It is estimated that U.S. military aircraft emit approximately 600,000 kilograms of particulate matter into the atmosphere each year. Most of this particulate matter is in the form of soot particles with diameters less than 2.5 microns (PM2.5). A growing body of evidence suggests that these small airborne particles pose both health and environmental risks. The National Ambient Air Quality Standards have a health-based regulation for particulate matter with diameters less than 10 microns (PM10). The Environmental Protection Agency (EPA) has since revised the PM10 regulation to include PM2.5 particles (see EPA Fact Sheet dated July 16, 1997). This joint university-government-industry project sought to reduce the PM2.5 emissions from military gas turbine engines in aircraft, helicopters, ships, and tanks using fuel additives.
The objective of this project was to develop a fundamental understanding of the complex interactions of nonmetallic fuel additives with the processes that lead to PM emissions from military gas turbine engines and to use that fundamental understanding to select and investigate the most promising additives for reducing PM emissions.
The use of fuel additives is a cost-effective approach that has the potential to reduce PM2.5 emissions in all engines of the fleet. However, the development of a PM2.5 emissions reduction additive poses a major challenge due to the complexity of the particulate formation process in gas turbine combustors. To simplify the problem, different laboratory burners were used to simulate the soot formation and burnout regions in the gas turbine combustor. These laboratory burners were used to study and evaluate the PM2.5 emissions reduction potential of additives. Fundamental experiments were conducted to provide insight into additive mechanisms for reducing PM2.5 emissions.
This project represents the largest and most significant coordinated effort yet attempted to understand soot formation and fuel-based pathways to reducing it. The methodologies developed led to a number of significant insights into the fundamental aspects of soot formation. The project demonstrated the value of tightly coupled experiments and modeling; unfortunately, it also showed that the lack of validated detailed kinetic models is a major stumbling block for such work. A major contribution to the soot modeling community is the large database, documented in this project’s final report and other publications, that provides significant opportunities to validate soot models.
Use of fuel additives has the potential to lead to significant emissions reduction, which is related directly to the amount of fuel consumed by a gas turbine engine. The fundamental insights regarding soot formation generated in this project, however, have not led to the discovery of a nonmetallic compound that is truly an additive (i.e., one that functions at 1,000 ppm or less in the fuel). Thus, it appears that metal additives remain the only known pathway to a true additive for reduction of PM from military engines. These additives have been shown to be effective at reducing soot in gas turbine and diesel engines as well as in many fundamental studies. However, the potential adverse effects of metal additives on the engines and the potential health effects make the use of metals a high-risk path. (Project Completed – 2006)