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Show Steam/Methane Reforming The traditional method of reforming natural gas is with steam. Referring to the process flow diagram in Figure 3, the glass melter flue gas exchanges heat with the steam/natural gas mixture as it is cooled from approximately 2,200°F to 1,lOO°F. It is not desirable to have a flue gas exit temperature from the reformer of much lower than 1,lOO°F since reforming will not take place to an appreciable extent below this temperature. Heat from the flue gas is further recovered downstream of the reformer through sequential heat exchange for steam/natural gas preheating, natural gas preheating, oxygen preheating and steam generation. All of the natural gas in the natural gas/steam mixture which enters the reformer at 900°F, is essentially converted to syngas. The syngas along with the excess steam exits the reformer at 1,600°F, blends with the bypassed natural gas and the mixture is fired as fuel in the glass melter. (It is also possible to fire the bypassed natural gas and the syngas separately to utilize their different flame characteristics at different locations within the furnace.) Since the reforming reaction is endothermic, higher temperatures will drive the reaction further towards completion. There is not enough heat available in the melter flue gas to reform all of the natural gas and some of it must bypass the reformer. The natural gas split to the reformer and bypass is, therefore, an important design and control parameter. It affects the syngas outlet temperature (and hence the degree of conversion), the approach at the cold end of the reformer and also the overall reformer size. Carbon Dioxide/Methane Reforming In the second reforming scheme, shown in Figure 4, carbon dioxide instead of water is blended with the natural gas for the reforming reaction. This is more energy efficient since in the steam reforming case the latent heat of vaporization of water is lost. The energy saved is used for cullet preheating. The heat recovery scheme is similar to the steam reforming case except for the presence of a cullet preheater instead of a boiler. After heat exchange with the reactant and feed preheaters, the flue gas has been cooled to approximately 500°F. At this point some of the flue gas is recycled and a stream rich in carbon dioxide after passing through a chiller, phase separator and compressor is blended with the natural gas to be fed to the reformer. The syngas exiting the reformer is blended with the bypassed natural gas and fired as fuel in the glass melter. As in the steam reforming case, the syngas and the bypassed natural gas can be fired separately if desired. Also, maintaining a high temperature in the reformer and the proper natural gas split to the reformer are important considerations as discussed earlier. - 5 - |
