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Coal and biomass cofiring: fundamentals and future trends 129

different fuels are involved, with biomass being much more reactive and having higher
volatiles and moisture content than coal. Combustion models based on coal need to be
modified to account for the effects of biomass cofiring on the overall combustion
behavior. To achieve good modeling results, the use of appropriate submodels for
the description of the behavior of biomass particles is very important. Some models
for burning blends of biomass and coal have been developed with a focus on predicting
combustion efficiency, fouling, and emission of pollutants for different fuels and their
mixtures in commercial-scale FBCs (Gayan et al., 2004).
Computacional fluid dynamics (CFD) and modeling techniques are becoming
increasingly important tools to assess the impact of biomass cofiring in the operation

of burners and boilers (Alvarez et al., 2014). They have been shown to be reasonably
effective in predicting the in-furnace temperature profiles and heat fluxes, as well as
slag deposition (Degereji et al., 2012). Future developments in the field of computer
simulation would be extremely useful in predicting ash deposition problems, without
the need for expensive and time-consuming field tests. The current state-of-the-art
CFD-based models are capable of solving the complex interdependent processes
such as fluid flow, turbulence, heat transfer, heterogeneous and homogeneous chemi-
cal reactions involved in cocombustion. However, the complete description of particle
trajectories, chemistry of devolatilization, char oxidation, and volatile combustion is
still, mainly, based on simple models (Tabet and G€ okalp, 2015).

5.5.4 Cofiring boiler types
There are some typical boilers used in existing coal-fired power plants that could be
used for cofiring: grated combustors, pulverized fuel combustors (PFCs), FBCs, and
cyclone boilers.
Grated combustion systems. They use grate-fired furnaces and underfeed stokers.
Fuel is directly combusted over a grate with no further processes or circulation of
air. It is the simplest and oldest design for combustion of solid fuels (Fig. 5.3). How-
ever, it is the least efficient and has high flue gas emissions. Temperatures in the com-
bustion chamber may reach 1300e1400 C, which can cause ash melting and

corrosion. Different types of grate furnaces (up to 20 MW th ) are available: fixed, mov-
ing, and vibrating. Underfeed stokers are used in small- and medium-scale systems up
to a nominal boiler capacity of 6 MW th (Dai et al., 2008). Water-cooled vibrating grate
boilers for fixed-bed combustion is a well-known technology for power generation
from wood residues. Based on natural circulation, these boilers are designed to burn
low-heating-value (LHV of about 13.8 MJ/kg) wood residues, with 30% humidity.
Wood fuels could be used in the grate combustor without big problems, but for other
biomass types, it is generally sensitive to changes in fuel quality and moisture. Mix-
tures of wood fuels can also be used, but mixtures of fuels with different combustion
behavior and ash melting points, such as blends of wood with straw or grass, would not
be possible. Although the grated boilers present low operational and maintenance
costs, their low thermal efficiency when compared with the FBC and PFC limits the
extensive application of this system.