The products of pyrolysis that come in contact with oxygen will undergo two kinds of combustion: glowing combustion (char oxidation) and flaming combustion (volatiles oxidation).

Figure 5.7. Schematization of char oxidation (Spliethoff, 2010).

Char oxidation is based on the reaction scheme shown in Figure 5.7.

Inside the particle or in the surface, oxygen, carbon dioxide and water vapor act as oxidants in the heterogeneous oxidation reactions:

C + 1 /2O2 ^ 2CO

C + CO2 ^ 2CO (Bouduard reaction)

C + H2O ^ CO + H2 (heterogeneous water-gas reaction)

In the gaseous phase the following homogeneous reactions happen:

CO + 1 /2O2 ^ CO2 H2 + 1 /2O2 ^ H2O

Char combustion reaction is influenced both by combustion kinetics and mass transport processes in which the limiting process depends essentially on temperature. As a function of temperature three areas can be distinguished (Fig. 5.8):

• chemical reactions (low temperature);

• pore diffusion (temperature rises);

• boundary film diffusion (high temperatures).

Combustion reactions generate very high temperatures on the surface of char particles (1370- 1650°C). With increasing temperatures the production of CO overcomes the production of CO2. For example at 1027°C the ratio between the production of CO and CO2 ranges from 5:1 to 21:1 (Matsui et al., 1986). The cause of the increase in production of CO is the limiting step in char oxidation represented by oxygen diffusion to the char particle surface. Carbon oxidation becomes a two-step reaction: first CO is produced and then it is oxidized away from the char particle.

Char combustion is exothermic (AH ~ 32kJ/kg), the activation energy is around 180kJ/mol and the frequency factor is around 1.4 x 1011 s-1. Char combustion results to be slower than volatiles combustion, which is why much of the char oxidation occurs after flaming combustion.

The overall reaction rate of char oxidation depends on oxygen partial pressure, expressed at atmospheric pressure, and the reaction order with respect to oxygen (nO2) (Anca-Couce et al.,

PORE DIFFUSION

CHEMICAL REACTION

Figure 5.8. Schematization of char combustion areas (Spliethoff, 2010).

2012; Janse etal., 1998):

where:

A = frequency factor [s-1]

E = activation energy [kJ/mol]

R = gas constant [J/K mol]

T = temperature [K] a = biomass conversion rate [—]

Xo2 = partial oxygen pressure [Pa] n = reaction order with respect to oxygen [—].

Figure 5.9 shows a thermogravimetry for a generic biomass. The peaks d1, d2 and ch represent respectively the devolatilization step (d1 and d2) and the char oxidation step (ch), obtained through deconvolution of thermogravimetric data (Fig. 5.9). The activation energy and frequency factor reported in Table 5.4 can be used for SFOR models (single first order reaction models).