| OCR Text |
Show Conceptual Designs Conceptual designs for a gas reburning system for the model furnaces were first developed to insure adequate reburning zone access along the gas path. By detennining where the reburning fuel and overfire air injectors can practically be placed, a more realistic assessment of the time-temperature history of the rebuming zone can be established. In addition, development of the conceptual designs pennits a more accurate estimate of the capital costs for retrofitting the model furnaces with a reburning system. The frrst step in the conceptual design process is development of material balances for baseline and reburning operation. For reburning, the primary design criterion is to achieve a stoichiometry in the reburning zone of about 0.90. This stoichiometry generally results in optimum NOx reduction at minimum reburning fuel usage. In this study, it was assumed that the reburning fuel would be added immediately downstream of the melter in the port neck to minimize impacts of rebuming on the melt. It may be possible to inject the reburning fuel into the end of the melter region; however, this possibility would need to be evaluated on a site specific basis. Reburning fuel injection downstream of the melt can decrease the overall furnace efficiency if the added heat input is not fully recovered in the regenerators. Therefore, to minimize the amount of reburning fuel injected and the impact on furnace efficiency, the primary stoichiometry should be minimized. For this study, the primary zone stoichiometry has been assumed to be 1.01 during reburning operation. It is expected that this level can, on average, be achievable on most glass furnaces, but may require some optimization of the melter frring rate and distribution. On furnaces where it is not possible to modify the melter stoichiometry without impacting the melter operation or glass quality, additional reburning fuel would be needed to offset the higher oxygen content of the flue gas. Recovery of the reburning fuel energy in the regenerators is expected to result in higher air preheat temperatures and lower primary fuel consumption, which can offset the reburning fuel consumption. The reburning process design involves utilizing empirical correlations of jet behavior in conjunction with process design parameters (zone residence times and temperatures) to specify the number, location, and size of the rebuming fuel and overfue air injectors. The correlations are used to specify injection systems which are expected to provide rapid mixing of the reburning and overfire air jets with the main flue gases. Rapid mixing of the rebuming gas and overfue air are necessary to minimize chemistry delay times due to mixing. In the conceptual designs, a dedicated fan was assumed to be required for the overfue air, which is at ambient temperatures. Thermal Analysis An overall heat transfer model of a glass furnace was developed to provide information about the potential impacts of reburning on refractory, gas and process temperatures, and on energy requirements. Specifically, the desired infonnation is the amount of energy associated with the rebuming fuel which is absorbed by the checkers, and the magnitude of any changes in the gas and brick temperatures. The temperature profiles through the regenerators are also of interest to characterize any potential changes in the nonna! operating profile. The processes considered in the thennal analysis model are illustrated in Figure 5. The thermal model calculates the mass and energy content of the gas stream as it passes through the furnace , from the entrance to the air preheat regenerator to the exit of the flue gas regenerator from known . measured process variables. The thennal analysis model also includes a time-dependent one-dimensional regenerator model and a simplified radiant equilibrium model of the melter . 5 |
