Assessment of the viability of VISR: a mid-wavelength infrared (MWIR) multispectral imaging-based approach for remote flare CE quantification

Abstract

In the upstream and downstream petrochemical industry, flaring is a common practice to dispose of the redundant by-products of crude oil extractions, including associated gas and highly reactive volatile organic compounds (HRVOCs), for a variety of regulatory, safety and economic purposes. The current consensus among government regulators and industry experts is that flaring typically occurs at a combustion efficiency (CE) higher than 98%. Recent studies, based on real-world observations and computational simulations, have called this into question. For example, a recent study, based on airborne sampling observations and unlit flare prevalence surveys, reported a mean flare CE of 91%, accounting for both inefficient flaring and unlit flares in the three largest basins in the US. This represents a five-fold increase in fugitive hydrocarbon emissions, primarily comprised of methane, compared to the presumed release rates, highlighting an opportunity for developing robust flare CE monitoring techniques to mitigate the adverse health and environmental impacts of the underappreciated flaring emissions. This study presents a numerical assessment of video imaging spectro-radiometry (VISR), a mid-wavelength infrared (MWIR) multispectral technique, proposed for remote flare combustion efficiency quantification applications. The present analysis utilizes a series of computational fluid dynamics (CFD) simulations of a crosswind steam-assisted industrial flare, with a focus on three aspects: how approximations in the radiometric model impact the local “pixel-wise” CE, the validity of the approach for computing flare global CE using inferred local CE values, and the ability and limitations of VISR instrument to capture fuel that may be aerodynamically stripped from the combustion zone under crosswind conditions. The current assessment is conducted on a simplified version of the VISR instrument model using simulated broadband images generated over spectral bands adjusted for the key absorption features of three main by-products of flare combustion reaction: CO2 (4.2–4.4 µm), CO (4.5–4.9 µm), and CH4 (3.2–3.4 µm). The results highlight the accuracy of the proposed simplified VISR approach in predicting local CE within the VISR region-of-interest (ROI) yet flawed in terms of converting these values into a flare global CE, potentially leading to large biases from the actual flare CE. Ultimately, the VISR technique, due to reliance on mid-wavelength infrared imaging, is inherently incapable of quantifying unburned (cold) methane, allegedly stripped from the flare stack, without participating in the combustion process, due to the presence of a high crosswind over the flare stack, leading to a considerable overestimation of the true flare performance.

Keywords: Flares, Combustion Efficiency, Remote Sensing, Verification and Validation, Radiometric Measurements, Uncertainty Analysis, Bayesian Inference.