Thermal processing of polyvinylchloride waste with oil shale ash to capture chloride.Introduction
Improving recycling and disposal methods of polyvinylchloride (PVC) waste to produce energy, fuel or raw material for petrochemical industry is of increasing interest as the PVC waste stream is expected to grow considerably in the foreseeable future . High chlorine content of this comparatively widespread plastic is a potential environmental problem, and its environmental impact should be minimized in the recycling process. Thermochemical conversion by pyrolysis, for example via stepwise pyrolysis, catalytic pyrolysis or co-pyrolysis including fuels such as coal, wood, petroleum residue or oil shale [2-8] is one class of possible recycling alternatives to recover the potential energy present in PVC waste streams. The distribution and fate of chlorine in the thermal processing of the PVC can be altered to more favorable endpoints with the additional presence of different catalytic and chemisorbing substances such as metal oxides or carbonate-rich materials [9-11]. The interest of this work is utilization of alkaline oil shale ash in the PVC pyrolysis process, as the presence of alkaline agents in the reaction mixture can facilitate the transfer of chlorine from the decomposing feedstock to the solid residue [12, 13]. This phenomenon in principle should favor PVC waste recycling via co-pyrolysis with carbonate-rich oil shales in solid heat carrier type industrial retorting units, where the feedstock is heated by mixing with ash from retorted feedstock combustion. There are many carbonate-rich oil shale deposits in the world resulting ash with considerable alkaline content, for example kukersite oil shale from Estonia [14, 15], El-Lajjun oil shale from Jordan  and Green River oil shale from USA .
This study was designed to investigate the potential for alkaline oil shale ash to capture chloride from PVC waste and oil shale co-pyrolysis as it can occur in the Galoter type retorting process. To simplify the study, the process was distributed into two sequential bench-scale steps: pyrolysis of PVC waste (70% PVC and 30% plasticizer) with oil shale ash in a Fischer retort followed by solid residue combustion in a muffle furnace. The effect of oil shale and therefore oil shale based carbonates on chloride emission is out of scope of this preliminary study.
Apparatus and materials
The plasticized PVC waste sample under study consisted of 70% PVC by weight and had chlorine content of 39.78% and ash residue 0.35%. Chlorine in this study is determined by standard methods .
The kukersite oil shale ash was produced by incinerating the oil shale with air for one hour in a muffle furnace at 900[degrees]C. Ash characteristics were as follows: chlorine content 0.16% and oxide content 99.67% (silicon dioxide, Si[O.sub.2] 23.68%; aluminium oxide, [Al.sub.2][O.sub.3] 5.79%; ferric oxide, Fe2[O.sub.3] 3.75%; calcium oxide, CaO 49.25%; magnesium oxide, MgO 8.43%; sodium oxide, [Na.sub.2]O 0.87%; titanium oxide, [TiO.sub.2] 0.28%; potassium oxide, [K.sub.2]O 3.27%; sulphur trioxide, S[O.sub.3] 4.35%).
PVC waste and kukersite oil shale ash co-pyrolysis experiments were carried out using a standard Fischer retort-based method . The solid residues of the co-pyrolysis were divided into three parts and combusted in a muffle furnace for one hour at different temperatures: 800, 830 and 860[degrees]C.
In the first step, the effect of alkaline ash on PVC pyrolysis was investigated by comparing retorting products and yields from PVC waste and PVC waste / ash mixture (29.9% PVC waste and 70.1% ash by weight) pyrolysis using the standard Fischer retort method. Tables 1 to 3 summarize the results.
Table 1 shows that the alkaline ash affects PVC waste pyrolysis by reducing the amount of oil produced (by 9.8% relative to the PVC waste or about 1.4 times), increasing the solid residue (by 34.3% or about 5 times), increasing the amount of pyrogenous water (by 12.3% or about 6.9 times) and decreasing the amount of gas produced (by 36.8% or about 3.5 times).
Table 2 presents comparative chlorine content of solid residues and oils. In addition oil densities and iodine numbers are shown as a rough measure of quality of oils.
The oil produced in the presence of ash is less dense, has higher iodine number, and contains less chlorine than the oil produced from PVC waste sample alone. The chlorine content of the solid residue is increased significantly. The effect of oil shale ash on decreasing the chlorine content of the oil could be limited based on the PVC/ash ratio shown above. The chlorine in oil was reduced only from 4.93% to 2.80%.
Using the above results one can track the fate of chlorine during PVC pyrolysis and co-pyrolysis with ash. Table 3 summarizes the chlorine distribution among the pyrolysis products for both PVC waste and mixture pyrolysis.
In the industrial Galoter process , the retorting or the thermal low-temperature decomposition of the feed-stock is performed via continuous mixing of feed-stock and solid heat carrier (combusted retorting solid residue) in the rotary drum reactor. The solid heat carrier is produced continuously by regenerating the solid residue of thermal processing (semicoke together with oil shale ash) via combustion in a heat regeneration furnace. Therefore, as a second step, the final fate of the chlorine from the co-pyrolysis of PVC with ash was tested by incineration of the co-pyrolysis residue in a muffle furnace at 800, 830 and 860[degrees]C. Table 4 presents the mass of solid residue remaining after incineration together with its chlorine content. Note that the solid residue from co-pyrolysis contains 13.33% chlorine (Table 2) which corresponds to 85.58% of total chlorine from the feedstock (Table 3).
These data indicate that 76.6-84.6% of the total amount of chlorine from the original PVC waste sample becomes bound in the incineration ash as Ca[Cl.sub.2] when co-pyrolysed with kukersite ash at about 30/70 PVC waste/ash ratio in a laboratory batch process. The fate of Ca[Cl.sub.2] in the combusted solid residue, that is to be removed from the Galoter process and deposited in an ash storage facility, depends on construction of latter and is out of scope of this study. It is only to bring out that the aqueous solubility of Ca[Cl.sub.2] is 42-43% at 20[degrees]C .
Thermal processing of PVC waste (containing 70% PVC) with oil shale ash (29.9% PVC waste and 70.1% ash) in a laboratory retort followed by the incineration of the solid retorting residue shows that from 76.6-84.6% of the total chlorine in PVC can be trapped through chemisorption by alkaline oil shale ash as inorganic compound Ca[Cl.sub.2]. As for the decreasing of the chlorine content in the retorting oil of PVC, the oil shale ash as a chemisorbent shows limited effect indicating that chlorine could be reduced only to a certain extent.
The financial support to this research was provided by the Estonian Science Foundation (Grants No. 4667 and 7222). The authors are very grateful to Dr D. S. Kellogg from PMUSA for her helpful advice and editorial comments. Authors also appreciate the contribution of Mr. David Schryer in preparation the manuscript.
Received February 22, 2008
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V. OJA *, A. ELENURM, I. ROHTLA, E. TEARO, E. TALI
Department of Chemical Engineering Tallinn University of Technology 5 Ehitajate Rd., 19086 Tallinn, Estonia
* Corresponding author: e-mail email@example.com
Table 1. Mass distribution of pyrolysis products, wt % Pyrolysis product PVC waste Mixture of PVC waste and oil shale ash total mixture PVC waste basis basis Pyrolysis oil 37.64 8.34 27.86 Solid residue 8.69 82.93 42.99 Pyrogenous water 2.09 4.31 14.40 Gas (by difference) 51.58 4.42 14.75 Table 2. Characterization of pyrolysis oils and residues Characteristic PVC waste Mixture of PVC waste and oil shale ash Cl content of residue, wt % * 2.35 13.33 Cl content of oil, wt % 4.93 2.80 Density of oil, g/cm3 1.0050 0.9220 Iodine number of oil, g [I.sub.2] 19.3 46.3 per 100 g oil * note that co-pyrolysis with ash produces 5 times more solid residue per PVC waste mass unit. Table 3. Distribution of chlorine in the pyrolysis products Pyrolysis product Weight percent of chlorine in: PVC waste Mixture of PVC waste and oil shale ash Pyrolysis oil 4.66 1.81 Solid residue 0.51 85.58 Gas and pyrolysis water 94.83 12.61 (by difference) Table 4. Influence of temperature on the incineration of pyrolysis residue from the 29.9% PVC / 70.1% ash mixture Temperature of incineration, Weight percent of: Percent of [degrees]C Chlorine in chlorine Ash retained the ash retained 800 92.80 14.22 99.00 830 92.21 13.55 93.73 860 91.85 13.00 89.58