Biodiesel combustion characteristics under reacting swirl spray condition

Abstract

Biodiesel portrays plenty of positive combustion properties, which presents itself as an essential alternative fuel to conventional fossil diesel. However, most of the reported works thus far are for automotive applications in internal combustion engines, with little being relevant for gas turbine applications. Therefore, in this study, biodiesel swirl flames are established using a model gas turbine swirl burner under atmospheric condition. The tested biodiesels are produced from palm, soybean, and coconut oils via transesterification process, while diesel is chosen as the baseline fuel. For neat biodiesel investigations, the flame spectroscopic analysis conducted shows that biodiesel swirl flames emit higher OH (Hydroxyl), CH (Carbyne), C2 (Diatomic Carbon) and CN (Cyanido) radical intensities when compared with baseline diesel. It is also observed that decreasing main air temperature results in substantial nitric oxide (NO) reduction at the expense of higher undesirable emissions of carbon monoxide (CO) for all types of fuels tested. The NO emissions from the tested biodiesels are also found to be approximately 20-60% higher than that of diesel from equivalence ratio (?) 0.65-0.9 at main air temperature of 250 °C. Besides this, the tested biodiesels showed higher CO emission than diesel at near stoichiometric combustion. Present research also unveils that biodiesels enhance flame stability as compared to diesel under lean combustion condition. Besides these, the study also investigated the potential use of biodiesels, being blended with natural gas (NG), under dual fuel combustion system. Introducing NG into palm biodiesel (PME) swirl flame at 90/10, 80/20 and 70/30 PME/NG, the obtained input thermal power proportion using the gas turbine swirl burner results in spectroscopic characteristic that resembles neat PME. However, radical emission intensities from PME/NG swirl flames are found to be typically higher than that of neat PME. 70/30 PME/NG combustions are observed to lower NO emission by a factor of 2-3.5 as compared to diesel and neat PME. Nonetheless, it is also found that the addition of NG reduces flame extinction limit when compared with neat PME and diesel. Finally, the effects of swirl angle variation on PME and PME/NG combustion characteristics were also examined. Increasing swirl angle from 45° to 60° lowers the NO and CO emissions by a factor of roughly 4. Conversely, NO and CO emissions increase by a factor of averaging 1.5-2 as swirl angle reduces from 45° to 30°. Novel empirical models are proposed for estimating NO and CO emissions from PME and PME/NG combustion at different NG proportions and swirl angles. Through this study, it is observed that highly unsaturated biodiesels show NO emission that is averaging 3.1 g/kWh higher than saturated biodiesels. Biodiesel flames also generally exhibit bluish flames, whereas diesel contains aromatics rings that lead to the production of sooty luminous orange-yellow flame brush. The sooty flame brush, however, vanishes when swirl angle increases from 45° to 60°. This research shows that PME/NG combustion using swirl vane angle 60° and 30% NG proportion is a promising way of reducing NO emission against neat PME. Biodiesels and biodiesel/NG can be viable alternative fuels for land-based gas turbine industrial applications, operating under lean combustion mode.