OCR Text |
Show point out the fact that nitrogen is eliminated from the combustion process. This will impact significantly on the thermal and aerodynamic properties of the flame. Figure 1 shows changes in the adiabatic flame temperature for combustion of natural gas with increasing oxygen concentration in the combustion air. Typical levels of oxygen addition (up to 6%) result in about 300°C increase of the flame temperature. At this level of the flame temperature no significant amount of dissociation of the combustion products occures and this effect is not disscused here. Radiative heat transfer is a major mode of energy transfer to the load in reheat furnaces. As it is propotional to the difference in the fourth power of the flame and the load temperature, one can expect increase in the radiative heat flux with the increased flame temperature due to oxygen enrichment. Futher increase of the radiative heat flux can potentially occur from the enhenced luminosity of the flame. One practical consequence of this is, that the throughput rate of heated material can be increased, or else the heating rate can be held constant and the firing rate reduced and fuel savings achieved. The increased radiative heat fluxes could however, impose high demand on furnace refractory lining melting points. The decreased amount of nitrogen in a flow of combustion air causes a significant reduction in the momentum of the air jets (Fig. 2) and consequently a smaller flame envelope. Means of mitigating this effect should exist in the process control system, otherwise the utilization of oxygen will have the increasingly negative outcomes such as local hot spots, non-uniform load heating, excess oxidation, decarburization and damage to the refractory lining and to the burners. The elimination of nitrogen has another important consequence in the reduction in the volume of combustion products, which will be more pronounced for very high levels of enrichment or for combustion with pure oxygen. Utilization of the oxygen enriched combustion air has a numerous environmental effects. First of all, since the NOx formation is dependant upon both flame temperature and oxygen concentration, elevated flame temperatures will increase nitrogen oxides emissions. Again, means to offset this effect should be sought and employed. On the other hand it can be expected, that the higher reaction rates in the flame with the increased oxygen concentration will ensure complete combustion of fuel, thus reducing carbon monoxide and unburned hydrocarbons in flue gas. THE CAGCT FURNACE AND LOW-NOx BURNER DESIGN. The reserach furnace at Queen's University Center for Advanced Gas Combustion Technology, was adapted for this single burner study. The furnace is described in detail in the presentation No.17 of this meeting. Modifying of the furnace for testing of a single burner involved a set-up of a rectangular 1m x 1.5m x 3m combustion chamber by putting a vertical partition between burners 1 and 2 (Fig. 3). The burner used in this study is a Bloom Eng. Company Low-Nox burner rated at 0.4 MW with 38.7 Sm3/hr natural gas at 13" WCDP and 425.2 Sm3/hr air at 18" WCDP 2 |