As plastic and paper products have become ubiquitous commodities in modern world, disposal of those derived wastes brings about great challenges for environment sustainability [1,2]. At the same time, excessive dependence on fossil fuels raises serious problems for energy security. Hence, research into finding effective ways to develop waste plastic and paper resources and convert them into alternative energy is strongly needed. Thermochemical measures are regarded as potential options owing to waste destruction with energy recovery [3]. Pyrolysis, as a typical thermochemical measure, is recognized as a promising thermochemical conversion method in transforming solid wastes into energy in a cleaner way than from conventional MSW incineration plants [4,5]. In a typical pyrolysis process, thermal degradation of the waste occurs in the total absence of air and produces recyclable products, including char, oil/wax and combustible gases [6]. Co-pyrolysis of different wastes has been drawing attentions due to the potential of the synthetic gas as supplementary or successor of fossil-based ones [7,8]. It has been found that pyrolysis of plastics and waste paper (WP) could generate a syngas with a high content of hydrogen or other higher calorific value gases which have been identified as a clean and highly-efficient future energy supply [9,10].
As primary constituents of packaging, mixed plastics and paper are the two main fractions of municipal solid waste (MSW) [11,12]. For typical waste paper (WP), cellulose is the main component, and most abundant plastics in MSW are polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), and polyvinylchloride (PVC). Specifically, PVC is one of the most commonly used raw materials for general synthetic resin [13]. It has been reported that the interactions/synergy between wastes depend on various factors including the chemical and structural compositions of the blends, reaction duration, temperature and heating rate, and catalysts etc [14]. In the previous study for co-pyrolysis of PVC and newspaper/glossy paper (mainly cellulose), a strong interaction between PVC and cellulose was observed due to the dehydrochlorination of PVC might increase the activity of cellulose by catalyzing acid hydrolysis reactions [15]. Generally, the benefits of co-pyrolysis was beneficial to improve the quantity of energy rich components while decreased energy lean components [16,17].
Synergistic effect of co-pyrolysis of PVC and cellulose containing paper could be generally understood as lowering the pyrolysis temperature of cellulose and reducing HCl emission [18]. It has been widely recognized that the use of catalysts can not only reduce the active energy of pyrolysis, but also produce more hydrogen and syngas [19]. Various catalysts were attempted in catalytic pyrolysis of waste plastic and paper in order to improve the product selectivity. Among those, zeolite-based catalysts (such as ZSM-5) are known for their high efficiency towards the improvement of pyrolysis products due to their high activity for C–C cracking and advanced pore structure [20,21]. Especially, active metal on a ZSM-5 support could enhance the catalytic activity by reducing catalyst coke deposition, owing to the reduction of strong Brønsted acidity which provides the major active sites for coke formation [22]. On the other hand, Yao et al. investigated that Fe-based catalysts presented excellent performance on H2-rich syngas production from waste plastic pyrolysis with high gas yields and hydrogen selectivity [23]. However, serious carbon deposition was also found occurred in the catalysts with high Fe content and solid acid support, which would not be conducive for stable catalytic gas production [24]. Comparative work between widely used ZSM-5 and Fe-based catalysts in the field of co-pyrolysis of plastic and paper is rarely studied in our knowledge.
Furthermore, multifunctional catalysts with alkali metal components have been proposed in previous studies in order to solve the catalyst deactivation caused by carbon deposition and obtain more hydrogen-rich gaseous products by regulating the balance of thermodynamic equilibrium (sorption-enhanced process) [[25], [26], [27]]. CaO-based materials are the most commonly used CO2 sorbents due to its low cost, high adsorption capacity and fast kinetics of CO2 sorption. Also, alkaline metal oxide catalysts (such as MgO, CaO) can further enhance the cracking of macromolecular volatiles during pyrolysis while fixing the generated CO2 in the form of carbonate [28,29].
In this work, PVC and paper were investigated as a typical municipal solid waste blend. In order to detailed investigate the synergetic effect between PVC and paper in co-pyrolysis process towards hydrogen-rich syngas production, various experimental and characterization methods are used. Typical ZSM-5 and Fe-based catalysts were synthesized to promote the performance.