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Coal and biomass cofiring: CFD modeling 95

From modeling point of view, different approaches exist for char reactions, e.g., as
follows:
• One-film model: There is no flame in the gas phase. Char is directly oxidized into CO 2 at the
particle surface. As a result, both the temperature and CO 2 peak at the particle surface and
decay when moving away from the particle surface.
• Two-film model: The principal product at the char particle surface is CO. CO diffuses always
from the particle surface and reacts with inward-diffusing O 2 ,CO þ 0.5O 2 / CO 2 , forming
a visible flame at a distance away from the surface.
• Continuous-film model: A flame zone is distributed within the boundary layer around the
char particle, rather than occurring in one or two sheets.
The one-film model is most widely used for char reactions in solid fuel combustion
CFD, in which different char surface reaction regimes can be identified: kinetics-
controlled (zone I), diffusion- and kinetics-controlled (zone II), and diffusion-
controlled (zone III).

4.3.5 Homogeneous reactions

In coal and biomass cofiring, the combustion of the released volatiles plays a vital role
in ignition, local stoichiometry, flame stability, and pollutant formation. Therefore,
gas-phase reaction modeling is also important. The overwhelming majority of indus-
trial combustion CFD relies on global combustion mechanisms. The commonly used
global mechanisms include the two-step hydrocarbon oxidation mechanism by West-
brook and Dryer (WD) (Westbrook and Dryer, 1981, 1984) and the four-step mecha-
nism developed by Jones and Lindstedt (JL) (Jones and Lindstedt, 1988) for alkane
hydrocarbons up to butane in mixtures with air in premixed and diffusion flames. Tak-
ing CH 4 as an example, the WD two-step mechanism is

CH 4 þ 1:5O 2 /CO þ 2H 2 O (R5)

CO þ 0:5O 2 4CO 2 (R6)

And the JL four-step scheme reads

CH 4 þ 0:5O 2 /CO þ 2H 2 (R7)

CH 4 þ H 2 O/CO þ 3H 2 (R8)

H 2 þ 0:5O 2 4H 2 O (R9)

CO þ H 2 O4CO 2 þ H 2 (R10)

which includes two competing fuel breakdown reactions into CO and H 2 and two
reversible reactions controlling the rate of reactions for CO and H 2 .