Pyrolysis of waste-derived fuel mixtures containing PVC

Fuel 81 (2002) 507±510

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Short Communication

Pyrolysis of waste-derived fuel mixtures containing PVC q Ron Zevenhoven a,*, Ernst Petter Axelsen b, Mikko Hupa c a

Laboratory for Energy Engineering and Environmental Protection, Helsinki University of Technology, P.O. Box 4400, FIN-02015 Espoo, Finland b Scancem International ANS, P.O. Box 73, N-3951 Brevik, Norway c Process Chemistry Group, AÊbo Akademi University, c/o LemminkaÈisenkatu 14-18 B, FIN-20520 Turku, Finland Received 24 February 2001; accepted 18 September 2001; available online 13 November 2001

Abstract This paper describes an experimental analysis of the pyrolysis of PVC and mixtures of PVC with wood (Finnish pine) and LDPE (low density polyethene) in nitrogen at 250±400 8C. The aim is to optimise the temperature range for producing low-chlorine or chlorine-free fuel in a dehydrochlorination reactor without pyrolysing any of the other combustible fractions. Results are presented for various process temperatures for PVC, PVC/wood and PVC/LDPE mixtures. It was found that the PVC tested is dehydrochlorinated at approximately 350 8C, and that secondary pyrolysis is suppressed when LDPE is present. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: PVC; Pyrolysis; Waste-derived fuels

1. Introduction Mass burning of solid waste as well as waste-to-energy processes with more emphasis on energy recovery are limited by too large fractions of chlorine-containing compounds. Problems are related mainly to corrosion, which enforces low steam parameters resulting in low power output. In Finland a classi®cation for recovered fuels (so-called `REF') is suggested which for chlorine de®nes Classes I±III with less than 0.15 wt% Cl, less than 0.5 wt% Cl, and less than 1.5 wt% Cl, respectively. At the same time, 90% of the chlorine found in the dry fraction of Finnish household waste (which does not contain paper, metal, glass or biowaste) is related to PVC [1]. Thus for Class I REF the mass fraction of PVC is limited to 0.25 wt%, enforcing dilution of a high-PVC waste with chlorine-free fractions. This makes PVC-containing wastes uninteresting from a wasteto-energy perspective, at a time when land ®lling is increasingly restricted. Fortunately, PVC behaves different from other commodity plastics. At temperatures in the range 200±400 8C PVC decomposes into HCl and a coke-like residue [2±6]. This residue may be burnt at higher temperatures without any chlorine-related limitations to ®ring temperature. This principle is used in a process (being developed at Helsinki * Corresponding author. E-mail address: ron.zevenhoven@hut.® (R. Zevenhoven). q Published ®rst on the web via Fuel®rst.comÐhttp://www.fuel®rst.com

University of Technology) that combines the combustion of high-PVC solid wastes with energy recovery and recovery of HCl gas. In this paper, experimental work supporting this development work is described. The results were applied to process optimisation using the software code PROSIM (used earlier to optimise, e.g. the biomass IGCC process at VaÈrnamo, Sweden), aiming at maximum heat recovery and thermal ef®ciency [7,8]. In short, the two-stage process involves the following chemistry: at low temperature : PVC 1 energy E1 ! HCl 1 hydrocarbon residue

…R1†

at high temperature : hydrocarbon residue 1 air ! energy E2 1 CO2 1 H2 O

…R2†

Besides moisture vaporisation, other components in the waste mixture are to remain unchanged during process (R1), to be combusted with the char residue from PVC at higher temperature. The process based on this principle is composed of two ¯uidised bed reactors plus a heat recovery system, see Fig. 1. Dehydrochlorination of the fuel takes places in a bed of hot sand, ¯uidised with nitrogen at 200±400 8C. Using an oxygen-free ¯uidisation gas blocks chemical routes to dioxines and furanes, for which the temperature level and the presence of catalysts such as (especially) copper, form the other prerequisites [9].

0016-2361/02/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0016-236 1(01)00168-5

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R. Zevenhoven et al. / Fuel 81 (2002) 507±510

PVC can be represented as ±(C2H3Cl)n ± (composed of units with molar mass 62.5 g/mole) gives the following energy balance for 1 kg PVC at standard conditions, where h 0 ˆ enthalpy at 298 K, 1 bar: h0PVC 1 E1 1 40h0O2 ˆ 16h0HCl 1 32h0CO2 1 16h0H2 O 1 HHV…residue†

Fig. 1. Simpli®ed process diagram for two-stage combustion of PVC with HCl recovery.

Chlorine is released as a gaseous mixture of HCl and moisture from the solid waste. One advantage of this process as compared to others dealing with waste containing PVC (e.g. Ref. [10]) is that it involves no hot HCl containing gases. The mixture of sand and chlorine-free waste-derived fuel is fed to a ¯uidised bed combustor operated at 700±900 8C. This heats up the sand and gives additional heat for steam and electricity generation. The sand is fed back to the ®rst reactor after heat exchange, reducing its temperature to that in the ®rst reactor. The temperature window in which a waste mixture containing PVC is decomposed into a chlorine-free fuel is the ®rst subject of this work. In a set of experiments PVC, PVC/wood and PVC/low density poly ethene (LDPE) mixtures were pyrolysed in nitrogen at temperatures in the range of 250±400 8C. The presence of carbonaceous decomposition products (CO, CxHy, tar, soot) in the pyrolysis off-gas was detected by oxidation with air (at 900 8C) followed by measurement of CO and CO2. HCl is known to hinder the oxidation of CO as a result of radical scavenging (e.g. Ref. [11]), therefore both CO and CO2 were measured. Information on morphological changes and possible melting behaviour were obtained from full-colour images of the solid pyrolysis residues. As to the second objective, for the thermal ef®ciency optimisation of the process, information is needed considering the thermodynamics of the ®rst conversion stage (R1), i.e. the value for E1 (unit: MJ/kg PVC). The (unknown) enthalpy of formation of the hydrocarbon residue from PVC decomposition can be eliminated by combining (R1) and (R2) and considering the lower or higher heating value (LHV or HHV) of the hydrocarbon residue. Noting that

…1†

(Residue HHV is used since liquid water is the combustion product at standard conditions). Using thermodynamic data for the species involved …h0PVC ˆ 12 784 J=mol-unit ˆ 204:54 kJ=kg [12], h0O2 ˆ 0 kJ=mole; h0HCl ˆ 292 300 J= mole; h0CO2 ˆ 2394 088 J=mole; h0H2 O ˆ 2242 174 J=mole† yields a relation between E1 and the LHV of the hydrocarbon residue: E1…MJ=kgPVC† ˆ 218:168 1 0:416LHV…MJ=kg residue† …2† noting that 1 kg of the PVC can yield 0.416 kg hydrocarbon residue (see Table 1). The LHV values were measured for residues from PVC dehydrochlorination in nitrogen. To some extent these were mixtures of PVC and hydrocarbon residue. These residues were also analysed for carbon, chlorine and hydrogen as to determine the degree of dehydroclorination in nitrogen with increasing temperature. 2. Experimental procedure Table 1 gives the composition of the PVC, LDPE and wood (Finnish pine) samples that were used in the tests. Ê bo Akademi The two test facilities used were located at A Ê University (AAU). The facility used for the gaseous release experiments with gas analysis was basically a vertical 1 in. quartz tube inside an electrically heated oven, allowing for temperatures up to 1100 8C. Reactor temperature was 250± 400 8C, pressure was 1 atm, the gas fed to the reactor was N2 at 100 l (STP)/h. Dried samples of approximately 100 mg PVC, PVC/wood or PVC/LDPE were entered into the gas stream at heating rates of the order of 1000 K/s. Pyrolysis product gases from the reactor were oxidised (at 900 8C) in a Ê AU. Gases were analysed converter reactor developed at A with respect to CO and CO2 using a Hartmann and Braun Uras 10E NDIR (non-dispersive infrared) analyser. Each measurement took approximately 10 min, all measurements were carried out three times. Pyrolysis residues for chemical analysis and determination of their LHV had to be prepared in larger amounts than used in the facility described above.

Table 1 Properties of the PVC, LDPE and wood (Finnish pine) tested Dry samples

C (wt%)

H (wt%)

Cl (wt%)

O (wt%)

N (wt%)

S (wt%)

Ash (wt%)

LHV (MJ/kg)

PVC LDPE Wood

40.1 85.7 48.9

5.1 14.3 6.0

53.8 ± ±

± 0.16 43.8

± ± 0.17

± ± 0.06

± ± 0.5

20.1 43.2 17.8

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less than 0.1%, i.e. the PVC has released approximately 99.5% of its chlorine. 3.2. Release of carbonaceous products during pyrolysis of PVC, PVC/LDPE, PVC/wood

The results of the chemical analysis of the residues from PVC pyrolysis in nitrogen are given in Fig. 2. The corresponding net calori®c value for these residues are given in Fig. 3, showing a trend that levels off at 38.2 MJ/kg. These ®gures show that already at 250 8C the decomposition of the PVC has started, showing a maximum rate at approximately 300 8C. At 350 8C the amount of Cl in the solid residue is

The pyrolysis experiments with PVC/wood and PVC/ LDPE mixtures in nitrogen at 250±400 8C show a slow pyrolysis of wood with release of carbon-containing gases. Considering kinetic data by Bockhorn et al. [4,5] this occurs at a rate of the same order as secondary decomposition of PVC, i.e. the further decomposition of the hydrocarbon residue from PVC dehydrochlorination into volatile hydrocarbons. The dehydrochlorination reaction itself is several orders of magnitude faster than that. The evidence that some CO and/or other carbon compounds are present in the product gas from the pyrolysis is given by the CO 1 CO2 measurements in the gas after product gas oxidation (at 900 8C), as shown in Fig. 4. The degree of PVC dehydrochlorination (%) versus temperature, calculated from the data given in Fig. 2 is included in the ®gure as well. It shows that combustible carbon compounds are released at temperatures above approximately 250 8C, where the PVC has released approximately 60% of its chlorine. At 350 8C, with more than 99.5% dehydrochlorination, the release of carbon compounds from the PVC by secondary pyrolysis is approximately 25%. This is presumably in the form of benzene [13]. For a mixture of PVC with wood the emission of combustible carbon compounds is slightly less but, more importantly, it levels off at approximately 25% of the fuel carbon. Most effective in retaining carbon in the fuel is adding LDPE to the PVC, it is shown that even at 400 8C the release of combustible carbon from LDPE pyrolysis and secondary PVC pyrolysis is less than 5%. These results con®rm earlier ®ndings by McGhee et al. [14] that the presence of chlorinated polymers tend to increase char yields during pyrolysis. Unfortunately, the results give no information on the in¯uence of wood or LDPE on the HCl release from PVC. Considering the two-stage process shown in Fig. 1, the gases that may be released with the HCl in the pyrolysis

Fig. 3. Net calori®c value (LHV) of the PVC dehydrochlorination residues (prepared in nitrogen, 20 min at temperature indicated, followed by cooling).

Fig. 4. Release of fuel-carbon versus pyrolysis temperature as determined by CO 1 CO2 measurement after product gas combustion. Also included is the dehydrochlorination ef®ciency for pure PVC.

Fig. 2. Chemical composition of PVC dehydrochlorination residues (prepared in nitrogen, 20 min at temperature indicated, followed by cooling).

A muf¯e furnace with an internal volume of approximately 30 l was used, after heating that to the desired temperatures under a continuous N2 purge, PVC samples of 10 g were introduced, in a ceramic container. After 20 min the heating power was switched off, allowing the pyrolysis residues to cool to room temperature in a nitrogen atmosphere. These chars were analysed for carbon, chlorine, hydrogen and LHV at an external laboratory. The visual analyses were made using a visible light microscope (Leica Wild MZ8, magni®cation 14.5±115 £ ) and photocamera (Leica Wild MPS 32) combination.

3. Results and discussion 3.1. Residues from PVC dehydrochlorination

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here (without signi®cant amounts of plasticiser) can be separated in HCl and chlorine-free char at pyrolysis temperatures of the order of 350 8C. HCl can be recovered as hydrochloric acid. Finally, it is noted that different types of PVC exist. Plasticisers, for example, were found to lower the temperature at which the PVC dehydrochlorination occurs [15], which is bene®cial for the process for highPVC solid waste treatment aimed here.

Acknowledgements

Fig. 5. Chars from PVC, PVC/LDPE and PVC/wood pyrolysis prepared in nitrogen, holding time. Sample mass approximately 150 mg, PVC fraction in mixtures approximately 20%. Grid scale is 1 mm.

product gas (e.g. CO, benzene, maybe, if the ®rst reactor is ¯uidised with air, dioxines/furanes) may be combusted in the second reactor after separating it from the hydrochloric acid by-product.

The work supported the project `Two-stage combustion of high-PVC solid wastes with HCl recovery' funded by Ekokem Oy (`apurahoitus' 1999). EPA was visiting at Ê bo Akademi University with full funds from Scancem A International ANS, Brevik, Norway. Mikael ForsseÂn from Ê bo Akademi University is acknowledged for technical A support. Helsinki University of Technology is gratefully acknowledged for ®nancing the PVC char analyses.

3.3. Visual analysis of the solid pyrolysis residues

References

How the morphology and composition of the PVCcontaining samples change during pyrolysis is shown in Fig. 5 for PVC, PVC/LDPE and PVC/wood mixtures, respectively, pyrolysed at 250, 340 and 400 8C. The ®gures show that the PVC has formed a black char residue already at 250 8C, looking the same as char formed at 400 8C. Co-pyrolysis with LDPE shows that the LDPE melts onto the PVC char, without showing any pyrolysis itself at temperatures up to 400 8C. Finally, the co-pyrolysis tests with wood show that at 250 8C nothing has happened yet with the wood while at 340 and 4008C clear signs of wood pyrolysis are seen. Altogether the images in Fig. 5 con®rm the measurements in Fig. 4.

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4. Conclusions The pyrolysis of PVC, PVC/LDPE and PVC/wood mixtures were analysed, aiming at optimising a temperature window where PVC-containing solid waste mixtures can be dehydrochlorinated without signi®cant pyrolysis of the other fuel components. It was found that the presence of wood or LDPE reduces the production of carbon-containing combustible gases at temperatures where the dehydrochlorination of the PVC approaches 100%. The PVC studied