OCR Text |
Show adjusted if limestone injection is employed. These approaches provide initial estimates of flyash and bottom ash from incineration of waste based fuels. 3.3. Trace Metal Emissions Emission rates for trace metals have been found to be largely independent of fuel type [2]. For metal emissions other than those which are diffusion controlled (Le. iron or aluminum from the combustion of MSW), the driving variables are combustion temperature and oxygen partial pressure [2, 8]. The work of Brittain and coworkers [8] is particularly imponant in this field, as it involves Knudsen cell mass spectrometry experiments to develop a data base, and then Gibbs Free Energy calculations to estimate the distribution of trace metal products from coal combustion. The data generated by Brittain and coworkers [8] include distribution of trace metals as a function of temperature, assuming a partial pressure of oxygen, Log P(OJ = -1. Selected data are shown in Table 5, and are used, with data from Banon et. al [2] as the basis for estimating trace metals in the bottom ash and in the gaseous products of combustion (including particulate). It should be noted that the estimation of trace metal concentrations in the bottom ash and in the gaseous products of combustion does not complete the analysis of the fate of trace metals. Condensation on flyash particles is particularly significant. A second model is under construction to evaluate this phenomena. It addresses the AQCS, and particularly the metals capture as a function of AQCS type and operating temperature. 3.4. Oxides of Nitrogen NO. emissions have been extensively studied for coal and lignite [4, 11, 12, 32, 38, 43, 50], wood and the other forms of biomass [26, 31, 38, 39, 52, 53, 55, 57, 63], and for wastes [10, 33, 36, 37, 40, 42, 64]. In all cases, while some thermal NO. generated by the Zeldovich mechanism may exist, the dominant pathway for NO. formation is the oxidation of reduced nitrogen species in the fuel or waste itself. This pathway is highlighted in Fig. 7. Given the dominance of the fuel nitrogen conversion pathway, the model approaches NO. estimation from a fuel nitrogen conversion perspective. Lignite. Estimating NO. formation from combustion of lignite in a spreader-stoker involves reliance upon extensive studies of NO. fonnation from coal. Such estimation recognizes that the nitrogen in lignite may come in amine functionalities. Alternatively it may be found in pyridine or quinole structures as well. Nitrogen in lignite can be more accessible, and more volatile, than nitrogen in the higher ranks of coal. For purposes of this model, most of the available literature is only of tangential importance, since it deals with pulverized coal combustion rather than grate fuing. The works of Munro [38], Langsjoen [32], and of Starley and coworkers [50] is particularly important, however, as they specifically address the grate fIred systems prevalent in industry. The research of Munro and of Starley and coworkers illustrates the influence of fuel nitrogen content, of grate or bed stoichiometry, and of total excess air. This leads to the following approximation equation: 11 |