FUNDAMENTALS OF PYROLYSIS

Подпись: 2.1INTRODUCTION

This chapter describes the underlying mechanisms and pathways of fast pyrolysis for liquids production before showing how these principles have been applied in working processes in subsequent chapters.

Подпись:Подпись: 2.2.1PYROLYSIS MECHANISMS AND PATHWAYS

Introduction

As biomass is heated, its various components become chemically unstable and thermally degrade or vaporise. A number of studies have shown that the main components of most biomass types, i. e. cellulose, hemicellulose and lignin, are chemically active at temperatures as low as 150°C (1). This has recently been indicated by the kinetic parameters determined by Bilbao et al. (2, 3, 4, 5, 6). Wood, is claimed to begin pyrolysis at 250°C (7). A review of the possible reaction pathways and mechanisms which the pyrolysis of wood may follow depending upon the reaction conditions are presented below. It is common to divide the reactions of lignocellulosic materials simplistically into primary and secondary pyrolysis reactions.

The component of wood which has received the most attention is cellulose. Cellulose occurs in most biomass types up to 50 wt % and has a well defined structure which allows its easy purification and separation (1, 8, 9,10, 11, 12, 13, 14, 15, 16, 17, 18, 19). This has been carried out at two different temperature ranges: up to 300°C and above 300°C. A reaction pathway for the pyrolysis of cellulose has been proposed by Shafizadeh (8, 20), Antal (9) and Kilzer et al. (10) as shown in Figure 2.

It is generally considered that primary pyrolysis of pure cellulose occurs by two competing pathways: one involving dehydration and the formation of char, CO2

and water and the second involves fragmentation and depolymerisation resulting in the formation of tarry products consisting mainly of levoglucosan as shown in Figure 2.1 (8, 9,10,11,12). At temperatures greater than 300°C, fragmentation or transglycosylation predominates which involve the conversion of cellulose into predominantly a liquid product consisting of levoglucosan and other anhydrosugars.

Two hypotheses have been proposed for the formation of levoglucosan: one by Tang (13) and Golova (14) that the glycostdic bonds are broken homolytically and that depolymerisation proceeds by a free radical mechanism. The second assumes a heterolytic transglycosylation reaction with depolymerisation proceeding by a carbonium ion intermediate (8, 9, 10, 11,15,16). The chemistry of the reactions has been reviewed in the literature (9,18).

image4,image5 image6

Water, char, СО, CO2

Tar (primarily levogiucosan)

image7Key:

10 — Primary Pyrolysis 2° — Secondary Pyrolysis CH4, ^ ^2^4

Figure 2.1 Pure Cellulose Pyrolysis Pathways 1965-1983 (8, 9, 10,

20)

Scott etal. (17) more recently have proposed the Waterloo model for the pyrolysis of cellulose taking into account two major competing pathways for the primary decomposition of cellulose by fast pyrolysis Each pathway is capable of minor rearrangement reactions to account for the variety of different products produced, due to the dependency on the cellulose morphology, degree of polymerisation, presence of alkali cations and the process parameters such as temperature, heating rate and pressure. These potential reaction pathways are shown in Figure

2.2

Подпись: Cellulose Подпись: Depolymerised Cellulose image8 Подпись: Decarbonylation Dehydration Fragmentation (Homolytic)
image9

Подпись:image10"Подпись: Formaldehyde

Подпись: Levogiucosan Cellobiosan -m Glucose
Подпись: Depolymerisa tion (Homolytic)

(17). Atypical analysis of liquids obtained from the flash pyrolysis of cellulose is given in Table 2.1.

Fructose

Подпись: Char, Gas, H2O■ці..- Other compounds

Figure 2.2 University of Waterloo Reaction Pathways 1988 (17)

Table 2.1 Composition

of Waterloo

Flash

Pyrolysis Li q

obtained From Cellulose (21)

Cellulose source

Commercial

Treated

Avicel

SS-144

SS-1440

Temperature, °С

500

502

500

Product yields (% based on moisture free feed)

Organic liquid*

72.5

83.5

87.1

Char

5.4

1.3

2.5

Water

10.8

6.1

3.1

Gas

7.8

3.9

8.9

Hydroxyacetaldehyde

15.3

6.2

8.6

Levoglucosan

7.0

31.8

26.9

Cellobiosan

4.0

11.5

10.1

Glucose

1.0

1.8

2.1

Fructose (?)

2.0

3.0

4.7

Glyoxal

3.5

5.5

6.5

Methylglyoxal

0.8

1.3

0.23

Formic acid

5.5

1.9

3.8

Acetic acid

4.9

0.1

1.4

Ethylene glycol

1.7

0.02

0.56

Formaldehyde

1.2

0.94

0.72

Acetol

2.2

0.12

0.04

Anhydroglucofuranose

N/A

5.5

Oligosaccharides

N/A

5.3

Ash

0.062

<0.01

* moisture free liquid

° pretreated with 5 mass % H2SO4 at 90°C for

5.5 hours