CFD modeling of varying load-influenced NOx, SOx emissions and combustion kinetics in a sub-critical coal-fired furnace

Abstract

This study investigates emissions and combustion behavior of the 125 MWe Barapukuria Thermal Power Plant (BTPP) by burning Bituminous coal (C₁₃₇H₉₇O₉NS). A 3D computational fluid dynamics (CFD) model is developed considering NO_x scheme through Extended Zeldovich mechanism and SO_x emission under various loads (50 % to 100 % fuel load). The study uses DDM-Discrete Droplet Method, DTRM-Discrete Transfer Radiation Model, and a standard k–ε turbulence model to study flow dynamics, particle dispersion, heat rate and burnout rate. It is studied that the highest temperatures in the burner throat range from 1850 K to 2200 K with emission profiles of thermal, prompt, and fuel NO_x at different load levels. The total amount of NO_x goes from 381 ppm at full load to 974 ppm at 50 % load at final exit. The amount of SO₂ mole fractions goes down 0.000387 to 0.000206 for reduced sulfur input and lower combustion temperatures, residence time (τ) which limit the oxidation of sulfur compounds. The lower load conditions have the highest levels of NO, NO₂, N₂O because the gases stay longer residence time (τ) and remain unburn partially. The study shows the reaction kinetics of faster devolatilization and longer residence char burnout rates. As load rises the volatilization rate ranges from 0.0001 to 0.00042 mol.m⁻³.s⁻¹ and the intense char burnout intensifies from 0.00025 to 0.00065 mol.m⁻³.s⁻¹.