Sam Cowart

Date of Award

January 2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Gautham Krishnamoorthy

Abstract

Conventional fuel-air combustion processes are widely used in the chemical processing industry, but often suffer from high NOx emissions and difficulties with CO2 capture from flue gases. Oxy-fuel combustion offers advantages over fuel-air combustion that addresses these shortcomings. One possible fuel for oxy-fuel combustion is ethylene, a gas that is produced in large quantities industrially. The numerical literature regarding ethylene-oxygen combustion is limited in scope.

This study focuses on obtaining flame characteristics of premixed and non-premixed ethylene-oxygen combustion using numerical methods and computational fluid dynamics (CFD) approaches. This study offers several contributions to the current research: (1) the use of a reduced chemical reaction mechanism for ethylene combustion that has not been widely used, (2) numerical resolution of soot and radiative heat transfer effects in lean (Φ = 0.2) premixed and non-premixed ethylene combustion systems, and (3) a methodology for obtaining accurate combustion characteristics while maintaining a low computational cost. Laminar flame speed, flame temperature, flame length, soot volume fraction, and radiant fractions are quantities of interest.

A commercially available CFD package is used to conduct simulations. Computational domains representative of experiments in ethylene combustion are designed and discretized to resolve flame characteristics while maintaining accumulation of numerical errors to less than 0.06%. Several unique inputs to the governing equations are added: a multi-step reaction mechanism, tailored radiation functions for CO2 and H2O, and the inclusion of acetylene as a precursor species for soot production.

In the premixed ethylene-oxygen study, flame velocities are reduced by up to 73% when radiative heat loss is accounted for and radiant fractions are in the range 0.12 – 0.17. In the non-premixed ethylene-oxygen study, soot profiles for oxygen indices between 21% - 90% are determined and compared against experimental measurements. Radiant fractions are in the range 0.09 – 0.26, depending on oxygen index. A model for the soot nucleation parameter is proposed that is validated against additional experiments.

This study shows that the application of a reduced reaction mechanism for premixed combustion of ethylene-oxygen is important for determination of flame characteristics that agree with experiment. This mechanism applied to non-premixed ethylene-oxygen combustion simulations performs equally well.

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