(419g) Oxy-Coal Combustion with near Zero Flue Gas Recycle

Oxy-fuel combustion produces much higher adiabatic flame temperatures than air-fuel combustion. In order to convert an existing air fired boiler to oxy-coal firing flue gas recirculation (FGR) is required. In fact all of the proposed oxy-coal boiler conversion projects are based on FGR to control the flame temperature and to maintain proper balancing of heat transfer to different parts of the steam cycle. High adiabatic flame temperature and the resulting high heat flux are often erroneously cited as the reason for the FGR requirement. As discussed in this paper, the actual reason is the configuration of the existing air?fired boiler design, i.e., the radiant furnace section followed by a convective section, which has been optimized in the last century in order to maximize the steam cycle efficiency for air firing. For the construction of a new oxy-coal fired boiler, there are no fundamental heat transfer issues requiring FGR in the boiler design. In fact, there are over 1,000 industrial furnaces that have been successfully converted from air-fuel firing to full oxy-fuel firing without FGR. They are typically high temperature process furnaces such as steel reheating, glass melting, aluminum melting, copper melting and hazardous waste incineration furnaces. Precise control of furnace temperature and heat flux profiles in these process furnaces is often more critical than for utility boilers. Both high flame temperature oxy-fuel burners and advanced oxy-fuel burners utilizing internal flue gas recirculation were successfully applied. The heat flux profile was often improved after the oxy-fuel conversion by selecting a proper burner design and optimizing the burner placement and the firing rate distribution without any changes in the furnace geometry. These commercial examples have clearly demonstrated that oxy-fuel combustion can reproduce the desired temperature and heat flux profiles in any air-fuel fired high temperature furnaces. An oxy-coal firing system that does not require flue gas is often called ?zero recycle oxy-coal combustion'. However, safety and operational concerns may require some recirculated flue gas to transport the pulverized coal into the boiler. Therefore a more accurate expression is ?near zero recycle oxy-coal combustion'. In order to be viable, the boiler and burner designs must mitigate the potential drawbacks associated with high flame temperatures and high pollutant concentrations. For example the design must manage the heat flux profile to avoid extreme material temperatures and slagging and corrosion problems. The design would also need to be able to handle high concentrations of pollutants such as SOx and particulates that could potentially lead to high corrosion and slagging/fouling rates. Finally, the significant equipment and design modifications required to mitigate these potential problems make it likely that near zero recycle boilers will be new build rather than retrofits. In this paper previous studies on oxy-fuel combustion for boilers without FGR will be reviewed first and key boiler design issues with near zero FGR will be discussed. Several new oxy-fuel boiler design concepts without FGR are discussed.