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The pyrolysis of different polymer-based materials (polyethylene, polypropylene, polystyrene, polyamide, acrylonitril-butadiene-styren and scrap tyres) and low volatile brown coal and also co-pyrolysis of their blends (5 wt.% of the polymer addition) was investigated using the thermogravimetric analyser. The experiments indicated that all polymers were decomposed in the temperature range 220 - 586 ° C while temperature range requisite for coal degradation was broader (203 - 728 °C). Overlapping degradation temperature interval (200 - 600 °C) of polymers and coal was chosen for investigation of synergic effect between these materials. Addition of all polymers caused increasing of weight loss of coal about 5 wt.% and shifting of temperature of maximum pyrolysis rate to lower value for blends with scrap tyres, polyamide and acrylonitril-butadiene-styren. The kinetic study showed that addition of different polymers to low volatile brown coal had not distinct effect on the kinetic parameters, the values of activation energy for pyrolysis of low volatile brown coal and its blends were very conformable.
Keywords: pyrolysis, co-pyrolysis, low volatile coal, plastics
Production, consumption and consequently waste of polymer-based materials increase very sharply every year because these materials have very excellent properties (resistant, light, workable, etc.) and nowadays are irreplaceable for people life.
Annual consumption of plastic in west Europe is about 60 million tons (Lopéz et all., 2010). Company OZO Ostrava s.r.o., which deals with cartage and waste disposal from Ostrava city (the third biggest city in the Czech Republic) and surrounding villages, manipulated with 86,61 thousands tons of municipal solid waste (MSW) in the year 2009 and plastics formed 17 wt.% of MSW. In this context, there are five main plastics in MSW - polyethylene of high and low density (18 wt.%), PET, polystyrene, polypropylene (OZO Ostrava, 2009), also rubber and other kind plastics in lower quantity (RoubíÄek, 2002).
Upgrading of polymer-based material waste is a necessity both for environmental protection and for sustainable development however nowadays waste disposal and incineration (connected with a number of environmental problems, e.g. formation of dioxines) of polymer-based waste are the most widely used method.
Addition of polymer-based material to feedstock of natural organic materials thermal processes (co-processing), can be perspective way for their conversion into valuable products and reduction of its volume. Pyrolysis and co-pyrolysis is thermal degradation (without oxygen agent) led to produce a char, oil and gas, which have potential as useful end products. Co-pyrolysis of coal with organic waste is the oldest method from group of co-processing methods which leaded to (i) enhancement of gas amount and its gross calorific value, (ii) improvement of thermoplastic properties of coal charge and (iii) improvem,ent of thermomechanical and thermochemical properties of blast furnace cokes. Waste rubber and plastics seem to be very suitable materials for co-pyrolysis with low volatile coal, because these materials are a rich source of hydrocarbons and play a vital role during coal liquefaction. Co-pyrolysis allows possible chemical interaction between polymer-based materials and coal leading to some positive changes in the yield and quality of the final materials (RoubíÄek, 2002).
Dynamic thermogravimetry (with linear temperature increase) is method widely used to study the thermal degradation of different type of polymer-based materials and to evaluate basic kinetics parameters such as a rate constant (k), activation energy (E), reaction order (n) and pre-exponential factor (A)(Vallová, 2003).
The science studies have shown that mixed plastic wastes were used as a minor component in coal blends without any detriment to coke quality (Zhou et al., 2009). Vivero et al. (2005) (Cai et al., 2008) studied the thermal decomposition of blends of coal and plastic such as high density polyethylene (HDPE) and polypropylene (PP) using thermogravimetric method. It was shown that plastic wastes have strong influence on the thermoplastic properties of coal as well as the structure and thermal behaviour of the semicokes. Cai et al. (2008) studied thermal behavior of low volatile coal (LVC), plastic (HDPE, low density polyethylene (LDPE) and PP) and their blends in ration 95 wt. % of LVC and 5 wt.% of plastics. LVC was decomposed at lower temperature than plastics; temperature range of organic matter devolatilisation region is broader for LVC than for plastic and this region is more complex for blends than for each component. Sharypov et al. (2007) concluded, that synergistic effect of polyolefin addition is observed preferentially with low rank coals due to their higher contents in thermally unstable C-O bonds. Lignin derived radicals promote polyolefinic macromolecules degradation, leading to an increase of the amount of distillable liquid fraction. Lignin structure has aromatic units combined together with oxygen containing chemical bonds less thermally stable than C-C bonds of plastic. For this reason a similar effect can occur during co-pyrolysis of brown coal and plastic blends. Brown coal is characterized by a high concentration of oxygen-containing chemical bonds. A synergistic effect of waste rubber tyres and coal was observed at temperature 430 °C without or with molybdenum catalysts and greater total conversion to liquids was yielded (Orr et al., 1996). Two-stage pyrolysis of 75 wt.% coal and 15 wt.% rubber led to produce gas with high amount of hydrogen and carbon monoxide and to solid phase consisting mainly of carbon (KÅ™íÅ¾ et al., 2008). Ishaq et. al. (2006) studied co-pyrolysis of polystyrene (PS), LDPE, HDPE and PP with coal in presence of a solvent and catalysts. Among the studied plastics, HDPE and PP gave higher weight loss.
On the other hand, some articles report no effect or "asynergic" effect of plastics and coal. Sakurovs (2003) discovered that PP did not influenced fluidity of coal during co-pyrolysis whereas PS decrease and polyacrilonitrile increase fluidity of all coking coal above 420 °C. Domínquez et al. (2001) reported slight reduction in fluidity when plastics waste (LDPE, HDPE) was added to coal pyrolysis because plastic degradation products were hydrogen acceptors. PS was suggested as strong hydrogen acceptor because it produced ethylbenzene as a major pyrolysis product when it was blended with coal and pitch (Sakurovs, 2003). Co-pyrolysis of brown and black coal and ABS led to enhancement of liquid pyrolysis phase. It can be interpreted by higher amount of hydrogen and low amount of oxygen in copolymer ABS (RoubíÄek, 2002).
From mentioned above, co-pyrolytic behavior of low volatile brown coal, chosen polymers and their blends were investigated using thermogravimetric analysis to obtain an overall understanding of interaction between coal and plastics.
Mechanism of co-pyrolysis of coal and plastics
Chemical structure of coal changes during coal pyrolysis. With heating up to around 400 °C, coal starts to soften, weaker bonds are cleaves and simultaneously molecular fragmentation produces species of low molecular weight in the form of free radicals, which are hydrogen rich donor species. These species take place in the recapping of free radicals from thermal decomposition and convert them into stabilized hydrocarbon molecules. During coal pyrolysis, aromatic compounds with naphtenic rings have nonplanar structure which is converted into planar and more condensed structure while hydrogen release. Created polyaromatic hydrocarbons are precursor for the high-quality cokes (RoubíÄek, 2002).
During co-pyrolysis of polymers and coal, structure of polymers is broken down and also smaller intermediate groups are formed. Formation of such groups could modify the thermal behaviour of coal, especially in the temperature range between 400 and 500 °C, where coal exist in a plastic state (Ishaq et al., 2006). Thermal degradation of polyolefins proceeds via radical initiation, chain propagation and radical termination. Developed small hydrocarbon species might have taken part in the recapping of the radicals generated from the coal and would stabilize the primary products from coal thermal degradation. Mechanism of co-pyrolysis of coal and plastic is supposed as coal cross-linked network disturbance: R-R ® 2R· and plastics is taken part in the recapping of the radicals generated from the coal: Plastic-H + R· ® Plastic· + R-H (Zhou et al., 2009) and low molecular mass R-H keeps the system at an optimum fluidity level. Formation and stabilization of free radicals by hydrogen transfer is influenced by chemical composition of the raw materials (Ishaq et al., 2006) and by temperature. With increasing temperature increases amount of hydrogen deficient species, which takes away hydrogen form plastics.
Determination of apparent kinetic parameters
Method of direct non-linear regression (Slovák, 2001) was used for calculation of apparent kinetic parameters of pyrolysis processes from thermogravimetric (TG) curves.
The calculation is based on the kinetic equation (1).
If the time step is set small enough, the derivatives in Eq (1) may be replaced by differences. We assume that the TG curve is composed of very small linear segments of the length Dt, in which the reaction rate is constant:
where a is the degree of conversion, t - time (s), T - absolute temperature (K), R -molar gas constant (8.314 JK-1mol-1), A - frequency factor (s-1), E - activation energy (Jmol-1) and n is the reaction order.
Assuming the a0 and t0 at the beginning of the a vs. t curve, further points of the curve can be calculated from the recurrence relation
If the TG curve consists of p various processes with kinetic parameters Aj, Eaj and nj (j=1 to p), Eq. (3) can be used for calculation of extent of conversion for individual reactions. In this case, the equation describing the whole curve can express as a sum of particular equations:
Last equation enables determination of apparent kinetic parameters of multistep reactions by non-linear optimalization.
In our calculations we assumed, similarly to other authors (Vivero et al., 2005; Ishaq et al., 2006), that the plastic pyrolysis is the first-order reaction.
The feedstock materials used in this work included a low volatile brown coal (LVC), as well as low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), scrap tyres (ST), polyamide (PA), polystyrene (PS), copolymer acrylonitril - butadiene-styren (ABS) and their blends (LVC+LDPE, LVC+HDPE, LVC+PP, LVC+ST, LVC+PA, LVC+PS, LVC+ABS) with the addition of polymers of 5 wt.%.
We used shredder commercial granules (under 1mm) of pure LDPE, PA, PS and ABS and PP in form of fine powder made in company HP-TREND s.r.o. and shredder shampoo's and soap's bottles (under 1mm) as a HDPE samples. As rubber sample it was used shredder scrap tyres (under 1mm) from personal cars. As low volatile coal we used brown coal (under 0.18 mm) from area Mostecká uhelná a.s. The experiments were carried out with such a small particle size due to elimination of temperature profiles inside the sample. Coal and polymer-based materials blends were homogenized by mixing in appropriate proportion. Some characteristic of the used polymers and coal is given in Table 1.
The thermal analysis was carried out using simultaneous TG-DTA apparatus NETZSCH STA 409 EP. All the experiments were conducted under the identical conditions: the samples (101 mg in weight) were heated up to 1000 °C in the crucibles (aluminium oxide) in a dynamic inert atmosphere of argon (with the flow rate of 100 cm3 min-1) at the heating rate of 10 °C min-1.
Proximate analysis was carried out by thermogravimetric analyser LECO TGA 601.
Results and discussion
Some characteristics of used polymers and coal are given in Table 1. Generally, with increasing amount of volatiles in the sample increases total pyrolysis weight loss.
Table 1. Proximate analysis of plastics and coal and elemental analysis of coal
Fig. 1 shows TG curves of pyrolysis of pure polymers and coal. It can be seen that with increasing temperature decreases sample weight (increases loss of sample weight). TG pyrolysis curves of (i) LDPE, HDPE and PP and (ii) ABS and PS are almost identical; this indicates that they have the same pyrolysis behaviors due to similar chemical bonds in their molecular structure (Aboulkas et al., 2010). Compared to TG curves of polymer samples, TG curve of pure coal is not so sharp and is finished at lower weight loss due to the lowest amount of volatiles (36 wt.%) and the highest amount of ash and fixed carbon in coal. Weight loss of ST is also lower because of lower amount of volatiles, higher amount of fixed carbon and addition of additives in the ST structure used during manufacturing of tyres. Compared to weight loss of LDPE and PP, total weight loss of HDPE sample is lower due to some pigments and additives used for shampoo's and soap's bottles.
As for blends, TG curves approach to TG curve for pure LVC. It is probably clarify by low addition (5 wt.%) of polymers.
The weight losses show that degradation of plastics is almost totally one-step process which illustrates presence of one peak at DTG curves (Fig. 2). On the contrary, thermal degradation of rubber proceeds in two stages. First peak at temperature 387 °C corresponds to oils, plasticators and additives vaporization. Second peak at temperature 460 °C fits into rubber decomposition.
Fig. 1. TG curves of pyrolysis of coal and plastics a) LVC (1), LDPE (2), HDPE (3), PP (4), b) ST (5), ABS (6), PS (7), PA (8).
Fig. 2. DTG curves of pyrolysis of coal and plastics a) LVC (1), LDPE (2), HDPE (3), PP (4), b) ST (5), ABS (6), PS (7), PA (8).
The characteristic temperatures determined from DTG curves are documented in Table 2, including these parameters: temperature of (i) initial weight loss (TI), (ii) the end of the reaction (TF) and (iii) maximum pyrolysis rate (Tmax). Tmax is related to material structure and usually plastics with similar structure have almost the same Tmax (Cai et al., 2008).
Table 2. Characteristic temperatures and weight loss of sample pyrolysis
1 Temperature of initial weight loss
2Temperature of the end of the reaction
3Temperature of maximum pyrolysis rate
4Weight loss in temperature range 200-600°C
Compared to DTG peak latitude, narrower latitudes were observed for polymers than for LVC. It means that decomposition of LVC is accomplished in broader temperature range and thermal degradation of the studied coal starts at lower temperature (203 °C) and ends at higher temperature (728 °C) than plastics which corresponds with literatures (Vivero et al., 2005; Sharypov et al., 2007). Temperature range for plastic thermal decomposition varies from 220 to 586 °C and in this temperature range the highest effect of polymers addition in coal could be occurred (Solomon et al., 1993). For this reason, synergic effect between polymers and coal was evaluated for temperature range 200 - 600 °C. Weight losses of pure polymers, coal and their blends in this temperature range are documented in Table 2. Addition of all polymers results slight enhancement (about 5 wt.%) of coal weight loss. The highest enhancement of coal weight loss was detected after (i) PS addition which is in discordance with literature (Sakurovs, 2003) and after (ii) ABS addition. Increasing of weight loss after ABS addition is interpreted by higher amount of hydrogen and lower amount of oxygen in copolymer ABS (RoubíÄek, 2003).
Fig. 3. DTG curves of pyrolysis of coal (1) and blends LVC+LDPE (2), LVC+HDPE (3), LVC+PP (4), LVC+ST (5), LVC+ABS (6), LVC+PS (7), LVC+PA (8).
Fig. 3. illustrates effect of polymer addition to coal. Addition of all polymers causes no shifting of TI and TF. For this reason decomposition temperature interval during heating is identical as for pure coal pyrolysis. Addition of (i) LDPE and HDPE caused separation of peak to two slight distinct subperiods, (ii) PP shifted Tmax to higher temperature and (iii) ST, PA, and ABS shifted Tmax to lower temperature. Thermal behaviour of the polymers and coal differs from each other, as well as the chemical composition of the volatile matter released during the pyrolysis of polymers could influence the fluidity of the coal (Vivero et al., 2005).
Apparent kinetic parameters, activation energy, pre-exponential factor and reaction order of pyrolysis processes of coal and its mixtures were calculated from a single TG curves by direct non-linear regression (Table 3).
Table 3. Apparent kinetic parameters, activation energy and pre-exponential factor
*R2 correlation coefficient
Kinetics parameters of pyrolysis of coal and polymers published in literature varied in wide range due to different process conditions of thermogravimetric measurement and applying differential methods (integral, differentia, approximate or special) (Carraso, 1993) .
The calculation of kinetic parameters was performed with assumption, that coal and polymer pyrolysis is a single process, with exception of pyrolysis of scrap tyres which is not a single process, but it proceeds in two partially overlapped stages. It corresponds with other authors (KoreÅˆová, 2008).
It can be seen that activation energy for pyrolysis of pure polymers is much higher than for pure coal and slope of straight line in Arrhenius plot is steeper. Activation energies and pre-exponential factors of coal/polymer blends are resembled that suggested that pyrolysis mechanismus for blends is also similar.
Table 4. Intersection region of Arrhenius plots of coal and polymers
Differences in the kinetics of coal and polymer pyrolysis are illustrated by Arrhenius plots (Fig. 4). Compared to coal Arrhenius plot, polymer linear dependency of ln k vs. 1/T is shifted to higher values of rate constant at higher temperature. Above temperature of Arrhenius plot crossing (Table 4) rate constant of pyrolysis is higher for polymers than for coal (Fig. 4). Addition of polymers to coal did not distinctly change the slope of straight line (minimal changes in activation energies). Comparable values of activation energies for pyrolysis of coal and blends indicate similar reactivity at different temperatures. Slope of straight line is identity for ABS and PS (Fig. 4b).
Fig. 4. Arrhenius plots of coal and plastics a) LVC (1), LDPE (2), HDPE (3), PP (4), b) ST (5), ABS (6), PS (7), PA (8).
From the thermogravimetric measurement of low volatile brown coal, plastics (LDPE, HDPE, PP, ST, PA, PS and ABS) and their blends it can be concluded: all polymers except ST proved comparable behaviour during the heating in a dynamic inert atmosphere of argon. The different weight losses for pyrolysis of ST and LVC were appeared which is connected with lower content of volatiles. The pyrolysis process of polymers proceeded in temperature range 220 - 586 °C and temperature range of organic matter devolatilisation for LVC was much broader (203 -728 °C).
Addition of plastics caused (i) increasing of the total weight loss of coal about 5 wt.% and (ii) shifting of temperature Tmax to lower value for blends with ST, PA and ABS only. On the basis of these results, it could be concluded that interaction between polymers and coal occurred during the thermal treatment.
The apparent kinetic parameters were calculated using the method of direct non-linear regression. Activation energy of pyrolysis for polymers was much higher than for coal and their addition into coal had only slight influence on its value. Compared to value of rate constant, at higher temperature higher value was observed for polymers than for coal.
Acknowledgements. This work was supported by the Grant Agency of Czech Republic - grant project No P106/10/P267.
A frequency factor s-1
E activation energy Jmol-1
k rate constant s
R molar gas constant 8.314 JK-1mol-1
t time s
T absolute temperature K
wt. weight %
a degree of conversion
ABS acrylonitril - butadiene-styren
HDPE high density polyethylene
LDPE low density polyethylene
LVC low volatile coal
MSW municipal solid waste
n reaction order
PET polyethylene terephthalate
R· free radical
ST srap tyres