Low NOx Combustion for Liquid Fuels: Atmospheric Pressure Experiments Using a Staged Prevaporizer-Premixer

Abstract

Low emissions of NOx are obtained for a wide range of liquid fuels by using a staged prevaporizing-premixing injector. The injector relies on two stages of air temperature and fires into a laboratory jet-stirred reactor operated at atmospheric pressure and nominal ϕ of 0.6. The liquid fuels burned are methanol, normal alkanes from pentane to hexadecane, benzene, toluene, two grades of light naphtha, and four grades of No. 2 diesel fuel. Additionally, natural gas, ethane, and industrial propane are burned. For experiments conducted for 1790 K combustion temperature and 2.3±0.1 ms combustion residence time, the NOx (adjusted to 15% O2 dry) varies from a low of 3.5 ppmv for methanol to a high of 11.5 ppmv for No. 2 diesel fuel. For the most part, the NOx and CO are positively correlated with the fuel carbon to hydrogen ratio (C/H). Chemical kinetic modeling suggests the increase in NOx with C/H ratio is caused in significant part by the increasing superequilibrium concentrations of O-atom created by the increasing levels of CO burning in the jet-stirred reactor. Fuel bound nitrogen also contributes NOx for the burning of the diesel fuel. This paper describes the staged prevaporizing-premixing injector, the examination of the injector, and the NOx and CO measurements and their interpretation. Optical measurements, using beams of He-Ne laser radiation passed across the outlet stream of the injector, indicate complete vaporization and a small variation in the cross-stream averaged fuel/air ratio. The latter is determined by measuring the standard deviation and mean of the transmission of the laser beam passed through the stream. Additional measurements and inspections indicate no pressure oscillations within the injector and no gum and carbon deposition. Thus, the NOx and CO measurements are obtained for fully vaporized, well premixed conditions devoid of preflame reactions within the injector.