Thermogravimetric pyrolysis of waste polyethylene-terephthalate and polystyrene: A critical assessment of kinetics modelling

Abstract

Pyrolysis is considered as possible technique to thermally convert waste plastics into chemicals and energy. Literature on experimental findings is extensive, although experiments are mostly performed in a dynamic heating mode, using thermogravimetric analysis (TGA) and at low values of the heating rate (mostly below 30 K/min). The present research differs from literature through the application of far higher heating rates, up to 120 K/min. The use of these dynamic results to define the reaction kinetics necessitates the selection of an appropriate reaction mechanism, and 21 models have been proposed in literature considering the rate limiting step being diffusion, nucleation or the reaction itself.

The current research studied the cracking of PET and PS by TGA at different heating rates (temperature ramps). Results were used to check the validity of the proposed mechanisms. Several conclusions are drawn: (i) to obtain fair results, the heating ramp should exceed a minimum value, calculated at 30 K/min for PET and 80 K/min for PS; (ii) application of the majority of the models to experimental findings demonstrated that they do not meet fundamental kinetic considerations and are questionable in their use; and (iii) simple models, with reaction order 1 or 2, provide similar results of the reaction activation energy.

A further comparison with literature data for dynamic and isothermal experiments confirms the validity of these selected models. Since TGA results are obtained on a limited amount of sample, with results being a strong function of the applied heating rate, the authors believe that isothermal experiments, preferably on a large scale both towards equipment and/or sample size, are to be preferred.

Introduction

The total plastic consumption in Western Europe exceeded 52 million tonnes in 2008 (Baeyens et al., 2010). These plastics include mostly polyethylene (PE – 37 wt%), polypropylene (PP – 19 wt%), polyvinyl chloride (PVC – 19 %), polystyrene (PS – 6 %) and polyethylene terephthalate (PET – 6 wt%). Thermoplastics are widely applied in industrial and commercial products, and usually contain stabilisers and additives. Household solid waste (HSW) consists of 7–8 wt%, or up to 20 vol% of the post-consumer plastic solid waste (PSW) (Al-Salem et al., 2010, Baeyens et al., 2010).

Treatment and disposal of post-consumer plastics have become an important environmental concern, with recovery and recycling, landfilling and (co-)incineration as most applied methods. Incineration gains interest, due to increasing landfilling constraints and taxes, and due to the difficulties encountered when attempting to recycle mixed plastic wastes (Al-Salem et al., 2009a, Al-Salem et al., 2009b, Al-Salem et al., 2010). The research focus has recently shifted towards pyrolysis or gasification as thermal treatments. Whereas high temperature gasification targets the production of a syngas, to be further upgraded to fuel-grade organics or directly used in electricity generation, pyrolysis takes place in the absence of oxygen at moderate temperatures and recycles plastic solid waste (PSW) into chemicals and/or fuel: the polymer structure is decomposed into smaller intermediate products, usable as fuel or as raw materials for the petrochemical industry. By producing value-added products (such as liquid and gaseous fuel, polymer monomers and/or a carbonaceous residue as a candidate for possible upgrading to activated carbon or carbon black), pyrolysis can overcome certain disadvantages of incineration and recycling (Brems et al., 2011). The low temperature of the process and the absence of oxygen, moreover, reduce emission problems as encountered during incineration, where large volumes of combustion gas and toxic pollutants are produced (Everaert and Baeyens, 2002, Bhandare et al., 1997). Co-incineration is moreover controlled by stringent emission standards as e.g. set by the EU Hazardous Waste Incineration Directive (EU, 2000). It is difficult to treat or recycle mixed PSW, due to its complex nature and composition, the contamination with various residues (organic, inorganic or biological), and the possible structural deterioration of the polymeric compounds.

When pyrolysing PSW, only 5% of the energy content is used in the endothermic cracking process (Brems et al., 2011). Pyrolysis is therefore considered as one of the most promising methods to recover material and energy from PSW. Furthermore, almost all types of plastics (commingled or mixed with other materials such as wood, paper, ink, paint) can be treated. Pigments, inorganic fillers, supports and other additives, possibly present in minor concentrations in the polymers, are mostly retained in the solid residue.

Due to possible interactions during decomposition and interactions between the components of the mixture and the low molecular weight products and free radicals formed by the scission of the polymeric chains, thermal degradation of polymer mixtures gets more complex than degradation of single polymers (Williams and Williams, 1999). These interactions can affect the quality of the products formed and are hence important when high quality standards have to be met for use as feedstock or fuel (Al-Salem et al., 2009b, Al-Salem et al., 2010).

The objectives of the present research were twofold, i.e. (i) to study the experimental pyrolytic degradation of 2 important thermoplastics, PET and PS; and (ii) to critically assess the multiple model treatment methods proposed in literature to determine the Arrhenius-based kinetic parameters of the pyrolysis reaction as often experimentally determined by TGA. There is indeed some literature controversy concerning the applicability of simple TGA experiments to define the pyrolysis kinetics, with results widely different according to the conversion model applied. TGA experiments are, however, easy and fast and can provide correct kinetic values, comparable with isothermal results, if the correct data treatment is applied (Brems et al., 2011, Al-Salem et al., 2010). A study of isothermal pyrolysis of polyethylene is e.g. recently proposed by Al-Salem and Lettieri (2010).