Numerical Study of Co-firing Biomass with Lean Coal under Air–Fuel and Oxy-fuel Conditions in a Wall-Fired Utility Boiler

Abstract

Co-firing biomass under oxy-fuel condition is one of the most attractive methods which is conducive to mitigating CO 2 emissions by combining the advantages of these two respective technologies. The combustion characteristics of a wall-fired utility boiler operating in this mode have been seldom investigated. The burnout behavior of the blended fuel is still controversial. By using the newly proposed combustion mechanisms, a numerical study was carried out in a 600 MW wall-fired boiler to evaluate the influences of oxy-fuel working condition and biomass share on flow, temperature, O 2 distributions, and burnout behavior in this combustion mode. Besides, the effect of biomass injection position was also explored, which has yet to be fully understood. The simulation results show that oxy-fuel working condition affected the combustion characteristics to some extent. The introduction of biomass led to a lower temperature but a better burnout within the furnace. O 2 distribution was also correlated to the biomass share due to the difference in fuel properties. The injection position of biomass presented crucial impacts on particle trajectories, temperature distribution, and O 2 distribution. In addition, due to the increase in residence time and the reduction in trapped particles, an enhanced burnout could be achieved as the biomass inlet was moved down.

Combustion and heat transfer characteristics of co-firing biomass and coal under oxy-fuel condition

“…The region of high temperature zone is quite similar between case B1 and case B3, while the peak temperature in main burning zone for case B4 is even higher than that for case B1. The result seems to refute the findings in Li et al that the temperature in main burning zone for oxy‐fuel cases (30% O 2 ) was lower than that for traditional air‐firing case. It could be attributed to different biomass fraction.…”

“…The results indicate that the CFD model used in this study is appropriate. The validation of the utilized model could also be found in our previous work …”

“…It could be attributed to different biomass fraction. The biomass fraction is 10 wt% in Figure , while the value was 30 wt% in Li et al In comparison, the burnout of biomass is quicker than that of coal so that most of biomass particles finish the combustion process in main burning zone with large amount of heat released. Additionally, the relatively low temperature zone brought by the injection of SOFA fades as the O 2 mole fraction increases because the SOFA nozzles are closed gradually.…”

“…The mesh system consists of 1 796 184 cells selected to conduct the numerical simulation in FLUENT 6.3 after the grid independence test. The results of the test can be referred in our previous work . Figure shows the chosen mesh system of whole furnace region and the refined O‐grid mesh structure near the nozzles.…”

“…The velocity of inlet gas can also be seen in Table , while the overall excess air coefficient is 1.2 for each case. Further, all biomass injections are located in the lower nozzles because the burnout ratio exhibits the best performance under this condition . The gas temperature of primary air (PA) maintains at 363 K in each case.…”