Table

Comparison of Characteristics of Combustion Methods

Variables

Combustion Method

Fixed Bed (Stoker)

Fluidized-Bed

Suspension

Particle size Approximate top size Average size System/bed temperature Particle heating rate

Reaction time Volatiles Char

Reactive element descriptiona

-100 sec -1000 sec

Diffusion-controlled combustion

103-104o/sec

10-50 sec 100-500 sec

Diffusion-controlled combustion

103-106o/sec

Chemically controlled combustion aDescribed in text and illustrated in Figure 5-8. Source: Adapted from van Krevelen [1] and Elliot [2].

Devolatilization of Pulverized Coal and Volatiles Combustion

The design of coal burners and furnaces is very dependent on the volatile matter released by the coal as it heats [15]. In flames, pulverized coal heats primarily by convective heat transfer with hot gases which are entrained and recirculated, with heating of only the coarsest particles being dominated by radiation from the hot regions of the flame. For large flames, in which coal remains for several hundred milliseconds, the extent of devlolatilization is strongly influenced by temperature rather than by limitations due to heating times or devolatilization kinetics. Studies have indicated that changes in heating rate (in the range of 1 to 50 x 103 °C/sec) have little effect on volatile yield and that the yield is more strongly influenced by the final temperature, with an increase in final temperature producing an increased yield [2]. Volatiles yield is also found to depend upon particle size, with smaller particles tending to yield more volatiles. Also, volatile yield can vary significantly within a given rank for coals that are similar in composition and mined adjacent to each other in the same coal basin (e.g., subbituminous coals from neighboring Powder River Basin coal mines) [16]. The combustion of the volatiles is generally assumed to be a homogenous reaction, although the possibility of volatile matter burning heterogeneously has been suggested by Howard and Essenhigh [17]. The burning of the volatiles is a very fast process that is measured in milliseconds [18].

Char Combustion

Char combustion is a much slower process than devolatilization and therefore determines the time for complete combustion in a furnace, which is on the order of several seconds for pulverized coal at furnace temperatures. Studies have shown that the combustion of the char begins with chemisorp-tion of oxygen at active sites on char surfaces and that the decomposition of the resultant surface oxides mainly generates carbon monoxide (CO) [2,19] (Some researchers think that an amount of CO2 may also be released during this step.) The CO is then oxidized to CO2 in a gaseous boundary zone around the char particle. Fresh reaction sites are continuously exposed as the surface oxides are decomposed. CO2 then either moves off into the gas stream or is reduced to CO if it impinges on the char. The overall reaction mechanism is complex [2,20], but the combustion of char involves at least four carbon-oxygen reactions [19]:

as well as the oxidation of non-carbon atoms, mainly:

which may be followed by

Some species of the mineral matter can be volatilized during combustion, while others are left behind as ash. In both cases, the mineral matter is usually altered in composition and mineralogy.

The rate of char combustion is a complicated process, as it is influenced by mass transfer by diffusion through the pores and the surface reactions. The diffusion coefficients are strongly dependent on pore diameters and pressure, and the surface reaction is influenced by the formation of activated adsorption complexes and their decomposition [1].

The rate of char combustion is controlled by two processes: the chemical reaction rate of carbon and oxygen on the char surface and the rate of mass transfer of oxygen from the bulk gas stream through the boundary layer surrounding the particle to the particle surface. This is illustrated in Figure 5-8 [20], where the general relationship between temperature and reaction rate for a heterogeneous gas/solid system is shown. At low temperature (Region I), the chemical reaction rate is slow compared to the rate of diffusion through the pores; therefore, oxygen completely penetrates the char matrix. Combustion then takes place within the porous char, and the density of the char rather than the diameter changes. In this case, the oxygen concentration at the particle surface would be the same as that in the bulk gas stream, and the overall reaction rate would be limited by the inherent rate of the chemical reaction. In Region I, the rate of surface reaction is rate determining, and oxygen molecules diffuse fast enough to reach the whole internal surface. The reaction rate is given by:

so that q a d, where q is the char combustion reaction rate (kg/m2/sec), d is the diameter of the particle (m), pv is particle density (kg/m3), and t is burning time (sec).

The rate of chemical reaction may be expressed by a generalized expression of the type:

where A is the true pre-exponential constant (kg C/m2/sec [atm O2]-m), T is the particle temperature (K), R is the universal gas constant, and, because

0 0

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