Page 147 - New Trends In Coal Conversion

110 New Trends in Coal Conversion

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flame zone. However, the refined JL 4-step scheme involves [H 2 ] in the reaction
rate of (R9), which can cause numerical instability or difficulty in CFD simulations,
e.g., in the zones without H 2 .
In the CFD analysis of a 0.8 MW natural gas oxy-fuel flame, three global mecha-
nisms are compared (Yin et al., 2011): (1) the original WD 2-step scheme (Westbrook
and Dryer, 1981), (2) the refined WD 2-step scheme (Andersen et al., 2009), and (3) a
newly refined JL 4-step scheme. The newly refined JL 4-step mechanism is generated
by using the H 2 oxidation model of Marinov et al. (1996) to replace the reversible H 2
oxidation reaction (R9) in the original JL 4-step mechanism. The Eddy Dissipation
Concept (EDC) is used for turbulenceechemistry interaction. The CFD results of
the three computational cases are compared with each other and also against the exper-
imental data. When applied to oxy-fuel combustion, the original WD 2-step scheme is
found to overpredict the flame temperature and also largely underpredict the CO level.
Both the refined WD 2-step and newly modified JL 4-step schemes can predict reason-
ably well the relatively high CO level in the furnace, in which the latter also predicts
reasonably well H 2 level and flame temperature (Yin et al., 2011).
Recently, Chen and Ghoniem (2014) perform a CFD study of a swirling diffusion
flame under air-fuel and oxy-fuel conditions, respectively. It is found the original WD
2-step global mechanism (Westbrook and Dryer, 1981) with either the Eddy Dissipa-
tion model or EDC for turbulenceechemistry interaction cannot reasonably predict the
CO concentrations. The WD quasi-global mechanism (12 species and 22 reactions)
(Westbrook and Dryer, 1981, 1984) combined with the EDC is found able to capture
the chemical effects of CO 2 in oxy-fuel combustion and show improved performance
in both air-fuel and oxy-fuel flame CFD simulations.
As recommended in (Yin and Yan, 2016), for large-scale oxy-fuel combustion
CFD, the 2-step or 4-step global mechanisms with kinetic parameters refined for
oxy-fuel conditions, in combination with the EDC for turbulenceechemistry interac-
tion, can be used. In case of need, more computationally expensive WD multiple-step
quasi-global mechanism (12 species and 22 reactions), coupled with the EDC, can also
be used. For nonpremixed oxy-fuel flame, the mixture fraction method and the steady
flamelet chemistry model are also alternative options.
Oxy-fuel conditions also have important impacts on char reactions. As reviewed
and recommended in (Yin and Yan, 2016), char gasification reactions become increas-
ingly important under oxy-fuel conditions due to the elevated concentrations of CO 2
and H 2 O. As a result, the traditional single-film model needs to be extended to account
for not only char oxidation (R1 and R2) but also char gasification reactions (R3 and
R4) to appropriately predict the reaction rates and char particle fate.

4.7 Concluding remarks

CFD modeling of coal and biomass cofiring has been discussed in detail in this chapter,
from which the key modeling strategies, modeling issues, and the recommendations
are summarized in Table 4.1.