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.
Highlights
Flaring in the upstream oil and gas industry is a process used to destroy unwanted combustible gas, typically in buoyancydriven, turbulent diffusion flames atop vertical stacks or in refractory-lined pits that are open to atmosphere
There is a general correlation between the species, as should be expected since the mean emission rate of each species is proportional to the overall rate of combustion, the results show considerable scatter about the central trend
The observed variability in black carbon (BC)/H2O species ratios, which was well beyond measurement uncertainty, reveals that the species ratio distributions are skewed towards higher BC/H2O values, suggesting that BC emissions are skewed by intermittent bursts of high BC production not well correlated with H2O
Summary
Flaring in the upstream oil and gas industry is a process used to destroy unwanted combustible gas, typically in buoyancydriven, turbulent diffusion flames atop vertical stacks or in refractory-lined pits that are open to atmosphere. In 2019, global gas flaring volumes were estimated to be 150 billion m3, an increase of 3 % from 2018, reaching levels not seen since 2009 (World Bank, 2020). Flaring is preferable to venting since the 20-/100-year global warming potential (GWP) of methane, a common constituent of flare gas, is 96/34 times higher than the CO2 produced by flaring (Gasser et al, 2017). Flaring can produce unwanted pollutants such as soot (primarily black carbon, which has its own important climate impacts), CO, oxides of nitrogen (NOx), volatile organic compounds (VOCs), and uncombusted flare gases. Of the pollutants produced by gas flaring, black carbon (BC; the carbonaceous, strongly light-absorbing component of particulate matter) and methane are both short-lived climate pollutants (SLCP) that, along with CO2, constitute the three most climate warming pollutants in the atmosphere Atmospheric BC directly warms the atmosphere, as it is a strong absorber of both in- and out-going radiation at all wavelengths
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