INVESTIGATION OF OXYGEN-ENRICHED COMBUSTION FOR APPLICATION TO A FUEL-STAGED OXY-COMBUSTION PROCESS

摘要

This paper presents experimental results of oxygen-enriched combustion of pulverized coal for the application to a fuel-staged oxy-combustion system. The process incorporates fuel-staging to minimize flue gas recycle while maintaining optimal heat flux and gas temperature. A techno-economic analysis of the process has demonstrated that when combining fuel staging with pressurized combustion, significant improvements are possible in plant efficiency, as compared to first-generation oxy-combustion approaches. High pressure allows the latent heat of the moisture in the flue gas to be recovered and utilized in the steam cycle. By introducing the fuel in stages, the overall radiative section can be increased, allowing for a greater percentage of the heat transfer in the boiler by radiation, as opposed to convection. Other advantages of the process include reduced gas volume, reduced parasitic power demand due to reduced flue gas recirculation, reduced oxygen demand and increased performance for lower rank fuels due to a higher local temperature. To investigate the combustion and heat transfer behavior of the fuel-staged process, experiments under oxygenenriched conditions and with high excess oxygen were conducted in a down-fired furnace at atmospheric pressure. The radiative heat flux profile at the furnace wall, along with axial temperature and gas (O_2, CO_2, CO, NO) profiles were obtained. Experiments were performed using up to 50% oxygen in the secondary oxidizer stream. When the oxygen concentration in the secondary oxidizer stream was increased to 50%, a 37% increase in peak radiative heat flux, when compared to air-firing was observed. In spite of the increased local flame temperature, the post-flame temperature remained largely unaffected by the oxygen enrichment, as would be predicted in the fuel-staged (SPOC) process, since conditions are run such that excess oxygen is available for dilution. The NO concentration is observed to be higher for oxygen-enriched cases due to enhanced formation of thermal and fuel NO_x. The enhanced conversion of fuel-bound N to NO_x is deemed beneficial for CO_2 compression and purification unit downstream, if the sour gas compression process is used. The sour gas process requires higher NO_x to SO_x ratio for efficient removal of NO_x and SO_x because NO is oxidized to NO_2, which then reacts with SO_2 to form SO_3. Modelling results for pure oxygen combustion conditions show that flame size can be varied to control the wall heat flux and avoid flame impingement on the wall. Furthermore, it is demonstrated that the burning of coal in nearly pure oxygen environment is feasible and that through fuel staging and elongation of the combustion zone, the heat flux can be maintained at levels comparable to existing state-of-the-art boilers.