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Show INTRODUCTION In the past, utility companies that operate coal-fired boilers entered into long-tenn contracts with their coal suppliers to secure a steady supply of similar fuels for smooth, trouble-free, day-today operations. In today's environment long-tenn contracts might seem unappealing because utility strategists would rather be free to switch to different coals to lower costs or meet emissions regulations. Coal switching is often an important pan of compliance strategies that seek an optimal balance among technology upgrades, bubble-based emissions averages across several boilers, and open-market trading allowances. Although switching is now motivated by S~ compliance, a new coal can affect many other operating characteristics, including pulverizer performance, heat rate, slagging and fouling, unburned carbon in ash, NOx emissions, and certainly cost. In ligh~ of the push for tighter NOx regulations before the tum of the century under Title I of the Clear Air Act Amendments, it is prudent for utilities to factor in the impact of a coal switch on NOx and unburned carbon emissions into their coal selection decisions. A coal switch in a utility boiler often entails a surprising number of adjustments to all the engineering subsystems devoted to coal handling, including storage, dust suppression, milling and drying, air delivery, and nearly all aspects of flame management and furnace operation. The most imporumt impacts are seen in the near-burner flame characteristics, where coal decomposition chemistry generates the gaseous fuel components that ignite and bum in the early mixing layers to stabilize the flame. Indeed, among the various stages of coal combustion chemistry, none is more sensitive to coal characteristics than devolatilization. With the advent of so-called coal network depolymerization models during the past decade, it is now possible to predict how the different properties of various coals will affect the initial stages of pulverized coal combustion. Three phenomenological network models are available (Niks~ 1995; Solomon 1993; Fletcher, 1992). All represent devolatilization as a depolymerization that disintegrates coal's macromolecular structure into smaller volatiles fragments with subsequent reintegration of larger intermediates into char. Whereas FLASHCHAIN and the CPD model are based on the same concise set of rate mechanisms, the FG-DVC model is all-encompassing. Each can generate predictions for yields, transient evolution rates, and various product characteristics based on coal-specific characterization data. The superior performance of these models is due to their elaborate submodels for coal's macromolecular configuration and chemical constitution. However, along with more illuminating structural features, these approaches incorporate far more input than classical, two-component models. FLASHCHAIN and CPD have comparable numbers of input parameters, but structural 2 |