Comparison of particulate-matter emissions from liquid-fueled pool fires and fire whirls

Abstract

In-situ burning (ISB) is one of the most effective means of removing oil spilled over open water. While current ISB practices can eliminate a large fraction of the spilled oil, they still result in significant airborne emissions of particulate matter. ISBs are classified as large, free-buoyant pool fires, from which black smoke consisting of particulate matter (PM, soot) emanates as a plume. An experimental investigation of soot emissions from pool fires (PF) and fire whirls (FW) was conducted using liquid hydrocarbon fuels, nheptane and Alaska North Slope (ANS) crude oil, in fuel pools 10 − 70 cm in diameter. Burning attributes such as burning rate, fuel-consumption efficiency, and emissions of PM, unburned hydrocarbons, carbon dioxide, and oxygen consumption were measured. For both fuels and all pool diameters, compared to PFs, FWs consumed fuel at a higher rate, had lower post-combustion residual mass and PM emission rates. Collectively, these resulted in consistently lower PM emission factors (EF PM ) for FWs at all scales. For FWs, EF PM decreased linearly with a nondimensional quantity defined as the ratio of inverse Rossby number to nondimensional heat-release rate. These results show that the addition of ambient circulation to free-burning PFs to form FWs can increase burning efficiency, reducing both burning duration and EF PM across length scales. The reduction in EF PM with increasing influence of circulation is attributed to a feedback loop of higher temperatures, heat feedback, burning rate and air-entrainment velocity, which in turn contributes to maintaining the structure of a FW. Boilover was observed for fires formed with ANS crude oil at the 70 cm scale, although the overall EF PM was not affected significantly. This investigation presents a foundation to evaluate the detailed mechanisms further, such that appropriate configurations can be developed help minimize the environmental impact of ISBs.

Effects of Natural and Forced Entrainment on PM Emissions from Fire Whirls

“…The value of Γ for NE does not vary much with W because of the logarithmic dependence on . The nondimensional heat-release rate, *, and nondimensional circulation, Γ*, were estimated as ,, …”

“…Comparing these quantities based on scaling analyses in the literature, , the ratio */Γ* is shown in Figure . In general, the quantity */Γ* is generally much lower for fire whirls under FE than NE.…”

“…The higher entrainment conditions (FE-2 and FE-3) result in a higher burning rate, which also results in higher heat flux feedback to the fuel surface. A consequence of this is that the water layer immediately below the fuel slick can vaporize suddenly, resulting in a condition called boilover. , The rise in the temperature of water immediately below the fuel slick is shown in Figure . The onset times of boilover for the different conditions are shown in Table .…”

“…For the FE conditions, U θ,avg was then used to estimate circulation as Γ = 4SU θ,avg . For NE, the value of circulation was estimated from the expression Γ = 0.81ln­( Q̇ )–0.75, where Γ is in [m 2 /s] and Q̇ is the heat-release rate in [kW] …”

“…Recent investigations − have shown that fire whirls, when compared to pool fires, emit lower quantities of particulate matter per unit mass of fuel burned. The objective of the present study is to investigate the effect of different air entrainment conditions in fire whirls on the emission of PM.…”