MODELING CHAR OXIDATION AS A FUNCTION OF PRESSURE ...

Background
1. Introduction
The rate of char oxidation is an important issue in coal utilization. Char oxidation
is the rate-determining primary step in coal combustion. The other primary step,
devolatilization, typically occurs about an order of magnitude faster (Smith, 1982). The
kinetics of char oxidation determine the pattern of heat release in combustors, which may
be important in influencing other variables, including NO x formation (Bar-Ziv et al.,
1989). Unburned carbon in fly ash affects many aspects of power plant performance and
economy, including boiler efficiency, electrostatic precipitation of fly ash particulates and
the value of fly ash as salable byproduct (Tyson and Blackstock, 1995; Hurt, 1998).
High carbon content prevents the sale of the fly ash to cement and construction
industries, and necessitates the disposal of the fly ash at the expense of the producer
(Hurt, 1998). Although acceptably low unburned carbon levels are achieved for many
traditional steam coals, combustion zone modifications for NO x control have made the
task of maintaining low residual carbon levels in boiler fly ash much more difficult
(Fiveland and Jamaluddin, 1992). In order to keep unburned carbon below a certain level,
knowledge of the char oxidation rate, especially during late burnout, is essential in boiler
design and operation.
Technologies have been under development to burn coal more efficiently and
cleanly. Among these technologies, two have attracted increasing interest around the
world: pressurized fluidized bed combustion and high pressure coal gasification (Balzhiser
and Yeager, 1987). In these processes, high pressure is used to reduce the required size of
reactors and to increase the conversion efficiency to electric power in combined-cycle
power plants.
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