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Hydrogen production by high density polyethylene steam gasification and in-line volatile reforming Gartzen Lopez, Aitziber Erkiaga, Maite Artetxe, Maider Amutio, Javier Bilbao, and Martin Olazar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02413 • Publication Date (Web): 16 Sep 2015 Downloaded from http://pubs.acs.org on September 20, 2015

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Industrial & Engineering Chemistry Research

Hydrogen production by high density polyethylene

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steam gasification and in-line volatile reforming

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Gartzen Lopez, Aitziber Erkiaga, Maite Artetxe, Maider Amutio, Javier Bilbao and

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Martin Olazar *

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Department of Chemical Engineering University of the Basque Country UPV/EHU,

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P.O. Box 644 - E48080 Bilbao (Spain). [email protected]

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Abstract

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Steam gasification (900 ºC) of high density polyethylene (HDPE) in a conical spouted

9

bed reactor (CSBR) followed by steam reforming in a fixed bed reactor (Ni commercial

10

catalyst) has been carried out. The effect reforming temperature in the 600-700 ºC range

11

has on product yields and gas composition as well as on the amount and nature of the

12

coke deposited on the catalyst has been studied. The reforming leads to a full

13

transformation of C2+ hydrocarbons and tar. The maximum H2 yield is obtained at 700

14

ºC (36 wt% by mass unit of HDPE in the feed, which accounts for 83 wt% of the

15

maximum H2 yield according to stoichiometry), with CH4 conversion being 94 % and

16

the coke amount deposited on the Ni catalyst being minimum (3.3 wt% by mass unit of

17

HDPE in the feed). The syngas obtained by reforming at 700 ºC has a CO/H2 volumetric

18

ratio of 6 and is suitable for hydrogenation reactions and further valorization to produce

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H2 .

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Keywords: hydrogen, gasification, reforming, conical spouted bed, catalyst, coke

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1. Introduction

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World production of plastics in 2013 was 299 million tons (57 million tons in Europe),

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with an annual growth of 3.9 % from 2012 1, which is a consequence of their use in

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numerous applications (packaging, building and construction, automotion, electricity

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and electronics, agriculture, consumer and household appliances) in order to maintain or

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improve our lifestyle. Nevertheless, plastics also bring about a large number of

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environmental hazards. In 2012, 25.2 million tons of post-consumer plastic wastes

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ended up in the waste upstream, with 26 % being recovered though recycling and 36 %

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by means of energy recovery processes, but 38 % still went to landfill 1. One of the

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more serious environmental problems associated with plastic landfill is the

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accumulation of waste plastics in oceans, which accounts for more than 4.8 million tons

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in 2012 higher 2. Accordingly, there is a growing interest by institutions to promote the

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implementation of recycling technologies, minimizing the landfill of waste plastics and

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avoiding the emissions in energy recovery processes. In addition, given that plastics are

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petroleum-derived materials, its recycling contributes not only to preserve the

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environment but also to intensify petroleum valorisation.

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Thermochemical routes (pyrolysis and gasification) have been considered to be the most

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interesting recycling routes for plastic wastes in order to recover monomers and obtain

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fuels or high value-added chemicals, and they have good perspectives concerning

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scaling up 3-5. Thermal or catalytic plastic pyrolysis has been widely studied using

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several reactor designs and catalysts 6-11. Plastic waste gasification studies are scarce;

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however this alternative has the advantage of being a more flexible technology,

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allowing to treat jointly plastic materials 12, 13 of different nature or plastic materials

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with biomass or coal 14, 15. The syngas obtained may be directly used as fuel in fuel cells

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or gas turbines 16, or may be used to obtain liquid fuels (via Fischer-Tropsch or 2 ACS Paragon Plus Environment

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methanol synthesis) or hydrogen, which is considered a renewable and sustainable

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energy source in the long term 17-19.

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Plastic thermal treatments, such as pyrolysis or gasification, have been widely studied in

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fluidized bed reactors 7, 12, 14, 20, 21, whose main feature lies in their high heat and mass

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transfer rates. Nevertheless, the sticky nature of plastic materials may cause

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agglomeration of particles by the fused plastic and subsequent bed defluidization. The

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cyclic solid circulation pattern characteristic of the conical spouted bed reactor (CSBR)

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gives way to the uniform coating of individual sand particles with fused plastic, which

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favours heat and mass transfer between phases 22 and so bed isothermicity. Furthermore,

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collisions between particles and a high gas velocity in the spout region avoid the

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formation of solid particle-fused plastic aggregates and subsequent bed defluidization 23.

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The low segregation is another interesting feature of the CSBR 24, which allows using a

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catalyst in situ25.

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In a previous paper 26 , the good performance of the CSBR has been proven in the steam

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gasification of HDPE, i.e., the limitations involving the physical steps prior to

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devolatilization as well as secondary reactions are minimized. Although the CSBR has

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the aforementioned advantages compared to other technologies, considerable tar yield

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has been obtained due to the reduced residence time, which is a serious problem for gas

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valorisation as fuel or synthesis gas (for the production of hydrocarbons, methanol or

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dimethyl ether) 27. Erkiaga et al. 26 used olivine and γ-Al2O3 in situ as a primary

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measure for tar elimination, but this strategy has a limited effect on tar reduction. In

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spite of the positive effect observed when temperature and S/P were increased, the yield

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of tar was still high (1.63 wt. %) at 900 ºC and with a steam/plastic (S/P) ratio of 2. The

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tar is mainly formed by the most stable non-substituted aromatic hydrocarbons, with

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benzene and naphthalene being the main compounds. Therefore, reforming of the 3 ACS Paragon Plus Environment


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volatiles is required in order to avoid the problems associated with the tar in the

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upgrading of the gaseous stream for energy or syngas 28.

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The objectives of this study lie in the minimization of the tar yield obtained by

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continuous HDPE gasification and production of a stream rich in H2. Therefore, a two-

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step reaction system has been proposed (Figure 1): i) steam gasification of HDPE in a

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conical spouted bed reactor, which is especially suitable for avoiding the limitations

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involving the physical steps; ii) in-line catalytic steam reforming of the volatiles in a

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fixed bed reactor on a Ni catalyst. The use of two reactors in-line increases the

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efficiency of the catalyst for tar reduction compared to in situ treatment, given that the

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optimum operating conditions (temperature, residence time) may be used in each reactor

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and the whole catalytic bed is available for reforming the tar contained in the stream

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leaving the gasification reactor.

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Ni catalysts are the most used reforming catalysts, mainly because of their lower cost

85

than noble metals (Pd, Pt, Ru and Rh), which are more active for breaking C-C bonds

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and more stable 29. Furthermore, Ni catalysts are more active for Water-Gas Shift

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(WGS) reaction at low temperature 30. Accordingly, they have been used in the

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reforming of the volatile products obtained by plastic pyrolysis by changing their

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composition (Ni content, supports and promoters) in order to increase the yield and

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selectivity to H2 and their stability by minimizing coke deposition 31-35. Figure 1

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2. Experimental section

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2.1. Materials


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The high density polyethylene (HDPE) was provided by Dow Chemical (Tarragona,

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Spain) in the form of chippings (4 mm), with the following properties: average

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molecular weight, 46.2 kg mol-1; polydispersity, 2.89 and density, 940 kg m-3. The

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carbon content in the polymer is approximately 86 % and that of hydrogen 14 %. The

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higher heating value, 43 MJ kg-1, has been determined by differential scanning

99

calorimetry (Setaram TG-DSC-111) and isoperibolic bomb calorimetry (Parr 1356).

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The olivine used in the gasification reactor has been provided by Minelco (Lulea,

101

Sweden). The material has been ground and sieved to the desired particle diameter,

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0.35-0.4 mm. Moreover, it has been calcined at 900 ºC for 10 h prior to use in order to

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enhance its reactivity in the tar cracking reaction. The surface properties have been

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measured by N2 adsorption-desorption (Micromeritics ASAP 2010). Calcined olivine

105

has a limited porosity, with a surface of only 0.18 m2 g-1.

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The catalyst used in the reforming reactor (G90LDP catalyst) is a commercial reforming

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catalyst, with a chemical formulation based on NiO, CaAl2O3 and Al2O3, which has

108

been provided by Süd Chemie. The catalyst has the form of perforated rings (19x16

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mm) with a metallic phase of Ni supported on Al2O3, which is doped with Ca. This

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catalyst was ground and sieved to 0.7-2 mm. Moreover, previously to reforming

111

reaction the catalyst has been subjected to an in situ reduction process at 710 ºC for 4 h

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under a 10% vol. H2 stream.

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Table 1 shows the NiO content of the catalyst and its physical properties, which have

114

been measured by N2 adsorption-desorption (Micromeritics ASAP 2010) (Figure 2). As

115

observed, it is a mesoporous material, with an average pore diameter of 122 Å and a

116

hysteresis characteristic of mesoporous materials in the N2 adsorption-desorption

117

isotherm. The values of BET surface area and pore volume are rather low.



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Figure 2

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Table 1

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Figure 3 shows the temperature programmed reduction (TPR) curve of the catalyst

121

measured by AutoChem II 2920 Micromeritics, which enables to establish the catalyst

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reduction temperature prior to use. The TPR shows a main peak at 550 ºC associated

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with NiO reduction, which interacts with α-Al2O3. Besides, another peak is observed at

124

700 ºC, which is probably related to NiAl2O4 according to the composition given by the

125

provider. As observed, the catalyst is completely reduced at 900 ºC, which is

126

characteristic of α-Al2O3 supports. Figure 3

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128

2.3. Equipment and reactors

129

Gasification and steam reforming runs have been carried out by continuously feeding

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HDPE in a bench scale plant whose scheme is shown in Figure 4. The main elements of

131

the plant are the CSBR and the fixed bed reactor arranged in-line. The design of the

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CSBR is based on previous hydrodynamic studies 24 and on the application of this

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technology to the continuous pyrolysis and gasification of different solid wastes, such as

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biomass 36-38, plastics 26, 39, 40 and waste tyres 41, 42. In addition this technology has been

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successfully scaled up to 25 kg/h for biomass pyrolysis process 43.

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The dimensions of the CSBR are: height of the conical section, 73 mm; diameter of the

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cylindrical section, 60.3 mm; angle of the conical section, 30 º; diameter of the bed

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bottom, 12.5 mm, and diameter of the gas inlet, 7.6 mm 26. The reactor is placed inside a

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1250 W radiant oven and the lower section of the reactor is a gas preheater filled with

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an inert ceramic material that increases the surface area for heat transfer, ensuring that


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steam is heated to the desired reaction temperature. Two K-type thermocouples are

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located inside the reactor, one in the bed annulus and the other one close to the wall.

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The fixed bed reactor is a cylindrical stainless steel vessel, with an internal diameter of

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38.1 mm and a total length of 440 mm. This reactor is located inside a 550 W radiant

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oven which provides the energy needed to maintain the reaction temperature, which is

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controlled by a thermocouple placed in the catalyst bed.

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Both reactors are located within an oven which is in turn placed in a forced convection

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oven maintained at 270 ºC to avoid the condensation of steam and the volatile stream

149

formed in the gasification reactor, which is fed in-line to the reforming reactor. A high-

150

efficiency cyclone and a sintered steel filter (5 µm) are located between the two reactors

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in order to retain the fine olivine particles entrained from the CSBR bed and the soot or

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char particles formed in the HDPE gasification.

153

Figure 4

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The plant is provided with a plastic feeding system, which consists of a vessel equipped

155

with a vertical shaft connected to a piston placed below the material bed. The plastic is

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fed into the reactor (in the 0.2-5 g min-1 range) by raising the piston at the same time as

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the whole system is vibrated by an electric engine. A very small nitrogen flow rate

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introduced into the vessel stops the volatile stream entering the feeding vessel. The pipe

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that connects the feeding system with the reactor is cooled with tap water to avoid

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plastic melting and blocking the system.

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Water has been fed into the CSBR by means of a Gibson 307 pump, which allows a

162

precise measuring of the flow rate, and has been vaporized by means of an 1250 W

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electric cartridge placed inside the forced convection oven. Moreover, N2 is used as



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fluidizing agent during the heating process and its flow rate is controlled by a mass flow

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controller that allows feeding up to 20 L min-1.

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The gases formed in the reforming reactor circulate through a volatile condensation

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system consisting of a condenser, a Peltier cooler and a coalescence filter, which ensure

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the total condensation and retention of the tar. The Peltier cooler consists of a 150 ml

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tank and a refrigerator that lowers the temperature to around 2 ºC, thereby efficiently

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condensing the volatile products. The non-condensable gas fraction leaves the Peltier

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through its upper part and the liquids are collected in the tank. The condenser is a

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double-shell tube cooled by tap water.

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2.3. Experimental conditions

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The conical spouted bed reactor contains 70 g of olivine (particle diameter in the 0.35-

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0.4 mm range) and the fixed bed reactor 12.5 g of Ni catalysts (0.7-2 mm). Water flow

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rate is 3 mL min-1 in all the studied conditions, corresponding to a steam flow rate of

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1.86 NL min-1, which is approximately 1.5 times that corresponding to the minimum

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spouting velocity 24. The runs have been carried out by continuously feeding 0.75 g min-

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1

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lower S/P ratios give way to severe operational problems of fixed bed blocking by coke

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formation. The gasification of HDPE (first step) has been carried out at 900 ºC and the

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effect of temperature on steam reforming (second step) has been studied in the 600-700

183

ºC range.

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All the runs have been performed in continuous mode for several minutes in order to

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ensure the steady state in the process. Moreover, the runs have been repeated sat least 3

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times under the same conditions in order to guarantee reproducibility of the results. The

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amount of char formed in the gasification step (below 0.5 wt. %) has been determined

of HDPE and using a steam/plastic ratio equal to 4. Previous experiments revealed that


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by weighing the mass retained in the cyclone and sintered steel filter, and its yield by

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monitoring also the amount of plastic fed into the reactor in each run.

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2.4. Product analysis

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The volatile stream leaving the reforming reactor has been analysed on-line by means of

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a GC Agilent 6890 provided with and HP-Pona column and a flame ionization detector

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(FID). The sample has been injected into the GC by means of a line thermostated at 280

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ºC, once the reforming reactor outlet stream has been diluted with an inert gas. The non-

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condensable gases have been analyzed in a micro GC (Varian 4900).

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The amount of the coke deposited on the catalyst has been measured by temperature

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programmed oxidation (TPO) in a TGA Q5000TA thermobalance (Thermo Scientific)

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connected on-line to a Blazer Instruments mass Spectrometer (Thermostar) monitoring

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the masses corresponding to CO and CO2 in order to avoid the disturbance caused by Ni

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oxidation. The following procedure was applied: i) signal stabilization at 200 ºC with

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He stream for 30 minutes; ii) a ramp of 5 ºC min-1 to 800 ºC under O2 diluted in He,

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finally this temperature was maintained for 30 minutes in order to ensure full coke

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combustion. Due to the non-homogeneous distribution of the coke in the fixed bed, the

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sample for the TPO analysis was taken subsequent to homogeneously mixing the whole

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catalyst amount. In addition, the nature of the coke has been studied by TEM

206

(transmission electron microscopy) images and SEM (scanning electron microscopy)

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imagines obtained by measn of Phillips CM200 and JEOL JSM-6400 devices,

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respectively.

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3. Results

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3.1. Products yields and concentrations



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Table 2 shows the effect of temperature in the reforming reactor on the yields of final

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products (by mass unit of HDPE in the feed). The yields of the compounds obtained in

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the gasification reactor (first step) 26 are also set out in the table in order to show the

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efficiency of the reforming step. It should be noted that the gasification results shown in

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Table 2 were obtained with an S/P ratio of 1 and in the present study this ratio has been

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increased to 4, however a limited effect could be expected at high S/P ratios in the

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gasification step 16, 26.

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Table 2

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As observed, the reforming step performs well, given that C2+ hydrocarbons and tar are

220

completely reformed. Furthermore, it should be noted that the catalytic reforming has a

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significant effect on the composition of the gas obtained, increasing the yields of H2 and

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CO2 and decreasing those of CO and CH4, and therefore improving considerably gas

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composition with respect to gasification. Although the conversion of CH4 is not

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complete, reaches 94 wt% at 700 ºC, i.e., its yield decreases from 16.09 wt% to 1.03 %.

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This significant effect of the reforming catalyst is due to the activation of the following

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reactions by Ni catalyst:

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Steam reforming: (CH2 )n + nH2O ⇔ 2nH2 + nCO

(1)

228

Dry reforming: (CH2 )n + nCO2 ⇔ nH2 + 2nCO

(2)

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Water-gas shift reaction: H2O + CO ⇔ H2 + CO2

(3)

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As a result of the enhancement of reforming and WGS reactions, there is an increase in

231

the amount of water reacted in the reforming step (84.27 g[water] 100 g[HDPE]-1 in the

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gasification step vs. 150.90 g[water] 100 g[HDPE]-1 in the reforming step at 600 ºC). This 10 ACS Paragon Plus Environment

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index has been determined based on the H2O mass flow rate in the feed and in the

234

product stream, (mH2O)i and (mH2O)oi, respectively, and on the HDPE mass flow rate fed

235

into the reactor (eq. 4).

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X H 2O =

(mH O )i − (mH O )o 2

2

mHDPE

100

(4)

237

An increase in temperature from 600 to 650 ºC gives way to a significant increase in the

238

amount of water reacted. However, above 650 ºC there is only a slight increase in the

239

amount of water reacted and a slight decrease in the yield of CO2. This is due to WGS

240

reaction thermodynamic limitation, which is a well known fact at this temperature in the

241

steam gasification of biomass 44, 45.

242

Furthermore, the decrease in the yield of CH4 as temperature is increased is an

243

indication of the favourable effect of temperature on the reforming of CH4. Higher

244

temperatures cause an increase in the yields of H2 and CO and the gasification of the

245

coke deposited on the catalyst. Accordingly, the yields of H2 and CO reach the values of

246

82 % and 36 %, respectively, at 700ºC.

247

As observed in Figure 5, the effect of temperature on the composition of the gaseous

248

components is not as significant as its effect on the yields of the components. Thus, the

249

concentration of H2 increases slightly as temperature is increased, from 71 % at 600 ºC

250

to 73 % at 700 ºC. The increase in the concentration of CO and the decrease in those of

251

CO2 and CH4 is of around 2 %. It is remarkable that CH4 concentration at 700 ºC is

252

almost negligible due to the high CH4 reforming efficiency. The concentrations of CO

253

and CO2 at this temperature are 12 % and 15.5 %, respectively.

254

Figure 5



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It is noteworthy that a high H2/CO molar ratio, 6.2, is obtained at 700 ºC, which is much

256

higher than the value of 2 recommended for standard syngas applications, such as the

257

production of fuels or chemical products, and it should therefore be further corrected. In

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addition, a low CO2 concentration (15.5 %) is obtained at 700 ºC, which makes this

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syngas especially suitable for hydrogenation processes (in those applications in which

260

CO2 and CO are not a problem) and for the production of H2 through the steps of WGS

261

reaction and CO2 separation process.

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These results are very similar to those obtained by Tsuji and Hataymana 46 in the

263

gasification/reforming of low density polyethylene (LDPE) on a Ni/Al2O3 catalyst used

264

in situ in a fluidized bed reactor at 750 ºC. The H2 concentration obtained was 72 % and

265

they concluded that H2 mainly comes from the water reacted (55-66 %). Furthermore,

266

they observed similar trends for CO and CO2 concentration with temperature, with the

267

concentrations of CO, CO2 and CH4 being 11 %, 16 % and 0.3 %, respectively, at 750

268

ºC

269

Given that H2 may be produced from different raw materials and processes, process

270

efficiency is best assessed by an index based on the maximum H2 yield defined by

271

stoichiometry 47-49:

272

YH = 2

mH 2 0 mH

(5)

100

2

273

and considering the ideal reforming stoichiometry:

274

CnHm + 2nH2O ↔ nCO 2 + (2n + m 2 )H2

275

the maximum H2 molar flow rate for the HDPE molar flow rate in the feed (m0H2) is:


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mH0 2 = mHDPE (2n + m / 2)

(6)

277

Figure 6 shows the effect of temperature on H2 yield and coke yield (g coke/100 g of

278

HDPE in the feed), which has been determined by TPO of the used catalysts. It is

279

observed that an increase in temperature gives way to an increase in H2 yield, reaching a

280

value of 83 % at 700 ºC, which is due to the enhancement of CH4 reforming reaction as

281

temperature is increased. This yield is close to that obtained by other authors in the

282

reforming of biomass-derived materials, such as DME and ethanol, using similar

283

steam/feed ratios 50. Figure 6

284

285

It should be noted that a high coke yield is obtained at 600 ºC (18 wt%), but it is

286

drastically reduced by increasing the reforming temperature to 700 ºC (3.3 wt%), which

287

is due to the partial gasification of the coke. Therefore, 700 ºC is a suitable reforming

288

temperature, given that it allows obtaining a high yield of gases (especially H2 and CO)

289

and coke gasification reaction rate, with a low space time by avoiding Ni sintering

290

occurring at higher temperatures 51.

291

Coke deposition is notable once a few reaction minutes have elapsed and is not

292

homogeneously distributed in the fixed bed reactor, i.e., there is a higher content in the

293

initial stretch of the reactor. This reveals that coke deposition is mainly caused by

294

secondary reactions in the catalytic reforming.

295

The high amount of coke deposited on the catalyst (mainly on the surface of the catalyst

296

particles) blocked the fixed bed reactor subsequent to 20-60 min operation, depending

297

on the reforming temperature (high temperature favoured coke gasification and gives

298

the possibility to operate for longer reaction times). Consequently, coke deposition



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299

hindered the study of the process for long reaction times. This problem may be solved

300

in future studies using two parallel reactors, with one in operation while the catalyst is

301

being regenerated in the other one. A fluidized bed reactor may also delay or avoid

302

reactor blockage by coke deposition, as has been proven by different authors in the bio-

303

oil reforming 52, 53. Furthermore, the industrial implementation of the catalytic

304

reforming would require a fluidized bed reactor with continuous catalyst feed and

305

deactivated catalyst removal.

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A comparison of the aforementioned results with those in the literature shows that the

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maximum H2 yield obtained, 83 wt% at 700 ºC, is similar to that obtained by Czernik et

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al 54 in the gasification-steam reforming (650-800 ºC) of polyethylene (PE) in a micro-

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reactor and of polypropylene (PP) in two fluidized bed reactors, the first one at 600-700

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ºC and the second one at 850 ºC, with H2 yield being 80 wt% in both cases. The same

311

authors obtained a H2 yield of 56 wt% using steam/air mixtures as gasifying agent and

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burning 25 wt% of PP in the feed in order to attain autothermal process.

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3.2. Coke characterization

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Based on the aforementioned results, the high coke deposition (with yields of 18, 6 and

315

3 % at 600, 650 and 700 ºC, respectively) is a determining factor for establishing the

316

optimum conditions for the reactor design. This fact is a common feature of Ni catalysts

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used in the reforming of hydrocarbons and oxygenates, but the type of the coke seems

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to play an even more significant role on deactivation. Thus, the main responsible for

319

catalyst deactivation seems to be the amorphous external coke (encapsulating coke),

320

whereas the filamentous coke, generally the prevailing one, grows towards the outside

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of the catalyst particle without contributing to the catalyst deactivation, but causing

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operational problems in the reactor, such as gas flow blockage 51, 55.


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Figure 7 and Figure 8 show SEM and TEM images of the coke deposited on the catalyst

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at the reforming temperatures of 600, 650 and 700 ºC. As observed, an increase in

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temperature changes the structure of the coke deposited. At 600 ºC (Figure 7a and

326

Figure 8a), the amount of coke deposited is highest, with its nature being filamentous

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and amorphous. At 650 ºC (Figure 7b and Figure 8b), the coke deposited is more

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structured and more fibres are observed. At 700 ºC (Figure 7c and Figure 8c), a less

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fibrous coke than at 650 ºC is observed due to the partial gasification of coke

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precursors. Besides, as temperature is increased the fibres are shorter and thicker.

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Furthermore, Ni crystals are observed in the SEM imagines (Figure 8), which are swept

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throughout the filamentous coke growth. These results are similar to those obtained by

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Wu and Williams 35 in the pyrolysis-reforming of PP on Ni-Al-Mg catalyst.

334

Figure 7

335

Figure 8

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In order to know the nature and location of the coke deposited, temperature

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programmed oxidation (TPO) has been carried out (Figure 9). The peaks reported in the

338

literature at low temperature (

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