THERMOCHEMICAL CONVERSION: DIRECT COMBUSTION - Biomass as a Sustainable Energy Source for the Future - page 272

long period compared to coal. For biomass, the pyrolysis stage may take 50% of the
total conversion time, while for coal, it is often negligible compared to the time for
char burnout to be completed. In addition, the heat released by the combustion of
volatile matter relative to the heat released by char combustion is much higher for
biomass than for coal, which could cause problems in the furnace design.

9.2.3 Combustion of Volatile Matter

The homogeneous combustion of the volatile components is characterized by a very
high reaction rate. The burning time of volatile matter is essentially determined by the
release rate of volatiles and their mixing with air. Volatile components formed on the
surface of the biomass particles diffuse into the bulk gas. Therefore, the concentration
of volatiles decreases with an increase of the distance to the biomass particle. In con-
trast, the temperature and oxygen concentration increase with an increase of the dis-
tance to the particle. At a certain distance from the particle, volatile matter and oxygen
are present at stoichiometric concentrations. This leads to stabilization of the combus-
tion and formation of a flame enveloping the particle. Under laminar flow conditions,
the diameter of this flame is about three to five times the diameter of the particle. In
pulverized fuel combustion and under more turbulent flow conditions, the volatile
matter combustion processes of the individual particles combine, forming a large
gas flame enclosing groups of particles. In fluidized bed operation, the flame cannot
form inside the bed but only above the bed surface.

9.2.4 Combustion of Char

The porous structure that remains after the volatile matter is released from the particle
consists almost only of carbon and ash and is called char. The carbon is oxidized with
oxygen, at a sufficiently high particle surface temperature, resulting in carbon mon-
oxide, carbon dioxide, and water vapor.
The reaction rate of the heterogeneous combustion of char is orders of magnitude
lower than the homogeneous combustion of volatile matter. The total combustion time
is therefore determined by the rate of char combustion. Together with the pyrolysis
stage, it is decisive for the design of biomass firing systems.
The course of char combustion of a single particle is shown in Figure 9.1. Inside the
particle or on the surface, the heterogeneous oxidation of carbon takes place with the
oxidants oxygen, carbon dioxide, and water vapor:

2C +O 2 ! 2CO ð RX : 9 : 1 Þ

C+CO 2 ! 2CO Boudouard reaction

ð Þ ð RX : 9 : 2 Þ

! CO+H 2 heterogeneous water

C+H 2 O ð - gas reaction Þ ð RX : 9 : 3 Þ

In practice, reactions (RX. 9.2) and (RX. 9.3) are also virtually irreversible. Today, it
is often assumed that first only conversion to carbon monoxide takes place directly on
the particle, either by incomplete combustion (RX. 9.1) or by gasification ((RX. 9.2)