| OCR Text |
Show Figure 3 shows the volume of oxidant required for combustion of methane-oxygen at 2% excess Ob which is approximately inversely proportional to the concentration of oxygen in oxidant. The key point is the huge amount of oxidant required in dilute oxygen combustion. For example, at 5% O2 about 70,000 if of oxidant is required to bum 1 x 10' Btu of methane, which is about seven times that required for the normal air combustion. It raises two technical problems for practical applications; (1) a large volume of flue gas recirculation is required and (2) fuel has to be mixed with a large volume of dilute oxidant for complete combustion. Figure 4 depicts the concept of DOC methods developed by Praxair Inc. An oxidant jet and a fuel jet are separately introduced into the furnace. The oxidant jet entrains furnace atmosphere by the aspiration effect of the turbulent jet and create an "oxidant mixing zone", in which oxygen concentration is reduced from the initial concentration to a value approaching that of the furnace gas. For example if air is the oxidant, the initial oxygen concentration is about 21%. If the average oxygen concentration of furnace gas entrained into the oxygen stream is 2% and 20 moles of furnace gas is mixed per mole of oxidant in the oxidant mixing zone, oxygen concentration after mixing becomes about 2.9%. When pure oxygen or oxygen enriched air is used as the oxidant, higher entrainment ratios of the furnace gas to oxidant are required to reduce the oxygen concentration to the same level as the air case. No combustion reaction takes place in this zone since the furnace atmosphere entrained into the oxidant jet is separated from the "fuel reaction zone". Fuel is introduced separately into the furnace from the opposite wall in this particular arrangement to avoid direct mixing of the fuel and oxygen jets. Fuel reacts with oxygen and other species contained in the furnace atmosphere spontaneously as the temperature of furnace gas is substantially above the auto-ignition temperature of fuel and oxygen. The peak flame temperature is kept low due to the low oxygen concentration of "oxidant". By providing a good recirculation pattern within the furnace the oxygen concentration in the furnace atmosphere outside of the oxidant and fuel mixing zones can be maintained close to that of flue gas, which typically contains only about 2 to 3 % O2• High momentum of the fuel jet is an important requirement for good combustion due to the large amount of oxidant that has to be mixed with fuel. For example, if the fuel is methane and the average concentration of oxygen in the furnace gas entrained into the fuel reaction zone is 3%, 67 volumes of furnace gas are required per volume of fuel to complete the combustion reactions. Small high velocity fuel jets can be used to entrain a large volume of furnace gas rapidly into the fuel jet. Thus the flame volume, or the fuel reaction zone within the furnace can be largely controlled by the turbulent jet mixing process. Although the actual chemical reactions of the methane jet with the furnace gas containing a low concentration of oxygen and high concentrations of water and carbon dioxide are very complex and potential kinetic limitations may exist depending on the furnace gas temperature, the simple conceptual arrangement of the DOC method make it easier to characterize and scale the flames for practical applications than most of the commercial burners. 3 |
