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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
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Hydrogen-rich gas as a product of two-stage co-gasification of lignite/waste plastics mixtures kova Pavel Straka*, Olga Bica ch 41, Institute of Rock Structure and Mechanics, v.v.i., Academy of Sciences of the Czech Republic, V Holesovicka 18209 Prague 8, Czech Republic
article info
abstract
Article history:
Under atmospheric pressure, mixtures of lignite with waste plastics were gasified on a
Received 15 February 2014
laboratory scale. The resulting tar was cracked in a thermal cracking reactor. For experi-
Received in revised form
ments, low-ash and low-sulfur lignite was used; the percentage of waste plastics in the
8 May 2014
mixtures was 10 and 20 wt.%. The main product of co-gasification was hydrogen-rich gas,
Accepted 11 May 2014
as by-products, soot and non-gasified solid residue were obtained. It was found that the
Available online xxx
higher heating value of obtained gas is fully comparable with that of industrial gas from lignite gasification. Probably, at least 20 wt.% of lignite can be replaced with mixed waste
Keywords:
plastics in this process. The effect of waste plastics addition on properties of the obtained
Co-gasification
gas and of the non-gasified solid residue was evaluated and discussed.
Waste plastics
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Lignite Hydrogen-rich gas
Introduction Wastes: production and utilization The current state of the environment is immediately related to the quality of individual areas including mainly air, water management, agriculture and waste management. Due to the large production of wastes, a need arises to replace the existing technologies with low- or no-waste technologies. In the current state of waste production, it is necessary to find a way of the maximum utilization of wastes. Waste materials as alternative fuels are receiving increased attention. Waste production in the individual categories in the Czech Republic in the years 2002e2012 is evident from Fig. 1 [1e3]. With regard to municipal waste, the production is continually growing since 2007. Whereas in the Czech Republic in 2003e2008, 4.6e3.8 Mt of municipal waste were produced
annually, in 2009e2012 this number already amounted to 5.3e5.4 Mt [2,3]. The generated waste contained and still contains a large portion of materials that may be used for power and heat production or to obtain useful liquid, solid or gaseous fuels. Municipal waste containing a number of further utilizable components is currently predominantly deposited at landfills or biologically modified to form compounds and mixtures which can be landfilled after such a modification. A smaller proportion of the produced municipal waste is used as fuels in incineration plants or other means to generate energy; a portion of municipal wastes is used for recultivation or composting or is deposited as technological material at secured landfills (Table 1). A part of municipal waste can be sold as secondary raw materials [3]. Municipal waste production in EU countries considerably varies, as arises from [4], but the total is quite high; so, wastes
* Corresponding author. Tel.: þ420 266 009 402; fax: þ420 284 680 105. E-mail address: [email protected] (P. Straka). http://dx.doi.org/10.1016/j.ijhydene.2014.05.054 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
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Fig. 1 e The total waste production by category in the Czech Republic, 2002e2012.
must be processed. In this connection, pyrolysis of municipal solid waste (MSW) was studied in order to evaluate the influence of the process conditions: temperature, catalyst, the residence time of the gaseous phase in reactor, and grain size [5e8]. Since waste plastics are an important part of MSW, it is necessary to deal intensively with their treatment. It was found that household municipal waste contains ca 12% plastics [9] which may be utilized, but currently are mostly landfilled along with other municipal waste. Plastics currently represent 11% of MSW in OECD countries [10]. Various ways of their processing are suggested. Wilk and Hofbauer [11] studied co-gasification of plastics with wood biomass in fluidized bed steam gasifier. Authors found more product gas than expected and more CO and CO2 were measured in the gas than resulted from mono-gasification of materials. Contrary, the tar content in the product gas was considerably lower than presumed. Sørum et al. studied pyrolysis of waste plastics [12] and Luo et al. [13] monitored the influence of the grain size on pyrolysis performance with plastics, kitchen garbage, and wood as the three most frequent components of MSW. Other authors verified a possible utilization of waste plastics by pressing of them with paper and lignite dust into briquettes, which in the municipal sphere can replace sorted lignite [14]. In summary, pyrolysis has the inherent advantage of high flexibility with respect to plastics with contaminants; gasification has the promising features, such as high conversion efficiency,
Table 1 e Municipal waste utilization in the Czech Republic in 2007e2012. Waste management (%) 2007 2008 2009 2010 2011 2012 Landfills 86.2 89.9 64.0 59.5 55.4 53.7 (codes D1 þ D5 þ D12) Material recovery 21.1 24.2 22.7 24.3 30.8 30.3 municipal wastea Energy production (code R1) 9.8 9.6 6.0 8.9 10.8 11.8 Incineration (code D10) 0.07 0.05 0.04 0.04 0.04 0.04 a
codes R2eR12; N1, N2, N8, N10eN13, N15.
effective processing of low-grade fuels and wastes, moreover, the producer gas (the gas obtained) can be effectively utilized in a variety of ways ranging from electricity production to chemical industry. In our case, gasification and co-gasification were considered as advanced methods of the thermal treatment of plastic wastes. Gasification processes may work with or without catalyst [15e19] in a moving or fluid bed or in an entrained flow reactor. A problem is the removal of the formed tar entrained by the gas flow. The currently most promising method for production of purified gas is the catalytic cracking of produced tar in a secondary reactor. For this reason, the first gasification stage was complemented with secondary catalytic reactor with calcined dolomite [20] working at 800e900 C or with catalysts on nickel basis [21] working at 700e800 C, which catalytically purify the raw gas. The use of a Ni-catalyst makes it possible to convert ca 90e99% of the tar in the secondary reactor into gases. Although the results are promising, the entire process must be more elaborate, both economically and technically, and have only one-stage, if possible. Other experiments, on the one-stage fluidized bed reactor, showed that Rh/CeO2/SiO2 catalysts containing 35% of CeO2 have an excellent influence on the conversion of carbon into gas at low temperature when compared with non-catalytic or dolomitecatalyzed reactions at high temperature [22]. The conversion of carbon into gaseous compounds may be enhanced by introduction of small amount of oxygen from the bottom of the reactor or a sufficient fluidization velocity. Mastral et al. [23] compared the pyrolysis and gasification of high density polyethylene (HDPE) in a fluidized bed reactor. It was found that the working temperature affects product distribution and gas composition, and that higher temperature increases a gas production and simultaneously reduces a tar production in both pyrolysis and gasification. In the partial oxidation during HDPE gasification, a higher reactivity of HDPE was observed. The gas composition depends on gasification temperature; with increasing temperature, the content of carbon monoxide and methane markedly increase while the content of carbon dioxide decreases. Pinto et al. [24,25] studied the co-gasification of coal with waste polyethylene
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
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and pinewood and of biomass/waste polyethylene mixture. The highest yield of gaseous products was achieved at 60 wt.% addition of waste polyethylene into the gasified charge. Xiao et al. [17] studied the effect of the equivalence ratio, static bed height, and fluidization velocity on the temperature of the bed during polypropylene gasification in a fluidized bed. They concluded that the equivalence ratio had the most significant effect on the reactor temperature and the gas production, and the products yield distribution. The selection of suitable equivalence ratio would depend on the final use of gas produced. A higher equivalence ratio reduced the content of tars and hydrocarbons, but also unfavorably affected the gas lower heating value. Similar conclusions concerning the relation between the equivalence ratio and the gas and tar yield were reported by Leung and Wang [26], who gasified pulverized waste rubber in a fluidized bed reactor. It seems that promising method of treatment of plastic wastes is their co-gasification with lignite [27,28] as the producer gas can be effectively utilized for electricity production as well as in chemical industry; moreover, fossil fuels can be saved by their partial replacement by waste plastics. In our case, low-ash and low-sulfur lignite from the Czech Rep. was used for co-gasification with waste plastics. In the Czech Republic, this lignite is commonly used for production of electrical power. Lignite is gasified in Lurgi generators and the produced gas utilized in IGCC. The treatment of waste tires through co-gasification with this lignite in a moving bed was studied on a commercial scale [29]. It was found that an addition of 20 wt.% of waste tires improved the lower heating value (LHV) of raw gas by 3% in comparison with the LHV of gas from gasification lignite alone. It was further discovered that an addition of waste tires reduced the H2S and CH3SH content in raw gas. Also co-gasification of mentioned lignite with polyethylene terephtalate (PET) in fluidized bed reactor was studied [28]. Authors found that (a) in the presence of PET the lignite particles are not significantly fragmented due to lower reactivity of lignite as compared with PET, which results in a higher bottom char formation than in single lignite gasification, (b) the tar content was approximately three times higher in co-gasification than in single lignite gasification, (c) the fluidized bed temperature exhibited considerable influence on the major components of producer gas. This article deals with possibility of conversion of mixed waste plastics containing low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), and polystyrene (PS) with lignite into hydrogen-rich gas through co-gasification. The aim of the work is to evaluate the effect of addition of mixed waste plastics on properties of producer gas and the nongasified solid residue.
Theory If syngas contains tar molecules, they have to be removed before syngas end-use, while tar can be physically removed (using filters and wet scrubbers) or chemically converted into gaseous compounds using thermal or thermal catalytic processes, which promote tar cracking reactions (eq. (6), see below) and endothermic reactions leading to the formation of H2 and CO, whose contents are expected to increase,
3
especially in the presence of waste plastics with high hydrogen and carbon contents. However, at temperatures up to 1000 C, tar chemical conversion releases also polycyclic aromatic hydrocarbons (PAHs) and soot. But at temperatures above 1000 C, notably at 1200 C, no PAHs were registered in resulting gas (using GC-MS), moreover, soot decreased. Due to this and the fact that practically useful catalysts are still in development, the variant with thermal cracking reactor working at 1200 C may be useful for waste plastic treatment and hydrogen-rich gas development. In our case, beside the main gasification rate controlling reactions C þ H2O(g) ¼ CO þ H2
C þ CO2 ¼ 2CO
DH ¼ 118.6 kJ/mol
DH ¼ 160.7 kJ/mol
(1)
(2)
and accompanied exothermic reactions, e.g. C þ 2H2 ¼ CH4
DH ¼ 87.5 kJ/mol
CO þ H2O(g) ¼ CO2 þ H2
CO þ 3H2 ¼ CH4 þ H2O
(3)
DH ¼ 42.3 kJ/mol
(4)
DH ¼ 205.9 kJ/mol
(5)
the cracking reactions in tar reformer took place. Three types of reactions were considered, (a) hydrocarbons decomposition, (b) cracking of oxygen-containing compounds and oxygen reactions, and (c) pyrogenetic water reactions. Formed higher aliphatic and cyclic hydrocarbons cracked to elemental carbon and hydrogen: CxHy ¼ xC þ 1/2yH2
(6)
Simultaneously, methane and hydrocarbons with short chains were formed: CxHy ¼ C1e4Hn þ Cx(1e4)H(yen)
(7)
Further, oxygen-containing compounds were cracked up to elemental oxygen, which easily exothermally reacted with carbon to carbon oxides: 2C þ O2 ¼ 2CO
C þ O2 ¼ CO2
DH ¼ 246.4 kJ/mol DH ¼ 406.4 kJ/mol
(8)
(9)
Carbon oxides were provided also by the reactions of pyrogenetic water with formed elemental carbon: C þ H2O(g) ¼ CO þ H2
C þ 2H2O(g) ¼ CO2 þ 4H2
DH ¼ 118.6 kJ/mol DH ¼ 16.2 kJ/mol
(10)
(11)
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
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At the temperature 1200 C, in the presence of carbon, a thermodynamic equilibrium between carbon oxides is established in favor of carbon monoxide as with increasing temperature the equilibrium of reaction C þ CO2 4 2CO shifts in favor of CO [30]. On the whole, the tar reformer can significantly enhance amounts of hydrogen and carbon monoxide in resulting producer gas.
determined by a standard-less X-ray analysis. The powder samples were fixed on a Cu-tape and then analyzed by the EDS Silicon Drift Detector Apollo X, EDAX Inc. In all the cases, the higher carbon content was found than expected, which was attributed to unburned carbon. Thus, the unburned carbon was then determined by subtracting ash from the non-gasified residue.
Experimental setup and gasification procedure
Experimental Materials Samples of low-ash and low-sulfur lignite from the Sokolov Lignite Basin, Czech Republic, were used for experiments. Based on previous experience [31] and taking into account the work [13] the lignite was ground to a grain size 1e4 mm. The samples of mixed waste plastics (WP) were obtained from MSW (source Transform AS, Czech Republic) by floating in water as a light fraction with density less than the density of water. The obtained fraction of WP contained LDPE, LLDPE, HDPE, PP, and PS, which was proved by DSC method (PerkineElmer Pyris DSC 7 analyzer, a nitrogen atmosphere, 10 C/ min, powdered sample) (Fig. 2). The separated fraction contained also wood and paper as a minority share. Regarding the work [13], the obtained WP fraction was homogenized and adjusted to the grain size 1e3 mm. The characteristics of the lignite and WP used are given in Table 2. Analyses were carried out according to standards ISO (687:2010, 562:2010, 1171:2010, 333:1996, 334:2013, 625:1996, 157:1996, 1928:2009). The X-ray diffractometry examination of crystallographic phases of non-gasified residues was performed on X'Pert PRO qeq powder diffractometer with para-focusing Bragg-Brentano geometry using CuKa radiation. An ultrafast X'Celerator detector was employed over the angular range of 7e70 (2q) with a step size of 0.017 (2q) and a counting time of 20.00 s step1. Data evaluation was performed using the software package High Score Plus V 2.2e PANalytical. Only the usual ash components were detected. Elemental composition was
The experiments were performed on a laboratory gasification unit preliminary tested in Ref. [32] (Fig. 3) under atmospheric pressure. A furnace with electrically heated vertical ceramic tube in a stainless-steel shell was used. A double-tube quartz reactor (Fig. 4) was placed in the furnace as a gasification reactor. The outer part of this reactor consists of a quartz tube (inner diameter 55 mm, length 480 mm) with sealed bottom, whereas the inner part is a quartz tube (inner diameter 40 mm, length 450 mm) with the bottom modified to a gasdistribution grate for the uniform distribution of gasification agents (H2O(g), CO2) to the gasifier bed. The gasification media are dosed into the area between the inner and outer parts of reactor allowing ensure both steam generation and the heating of gasification media to a temperature identical with temperature in the bed. The output for the developed raw gas in the upper part of reactor is immediately connected to the tar reformer inlet. In principle, the tar reformer is a horizontal quartz reactor (inner diameter 43 mm, length 740 mm) placed in an electro-resistance furnace with temperature 1200 C. In this horizontal cracking module (Fig. 3), the emerging liquid hydrocarbons, higher gaseous hydrocarbons, and partially also methane were thoroughly cracked to hydrogen and soot. The obtained gas was further conducted through a vertical cooler into a gas holder. The cooler was connected to an external circulation circuit with ethanol cooled to a temperature of 10 C. Having been cooled, the gas was collected in the gas holder with capacity 130 dm3, equipped with a circulation loop with a continuously operating infrared gas analyzer (Teledyne Analytical Instruments, Model 7500),
Fig. 2 e DSC curve of mixed waste plastics. 1 e LDPE (melting point 108 C), 2 e LLDPE (m.p. 124 C), 3 e HDPE (m.p. 130 C), 4 e PP (m.p. 164 C), 5 e PS (m.p. 245 C). a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
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Table 2 e The characteristics of the lignite and mixed waste plastics used (WP). Sample
Proximate analysis (wt %)
Lignite WP
Ultimate analysis (wt %, daf)
HHV
W
Ad
Vdaf
Sdt
C
H
N
So
Od
(MJ/kg)
12.1 0.4
7.8 2.7
51.9 98.9
0.51 0.10
76.3 82.3
5.6 12.3
0.9 0.3
0.6 0.1
16.6 5.0
30.2 42.2
W e water; Ad e ash (dry basis); Vdaf e volatile matter (dry ash-free basis); Sdt e total sulphur (dry basis); So e organic sulfur; Od e oxygen by difference; HHV e higher heating value.
through which the changes in the concentration of the generated gaseous components CO, CO2, and CH4 were monitored at sampling of signals 3 s. Hydrogen was determined by GC (see later) when sampling the gas from the gas holder was 5 min. Samples of lignite alone and the lignite/WP mixtures with 10 and 20 wt.% WP were gasified. The weight of gasified sample in all the cases was constant, 30 g. The sample placed in the inner part of double-tube quartz gasification reactor was heated at a rate 5 C/min from the ambient temperature to the final temperature 900 C; the soaking time at this temperature was 95 min. In the gasification reactor, pyrolysis occurs first, yielding gas, liquid, and gaseous products. The development of liquid products and the char formation were terminated at a temperature of 700 C, at which water was fed into the reactor by a dosing pump at a flow rate 30 cm3/h. Subsequently, from 750 C, carbon dioxide was dosed at a flow rate 5 dm3/h. The volume flow rates of water and CO2 were chosen based of experience with co-gasification of lignite/ rubber mixtures under atmospheric pressure [31]. At 900 C a steady state was reached lasting for 95 min at the tar reformer temperature 1200 C. As a result, hydrogen-rich gas and the non-gasified solid residue (unburned carbon along with ash) were obtained. Different evolutions of gas during gasification of lignite alone and that with WP are shown in Fig. 5. Each
experiment was repeated 3 times. Standard deviations are shown in Tables 3 and 7. The ongoing gas-component concentrations, pressure, volume and temperature of gas in the gas holder, further, the sample and gasification media temperatures were monitored and recorded on a PC with sampling 3 s. After the end of the experiment, the total gas was analyzed by Agilent Technologies 6890N gas chromatographs with TCD and FID detectors. O2, N2, and CO were analyzed on an HP-MOLSIV capillary column (40 C) with helium as carrier gas (5 cm3/min) using TCD. Methane and other hydrocarbons were determined on a GS-Gaspro capillary column (60 C) with nitrogen as carrier gas (20 cm3/min) using FID (air 400 cm3/min, H2 30 cm3/min, N2 20 cm3/min); carbon dioxide on the GS-Gaspro column (40 C) with helium as carrier gas (5 cm3/min) using TCD; and hydrogen on an HP-5 capillary column (40 C) with nitrogen as carrier gas (7 cm3/min) using TCD. Each determination was repeated 9 times (3 determinations for each of the three experiments). Standard deviations for the gaseous components determination are mentioned further. The amounts of ash and unburned carbon as non-gasified solid residues, soot in tar reformer, and water, both formed (pyrogenetic) and reacted (consumed), were assessed as well.
Results and discussion Continuous analyzer of gas components
The main product of the process was hydrogen-rich energetic gas, by-products were ash-containing non-gasified residue (remained in the gasification reactor) and a less significant amount of soot (remained in the tar reformer). The losses Cooler H2O + CO2 dosing
Raw gas output H2O + CO2 dosing
plynojem Gas holder
PC
Cracking module
Thermocouples
Flask Temperature controller
Outer part Gasified sample
Furnace with a double-tube quartz reactor
Stream of products Signals
Inner part Gas-distribution grate
Fig. 3 e A scheme of a laboratory gasification unit. A flask is calibrated.
Fig. 4 e A double-tube quartz reactor.
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
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80 10%
70
0%
60
Sample
3
Total volume (dm )
Table 4 e Composition of the non-gasified residue (WPmixed waste plastics, L-lignite).
2
1
20%
W (wt %)
Ad (wt %)
UCd (wt %)
2.08 2.56 2.60
53.32 38.96 37.60
46.68 61.04 62.40
40
L (100%) L þ 10% WP L þ 20% WP
30
W e water; Ad e ash (dry basis); UCd e unburned carbon (dry basis).
50
3
20 10 0 0
50
100
150
200
250
300
Time (min)
Fig. 5 e Evolution of producer gas during the gasification of lignite alone (line 1, black), with 10% WP (line 2, green), and with 20% WP (line 3, red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
were 4e6%. The basic products of gasification experiments and the reacted water are shown in Table 3. The values are related to the initial weight of the gasified charge (to 1 g the gasified mixture). It arises from Table 3 that the amount of the consumed water decreased with increasing amount of plastic admixtures, contrary, an amount of non-gasified residue increased. Probably, this phenomenon was caused by the lower rate of gasification of formed lump chars from the lignite/plastics mixtures in comparison with the rate of gasification of formed char from lignite alone. A similar phenomenon was observed at powdered chars from lignite/ plastics mixtures and lignite alone at 800 C [33]. The same holds true for the rate of gasification with CO2. The lower steam and CO2 reactivities meant also less gasified carbon, so that the content of unburned carbon in resulting non-gasified residue was higher in the case of plastic admixtures in comparison with that of lignite alone (Table 4). Composition of producer gas was given by development of single gas-components. Evolution of majority gases, H2 and CO, is shown in Fig. 6; evolution of minority gases, CO2 and CH4, is pictured in Fig. 7. From Fig. 6 it is clear that the evolution of H2 was practically independent on the addition of WP. Contrary, the CO evolution was influenced by WP, as, from about 100 min, the CO concentration in the gas from lignite/ WP mixtures was always lower than that from lignite alone. From Fig. 7 it follows that also development CO2 was influenced by WP. The reason is that present lignocellulosic material starts to decompose already around 200 C (in our case ~40 min) and the decomposition continues until 500 C [34]
(~100 min), but lignite alone decomposes from about 400 C (~80 min). Due to this, CO2 resulting from lignite/WP mixtures with lignocelluloses evolved from ~40 min, but CO2 resulting from lignite alone developed from ~80 min. At ~100 min the lignocelluloses were completely decomposed, therefore the development CO2 from lignite/WP mixtures decreased (Fig. 7, lines 2, 3). Contrary, the development CO2 from lignite alone continued (Fig. 7, line 1) until gasification CO2 dosage in ~150 min. No significant changes in the evolution of CH4 due to WP additions were observed (Fig. 7, lines 4, 5, 6). To demonstrate an effect of tar reformer on the gas composition, GC analysis of gas was taken before and after cracking in the reformer. The results obtained are shown in Table 5 (before) and Table 6 (after). A comparison of the gas concentrations shows that, after cracking, the CH4 concentration decreased and concentration of CO2 even significantly decreased. On the contrary, the concentration of CO increased significantly. The H2 concentration also increased while a significant increase was observed at a 20% WP in gasified mixture, from 51.87 to 55.73 vol.%. No ethane, C3 and C4 hydrocarbons were registered after cracking. On the whole, the tar reformer improved the gas composition. It was demonstrated that the concentration of H2 increases with increasing portion of WP in the gasified mixture. The data presented in Tables 5 and 6 are reliable because in Table 5 the standard deviations (n ¼ 9) were as follows: CH4 0.01e0.03, CO 0.04e0.13, CO2 0.06e0.45, H2 0.07e0.38; in Table 6 the standard deviations (n ¼ 9) were as follows: CH4 0.01e0.08, CO 0.38e0.63, CO2 0.35e0.38, H2 0.43e0.97. Table 6 shows the composition of the gas obtained with a hydrogen content of 55e56 vol.%. Analogously, Pinto et al. [25] discovered that during co-gasification of biomass with 40% polyethylene at 885 C the hydrogen content increased up to 52 vol.%. But simultaneously, at this temperature, the hydrocarbons content decreased, which led to the reduction of the gas higher heating value (HHV). Therefore, HHV together with the gas density as important utility property were calculated and compared in order to describe an influence of waste plastics on these properties. Table 7 shows HHV, density, and the relative density of the obtained gas. (Relative density is the ratio of the density of the gas generated and the air density at temperature and under pressure considered (25 C, 100 kPa)).
Table 3 e The basic products and reacted (consumed) water of gasification experiments (WP-mixed plastics, L-lignite) (g/g charge). Sample L (100%) L þ 10% WP L þ 20% WP
Reacted water
Hydrogen-rich gas (g/g charge)
Soot
Non-gasified residue
0.87 ± 0.02 0.83 ± 0.02 0.80 ± 0.04
1.79 ± 0.03 1.71 ± 0.03 1.69 ± 0.01
0.07 ± 0.02 0.09 ± 0.02 0.09 ± 0.01
0.15 ± 0.01 0.16 ± 0.02 0.18 ± 0.01
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
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60
0%
50
H2
3
Table 5 e The composition of the gas obtained (vol.%) from lignite alone (L) and with additions of mixed plastics (WP) before cracking in the module.
2
10%
1
20%
vol %
40
30
CO
4
5
6
20
0 50
100
150
200
250
300
Time (min)
Fig. 6 e Evolution of H2 and CO during the gasification of lignite alone, with 10% WP, and with 20% WP. Lines 1 (black), 2 (green), and 3 (red) belong the H2 evolutions from lignite alone, lignite with 10% WP, and lignite with 20% WP, respectively. Lines 4 (black), 5 (green), and 6 (red) belong the CO evolution from lignite alone, lignite with 10% WP, and lignite with 20% WP, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
It arises from Table 6 that the obtained gas consists of mainly three components, with the majority in all three cases being comprised of hydrogen, followed by carbon monoxide and carbon dioxide, while H2 and CO have a dominant influence on HHV (11 MJ/m3). The mentioned components form the 18
CO2
0%
16
10 %
14
20%
vol %
12 3
10
1
8 2
6
6
4
4
CH4
5
2 0 0
50
100
150
200
CH4
C2H4
SC2eC*4
N2,f
CO
CO2
H2
L (100%) L þ 10% WP L þ 20% WP
3.14 3.33 3.39
0.06 0.16 0.29
0.24 0.63 1.16
0.60 0.50 0.75
19.54 16.62 15.55
21.44 25.54 26.99
54.99 53.22 51.87
f
10
0
Sample
250
300
Time (min)
Fig. 7 e Evolution of CO2 and CH4 during the gasification of lignite alone, with 10% WP, and with 20% WP. Lines 1 (black), 2 (green), and 3 (red) belong the CO2 evolutions from lignite alone, lignite with 10% WP, and lignite with 20% WP, respectively. Lines 4 (black), 5 (green), and 6 (red) belong the CH4 evolutions from lignite alone, lignite with 10% WP, and lignite with 20% WP, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
e formed; SC2-C*4 e ethane, propane, propene, C4 hydrocarbons.
basis of the hydrogen-rich gas for its further application. The gas further contains a small amount of methane as a residue after its imperfect cracking or as a fission product of cracking of higher hydrocarbons in the tar reformer. The amount of ethylene, which is of the same origin as the methane, is negligible. Higher gaseous hydrocarbons were not found in the mixture. A small amount of N2,f could be formed by a decomposition of ammonia released from lignite [35] or by decomposition of secondary aromatic amines present in WP as antioxidants. On the whole, the evaluation of the experiments has shown that an addition of mixed waste plastics up to 20 wt.% does not have much effect on the composition of the gas obtained and on its key properties. As the gas exhibits stable properties, it is possible to consider partially replace lignite with mixed waste plastics when it is gasified under described conditions. Besides the evaluation of the gas composition, it is worth noting the evaluation of the total volume of the gas obtained and the individual components (at 25 C, 101.325 kPa). Based on the data in Table 6 and measurement of the total volume of gas it was found that the total gas volume was 74.92 Ndm3 in the case of lignite alone, and 71.70 and 70.82 Ndm3 for 10 and 20% WP portions, respectively. It is evident that with increasing amount of WP the total volume of producer gas slightly decreases, which was caused mainly by the reduced volumes of hydrogen (from 41.34 Ndm3 for lignite alone to 39.47 Ndm3 for 20% WP portion) and carbon monoxide (from 19.22 Ndm3 for lignite alone to 17.15 Ndm3 for 20% WP portion). The volume of carbon dioxide was considered as constant (12.13e11.88 Ndm3), similarly for methane (1.45e1.18 Ndm3). It arises from Table 2 that WP contain a higher amount of C and H than the lignite used. Therefore, it would be possible to expect an increase in the amount of hydrogen in the gas obtained from the mixture with WP. The observed opposite trend seems to be caused by a lower rate of the reaction of chars from the lignite/WP mixtures with the gasification media in comparison with the char from lignite alone. The study of chars performed by Raman spectroscopy
Table 6 e The composition of the gas obtained (vol.%) from lignite alone (L) and with additions of mixed plastics (WP) after cracking in the module. Sample
CH4
C2H4
N2,f
CO
CO2
H2
L (100%) L þ 10% WP L þ 20% WP
1.93 1.64 1.97
0.08 0.05 0.06
0.96 0.99 1.13
25.66 25.13 24.21
16.19 16.57 16.90
55.18 55.62 55.73
f
e formed.
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
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Table 7 e HHV, density, and the relative density (WPmixed waste plastics, L-lignite). Sample L (100%) L þ 10% WP L þ 20% WP
HHV (MJ/m3)
Density (kg/m3)
Relative density ()
11.110 ± 0.075 10.964 ± 0.025 11.000 ± 0.022
0.717 ± 0.005 0.717 ± 0.017 0.716 ± 0.007
0.555 ± 0.004 0.554 ± 0.013 0.554 ± 0.008
[33] indicates that the ratio of disordered carbon (D) to ordered (graphite) carbon (G) is higher in the case of char from lignite alone (D/G 3.4) in comparison with that of chars from the lignite/plastics mixtures (D/G 3.0). It can be expected that the disordered structures will be more reactive than the ordered. Therefore, it is suggested that the steam and CO2 reactivity of char from lignite alone is higher as compared with that of chars from the lignite/WP mixtures. Consequently, the main gasification reactions (1) and (2) proceed more slowly with these chars and amounts of formed H2 and CO are somewhat lower. This conclusion is in agreement with the increase in the amount of unburned carbon found with the increasing content of WP in the mixture with lignite (Table 4). A similar reduction of gasification rate was recorded in the lignite/rubber mixtures [31]. It seems that the co-gasification of lignite with mixed waste plastics and the subsequent cracking of volatile products provides the gas, whose HHV 11 MJ/m3 is fully comparable with HHV of above-mentioned energetic gas from the Lurgi generators (HHV 10.7e11.3 MJ/m3). Therefore, probably at least 20 wt.% of lignite can be replaced with mixed waste plastics.
Conclusions Mixed waste plastics may be processed by their co-gasification with lignite to obtain energetic gas with high hydrogen content. In this process, at least 20 wt.% of lignite can be replaced with mixed waste plastics. This substitution does not have much effect on the composition, properties and amount of the energetic gas obtained. The higher heating value of the gas is fully comparable with that of industrial gas from lignite gasification in Lurgi generators.
Acknowledgments This work has been supported by the Research Program for 2012e2017 of Institute of Rock Structure and Mechanics, v.v.i., Academy of Sciences of the Czech Republic, #USMH-130/135/ 2011.
references
[1] Statistical environmental yearbook of the Czech Republic 2008. Prague: Ministry of the Environment of the CR and Czech Statistical Office; 2008. pp. 86e120.
[2] Statistical environmental yearbook of the Czech Republic 2010. Prague: Ministry of the Environment of the CR and Czech Statistical Office; 2010. pp. 92e116. [3] Statistical environmental yearbook of the Czech Republic 2012. Prague: Ministry of the Environment of the CR and Czech Statistical Office; 2012. pp. 46e64. [4] Eurostat, the Statistical Office of the EU. Generation of waste by waste category. update 2013. http://epp.eurostat.ec. europa.eu/tgm/table.do?tab¼table&init¼1&language¼en& pcode¼ten00108&plugin¼1 and http://appsso.eurostat.ec. europa.eu/nui/show.do?dataset¼env_wasmun&lang¼en. [5] He M, Xiao B, Hu Z, Xiao B, Li J, Guo X, et al. Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): influence of catalyst and temperature on yield and product composition. Int J Hydrogen Energy 2009;34:195e203. [6] Kwak TH, Lee S, Maken S, Shin HC, Park JW, Yoo YD. A study of gasification of municipal solid waste using a double inverse diffusion flame burner. Energy Fuels 2005;19:2268e72. [7] He MY, Xiao B, Hu ZQ, Liu SM, Guo XJ, Luo SY. Syngas production from catalytic gasification of waste polyethylene: influence of temperature on gas yield and composition. Int J Hydrogen Energy 2009;34:1342e8. [8] Chiemchaisri C, Charnnok B, Visvanathan C. Recovery of plastic wastes from dumpsite as refuse-derived fuel and its utilization in small gasification system. Bioresour Technol 2010;101:1522e7. [9] Waste Manager. The proportion of the occurrence of the individual commodities in domestic waste. EKO-KOM, AS; March 5, 2010. http://www.odpadovyhospodar.cz/ str¼komunalniOdpad. [10] Hoornweg D, Bhada-Tata P. What a waste: a global review of solid waste management. Washington, D.C: Urban Development and Local Government Unit, World Bank; 2012. [11] Wilk V, Hofbauer H. Co-gasification of plastics and biomass in a dual fluidized-bed steam gasifier: possible interactions in fuels. Energy Fuels 2013;27:3261e73. [12] Sørum L, Grønli MG, Hustad JE. Pyrolysis characteristics and kinetics of municipal solid wastes. Fuel 2001;80:1217e27. [13] Luo S, Xiao B, Hu Z, Liu S. Effect of particle size on pyrolysis of single-component municipal solid waste in fixed bed reactor. Int J Hydrogen Energy 2010;35:93e7. a c ek P, Saf rova M, Vale [14] Sedla s J, Kusy´ J. Multi-component fuel based on coal and wastes. Proceedings of the international conference on coal science and technology; Okinawa, Japan, October 9e14, 2005; Session II Beneficiation (I), paper 2C03. [15] Straka P. Kinetics of Fe-catalyzed gasification of chars from lignite admixtures. Acta Geodyn Geomater 2011;8:79e88. z E. Plastic waste [16] Aznar MP, Caballero MA, Sancho JA, France elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant. Fuel Process Technol 2006;87:409e20. [17] Xiao R, Jin B, Zhou H, Zhong Z, Zhang M. Air gasification of polypropylene plastic waste in fluidized bed gasifier. Energy Convers Manage 2007;48:778e86. ski A, Stan czyk K, Pichlak M. Steam co[18] Howaniec N, Smolin gasification of coal and biomass derived chars with synergy effect as an innovative way of hydrogen-rich gas production. Int J Hydrogen Energy 2011;36:14455e63. [19] Wu Ch, Williams PT. Pyrolysis-gasification of post-consumer municipal solid plastic waste for hydrogen production. Int J Hydrogen Energy 2010;35:949e57. rez P, Aznar MP, Caballero MA, Gil J, Martín JA, Corella J. [20] Pe Hot gas cleaning and upgrading with a calcined dolomite located downstream a biomass fluidized bed gasifier
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
[21]
[22]
[23]
[24]
[25]
[26] [27]
operating with steam-oxygen mixtures. Energy Fuels 1997;11:1194e203. Corella J, Orío A, Toledo J-M. Biomass gasification with air in a fluidized bed: exhaustive tar elimination with commercial steam reforming catalysts. Energy Fuels 1999;13:702e9. Asadullah M, Ito S-i, Kunimori K, Tomishige K. Role of catalyst and its fluidization in the catalytic gasification of biomass to syngas at low temperature. Ind Eng Chem Res 2002;41:4567e75. Mastral FJ, Esperanza E, Berrueco C, Juste M, Ceamanos J. Fluidized bed thermal degradation products of HDPE in an inert atmosphere and in airenitrogen mixtures. J Anal Appl Pyrolysis 2003;70:1e17. RN, Tavares C, Dias M, Gulyurtlu I, Pinto F, Franco C, Andre et al. Effect of experimental conditions on co-gasification of coal, biomass and plastics wastes with air/steam mixture in a fluidized bed system. Fuel 2003;82:1967e76. RN, Miranda M, Gulyurtlu I, Pinto F, Franco C, Andre Cabrita I. Co-gasification study of biomass mixed with plastic wastes. Fuel 2002;81:291e7. Leung DYC, Wang CL. Fluidized-bed gasification of waste tire powders. Fuel Process Technol 2003;84:175e96. Kern SJ, Pfeifer Ch, Hofbauer H. Cogasification of polyethylene and lignite in a dual fluidized bed gasifier. Ind Eng Chem Res 2013;52:4360e71.
9
P, Punc ocha r M, Skoblja S, [28] Pohorely´ M, Vosecky´ M, Hejdova Staf M, et al. Gasification of coal and PET in fluidized bed reactor. Fuel 2006;85:2458e68. ko Z. Co-gasification of a lignite/waste-tyre [29] Straka P, Buc mixture in a moving bed. Fuel Process Technol 2009;90:1202e6. [30] Zhou H, Jing B, Zhong Z, Huang Y, Xiao R. Air and steam coal partial gasification in an atmospheric fluidized bed. Energy Fuels 2005;19:1619e23. ko Z. Co-gasification of rubber with V, Buc [31] Straka P, Kríz brown coal. Acta Geodyn Geomater 2008;151:329e34. ko Z. Alternative utilization of organic wastes e V, Buc [32] Kríz their co-processing with coal. In: Proceedings 10th conference on environment and mineral processing, part I, Juni 22e24, Ostrava; 2006. pp. 7e10. [33] Straka P. Study of the catalytic effect of magnetite in the gasification of chars from the lignite/rubber mixtures. In: Baker MD, editor. Gasification: chemistry, processes and applications. New York: Nova Science Publishers; 2012. pp. 315e27. rhegyi G, Faix O. Thermal decomposition of [34] Jakab E, Va polypropylene in the presence of wood-derived materials. J Anal Appl Pyrolysis 2000;56:273e84. [35] Xu C, Tsubouchi N, Hashimoto H, Ohtsuka Y. Catalytic decomposition of ammonia gas with metal cations present naturally in low rank coals. Fuel 2005;84:1957e67.
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
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Hydrogen-rich gas as a product of two-stage co-gasification of lignite/waste plastics mixtures kova Pavel Straka*, Olga Bica ch 41, Institute of Rock Structure and Mechanics, v.v.i., Academy of Sciences of the Czech Republic, V Holesovicka 18209 Prague 8, Czech Republic
article info
abstract
Article history:
Under atmospheric pressure, mixtures of lignite with waste plastics were gasified on a
Received 15 February 2014
laboratory scale. The resulting tar was cracked in a thermal cracking reactor. For experi-
Received in revised form
ments, low-ash and low-sulfur lignite was used; the percentage of waste plastics in the
8 May 2014
mixtures was 10 and 20 wt.%. The main product of co-gasification was hydrogen-rich gas,
Accepted 11 May 2014
as by-products, soot and non-gasified solid residue were obtained. It was found that the
Available online xxx
higher heating value of obtained gas is fully comparable with that of industrial gas from lignite gasification. Probably, at least 20 wt.% of lignite can be replaced with mixed waste
Keywords:
plastics in this process. The effect of waste plastics addition on properties of the obtained
Co-gasification
gas and of the non-gasified solid residue was evaluated and discussed.
Waste plastics
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Lignite Hydrogen-rich gas
Introduction Wastes: production and utilization The current state of the environment is immediately related to the quality of individual areas including mainly air, water management, agriculture and waste management. Due to the large production of wastes, a need arises to replace the existing technologies with low- or no-waste technologies. In the current state of waste production, it is necessary to find a way of the maximum utilization of wastes. Waste materials as alternative fuels are receiving increased attention. Waste production in the individual categories in the Czech Republic in the years 2002e2012 is evident from Fig. 1 [1e3]. With regard to municipal waste, the production is continually growing since 2007. Whereas in the Czech Republic in 2003e2008, 4.6e3.8 Mt of municipal waste were produced
annually, in 2009e2012 this number already amounted to 5.3e5.4 Mt [2,3]. The generated waste contained and still contains a large portion of materials that may be used for power and heat production or to obtain useful liquid, solid or gaseous fuels. Municipal waste containing a number of further utilizable components is currently predominantly deposited at landfills or biologically modified to form compounds and mixtures which can be landfilled after such a modification. A smaller proportion of the produced municipal waste is used as fuels in incineration plants or other means to generate energy; a portion of municipal wastes is used for recultivation or composting or is deposited as technological material at secured landfills (Table 1). A part of municipal waste can be sold as secondary raw materials [3]. Municipal waste production in EU countries considerably varies, as arises from [4], but the total is quite high; so, wastes
* Corresponding author. Tel.: þ420 266 009 402; fax: þ420 284 680 105. E-mail address: [email protected] (P. Straka). http://dx.doi.org/10.1016/j.ijhydene.2014.05.054 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
2
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Fig. 1 e The total waste production by category in the Czech Republic, 2002e2012.
must be processed. In this connection, pyrolysis of municipal solid waste (MSW) was studied in order to evaluate the influence of the process conditions: temperature, catalyst, the residence time of the gaseous phase in reactor, and grain size [5e8]. Since waste plastics are an important part of MSW, it is necessary to deal intensively with their treatment. It was found that household municipal waste contains ca 12% plastics [9] which may be utilized, but currently are mostly landfilled along with other municipal waste. Plastics currently represent 11% of MSW in OECD countries [10]. Various ways of their processing are suggested. Wilk and Hofbauer [11] studied co-gasification of plastics with wood biomass in fluidized bed steam gasifier. Authors found more product gas than expected and more CO and CO2 were measured in the gas than resulted from mono-gasification of materials. Contrary, the tar content in the product gas was considerably lower than presumed. Sørum et al. studied pyrolysis of waste plastics [12] and Luo et al. [13] monitored the influence of the grain size on pyrolysis performance with plastics, kitchen garbage, and wood as the three most frequent components of MSW. Other authors verified a possible utilization of waste plastics by pressing of them with paper and lignite dust into briquettes, which in the municipal sphere can replace sorted lignite [14]. In summary, pyrolysis has the inherent advantage of high flexibility with respect to plastics with contaminants; gasification has the promising features, such as high conversion efficiency,
Table 1 e Municipal waste utilization in the Czech Republic in 2007e2012. Waste management (%) 2007 2008 2009 2010 2011 2012 Landfills 86.2 89.9 64.0 59.5 55.4 53.7 (codes D1 þ D5 þ D12) Material recovery 21.1 24.2 22.7 24.3 30.8 30.3 municipal wastea Energy production (code R1) 9.8 9.6 6.0 8.9 10.8 11.8 Incineration (code D10) 0.07 0.05 0.04 0.04 0.04 0.04 a
codes R2eR12; N1, N2, N8, N10eN13, N15.
effective processing of low-grade fuels and wastes, moreover, the producer gas (the gas obtained) can be effectively utilized in a variety of ways ranging from electricity production to chemical industry. In our case, gasification and co-gasification were considered as advanced methods of the thermal treatment of plastic wastes. Gasification processes may work with or without catalyst [15e19] in a moving or fluid bed or in an entrained flow reactor. A problem is the removal of the formed tar entrained by the gas flow. The currently most promising method for production of purified gas is the catalytic cracking of produced tar in a secondary reactor. For this reason, the first gasification stage was complemented with secondary catalytic reactor with calcined dolomite [20] working at 800e900 C or with catalysts on nickel basis [21] working at 700e800 C, which catalytically purify the raw gas. The use of a Ni-catalyst makes it possible to convert ca 90e99% of the tar in the secondary reactor into gases. Although the results are promising, the entire process must be more elaborate, both economically and technically, and have only one-stage, if possible. Other experiments, on the one-stage fluidized bed reactor, showed that Rh/CeO2/SiO2 catalysts containing 35% of CeO2 have an excellent influence on the conversion of carbon into gas at low temperature when compared with non-catalytic or dolomitecatalyzed reactions at high temperature [22]. The conversion of carbon into gaseous compounds may be enhanced by introduction of small amount of oxygen from the bottom of the reactor or a sufficient fluidization velocity. Mastral et al. [23] compared the pyrolysis and gasification of high density polyethylene (HDPE) in a fluidized bed reactor. It was found that the working temperature affects product distribution and gas composition, and that higher temperature increases a gas production and simultaneously reduces a tar production in both pyrolysis and gasification. In the partial oxidation during HDPE gasification, a higher reactivity of HDPE was observed. The gas composition depends on gasification temperature; with increasing temperature, the content of carbon monoxide and methane markedly increase while the content of carbon dioxide decreases. Pinto et al. [24,25] studied the co-gasification of coal with waste polyethylene
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
and pinewood and of biomass/waste polyethylene mixture. The highest yield of gaseous products was achieved at 60 wt.% addition of waste polyethylene into the gasified charge. Xiao et al. [17] studied the effect of the equivalence ratio, static bed height, and fluidization velocity on the temperature of the bed during polypropylene gasification in a fluidized bed. They concluded that the equivalence ratio had the most significant effect on the reactor temperature and the gas production, and the products yield distribution. The selection of suitable equivalence ratio would depend on the final use of gas produced. A higher equivalence ratio reduced the content of tars and hydrocarbons, but also unfavorably affected the gas lower heating value. Similar conclusions concerning the relation between the equivalence ratio and the gas and tar yield were reported by Leung and Wang [26], who gasified pulverized waste rubber in a fluidized bed reactor. It seems that promising method of treatment of plastic wastes is their co-gasification with lignite [27,28] as the producer gas can be effectively utilized for electricity production as well as in chemical industry; moreover, fossil fuels can be saved by their partial replacement by waste plastics. In our case, low-ash and low-sulfur lignite from the Czech Rep. was used for co-gasification with waste plastics. In the Czech Republic, this lignite is commonly used for production of electrical power. Lignite is gasified in Lurgi generators and the produced gas utilized in IGCC. The treatment of waste tires through co-gasification with this lignite in a moving bed was studied on a commercial scale [29]. It was found that an addition of 20 wt.% of waste tires improved the lower heating value (LHV) of raw gas by 3% in comparison with the LHV of gas from gasification lignite alone. It was further discovered that an addition of waste tires reduced the H2S and CH3SH content in raw gas. Also co-gasification of mentioned lignite with polyethylene terephtalate (PET) in fluidized bed reactor was studied [28]. Authors found that (a) in the presence of PET the lignite particles are not significantly fragmented due to lower reactivity of lignite as compared with PET, which results in a higher bottom char formation than in single lignite gasification, (b) the tar content was approximately three times higher in co-gasification than in single lignite gasification, (c) the fluidized bed temperature exhibited considerable influence on the major components of producer gas. This article deals with possibility of conversion of mixed waste plastics containing low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), and polystyrene (PS) with lignite into hydrogen-rich gas through co-gasification. The aim of the work is to evaluate the effect of addition of mixed waste plastics on properties of producer gas and the nongasified solid residue.
Theory If syngas contains tar molecules, they have to be removed before syngas end-use, while tar can be physically removed (using filters and wet scrubbers) or chemically converted into gaseous compounds using thermal or thermal catalytic processes, which promote tar cracking reactions (eq. (6), see below) and endothermic reactions leading to the formation of H2 and CO, whose contents are expected to increase,
3
especially in the presence of waste plastics with high hydrogen and carbon contents. However, at temperatures up to 1000 C, tar chemical conversion releases also polycyclic aromatic hydrocarbons (PAHs) and soot. But at temperatures above 1000 C, notably at 1200 C, no PAHs were registered in resulting gas (using GC-MS), moreover, soot decreased. Due to this and the fact that practically useful catalysts are still in development, the variant with thermal cracking reactor working at 1200 C may be useful for waste plastic treatment and hydrogen-rich gas development. In our case, beside the main gasification rate controlling reactions C þ H2O(g) ¼ CO þ H2
C þ CO2 ¼ 2CO
DH ¼ 118.6 kJ/mol
DH ¼ 160.7 kJ/mol
(1)
(2)
and accompanied exothermic reactions, e.g. C þ 2H2 ¼ CH4
DH ¼ 87.5 kJ/mol
CO þ H2O(g) ¼ CO2 þ H2
CO þ 3H2 ¼ CH4 þ H2O
(3)
DH ¼ 42.3 kJ/mol
(4)
DH ¼ 205.9 kJ/mol
(5)
the cracking reactions in tar reformer took place. Three types of reactions were considered, (a) hydrocarbons decomposition, (b) cracking of oxygen-containing compounds and oxygen reactions, and (c) pyrogenetic water reactions. Formed higher aliphatic and cyclic hydrocarbons cracked to elemental carbon and hydrogen: CxHy ¼ xC þ 1/2yH2
(6)
Simultaneously, methane and hydrocarbons with short chains were formed: CxHy ¼ C1e4Hn þ Cx(1e4)H(yen)
(7)
Further, oxygen-containing compounds were cracked up to elemental oxygen, which easily exothermally reacted with carbon to carbon oxides: 2C þ O2 ¼ 2CO
C þ O2 ¼ CO2
DH ¼ 246.4 kJ/mol DH ¼ 406.4 kJ/mol
(8)
(9)
Carbon oxides were provided also by the reactions of pyrogenetic water with formed elemental carbon: C þ H2O(g) ¼ CO þ H2
C þ 2H2O(g) ¼ CO2 þ 4H2
DH ¼ 118.6 kJ/mol DH ¼ 16.2 kJ/mol
(10)
(11)
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
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At the temperature 1200 C, in the presence of carbon, a thermodynamic equilibrium between carbon oxides is established in favor of carbon monoxide as with increasing temperature the equilibrium of reaction C þ CO2 4 2CO shifts in favor of CO [30]. On the whole, the tar reformer can significantly enhance amounts of hydrogen and carbon monoxide in resulting producer gas.
determined by a standard-less X-ray analysis. The powder samples were fixed on a Cu-tape and then analyzed by the EDS Silicon Drift Detector Apollo X, EDAX Inc. In all the cases, the higher carbon content was found than expected, which was attributed to unburned carbon. Thus, the unburned carbon was then determined by subtracting ash from the non-gasified residue.
Experimental setup and gasification procedure
Experimental Materials Samples of low-ash and low-sulfur lignite from the Sokolov Lignite Basin, Czech Republic, were used for experiments. Based on previous experience [31] and taking into account the work [13] the lignite was ground to a grain size 1e4 mm. The samples of mixed waste plastics (WP) were obtained from MSW (source Transform AS, Czech Republic) by floating in water as a light fraction with density less than the density of water. The obtained fraction of WP contained LDPE, LLDPE, HDPE, PP, and PS, which was proved by DSC method (PerkineElmer Pyris DSC 7 analyzer, a nitrogen atmosphere, 10 C/ min, powdered sample) (Fig. 2). The separated fraction contained also wood and paper as a minority share. Regarding the work [13], the obtained WP fraction was homogenized and adjusted to the grain size 1e3 mm. The characteristics of the lignite and WP used are given in Table 2. Analyses were carried out according to standards ISO (687:2010, 562:2010, 1171:2010, 333:1996, 334:2013, 625:1996, 157:1996, 1928:2009). The X-ray diffractometry examination of crystallographic phases of non-gasified residues was performed on X'Pert PRO qeq powder diffractometer with para-focusing Bragg-Brentano geometry using CuKa radiation. An ultrafast X'Celerator detector was employed over the angular range of 7e70 (2q) with a step size of 0.017 (2q) and a counting time of 20.00 s step1. Data evaluation was performed using the software package High Score Plus V 2.2e PANalytical. Only the usual ash components were detected. Elemental composition was
The experiments were performed on a laboratory gasification unit preliminary tested in Ref. [32] (Fig. 3) under atmospheric pressure. A furnace with electrically heated vertical ceramic tube in a stainless-steel shell was used. A double-tube quartz reactor (Fig. 4) was placed in the furnace as a gasification reactor. The outer part of this reactor consists of a quartz tube (inner diameter 55 mm, length 480 mm) with sealed bottom, whereas the inner part is a quartz tube (inner diameter 40 mm, length 450 mm) with the bottom modified to a gasdistribution grate for the uniform distribution of gasification agents (H2O(g), CO2) to the gasifier bed. The gasification media are dosed into the area between the inner and outer parts of reactor allowing ensure both steam generation and the heating of gasification media to a temperature identical with temperature in the bed. The output for the developed raw gas in the upper part of reactor is immediately connected to the tar reformer inlet. In principle, the tar reformer is a horizontal quartz reactor (inner diameter 43 mm, length 740 mm) placed in an electro-resistance furnace with temperature 1200 C. In this horizontal cracking module (Fig. 3), the emerging liquid hydrocarbons, higher gaseous hydrocarbons, and partially also methane were thoroughly cracked to hydrogen and soot. The obtained gas was further conducted through a vertical cooler into a gas holder. The cooler was connected to an external circulation circuit with ethanol cooled to a temperature of 10 C. Having been cooled, the gas was collected in the gas holder with capacity 130 dm3, equipped with a circulation loop with a continuously operating infrared gas analyzer (Teledyne Analytical Instruments, Model 7500),
Fig. 2 e DSC curve of mixed waste plastics. 1 e LDPE (melting point 108 C), 2 e LLDPE (m.p. 124 C), 3 e HDPE (m.p. 130 C), 4 e PP (m.p. 164 C), 5 e PS (m.p. 245 C). a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
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Table 2 e The characteristics of the lignite and mixed waste plastics used (WP). Sample
Proximate analysis (wt %)
Lignite WP
Ultimate analysis (wt %, daf)
HHV
W
Ad
Vdaf
Sdt
C
H
N
So
Od
(MJ/kg)
12.1 0.4
7.8 2.7
51.9 98.9
0.51 0.10
76.3 82.3
5.6 12.3
0.9 0.3
0.6 0.1
16.6 5.0
30.2 42.2
W e water; Ad e ash (dry basis); Vdaf e volatile matter (dry ash-free basis); Sdt e total sulphur (dry basis); So e organic sulfur; Od e oxygen by difference; HHV e higher heating value.
through which the changes in the concentration of the generated gaseous components CO, CO2, and CH4 were monitored at sampling of signals 3 s. Hydrogen was determined by GC (see later) when sampling the gas from the gas holder was 5 min. Samples of lignite alone and the lignite/WP mixtures with 10 and 20 wt.% WP were gasified. The weight of gasified sample in all the cases was constant, 30 g. The sample placed in the inner part of double-tube quartz gasification reactor was heated at a rate 5 C/min from the ambient temperature to the final temperature 900 C; the soaking time at this temperature was 95 min. In the gasification reactor, pyrolysis occurs first, yielding gas, liquid, and gaseous products. The development of liquid products and the char formation were terminated at a temperature of 700 C, at which water was fed into the reactor by a dosing pump at a flow rate 30 cm3/h. Subsequently, from 750 C, carbon dioxide was dosed at a flow rate 5 dm3/h. The volume flow rates of water and CO2 were chosen based of experience with co-gasification of lignite/ rubber mixtures under atmospheric pressure [31]. At 900 C a steady state was reached lasting for 95 min at the tar reformer temperature 1200 C. As a result, hydrogen-rich gas and the non-gasified solid residue (unburned carbon along with ash) were obtained. Different evolutions of gas during gasification of lignite alone and that with WP are shown in Fig. 5. Each
experiment was repeated 3 times. Standard deviations are shown in Tables 3 and 7. The ongoing gas-component concentrations, pressure, volume and temperature of gas in the gas holder, further, the sample and gasification media temperatures were monitored and recorded on a PC with sampling 3 s. After the end of the experiment, the total gas was analyzed by Agilent Technologies 6890N gas chromatographs with TCD and FID detectors. O2, N2, and CO were analyzed on an HP-MOLSIV capillary column (40 C) with helium as carrier gas (5 cm3/min) using TCD. Methane and other hydrocarbons were determined on a GS-Gaspro capillary column (60 C) with nitrogen as carrier gas (20 cm3/min) using FID (air 400 cm3/min, H2 30 cm3/min, N2 20 cm3/min); carbon dioxide on the GS-Gaspro column (40 C) with helium as carrier gas (5 cm3/min) using TCD; and hydrogen on an HP-5 capillary column (40 C) with nitrogen as carrier gas (7 cm3/min) using TCD. Each determination was repeated 9 times (3 determinations for each of the three experiments). Standard deviations for the gaseous components determination are mentioned further. The amounts of ash and unburned carbon as non-gasified solid residues, soot in tar reformer, and water, both formed (pyrogenetic) and reacted (consumed), were assessed as well.
Results and discussion Continuous analyzer of gas components
The main product of the process was hydrogen-rich energetic gas, by-products were ash-containing non-gasified residue (remained in the gasification reactor) and a less significant amount of soot (remained in the tar reformer). The losses Cooler H2O + CO2 dosing
Raw gas output H2O + CO2 dosing
plynojem Gas holder
PC
Cracking module
Thermocouples
Flask Temperature controller
Outer part Gasified sample
Furnace with a double-tube quartz reactor
Stream of products Signals
Inner part Gas-distribution grate
Fig. 3 e A scheme of a laboratory gasification unit. A flask is calibrated.
Fig. 4 e A double-tube quartz reactor.
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
80 10%
70
0%
60
Sample
3
Total volume (dm )
Table 4 e Composition of the non-gasified residue (WPmixed waste plastics, L-lignite).
2
1
20%
W (wt %)
Ad (wt %)
UCd (wt %)
2.08 2.56 2.60
53.32 38.96 37.60
46.68 61.04 62.40
40
L (100%) L þ 10% WP L þ 20% WP
30
W e water; Ad e ash (dry basis); UCd e unburned carbon (dry basis).
50
3
20 10 0 0
50
100
150
200
250
300
Time (min)
Fig. 5 e Evolution of producer gas during the gasification of lignite alone (line 1, black), with 10% WP (line 2, green), and with 20% WP (line 3, red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
were 4e6%. The basic products of gasification experiments and the reacted water are shown in Table 3. The values are related to the initial weight of the gasified charge (to 1 g the gasified mixture). It arises from Table 3 that the amount of the consumed water decreased with increasing amount of plastic admixtures, contrary, an amount of non-gasified residue increased. Probably, this phenomenon was caused by the lower rate of gasification of formed lump chars from the lignite/plastics mixtures in comparison with the rate of gasification of formed char from lignite alone. A similar phenomenon was observed at powdered chars from lignite/ plastics mixtures and lignite alone at 800 C [33]. The same holds true for the rate of gasification with CO2. The lower steam and CO2 reactivities meant also less gasified carbon, so that the content of unburned carbon in resulting non-gasified residue was higher in the case of plastic admixtures in comparison with that of lignite alone (Table 4). Composition of producer gas was given by development of single gas-components. Evolution of majority gases, H2 and CO, is shown in Fig. 6; evolution of minority gases, CO2 and CH4, is pictured in Fig. 7. From Fig. 6 it is clear that the evolution of H2 was practically independent on the addition of WP. Contrary, the CO evolution was influenced by WP, as, from about 100 min, the CO concentration in the gas from lignite/ WP mixtures was always lower than that from lignite alone. From Fig. 7 it follows that also development CO2 was influenced by WP. The reason is that present lignocellulosic material starts to decompose already around 200 C (in our case ~40 min) and the decomposition continues until 500 C [34]
(~100 min), but lignite alone decomposes from about 400 C (~80 min). Due to this, CO2 resulting from lignite/WP mixtures with lignocelluloses evolved from ~40 min, but CO2 resulting from lignite alone developed from ~80 min. At ~100 min the lignocelluloses were completely decomposed, therefore the development CO2 from lignite/WP mixtures decreased (Fig. 7, lines 2, 3). Contrary, the development CO2 from lignite alone continued (Fig. 7, line 1) until gasification CO2 dosage in ~150 min. No significant changes in the evolution of CH4 due to WP additions were observed (Fig. 7, lines 4, 5, 6). To demonstrate an effect of tar reformer on the gas composition, GC analysis of gas was taken before and after cracking in the reformer. The results obtained are shown in Table 5 (before) and Table 6 (after). A comparison of the gas concentrations shows that, after cracking, the CH4 concentration decreased and concentration of CO2 even significantly decreased. On the contrary, the concentration of CO increased significantly. The H2 concentration also increased while a significant increase was observed at a 20% WP in gasified mixture, from 51.87 to 55.73 vol.%. No ethane, C3 and C4 hydrocarbons were registered after cracking. On the whole, the tar reformer improved the gas composition. It was demonstrated that the concentration of H2 increases with increasing portion of WP in the gasified mixture. The data presented in Tables 5 and 6 are reliable because in Table 5 the standard deviations (n ¼ 9) were as follows: CH4 0.01e0.03, CO 0.04e0.13, CO2 0.06e0.45, H2 0.07e0.38; in Table 6 the standard deviations (n ¼ 9) were as follows: CH4 0.01e0.08, CO 0.38e0.63, CO2 0.35e0.38, H2 0.43e0.97. Table 6 shows the composition of the gas obtained with a hydrogen content of 55e56 vol.%. Analogously, Pinto et al. [25] discovered that during co-gasification of biomass with 40% polyethylene at 885 C the hydrogen content increased up to 52 vol.%. But simultaneously, at this temperature, the hydrocarbons content decreased, which led to the reduction of the gas higher heating value (HHV). Therefore, HHV together with the gas density as important utility property were calculated and compared in order to describe an influence of waste plastics on these properties. Table 7 shows HHV, density, and the relative density of the obtained gas. (Relative density is the ratio of the density of the gas generated and the air density at temperature and under pressure considered (25 C, 100 kPa)).
Table 3 e The basic products and reacted (consumed) water of gasification experiments (WP-mixed plastics, L-lignite) (g/g charge). Sample L (100%) L þ 10% WP L þ 20% WP
Reacted water
Hydrogen-rich gas (g/g charge)
Soot
Non-gasified residue
0.87 ± 0.02 0.83 ± 0.02 0.80 ± 0.04
1.79 ± 0.03 1.71 ± 0.03 1.69 ± 0.01
0.07 ± 0.02 0.09 ± 0.02 0.09 ± 0.01
0.15 ± 0.01 0.16 ± 0.02 0.18 ± 0.01
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
7
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60
0%
50
H2
3
Table 5 e The composition of the gas obtained (vol.%) from lignite alone (L) and with additions of mixed plastics (WP) before cracking in the module.
2
10%
1
20%
vol %
40
30
CO
4
5
6
20
0 50
100
150
200
250
300
Time (min)
Fig. 6 e Evolution of H2 and CO during the gasification of lignite alone, with 10% WP, and with 20% WP. Lines 1 (black), 2 (green), and 3 (red) belong the H2 evolutions from lignite alone, lignite with 10% WP, and lignite with 20% WP, respectively. Lines 4 (black), 5 (green), and 6 (red) belong the CO evolution from lignite alone, lignite with 10% WP, and lignite with 20% WP, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
It arises from Table 6 that the obtained gas consists of mainly three components, with the majority in all three cases being comprised of hydrogen, followed by carbon monoxide and carbon dioxide, while H2 and CO have a dominant influence on HHV (11 MJ/m3). The mentioned components form the 18
CO2
0%
16
10 %
14
20%
vol %
12 3
10
1
8 2
6
6
4
4
CH4
5
2 0 0
50
100
150
200
CH4
C2H4
SC2eC*4
N2,f
CO
CO2
H2
L (100%) L þ 10% WP L þ 20% WP
3.14 3.33 3.39
0.06 0.16 0.29
0.24 0.63 1.16
0.60 0.50 0.75
19.54 16.62 15.55
21.44 25.54 26.99
54.99 53.22 51.87
f
10
0
Sample
250
300
Time (min)
Fig. 7 e Evolution of CO2 and CH4 during the gasification of lignite alone, with 10% WP, and with 20% WP. Lines 1 (black), 2 (green), and 3 (red) belong the CO2 evolutions from lignite alone, lignite with 10% WP, and lignite with 20% WP, respectively. Lines 4 (black), 5 (green), and 6 (red) belong the CH4 evolutions from lignite alone, lignite with 10% WP, and lignite with 20% WP, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
e formed; SC2-C*4 e ethane, propane, propene, C4 hydrocarbons.
basis of the hydrogen-rich gas for its further application. The gas further contains a small amount of methane as a residue after its imperfect cracking or as a fission product of cracking of higher hydrocarbons in the tar reformer. The amount of ethylene, which is of the same origin as the methane, is negligible. Higher gaseous hydrocarbons were not found in the mixture. A small amount of N2,f could be formed by a decomposition of ammonia released from lignite [35] or by decomposition of secondary aromatic amines present in WP as antioxidants. On the whole, the evaluation of the experiments has shown that an addition of mixed waste plastics up to 20 wt.% does not have much effect on the composition of the gas obtained and on its key properties. As the gas exhibits stable properties, it is possible to consider partially replace lignite with mixed waste plastics when it is gasified under described conditions. Besides the evaluation of the gas composition, it is worth noting the evaluation of the total volume of the gas obtained and the individual components (at 25 C, 101.325 kPa). Based on the data in Table 6 and measurement of the total volume of gas it was found that the total gas volume was 74.92 Ndm3 in the case of lignite alone, and 71.70 and 70.82 Ndm3 for 10 and 20% WP portions, respectively. It is evident that with increasing amount of WP the total volume of producer gas slightly decreases, which was caused mainly by the reduced volumes of hydrogen (from 41.34 Ndm3 for lignite alone to 39.47 Ndm3 for 20% WP portion) and carbon monoxide (from 19.22 Ndm3 for lignite alone to 17.15 Ndm3 for 20% WP portion). The volume of carbon dioxide was considered as constant (12.13e11.88 Ndm3), similarly for methane (1.45e1.18 Ndm3). It arises from Table 2 that WP contain a higher amount of C and H than the lignite used. Therefore, it would be possible to expect an increase in the amount of hydrogen in the gas obtained from the mixture with WP. The observed opposite trend seems to be caused by a lower rate of the reaction of chars from the lignite/WP mixtures with the gasification media in comparison with the char from lignite alone. The study of chars performed by Raman spectroscopy
Table 6 e The composition of the gas obtained (vol.%) from lignite alone (L) and with additions of mixed plastics (WP) after cracking in the module. Sample
CH4
C2H4
N2,f
CO
CO2
H2
L (100%) L þ 10% WP L þ 20% WP
1.93 1.64 1.97
0.08 0.05 0.06
0.96 0.99 1.13
25.66 25.13 24.21
16.19 16.57 16.90
55.18 55.62 55.73
f
e formed.
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
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Table 7 e HHV, density, and the relative density (WPmixed waste plastics, L-lignite). Sample L (100%) L þ 10% WP L þ 20% WP
HHV (MJ/m3)
Density (kg/m3)
Relative density ()
11.110 ± 0.075 10.964 ± 0.025 11.000 ± 0.022
0.717 ± 0.005 0.717 ± 0.017 0.716 ± 0.007
0.555 ± 0.004 0.554 ± 0.013 0.554 ± 0.008
[33] indicates that the ratio of disordered carbon (D) to ordered (graphite) carbon (G) is higher in the case of char from lignite alone (D/G 3.4) in comparison with that of chars from the lignite/plastics mixtures (D/G 3.0). It can be expected that the disordered structures will be more reactive than the ordered. Therefore, it is suggested that the steam and CO2 reactivity of char from lignite alone is higher as compared with that of chars from the lignite/WP mixtures. Consequently, the main gasification reactions (1) and (2) proceed more slowly with these chars and amounts of formed H2 and CO are somewhat lower. This conclusion is in agreement with the increase in the amount of unburned carbon found with the increasing content of WP in the mixture with lignite (Table 4). A similar reduction of gasification rate was recorded in the lignite/rubber mixtures [31]. It seems that the co-gasification of lignite with mixed waste plastics and the subsequent cracking of volatile products provides the gas, whose HHV 11 MJ/m3 is fully comparable with HHV of above-mentioned energetic gas from the Lurgi generators (HHV 10.7e11.3 MJ/m3). Therefore, probably at least 20 wt.% of lignite can be replaced with mixed waste plastics.
Conclusions Mixed waste plastics may be processed by their co-gasification with lignite to obtain energetic gas with high hydrogen content. In this process, at least 20 wt.% of lignite can be replaced with mixed waste plastics. This substitution does not have much effect on the composition, properties and amount of the energetic gas obtained. The higher heating value of the gas is fully comparable with that of industrial gas from lignite gasification in Lurgi generators.
Acknowledgments This work has been supported by the Research Program for 2012e2017 of Institute of Rock Structure and Mechanics, v.v.i., Academy of Sciences of the Czech Republic, #USMH-130/135/ 2011.
references
[1] Statistical environmental yearbook of the Czech Republic 2008. Prague: Ministry of the Environment of the CR and Czech Statistical Office; 2008. pp. 86e120.
[2] Statistical environmental yearbook of the Czech Republic 2010. Prague: Ministry of the Environment of the CR and Czech Statistical Office; 2010. pp. 92e116. [3] Statistical environmental yearbook of the Czech Republic 2012. Prague: Ministry of the Environment of the CR and Czech Statistical Office; 2012. pp. 46e64. [4] Eurostat, the Statistical Office of the EU. Generation of waste by waste category. update 2013. http://epp.eurostat.ec. europa.eu/tgm/table.do?tab¼table&init¼1&language¼en& pcode¼ten00108&plugin¼1 and http://appsso.eurostat.ec. europa.eu/nui/show.do?dataset¼env_wasmun&lang¼en. [5] He M, Xiao B, Hu Z, Xiao B, Li J, Guo X, et al. Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): influence of catalyst and temperature on yield and product composition. Int J Hydrogen Energy 2009;34:195e203. [6] Kwak TH, Lee S, Maken S, Shin HC, Park JW, Yoo YD. A study of gasification of municipal solid waste using a double inverse diffusion flame burner. Energy Fuels 2005;19:2268e72. [7] He MY, Xiao B, Hu ZQ, Liu SM, Guo XJ, Luo SY. Syngas production from catalytic gasification of waste polyethylene: influence of temperature on gas yield and composition. Int J Hydrogen Energy 2009;34:1342e8. [8] Chiemchaisri C, Charnnok B, Visvanathan C. Recovery of plastic wastes from dumpsite as refuse-derived fuel and its utilization in small gasification system. Bioresour Technol 2010;101:1522e7. [9] Waste Manager. The proportion of the occurrence of the individual commodities in domestic waste. EKO-KOM, AS; March 5, 2010. http://www.odpadovyhospodar.cz/ str¼komunalniOdpad. [10] Hoornweg D, Bhada-Tata P. What a waste: a global review of solid waste management. Washington, D.C: Urban Development and Local Government Unit, World Bank; 2012. [11] Wilk V, Hofbauer H. Co-gasification of plastics and biomass in a dual fluidized-bed steam gasifier: possible interactions in fuels. Energy Fuels 2013;27:3261e73. [12] Sørum L, Grønli MG, Hustad JE. Pyrolysis characteristics and kinetics of municipal solid wastes. Fuel 2001;80:1217e27. [13] Luo S, Xiao B, Hu Z, Liu S. Effect of particle size on pyrolysis of single-component municipal solid waste in fixed bed reactor. Int J Hydrogen Energy 2010;35:93e7. a c ek P, Saf rova M, Vale [14] Sedla s J, Kusy´ J. Multi-component fuel based on coal and wastes. Proceedings of the international conference on coal science and technology; Okinawa, Japan, October 9e14, 2005; Session II Beneficiation (I), paper 2C03. [15] Straka P. Kinetics of Fe-catalyzed gasification of chars from lignite admixtures. Acta Geodyn Geomater 2011;8:79e88. z E. Plastic waste [16] Aznar MP, Caballero MA, Sancho JA, France elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant. Fuel Process Technol 2006;87:409e20. [17] Xiao R, Jin B, Zhou H, Zhong Z, Zhang M. Air gasification of polypropylene plastic waste in fluidized bed gasifier. Energy Convers Manage 2007;48:778e86. ski A, Stan czyk K, Pichlak M. Steam co[18] Howaniec N, Smolin gasification of coal and biomass derived chars with synergy effect as an innovative way of hydrogen-rich gas production. Int J Hydrogen Energy 2011;36:14455e63. [19] Wu Ch, Williams PT. Pyrolysis-gasification of post-consumer municipal solid plastic waste for hydrogen production. Int J Hydrogen Energy 2010;35:949e57. rez P, Aznar MP, Caballero MA, Gil J, Martín JA, Corella J. [20] Pe Hot gas cleaning and upgrading with a calcined dolomite located downstream a biomass fluidized bed gasifier
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
[21]
[22]
[23]
[24]
[25]
[26] [27]
operating with steam-oxygen mixtures. Energy Fuels 1997;11:1194e203. Corella J, Orío A, Toledo J-M. Biomass gasification with air in a fluidized bed: exhaustive tar elimination with commercial steam reforming catalysts. Energy Fuels 1999;13:702e9. Asadullah M, Ito S-i, Kunimori K, Tomishige K. Role of catalyst and its fluidization in the catalytic gasification of biomass to syngas at low temperature. Ind Eng Chem Res 2002;41:4567e75. Mastral FJ, Esperanza E, Berrueco C, Juste M, Ceamanos J. Fluidized bed thermal degradation products of HDPE in an inert atmosphere and in airenitrogen mixtures. J Anal Appl Pyrolysis 2003;70:1e17. RN, Tavares C, Dias M, Gulyurtlu I, Pinto F, Franco C, Andre et al. Effect of experimental conditions on co-gasification of coal, biomass and plastics wastes with air/steam mixture in a fluidized bed system. Fuel 2003;82:1967e76. RN, Miranda M, Gulyurtlu I, Pinto F, Franco C, Andre Cabrita I. Co-gasification study of biomass mixed with plastic wastes. Fuel 2002;81:291e7. Leung DYC, Wang CL. Fluidized-bed gasification of waste tire powders. Fuel Process Technol 2003;84:175e96. Kern SJ, Pfeifer Ch, Hofbauer H. Cogasification of polyethylene and lignite in a dual fluidized bed gasifier. Ind Eng Chem Res 2013;52:4360e71.
9
P, Punc ocha r M, Skoblja S, [28] Pohorely´ M, Vosecky´ M, Hejdova Staf M, et al. Gasification of coal and PET in fluidized bed reactor. Fuel 2006;85:2458e68. ko Z. Co-gasification of a lignite/waste-tyre [29] Straka P, Buc mixture in a moving bed. Fuel Process Technol 2009;90:1202e6. [30] Zhou H, Jing B, Zhong Z, Huang Y, Xiao R. Air and steam coal partial gasification in an atmospheric fluidized bed. Energy Fuels 2005;19:1619e23. ko Z. Co-gasification of rubber with V, Buc [31] Straka P, Kríz brown coal. Acta Geodyn Geomater 2008;151:329e34. ko Z. Alternative utilization of organic wastes e V, Buc [32] Kríz their co-processing with coal. In: Proceedings 10th conference on environment and mineral processing, part I, Juni 22e24, Ostrava; 2006. pp. 7e10. [33] Straka P. Study of the catalytic effect of magnetite in the gasification of chars from the lignite/rubber mixtures. In: Baker MD, editor. Gasification: chemistry, processes and applications. New York: Nova Science Publishers; 2012. pp. 315e27. rhegyi G, Faix O. Thermal decomposition of [34] Jakab E, Va polypropylene in the presence of wood-derived materials. J Anal Appl Pyrolysis 2000;56:273e84. [35] Xu C, Tsubouchi N, Hashimoto H, Ohtsuka Y. Catalytic decomposition of ammonia gas with metal cations present naturally in low rank coals. Fuel 2005;84:1957e67.
a kova O, Hydrogen-rich gas as a product of two-stage co-gasification of lignite/ Please cite this article in press as: Straka P, Bic waste plastics mixtures, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.05.054