Pyrolysis of tyres. Influence of the final temperature of the process on emissions and the calorific value of the products recovered

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Abstract

A study was made of the pyrolysis of tyre particles, with the aim of determining the possibilities of using the products resulting from the process as fuel. Three final temperatures were used, determined from thermogravimetric data. The design of the experiment was a horizontal oven containing a reactor into which particles of the original tyre were placed. After the process, a solid fraction (char) remained in the reactor, while the gases generated went through a set of scrubbers where most of the condensable fraction (oils) was retained. Finally, once free of this fraction, the gases were collected in glass ampoules. Solid and liquids fractions were subjected to thermogravimetric analyses in order to study their combustibility. The gas fraction was analysed by means of gas chromatography to establish the content of CO, CO2, H2 and hydrocarbons present in the samples (mainly components of gases produced in the pyrolysis process). A special study was made of the sulphur and chlorine content of all the fractions, as the presence of these elements could be problematic if the products are used as fuel. Tyre pyrolysis engenders a solid carbon residue that concentrates sulphur and chorine, with a relatively high calorific value, although not so high as that of the original tyre. The liquid fraction produced by the process has a high calorific value, which rises with the final temperature, up to 40 MJ/kg. The chlorine content of this fraction is negligible. Over 95% of the gas fraction, regardless of the final temperature, is composed of hydrocarbons of a low molecular weight and hydrogen, this fraction also appearing to be free of chlorine.

Introduction

One of the many problems recently observed in the area of waste is that of scrap tyres, a waste with special characteristics of its own, among them their complex nature, which makes them extremely difficult to recycle, and their high calorific value, an important fact for assessing their value.

In the European Union over 2,500,000 tonnes of tyres are produced per year, with a very similar figure for the United States. The management of this type of waste leaves much to be desired, as in the European Union almost 40% of tyres are thrown away untreated. There are alternatives, such as re-treading, shredding, etc., although with many limitations (Mastral et al., 2000). Yet it is possible to evaluate their energy potential, as there are great possibilities for this type of waste.

The only way of assessing energy so far put into practice is incineration, mainly using tyres as a back-up fuel, especially in cement works, paper factories and power stations. There are other techniques, such as gasification and pyrolysis. The pyrolysis process for the treatment of tyres has hardly been developed at all, although there have been several studies concerning the great possibility of this process for assessing energy potential (Sharma et al., 2000).

The process of pyrolysis consists in a decomposition of the material by means of temperature, in the total absence of oxygen, which is why it is sometimes called thermolysis. It leads to the production of a solid carbon residue, a condensable fraction and gases. The solid fraction contains the mineral matter initially present in the tyre (Napoli et al., 1997). The liquid by-products of tyre pyrolysis consist of a very complex mixture of organic compounds of 5–20 carbons with a very high proportion of aromatics (Laresgoiti et al., 2000). Finally, the gas fraction is composed mainly of CO, CO2, H2 and light hydrocarbons (Cunliffe, and Williams, 1998).

The main aim of the present study is to assess the influence of the final temperature on the process of pyrolysis, with a view to evaluating the energy (combustion) potential of the products of the process (Inguanzo et al., 2002). Moreover, a special study was made of elements in the samples that could pose problems from the point of view of this application, such as sulphur and chlorine. The final temperatures chosen were obtained from thermogravimetric data.

Section snippets

Experimental work

The design used for the experiment is shown in Fig. 1. A horizontal oven was used, in which a reactor was placed, consisting of a quartz tube 40 cm long and 7 cm in diameter, which was loaded with the sample. The sample was of tyre material ground mechanically to pass through a 420 μm screen and was free of metal reinforcing. About 50 g of the sample was used in each experiment.

The oven was heated electrically with a heating gradient that was not constant but which increased with temperature,

Yields of the pyrolysis products

Fig. 2 shows a derived thermogravimetry (DTG) profile corresponding to the process of pyrolysis of the tyre dust, revealing its mass loss rate at temperatures similar to those used in the reactor. From the figure three stages are to be observed: 200–350, 350–450 and 450–550 °C. For this reason, 350, 450 and 550 °C were chosen as the final temperatures for reactor tests. It can be said that after 550 °C the pyrolysis process is complete (Sørum et al., 2001). Some authors (Leug, 1999) attribute

Solid fraction (char)

The results of the elemental, proximate and calorific value analyses of the initial sample and of the solid fraction are given in Table 2. No major differences are to be observed between the char yields for each temperature, but the drop in the volatile matter content with the increase in the final temperature is significant, because pyrolysis does not finish until 550 °C is reached. There is also a noticeable decrease in the hydrogen content as temperature increases, probably due to the great

Conclusion

Tyre pyrolysis leads to the production of a solid waste, liquids and gases. The amount recovered of each of them depends on process conditions, mainly the final temperature. A previous study to determine the final temperatures uses was carried out in a simple and reliable way by means of thermogravimetric analysis.

The solid fraction is a carbonic residue, with a carbon content of around 80%. Its properties as a fuel can be compared with those of good quality coals, the carbon content and the

Acknowledgments

The authors wish to thank the University of León for funding the project with grant ULE-2001-05 B. We also thank the MAPFRE Trust for co-funding the project with a grant to assist research.

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