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Introduction

Polyvinyl chloride (PVC) is a widely used chlorinated polymer present in many applications of daily life such as 

pipes, window frames, cables, flooring, packaging, medical products or car under-floor protection. In 1999, 4.1 

million tons of PVC waste were generated in the European Community, and this is predicted to increase 80% up to 

7.3 million tons in 2020 1. Currently, most of PVC waste is landfilled, a small fraction is mechanically recycled and 

incineration accounts for about 15% of total PVC waste. New technologies are also being developed that will make 

pyrolysis+combustion treatments (with initial dehydrochlorination) economically viable 2.

Thermal decomposition of PVC can be represented by two main steps. In the first one, dehydrochlorination takes 

place and as soon as hydrogen chloride starts to be generated, benzene is produced through intramolecular 

cyclization of polyene radicals formed from direct scission of the remaining polyene chains. In the second step, the 

polyene chains are believed to react through intermolecular reactions and these crosslinked chains undergo further 

reactions to form alkyl aromatic hydrocarbons and char residue 3,4.

Incineration presents some advantages compared to other options, such as energy recovery and great volume 

reduction. Several disadvantages are also involved, like heavy metal and toxic chloroaromatic compounds 

emissions (PCDDs, PCDFs, PCBs, chlorobenzenes, chlorophenols,...) due to the chlorine-containing waste. In spite 

of this, it has been proven that disturbances in the combustion conditions are much more important than the chlorine 

content of the fuel or the chlorine source (PVC, NaCl,...) for the formation of some of these compounds 5,6. PCDDs, 

PCDFs and PCBs cannot be easily monitored in industrial incinerators, therefore other compounds like 

chlorobenzenes (CBzs), that are present in higher quantities, have been proposed as indicators 7. Chlorobenzenes 

have been chosen as indicator since they have shown good correlation with PCDD/F emissions and are known 

precursors in PCB formation. This laboratory study deals with the formation of chlorobenzenes during PVC thermal 

decomposition and the effect of different operating parameters on their formation.

Materials and Methods

Experiments were performed with PVC resin Etinox-450 free of additives obtained by emulsion polymerization, 

which is commonly used in low viscosity plastisols. It is a white powder with a medium molecular weight and a very 

small particle size ranging from 1.4 to 24.4 mm. Table 1 shows the chemical composition of the PVC obtained by 

elemental analysis (C, H, S) and X-ray fluorescence.

Table 1

Chemical composition of PVC resin

Influence of Chlorine and Oxygen on the Formation of Chlorobenzenes during PVC 

Thermal Decomposition 

Ignacio Aracil1, Andres Fullana1, Juan Conesa1, Sukh S Sidhu2

1Department of Chemical Engineering. University of Alicante

2Environmental Engineering. University of Dayton

Element wt % Element wt % Element wt %

Cl 55.2 Na 0.12 Ca 0.017

C 38.4 O 0.076 Si 0.0095

H 4.80 Zn 0.034 Al 0.0057

S 1.40 K 0.018

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Fly ash obtained from the US EPA was used as catalyst in some of the experiments. This fly ash, denoted as OX fly 

ash, had been collected from an electrostatic precipitator of a grate-fired MSW incinerator and thermally treated at 

500oC in air for 24h, and its organic carbon content (equals the total carbon content) was 0.03%. All the chemical 

properties of this fly ash can be found elsewhere 8. 

Figure 1 shows a drawing of the system designed to conduct the experiments. For each experiment, 25 mg of PVC 

were placed in a tubular glass reactor (100 mm length, 7 mm ID) and heated to 500oC at 30oC/min in a 25 mL/min 

helium (He) stream. In the reference experiment, the compounds evolved entered the second reactor at 750oC, 

where high-temperature reactions among compounds occurred. A 25 mL/min of He flow was added to the second 

reactor exit gas stream before it entered the low-temperature zone (300oC). The products were then trapped in an 

adsorptive (XAD-2) trap. Simultaneously, using a 1 mL/min split, products were also monitored on-line with a mass 

spectrometer. Six deuterated aromatic compounds (dichlorobenzene-d4, naphthalene-d8, acenaphthene-d10, 

phenanthrene-d10, chrysene-d12 and perylene-d12) were added to the XAD-2 resin before starting the experiments 

and these were later used as internal standards to quantify the reaction products. All the compounds trapped in the 

resin were extracted with methylene chloride and analyzed by GC/MS. Major products (aromatic hydrocarbons) were 

identified using MS spectral library matches, while standards of CBz isomer were used to quantify CBz yields.

Figure 1. Schematic drawing of the system designed for the experiments.

In some of the experiments, calcium hydroxide (75 mg) was placed after the first reactor to trap the hydrogen chloride 

coming from PVC thermal decomposition. The effect of the oxygen (about 11% v/v in the total flow) in the lowtemperature zone was studied by changing the secondary flow from He to air. Several experiments were conducted 

where hydrogen chloride gas was added to the secondary flow in the same quantity (10 mL) and during the same 

time interval in which HCl was generated during PVC decomposition. Finally, some experiments were performed 

where the gas stream in the low-temperature zone was passed through a fixed bed of OX fly ash (100 mg).

This experimental design enabled us to investigate the impact of the following four parameters:

1) The presence/absence of HCl in the high-temperature zone.

2) The presence of oxygen in the low-temperature zone.

3) The addition of hydrogen chloride to affect only reactions in the low-temperature zone.

4) The presence of OX fly ash in the low-temperature zone to act as catalyst.

By combining these four parameters, a total of sixteen experiments were performed. 

Results and Discussion

Figure 2 shows the total yields of non-chlorinated aromatics detected. The major ones were, in this order, benzene, 

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toluene, naphthalene, indene, phenanthrene, 2-methylnaphthalene and 1-methylnaphthalene. There was no significant 

difference in the total yields and no clear trend could be observed. This indicates that the parameters investigated in 

this study have little effect on the formation of non-chlorinated aromatic compounds. However, the behavior of 

chlorobenzenes was influenced by the parameters investigated in this study. It can be noted from Figure 3 that 

oxygen was the most important parameter enhancing CBz formation; the only case in which this did not occur is 

when neither fly ash nor HCl were present (this only happened when Ca(OH)2 was present after 1st reactor and no 

additional HCl was injected after 2nd reactor). This is logical, since if no chlorine source is present (HCl or fly ash), no 

difference should be expected on the total CBz yields in the presence or absence of oxygen. When oxygen was 

present, chlorination of benzene was higher in the presence of metals (OX fly ash). This is probably due to 

conversion of HCl to Cl2 (metal catalyzed Deacon reaction) or the presence of metal hypochlorites (Cu2OCl2), both 

of which are highly efficient chlorinating agents 9. Table 2 details the results of the different congeners of 

chlorobenzenes in each experiment and the conditions in which they were formed.

Table 2

Yields (in ppm) of the different CBz congeners for each experiment and conditions

When comparing two by two the yields of total chlorobenzenes in experiments which only differed in one parameter 

between them, some interesting results arise, although it was not always easy to interpret that because the behavior 

of the congeners was different. Thus, oxygen clearly rises CBz yields, as mentioned above, but according to the 

results, this effect is almost independent on the presence of fly ash as long as HCl is present in the low-temperature 

zone, which seems to contradict the mechanism by which oxychlorination reaction is catalyzed by metals. Maybe the 

explanation is that oxygen was equally able to react with HCl on the walls of the tubes.

The addition of HCl in the secondary flow resulted in an increase of total chlorobenzenes only when there was Ca

Figure 2. Yields of total non-chlorinated 

aromatics Figure 3. Yields of total chlorobenzenes

Experiment 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

+Ca(OH)2 x x x x x x x x

+HCl x x x x x x x x

+OX fly ash x x x x x x x x

+O2 x x x x x x x x

monoCBz 95 112 65 97 97 71 41 92 196 449 125 290 116 210 333 321

total diCBz 64 66 42 52 79 54 19 16 36 54 25 31 11 24 14 87

total triCBz 88 138 62 113 90 123 27 25 39 92 27 63 13 31 5 105

total 

tetraCBz 68 212 63 146 69 214 28 9 43 197 21 110 16 60 2 82

pentaCBz 38 214 45 193 48 248 18 9 29 289 18 130 17 90 2 95

hexaCBz 13 124 23 71 17 156 9 2 8 109 15 95 9 52 2 39

total 366 866 300 672 400 865 142 153 352 1190 230 719 183 469 359 729

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(OH)2

 but no fly ash, and when there was fly ash but no Ca(OH)2. This means that, in the presence of fly ash, CBz 

yields increased when HCl was present at the highest concentration (no Ca(OH)2 and HCl addition). On the other 

hand, when no fly ash was present, the addition of HCl only increased total yields when chlorobenzenes had not been 

previously formed at high temperatures (Ca(OH)2 present), so in this case, the rise in the yields appeared at 

intermediate concentrations. In general, the presence of fly ash increased total yield of chlorobenzenes, but the 

opposite occurred when there was no Ca(OH)2 and no HCl was added. This suggests that fly ash can dechlorinate 

chlorobenzenes at intermediate HCl concentrations. 

The amount of data generated makes it difficult to study product trends for each CBz congener. To resolve this 

problem, Principal Component Analysis (PCA) was used. This is a statistical method that provides a group of new 

variables (principal components or PCs) whose linear combination generates the original variables. Figure 4 shows 

the representation of the different CBz congeners, total chlorobenzenes and the four parameters studied versus the 

two PCs that most contribute to the data variance. In this study, five PCs explain 96% of this variance. The 

distribution with respect to the two axes gives an idea of the relationship between variables. For example, in this 

case, monochlorobenzene and Ca(OH)2 are situated in opposite directions along the PC2, for which it is deduced 

that this congener was mainly produced when no Ca(OH)2 was present; i.e., in the high-temperature zone by the 

reaction of benzene and hydrogen chloride. The rest of the congeners (hexa-, penta-, tetra- and a bit further, tri- and 

dichlorobenzene), together with total chlorobenzenes, are located in a cluster near the PC1 axis where the oxygen is 

also present. Thus, it is expected that the presence of oxygen enhances the yields of these chlorobenzenes. Finally, 

both fly ash and additional HCl are very close and just over the PC1 axis, which indicates that the effect of these two 

variables is very similar; for example, monochlorobenzenes were hardly affected by the two.

Figure 5. Loading plot from PCA showing the relationship among chlorobenzenes and the parameters studied

Several correlations were investigated, and a linear correlation between total chlorobenzenes and total chlorinated 

compounds detected was found. However, no correlation was found between the different CBz congeners and 

benzene. Other major chlorinated compounds apart from chlorobenzenes were chloronaphthalenes, 

chloroanthracenes and tetrachloroethene.

Despite some difficulties when interpreting certain results, some important conclusions can be drawn from this work:

a) Ca(OH)2 trapped HCl but this did not completely inhibit CBz formation. The chlorobenzenes were either formed 

directly from the PVC chain during thermal decomposition or were formed from a reaction between HCl and benzene 

in tubes prior to Ca(OH)2 .

b) The highest yields of chlorobenzenes appeared when OX fly ash, oxygen and additional HCl were present and no 

Ca(OH)2 was used; i.e. most chlorinating conditions.

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c) The presence of HCl in the high-temperature zone (750oC); i.e. when no Ca(OH)2

 was used, enhanced 

monochlorobenzene formation, which suggests gas-phase formation.

d) Oxygen in the low-temperature zone promoted the formation of higher chlorinated benzenes except in the absence 

of both HCl and OX fly ash.

e) OX fly ash and additional HCl enhanced the formation of higher chlorinated benzenes. 

Acknowledgements

Support for this work was provided by the Ministry of Education and Science of Spain and research projects 

PPQ2002-00567 and PPQ2002-10548-E.

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