Spray characteristics and combustion performance of unheated and preheated liquid biofuels

Abstract

Recent increases in fuel costs, concerns for global warming, and limited supplies of fossil fuels have prompted wide spread research on renewable liquid biofuels produced domestically from agricultural feedstock. In the present research diesel, Vegetable Oil (VO), two types of biodiesel produced from VO and animal fat are investigated as potential fuels for gas turbines to generate power. Experiments are performed using a laboratory scale burner simulating gas turbine combustor operated at atmospheric pressure. A commercially available air blast (AB) atomizer is used to create the fuel spray. A parametric study of combustion performance (CO and NOx emissions) and spray characteristics (droplet diameter, drop size distribution, and mean and RMS axial velocities) is carried out by varying air to liquid mass ratio (ALR),and fuel inlet temperature in cold spray and spray flame with/without swirl air and without/with enclosure. The problems of high viscosity and poor volatility of VO (soybean oil) were addressed by using diesel-VO blends with up to 30% VO by volume. Gas chromatography/mass spectrometry, thermogravimetric analysis, and density, kinematic viscosity, surface tension and water content measurements are used to characterize the fuel properties. Characteristics of the resulting spray are measured using a laser sheet visualization system and a Phase Doppler Particle Analyzer system (PDPA). However, several operational and durability problems of using straight VO's for direct combustion occur because of their higher viscosity and low volatility compared to diesel fuel. The high kinematic viscosity of vegetable oil (VO) makes it unsuitable for direct combustion using conventional fuel preparation systems. Thus, we preheat the fuel to reduce its kinematic viscosity and to improve fuel atomization. Measurements are obtained for fuel inlet temperature varying from 40 to 100°C and for ALR varying from 2 to 4. Results show that an increase in the fuel inlet temperature decreases NOx and CO emissions, which can be attributed to improved fuel atomization resulting from decreased kinematic viscosity at higher fuel temperatures. Results also show a decrease in Sauter Mean Diameter (SMD) with an increase in VO temperature, regardless of the ALR at any given axial location in the spray. A significant difference in the distributions of mean and root mean square (RMS) axial velocity occurs with an increase in VO inlet temperature for a fixed ALR, presence of swirling air, and presence of flame. In general, the radial profiles show larger droplets distributed towards the edge of the spray and smaller droplets in the interior spray region. Higher VO inlet temperature and higher ALR produced a narrower spray with smaller diameter droplets and higher peak axial velocities. Swirling air flow and of high temperatures in flames facilitates secondary breakup of larger droplets to significantly reduce the SMD. Finally the effect of enclosure is also studied since it represents a more realistic combustor design for any continuous flow system. The insulated enclosure eliminated the ambient air entrainment and minimized hear loss to the ambient air to create a fine spray flame with characteristics similar to those of an open flame.