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Coal and biomass cofiring: CFD modeling 105

combustibles leaving the dense bed into the freeboard and radiative heat flux incident
from the freeboard onto the fuel bed. As seen in Fig. 4.9, the coupled modeling strat-
egy needs to iteratively switch between the dense fuel bed conversion modeling and
the freeboard reacting flow simulation until there is no remarkable change in both
the combustibles leaving the fuel bed and the incident radiative heat flux onto the
fuel bed.

Freeboard dilute flow CFD
SA (incl. OFA) (CFD modeling of reacting, dilute
& wall BCs gas-solid flows in freeboard, using
eulerian-lagrangian approach)

Iteratively switch between the
dense bed and freeboard
modeling, until no big change in Combustibles
● Combustibles leaving fuel bed T(x), V(x), Y (x) Radiative heat
i
● Incident radiation on fuel bed flux, Q rad (x)

Coal/biomass Dense fuel-bed modeling
(mass, property)
x = 0 x = L
PA: T(x), V(x)
Figure 4.9 Coupled modeling strategy for cofiring in fluidized bed or grate boilers. CFD,
computational fluid dynamics; OFA, over fire air; SA, secondary air; PA, primary air.

4.5.2 Special modeling issue: solid fuel conversion in a dense
fuel bed

Different approaches exist for modeling of solid fuel conversion in a dense fuel bed.
In the first approach, the porous zone model of commercial CFD package can be
used for the dense fuel bed conversion. The fuel bed itself can be included in the
CFD of the boiler (Collazo et al., 2012; G omez et al., 2014) or be modeled separately
(Nasserzadeh et al., 1991, 1993). In the former, the mass, momentum, species, and
energy source terms in the porous zone need to be properly evaluated and included
in the transport equations. In the latter, the dense fuel bed is not a part of the CFD
of the boiler, and the results got from the porous zone model are used as the inlet
conditions for the freeboard CFD.
In the second approach, empirical models are used to predict the conversion of the
dense fuel bed. For example, the dense fuel bed is treated as a 0D system, in which the
thermochemical processes are divided into two successive sections: drying and chem-
ical conversion. Phenomenological laws are used to characterize the syngas release as
a function of the main governing parameters (Costa et al., 2014). Modeling of the