CFD technology helps solve petrochem industry problems

Paul Boughton

Joseph D Smith outlines how the effective use of computational fluid dynamics has become an indispensable tool in helping meet tighter environmental regulations.

Ever increasing energy costs along with tighter environmental restrictions makes computational fluid dynamics (CFD) an indispensable tool to engineers developing Low-NOx burners; Haz-waste incinerators, and refinery gas flares.
Effective use of CFD requires a working knowledge of computational combustion together with actual plant experience. Using CFD allows the engineer with the aright' experience to: meet tighter environmental regulations; identify the abest' combustion equipment; optimise operating plants and help reduce downtime during plant shutdowns.
Two examples illustrate how CFD was used to improve heater performance. The first illustrates how to balance air-flow fed to several burners located in a cabin heater with a single blower. The second shows how to analyse flame shape and estimate radiant flux profile with potential flame impingement on process tubes. Example1: Balancing Air flow from a single blower to horizontally fired Low-NOx burners. CFD was effectively used to optimise the air flow through ductwork leading to individual burners in an industrial cabin heater. Here, air fed from a single blower through two manifolds is supplied to burners through individual ductwork. Flow mal-distribution causes non-uniform flames and high emission levels. Non-uniform flames produce skewed heat flux profiles on process tubes which results in de-rating the heater and lost revenues.
To fix this, CFD helped to: evaluate flow through common air plenum to burners; design restrictor baffles to modify overall eP through ductwork and balance burner air flow. This successfully led to the heater being operating at full capacity and meeting their permitted emission levels. Example2: Predicting Flame-shape and radiant flux profiles. CFD also effectively analysed the flame shape inside a cabin furnace where the air flow distribution had been balanced. After successfully balancing the air flow, the detailed Low-NOx burner was modelled. The main concern was flame coalescence between adjacent burners and long flames impinging on process tubes and the resulting heat flux profile which affects heater efficiency and service lifetime.
To analyse flame shape and heat flux profile, CFD was used to: predict flame shape and height; analyse combustion and furnace flow characteristics; assess burner/burner interaction and potential for flame impingement on tubes.
Results illustrate the expected flame shape/height and the potential for flame coalescence. Based on these results the burners were modified prior to installation and the heater is now operating at full capacity and meeting permitted emission levels.
Here CFD helped reduce the risk and cost of upgrading combustion equipment and helped improve efficiency and safety of two heaters. This required a CFD engineer with the appropriate experience at solving these types of problems.

Joseph D Smith is with CDa-acces, Tulsa, OK, USA. " target="_blank">