Flaring efficiencies and NO_x emission ratios measured for offshore oil and gas facilities in the North Sea

Abstract

Abstract. Gas flaring is a substantial global source of carbon emissions to atmosphere and is targeted as a route to mitigating the oil and gas sector carbon footprint due to the waste of resources involved. However, quantifying carbon emissions from flaring is resource-intensive, and no studies have yet assessed flaring emissions for offshore regions. In this work, we present carbon dioxide (CO2), methane (CH4), ethane (C2H6), and NOx (nitrogen oxide) data from 58 emission plumes identified as gas flaring, measured during aircraft campaigns over the North Sea (UK and Norway) in 2018 and 2019. Median combustion efficiency, the efficiency with which carbon in the flared gas is converted to CO2 in the emission plume, was 98.4 % when accounting for C2H6 or 98.7 % when only accounting for CH4. Higher combustion efficiencies were measured in the Norwegian sector of the North Sea compared with the UK sector. Destruction removal efficiencies (DREs), the efficiency with which an individual species is combusted, were 98.5 % for CH4 and 97.9 % for C2H6. Median NOx emission ratios were measured to be 0.003 ppm ppm−1 CO2 and 0.26 ppm ppm−1 CH4, and the median C2H6:CH4 ratio was measured to be 0.11 ppm ppm−1. The highest NOx emission ratios were observed from floating production storage and offloading (FPSO) vessels, although this could potentially be due to the presence of alternative NOx sources on board, such as diesel generators. The measurements in this work were used to estimate total emissions from the North Sea from gas flaring of 1.4 Tg yr−1 CO2, 6.3 Gg yr−1 CH4, 1.7 Gg yr−1 C2H6 and 3.9 Gg yr−1 NOx.

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

“…Our emissions factors are more comparable with air-assisted test flares sampled by Torres et al (0.037–0.083 lb NOx/10 6 BTU) compared to steam-assisted flares (0.009–0.033 lb NOx/10 6 BTU) measured in that work . Field measurements of NOx from flares in oil and gas production regions are limited; however, recent airborne measurements of 58 flare plumes in the North Sea found a mean NOx:CO 2 of 0.004 ppm/ppm (4 ppb/ppm), which is considerably higher than our average observed signals, but within the range of our measurements (see Table S3). This difference in average value is possibly due to the high fractional methane content of the gas in their study region (median of 0.845) compared to our U.S. regions (Table ), as well as the higher expected temperatures of these large offshore flares.…”

“…24 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. 28 For regulatory purposes, NOx from flares is inventoried in the U.S. by the EPA using an emission factor (EF = 0.068 lb of NOx/10 6 BTU) in conjunction with reported activity data (A = flow rate × heating value of flare gas). This emission factor value was derived in a 1983 EPA study of air-and steamassisted flares.…”

“…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 …”

“…The TCEQ framework applies four emissions factors depending on the flare type and gas heating value, with values ranging from 0.0485 to 0.138 lb NOx/10 6 Btu . Overall, few assessments of the real-world operation of flares have been made due to the challenging nature of in situ sampling of installed flares and the geographical spread of flares across O&G regions. ,− …”

Quantification of Gas Flaring from Satellite Imagery: A Comparison of Two Methods for SLSTR and BIROS Imagery

“…The variability observed could thus be traced back to a variability in the combustion efficiency, which has been observed in other studies [13,18,34,88]. Several factors affect the combustion efficiency of a flare, such as ambient conditions (atmospheric temperature and pressure, wind), operating conditions (air-fuel ratio in the burner, composition and mass flow of the gas) or possibly the design of the infrastructure (stack diameter) [10,13,21,83,89]. The average mass flow derived from the SLSTR-based detections was 2508 m 3 h −1 (3817 kg h −1 ), with a standard deviation of 3096 m 3 h −1 (4713 kg h −1 ).…”