Performance of Flare Flames in a Crosswind With Nitrogen Dilution

Abstract

Abstract Flares are used for the safe, clean, and economical disposal of waste gases, whether in upstream oil production (solution gas flares), refineries, gas plants, or other chemical processing facilities. Elevated flares are exposed to a range of weather patterns; perhaps the most important being the crosswind. The CETC Flare Test Facility (FTF) was constructed to address the question of performance of solution gas flares, in response to limited field trials that indicated the possibility of very low combustion efficiencies under certain conditions. The FTF produces crosswind speeds up to 45 km/h with very low turbulence intensity, as designed. Model solution gases are produced from natural gas, propane, inert diluents (carbon dioxide and nitrogen), and volatile liquids. Atmospheric wind is a turbulent shear flow with an intensity of around 7﹪. To more closely replicate the turbulence properties of atmospheric wind, a range of turbulence intensities and integral length scales were imposed in the FTF by using grids. The flare flame efficiency is measured by the conversion of carbon in the solution gas to carbon dioxide. Efficiency is lowest for pure natural gas, and increases with the amount of propane. Conversion efficiency decreases significantly with the increase of turbulence intensity of the crosswind, which has implications for existing and novel strategies to improve flare performance. The primary product of incomplete combustion is methane, a significant greenhouse gas. The combined effect of inert diluents and turbulence intensity on flame stability shows that turbulence amplifies the destabilizing effect of fuel dilution. Introduction Flares are used for the safe, clean, and economical disposal of waste gases, whether in upstream oil production (solution gas flares), refineries, gas plants or other chemical processing facilities. Elevated flares are exposed to all the weather patterns; perhaps the most important is the crosswind. The flaring of solution gas (also called associated gas) was a common practice in upstream oil extraction. This gas is a mixture of light hydrocarbons, mostly methane but including possibly significant amounts of ethane, propane, butane, hydrogen, perhaps some inert gases (nitrogen and carbon dioxide), and sometimes hydrogen sulphide (sour gas). The composition and amount of solution gas can vary a great deal between production sites. Over 800 million m3 of gas was flared in Alberta in 2000, which is almost a 50﹪ reduction from 1996(1). The current global flaring rate is estimated between 100 and 126 billion m3 of solution gas each year(2). Strosher(3) conducted laboratory-scale investigations on an enclosed flare and an open-air flare, and on two commercial flares in the field. He found in the field investigations that combustion efficiency is typically about 70%, sometimes as low as 62﹪. The unburned portion consisted of methane, unsaturated hydrocarbons, light aromatics such as benzene, and polycyclic aromatic hydrocarbons (PAH). This work has led to much of the current interest in flare research by industry, government, academia, and the public. An important parameter in the flare flame behaviour is the ratio of the fuel jet momentum flux to the crosswind momentum flux.