03.08.2015   Pretreatment Techniques for Biofuels and Biorefineries

Effects of Pyrolysis Conditions on the Kinetics

The pyrolysis conditions affect the global kinetics by promoting specific elemental reactions. The main operational parameters for pyrolysis are temperature (and heating rate), pressure, co-feeding of different feedstocks and presence of catalysts.

11.3.3.1 Temperature and Heating Rate

Pyrolysis is governed by many parallel and series reactions characterized by their specific kinetics and the relative importance of each of these reactions will depend on the temperature of the system [41, 42]. Also, a slow heating process implies that the biomass remains at every temperature for a longer time period. As pyrolysis kinetics and heat transfer compete, pyrolysis occurs during the heating of the particles and might even be completed (at thermodynamic equilibrium) before reaching the temperature set-point. At low heating rate, more decomposition happens at low temperature such that more bio-char and less volatiles (condensable (bio-oil) and non-condensable gases) are produced.

Table 11.2 Pyrolysis kinetics parameters for selected materials from literature

Biomass

Model

A (min 1)

E (kJ/mol)

n

Temperature

Reference

range of validity (°K)

Poplar

One step de-

2.14

x 1012

153.9

1

<673

[22]

composition

x 1012

Wheat straw

Three pseudo-

2.57

69

2.3

<873

[24]

components

linear

3.97

x 107

78

0.65

<873

3.17

x 106

80

2.7

<873

combination

Rice husk

Two pseudo-

1.02

x 102

33.1

1.5

<623

[26]

components

linear

combination

3.3 X

101

28.3

2

623-823

Rice husk

Two pseudo-

7.25

x 103

30

0.91

<640

[25]

components

linear

combination

5.14

x 102

16.3

0.3

640-813

Cellulose

Two pseudo-

4.69

x 105

82.7

1

<623

[38]

components

linear

combination

1.33

x 1023

282

2

623-673

Cellulose

One step de-

1.6 X

1010

244

1

<623

[35]

composition

Cellulose

Two pseudo-

7 X 107

126

1

—

[39]

components

linear

4к 1017

234

1

—

combination

Lignin

Two pseudo-

5.39

x 104

67

1

—

[39]

components

linear

2.1 X

105

70.7

2

combination

Klason

One step de-

1.21

x 1012

156.5

1.53

<1000

[40]

lignin

composition

One of the main mechanisms controlling the interaction between the temperature and the heating rate is the stabilization and reorganization of the macromolecular solids. Thermal decomposition brings lighter molecules to unbind from the solids (biomass or waste in the present case) to form a volatile phase. In parallel, this creates physicochemical instabilities that lead to a molecular rearrangement. The kinetics associated to these intra-molecular modifications then inhibits the volatile formation kinetics. If the heating rate is slow, stabilization occurs and higher char yield is obtained. On the other hand, heating faster will impede stabilization and volatile production will be promoted. Temperature has a different effect on the pyrolysis products. Char production decreases with increasing temperature and the yield of gas increases (both condensable and non-condensable). The extent of the gas thermal cracking determines the yield of non-condensable gas and average molecular weight of the volatile fraction. Thermal cracking kinetics becomes important with increasing temperature and gas residence time.