Secondary pyrolysis kinetics has been studied by number of researchers to account for the conversion of primary liquids to secondary products such as char, tar and gases. Antal (45, 46) proposed the conversion of primary vapours to either gases or tars by a temperature based competitive reaction scheme (Figure 2.8).

Primary Pyrolysis Secondary Pyrolysis

The first reaction produced more permanent gases by cracking the reactive volatile matter to smaller, less reactive species. The second reaction produced refractory condensable materials, which may be tar or some combination of water-soluble organic compounds.

where:

Cb the mass fraction of carbon in biomass

Cv the mass fraction of carbon composing the reactive volatile matter (carbon in volatile matter/carbon In sample pyrolysate)

Cgi, Cg2 the mass fraction of carbon composing the permanent gases

C the mass fraction of carbon composing the refractory condensable materials, including the tars.

Recently kinetic models for the secondary decomposition of primary pyrolysis tars have been proposed by Liden (80), Diebold (56) and Knight et al. (81). Liden (80) and Diebold (56) using different reactor configurations, proposed similar kinetic models. The reaction scheme used is shown in Figure 2.9.

The kinetic expression used for the estimation of the yield of liquid products and the values of the kinetic parameters used for each model are listed below:

1-exp(-k3)> k3 q j

where

k3: the reaction rate constant for the oil decomposition step [s-1]

q: the mean residence time for the oil decomposition [s]

x0: the theoretical ‘ultimate’ oil yield

with: k3 = 4.28 x 106 exp ^ ^ j s-1; x0 = 0.703

and; k3 = 1.55 x 1 o5 exp f’8^.34) s-1; x0 = 0.78 or 0.76

for Liden and Diebold respectively.

The reaction scheme used by Knight et al. (81) is shown below in Figure 2.10 with the following reaction rate expression :

k1b5

(k2-ki )[exp (-k2t)]

with

kj = 1.483 x 106 exp ^ s-1 k2 = 23.12 exp s_^ b§ = 0.811

where,

ki: reaction rate constant for the first order production of oil

k2: reaction rate constant for the first order decomposition of oil

b5: maximum fractional conversion of wood to oil

ki

W00d——————— ► Gas + Oil + Char

4%

Gas + Char

Figure 2.10 The Reaction Scheme of Knight et al. (81)

VasaJos et al. (82) and Scott et al. (83) tested the above models using their own experimental data. Vasalos et al. (82) found that using Liden’s parameters, they obtained a better fit of the liquid yields of -20% for the particle size range 300-425 mm, while Diebold’s parameters gave a better fit of ± 10% for the particle size range 500-600 mm. Knight’s et al. model did not predict satisfactorily liquid yields for either particle size range. They concluded that the variations between the predicted values and the experimental results could be attributed to the exclusion of the water yield in the reaction mechanism, the residence times used, and the type and size of biomass tested.

Scott and Piskorz (83) found when testing the models of both Liden and Diebold, that the predicted liquid yields agreed with achieved yields within ± 10% for the temperature range 500-700°C with residence times of up to 1 second. Low predictions of liquid yields at the highest temperature were attributed to the assumed water yield, the constant x0 parameter or the total yield being normalised

to 100%.

2.3.4.1 Summary

As pyrolysis is a very complex process and the different intermediates formed are difficult to collect and identify, various approaches have been used to develop kinetic models. Most predict weight loss rather than product yield and distribution. The kinetic parameters vary from one model to another because they are very sensitive to experimental conditions. One research group found that even a decrease of 1 kCal/mol (from 31.8 to 30.8) in the activation energy of tars caused the predicted value of the liquid yield to increase by approximately 16 % (84).

Stagewise models have been discussed earlier with regards to pyrolysis pathways and mechanisms.