Recent advances in the gasification of waste plastics. A critical overview

Renewable and Sustainable Energy Reviews 82 (2018) 576–596

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Recent advances in the gasification of waste plastics. A critical overview ⁎

MARK

Gartzen Lopez , Maite Artetxe, Maider Amutio, Jon Alvarez, Javier Bilbao, Martin Olazar Department of Chemical Engineering, University of the Basque Country UPV/EHU, P.O. Box 644, E48080 Bilbao, Spain

A R T I C L E I N F O

A B S T R A C T

Keywords: Waste plastics Gasification Reforming Hydrogen Syngas Review

The current review provides an assessment of the main waste plastics valorization routes to produce syngas and H2, thus covering different gasification strategies and other novel alternative processes, such as pyrolysis and inline catalytic steam reforming. The studies dealing with plastics gasification are in general scarce. However, due to the knowledge acquired on biomass and coal gasification, the state of development of plastic gasification technologies is considerable and, in fact, several gasification studies have been performed at pilot scale units. Air gasification is the most studied and developed strategy and pursues the production of a syngas for energy purposes. In spite of the higher H2 content and heating value of the gas produced by steam gasification, this alternative faces significant challenges, such as the energy requirements of the process and the tar content in the syngas. Moreover, the co-gasification of plastics with coal and biomass appears to be a promising valorization route due to the positive impact on process performance and greater process flexibility. Other promising alternative is the pyrolysis and in-line reforming, which allows producing a syngas with high hydrogen content and totally free of tar.

1. Introduction Plastics have become a basic support for the modern style of living due to their low production cost and wide range of suitable properties, such as low density, durability and resistence to corrosion, and have therefore caused a displacement of traditional materials, such as wood, metals and ceramics [1]. In fact, plastics global production has steadily increased in recent years (Fig. 1), reaching a global annual production of 322 million tons (Mt) in 2015 [2]. The increase in plastics consumption corresponds to both the traditional plastics and the new plastic composites, with their main sectors of application being packaging, building, automotive, electrical and electronics, and agriculture [2]. Fig. 2a shows a detailed distribution of plastics demand in Europe according to the different sectors and Fig. 2b shows the distribution of the different plastic types. As observed, polyolefins account for half of the plastics produced, but polyvinyl chloride (PVC), polyurethane (PUR), polyethylene terephtalate (PET) and polystyrene (PS) are also produced in considerable amount. Plastic waste management poses a great challenge that must be urgently addressed. Thus, the low degradability of the plastics causes serious environmental problems, especially in marine environments [3]. In addition, the inadequate management of waste plastics leads to sustainability problems due to the loss of valuable and scarce resources derived from petroleum. Accordingly, public policies have been promoted over the last years for improving waste plastics management. In ⁎

Corresponding author. E-mail address: [email protected] (G. Lopez).

http://dx.doi.org/10.1016/j.rser.2017.09.032 Received 24 May 2017; Received in revised form 18 July 2017; Accepted 13 September 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

fact, the fraction of waste plastics recycled and the one used for energy valorization in Europe over the last decade have increased by 64% and 46%, respectively, whereas the amount of waste plastics sent to landfill was reduced by 38% [2]. Although waste plastics management scenario is slowly improving in developed countries, its current situation is far from being satisfactory, and plastics management in developing countries is clearly less promising. With the aim of reducing the amount of waste sent to landfill, different approaches are considered, namely, waste minimization, reuse, recycling and energy recovery. However, in the case of waste plastics, both minimization and reuse have been hardly applied [4]. Given their high calorific value, combustion is a feasible valorization route, but this alternative is hindered by the emissions produced [5]. Amongst recycling routes, chemical ones have the best perspectives for large scale implementation because they allow for the production of fuels, chemicals and syngas/hydrogen from waste plastics. The main chemical valorization routes of waste plastics are summarized in Fig. 3. Waste plastic pyrolysis is regarded as the main route for the production of fuels and chemicals from waste plastics [1,4,6–13]. In fact, different plastic pyrolysis processes aimed at the selective production of waxes [14–16], light olefins [17–21] and monomers [22,23] have been developed. Moreover, the co-pyrolysis of waste plastics and biomass is gaining increasing attention in recent years [10,24,25]. In spite of the interest in plastics pyrolysis process, its state of development and fullscale implementation is limited [6,10]. Furthermore, waste plastics or

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Fig. 1. Evolution of plastics global and European (EU-28) production during the last decade [2].

their derived products, such as pyrolysis waxes, may also be fed into conventional refinery units to produce fuels [10,26–28]. Gasification of waste plastics leads to the production of a stream made up of mainly H2, CO, CO2, CH4 and N2. Thus, the interest of gasification processes lies in the feasibility of producing energy, energy carries (such as H2) and chemicals from the syngas produced (fuels, DME, methanol and so on) [29]. A remarkable advantage of gasification compared to pyrolysis is the greater flexibility to jointly valorize plastics of different composition or mixtures or plastics mixed with other feedstocks. The composition, and therefore applications, of the gas produced depends on the gasifying agent used. Thus, air gasification of waste plastics leads to a syngas with an average heating value in the 6–8 MJ m−3 range [30–33], with its main interest being energy production. Nevertheless, steam gasification allows for producing a N2 free syngas with a heating value above 15 MJ m−3 [34,35], with its composition being suitable for synthesis applications. The main challenge of waste plastics gasification is the high tar content in the gas product, i.e., usually higher than those reported in biomass gasification [34,36–38]. Thus, a very efficient gas cleaning system is needed to meet the requirements for applying the syngas to chemical production [39–42]. Currently, the pyrolysis and in-line catalytic steam reforming [43–47] and the reforming of dissolved plastics [48–50] strategies are gaining growing attention. The interest of the pyrolysis-reforming process lies in the high H2 production, usually above 30 g 100 gplastic−1 [45,46,51]. Moreover, the gaseous product obtained in the pyrolysisreforming process is free of tars, and therefore they avoid the major problem involving gasification processes. Therefore, this review analyses the state-of-the-art of the main waste plastic conversion technologies aimed at the production of syngas and H2 including air gasification, steam gasification, co-gasification of plastics with other feedstocks, and pyrolysis-reforming. Thus, gasification and pyrolysis-reforming technologies are presented and their main features discussed. Moreover, their development state is critically evaluated and their scale-up possibilities assessed. The influence the

Fig. 3. Scheme of the main routes for chemical valorization of waste plastics.

main operating conditions have on conversion efficiency is evaluated, and so the role played by temperature, ER ratio, S/P (or S/C) ratio, feed composition and tar cracking or reforming catalysts is analyzed in depth. Moreover, focus is placed on key process parameters that allow assessing and comparing real process performance, such as the gas and H2 productions and the quality of the syngas obtained (H2 concentration, tar content, heating value and so on). Furthermore, the results obtained by the different valorization strategies have also been compared and their potential interest critically discussed. 2. Plastic waste management legislation In spite of the great volume of waste plastics produced and the environmental problems associated with their inadequate handling, plastic waste management is not addressed by any specific EU legislation, but the Waste Framework Directive [52] and the Directive on Packaging and Packaging Waste [53] regulate their management. In Europe, waste management was legislated for the first time in 1975 (Directive 75/442/EEC) with the aim of reducing waste production and its harmfulness [54]. This directive was modified throughout time [55] and currently waste management is legislated by the Waste Framework Directive 2008/98/EC [52]. Given that environment and human health protection are the main aims of this directive, the reduction of adverse impacts of waste generation and management is promoted, as well as the overall impacts involving the use of resources. The mentioned waste directive includes a waste management hierarchy, establishing the following priority order in waste prevention and management legislation and policy: i) prevention, ii) preparing for reuse; iii) recycling; iv) valorization; v) elimination. Besides, the Waste Framework Directive extended producer responsibility and marked the Fig. 2. Current plastic distribution demand in Europe according to their application (Fig. 1 a) and type of polymer (Fig. 1b) [2].

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technologies is described in detail in the following sections.

objective of recycling 50% of household waste by 2020, in which plastic wastes are also included. Regarding packaging waste, the current legislation is Directive 94/ 62/EC [53], to which several amendments have been made, with the last one being Directive (EU) 2015/270 [56] focused on the consumption of lightweight plastic carrier bags. Directive 94/62/EC aimed at limiting the production of packaging waste and promoting re-use, recycling and valorization of this material in order to minimize its impact. The objective of Directive 2015/270 is to limit the consumption of lightweight carrier bags, focusing on the prevention of wastes instead of their recycling. Thereby, this directive drives member states to take measures in order to ensure that the annual consumption level does not exceed 90 lightweight plastic carrier bags per person by 31 December 2019 and 40 lightweight plastic carrier bags per person by 31 December 2025, or equivalent targets set in weight. It is to note that in 2013 the European Commission wrote the Green Paper on a European Strategy on Plastic Waste in the Environment [57] to launch a structured discussion about how to make plastic products more sustainable throughout their life cycle and reduce the impact of plastic waste on the environment. This document reviews the environmental issues caused by plastic wastes and the current EU legislation for its management, and points out the lack of specific legislation concerning plastic wastes. Besides, it is mentioned that plastic recycling gives the opportunity to improve the objectives proposed in the Roadmap to a Resource Efficient Europe [58], in which one milestone is to manage waste as a resource by 2020. Moreover, it is stated that plastic waste should be considered a material with value and the amount littered should be decreased considerably following the waste management priority order. Bearing in mind this idea, a gradual phasing out or ban has been proposed for plastic waste landfill by modifying the Landfill Directive 1999/31/EC [59]. It should be noted that this directive already banned tires, liquids or explosives landfill. In this scenario, The Green Paper on a European Strategy on Plastic Waste in the Environment lays out several questions with the aim of both initiating a broad reflection on the plastic waste issue and approaching a new public policy for its management.

3.1. Fluidized bed reactors Traditionally two types of fluidized beds have been used in gasification processes, namely, bubbling fluidized beds and circulating fluidized beds [60,63]. In spite of the interesting features of circulating fluidized beds for gasification processes, especially the fact they allow obtaining high conversion and low tar yields [74], plastic gasification studies have exclusively been carried out in bubbling reactors. The main advantages of bubbling fluidized bed reactors deal with their high heat and mass transfer rates, excellent gas-solid contact, good control of temperature, solid mixing regime and flexibility. Their main shortcomings are related to their high investment cost, limitations in particle size in both bed and feed, defluidization problems and entrainment of unreacted material [60]. These reactors operate in continuous regime and their scale and development degree is in general high, with several studies being performed in pilot plant scale units [31,32,34,66,75–78]. Fluidized bed gasifiers are commonly operated with air as gasyifing agent in the gasification of plastic wastes or in their co-gasification with biomass or coal [30,31,67,68,77,79–81]. In spite of the low heating value of the gas produced, this strategy involves operational advantages, such as those related to auto-thermal process and lower tar content in the gaseous product [40,82]. The research team headed by Arena et al. [32,83,84] performed studies in a pilot plant with continuous feed of up to 100 kg h−1, and other researchers have also used gasifiers operating in continuous regime with plastic feed rates in the 1–4 kg h−1 range [30,33,67]. Steam gasification is highly endothermic, and therefore has a high energy demand, which has been solved in the gasification of biomass by using dual fluidized bed reactors, i.e., by combining a steam-blown fluidized bed with a fast fluid bed blown with air, where the residual char is burnt [85,86]. This operating strategy has been applied to the gasification of waste plastics by the research group headed by Prof. Hofbauer using a pilot plant of 15 kg h−1 [34,76]. However, problems may arise due to the low char yield and the difficulty to maintain the heat balance between gasification and combustion processes. Schemes for bubbling, circulating and dual fluidized beds are shown in Fig. 4.

3. Chemical reactors for waste plastic gasification The plastic gasification technologies are essentially those already developed for the gasification of other feedstocks, such as biomass and coal. However, the particular characteristics of waste plastics, especially the low thermal conductivity, sticky behavior, high volatile content and remarkable tar formation, hinder their treatment in conventional gasification technologies and involve a serious challenge for the process implementation. Accordingly, a suitable gasifier design for handling plastic has to combine the following features: i) be able to provide high heat transfer rates in order to promote a fast plastic waste depolymerisation, ii) avoid operational problems related to the sticky nature of plastics by ensuring a good control of operating conditions, iii) appropriate residence time distribution to favor tar cracking and iv) allow using primary catalyst in situ providing a good contact with this catalyst. The main reactors conventionally used in biomass gasification are entrained flow, fixed bed, updraft, downdraft, fluidized bed, rotary kiln and plasma reactor [29,60–64]. However, the complex characteristics of waste plastics have limited the use of some of these technologies. Thus, the low or even null fixed carbon content hinders the gasification of plastics in updraft and downdraft gasifiers. Likewise, the sticky nature of the polymers and the difficulties for their heating up also hinder the use of rotary ovens and, to a lesser extent, rotary kilns. Fluidized beds have suitable features for avoiding these disadvantages, and have therefore been widely used in the gasification of waste plastics [34,38,65–68]. In addition, other reactor designs, such as fixed beds [69,70], spouted beds [35,71] and plasma reactors [72,73] have been applied to waste plastic gasification processes. The application of these

3.2. Fixed beds The use of fixed bed reactors in plastic gasification processes is related to their easy design and operation and limited investment cost, with their main challenges being scaling up, operation in continuous regime, poor heat transfer rate and limited gas-solid contact. Amongst the fixed bed reactors there is a wide variety of designs, with their common point being their use at small scale units [87–93]. In general, the gasification of waste plastics [69,70,94–96] or their co-processing with coal [97] and biomass [98,99] has been scarcely studied in fixed bed reactors. Thus, a laboratory fixed bed reactor operating in batch regime has been used by Ahmed et al. for the steam cogasification of plastic-wood samples [98] and polystyrene [93]. Furthermore, runs have been carried out in a bench scale fixed reactor developed by He et al. [69] operating with a continuous plastic feed rate of 0.3 kg h−1 and the effect of using a Ni/Al2O3 reforming catalyst in situ was studied. A similar experimental equipment also running in continuous regime was developed by Li et al. [100] for the steam gasification of municipal solid waste (MSW). The study performed by Lee et al. [70] was carried out in a laboratory semi-batch reactor under steam atmosphere. Guo et al. [101,102] studied the air gasification of polyurethane in a bench scale fixed bed reactor using different in situ catalysts.

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Fig. 4. Different types of fluidized beds used in gasification processes. Bubbling fluidized bed (a), circulating fluidized bed (b) and dual fluidized beds (c).

temperature reached, which promotes an almost complete cracking of tar compounds, and therefore high gas yields [72] by enhancing the elimination of harmful and toxic compounds [119]. Plasma technologies are divided into three categories according to the plasma discharge techniques, namely, direct current, radio frequency and microwave. This technology has been scarcely applied to the gasification of plastic wastes and, furthermore, the studies have generally been performed in small scale units [72,120–122]. However, the degree of development of the gasification unit by Hlina et al. [73] is remarkable, which operates in continuous regime with a plastic feed rate of 11 kg h−1. The process proposed by Park et al. [123] combines continuous pyrolysis (1.3 kg h−1) with in-line gasification of pyrolysis volatiles in a plasma reactor.

3.3. Spouted beds The peculiar features of spouted beds make them an alternative to fluidized beds for waste valorization processes. Thus, these reactors are characterized by their high heat and mass transfer rates, good solid mixing and suitable gas-solid contact [103]. In addition, their vigorous cyclic solid circulation avoids defluidization problems and eases the handling of irregular particles and those with a wide size distribution and sticky materials. The main limitations for their application on gasification processes deal with the short residence of the volatiles, which hinders tar cracking reactions [104]. This technology has been widely used in the pyrolysis of different solid wastes in bench scale units [17,105–108]. In addition, it has been successfully scaled up to 25 kg h−1 for the biomass pyrolysis process [109,110]. The original application of spouted beds in gasification processes used coal as feed [111–114]. More recently the application of this technology has been extended to the gasification of other feedstocks, such as biomass and waste plastics [35,104,115–118]. In order to enhance process efficiency and reduce tar content in the gaseous product, different primary catalysts have been studied in situ [35,117] or secondary catalyst have been used in a second reactor [71]. A scheme of a conical spouted bed reactor is shown in Fig. 5.

3.5. Other reactors for plastic gasification The features of waste plastics during their thermal degradation, especially its sticky nature and high volatile content, have hindered the application of certain conventional gasification technologies. Fig. 6 shows a scheme of an updraft and a downdraft reactor. The use of downdraft and updraft gasifiers is very limited in waste plastics gasification. Kungkajit et al. [124] studied the air gasification of plastics from refuse derived fuel in a downdraft batch reactor. Recently, Madadian et al. [125] studied the co-gasification of waste plastics (up to 10 wt% in the fed) with a fibrous fraction from MSW in a downdraft gasifier with a nominal thermal output of 10 kW. Ponzio et al. [94] studied the gasification of a waste made up of plastics, paper and fabric fibers, and wood in an updraft gasifier with a continuous feed of 60 kg h−1. The moving-grate technology, originally developed for the incineration of MSW, was adapted by Lee et al. [126] for the gasification of plastic wastes using air as gasifying agent and the pilot plant constructed has a capacity of 500 kW.

3.4. Plasma reactors Plasma gasification processes consist in the transformation of waste materials into gaseous products under oxidant environment. The main advantage of plasma reactors for plastic gasification is the high

3.6. Reactors used in pyrolysis-reforming processes This process aimed at the production of hydrogen is carried out in two reactors connected in-line for the pyrolysis and reforming steps. This novel strategy has been studied mainly in bench and laboratory scale units with a wide variety of reactor configurations, Fig. 7. The pioneering studies were carried out by Czernik and French [127] in a system operating in continuous regime by feeding 0.06 kg h−1 of plastic, with the experimental unit being made up of a fluidized bed for plastic pyrolysis and a fixed bed for the catalytic reforming of the volatiles produced. The continuous process developed by Yoshikawa et al.

Fig. 5. Diagrammatic representation of a conical spouted bed gasifier.

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Fig. 6. Schemes of downdraft (a) and updraft (b) gasifiers for plastic treatment.

[45,51] consists of two fixed bed reactors connected in-line, with the plastic feed being also of 0.06 kg h−1. Williams et al. [88–90,128–130] have studied in detail the pyrolysis and steam reforming of plastics in a batch unit made up of two fixed bed reactors. Erkiaga et al. [43] combined a conical spouted bed reactor for the continuous pyrolysis (0.05 kg h−1 of plastic in the feed) with a fixed bed reactor for the inline reforming. The performance of this system was improved in subsequent studies by replacing the fixed bed with a fluidized bed for the reforming process [46,131]. Ouadi et al. [132] developed a pilot plant operating in continuous regime (2 kg h−1 feed) for the pyrolysis and inline reforming of MSW. The pyrolysis step was carried out in an auger reactor at 450 °C and the reforming of volatiles in a fixed bed reactor on pyrolysis char.

4. Plastic gasification mechanism The gasification of plastics pursues the maximum conversion to a gas product or syngas, with tar and char being the main undesirable byproducts. Gasification involves several steps and complex chemical reactions but it can be summarized in the following steps: drying, pyrolysis, cracking and reforming reactions in the gas phase, and heterogeneous char gasification. These steps are set out in Fig. 8. The significance of these steps on the process performance and their kinetics depends on the feedstock characteristics and gasification conditions. In fact, the moisture content of waste plastics is usually much lower than those of other commonly gasified feedstocks, such as biomass and coal and, furthermore, this moisture is external and its drying is fast, as Fig. 7. Different reactor configurations used in the pyrolysis and in-line reforming process. Fluidized bed and fixed bed (a), fixed bed and fixed bed (b), spouted bed and fixed bed (c) and spouted bed and fluidized bed (d).

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Fig. 8. Scheme of the main steps occurring in plastics gasification.

it is not under diffusional restrictions. Accordingly, the drying process usually plays a negligible role on the overall conversion. The pyrolysis step involves a series of complex chemical reactions of endothermic nature and leads to volatiles (gases and tars) and a solid residue or char. The physical and chemical characteristics of waste plastics make pyrolysis a key step in their gasification. The poor heat conductivity and the sticky nature of the fused plastics reduce their thermal degradation kinetics, especially when the gasifier cannot provide high heat transfer rates and avoid the formation of fused plastic agglomerates. The formation of these agglomerates in fluidized bed gasifiers has been described in detail by Aznar et al. [133]. Another feature of waste plastics is their high volatile content and, indeed, most common polymers, such as polyolefins or PS, can be almost completely converted into volatiles when the pyrolysis is carried out under fast heating conditions [9,10,15,134–137], as is the case in most common gasification technologies. Similarly, other polymers, such as PET or PVC, lead to a low char yield in their degradation [138–140]. In fact, a significant formation of char only occurs when plastic wastes contain other materials, such as biomass, fibers or cardboard. Consequently, the low char yield obtained in the pyrolysis of plastics reduces the significance of its heterogeneous conversion step on the overall gasification process kinetics. This fact involves a remarkable difference compared to coal and biomass gasification, in which char gasification is the controlling step and strongly conditions the gasifier design [141–144]. The plastics degradation mechanism plays a significant role in the composition of the volatiles formed, and therefore in their subsequent cracking and reforming reactions undergone in the gaseous phase. The main thermal degradation mechanism of polyolefins is random scission [145,146], which leads to a wide product distribution [14,19,147]. However, it should be noted that, at temperatures above 800 °C, the prevailing degradation mechanism shifts to end chain scission, with the most probable routes being: direct scission, 1,5-radical transfer scission, and multiple step-radical transfer scissions [148]. Furthermore, the degradation of polyolefins via this latter mechanism produces mainly light olefins. In fact, the cracking at high temperatures combined with short residence times has been regarded as a feasible route for the selective production of light olefins from polyolefins [18,149]. Other polymers, such as PS, are degraded mainly by random chain scission producing oligomers of varying length [145]. These oligomers may evolve to styrene or other monoaromatic and polyaromatic species in the gas phase depending on temperature and residence time [150]. In the case of PET, its degradation follows a random scission degradation producing both carboxyl and vinyl groups [145], leading to a wide product distribution including carbon monoxide and dioxide and several oxygenated compounds, such as benzoic acid, terephthalic acid, aldehydes, acetophenone, and so on [139,151]. The homogeneous gasification reactions include a wide variety of reactions, with the balance and the extent of these reactions depending mainly on the gasifying agent used, its ratio in the feed (ER in the case of air/O2 or S/P in the case of steam) and temperature. These reactions are as follows:

Methane reforming: CH 4 + H2 O ⇔ 3H2 + CO ΔH=206 kJ mol−1

(2)

Char steam gasification: C + H2 O → H2 + CO ΔH=131 kJ mol−1

(3)

Dry reforming of hydrocarbons: Cn Hm + nCO2 →(m /2)H2 + 2nCO ΔH > 0 (4) Boudouard reaction: C + CO2 ⇔ 2CO ΔH=172 kJ mol−1

(5)

Water-gas shift reaction: H2 O + CO ⇔ H2 + CO2 ΔH=−41 kJ mol−1 (6) It should be noted that gasification reactions are only those involving H2O and CO2 [63], because O2 only promotes combustion and partial oxidation reactions that produce CO, CO2 and H2O. In addition, the exothermic nature of oxidation reactions provide the energy required for the highly endothermic steam and CO2 reforming (eqs. (1)–(4)) and Boudouard (eq. (5)) reactions. Steam improves H2 production by means of steam reforming reactions (eqs. 1 and 2) and also by enhancing the water-gas shift (eq. (6)) equilibrium. High temperatures are required for promoting char gasification, especially CO2 gasification, whose kinetics is between 2 and 5 times slower than under steam atmosphere and does not occur below 730 °C [152]. The main drawback of high temperatures in gasification processes is the thermodynamic restriction in the WGS reaction. In order to enhance the conversion efficiency, reforming catalysts are used in gasification processes, with Ni based ones being the most common [69,71,78,153]. These catalysts are highly active for the reforming of tar and other hydrocarbons (eqs. (1) and (2)) and, moreover, also improve H2 production by promoting the WGS reaction (eq. (6)). The high volatile content of plastics is related to the higher tar formation compared to other feedstocks such as biomass and coal [34,37,38]. The tar formation mechanism, especially its evolution in the reaction environment in plastic waste gasification, significantly differs from those of coal and biomass mainly due to the different composition of the volatiles produced in the pyrolysis step. Thus, the tar formation in biomass gasification has been related to two different pathways: i) the direct release of aromatic structures in lignin devolatilization and ii) a mechanism of hydrogen abstraction and acetylene addition [154]. In the case of coal, similar routes have been described for tar formation, i.e., the direct production of aromatic hydrocarbons in the pyrolysis step and tar formation by homogenous reactions of hydrogen abstraction and acetylene addition in the gas phase [155]. In the case of plastics wastes, the tar formation and evolution mechanism depends on the plastic waste composition (Fig. 9). In fact, a direct production of primary tar of aromatic nature only occurs in the degradation of polymers with aromatic rings in their structure, basically PS and PET. Furthermore, the volatiles or primary tar derived from polyolefin degradation are alkanes and alkenes of varying chain length. These hydrocarbons are characterized by their low thermal stability and, under gasification temperatures, are rapidly cracked into lighter compounds; in fact, linear hydrocarbons are not detected in the tar

Steam reforming of hydrocarbons: Cn Hm + nH2 O → (n + m /2)H2 + nCO ΔH > 0 (1) 581

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Fig. 9. Tar formation and evolution pathways in the gasification of plastics of different nature.

An alternative of air and steam is the gasification with pure O2, which combines the advantages of both gasifying agents. However, the high fixed assets and operating cost for air separation makes this option more complex and expensive [33], especially for medium scale applications. More recently, the pyrolysis and in-line reforming of pyrolysis volatiles has been proposed as a promising valorization route of waste plastics for H2 production [45,46,51,88,127,159]. Moreover, this alternative takes advantage of highly active reforming catalysts, which allow producing a syngas completely free of tar, and therefore overcoming the main challenge in the conventional gasification of plastics.

produced in the gasification of polyolefins [34,35,126,156]. Amongst the light compounds formed in plastics thermal degradation, light olefins play a critical role in tar formation. Thus, C2-C4 olefins, and especially acetylene, are tar precursors, with the most presumable tar formation routes being: i) hydrogen abstraction and acetylene addition and ii) dehydrogenation and Diels-Alder condensation reactions. In fact, the higher light olefin concentration in the gasification of plastics is responsible for the higher tar yield compared to those obtained in biomass or coal gasification. Therefore, the increase in tar yield observed in plastic gasification is a consequence of the higher contents of light hydrocarbons in the gas product [37,38,77]. Although the primary degradation of PS leads to styrene and styrene oligomers [136,157], the unstable character of these compounds causes a fast evolution towards more stable structures (secondary and tertiary tars), and therefore they are not usually detected in the tar produced in PS gasification [34]. Similarly, the degradation of PET gives way to primary tars as benzoic acid and benzoil formic acid, but under the severe conditions in the gasifiers they also evolve to more stable species in the range of secondary and tertiary tars. The conversion of stable secondary and tertiary tars involves a great challenge because their effective thermal cracking requires temperatures of around 1250 °C and residence times above 0.5 s [154].

5.1. Air gasification Studies on waste plastic air gasification have been carried out essentially in fluidized bed reactors with a significant development in the experimental units, mainly pilot or bench scale plants operating in continuous regime. The main results obtained in plastics air gasification, namely, those involving gas yield and composition, tar content, and LHV, are summarized in Table 1. Independently of the gasifying agent used, the main challenge of plastic gasification processes lies in the tar content of the gas product, although it is accepted that tar contents are lower when air or O2 is used instead of steam [40,82]. Thus, the tar content must be lower than 10 mg Nm−3 for use of the syngas for energy production in engines and turbines, but this content must be much lower for synthesis applications [40]. Moreover, the characteristics of the tar, especially its dew point, play a significant role in the problems it may cause, as are deposition in heat exchangers and other process equipment [41]. The dew point depends on the tar concentration, but also on its composition, as singlering aromatic compounds are incondensable even at concentrations as high as 10 g Nm−3. However, polyaromatics of more than 4 rings condense at a concentration of only 1 mg Nm−3, causing severe operational problems in process equipment [39]. The research team by Prof. Arena have widely studied the air gasification of different plastics and mixtures of plastics in a bubbling fluidized bed pilot plant with an approximate capacity between 30 and 100 kg h−1 [31,32,66,83,84]. Their early studies dealt with the gasification of waste PE at around 850 °C with equivalence ratios (ER) in the 0.2–0.34 range in order to study the effect olivine has as primary catalyst for tar reduction [83,84]. The use of olivine greatly improved the efficiency of the gasification process causing a dramatic reduction in tar content in the gas product, with this result being related not only to the direct tar cracking but also to the elimination of its promoters, i.e., light

5. Waste plastic gasification processes The valorization of waste plastics by gasification processes has been addressed following a wide range of strategies and pursuing the production of syngases of different composition and potential applications. The interest in the research on plastic waste gasification is relatively recent and the number of studies is also limited. However, several studies of co-gasification with coal and biomass have been carried out. Amongst them, direct air gasification is the most studied one, with this alternative leading to a gas product of relatively low heating value due to the diluting effect of N2. The main advantage of air gasification is the simplicity of the process, as there are no external energy requirements. Furthermore, tar content in the gas product is usually lower than in steam gasification [82]. Accordingly, this gas is mainly used for energy production [32,158]. Steam gasification allows producing a H2 rich syngas with high H2/ CO ratios, which is more appropriate than the syngas from direct air gasification for chemical synthesis applications [35]. The main challenge of this alternative lies in the heat input into the reactor to carry out the endothermic steam reforming reactions.

582

PE

waste plastic mixture

Bubbling fluidized bed (100 kg h−1)

Bubbling fluidized bed (100 kg h−1)

)

)

583

waste polyolefins

Bubbling fluidized bed (1 kg h−1)

b

a

HHV. Tar yield expressed in wt%.

Fluidized bed + fixed bed (0.5 kg h−1) Fixed bed (0.06 kg h−1) Moving grate (80 kg h−1) * fueled with pure O2

Bubbling fluidized bed (1 kg h ) Bubbling fluidized bed (1 kg h−1) Fluidized bed + fixed bed (0.5 kg h−1) Fluidized bed + fixed bed (0.5 kg h−1)

waste plastic mixture waste plastic waste plastic mixture

PE waste PE waste plastic mixture waste plastic mixture

PP PP

Fluidized bed ( 1 kg h−1) Fluidized bed ( 4 kg h−1)

−1

PP

Fluidized bed ( 1 kg h−1)

Bubbling fluidized bed (100 kg h−1) Fluidized bed ( 1 kg h−1) Fluidized bed ( 1 kg h−1)

Bubbling fluidized bed (5 kg h−1)

Bubbling fluidized bed (100 kg h

−1

Bubbling fluidized bed (100 kg h

mixed plastic wastes (mainly polyolefins) mixed plastic and cellulosic material recycled plastic (from packaging) mixture of waste polyolefins PP PP

PE

Bubbling fluidized bed (100 kg h−1)

−1

Feedstock

Reactor configuration

Table 1 Results obtained by different authors in waste plastic air gasification.

silica sand silica sand silica sand/dolomite silica sand/active carbon olivine/active carbon – –

silica sand

olivine sand 70% sand−30% dolomite 70% sand−30% olivine olivine bottom ash

silica sand

olivine

olivine

olivine

olivine

sand

Bed material

0.3, 0.3, 0.2, 0.2,

T: T: T: T:

750 750 800/800 800/800

ER: 0.2, T: 800/830 ER: 0.4, T:700–900 ER: 0.15–0.6 T:700–900

ER: ER: ER: ER:

ER: 0.32–0.36, T: 850 ER: 0.2–0.45, T: 690–950 ER: 0.25–0.35, T: 750

ER: 0.32–0.36, T: 850

ER: 0.25, T: 887 ER: 0.32–0.36, T: 850 ER: 0.32–0.36, T: 850

ER: 0.25, T: 877

ER: 0.24, T: 869

ER: 0.20–0.31, T: 845–897 ER: 0.2–0.29, T: 807–850 ER: 0.22–0.31, T: 869–914 ER: 0.25, T: 887

Conditions(-, ºC)

– – 1.2–1.5

3.6 3.7 – –

3.2–4.4

6 2.0–3.8

2.9

3.3 4.5 5.3

3.5

2.73

3.3

2.5–3.2

4.2–6.2

3–4.3

Gas yield (m3 kg−1)

H2: 27.1, CO: 6.7, CO2: 8.5, CH4: 6.4 H2: 0–2, CO: 0.2–4, CO2: 5–7, CH4: 21–20 H2: 41–29, CO: 22–33, CO2: 8.2–22, CH4: 4.3–10

H2: 3, CO: 8.5–10, CO2: 7.8–6.5, CH4: 8.5–10 H2: 2.7, CO: 6.1, CO2: 8.8, CH4: 7.0 H2: 3, CO: 8.7, CO2: 7.4, CH4: 8.7 H2: 14.2, CO: 6.6, CO2: 4.0, CH4: 15.7 H2: 15.2, CO: 6.7, CO2: 4.5, CH4: 14.8

H2: 10, CO: 8, CO2: 11, CH4: 7 H2: 4–5, CO: 20–15, CO2: 9–15, CH4: 6–4

H2: 5, CO: 4, CO2: 14, CH4: 7

H2: 6.0, CO: 4.5, CO2: 10.3, CH4: 6.6 H2: 5, CO: 5, CO2: 12, CH4: 3 H2: 6, CO: 7, CO2: 16, CH4: 8

H2: 6.0, CO: 6.6, CO2: 12.7, CH4: 6.5

H2: 6.0, CO: 6.6, CO2: 12.7, CH4: 6.5

H2: 9.1–9.5, CO: 2.8–2.2, CO2:9.1–10.4, CH4: 10.4–7.1 H2: 30.1–29.1, CO: 18.4–20.9, CO2:1.6–1.2, CH4: 3.4–1.5 H2: 6.8–6.6, CO: 3.7–4.8, CO2: 11.1–11.6, CH4: 7.3–6.3 H2: 5.9, CO: 4.5, CO2: 10.3, CH4: 6.6

Gas composition (% vol)

– 18–12b –

128 102 – –

150–55

2 40–1.3

10

59 17 1.5

46

34

59

99–56

0

160–81

Tar (g m−3)

5.8 7.8–8 9.0–11.8

3.9 4.9a 13.4 13.2

a

4.9–5.7a

6 11.3–5.2a

5.8

6.6 2.9 7.4

7.9

7.4

6.6

6.8–5.2

7.6–6.3

7.9–6.3

LHV (MJ m−3)

[160] [164] [126]

[80] [80] [68] [68]

[80]

[30] [33]

[30]

[163] [30] [30]

[162]

[31]

[31]

[32]

[32]

[32]

Reference

G. Lopez et al.

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influential variable studied, as an increase in ER from 0.2 to 0.45 caused a significant increase in the gasifier temperature from 703 to 915 °C. Moreover, the tar content in the gas product was reduced from 40.3 to 0.25 g Nm−3 in the ER range studied. This reduction was a direct consequence of the tar cracking (higher temperature) and of the higher gas yield. The authors concluded that ER should be carefully adjusted in order to avoid a reduction in the gas product heating value for high ER values. Martinez-Lera et al. [81] studied the air gasification of pure PE and PP and a waste PE in a 1 kg h−1 bench scale bubbling fluidized bed gasifier. The experiments were performed at 750 °C with an ER between 0.25 and 0.35, and a bed made up of inert silica sand. The gasification of the pure PE and PP produced a similar gas yield and composition. However, better results were observed in the gasification of waste PE than in the gasification of pure polyolefins. Thus, the gas yield obtained with pure PE was 90.8% and that of the waste PE 92.5%, with the difference in the tar content being even more significant. Thus, the gas obtained from the waste plastic had a tar content of 102 g Nm−3, whereas that obtained from pure PE had 128 g Nm−3. Although the ER was varied in a limited range (0.25–0.35), its effect on process performance, and especially on tar yield, was remarkable. Thus, in the case of waste PE it was reduced from approximately 150 g Nm−3 to below 60 g Nm−3. Interestingly, Martinez-Lera and Pallares [161] developed a semiempirical model for the gasification of polyolefin in fluidized bed reactor, with the predictions of this model being validated with the previously mentioned results and others from the literature dealing with steam gasification. Lee et al. [126] studied the plastic waste gasification in a 80 kg h−1 moving grate gasifier pilot plant using pure oxygen as gasifying agent. The optimum ER was in the 0.3–0.45 range and, under these conditions, the heating value of the gas produced was above 10 MJ m−3 and the gas yield ranged between 1.36 and 1.49 m3 kg−1. Air gasification of waste plastics is an interesting alternative for the production of a gas stream suitable for different energy applications, with the production of electricity in turbines and engines being the most feasible one [158]. The heating value of the gas produced varies in the range from 3 to 12 MJ m−3, and is mainly affected by the plastic waste composition and especially by the ER used, which is usually between 0.2 and 0.45 (Fig. 10). The average heating value obtained in waste plastic gasification is around 6–8 MJ m−3 (Table 1). Regarding the effect operating conditions have on air gasification performance, the most influential parameter is undoubtedly ER, since it determines the gas yield and its composition [33,81]. An increase in ER leads to higher gas yields (Fig. 11), but its heating value is also reduced

olefins. Furthermore, the gas composition was also improved by enhancing the reforming reactions, and therefore considerably increasing H2 content. Thus, the tar content in the gas product in the experiments performed with inert silica sand was of around 100 g Nm−3, whereas the tar were almost fully eliminated when calcined olivine was used in situ as catalyst. The overall improvement of the process performance using olivine has been proven based on the carbon conversion efficiency, i.e., the carbon fraction in the feed converted into products in the outlet stream. This parameter increased from 59% to 65% at low ERs and from 69% to 81% at high ERs. Moreover, an increase in ER led to a positive effect on the tar content in the gas product, but this fact could also be related to the dilution effect due to the increase in the gas production for high ER values. In a subsequent study, the same authors compared the gasification performance of different waste plastics and mixtures of plastics recovered from MSW and post-consumer packaging [32]. The gasification of a waste polyolefin mixture with olivine in situ produced a similar gas fraction composition, tar yield and process efficiency to that obtained with pure PE, which is evidence of the flexibility of this valorization route. However, the results obtained in the gasification of complex mixtures of several plastics were poorer, i.e., higher tar yields and lower process efficiencies, and the authors related this fact to the reduction of olivine catalytic performance. Kim et al. [68] studied the air gasification of a mixture of waste plastics made up of polyolefins and other plastics, such as PS, PVC and PET, in a two-step bench scale unit with a continuous feed of 0.5 kg h−1. Both steps were carried out in fluidized bed reactors at around 800 °C, with the first one containing sand and the second one tar cracking catalysts. Amongst the catalysts studied, there are activated carbon and dolomite, with the former having proven to be a better alternative for tar elimination. The use of active carbon as cracking catalyst not only reduced tar formation below one half, but also improved the H2 content in the gas product. The tar yields were in the 3-7 wt% range depending on the experimental conditions, with the effect of catalyst bed mass being remarkable. In a subsequent study operating under comparable conditions, the same authors proposed a similar strategy, but in the first bed they replaced sand with olivine and dolomite as primary catalyst [79,160]. The combination of each one of these catalysts in the first bed with active carbon in the second one produced a tar yield below 2 wt%. In addition, the use of dolomite significantly improved the gas fraction composition. Sancho et al. [30] studied the air gasification of PP in a 1 kg h−1 continuous bench scale fluidized bed reactor at 850 °C with an ER of around 0.35. The catalytic performance of olivine and dolomite as primary catalysts was assessed in this study, and the results were compared with those obtained with inert sand. The authors observed that the utilization of dolomite is limited by its poor physical strength, which causes its elutriation from the gasifier. Furthermore, olivine has suitable physical properties for use in fluidized beds, with its catalytic activity being only slightly lower than that of dolomite. Thus, the use of olivine caused a reduction in the tar content in the gas product from 17 g Nm−3 obtained with sand to 2 g Nm−3. In addition, olivine also promotes hydrocarbon reforming reactions, which improve the H2 content in the syngas produced. In a subsequent study, the same authors delved into the application of olivine in the air gasification of PP [67] and shown olivine stability for long gasification runs. In addition, the ER was reduced from 0.38 to 0.25 in order to increase the gas product heating value and, at the same time, maintain the low tar content (2 g Nm−3). This objective was achieved by increasing the temperature in the freeboard region of the gasifier to 910 °C providing external heat supply. Xiao et al. [33] analyzed the effect of different operating parameters such as ER, residence time and gas velocity in the air gasification of PP in a 4 kg h−1 bench scale bubbling fluidized bed gasifier. The bottom ash from a boiler was used as a primary catalyst and the presence of Al, Fe Ca and Mg was responsible for tar cracking activity. ER was the most

Fig. 10. Effect of ER on gas product LHV obtained by different authors in the air gasification of waste plastics. Arena et al. [84], Arena et al. [32], Kim et al. [68], Cho et al. [160], Martinez-Lera et al.* [80], Xiao et al.* [33], Toledo et al. [67] and Lee et al. [126]. * the values reported are of HHV.

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studied in different types of reactors (Table 2), such as are fluidized beds [34], fixed bed reactors [69,70,87,95] and conical spouted beds [35,71]. Apart from the tar content in the gas product, steam gasification faces another significant challenge, as is the heat requirement of the process. In order to overcome this limitation, steam gasification of different plastics has been studied in a dual fluidized bed reactor, with a pilot plant of 100 kW having been developed by Wilk and Hofbauer [34]. The gasification reactor operates at 850 °C with an S/P ratio of 2 and using olivine as in situ primary catalyst. The gasification of PE and PP produced a syngas with a H2 concentration of up to 40%, with H2 production being 4 and 3 wt% (gH2 100 gplastic−1), respectively. However the most remarkable point regarding gas product composition is the high concentration of methane, 30% and 40%, and ethylene, 15% and 11%, in the gasification of PE and PP, respectively. The high content of hydrocarbons enhanced the heating value of the gas produced up to 25 MJ m−3. However, as previously reported by other authors, the high concentration of CH4 and light hydrocarbons is a clear indication of tar presence [37,38,83,173] and, in fact, the tar content values reported were higher than 120 g m−3 for both plastics, with naphthalene being the prevailing compound. The same authors reported an order of magnitude lower tar values in the gasification of biomass using similar experimental conditions [85]. Erkiaga et al. [35] studied the steam gasification of HDPE in a 0.1 kg h−1 continuous bench scale conical spouted bed reactor between 800 and 900 °C. Operating above 850 °C with an S/P of 1 the H2 content of the product stream was slightly higher than 60% which accounts for a production of 18 wt%. The heating value of the gas decreased from 19.2 to 15.5 MJ m−3 as gasification temperature was increased due to the reduction in the hydrocarbon content. A minimum tar content of 16.7 g m−3 was obtained at the highest temperature studied with a bed of inert sand and, interestingly, this tar was mainly composed of singlering aromatics. The use of olivine and γ-alumina as primary catalysts reduced only slightly the tar content in the syngas and had a limited influence on gas composition. In a subsequent study, the same authors [71] used a fixed bed reactor with a Ni commercial reforming catalyst connected in-line with the conical spouted bed gasifier. The fixed bed operated between 600 and 700 °C, with the experimental conditions in the gasification being similar to those used in the previous study. The addition of the catalytic reforming step allowed for full reforming of hydrocarbons and tar, increasing the H2 production up to 36.4 wt%. Recently, Dou et al [174] developed a continuous reaction system at laboratory scale made up of a fluidized bed gasifier followed by a steam reforming/CO2 adsorption in a moving bed reactor. The combined effect of steam reforming on Ni-Al2O3 catalyst and CO2 retention on CaO led to high H2 productions, but they noted that CO2 adsorption is only effective operating below 700 °C. He et al. [69] studied the gasification of 0.3 kg h−1 of PE in a fixed bed reactor between 700 and 900 °C on a Ni-Al2O3 catalyst with an S/P ratio of 1.33. An increase in temperature improved plastic conversion, with the yield of gases reaching 2.04 m3 kg−1 at 900 °C and H2 concentration and production significantly improving to 37 and 6.7 wt%, respectively. The heating value of the gas product was between 12.4 and 11.3 MJ m−3, with the highest value being obtained at the lowest temperature studied. Moreover, the Ni based reforming catalyst showed no deactivation after 3 h time on stream. Friengfung et al. [87] studied the steam/O2 gasification of different plastics in a laboratory scale (0.1 g of sample) fixed bed batch reactor. The experiments were carried out at 850 °C without catalyst and using dolomite and dolomite impregnated with Ni. The results obtained with different polyolefins, HDPE, LDPE and PP, were poor without any catalyst, with tar yields being higher than 80 wt% in all cases. Although using dolomite the gasification performance improved, the tar yields were higher than 50 wt%. Only when the Ni-dolomite catalyst was used suitable results were obtained, especially in the case of HDPE, for which tar yields below 10 wt% were obtained.

Fig. 11. Effect of ER on gas yield reported by different authors. Arena et al. [84], Arena et al. [32], Martinez-Lera et al. [80], Xiao et al. [33], Toledo et al. [67] and Lee et al. [126].

(Fig. 10). Furthermore, an increase in ER increase N2 presence in the gas product, and it also extents the combustion of H2, CO and CH4 and the subsequent increase in CO2 concentration. The tar content in the gas product is usually reduced as ER is increased [33,81,84], which is due not only to the enhancement of tar cracking reactions by increasing O2 concentration in the reaction environment, but also to the increase in the volumetric gas yield and gasifier temperature. The direct utilization of the gas produced is seriously conditioned by tar content, and therefore different strategies have been proposed for its elimination or reduction. Thus, the use of a primary catalyst in situ allows for significantly reducing the tar content in the gas product [30,67,84]. Concerning the primary catalyst studied in plastic waste air gasification, although dolomite is more active for tar cracking than olivine [30,165–167], the better mechanical properties of olivine boosted its use in fluidized bed reactors [30–32,67]. The catalytic role of olivine is usually related to Fe(II) content [168], with its interest being related not only to its capacity for direct tar cracking, but also for enhancing the elimination of tar promoters, such as light olefins [32,85], which avoids further tar formation in the gasification environment. Tar catalytic cracking in secondary beds has been proposed and different catalyst have been reported, such as dolomite [68,160], zeolite [156], active carbon [68,79,160] and active carbon loaded with Ni [169]. Moreover, the use of filters and electrostatic precipitators is also recommended for tar removal from the gas product [68,79]. The design of the gasifier is also crucial for improving the tar elimination efficiency. Thus, in the case of fluidized bed reactors, an increase in residence time and temperature in the freeboard region is usually pursued in order to favor tar cracking [67,80]. In the same line, the location of the feed has also influence on tar yield in fluidized bed gasifiers [78,170]. The use of secondary air injections in the freeboard is also a common strategy to increase temperature and improve tar cracking in this region of the gasifier [171,172]. The tar contents in the gas produced in the air gasification of waste plastics by different authors vary in a wide range depending on several factors, such as reactor design, the catalyst used, the plastic composition and experimental conditions, such as temperature, residence time and ER (Table 1). These tar contents are in general higher than those reported in biomass gasification [36–38], whose average value is of 10 g m−3 in fluidized bed reactors [39]. 5.2. Steam gasification The steam gasification of plastics has been scarcely studied in the literature. Unlike air gasification studies carried out almost exclusively in fluidized bed reactors, steam gasification of waste plastics has been 585

586

PP

Plastic waste PET waste plastics

Batch fixed bed (0.1 g)

Batch fixed bed (0.1 g)

Batch fixed bed (6 g) Semibatch fixed bed Plasma

Tar yields calculated in carbon moles.

HDPE

Batch fixed bed (0.1 g)

a

refuse paper and plastic waste PS

Batch fixed bed (7 g)

waste PE

PE PE PE

PE PP PP+PE PE+PET PE+PS PE

Dual fluidized bed (15 kg h−1) Dual fluidized bed (15 kg h−1) Dual fluidized bed (15 kg h−1) Dual fluidized bed (15 kg h−1) Dual fluidized bed (15 kg h−1) Spouted bed (0.1 kg h−1)

Spouted bed (0.1 kg h−1) Spouted bed (0.1 kg h−1) Two steps: Spouted bed + fixed bed (0.1 kg h−1) Fixed bed (0.3 kg h−1)

Feedstock

Reactor configuration

– – –

Ni-Dolomite

Ni-Dolomite

Ni-Dolomite

–

Ni/γ-Al2O3

olivine γ-alumina olivine/NiCaAl2O4

olivine olivine olivine olivine olivine sand

Bed material

Table 2 Results obtained by different authors in waste plastic steam gasification.

2, T: 850 2, T: 850 2, T: 835 1.2, T: 850 1.8, T: 850 1, T: 800–900

T:850, Gasifying agent: stam/ O2 1:1 T:850, Gasifying agent: stam/ O2 1:1 T:850, Gasifying agent: stam/ O2 1:1 T: 850 (15 °C min−1) T: 1000 T: 1200, Gasifying agent: stam/ O2

T: 900

S/P: 1.33, T: 700–900

S/P: 1, T: 900 S/P: 1, T: 900 S/P: 1, T: 900/600–700

S/P: S/P: S/P: S/P: S/P: S/P:

Conditions (-, ºC)

H2: 38, CO: 45, CO2: 8, CH4: 9 H2: 44, CO: 19, CO2: 13, CH4: 20 H2: 61, CO: 6, CO2: 12, CH4: 2 H2: 62, CO: 34, CO2: -, CH4: -

– – 3.5

H2: 35, CO: 43, CO2: 10, CH4: 11

H2: 29, CO: 43, CO2: 26, CH4: 1.7

H2: 38, CO: 7, CO2: 8, CH4: 30 H2: 34, CO: 4, CO2: 8, CH4: 40 H2: 46, CO: 22, CO2: 5, CH4: 16 H2: 27, CO: 20, CO2: 29, CH4: 15 H2: 52, CO: 24, CO2: 7, CH4: 12 H2: 57–60, CO: 24–28, CO2: 3–1, CH4: 6–7 H2: 58, CO: 27, CO2: 3, CH4: 7 H2: 59, CO: 26, CO2: 2, CH4: 8 H2: 71–73, CO: 8–12, CO2: 17–15, CH4: 3–0.3 H2: 17–37, CO: 20–27, CO2: 35–21, CH4: 21–10 H2: 38, CO: 22, CO2: 17, CH4: 12

Gas composition (% vol)

1.9

2.4

1.3

0.9

1.22–2.04

3.2 3.3 4.4–5.6

1.2 1 2.1 1 1.4 2.5–3.4

Gas yield (m3 kg−1)

– – –

[99] [70] [72]

[87]

– 140

20.4 7.8 10.1

[87]

– a

a

[87]

– 290a 17

[175]

17.9

–

[69]

[35] [35] [71]

[34] [34] [34] [34] [34] [35]

Reference

12.4–11.3

16.2 16.2 –

25.8 27.2 19.4 16.4 17 19.2–15.5

LHV (MJ m−3)

106–13

15 16.1 0

190 180 30 160 110 29.5–16.7

Tar content (g m−3)

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5.3. Co-gasification A remarkable difference between pyrolysis and gasification processes is the degree of dependence of product distribution on the composition of the feed. Thus the product yields and composition obtained in the pyrolysis of different solid wastes are highly diverse. Furthermore, the differences observed in the gasification of different feedstocks are restricted to gas composition and the yield of minor byproducts, such as tar and char. This versatility of the gasification process together with the higher development degree of biomass and coal gasification processes has encouraged the study of plastic waste cogasification. Although waste plastics have been mainly co-gasified with biomass [65,75,76,98,115,130,162], they have been co-processed with coal [38,97,177] and ternary mixtures [133,162,178] have also been proposed. Regarding the gasifying agent, these studies were carried using air, steam or their mixtures. The main results in plastic waste co-gasification are shown in Table 3. The continuous steam gasification of different PE/biomass mixtures was analyzed by Pinto et al. [65] in a fluidized bed reactor. These authors observed that the maximum PE content studied (60%) led to efficient conversion, as evidenced by an specific gas yield of 1.96 m3 kg−1 and a heating value of 18.3 MJ m−3. Moreover, an increase in PE in the feed caused, on the one hand, an increase in H2 (to 50%) and methane concentration, but, on the other hand, a reduction in that of CO and CO2. In a later study, Pinto et al. [178] approached the air/steam cogasification of coal with lower amounts of biomass (20%) and PE (20%) in a 5.5 kg h−1 fluidized bed gasifier. The co-feeding of plastics caused an increase in the hydrocarbon concentration in the gas product, but this effect could be avoided operating at higher temperatures or ERs. A similar trend was reported regarding tar formation. Thus, the results emphasized the flexibility of the co-gasification strategy and the authors concluded that the operating conditions in the gasifier should be carefully adjusted in order to attain a suitable performance for each feedstock mixture. Full tar elimination was achieved by the same authors by means of two secondary tar cracking fixed bed reactors, using dolomite in the first one and Ni-Al2O3 in the second one [37]. Interestingly, the retention of undesired sulphur and halogen compounds in the dolomite bed improved the performance and durability of the Ni based catalyst. Recently, Pinto et al. studied the co-gasification of rice husk (80%)/PE(20%) mixtures in a fluidized bed gasifier using different gasifying agents, such as steam, air, pure oxygen and their mixtures [36]. The results obtained showed that the gas with best features is attained by operating with pure oxygen and steam, but the use of the former alternative is limited by the high cost of oxygen production, and therefore enriched air was regarded as a feasible alternative. Wilk and Hofbauer [75] studied the steam co-gasification of biomass pellets with different types of plastic wastes (including PE) and their mixtures in a 15 kg h−1 dual fluidized bed gasifier using olivine as bed material. Thus, for a HDPE/ biomass blending ratio of 1/1, a gas yield of 1.6 m3 kg−1 with a LHV value of 16 MJ m−3 was reported, with the tar content being 39 g m−3, which is considerably lower than that obtained with pure plastic. In fact, a synergistic effect on tar formation was observed in the co-gasification of plastics and biomass, with tar contents being lower than those expected considering their individual gasification. In addition, an increase in plastic content in the feed also influenced tar composition by decreasing the content of phenols and furans and increasing that of naphthalene. Similarly, the gas product composition could not be estimated directly from the results obtained with individual feedstocks, as nonlinear trends are observed when using with different blending ratios. The same authors analyzed the influence of lignite co-feeding in the steam gasification of PE [179]. Furthermore, a synergistic effect on the cold gas efficiency was reported, and lignite co-feeding also allowed for reducing tar content compared to the results obtained with pure plastic.

Fig. 12. Effect of temperature on gas yield in waste plastic steam gasification. Wilk and Hofbauer [34], Erkiaga et al. [35], Hwang et al. [175] and He et al. [69].

Fig. 13. Effect of temperature on hydrogen production in waste plastics steam gasification. Wilk and Hofbauer [34], Erkiaga et al. [35], Hwang et al. [175] and He et al. [69].

Steam gasification of polyolefins leads to a syngas with high H2 concentrations, with H2 production values in the literature ranging from 3 to 18 wt% (g 100 gplastic−1) [34,35,69]. Moreover, the syngas produced is interesting for the synthesis of different fuels (hydrocarbons, methanol, DME) [176]. The most influential variable on plastic steam gasification is temperature. Its increase promotes the endothermic reactions of reforming and cracking involving light hydrocarbons and tar, which increases the production of both gas (Fig. 12) and H2 (Fig. 13). However, the tar content in the gaseous stream must be drastically reduced in order to meet the strict tar content limitations for synthesis applications [40]. As reported, plastic waste steam gasification leads to high tar concentrations in the gas product, even above 100 g m−3 [34,69]. In fact, it is generally accepted that steam gasification produces more tar than air gasification [40,82], and plastic waste gasification leads to higher tar contents than biomass or coal gasification [34,37,38]. Although the absence of N2 enhances the heating value of the gas above 15 MJ m−3 [34,35,175], steam gasification faces a significant challenge for its full scale development, as is the high heat requirement of the process. In fact, the well developed dual fluidized beds technology [34,75,85] is also limited by the low fixed carbon of plastic wastes which hinders process heat balance closure. Overall, plastic waste steam gasification has been scarcely studied and developed and cannot be considered as mature as the air gasification strategy.

587

588

)

a

HHV.

Fixed bed/fixed bed (0.04 g) Updraft (60 kg h−1)

Bubbling Fluidized bed (5 kg h−1) Bubbling Fluidized bed (5 kg h−1) Fluidized bed/fixed bed (2 kg−1) Spouted bed (0.1 kg h−1) Fixed bed

PP (0.5)/biomass (0.5)

waste polyolefins (0.55)/biomass paper fiber and wood (0.45)

air

steam steam

steam

HDPE(0.5)/wood (0.5) rice straw (0.5)/PE (0.5)

steam

coal (0.6)/waste polyolefins (0.2)/biomass (0.2) recycled plastic (0.8)/wood (0.2)

coal (0.6)/waste polyolefins (0.4)

olive husk (0.75)/PET (0.25)

coal (0.5)/mixed polyolefins (0.5) coal (0.5)/mixed polyolefins (0.3)/wood (0.2) olive husk (0.75)/PET (0.25)

pine wood(0.8)/PE (0.2)

coal (0.5)/Recycled plastic (0.3)/ wood (0.2) coconut shell (0.5)/PE(0.5)

air

air

air

Fluidized bed (4 kg h−1)

steam/air

air

)

steam/air

Fluidized bed (4 kg h−1)

Fluidized bed (5 kg h

−1

Fluidized bed (5 kg h

O2 enriched air

−1

Fluidized bed

air

air

)

Fluidized bed

Fluidized bed (5 kg h

pine wood(0.8)/PE (0.2)

air

−1

wood pellets (0.5)/plastics from MSW(0.5) lignite (0.66)/PE(0.33)

steam

steam

wood pellets (0.7)/PE(0.3)

steam

Dual fluidized bed (15 kg h−1) Dual fluidized bed (15 kg h−1) Dual fluidized bed (15 kg h−1) Fluidized bed (5 kg h−1)

wood (0.5)/PET (0.5)

wood (0.58)/PE (0.32)

rice husk (0.8)/PE(0.2)

rice husk (0.8)/PE(0.2)

air

oxygen

air

Fluidized bed

)

)

steam/air

−1

rice husk (0.8)/PE(0.2)

coal (0.6)/pine wood (0.2)/PE (0.2) coal (0.9)/PE (0.1)

pine wood (0.9)/PE (0.1)

Feedstock

Fluidized bed

Fluidized bed (0.3 kg h

Fluidized bed (0.3 kg h

−1

steam

Fluidized bed (0.3 kg h−1)

steam/air

steam/air

)

steam

Gasifying agent

Fluidized bed (6 kg h−1)

Fluidized bed (5.5 kg h

−1

Fluidized bed (0.75 kg h−1)

Reactor configuration

T: 800–930, ER: 0.19–0.24

–

2.6–3.4

2.55

T: 850/700 Fe-CeO2

2.7

2.7

3.4

3

2.9

1.4

1.3

2.8–1.8

2.1–2.8

–

–

–

2.64 1.1

T: 800/600, S/F: 2

ER: 0.25, T: 868

ER: 0.25, T: 872

T: 850, ER: 0.36

T: 850, ER: 0.36

T: 845, ER: 0.1, S/F: 0.62

T: 850, ER: 0.25, O2 content: 21–35% T: 752, ER: 0.1, S/F: 0.76

T: -, ER: 0.2–0.3

T: 780, ER: 0.23

T: 780, ER: 0.23

T: 850, S/F: 0.9

1.1

1.9

–

–

1.0

1.3

0.35

1.3

0.6–1.35

0.63–1.28

Gas yield (m3 kg−1)

T: 900, S//F: 1 T: 900

Ni commercial catalyst/dolomite olivine –

silica sand

silica sand

sand-dolomite

sand-dolomite

Ni-γAl2O3

γ-Al2O3

quartz sand

quartz sand

Ni-γAl2O3

quartz sand

olivine

olivine

olivine T: 850, S/F: 0.94

T: 900, S/F: 0.42, ER: 0.14 T: 725–875, ER: 0.19–0.31 T: 850, S/F: 1.6

γ-Al2O3 olivine

T:850, ER:0.2

T:850, ER:0.2

T:850, S/F: 1

T: 740–880, S/F: 1, Air/ F: 1.14 T: 850, S/F: 0.85, ER: 0.2

T: 740–880, S/F: 0.8

Conditions (ºC, -)

–

–

–

–

–

–

Bed material

Table 3 Results obtained by different authors in the co-gasification of waste plastics with other feedstocks.

H2: 10–15, CO: 15–14, CO2: 8, CH4: 6–5

H2: 57, CO: 27, CO2: 7, CH4: 6 H2: 46, CO: 30, CO2: 12, CH4: 12 H2: 40, CO: 5, CO2: 16, CH4: 6

H2: 14, CO: 13, CO2: 14, CH4: 2 H2: 82, CO: 9, CO2: 2, CH4: 7

H2: 25–44, CO: 34–31, CO2: 14–9, CH4: 15–10 H2: 25–40, CO: 18–17, CO2: 24–20, CH4: 18–15 H2: 40, CO: 17, CO2: 16, CH4: 17 H2: 41, CO: 15, CO2: 24, CH4: 11 H2: 19, CO: 24, CO2: 33, CH4: 13 H2: 38, CO: 12, CO2: 37, CH4: 12 H2: 32, CO: 23, CO2: 23, CH4: 15 H2: 4.3–5.4, CO: 13–9, CO2: 17, CH4: 3–2.7 H2: 41, CO: 23, CO2: 16, CH4: 14 H2: 35, CO: 24, CO2: 19, CH4: 6 H2: 45, CO: 24, CO2: 10, CH4: 8 H2: 17, CO: 16, CO2: 15, CH4: 12 H2: 30, CO: 14, CO2: 10, CH4: 3 H2: 8–6, CO: 5, CO2: 10, CH4: 8–6 H2: 8–14, CO: 11–19, CO2: 13–16, CH4: 4–6 H2: 33, CO: 13, CO2: 19, CH4: 9 H2: 40, CO: 22, CO2: 16, CH4: 5.5 H2: 11, CO: 12, CO2: 14, CH4: 2 H2: 14, CO: 13, CO2: 14, CH4: 2 H2: 10, CO: 7, CO2: 11, CH4: 8

Gas composition (% vol)

a

9.5–79

35.5b

– 22–11.2

[115] [92]

– 13.8 9.7 –

[94]

[91]

[180]

12.4

0

[162]

[162] 6

7

34

[133]

[133]

[78]

[78]

[182]

[38]

[77]

[77]

[179]

[75]

[75]

[184]

[183]

[36]

[36]

[36]

[37]

[178]

[65]

Reference

41

5.5

8.3

9

10.2

1

1.3

29

90

5.1–8.9

7.7–6.0

47–25 13–22

6.5

7.3 27

60

13

16

39 9

16

4.5–3.5

–

13

8

a

13a

–

24–18

21–15

LHV (MJ m−3)

47

145–63

–

12

12

15

19

–

–

Tar content (g m−3)

G. Lopez et al.

Renewable and Sustainable Energy Reviews 82 (2018) 576–596

Renewable and Sustainable Energy Reviews 82 (2018) 576–596

G. Lopez et al.

the former led to better results in terms of tar content and gas composition. Moreover, a significant improvement in the gasifier performance was observed when the results reported by feeding though a middle point in the bed are compared with those reported by feeding from the top of the bed. Park et al. [123] developed a two-step gasification system consisting of an oxidative pyrolysis (527 °C) followed by a thermal plasma reactor (627 °C). The co-gasification of HDPE and biomass was studied under different blending ratios and ERs. The optimum results were obtained for a 70% of biomass in the feed and an ER of 0.47. Mastellone et al. [38,162,182] studied the air gasification of coal and plastic mixtures, as well as ternary mixtures made up of coal, plastic mixtures and biomass, in a pre-pilot fluidized bed gasifier. The main effect of co-feeding plastics was an increase in the gas yield and its heating value, mainly due to the higher content of light hydrocarbons. They observed a lower H2 concentration [162] and an increase in tar formation when plastics were included in the feed, i.e., tar contents in the coal/plastics co-gasification ranged from 25 to 47 g m−3 for different ERs (0.2–0.3). Interestingly, the effect of biomass was the opposite of what was expected to happen (reduction in tar formation). Accordingly, the authors concluded that, using suitable proportions of the components in the feed, synergistic effects can be promoted, increasing the viability of the process. The air co-gasification of binary and ternary mixtures made up of coal, biomass and waste plastics (PE and PP) was studied by Aznar et al. [133] in a fluidized bed reactor. The plastic contents in the feed were relatively low in the ternary mixtures (10–20%) but rather high in the binary ones (40%). Plastics in the feed increased the hydrocarbon concentration in the gas product and reduced that of CO, CO2 and H2. Moreover, a positive effect of plastics in the feed on the gas heating value and tar content was observed, but this effect on tar formation is opposite to that reported by other authors [75,77,162]. Thus, operating at 850 °C, with an ER of 0.36 and using dolomite as primary catalyst, very low tar contents (1.3–1 g m−3) were obtained for binary and ternary mixtures, with the heating values being in the 5–8 MJ m−3 range due to the high ER used. Based on the above mentioned results, the co-gasification of waste plastics with different feedstocks leads to interesting synergies, which emphasize the interest of this strategy [75,98,115]. These synergies are commonly attributed to the interactions between biomass chars with the polymer degradation products [185], with the positive interactions in their joint thermal degradation being well documented [10,186]. Thus, an increase in plastic content in its co-gasification with biomass and coal improves both gas yield and H2 production, as observed in Figs. 14 and 15, respectively. These results are explained by the higher

Fig. 14. Effect of plastic content in the feed on gas yields in the co-gasification of plastics with biomass and coal. Baloch et al. [92], Zaccariello and Mastellone [162], Pinto et al. [37], Wilk and Hofbauer, [75], Lopez et al. [115] and Kern et al. [179].

Ahmed et al. [98] studied the steam co-gasification of PE and woodchips in a laboratory batch fixed bed reactor at 900 °C. These authors also mentioned the synergistic effect on the yields of H2, hydrocarbons and gas, and on the thermal efficiency, in the co-processing of plastics and biomass. Moreover, the optimum plastic content in the feed was determined to be between 60% and 80%. The PE and biomass co-gasification study by Lopez et al. [115] in a 0.1 kg h−1 conical spouted bed gasifier also confirms the previously reported synergistic effects. This effect is especially remarkable for a blending ratio of 1/1. Thus, operating at 900 °C with an S/F ratio of 1 and using olivine as primary catalyst, the tar content in the gas product in the gasification of a 1/1 mixture of PE and biomass was reduced to 9.7 g Nm−3 (58.2 and 5.1 g N m−3 in the individual gasification of biomass and PE, respectively). Moreover, although the gas yield (2.64 m3 kg−1) is approximately the theoretical value expected based on the results obtained for the individual feeds of PE and biomass, a synergistic effect was observed on the H2 content in the syngas and on the reduction of char yield. Recently, Esfahani et al. [180,181] proposed a novel process made up of two stages for the steam gasification of HDPE and coconut shell/ palm kernel shell mixtures in a fluidized bed reactor (between 650 and 870 °C) using Ni powder as in situ catalyst followed by tar cracking in a dolomite fixed bed reactor (600 °C). This process allows obtaining a high H2 content in the syngas produced and efficient tar elimination. Thus, operating at the highest gasification temperature of 870 °C, a H2 production of 28.4 wt% was reported, with its concentration being up to 86% vol. Ruoppolo et al. [77] studied the gasification of pellets made up of PE (20%) and pine wood (80%) in a pilot scale fluidized bed gasifier, and compared the results with those obtained with pure biomass. Two different bed materials were used, inert quartzite and Ni-Al2O3 catalyst. The gasifying agents used were air and air/steam mixtures, and they observed that air/steam mixtures led to an increased in H2 concentration and a reduction in tar content by enhancing the reforming reactions. The most promising result in the gasification of the pellets containing PE was the high H2 concentration obtained (30% vol.). However, the tar contents reported were considerably higher (of around 45 g Nm−3) than those obtained with biomass (below 30 g Nm−3), even though they used relatively low plastic contents in the pellets and a Ni-Al2O3 catalyst. Thus, the aforementioned synergies in the steam gasification seem to be less marked using air as gasifying agent. In a subsequent study, the same authors studied the gasification of pellets made up of olive husk (75%) and PET (25%) with steam/air mixtures, but using low ERs in order to improve syngas quality [78]. The performance of a Ni-Al2O3 catalyst was compared with that of Al2O3, and

Fig. 15. Effect of plastic content in the feed on H2 production in the steam co-gasification of plastics with biomass and coal. Baloch et al. [92], Pinto et al. [37], Wilk and Hofbauer, [75], Lopez et al. [115] and Kern et al. [179].

589

Renewable and Sustainable Energy Reviews 82 (2018) 576–596

G. Lopez et al.

An independent temperature optimization may also be carried out in the thermal degradation and catalytic steam reforming steps [51]. In addition, the process temperature is significantly reduced in relation to direct gasification, minimizing material costs and sintering problems of the reforming catalyst [46,159]. Therefore, this process takes advantage of the highly active reforming catalyst to fully eliminate tars from the gas product, which pose a serious drawback in conventional plastic gasification process. The relevance of this strategy has also been reported in the production of H2 from biomass [131,189,190]. Moreover, recently a novel alternative for the plastic pyrolysis-reforming process has been proposed, which consist in the combined production of H2 and carbon nanotubes using different Ni and Fe based catalysts [191–195]. The pioneering studies of pyrolysis and in-line steam reforming of plastics were carried out by Czernik and French [127] in an experimental unit made up of two fluidized bed reactors for the pyrolysis and reforming steps, with a Ni based commercial catalyst used in the reforming step. Operating with continuous PP feed (1 g min−1), at 650 and 800 °C in the pyrolysis and reforming steps, respectively, a H2 production of 34 wt% (34 g 100 gPP−1) was obtained, and the plastic derived volatiles were fully converted into a gaseous stream completely free of tar. This yield accounts for 80% of the maximum allowable by stoichiometry. H2 production was reduced to 24 wt% when operating under autothernal conditions by co-feeding air (ER=0.25) into the reforming step. Interestingly, the process was able to operate under steady state conditions for 10 h without detectable catalyst deactivation. The research group headed by Prof. Williams studied in detail the pyrolysis and in-line reforming of waste plastics on different catalysts [44,88,90,196–200]. The studies were carried out in a laboratory scale reactor made up of two fixed bed reactors for the pyrolysis and reforming steps, and operating in batch regime. A plastic sample of around 1 g was heated in the pyrolysis reactor (40 °C min−1 to 500 °C), and the volatiles formed were then treated in the fixed bed reforming reactor at 800 °C. Using a Ni-Mg-Al catalyst, the H2 productions were 26.6 and 18.5 wt% when PP and PS were fed, respectively [90], with these values being below those reported by Czernik and French [127]. Similarly, in other studies by the same authors using Ni based catalysts for the reforming of PP derived volatiles, the maximum H2 yield was of around 65% of the one allowable by stoichiometry [197,201]. Recently, the same authors replaced steam by CO2 in order to study the dry reforming of plastic pyrolysis volatiles [202]. This novel route is an interesting CO2 valorization strategy, as almost full conversion is achieved, with the syngas produced being mainly made up of H2 and CO. Operating at 500 and 800 °C in the pyrolysis and reforming steps and using a Ni-Mg-Al catalyst, the H2 production values were 15.0, 13.6, 7.6 and 2.5 wt% when feeding PE, PP, PS, PET, respectively. These values are clearly lower than those usually obtained in the pyrolysis and in-line steam reforming process. The two-step process developed by Park et al. [45,51] for the pyrolysis-reforming of PP was based in two fixed bed reactors operating in continuous regime (1 g min−1). The pyrolysis step was performed between 400 and 600 °C and the reforming on a commercial Ru-Al2O3 catalyst between 580 and 680 °C. The reforming temperature showed a remarkable effect on the process performance, with the optimum results being obtained at an intermediate temperature studied (630 °C) due to the significant increase in coke yield at high temperatures. Thus, at 630 °C the H2 production reached a value of 34.2 wt%, and the liquid hydrocarbons were totally converted into gaseous products (and coke). The same authors studied the pyrolysis and in-line reforming of PS in the same experimental unit and under similar conditions [45], and the H2 production reported was 33.0 wt%, which is slightly lower than that obtained by feeding PP. The continuous experimental setup developed by Erkiaga et al. [43] was based on a conical spouted bed reactor for the pyrolysis step and a fixed bed reactor for the catalytic steam reforming. The pyrolysis of

Fig. 16. Effect of plastic content in the feed on the tar content of the gas produced in the co-gasification of plastics with biomass and coal. Zaccariello and Mastellone [162], Pinto et al. [37], Wilk and Hofbauer, [75], Lopez et al. [115] and Kern et al. [179].

Fig. 17. Effect of plastic content in the feed on the heating value of the gas produced in the co-gasification of plastics with biomass and coal. Baloch et al. [92], Zaccariello and Mastellone [162], Wilk and Hofbauer [75] and Kern et al. [179].

hydrogen and carbon contents in plastic wastes than in the biomass and coal and, furthermore, char production is also lower or even negligible. As observed in Fig. 16, the main drawback of co-feeding plastics is usually related to the increase in tar formation. The improvement in the heating value of the gas produced compared to that obtained in biomass gasification, Fig. 17, also reinforces the interest of plastic co-feeding [36]. This fact is of great interest for the implementation of advanced technology units for biomass gasification, as the seasonal availability of biomass can be partially avoided by co-gasifying it with plastics. Finally, an additional advantage of biomass and plastic co-gasification lies in the minimization of operational problems in the gasification of plastics, such as those related to their feeding into the reactor and the formation of fine char particles [65]. 5.4. Pyrolysis and in-line steam reforming Apart from the direct gasification, other alternative processes have been recently proposed for H2 and syngas production from waste plastics (Table 4). Amongst them, the two-step process of pyrolysis and in-line catalytic reforming of volatiles is probably the most promising one due to its operational advantages and the high H2 productions achieved [45,88,159,187,188]. Furthermore, the impurities contained in the plastic waste remain in the pyrolysis reactor, which avoids their contact with the reforming catalyst, and therefore its deactivation [90]. 590

591

a

Fluidized bed (0.08 kg h−1) Fluidized bed (0.08 kg h−1) Fixed bed/fixed bed (0.06 kg h−1) Fixed bed/fixed bed (0.06 kg h−1) Fixed bed/fixed bed (0.06 kg h−1) Spouted bed/fixed bed (0.05 kg h−1) Spouted bed/fluidized bed (0.05 kg h−1) Spouted bed/fluidized bed (0.05 kg h−1) Fluidized bed/fluidized bed (0.06 kg h−1) Fluidized bed/fluidized bed (0.06 kg h−1)

Steam reforming of plastic pyrolysis oil Steam reforming of plastic pyrolysis oil Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line autothermal steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line steam reforming Pyrolysis and in line dry reforming Pyrolysis and in line dry reforming Pyrolysis and in line dry reforming Pyrolysis and in line dry reforming Plasma gasification with CO2 T: 650/850, S/C: 4.6 (molar) T: 650/850, S/C: 4.6 (molar), ER: 0.25 T: 500/600–900

Ni-Al2O3 -/Ru-Al2O3 -/Ru-Al2O3 -/Ru-Al2O3 sand/Ni-CaAl2O4 sand/Ni-CaAl2O4 sand/Ni-CaAl2O4 sand/Ni commercial catalyst sand/Ni commercial catalyst -/Ni-CeO2 ZSM−5

PS pyrolysis oil PP PP PS PE PE PS PP

PE waste plastics (from MSW) PP PE PP PS PET

Fixed bed/fixed bed (1 g)

Fixed bed/fixed bed (1 g)

Fixed bed/fixed bed (0.04 g) Fixed bed/fixed bed (1 g)

Fixed bed/fixed bed (1 g)

Fixed bed/fixed bed (1 g)

Fixed bed/fixed bed (1 g)

H2 yield reported as a percentage of the maximum allowable by stoichiometry.

mixture of PE, PP and PET

-/Ni-Co-Al

PS

Fixed bed/fixed bed (1 g)

Plasma (11 kg h−1)

-/Fe-CeO2

PP

Fixed bed/fixed bed (1 g)

–

-/Ni-Co-Al

-/Ni-Co-Al

-/Ni-Co-Al

-/Ni-Mg-Al

-/Ni-Mg-Al

-/Ni-Mg-Al

-/Ni-Mg-Al

T: 1200–1400

T: 500/800

T: 500/800

T: 500/800

–

–

H2:42, CO: 50, CO2: 7, CH4: 0

–

–

–

–

–

–

3.94

T: 850/700 T: 500/800

3.94

4.35

3.57

4.65

T: 500/800

T: 500/800

T: 500/800

T: 500/800

–

T: 500/800

-/Ni-CeO2

PP

Fixed bed/fixed bed (1 g)

–

T: 500/800

PP

Fixed bed/fixed bed (1 g)

-/Ni-Al2O3

–

T: 500/600–900

H2: 64, CO: 24, CO2: 10, CH4: 1 H2: 58, CO: 25, CO2: 10, CH4: 1 H2: 67, CO: 24, CO2: 9, CH4: 1 H2: 67, CO: 20, CO2: 12, CH4: 1 H2: 67, CO: 20, CO2: 12, CH4: 1 –

H2:62–67, CO: 8–26, CO2: 16–4, CH4: 7–4 H2:62–65, CO: 9–27, CO2: 18–4, CH4: 4–1 H2:56, CO: 20, CO2: 9, CH4: 6 H2:75, CO: 6, CO2: 7, CH4: 5

–

4.1

5.4

5

5.4

5.4

4.2–5.2

5.4–5.6

5.4–8.8

4.6–5.2

H2:70, CO: 7–18, CO2: 19–12, CH4: < 1 H2:68, CO: 8–16, CO2: 20–18, CH4: < 1 H2:71–70, CO: 9–11, CO2: 19–16, CH4: 1.5–1.4 H2:71–72, CO: 9–8, CO2: 19, CH4: 1.5–0.9 H2:69–68, CO: 5–10, CO2: 25–21, CH4: 0 H2:71, CO: 11, CO2: 17, CH4: < 1 H2:71, CO: 11, CO2: 17, CH4: < 1 H2: 65, CO: 14, CO2: 21, CH4: < 0.1 H2:71, CO: 12, CO2: 16, CH4: 1.2 H2:65, CO: 12, CO2: 21, CH4: 1.6

Gas composition (% vol)

4.7–5.8

Gas yield (m3 kg−1)

-/Ni-CeO2 Al2O3

PP

PP

PP

T: 570–800, S/C: 3.5(molar) T: 600–800, S/C: 3.5(molar) T:400/580–680, S/C: 3.6 (molar) T:400–600/630, S/C: 3.6 (molar) T:400/580–680, S/C: 3.7 (molar) T: 500/700, S/C: 3.1

Ni-Al2O3

PE pyrolysis oil

T: 500/600–700, S/C: 3.1 T: 500/700, S/C: 2.89

Conditions (ºC, -)

Bed material

Feedstock

Fixed bed/fixed bed (1 g)

Fixed bed/fixed bed (1 g)

Reactor configuration

Strategy

Table 4 Results obtained by different authors in the H2 production from plastic wastes in alternative processes to gasification.

31.5

–

26.0 23.6

0 0

< 0.001

0

0

0

0

–

2.5

7.6

13.6

15.0

23.6

18.5

0

0

26.6

27a

27ª

13–52ª

27–61ª

24.0

34.0

29.1

37.3

34.5

33.0

36.0

0

0

0

0

0

0

0

0

0

0.11

0

0

36.5

37

–

0

H2 production (g 100 gplast−1)

Tar content (g m−3)

[73]

[202]

[202]

[202]

[202]

[90]

[90]

[90]

[90]

[90]

[196]

[196]

[201]

[197]

[127]

[127]

[159]

[46]

[43]

[45]

[51]

[51]

[153]

[153]

Reference

G. Lopez et al.

Renewable and Sustainable Energy Reviews 82 (2018) 576–596

Renewable and Sustainable Energy Reviews 82 (2018) 576–596

G. Lopez et al.

HDPE was carried out at 500 °C and the reforming on a Ni commercial catalyst at 700 °C. The reforming catalysts promoted the full conversion of pyrolysis volatiles to yield gases, with H2 production being 34.5 wt%, which accounts for 81.5% of the maximum allowable by stoichiometry. The main challenge to overcome in this process is coke formation (4.4 wt% of the plastic in the feed), which hinders reactant flow in the fixed bed reforming reactor. These operational problems had been avoided in a subsequent study in which the fixed bed was replaced with a fluidized bed reactor for the reforming step [46]. Thus, when operation following this strategy was conducted at the same temperatures (in the pyrolysis and reforming steps) as in the old one (fixed bed reformer), H2 production increased to 38.1 wt% (92.5% of the allowable by stoichiometry), which is evidence of the advantages of using a fluidized bed reactor for the reforming step. The high performance of this combination (a conical spouted bed and a fluidized bed) for the pyrolysis-reforming process was confirmed in a subsequent study by feeding PS [159]. In line with the results reported by Park et al [45], the H2 production in the pyrolysis-reforming of PS was below that reported for HDPE, 29.1 vs. 38.1 wt%. This result is related to the different H2 content of these polymers. In the case of both PE and PS, there is a significant catalyst activity decay due to coke formation, with the deactivation rate being higher in the case of PS. Thus, both the nature of the deposited coke and the deactivation kinetics depend on the nature of the hydrocarbons formed in the polymer degradation [159,203]. Although the indirect route of H2 production by the reforming of biomass pyrolysis oil (bio-oil) has been widely studied [204–206], this alternative has been scarcely studied in the case of waste plastics. Thus, the single study proposing an indirect route for H2 production from waste plastics is that by Tsuji and Hatayama [153]. The oil previously produced by LDPE pyrolysis was vaporized and subjected to steam catalytic reforming on a Ni-Al2O3 catalyst in a fluidized bed reactor between 600 and 800 °C. The gas produced has a H2 content of around 70% vol., which is close to the equilibrium one and accounts for a production of 37.0 wt%. The reforming of the oil obtained in the pyrolysis of PS has also been studied, with H2 production being in this case 31.5 wt%. Moreover, operating with an S/C ratio higher than 3, coke formation was avoided and the catalytic performance was stable after several hour operation. The two-step process of pyrolysis and in-line reforming of volatiles allows obtaining full conversion to yield a gaseous stream rich in H2 and free of liquid hydrocarbons or tar, which means a significant advantage compared to conventional gasification. Thus, H2 production values above 30 wt% have been reported by different authors [43,45,46,51,153,159,188]. The most influential variables on the pyrolysis-reforming performance are the reforming step temperature and

Fig. 19. Effect of steam/carbon (molar) ratio on the hydrogen production in waste plastics reforming process. Li et al. [100], Barbarias et al. [46], and Tsuji and Hatayama [153].

the steam/carbon ration. The effect of both variables on H2 production is shown in Figs. 18 and 19, respectively. As observed in Fig. 18, an increase in the reforming temperature improves H2 production by enhancing the endothermic steam reforming reactions involving hydrocarbons, even though this improvement is limited by the WGS reaction equilibrium (hindered by temperature). An increase in the S/C ratio increases the steam partial pressure in the reaction environment, which enhances both the reforming and the WGS reactions favoring H2 production, but this effect is attenuated at high S/C ratios, Fig. 19. These H2 production values obtained in the pyrolysis-reforming process are noticeably higher than those typically obtained in the steam gasification of plastics, below 20 wt% even under optimum conditions [34,35,69]. In the same line, and due to the higher carbon and H2 content in the plastics, the H2 productions obtained in the steam gasification and pyrolysis-reforming of biomass are also considerably lower, in the 2–8 wt% [104,166,207,208] and 4–11 wt% [131,189,190] ranges, respectively. Accordingly, the valorization of waste plastics by the pyrolysis-reforming strategy is a promising route. In spite of the potential interest of the two-step process, its development degree is limited; in fact, the studies in the literature are currently limited to laboratory scale units. Accordingly, several challenges should be faced prior to industrial implementation, such as the operation in pilot-plant scale, detailed studies dealing with catalyst deactivation/regeneration or the kinetic modelling of the volatiles reforming step. 6. Conclusions and final remarks Although the gasification of waste plastics has been less studied than other valorization routes, such as pyrolysis, the development degree of gasification technologies is high, especially in the case of fluidized bed reactors, with numerous studies having been performed in pilot plant gasifiers. In fact, this technology has already been applied to the gasification of other feedstocks, such as coal or biomass. Accordingly, the suitable development degree of waste plastics gasification technologies improves the scale up and implantation possibilities of this valorization route. The main challenge of plastic gasification is tar formation, which causes serious operational problems leading to a reduction in the overall process efficiency and applications of the gas produced. Accordingly, significant research efforts have been made pursuing their elimination or reduction, including the evaluation of a wide variety of primary and secondary catalysts and modification in the reactor design. Air gasification appears to be an interesting option for the production of a gas product of interest for energy production in medium and small

Fig. 18. Effect of reforming temperature on the hydrogen production in plastics pyrolysisreforming process. Namioka et al. [45], Park et al. [51], Barbarias et al. [46], Wu and Williams [90] and Tsuji and Hatayama [153].

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scale power plants. The average lower heating value obtained in waste plastic air gasification is in the 6–8 MJ m−3 range, although this value strongly depends on the equivalence ratio and plastic waste properties. The tar contents reported in the literature vary depending on operating conditions, reactor design and feed characteristics. Overall, these values are clearly above those obtained in biomass or coal gasification, i.e., they are usually above 100 g m−3. Steam gasification of waste plastics allows producing a H2 rich syngas of interest for both synthesis and energy applications. The H2 production values reported in the literature are between 3 and 18 wt%. Moreover, the absence of N2 enhances syngas heating power to values above 15 MJ m−3. In spite of the higher quality of the syngas obtained in plastics steam gasification, this process should overcome two main limitations, the endothermicity of the process and the higher tar content in the gas product. The flexibility of the gasification process is significantly increased when plastics are co-fed with other feedstocks (especially biomass and coal), which has been attributed to synergistic effects in the reaction medium. Thus, an increase in plastic content in the feed improves H2 content and gas product heating value, but the tar content is also increased. A novel strategy of pyrolysis and in-line reforming to produce H2 from waste plastics is gaining increasing attention. Operating at lower temperatures than conventional gasification, this process is able to produce a H2 rich gas product free of tars. Thus, H2 productions as high as 30 wt% have been reported. In spite of the interest and promising results of this alternative, detailed studies are required prior to its industrial implementation, especially those involving catalyst stability and regeneration. In fact, the development degree of pyrolysis-reforming strategy is low, with current studies being limited to laboratory reactors or bench scale units. A significant problem facing all plastic conversion technologies is the variable quality and composition of the feed. Given the cost entailed in the elimination of plastic contaminants, the commercial success of any plastic valorization route should not rely only on the added value of the products obtained, but also on its capacity for accepting feeds of high impurity content without conditioning process performance. Thus, in order to develop successful waste plastic valorization routes, the presence of inorganic components and complex plastics, such as PVC, should be addressed. Acknowledgements This work was carried out with financial support from the Ministry of Economy and Competitiveness of the Spanish Government (CTQ2016-75535-R (AEI/FEDER, UE) and CTQ2014-59574-JIN (AEI/ FEDER, UE)), the Basque Government (IT748-13) and the University of the Basque Country (UFI 11/39). Jon Alvarez also thanks the University of the Basque Country UPV/EHU for his postgraduate Grant (ESPDOC 2015). References [1] Wong SL, Ngadi N, Abdullah TAT, Inuwa IM. Current state and future prospects of plastic waste as source of fuel: a review. Renew Sustain Energy Rev 2015;50:1167–80. [2]. Plastics – the Facts 2016. An analysis of European plastics production, demand and waste data; 2016. [3] Moore CJ. Synthetic polymers in the marine environment: a rapidly increasing, long-term threat. Environ Res 2008;108:131–9. [4] Aguado J, Serrano DP, Escola JM. Fuels from waste plastics by thermal and catalytic processes: a review. Ind Eng Chem Res 2008;47:7982–92. [5] Katami T, Yasuhara A, Okuda T, Shibamoto T. Formation of PCDDs, PCDFs, and coplanar PCBs from polyvinyl chloride during combustion in an incinerator. Environ Sci Technol 2002;36:1320–4. [6] Butler E, Devlin G, McDonnell K. Waste polyolefins to liquid fuels via pyrolysis: review of commercial state-of-the-art and recent laboratory research. Waste Biomass- Valor 2011;2:227–55. [7] Al-Salem SM, Lettieri P, Baeyens J. Recycling and recovery routes of plastic solid

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