Species correlation measurements in turbulent flare plumes: considerations for field measurements

Abstract

Abstract. Field measurement of flare emissions in turbulent flare plumes is an important and complex challenge. Incomplete combustion from these processes results in emissions of black carbon, unburnt fuels (methane), CO2, and other pollutants. Many field measurement approaches necessarily assume that combustion species are spatially and/or temporally correlated in the plume, such that simple species ratios can be used to close a carbon balance to calculate species emission factors and flare conversion efficiency. This study examines the veracity of this assumption and the associated implications for measurement uncertainty. A novel tunable diode laser absorption spectroscopy (TDLAS) system is used to measure the correlation between H2O and black carbon (BC) volume fractions in the plumes of a vertical, turbulent, non-premixed, buoyancy-driven lab-scale gas flare. Experiments reveal that instantaneous, path-averaged concentrations of BC and H2O can vary independently and are not necessarily well correlated over short time intervals. The scatter in the BC/H2O ratio along a path through the plume was well beyond that which could be attributed to measurement uncertainty and was asymmetrically distributed about the mean. Consistent with previous field observations, this positive skewness toward higher BC/H2O ratios implies short, localized, and infrequent bursts of high BC production that are not well correlated with H2O. This demonstrates that the common assumption of fixed species ratios is not universally valid, and measurements based on limited samples, short sampling times, and/or limited spatial coverage of the plume could be subject to potentially large added uncertainty. For BC emission measurements, the positive skewness of the BC/H2O ratio also suggests that results from small numbers of samples are more likely to be biased low. However, a bootstrap analysis of the results shows how these issues can be avoided with sufficient sample size and provides initial guidance for creating sampling protocols for future field measurements using analogous path-averaged techniques.

Inefficient and unlit natural gas flares both emit large quantities of methane

“…Although VIIRS provides global flare volume information for flares large enough to be viewed from space, this product does not evaluate flare performance. The operation of individual flares in response to specific environmental and gas composition variables has been evaluated in laboratory and testing facility studies (19)(20)(21)(22)(23)(24)(25), however, only a handful of in situ measurements to characterize performance (i.e., efficiency) have been made of real-world flares (26)(27)(28)(29)(30). A smaller subset of these studies estimate flaring efficiency in terms of CH 4 removal.…”

In Situ Sampling of NOx Emissions from United States Natural Gas Flares Reveals Heavy-Tail Emission Characteristic

“…Gas volumes from flares observable from space are estimated across the globe using measurements from the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument. , Using nitrogen dioxide (NO 2 ) as a proxy for NOx, flaring trends have also been investigated using satellite retrievals in regions such as the Artic, Mexico, and Russia . Investigations into the relationships between flare performance and parameters such as flare design, fuel composition, and crosswinds have been mostly conducted in laboratory and test facilities. − Quantifying NOx flaring emission rates as part of a Texas Commission on Environmental Quality (TCEQ) 2010 study, Torres et al measured NOx formation from industrial flares under low flow conditions (0.1%–0.25% of maximum flow), finding emission differences between steam and air-assisted flares . More recently, Shaw et al characterized flaring efficiencies and NOx emission ratios for 58 plumes from offshore facilities in the North Sea, with the largest from floating production and offloading vessels …”

Efficiency and Emission Rates of Flares in a Turbulent Crosswind

“…BC emission rates from individual flares can vary by scale several orders of magnitude (Conrad and Johnson 2017;Weyant et al 2016) and likely vary strongly with operating conditions and flare gas composition. In addition, differences in estimated magnitudes of emissions rates among studies may be at least partially related to under sampling in techniques relying on extractive samples from the plume (Seymour and Johnson 2021). However, it seems likely that BC emissions account for less than 0.5% of the total carbon mass in the fuel for propanefueled flares (Torres et al 2012b;Ellzey et al 1990;Pohl et al 1986) and < 0.1% for flares burning methane-dominated fuel mixtures (McEwen and Johnson 2012).…”

“…McEwen and Johnson (2012), citing other authors, suggested that efficiency measurements using single point sampling of an inhomogeneous flare plume may be challenged to accurately represent overall combustion efficiency of vertical, pilot scale flares (Pohl et al 1986) and laboratory scale flares in a crosswind (Howell 2004;Poudenx 2000). Recent experiments and analysis by Seymour and Johnson (2021) have shown that H2O and BC volume fractions can independently vary within in a turbulent plume, such that single point sampling methods relying on fixed ratios of species to complete a mass balance can be prone to error if sampling periods are short and/or sample numbers are small.…”

“…Quantitative understanding of flare performance over the broad range of operating conditions encountered in the field is crucial from an environmental and regulatory perspective.Although many different approaches for measuring flare emissions have been attempted, including remote optical techniques(Zeng et al 2016;Wormhoudt et al 2012;Spellicy et al 2006;Yang and Chang 2006), aircraft sampling techniques(Gvakharia et al 2017;Caulton et al 2014), extractive probe sampling from full-sized flare plumes(Herndon et al 2012;Torres et al 2012b; (especially the influence of crosswind) remain elusive. For example, in airborne measurements of flares in North Dakota, Caulton et al(Caulton et al 2014) measured destruction efficiencies generally >98.8% (lower quartile) whereas Gvakharia et al(Gvakharia et al 2017) reported a also a direct reflection of the difficulty of the problem, where the time-varying and spatially inhomogeneous nature of the unconfined turbulent combustion plume complicates accurate measurements(Seymour and Johnson 2021;Wormhoudt et al 2012). These challenges are exacerbated in high crosswind conditions, where wind tunnel experiments(Johnson et al 2001;Poudenx 2000) and flow visualization(Johnson and Kostiuk 2002a) suggest that methane can be emitted on the underside of the wind-deflected flame, separate from the main plume of hot products.…”