Pyrolysis–gasification of post-consumer municipal solid plastic waste for hydrogen production

international journal of hydrogen energy 35 (2010) 949–957

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Pyrolysis–gasification of post-consumer municipal solid plastic waste for hydrogen production Chunfei Wu, Paul T. Williams* Energy & Resources Research Institute, The University of Leeds, Leeds LS2 9JT, UK

article info

abstract

Article history:

Post-consumer plastic waste derived from municipal solid waste was investigated using

Received 7 October 2009

a two-stage, catalytic steam pyrolysis–gasification process for the production of hydrogen.

Received in revised form

The three important process parameters of catalyst:plastic ratio, gasification temperature

6 November 2009

and water injection rate were investigated. Temperature-programmed oxidation (TPO) and

Accepted 10 November 2009

scanning electron microscopy (SEM) methods were used to analyse the reacted catalysts.

Available online 27 November 2009

The results showed that there was little influence of catalyst:plastic ratio between the range 0.5 and 2.0 (g/g) on the mass balance and gas composition for the pyrolysis–gasifi-

Keywords:

cation of waste plastics; this might be due to the effective catalytic activity of the Ni–Mg–Al

Plastic

catalyst. However, increasing the gasification temperature and the water injection rate

Catalyst

resulted in an increase of total gas yield and hydrogen production. The coke formation on

Pyrolysis

the catalyst was reduced with increasing use of catalyst; however, a maximum coke

Gasification

formation (9.6 wt.%) was obtained at the gasification temperature of 700  C when the

Hydrogen

influence of gasification temperature was investigated. The maximum coke formation was obtained at the water injection rate of 4.74 g h1, and a more reactive form of coke seemed to be formed on the catalyst with an increase of the water injection rate, according to the TPO experiments. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The plastic content of municipal solid waste represents about 9.5 wt% in the EU and about 11.7 wt% in the US [1,2], representing a major proportion of the post-consumer waste stream. There are six main plastics which arise in West European municipal solid waste which are, high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC) and polyethylene terephthalate (PET). The majority of waste plastic is disposed of with the mass of municipal solid waste through landfilling or incineration and the amount of plastic waste recycled is low, typically less than 10%. Therefore, process routes which can treatment the waste plastics and generate useful fuels or petrochemical feedstocks are

generating increased interest. In addition, in many countries, waste plastic is collected as a separate waste fraction for ease of subsequent processing. Chemical recycling of the waste plastics, via pyrolysis and gasification, to generate useful hydrocarbons has been recognized as a promising technology [3–6]. The gasification of plastics produces a syngas which has a high content of hydrogen. Hydrogen has been identified as a fuel which might play an important role in future energy supply. Therefore, hydrogen production from the gasification of plastics represents an area of active research [7–9]. In addition, the plastics have a higher heating value and hydrogen content compared to biomass or some other municipal solid wastes; this ensures a higher hydrogen production from the gasification of plastics [3].

* Corresponding author. Tel.: þ44 1133432504. E-mail address: [email protected] (P.T. Williams). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.11.045

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international journal of hydrogen energy 35 (2010) 949–957

Different process conditions such as gasification temperature, introduction of a catalyst and introduction of steam etc. have an influence on the production of hydrogen from the gasification of plastics [10,11]. Nickel catalysts are commonly used in the gasification process due to their lower price and effective catalytic activity [12–14]. Furthermore, in order to obtain high concentrations of hydrogen in the product gases, multiple stage reaction systems have been applied to the gasification process [8–10]. Single stage gasification processes tend to generate lower quality gases which are commonly used for direct thermal use [15]. In this paper, a two-stage process was used for hydrogen production from the catalytic steam pyrolysis–gasification of waste plastics. Three important parameters, namely, catalyst to plastic ratio, gasification temperature and water injection rate, were investigated for their influence on the production of hydrogen from real-world waste plastics derived from municipal solid waste in the presence of a Ni– Mg–Al catalyst.

2.

Experimental

2.1.

Materials

The waste plastic was post-consumer municipal solid waste, mixed plastic from Belgium collected, sorted and fractionated by the Fost Plus Company, Belgium. The collected waste plastic fraction was flaked to approximately 5 mm size range and was separated into a low density fraction through air separation. This lower density fraction was used in this research. The Fost Plus plastic consists of mainly high density polyethylene and polyethylene terephthalate. The Ni–Mg–Al catalyst was prepared using the rising pH technique according to the method reported by Garcia et al. [16]. The precipitant 1 M NH4(OH) was added to 200 ml of an aqueous solution containing Ni(NO3)$6H2O, Al(NO3)3$9H2O and Mg(NO3)2$6H2O. The precipitation was carried out at 40  C with moderate stirring until the final pH (8.3) was obtained. The precipitates were filtered with water (40  C), followed by drying at 105  C overnight, and then were calcined at 750  C for 3 h. The initial Ni–Mg–Al molar ratio was 1:1:1. The Ni–Mg– Al catalysts were crushed and sieved to granules with a size range between 0.065 and 0.212 mm.

2.2.

Characterization of materials

The elemental analysis of the waste plastics was determined using a CE Instruments CHNS-O analyser and the results are shown in Table 1. The temperature-programmed oxidation (TPO) of the reacted catalysts was carried out using a Stanton–Redcroft thermogravimetric analyser (TGA) to determine the properties of the coked carbons deposited on the reacted catalysts. The differential thermo-gravimetry (DTG) results from the experiment of TPO are also discussed in this paper. About 100 mg of the reacted catalyst was heated in an atmosphere of air at a heating rate of 15  C min1 to a final temperature of 800  C, with a dwell time of 10 min at 800  C.

A high resolution scanning electron microscope (SEM, LEO 1530) was used to characterize and examine the characteristics of the carbon deposited on the coked catalysts.

2.3.

Experimental system

The two-stage, pyrolysis–gasification reaction system consisted a first stage pyrolysis reactor heated by a tube furnace and a second gasification reactor separately heated by a second tube furnace. Both furnaces were thermally controlled separately. The waste plastics were put in the sample boat which was placed in the first reactor. The evolved pyrolysis gases passed through the second reactor containing the catalyst and where steam was introduced and the reforming reactions were carried out. A schematic diagram of the reaction system was shown in our previous paper [17]. During the experiment, 0.5 g of waste plastics was used and a certain amount of the Ni–Mg–Al catalyst was placed in the second reactor. The carrier gas was N2. The second furnace was first heated to the desired gasification temperature. The first stage pyrolysis furnace then started to be heated to the final pyrolysis temperature of 500  C with a heating rate of 40  C min1. As the temperature of the pyrolysis reactor increased to 500  C, the evolved pyrolysis gases were passed to the second catalytic gasification reactor with the addition of steam. During steam gasification, water was injected into the gasification reactor using a syringe pump. The products from the gasification reactor passed through two condensers, where the first condenser was cooled by air and the second was cooled by dry ice. The liquid products were collected in the condensers and the non-condensible gases were collect with a 25 L Tedlar gas sample bag. The reproducibility of the reaction system has been tested and proved to be acceptable in our previous works using polypropylene [10]. In this paper, repeated experiments using waste plastics were carried out. The mass balance repeatability showed good results, ranging from 94 to 102 wt.% and a standard deviation of 0.042 wt% for the gas yield was obtained for our repeated experiments. Although some variations were also obtained these, might be due to the differing content of waste plastics for each batch of experiments. The gases collected in the sample bag were analysed offline by packed column gas chromatography (GC). Hydrocarbons (C1–C4) were analysed using a Varian 3380 gas chromatograph with a flame ionisation detector, with a 80–100 mesh Hysep column and nitrogen carrier gas. Permanent gases (H2, CO, O2, N2 and CO2) were analysed by a second Varian 3380 GC with two separate columns. Hydrogen, oxygen, carbon monoxide and nitrogen were analysed on

Table 1 – Elemental analysis of the waste plastics. Element C H O N

Wt.% 77.1 11.5 11.2 0.2

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international journal of hydrogen energy 35 (2010) 949–957

Results and discussion

3.1.

The influence of catalyst:plastic ratio

In this paper, four catalyst:plastic ratios, 0.5, 1.0, 1.5 and 2.0 (g/g) were investigated. The aim of the experiments was to determine the influence of catalyst:plastic ratio on hydrogen production from the pyrolysis–gasification of waste plastics. In addition, experiments were carried out in the absence of a catalyst; in this case, sand was placed in the gasification reactor instead of the Ni–Mg–Al catalyst. The other process parameters of gasification temperature and water injection rate were maintained at 800  C and 4.74 g h1, respectively. The product yield in relation to different catalyst:plastic ratio is shown Table 2. The results suggest that around 40 wt.% of product oil related to the mass of plastic was collected for the experiment without the presence of any catalyst. In the presence of the Ni–Mg–Al catalyst (0.25 g was used), all the products were converted to gases and a solid product. In this paper, the solid fraction included char residue in the sample boat and the coke formed in the gasification reactor. The weight of solid fraction was obtained by the mass difference of the reactor before and after the experiment. The gas yield in relation to the mass of plastics markedly increased from 49.4 wt% to 225.1 wt.% when 0.25 g of Ni–Mg–Al catalyst was introduced into the gasification reactor. From Table 2, only 0.02 g of water was consumed for the non-catalytic steam pyrolysis–gasification of waste plastics. The amount of reacted water increased to 1.32 g when 0.25 g of Ni–Mg–Al catalyst was used. However, with the further increasing of catalyst:plastic ratio, it seems that there was a slight change in the product distribution and the reacted water amount (Table 2). When no oil was collected, the consumed amount of water was obtained by the difference in the amount of injected water and condensed water. While there was oil produced and collected in the condensers, the consumed water amount during the gasification experiment was calculated from the content of oxygen which was assumed to be converted into the CO and CO2 gases.

Table 2 – Mass balance for the different catalyst:plastic ratio. Catalyst:plastic ratio Mass balance in relation to only plastic (wt.%) Gas yield (wt.%) Oil yield (wt.%) Solid yield (wt.%) Mass balance (wt.%) Reacted water (g/g plastics)

0.0

49.4 40.0 13.0 102.4 0.02

0.5

225.1 0.0 13.4 238.5 1.32

1.0

1.5

205.7 0.0 19.4 225.1

206.2 0.0 18.6 224.8

1.3

1.3

2.0

228.6 0.0 5.2 233.8 1.28

0.30

70 60

0.25 CO H2

50

0.20

CO 2

40

CH 4 C 2-C 4

30

0.15 0.10

20 10

0.05

0 0.0

0.5

1.0

1.5

2.0

0.00

Catalyst:plastic ratio (g/g)

Fig. 1 – Gas composition and hydrogen production for different catalyst:plastic ratio.

H2 production (g H2/g plastics)

3.

The gas composition derived from the different catalyst:plastic ratio in the catalytic steam pyrolysis–gasification of waste plastics is shown in Fig. 1. The results show that the H2 concentration was largely increased, CO2 was decreased, CO was increased and the C2–C4 hydrocarbons were decreased, when the Ni–Mg–Al catalyst was used compared to the noncatalytic steam gasification. As was found for the product yield, little influence of catalyst:plastic ratio on the gas composition was observed from Fig. 1. The hydrogen production, presented in this paper (Fig. 1), was calculated by using the produced hydrogen weight divided by the weight of waste plastic used in the experiment (0.5 g). Furthermore, there was no significant change in the production of hydrogen in relation to increasing of catalyst:plastic ratio. It has been reported that the catalytic activity of a catalyst is reduced with decreasing catalyst weight during the gasification of hydrocarbons materials [16–20]. However, in this paper, increasing the catalyst weight in the range of 0.25– 1.0 g in relation to a mass of plastic of 0.5 g, representing a catalyst:plastic ratio of 0.5:1.0, 1.0:1.0, 1.5:1.0 and 2.0:1.0 showed that the catalytic performance of the Ni–Mg–Al catalyst seems not to be influenced greatly. This might be due to the effective catalytic activity of the Ni–Mg–Al catalyst; where 0.25 g of catalyst is adequate enough for the catalytic steam reforming of the hydrocarbons derived from the pyrolysis of 0.5 g waste plastics. The performance of Ni–Mg– Al catalysts in the gasification process has already been reported [17,21,22]. In this paper, the characterization of the reacted Ni–Mg–Al catalysts from the gasification experiments was carried out. The TGA-TPO and DTG-TPO results for the reacted catalysts from different catalyst:plastic ratio experiments are shown in Fig. 2. The results of DTG-TPO show that the there is a weight loss peak around 100  C; this weight loss stage was regarded as the vaporization of water contained in the reacted catalyst. The weight increasing peak happed around 400  C in the DTG-TPO was suggested to be the oxidation of Ni phases of the catalyst, which was discussed in our previous work [9]. From Fig. 2, the coke formed on the catalyst seems to be reduced with the increasing of catalyst:plastic ratio. The amount of coke formed on the catalyst was calculated as follows:

Gas composition (vol. )

a 60–80 mesh molecular sieve column with argon carrier gas. Carbon dioxide was analysed on a Hysep 80–100 mesh column with argon carrier gas.

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1.08

1.5

Weight ratio

1.04 1.00

2.0 1.0

0.96 0.92

0.5

0.88 0

100

200

300

400

500

600

700

800

-1

Derivative weight (°C )

0.0006 0.0004 0.0002

1.5

2.0

0.0000 -0.0002 -0.0004 -0.0006

1.0

-0.0008 -0.0010

0.5 0

100

200

300

400

500

600

700

800

Temperature (°C) Fig. 2 – TGA-TPO and DTG-TPO results of the reacted catalysts derived from different catalyst:plastic ratio.

W1  W2 w¼ $100 W1 where w is the amount of coke deposited on the catalyst (wt.%); W1 is the weight of coked catalyst after moisture evaporation in the TGA; W2 is the final weight of coked catalyst after oxidation in the TGA. The calculated amount of coke deposited on the catalyst was 11.4, 4.1, 1.6 and 1.6 wt.% for the catalyst weight of 0.25, 0.5, 0.75 and 1.0 g, respectively. From the DTG-TGA results in Fig. 2, it is suggested that the coke oxidation started at a temperature of approximately 400  C. The oxidation peak at around 600  C was assigned to filamentous type carbons deposited on the catalyst, where such carbons are shown by the SEM results in Fig. 3. From Fig. 2, the oxidation peak of the filamentous type carbons seems to be moved to higher temperature with the use of increasing catalyst weight from 0.25 to 0.75 g, but moved to lower oxidation temperature at the catalyst weight of 1.0 g for 0.5 g of waste plastic. From the TPO experiments of the reacted catalysts in relation to different catalyst weights (catalyst:plastic ratio), it is suggested that the coke formation was prohibited with the increase of catalyst weight during the gasification process. Similar results were obtained in our previous work, where the catalyst ratio in the gasification of polypropylene was investigated with a Ni/CeO2/Al2O3 catalyst [10].

3.2.

The influence of gasification temperature

The gasification temperatures of 600, 700, 800 and 900  C were investigated for the production of hydrogen from the

pyrolysis–gasification of waste plastics. The catalyst:plastic ratio and water injection rate in this series of experiments was maintained at 1.0 g/g and 4.74 g h1, respectively. The product yield and the gas composition from the catalytic steam pyrolysis–gasification of waste plastics at different gasification temperatures are shown in Table 3 and Fig. 4, respectively. From Table 3, it seems that the gas yield increased and the amount of reacted water was also increased with increasing gasification temperature. For example, the gas yield corresponding to the mass of waste plastics increased from 112.4 to 234.6 wt.% and the amount of reacted water increased from only 0.36–1.39 g/g plastics, when the gasification temperature was increased from 600 to 900  C. The increase of gas yield with increasing gasification reaction temperature has also been reported before [23–25]. It is suggested that more secondary reactions might be occurring at higher gasification temperatures together with more water reaction during the gasification process. However, the concentration of hydrogen was not increased although there was an increase in gas yield in relation to the increasing gasification temperature (Fig. 4). From Fig. 4, the H2 concentration increased slightly when the gasification temperature was increased from 600 to 700  C, then the H2 concentration decreased with a further increase in gasification temperature to 900  C. At the same time, the CO concentration increased, the CO2 and hydrocarbon gases decreased, when the gasification temperature was increased (Fig. 4). A maximum H2 concentration was obtained at 700  C with the increasing of the reaction temperature from 650 to 780  C, when lignocellulosic residues were steam gasified in

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international journal of hydrogen energy 35 (2010) 949–957

Fig. 3 – SEM results of the reacted catalysts from different catalyst:plastic ratio.

gasification temperature and resulted in higher the total gas yield. Therefore, the total hydrogen production was still increased from 0.204 to 0.256 g H2/g waste plastics (Fig. 4), although the H2 concentration was reduced while the gasification temperature increased from 700 to 900  C. The reacted Ni–Mg–Al catalyst from different gasification temperatures was analysed by TPO, in order to obtain the influence of gasification temperature on the formation of coke on the catalyst. The TGA-TPO and DTG-TPO results from different gasification temperatures are shown in Fig. 5. From the TGA-TPO results, the coke formed on the catalyst obtained from gasification temperatures of 600, 700, 800 and 900  C was 3.4, 9.6, 4.1 and 1.3 wt.%, respectively. It was found that the highest coke formation was obtained at

0.30

70

Table 3 – Mass balance for the different gasification temperature. Gasification temperature ( C) Mass balance in relation to only plastic (wt.%) Gas yield (wt.%) Oil yield (wt.%) Solid yield (wt.%) Mass balance (wt.%) Reacted water (g/g plastics)

600

112.4 0.0 16.0 128.4 0.36

700

166.8 0.0 36.0 202.8 1.14

800

205.7 0.0 19.4 225.1 1.30

900

234.6 0.0 5.6 240.1 1.39

Gas composition (vol. )

60

0.25

50

CO H2

40

0.20

CO 2

0.15

CH 4

30

C 2-C 4

0.10

20 10

0.05

0 600

700

800

900

0.00

Gasification temperature (°C)

Fig. 4 – Gas composition and hydrogen production for different gasification temperature.

H2 production (g H2/g plastics)

a fluidized bed [26]. Herguido et al. [26] found that the gasification process would be dominated by the water gas shift reaction above 670  C. Wallawender et al. [27] also proposed that the first temperature interval of the gasification process for cellulose was dominated by the cracking of volatile matter and a second temperature interval dominated by the shift reaction. In this paper, it is suggested that the hydrogen concentration was mainly controlled by two reactions: decomposition of hydrocarbons and the water gas shift reaction. When the gasification temperature was lower than 700  C, the reaction of decomposition of hydrocarbons might be more dominant than the water gas shift reaction, and resulted in a higher H2 concentration. While the gasification temperature was higher than 700  C, the exothermic water gas shift reaction became more dominant for the production of hydrogen and resulted in lower concentrations of H2 and also the CO2 at higher reaction temperature; however, the reaction rate was increased at higher

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international journal of hydrogen energy 35 (2010) 949–957

1.08

Weight ratio

1.04

900 °C

1.00

800 °C 0.96 0.92

600 °C

0.88

700 °C 0

100

200

300

400

500

600

700

800

-1

Derivative weight (°C )

0.0006 0.0004

900 °C

0.0002

800 °C 0.0000 -0.0002

600 °C 700 °C

-0.0004 0

100

200

300

400

500

600

700

800

Temperature (°C) Fig. 5 – TGA-TPO and DTG-TPO results of the reacted catalysts derived from different gasification temperature.

the gasification temperature of 700  C. It is interesting to note that the highest H2 concentration was obtained at the gasification temperature of 700  C. It has been reported that the coke formed on the catalyst is balanced through two competing reactions, the coke formation reaction and the coke gasification reaction [28]. Therefore, the decomposition of hydrocarbons might be a dominant reaction at the gasification temperature of 700  C, and resulted in the highest H2 concentration and also might correspond to the highest coke formation. When the gasification temperature is higher than 700  C, the coke gasification reactions might be prominent with the Ni–Mg–Al catalyst, and less coke formation was obtained. From the DTG-TPO results in Fig. 5, the oxidation peak assigned to the filamentous type carbons

was moved to higher temperature, when the gasification temperature was increased from 600 to 900  C. It is suggested that more reactive carbons were gasified at the higher gasification temperature during the gasification process, and less reactive carbons were left deposited on the surface of the catalyst.

3.3.

Influence of water injection rate

The introduction of steam into the gasification system has an important role for the production of hydrogen and the reduction of coke formation. It is suggested that 1 g of 0 .4 0 70

Water injection rate (g h1) Mass balance in relation to only plastic (wt.%) Gas yield (wt.%) Oil yield (wt.%) Solid yield (wt.%) Mass balance (wt.%) Reacted water (g/g plastics)

1.90

182.6 0.0 14.6 197.2 0.90

4.74

205.7 0.0 19.4 225.1 1.30

9.49

269.6 0.0 5.6 275.2 1.98

14.20

294.6 0.0 12.0 306.6 2.22

0 .3 0

Gas composition (vol. )

Table 4 – Mass balance for the different water injection rate.

50

CO H2

40

0 .2 5

C O2

0 .2 0

C H4

30

C2-C4

0 .1 5

20 0 .1 0 10 0 .0 5 0

2

4

6

8

10

12

14

0 .0 0

-1

Water injection rate (g h )

Fig. 6 – Gas composition and hydrogen production for different water injection rate.

H2 production (g H2/g plastics)

0 .3 5 60

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international journal of hydrogen energy 35 (2010) 949–957

1 .0 6

1.90

Weight ratio

1 .0 4

4.74

1 .0 2

9.49

1 .0 0

14.20

0 .9 8

0 .9 6

0

100

200

300

400

500

600

700

800

-1

Derivative weight (°C )

0 .0 0 0 6 0 .0 0 0 4

1.90

0 .0 0 0 2

9.49 0 .0 0 0 0

14.20

-0 .0 0 0 2

4.74

-0 .0 0 0 4

0

100

200

300

400

500

600

700

800

Temperature (°C) Fig. 7 – TGA-TPO and DTG-TPO results of the reacted catalysts derived from different water injection rate.

polypropylene or polyethylene reacts with a maximum of 2.57 g of water and generate 0.429 g of H2 [7]. Therefore, the steam content in the gasification reactor might have a great influence on hydrogen production. In this section, different water injection rates, resulting in different steam contents, was investigated for hydrogen production from the pyrolysis–gasification of waste plastics. The investigated water injection rates were, 1.90, 4.74, 9.49 and 14.20 g h1. The catalyst weight and gasification temperature were kept at 0.5 g (catalyst:plastic ratio of 1:1) and 800  C, respectively. The product yield and the gas composition results are shown in Table 4 and Fig. 6, respectively. From Table 4, it is shown that the gas yield corresponding to the waste plastics was increased with increasing water injection rate. For example, when the water injection rate was increased from 1.90 to 14.20 g h1, the gas yield corresponding to the weight of plastics increased from 182.6 to 294.6 wt.% and the amount of reacted water also increased from 0.90 to 2.22 g. The water injection rate was also investigated for the catalytic steam pyrolysis–gasification of polypropylene with a Ni/ZSM-5 catalyst [11]. It is reported that a maximum gas yield corresponding to the weight of plastics was obtained at the water injection rate of 9.49 g h1 [11]. It is suggested that the catalyst might change the saturation point of steam [29] in the gasification process; while in this paper, the saturation point of steam might be moved to a higher steam content. From Fig. 6, the H2 concentration increased from 62.4 to 69.1 vol.% when the water injection rate was increased from

1.90 to 4.74 g h1, and the H2 concentration seems to be stable with a further increase of the water injection rate to 14.20 g h1. The CO concentration decreased from 27.6 to 8.2 vol.%, the CO2 increased from 5.3 to 19.9 vol.%, CH4 decreased from 4.7 to 0.3 vol.% and the C2–C4 gases concentrations were nearly zero, when the water injection rate was increased (Fig. 6). The increasing concentration of CO2 and the decreasing of CO concentration might be due to the promoting of the water gas shift reaction at the higher partial pressure of steam. Probably due to the large increase in the total gas yield, the hydrogen production was increased from 0.190 to 0.334 g H2/g waste plastics, when the water injection rate was increased from 1.90 to 14.20 g h1. The reacted Ni–Mg–Al catalyst derived from the experiments related to the different water injection rates was analysed by TPO. The results of TGA-TPO and DTG-TPO experiments are shown in Fig. 7. From the TGA-TPO results, the amount of coke formed on the catalyst was 1.9, 4.1, 1.7 and 1.3 wt.% for the water injection rate of 1.90, 4.74, 9.49 and 14.20 g h1, respectively. From the DTG-TPO results in Fig. 7, it appears that the oxidation peak for filamentous type carbons was moved to lower temperature, when the water injection rate was increased during the catalytic steam pyrolysis–gasification of waste plastics. It is suggested that the coke formed on the catalyst during a higher water injection rate was more reactive that it could be more easily gasified with air. Due to the highest coke formation obtained at the water injection rate of 4.74 g h1, it could be suggested that less water was reacted for the carbon gasification when the water injection rate was lower than 4.74 g h1. When the water injection rate

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international journal of hydrogen energy 35 (2010) 949–957

was higher than 4.74 g h1, the extra steam might be applied to the coke gasification reactions resulting in lower carbon deposition on the catalyst.

4.

Conclusions

In this paper, a post-consumer plastic waste derived from municipal solid waste was used to produce hydrogen by a two-stage pyrolysis–gasification process. Three important parameters, catalyst:plastic ratio, gasification temperature and water injection rate were investigated for their influence on hydrogen production and coke formation. The main conclusions were: (1) With the presence of 0.25 g of Ni–Mg–Al catalyst in the gasification bed, the hydrogen production was significantly increased to 0.258 g H2/g waste plastics from 0.015 g H2/g waste plastics in the absence of a catalyst, when the gasification temperature and water injection rate was 800  C and 4.74 g h1, respectively. (2) With increasing catalyst:plastic ratio in the gasification bed, there were no significant changes in product yield and the gas composition; however, the amount of coke deposited on the catalyst was decreased from about 11.4 to 1.6 wt.%. It is suggested that the coke formation on the catalyst was prohibited with the increasing use of catalyst. (3) The gasification temperature showed an obvious influence on the product yield and gas composition for the catalytic steam pyrolysis–gasification of waste plastics. The total gas yield and the production of hydrogen increased with increasing gasification temperature, although the H2 concentration in the produced gases did not show a similar trend. The amount of coke deposited on the catalyst was firstly increased and then decreased, when the gasification temperature was increased from 600 to 900  C. (4) With increasing water injection rate from 1.90 to 14.20 g h1, the total gas yield corresponding to the mass of waste plastics was increased from 182.6 to 294.6 wt.%. The H2 concentration showed no significant change, the CO concentration decreased, the CO2 concentration increased, and the total hydrogen production increased. The maximum coke formation was obtained at the water injection rate of 4.74 g h1. A more reactive coke seems to be formed on the catalyst with the increasing of the water injection rate.

Acknowledgements The authors are grateful for the financial support of the UK Overseas Research Student Award Scheme and the University of Leeds, International Research Studentship (Chunfei Wu). The authors also thank Mr. Ed Woodhouse for his technical support and the analytical support from Dr. Jude Onwudili.

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