Modelling of flameless oxy-fuel combustion with emphasis on radiative heat transfer for low calorific value blast furnace gas

Abstract

Using low calorific value (LCV) gases such as blast furnace gas (BFG) issued from steelmaking industry in steelmaking heating process itself has a very positive impact on the environment and the energy saving. A burner of 325kW using flameless oxy-fuel combustion technology combining with central BFG preheating has been designed for efficient utilization of BFG and less pollutant emissions. In view of industrial application, such technology is one of the potential candidates due to easy furnaces retrofitting, carbon capture potential, low NOx emissions and limited capital expenditures. Modelling such flameless oxy-fuel LCV BFG combustion is still very challenging because finite rate chemistry should be taken into account and radiative heat transfer from fumes issued from BFG combustion should be described properly. If we focus on the latter challenge, most efforts in the literature were spent on developing gaseous radiative properties for high calorific value (HCV) gases such as natural gas, and very few studies were reported on radiation of LCV gases. However, in industrial combustion systems, radiative heat transfer represents more than 80% of the total heat transfer transmitted into charge. Therefore, calculating radiation with accuracy in industrial combustion systems firing on LCV gases is crucial. Along this purpose, the study was devoted to the application of a new gaseous radiative properties model called Spectral Line-based Weighted-Sum-of-Gray-Gases (SLWSGG) adapted for BFG combustion. Full coupling combustion radiation CFD with this newly adapted radiative properties model implemented into Ansys Fluent® was performed for a semi-industrial furnace incorporating the aforementioned oxy-fuel BFG burner. CFD results were compared to the experimental data such as temperature and species concentrations. It was seen that flameless characteristics homogeneous temperature field and maximum temperature of 1400°C were reproduced by numerical simulation. At some measurement points located on the transverse temperature profiles, it was noted that CFD results obtained with the newly adapted SLWSGG model were closer to the pyrometer measurements as compared to the CFD results obtained with the standard WSGG model available in Ansys Fluent®. This can be explained by the fact that the new radiation model evaluated the BFG-air combustion products radiative properties with lower emissivity compared to the higher emissivity of the rich gas like natural gas and that the standard radiation model's coefficients in Fluent® were a priori fitted for radiative properties of rich gas combustion. It can be concluded that newly adapted radiation model should be used to calculate the radiative heat transfer with more accuracy in industrial furnaces for LCV BFG.