OCR Text |
Show Base Model Adaptation. The base model chosen, however, required adaptation in order to handle the highly oxygenated solid fuels and wastes being analyzed. The specific code created difficulties in considering the oxygen in the fuel. Further, and more significantly, the code assumed that all reactions go to completion. Carbon conversion is essentially 100 percent in the G-M model. The adaptations included adjusting the equivalence ratio (Q» for the oxygen in the fuel by calculating an apparent equivalence ratio (<p') which is substantially lower than the equivalence ratio considered by the system. The sequence of calculations used for this calculation are as follows: SR' = SR - O2 (fuel) (1) Where SR' is the apparent stoichiometric ratio (adjusting for oxygen in the fuel), SR is the stoichiometric ratio, and 02 is the moles of Oxygen contained in the fuel. SR is further defmed as SR = moles O2 (actual)/moles O2 (stoichiometric) (2) As a practical matter, equivalence is then calculated as follows: ~ = I/SR (3) and <p' = I/SR' (4) This correction in equivalence ratio is a mechanical adjustment for the code of the existing model, which apparently does not consider oxygen bound in the fuel itself. With this correction, the output of the model reproduces conventional stoichiometry models used throughout the power generation industry. The correction made for incomplete solid fuel combustion recognizes that existing grate fired solid fuel boilers typically do not achieve 100 percent carbon conversion. Junge [27], for example, places the carbon conversion for wood at 92 - 97 percent. Other authors [52, 53] have reported carbon conversions for biomass combustion at about 98 percent. Schwieger [43] has estimated carbon conversion rates for coal and lignite stoker-fired boilers at 96 - 97 percent. Langsjoen [32] reports carbon conversion levels for coal fued stoker systems as follows: spreader-stokers with flyash reinjection, 94 - 98 percent; spreader-stokers without flyash reinjection, 93 - 96 percent; and overfeed pile burners without reinjection, 96 - 98 percent. Carbon conversion rates for MSW incinerators may be as high as 98.5 percent [33], although typical guarantees are on the order of 95 - 96 percent [see, for example, 64]. Carbon conversions measured for MSW combustion have typically been on the order of 96 - 97 percent [40, 42]. This correction has a substantial impact upon solid products of combustion (see Fig. 4), and upon the flame temperature as calculated. 6 |