CFD investigation of a Stirling engine flexi-fuel burner

A. Abou-Taouk, P. Wettrell and L. E. Eriksson (Chalmers University of Technology)

CHALLENGE - Several different reduced reaction mechanisms of methane-air mixture and propane can be found in literature. However, the number of reduced reaction mechanisms for LCV (Low Caloric Value) mixtures are very limited. Cleanergy, the worlds’s leading supplier of sustainable energy solutions based on the Stirling engine, has a newly developed and manufactured burner able to burn landfill gas and other types of LCV mixtures with very low energy content. This study focuses on the development of reduced chemistry for landfill gas that can be used with CFD in a more cost effective manner.  

SOLUTION - This paper presents comparisons of results from tests and 3D CFD combustion simulations based on both RANS and hybrid URANS/LES (SAS-SST model) turbulence models applied to an industrial Stirling engine combustion chamber at atmospheric pressure. The combustor is designed to operate in the MILD combustion mode which is characterized by low flame temperatures and low NOX emissions. A 4-step reduced reaction mechanism, named AAT4NR, involving seven species was developed to represent the landfill gas. The software tool CHEMKIN has been chosen to solve the freely propagating premixed flame equations, and it was coupled with the optimization toolbox modeFRONTIER.






BENEFITS - A new optimized global reaction mechanism, AAT4NR, was developed for the present land- fill mixture (24.2% CH4, 21.6% CO2, 2.0% O2 and 52.2% N2 by volume). Comparisons with detailed chemistry solutions of a planar propagating flame front show that the laminar flame speed, the adiabatic flame temperature, the ignition delay time and the species concentration at equilibrium are adequately predicted. There is good agreement between the quantities predicted with URANS/LES and experimental data, in terms of flow and flame dynamics, averaged temperatures, NOX-levels and the concentrations of some major species.