Researchers explore catalytic partial oxidation reformation of diesel, gasoline, and natural gas for “single-fuel RCCI”
Researchers at Stony Brook University, with colleagues from The City College of New Tyork, Alloy Surfaces and Innoveering, explored the catalytic partial oxidation (CPOX) reforming of three potential transportation-relevant fuels—gasoline, diesel, and natural gas—for use in low-temperature combustion (LTC) engines. They report their results in a paper in the journal Fuel.
Low Temperature Combustion (LTC) strategies have been researched extensively in recent years due to their potential to achieve high thermal efficiencies while producing significantly lower NOx and soot emissions compared to conventional combustion modes. Although they have the aforementioned advantages, most LTC strategies, such as Homogeneous Charge Compression Ignition (HCCI) or Premixed Charge Compression Ignition (PCCI), also have drawbacks; namely, the lack of a direct control over combustion. To address this issue, Reactivity Controlled Compression Ignition (RCCI) was introduced.
In RCCI, a low reactivity fuel is premixed with air while a high reactivity fuel is direct injected into the combustion chamber during the compression stroke, introducing a reactivity gradient within the combustion chamber. Combustion can be directly controlled by varying the ratio of the two fuels and/or adjusting the injection timing of the high reactivity fuel. In RCCI, gasoline and diesel are the most commonly used low and high reactivity fuels, respectively. Although RCCI addresses the issue of combustion control, the requirement of two separate fuel systems is a significant shortcoming. This has led to a specific adaptation technology—“single-fuel RCCI” where the potential to enable RCCI combustion from a single parent fuel is being investigated.
… “single-fuel RCCI” can be enabled from a single parent fuel with onboard fuel reformation. In this variant of single-fuel RCCI, a branch of the fuel stream would be fed to the engine unaltered, while a separate branch of the fuel stream would be reformed using an onboard fuel reformer. The fuel reformer would react the parent fuel, changing its composition, and creating a new fuel stream whose chemical composition and autoignition properties are distinct from the parent fuel. … With the parent fuel and the reformate, it is possible to enable single-fuel RCCI through onboard fuel reformation. However, the composition, properties, and energy balance of the fuel reformer needs to be precisely quantified. This concept is very attractive due to its ability to enable RCCI combustion from a single parent fuel and therefore is a new, active topic of research.
This paper aims to provide new knowledge concerning the reforming process and the reformate mixtures that are produced through the catalytic partial oxidation of diesel, gasoline, and natural gas, as they relate to onboard reforming strategies for transportation applications.—Hariharan et al.
The researchers performed reformation at low and high pressure levels for each parent fuel for equivalence ratios ranging from 3.7 to 7.6. They then compared the composition of the reformation products and the energy released during reformation.
Among the findings:
For natural gas, methane conversion during reformation at low- pressure is higher than at high-pressure. H2 and CO formation are inversely proportional to the equivalence ratio. The energy released in natural gas reformation decreases as equivalence ratio increases.
During the low-pressure reformation, oxidation of gasoline is inversely proportional to the equivalence ratio. During the high- pressure reformation, oxidation of gasoline is near constant. The energy released during the low-pressure reformation of gasoline increases as equivalence ratio decreases.
The oxidation of diesel is constant, regardless of equivalence ratio or pressure. This results in a relatively constant fraction of energy re- leased during reformation, irrespective of pressure and equivalence 4. ratio, possibly due to the low temperature chemistry of diesel fuel.
The team then selected two reformate fuels for each parent fuel and tested combustion characteristics in HCCI combustion. The researchers concluded:
The lower heating value of each selected reformate fuel was significantly lower than their parent fuel due to the diluents associated with a CPOX process (N2, CO2, and H2O). These diluents then affected the peak bulk temperature, NOx, and CO emissions consistent with the known trends of LTC.
In both the gasoline and diesel reformates, the peak heat release rate increases when a fraction of N2 concentration in the reformate fuel is replaced by CO. The required intake temperature subsequently decreased to maintain the combustion phasing.
However, in the natural gas reformates, when a fraction of N2 is replaced by CH4, the required intake temperature increased due to the high-octane rating of methane.
Unlike some of the parent fuels, none of the reformate fuels show any low- or intermediate-temperature heat release. Instead, they all underwent single-stage heat release.
The effective octane rating of the six reformate fuels was determined by their intake temperature requirement to achieve autoignition. They were all similar and they all were slightly harder to autoignite than gasoline or iso-octane, but significantly easier than natural gas.
Deivanayagam Hariharan, Ruinan Yang, Yingcong Zhou, Brian Gainey, Sotirios Mamalis, Robyn E. Smith, Michael A. Lugo-Pimentel, Marco J. Castaldi, Rajinder Gill, Andrew Davis, Dean Modroukas, Benjamin Lawler (2019) “Catalytic partial oxidation reformation of diesel, gasoline, and natural gas for use in low temperature combustion engines,” Fuel Volume 246, Pages 295-307 doi: 10.1016/j.fuel.2019.02.003