Optimized Combustion of GTL Diesel Can Reduce Emissions, Improve Thermal Efficiency
18 October 2006
|NOx, BSN (Bosch Smoke Number), brake specific fuel consumption (BSFC), and thermal efficiency for diesel, GTL, GTL-diesel 50-50 blend, and GTL with advanced injection timing. Click to enlarge.|
In a comparative study of Gas-to-Liquids (GTL) diesel and conventional Ultra Low-Sulfur Diesel (ULSD) (and blends), researchers at the University of Birmingham (UK) and Brunel University (UK) have found that the optimized use of GTL and GTL blends can reduce NOx and PM emissions simultaneously while providing improvements in engine thermal efficiency.
In a paper just published in Energy & Fuels, the team also describes their finding that the use of GTL with exhaust-gas reforming—passing the exhaust gas through a minireformer installed at the EGR to generate hydrogen for injection into the fuel charge—increased fuel conversion to hydrogen and reduced methane production.
The researchers used a single-cylinder direct-injection diesel engine with a pump-line-nozzle-type fuel injection system. (Results may vary for engines equipped with common rail or unit injection systems.) The fuels used were ULSD and GTL provided by Shell Global Solutions, UK, and a 50-50 blend of GTL and ULSD.
The team found that use of GTL with unoptimized injection timing resulted in a significant reduction of NOx, but with an increase in smoke emissions due to shorter premixed combustion and longer diffusion combustion—especially at high loads.
Use of the 50-50 blend in the same unoptimized engine resulted in reduction in smoke without significant changes in the NOx values. Both neat GTL and the blend showed enhanced fuel economy on a gravimetric basis and better engine thermal efficiency.
However, by advancing the injection by 4° crank angle and allowing the combustion of GTL to shift to earlier stages, they found that both NOx and smoke emissions decreased simultaneously, while improving engine thermal efficiency compared to the ULSD.
The reduction of the premixed combustion phase in the case of GTL fuel allows injection timing to be advanced, which results in improved engine efficiency while still maintaining NOx and combustion noise at low levels. In the case of ULSD, advancing the injection timing can lead to improved engine thermal efficiency but this will result in the considerable increase of NOx emissions and engine combustion noise. At the lowest load condition...the results suggest that the same engine power could be generated by 10% less fuel energy when GTL is employed. This benefit of GTL has been overlooked in a recent influential well-to-wheels study [CONCAWE, earlier post].
|Major components of the REGR (reforming and exhaust-gas recirculation) system. Click to enlarge.|
The use of the REGR (reforming and exhaust-gas recirculation) system, which puts a small minireformer inline with the EGR path, is part of a separate, ongoing study. The reformer is a largely passive, adiabatic reactor, and must be able to function under variable feed rates and inlet temperatures with minimal external control.
In using the REGR, the researchers found that GTL exhaust delivered more hydrogen with greater process efficiency than the ULSD. As a further benefit in such an application, GTL’s sulfur- and aromatic-free properties can also increase the long-term performance and durability of the catalyst.
The fuel properties (e.g., density, viscosity, CN, aromatic HC) and hence combustion of the new fuels such as GTL and biofuels are different from conventional fuels, and this will affect the engine performance and emissions. Similarly, the catalyst and reformer design have to be optimized according to the specifications of the new fuels in order to value the beneficial effects.
For this reason, and those related to the production of small volumes, these fuels will be likely to be used as blends with conventional fuels. Although, modern diesel engine injection systems such as common rail (CR) are less sensitive to fuel properties (e.g., CN) compared to pump-line-nozzle injection systems, engine tuning will be required in order to accomplish the highest improvements in engine performance.
“Effect of Gas-to-Liquid Diesel Fuels on Combustion Characteristics, Engine Emissions, and Exhaust Gas Fuel Reforming. Comparative Study”; A. Abu-Jrai, A. Tsolakis, K. Theinnoi, R. Cracknell, A. Megaritis, M. L. Wyszynski, and S. E. Golunski; Energy Fuels, ASAP Article 10.1021/ef060332a S0887-0624(06)00332-X
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