THERMOCHEMICAL CONVERSION: DIRECT COMBUSTION - Biomass as a Sustainable Energy Source for the Future - page 274

fuel). Moreover, diffusion of the oxidizer to the outer surface of the solid fuel, where
the surface can oxidize, has to be taken into account, as well as diffusion of the fuel
through the pores where both homogeneous and heterogeneous reactions may take
place. Also, the solid fuel will not only vaporize, but depending on the fuel, it might
melt and eventually pyrolyze as well.
In order to understand the conversion process in a quantitative way, one could
analyze one-dimensional systems like a propagating front. Studies are carried out
on a planar traveling conversion wave or spherical particles, employing this strategy.
The plane configuration is relevant for fixed bed conversion as explained by Van
Kuijk et al. (2008), whereas the model of spherical particles applies to larger-scale
fluidized beds and entrained flow reactors. Here, we will look at the combustion of
the remaining (porous) solid particles that are composed of carbon, with mass fraction
Y C , and inert ashes that are not converted. It is assumed that reactions exclusively take
place within the particle.
Figure 9.2 shows three simplified modes for the conversion of char. In these
models, the char is converted either at the outside surface or inside the particle. Also,
a practical particle conversion situation in which an ash layer builds up is considered
in this figure. Models with more details and fewer assumptions can be found in the
literature (see, e.g., Goméz-Barea and Leckner, 2010). A general discussion on this
subject can be found in, e.g., Levenspiel (2012).
It is often assumed that the chemistry of this combustion process can be modeled
with a single reaction:

C+O 2 ! CO 2 ð RX : 9 : 6 Þ

At high temperatures, the observed behavior does not correspond to the reaction
model shown in the equation above; it is found that the mass fraction of oxygen

Shrinking sphere

Shrinking core

Shrinking density

Conversion time

FIGURE 9.2 Simplified char particle combustion modes. (Source: Reproduced with

permission from Thunman and Leckner (2007). © H. Thunman.)