Quantifying the carbon conversion efficiency and emission indices of a lab-scale natural gas flare with internal coflows of air or steam

Abstract

To better understand the effect of air- and steam-assist on flaring operations on carbon conversion efficiency and pollutant emissions, a lab-scale coflow burner constructed of two concentric circular tubes was used. Natural gas at 20 SLPM flowed through the annular space and increasing amounts of air or steam were added through the inner tube until flame extinction was invoked. The combustion products were captured by an exhaust hood and analyzed using a gas chromatograph to measure the concentrations of CH4, C2H6, C3H8, C4H10, CO and CO2. A photoacoustic extinctiometer and a NOx analyzer were used to measure black carbon and NOx concentrations, respectively. A challenge associated with the experiments was to accurately resolve stark transitions in conversion efficiency and orders of magnitude change in key emissions. Results indicated that the addition of air or steam to a natural gas jet diffusion flame caused a sudden collapse in carbon conversion efficiency, occurring at a coflow-fuel gas mass flow ratio of 1.8 for steam and 5.0 for air. As was expected, the CO2 emission indices for both air and steam coflow followed their respective carbon conversion efficiency trends. Conversely, the total unburned hydrocarbon emission indices increased with the reduction in carbon conversion efficiency. NOx emission indices for air and steam coflow were shown to decrease by an order of magnitude compared to the unassisted flame. For air coflow this occurred before the collapse in carbon conversion efficiency, while for steam coflow this occurred concurrently. Black carbon emission indices decreased rapidly by three orders of magnitude for both air and steam coflow. For both these lab-scale air and steam assisted flares there was a range of mass flow rates of the assist fluid that resulted in high combustion efficiency and low pollutant emissions.