Introduction

The carbonization of a product consists in heating it in a confined atmosphere, i.e. in the absence or lack of oxygen. When heating a material above 300°C in the absence of air, the organic material decomposes in three phases: gas, liquid and solid.

In the pyrolysis processes, a thermal degradation occurs between 400 and 600°C in the complete absence of oxygen. These processes are characterized by the indirect heating of the material through the furnace wall (or pipes). The pyrolitic products, the solid (mix of char metals and mineral matter) and the hot gases (condensable and noncondensable mixture), are collected. Their relative proportions depend on the nature of the material, the applied technology and the pyrolysis conditions, i.e. temperature, pressure, heating rate, etc. The reductive atmosphere of the furnace is mainly a function of the pyrolitic gas composition.

In the gasification processes, the material is directly heated at higher temperatures (800-1000°C) by partial combustion of the contained carbon into carbon monoxide. A limited amount of air (or oxygen) is admitted to the furnace according to the stoichiometry (formation of CO2 and H2O) so that the reductive atmosphere in the furnace is governed by the ratio CO/CO2.

Wood carbonization is a very well known process, used in the past for the production of charcoal in primitive extractive metallurgy. In this case, the volatile matter and the water are extracted from the wood, leaving solid charcoal.

The carbonization of a sample proceeds progressively from the outside to the inside: it will depend on the heat transfer to the material, but also on the heat transfer inside the material. The residence time at the selected temperature is then of major importance in order to complete the carbonization. For each kind of material, the residence time has to be increased with its dimensions. If the residence time is too low, incompletely carbonized material will be found at the heart of the piece.

Slow reactions at very low temperature maximize the solid yields, as in charcoal production since antiquity. Changing the heating rate, temperature, pressure and residence time leads to substantial modifications in the proportions of the gas, liquids and solids. High heating rates (up to 1000 K/min) minimize the char formation and rapid quenching favours the condensation of the liquid phase before the cracking into gaseous products. Table 10.1 gives a general overview of the different kinds of processes (hydrogenation and reactive processes are not included).

It is then very difficult to compare the gas, liquid and solid yields obtained for a specific material by different authors as the operating conditions can be very different (mass between a few milligrams to few grams, heating rate from a few K/min to tens of K/sec, etc.).

The use of polymers has increased by a factor of about three since 1980 and life-cycle analysis shows that energy and raw material saving could be reached by the reprocessing of plastics recovered from waste streams. Today, PVC, PE and PET are easily sorted from different waste streams for reprocessing, but all sorting plants evolve large quantities of mixed plastics. The main conditions for efficient material reprocessing concern the purity of the recycled feedstock, i.e. the quality of the sorting process. For a lot of waste streams, as well as for sorting plant refuse, it is not economic to separate the different kinds of polymers. This explains the large amount of plastic waste landfilling in Europe. When easy and cheap plastic waste sorting is feasible to produce high-quality secondary products, material reprocessing is the best solution. When wastes containing large quantities of plastics are not suitable for sorting, the main alternative to landfilling is incineration, but the high net calorific value of this kind of waste could be a major problem. For mixed plastics low in PVC, energy valorization (upgrading) in cement kilns or in the steel industry is useful. An alternative way consists in pelletizing the mix in order for it to be gasified, but the preparation costs are relatively high.

Another cleaner alternative consists in producing solid, liquid and gaseous fuels by pyrolysis. The solid fuel could be upgraded by mechanical separation of metals and minerals in order to produce a cheap feedstock to a classical gasifier. Moreover, selected additions during pyrolysis could entrap pollutants such as chlorine and heavy metals [1-3].

The main advantage of pyrolysis over direct combustion in a waste-to-energy unit is a tremendous reduction in the volume of product gases (10- "to" 20-fold). This leads to a significant reduction in the complexity of the exhaust gas purification system. Moreover, pyrolysis of waste containing plastics could be performed with less charge preparation, so that minerals and metals are easily separated during the solid fuel conditioning and less ash are produced.

The thermal degradation of polymers has been extensively studied for different purposes, performed by analytical pyrolysis for information on the kinetics and mechanisms of degradation and has been extended during the last few decades to studies of the catalytic effects taking place under pyrolysis of complex waste materials [4,5].

Table 10.1 Pyrolysis processes Z

Process

Feed size

Moisture

Heating

Residence

Temperature

Pressure

Products

rate

time

(°C)

Slow carbonization

Large

Very low

Days

450-600

Atmospheric

Charcoal

Slow pyrolysis

Medium <200 mm

<15%

10-100 K/min

10-60 min

450-600

Atmospheric/vacuum

Gas, oil, char

Fast pyrolysis

Small <1 mm

<10%

Up to 1000 K/s

0.5-5 s

550-650

Atmospheric

Gas, oil, (char)

Flash pyrolysis

Small <1 mm

<10%

Up to 10000 K/s

<1 s

450-900

Atmospheric

Gas, oils, (char)

Table 10.2 Influence of operating conditions on carbonization yields of polyethylene

Slow

Fast

Slow

Fast

(500oC)

(500oC)

(700oC)

(700oC)

Gas (wt%)

l0

l0

l5

50

Liquid (wt%)

SS

90

S5

50

Solid (wt%)

2

0

0

0

This section is dedicated to a survey of the literature on carbonization product yields. Very large discrepancies can be seen between the results obtained by different authors in the carbonization yields for the same material (Table 10.2).

The following section reviews the literature data summarizing the behaviour during carbonization of five individual polymers, i.e. polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC) and polyethylene terephthalate (PET). For each polymer, results will first be presented for flash pyrolysis then for slow pyrolysis by the isothermal and dynamic methods.

In slow pyrolysis, temperatures are generally lower than for flash pyrolysis, but residence time is longer. Meanwhile, there are two different methods. One method is the isothermal-static method where the charge is directly heated at a given temperature and maintained during a certain time while in the dynamic method (TGA) the charge is progressively heated.

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