Waste Management 37 (2015) 116–136

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Waste Management journal homepage: www.elsevier.com/locate/wasman

Review

Reprint of: Pyrolysis technologies for municipal solid waste: A review q Dezhen Chen a,⇑, Lijie Yin a, Huan Wang a, Pinjing He b a b

Thermal & Environmental Engineering Institute, Tongji University, Shanghai 200092, China State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China

a r t i c l e

i n f o

Article history: Received 23 February 2014 Accepted 1 August 2014 Available online 17 February 2015 Keywords: Municipal solid waste (MSW) Pyrolysis Reactor Product Environmental impact

a b s t r a c t Pyrolysis has been examined as an attractive alternative to incineration for municipal solid waste (MSW) disposal that allows energy and resource recovery; however, it has seldom been applied independently with the output of pyrolysis products as end products. This review addresses the state-of-the-art of MSW pyrolysis in regards to its technologies and reactors, products and environmental impacts. In this review, first, the influence of important operating parameters such as final temperature, heating rate (HR) and residence time in the reaction zone on the pyrolysis behaviours and products is reviewed; then the pyrolysis technologies and reactors adopted in literatures and scale-up plants are evaluated. Third, the yields and main properties of the pyrolytic products from individual MSW components, refusederived fuel (RDF) made from MSW, and MSW are summarised. In the fourth section, in addition to emissions from pyrolysis processes, such as HCl, SO2 and NH3, contaminants in the products, including PCDD/F and heavy metals, are also reviewed, and available measures for improving the environmental impacts of pyrolysis are surveyed. It can be concluded that the single pyrolysis process is an effective waste-toenergy convertor but is not a guaranteed clean solution for MSW disposal. Based on this information, the prospects of applying pyrolysis technologies to dealing with MSW are evaluated and suggested. Ó 2015 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pyrolysis behaviours of MSW with respect to products and the influential factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Terminology and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Factors influencing MSW pyrolysis behaviours and products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pyrolysis technologies and the reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Pyrolysis technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Pyrolysis reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Fixed-bed reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Rotary kiln reactors and their systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Fluidised-bed reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4. Tubular reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.5. Other pyrolysis reactors and technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Products from pyrolysis of typical MSW components and MSW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Pyrolysis products from waste paper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

117 117 117 118 118 118 125 126 126 127 127 127 128 128

Abbreviations: HCs, hydrocarbons; HDPE, high-density polyethylene; HHV, higher heat value, the same meaning as gross calorific value; LDPE, low-density polyethylene; LHV, lower heat value; MSW, municipal solid wastes; MPW, mixed plastic waste; PCDD/F, polychlorinated dibenzodioxins/furans; PET, polyethylene terephthalate; PP, polypropylene; PS, polystyrene; MPW, mixed plastics waste; PVC, polyvinyl chloride; RDF, refused-drive fuel; rpm, revolution per minute; TG-FTIR, thermogravimetric analysis-Fourier transform infrared spectrometer. DOI of original article: http://dx.doi.org/10.1016/j.wasman.2014.08.004 q This article is a reprint of a previously published article. The article is reprinted here for the reader’s convenience and for the continuity of the special issue. For citation purposes, please use the original publication details; D. Chen et al./Waste Management 34 (2014) 2466–2486 DOI of original item: 10.1016/j.wasman.2014.08.004. ⇑ Corresponding author. Tel.: +86 21 6598 5009. E-mail address: [email protected] (D. Chen).

http://dx.doi.org/10.1016/j.wasman.2015.01.022 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.

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4.2. 4.3. 4.4. 4.5. 4.6.

5.

6.

Pyrolysis products from MPW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pyrolysis products from wood and woody mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pyrolysis products from fallen leaves and vegetables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pyrolysis products from RDF from MSW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pyrolysis products from MSW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.1. Properties of gas products from MSW. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.2. Liquid products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.3. Solid products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.4. Standardized product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7. Considerations for pyrolysis technology and product choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental effects of MSW pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Emissions and contaminants associated with the MSW pyrolysis process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Pyrolysis-associated emission control technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The treatment, management and disposal of municipal solid waste (MSW) are common concerns in every country. MSW pyrolysis is considered as an innovative alternative for treating MSW that obtains different chemicals and fuels (Schaefer, 1975; Malkow, 2004). In a pyrolysis-involved process, energy can be obtained in a cleaner way than from conventional MSW incineration plants as lower amounts of nitrogen oxides (NOx) and sulphur oxides (SO2) are produced as a consequence of the inert atmosphere in the pyrolysis processes and the opportunity to wash syngas before its combustion. In addition to reduced gas emissions, better quality of solid residues can be also expected from pyrolysis-involved treatment technique for MSW (Saffarzadeh et al., 2006). In general, pyrolysis represents a process of thermal degradation of the waste in the total absence of air that produces recyclable products, including char, oil/wax and combustible gases. Pyrolysis has been used to produce charcoal from biomass for thousands of years. When applied to waste management, MSW can be turned into fuel and safely disposable substances (char, metals, etc.), and the pyrolysis process conditions can be optimised to produce either a solid char, gas or liquid/oil product, namely, a pyrolysis reactor acts as an effective waste-to-energy convertor. Compared to the conventional incineration plant which runs in capacity of kiloton per day; the scale of pyrolysis plant is more flexible. Recently, MSW pyrolysis is receiving increasing attention in small cities and towns due to the desire to prevent long-distance transportation; and it is also demanded in big cities as a distributed MSW treatment method due to the increased difficulty in finding new sites for incinerators and landfills. Generally distributed MSW treatment facilities are difficult to ensure environmental safety due to capital cost limitations; while pyrolysis plants of proper capacity with energy products output are suitable alternative when the quality of char, oil/wax and combustible gases is under fine control. A variety of pyrolysis studies have been conducted on industrial wastes such as tyres and plastics, and several reviews have reported on the characterization of the development of pyrolysis technologies in terms of different aspects, for example, reactor development and product characterization (Sannita et al., 2012; Williams, 2013; Yang et al., 2013); conditions for oil production, oil characteristics and upgrading (Quek and Balasubramanian, 2013); the heating rate and other governing variables affecting pyrolysis process and pyrolysis products of tyres (Martínez et al., 2013); and the mechanism investigation or kinetics modelling of the pyrolysis process (Al-Salem et al., 2010; Quek and

129 129 130 130 131 131 131 131 131 132 132 132 133 133 134 134

Balasubramanian, 2012). These reviews facilitate making the state-of-the-art of the development of pyrolysis of waste tyres and industrial plastics well known. As for municipal wastes, the pyrolysis of sewage sludge has been investigated for decades for liquid production, and the state-of-the-art of this technology has also been addressed in a recent review (Fonts et al., 2012). However, compared to waste types such as tyres, plastics and sewage sludge, MSW is more heterogeneous in composition and size. Currently, for MSW pyrolysis, information on technology development, characterization of products and correlated pollution is not sufficient to support technology application and system design, especially in regards to environmental impacts, which is fundamental for a single step MSW pyrolysis application. Therefore, this work reviews pyrolysis technologies for MSW, with focus on reactors, the products from MSW pyrolysis, the pollutants involved with the MSW pyrolysis process and product applications, and the reported measures to alleviate the associated environmental impacts. The final aim is to provide essential information for understanding the pyrolysis process applied to MSW and to standardize its application as an energy converter. 2. Pyrolysis behaviours of MSW with respect to products and the influential factors 2.1. Terminology and scope Reactions take place in a recognized pyrolysis process can be expressed as:

CxHyOz þ Q ! Char þ Liquid þ Gas þ H2 O

ð1Þ

where Q is the heat that needs to be input to the reactor for the reactions to take place, it includes three portions:  Moisture vaporization Q1 During pyrolysis the feedstock will not undergo thermal decomposition before its moisture is vaporized, and Q1 can be calculated as:

Q 1 ¼ W  2260; kJ kg

1

ð2Þ

where W is the water content of the feedstock to the reactor, %; therefore to reduce this part of energy, MSW components with high moisture content such as food wastes, biomass are suggested to be separated before pyrolysis. In addition, in order to reduce this portion of energy a drying step is usually adopted in front of pyrolysis reactor.  Caloric requirement of pyrolysis Q2

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The caloric requirement of the pyrolysis is calculated using the following equation (Raveendran et al., 1996):

Q 2 ¼ C p;M

Z

mM dT þ C p;ch

Z

mch dT þ C p;v

Z

mv dT þ Q p ; kJ kg

1

ð3Þ where Cp,M, Cp,ch, Cp,v are the specific heat capacities of the dry materials, char and volatile generated, J kg1 °C1; mM, mch and mp,v are the mass ratio of the dry materials, char and volatile to the feedstock, %; the former three items at the right side of Eq. (3) are the energy required for heating feedstock and the products to the pyrolysis temperature; and Qp is the reaction heat of the pyrolysis, J kg1. Q2 can be obtained through differential scanning calorimetry (DSC) or differential thermal analysis (DTA) techniques for different MSW components (He et al., 2006), and it has been found that for reactor design purpose Qp can be neglected and the Q2 is mainly decided by pyrolysis temperature (Boukis et al., 2007; Wang et al., 2012). The higher pyrolysis temperature does not only result in a higher energy requirement, but also cause challenge to the safety of reactors.  The radiation loss from the reactor to the surrounding Q3 As heat is transferred from outside to the inner side of the reactor, Q3 can be ignored for heat transfer surface design; but on the contrary, when the reactor is heated opposite, the heat transfer surface should bear this portion of energy to maintain the desired temperature of the reactor. Better insulation will reduce this radiation loss Q3. A pyrolysis system consisting of the pyrolysis reactor and other appurtenances serves the purposes of supplying the heat Q to the reactor, controlling the proper conditions for the desired products, namely liquid, gas and char shown in Eq. (1) and preventing side effects such as pollution. Numerous studies have been performed based on Eq. (1) to investigate the pyrolysis kinetics, pyrolysis behaviours vs. the reaction conditions and to characterize the products. But with the focus on giving instruction in setting up a proper pyrolysis technology for MSW, only those literatures most relevant to MSW pyrolysis technologies and products are reviewed here. 2.2. Factors influencing MSW pyrolysis behaviours and products MSW consists mainly of paper, cloth material, yard waste (including fallen leaves and branches, etc.), food wastes, plastics and a small amount of leather and rubber, metals, glass, ceramic, earthen materials and miscellaneous other materials. The fractions subjected to pyrolysis mainly include paper, cloth, plastics and yard wastes. Highly wet food wastes should be separated to reduce Q1. Numerous studies have been performed in laboratories to determine pyrolysis behaviours and product properties of individual MSW fractions such as paper, plastics, fabric, lignocellulosic materials and putrescibles (Di Blasi, 1996; Wu et al., 2002, 2003; Wu et al., 2005; Zhang et al., 2008; Zheng et al., 2009; Luo et al., 2010a; Atesß et al., 2013; Miskolczi et al., 2013), refuse-derived fuel (RDF) pellets (Cozzani et al., 1995; Garcia et al., 1995a,b; Lin et al., 1999; Buah et al., 2007; Dou et al., 2007; Grammelis et al., 2009; Bosmans et al., 2013) and real MSW ( Islam and Beg, 2004; Islam et al., 2005; He et al., 2010; Luo et al., 2010b; Zhao et al., 2011; Atesß et al., 2013; Miskolczi et al., 2013). The interactions between the different individual fractions and the pyrolysis products have also been explored (Williams and Williams, 1997; Williams and Williams, 1999b; Sørum et al., 2001; Grieco and Baldi, 2012; Ding et al., 2013), but not as extensively. The previous researches indicated that the important parameters affecting MSW pyrolysis included temperature, heating rate (HR), residence time in the reaction zone and materials’ size, etc.; and their effects on

pyrolysis behaviours and the products can be deduced from the summary in Table 1. From Table 1 it can be seen that the reported pyrolysis temperature varied from 300 to 900 °C, but the typical running temperature is around 500–550 °C with liquid products as major portion of products. At temperatures higher than 700 °C, syngas is the vital product. Most of the researches paid more attention to the liquid and syngas products than the char due to the fact that the oil and syngas are more valuable; and the yields and composition of pyrolysis oil and syngas are mainly changing with temperature. The residence time of the materials in the reaction zone is another important parameter; which was reported to be in the range of a few seconds to 2 h as shown in Table 1. It was well recognized that longer residence time would enhance the tar cracking and result in higher gas yields; but at the same time longer residence time reduced water content and waxy material in the liquid products (Velghe et al., 2011), conduced to the improved quality. The complex impact of residence time on products can be deduced from Eq. (1): in a longer residence time, more heat is input into the reactor, the extra heat will evaporate the water produced, crack the tar into small molecules, and vaporise more organics from the char, all of this would result in higher gas yield. But longer residence time will lead to lower treatment capacity of the reactor. The third important factor affecting MSW pyrolysis is heating rate (HR) and it was reported to vary from about 4 °C min1 to 670 °C S1 as shown in Table 1. Higher HR is corresponded to a higher volatile matter (tar and gas) yield, accordingly less char will be left (Li et al., 1999a,b; Yi, 2007; Velghe et al., 2011); together with a longer residence time, higher yield of syngas will be obtained due to tar cracking; while as the volatile matter is extracted immediately and cooled down, higher yield of liquid products can be obtained (Font et al., 1995a,b). Theoretically the HR can be calculated as:

HR ¼

DT  a  ; C S1 m  Cp

ð4Þ

where DT is the temperature difference between the reactor wall and the feedstock, °C; a is heat transfer coefficient inside the reactor, W m2 °C1; m is the mass of feedstock heated by per m2 of heat transfer surface (reactor wall), kg m2; Cp is the is the specific heat of the feedstock, J kg1 °C1. According to Eq. (4), HR is decided by DT, a and the mass load of the reactor. In the laboratory researches, the HR was setting by adjusting DT or mass fed to the reactor to realize slow or flash pyrolysis, as shown in Table 1. While in a practical and continuously running reactor HR is decided by the reactor type which is characterized with its special temperature difference (DT) and heat transfer coefficient (a) features. The pyrolysis reactors reported in the literatures are mainly fixed-bed reactors, rotary kilns and fluidised bed reactors, as shown in Table 2; they are of a sequence in ascending order according to HR. The fluidised bed reactors are characterized with very high heat transfer coefficient of 112–559 (J/s m2 K) (Garcia et al., 1995a). But very few reports are available on heat transfer coefficients for MSW pyrolysis in rotary kilns and fixed-bed reactors, as they are affected by many factors; and they are also the company’s private know-how for designing and running pyrolysis reactors. The transport tube thermo-chemical convertor in Table 2 shows a new reactor type, it belongs to a family of tubular reactor, which will be discussed later. As for the influence of particle size on the MSW pyrolysis, it was not extensively investigated; but in general smaller particles contributed to a larger surface area and faster HRs (Di Blasi, 1996), therefore smaller particle size caused higher yields of liquid or gas products. Coarser particles correspond to the lower HR therefore longer residence time is required. However, once the temperature is high enough, the difference in yields of products

Table 1 Pyrolysis behaviours and products under different reaction conditions. Reaction conditions

Feedstock

Reactor type

Main results respect to pyrolysis behaviours and the products

Reference

MSW was decomposed in the 500–900 °C temperature range, and the lignin was decomposed in the range of 600–900 °C Sample mass for the fluidised-bed reactor was 1–3 g (at temperatures greater than 700 °C) or 0–30 g in the remaining cases. Particle size of PE was 0.5 cm. Almond shells was 0.297–0.500 mm in particle size. MSW sample was 0.8–5 g and ground No information on residence time and heating rate (HR)

MSW, almond shells, lignin and polyethylene

An analytical Pyroprobe apparatus, and a fluidised sand bed reactor

When to investigate kinetics of the primary decomposition and the tar cracking, it was found that when there was no tar cracking, the increase in yields are linear with time; and that tar cracking took place from the top of the sand bed to the reactor head. In addition, the sample could continue decomposing during the cooling period if the temperature is not too low, and this explains why flash pyrolysis in fluidised-bed reactor results in a high gas yield For pyrolysis products in the fluidised bed at 850 °C, almond shells corresponded to 89% of total gas with 8.6% of methane and 4.2% of ethylene; MSW had 47% of total gas with 4.2% methane and 3.4% of ethylene. At 800 °C, polyethylene had 95% of total conversion with 19% of methane, 4% of ethane, 37% of ethylene, 5% of propylene, 5.5% of butylenes, 5.0% of pentane, 25% of benzene and 2.1% of toluene. By using the Pyroprobe and a secondary reactor, the yields for Kraft lignin were 9% of methane, 12% of aromatics and 1.5% of ethylene The heat transfer coefficients in the sand fluidised-bed reactor for MSW pyrolysis was reported to be 112–559 (J/s m2 K) Around 70–80% of the primary tars can be cracked to low molecular weight gases When to study the production of gases from MSW pyrolysis at high nominal temperatures (700–850 °C) and to explore the influence of pyrolysis temperature and residence time of volatiles in the hot zone of reactor on the yields of pyrolysis compounds, it was found that biomass decomposition followed by tar cracking reactions took place inside the reactor. The primary tars evolved cracked at approximately 70% at 800 °C, increasing the gas yield (from 23.8% (primary yields) to 48.0% (secondary yields)) At the highest operating temperature (850 °C) and high residence times, when the cracking of tar was complete, the gas components were CH4 6.1%, C2H6 0.32%, C2H4 4.5%, C3H8 0.001%, C3H6 0.079%, C2H20.19%, C4H80.002%, CO 26.1%, CO215.7% and H2 2.4% When investigate the yield and the calorific value of the syngas changing with individual MSW components, the size of raw materials and the heating modes, the authors found that the conversion of energy contained in MSW components into syngas was much higher under the fast heating mode; PE was corresponded to the highest syngas yield of 720 L kg1 and vegetables had the lowest syngas yield of 51 L kg1, but the latter corresponded to the highest tar and water yield of 0.888 kg kg1. The calorific value of the syngas was in the range of 13000–23000 kJ kg1. Smaller size did not match the higher syngas yield When investigate the yields of pyrolysis products varying with final temperature, the researchers found that the char yield decreased with the increase of temperature, especially within the temperature range 300–550 °C, and that the pyrolysis liquid yield increased with the increase of temperature till 550 °C, whereas, afterward, it began to decline. The gas yield steadily increased with the increase of temperature in the whole test temperature range When investigate the production distribution, the gas characteristics, the pyrolysis gas quality, the characteristics of the pyrolysis liquids and char, the researchers found that the pyrolysis temperature is the top-drawer parameter affecting the pyrolysis of the MSW. With temperature increases, the gaseous yield increased, and the yield of pyrolysis liquids and char yield decreased accordingly. The main components of pyrolysis gas were CO, H2, CH4 and CO2. The CO and H2content increased in the gaseous component with the increase in temperature; the calorific value of pyrolysis gas has a maximum within the testing range, whereas the calorific value of char displays an increasing trend. The liquid products are rich in moisture and volatiles, and the gross content of the fixed carbon and ash in liquid is less than 3%

Font et al. (1995a,b)

The final temperature in the reactor was 850 °C. For fast pyrolysis, the reactor was heated to 850 °C first, and then, the sample was fed. For slow pyrolysis the MSW components was heated together with the reactor wall HR: 5–60 °C min1* Sample mass: 1000 g per batch Residence time: 7–15 min

A helium fluidised sand-bed reactor

9 components in MSW, An externally heated rotary pre-treated kiln running at 3r min1

The reactor was heated up after materials were fed. The final 9 components in MSW, An electrically heated fixedbed reactor with N2 flush in crushed to less than temperature was changed from 300 to 700 °C 10 mm and then mixed the beginning HR: not mentioned Sample mass: 600 g per batch Residence time: 55–180 min depending on the temperature

The bed temperature was varied from 500 to 900 °C Sample mass: 1000 g per batch Sample size: less than 5 cm HR: not mentioned Residence time: up to 50 min

Wood, paper, municipal plastic waste and crushed MSW

An electrically heated fixedbed reactor

Garcia et al. (1995a,b)

Li et al. (1999a)

D. Chen et al. / Waste Management 37 (2015) 116–136

Dry pellets of MSW The reactor was running at high nominal temperatures (700–850 °C) batch by batch The HR of the sample was evaluated to be approximately 670 °C/s when the nominal temperature was 850 °C and approximately 425 °C/s if the nominal temperature is 700 °C Sample mass: 0.8–5 g Residence time: approximately 20 min

Wang et al. (2005a,b)

Jiang, 2006

119

(continued on next page)

120

Table 1 (continued) Reaction conditions

Feedstock

Reactor type

Main results respect to pyrolysis behaviours and the products

Reference

The pyrolysis temperature was increased from 400 to 700 °C HR: 10 K min1 Sample mass: 15 g per batch Residence time: estimated to be 40–70 min*

MSW in the form of refuse-derived fuel (RDF) pellets with a particle size of approximately 8 mm

A fixed-bed reactor constructed of stainless steel and it was externally heated by an electrical ring furnace

Buah et al., 2007

The reactor bed temperature was varied from 600 to 900 °C. When the desired temperatures were achieved in the reactor, the screw feeder was turned on to feed material into the reactor with a flow rate of 5 g min1 No information was available for the residence time and HR

Prepared MSW of uniform size in three groups: smaller than 5 mm, between 5 and 10 mm and between 10 and 20 mm

A fixed-bed reactor heated by an electrical ring furnace with a screw feeder at top of the reactor

Slow pyrolysis was performed up to 550 °C (with HR of 4 °C/ min) in a 2 h process Fast pyrolysis was performed at 450, 480, 510 and 550 °C with few seconds of residence. Sample mass: 70 g MSW in a flow rate of 24 g/min or 12 g/min

MSW cut in 2 mm pieces and oven dried at 110 °C before pyrolysis

A semi-batch reactor filled with hot sand as a heat transfer medium under N2 atmosphere; A Archimedes’ screw ensures good mixing of the samples and the sand and creates a ‘fluidized bed effect’

Pyrolysis was performed up to 550 °C (with HR of 20–25 °C/ min) in a 30 min process Sample mass: 1000 g per batch

MSW fractions and mixed waste plastics

Three-section reactor, with individual temperature controls at different sections, batch run or continuous operation

The temperature inside the reactor was electrically heated up to 700, 800 and 900 °C respectively before feeding samples HR: assumed to be up to 1000 K min1 at 900 °C Sample mass: 4 g Residence time: 10 min

PE (representative for plastic materials in MSW), paper and bamboo (representative for wood & biomass in MSW)

A static batch reactor under a N2 atmosphere

It has been found that the yield and composition of the pyrolysis products depended on temperature. The char yield fell as the pyrolysis temperature was increased from 400 °C to 700 °C, whereas that of oil/wax and gaseous products rose. The chars recovered were also found to have properties that depended on the size fraction analysed. The gaseous products obtained from the pyrolysis consisted mainly of CO2, CO, H2, CH4, C2H6 and C3H8 with lower concentrations of other hydrocarbon gases. Both the calorific value of the gases and the surface areas of the chars increased with pyrolysis temperature. Carboxylic acids and their derivatives, alkanes, alkenes, mono- and polycyclic and substituted aromatic groups were present in the oils, and the oils displayed an increase in aromatic groups and a decrease in aliphatic groups as the temperature rose. The surface area, moisture, ash and volatile content of the chars were dependent on the size range used for analysis When to evaluate the effects of particle size at different bed temperatures on product yield and composition during MSW pyrolysis, it was found that higher temperature resulted in higher gas yields with less tar and char, and dry gas yield increased with a decrease in particle size, and the char and tar yield decreased. The differences due to particle sizes in pyrolysis and gasification performance practically disappeared at the highest temperatures tested. Smaller particle sizes resulted in higher H2 and CO contents for both pyrolysis and gasification of MSW. Minimising the size of raw materials is an alternative method to improve the gas quality of MSW pyrolysis and gasification This research was to investigate the effect of reactor temperature and HR on composition and yield/distribution of pyrolysis products; to investigate the distribution of metals present in MSW towards the pyrolysis products and to investigate the obtained products for their use as fuel for energy production or as raw chemical feedstock. The important findings included: (1) The volatile matter was degraded between 200 °C and 560 °C at a HR of 20 °C min1, as revealed by thermogravimetric analyses. (2) The highest yield of solid products was obtained by slow pyrolysis up to 550 °C, but under this condition the liquid fractions were low with very low water content, free of waxy material and a good candidate as chemical feedstock and fuel. (3) The fast pyrolysis process induces the presence of waxy material in the liquid products; and incomplete breakdown may happen within very limited residence time. (4) The syngas gases contain mainly hydrocarbons and have an averaged LHV of around 13– 20 MJ N m3 and ever higher at higher feeding rate. (5) The distribution of metal ions towards oils is negligible This research was to investigate the impact of impurities such as food wastes, paper, textile and especially soil on the pyrolysis of waste plastics. Emissions, gas and liquid products from pyrolysis of waste plastics and impurities were studied. In addition, the transfer of elemental N, Cl, and S from the substrates to the pyrolysis products was investigated. It was found that the presence of food waste reduced the heat value of the pyrolysis oil and increased the moisture in the liquid products. Therefore, the food residue should be removed, but the soil enhanced the waste plastic pyrolysis by improving the quality of gas and oil products. The presence of food residue, textile and paper led to higher gas emissions This research was to investigate the effects of CaO additives and heating rates on tar generation during MSW pyrolysis. It was found that CaO additives can reduce the content of acids, phenols and hydrocarbons in tar from bamboo and paper pulp pyrolysis, but there was no obvious effect on PE. A higher heating rate resulted in more tar formation, consisting of acids, phenols and hydrocarbons within a wider temperature range The LHV of syngas from PE pyrolysis varies between 37 and 51 MJ Nm3, more than twice of that of bamboo or paper

Luo et al., 2010b

Zhao et al. (2011)

Pan, 2012

D. Chen et al. / Waste Management 37 (2015) 116–136

Velghe et al., 2011

3. Pyrolysis technologies and the reactors 3.1. Pyrolysis technologies The concept of pyrolysis technology can be represented by RWE-ConThermÒ process (Hauk et al., 2004), as shown in Fig. 1, which involves a rotary kiln as pyrolysis reactor with a shredder in front, exports pyrolysis gas and char to a conventional boiler. Malkow (2004) summarised the typical pyrolysis-involved processes and technologies available in European market from the point view of energy efficiency, some of these processes with their updated information and the presently available commercial, demonstration or pilot technologies are summarized in Table 3 from the point view of technologies details and emissions abatement. It can be seen that the existing commercial pyrolysis technologies are characterized with the following configurations:

The same as above

Estimated by this article author according to the substance in reference text.

 Combined technologies

*

The same as above

121

was minimal (Luo et al., 2010b) due to the lower HR effect correlated to coarser particle size was compensated by the increase of the radiation contribution to heat transfer. Basically the influence of particle size is a combination of HR and the interaction between the volatile matter and the char, the latter takes place when the volatile matter diffuses from inner particle to the outside. Finally, as shown in Table 2, all of the experimental pyrolysis systems are equipped with products cooling and collection devices, and most of them have N2 flush to keep inert atmosphere, both of them affect the final product yields. Tar would continue cracking during the cooling period if the temperature is not too low (Font et al., 1995a), therefore a slow cooling of volatile from a high temperature reactor would result in higher gas yield; fast and deep cooling of the volatile would gain higher liquid yield and it is also preferable to obtain cleaner gas. As for N2 flush, it changed the flow rate and residence time of the volatile in the reactor, and the cooling of the volatile was interfered too, so data obtained from those systems with N2 flush may deviate from practice.

Miskolczi et al., 2013 It has been found that viscosity, average molecular length and contaminant amounts of pyrolysis oils were decreased when using catalysts. Comparing the two raw materials, the catalysts displayed higher activity using MPW than MSW. Both the carbon frame and double bond isomerisation occurred during thermo-catalytic pyrolysis. The morphology and texture of the chars were modified by catalysts, especially by Ni–Mo catalysts

Reference

Atesß et al. (2013)

Main results respect to pyrolysis behaviours and the products

A batch fixed reactor, N2 Crushed MSW and mixed plastic waste flushed (MPW) in particles less than 10 mm in main dimension with & without catalysts

The reactor was heated to 500, 550 and 600 °C for thermal pyrolysis and 500 °C for thermo-catalytic pyrolysis Sample mass: 50 g per batch No information on HR Residence time: 5239 s for MSW at 500 °C in absence of catalyst; in the range of 4331–5110 s using catalysts. Fully decomposition of MPW pyrolysis have higher time necessity than MSW in absence of catalyst; 6652 s at 500 °C, 5299 s at 550 °C and 4187 s at 600 °C The same as above

Table 1 (continued)

Reactor type Feedstock Reaction conditions

This research was to characterize product yields, gas and pyrolysis oil properties for both thermal pyrolysis and thermo-catalytic pyrolysis. It was found that gases contained hydrogen and hydrocarbons; CO and CO2 were obtained only from MSW; aromatic and cyclic compounds predominantly formed in the presence of catalysts, and the catalytic effect was more significant using MPW

D. Chen et al. / Waste Management 37 (2015) 116–136

The existing commercial pyrolysis technologies seldom run alone with gas, tar and char output as end products, most of them are combined with gasification, combustion and smelting; and the combination with gasification produces moderate-calorific-valued fuel gas, it still receives extensive attention in waste-to-energy processes in recent studies (Smith et al., 2001; Li et al., 2007; Yi, 2007; Ohmukai et al., 2008) and will be a competitive choice in the future. But at the same time, all the combined technologies in Table 3 are expensive and may not be affordable in places where the pyrolysis technology is needed.  Fed with pre-treated materials All of those MSW pyrolysis technologies accept pre-treated MSW instead of raw MSW in its original state, suggesting pretreatment is a necessary step for pyrolysis technology. The pretreatment generally includes separation of undesirable materials and size reduction; sometime a drying step in front of pyrolysis reactor is needed to reduce moisture content of materials feeding to the reactor. The above RWE-ConThermÒ plant (Hamm) is no longer in operation after the chimney collapse in December 2009. The accident was analysed to be caused by the feeding waste material, for the feed composition did not match the process as designed, resulting in process temperatures beyond tolerable limits. However preparation of input waste stream to match the process could be too expensive to be beyond profitability. A process flexible to the waste input is more practical, and combination of pyrolysis and

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Table 2 Typical MSW pyrolysis reactors and system in literatures. Pyrolysis reactor and system in researches

Reference

waste collection

Atesß et al. (2013), Miskolczi et al. (2013)

waste separation

Metals, paper, organic waste, etc

Shredding Crushing

Gases

3 6 4

5

2

7

Pyrolysis oil 4

1 Char

Water Fixed-bed reactor and pyrolysis system (1-N2 bottle; 2-reactor; 3-heat exchanger; 4-separation unit, 5-water trap; 6-gas flow meter; 7-rotameter) Wang et al. (2005b)

7

10

3

9

5

8 8

8 4

11

1 6

2

Fixed bed pyrolysis system (1-furnace; 2-pyrolysis reactor; 3-thermocouple; 4-temperature controller; 5-N2 pipe; 6-liquid gathering tank; 7-thermometer; 8-condenser; 9-pressure gauge; 10-sampling vent) Li et al. (1999a,b), Li et al. (2000a,b)

Rotary kiln pyrolysis system (1-thermometer; 2-bearing; 3-gear transmission; 4-electrical furnace; 5-rotary kiln; 6-temperature controller; 7-seal; 8-two-steps condenser; 9-filter; 10-accumulative flowmeter; 11-computer; 12-gas sampling device; 13-feed and discharge opening; 14-speed adjustable electrical machinery) Williams and Williams (1999a)

Feed Hopper

Gas sample bag

TC De-mister Pyrolysis reactor

Fluidised bed

TC TC TC

Cyclone

TC Distributer

Glass wool trap

Catch pot CO2/ acetone condensers

Gas pre-heater

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D. Chen et al. / Waste Management 37 (2015) 116–136 Table 2 (continued) Pyrolysis reactor and system in researches

Reference

Fluidised-bed pyrolysis system

Alimentation with divided solid

Natural gas towards burner

Marculescu et al. (2007)

Process gases analysis and treatment

Heated case

Transport tube

Treatment gas inlet (comburant or inert) Burned gases Functional scheme of the transport tube thermo-chemical convertor

Solid residue analysis

Integrated Pyrolysis for Power Plants Pyrolysis of high calorific solid recoverd fuels Flue gas to boiler pyrolysis gas to boiler

1.Tipping bunker 2. Shredder 3.Fine material bunker 4.Crane system 5.Material sluice 6.Rotary pyrolysis 7.Burner system 8.Solid residue discharge 9.Fan 10.Cyclone(dedusting)

Inerts Metals

Pyrolysis coke to boiler

Fig. 1. Schematic flow sheet of RWE-ConThermÒ process (Tech Trade, 2014).

gasification stage and independent utilization of the pyrolysis products serve the necessity of process flexibility.  Installed with secondary treatment of products The commercial technologies listed in Table 3 are characterized with the immediate utilization of their products in a combustion chamber or a gas engine, suggesting that those products are not standardized and not ready for the market. In those technologies with pyrolysis gases exported to gas engine, gasification acts as primary processing, and gas scrubbing is followed to ensure the quality of the syngas. The char should have secondary treatment measures such as quenching, screening and separation of metals when sold as carbon-rich material for industrial purposes. Sometime secondary treatment of pyrolysis products could be very rigid depending on the feeding materials and the way to use products. To avoid extensive secondary treatment of products, high temperature combustion inside the system is a suitable choice.  Equipped with emission abatement devices As shown in Table 3, all of the commercial pyrolysis processes are equipped with emissions’ abatement devices similar to

incineration plant, ensuring the pyrolysis-involved process is a clean one. Typical exhaust flue gas emission control devices include particulate filters, cooling tower, wet scrubbers, etc., but when compared to those adopted in incineration plants, they are reduced in dimension. In addition to the commercial and semi-commercial pyrolysis technologies, some pilot or demonstration technologies are newly developed with purpose to simply the pyrolysis system for MSW through applying pyrolysis alone. Fig. 2 shows a pilot MSW pyrolysis facility developed in Tianjin, China (Li et al., 2007) with a capacity of 5 tons per day. Dealing with unsorted MSW, it consisted of a main reactor and a subsidiary reactor. The main pyrolysis reactor was a tubular reactor with a screw inside to push the MSW forward; the tube was heated from outside by burning solid fuels such as coal or the charcoal produced in the pyrolysis process. The hot flue gas leaving the main reactor was then conveyed to the subsidiary reactor, and, after cooling in a radiation duct, it mixed together with pyrolysis gas and contacted MSW directly to heat up the latter for decomposition and gasification. The pilot tests were based on an artificially assembled MSW with the following composition: rubber 8.27%, PVC 8.51%, PE 8.51%, kitchen waste 24.33%, pericarp14.60%, vegeta-

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Table 3 Main pyrolysis involved units and technologies used in pilot, demonstration and industrial plants. Process name

Reactor & operation conditions

Materials input & products

Technologies

Application example

Environmental protection device

PYROPLEQÒ process (Bracker et al., 1998; Modern Power Systems, 2014)

Rotary drum, pyrolysis at 450– 500 °C; syngas was burnt at 1200 °C and the hot gas from the combustion chamber heats the pyrolysis drum from the outside

Input: Pre-treated domestic refuse; MPW Output: The char with calorific value of around 10 GJ t1. Power from steam turbine

Pyrolysis & combustion

The Ticino Canton waste treatment centre in Switzerland

Input: Shredded MSW, automotive shredder residues as well as up to 50% waste plastics Output: Power from steam turbine Input: MSW, industrial waste, spent tyres, industrial and plastic waste as well as contaminated soil

Pyrolysis & combustion

The Hamm powerplant, Germany, with capacity of 100 kt/ year. Stopped

Hot-gas filtration by means of ceramic filter cartridges. Lime is added to the reactor for acid gas removal. Conventional flue gas clean up with a bag filter for fly ash collection with sodium bicarbonate injection for acid gas emission control is used. The ash and acid gas clean-up solids are mixed and sent for landfill disposal The pyrolysis gas passed through a cyclone before boiler Flue gas scrubbing system of the coal-fired power plant

A combination of pyrolysis, gasification (thermal cracking), and smelting

Aalen, Germany, with Gas scrubbing unit to wash out inorganic acid pollutants, capacity of 25,000 followed by a bag filter to tons per year remove the remaining dust; and a basic and a biological washer to remove H2S. Finally, an active coal filter to absorb dioxins, furans, and Hg

Pyrolysis & combustion

Arras, France, with The flue gases produced by the capacity of 50,000 tpa combustion of the thermolysis gases need a deduster and rational scrubbing system

Pyrolysis and entrained flow gasification

Freiberg, Germany, with capacity of 12,000 tpa sewage sludge + 5760 tpa MSW. Demonstration plant

Pyrolysis, gasification & combustion

Keflavic, Iceland, with Rational scrubbing techniques for fuel gas and flue gas capacity of 45 tons per day

A combination of pyrolysis and high temperature combustion

Fürth, Germany, with capacity of 100.000– 150.000 kt/y, Shut down August 1998 after accident with pyrolysis gas

ConThermÒ technology Rotary kilns, pyrolysis taking place at 500–550 °C for about (Tech Trade, 2014; 1 h, gas combustion in a Hauk et al., 2004) pulverised coal (PC)-fired boiler, see Fig. 1 Rotary kiln, pyrolysis at 500– 550 °C, for approximately 45– 60 min, the kiln was externally heated with part of the gas produced in the process The pyrolysis gases containing tars are subsequently gasified in a high temperature (1200– 1300 °C) gasifier; char is smelted (at 1400–1500 °C) A rotary kiln running at 450– EDDITh process (Martin et al., 1998; 600 °C for 45 min; gas is combusted at approximately Malkow 2004) 1100 °C with air coming from the dryer while the char undergoes separation and materials filtering

Gibros PEC Process or PKA technology in Germany (IEA Bioenergy, 2004)

Output: CO/H2-rich fuel gas, metals, basalt

Input: Shredded MSW, industrial waste and sludge Output: Gas (12 MJ kg1), coke (16 MJ kg1, CARBORÒ), metals, salts, mainly CaCl2 and NaCl, APC residues. Gas is burnt for heat and power generation Input: MSW, other feedstocks A rotary kiln and a gasifier, Noell-KRC conversion (dried sewage sludges) may process (now Future pyrolysis at approximately be co-gasified 550 °C, gasification at 1400– Energy) (Malkow, 2000 °C and 2–50 bars 2004; Jaeger and Output: Medium calorific Mayer, 2000) value gas; a part of the cleaned gas is used to heat the kiln. Metals and slag can be used as construction materials Serpac technology Two interconnected chambers of Input: Mixed waste, MSW, (Malkow, 2004) cylindrical and conical shape, industrial and hospital waste inclined and rotary. Pyrolysis at Output: steam; ash and 600–700 °C, char gasification metals with air at about approximately 800 oC; gas is combusted at approximately 1100–1200 °C Input: Tyres, MSW, sewage Siemens Schwel-Brenn A rotating drum and a high sludge, industrial wastes temperature furnace coupled technology with a steam boiler, pyrolysis at Output: Energy in form of (Malkow, 2004; steam or power. Metals, glass 450 °C for approximately 1 h; Richers and the finer carbon-enriched (30%) and ash Bergfeldt, 1996) fraction is ground to 0.1 mm and combusted together with the gas in the slagging furnace at 1300 °C Rotary drum running at 450 °C, Input: MSW is shredded into Mitsui R21 Process, a gas combustion at 1300 °C less than 200 mm in length branch of Siemens Output: power generation Schwel-Brenn from pyrolysis gas technology. (IEA combustion; char, ferrous and Bioenergy, 2002) non-ferrous metals; production of a fused ash product. Input: MSW, industry waste, Rotary kiln and ash-melting Takuma SBV(Kawai, 2009) (Derived from system. Pyrolysis at 500–550 °C sewage sludge, etc. in the rotary kiln; pyrolysis gas is Output: Energy (power & the above Siemens steam), iron, aluminium burnt in a high temperature Schwel-Brenn chamber process)

A combination Yame Seibu Plant, Japan, with capacity of pyrolysis of 220 t/d, etc. gasification & melting process

A combination Kakegawa Plant, of pyrolysis and Japan, with capacity of 140 t/d MSW gasification & melting process

Pyrolysis gas is dedusted and dewatered before entering the gasifier. There are two scrubbers to clean gas from the gasifier. The first stage removes H2S and heavy metals, and the second stage washes all of the other contaminants

Dust collection and flue gas scrubber

Flue Gas cooler followed by two bag filters in series: No.1 for particulate collection and recycling of ashes to the combustor, and No. 2 with dry lime injection for acid gas emission control, with landfill disposal of the solid residues Flue gas quencher followed by two scrubber stag

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D. Chen et al. / Waste Management 37 (2015) 116–136 Table 3 (continued) Process name

Reactor & operation conditions

Materials input & products

Thermoselect process (Malkow, 2004; Thermoselect S.A., 2000)

Moving channel bed. The process consists of shredding, in the ‘channel’ MSW is heated from 50 to 600 °C and pyrolyses, followed by gasification at 1200– 2000 °C

Input: Unsorted domestic waste, shredded Output: Medium calorific value gas, a part of cleaned gas is used to heat the channel. Mineral and metals from the smelt slag

A combination Mutsu, Japan, with of pyrolysis and capacity of 50,000 tpa gasification & melting process

Von Roll RCP technology (Malkow, 2004; Hesseling, 2002)

A forward reciprocating grate furnace as a pyrolysis (degassing) chamber; the pyrolysis temperature in the chamber is heated by partial combustion of the gas with oxygen to approximately 500– 900 °C; product gas and char are sent to a smelting reactor at 1400 °C and then to a circulating fluidised-bed furnace Pyrolysis in the two tubular reactors at 800 °C; the char is reacted with steam and air in a fixed bed gasifier, and gas combustion is in a cyclone chamber at 1200–1250 °C

Input: Dewatered sewage sludges, pre-treated MSW, clinical wastes, scrap tire crumbs; Output: Energy in form of steam or power; Char/ash material from the gasification unit Input: pre-treated MSW Output: syngas with moderate to low calorific value, char, metals and ash

Pyrolysis, gasification and high temperature combustion

Avonmouth, UK, with capacity of 8000 tpa. (Mainly clinical waste now)

Pyrolysis & partial gasification

Tianjin, China, with Gas cooler and filter capacity of 200 kg h1

Compact Power process (now Ethos Renewables Avonmouth (ERA) Limited) (Malkow, 2004)

A gasification-coupled pyrolysis process. The main reactor is a screw-bed reactor, and gasification takes place in the subsidiary reactor. No information on pyrolysis temperature. See Fig. 2 Honghoo technology Multi-sectional rotary kilns, (Chen et al., 2013) pyrolysis at lower temperature of approximately 400–450 °C, none-catalytic pyrolysis, indirect heat transfer; the gas is burnt online to supply the heat. See Fig. 3 CNRS thermo-chemical A tubular rectilinear reactor heated by circulation of hot flueconvertor gases (natural gas burner) within (Marculescu et al., an external double envelope. The 2007) solid continuously advances by vibro-fluidised transport Flow rate up to 50 kg/h Running from pyrolysis to combustion with temperature changing from 400 to 1000 °C. See Table 2

Pilot pyrolysis process in Tianjin, China (Li et al., 2007)

Technologies

Application example

Environmental protection device

A water jet quenching section is used to avoid the formation of dioxins, and an acid gas scrubber unit is used to remove the HCl and HF; an alkaline scrubber unit with an aqueous sodium hydroxide solution at higher pH is used to remove residual traces of CO2 and SO2, and the desulphurisation stage removes H2S from the gas; an activated carbon filter is installed to act as a final polishing unit for the synthesis gas Particulate emission control A combination Bremerhaven, Input: Pre-treated MSW, systems and acid gas scrubbing residual waste from recycling, of pyrolysis and Germany, a pilot plant with capacity of equipment used in a common melting & industrial waste Output: Metal, slag; energy in combustion power generation system 6 t/h MSW form of steam or power

Dry scrubber with sodium bicarbonate and Selective Catalytic NOx Reduction (SCR). The solid residues from the dry scrubbing unit are sent for landfill disposal

Input: Raw MSW with bottles, Pyrolysis alone stones, bricks and glass separated Output: Oil, char, cleaned gas (for power generation)

Shanghai, China, with Pyrolysis gas was scrubbed capacity of 100 ton/d, before burning. Char was quenched and separated from demonstration plant metals

Input: Ground MSW Output: Syngas or flue gas depending on whether comburant or inert gas is supplied; accordingly char or ash output

Pilot test facility

bles 7.30%, cloth 5.35%, paper 18.25% and sawdust 4.87%. The results revealed that as the temperature in the main reactor changed from 500 to 700 °C, the gas yield increased from 24.4% to 38.9%, and the oil yields changed from 21% to 16.4%. At around 600 °C, the averaged calorific value of the gas was 7.5464 MJ m3. The H2 content was 15.2%, and the total C2H4, C2H6, C3H6 and CH4 content was 10%. No recommendation was given for the end products; but syngas seemed to be the most valuable product. There was none sequel to this pilot MSW pyrolysis facility, possibly due to the fact that tubular reactor with a screw inside cannot deal with unsorted and unprepared MSW. The Honghoo technology shown in Fig. 3 is a ‘‘pure’’ pyrolysis process with multi-section rotary reactors, which run under the

Pyrolysis alone

Not mentioned

temperatures from 300 °C in section I to 500 °C in section III. The demonstration plant was feeding with unsorted MSW, syngas, oil and char are output as products. According to a previous testing operation analysis (Chen et al., 2013), the moisture-free oil yield and the char yield were approximately 3% (moisture separated) and 20% of the feedstock, respectively, whereas the moisture and syngas gases accounted for approximately 77% of the feedstock. However, as the input wastes were raw MSW with a moisture content of approximately 60%, the moisture condensate in the liquid products was more than 50% of its income in the feedstock, and the vaporisation heat of the moisture in the reactor consumed most of the reaction heat, reducing the treatment capacity from the designed 100 tons per day to the actual value of less than

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Fig. 2. Schematic of the pilot MSW pyrolysis plant in Tianjin, China (Li et al., 2007).

Drying section

T

Collecting and seperating systems for gas,oil and water

T

T

Purified Gas

Oil

Wastes Gas engine Gas

Chartank Char discharge

P

Automatic Control System

Fig. 3. Honghoo technology schematic (Chen et al., 2013) (I, II and III are the first, second and third section of the pyrolysis reactor, respectively, and IV is the char cooling drum).

30 tons per day. The end products are syngas and the char; the syngas was cleaned and used for power generation. Oil was planned to replace the cleaned gas for heating the system after being upgraded with distillation, the heavy residue would be sent back to the reactor. Presently Honghoo technology is on the way to be commercialized, yet its char product needs to be standardized and it should be equipped with MSW pre-treatment devices to ensure the stable operation and improve the quality of products.

3.2. Pyrolysis reactor The reported reactors for MSW pyrolysis include fixed-bed reactors, rotary kilns, fluidised bed reactors and some innovative reactors, the concepts of these reactors are shown in Table 2. In regards to the reaction conditions, most pyrolysis processes have been conducted at atmospheric pressure. Vacuum pyrolysis has only been reported in studies of special wastes such as printed circuit board disposal (Peng et al., 2006; Li et al., 2009). Although vacuum pyrolysis is reported to shorten the residence time of volatile products in the high-temperature zone, reducing the secondary decomposition and increasing the heat value of the gas products,

D. Chen et al. / Waste Management 37 (2015) 116–136

achieving vacuum pyrolysis is difficult in practice. Therefore, all of the following discussions in this review are based on atmospheric pressure. The reactors used in studies and at industrial scale are summarised. 3.2.1. Fixed-bed reactor The fixed-bed reactor is characterized by a low HR, as the result of its low heat transfer coefficient, as shown by Eq. (4). Therefore, when a greater sample mass is tested, the temperature is not uniform inside the sample (Wang et al., 2006), and the feedstock is decomposed at different temperatures simultaneously. Fixed-bed reactors have mainly been used to identify governing parameters affecting pyrolysis products, as discussed previously and shown in Table 1, but few comparisons have been reported with changed sample masses. Due to its inefficiency this reactor is seldom adopted in scale-up facilities. 3.2.2. Rotary kiln reactors and their systems The rotary kiln is more efficient than the fixed-bed reactor in heating up the feedstock. The slow rotation of an inclined kiln enables good mixing of wastes. The rotary kiln reactors are widely used, but they are typical reactors used for conventional pyrolysis (slow pyrolysis), which proceeds under a slow HR with significant product portions of char, liquid and gas. The reported HRs are not higher than 100 °C min1 as shown in Table 1 and the residence time is up to 1 h, as shown in Table 3. This is because during pyrolysis, only the reactor wall serves to transport heat from outside to the particles; the small wall surface distributed to the unit mass of feedstock and coarse size of the particles result in a low HR. However, most reported MSW pyrolysis technologies are based on rotary kiln pyrolysers, as shown in Table 3, because the rotary kiln reactor has many unique advantages over other types of reactors. In addition to the good mixing of wastes, the flexible adjustment of residence time and larger channel for the waste stream allow feeding of heterogeneous materials, and thus, extensive pre-treatment of wastes is not required, and its maintenance is also simple. Rotary kilns for pyrolysis are externally heated using combusted pyrolysis gas. The calculation of heat and mass transfer and design of the kiln have been extensively investigated for homogeneous materials (Donald and Rosseman, 1962; Rutgers, 1965; Bridgewater, 1985; Boateng and Barr, 1996; Li et al., 2005). For MSW pyrolysis, Li et al. (2002) performed comparative studies of homogeneous sand and irregular MSW in a rotary kiln in a cold state; examined the impacts of material characteristics, kiln geometry characteristics (i.e., roughness of kiln wall, exit end dam and internal structures) and operational parameters (i.e., kiln inclination and rotational speed) on both material residence time (MRT) and material volumetric flow (MVF); and then proposed a set of simplified formulas for the proper prediction of the MRT and MVF of MSW during pyrolysis (Li et al., 2002). But there is not much information in the literature on their heat transfer coefficients for heterogeneous MSW with changing particle size and composition. 3.2.3. Fluidised-bed reactors Fluidised-bed reactors are characterized by a high HR and good blending of the feedstock. Therefore, such reactors are more frequently used to describe the influence of temperature and residence time on pyrolysis behaviour and products (Williams and Williams, 1999a; Mastral et al., 2002, 2003; Dai et al., 2001a,b). Typically, fluidised-bed reactors are used to investigate the behaviours of fast pyrolysis (or flash pyrolysis) and to explore the secondary cracking of tar at longer residence times. Although fluidised-bed reactors have been extensively adopted in laboratory studies, their industrial application is not common for MSW pyrolysis. The reason is that the separation of bed mate-

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rial from coke, along with its external heating and recirculation, is complicated. However, providing uniform products is usually expected for a polymer cracking process, and polymer pyrolysis in a fluidised-bed reactor can provide remarkable advantages over other reactors wherein heat is not as properly transferred for the cracking of polymers because polymers have a very low thermal conductivity and high viscosity. Therefore, fluidised-bed reactors are widely adopted as pyrolysis reactors for MPW, such as in, for example, the Hamburg process developed by Kaminsky (1992, 2006) and BP polymer cracking process (Botom, 1993; Al-Salem et al., 2010). Arena and Mastellone (2006) summarised all of those processes for MPW pyrolysis and their commercial states, concluding that the fluidised-bed technology appears mature and particularly attractive for plastic waste pyrolysis. As for its application to MSW pyrolysis, in addition to the problems of bed material separation, external heating and recirculation, fluidising agent (gas) choice and MSW pretreatment should be dealt with thoroughly, as they are expensive steps. 3.2.4. Tubular reactors Tubular reactors include a family of reactors with fixed walls in a tube shape, but for which, the materials move inside via various driving modes. Tubular reactors are generally heated externally, and they studies featuring them reported different forms, such as a screw pyrolyser (Aguado et al., 2002), a tubular rectilinear reactor with solid driven forward by a vibro-fluidised transport (Marculescu et al., 2007), as shown in Fig. 4; and a tube with an inner mixer (Walendziewski, 2002). In Table 3, the pyrolysis reactors in the Thermoselect process, the Compact Power process and CNRS thermo-chemical convertor are all tubular reactors. The advantages of these systems include continuous coke and gas removal from the reactor tubes free from leakage, larger heat transfer surface in a unit volume and convenience for syngas reforming. It is easy to design and run a tubular reactor if the heat transfer coefficient is known because of its simplicity and safety. As a typical tubular reactor, the screw tube, with its lower construction and operation costs, has great future prospects. For this design, the screw speed can be varied within 0.5–25 rpm, thereby changing the residence time of the materials; this reactor system has been found to be useful for both the thermal and catalytic cracking of waste plastics (Aguado et al., 2002). However, tubular reactors have the same rigid requirements for MSW pretreatment as the fluidised-bed reactors due to the small channel for passage of MSW. In addition, erosion caused by sand and other hard solids contained in the MSW can be a risk for this reactor, and heat transfer coefficients are not well defined for different waste types. In a conclusion, adaptability of different reactors to MSW pyrolysis can be summarised in Table 4, from Table 4 it can be predicted that rotary kiln will still serve as the main reactor type for MSW pyrolysis. However, for small and moderate scale, tubular reactor can be competitive. The operation temperatures are dependent on product choice. 3.2.5. Other pyrolysis reactors and technologies In addition to the above single-stage reactor types, whose products must be post-treated outside the reactors, some multi-stage reactor types have also been adopted, and they are mainly for reforming or improving products. For example, a two-stage tube reactor was used to investigate the effect of steam reforming on the tar produced in the first reactor stage (Ohmukai et al., 2008), as the two stages can be controlled under different conditions. For a similar purpose, Zhao et al. (2011) proposed a three-stage reactor, in the first stage, HCl can be extracted immediately, whereas the volatiles generated in the second stage can be extracted out or passed through the third stage and vice versa

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Table 4 Adaptability of different reactors to MSW pyrolysis. Reactor type

Running experiences

Requirements on material preparation

Capacity

Maintenance requirement

Flexibility to operation parameters’ change

Application recommendation

Fixed-bed reactor

Running in batch, only in laboratory researches

Small, may not exceed several tons per day

Not recommended for industrial application

Most common

Low, but batch by batch operation demanding manual labour Low to moderate

Excellent flexibility

Rotary kiln

Almost no requirements except for energysaving purpose Not rigid

Good flexibility

Fluidised-bed reactor

Only for laboratory researches, no practical experiences for MSW A few

Very rigid

Large theoretically

Highest

Rigid

Moderate, up to 50,000 tpa

Moderate to high

Very limited to size change; good flexibility to temperature, etc. Limited to size and temperature change

Recommended but efficiency should be improved; multisectional reactor suggested Heating system should be improved before application to MSW pyrolysis

Tubular reactor

Large, up to 150,000 tpa

for product reforming. Those multi-stage reactors give hope for the independent running of pyrolysis technology. In a traditional thermal pyrolysis, in which process the heat Q in Eq. (1) is supplied externally by hot flue gas, alternatively Q can be supplied through volumetric heating as performed in the new technologies such as plasma pyrolysis (George, 1994; Hrabovsky et al., 2006) and microwave pyrolysis (Cho et al., 2009; Macquarrie et al., 2012) . Plasma pyrolysis is the process of heating waste to a very high temperature over 1000 °C using plasma torches without an air supply, which converts waste into a synthetic gas (mainly CO and H2) and other end-products, such as a vitrified matrix. The highly efficient delivery of heat for simultaneous rapid promotion of both physical and chemical changes in waste materials results in better control of process temperature, higher process rates, lower reaction volume and especially optimum composition of produced syngas. The properties of plasma pyrolysis products are predictable, suitable for energy and material recycling, and harmless in terms of public health and the environment (Huang and Tang, 2007), but plasma pyrolysis requires a large amount of secondary energy, for example,1 kW h kg1 for arc plasma technology (George, 1994; Wang and Huang, 2008). Presently, plasma pyrolysis is being tested mainly for hazardous waste disposal. For its MSW application, a life cycle assessment comparison of the whole process with a thermal cracking system is suggested, even a facility is available. Microwave pyrolysis is being investigated mainly for homogenous wastes such as sludge, shredded plastics and tyres. It is a type of microwave dielectric heating method, and its main advantages include rapid, efficient in-core volumetric heating for direct coupling of microwave energy with the molecules that are present in the reactants, easily controlled and maintained desired temperature of pyrolysis for desired product raising, lower temperatures for the reaction vessel (or material surface) and higher temperatures for the reaction mixture (or material interior), etc. (Baghurst and Mingos, 1992). However, to achieve the required rapid heating rate, the feedstock particles have to be very fine to fulfil the requirements, and achieving accurate temperature control in a microwave reactor is dependent on accurate dielectric data in the microwave frequency range as a function of temperature, which is not available for most of waste components. In addition, solid-laden vapour has to be swept out of a microwave reactor very rapidly to reduce secondary cracking of organic vapour in the freeboard and to drive water vapour and water-soluble small polar molecules out of the reactor (Yin, 2012). Together with its relatively small treatment capacity, the application of microwave pyrolysis to heterogeneous MSW is not practical in the near future.

Recommended, especially the multi-sectional tubular reactor

In addition pyrolysis with various catalysts was adopted to improve products. Process catalysts have been widely used in MPW pyrolysis in literatures. Pure polymer thermal degradation in the absence of catalyst produces a high boiling point wax-like mixture that requires further upgrading via conventional refinery processes. The use of catalysts in pyrolysis presents some advantages compared with simple thermal processes: i.e., lower energy consumption, shorter reaction time and good selectivity to higher-valued products. In addition, the liquid products formed are in the boiling point range of commercial motor engine fuel, eliminating the necessity for an upgrade process as well as the necessity for the whole process to be installed near existing refineries (Gulab et al., 2010). For MSW pyrolysis, the summary listed in Table 1 shows that catalysts improved oil quality and enhanced gas generation, but only cheap catalysts such as calcined dolomite (Yi, 2007; He et al., 2010) and CaO (Pan, 2012) can be used, as the catalysts cannot be subsequently recycled. In addition, the catalytic effects are usually not sufficient; more significant effects can be achieved for MPW than for MSW under the same conditions (Miskolczi et al., 2013) because of the poor contact between the MSW and the catalysts. To improve the MSW pyrolysis process, cheap catalysts that can absorb acidic gases at the same time are more preferred than highly effective and expensive catalysts. 4. Products from pyrolysis of typical MSW components and MSW A great advantage of pyrolysis technology over incineration is to export high quality products of oil or gas instead of heat, especially for the small-scaled systems. As the information on MSW pyrolysis products is very limited, and composition of MSW changed from place to place, a brief survey of products of principal MSW components will help to elucidate the products from practical MSW pyrolysis process. 4.1. Pyrolysis products from waste paper Paper represents a mainstream combustible in MSW. As its principal components are semicellulose and cellulose, waste paper is an important representative of biomass in MSW. Wu et al. (2002, 2003) investigated newspaper, uncoated printing and writing paper in MSW and found that paper begins to decompose around 488 K with the HR of 5 K min1, and at 583 K essential matrix decomposition takes place already. The major pyrolysis in their studies included non-hydrocarbons (HCs) (H2, CO, CO2, and H2O) and hydrocarbons (C1–3, C4, C5, C6, 1-ring, C10–12, levoglucosan,

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C13–15, and C16–18), and the concentrations of both non-HC and HC products increased with temperature. The percentage of volatiles increased from 4.75% to 90.56% when the temperature increased from 310 °C (583 K) to 665 °C (938 K) at a low heating rate of 5 min K1. Tar formation occurred within the moderate temperature range of 290–540 °C (563–813 K). The tar yield was approximately 47.03% at 450 °C and a heating rate of 10 °C min1. There were four main different compounds they identified in the tar: anhydrosugars, carboxyl compounds, carbonyl compounds and aromatic compounds. As temperature increased to 665 °C (938 K), the char was reduced to less than 10% for the newspaper. Ahmed and Gupta (2009) founded that waste paper pyrolysis had a considerable overlap with its gasification, especially at higher temperature, but pyrolysis was more flexible compared with gasification regarding the temperature. In a relatively large-scale laboratory study based on a fixed-bed reactor, Jiang (2006) found the LHV of syngas from waste paper pyrolysis reached a maximum of 17.5 MJ N m3 at approximately 600 °C with a yield of 301.4 L kg1, and the gas yield increased from 25.12% to 47.14% as the pyrolysis temperature increased from 600 to 900 °C. The liquid product yield decreased from 44.47% to 32.08% at the same time, but 67.4% of liquid product was moisture at 600 °C. As the temperature increased, the moisture fraction also increased. The LHV of the syngas was between 10 and 17.5 MJ N m3 within the temperature range of 400–850 °C, which is much higher than the LHV of syngas from gasification, the latter was reported to vary from 1.8 to 2.5 MJ N m3 within the temperature range of 400– 700 °C (Xiao et al., 2007). All of these results indicate that syngas will be an important product from waste paper pyrolysis and the reaction temperature should not be lower than 600 °C. 4.2. Pyrolysis products from MPW MPW is the most important energy contributor to MSW. A number of important studies on polymers and MPW pyrolysis were summarised by Al-Salem et al. (2010). The main purpose of MPW pyrolysis is to recover liquid products in many processes. For example, the famous BP polymer cracking process, Fuji process (Fuji Electric, 2001) and Hamburg process are all designed to recover oil products. To obtain high-quality, market-ready oil, the pyrolysis recycling of MPW usually consists of two processes. The first is the degradation of MPW for the production of heavy oils, and the second is a catalytic cracking process that converts

the heavy oils into useful hydrocarbons. Many catalytic cracking processes have been tested and were summarised by Masuda and Tago (2006). According to their recommendation, catalytic hydrolysis reactors with steam as a carrier gas and FeOOH as a catalyst followed by a catalytic cracking reactor with zeolite as a catalyst could be a promising solution for fuel oil upgrading. In regards to components, there are six main plastics in MSW: high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC) and polyethylene terephthalate (PET). PE (including LDPE and HDPE) and PP account for 70 wt% of the waste plastics stream in MSW in China (Wang et al., 2013), most important plastic components in UK (Waste watch, 2008) and also worldwide (APME, 2004). Table 5 gives the product yield from the pyrolysis of PE and PP plastics, as summarised by Williams (2006). It can be seen that single PE and PP pyrolysis results in a yield of liquid products varying from 5% to 95%. In vacuum reactor or a fluidised-bed reactor running at moderate temperature (approximately 500 °C), the volatiles can be extracted immediately before the secondary cracking takes place, and a higher yield of liquid products can be expected. Whereas in a high-temperature fluidised bed (for example higher than 800 °C) or a fast pyrolysis reactor, the volatiles will be subjected to secondary cracking before leaving the hot zone, resulting in high gas yields. Grieco and Baldi (2012) showed that the oil yield of PE pyrolysis performed in a fixed-bed reactor decreased from 90.9% to 86.2% as the HR increased from 0.1 °C s1 to 1 °C s1, while the gas yield increased from 9.1% to 13.7%. However, a very slow HR would result in a higher char product, as shown in Table 5 for the two lower temperature cases (430 °C for HDPE and 380 °C for PP). The major gaseous products were H2, methane, ethane, ethylene, propylene, butadiene, benzene and toluene without CO, CO2 or HCl appearance for PE, PP and PS. Therefore, gas products are of high calorific value. PVC is regarded as a harmful component in the MPW, as HCl evolves during pyrolysis, contaminating the products and causing apparatus corrosion, its separation is desired. All of the reported polymer cracking processes include an HCl abatement step to separate chlorine from HCs. In contrast to biomass, MPW in MSW requires a higher temperature to finish its pyrolysis. Within the conventional pyrolysis temperature range of 500–550 °C for rotary kilns oil is the main product of MPW; and MSW rich in MPW components would inevitably have oil/liquid products generated.

Table 5 Product yield from the pyrolysis of polyalkene plastics (Williams, 2006).

a

Feedstock

Reactor type

Temperature (°C)

Gas (wt%)

Oil/wax (wt%)

Char (wt%)

PE PE LDPE LDPE LDPE LDPE HDPE LDPE LDPE HDPE HDPE HDPE LDPE LLDPE LLDPE PP PP PP PP PP

Fluidised-bed Fluidised-bed Fluidised-bed Fluidised-bed Fluidised-bed Fixed-bed(batch) Fixed-bed(batch) Fixed-bed(batch) Ultra-fast pyrolysis Fixed-bed(batch) Fixed-bed(batch) Vacuum Vacuum Fluidised-bed Fluidised-bed Fixed-bed(batch) Fixed-bed(batch) Fluidised-bed Vacuum Fixed-bed(batch)

760 530 700 600 500 700a 700a 500a 825 450 430 500 500 730 515 380 700a 740 500 500a

55.8 7.6 71.4 24.2 10.8 15.1 18.0 37.0 92.9 13.0 9.6 0.9 2.7 58.4 0.0 24.7 15.3 49.6 3.5 55.0

42.4 92.3 28.6 75.8 89.2 84.3 79.7 67.0 5 84 69.3 97.7 96.0 31.2 89.8 64.9 84.4 48.8 95 45.0

1.8 0.1 0.0 0.0 0.0 0.0 0.0 0.0 2 3 21.1 0.8 1.0 2.1 5.9 10.4 0.2 1.6