title: Thermal Radiation, Scaling and Flame Stability in Semi-Industrial Oxyfuel Combustion
Creators: name: Richter, Marcel
Publisher: name: UNSPECIFIED
Year: 2025

description: The oxyfuel technology enables combustion processes producing exhaust gases consisting almost entirely of CO₂. Thus, CO₂ can be efficiently captured from the combustion off-gases by using Carbon Capture and Storage (CCS) to prevent CO₂ from being emitted into earth’s atmosphere. Compared to conventional firing with air, the main gas component is CO₂, which has different material properties than N₂ and therefore changes the combustion characteristics. This concerns, e.g. the ignition behavior of the fuel, the pollutant emissions and the heat transfer. Especially the heat transfer changes due to the strongly absorbing properties of CO₂ in the thermal spectrum. While many studies investigated this combustion process on a laboratory scale using coal as fuel, a semi-industrial scale is considered here for the combustion of pulverized biomass under oxyfuel conditions. First, an existing laboratory oxyfuel burner was scaled up to a semi-industrial size. Numerical simulations and dimensionless quantities were used for this purpose. Based on the geometric results of this procedure, a 500 kW burner was manufactured and installed in a semi-industrial
combustion chamber. The results suggest that regardless of the fuel, a strong recirculation
zone near the burner is important to aerodynamically stabilize the flame. This means that
all combustion parameters, such as a higher thermal power or less O₂ content in the oxidant, which both lead to a higher momentum ratio between secondary and primary flow, intensify the recirculation zone. Also, the velocity of the secondary stream should be high enough to carry the particles. However, the O₂ content in the oxidant cannot be reduced arbitrarily. CO emissions increase significantly for biomass below 27 vol%, whereas coal requires lower O₂ enrichment in the oxidant. It is assumed that this is mainly due to the smaller particles of the coal. A heat flux sensor was used to measure the radiative heat flux in absolute terms at the combustion chamber wall. The combustion of biomass shows a higher radiative heat flux for all O₂ concentrations in the case of oxyfuel than in the case of combustion with air. For lignite, on the other hand, the measurement results for oxyfuel are all below those of air combustion. When comparing the thermal radiation of biomass and lignite, the biomass shows significantly higher radiative heat transfer; it is assumed that the higher volatile content in the biomass leads to a stronger local heat release and therefore a higher radiative heat flux. In case of a natural gas flame, the radiative heat flux for all oxyfuel conditions is higher than for air-firing. This can probably be attributed to a local displacement of the main reaction zone. In order to efficiently calculate the radiative exchange in oxyfuel combustion with numerical simulations, several gas radiation models were implemented and tested in the open-source code OpenFOAM. The Full Spectrum Correlated-k (FSCK) and Weighted Sum of Gray Gases (WSGG) models showed good approximations of the wall-incident radiative heat flux with the measured data in the combustion chamber, whereby the WSGG has a lower computing time than the FSCK.