Subsequent studies by researchers from The University of Birmingham (UK) and specialty chemicals company suggested that DMF is very promising as a new biofuel; not only is the combustion performance similar to commercial gasoline, but the regulated emissions are also comparable. (Earlier post.)

However, the RWTH Aachen and FEV team noted in their paper, the catalytic multiphase reaction sequence developed for that pathway—starting from fructose and yielding 2,5-dimethylfuran after dehydration and subsequent hydrogenolysis/hydrogenation steps—requires several equivalents of hydrogen.

Based on these findings a transformation of fructose to furans without the need of an external source of hydrogen was envisaged. A selective catalytic defunctionalization with homogeneous transition metal catalyst in a multiphase reaction system should enable a hydrogen free pathway to furans. The novel reactions sequence starts with fructose and yields after acid catalyzed dehydration 5-hydroxymethylfurfural.

Subsequent Palladium-catalyzed dehydroxylation in a two-phase reaction system and successive decarbonylation results in 2-methylfuran. This molecule is even more compact than 2,5-dimethylfuran, thus a higher knock resistance can be expected allowing for higher engine efficiencies. The lower boiling temperature is expected to be beneficial with regard to mixture formation, however, may require a pressurized vehicle tank system in order to avoid excessive evaporation under high ambient temperatures.

Both fuels, 2-methylfuran and 2,5-dimethylfuran, are excellent plastic material solvents. Therefore, blending of these molecules to conventional fuel and usage in today’s vehicle fleet is not possible. Consequently the requirement for a pressurized tank system is not necessarily a big disadvantage for 2-methylfuran compared to 2,5-dimethylfuran as usage of any of these two fuels would require a consensus within politics, society and automotive industry in order to develop suitable vehicles prior to the availability of these fuels in the market.

Despite the developed reaction sequence which may enable an efficient and cost effective mass production of 2-methylfuran, there is still little knowledge about the characteristics of this fuel when burned in an internal combustion engine.

—Thewes et al.

The RWTH Aachen and FEV study. The team used RON 95, ethanol and 2-methylfuran in a single-cylinder research engine. The outward-opening direct-injection gasoline injector has a nominal spray cone angle of 90° ± 3° and a maximum needle lift of ~30 µm which is directly induced by a piezo stack. The spark plug is installed between the exhaust valves while the injector is installed between the intake valves. The intake ports are designed as symmetrical high tumble ports.

A volume flow controlled 20 MPa high pressure fuel pump pressurized the fuel. A conventional low pressure fuel pump supplied the RON 95 and ethanol to this high pressure pump. A vessel pressurized by nitrogen is used for 2-methylfuran to eliminate problems in material incompatibility.

Among their findings and conclusions were:

• The initial evaporation of 2-methylfuran is quicker than with ethanol. The lower boiling temperature and high vapor pressure seem to overcompensate the negative impacts of higher viscosity and surface tension. The total evaporation duration is comparable to the other two fuels.

• The burn rates and the resulting combustion duration of 2-methylfuran in warm engine conditions are very similar to the other two fuels as long as the center of combustion is controlled to the optimal value via spark timing adaptation.

• The combustion delay of 2-methylfuran is shorter, especially in cold conditions, requiring retarded spark timings to achieve the same combustion phasing as with ethanol and RON 95 fuel.

• The onset of autoignition when using 2-methylfuran occurs at lower engine loads than for ethanol, however, at higher loads than for RON 95. Moreover the required combustion retardation has a significantly shallower gradient with engine load than when using RON 95. Thus the compression ratio can be raised by more than 3.5 units which enables efficiency improvements ranging from 2.5 % to 9.9 % compared to RON 95.

• A drawback of the combustion of 2-methylfuran is the higher adiabatic flame temperature resulting from the higher CO₂ content in the exhaust gas and leading to higher emissions of NO_x at the same air/fuel-ratio. In lean burn combustion systems the higher enleanment capability of 2-methylfuran compared to RON 95 can overcompensate this, resulting in lower NO_x emissions combined with higher efficiency at the lean burn limit. Therefore 2-methylfuran has a higher potential for a homogeneous lean burn combustion systems than conventional fuel.

• HC emissions of 2-methylfuran are mostly lower compared to RON 95 and ethanol. In comparison to ethanol this is especially pronounced in cold conditions and at high engine loads. Combined with the findings of the optical measurements this indicates that the lower HC emissions could result from a reduced amount of fuel impinged on the liner or contained in quench zones.

• Particles emitted after the combustion of 2-methylfuran under cold boundary conditions representing catalyst heating are lower than for RON 95 by number and also by mass despite for one operation point. However they are not as low as the particle emissions of ethanol. In warm conditions 2-methylfuran tends to emit higher particle emissions by mass indicating that the resulting particles agglomerate to bigger particles.

These results indicate that 2-methylfuran is a promising potential bio-fuel candidate which should be studied in more detail. Future research should be conducted in order to understand the particle formation and agglomeration mechanisms in 2-methylfuran flames. Moreover the effects of EGR on NO_x emissions from the combustion of 2-methylfuran should be studied as this poses the most important measure to reduce these emissions in stoichiometric combustion systems. Measurements of laminar burning velocities or ignition delay times at elevated pressures could broaden the understanding of the observed autoignition and combustion delay behavior in this study.

The future research of the authors will include 2,5-dimethylfuran as a further potential bio-fuel where combustion has not yet been studied in detail at high engine loads and in cold conditions. The investigations will also be extended to controlled auto ignition (CAI) operation in part load. Another focus will be laid on a more detailed characterization of the exhaust gas emission spectrum, especially with regard to potentially toxic hydrocarbon emissions. Assuming that 2-methylfuran can still be considered a promising bio-fuel after all this future research it has to be studied how blends of 2-methylfuran and conventional gasoline influence combustion.

—Thewes et al.

Resources

• Matthias Thewes, Martin Muether, Stefan Pischinger, Matthias Budde, André Brunn, Andreas Sehr, Philipp Adomeit, and Juergen Klankermayer (2011) Analysis of the Impact of 2-Methylfuran on Mixture Formation and Combustion in a Direct-Injection Spark-Ignition Engine. Energy & Fuels. DOI: 10.1021/ef201021a