3 have no function, while 4-6, operating on the stock engine control unit (ECU) and conventional fuel, were responsible only for setting and maintaining the desired engine speed.

The team used five fuels: neat GTL, three GTLine blends (e.g., 10, 20 and 50 vol.% gasoline), and EN590 diesel as a reference fuel.

All tests were conducted with a single injection per cycle, at a fixed engine speed of 1200 rpm, and a constant engine oil and coolant temperatures of 90 and 85 °C, respectively. For all fuels, EGR rates were varied from 0 to 50%.

Based on the experimental data, the team concluded:

  • The higher the CN, the higher the amount of EGR required to sufficiently extend the ID into PPC territory and achieve associated low PM-NOx emissions, whereby the ignition dwell should be somewhat greater than zero.

  • A CN higher than 60 with EGR rates above 30–40 wt% have a negative impact on indicated thermal efficiency, most likely as a result of incomplete combustion.

  • Blending in relatively low cost and abundantly available gasoline improves the efficiency and emissions behavior of GTL when operating in PPC.

Resources

  • Jos Reijnders, Michael Boot, Bengt Johansson, Philip de Goey (2018) “GTLine – Gasoline as a potential CN suppressant for GTL,” Fuel, Volume 222, Pages 278-286 doi: 10.1016/j.fuel.2018.02.111

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