Particulate matter less than 2.5 microns in diameter (PM2.5) is a target pollutant in the National Ambient Air Quality Standard and has been linked to human health issues. Particulate emissions have the potential to impact both the environment and the health of personnel in and around military installations and equipment powered by gas turbines. Finding methods of reducing or eliminating PM2.5 emissions from gas turbine engines is a high priority within the Department of Defense. The formation of particulate matter during the gas turbine combustion process is related directly to the fuel droplet size entering the combustion zone. The influence of mean droplet size on particulate emissions derives from the manner in which each individual droplet evaporates. Published data shows that droplet size reduction will have a first order effect on PM2.5 reduction. Electrostatic fuel atomization has the ability to generate ultra fine (less than 10 microns) droplet distributions with maximum self-dispersal qualities.
The objective of this project was to develop electrostatic fuel atomization technology to achieve an 80% reduction in PM2.5 emissions from military gas turbine engines.
In an electrostatic fuel atomizer, two submerged electrodes are used to form a self-contained field emission electron gun assembly. The centrally located emitter electrode is positioned immediately upstream of a grounded orifice plate through which the fuel to be atomized exits. When a negative voltage is applied to the emitter, free charge is driven into the exiting fuel. The now charged fuel, once free of the confines of the device, automatically atomizes. Droplet development and dispersion is purely an electrodynamic effect that is unencumbered by variations in fuel properties and flow rate. Aerodynamic or fluid dynamic forces are not involved, nor are they needed. The mean droplet size will be determined solely by the free charge density established within the fuel. Electrostatic atomization enables the electronic control of fuel droplet size and dispersion.
Electrostatic atomizers using an induction charging approach were investigated and characterized. Diesel fuel was successfully charged and atomized up to a flow rate of 250 pounds per hour. Designs that integrate electrostatic atomization technology into a fuel nozzle/injector suitable for use in a gas turbine engine were developed along with a controller. Prototype electrostatic fuel nozzles suitable for engine installation were fabricated and tested in both an ambient and engine test bed. Ambient test results indicated the potential to reduce gaseous emissions with electrostatic charging. Numerical simulations have indicated that electrostatic atomization has the potential to reduce particulate emissions by 80% from current engine levels. Actual engine testing was inconclusive since the nozzles were not run above the engine idle condition long enough to collect emissions.
This project has demonstrated that electrostatic atomization technology could improve fuel spray characteristics. At low to moderate flow rates, electrostatic atomization has a dramatic effect on droplet size and dispersion. At high flow rates, these effects are less pronounced. However, at high flow rates electrostatics continue to improve overall atomization through improved secondary droplet breakup due to the charge on the fuel droplets. The atomization charging process is simple, rugged, compact, and low cost, thereby enabling this technology to be retrofitted to existing gas turbine engines. The insensitivity to flow rates and fuel properties offers maximum technology flexibility within the gas turbine community. Although the engine test results were inconclusive, the potential of electrostatic charging in the fuel ejection process offers promise with respect to enhancing combustion performance while reducing emissions, particularly particulate emissions. (Project Completed – 2006)