On Numerical Simulation of Fuel Mixed With Steam or Air to Investigate Soot and Other Emissions

Abstract

Soot emissions (PM 2.5) as well as CO and NOx from industrial flares and other industrial processes or sources pose a substantial risk to human being health and the environment, and now are subject to new and tougher EPA regulations. Flaring is used widely used in many industries to dispose unwanted combustion gases by burning them as a flame. However, flaring produces significant amount of particulate matter in the form of soot, along with other harmful gas emissions. Although many experimental and numerical studies have previously been done on flames burning in a controlled condition, relatively few studies have been conducted with fuel-steam mixture. In practice, air and steam are commonly used to assist the flaring processes — control the smoke and the combustion efficiency. This study aims to investigate soot, CO and NOx emissions of turbulent diffusion methane and propane flame mixed with air or superheated steam. To study such effect numerically, the computational fluid dynamics software ANSYS Fluent 14.5 is used with non-premixed probability density function (PDF) model. The laminar flamelet is generated with automated grid refinement. For the soot generation, the Moss-Brookes soot model with Lee sub-model is considered. The combustion mechanism is developed by the authors’ research group from the combined GRI and USC mechanisms. Two types of fuel, methane and propane, are used. The amount of super-heated steam varied from four percent to twenty percent (4%, 8 %, 12%, 16%, and 20%), and the behavior of the flame is analyzed. For the baseline case, the jet has a diameter of 50.8 mm or 2 inches, and the jet velocity is kept to 1.0 m/s. A co-flow air is supplied at a velocity of 0.2 m/s. The temperature distribution of methane and propane are compared with different contents of steam or air assists. The NOx, Soot and CO yields (kg/kg) varying with steam or air percentages are also presented. The results indicate that the soot yield is dependent on fuel type strongly and the percentage of steam or air affects the soot yield differently as the fuel type varies.