Effect of Combustion Characteristics on Wall Radiative Heat Flux in a 100 MWe Oxy-Coal Combustion Plant

Oxy-coal combustion exhibits different reaction, flow and heat transfer characteristics from air-coal combustion due to different properties of oxidizer and flue gas composition. This study investigated the wall radiative heat flux (WRHF) of air- and oxy-coal combustion in a simple hexahedral furnace and in a 100 MWe single-wall-fired boiler using computational modeling. The hexahedral furnace had similar operation conditions with the boiler, but the coal combustion was ignored by prescribing the gas properties after complete combustion at the inlet. The concentrations of O₂ in the oxidizers ranging between 26 and 30% and different flue gas recirculation (FGR) methods were considered in the furnace.

In the hexahedral furnace, the oxy-coal case with 28% of O₂ and wet FGR had a similar value of T_af with the air-coal combustion case, but its WRHF was 12% higher. The mixed FGR case with about 27% O₂ in the oxidizer exhibited the WRHF similar to the air-coal case. During the actual combustion in the 100 MWe boiler using mixed FGR, the reduced volumetric flow rates in the oxy-coal cases lowered the swirl strength of the burners. This stretched the flames and moved the high temperature region farther to the downstream. Due to this reason, the case with 30% O₂ in the oxidizers achieved a WRHF close to that of air-coal combustion, although its adiabatic flame temperature (T_af) and WHRF predicted in the simplified hexahedral furnace was 103 K and 10% higher, respectively. Therefore, the combustion characteristics and temperature distribution significantly influences the WRHF, which should be assessed to determine the ideal operating conditions of oxy- coal combustion. The choice of the weighted sum of gray gases model (WSGGM) was not critical in the large coal-fired boiler.