Emission and Efficiency Comparison of Different Firing Modes in a Furnace with Four HiTAC Burners

Abstract

Combustion in a furnace equipped with two HiTAC burner pairs, with a thermal power of 100 kW_th each, has been investigated experimentally and computationally. The objective of this study is (1) to observe differences in the performance of the furnace operating in two different firing modes, parallel and staggered, and (2) to explain these differences using detailed CFD simulations. Besides the permanent measurements of temperature, flow and pressure, in-furnace probe measurements of temperature, oxygen and emissions (NO and CO) have been performed. Experimental results show that the efficiency of the furnace was higher in parallel mode compared to staggered mode, 48% and 41% respectively. The values of CO emitted were equal for both firing modes. However, in parallel mode the NO_x production was 39 ppm _v @3%O₂, whereas in staggered mode 53 ppm _v @3%O₂ NO_x was produced. Considering both efficiency and emissions, parallel firing mode performs better than staggered mode.

Next, CFD simulations of the furnace were performed in order to explain the observed differences. The simulations were validated with the in-furnace measurements. It was confirmed that the furnace firing in parallel mode achieved a higher efficiency. The radiative heat transfer was higher due to formation of a larger zone with gases with improved radiative properties. In addition, higher velocities along the cooling tubes, due to lower momentum destruction, led to higher convective heat transfer. Also, the lower production of NO_x in parallel mode was reproduced by the simulations. This is due to the fact that in parallel mode the fuel jets are merging slower with the combustion air jet, leading to less intense combustion zones. Thus, lower peak temperatures and radical concentrations are achieved, and the NO_x production via the thermal and N₂O pathways was lower.

Keywords:

• Efficiency
• Emission
• Firing modes
• Furnace
• HiTAC

ACKNOWLEDGMENTS

The European Union is acknowledged for its financial support in the framework of SUSPOWER. Also, many thanks go to Bart Merci, Karel Laureys, and Luuk Thielen for their advice on the numerical work.